JP4646095B2 - Semiconductor light emitting device, manufacturing method thereof, and optical information recording / reproducing device - Google Patents

Semiconductor light emitting device, manufacturing method thereof, and optical information recording / reproducing device Download PDF

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JP4646095B2
JP4646095B2 JP2001121824A JP2001121824A JP4646095B2 JP 4646095 B2 JP4646095 B2 JP 4646095B2 JP 2001121824 A JP2001121824 A JP 2001121824A JP 2001121824 A JP2001121824 A JP 2001121824A JP 4646095 B2 JP4646095 B2 JP 4646095B2
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nitride semiconductor
layer
region
defect density
emitting device
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JP2002319733A (en
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茂稔 伊藤
智輝 大野
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シャープ株式会社
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Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor light emitting device which is used for information recording, reproduction, etc. of an optical disc, has a stable horizontal transverse mode and has self-oscillation characteristics for noise reduction, a manufacturing method thereof, and an optical information recording / reproducing device About.
[0002]
[Prior art]
As the storage capacity of an optical disk increases, a light source with a wavelength of around 400 nm is required as a light source for an optical disk, which can reduce the light collection diameter and can record information at a higher density. In addition, in the optical disk system, use of an inexpensive plastic material is being studied for lenses, disks, and the like in order to reduce costs. However, since such a plastic material has a light absorption edge with a maximum wavelength of about 390 nm, it is necessary to further shorten the wavelength as a light source for optical disks. For this reason, it is necessary to study materials as a light source, and it is not easy to deal with mass production. As a light source of such an optical disk system, a semiconductor laser has been conventionally used, and a typical material of a semiconductor laser having a wavelength of about 400 nm is a gallium nitride compound semiconductor.
[0003]
When a nitride semiconductor laser is used in an optical disk system or the like, a structure having self-excited oscillation characteristics is used in order to reduce return light noise from a reflection point of the optical disk or the like.
[0004]
Japanese Laid-Open Patent Publication No. 10-294532 discloses such a nitride semiconductor laser, and FIG. 4 is a sectional view showing a typical structure of the nitride semiconductor laser. In this publication, in an adjacent region of an active layer containing InGaN, an island-like region of In, which is a non-light emitting region, is a region having a saturable absorption characteristic in which light absorption is saturated (hereinafter referred to as a saturable absorption region). ), A self-excited oscillation function is obtained, and a semiconductor laser with reduced return light noise is disclosed.
[0005]
As shown in FIG. 4, in this semiconductor laser, an n-type GaN buffer layer 71 and an n-type GaN contact layer 72 are formed on a sapphire substrate 70, and on a predetermined region of the n-type GaN contact layer 72, An n-type AlGaN cladding layer 73, an n-type InGaN / GaN multiple quantum well adjacent layer 74 having an In island region, an InGaN / GaN multiple quantum well active layer 75, a p-type GaN adjacent layer 76, and a p-type AlGaN cladding layer 77 They are stacked in order. A stripe-shaped waveguide region 78a constituting a laser resonator is provided in the central portion on the p-type AlGaN cladding layer 77, and no current is injected outside the stripe-shaped waveguide region 78a other than the waveguide. An n-type GaN conduction barrier layer 79 is formed. A p-type GaN contact layer 78 is formed on the striped waveguide region 78 a and the n-type GaN conduction barrier layer 79. A p-side electrode 80 is provided on the p-type GaN contact layer 78, and an n-side electrode 81 is formed on the n-type GaN contact layer 72 in a portion other than the predetermined region. Yes.
[0006]
[Problems to be solved by the invention]
The nitride semiconductor laser disclosed in Japanese Patent Laid-Open No. 10-294532 uses an In island region 82 which is a non-light emitting region as a saturable absorption region in a region adjacent to an active layer. It is not easy to form an element while controlling the process so that the In island region 82 that generates absorption maintains good absorption characteristics. As a result, it is difficult to control the process to obtain good self-oscillation characteristics. There is a risk.
[0007]
Further, such an In island-like region 82 is not formed so as to positively have saturable absorption characteristics, and is formed below the region (current-carrying barrier layer) where current outside the stripe-shaped waveguide is not injected. A technique of a low-noise semiconductor laser that enables a self-oscillation operation by using the formed active layer region as a saturable absorption region is known. In this case, in order to effectively sustain the self-excited oscillation of the low-noise semiconductor laser, it is necessary to shorten the carrier lifetime of the active layer other than the current injection region relative to the carrier lifetime of the active layer in the current injection region. However, since a nitride semiconductor has a small carrier diffusion coefficient, carriers generated by light absorption in the saturable absorption region are difficult to diffuse, and it is not easy to shorten the apparent carrier lifetime.
[0008]
Further, in the conventional semiconductor laser, it is difficult to sufficiently stabilize the horizontal and transverse modes during high output operation, and noise, which is a fluctuation in optical output, may occur.
[0009]
The present invention solves such problems, and its purpose is to shorten the carrier life of the active layer other than the current injection region and to obtain a self-oscillation characteristic in which the horizontal transverse mode is stable even during high output operation. An object of the present invention is to provide a semiconductor light emitting device, a manufacturing method thereof, and an optical information recording / reproducing device.
[0010]
[Means for Solving the Problems]
The semiconductor light emitting device of the present invention is A semiconductor light-emitting device that performs self-excited oscillation of laser light, the semiconductor substrate And the semiconductor substrate Multiple nitride semiconductor layers on top The Laminated And a resonator that performs laser oscillation is formed in the stacked structure. Striped waveguide is formed And a current confinement structure for confining a current supplied to the active layer of the plurality of nitride semiconductor layers, and the stacked structure includes the stripe-shaped waveguide into which current in the active layer is injected. The plurality of nitride layers are formed so that the defect density in the saturable absorption region other than the current injection region of the active layer located on both sides of the stripe-shaped waveguide is larger than the defect density in the current injection region constituting the structure. It is a grown semiconductor layer It is characterized by that.
[0011]
The semiconductor substrate is formed on the surface thereof with a concave groove parallel to the stripe-shaped waveguide so that the stripe-shaped waveguide is positioned between a central portion of the concave groove and one end side portion of the concave groove. In the stacked structure, a nitride semiconductor layer is formed on the semiconductor substrate by lateral growth from both side surfaces of the concave groove, and defects are concentrated in the saturable absorption region. Is a thing .
[0012]
The semiconductor light emitting device of the present invention is The semiconductor substrate is selectively grown on the surface thereof to suppress growth of the nitride semiconductor layer on the semiconductor substrate, or to promote growth of the nitride semiconductor layer on the semiconductor substrate. In the stacked structure, the formation of the nitride semiconductor layer on the semiconductor substrate is selectively suppressed or promoted by the growth suppressing film or the growth promoting film, and the defects are formed. Concentrated in the saturable absorption region It is characterized by that.
[0013]
Above In the saturable absorption region with high defect density Defect density is the stripe waveguide Configure current injection region so The defect density is 10 times or more.
[0014]
Above In the saturable absorption region with high defect density The defect density is 10 8 / Cm 2 That's it.
[0015]
High defect density Saturable absorption Region and said striped waveguide Current injection region that constitutes Is 0.5 μm to 4 μm.
[0017]
The stripe width of the striped waveguide is 0.5 μm to 8 μm.
[0019]
A method for manufacturing a semiconductor light emitting device of the present invention includes: A method of manufacturing the above-described semiconductor light emitting device of the present invention, which is the semiconductor substrate. On the nitride semiconductor substrate of the first conductivity type Concave groove Forming a nitride semiconductor substrate of the first conductivity type, So that the concave groove is embedded Forming a nitride semiconductor layer of a first conductivity type at a first growth temperature; and a second growth temperature different from the first growth temperature on the nitride semiconductor layer of the first conductivity type. Forming a first conductivity type nitride semiconductor crack prevention layer, a first conductivity type nitride semiconductor clad layer, and a first conductivity type nitride semiconductor guide layer in order according to the first growth temperature; And a third growth temperature different from the second growth temperature on the nitride semiconductor guide layer of the first conductivity type. As the active layer Forming a first conductivity type nitride semiconductor active layer; and a second conductivity type nitride semiconductor barrier layer on the first conductivity type nitride semiconductor active layer by the first growth temperature. Forming a second conductive type nitride semiconductor guide layer, a second conductive type nitride semiconductor clad layer, and a second conductive type nitride semiconductor contact layer in sequence; The second conductivity type nitride contact layer and the second conductivity type nitride semiconductor cladding layer; By dry etching process Etching is performed so as not to reach the nitride semiconductor guide layer of the second conductivity type, thereby defining the striped waveguide. Forming a ridge structure.
[0020]
The optical information recording / reproducing apparatus of the present invention is Of the present invention described above A semiconductor light emitting device is used as a light source.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
The self-oscillation of a semiconductor laser is based on mutual interaction between carriers and photons in an active layer (gain region) in which an inversion distribution is generated by carriers injected into the semiconductor laser and a saturable absorption region that is a region having saturable absorption characteristics. Caused by action. The characteristic required for the saturable absorption region is that the substantial band gap is the same as or slightly narrower than the band gap of the active layer (gain region), and is required for the saturable absorption region. The second characteristic is that the carrier life in the saturable absorption region is shorter than the carrier life in the active layer and the light absorption is easily saturated in order to effectively cause self-excited oscillation.
[0022]
In the nitride semiconductor laser, the lifetime characteristic of carriers in the saturable absorption region, which is the second characteristic required for the saturable absorption region, is particularly important. It is known that the lifetime of a nitride-based semiconductor carrier is as short as a few ns at minimum, and in order to make a clear difference in the carrier lifetime between the active layer and the saturable absorption region, Impurity elements (for example, Mg) are added (doping) at a high concentration, and carrier diffusion from the light absorption region to the outside of the light absorption region in the saturable absorption region is promoted to efficiently recombine carriers. Thus, it is conceivable to shorten the apparent carrier life.
[0023]
However, in the conventional nitride semiconductor laser, the current injection region of the active layer (the oscillation region corresponding to the striped waveguide) is the gain region, and the other than the current injection region of the active layer is the saturable absorption region having saturable absorption characteristics. In this case, it is not easy to make a clear difference in the amount of doping (doping) of the impurity element in both the gain region and the saturable absorption region. In addition, in order to apparently shorten the lifetime of carriers generated in the saturable absorption region by light absorption, it is possible to have a large diffusion coefficient such that the generated carriers diffuse out of the light absorption region in the order of several ns. The material characteristics of the saturated absorption region are necessary. However, in the saturable absorption region formed of InGaN or the like, generally, carriers are generated by sufficiently diffusing carriers generated in the saturable absorption region because the diffusion coefficient is small. It is difficult to obtain the effect of recombining and shortening the lifetime of the carrier.
[0024]
In the present invention, as a result of repeated investigations on this point, by using a process of forming a step in the GaN substrate, the defect density is periodically formed in the region where the gain region and the saturable absorption region of the semiconductor laser are formed. A region having a high density and a region having a low defect density are formed, and a current injection region that is a gain region (an oscillation region corresponding to a striped waveguide) is disposed in a region having a low defect density, and the current injection region that is a gain region By disposing a saturable absorption region having saturable absorption characteristics in a region having a high defect density so as to be close to each other, it is possible to quickly recombine carriers by non-radiative transition. As a result, in the formation region of the semiconductor laser, the effective carrier lifetime in the saturable absorption region, which is a high defect density region outside the current injection region (oscillation region corresponding to the striped waveguide) that is the gain region, is increased. A nitride semiconductor laser that can be self-excited up to a high output can be obtained because the self-excited oscillation is easily maintained in the gain region while being shortened.
[0025]
Also, in the vicinity of the current injection region (oscillation region corresponding to the striped waveguide) which is a gain region having a low defect density, as compared to the current injection region (oscillation region corresponding to the striped waveguide), By disposing a region having a saturable absorption characteristic having a high defect density and a short carrier lifetime, light absorption in the saturable absorption region having a high defect density can be sufficiently maintained during operation of the semiconductor laser. As a result, in the nitride semiconductor laser, the difference in defect density between the current injection region (oscillation region corresponding to the striped waveguide) which is a gain region having a low defect density and the saturable absorption region having a high defect density is predetermined. When the nitride semiconductor laser is operated, a saturable absorption region with a high gain and defect density in a current injection region (an oscillation region corresponding to a striped waveguide) which is a gain region with a low defect density is obtained. This makes it possible to clearly set the difference between absorption and horizontal horizontal mode until high output.
[0026]
The present invention is based on such knowledge.
[0027]
In a nitride semiconductor laser with a groove forming a step on the surface of the GaN substrate, a land which is a convex region of the step and a groove which is a concave region of the step are periodically and alternately formed on the surface of the GaN substrate. ing. If the land width, the groove width, the groove depth, and the thickness of the regrowth layer formed on the GaN substrate having the step structure are defined by predetermined conditions, the regrowth layer is regenerated according to the step structure. It was confirmed that the growth layer had a region with a high defect density and a region with a low defect density. By observing the regrowth layer with etch pits and TEM (transmission electron microscope), it was confirmed that a region having a high defect density was formed on the central portion of the groove upper surface and the land upper surface. This means that when the regrowth layer is regrowth from a GaN substrate with a step formed, the growth from the side of the land is selected to be superior to the growth from the top surface of the groove, and as a result, threading dislocations are formed on the groove. It is considered that defects such as those are focused on the central portion on the groove, and defects such as threading dislocations are reduced in regions other than the central portion on the groove.
[0028]
In addition, when the regrowth layer is thin, the top surface of the land or the central portion of the groove may not be completely filled with the regrowth layer, and a stepped groove may remain. Then, regions with various defect densities were formed, and the changes in the self-excited oscillation characteristics of the nitride semiconductor laser and the stability of the horizontal transverse mode during high output operation were investigated.
[0029]
The region with low defect density has good crystallinity, and when a striped waveguide with a ridge structure (current injection region that is a gain region) is formed in this region, the injected carriers are effectively recombined by radiation. A nitride semiconductor laser having a high slope efficiency, which is a rate of increase in light intensity with respect to a rate of increase in injection current, was obtained. In the region where the defect density is high, the rate of non-radiative recombination and the rate of relaxation to the low energy level due to the defect increase, and it is generated by light absorption in the active layer other than the current injection region which is the gain region. Recombination of carriers is promoted. As a result, the lifetime of carriers in the saturable absorption region, which is a region with a high defect density, is shortened, which is effective for sustaining self-excited oscillation in the nitride semiconductor laser and stabilizing the horizontal transverse mode at high output.
[0030]
As described above, as a substrate required for a nitride semiconductor laser having a self-excited oscillation characteristic, it is optimal that the difference in defect density between a portion with a high defect density and a portion with a low defect density is optimal. It is better that the difference in defect density changes sharply at the boundary.
[0031]
In the present invention, the conditions for forming a step on the GaN substrate were examined for such points. As a result, the groove width was 4 μm to 30 μm, the groove depth was 0.1 μm to 5 μm, and the regrowth layer was 1 μm to 10 μm. Then, it was confirmed that it is effective for a nitride semiconductor laser having self-excited oscillation characteristics. Here, if the depth of the groove is A and the thickness of the regrowth layer on the groove is B, the above-described effects can be obtained if 20A ≧ B ≧ 2A. As a result of observing the surface of the GaN substrate having such a step structure, a region with a high defect density and a region with a low defect density are generated at the center of the upper surface of the groove on the GaN substrate and the upper surface of the land, respectively. It was. When lands and grooves are periodically arranged on the GaN substrate, the land width may be set to about 3 μm to 20 μm. At this time, the groove may be formed on the surface of the GaN substrate so as to face the striped waveguide. Further, it is preferable that the region having a high defect density is arranged symmetrically with respect to the stripe-shaped waveguide having a ridge structure (current injection region which is a gain region), and the light distribution in the horizontal and transverse modes is targeted. As a result, the groove width may be 10 μm to 20 μm. Further, the defect density in the high defect density region is 10 times or more the defect density in the low defect density region, and the defect density in the high defect density region is 10 times. 8 cm -2 That is all you need.
[0032]
Next, the arrangement of the current injection region (oscillation region corresponding to the striped waveguide), which is the gain region, is used to control the self-excited oscillation characteristics and to stabilize the light in the saturable absorption region for stabilizing the horizontal transverse mode. Since it is necessary to control the amount of absorption, the thickness of the active layer, the distance from the saturable absorption region to the end of the stripe-shaped waveguide of the ridge structure, the stripe width (average width of the upper and lower parts of the ridge structure), etc. Design is required. Good results are obtained when the stripe width is 0.5 μm to 8 μm, and good results are obtained when the distance from the saturable absorption region having a high defect density to the end of the striped waveguide having the ridge structure is 0.5 μm to 4 μm. was gotten. When the stripe width is less than 0.5 μm, the horizontal transverse mode light distribution in the gain region becomes small, and a sufficient gain as a semiconductor laser cannot be obtained. If the stripe width is increased, the light distribution in the active layer other than the gain region corresponding to the saturable absorption region can be set to an appropriate range, and the effect of shortening the carrier life can be easily obtained. If it exceeds the threshold current, the threshold current that becomes the laser oscillation start point becomes high, and a long life as a semiconductor laser cannot be obtained.
[0033]
In addition, the high defect density region shortens the lifetime of carriers generated in the active layer other than the current injection region, which is the gain region, by non-radiative recombination and relaxation to a low energy level caused by defects. The carriers generated at the bottom of the light distribution in the horizontal transverse mode in the laser oscillation state or generated in a time shorter than the carrier lifetime (several ns) of the active layer in the current injection region which is the gain region Must be located within a distance that can be diffused. When the distance from the saturable absorption region having a high defect density to the end of the ridge-structured striped waveguide is less than 0.5 μm, the current density in the current-injected region (oscillation region) of the striped waveguide is within the defect density. The threshold current that becomes the laser oscillation start point is remarkably increased, and there is a possibility that continuous oscillation does not occur at room temperature.
[0034]
In addition, if the distance from the saturable absorption region having a high defect density to the end of the ridge-structured striped waveguide exceeds 4 μm, self-pulsation characteristics as a semiconductor laser cannot be obtained. The stability of the light distribution in the horizontal and transverse modes cannot be obtained. This is because, since the bottom of the horizontal transverse mode light distribution in the laser oscillation state is far away from the saturable absorption region, which is a region with high defect density, carriers generated by light absorption may have high defect density. This is because it cannot diffuse into the saturated absorption region in a short time, and effects such as non-radiative recombination cannot be obtained.
[0035]
The active layer preferably has a multiple quantum well structure. If the thickness of the active layer is 5 nm to 200 nm as the sum of the quantum well layer and the barrier layer, self-oscillation characteristics can be obtained in the nitride semiconductor laser. . When the thickness of the active layer is thick, if the ratio Γ / d between the optical confinement factor Γ in the vertical direction and the thickness d of the active layer is made constant, the light absorption is larger at a constant light output than when the thickness of the active layer is thin. The amount increases. In this case, the carrier density in the active layer in the current injection region, which is the gain region, is also reduced, so that the lifetime of the carriers is increased and the self-oscillation is likely to occur in the nitride semiconductor laser. However, if the thickness of the active layer becomes too thick, it is difficult to obtain a gain in the current injection region, which is the gain region. Therefore, the threshold current that becomes the laser oscillation start point becomes high, and the injection power increases. If the thickness exceeds 200 nm, oscillation will stop in a short time even if continuous oscillation is performed.
[0036]
As described above, in the present invention, by providing a groove for forming a step on the surface of the GaN substrate, a structure in which horizontal transverse mode is stabilized in a wide range of light output and the self-oscillation is stabilized, and noise induced by return light is reduced. A nitride semiconductor laser having the following characteristics was obtained.
[0037]
FIG. 1 is a cross-sectional view of a nitride semiconductor laser which is a semiconductor light emitting device according to a first embodiment of the present invention.
[0038]
An n-type GaN regrowth layer 23, an n-type GaN layer 12, an n-type InGaN crack prevention layer 13, an n-type AlGaN clad layer 14, An n-type GaN guide layer 15, an n-type InGaN active layer 16, a p-type AlGaN barrier layer 17, and a p-type GaN guide layer 18 are sequentially stacked. A p-type AlGaN cladding layer 19 is laminated on the p-type GaN guide layer 18, and the p-type AlGaN cladding layer 19 has a ridge structure in which a central portion in the width direction orthogonal to the stripe direction protrudes. A p-type GaN contact layer 20 is stacked thereon. An insulating film 21 is provided on the p-type AlGaN cladding layer 19 and on the side surfaces of the p-type AlGaN cladding layer 19 and the p-type GaN contact layer 20 except for the upper surface of the p-type GaN contact layer 20. A p-type electrode 22 is provided on the upper surface of the p-type GaN contact layer 20. An n-type electrode 10 is formed on the n-type GaN substrate 11 side.
[0039]
As described above, the nitride semiconductor laser according to the first embodiment of the present invention shown in FIG. 1 has a striped refractive index waveguide using a ridge structure.
[0040]
FIG. 2 is a schematic diagram showing the positional relationship between the striped waveguide that is the current injection region of the nitride semiconductor laser shown in FIG. 1 and the high defect density region that is the saturable absorption region. In FIG. 2, a region A indicates a low defect density region having a stripe-shaped waveguide which is a current injection region having a low defect density, and a region B indicates a high defect density region which is a saturable absorption region having a high defect density. , L indicates the distance from the end of the striped waveguide to the center of the high defect density region. In FIG. 2, the stripe width is 2 μm and L is 1 μm. The cavity length of the nitride semiconductor laser is 450 μm, the front reflectance of the resonator is 20%, and the rear reflectance of the resonator is 85%. The depth of the ridge is adjusted so that the optical confinement factor in the horizontal direction is 0.88 to 0.97.
[0041]
A method for manufacturing a nitride semiconductor laser according to the first embodiment of the present invention will now be described with reference to FIG. The epitaxial growth method shown below is a method for growing a crystal film on a substrate, and includes a VPE (vapor phase epitaxial) method, a CVD (chemical vapor deposition) method, a MOVPE (organometallic vapor phase epitaxial) method, MOCVD (organic metal chemical vapor deposition) method, Halide-VPE (halogen chemical vapor deposition) method, MBE (molecular beam epitaxial) method, MOMBE (organometallic molecular beam epitaxial) method, GSMBE (gas source molecular beam epitaxial) And CBE (Chemical Beam Epitaxial) method.
[0042]
First, the n-type GaN substrate 11 is formed. The n-type GaN substrate 11 is provided with a groove having a pitch interval of 20 μm and a depth of 2.5 μm and a width of approximately 15 μm on a GaN single crystal film having a thickness of approximately 500 μm.
[0043]
Next, each gallium nitride semiconductor layer constituting the nitride semiconductor laser is stacked on the n-type GaN substrate 11 by an epitaxial growth method. In this case, first, an n-type GaN substrate 11 is set in a furnace of a MOCVD (metal organic chemical vapor deposition) apparatus, and a group V raw material NH Three And a group III raw material TMGa (trimethylgallium), a low temperature GaN buffer layer is grown at a growth temperature of 550 ° C., and a low temperature GaN buffer layer having a thickness of 25 nm is formed on the n-type GaN substrate 11. On this low-temperature GaN buffer layer, the temperature is raised to a growth temperature of 1075 ° C. Four In addition, an n-type GaN regrowth layer 23 having a thickness of about 3.5 μm is newly deposited by an epitaxial growth method. Further, on the n-type GaN regrowth layer 23, an n-type GaN layer 12 having a thickness of 0.5 μm (Si impurity concentration 1 × 10 18 / cm Three ).
[0044]
Subsequently, the growth temperature is lowered to about 700 ° C. to 800 ° C., and TMIn (trimethylindium), which is a group III material, is supplied to the n-type GaN layer 12 with the n-type In 0.07 Ga 0.93 An N layer is grown to form an n-type InGaN crack prevention layer 13 having a thickness of 50 nm. Thereafter, the growth temperature is again raised to 1075 ° C., and a group III raw material TMAl (trimethylaluminum) is used to form n-type Al on the n-type InGaN crack prevention layer 13. 0.1 Ga 0.9 N layer (Si impurity concentration 1 × 10 18 / cm Three ) To form an n-type AlGaN cladding layer 14 having a thickness of 0.95 μm, and further, an n-type GaN guide layer 15 having a thickness of 0.1 μm is formed on the n-type AlGaN cladding layer 14.
[0045]
Thereafter, the growth temperature is lowered to 730 ° C., and an In-type film having a thickness of 4 nm is formed on the n-type GaN guide layer 15. 0.15 Ga 0.85 N quantum well layer and 6 nm thick In 0.05 Ga 0.95 An n-type InGaN active layer 16 is formed by alternately forming N barrier layers and growing an active layer having a multiple quantum well structure in which four barrier layers and three quantum well layers are periodically stacked. To do. In addition, the n-type InGaN active layer 16 is 1 second to 180 seconds after the barrier layer is stacked and before the quantum well layer is stacked, or after the quantum well layer is stacked and the barrier layer is stacked. The crystal growth may be interrupted. By this operation, the flatness of each layer of the n-type InGaN active layer 16 is improved, and the light emission half width is reduced.
[0046]
Next, the growth temperature is raised again to 1050 ° C., and the p-type Al is formed on the n-type InGaN active layer 16. 0.2 Ga 0.8 An N layer is grown to form a p-type AlGaN barrier layer 17 having a thickness of 18 nm, and a p-type GaN guide layer 18 having a thickness of 0.1 μm is formed on the p-type AlGaN barrier layer 17. In the p-type AlGaN barrier layer 17 and the p-type GaN guide layer 18, 5 × 10 5 of Mg is used as a p-type impurity element. 19 / Cm Three ~ 2x10 20 / Cm Three Add at a concentration of Subsequently, the p-type Al is formed on the p-type GaN guide layer 18. 0.1 Ga 0.9 An N layer is grown to form a p-type AlGaN cladding layer 19 having a thickness of 0.5 μm, and a p-type GaN contact layer 20 having a thickness of 0.1 μm is formed on the p-type AlGaN cladding layer 19. In the p-type AlGaN cladding layer 19 and the p-type GaN contact layer 20, 5 × 10 5 of Mg is used as a p-type impurity element. 19 / Cm Three ~ 2x10 20 / Cm Three Add at a concentration of As described above, TMGa, TMAl, TMIn, NH3, and the like are used as the raw materials of the elements constituting each layer of the nitride semiconductor laser, and the impurity elements (dopants) added to the respective layers are used as the raw materials. , Cp2Mg (biscyclopentadienylmagnesium), SiH Four Etc. are used.
[0047]
When the nitride semiconductor laser wafer thus formed is observed, each layer formed on the n-type GaN substrate 11 corresponds to a step structure having a groove on the n-type GaN substrate 11 as described above. Thus, a region having a high defect density and a region having a very low defect density are periodically repeated. By the selective growth in the lateral direction from the n-type GaN regrowth layer 23 formed in a step shape. It is considered a thing. Many defects due to threading transition occur along the central portion of the groove (groove) on the n-type GaN substrate 11 and the top surface of the land, and the defect density is divided into a range of about 0.1 μm in width parallel to the groove. Was evaluated, the central portion of the groove had a defect density of 10 Ten cm 2 The above-described region having an extremely high defect density, and regions within about 1 μm from the center on both sides of the central portion of the groove, each has a defect density of 10 8 cm 2 The above-described high defect density region. Similarly, the land upper surface also has a defect density of 10 8 cm 2 This is the above high-density defect region. In contrast to this result, there are very few defects in other regions in the groove (10 7 cm 2 It was in a high quality crystal state.
[0048]
Further, after the p-type GaN contact layer 20 is formed on the nitride semiconductor laser wafer, the p-type AlGaN cladding layer 19 and the p-type GaN contact layer 20 are left only in the center in the width direction. The ridge structure is formed so that the end portion of the striped waveguide is disposed at a position of 2 μm from the center of the groove (groove) by removing by dry etching. As a result, the distance L between the end of the striped waveguide and the high defect density region near the center of the groove is 1 μm. Thereafter, the p-type AlGaN cladding layer 19 and the p-type GaN contact layer 20 are covered with an insulating film 22 so that only the upper surface of the p-type GaN contact layer 20 is exposed. Further, a p-type electrode (Pd / Mo / Au) 22 is formed over the exposed upper surface of the p-type GaN contact layer 20 and the upper surface of the insulating film 21. The p-type electrode 22 is electrically connected to the upper surface of the p-type GaN contact layer 20.
[0049]
Thereafter, the back side of the n-type GaN substrate 11 is polished or etched to remove a part of the n-type GaN substrate 11 and adjust the thickness of the wafer to about 100 to 150 μm. This operation is an operation for making it easy to divide the wafer into individual semiconductor laser chips in a subsequent process. In particular, when the laser end face mirror is formed at the time of division, it is desirable to adjust the thickness to about 80 to 120 μm. In the first embodiment of the present invention, the thickness of the wafer is adjusted to 100 μm using a grinding machine and a polishing machine, but may be adjusted using only the polishing machine. The back surface of the wafer is flat because it is polished by a polishing machine.
[0050]
After polishing the back surface of the n-type GaN substrate 11, a thin metal film is deposited on the back surface of the n-type GaN substrate 11 to form the n-type electrode 10 having a stacked structure of Hf / A1 / Mo / Au. As a method for forming such a thin metal film while controlling the film thickness, a vacuum deposition method is suitable, and this method is also used in the first embodiment of the present invention. However, as a method for forming the n-type electrode 10, other methods such as an ion plating method and a sputtering method may be used. The p-type electrode 23 and the n-type electrode 10 are each annealed at a temperature of 500 ° C. after forming the metal film in order to form an ohmic electrode with good conduction.
[0051]
The semiconductor element manufactured in this way is divided by the following method. First, a scribe line is inserted at the diamond point from the surface of the wafer, and a force is appropriately applied to the wafer to divide the wafer along the scribe line. The scribe line may be inserted from the back surface of the wafer. Other methods for dividing the wafer include a dicing method in which a wire saw or a thin blade is used to cut or cut, a laser beam irradiation heating such as an excimer laser, and subsequent rapid cooling to cause a crack in the irradiation part. Laser scribing using a scribe line, laser ablation that irradiates a laser beam with high energy density, and evaporates this part to perform grooving can be applied. can do.
[0052]
Furthermore, in the nitride semiconductor laser according to the first embodiment of the present invention, a reflection film having a reflectivity of about 10% is formed on one end face on the two end faces of the semiconductor laser element, and 80% on the other end face. A reflective film having a reflectivity of about% is formed and an asymmetric coating is applied.
[0053]
Next, a nitride semiconductor laser chip is mounted on a heat sink such as a stem by a die bonding method to obtain a nitride semiconductor laser device. The nitride semiconductor laser chip was firmly bonded by junk-up in which the n-type electrode 10 was bonded to the heat sink. Here, the heat sink is a stem, a Si submount, a Cu submount, or the like, and a light receiving element or the like may be formed on the Si submount.
[0054]
When various characteristics of the nitride semiconductor laser thus manufactured were examined, the following results were obtained. The cavity length of the nitride semiconductor laser is 450 μm, and the stripe width is 2 μm. This nitride semiconductor laser oscillated continuously at a threshold current of 32 mA as a laser oscillation starting point at a room temperature of 25 ° C., and the oscillation wavelength at that time was 405 ± 5 nm. Further, self-oscillation was obtained when the optical output was in the range of 2 mW to 21 mW. Further, when the optical output is increased and the kink indicating the level at which the laser oscillation horizontal transverse mode becomes unstable is examined, the optical output of the kink is 70 mW or more, and this nitride semiconductor laser has an optical output from 2 mW. It was confirmed that the self-excited oscillation operation was performed in the range of 21 mW.
[0055]
Next, in the nitride semiconductor laser according to the first embodiment of the present invention, the result of confirming the optical output range in which self-oscillation occurs by changing the position where the striped waveguide having the ridge structure is formed is shown. It is shown in 1. In the n-type GaN substrate 11 which is a nitride semiconductor laser wafer, when the position where the stripe waveguide is formed is changed, the distance L from the end of the stripe waveguide to the center of the high defect density region is 5 μm. This is the same as the case where a groove (groove) structure for forming a step is not provided on the n-type GaN substrate 11 (L = ∞). Compared with the nitride semiconductor laser of the first embodiment, self-oscillation is The optical output becomes narrower in the range of 3 to 6 mW, and the kink generation output is reduced to 30 mW or more.
[0056]
When L is 4 μm, the self-excited oscillation has an optical output in the range of 2 to 15 mW, the kink generation output is 50 mW or more, and the effect of providing a groove (groove) structure that forms a step on the n-type GaN substrate 11 is obtained. can get. When L is 0.5 μm, the self-excited oscillation has a light output in the range of 1 to 26 mW, a kink generation output is 70 mW or more, and a groove (groove) structure that forms a step on the n-type GaN substrate 11 is provided. The effect is noticeable.
[0057]
Furthermore, when L is smaller than 0.5 μm and L = 0.3 μm, continuous oscillation is not observed. This is because the stripe-shaped waveguide is partially covered by the high defect density region, so that a large number of carriers injected into the stripe waveguide reach the high defect density region due to diffusion, and there is no radiation recombination. This is considered to be due to a decrease in gain.
[0058]
From this, the existence of defects affects the self-excited oscillation characteristics of the nitride semiconductor laser because the distance that carriers reach the defects by diffusion is about 0.5 μm, so the average distance between the defects is Since it is considered to be on the order of 1 μm or less, the defect density in the high defect density region where recombination of carriers is promoted is one defect per 1 μm square (1 / μm 2 = 10 8 / Cm 2 ) That's all you need.
[0059]
In order to expand the range of light output in which self-excited oscillation occurs by giving a clear difference in carrier recombination between the high defect density region and the striped waveguide region, the striped waveguide The defect density in this area needs to be one digit or more lower than the defect density in the high defect density area. Actually, in the manufacturing process of the nitride semiconductor laser according to the first embodiment, when the thickness of the n-type regrowth layer 23 formed on the upper surface of the groove on the n-type GaN substrate 11 is increased, the defect density in the high defect density region is increased. And the defect density of the striped waveguide which is the low defect density region is reduced. In this way, the n-type regrowth layer 23 is thickened so that the defect density in the stripe-shaped waveguide region is 3 × 10 6. 7 / Cm 2 When an element of the same level is manufactured, the range of light output capable of self-oscillation is 3 to 7 mW, the kink generated light output is 20 mW or more, and no groove (groove) structure is provided on the n-type GaN substrate 11 It was confirmed that there was no difference from (L = ∞).
[0060]
[Table 1]
As a result, in the nitride semiconductor laser according to the first embodiment of the present invention, the defect density in the high defect density region serving as the saturable absorption region is the same as the defect density in the low defect density region where the striped waveguide is formed. The defect density in the high defect density region is 10 times or more. 8 / Cm 2 By doing so, the effect of expanding the range of light output capable of self-oscillation was obtained. As described above, the nitride semiconductor laser according to the present invention can be operated with low noise up to a high output power range. For example, if it is used as a system light source for an optical disk, it is very useful for system application.
[0061]
Further, as shown in Table 1, in the nitride semiconductor laser of the present invention, no kink occurred up to a high output corresponding to the range of optical output capable of self-oscillation. This is because a high defect density region serving as a saturable absorption region is arranged at a predetermined distance L from the end portion of the stripe-shaped waveguide, so that the light distribution is higher than that of the fundamental mode. This is because the oscillation efficiency for the next mode is relatively lower than the oscillation efficiency for the fundamental mode, and the appearance of higher-order modes is suppressed. Such a nitride semiconductor laser having a stable horizontal transverse mode at the time of high output operation, particularly when used as a light source for an optical disc system, is particularly a noise (due to instability of the horizontal transverse mode, which is a concern at the time of high output operation ( Instability of operation) is suppressed, which is very useful.
[0062]
Next, a nitride semiconductor laser according to a second embodiment of the present invention will be described. In the second embodiment, the quantum well layer of the n-type InGaN active layer 16 of the nitride semiconductor laser of the first embodiment is formed of GANP, GANAS, InGaNP, InGaNAs or the like having an oscillation wavelength of 360 to 550 nm. Other configurations are the same as those of the nitride semiconductor laser according to the first embodiment shown in FIG. As for the nitride semiconductor laser of the second embodiment, as in the nitride semiconductor laser of the first embodiment, stabilization of the horizontal transverse mode and expansion of the range of light output capable of self-oscillation were obtained.
[0063]
In the nitride semiconductor laser of the first embodiment and the nitride semiconductor laser of the second embodiment, the positional relationship between the stripe-shaped waveguide having the ridge structure and the high defect density region on the upper surface of the groove on one side Although the high defect density region on the upper surface of the land on the other side is also arranged at the same distance as described above from the striped waveguide, it becomes a right and left object with the striped waveguide as the center, and a more preferable configuration Nitride semiconductor laser can be obtained.
[0064]
In the present invention, a method for forming a region having a high defect density and a region having a low defect density is provided with a groove (groove) for forming a step on the n-type GaN substrate 11 and an n-type GaN regrowth layer thereon. However, the method is not limited to this method. Instead of the groove (groove), SiO 2 is used. 2 Alternatively, a method of re-growth on a substrate such as the n-type GaN substrate 11 may be used so that a growth suppressing film such as W or W or a growth promoting film such as AlN is disposed on the stripe. Further, the substrate material is not limited to GaN, and a substrate material used as another nitride semiconductor substrate such as sapphire, SiC, or silicon may be used.
[0065]
Next, the noise characteristics with respect to the return light when the nitride semiconductor laser of the present invention was used as a system light source for an optical disk were examined. FIG. 3 is a conceptual diagram showing an optical information recording / reproducing apparatus using the nitride semiconductor laser of the present invention. The optical information recording / reproducing apparatus includes a base 121, a nitride semiconductor laser 122 installed on the base 121, a collimator lens 123, a branch element 124, an objective lens 125, a lens 127 for collecting reflected light, a light It comprises a detector 128 and the like.
[0066]
In the optical information recording / reproducing apparatus, an optical information recording disk (optical disk) 126 is set at the condensing point position of the objective lens 125. During reproduction, the light emitted from the nitride semiconductor laser 122 is collimated by the collimator lens 123, The light is condensed on the information recording surface of the optical information recording board 126 by the objective lens 125. Information is written on the information recording surface of the optical information recording board 126 by means of unevenness, magnetic modulation, refractive index modulation or the like. The laser beam condensed on the information recording surface of the optical information recording board 126 is modulated and reflected there, and is guided to the photodetector 128 side by the branch element 124 through the objective lens 125 and is incident on the photodetector 128. The The optically detected signal is converted into an electrical signal by the photodetector 128, and the recorded information is read.
[0067]
At the time of recording, the light emitted from the nitride semiconductor laser 122 is similarly focused on the information recording surface of the optical information recording board 126 by the collimator lens 123 and the objective lens 125, and the laser light itself is modulated according to the information. As a result, the refractive index or magnetic field of the information recording surface of the optical information recording board 126 is modulated, so that information is written or the optical information recording is partially performed by the influence of the focused laser beam. The information recording surface of the board 126 is heated, and at the same time, a magnetic field is applied to the information recording surface, thereby modulating the magnetic field of the information recording surface of the optical information recording board 126 and writing information.
[0068]
The noise of the optical information recording / reproducing apparatus using the nitride semiconductor laser of the present invention was measured by the following method. The laser light was continuously oscillated, and the information recording surface of the optical information recording board 126 was slightly oscillated to evaluate the relative noise intensity (RIN: Relative Intensity Noise). Continuously oscillating lasers are known to have unstable output due to interference with return light (light returning to the laser from the information recording surface of the optical information recording board 126) and have low noise characteristics. For this purpose, self-excited oscillation may be performed at a specific period. First, when the optical output was 5 mW, the noise when the return light was 0.1% to 10% was examined, and it was found that RIN = −130 [dB / Hz] or less. Next, in order to investigate the noise characteristic with respect to the return light when the optical output is high, when the optical output is reduced to about 15 mW, similarly, RIN = −135 [dB / Hz] or less, and the low noise characteristic is maintained. Was. As a result, it was confirmed that the nitride semiconductor laser of the present invention has stable self-oscillation characteristics and is suitable as a light source for an optical disc system.
[0069]
In order to compare with the nitride semiconductor laser of the present invention, the semiconductor laser in the comparative example column of Table 1 was selected and mounted instead of the light source of the optical information recording / reproducing apparatus described above, and the light output was 15 mW. In some cases, the maximum RIN is about −110 [dB / Hz]. With such an optical output, it has been found that there is a lot of noise and it is not suitable for use in an optical disc system or the like.
[0070]
In addition, we examined the operation at high output corresponding to the writing operation. When the return light is 0.1%, the nitride semiconductor laser of the present invention operates stably up to an optical output of 40 mW or more, and the relative noise intensity is also a maximum RIN = −130 [dB / Hz] or less. When the semiconductor laser in the comparative example column of Table 1 is selected and mounted instead of the light source of the optical information recording / reproducing apparatus, the relative noise intensity is maximum RIN = −100 in the vicinity of the kink generated light output. It became [dB / Hz] or more, and the operation was extremely unstable.
[0071]
Therefore, with the nitride semiconductor laser of the present invention, it is possible to construct a low noise system and realize an optical information recording / reproducing apparatus with few read / write operation failures.
[0072]
【The invention's effect】
In the semiconductor light emitting device of the present invention, a plurality of semiconductor layers are stacked on a substrate to provide a striped waveguide, and a concave region is formed on the surface of the substrate in a region facing the striped waveguide. Thus, a pair of convex regions are formed, and one convex region is formed in the vicinity of the concave region, so that the carrier life of the active layer other than the current injection region is shortened, and horizontal and horizontal even during high output operation. A self-oscillation characteristic with a stable mode can be obtained.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a semiconductor light emitting device according to a first embodiment of the present invention.
2 is a schematic diagram showing a positional relationship between a striped waveguide and a high defect density region of the semiconductor light emitting device of FIG.
FIG. 3 is a schematic view of an optical information recording / reproducing apparatus using the semiconductor light emitting device of the present invention.
FIG. 4 is a cross-sectional view of a nitride semiconductor laser which is a conventional semiconductor light emitting device.
[Explanation of symbols]
10 N electrode
11 n-type GaN substrate
12 n-type GaN layer
13 n-type InGaN crack prevention layer
14 n-type AlGaN cladding layer
15 n-type GaN guide layer
16 n-type InGaN active layer
17 p-type AlGaN barrier layer
18 p-type GaN guide layer
19 p-type AlGaN cladding layer
20 p-type GaN contact layer
21 Insulating film
22 P electrode
23 n-type GaN regrowth layer

Claims (9)

  1. A semiconductor light emitting device that performs self-excited oscillation of laser light,
    A semiconductor substrate ;
    A stacked structure in which a plurality of nitride semiconductor layers are stacked on the semiconductor substrate ;
    The laminate structure comprises so that stripe-shaped waveguides constituting a resonator performing laser oscillation is formed, and a current constriction structure for constricting a current supplied to the active layer of the plurality of nitride semiconductor layers ,
    The laminated structure is
    Compared with the defect density in the current injection region constituting the stripe-shaped waveguide into which the current in the active layer is injected, the potential other than the current injection region of the active layer located on both sides of the stripe-shaped waveguide is determined. A semiconductor light emitting device , wherein the plurality of nitride semiconductor layers are grown so that the defect density in the saturated absorption region is increased .
  2. The semiconductor substrate is formed on the surface thereof with a concave groove parallel to the stripe-shaped waveguide so that the stripe-shaped waveguide is positioned between a central portion of the concave groove and one end side portion of the concave groove. Is,
    In the stacked structure, a nitride semiconductor layer is formed on the semiconductor substrate by lateral growth from both side surfaces of the concave groove, and defects are concentrated in the saturable absorption region. Item 14. The semiconductor light emitting device according to Item 1.
  3. The semiconductor substrate is selectively grown on the surface thereof to suppress growth of the nitride semiconductor layer on the semiconductor substrate, or to promote growth of the nitride semiconductor layer on the semiconductor substrate. Which is an accelerated film,
    In the stacked structure, the formation of a nitride semiconductor layer on the semiconductor substrate is selectively suppressed or promoted by the growth suppressing film or the growth promoting film, and the defects are concentrated in the saturable absorption region. the semiconductor light emitting device according to claim 1, characterized in that.
  4. Defect density at high saturable absorption region of the defect density, the semiconductor light emitting device according to claim 1 is at least 10 times the defect density in the current injection region constituting the stripe-shaped waveguides.
  5. 2. The semiconductor light emitting device according to claim 1 , wherein a defect density in the saturable absorption region having a high defect density is 10 8 / cm 2 or more.
  6. A high saturable absorption region of said defect density, the semiconductor light-emitting device according to claim 1 which interval is in 0.5μm~4μm of the current injection region constituting the stripe-shaped waveguides.
  7. The semiconductor light emitting device according to claim 1 , wherein a stripe width of the stripe waveguide is 0.5 μm to 8 μm.
  8. A method of manufacturing the semiconductor light emitting device according to claim 1, comprising:
    Forming a concave groove on the first conductive type nitride semiconductor substrate which is the semiconductor substrate;
    A first conductive type nitride semiconductor substrate, forming a first conductive type nitride semiconductor layer of the first growth temperature so that the concave groove is embedded,
    On the first conductivity type nitride semiconductor layer, a first conductivity type nitride semiconductor crack prevention layer is formed at a second growth temperature different from the first growth temperature, and the first growth temperature is the first at the first growth temperature. A step of sequentially forming a nitride semiconductor cladding layer of the first conductivity type and a nitride semiconductor guide layer of the first conductivity type;
    Forming a first conductivity type nitride semiconductor active layer as the active layer on the first conductivity type nitride semiconductor guide layer at a third growth temperature different from the second growth temperature;
    On the first conductive type nitride semiconductor active layer, the second conductive type nitride semiconductor barrier layer, the second conductive type nitride semiconductor guide layer, and the second conductive type are formed by the first growth temperature. Forming a nitride semiconductor cladding layer of a type and a nitride semiconductor contact layer of a second conductivity type in order;
    Etching the second conductivity type nitride contact layer and the second conductivity type nitride semiconductor cladding layer by dry etching so as not to reach the second conductivity type nitride semiconductor guide layer, Forming a ridge structure defining a striped waveguide; and
    A method of manufacturing a semiconductor light emitting device, comprising:
  9. Optical information recording and reproducing apparatus characterized by the semiconductor light-emitting device according to any one of claims 1 to 7 is used as a light source.
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TWI347054B (en) 2003-07-11 2011-08-11 Nichia Corp Nitride semiconductor laser device and method of manufacturing the nitride semiconductor laser device
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JP4193867B2 (en) 2006-05-02 2008-12-10 ソニー株式会社 GaN semiconductor laser manufacturing method
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