JP2008053760A - Nitride semiconductor laser element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride semiconductor element so constructed that an active layer having a nitride semiconductor including In is sandwiched between a p-type clad layer and an n-type clad layer, especially that threshold current is reduced in one emitting light over 440 nm wavelength. <P>SOLUTION: The nitride semiconductor element includes a first nitride semiconductor layer 31 composed of a nitride semiconductor including In between the n-type clad layer 25 and the active layer 12, and also includes a second nitride semiconductor layer 32 composed of a nitride semiconductor not including In between the p-type clad layer 30 and the active layer 12. The element is so constructed to have an unsymmetrical waveguide path, avoiding, by utilizing the second nitride semiconductor layer 32, crystallinity deterioration and loss of light with In by mounting the nitride semiconductor including In on a p-type layer 13 side, enlarging an index of refraction inside the waveguide path by the first nitride semiconductor layer 31, and reducing threshold current density. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention is used for light-emitting elements such as light-emitting diode elements (LEDs) and laser diode elements (LD), light-receiving elements such as superphotoluminescence diodes, solar cells, and photosensors, or electronic devices such as transistors and power devices. The present invention relates to a nitride semiconductor device using a nitride semiconductor (In _X Al _Y Ga _1-XY N, 0 ≦ X, 0 ≦ Y, X + Y ≦ 1), and in particular, a nitride semiconductor containing In having a light wavelength of 440 nm or more. The present invention relates to a nitride semiconductor device having a layer as an active layer, an edge emitting device having a waveguide structure sandwiched between optical confinement cladding layers, and a laser device.

Nowadays, semiconductor lasers using nitride semiconductors are increasingly demanded for use in optical disk systems capable of recording and reproducing information with a large capacity and high density, such as DVDs. For this reason, research on semiconductor laser elements using nitride semiconductors has been actively conducted. Semiconductor laser devices using nitride semiconductors are thought to be capable of oscillating in a wide range of visible light from the ultraviolet region to the red region, and their application range is not limited to the light source of the optical disk system described above. It is expected to be a wide variety of light sources such as optical networks. In addition, the applicant is 405
We announced a laser exceeding 10,000 hours under continuous oscillation conditions of nm, room temperature and 5 mW.

In addition, a light emitting element, a light receiving element, and the like using a nitride semiconductor have a structure in which an active layer is formed using a nitride semiconductor containing In. This is important in improving the characteristics.

In a nitride semiconductor laser element or a light emitting element, in order to obtain long wavelength light emission, the In mixed crystal ratio in the nitride semiconductor containing In in the active layer or the light emitting layer is
By changing the emission wavelength, the emission wavelength can be changed. In particular, when the In mixed crystal ratio is increased, the emission wavelength can be increased. In the edge emitting device and laser device,
When the active layer has a structure sandwiched between the upper and lower cladding layers, the refractive index of both cladding layers is reduced and the refractive index in the waveguide sandwiched between the upper and lower cladding layers is increased. Light is efficiently confined in the waveguide, and as a result, the laser element contributes to a decrease in threshold current density.

Conventionally, in a nitride semiconductor device having such a cladding layer, 440 nm
As a structure for obtaining light emission of the above long wavelength, for example, in a laser element, an SCH structure using InGaN for a guide layer and AlGaN for a cladding layer has been proposed.

However, as the long wave becomes longer, the refractive index difference between AlGaN and InGaN becomes smaller, that is, a loss due to light absorption occurs in the guide layer in the waveguide,
The threshold current increases. Further, when the upper clad layer is a p-type nitride semiconductor and the lower clad layer is an n-type nitride semiconductor, p-type InGaN is used for the upper guide layer. In this case, the device characteristics are adversely affected, and the crystallinity of the upper cladding layer of AlGaN formed thereon is also deteriorated, resulting in a problem of deterioration of the device characteristics.

In the present invention, a nitride semiconductor device having a structure in which an active layer is sandwiched between an upper clad layer and a lower clad layer. In the waveguide, it is necessary to suppress light absorption to a low level, efficiently confine light in the waveguide including the active layer, and form an element structure with better crystallinity.

The present invention has been made in view of the above circumstances, and provides a nitride semiconductor device having excellent device characteristics such as threshold current density and good crystallinity.

That is, the semiconductor device of the present invention can achieve the object of the present invention with the following configuration.

In a nitride semiconductor device having an active layer sandwiched between a p-type layer and an n-type layer, the p-type layer having a p-type cladding layer, and the n-type layer having an n-type cladding layer, the active layer The layer has a nitride semiconductor containing In, and has a first nitride semiconductor layer made of a nitride semiconductor having an In mixed crystal ratio z> 0 between the n-type cladding layer and the active layer, and p A second nitride semiconductor layer in which an In mixed crystal ratio u is z> u is provided between the mold cladding layer and the active layer. With this configuration, the n-type layer has an In mixed crystal ratio z (z> u).
The first nitride semiconductor layer having a large size is provided, and the second nitride semiconductor layer having a smaller In mixed crystal ratio u (including u = 0) than the first nitride semiconductor layer is provided in the p-type layer. In the waveguide structure described later, an asymmetric structure sandwiching an active layer in the waveguide is formed. As another invention, in a nitride semiconductor device having a structure in which an active layer is sandwiched between a p-type cladding layer and an n-type cladding layer, the active layer has a nitride semiconductor containing In, and the n-type cladding layer A first nitride semiconductor layer made of a nitride semiconductor containing In between the active layer and the active layer, and the In layer between the p-type cladding layer and the active layer
It has the 2nd nitride semiconductor layer whose mixed crystal ratio is 0, It is characterized by the above-mentioned. With this configuration, by using a nitride semiconductor (second nitride semiconductor layer) that does not contain In or has a small In mixed crystal ratio between the active layer and the p-type cladding layer, crystallinity is deteriorated. And a nitride semiconductor containing In (first layer) between the active layer and the n-type cladding layer.
Thus, an appropriate difference in refractive index can be provided between the waveguide sandwiched between both cladding layers and the cladding layer. In particular, in a nitride semiconductor device that emits light having a long wavelength of 440 nm or more in the active layer, a laser device having a low threshold current can be obtained, resulting in a nitride semiconductor device having excellent device characteristics. This is p-type and n-type
In order to guide light having a long wavelength of 440 nm or more with an appropriate spread in a waveguide sandwiched between mold-type cladding layers, an In mixed crystal ratio or less of an In-containing nitride semiconductor used for an active layer is used. It has been considered that a nitride semiconductor containing, for example, a light guide layer, which will be described later, is preferably provided in the waveguide. However, a nitride semiconductor containing In doped with Mg, which is a p-type impurity, has high crystallinity. This is because the device characteristics are deteriorated. That is, in the present invention, the waveguide sandwiched between the p-type cladding layer and the n-type cladding layer has the first nitride semiconductor layer and the second nitride semiconductor layer having different compositions and sandwiching the active layer. Thus, a nitride semiconductor device having an asymmetric waveguide structure and excellent device characteristics at a long wavelength can be obtained. In addition, the n-type and p-type clad layers are provided as optical confinement clad layers so as to form such a waveguide structure, and in devices that do not have waveguides, as carrier confinement layers. It can also be a functioned structure. As such a cladding layer, the band gap energy is made larger than that of the active layer, and when the active layer has a quantum well structure, the band gap energy is made larger than that of the well layer, and preferably, the band gap energy is larger than that of the barrier layer. Increase gap energy.

Furthermore, in the element structure in which an active layer is provided between the first nitride semiconductor layer in the n-type layer and the second nitride semiconductor layer in the p-type layer, the active layer is in the active layer. N-side barrier layer (2a) arranged closest to the n-type layer, p-side barrier layer (2c) arranged closest to the p-type layer, and n-side barrier layer ( 2a) and a p-side barrier layer (2b) having a well layer made of a nitride semiconductor containing at least one In, and the n-type impurity concentration of the p-side barrier layer (2c) (2a)
It is preferable that the n-type impurity concentration be lower. As will be described later, when the p-side barrier layer (2c) serves as a carrier injection port, and the n-type impurity is highly doped in the p-side barrier layer (2c), holes are injected into the active layer. Since the n-type impurity concentration is smaller than that of the n-side barrier layer (2a), the n-side, p-type
The function of the side barrier layer can be made different, and carrier injection can be made good. On the other hand, the n-side barrier layer (2a) can be structured to promote carrier injection from the n-type layer by making it higher than the n-type impurity concentration of the p-side barrier layer. The p-side barrier layer (
Since the n-type impurity concentration of 2c) is formed close to or in contact with the p-type layer, p-type impurity diffusion may occur. In this case, the p-side barrier layer (2c)
When n is doped with n-type impurities, a barrier layer having n-type and p-type impurities is formed, and therefore the carrier injection function of the p-side barrier layer (2c) tends to be lowered. for that reason,
In such a case, preferably, when the n-type impurity concentration of the p-side barrier layer (2c) is made smaller than the p-type impurity concentration, such functional deterioration can be avoided. In any of the p-side barrier layers, it is preferable that the n-type impurity concentration be low. Specifically, by setting the n-type impurity concentration to less than 5 × 10 ¹⁶ / cm ³ , the p-side barrier layer (2c ) Can be improved.

In the nitride semiconductor device of the present invention, the p-type layer is made of a nitride semiconductor containing Al between the active layer and the second nitride semiconductor layer, or between the active layer and the p-type cladding layer. It is preferable to have a p-side electron confinement layer. This p-side electron confinement layer functions as a layer for confining carriers from the n-type layer in the active layer. When the p-type clad layer is an optical confinement layer, the p-side electron confinement layer is more active than the p-type clad layer. The p-side electron confinement layer arranged nearby functions mainly as a carrier confinement, and the cladding layer functions mainly as an optical confinement, and can be used for an edge emitting device and a laser device. Further, in an element in which the p-type cladding layer does not need to function as optical confinement, for example, a light-emitting element, the p-type cladding layer and the p-side carrier confinement layer have a carrier confinement structure. As the p-side electron confinement layer, the band gap energy is made larger than that of the active layer as in the cladding layer. In the active layer having the quantum well structure, the band gap energy is made larger than that of the well layer. However, it is preferable to increase the band gap energy. When the p-type cladding layer is an optical confinement layer, the barrier can be increased by increasing the band gap energy as compared with the p-type cladding layer, as shown in the examples, and efficient electron confinement is possible. On the other hand, when the Al mixed crystal ratio increases, the resistance value of the p-side electron confinement layer described later tends to increase. In such a case, the Al mixed crystal ratio and the band gap energy are This is preferable because it can be made smaller than the p-type cladding layer to suppress the heat generation by the high resistance layer and to enhance the function of the active layer.

Further, the position of the p-side electron confinement layer is preferably provided in contact with the active layer or through the buffer layer, whereby a structure with an enhanced electron confinement function can be obtained. For the buffer layer, as will be described later, a large piezoelectric field due to the nitride semiconductor containing Al and a nitride semiconductor further containing Al are provided near the nitride semiconductor containing In used for the active layer and the well layer. It is formed so as to suppress an adverse effect on the active layer due to internal stress caused by and to obtain crystallinity suitable as a base layer during growth. As a specific composition of the buffer layer, as described later, G
It is preferable to use aN or a nitride semiconductor containing Al whose Al mixed crystal ratio is smaller than that of the p-side electron confinement layer. In addition, it has been shown that the adverse effect of such a p-side electron confinement layer on the active layer, particularly the well layer, can be avoided by increasing the distance between the two, but the buffer layer also has a p-side barrier layer (2c). Similarly to the above, it is possible to provide such a function as a spacer. That is, the active layer has a well layer (1b) provided between the n-side barrier layer (2a) and the p-side barrier layer (2b) closest to the p-side electron confinement layer, With a configuration in which the distance between the well layer (1b) and the p-side barrier layer is 100 mm or more, a device having excellent device characteristics can be obtained. What determines the distance between the well layer (1b) and the p-side barrier layer is determined by the layer interposed between them, specifically, the p-side barrier layer, the active layer, and the p-side electron. It is a buffer layer interposed between the confinement layers, and the device characteristics can be improved by adjusting the film thickness of these layers. The upper limit of this distance is set to 400 mm or less as will be described later.
Further, when the p-side barrier layer (2c) is made of a nitride semiconductor containing In, similarly to the first nitride semiconductor layer, the refractive index in the vicinity of the waveguide, particularly in the active layer, is increased, and light confinement is achieved. By increasing the difference in refractive index between the cladding layer and the cladding layer, it is possible to form an element structure excellent in a long wavelength laser element and an edge emitting element. The difference between the second nitride semiconductor layer and the layer interposed between the p-side electron confinement layer and the well layer (1b) such as the buffer layer and the p-side barrier layer (2c) In the vicinity of the p-side electron confinement layer provided near the active layer, a pn junction is formed, so that a buffer layer and a p-side barrier layer are disposed closer to the active layer than the pn junction. (2c) is I on the p-type layer side.
This is because the adverse effect of providing a nitride semiconductor containing n tends to be avoided.

The n-side barrier layer (2a) described above may have a structure in which the n-side barrier layer (2a) and / or the p-side barrier layer (2c) are arranged on the outermost side in the active layer.
The function of the p-side barrier layer (2c) can be improved, which is preferable.

The p-type cladding layer and the n-type cladding layer have a nitride semiconductor containing Al. With this configuration, it is possible to provide a large refractive index difference between the waveguide sandwiched between both cladding layers and each cladding layer, and a waveguide structure excellent in light guiding is formed. A nitride semiconductor device excellent in the above can be obtained. here,
Preferably, as a nitride semiconductor containing Al, a nitride semiconductor having an In mixed crystal ratio of 0 and containing no In is excellent in crystallinity and can provide a larger refractive index difference. Further, Al _x Ga ₁ It is more preferable to use a nitride semiconductor represented by _-xN (0 <x ≦ 1).

The active layer has a quantum well structure having a well layer made of a nitride semiconductor containing In, and the In mixed crystal ratio of the first nitride semiconductor layer is smaller than the In mixed crystal ratio of the well layer, To do. With this configuration, the active layer having a quantum well structure promotes luminescence recombination, lowers the threshold current, improves the output, and improves the device characteristics as compared with the case where the quantum well structure is not used. An element is obtained. In addition, by making the In mixed crystal ratio of the nitride semiconductor used for the first nitride semiconductor layer smaller than that of the well layer, the band gap energy difference with the well layer can be increased and carrier injection can be improved. This leads to improved device characteristics. In addition, since In absorbs and scatters light in the waveguide of light, the first nitride semiconductor layer of the nitride semiconductor having a low In mixed crystal ratio and the second layer containing no In are included. By using the nitride semiconductor layer, a nitride semiconductor element in which the loss of light is suppressed and the threshold current and the drive current are reduced is obtained. Here, even when the active layer does not have a quantum well structure, the same effect can be obtained by making the In mixed crystal ratio of the first nitride semiconductor layer smaller than the In mixed crystal ratio of the nitride semiconductor used for the active layer. can get.

The first nitride semiconductor layer is provided in contact with the active layer. With this configuration, as shown in FIGS. 5 and 6, a band gap energy difference is gradually reduced from the n-type cladding layer and connected to the active layer, and carriers are efficiently injected into the active layer and the well layer. A nitride semiconductor device having excellent device characteristics can be obtained. Further, as described above, since light loss due to In occurs, the light distribution in the waveguide overlaps the active layer by providing the first nitride semiconductor layer in contact with the active layer. It is considered that the structure has a large peak and a large amount of light is distributed in the active layer, and functions as a layer substantially similar to the active layer, and the loss of light in the waveguide can be kept low. . On the other hand, as shown in FIGS. 8 and 10, the In mixed crystal ratio z of the first nitride semiconductor layer is made substantially the same as or smaller than the In mixed crystal ratio v of the n-side barrier layer (2a). In other words, by satisfying z ≦ v, the In mixed crystal ratio can be made larger than that of the n-side barrier layer, whereby a structure in which the refractive index of the waveguide is increased can be obtained.
In this case, the carrier confinement from the p-type layer is mainly performed by the n-side barrier layer (2b). However, in the nitride semiconductor in which the hole diffusion length is smaller than the electrons, the p-side electron confinement layer Thus, it is possible to confine carriers in the active layer without providing a large barrier. That is, in the carrier confinement from the p-type layer, the simplification is realized by minimizing the structure so that the n-side barrier layer (2a) having a large bandgap energy takes on the first structure. Since the nitride semiconductor layer has a large In mixed crystal ratio, it is possible to increase the refractive index of the active layer and the central portion of the waveguide and to form an element structure excellent in light guiding.

A p-type light guide layer made of the second nitride semiconductor layer between the p-type cladding layer and the active layer, and a p-type made of a nitride semiconductor containing Al between the p-type light guide layer and the active layer. It has a side electron confinement layer. With this configuration, the waveguide has a p-type light guide layer composed of a p-side electron confinement layer and a second nitride semiconductor layer between the active layer and the p-type cladding layer, thereby providing a crystalline property. Deterioration and light loss due to In are suppressed, and device characteristics are improved. At this time, by having a p-side electron confinement layer,
Electrons are efficiently confined in the active layer and the device characteristics are improved, and in order to operate in this manner, it is necessary to use a nitride semiconductor containing Al for the p-side electron confinement layer, In order to suppress the loss of light, it is preferable to use a nitride semiconductor that does not contain In, and more preferably, Al _z Ga _1-z N (0 <z ≦ 1).
) Is used.

An n-type light guide layer made of a nitride semiconductor having an In mixed crystal ratio of 0 is provided between the active layer and the first nitride semiconductor layer. With this configuration, since the n-type light guide layer does not contain In in the waveguide, it becomes a guide layer without loss of light, and the first nitride semiconductor layer is refracted between the waveguide and the cladding layer. It functions as a layer that increases the rate difference, and the device characteristics are improved.

The film thickness of the first nitride semiconductor layer is 300 mm or more. With this configuration, the effect of increasing the refractive index in the waveguide by having the above-described first nitride semiconductor layer is increased, the refractive index difference from the cladding layer can be increased, and the device characteristics are improved.

The active layer has an n-side barrier layer as a layer closest to the n-type layer, and a sum of film thicknesses of the n-side barrier layer and the first nitride semiconductor layer is 300 mm or more. And

As another embodiment of the present invention, the p-type cladding layer and the n-type cladding layer are optically confined cladding layers, and at least one of the p-type cladding layer and the n-type cladding layer is A multilayer clad layer in which a first layer having a nitride semiconductor containing at least Al and a second layer having a band gap energy different from that of the first layer is alternately stacked is used as a light confinement layer. That is. When the clad layer is formed of a multilayer film, a plurality of nitride semiconductors having different compositions are stacked. Specifically, a plurality of nitride semiconductors having different Al composition ratios are stacked. Thus, when it forms with a multilayer film, it becomes possible to suppress the deterioration of crystallinity and generation | occurrence | production of a crack in the case of a single film. Specifically, as the multilayer film, a first layer and a second layer having a different composition are stacked, and a plurality of layers having different refractive indexes and band gap energies are provided. For example, a multilayer film having a structure in which a first layer having an Al composition ratio x1 and a second layer having an Al composition ratio x2 (x1 ≠ x2) may be stacked. At this time, the Al composition ratio is x1> x2 (0 ≦ x2
X1 ≦ 1), the first layer having a large Al composition ratio reduces the refractive index, increases the band gap energy, and the second layer having a small Al composition ratio forms the first layer. Deterioration of crystallinity due to this can be suppressed. Also, the first layer and the second layer are laminated, and the fifth layer having a composition different from that of the second layer is laminated.
Further, a plurality of layers having different compositions may be stacked. Alternatively, a structure in which a plurality of first layers and second layers are alternately stacked may be employed, and a structure in which a plurality of pairs each having at least a first layer and a second layer are formed may be employed. In such a multilayer film structure, it is possible to suppress the deterioration of crystallinity of the nitride semiconductor containing Al and increase the film thickness, so that it is possible to obtain a film thickness that is important in light confinement.

It is preferable that a clad layer having a multilayer structure has a superlattice structure so that the crystallinity can be further improved and the clad layer can be formed. Here, the superlattice structure is provided on at least a part of the cladding layer, and preferably the superlattice structure is provided on all of the cladding layers, whereby the cladding layer can be formed with good crystallinity. At this time, as the superlattice structure, as in the case of the light guide layer, at least the first layer and the second layer are alternately stacked, or at least the first layer and the second layer are stacked. Having a pair,
A structure in which a plurality of pairs are provided. As the film thickness of each layer constituting the superlattice structure, the film thickness varies depending on the composition and the combination of each layer. Specifically, it is 10 nm or less, preferably 7.5 nm or less. The crystallinity can be kept good, and more preferably 5 nm or less, whereby better crystallinity can be obtained. At this time, at least one of the first and second layers is in the above-mentioned film thickness range, and preferably both film thicknesses are in the above-mentioned film thickness range, whereby the formation of a clad layer with a thick film has good crystallinity.

The cladding layer is preferably doped with impurities of at least each conductivity type,
Similar to the light guide layer, it may be entirely doped or partially doped. Also,
Also in the case of a multilayer film, similarly to the light guide layer, for example, a multilayer film having the first layer and the second layer may be doped, or both the first layer and the second layer may be doped. The modulation dope may be different from each other or may be doped on one side and undoped on the other side. For example, the first layer / second layer may be Al _x1 Ga _1-x1 N (0 <x1 ≦
1) / Al _x2 Ga _1-x2 N (0 ≦ x2 ≦ 1, x1> x2) in a superlattice multilayer film structure, the second layer having a small Al composition ratio is doped with impurities, By making the layer undoped, the crystallinity can be improved similarly to the light guide layer.

Although it does not specifically limit as a film thickness of a clad layer, 10 nm or more and 2 micrometers or less, 5
It is formed in the range of 0 nm to 1 μm. This makes it possible to confine carriers when the thickness is 10 nm or more, and suppresses deterioration of crystallinity when the thickness is 2 μm or less, and further enables light confinement when the thickness is 50 nm or more. The clad layer can be formed with good crystallinity when the thickness is 1 μm or less.

Furthermore, in addition to the optical confinement cladding layer, an optical guide layer is provided between at least one of the p-type cladding layer and the n-type cladding layer and the active layer. The third layer having a nitride semiconductor containing, and the third layer may have a structure provided with a multilayer light guide layer in which fourth layers having different band gap energies are alternately stacked. As for the composition of the multilayer light guide layer, it is preferable to use a multilayer film having a superlattice structure as in the multilayer clad layer. Specifically, a plurality of nitride semiconductors having different In composition ratios are stacked. When the multilayer film is formed as described above, as shown in FIG. 10, the first layer disposed in the vicinity of the active layer is disposed on the n-type layer side.
The nitride semiconductor layer and the multilayer light guide layer disposed on the clad layer side can be provided, and the structure can reduce the refractive index of the waveguide, and is made of a nitride semiconductor containing a single film of In. It is possible to suppress the deterioration of crystallinity when the light guide layer is provided. Specifically, as the multilayer film, a sixth layer and a fourth layer having a different composition are stacked, and a plurality of layers having different refractive indexes and band gap energies are provided. For example, a multilayer film having a structure in which a third layer having an In composition ratio y1 and a fourth layer having an In composition ratio y2 (y1 ≠ y2) may be stacked. At this time, the In composition ratio may be y1> y2 (
When the configuration is 0 ≦ y2, y1 ≦ 1), the third layer having a large In composition ratio has a large refractive index, the band gap energy is increased, and the fourth layer having a small In composition ratio has a third Deterioration of crystallinity due to the formation of the layer can be suppressed. Further, a plurality of layers having different compositions may be stacked by stacking a third layer and a fourth layer, and stacking a sixth layer having a composition different from that of the fourth layer. Further, a structure in which a plurality of third layers and fourth layers are alternately stacked may be employed, and a structure in which a plurality of pairs each including at least the third layer and the fourth layer are formed may be employed. In such a multilayer film structure,
It is possible to suppress the deterioration of crystallinity of the nitride semiconductor containing In and increase the refractive index of the waveguide, and to increase the refractive index difference from the cladding layer.

In a light guide layer having a multilayer structure, a superlattice structure is preferable because the light guide layer can be formed with better crystallinity. Here, the superlattice structure is provided in at least a part of the light guide layer, and preferably, the light guide layer can be formed with good crystallinity by providing the superlattice structure in all. At this time, as the superlattice structure, as in the case of the cladding layer, at least a third layer and a fourth layer are alternately stacked, or at least a third layer and a fourth layer are provided. Pair
A structure in which a plurality of pairs are provided. As the film thickness of each layer constituting the superlattice structure, the film thickness varies depending on the composition and the combination of each layer. Specifically, it is 10 nm or less, preferably 7.5 nm or less. The crystallinity can be kept good, and more preferably 5 nm or less, whereby better crystallinity can be obtained. At this time, at least one of the first and second layers is in the above-mentioned film thickness range, and preferably both film thicknesses are in the above-mentioned film thickness range, so that the light guide layer is formed with good crystallinity.

The cladding layer is preferably doped with impurities of at least each conductivity type,
Similar to the light guide layer, it may be entirely doped or partially doped. Also,
Also in the case of a multilayer film, similarly to the light guide layer, for example, the multilayer film having the third layer and the fourth layer may be doped, or the third layer and the fourth layer may be doped. The modulation dope may be different from each other or may be doped on one side and undoped on the other side. For example, the third layer / fourth layer may be In _y1 Ga _1-y1 N (0 <y1 ≦
1) / In _y2 Ga _1-y2 N (0 ≦ y2 ≦ 1, y1> y2) superlattice multilayer film structure, the fourth layer having a small In composition ratio is doped with impurities, By making the layer undoped, the crystallinity can be improved similarly to the cladding layer.

The film thickness of the light guide layer is not particularly limited, but is 10 nm.
When the thickness is 1 μm or less, preferably 50 nm or more and 500 nm or less, an excellent waveguide structure is formed in the structure in which the first and second nitride semiconductor layers are combined. More preferably, by setting the thickness to 100 nm or more and 300 nm or less, a suitable optical waveguide is formed in a configuration in which the first nitride semiconductor layer, the second nitride semiconductor layer, and the light guide layer are used in combination. Thus, the light is confined and the threshold current can be reduced.

The n-type layer has a light guide layer, and the first layer is interposed between the light guide layer and the active layer of the n-type layer.
By having this nitride semiconductor layer, a good waveguide structure is formed.
The p-type layer has a light guide layer, and the light guide layer of the p-type layer has the second nitride semiconductor layer, thereby suppressing deterioration of crystallinity due to the nitride semiconductor containing In. A structure in which the refractive index of the waveguide is increased can be obtained.

With the nitride semiconductor device of the present invention, a laser device with a low threshold current and an edge emitting device with excellent device characteristics can be obtained in the long wavelength region. In particular, as shown in FIG. 9, compared to the structure of the reference example using the nitride semiconductor containing In as the guide layer, the structure in the waveguide is asymmetrical as in the present invention, so that the wavelength of 440 nm is reduced. A nitride semiconductor device having excellent device characteristics in a long wavelength region can be obtained.

Examples of the nitride semiconductor used in the nitride semiconductor device of the present invention include GaN, AlN,
Alternatively, InN or a gallium nitride compound semiconductor (In _x
Al _y Ga _1-xy N, 0 ≦ x, 0 ≦ y, x + y ≦ 1). In addition, a mixed crystal in which a part of the gallium nitride compound semiconductor is replaced with B or P may be used. Further, a nitride semiconductor containing In used for an active layer, a well layer, a barrier layer, etc., specifically,
_{In x Al y Ga 1-xy} N (0 <x, 0 ≦ y, x + y ≦ 1) is to use a nitride semiconductor represented by. As a nitride semiconductor containing Al, specifically, I
_{n x Al y Ga 1-xy} N (0 ≦ x, 0 <y, x + y ≦ 1) is to use a nitride semiconductor represented by.

(Active layer)
The active layer in the present invention has a nitride semiconductor containing at least In,
In particular, it emits light having a wavelength of 440 nm or more. Here, the composition of the nitride semiconductor containing In is not particularly limited, but is preferably In _x Ga _1-x N (0
It is to use a nitride semiconductor represented by <x ≦ 1). At this time, the nitride semiconductor containing In may be any of non-doped, n-type impurity doped, and p-type impurity doped. Preferably, the active layer is a non-doped or undoped nitride semiconductor containing In. By providing it inside, nitride semiconductor devices such as laser devices and light emitting devices can achieve high output. When the active layer has a quantum well structure, the nitride semiconductor containing In is used for at least the well layer. Here, the quantum well structure may be either a multiple quantum well structure or a single quantum well structure. Preferably, by using a multiple quantum well structure, the output is improved,
It is possible to reduce the oscillation threshold. As the quantum well structure of the active layer,
A layer in which a well layer and a barrier layer described later are stacked can be used. At this time, when the quantum well structure is used, the number of well layers is preferably 1 or more and 4 or less, so that, for example, in a laser element, the threshold current can be lowered, and more preferably, the number of well layers By using a multi-quantum well structure with 2 or 3, high-power laser elements and light-emitting elements tend to be obtained.

Further, in the multiple quantum well structure, the barrier layer sandwiched between the well layers is not limited to a single layer (well layer / barrier layer / well layer), and two or more barrier layers. May be provided with a plurality of barrier layers having different compositions and impurity amounts, such as “well layer / barrier layer (1) / barrier layer (2) / barrier layer (3) /.. ./Well layer”. . For example, a structure in which an upper barrier layer made of a nitride semiconductor containing Al and a lower barrier layer having an energy band gap smaller than that of the upper barrier layer is provided on the well layer. Specifically, by providing an upper barrier layer made of a nitride semiconductor containing Al disposed on the well layer, in the well layer, In segregation, In-plane distribution of In concentration is induced, quantum dots, Since the quantum wire effect tends to be obtained, this may be used. At this time, as a nitride semiconductor containing Al, specifically, In _x Al _y G
a nitride semiconductor represented by a _1-xy N (0 ≦ x, 0 <y, x + y ≦ 1), preferably ternary mixed crystal Al _z Ga _1-z N (0 <z ≦ Use of 1) is preferable because it allows growth with good crystallinity and controllability. Further, the nitride semiconductor containing Al is not limited to the upper barrier layer, but may be a lower barrier layer disposed under the well layer, and a barrier layer (2) sandwiched between the barrier layers (1) and (3). ) May be provided.
Preferably, it is used other than the lower barrier layer provided in contact with the lower portion of the well layer, because the well layer tends to be formed with good crystallinity, and the above-described quantum effect tends to be obtained. Because there is. As the lower barrier layer in contact with the well layer, a nitride semiconductor containing no Al is preferably used, and In _x Ga _1
It is preferable to use a nitride semiconductor of _−xN (0 ≦ x ≦ 1) from the viewpoint of the crystallinity of the well layer. Furthermore, it is more preferable to use InGaN with an In mixed crystal ratio x larger than 0. The formation effect is obtained, which is preferable.

(Well layer)
As well layer of the present invention, it is preferable to use a nitride semiconductor layer containing In, as the case specific composition, In _α Ga _1-α N can (0 <α ≦ 1) be used preferably . As a result, a well layer that enables good light emission and oscillation is obtained. At this time, the emission wavelength can be determined by the In mixed crystal ratio.

Moreover, as the film thickness of the well layer and the number of well layers, the film thickness and the number of well layers can be arbitrarily determined. As a specific film thickness, Vf and threshold current density can be reduced by setting the thickness in the range of 10 to 300 mm, preferably in the range of 20 to 200 mm. Further, from the viewpoint of crystal growth, if the thickness is 20 mm or more, a layer having a relatively uniform film quality without large unevenness can be obtained. It becomes possible. The number of well layers in the active layer is not particularly limited and is 1 or more. At this time, when the number of well layers is 4 or more, the active layer becomes thicker as the thickness of each layer constituting the active layer increases. Since the entire film thickness is increased and Vf is increased, it is preferable to keep the film thickness of the active layer low by setting the film thickness of the well layer to a range of 100 mm or less.

The well layer of the present invention may or may not be doped with an n-type impurity, like the nitride semiconductor containing In in the active layer. However, since the well layer is made of a nitride semiconductor containing In and tends to deteriorate the crystallinity as the n-type impurity concentration increases, the well layer should have a good crystallinity by keeping the n-type impurity concentration low. Is preferred. Specifically, the well layer is grown undoped in order to maximize the crystallinity. At this time, the n-type impurity concentration is 5 × 10 ¹⁶ / cm ³ or less, which is substantially n-type. The well layer does not contain impurities. N well layer
When doping the n-type impurity, if the n-type impurity concentration is in the range of 1 × 10 ¹⁸ or less and 5 × 10 ¹⁶ or more, the deterioration of crystallinity can be suppressed and the carrier concentration can be increased. , Threshold current density and Vf can be reduced.
At this time, since the n-type impurity concentration of the well layer is approximately the same as or smaller than the n-type impurity concentration of the barrier layer, light emission recombination in the well layer tends to be promoted and light emission output tends to be improved. preferable. At this time, the well layer and the barrier layer may be grown undoped to constitute a part of the active layer.

In particular, when the element is driven with a large current (high output LD, high power LED, super photoluminescence diode, etc.), the well layer is undoped and substantially free of n-type impurities, so that the well layer Promotes carrier recombination at
Emission recombination with high efficiency is realized, and conversely, when the n-type impurity is doped in the well layer, the carrier concentration in the well layer is high, so the probability of luminescence recombination decreases, and it is driven at a constant output. A vicious circle that causes an increase in current and drive current occurs, and the reliability (element life) of the element tends to be greatly reduced. For this reason, in such a high-power element, the n-type impurity concentration of the well layer should be at least 1 × 10 ¹⁸ / cm ³ or less, preferably a concentration that is undoped or substantially free of n-type impurities. As a result, a nitride semiconductor device capable of stable driving with high output is obtained. Further, in a laser element in which an n-type impurity is doped in the well layer, the spectral width of the peak wavelength of the laser light tends to be widened, so that it is not preferable 1 × 10 ¹⁸ / cm ³ , preferably 1
× 10 ¹⁷ / cm ³ or less.

(Barrier layer)
In the present invention, the composition of the barrier layer is not particularly limited, but a band gap energy difference is provided between the well layer and the band gap energy is larger than that of the well layer. A nitride semiconductor containing In having a low ratio or a nitride semiconductor containing GaN or Al can be used. Specific compositions include In _β Ga _1-β N (0 ≦ β <1, α> β), GaN, Al _γ
Ga _1-γ N (0 <γ ≦ 1) or the like can be used. Here, in the case of a barrier layer (lower barrier layer) that is in contact with the well layer and serves as a base layer, it is preferable to use a nitride semiconductor that does not contain Al. This is because a well layer made of a nitride semiconductor containing In is formed of Al.
This is because, when grown directly on a nitride semiconductor containing Al such as GaN, the crystallinity tends to be lowered, and the function of the well layer tends to be deteriorated.

Further, the barrier layer may be doped with p-type impurities and n-type impurities, or may be non-doped, but preferably it is doped with n-type impurities, or is undoped or undoped. At this time, when the n-type impurity in the barrier layer is doped, the concentration is at least 5 × 10 ¹⁶ / cm ³ or more. Specifically, in the case of an LED, for example, 5 × 10 ¹⁶ / cm
It has n-type impurities in the range of ³ or more and 2 × 10 ¹⁸ / cm ³ or less, and for higher output LEDs and higher output LDs, it is 5 × 10 ¹⁷ / cm ³ or more and 1 × 10 ²⁰ / cm. ³ following range, when preferably is preferably is doped with a range of 1 × 10 ¹⁸ / cm ³ or more 5 × 10 ¹⁹ / cm ³ or less, is thus doped with high concentration, the well layer n It is preferable that the substrate is substantially free of type impurities or grown undoped.

On the other hand, as shown in FIGS. 3 and 5 to 8, in the active layer, on the outermost side, the most p-type layer 1.
The barrier layer 2c located on the 3 side preferably has substantially no n-type impurities, whereby the carrier injection from the p-type layer 13 becomes good and the device life tends to be improved. This is because the most p-side barrier layer 2c is provided in contact with the p-type layer, and p
In the case of having an n-type impurity as a carrier injection port from the type layer, it is considered that the carrier injection is prevented, and the p-type layer 1 is substantially free of the n-type impurity.
This is probably because the carriers from No. 3 are stably and efficiently injected into the deeper portion of the well layer far from the p-type layer. This tends to significantly improve the element life, particularly in a large current driven high output LD, LED, etc. that injects a large amount of carriers with a large current. At this time, the fact that the n-type impurity is substantially not included means that the n-type impurity concentration of the most p-side barrier layer 2c is 5 × 10 ^16.
To be less than / cm ³ . The most p-side barrier layer 2c is
Preferably, it is formed on the outermost side in the active layer. However, in the case where the effect is small but not on the outermost side, for example ... well layer / barrier layer / well layer /
Even if the p-type layer 13 is laminated in this order, the effect can be expected. The position of the most p-side barrier layer 2c is preferably arranged on the outermost side in the active layer, and more preferably provided in contact with a p-side electron confinement layer, which will be described later. The injection of carriers from the p-type layer becomes more efficient. Furthermore, since the most p-side barrier layer 2c contains the p-type impurity, carriers from the p-type layer 13 are also absorbed in the deeper well layer and the well layer located far from the p-type layer 13. It is preferable that the barrier layer does not substantially contain n-type impurities and contains p-type impurities since it is efficiently implanted and the device lifetime tends to be improved. At this time, the p-type impurity amount is in the range of 5 × 10 ¹⁶ / cm ^{3 to} 1 × 10 ²⁰ / cm ³ , preferably in the range of 5 × 10 ¹⁶ / cm ^{3 to} 1 × 10 ¹⁸ / cm ^3. It is. This is 1
Even if the p-type impurity is increased to × 10 ²⁰ / cm ³ or more, the carrier concentration hardly changes. Therefore, the crystallinity deteriorates due to the inclusion of the impurity, the loss due to the light scattering action by the impurity increases, and it is rather active. Reduce the luminous efficiency in the layer. Furthermore,
When it is 1 × 10 ¹⁸ / cm ³ or less, it is possible to suppress a decrease in light emission efficiency due to the increase in impurities and to keep the carrier concentration from the p-type layer into the active layer stably high. In addition, as a lower limit of the p-type impurity, it is preferable that the p-type impurity has a slight amount, but this has a higher probability when the impurity is low in concentration than in the case of high concentration when the impurity is low in concentration. This is because impurities tend to function as carriers.

The thickness of the barrier layer is not particularly limited, and can be 500 mm or less, and more specifically, the range of 10 to 300 mm can be applied as in the case of the well layer.

(Waveguide structure)
In the nitride semiconductor device of the present invention, the device structure has at least a structure in which an active layer is sandwiched between a p-type nitride semiconductor layer and an n-type cladding layer and a p-type cladding layer in the n-type nitride semiconductor layer. It becomes. At this time, it is preferable to use a nitride semiconductor containing In for the active layer, and it is preferable to have an In mixed crystal ratio in which light emission with a wavelength of 440 nm or more can be obtained in the active layer. Further, a light guide layer sandwiching the active layer may be provided between the cladding layer and the active layer. Here, a region sandwiched between the p-type cladding layer and the n-type cladding layer is called a waveguide.

Here, as the n-type cladding layer and the p-type cladding layer, a nitride semiconductor containing Al is preferably used, so that a large difference in refractive index can be obtained between the waveguide and both cladding layers. . At this time, the nitride semiconductor of the cladding layer includes In
It is preferable that a nitride semiconductor containing In has a tendency to deteriorate in crystallinity as compared with a case where In is not contained, and in particular, in a structure having a p-type cladding layer on an active layer. When a nitride semiconductor containing In is used for the p-type cladding layer, the crystallinity is greatly deteriorated and the device characteristics are greatly deteriorated. At this time, as a nitride semiconductor used for the cladding layer, specifically, Al _b Ga _1-b N (0 <
b <1) is preferably used.

In the present invention, the layer between the clad layer and the active layer is important in the formation of the waveguide, like the light guide layer shown in FIG. This is because, in order to confine light in the waveguide, the refractive index of the cladding layer is lowered relative to the waveguide and the refractive index difference is increased or the refractive index in the waveguide is increased. However, when the wavelength of light from the active layer becomes long, a difficult problem occurs. AlGaN and InG
This is because the refractive index difference from aN has a large refractive index difference in a short wavelength region, for example, around 400 nm, but the refractive index difference decreases as the wavelength increases. For this reason, the Al mixed crystal ratio of the nitride semiconductor used for the cladding layer is increased to reduce the refractive index of the cladding layer, or the nitride semiconductor containing In is used in the light guide layer. It is necessary to reduce the refractive index and increase the refractive index difference between the waveguide and the cladding layer. However, when the Al mixed crystal ratio of the cladding layer is increased, the crystallinity is greatly deteriorated, cracks and the like are generated, and the element characteristics are deteriorated such as causing a leakage current.
It is difficult to provide a thick Al nitride semiconductor with a high Al mixed crystal ratio in the device structure.
Furthermore, the nitride semiconductor layer in the waveguide excluding the active layer, for example, the light guide layer, has In
In a structure in which the refractive index of the waveguide is increased by using a nitride semiconductor containing, light absorption occurs by the nitride semiconductor containing In, which causes light loss in the waveguide and causes a threshold current of Deterioration of device characteristics such as increase occurs. As described above, for example, when a nitride semiconductor containing In is used in the p-type light guide layer shown in FIG. 2 as the p-type nitride semiconductor, Mg that is preferably used as a p-type impurity is contained. Crystallinity is greatly deteriorated, resulting in deterioration of device characteristics.

In the present invention, since the first nitride semiconductor layer and the second nitride semiconductor layer sandwiching the active layer as the waveguide sandwiched between both cladding layers, the waveguide at a long wavelength and the problem of crystallinity Has solved. That is, between the n-type cladding layer and the active layer,
By providing the first nitride semiconductor layer made of a nitride semiconductor containing In, the refractive index in the waveguide is relatively larger than that of the cladding layer, while the p-type cladding layer and the active layer And a second nitride semiconductor layer made of a nitride semiconductor not containing In (the In mixed crystal ratio is 0) is provided to solve the problem of deterioration of crystallinity on the p-type layer side. Is. Therefore, the waveguide has a structure in which the active layer is sandwiched between the first nitride semiconductor layer containing In and the second nitride semiconductor layer not containing In, and the composition is asymmetric.

Conventionally, the waveguide structure in which the emission wavelength of the active layer is a long wavelength includes the above-described problems of the difference in refractive index between InGaN and AlGaN in the long wavelength region and the problem of deterioration of crystallinity due to high Al mixed crystals. A structure using a nitride semiconductor containing In, such as InGaN, in the waveguide as the light guide layer, for example, an InGaN single film, InGaN / GaN
A multilayer film (superlattice layer) has been considered. However, the p-type light guide layer is
As an nGaN / GaN superlattice multilayer film, it is difficult to obtain crystallinity to such an extent that the device characteristics are not affected even if the deterioration of crystallinity is suppressed to a low level. It becomes a big cause of characteristic deterioration. This is because the longer the emission wavelength of the active layer, the smaller the refractive index difference between InGaN and AlGaN, and the In mixed crystal ratio of the nitride semiconductor used for the light guide layer and the like to increase the refractive index in the waveguide. Must be increased, but if the In mixed crystal ratio increases, the crystallinity,
Deterioration of device characteristics due to light loss and the like is also increased.

However, in the present invention, in the waveguide, the refractive index of the entire waveguide is made larger than that of the cladding layer by using the first nitride semiconductor layer containing In on the n-type cladding layer side. By increasing the refractive index difference and providing the second nitride semiconductor layer on the p-type cladding layer side, it is possible to avoid deterioration of crystallinity and light loss due to the nitride semiconductor containing In, and to improve the device characteristics. A physical semiconductor device is obtained. Less than,
Each layer will be described.

(First nitride semiconductor layer)
The first nitride semiconductor layer in the present invention is disposed between the active layer and the n-type cladding layer in the waveguide, and is made of a nitride semiconductor containing In. Here, the composition of the first nitride semiconductor layer is preferably a nitride semiconductor that does not contain Al, whereby the difference in refractive index from the cladding layer that uses the nitride semiconductor containing Al. In other words, in the clad layer and the waveguide sandwiched between them, the refractive index can be relatively increased in the waveguide, and In _z Ga _{1 -z} N (0 <z ≦ 1). The first nitride semiconductor layer with good crystallinity can be obtained by forming the nitride semiconductor represented by Further, another layer may or may not be provided between the first nitride semiconductor layer and the active layer or the n-type cladding layer, that is, the first nitride semiconductor layer may be the active layer or n-type cladding layer,
Alternatively, it may be provided in contact with both, or may be provided separately on one or both. Further, a multilayer film structure in which a plurality of first nitride semiconductor layers are stacked by alternately stacking layers having different compositions from the first nitride semiconductor layer may be used. Since the first nitride semiconductor layer of the present invention is located between the active layer and the n-type cladding layer and is in the waveguide, it functions as a light guide layer, while containing In, the entire waveguide is refracted. Since this increases the rate and contributes to the confinement of light in the waveguide, it also functions as a second light confinement layer compared to the second nitride semiconductor layer on the p-type layer side. It is thought that there is.

The In mixed crystal ratio z of the first nitride semiconductor layer is the In mixed crystal ratio of the nitride semiconductor containing In in the active layer, or the In mixed crystal of the well layer in the case of an active layer having a quantum well structure. When the ratio is w, preferably z ≦ w, and more preferably z <
w. For example, as shown in FIGS. 5 and 6, the In mixed crystal ratio z of the first nitride semiconductor layer 31 is set to z ≦ w compared to the mixed crystal ratio w of the well layer 1 in the active layer 12.
As shown in the figure, a stepwise bandgap energy structure can be formed, and carriers can be efficiently injected into the active layer in the waveguide, particularly the n-type layer 1.
This contributes to carrier injection from one side. At this time, since z <w, a large bad gap energy difference can be provided between the nitride semiconductor layer containing In in the well layer or the well layer and the first nitride semiconductor layer. The carrier injection efficiency can be improved. Further, the first nitride semiconductor layer 31
Is provided adjacent to the active layer 12, disposed on the outermost side of the active layer, on the most n-type layer 11 side, and provided with the barrier layer 2 a adjacent to the first nitride semiconductor layer 31. It is more preferable that the In mixed crystal ratio z of the first nitride semiconductor layer 31 is set to satisfy z ≦ v as compared with the In mixed crystal ratio v of the barrier layer 2a, and further, z <v. preferable. As shown in FIGS. 5 and 6, when z ≦ v, the band gap is gradually increased in the vicinity of the junction between the active layer 12 and the n-type layer 11 from the n-type layer 11 to the active layer 12. The structure can be reduced in energy, the carrier can be efficiently injected from the n-type layer 11 into the active layer, and the band gap structure can be further stepped as shown in FIG. Because it becomes.

On the other hand, as shown in FIG. 10, when the In mixed crystal ratio z of the first nitride semiconductor layer is substantially the same as or larger than the In mixed crystal ratio v of the barrier layer (z ≧ v). Preferably, it is increased (z> v), and the band gap energy is smaller than that of the barrier layer, but the In mixed crystal ratio is large, and the film thickness is larger than that of the barrier layer (n-side barrier layer). The first nitride semiconductor layer increases the refractive index of the waveguide,
The refractive index difference from the optical confinement cladding layer can be increased. In this case, when carriers from the n-type layer are injected, a barrier is provided between the first nitride semiconductor layer and the n-side barrier layer, but the barrier is small in the bias ground. The effect is lessened. On the other hand, the n-side barrier layer 2a mainly serves as a confinement layer for carriers from the p-type layer. However, in the nitride semiconductor, the hole diffusion length is small, so that the barrier is small and the film thickness is reduced. Is relatively low. As for the In mixed crystal ratio of the light guide layer and the first nitride semiconductor layer, as shown in FIG.
It is preferable to reduce the band gap energy and increase the In mixed crystal ratio as compared with the optical guide layer. This is because the first nitride semiconductor layer is provided to increase the refractive index, and it is preferable to increase the refractive index in the vicinity of the active layer corresponding to the central portion of the waveguide structure. By providing the enlarged first nitride semiconductor layer closer to the active layer than to the light guide layer, a more excellent waveguide structure can be formed. Here, in the present invention, when the clad layer and the light guide layer are composed of multilayer films, particularly when composed of a superlattice multilayer film, the In mixed crystal ratio, Al mixed crystal ratio, and band gap energy of each layer are The average composition and average energy are compared with other layers. The average composition and average energy are the first layer (third layer) and the second layer (fourth layer) constituting the multilayer film. It is a value obtained by weighted averaging of Al, In composition, and energy at each layer thickness. For example, the light guide layer has an In mixed crystal ratio y1,
A third layer of thickness d _3, In composition ratio y2, has a fourth layer having a thickness d ₄ In the superlattice structure are alternately laminated, the average mole fraction y _m of In, y _m = [(d ₃ × y1) +
(D ₄ × y 2)] / (d ₃ + d ₄ )

As shown in FIGS. 5, 6, and 10, light guide layers 26 and 29 are provided between the cladding layers 25 and 30 and the active layer 12, and on the n-type layer 11 side, a light guide is provided. When the first nitride semiconductor layer of the layer 26 and the active layer is provided, as shown in FIG. 10, the n-side disposed outside the active layer with respect to the In mixed crystal ratio or the average composition of the light guide layer It is preferable to increase the In composition z of the barrier layer 2a. This is to suitably draw out the function of increasing the refractive index by the first nitride semiconductor layer described above,
Specifically, by increasing the In mixed crystal ratio of the first nitride semiconductor layer provided closer to the active layer than the light guide layer 26 and increasing the refractive index near the active layer, the first nitride semiconductor layer is positioned near the center of the waveguide. By providing a layer having a large refractive index with the active layer as a center, a good light distribution can be realized. On the other hand, on the p-type layer side, the p-side barrier layer plays this role, that is, the In composition ratio of the p-side barrier layer 2c is made larger than the In composition ratio of the light guide layer 29 of the p-type layer. Thus, the same function as that of the first nitride semiconductor layer can be obtained.

Here, referring to the respective forms at the position of the first nitride semiconductor layer, in the form in which the first nitride semiconductor layer is provided in contact with the active layer and the n-type cladding layer,
Since it is difficult to form a nitride semiconductor containing In with a thick film with good crystallinity, if it is formed with a sufficient film thickness as a waveguide, the device characteristics deteriorate due to the deterioration of crystallinity. If the film thickness is such that the device characteristics do not deteriorate the device characteristics, the film properties are insufficient to function as a waveguide, and the device characteristics tend to deteriorate due to loss due to light leakage to the outside of the cladding layer. The first nitride semiconductor layer is divided into an active layer and n
When a multi-layered film is formed between the mold clad layer and the multi-layered film, for example, a superlattice structure is laminated together with a nitride semiconductor not containing In to form a thick film while suppressing deterioration of crystallinity. be able to. For example, an InGaN / GaN multilayer film or a structure in which the composition gradient is made so that the In mixed crystal ratio increases as it approaches the active layer from the n-type cladding layer can be employed. On the other hand, in order to make the refractive index of the waveguide equal to that of a single film, the multilayer film becomes thick, and the first nitride semiconductor layer containing In is scattered in the multilayer film. Therefore, loss of light due to In occurs in a multilayer film that is thicker than a single film, so that the loss tends to be larger than that of a single film. In addition, the position of the first nitride semiconductor layer is specifically between the n-type cladding layer and the active layer, and a light guide layer is provided between the n-type cladding layer and the active layer. The first nitride semiconductor layer is provided between the light guide layer and the active layer as shown in FIGS. 5, 6, or 10, or inside the light guide layer or inside the light guide layer as shown in FIGS. Can be provided. In the present invention, the above-described various forms can be applied, but it tends to be preferably arranged close to the active layer, more preferably in contact with the active layer. Although it is unknown in detail, as shown in FIGS. 5 and 6, etc., the band gap structure is made to be stepwise in the region from the n-type cladding layer to the active layer, and from the n-type layer side 11. It is thought that promoting carrier injection has an influence.

The film thickness of the first nitride semiconductor layer is not particularly limited, but is set to at least 1500 mm or less, preferably 300 mm, in consideration of the generation of light loss due to In as described above. With the above, the refractive index of the entire waveguide is increased, a large refractive index difference can be formed with the n-type cladding layer, and an excellent waveguide with reduced loss and reduced threshold current is formed. Is done. At this time, as will be described later, the function as a waveguide is affected by the total thickness of the region sandwiched between the clad layer and the active layer, so that the region sandwiched between the n-type clad layer and the active layer The first nitride semiconductor layer may be determined in consideration of the total film thickness. As shown in FIGS. 5 and 6 and the like, when the first nitride semiconductor layer is disposed adjacent to the barrier layer 2a closest to the n-type layer, the first nitride semiconductor layer has a barrier. In this case, the thickness of the first nitride semiconductor layer is set so that the total thickness of the first nitride semiconductor layer and the barrier layer 2a is 300 mm or more. The upper limit of the film thickness at this time is preferably 1500 mm or less.

The first nitride semiconductor layer may or may not be doped with an n-type impurity, but preferably has an n-type conductivity by doping with an n-type impurity. . At this time, since the first nitride semiconductor layer is a nitride semiconductor containing In, the crystallinity is deteriorated by doping the n-type impurity, although not as much as the p-type impurity as described above. Preferably, the doping amount is 1 × 10 ¹⁹ / c
By setting it as the range of m < ³ > or less, the deterioration of the crystallinity in the nitride semiconductor containing In can be suppressed.

(Second nitride semiconductor layer)
In the present invention, a nitride semiconductor having an In mixed crystal ratio of 0 is used as the second nitride semiconductor layer, and the second nitride semiconductor layer is interposed between the p-type cladding layer and the active layer. By providing, it becomes the layer which is excellent in crystallinity and functions as a waveguide. This is because a structure in which an active layer is sandwiched between the first nitride semiconductor layer and the second nitride semiconductor layer is provided in the waveguide, that is, the first nitride semiconductor layer on the n-type layer side and p A second nitride semiconductor layer on the mold layer side, both layers are arranged opposite to each other with an active layer interposed therebetween, and an asymmetric waveguide structure having different functions in the waveguide is obtained by having a different composition It is. By setting the In mixed crystal ratio u of the nitride semiconductor used for the second nitride semiconductor layer to u = 0, a layer having excellent crystallinity can be formed, and Vf,
An increase in threshold current can be avoided. This is because a nitride semiconductor containing In tends to deteriorate in crystallinity as compared with a semiconductor containing no In. Further, unlike the first nitride semiconductor layer, the second nitride semiconductor layer needs to be doped with p-type impurities to have p-type conductivity, and crystallinity deteriorates due to impurity doping. p
Mg, which is preferably used as a type impurity, causes a significant deterioration in crystallinity. This is a significant deterioration in crystallinity in nitride semiconductors containing In as compared with those not containing In.

As shown in FIGS. 2 to 4, nitride semiconductor elements such as LEDs and LDs usually have a structure in which an n-type layer 11 / active layer 12 / p-type layer 13 are stacked in this order on a substrate. However, in such a case, the n-type layer 11 disposed below the active layer 12 using the nitride semiconductor containing In and the p-type layer 13 disposed above are normally grown. The p-type layer 13 arranged under the different conditions and disposed above the active layer normally needs to be grown under a temperature condition that does not deteriorate the crystallinity due to decomposition of In in the active layer. 11 has no such limitation. Therefore, the p-type layer 13 grown at a low temperature may be difficult to grow under good crystal growth conditions.

Specifically, as shown in FIG. 2, the active layer 11 is interposed between the n-type layer 11 and the p-type layer 13.
Is provided on a substrate (not shown) or the like. As a basic configuration of the present invention, the n-type layer 11 is provided with an n-type cladding layer 26, and the p-type layer 13 is provided with a p-type layer 13. Is p
There is a structure in which a mold cladding layer 30 is provided. Further, in each conductive type layer, an n-type contact layer 25 and a p-type contact layer may be formed at a position farther from the active layer than these cladding layers 26 and 30, and an electrode may be provided on the surface thereof. On top of the n-type layer 1
In the structure in which the active layer 12, the p-type layer 13 are laminated in order, an electrode is provided on the side of the substrate facing the n-type layer, the n-type contact layer 25 is used as a charge injection layer, and the n-type cladding layer 26 It is also possible to provide a layer doped with n-type impurity doping at a higher concentration. Further, the clad layer may also serve as a contact layer. In such a basic structure, the first nitride semiconductor layer 31 is formed as shown in FIG.
The second nitride semiconductor layer 32 is provided between the n-type cladding layer 26, the p-type cladding layer 30, and the active layer 12, respectively. As shown in FIGS.
When the light guide layers 26 and 29 are provided between the active layer and the clad layer, the first and second nitride semiconductor layers may be provided between the light guide layer and the active layer, The light guide layer may have a structure including a first nitride semiconductor layer or a second nitride semiconductor layer. For this reason, when the light guide layer has a multilayer film structure, a multilayer film structure having the first nitride semiconductor layer or the second nitride semiconductor layer can be obtained.

The composition of the second nitride semiconductor layer is to use a nitride semiconductor that does not contain In, and preferably a nitride semiconductor represented by Al _t Ga _1-t N (0 ≦ t <1). Is to use. At this time, in order to provide a refractive index difference from the p-type cladding layer, it is preferable to make the Al mixed crystal ratio t of the second nitride semiconductor smaller than the Al mixed crystal ratio of the p-type cladding layer. Furthermore, in consideration of the refractive index difference between the clad layer and the waveguide, in order to form a low Al mixed crystal ratio with t ≦ 0.5, or to maximize the refractive index in the waveguide, Most preferably, GaN with t = 0 is used. Second
The nitride semiconductor layer preferably has a p-type impurity and can function as a p-type layer having good conductivity by containing the p-type impurity and having p-type conductivity. At this time, the doping amount of the p-type impurity is not particularly limited, but even if the nitride semiconductor does not contain In, the deterioration of crystallinity is small compared to the case where In is contained, but the crystallinity becomes smaller as the doping amount is smaller 1x because it tends to be better
By setting it to a range of 10 ¹⁸ / cm ³ or less, a second nitride semiconductor layer having good crystallinity can be obtained. In the examples described later, the second nitride semiconductor layer is grown undoped and doped with p-type impurities by diffusion from the adjacent layer. However, the present invention is not limited to this method, and the same applies to other layers. However, either diffusion after growth or a method of growing while doping may be used.

The second nitride semiconductor layer may be formed of a single film or a multilayer film. The multilayer film may be a multilayer film in which a plurality of AlGaN / GaN layers are stacked.
A layer with a composition gradient that increases the l-mixed crystal ratio as the distance from the active layer increases.

When the second nitride semiconductor layer is used for the p-type light guide layer, the p-type light guide layer may be composed of only the second nitride semiconductor layer. The composition of the second nitride semiconductor layer is You may comprise combining with a different layer. At this time, it is preferable to use only the second nitride semiconductor layer that does not contain In because light loss due to In in the waveguide is avoided. At this time, the film thickness of the p-type light guide layer is not particularly limited. However, by forming the p-type light guide layer with a film thickness of at least 200 mm, it is possible to guide light with good loss and low loss. As a result, the upper limit of the film thickness at this time is set to 4000 mm or less, so that an increase in the threshold current and Vf can be suppressed. Preferably, the threshold current is set to 500 to 2000 mm. Vf
Thus, a waveguide having a thickness suitable for light guiding can be formed. About this film thickness
The present invention can also be applied to the thickness of the region sandwiched between the clad layer and the active layer, that is, the region sandwiched between the n-type clad layer and the active layer. Specifically, in the case where the first nitride semiconductor layer is provided between the n-type cladding layer and the active layer, the thickness of the first nitride semiconductor layer is different from that of the first nitride semiconductor layer and the n-type light guide layer. When it has a layer, it can apply about the sum total of the film thickness of those layers. As described above, the thickness of the region sandwiched between the clad layer and the active layer is made substantially the same on both the p-type layer side and the n-type layer side, so that the waveguide structure is symmetrical with respect to the thickness through the active layer. It is also possible to make the thicknesses of the both different so that a waveguide structure with an asymmetrical thickness is used, and the thickness may be appropriately selected in consideration of the characteristics of the obtained nitride semiconductor device.

Further, according to another aspect of the present invention, in a configuration in which the In mixed crystal ratio u of the second nitride semiconductor layer is smaller than the In mixed crystal ratio z of the first nitride semiconductor layer (u <z), p
A layer that increases the refractive index of the waveguide can be formed on the p-type layer side while suppressing a decrease in crystallinity due to the nitride semiconductor containing In being provided on the mold layer side.
It can be set as the structure which suppressed shifting to the n-type layer side.

(P-side electron confinement layer)
In the present invention, it is preferable to provide a p-side electron confinement layer as the p-type nitride semiconductor layer, particularly in a laser device and an edge emitting device. As this p-side electron confinement layer, a nitride semiconductor containing Al is used. Specifically, Al _γ Ga _1−
γ N (0 <γ <1) is used. At this time, the Al mixed crystal ratio γ needs to have a sufficiently large band gap energy (take an offset) so as to function as an electron confinement layer, and at least 0.1 ≦ γ <1. The range is preferably 0.2 ≦ a <0.5. Because
If γ is 0.1 or less, the laser device does not function as a sufficient electron confinement layer, and if it is 0.2 or more, sufficient electron confinement (carrier confinement) is achieved, and carrier overflow is suppressed. In addition, when it is 0.5 or less, it is possible to grow while suppressing generation of cracks, and more preferably, when γ is 0.35 or less, it is possible to grow with good crystallinity. At this time, the Al mixed crystal ratio is preferably larger than that of the p-type cladding layer, and this requires a nitride semiconductor having a higher mixed crystal ratio than the cladding layer that confines light to confine carriers. Because. This p-side electron confinement layer can be used for the nitride semiconductor device of the present invention, and in particular, like a laser device,
When driving with a large current and injecting a large amount of carriers into the active layer, it is possible to confine carriers more effectively than when the p-side electron confinement layer is not provided. It can also be used for high output LEDs.

The film thickness of the p-side electron confinement layer of the present invention is at least 1000 mm or less, preferably 400 mm or less. This is because the nitride semiconductor containing Al has a larger bulk resistance than other nitride semiconductors (not containing Al), and the Al mixed crystal ratio of the p-side electron confinement layer is set high as described above. Therefore, if it is provided in the element exceeding 1000 Å, it becomes a very high resistance layer, resulting in a significant increase in the forward voltage Vf. It is possible to further reduce the pressure by setting it to 200 mm or less. Here, as a lower limit of the film thickness of the p-side electron confinement layer,
When it is at least 10 mm or more, preferably 50 mm or more, it functions well as electronic confinement.

In the laser element, as shown in FIGS. 3 and 4, the p-side electron confinement layer is provided between the active layer and the cladding layer in order to function as an electron confinement layer. Between the nitride semiconductor layer and the active layer. Also,
When the nitride semiconductor device has a waveguide structure and has a light guide layer between the cladding layer and the active layer, the p-side electron confinement layer is provided between the light guide layer 29 and the active layer 27. As a result, a p-side electron confinement layer is provided in the vicinity of the active layer, so that a suitable carrier confinement structure can be realized. As another form, the p-side electron confinement is provided inside the light guide layer. The p-side electron confinement layer and the active layer can be separated from each other, and the internal stress, the piezoelectric field, and the heat generated by the proximity of the p-side electron confinement layer to the active layer. The action can be avoided, which is preferable. At this time, when the distance between the active layer and the p-side electron confinement layer is at least 1000 mm or less, it functions as carrier confinement, and preferably 500 cm or less enables good carrier confinement. . That is, the closer the p-side electron confinement layer is to the active layer, the more effectively the carrier confinement functions.
In most cases, no other layer is required between the side electron confinement layer and, therefore, it is usually most preferable that a p-side electron confinement layer can be provided in contact with the active layer. At this time, a layer located closest to the p-type nitride semiconductor layer in the active layer of the quantum well structure, and p
In order to avoid the case where the crystallinity deteriorates when the side electron confinement layer is provided in contact with each other, a buffer layer for crystal growth can be provided between the both. For example,
Between the p-side electron confinement layer of InGaN and AlGaN, the most p-side layer of the active layer,
Provide a buffer layer made of GaN, or Al lower than the p-side electron confinement layer
There is a buffer layer made of a nitride semiconductor containing Al having a mixed crystal ratio.

Here, as the p-side electron confinement layer, specifically, the threshold current density is lowered as the p-side electron confinement layer is closer to the active layer, but the device life is lowered as the p-side electron confinement layer is closer. As described above, since the p-side electron confinement layer is a layer having a very high resistance compared to other layers, the amount of heat generated during device driving is large, that is, a high temperature is exhibited in the device. This is considered to have an adverse effect on the active layer and the well layer which are vulnerable to heat and greatly reduce the device lifetime. On the other hand, as described above, the p-side electron confinement layer responsible for carrier confinement is
The closer to the active layer, in particular, the well layer, the more effective the confinement of carriers, so that the effect is weakened away from the active layer. For this reason, the p-side electron confinement layer has a larger band gap energy than the active layer and preferably larger than the band gap energy than at least one barrier layer in the active layer so as to function suitably as carrier confinement. More preferably, the composition is selected so that the band gap energy is larger than all the barrier layers in the active layer. Further, in the edge-emitting device and laser device having a waveguide structure, the p-side electron confinement layer has a band gap energy larger than that of a part of the light guide layer, preferably all of the light guide layer. In the carrier layer, the p-side electron confinement layer disposed closer to the active layer than the guide layer can preferably realize carrier confinement in the active layer. When the band gap energy is made larger than a part or all of the structure, a large barrier is arranged in the vicinity of the active layer, realizing suitable carrier confinement, and reducing the thickness of the p-side electron confinement layer. However, the function can be maintained, which is preferable.

Therefore, in FIG. 5 and FIG. 6, the distance from the well layer 1b closest to the p-side electron confinement layer in the active layer to the p-side electron confinement layer 28 in FIG. And preferably is 120 mm or more,
More preferably, it is 140 mm or more. This is because when the distance between the well layer and the p-side electron confinement layer is shorter than 100 mm, the device life tends to be drastically reduced. When the distance is 120 mm or more, the device life can be significantly improved. , 15
If it is 0% or more, the device life tends to be further improved, but the threshold current density tends to gradually increase. Furthermore, when the distance is larger than 200 mm, a clear increase tendency of the threshold current density is observed, and when the distance is larger than 400 mm, the threshold current density tends to increase rapidly. 4
It is set to 00 cm or less, preferably 200 cm or less. This is because the p-side electron confinement layer is separated from the well layer, thereby reducing the efficiency of carrier confinement. This is mainly caused by an increase in threshold current density and a decrease in light emission efficiency. it is conceivable that.

The p-side electron confinement layer of the present invention is usually doped with a p-type impurity, and a laser device,
In the case of driving with a large current such as a high power LED, doping is performed at a high concentration in order to increase the mobility of carriers. The specific doping amount is at least 5 × 10
Doping of ¹⁶ / cm ³ or more, preferably 1 × 10 ¹⁸ / cm ³ or more, and in the large current driving element, 1 × 10 ¹⁸ / cm ³ or more, preferably 1 × Doping is at least 10 ¹⁹ / cm ³ . The upper limit of the p-type impurity amount is not particularly limited, but is 1 × 10 ²¹ / cm ³ or less. However, if the amount of p-type impurities increases, the bulk resistance tends to increase, resulting in an increase in Vf. Therefore, in order to avoid this, it is preferable that the minimum carrier mobility can be ensured. P-type impurity concentration. It is also possible to dope the p-side electron confinement layer at a low concentration, for example, dope at a lower concentration than the layers near the p-side electron confinement layer such as the guide layer and the clad layer. It can also be.

In the nitride semiconductor device of the present invention, as shown in the embodiment, after providing a ridge as a striped waveguide, an insulating film serving as a buried layer is formed on the side surface of the ridge. At this time, as the buried layer, the material of the second protective film is a material other than SiO ₂ , preferably at least one selected from the group consisting of Ti, V, Zr, Nb, Hf, and Ta. It is desirable to form with at least one of oxides containing elements, SiN, BN, SiC, and AlN, and among them, oxides of Zr and Hf, BN, Si
It is particularly preferable to use C. Further, as the buried layer, semi-insulating, i-type nitride semiconductor, conductivity type opposite to the ridge portion, in the embodiment n-type nitride semiconductor, Al, such as AlGaN, is used for the current confinement layer. Including nitride semiconductors can be used. Further, without providing a ridge by etching or the like, ions such as B and Al can be implanted to form a non-implanted region in a stripe shape and a region through which a current flows. The nitride semiconductor used at this time is In _x Al _1-y Ga
A nitride semiconductor represented by _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y = 1) can be preferably used.

Further, by setting the ridge width to 1 μm or more and 3 μm or less, preferably 1.5 μm or more and 2 μm or less, an excellent spot-shaped or beam-shaped laser beam can be obtained as a light source of the optical disk system.

Here, each figure will be described below. 2 and 3 are schematic cross-sectional views according to an embodiment of the present invention. In particular, in the laser element structure and the light-emitting element structure, the active layer 12 is shown.
Shows a structure sandwiched between the n-type layer 11 and the p-type layer 13. FIG. 2 shows an element structure in which an active layer 12 is sandwiched between an upper clad layer 30 and a lower clad layer 25, and a p-side electron confinement layer 28 as an electron confinement layer is provided between the active layer 12 and the upper clad layer 30. Is described. 3 and 4, as a feature of the present invention, the first nitride semiconductor layer and the second nitride semiconductor layer are provided in a waveguide in a region sandwiched between upper and lower cladding layers. The nitride semiconductor layer is formed in an n-type cladding layer (not shown), between the n-type light guide layer 26 and the active layer 12 (shown in FIGS. 3 and 4), and between the n-type light guide layer 26 and the n-type cladding. The second nitride semiconductor layer is provided between the layers 25 (not shown), and is used for the p-type light guide layer 29. FIG. 3 illustrates the quantum well structure of the active layer 12, which has a structure in which the barrier layer 2a / well layer 1a are repeatedly stacked as a pair, and finally the barrier layer 2c is provided. 5 to 8 show the active layer 12, the waveguide structure in the region sandwiched between the upper and lower cladding layers 26 and 30, and the laminated structure 20 around the active layer, and the laminated structure 20 according to the embodiment of the present invention. A corresponding energy band gap 21 is shown below. 10 also shows the laminated structure 20 and the energy band diagram 21 corresponding to the laminated structure as in FIGS. 5 to 8, and in addition to these, the Al composition ratio 41, I in each layer
A schematic diagram showing one embodiment of the n composition ratio 42 is shown.

The doping amount of each layer in the nitride semiconductor device of the present invention will be described below with reference to FIG. Regarding the impurity doping of the light guide layer of the present invention, as shown in FIG. 11 as the doping amount change 43, in the first and second light guide layers 226 and 229,
Impurity doping amount is decreased as approaching the active layer, or
By reducing the amount of doping in the region closer to the active layer than in the region far from the active layer, light loss can be further reduced in the waveguide, particularly in the light guide layer, and good light guiding can be realized. It is possible to reduce current density and drive current. This is because when light is guided through the impurity-doped region, light is lost due to light absorption by the impurity. In addition to this, the waveguide, as mentioned above,
It has at least a structure in which the active layer 227 is sandwiched between the first light guide layer 226 and the second light guide layer 229, and further, the outer side of the guide layer or the waveguide has an upper part with a lower refractive index than the guide layer. The light is confined in the waveguide by the structure sandwiched between the lower clad layers 225 and 230, and a large amount of light is distributed in the vicinity of the active layer 27 and the active layer in the waveguide. By reducing the impurity doping amount in FIG. 5, light loss in a region where a large amount of light is distributed is reduced, resulting in a waveguide with little light loss. Specifically, the first light guide layer 226, the second
In the light guide layer 229, when the region near the active layer and the region far from the active layer are considered by dividing the region by half the thickness of each layer, the conductivity type impurity concentration in the region near the active layer is changed to the impurity concentration in the region far from the active layer Is to make it smaller. The impurity concentration of the light guide layer is not particularly limited, but specifically, 5 × 10 ¹ in a region close to the active layer.
⁷ / cm ³ or less. Here, the impurity doping means that the first light guide layer is doped with a first conductivity type impurity, and the second light guide layer is doped with a second conductivity type impurity.

As a specific example of the form in which the doping amount is changed in the light guide layer, in each light guiding layer, a form in which the doping amount is gradually decreased gradually as the active layer is approached (43a), and the steps are discontinuous. To make the doping amount small (43b)
In addition, the step (43c) in which the stepwise change in the doping amount is made fine and the doping amount change is partially provided in the light guide layer may be used, or a combination thereof may be used. Preferably, in the light guide layer, the distance from the active layer side is 5
By making the region of 0 nm or less a lightly doped region (226b, 229a), preferably undoped, it is possible to reduce the loss of light. Preferably, the region of 100 nm or less is made a lightly doped region (226b, 229a). Good optical loss reduction, threshold current density, and driving current can be reduced. At this time, the film thickness of the light guide layer is
When the lightly doped region (226b, 229a) is a region of 50 nm or less, the film thickness is 50 nm or more, and when the region is 100 nm or less, 100 nm.
Needless to say, the above film thickness is used. At this time, the lightly doped region (22
6b, 229a) is preferably used in combination with the light guide layer having the above-described composition gradient structure, as shown in FIG. 11, the band gap energy is applied to the active layer. This is because the band structure that becomes smaller as it approaches, even if a region that is not doped with an impurity is provided in the vicinity of the active layer, a light guide layer that suppresses a decrease in carrier injection efficiency is formed. At this time,
The light guide layer having a composition gradient preferably has a GRIN structure as described above, and is effective in forming a lightly doped region even if the multilayer film structure has a band gap energy that decreases as it approaches the active layer. is there. Here, in each light guide layer, even if the impurity is not doped at the time of growth, that is, even if the light guide layer is grown by low concentration doping, impurities may diffuse from the adjacent layer. Even in the region grown by doping, impurities are doped.
Specifically, Mg that is preferably used as a p-type impurity is likely to cause such a diffusion phenomenon, and 43a schematically illustrates a form in which the impurity diffuses from the p-side electron confinement layer 228 to the adjacent layer by diffusion. A light guide layer (region 229a) adjacent to the heavily doped p-side electron confinement layer 229, an active layer (near the p-side barrier layer)
Then, a concentration gradient occurs and a form of diffusion is observed. Further, as shown in the first embodiment, even if the p-side light guide layer is formed by low concentration doping, the p-type impurity is doped by diffusion from the electron confinement layer and the cladding layer of the adjacent layers. Thus, when impurity doping is performed by diffusion, as described above, the impurity concentration in the region near the active layer is made smaller than that in the far region. Such a doped region is preferably provided in at least one of the light guide layers, and more preferably provided in both of the light guide layers to provide a waveguide with reduced light loss. In addition, 51 in the figure,
Reference numeral 52 denotes a change in doping amount in each light guide layer.

Further, the layer structure, impurity doping form, composition, film thickness, etc. in the light guide layer may be the same or different between the first light guide layer and the second light guide layer. . For example, the first light guide layer may be a single film, the second light guide layer may be a multilayer film, and the light guide layers may have different layer configurations.
In the present invention, a highly doped region (226a, 229b) disposed on the cladding layer side between the cladding layers 225, 230 and the active layer 227, and doped at a lower concentration than the heavily doped region, A lightly doped region (2
26b, 229a), the optical loss in the waveguide can be reduced. More preferably, between the lightly doped region (226b, 229a) and the active layer, that is, on the active layer side of the light guide layer, the heavily doped layer (23
1,228) are preferably provided. Here, the high-concentration doped layer can be provided as a part or all of the p-side electron confinement layer 228 and the first nitride semiconductor layer 231 located in the vicinity of the active layer, and the high-concentration doped layers 231 and 228 can be provided. The doped amount of each of the lightly doped regions 226b,
229a is doped at a higher concentration, and preferably, the doping amount of the high-concentration doped layer 228 in the p-type layer is larger than that in the high-concentration doped region 229b in the p-type layer. In the concentration doped layer, a pn junction is formed, which is excellent in carrier injection from the p-type layer side, and can be provided with a change in impurity doping amount and a change in carrier concentration. Here, 225 to 230 in FIG. 1 correspond to the respective layers 25 to 30 in the stacked structure 20 in FIG. 10.

[Example 1]
Hereinafter, as an example, a laser element using a nitride semiconductor will be described with respect to the laser element structure shown in FIG. 1 and the waveguide structure shown in FIG.

Here, although the GaN substrate is used in this embodiment, a different substrate different from the nitride semiconductor may be used as the substrate. Examples of the heterogeneous substrate include, for example, sapphire, spinel (including an insulating substrate such as MgA1 ₂ O ₄ , SiC (including 6H, 4H, and 3C) having any one of the C-plane, R-plane, and A-plane. ZnS, ZnO, GaAs, S
It is known conventionally that a nitride semiconductor such as i and an oxide substrate lattice-matched with the nitride semiconductor can be grown, and a substrate material different from the nitride semiconductor can be used. Preferable heterogeneous substrates include sapphire and spinel.
Further, the heterogeneous substrate may be off-angle, and in this case, it is preferable to use a step-off-angle substrate because growth of the underlayer made of gallium nitride grows with good crystallinity. Further, in the case of using a different substrate, after growing a nitride semiconductor serving as an underlayer before forming the element structure on the different substrate, the different substrate is removed by a method such as polishing to obtain a single substrate of the nitride semiconductor. An element structure may be formed, or a method of removing the heterogeneous substrate after the element structure is formed may be used.

When a heterogeneous substrate is used, an element structure is formed through a base layer made of a buffer layer (low temperature growth layer) and a nitride semiconductor (preferably GaN), and the nitride semiconductor grows well. . In addition, when a nitride semiconductor grown by ELOG (Epitaxially Laterally Overgrowth) is used as a base layer (growth substrate) provided on a different substrate, a growth substrate having good crystallinity can be obtained. As a specific example of the ELOG layer,
A mask region formed by growing a nitride semiconductor layer on a heterogeneous substrate and providing a protective film on the surface of which a nitride semiconductor is difficult to grow, and a non-mask region for growing a nitride semiconductor are striped. In addition to the growth in the film thickness direction, the growth in the lateral direction is achieved by growing the nitride semiconductor from the non-mask region, so that the nitride semiconductor also grows in the mask region. There are deposited layers. In another form, the nitride semiconductor layer grown on the different kind of substrate may be provided with an opening, and the film may be formed by lateral growth from the side of the opening.

(Substrate 101) As a substrate, a nitride semiconductor grown on a different substrate, GaN in this example, was grown as a thick film (100 μm), and then the different substrate was removed to obtain 80 μm.
A nitride semiconductor substrate made of m GaN is used. A detailed method of forming the substrate is as follows. A different type substrate made of sapphire with a 2 inch φ and C-plane as the main surface is M
Set in an OVPE reaction vessel, set the temperature to 500 ° C., and trimethylgallium (
Using TMG) and ammonia (NH ₃ ), the buffer layer made of GaN is 200 mm.
Then, the temperature is raised, and undoped GaN is grown to a thickness of 1.5 μm to form an underlayer. Next, a plurality of striped masks are formed on the surface of the underlayer, and a nitride semiconductor, GaN in this embodiment is selectively grown from the mask opening (window), and growth accompanied by lateral growth (ELOG) The nitride semiconductor layer formed by the above method is further grown in a thick film, and the heterogeneous substrate, the buffer layer, and the base layer are removed to obtain a nitride semiconductor substrate. At this time, the mask during selective growth is made of SiO ₂ and has a mask width of 15 μm and an opening (window) width of 5 μm.

(Buffer layer 102) The temperature is set to 1050 ° C. on the nitride semiconductor substrate, and T
A buffer layer 102 made of Al _0.05 Ga _0.95 N is grown to a thickness of 4 μm using MG (trimethyl gallium), TMA (trimethyl aluminum), and ammonia. This layer functions as a buffer layer between the AlGaN n-type contact layer and the nitride semiconductor substrate made of GaN. Next, each layer which becomes an element structure is laminated | stacked on the base layer which consists of nitride semiconductors.

(N-type contact layer 103)
Next, an n-type contact layer 103 made of Al _0.05 Ga _0.95 N doped with Si at 1050 ° C. is grown to a thickness of 4 μm on the obtained buffer layer 102 using TMG, TMA, ammonia, and silane gas as an impurity gas. n-type contact layer,
Or a nitride semiconductor containing Al in the underlayer such as a buffer layer, specifically, Al
_{By using x} Ga _1-x N (0 <x ≦ 1), the deterioration of crystallinity due to the use of ELOG, particularly the generation of pits, is suppressed compared to nitride semiconductors that do not contain Al, such as GaN. Therefore, it is preferable to use a nitride semiconductor containing Al.

(Crack prevention layer 104) Next, TMG, TMI (trimethylindium),
A crack prevention layer 104 made of In _0.06 Ga _0.94 N is grown to a thickness of 0.15 μm using ammonia at a temperature of 800 ° C. This crack prevention layer can be omitted.

(N-type cladding layer 105) Next, the temperature is set to 1050 ° C., and TMA is used as the source gas.
Then, an A layer made of undoped Al _0.05 Ga _0.95 N is grown to a thickness of 25 mm using TMG and ammonia. Subsequently, TMA is stopped, silane gas is used as an impurity gas, and Si is 5 × 10 ¹⁸ / cm ^3. A B layer made of doped GaN is grown to a thickness of 25 mm. Then, this operation is repeated 200 times, and the A layer and the B layer are laminated to grow an n-type cladding layer 106 made of a multilayer film (superlattice structure) having a total film thickness of 1 μm. At this time, if the Al mixed crystal ratio of undoped AlGaN is in the range of 0.05 or more and 0.3 or less, a refractive index difference that sufficiently functions as a cladding layer can be provided.

(N-type light guide layer 106) Next, at the same temperature, using TMG and ammonia as source gases, an n-type light guide layer 106 made of undoped GaN is grown to a thickness of 0.1 μm. Further, an n-type impurity may be doped.

(First Nitride Semiconductor 131) Next, as shown in FIG. 6, the temperature is set to 800 ° C., TMI (trimethylindium) and TMG are used as source gases, Si-doped In _0.05 Ga _0.95 N, film thickness A first nitride semiconductor layer of 500 Å is formed.

(Active layer 107) Next, the temperature was set to 800 ° C., and as shown in FIG. 6, TMI (trimethylindium) and TMG were used as source gases, and undoped In _0.05 Ga.
_A barrier layer made of _0.95 N and a well layer made of undoped In _0.32 Ga _0.68 N are stacked in this order: barrier layer 2 a / well layer 1 a / barrier layer 2 b / well layer 1 b / barrier layer 2 c. At this time, as shown in FIG. 6, the barrier layers 2a, 2b and 2c are formed with a thickness of 130 mm, and the well layers 1a and 1b are formed with a thickness of 25 mm. The active layer 107 has a multiple quantum well structure (MQW) with a total film thickness of about 440 mm.

(P-side electron confinement layer 108) Next, at the same temperature, TMA and TMG are used as source gases.
And p-type electron confinement layer 108 made of Al _0.3 Ga _0.7 N doped with 1 × 10 ¹⁹ / cm ^{3 of} Mg using Cp ₂ Mg (cyclopentadienylmagnesium) as an impurity gas and ammonia. Grow with thickness. Although this layer does not need to be provided in particular, it functions as electron confinement and contributes to lowering the threshold.

(P-type light guide layer 109: second nitride semiconductor layer) Next, the temperature is set to 1050 ° C., TMG and ammonia are used as source gases, and p made of undoped GaN is used.
The mold light guide layer 109 is grown to a thickness of 0.15 μm.

The p-type light guide layer 109 is grown as an undoped layer, but the Mg concentration becomes 5 × 10 ¹⁶ / cm ³ due to diffusion of Mg from adjacent layers such as the p-side electron confinement layer 108 and the p-type cladding layer 109. p-type. This layer may be intentionally doped with Mg during growth.

(P-type cladding layer 110) Subsequently, undoped Al _0.05 Ga _0.95 at 1050 ° C.
A layer made of N is grown to a thickness of 25 mm, then TMA is stopped, a layer made of Mg-doped GaN is grown to a thickness of 25 mm using Cp ₂ Mg, and this is repeated 90 times to obtain a total film thickness. A p-type cladding layer 110 made of a 0.45 μm superlattice layer is grown. When the p-type cladding layer is made of a superlattice in which at least one nitride semiconductor layer containing Al is included and nitride semiconductor layers having different band gap energies are stacked, impurities are heavily doped in either one of the layers. Although so-called modulation doping tends to improve the crystallinity, both may be doped in the same manner. The clad layer 110 is a nitride semiconductor layer containing Al, preferably Al _x Ga ₁ -xN (0
A superlattice structure including <X ≦ 1) is desirable, and a superlattice structure in which GaN and AlGaN are stacked is more preferable. Since the p-side cladding layer 110 has a superlattice structure, the Al mixed crystal ratio of the entire cladding layer can be increased.
Since the refractive index of the cladding layer itself is reduced and the band gap energy is increased, it is very effective for lowering the threshold value. Furthermore, since the superlattice is used, the number of pits generated in the clad layer itself is less than that not formed in the superlattice, so that the occurrence of short circuit is also reduced.

(P-type contact layer 111) Finally, a p-type contact layer 111 made of p-type GaN doped with 1 × 10 ²⁰ / cm ^{3 of} Mg is formed on the p-type cladding layer 110 at 1050 ° C. to a thickness of 150 mm. Grow. The p-type contact layer 111 is p-type In _X
Al _Y Ga _1-XY N (0 ≦ X, 0 ≦ Y, X + Y ≦ 1) can be formed, and the most preferable ohmic contact with the p-electrode 120 can be obtained by using GaN doped with Mg. Since the contact layer 111 is a layer for forming an electrode, 1 × 10 ¹⁷
A high carrier concentration of / cm ³ or higher is desirable. If it is lower than 1 × 10 ¹⁷ / cm ^3, it tends to be difficult to obtain a preferable ohmic with the electrode. Furthermore, when the composition of the contact layer is GaN, a preferable ohmic with the electrode material is easily obtained. After the completion of the reaction, the wafer is annealed in a reaction vessel at 700 ° C. in a nitrogen atmosphere to further reduce the resistance of the p-type layer.

After growing the nitride semiconductor and laminating the layers as described above, the wafer is taken out from the reaction vessel, and a protective film made of SiO ₂ is formed on the surface of the uppermost p-type contact layer, and RIE (reactive ions) is formed. etched with SiCl ₄ gas using the etching), as shown in FIG. 1 to expose the surface of the n-type contact layer 103 for forming the n electrode. Thus, SiO ₂ is optimal as a protective film for deep etching of the nitride semiconductor.

Next, a ridge stripe is formed as the above-described stripe-shaped waveguide region. First, PVD is formed on almost the entire surface of the uppermost p-type contact layer (upper contact layer).
A first protective film 161 made of Si oxide (mainly SiO ₂ ) is formed with a thickness of 0.5 μm by an apparatus, and then a mask having a predetermined shape is applied on the first protective film, and RIE ( The first protective film 161 having a stripe width of 1.6 μm is formed by a photolithography technique using CF ₄ gas by a reactive ion etching apparatus.
At this time, the height (etching depth) of the ridge stripe is the p-type contact layer 11.
1 and a part of the p-type cladding layer 109 and the p-type light guide layer 110 are etched to a depth at which the thickness of the p-type light guide layer 109 is 0.1 μm.
Form.

Next, after forming the ridge stripe, the Zr oxide (
A second protective film 162 mainly made of ZrO ₂ ) is continuously formed with a film thickness of 0.5 μm on the first protective film and on the p-type light guide layer 109 exposed by etching.

After forming the second protective film 162, the wafer is heat-treated at 600 ° C. Thus Si
When a material other than O ₂ is formed as the second protective film, 3 is formed after the second protective film is formed.
00 ° C. or higher, preferably 400 ° C. or higher, and below the decomposition temperature of the nitride semiconductor (1200
It is more desirable to add this step, because the second protective film becomes difficult to dissolve in the dissolved material (hydrofluoric acid) of the first protective film by heat treatment at a temperature of [deg.] C.).

Next, the wafer is immersed in hydrofluoric acid, and the first protective film 161 is removed by a lift-off method. As a result, the first protective film 161 provided on the p-type contact layer 111 is removed, and the p-type contact layer is exposed. As described above, as shown in FIG. 1, the second protective film 162 is formed on the side surface of the ridge stripe and on the plane continuous therewith (exposed surface of the p-type light guide layer 109).

Thus, after the first protective film 161 provided on the p-type contact layer 112 is removed, the exposed surface of the p-type contact layer 111 is made of Ni / Au as shown in FIG. A p-electrode 120 is formed. However, the p-electrode 120 is 100 μm
A stripe width of m is formed over the second protective film 162 as shown in FIG. After forming the second protective film 162, the n-type contact layer 103 already exposed is exposed.
A striped n-electrode 121 made of Ti / Al is formed in the direction parallel to the stripe.

Next, the p and n electrodes are etched and exposed to form an n electrode.
A desired region is masked to provide an extraction electrode, and a dielectric multilayer film 164 made of SiO ₂ and TiO ₂ is provided, and then Ni—Ti—Au (1000Å−) is formed on the p and n electrodes.
Extraction (pad) electrodes 122 and 123 each having a thickness of 1000 to 8000 cm were provided. At this time, the width of the active layer 107 is 200 μm (width in the direction perpendicular to the resonator direction), and a dielectric multilayer film made of SiO ₂ and TiO ₂ is also provided on the resonator surface (reflection surface side). It is done.

After forming the n-electrode and the p-electrode as described above, a bar is formed on the nitride semiconductor M-plane (GaN M-plane, (1 1-0 0), etc.) in a direction perpendicular to the stripe-shaped electrode. Then, a bar-shaped wafer is further divided to obtain a laser element. At this time, the resonator length is 650 μm. The laser element obtained in this way has a laminated structure 20 shown in FIG. 6 and a band gap energy diagram.

The obtained laser device has a threshold current density of 2.8 kA / cm ² and a wavelength of 448 n.
In comparison with the case where the light guide layer of Reference Example 1 is made of InGaN, a laser having a low threshold current density can be obtained in the long wavelength region.

FIG. 8 shows the wavelength of 425 in Example 1 by changing the In mixed crystal ratio of the well layer 1.
A laser element having a wavelength of ˜450 nm is manufactured and the threshold current density Jth is measured to show the wavelength dependence of the threshold current density. As is apparent from FIG. 8, 430 nm
In the following short wavelength region, the upper part having a nitride semiconductor containing In as in Reference Example 1,
When the structure in which the active layer is sandwiched between the lower light guide layers is used for the waveguide structure, the threshold current density tends to be lower. In the long wavelength region of 440 nm or more, Example 1
Shows a gentle upward trend, whereas in Reference Example 1, a rapid upward trend is observed. As in Example 1, the first nitride semiconductor layer that is a feature of the present invention,
By providing a structure in which the active layer is sandwiched between the second nitride semiconductor layer in the waveguide,
It can be seen that the above-described light loss due to In and the crystallinity problem of the p-type light guide layer can be improved, and a nitride semiconductor device having excellent device characteristics in a long wavelength region can be obtained.

[Example 2]
In Example 1, as shown in FIG. 5, a first nitride semiconductor layer made of undoped In _0.025 Ga _0.975 N having a lower crystal ratio than that of the barrier layer 2 is formed to a thickness of 500 mm. The resulting laser element has a waveguide sandwiched between the upper and lower cladding layers because the In mixed crystal ratio of the first nitride semiconductor layer 31 is smaller than that of the first embodiment.
In this embodiment, since the difference in refractive index between the n-type light guide layer and the region sandwiched by the p-type light guide layer and the cladding layer is smaller than that in the first embodiment, the threshold current increases, but the long wavelength region. The laser device having excellent characteristics can be obtained.

[Example 3]
In the second embodiment, as shown in FIG. 8, the first nitride semiconductor layer 31 is provided at a distance of 200 mm from the active layer. At this time, the laminated structure sandwiched between the n-side cladding layer and the active layer is n-type cladding layer 25 / first n-type light guide layer 26a / first nitride semiconductor layer 31 / second n-type light. The guide layer 26b / active layer 12 are stacked in this order, and the first n-type light guide layer 26a is 800 ２６ with undoped GaN, and the second n-type light guide layer 26b is 200 で with undoped GaN. In the obtained laser element, since the first nitride semiconductor layer is separated from the active layer as compared with Example 2, the light confinement and carrier injection effects by the first nitride semiconductor layer are weakened. The distribution of light in the waveguide is more distributed on the n-type cladding layer side than in Example 1, stimulated emission in the active layer is reduced, and light loss due to the first nitride semiconductor also occurs. The threshold current tends to be larger than in the second embodiment.

[Example 4]
In Example 1, as shown in FIG. 7, as an n-type light guide layer, undoped In
_A laser element is obtained in the same manner as in Example 1 using a first nitride semiconductor composed of _0.05 Ga _0.95 N and a film thickness of 0.15 μm. The resulting laser element is compared to Example 1,
The n-type light guide layer made of InGaN increases the refractive index difference between the waveguide and the clad layer, but the absorption of light by the thick n-type light guide layer increases, and the light distribution in the waveguide also increases. This is considered to be broadly distributed in the region from the active layer to the n-type cladding layer, the stimulated emission in the active layer is reduced, and the threshold current is increased as compared with Example 1. At this time, the n-type light guide layer (first nitride semiconductor layer) is made of In.
Even though a GaN / GaN superlattice multilayer film is formed, the crystallinity of the film is improved as compared with the case of forming a single film, but the problems of light distribution and waveguide refractive index are This is equivalent to the case of a single film, and the obtained laser element has the same tendency.

[Reference Example 1]
In Example 4, the p-side light guide layer is made of the same undoped I as the n-side light guide layer.
A laser element is obtained in the same manner as in Example 4 _{except that} n _0.05 Ga _0.95 N is used.
The obtained laser element has the same thickness as that of the p-type light guide layer without providing the first nitride semiconductor as compared with the waveguide structure of the first embodiment. The guide layer also has a structure using a nitride semiconductor containing In. The laser element obtained in this manner is significantly deteriorated in crystallinity due to the use of a nitride semiconductor containing In in the p-type light guide layer, and further, light absorption by the light guide layer occurs. Compared to Example 1, the threshold current becomes larger. FIG. 9 shows a reference example 1 in which a laser element having a wavelength of 425 nm to 450 nm is manufactured by changing the In mixed crystal ratio of the well layer, the threshold current density Jth is measured, and the wavelength dependence of the threshold current density is measured. It shows sex. FIG.
As is clear from the above, in the structure using the nitride semiconductor containing In in the upper and lower light guide layers, the threshold current density rapidly increases as the wavelength increases from around 430 nm, and the long wavelength region of 440 nm or more. Then, compared with Example 1, it turns out that the difference becomes large as a threshold current density becomes large and wavelength becomes longer than it.

[Example 5]
In Example 1, an n-type layer, a p-type clad layer, a guide layer, and an active layer are formed as follows, and a laser device having a structure shown in FIG. 10 is manufactured.

(N-type cladding layer 105) n-type contact layer 103, crack prevention layer 104 (
On top of the undoped Al _0.1 Ga _0.9 N as an n-type cladding layer
A first layer of a more is grown to a thickness of 25 Å, followed by stopping the TMA, using a silane gas as the impurity gas, and 5 × 1018 / cm3 doped with Si _{Al 0.0}
A second layer made of ₅ Ga _0.95 N is grown to the thickness of 25 Å. Then, this operation is repeated 200 times, and the first layer and the second layer are alternately laminated to grow the n-type cladding layer 106 made of a multilayer film (superlattice structure) having a total film thickness of 1 μm.
When this n-type cladding layer is a lower cladding layer provided under the active layer, it does not have to be composed of a superlattice multilayer film, but can be a single film or a multilayer film having a thickness of 100 mm or more. A layer can be formed.

(N-type light guide layer 106) A third layer made of Si-doped GaN is formed with a thickness of 15 mm.
Then, a fourth layer made of undoped In _0.05 Ga _0.95 N is grown to a thickness of 15 mm. Then, this operation is repeated 60 times, and the third layer and the fourth layer are alternately stacked. Growing on the n-type cladding layer.

(First Nitride Semiconductor 131) Next, Si-doped In _0.05 Ga _0.95
A first nitride semiconductor layer having a thickness of 530 mm is formed on the n-type light guide layer.

(Active layer 107) As shown in FIG. 10, an n-side barrier layer 2a made of undoped GaN having a thickness of 130 mm, and undoped In _0.25 Ga _0.75 N having a thickness of 25 mm.
Well layer 1a, barrier layer 2b made of undoped GaN with a thickness of 100 、, and further, well layer 1b same as well layer 1a, undoped In _{0.05 with a} thickness of 530
The p-side barrier layer 2c made of Ga _0.95 N is replaced with a barrier layer 2a / well layer 1a / barrier layer 2
The layers are stacked in the order of b / well layer 1b / barrier layer 2c. The active layer 107 has a total film thickness of about 810.
A multi-quantum well structure (MQW) is formed, and is formed on the first nitride semiconductor layer. Further, when the n-side barrier layer 2a is in contact with the first nitride semiconductor layer, the first nitride semiconductor layer can also serve as the n-side barrier layer 2a. In this case, the n-side barrier layer 2a The first nitride semiconductor layer in contact with the active layer also functions as the n-side barrier layer 2a.

(P-side electron confinement layer 108) Next, at the same temperature, TMA and TMG are used as source gases.
And ammonia, Cp2Mg (cyclopentadienylmagnesium) as an impurity gas, and Al _0.3 Ga _0.7 N doped with Mg at 1 × 10 19 / cm 3
A p-type electron confinement layer 108 is grown to a thickness of 100 mm. Although this layer does not need to be provided in particular, it functions as electron confinement and contributes to lowering the threshold.

(P-type light guide layer 109) A third layer made of Mg-doped GaN is formed with a thickness of 15 mm.
Then, a fourth layer made of undoped In _0.05 Ga _0.95 N is grown to a thickness of 5 mm. Then, this operation is repeated 90 times, and the third
Layer and fourth layer are alternately stacked to form a multilayer film (superlattice structure) with a total film thickness of 0.18 μm
A p-type light guide layer 109 is grown on the p-side electron confinement layer. At this time, the second nitride semiconductor layer of the present invention is formed as a third layer.

(P-type cladding layer 110) A first layer made of undoped Al _0.1 Ga _0.9 N
A layer of 25 mm thick and subsequently Mg doped Al _0.05 Ga _{0
. A} second layer of ₉₅ N is grown to a thickness of 25 mm. Then, this operation is repeated 90 times, and the first layer and the second layer are alternately laminated to obtain a total film thickness of 0.45.
A p-type cladding layer 110 made of a μm multilayer film (superlattice structure) is grown on the p-type light guide layer.

Thus, the n-type cladding layer 25, the n-type light guide layer 27, the first nitride semiconductor layer 31, the active layer 27, the p-side electron confinement layer 28, the p-type light guide layer 29, and the p-type cladding layer 30. However, as shown in FIG. 10, the stacked structure is obtained, and the In composition and the Al composition in each layer at that time are structures as shown in 41 and 42. In this embodiment, in the light guide layer, the fourth layer constituting the multilayer n-type light guide layer is formed thicker than the fourth layer of the p-type light guide layer. The nitride semiconductor layer containing In is provided on the p-type layer side because the In mixed crystal ratio (average composition) of the n-type light guide layer is larger than that of the p-type light guide layer. The crystallinity deterioration of the structure is reduced. Also, the In mixed crystal ratio is p higher than that of the n-type light guide layer.
As a configuration for reducing the type light guide layer, in addition to reducing the film thickness, it is also possible to reduce the In mixed crystal ratio of the third and fourth layers constituting the multilayer film.
The laser element thus obtained has a threshold current density of 1.9 kA / cm ^2.
Thus, a nitride semiconductor laser element capable of continuous oscillation at a wavelength of 453 nm and room temperature and having a device lifetime of 10,000 hours at 60 ° C. and 5 mW continuous oscillation can be obtained.

[Example 6]
In Example 5, a first multilayer film comprising an n-type cladding layer and a p-type cladding layer is formed.
The layer is undoped Al _0.05 Ga _0.95 N, and the second layer is S
A laser element is obtained in the same manner as in Example 5 except that i and Mg-doped GaN are used.

In the element structures of Examples 1, 5, and 6, the threshold current change when the wavelength of the well mixed layer is changed by changing the In mixed crystal ratio of the well layer is shown in FIG.
Example 5 is indicated by a white triangle Δ and Example 6 is indicated by a white square □. As can be seen from FIG. 12, in the long wavelength region of 440 nm or longer, Examples 5 and 6 provide laser elements with reduced threshold currents, and Example 5 is superior to Examples 5 and 6 in comparison. It can be seen that a laser element having the above characteristics can be obtained.
In Example 1 and Examples 5 and 6, the thicknesses of the p-side barrier layer and the n-side barrier layer are greatly different. Both barrier layers are 200 mm or more, preferably 300 mm or more, more preferably 400 mm or more. Thus, a tendency to reduce the threshold current is observed, and in particular, the distance between the p-side barrier layer or the p-side electron confinement layer and the well layer 1b closest to the p-type layer in the active layer is increased, that is, 200Å. As described above, it is considered that a favorable waveguide structure in a long wavelength region is formed by setting the thickness to 300 mm or more, more preferably 400 mm or more, and the threshold current characteristics as shown in FIG. 12 are obtained. Another difference in the configuration is that the light guide layer has a multilayer film structure including a nitride semiconductor layer containing In. In Examples 5 and 6, this improves the refractive index in the waveguide. This is thought to have led to improved device characteristics.
In Example 5 and Example 6, the Al mixed crystal ratio (average composition) of the cladding layer is different, and the Al mixed crystal ratio (average composition) of the cladding layer is 0.05 or more. In the long wavelength region, it is considered that the characteristic improvement was obtained by forming an excellent waveguide structure. At this time, as the upper limit of the Al mixed crystal ratio (average composition) of the cladding layer, the crystallinity was considered, When the clad layer of the multilayer film is used, the nitride semiconductor layer containing Al (first layer) and Al having an Al mixed crystal ratio smaller than that of the first layer are included. A multilayer film structure in which nitride semiconductor layers (second layers) are stacked at least alternately is preferable, and the Al mixed crystal ratio x1 of the first layer is the Al mixed crystal ratio x2 of the second layer. Larger, x1> x2 (x2> 0) is a laser element in the long wavelength region, It can be seen that excellent device characteristics in the device can be obtained.
In Example 5, a laser element in which the element lifetime at the oscillation wavelengths of 465 and 470 reaches 10,000 hours and 3,000 hours under the same conditions as in Example 5 is obtained.

1 is a schematic cross-sectional view illustrating an embodiment of the present invention. 1 is a schematic cross-sectional view illustrating an embodiment of the present invention. 1 is a schematic cross-sectional view illustrating an embodiment of the present invention. The schematic diagram explaining one Embodiment of this invention. The schematic diagram explaining the laminated structure 20 which concerns on one Embodiment of this invention, and the band structure 21 corresponding to the laminated structure. The schematic diagram explaining the laminated structure 20 which concerns on one Embodiment of this invention, and the band structure 21 corresponding to the laminated structure. The schematic diagram explaining the laminated structure 20 which concerns on one Embodiment of this invention, and the band structure 21 corresponding to the laminated structure. The schematic diagram explaining the laminated structure 20 which concerns on one Embodiment of this invention, and the band structure 21 corresponding to the laminated structure. The figure which shows the wavelength dependence of the threshold current density in one Embodiment of this invention and embodiment of the reference example 1. FIG. The schematic diagram explaining the laminated structure 20 which concerns on one Embodiment of this invention, and the band structure 21, Al composition ratio 41, and In composition ratio 42 corresponding to the laminated structure. The schematic diagram explaining the impurity concentration change (51, 52) corresponding to the laminated structure 20 of FIG. 10 which concerns on one Embodiment of this invention. The figure which shows the wavelength dependence of the threshold current in each embodiment (Example 1, 5, 6) of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Well layer, 2 (2b) ... Barrier layer, 2a ... N side barrier layer,
2c ... p-side barrier layer, 11 ... n-type nitride semiconductor layer, 12 ...
Active layer, 13... P-type nitride semiconductor layer, 20.
01 ... Substrate (GaN substrate) 102 ... Buffer layer, 103 ...
n-type contact layer, 104... crack prevention layer, 105, 25, 22
5 ... n-type cladding layer (lower cladding layer), 106, 26, 226 ...
n-type light guide layer (lower light guide layer), 107, 27, 227... active layer,
108, 28, 228... P-side electron confinement layer, 109, 29, 229
... p-type light guide layer (upper light guide layer), 110, 30, 230 ... p
Type cladding layer (upper cladding layer), 111... P-type contact layer, 1
20 ... p electrode, 121 ... n electrode, 122 ... p pad electrode,
123 ... n pad electrode 131, 31, 231 ... first nitride semiconductor layer 32 232 ... second nitride semiconductor layer 163 ... third protective film 164 ..Insulating film

Claims (23)

In a nitride semiconductor device having an active layer sandwiched between a p-type layer and an n-type layer, the p-type layer having a p-type cladding layer, and the n-type layer having an n-type cladding layer,
The active layer has a nitride semiconductor containing In, and has a first nitride semiconductor layer made of a nitride semiconductor having an In mixed crystal ratio z> 0 between the n-type cladding layer and the active layer. And a second nitride semiconductor layer having an In mixed crystal ratio u>z> u between the p-type cladding layer and the active layer. In a nitride semiconductor device having an active layer sandwiched between a p-type layer and an n-type layer, the p-type layer having a p-type cladding layer, and the n-type layer having an n-type cladding layer,
The active layer has a nitride semiconductor containing In, and has a first nitride semiconductor layer made of a nitride semiconductor containing In between the n-type cladding layer and the active layer, and is active with the p-type cladding layer. A nitride semiconductor device having a second nitride semiconductor layer having an In mixed crystal ratio of 0 between the layers. Among the barrier layers in the active layer, the active layer is an n-side barrier layer (2a) disposed closest to the n-type layer, and a p-side barrier layer (2c) disposed closest to the p-type layer. And a well layer made of a nitride semiconductor containing at least one In between the n-side barrier layer (2a) and the p-side barrier layer (2b),
The nitride semiconductor device according to claim 1 or 2, wherein the n-type impurity concentration of the p-side barrier layer (2c) is smaller than the n-type impurity concentration of the n-side barrier layer (2a).   The p-type layer has a p-side electron confinement layer made of a nitride semiconductor containing Al between the active layer and the second nitride semiconductor layer, or between the active layer and the p-type cladding layer. The nitride semiconductor device according to any one of claims 1 to 3.   The p-type layer has a p-side electron confinement layer made of a nitride semiconductor containing Al, and the p-side electron confinement layer is provided in contact with the active layer or through a buffer layer. The nitride semiconductor device according to claim 1, wherein the nitride semiconductor device is a semiconductor device.   6. The nitride semiconductor device according to claim 5, wherein the buffer layer is made of a nitride semiconductor containing Al having an Al mixed crystal ratio lower than that of the p-side electron confinement layer, or GaN.   The active layer has a well layer (1b) provided between the n-side barrier layer (2a) and the p-side barrier layer (2b) closest to the p-side electron confinement layer in the active layer. 7. The nitride semiconductor device according to claim 4, wherein a distance between the well layer (1b) and the p-side barrier layer is 100 mm or more.   7. The nitride semiconductor device according to claim 1, wherein the n-side barrier layer (2a) and / or the p-side barrier layer (2c) is disposed on the outermost side in the active layer.   9. The nitride semiconductor device according to claim 1, wherein the first nitride semiconductor layer is provided in contact with the active layer.   10. The nitride semiconductor device according to claim 1, wherein the thickness of the first nitride semiconductor layer is 300 mm or more.   11. The nitride semiconductor device according to claim 7, wherein a distance between the well layer (1b) and the p-side barrier layer (2c) is 400 mm or less.   The active layer has an n-side barrier layer (2a) as the layer disposed closest to the n-type layer, and the sum of the film thicknesses of the n-side barrier layer (2a) and the first nitride semiconductor layer The nitride semiconductor device according to claim 9, wherein is not less than 300 mm.   10. The In mixed crystal ratio z of the first nitride semiconductor layer and the In mixed crystal ratio v of the n-side barrier layer (2a) satisfy z ≦ v. Nitride semiconductor device.   11. The nitride semiconductor device according to claim 1, wherein the p-side barrier layer (2c) has a p-type impurity.   15. The nitride semiconductor device according to claim 1, wherein the n-type impurity concentration of the p-side barrier layer (2c) is smaller than the p-type impurity concentration. ^16. The nitride semiconductor device according to claim 1, wherein an n-type impurity concentration of the p-side barrier layer (2c) is less than 5 × 10 ¹⁶ / cm ³ .   14. The nitride semiconductor device according to claim 1, wherein each of the p-type cladding layer and the n-type cladding layer is a light confinement cladding layer and includes a nitride semiconductor containing Al.   The active layer has a quantum well structure having a well layer made of a nitride semiconductor containing In, and the In mixed crystal ratio of the first nitride semiconductor layer is smaller than the In mixed crystal ratio of the well layer, The nitride semiconductor device according to claim 1.   20. The nitride according to claim 1, further comprising an n-type light guide layer made of a nitride semiconductor having an In mixed crystal ratio of 0 between the active layer and the first nitride semiconductor layer. Semiconductor element.   The p-type cladding layer and the n-type cladding layer are optical confinement cladding layers, and at least one of the p-type cladding layer and the n-type cladding layer includes a nitride semiconductor containing at least Al. 18. The nitride semiconductor device according to claim 17, wherein the first semiconductor layer is a multilayer clad layer in which first layers and second layers having different band gap energies are alternately stacked.   A light guide layer between at least one of the p-type clad layer and the n-type clad layer and the active layer, and the light guide layer includes a third layer including a nitride semiconductor containing at least In; 18. The nitride semiconductor device according to claim 16, wherein the third layer is a multilayer light guide layer in which fourth layers having different band gap energies are alternately laminated.   The nitride according to claim 22, wherein the n-type layer includes a light guide layer, and the first nitride semiconductor layer is provided between the light guide layer and the active layer of the n-type layer. Semiconductor element.   The nitride semiconductor device according to claim 22 or 23, wherein the p-type layer includes a light guide layer, and the light guide layer of the p-type layer includes the second nitride semiconductor layer.

Images