JP2006049622A - Nitride semiconductor laser element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a group III-V nitride semiconductor laser element wherein a stimulation threshold value is stably low and a drive current is not secularly increased even when the element is continuously driven at a high output of 100 mW or over. <P>SOLUTION: A band gap energy Ecl of a lower clad layer (102), the band gap energy Enl of a lower adjacent layer (104), the band gap energy Ew of the well layer of a quantum well active layer (105), the band gap energy Eb of a barrier layer, the band gap energy Enu of an upper adjacent layer (106), the band gap energy E1 of a first layer (107), the band gap energy E2 of a second layer (108), the band gap energy E3 of a third layer (109), and the band gap energy Ecu of an upper clad layer (110) hold a relation of Ew < Ecl, Enl, Eb, Enu, E1, E2, Ecu <E3. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a semiconductor laser device made of a III-V group nitride semiconductor.

  A short wavelength light source using a group III-V nitride semiconductor laser element has attracted attention as a technique for improving the information reading rate and writing rate for an optical recording medium. In addition, the development of technology for improving the light emission luminance is active, and a technology for making the active layer of a nitride semiconductor laser element a quantum well structure in which at least one well layer and a barrier layer are stacked is being studied. .

  The advantage of the quantum well structure is that the state density function ρ of electrons and holes in the active layer can be controlled artificially narrowly. This makes it possible to achieve an active layer structure that is not a quantum well structure such as a bulk type. Therefore, it is considered that the emission luminance of the element is drastically improved because the oscillation threshold value of the laser element is greatly reduced.

  By the way, in a semiconductor laser device having an active layer having such a quantum well structure, p-type impurities in a semiconductor layer provided close to the active layer are diffused into the active layer to raise the oscillation threshold of the device. There is a problem that it ends up. Therefore, for the purpose of preventing the diffusion of such p-type impurities into the active layer, as shown in FIG. 4, the n-type is interposed between the active layer 407 and the p-type nitride semiconductor layer (p-type light guide layer 409). There is a technique of providing a nitride semiconductor layer (diffusion prevention layer 414) (see, for example, Patent Document 1).

JP-A-10-200214 (2nd page)

  However, even if a nitride semiconductor laser device is manufactured using the technique described in Patent Document 1, the oscillation threshold value of the device cannot be lowered sufficiently stably, and it is continuously driven at a high output of 100 mW or more. In this case, there is a problem that the operating current is short because the driving current increases with time.

  The present invention solves the above-described problems in a nitride semiconductor laser device. The oscillation threshold is stable and low, and the drive current does not increase with time even when continuously driven at a high output of 100 mW or more. An object of the present invention is to provide a group-based nitride semiconductor laser device.

In order to solve the above problems, a III-V nitride semiconductor laser device according to the present invention includes a substrate, a lower cladding layer doped with an n-type impurity provided on the substrate, and the lower cladding. An undoped lower adjacent layer not doped with a conductive impurity provided on the layer, an undoped barrier layer provided on the lower adjacent layer, and an undoped type sandwiching the barrier layer vertically A quantum well active layer composed of a plurality of well layers; an undoped upper adjacent layer provided on the quantum well active layer; and an n-type impurity provided on the upper adjacent layer. A first layer; an undoped second layer provided on the first layer; and an upper cladding layer doped with a p-type impurity provided on the second layer; A lowermost layer in the quantum well active layer A group III-V nitride semiconductor laser device in which a door layer is in contact with the lower adjacent layer and an uppermost well layer is in contact with the upper adjacent layer, and the band gap energy Ecl of the lower cladding layer, The band gap energy Enl of the lower adjacent layer, the band gap energy Ew of the well layer of the quantum well active layer, the band gap energy Eb of the barrier layer, the band gap energy Enu of the upper adjacent layer, and the first layer The band gap energy E1 of the second layer, the band gap energy E2 of the second layer, the band gap energy E3 of the third layer, and the band gap energy Ecu of the upper cladding layer are expressed by the following relational expression (1). It is characterized by satisfying.
[Equation 1]
Ew <Ecl, Enl, Eb, Enu, E1, E2, Ecu <E3 (1)

  The above “dope” means intentional addition of conductive impurities into the semiconductor crystal, and the doped semiconductor layer means a semiconductor layer to which conductive impurities are intentionally added. .

The “undoped type” means a state of a semiconductor crystal to which no conductive impurities are intentionally added, and has almost no influence on impurities such as C, H, O, Cl, and conductivity. Even a semiconductor crystal in which a conductivity control impurity of such a level that the concentration is not given (less than about 5 × 10 ¹⁶ / cm ³ ) is inevitably mixed during crystal growth is included.

  Since the band gap energy of the third layer is set to be significantly higher than that of the well layer in the active layer in the above-described configuration according to the III-V group nitride semiconductor laser device of the present invention, the active layer It is possible to prevent overflow of electrons injected into the semiconductor layer, improve carrier injection efficiency into the active layer, and lower the oscillation threshold of the device.

  Furthermore, with the above structure, since the active layer is an undoped type, a decrease in crystallinity due to the addition of impurities is not caused. Thereby, since the durability of the active layer is increased, even if the light energy density is increased by high-power laser oscillation of 100 W or more, the crystallinity is not rapidly deteriorated and the drive current value does not increase with time.

  In the above structure, since the third layer is doped with p-type impurities, the conductivity of the third layer is high. This facilitates the injection of holes from the p-doped upper cladding layer into the active layer, that is, improves the efficiency of carrier injection into the active layer, thereby lowering the oscillation threshold of the device.

  Also, with the above configuration, by providing the first layer doped with n-type impurities between the p-doped third layer and the active layer, the pn junction position in the element is stabilized, Since resistance to perturbations of growth conditions that can occur during crystal growth is increased, devices can be manufactured with high yield.

In the III-V nitride semiconductor laser element of the present invention, a lower guide layer is further provided between the lower cladding layer and the lower adjacent layer, and a band gap energy Egl of the lower guide layer is as follows. It can be set as the structure which satisfy | fills relational expression (2). In the III-V nitride semiconductor laser element of the present invention, an upper guide layer is further provided between the third layer and the upper clad layer, and the band gap energy Egu of the upper guide layer is provided. Can satisfy the following relational expression (3).
[Equation 2]
Ew <Egl <Ecl (2)
[Equation 3]
Ew <Egu <Ecl (3)

  With these configurations, since the light confinement coefficient of the active layer is increased by providing the light guide layer, the oscillation threshold of the element can be further reduced.

The III-V group nitride semiconductor laser device of the present invention further has a configuration in which the distance L between the uppermost well layer and the third layer in the quantum well active layer is 50 nm or more and 200 nm or less. can do. The distance L and the distance D between the uppermost well layer and the first layer in the quantum well active layer may satisfy the following relational expression (4).
[Equation 4]
2D ≦ L (4)

  The p-doped layer has a property of absorbing laser light to some extent, and if a p-doped layer is provided in the vicinity of the active layer where the light distribution of the laser light is large, waveguide loss is caused. Further, since the nitride semiconductor has a wurtzite crystal structure and is not isotropic, an electric field exists inside the crystal, and when a semiconductor layer having a large band gap difference from the active layer is arranged in the vicinity of the active layer, The band of the active layer is bent, a band flat state cannot be obtained, and the carrier recombination probability in the active layer is lowered.

  However, in the above configuration according to the present invention, since the distance (L) between the lower surface of the third layer and the upper surface of the well layer provided at the uppermost stage of the active layer is 50 nm or more, significant waveguide loss And a sufficient band flat state of the active layer can be obtained. On the other hand, since the upper limit of the separation interval (L) is 200 nm or less, hole injection from the p-doped upper cladding layer to the active layer does not become difficult in terms of distance.

  The III-V group nitride semiconductor laser device of the present invention may be configured such that the thickness of the first layer is not less than 5 nm and not more than 50 nm.

  With this configuration, since the n-doped first layer has a thickness of 5 nm or more, n-type conductivity can be established in the layer, and the active layer is sandwiched between the n-doped layers and activated. The layer can be in a band flat state. On the other hand, since the layer thickness is 50 nm or less and the content of n-type impurities is limited, hole injection from the p-doped upper cladding layer to the active layer is not electrically inhibited by the first layer. .

  The III-V group nitride semiconductor laser device of the present invention may be configured such that the lower adjacent layer or the upper adjacent layer has a layer thickness of 0.5 nm or more.

  In the range of about 0.5 nm above and below the active layer, carriers may leak out in a quantum manner, but with the above configuration according to the present invention, the lower adjacent layer or the upper adjacent layer provided in contact with the upper and lower sides of the active layer Since the layer thickness of the layer is 0.5 nm or more and the semiconductor layer doped with the conductive impurity is provided at a distance of 0.5 nm or more from the active layer, such a seepage carrier is trapped and the carrier injection efficiency is increased. There is no decline.

  According to the present invention, the pn junction position in the III-V group nitride semiconductor laser element is stabilized, the crystallinity of the active layer is hardly lowered, and the carrier injection efficiency is improved. The driving current does not increase with time even when continuously driven at a high output of 100 mW or more.

  The best mode of the nitride semiconductor laser device of the present invention will be described based on the first to eighth embodiments.

<Embodiment 1>
FIG. 1 is a schematic cross-sectional view of a group III-V nitride semiconductor laser element according to Embodiment 1 of the present invention. As shown in FIG. 1, this nitride semiconductor laser device includes an n-type GaN substrate 100 and an n-doped GaN layer 101 (thickness) doped with silicon (Si) provided in contact with one main surface of the substrate. 0.5 μm), a silicon-doped n-doped Al _0.07 Ga _0.93 N lower cladding layer 102 (thickness 2 μm) provided on and in contact with the GaN layer 101, and the lower cladding layer 102 An n-doped GaN lower guide layer 103 (thickness 0.1 μm) doped with silicon, which is provided in contact with the electrode, and a conductive impurity which is provided in contact with the lower guide layer 103 and is not doped. An undoped GaN lower adjacent layer 104 (thickness 20 nm), an active layer 105 (details will be described later) provided in contact with the lower adjacent layer 104, and a contact with the active layer 105 An undoped GaN upper adjacent layer 106 (thickness 20 nm) provided on the upper adjacent layer 106, and an n-doped GaN layer 107 (thickness 20 nm) as a first layer provided on the upper adjacent layer 106. The undoped GaN layer 108 (thickness 30 nm) as the second layer provided in contact with the first layer 107 and the second layer 108 provided in contact with the first layer 107. A p-doped Al _0.1 Ga _0.9 doped with magnesium (Mg) provided on and in contact with the p-doped Al _0.2 Ga _0.8 N layer 109 (thickness 15 nm) as the third layer. N upper clad layer 110 (thickness 0.6 μm), p-doped GaN contact layer 111 (thickness 0.1 μm) doped with magnesium provided on and in contact with upper clad layer 110, and this contact A p-side electrode 121 provided over and in contact with the 111, and an n-side electrode 120 provided on the back surface of the substrate on one main surface opposite to the substrate.

  The upper cladding layer 110 and the contact layer 111 form a striped ridge stripe waveguide extending in the resonator direction. Then, a portion other than the ridge stripe structure is buried with the insulating film 122 to realize current confinement. The width of the ridge stripe is 1.6 μm and the resonator length is 600 μm. Further, an AR (anti-reflective) coating is applied to the front surface of the element, and an HR (hydroreflective) coating is applied to the rear surface.

The concentration of silicon doped in each n-doped layer described above is in the range of 5 × 10 ^{16 to} 5 × 10 ¹⁸ / cm ³ , and the concentration of magnesium doped in the p-doped layer is 2 × 10 ¹⁸ to 2 It is in the range of × 10 ²⁰ / cm ³ .

In Example 1, the silicon concentration doped in the first layer 107 is 5 × 10 ¹⁷ / cm ³ , but the concentration may be changed in the range of 5 × 10 ^{16 to} 5 × 10 ¹⁸ / cm ^3. A range of 1 × 10 ¹⁷ / cm ³ to 2 × 10 ¹⁸ / cm ³ is preferable. The magnesium concentration doped in the third layer 109 is 1 × 10 ²⁰ / cm ³ , but the concentration may be changed in the range of 5 × 10 ¹⁹ to 2 × 10 ²⁰ / cm ³ . In such a doping concentration range, the conductivity type of the layer itself can be established by the doped conductive impurity, and the pn junction position in the element is sufficiently stabilized, while the crystallinity of the layer due to the impurity inclusion The decrease is not caused and effective carrier injection into the active layer is not inhibited.

  The n-type impurity is not limited to the silicon (Si), but is a group IV such as germanium (Ge), tin (Sn), sulfur (S), oxygen (O), titanium (Ti), zirconium (Zr), etc. Alternatively, a group VI element can be used, but one of silicon (Si), germanium (Ge), and tin (Sn) is selected which is relatively less likely to cause a decrease in crystallinity of the nitride semiconductor layer due to impurities. Is preferred.

  The layers adjacent to the lowermost layer and the uppermost well layer 131 (the lower adjacent layer 104 and the upper adjacent layer 106) need to be undoped as described above. This is because carriers may ooze out from the active layer to the layers adjacent to the top and bottom, and if the layers adjacent to the top and bottom contain conductive impurities, the carriers ooze out and are trapped in the layer. This is to reduce the carrier injection efficiency. Further, since the extent of this bleeding is about 0.5 nm, the layer thicknesses of the lower adjacent layer 104 and the upper adjacent layer 106 are set to at least about 0.5 nm, preferably about 1 nm or more, respectively.

  The composition of the first layer 107 and the second layer 108 is not limited to the above GaN, but if the composition of these two layers is matched as in the above structure, the lattice distortion between the layers due to stacking is less likely to occur. Therefore, the variation in electrical characteristics is reduced, and the crystallinity of the layer is increased. As a result, the operating life of the element can be improved, which is preferable.

The substrate is most preferably a GaN substrate as described above from the side face that suppresses lattice mismatch with respect to the nitride semiconductor layer to be laminated, but instead, AlGaN, SiC, spinel, Ga ₂ O ₃ , sapphire Alternatively, a substrate made of ZrB ₂ or the like may be used.

The lower cladding layer 102 and the upper cladding layer 110 may have a superlattice structure. Further, the composition is not limited to the above, and for example, Al _x Ga _1-x N in which x is defined in the range of 0.05 or more and 0.15 or less can be used. The reason for defining x in such a range is that if x is less than 0.05, the refractive index difference from the active layer decreases, optical confinement deteriorates, and the operating current increases. Is given. Further, when x is 0.15 or more, it is difficult to obtain a low-resistance crystal, and the operating voltage increases.

Here, a detailed structure of the active layer 105 will be described with reference to FIG. 2 showing an element structure relating to the active layer and its neighboring layers. This active layer 105 is composed of an undoped In _0.15 Ga _0.85 N well layer 131 (thickness 4 nm) and an undoped GaN barrier layer 132 (thickness 8 nm), a well layer, a barrier layer, a well layer, a barrier layer, A multi-quantum well structure having three wells stacked in order of well layers, with the lowermost well layer in contact with the lower adjacent layer 104 and the uppermost well layer in contact with the upper adjacent layer A layer 106 is provided.

As long as the band gap energy of the barrier layer is adjusted to be larger than that of the well layer, the well layer and the barrier layer are not limited to the above composition, and InxGa ₁ -xN (0 ≦ x <1), AlxGa _{1 -x N (0 ≦ x <1} ), InGaAlN, GaN 1-x As x (0 <x <1), GaN 1-x P x (0 <x <1) or a nitride semiconductor made of these compounds It is good.

  In addition, it is preferable that the active layer has such a multiple quantum well structure (MQW structure) because the oscillation threshold of the device is lowered, but one barrier layer is in contact with the lowermost well layer and the upper adjacent layer 106 It does not exclude the single quantum well structure (SQW structure) provided in contact with the bottom.

FIG. 3 is a distribution diagram showing the band gap energy of each layer around the active layer of the nitride semiconductor laser device according to the first embodiment. In the semiconductor laser device of the present invention, as shown in FIG. 3, the band gap energy Ecl of the lower cladding layer 102, the band gap energy Enl of the lower adjacent layer 104, the band gap energy Ew of the well layer 131, and the barrier layer 132, the band gap energy Enu of the upper adjacent layer 106, the band gap energy E1 of the first layer 107, the band gap energy E2 of the second layer 108, and the band of the third layer 109. The gap energy E3 and the band gap energy Ecu of the upper cladding layer 110 satisfy the following relational expression (1).
[Equation 5]
Ew <Enl, Eb, Enu, E1, E2 <Ecl, Ecu <E3 (1)

Further, the band gap energy Egl of the lower guide layer 103, the band gap energy Ew of the well layer 131, and the band gap energy Ecl of the lower cladding layer 102 satisfy the following relational expression (2).
[Equation 6]
Ew <Egl <Ecl (2)

  A III-V group nitride semiconductor laser device having the above-described structure is formed by a known crystal growth method such as metal organic chemical vapor deposition (MOCVD) or a method of forming a ridge stripe structure by etching using a photoresist mask. It was made using.

[Example 1]
Example 1 is a group III-V nitride semiconductor laser device having the above structure. When this laser element was driven at room temperature (25 ° C.) under atmospheric pressure (1.01325 × 10 ⁵ Pa), the threshold current value was about 30 mA, and the slope efficiency was about 1.7 W / A. The drive current value was about 100 mA when the output was 120 mW.

  In addition, when an aging test was performed in an atmosphere of 60 ° C. with a pulse condition of 30% and a high output of 120 mW, the drive current value hardly increased even after 24 hours from the start of the test. It was not observed and was suppressed to an increase of 5 mA or less. And even if 1000 hours passed from the start of the test, the drive current value did not increase further from there.

  Such element characteristic values according to Example 1 were stably obtained in all manufactured elements without variation in the wafer every crystal growth.

[Comparative Example 1]
This comparative example 1 is a laser element similar to that of the above-described example 1 except that the first layer 107 is an undoped type. The laser element according to Comparative Example 1 has a very large variation in the characteristic value of each manufactured element, and the threshold current value is about 35 mA and the slope efficiency is about 1 even if the element has the best characteristic value. The drive current value was about 120 mA or more when the output was 120 mW. Moreover, it was observed by the aging test that the drive current value after 24 hours from the start of the test greatly increased by 10 mA or more from the initial value. When the test time was further extended, the increase rate of the drive current value became moderate, but the drive current value continued to increase, and a stable operating current could not be obtained.

[Comparative Example 2]
The present comparative example 2 is the same laser element as in the first embodiment except that the second layer 108 is not provided and the third layer 109 is provided on and in contact with the first layer 107. The periphery of the active layer has substantially the same structure as the above-described laser element according to the prior art (see FIG. 4). The laser element according to Comparative Example 2 had a threshold current value of about 35 mA, a slope efficiency of about 1.0 W / A, and a drive current value of about 160 mA when the output was 120 mW. Further, it was observed by the aging test that the drive current value 24 hours after the start of the test increased by 10 mA from the initial value. When the test time was further extended, the increase rate of the drive current value became moderate, but the drive current value continued to increase, and a stable operating current could not be obtained.

[Comparative Example 3]
The present comparative example 3 is the same as the first embodiment except that the first layer 107 is undoped and the active layer 105 is doped with n-type impurities at a concentration of 1 × 10 ¹⁷ to 1 × 10 ¹⁸ / cm ^3. The same laser element. The laser element according to Comparative Example 3 had a threshold current value of about 50 mA, a slope efficiency of about 1.4 W / A, and a drive current value of about 135 mA when the output was 120 mW. In addition, it was observed by the aging test that the drive current value 24 hours after the start of the test increased by 15 mA or more from the initial value. And when test time was extended, the raise of the further drive current value was observed.

[Comparative Example 4]
The present comparative example 4 is a laser device similar to that of the first embodiment except that the active layer 105 is doped with an n-type impurity at a concentration of 1 × 10 ¹⁷ to 1 × 10 ¹⁸ / cm ³ . It was observed that the laser element according to Comparative Example 4 exhibited the same element characteristic values as in Comparative Example 3 above.

[Comparative Example 5]
The present comparative example 5 does not have the first layer 107 and the second layer 108, and is the same as the example 1 except that the third layer 109 is provided on and in contact with the upper adjacent layer 106. The same laser element. Although the laser device according to Comparative Example 5 is not as large as Comparative Example 1 above, the variation in the characteristic value of each manufactured element is large, and the threshold current value is about even if the element has the best characteristic value. 35 mA, the slope efficiency was about 1.0 W / A, and the drive current value was about 120 mA or more when the output was 120 mW. Moreover, it was observed by the aging test that the drive current value 24 hours after the start of the test increased by 5 to 10 mA from the initial value.

[Comparative Example 6]
The comparative example 6 is a laser element similar to the above-described example 1 except that the third layer 109 is an undoped type. The threshold current value of the laser element according to Comparative Example 6 was about 100 mA. Further, the other element characteristics showed the worst values among the comparative examples.

  From the above, it was found that in Comparative Examples 1 to 6, the threshold current value was about 35 mA or more, whereas in Example 1, it was lowered to 30 mA. Further, it was found that the slope efficiency of each comparative example is about 1.4 W / A or less, whereas in Example 1, the slope efficiency can be improved to 1.7 W / A. Further, in each comparative example, the driving current value when the output was 120 mW was about 120 mA or more, whereas in Example 1, it was found that the driving could be performed at a lower current of 100 mA.

  Further, in each comparative example, the drive current value increases more than 5 mA after 24 hours from the aging test, whereas in Example 1, the increase is suppressed to 5 mA or less, and even after 1000 hours, no more than that. It turns out that it hardly rises.

  In addition, it was found that, in each comparative example, the variation in the characteristics of the manufactured elements was large, whereas in Example 1, all the manufactured elements were stably obtained without variation in the wafer for each crystal growth.

  From these facts, it was found that in the nitride semiconductor laser device according to the first embodiment, the oscillation threshold is stably low, and the drive current does not increase with time even when continuously driven.

  Here, each comparative example and Example 1 are compared in detail, and the reason why the above-described excellent device characteristics are obtained in the first embodiment will be considered.

  First, in Comparative Example 3 and Comparative Example 4 in which the active layer is doped with n-type impurities, the threshold current value is about 50 mA, which is significantly higher than Example 1 (about 30 mA). In addition, in 24 hours after the start of the aging test, the driving current value increased by 15 mA or more in Comparative Examples 3 and 4, and the aging characteristics were unstable compared to Example 1 (about 5 mA or less). It was. For these reasons, in order to lower the drive threshold of the laser element and improve the aging characteristics, it is considered important that both the barrier layer and the well layer are undoped layers that are not doped with impurities.

  The reason why the device characteristics of Comparative Example 3 and Comparative Example 4 are inferior to that of Example 1 is that the active layer is doped with impurities, and the crystallinity of the active layer is reduced as compared with the undoped type. can give. Further, when one of the purposes is to increase the output of the laser as in the present invention, the energy density of the laser beam is naturally increased. However, if the crystallinity of the active layer is reduced in this way, continuous driving is performed. On the other hand, the deterioration of element characteristics such as an increase in the drive current value occurs.

Next, consideration will be given based on Comparative Example 6 and Example 1. In Comparative Example 6 in which the third layer 109 made of Al _0.2 Ga _0.8 N is an undoped type, the threshold current value is about 100 mA, and a drastic increase in the threshold current value is observed compared to Example 1 (about 30 mA). It was done. For this reason, it is considered important to dope the third layer with p-type impurities.

  The reason why the device characteristics of Comparative Example 6 are inferior to those of Example 1 is that the third layer is injected into the active layer by setting the band gap energy to be significantly higher than that of the well layer in the active layer. At the same time, the band gap energy is set to be higher than that of the p-doped upper cladding layer, so that the third layer is doped with p-type impurities and becomes conductive. If the height is not increased, it is difficult to inject holes from the p-doped upper cladding layer into the active layer.

  Next, consideration will be given based on Comparative Example 1 and Example 1. In Comparative Example 1 in which the first layer 107 was an undoped type, the device characteristics varied greatly, and even if the best characteristics were shown, the characteristic values were inferior to those of Example 1. For these reasons, it is considered important that the first layer is doped with an n-type impurity.

  The reason why the device characteristics of Comparative Example 1 vary greatly is that the first layer is an undoped type, and the p-doped layer and the n-doped layer that are closest to each other in the device are arranged far apart from each other. Layer 109 and the n-doped GaN guide layer 103, and the pn junction position in the device is difficult to be determined in terms of the device structure. In this case, since the operation of the active layer is greatly affected by the position of the pn junction, even if perturbation of the normal level growth conditions that may occur during crystal growth, it is extremely difficult to determine the device characteristics. Can have a major impact on

  In addition, when the present inventors further examined, when the p-doped layer and the n-doped layer are brought close to each other by making the first layer p-type, the slope efficiency is 1.0 W / A or less. It turned out to get worse. This is because the p-doped layer has the property of absorbing laser light to a slight extent, and if a p-doped layer is provided near the active layer where the light distribution of the laser light is large, waveguide loss occurs.

  Next, consideration will be given based on Comparative Example 5 and Example 1. In Comparative Example 5 in which the first layer 107 and the second layer 108 are not provided, and the third layer 109 is provided on and in contact with the upper adjacent layer 106, the device characteristics are not as high as those in Comparative Example 1. Even if the characteristics were severely varied and the best characteristics were exhibited, the characteristic values were inferior to those of Example 1, and the slope efficiency (1.0 W / A) was particularly deteriorated. For these reasons, it is considered important to provide the first layer 107 and the second layer 108 and dispose the third layer 109 at least 50 nm away from the active layer.

  The reason why the slope efficiency of the comparative example 5 is particularly deteriorated is that an adverse effect due to the close arrangement of the active layer 105 and the third layer 109, that is, near the active layer where the light distribution of the laser beam is large as described above. Waveguide loss due to the provision of the p-doped layer is raised. In addition, since the nitride semiconductor has a wurtzite crystal structure and is not isotropic, an electric field exists inside the crystal, and a third layer having a large band gap difference from the active layer is arranged near the active layer. As a result, the band of the active layer is bent, and the active layer cannot obtain a band flat state. As a result, the probability of carrier recombination in the active layer decreases.

  The active layer is particularly susceptible to such adverse effects when it is an undoped type, and a nitride semiconductor containing Al atoms as in the third layer 109 and In atoms as in the active layer 105 are used. When the nitride semiconductor is included, a large band bending occurs. Therefore, as described above, the distance (L) between the lower surface of the third layer 109 and the upper surface of the well layer disposed on the uppermost stage of the active layer is set. It is important to reduce this adverse effect by setting it to at least 50 nm or more. Further, the separation interval (L) is preferably 200 nm or less. This is because hole injection from the p-doped upper cladding layer 110 to the active layer becomes difficult in terms of distance if both layers are arranged apart from 200 nm.

  Further, the deterioration of the slope efficiency in the laser element is less affected when a low-power laser is used, but it is a very big problem when the purpose is to obtain a high output of 100 mW or more as in the present invention. Become. Therefore, as in the first embodiment, a sufficient distance between the p-doped layer and the active layer is provided, and the n-doped first layer 107 is disposed between them, and the n-doped layer is formed above and below the active layer. By adopting an element structure sandwiched between the two, it is preferable that the active layer be in a band flat state while an undoped active layer is employed.

  Here, in order to put the active layer in a band flat state by sandwiching the upper and lower sides of the active layer with the n-doped layer, at least the n-doped first layer 107 has a thickness of 5 nm or more, and the n-type is formed in the first layer. It is necessary to establish the conductivity. On the other hand, in order not to electrically inhibit hole injection from the p-doped upper cladding layer 110 to the active layer, it is necessary to limit the content of the n-type impurity by setting the thickness of the first layer 107 to 50 nm or less. is there.

Further, another undoped nitride semiconductor layer may be provided between the active layer 105 and the first layer 107 in addition to the upper adjacent layer 106, or the layer thickness of the upper adjacent layer 106 may be changed. However, in order to obtain a band flat state of the active layer by sandwiching the upper and lower sides with n-doped layers, the distance (D) between the lower surface of the first layer 107 and the upper surface of the well layer disposed on the uppermost stage of the active layer is The first layer is disposed such that the distance (L) between the lower surface of the third layer 109 and the upper surface of the well layer disposed on the uppermost stage of the active layer satisfies the following relational expression (4): It is preferable.
[Equation 7]
2D ≦ L (4)

  The D value is preferably 60 nm or less, and more preferably 30 nm or less. However, as described above, the layer thickness of the upper adjacent layer 106 needs to be at least about 0.5 nm or more in order to avoid the trapping of the carriers exuding quantum from the active layer. .

  Next, consideration will be given based on Comparative Example 2 and Example 1. In Comparative Example 2 in which the second layer 108 was not provided and the third layer 109 was provided on and in contact with the first layer 107, the slope efficiency was particularly deteriorated (1.0 W / A). Therefore, it is considered important to provide the undoped second layer 108 between the n-doped first layer 107 and the p-doped third layer 109.

  The reason why the slope efficiency of Comparative Example 2 is particularly deteriorated is that the n-doped first layer 107 and the p-doped third layer 109 are arranged in contact with each other, so that many lasers are near the interface between them. The light is absorbed.

  As described above, the n-doped cladding layer, the undoped active layer, the n-doped first layer, the undoped second layer, the p-doped third layer having a large band gap energy, and the p-doped In the nitride semiconductor laser device according to the first embodiment in which the cladding layers are stacked in this order and the separation distance between the active layer and the third layer is 50 nm to 200 nm, the oscillation threshold is low and the slope efficiency is high. Since an element excellent in aging characteristics can be stably obtained, a laser element capable of single transverse mode oscillation with a high output of 100 mW or more is realized.

<Embodiment 2>
The second embodiment is a nitride semiconductor laser element similar to the first embodiment except that the composition of the first layer 107 is Al _0.05 Ga _0.95 N. With this configuration, the band gap energy of the first layer 107 is larger than that of the upper adjacent layer 106 and the carrier injection efficiency into the active layer is improved, so that the threshold value of the element is further increased than in the first embodiment. Pull down.

Here, the composition of the first layer is not limited to Al _0.05 Ga _0.95 N, but is represented by Al _x Ga _1-x N (where x is a real number that satisfies the range 0 <x <1). Although the composition can include an Al atom, when the band gap energy of the first layer is larger than that of the second layer, holes that cannot exceed this energy gap are generated in the second layer. Since the efficiency of hole injection into the active layer is reduced, the composition ratio of the first layer may be adjusted in a range where the band gap energy of the first layer is lower than that of the second layer. preferable.

<Embodiment 3>
The third embodiment is a nitride semiconductor similar to the first embodiment except that the composition of the barrier layer 132, the upper adjacent layer 106, and the lower adjacent layer 104 in the active layer 105 is In _0.03 Ga _0.97 N. It is a laser element. With this configuration, first, the band gap energy of the first layer 107 is larger than that of the upper adjacent layer 106, so that the carrier injection efficiency into the active layer is improved. Furthermore, since the refractive index of the layer near the well layer 131 is increased, the optical confinement factor in the active layer is improved. As a result, the threshold value of the element is further lowered than in the first embodiment.

The composition of the barrier layer 132, the upper adjacent layer 106, and the lower adjacent layer 104 is not limited to In _0.03 Ga _0.97 N, but In _x Ga _1-x N (where x is 0 <x ≦ 0 However, in order to increase the refractive index and increase the efficiency of light confinement in the active layer, the value of x should be 0.001 or more. preferable. On the other hand, the value of x is desirably 0.05 or more smaller than the In composition of the well layer. This is because if x exceeds this range, the confinement of carriers in the well layer may be insufficient.

<Embodiment 4>
The fourth embodiment is the same as the first embodiment except that the first layer 107 includes at least two n-doped GaN layers and an undoped GaN layer is provided between the two n-doped GaN layers. This is the same nitride semiconductor laser element as in the first embodiment. Even with this configuration, excellent element characteristics similar to those of the first embodiment are exhibited.

The composition of the two or more n-doped layers and undoped layers is not limited to the GaN as long as their band gap energy is defined larger than the well layer and smaller than the third layer, Al _x Ga _y In _z N (where x, y, and z are real numbers satisfying 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, x + y + z = 1) Good.

<Embodiment 5>
In the fifth embodiment, a p-doped upper guide layer (p-type impurity concentration: 2 × 10 ¹⁸ to 1 ×) made of GaN having a thickness of 5 to 30 nm is provided between the third layer 109 and the upper cladding layer 110. 10 ²⁰ / cm ³ ), and the nitride semiconductor laser element is the same as that of the first embodiment except that the band gap energy Egu of the upper guide layer satisfies the following relational expression (3). .
[Equation 8]
Ew <Egu <Ecl (3)

  With this configuration, the optical confinement coefficient of the active layer can be increased, and the oscillation threshold of the device can be further reduced.

  In the fifth embodiment, the p-doped guide layer has a thickness of 5 to 30 nm as long as the energy gap is not smaller than that of the second layer and is smaller than that of the p-type cladding layer. Excellent device characteristics are exhibited as in the case of a layer made of GaN.

<Embodiment 6>
In the sixth embodiment, the barrier layer 132 in the active layer 105 has a double structure including a GaN layer and an In _0.03 Ga _0.97 N layer, or an In _0.03 Ga _0.97 N layer, a GaN layer, and In _0.03 Ga _0.97 N. The nitride semiconductor laser element is the same as that of the third embodiment except that it has a triple structure composed of layers.

  With this configuration, the optical confinement factor in the active layer is improved to the same level as in the third embodiment. Furthermore, by interposing the GaN layer, it is possible to suppress the compositional deviation caused by the memory effect that occurs when the InGaN layer is continuously stacked. Therefore, the device characteristics are included while including the InGaN layer in the active layer. Can be further reduced, that is, the production yield can be improved.

<Embodiment 7>
The seventh embodiment is a nitride semiconductor laser element similar to that of the first embodiment except that the first layer 107 is an In _0.03 Ga _0.97 N layer. Even with this configuration, excellent element characteristics similar to those of the first embodiment are exhibited.

Here, the composition of the first layer is not limited to In _0.03 Ga _0.97 N, but is represented by In _x Ga _1-x N (where x is a real number satisfying a range of 0 <x <1). Although the composition can include In atoms, if the band gap energy of the first layer is smaller than that of the upper adjacent layer, electrons are likely to accumulate in the first layer, and carriers of the active layer Since the injection efficiency is lowered, it is preferable to adjust the respective composition ratios in a range where the band gap energy of the first layer is higher than that of the upper adjacent layer.

<Other matters>
(1) Each nitride semiconductor layer constituting the III-V group nitride semiconductor laser element of the present invention is not limited to the composition shown in the first to seventh embodiments, but the relative physical properties described above. As long as the relationship is satisfied, Al _x Ga _y In _z N (where x, y, and z are real numbers satisfying 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, x + y + z = 1) and can do. Further, a composition in which about 10% or less (but hexagonal) of nitrogen element is substituted with any element of arsenic (As), phosphorus (P), and antimony (Sb) may be used.

Here, the n-doped first layer 107, the undoped second layer 108, and the p-doped third layer 109 are In _x Ga _1-x N (wherein _x satisfies 0 <x ≦ 1). (Real number), each layer can be formed continuously by simply changing the element ratio from InGaN, which is a general active layer composition, so that the influence of carrier recombination at the interface can be suppressed.

On the other hand, when these layers are Al _x Ga _1-x N (where x is a real number satisfying 0 ≦ x ≦ 1), the composition is In _x Ga _1-x N (where x is 0). The growth conditions of each layer are more stable than in the case of <a real number satisfying x ≦ 1). As a result, the controllability of the band gap and electrical characteristics of each device is further improved, and the in-plane distribution in the wafer is made uniform, so that the device manufacturing yield can be further improved. In addition, since the composition of the p-doped cladding layer is AlGaN-based and the composition of the active layer is often InGaN-based, lattice distortion caused by the difference in lattice constant from the p-doped cladding layer can be reduced.

  (2) In the first embodiment, the distribution diagram (FIG. 3) in which the band gap energy Ecu of the upper cladding layer 110 is larger than the band gap energy E2 of the second layer 108 is shown. As long as it is possible to prevent overflow of electrons injected into the active layer by this layer, the distribution of the band gap energy of each layer around the active layer is not limited to this. For example, the energy distribution may be such that E2 is larger than Ecu. .

(3) The semiconductor composition of the upper cladding layer 110 is Al _p Ga _1-p N (where p is a real number satisfying 0 <p ≦ 1), and the semiconductor composition of the third layer 109 is Al _q Ga _{1− When qN} (where q is a real number satisfying 0 <q ≦ 1), these parameters p and q preferably satisfy the relationship q ≧ p + 0.05. In order to prevent overflow of electrons injected into the active layer by the third layer, it is necessary to increase the content of Al atoms in the third layer as compared to the p-cladding layer. This is because if it is 0.05 or more, an increase in the drive current value at a high temperature can be sufficiently suppressed.

  Further, the absolute value of q is preferably 0.1 to 0.3. This is because when the Al composition ratio is 0.3 or less, the drive voltage value tends to decrease, and when the Al composition ratio is 0.1 or more, an increase in the drive current value when the temperature is increased tends to be suppressed.

  (4) As long as the active layer according to the present invention has a multiple quantum well structure or a single quantum well structure, the number of well layers can be changed in the range of 1 to 10 layers, and 2 to 4 layers are sufficient. It has been confirmed that long life can be demonstrated. In order to obtain a sufficient gain for laser oscillation at a low threshold, the total thickness of the active layer is preferably set in the range of 10 to 70 nm.

  In addition, as long as the band gap energy of the barrier layer is made larger than that of the adjacent well layer and carriers injected into the well layer can be sufficiently confined, the band gap energy of the plurality of barrier layers included in the active layer is They may be different from each other.

  As described above, according to the present invention, the position of the pn junction in the group III-V nitride semiconductor laser element is stabilized, the crystallinity of the active layer is hardly lowered, and the carrier injection efficiency is improved. The oscillation threshold of the laser element can be stably reduced, and the drive current does not increase with time even when continuously driven at a high output of 100 mW or more. Therefore, since it can be used for extending the life of the laser element, its industrial applicability is great.

FIG. 1 is a schematic cross-sectional view of a nitride semiconductor laser device which is an example of the present invention. FIG. 2 is a schematic cross-sectional view showing a more detailed structure around the active layer of the nitride semiconductor laser device shown in FIG. FIG. 3 is a distribution diagram showing the band gap energy of each layer around the active layer of the nitride semiconductor laser device shown in FIG. FIG. 4 is a schematic cross-sectional view of a nitride semiconductor laser device according to the prior art.

Explanation of symbols

100 n-type GaN substrate 101 GaN layer doped with n-type impurities 102 lower clad layer doped with n-type impurities 103 GaN guide layer doped with n-type impurities 104 undoped GaN lower adjacent layer 105 active layer 106 undoped type GaN upper adjacent layer 107 first layer doped with n-type impurity 108 undoped GaN second layer 109 third layer doped with p-type impurity 110 upper cladding layer doped with p-type impurity 111 p-type GaN contact layer doped with impurities 120 n-type electrode 121 p-type electrode 122 insulating film 131 undoped InGaN well layer 132 undoped GaN barrier layer

Claims (9)

A substrate,
A lower clad layer provided on the substrate and doped with an n-type impurity;
An undoped lower adjacent layer not doped with a conductive impurity provided on the lower cladding layer; and
A quantum well active layer formed on the lower adjacent layer and comprising an undoped barrier layer and an undoped well layer sandwiching the barrier layer vertically;
An undoped upper adjacent layer provided on the quantum well active layer;
A first layer provided on the upper adjacent layer and doped with an n-type impurity;
An undoped second layer provided on the first layer;
An upper cladding layer provided on the second layer and doped with a p-type impurity;
A group III-V nitride semiconductor laser device in which the lowest well layer in the quantum well active layer is in contact with the lower adjacent layer, and the uppermost well layer is in contact with the upper adjacent layer,
Band gap energy Ecl of the lower cladding layer;
The band gap energy Enl of the lower adjacent layer;
The band gap energy Ew of the well layer of the quantum well active layer, the band gap energy Eb of the barrier layer,
The band gap energy Enu of the upper adjacent layer;
Band gap energy E1 of the first layer;
The band gap energy E2 of the second layer;
The band gap energy E3 of the third layer;
The band gap energy Ecu of the upper cladding layer is
The III-V group nitride semiconductor laser element satisfying the following relational expression (1).
[Equation 1]
Ew <Ecl, Enl, Eb, Enu, E1, E2, Ecu <E3 (1) A lower guide layer is further provided between the lower cladding layer and the lower adjacent layer;
The III-V group nitride semiconductor laser device according to claim 1, wherein the band gap energy Egl of the lower guide layer satisfies the following relational expression (2).
[Equation 2]
Ew <Egl <Ecl (2) An upper guide layer is further provided between the third layer and the upper cladding layer;
The III-V group nitride semiconductor laser device according to claim 1, wherein the band gap energy Egu of the upper guide layer satisfies the following relational expression (3).
[Equation 3]
Ew <Egu <Ecl (3) The III-V group nitride semiconductor laser device according to claim 1, wherein a distance L between the uppermost well layer and the third layer in the quantum well active layer is 50 nm or more and 200 nm or less. . 5. The III according to claim 4, wherein the distance L and the distance D between the uppermost well layer and the first layer in the quantum well active layer satisfy the following relational expression (4): -Group V nitride semiconductor laser element.
[Equation 4]
2D ≦ L (4) The group thickness of the first layer is 5 nm or more and 50 nm or less. The group III-V nitride semiconductor laser element according to claim 1. 2. The group III-V nitride semiconductor laser device according to claim 1, wherein a thickness of the lower adjacent layer or the upper adjacent layer is 0.5 nm or more. The group III-V nitride according to claim 1, wherein the n-type impurity doped in the first layer is silicon, and the p-type impurity doped in the third layer is Mg. Semiconductor laser element. The first layer is made of In _x1 Ga _1-x1 N (wherein x1 is a real number satisfying a range of 0 ≦ x1 ≦ 0.5);
Wherein the second layer _{In x2 Al y2 Ga 1- (x2} + y2) N consists (x2 in the formula 0 ≦ x2 ≦ 0.5, y2 is a real number satisfying a range of 0 ≦ y2 ≦ 0.5) ,
3. The group III-V nitride semiconductor according to claim 1, wherein the third layer is made of Al _y3 Ga _1-y3 N (wherein y3 is a real number satisfying a range of 0 <y3 ≦ 1). Laser element.

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