JP2003163418A - Nitride gallium system compound semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem of a short-wavelength laser for blue-green color and green color among the nitride gallium system compound semiconductor lasers that threshold value becomes high because of bad confinement of light beam. <P>SOLUTION: This nitride gallium system compound semiconductor laser is an isolation confinement type nitride gallium system compound semiconductor laser comprising an optical guide layer. This optical guide layer is provided between an active layer consisting of a nitride gallium system compound semiconductor and a clad layer consisting of the nitride gallium system compound semiconductor, and is comprised of the nitride gallium system compound semiconductor having a refraction index which is larger than that of the clad layer and having a band gap which is larger than that of the active layer in order to form an optical waveguide in combination with the active layer. In the optical guide layer, impurity is doped to raise the conductivity during the growth thereof. However, the optical guide layer is grown without doping of impurity in the area thereof isolated at least by 0.05 μm or more from the end face in the active layer side. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gallium nitride-based compound semiconductor laser, and more particularly to a separate confinement type gallium nitride-based compound semiconductor laser having a quantum well type active layer and an optical guide layer.

[0002]

2. Description of the Related Art Today, a semiconductor laser using a nitride semiconductor is required to be used in an optical disk system capable of recording / reproducing information with a large capacity and a high density such as a DVD. In particular, next-generation DV that handles digital image data
A blue laser with a short wavelength is considered to be indispensable for D. As a blue semiconductor laser, a gallium nitride-based semiconductor compound laser is the most promising and the development thereof is progressing.

One of the important factors in laser oscillation is
Efficient optical confinement in the optical waveguide, which is the region that amplifies light. If the light is not well confined in this region, defects in device characteristics such as an increase in threshold current and a decrease in luminous efficiency occur. Originally, the active layer was used as it was as an optical waveguide, but the active layer moved to the quantum well structure and the thickness of the active layer became thin, so that it was difficult to confine light only by the active layer. Therefore, a separated light confinement type (SCH) in which an optical guide layer having a band gap larger than that of the active layer is arranged on one side or both sides of the active layer has been adopted. In the basic SCH, the optical waveguide is composed of an active layer and an optical guide layer, and light is confined inside the optical waveguide by providing a refractive index difference between the cladding layer and the optical guide layer. Some applications of the optical waveguide include a layer such as an electron confinement layer in addition to the active layer and the light guide layer.

[0004]

However, among the gallium nitride-based compound semiconductor lasers, in the blue-green or green short-wavelength laser, the difference in the refractive index between the optical guide layer and the cladding layer becomes small, so that the light guide layer and the clad layer have a small refractive index. Light confinement is not enough,
The threshold voltage becomes high. This is because the refractive index of the material changes depending on the wavelength of light.

In order to improve the light confinement, a method of increasing the Al composition ratio in the cladding layer to increase the difference in refractive index between the optical guide layer and the cladding layer can be considered. However, if the Al composition ratio of the clad layer is increased, the crystallinity of the clad layer is deteriorated and cracks may occur in some cases, which is not preferable.

On the other hand, if the light guide layer is made thicker, the light confinement can be improved without increasing the Al composition ratio so much, but the voltage loss in the light guide layer becomes large. Therefore, an increase in the threshold voltage cannot be sufficiently suppressed by simply increasing the thickness of the light guide layer. Therefore, an object of the present invention is to provide a gallium nitride-based compound semiconductor laser capable of realizing a low threshold voltage even in a short wavelength such as blue-green or green.

[0007]

A gallium nitride-based compound semiconductor laser according to the present invention has a refractive index higher than that of the clad layer between an active layer made of a gallium nitride-based compound semiconductor and a clad layer made of a gallium nitride-based compound semiconductor. A separate confinement type gallium nitride compound semiconductor laser comprising a gallium nitride compound semiconductor that is large and has a bandgap larger than that of the active layer, and has an optical guide layer that forms an optical waveguide with the active layer, the optical guide layer comprising: Is doped with impurities that increase the conductivity during growth, but at least 0.05 μm or more from the end face on the active layer side of the light guide layer is grown without being doped with the impurities. Is characterized by.

By doping the light guide layer with impurities to increase the conductivity, it is possible to suppress an increase in voltage loss in the light guide layer when the light guide layer is formed as a thick film. Therefore, the light confinement ability can be improved and the threshold voltage of the laser can be lowered without increasing the voltage loss of the light guide layer. . However, when the light guide layer is doped with impurities, the light absorption coefficient of the light guide layer itself rises and the light emitted from the active layer is absorbed. As a result, the crystallinity of the light guide layer decreases, resulting in a decrease in the crystallinity of the active layer. However, there is a problem that impurities are diffused from the light guide layer to the active layer due to thermal diffusion, and band edge emission of the active layer is hindered. Therefore, in the present invention, the optical guide layer is grown without doping in a range of at least 0.05 μm, preferably 0.1 μm from the end face on the active layer side.

In the optical guide layer, the range of growth without doping is too large and the voltage loss becomes large. Therefore, the range from the end face on the active layer side is 0.3 μm or less, more preferably 0.15 μm or less. Is desirable.

Further, it is preferable that the thickness of the light guide layer is 0.15 μm or more and 0.5 μm or less on the p-side and the n-side, respectively, because the effect of confining light becomes remarkable.
It is particularly preferable that the thickness is 0.2 μm or more and 0.3 μm or less.

When the optical guide layer is doped with impurities to increase the conductivity, the doping state is not particularly limited, and the doping amount may be uniform or non-uniform. From the viewpoint of simplicity of formation, it is preferable that the layer is formed of a single layer having a uniform doping amount, but there is no problem even if the impurity doping amount is nonuniform. For example, the doping amount may be increased or decreased by a small amount so that there is no boundary of concentration change, or the boundary of concentration change may be clear by stacking a plurality of layers having different concentrations. Also,
It is preferable that the amount of doping increases as the distance from the active layer increases, because the intensity distribution with the peak in the active layer and the change in the light absorption rate due to the doping make the structure less likely to absorb strong light.

[0012] When doped with impurities doped region of the optical guide layer, an impurity concentration ¹ × ¹⁰ 1 5 / cm ³ to 1 ×
It is preferable to grow so as to be 10 ²⁰ / cm ³ or less because sufficient conductivity can be obtained and further deterioration of crystallinity due to impurity doping can be suppressed within an allowable range.

From the active layer of the light guide layer, the range from 0.05 to
Although 0.3 μm is formed undoped, when it is formed adjacent to a layer formed by impurity doping, impurities may intrude from the adjacent layer depending on the temperature at the time of stacking the gallium nitride compound semiconductor. . When the impurity-doped layer is adjacent to the active layer side of the light guide layer, the active layer side of the unguided light guide layer is doped with the impurity. It is estimated that the concentration of impurities doped at that time will be one-half or less of that of the adjacent layer, and the penetration depth of impurities will be 0.02 μm or less. Although the undoped region also absorbs light due to the impurity being doped by thermal diffusion, the amount of absorption is smaller than the loss due to the optical waveguide during laser oscillation, so that it is not a big problem.

An undoped region and a doped region of the optical guide layer are adjacent to each other because they are sequentially stacked, and impurities are diffused by thermal diffusion to the cladding layer side of the undoped region. May be introduced.
However, the position where impurities are introduced by thermal diffusion is farthest from the active layer in the undoped region, and since the penetration depth is estimated to be 0.02 μm or less, the influence on light absorption is small, It is very small compared to other losses and is of little concern.

In the gallium nitride compound semiconductor laser of the present invention, it is preferable that the light guide layer is made of GaN. Since the light guide layer is formed in the vicinity of the active layer, its crystallinity is desirable, and binary GaN with good crystallinity is preferable to ternary gallium nitride compound semiconductor with poor crystallinity. .

The active layer of the gallium nitride-based compound semiconductor laser of the present invention is preferably formed of In _x Ga _1-x N (0 <x ≦ 1). Since the laser of the present invention has a structure particularly suitable for blue to pure green lasers, it is preferable to use a substance that emits blue to pure green light in the active layer. In In x _Ga 1-x _N, it is possible to change a range of colors for emitting blue to pure green by changing the value of x. Furthermore, a quantum well structure having an In _x Ga _1-x N (0 <x ≦ 1) well layer is preferable because a high-performance laser is obtained.

Further, the clad layer is preferably formed of a material having a lower refractive index than the light guide layer in order to confine light in the optical waveguide. When forming the optical guide layer of GaN, the cladding layer is _{Al y Ga 1-y N (} 0 ≦ y
A nitride semiconductor containing ≦ 1) is preferable because it has a lower refractive index than GaN. At this time, the larger the value of y, the lower the refractive index tends to be.

However, Al_yGa_1-yN is the value of y
If it is too large, the crystallinity deteriorates.
The rack can be easily inserted. Therefore, the clad layer is made of Al
_yGa_1-yN (0 <y ≦ 1) and GaN are alternately laminated
As a superlattice structure _yGa_1-yThe thickness of the N layer
Form a thin film that exceeds the critical thickness for crack formation, and crystallize
Prevents crack formation by sandwiching between GaN with good properties
Is preferable.

Impurity elements introduced into the doped region of the optical guide layer formed on the p-side and the cladding layer are
It is preferable that the p-type dopant is Mg. Further, the Mg doping amount of the doped region of the p-side optical guide layer is preferably equal to or less than the average doping amount of Mg of the p-side cladding layer, and more preferably 1/2 of the average doping amount of Mg of the p-side cladding layer.
It is the following.

Impurity elements introduced into the doped region of the optical guide layer formed on the n side and the cladding layer are
The n-type dopant Si is preferable. Further, the Si doping amount of the doped region of the n-side optical guide layer may be equal to or less than the average doping amount of Si of the n-side cladding layer, and more preferably, 1/2 of the average doping amount of Si of the n-side cladding layer.
It is the following.

Further, according to the present invention, between the active layer 106 made of a gallium nitride-based compound semiconductor and the cladding layers 106, 109 made of a gallium nitride-based compound semiconductor, the refractive index is larger than that of the cladding layer and the band gap is larger than that of the active layer. Is a large gallium nitride-based compound semiconductor and has a light-guiding layer that forms an optical waveguide with the active layer. A range of 0.3 μm or less is a method for manufacturing a gallium nitride-based compound semiconductor laser, which is characterized in that it is grown undoped. By growing undoped in the vicinity of the active layer, strong light is prevented from being absorbed.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1. Examples of the gallium nitride-based compound semiconductor used in the gallium nitride-based compound semiconductor laser of the present invention include GaN, AlN, or In.
N, or gallium nitride-based compound is these mixed crystal semiconductor _{(In x Al y Ga 1-} x-y 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 and P may be used.

FIG. 4 is a sectional view showing an example of a gallium nitride-based compound semiconductor laser according to the present invention. On the GaN substrate 101, the active layer 107 made of In _x Ga _1-x N (0 ≦ x <1) is the n-side light guide layer 1 made of a GaN layer.
06 and sandwiched between the p-side optical guide layer 109 made of GaN layer to form an optical waveguide, further both sides n-side _Al y G
a _1-y N (0 ≦ y <1) layers 103 to 107 (the value of y is different for each layer) and p-side Al _z Ga _1-z N (0 ≦ z
<1) Layers 110 and 111 (z value is different for each layer)
And a so-called optical-carrier separated confinement heterostructure is formed.

FIG. 1 is a diagram showing the laminated portion of the optical waveguide and the clad layer extracted from the laminated structure of the semiconductor laser shown in FIG. Light guide layers 106 and 10
Reference numeral 9 denotes undoped regions 6a, 9a and doped regions 6b, 9
and b are laminated as shown in the figure.

Since the non-doped gallium nitride compound semiconductor has low conductivity, if the optical guide layer is formed as a thick film to thicken the optical waveguide, the resistance of the optical guide layers 106 and 109 becomes high. Therefore, in order to increase the thickness of the optical waveguide and improve the light confining ability, it is preferable to form a thick optical guide layer having good conductivity. Therefore, the light guide layer 10
6 and 109 are doped with impurities to increase conductivity.

Since the optical absorption coefficient becomes high when impurities are doped, it is not desirable to dope the inside of the optical waveguide in order to reduce the amount of absorption of light. However, in light of the light confining ability, the optical guide layer is doped. It was found that the laser performance was improved by forming a thick optical waveguide. Therefore, by growing the undoped regions 6a and 9a of 0.05 μm or more and 0.3 μm or less on the active layer side and growing the doped region 9b on the cladding side, strong light near the active layer is not absorbed, and The conductivity of the guide layers 106 and 109 can be improved. Furthermore, the undoped regions 6a and 9
It is more preferable to grow a by 0.1 μm or more and 0.15 μm or less.

In order to efficiently confine blue to pure green light, the thickness of each of the light guide layers 106 and 109 formed on the p-side and the n-side should be 0.15 μm or more and 0.5 μm or less. Preferably, it is formed, especially 0.
The thickness is preferably 2 μm or more and 0.3 μm or less.

Since the effect of improving the conductivity is the same regardless of where the doped regions are formed in the light guide layers 106 and 109, it is preferable to dope the light absorbing layer at a position where light absorption is small. During laser oscillation, the active layer has the strongest light intensity, and the light intensity decreases as the distance from the active layer increases. Since the absorbed amount of light that has already weakened is smaller than that of absorbed light of high intensity, the doped region 6 b is located at a position apart from the active layer in the light guide layers 106 and 109. , 9b are preferably formed.

It is sufficient that the doped regions 6b and 9b of the light guide layer have high conductivity, and the doping amount in the doped regions 6b and 9b may be intentionally changed or may not be changed. If you want to grow without changing the doping amount,
Since it is easy to control the concentration of impurities, the formation becomes simple, which is preferable. On the other hand, depending on the intensity distribution of light, the amount of doping may be intentionally changed so that the amount of doping is increased as the distance from the active layer increases, and if it is formed in this way, the amount of absorption of light is further reduced. It is possible because it is possible.

When the impurity doping amount is intentionally changed in the doped regions 6b and 9b of the light guide layer, the impurity concentration in the growth direction of the layer changes smoothly.
There is a form that gradually changes. The stepwise changing form is, specifically, a form in which a plurality of layers each having a different impurity doping amount are laminated to form the doped regions 6b and 9b. Whichever concentration change
It is applicable to the present invention.

Considering the balance between the increase in conductivity due to the doping of impurities and the deterioration in crystallinity, the light guide layer 106,
When 109 is doped with impurities to grow, the impurity concentration of the grown portion is 1 × 10 ¹⁵ / cm ³ to 1 × 10.
It is preferably doped so as to have a concentration of ²⁰ / cm ³ . Within this range, the resistance does not sharply increase even if the optical waveguide is formed in a thick film, and the crystallinity required for the optical waveguide can be obtained, which is preferable.

Of the optical guide layers 106 and 109, 0.05 μm or more and 0.3 μm or less from the active layer are grown as undoped regions 6a and 9a without doping, but if adjacent layers are doped with impurities, the device Impurities in the adjacent layers are thermally diffused by the temperature at the time of growth to cause undoped regions 6a and 9
May invade a. When the impurity-doped layer is adjacent to the active layer side of the light guide layer, the active layer side of the unguided light guide layer is doped with the impurity. At that time, the concentration of impurities doped is one-half or less of that of the adjacent layer, and the penetration depth of impurities is 0.
It is estimated that it will be 02 μm or less. Although the undoped region also absorbs light due to the impurity being doped by thermal diffusion, the amount of absorption is smaller than the loss due to the optical waveguide during laser oscillation, so that it is not a big problem.

An undoped region and a doped region of the optical guide layer are adjacent to each other because they are sequentially stacked, and impurities are diffused by thermal diffusion to the cladding layer side of the undoped region. May be introduced.
However, the position where impurities are introduced by thermal diffusion is farthest from the active layer in the undoped region, and since the penetration depth is estimated to be 0.02 μm or less, the influence on light absorption is small, It is very small compared to other losses and is of little concern.

In the gallium nitride-based compound semiconductor laser of the present invention, the light guide layers 106 and 109 are preferably made of GaN among the gallium nitride-based compound semiconductors. The light guide layers 106 and 109 are preferably formed in the vicinity of the active layer and, since they are a part of the optical waveguide, have good crystallinity. Therefore, it is preferable that the light guide layer is formed of GaN having high crystallinity among the gallium nitride compounds.

The active layer 107 of the gallium nitride-based compound semiconductor laser of the present invention is made of In _x Ga _{1 -x} N (0 <x ≦.
It is preferably formed in 1). Since the laser of the present invention has a structure particularly suitable for a blue to pure green laser, a substance emitting blue to pure green light may be used for the active layer 107. In In x _Ga 1-x _N, it is possible to change a range of colors for emitting blue to pure green by changing the value of x. _Furthermore, In _{x Ga 1-x} N
A quantum well structure having a well layer of (0 <x ≦ 1) is preferable because a threshold current can be expected to be low and a high-performance laser can be expected.

The cladding layers 105 and 110 are made of a material having a lower refractive index than the light guide layer in order to confine light in the optical waveguide. Light guide layers 106, 10
When 9 is formed of GaN, the cladding layers 105, 1
10 is preferably formed of a nitride semiconductor containing Al _y Ga _1-y N (0 ≦ y ≦ 1) because it can have a lower refractive index than GaN. Increasing the value of y lowers the refractive index, and thus improves the light confining ability.

Al _y Ga _1-y N has poorer crystallinity than GaN, and cracks easily occur particularly when the y value is increased to form a thick film. Therefore, the cladding layer 10
5,110 is Al _y Ga _1-y N (0 <y ≦ 1) and G
As a superlattice structure in which aN and aN are alternately stacked, Al _y Ga
_It is preferable to prevent the formation of cracks by forming the _1-y N layer to be thinner than the limit thickness of crack formation and further sandwiching it with GaN having good crystallinity. Clad layers 105, 1
When 10 is formed in such a superlattice structure, the y value of the Al _y Ga _1-y N layer can be increased without generating cracks, so that the average Al content of the entire cladding layer increases and the refractive index increases. It is possible to lower it.

Impurity elements introduced into the optical guide layer 109 and the cladding layer 110 formed on the p side are
It is preferable that the p-type dopant is Mg. In addition, the Mg doping amount of the p-side optical guide layer 109 is set to the p-side cladding layer 1
The average doping amount of Mg is preferably 10 or less, and more preferably 1/2 or less of the average doping amount of Mg of the p-side cladding layer 110.

Impurity elements introduced into the optical guide layer 106 and the cladding layer 105 formed on the n-side are
The n-type dopant Si is preferable. In addition, the Si-doped amount of the n-side optical guide layer 106 is set to the n-side cladding layer 1
It is preferable that the average doping amount of Si is 0.05 or less, and more preferably, the average doping amount of Si of the n-side cladding layer 105 is 1/2 or less.

The structure of the gallium nitride compound semiconductor laser shown in FIG. 4 will be described below in detail. GaN is preferably used as the substrate 101,
A different substrate different from the gallium nitride-based compound semiconductor may be used. Examples of the dissimilar substrate include sapphire and spinel (M
Insulating substrate such as gA1 ₂ O ₄ , SiC (6H, 4H,
(Including 3C), ZnS, ZnO, GaAs, Si, and gallium nitride-based compound semiconductors, and oxide substrates that are lattice-matched with gallium nitride-based compound semiconductors are known to be able to grow and are conventionally known. A substrate material different from that of the compound semiconductor can be used. Examples of preferable different substrates include sapphire and spinel.
The heterogeneous substrate may be off-angled, and in this case, it is preferable to use a step-shaped off-angled substrate because the underlayer made of gallium nitride grows with good crystallinity. Further, when using a heterogeneous substrate, after growing a gallium nitride-based compound semiconductor which is an underlayer before forming a laser structure on the heterogeneous substrate, the heterogeneous substrate is removed by a method such as polishing to remove the gallium nitride-based compound semiconductor. The laser structure may be formed as a single substrate of a semiconductor, or a method of removing a different type substrate after forming the laser structure may be used.

When a heterogeneous substrate is used, when a laser structure is formed through a buffer layer (low temperature growth layer) and an underlayer made of a gallium nitride compound semiconductor (preferably GaN), growth of the gallium nitride compound semiconductor is achieved. Will be good. In addition, as an underlayer (growth substrate) provided on a different type of substrate, ELOG (Epitaxially Laterally
When a gallium nitride-based compound semiconductor that has been grown is used, a growth substrate with good crystallinity can be obtained. As a specific example of the ELOG growth layer, a mask region formed by growing a gallium nitride-based compound semiconductor layer on a heterogeneous substrate and providing a protective film on the surface of which a growth of a gallium nitride-based compound semiconductor is difficult, By providing a non-mask region for growing a gallium nitride-based compound semiconductor in a stripe shape and growing a gallium nitride-based compound semiconductor from the non-masked region, in addition to growth in the film thickness direction, growth in the lateral direction can be achieved. By being formed, the mask region also has a layer in which a gallium nitride-based compound semiconductor is grown and deposited. In another form, the gallium nitride-based compound semiconductor layer grown on a heterogeneous substrate may be provided with an opening, and the film may be grown by lateral growth from the side surface of the opening.

On the substrate 101, the n-side contact layer 103, which is an n-type gallium nitride compound semiconductor layer, the crack prevention layer 104, the n-side cladding layer 105, and the n-side light guide layer 106, are arranged with the buffer layer 102 interposed therebetween. Are formed. The layers other than the n-side cladding layer 105 and the n-side light guide layer 106 may be omitted depending on the laser. The n-type gallium nitride compound semiconductor layer needs to have a bandgap wider than that of the active layer at least in a portion in contact with the active layer, and therefore, a composition containing Al is preferable. Each layer except the n-side light guide layer is grown while being doped with n-type impurities.
It may be the side or the non-doped n-side.

N-type gallium nitride-based compound semiconductor layer 103
An active layer 107 is formed on the layers 106 to 106. As described above, the active layer 107 includes the In _x1 Ga _1-x2 N well layer (0 <x ₁ <1) and the In _x2 Ga _1-x2 N barrier layer (0
≦ x ₂ <1, x ₁ > x ₂ ) has an MQW structure in which an appropriate number of layers are alternately and repeatedly stacked, and both ends of the active layer are barrier layers. Well layer is formed undoped, all of the barrier layer is Si, Sn or the like of the n-type impurity is preferably ^{1 × 10 17 ~1 × 10 1} 9 cm
It is formed by doping with a concentration of ^-3 .

On the final barrier layer, p-type electron confinement layers 108 and p are formed as p-type gallium nitride compound semiconductor layers.
A side light guide layer 109, a p-side cladding layer 110, and a p-side contact layer 111 are formed. p-side clad layer 11
The layers other than the 0 and p-side light guide layers 109 can be omitted depending on the laser. The p-type gallium nitride compound semiconductor layer needs to have a bandgap wider than that of the active layer at least in a portion in contact with the active layer, and therefore, a composition containing Al is preferable. Further, each layer except the p-side optical guide layer may be grown as a p-side while being doped with a p-type impurity, or as a result of p-type impurities being diffused from another adjacent layer after being grown undoped. It may be the doped p-side.

The p-type electron confinement layer 108 is made of a p-type gallium nitride-based compound semiconductor having a higher Al mixed crystal ratio than the p-side clad layer 110, and is preferably Al _x Ga _{1 -x.}
It has a composition of N (0.1 <x <0.5). Also, M
A high concentration of p-type impurities such as g, preferably 5 × 10 ¹⁷
Doped at a concentration of ˜1 × 10 ¹⁹ cm ⁻³ . As a result, the p-type electron confinement layer 108 can effectively confine electrons in the active layer, and lowers the threshold value of the laser.

In the p-type gallium nitride-based compound semiconductor layer, a ridge stripe is formed up to the middle of the p-side optical guide layer 109, and further, protective films 161, 162, p-type electrode 120, n-type electrode 121, p-pad electrode. 122, and n
The pad electrode 123 is formed to form a semiconductor laser.

Embodiment 2. Embodiment 2 is shown in FIG. Between the active layer 107 and the p-side light guide layer 109, p
It is formed in the same manner as in Embodiment 1 except that the side electron confinement layer 108 is formed. p-side electron confinement layer 10
In No. 8, Mg is grown in a high concentration.

It is said that Mg has a large mobility in a gallium nitride-based compound semiconductor and thermal diffusion easily occurs. In the present invention, each layer is laminated at a high temperature, but in an arrangement in which the undoped layer and the Mg-doped layer are adjacent to each other, Mg may be thermally diffused from the doped layer to the undoped layer due to heat during lamination. As a result, even though the layer is grown undoped, it becomes Mg-doped after the element is formed.

In the present invention, the undoped region 9a of the p-side optical guide layer grown undoped is also Mg-doped due to the stacking of other layers after its formation. In that case, the distribution state of Mg is Mg during growth. Different from dope. The characteristics of the actually manufactured laser element will be described by analyzing the distribution state of Mg in the stacking direction.

FIG. 3 shows a graph in which the contents of Mg and Al of the laser formed in the second embodiment of the present invention are analyzed. A
l is doped when growing the n-side clad layer 105, the p-side clad layer 110 and the electron confinement layer 108, and since Al is less likely to undergo thermal diffusion, it is an index for knowing the layer boundary after element formation. . FIG. 3 also shows the layer structure corresponding to the boundary line of each layer determined from the change in Al content and the layer thickness set at the time of growth.

M of the undoped region 9a of the p-type light guide layer
The g concentration distribution shows a high concentration at the boundary with the adjacent layer, and the concentration tends to decrease as the distance from the boundary increases. Also,
The graph has an acute-angled depression ((X) in the figure) showing the minimum Mg concentration in the undoped region 9a of the p-type light guide layer. Next, the characteristics of the Mg distribution state of the electron confinement layer 108 and the doped region 9b of the p-type optical guide layer will be described. Both layers are grown without intentionally changing the concentration, but after the element is formed, the Mg concentration decreases so as to approach the undoped region 9a of the p-type optical guide layer.

[0052]

EXAMPLES [Examples] In the following, as an example, a laser using a gallium nitride-based compound semiconductor having a laser structure as shown in FIG. 4 is formed by further growing the cladding layer and the optical waveguide shown in FIG. The laser will be described.

(Substrate 101) As the substrate, a gallium nitride-based compound semiconductor grown on a heterogeneous substrate, G in this embodiment is used.
After growing aN with a thick film (100 μm), the heterogeneous substrate is removed and a gallium nitride-based compound semiconductor substrate made of GaN having a thickness of 80 μm is used. The detailed method of forming the substrate is as follows. A dissimilar substrate made of sapphire having a 2-inch φ and a C-plane as a main surface is set in a MOVPE reaction vessel, and the temperature is set to 500 ° C.
G), ammonia (NH ₃ ) is used to grow a buffer layer made of GaN to a film thickness of 200 Å, and then the temperature is raised to grow undoped GaN to a film thickness of 1.5 μm to form an underlayer. To do. Next, a plurality of stripe-shaped masks are formed on the surface of the underlayer, and gallium nitride-based compound semiconductor, GaN in this embodiment, is selectively grown from the mask openings (windows) to grow with lateral growth. (ELOG)
The gallium nitride-based compound semiconductor layer formed by (1) is grown to a thicker film, and the heterogeneous substrate, the buffer layer, and the underlayer are removed to obtain a gallium nitride-based compound semiconductor substrate.
At this time, the mask for 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) After the growth of the buffer layer on the gallium nitride compound semiconductor substrate, the temperature is set to 105.
TMG (trimethylgallium), TMA at 0 ℃
(Trimethylaluminum), using ammonia, Al
_A buffer layer 102 made of _0.05 Ga _0.95 N is grown to a film thickness of 4 μm. This layer functions as a buffer layer between the n-side contact layer of AlGaN and the gallium nitride-based compound semiconductor substrate made of GaN. Next, each layer having a laser structure is laminated on the underlayer made of a gallium nitride-based compound semiconductor.

(N-side contact layer 103) Next, on the obtained buffer layer 102, n-side contact layer made of Al _0.05 Ga _0.95 N Si-doped at 1050 ° C. using TMG, TMA, ammonia and silane gas as an impurity gas. 103 is grown to a film thickness of 4 μm.

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

(N-side clad layer 105) Next, the temperature is set to 1
The temperature is set to 050 ° C., TMA, TMG, ammonia and silane gas are used as raw material gases, and Si is 1 × 10 ¹⁹ / cm ^3.
A layer of doped Al _0.08 Ga _0.92 N is 25 Å
Then, the TMA and the silane gas are stopped, and the B layer made of undoped GaN is grown to a film thickness of 25 Å. Then, this operation is repeated to stack the A layer and the B layer to grow the n-side cladding layer 105 made of a multilayer film (superlattice structure) having a total film thickness of 1.2 μm. At this time, S
As the Al mixed crystal ratio of the i-doped AlGaN layer (A layer),
Within the range of 0.05 or more and 0.3 or less, it is possible to sufficiently provide a refractive index difference that functions as a cladding layer. When the n-side clad layer is formed with a superlattice structure in which nitride semiconductors having different bandgap energies are stacked,
Impurity tends to be improved in crystallinity when one of the layers is heavily doped, that is, so-called modulation doping.

(N-side light guide layer 106, 6a, 6b) 1
TMA, TMG, ammonia, and silane gas were made to flow at 050 ° C., and the doped region 6b of the n-side optical guide layer made of n-type GaN doped with Si at 2.5 × 10 ¹⁸ / cm ³ was set to 0.
Grow with a film thickness of 1 μm. Then, the silane gas is stopped, and the undoped region 6a of the n-side optical guide layer made of undoped GaN is grown at 1050 ° C. to a film thickness of 0.15 μm.

(Active layer 107) Next, the temperature is set to 800 ° C., TMI and TMG are used as source gases, and undoped I
a barrier layer made of n _0.05 Ga _0.95 N, and
Undoped In _{0. A} well layer made of ₃₂ Ga _0.68 N is laminated in the order of barrier layer / well layer / barrier layer / well layer / barrier layer. At this time, the barrier layer is formed with a film thickness of 130 Å and the well layer is formed with a film thickness of 25 Å. The active layer has a multiple quantum well structure (MQW) with a total film thickness of about 440Å.

(P-side electron confinement layer 108) Next, at the same temperature, TMA, TMG and ammonia are used as source gases, Cp ₂ Mg (cyclopentadienyl magnesium) is used as an impurity gas, and Mg is 1 × 10 ¹⁹ / cm ³
P-type electron confinement layer 1 made of doped Al _0.3 Ga _0.7 N
08 is grown to a film thickness of 100Å. This layer does not have to be provided in particular, but by providing it, it functions as electron confinement and contributes to the reduction of the threshold value.

(P-side light guide layer 109, 9a, 9b) Subsequently, Cp ₂ Mg and TMA are stopped, and at 1050 ° C., the undoped region 9 of the p-side light guide layer made of undoped GaN.
a is grown to a film thickness of 0.15 μm. Then Cp ₂ M
by flowing g, at 1050 ℃, 2.5 the Mg × 10 ¹⁹ /
A doped region 9b of the p-side light guide layer made of cm ³ doped GaN is grown to a film thickness of 0.1 μm.

(P-side clad layer 110) Subsequently, 105
Using Cp ₂ Mg at 0 ° C., Mg was added to 1 × 10 ²⁰ / cm ³
25 layers of doped p-type Al _0.08 Ga _0.92 N
The p-side clad layer 110 made of a superlattice layer having a total film thickness of 0.6 μm is grown to a film thickness of Å, then only TMA is stopped, and a layer made of undoped GaN is grown to a film thickness of 25 Å.
Grow. At least one of the p-side cladding layers is Al
When a superlattice including a gallium nitride-based compound semiconductor layer containing a layered gallium nitride-based compound semiconductor layer having different bandgap energies is used, a large amount of impurities is doped in either one of the layers, and so-called modulation doping is performed. Doing so tends to improve the crystallinity.

(P-side contact layer 111) Finally, 10
Mg at 1 × 1 on the p-side cladding layer 110 at 50 ° C.
0²⁰/cm ³P-side contact made of doped p-type GaN
The growth layer 111 is grown to a film thickness of 150Å. p side contour
Layer 111 is p-type In_XAl_YGa_1-XYN (0 ≦ X,
0 ≦ Y, X + Y ≦ 1), and preferably
If GaN doped with Mg is used, the p-electrode 120 and the
A favorable ohmic contact is obtained. Contact layer 11
Since 1 is a layer forming an electrode, 1 × 10¹⁷/cm³Since
It is desirable to have the above high carrier concentration. 1 x 10¹⁷
/cm³Lower than that will get a good ohmic with the electrode
Tends to be difficult. Furthermore, the composition of the contact layer
When GaN is used, an electrode material and a preferable ohmic contact can be obtained.
It becomes easy to be damaged. After the reaction is completed, the wafer is placed in the reaction vessel.
C is annealed at 700 ° C in a nitrogen atmosphere, and
The resistance of the mold layer is further reduced.

After growing the gallium nitride-based compound semiconductor as described above and stacking the respective layers, the wafer is taken out from the reaction container and SiO 2 is formed on the surface of the p-side contact layer which is the uppermost layer.
_A protective film made of ₂ is formed and etched with SiCl ₄ gas using RIE (reactive ion etching),
As shown in FIG. 4, the surface of the n-side contact layer 103 on which the n-electrode is to be formed is exposed. Thus, SiO ₂ is most suitable as the protective film for deeply etching the gallium nitride compound semiconductor.

Next, a ridge stripe is formed as the above-mentioned striped waveguide region. First, the top p
P on almost the entire surface of the side contact layer (upper contact layer)
After forming a first protective film 161 made of Si oxide (mainly SiO ₂ ) with a film thickness of 0.5 μm by a VD device, a mask having a predetermined shape is applied on the first protective film 161. By RIE (Reactive Ion Etching) device,
A first protective film 161 having a stripe width of 1.6 μm is formed by a photolithography technique using CF ₄ gas. At this time, the height of the ridge stripe (etching depth) is
p-side contact layer 111 and p-side clad layer 10
9, part of the p-side light guide layer 110 is etched to form p
The side light guide layer 109 is formed by etching to a depth of 0.1 μm.

Next, from the top of the first protective film 161, Zr
Second protective film 16 made of oxide (mainly ZrO ₂ ).
2 is continuously formed with a film thickness of 0.5 μm on the first protective film 161 and on the p-side optical guide layer 109 exposed by etching.

After forming the second protective film 162, the wafer 60 is formed.
Heat treatment at 0 ° C. When a material other than SiO ₂ is formed as the second protective film as described above, after the second protective film is formed, the temperature is 300 ° C. or higher, preferably 400 ° C. or higher and the decomposition temperature of the gallium nitride-based compound semiconductor or lower (1200 ° C. or lower). It is more desirable to add this step, because the second protective film is less likely to be dissolved in the dissolving material (hydrofluoric acid) of the first protective film by heat treatment in (1).

Next, the wafer is immersed in hydrofluoric acid, and the first protective film 161 is removed by the lift-off method. As a result, the first protective film 161 provided on the p-side contact layer 111 is removed and the p-side contact layer is exposed. As described above, as shown in FIG. 4, the second protective film 162 is formed on the side surface of the ridge stripe and the plane (the exposed surface of the p-side light guide layer 109) continuous with the side surface.

After the first protective film 161 provided on the p-side contact layer 112 is thus removed, as shown in FIG. 4, the exposed p-side contact layer 111 is exposed.
A p-electrode 120 made of Ni / Au is formed on the surface of.
However, the p-electrode 120 has a stripe width of 100 μm,
As shown in FIG. 4, it is formed over the second protective film 162. After the formation of the second protective film 162, the already exposed n
On the surface of the side contact layer 103, a striped n-electrode 121 made of Ti / Al is formed in a direction parallel to the stripe.

Next, the surface exposed by etching to form the n-electrode is masked on the p- and n-electrodes and on the desired region to provide the take-out electrode, and a dielectric multilayer film of SiO ₂ and TiO ₂ is formed. After providing 164, Ni on the p and n electrodes
-Ti-Au (1000Å-1000Å-8000Å)
The take-out (pad) electrodes 122 and 123 are respectively provided. At this time, the width of the active layer 107 is 200 μm.
(The 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).

After forming the n-electrode and the p-electrode as described above, the M-plane of the gallium nitride-based compound semiconductor (M-plane of GaN, (1 1-0
(0) etc.) to divide into bar-shaped wafers, and the bar-shaped wafer is further divided to obtain lasers. At this time, the resonator length is 650μ.
m.

The obtained laser device was 20 mA,
The threshold voltage is 3.9 V, the threshold current is also lower than that of the comparative example, and the oscillation wavelength is around 440 nm (435 nm to 445 n
m) continuous oscillation was observed at room temperature. The far field pattern (FFP) in the vertical and horizontal mode is shown in FIG.
The ripple was significantly reduced compared to the comparative example.

[Comparative Example] In the example, the n-side optical guide layer 106 was made of G doped with Si2.5 × 10 ¹⁸ / cm ³
A gallium nitride-based compound semiconductor laser was prepared in the same manner except that aN was grown to a film thickness of 0.1 μm and undoped GaN was grown to a film thickness of 0.1 μm for the p-side optical guide layer. The obtained laser device is at 20 mA,
The threshold voltage is 4.5 V, and the oscillation wavelength is around 440 nm (4
Continuous wave (35 nm to 445 nm) was observed at room temperature. Further, the FFP in the vertical and transverse mode is shown in FIG.

[0074]

In the separation-confinement type gallium nitride compound semiconductor laser of the present invention, the optical guide layer is doped with impurities so that the active layer side is less and the cladding layer side is more, and the optical guide layer is thicker. By
The improvement of the light confining ability in the optical waveguide and the reduction of the threshold voltage and the threshold current are both achieved. Further, since the light leaking to the cladding layer of the light guide layer is reduced, the vertical transverse mode FFP is improved.

[Brief description of drawings]

FIG. 1 is a diagram in which only a configuration of an optical waveguide region and its periphery is extracted in a first embodiment of the present invention.

FIG. 2 is a diagram in which only the configuration of an optical waveguide region and its periphery is extracted in the second embodiment of the present invention.

FIG. 3 is a graph showing the analysis results of the impurity concentration in the layer growth direction of the laser produced in the example of the present invention.

FIG. 4 is a schematic sectional view of a semiconductor laser for explaining an embodiment of the present invention.

FIG. 5 is a vertical-transverse-mode FFP of a laser produced in an example of the present invention.

FIG. 6 is a vertical transverse mode FFP of a laser produced in a comparative example.

[Explanation of symbols]

101 ... Substrate (GaN substrate) 102 ... buffer layer 103 ... n-side contact layer 104 ... Crack prevention layer 105 ... N-side clad layer 106 ... n-side light guide layer 6a ... Undoped region of n-side light guide layer 6b ... Doped region of n-side light guide layer 107 ... Active layer 108 ... p-side electron confinement layer 109 ... p-side light guide layer 9a ... Undoped region of p-side light guide layer 9b ... Doped region of p-side light guide layer 110: p-side clad layer 111 ... p-side contact layer 120 ... p electrode 121 ... n electrode 122 ... p pad electrode 123 ... n pad electrode 163 ... Third protective film 164 ... Insulating film

Claims (16)

[Claims] 1. A gallium nitride compound having a refractive index larger than that of the cladding layer and a bandgap larger than that of the active layer between an active layer made of a gallium nitride compound semiconductor and a cladding layer made of a gallium nitride compound semiconductor. A separate confinement type gallium nitride-based compound semiconductor laser comprising a semiconductor and having an optical guide layer that forms an optical waveguide together with the active layer, wherein the optical guide layer is doped with an impurity that increases conductivity during growth. The gallium nitride-based compound semiconductor laser is characterized in that, in the optical guide layer, at least 0.05 μm from the end face on the active layer side is grown without doping the impurities. 2. The gallium nitride-based compound semiconductor laser according to claim 1, wherein at least a range of 0.1 μm from the end face of the active layer in the optical guide layer is grown without doping impurities. . 3. The gallium nitride-based compound semiconductor laser according to claim 1, wherein the light guide layer is formed with a film thickness of 0.15 μm or more and 0.5 μm or less. . 4. The gallium nitride-based compound semiconductor laser according to claim 1, wherein the light guide layer is formed to have a film thickness of 0.2 μm or more and 0.3 μm or less. . 5. The impurity concentration of a region grown by doping in the optical guide layer is 1 × 10 ¹⁵ / cm ³ or more 1
5. The gallium nitride-based compound semiconductor laser according to claim 1, wherein the gallium nitride-based compound semiconductor laser has a density of x10 ²⁰ / cm ³ or less. 6. The region of the optical guide layer grown by undoping contains impurities due to impurity intrusion from an adjacent layer, and the impurity concentration due to the impurity intrusion is lower than the impurity concentration of the adjacent layer. The gallium nitride-based compound semiconductor laser according to any one of claims 1 to 5, wherein 7. The gallium nitride-based compound semiconductor laser according to claim 1, wherein the light guide layer is made of GaN. 8. The active layer comprises In _x Ga _1-x N (0
A quantum well structure having <x ≦ 1) in a well layer, wherein the cladding layer is made of a nitride semiconductor containing Al _y Ga _1-y N (0 ≦ y ≦ 1). 8. The gallium nitride-based compound semiconductor laser according to any one of 1 to 7. Wherein said cladding _layer, Al _{y Ga 1-y} N
9. The gallium nitride-based compound semiconductor laser according to claim 8, having a superlattice structure in which (0 <y ≦ 1) and GaN are alternately laminated. 10. The impurity element introduced into the light guide layer and the cladding layer formed on the p-side is M
The gallium nitride-based compound semiconductor laser according to any one of claims 1 to 9, wherein g is g. 11. The Mg-doped amount in the Mg-doped region of the p-side light guide layer is M of the p-side cladding layer.
The average doping amount of g is less than or equal to the average doping amount of g.
0. The gallium nitride-based compound semiconductor laser according to item 0. 12. The Mg-doped amount in the Mg-doped region of the p-side optical guide layer is ½ or less of the average Mg-doped amount of the p-side cladding layer. Gallium nitride compound semiconductor laser. 13. The impurity element introduced into the optical guide layer and the cladding layer formed on the n-side is S
The gallium nitride-based compound semiconductor laser according to any one of claims 1 to 12, wherein i is i. 14. The Si-doped amount in the Si-doped region of the n-side light guide layer is S of the n-side cladding layer.
The average doping amount of i is less than or equal to the average doping amount of i.
3. A gallium nitride-based compound semiconductor laser according to item 3. 15. The amount of Si doping in the Si-doped region of the n-side optical guide layer is S of the n-side cladding layer.
15. The gallium nitride-based compound semiconductor laser according to claim 14, wherein the average doping amount of i is 1/2 or less. 16. A gallium nitride compound having a refractive index larger than that of the cladding layer and a bandgap larger than that of the active layer between an active layer made of a gallium nitride compound semiconductor and a cladding layer made of a gallium nitride compound semiconductor. A method of manufacturing an isolated confinement type gallium nitride-based compound semiconductor laser, which comprises a semiconductor and has an optical guide layer that forms an optical waveguide together with the active layer, wherein the optical guide layer contains impurities that increase conductivity during growth. A method for manufacturing a gallium nitride-based compound semiconductor laser, wherein doping is performed, but in the optical guide layer, at least 0.05 μm or more from the end surface on the active layer side is grown without being doped with the impurities.