WO2009139239A1 - Nitride semiconductor laser and method for manufacturing the same - Google Patents

Nitride semiconductor laser and method for manufacturing the same Download PDF

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WO2009139239A1
WO2009139239A1 PCT/JP2009/057168 JP2009057168W WO2009139239A1 WO 2009139239 A1 WO2009139239 A1 WO 2009139239A1 JP 2009057168 W JP2009057168 W JP 2009057168W WO 2009139239 A1 WO2009139239 A1 WO 2009139239A1
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layer
quantum well
semiconductor laser
nitride semiconductor
layers
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French (fr)
Japanese (ja)
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みつき 松舘
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日本電気株式会社
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
    • H01S5/343—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser
    • H01S5/34333—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser with a well layer based on Ga(In)N or Ga(In)P, e.g. blue laser
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S2304/00—Special growth methods for semiconductor lasers
    • H01S2304/04—MOCVD or MOVPE
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
    • H01S5/22—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
    • H01S5/3403—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers having a strained layer structure in which the strain performs a special function, e.g. general strain effects, strain versus polarisation

Definitions

  • the present invention relates to a nitride semiconductor laser and a method for manufacturing the same.
  • III-V nitride semiconductor light emitting devices based on gallium nitride (GaN)
  • semiconductor lasers emitting blue-violet light have been put into practical use as next-generation high-density optical disk light sources.
  • the market is expected to expand further in the future.
  • this material system can emit light in the visible wavelength region from blue to red by controlling the indium composition of the indium gallium nitrogen (InGaN) active layer. Therefore, high-intensity light emitting diodes have been developed.
  • InGaN indium gallium nitrogen
  • the indium composition of the InGaN quantum well active layer should be changed. That's fine. For example, in order to emit light in a pure blue band having a wavelength of about 450 nm, it is necessary to increase the indium composition of the well layer to about 0.2 or more. In order to emit light in the green band, a higher indium composition is required.
  • the laser element using the InGaN quantum well as the active layer the following problems occur due to the high concentration of the indium composition.
  • the lattice constant difference between InN and GaN is as large as about 11%, the compressive strain increases as the indium composition increases, and the strain critical film thickness decreases. Therefore, it becomes difficult to produce an active layer having a favorable crystal. Above a certain wavelength, it is even difficult to obtain a layer thickness that functions as a light emitting layer.
  • the optical transition probability when the optical transition probability is lowered in a light emitting element, the light emission recombination lifetime of the carrier is increased and the light emission element is easily affected by non-light emission recombination, so that the light emission efficiency is lowered.
  • the probability of non-radiative recombination is made as small as possible due to good crystal quality, such a decrease in light emission efficiency can be suppressed to some extent.
  • a decrease in active layer gain due to a decrease in optical transition probability is unavoidable, resulting in an increase in oscillation threshold.
  • the gain peak wavelength with respect to the carrier density is shortened compared to other material systems.
  • the transition energy between quantum levels is reduced due to the quantum Stark confinement effect at a low carrier density. This is because the electric field is shielded as the injected carrier density increases, and the transition energy becomes shorter.
  • the state density of the quantum well has a shape with a tail toward the low energy side, and the gain peak wavelength and the absorption edge wavelength are continuously shifted by a short wave together with the injected carrier density.
  • the indium composition of the InGaN well layer is increased or an AlGaN layer containing Al is used for the barrier, the valence band discontinuous energy is further increased, and the influence of the carrier nonuniformity is further increased. Therefore, in a region where the indium composition as required at a wavelength of the blue band or more is about 0.2 or more, it is very difficult to multiplex quantum wells using InGaN well layers and AlGaN barrier layers.
  • Patent Document 2 p-type doping is applied to the barrier layer in order to reduce carrier nonuniformity.
  • Mg which is a general p-type dopant of nitride-based crystals, with the current crystal growth technology.
  • Mg can easily diffuse during laser operation, it is not practical from the viewpoint of reliability to apply Mg doping in the very vicinity of the quantum well layer as the light emitting layer.
  • Patent Document 3 proposes a technique of limiting the number of quantum well layers or limiting the barrier layer thickness in order to reduce the influence of carrier nonuniformity.
  • a quantum well with a high indium composition must have a multilayered well layer to compensate for a decrease in gain, and there is a limit to improving carrier nonuniformity even if the barrier layer thickness is reduced. Thus, a sufficient effect cannot be obtained in a quantum well active layer having a high indium composition.
  • patent documents 4 and 5 can be mentioned as another related technique.
  • the present invention has been made in view of the above, and even when an active layer having a high indium composition is used to increase the wavelength, a quantum well is multiplexed to generate a high gain, and laser oscillation characteristics at a long wavelength
  • An object of the present invention is to provide a nitride semiconductor laser having a good resistance and a method for manufacturing the same.
  • the nitride semiconductor laser according to the present invention is First and second cladding layers; A quantum well active layer provided between the first and second cladding layers, The quantum well active layer has a plurality of quantum wells composed of a plurality of quantum well layers and a barrier layer sandwiching the quantum well layers, Each quantum well layer contains In, Ga, and N, and the gain peak wavelength of the emission wavelength of each quantum well layer depends on the carrier density in each quantum well layer during operation of the nitride semiconductor laser. It is characterized by being controlled.
  • a method for manufacturing a nitride semiconductor laser according to the present invention includes: Forming first and second cladding layers; Forming a quantum well active layer provided between the first and second cladding layers;
  • the quantum well active layer has a plurality of quantum wells composed of a plurality of quantum well layers and a barrier layer sandwiching the quantum well layers,
  • Each quantum well layer contains In, Ga, and N, and the gain peak wavelength of the emission wavelength of each quantum well layer depends on the carrier density in each quantum well layer during operation of the nitride semiconductor laser. It is characterized by being controlled.
  • a nitride semiconductor that has a good laser oscillation characteristic at a long wavelength by generating a high gain by multiplexing quantum wells A laser and a manufacturing method thereof can be provided.
  • FIG. 1 is a cross-sectional view of a semiconductor laser device according to a first embodiment. It is sectional drawing of the active layer of the semiconductor laser apparatus of FIG. It is a schematic diagram which shows the manufacturing process of the semiconductor laser apparatus which concerns on 1st Embodiment. It is a schematic diagram which shows the manufacturing process of the semiconductor laser apparatus which concerns on 1st Embodiment. It is a schematic diagram which shows the manufacturing process of the semiconductor laser apparatus which concerns on 1st Embodiment. It is sectional drawing of the active layer of the nitride-type semiconductor laser apparatus which concerns on a comparative example. It is a figure which shows the relationship between the injection
  • FIG. 1 is a sectional view showing a nitride semiconductor laser device according to the first embodiment of the present invention.
  • the nitride semiconductor laser device according to the present embodiment is a ridge stripe type.
  • This semiconductor laser device includes an n-type GaN substrate 101, an n-type AlGaN cladding layer 102 (first cladding layer) provided on the substrate 101, and an InGaN lower optical waveguide layer sequentially provided on the n-type AlGaN cladding layer 102.
  • an InGaN / AlGaN quantum well active layer 104 an InGaN upper optical waveguide layer 105
  • a p-type AlGaN cladding layer 106 (second cladding layer) provided on the InGaN upper optical waveguide layer 105 and constituting a current confinement portion.
  • the p-type AlGaN cladding layer 106 and the p-type GaN contact layer 107 provided thereon are processed into a ridge stripe shape, and this ridge stripe functions as a current confinement portion.
  • the ridge stripe also functions as a horizontal refractive index waveguide mechanism.
  • the width of the ridge stripe is 2.0 ⁇ m, for example.
  • a p-side electrode 108 is provided on the contact layer 107, and an n-side electrode 109 is provided below the GaN substrate 101. Further, the surface of the semiconductor laser device of FIG. 1 is covered with an insulating film 110 except for the portion where the ridge stripe or electrode 108 is provided.
  • the quantum well active layer 104 includes a three-period quantum well, and includes three InGaN quantum well layers 121a, 121b, and 121c and four AlGaN barrier layers 122 sandwiching the InGaN quantum well layers 121a, 121b, and 121c.
  • this semiconductor laser device is designed to oscillate at about 510 nm, it is necessary to increase the indium composition of the quantum well layers 121a, 121b, and 121c. Therefore, AlGaN having tensile strain is used as the barrier layer 122 in order to stabilize the compressive strain of the quantum well layer.
  • Each layer of the barrier layer 122 is AlGaN with an aluminum composition of 5%, and the layer thickness is 10 nm.
  • the layers 121a, 121b, 121c of the InGaN quantum well are all the same in thickness as 2.6 nm, and the composition thereof is determined so that each layer has substantially the same gain peak wavelength according to the injected carriers during laser operation. It is characterized by having.
  • It is InGaN having a composition.
  • the transition wavelengths between the ground levels in these quantum well layers 121a, 121b, and 121c are 505 nm, 480 nm, and 456 nm, respectively. That is, the band gap is smaller as the quantum well layer 121 is closer to the p-type cladding layer 106.
  • the transition wavelength between the ground levels corresponds to the energy between the ground levels of electrons and holes when it is assumed that an internal electric field is not applied to the quantum well.
  • the influence of the internal electric field is small, it corresponds to a general low excitation photoluminescence peak wavelength.
  • the transition wavelength is obtained from the indium composition and the layer thickness.
  • an n-type cladding layer 102, a lower optical waveguide layer 103, an InGaN / GaN quantum well activity are formed on the n-type GaN substrate 101 by using a metal organic chemical vapor deposition method (MOVPE method) or the like.
  • MOVPE method metal organic chemical vapor deposition method
  • the layer 104, the upper optical waveguide layer 105, the p-type cladding layer 106, and the contact layer 107 are sequentially stacked (step 1, FIG. 3A).
  • a stripe-shaped etching mask having a width of about 2 ⁇ m is formed using a normal photolithography process, and etching is performed partway through the contact layer 107 and the p-type cladding layer 106 by dry etching using a chlorine-based gas.
  • a ridge stripe having a width of about 2 ⁇ m is formed.
  • the value of the ridge width and the etching depth of the p-type cladding layer 106 are not directly related to the present invention, but affect the current-light output characteristics and current-voltage characteristics including the horizontal transverse mode characteristics of the semiconductor laser device. Therefore, an optimum value is selected in consideration of required device characteristics and the like. (Step 2, FIG. 3B).
  • an insulating film 110 such as a silicon oxide film is formed on the entire element by using a CVD method or the like. Then, the insulating film 110 in the p-side electrode 108 formation portion is removed using a normal photolithography process. Thereafter, titanium and gold are vapor-deposited, and heated under appropriate conditions to perform an alloy process, thereby forming the p-side electrode 108. Further, titanium and gold are vapor-deposited on the back surface of the substrate 101, and heated under appropriate conditions to perform an alloy process, thereby forming the n-side electrode 109. Finally, a laser mirror end face is formed by cleavage (step 3, FIG. 3C).
  • the gain improvement effect obtained by the present invention will be described with reference to a comparative example.
  • the gain reduction in the layer structure of the nitride semiconductor laser device having the structure of the comparative example will be described.
  • the structure of the quantum well active layer is different between the comparative example and the present invention.
  • FIG. 4 shows details of the quantum well active layer structure of the comparative example.
  • the quantum well active layer includes three periods of quantum wells, and is composed of three InGaN quantum well layers 21 and four barrier layers 22 sandwiching them.
  • Each quantum well layer 21 has the same layer thickness and indium composition, and is made of InGaN having a layer thickness of 2.6 nm and an indium composition of 30%.
  • Each barrier layer 22 has a thickness of 10 nm and is made of AlGaN with an aluminum composition of 5% in order to compensate for the compressive strain of the quantum well layer 21.
  • the transition wavelength between the ground levels in these quantum well layers 21 is about 480 nm.
  • the valence band discontinuity energy between the quantum well layer 21 and the barrier layer 22 is as large as about 450 meV, so that confinement of holes is large, and hole transport during forward bias is limited,
  • the well layer closer to the p-type cladding layer has a higher hole concentration.
  • the concentration distribution is the same as that of the hole due to the Coulomb force, that is, the injected carriers between the quantum wells are not uniform.
  • the carrier density ratio of each well layer is about 1: 6: 25, which is the smallest in the well layer close to the n-type cladding layer, and the p-type cladding layer It is the largest in the well layer near.
  • FIG. 5 shows the relationship between the gain peak wavelength of the quantum well used here and the injected carrier density. Since the indium composition of the quantum well layer is as high as 30%, the gain peak wavelength is largely blue-shifted with the carrier density due to the influence of the state density distribution due to the composition fluctuation and the internal electric field shielding effect accompanying the increase in the carrier density.
  • FIG. 6 shows gain spectra of the three well layers 21 when carrier nonuniformity occurs in such a situation.
  • the gain peak wavelengths of the quantum well layers having different injected carrier densities are greatly different from about 545 nm, 520 nm, and 495 nm, respectively, and a wavelength band in which a high gain is obtained in the well layer close to the p-type cladding layer. It can be seen that the well layer near the n-type cladding layer is absorbing.
  • the solid line in FIG. 7 shows the gain spectrum of the entire active layer obtained by adding the respective optical confinement coefficients to the spectrum of each well layer.
  • a gain spectrum when an average injected carrier density is assumed is indicated by a broken line. Compared to this, the peak gain is greatly reduced to about 50%. That is, it can be seen that the injected carriers, that is, the threshold current density in the entire active layer for obtaining a gain necessary for laser oscillation greatly increases.
  • the valence band discontinuous energy of the quantum well layer 121 and the barrier layer 122 is similarly large, carrier nonuniformity occurs as in the comparative example.
  • the valence band discontinuity energy between each quantum well layer 121 and the n-side barrier layer 122 changes, the carrier density distribution slightly changes. That is, when the injection current density is about 2 kA / cm 2 , the carrier density ratio of each well layer is about 1: 5: 25, which is the smallest in the well layer 121c close to the n-type cladding layer 102, p The well layer 121a closest to the mold cladding layer 106 is the largest.
  • FIG. 8 shows the gain spectrum of each quantum well layer in this case, as in FIG. Referring to FIG. 8, the peak gain value of each well layer is almost the same as that of the comparative example of FIG. 6, and is affected by nonuniformity of injected carriers. However, it can be seen that the gain peak wavelengths of the three well layers are substantially the same, and each of them does not cancel each other as in the comparative example.
  • FIG. 9 shows the relationship between the carrier density and the gain peak wavelength in each quantum well layer.
  • the composition is determined so as to have a gain peak wavelength substantially equal to the carrier density of each well layer.
  • the injected carriers indicated by arrows And has a peak wavelength approximately equal to 520 nm.
  • FIG. 10 shows the gain spectrum of the whole active layer obtained by adding the respective optical confinement coefficients to the spectrum of each well layer.
  • FIG. 10 also shows a gain spectrum (broken line) on the assumption that average carrier injection is performed in quantum wells having the same composition.
  • a peak gain comparable to this is obtained.
  • it is about 1.9 times the peak gain value of the comparative example (dotted line).
  • the current density required for the required threshold gain can be reduced as compared with the comparative example. Furthermore, by reducing the threshold carrier density, the gain peak wavelength at the threshold can be lengthened even if the average composition of the quantum wells is the same. Therefore, it can be said that the effect is great from the viewpoint of the oscillation wavelength and the distortion amount.
  • the degree of carrier non-uniformity was estimated and designed in each case, but this depends on the operating level of the laser element and the structure and characteristics of the quantum well layer. It is desirable to design accordingly.
  • the relationship between carrier density and gain peak wavelength can be estimated experimentally to some extent. However, in general, it is difficult to experimentally estimate the numerical value of the degree of carrier nonuniformity. Therefore, numerical calculation is performed using a device simulator or the like, or a method of experimentally producing a multiple quantum well structure in which the transition wavelength is changed to some extent as in the present invention and measuring the gain characteristic is used. It is most desirable to optimize the active layer structure.
  • One of the criteria for estimating the degree of carrier non-uniformity is the probability of thermionic emission from holes.
  • the transport of holes between quantum wells is mainly governed by thermionic emission, diffusion in the barrier layer, trapping process to adjacent quantum wells, or tunneling to adjacent quantum wells. Therefore, if the hole escape probability due to these becomes larger than the light emission or non-light emission recombination probability in the quantum well, the quasi-Fermi levels between the quantum wells are almost the same, and uniform carrier injection is considered. .
  • the greatest cause of carrier non-uniformity in the nitride semiconductor is considered to be a decrease in the probability of thermal electron emission. This is caused by a large valence band discontinuity energy and a large effective mass of holes. Therefore, if an optimum quantum well design of the present invention is obtained in a certain structure, the degree to which the thermionic emission probability changes for a new structure is determined, and thereby the transition wavelength variation range of the present invention is determined. It can be determined to some extent.
  • the gain peak wavelength corresponding to the degree of non-uniformity cannot be accurately determined.
  • a multi-quantum well structure in which the transition wavelength is inclined as in the present invention along the tendency of non-uniformity may be used.
  • the effect of canceling the gain between the quantum wells can be reduced and the gain reduction of the entire active layer can be suppressed.
  • the device cannot be designed to match the gain peak wavelength due to restrictions on device fabrication such as crystal growth, it is possible to obtain the effect of suppressing gain reduction by providing a composition gradient along the tendency of carrier nonuniformity. Is possible.
  • the transition wavelength when the degree of nonuniformity of the carrier is not clear, it is desirable to have a wide range within the assumed range. The reason is that even if the gain peak wavelength of the quantum well close to the n-type cladding layer 102 becomes too short compared to the quantum well close to the p-type cladding layer 106, this well is close to the quantum well close to the p-type cladding layer 106. This is because it becomes transparent with respect to the gain peak wavelength of the well, so there is no fear of absorption and lowering of the gain. However, in this case as well, if the gain peak wavelength difference becomes too large, the effect of increasing the peak gain of the entire active layer is reduced, so it is desirable to design the peak wavelength as close as possible.
  • the number of periods of the quantum well is not limited to this and may be two layers.
  • the gain per period is small, such as a long wavelength active layer with a high indium composition
  • the present invention is applied in order to obtain the maximum effect by further multilayering the quantum well layer. This is particularly effective.
  • the indium composition of the quantum well is as high as 30%, but the composition of the quantum well and the emission wavelength range are not limited to this, and the present invention can be applied to lasers in the 405 nm band or 450 nm band. Is possible. This is because the combined effect of the carrier non-uniformity and the shortening of the gain peak due to carrier injection, which is the subject of the present invention, is present in a general nitride semiconductor laser, although its influence depends on the wavelength band. is there.
  • the blue shift of the gain peak wavelength is considered to be caused by the shielding effect accompanying the internal electric field and carrier injection, and the tailing of the state density due to the composition fluctuation in the plane or in the layer.
  • the internal electric field increases with the amount of strain, and the greater the well layer thickness, the greater the potential difference across the quantum well layer. Therefore, the blue shift becomes more pronounced as the In composition increases or the well layer thickness increases. Further, the composition fluctuation tends to increase as the In composition increases or the well layer thickness increases.
  • the InGaN quantum well layer has an indium composition of 25% or more, the indium composition is 15% or more and the layer thickness is 2.5 nm or more, or the layer thickness is 5 nm or more, The application effect can be obtained particularly effectively. That is, when the quantum well layer is made of Al y1 In x1 Ga 1-x1-y1 N (0 ⁇ y1 ⁇ 1, 0 ⁇ x1 ⁇ 1) and x1 is 0.25 or more, x1 is 0.15 or more.
  • each well layer is 2.5 nm or more, and when the layer thickness is 5 nm or more, the effect of applying the present invention is particularly effectively obtained. Further, in the quantum well having a high indium composition and a large layer thickness as shown here, the influence of the carrier nonuniformity due to the valence band discontinuity is further increased, so that the application effect of the present invention is further enhanced.
  • the material of the barrier layer is not limited to AlGaN, and GaN may be used, and the best light emission characteristics can be obtained. Even InGaN having an indium composition smaller than that of the well layer can be applied.
  • the use of AlGaN having tensile strain for the barrier layer 124 is very important in stabilizing the compressive strain of the InGaN quantum well.
  • the use of AlGaN having a high band gap for the barrier layer 124 increases the band discontinuity energy in the valence band, thereby further enhancing carrier nonuniformity. Therefore, the application effect of the present invention can be obtained particularly effectively in a structure using AlGaN for the barrier layer 124 among InGaN quantum well structures.
  • the effect of the present invention in the case where non-uniform injection of holes occurs in a nitride semiconductor has been described.
  • the gain peak shift accompanying the carrier density is large, and non-uniformity between holes or electrons between quantum wells occurs remarkably.
  • a structure similar to that of the present invention can be designed even in that case.
  • the carrier non-uniformity is rate-determined by the electron transport effect, contrary to the case described here, the distribution is such that the carrier density of the quantum well layer close to the n-type cladding layer increases. Therefore, the transition wavelength of each quantum well may be designed to be the longest wavelength in the quantum well layer close to the n-type cladding layer.
  • Embodiment 2 Next, a second embodiment of the semiconductor laser device according to the present invention will be described with reference to FIG.
  • the second embodiment is different from the first embodiment in the structure of the quantum well active layer, the details of which are shown in FIG.
  • the quantum well active layer includes four periods of quantum wells, and includes four layers of InGaN quantum well layers 221a, 221b, 221c, and 221d and five barrier layers 222 sandwiching the layers. All the barrier layers are made of AlGaN having an aluminum composition of 5%, and the layer thickness is 10 nm. Each layer of the InGaN quantum well is made of InGaN having the same indium composition as 30%, and the layer thickness is determined so that each layer has substantially the same gain peak wavelength according to the injected carrier during laser operation. It is characterized by.
  • the thickness of each quantum well layer is set to 3.3 nm for 221a, 2.6 nm for 221b, 2.0 nm for 221c, and 1.5 nm for 221d.
  • the transition wavelengths between the ground levels in these quantum wells are 488 nm for 221a, 480 nm for 221b, 468 nm for 221c, and 454 nm for 221d.
  • FIG. 12 shows the relationship between the sheet carrier density and the gain peak wavelength in each quantum well layer.
  • the layer thickness is determined so as to have a gain peak wavelength of about 510 nm in accordance with the carrier density of each well layer.
  • FIG. 13 shows the gain spectrum of the entire active layer when current injection of about 2 kA / cm 2 is performed in this embodiment.
  • FIG. 13 shows the gain spectrum of the entire active layer when current injection of about 2 kA / cm 2 is performed in this embodiment.
  • the peak gain value is improved by 1.25 times compared to the comparative structure, and a peak gain close to that assumed when average carrier injection is assumed is obtained. Yes.
  • This embodiment has an advantage in crystal production as compared with the first embodiment. Since indium uptake in MOVPE growth of InGaN crystals depends greatly on the growth temperature, the optimum growth temperature differs depending on the target indium composition. Therefore, in the first embodiment, some of the quantum well layers must be grown at a temperature deviating from the optimum growth temperature when the growth temperature is changed during the growth of the active layer, or when growing at a constant temperature. In order to obtain high-quality crystals in the entire active layer, advanced techniques are required. However, in this embodiment, since the indium composition of each well layer is the same, the growth time may be changed while the growth temperature is kept optimal.
  • Embodiment 3 Furthermore, a third embodiment of the semiconductor laser device according to the present invention will be described.
  • the third embodiment is different from the first and second embodiments in the structure of the quantum well active layer, and details thereof are shown in FIG.
  • the quantum well active layer includes four-period quantum wells, and is composed of four InGaN quantum well layers 321a, 321b, 321c, and 321d and five barrier layers 322a, 322b, 322c, 322d, and 322e sandwiching them. Has been.
  • Each layer of the barrier is made of AlGaN having a thickness of 10 nm.
  • the aluminum composition is 12% in the barrier layer 322a closest to the p-type cladding layer and the barrier layer 322e closest to the n-type cladding layer, and the quantum well layer.
  • the barrier layers 322b, 322c, and 322d that are sandwiched are 6%, 8%, and 10%, respectively.
  • Each of the layers 321a, 321b, 321c, and 321d of the InGaN quantum well has the same layer thickness as 2.6 nm, and its indium composition so that each layer has substantially the same gain peak wavelength according to the injected carriers during operation. Is characterized by the fact that
  • the composition of the AlGaN barrier layer which is tensile strain, is designed to be larger than that of the first embodiment so as to withstand the compressive strain of the InGaN quantum well layer, and the strain amount of the entire active layer is reduced. It has a structure.
  • the valence band band discontinuity energy becomes very large in the quantum well closest to the p-type cladding layer. Therefore, the carrier non-uniformity is further enhanced, and there is a possibility that carrier injection necessary for the inversion distribution does not occur in the quantum well layer farthest from the p-type conductive layer and close to the n-type cladding layer.
  • the quantum well layer closest to the n-type cladding layer functions only as an absorption layer, and the multilayer effect cannot be obtained.
  • the aluminum composition of each barrier layer is changed according to the position to improve carrier nonuniformity.
  • the aluminum composition used here there is no effect to eliminate the carrier non-uniformity.
  • the escape probability of holes from the well layer greatly depends on the band discontinuity energy of the valence band, it is possible to slightly improve the carrier nonuniformity.
  • the carrier density ratio of each well layer is expected to be about 1: 3: 5: 10, which is the highest in the quantum well layer 321d close to the n-type cladding layer 102.
  • the quantum well layer 321a which is small and close to the p-type cladding layer 106 is the largest.
  • the indium composition of each quantum well layer is 32% (from the well layer 321a on the side close to the p-type cladding layer 106 so that each layer has substantially the same gain peak wavelength in accordance with the improved injected carriers during operation.
  • Quantum well layer 321a), 30% (quantum well layer 321b), 28% (quantum well layer 321c) and 26% (quantum well layer 321d) are set, and the transition wavelength between the ground levels in these quantum wells Are 489 nm, 480 nm, 470 nm, and 461 nm, respectively.
  • the transition wavelength is determined in consideration of this.
  • FIG. 15 shows a gain spectrum of the entire active layer when current injection of about 2 kA / cm 2 is performed in the present embodiment.
  • gain spectra are combined for cases where carrier inhomogeneity occurs in quantum wells with the same indium composition (comparative example) and when average carrier injection is realized in quantum wells with the same indium composition. It shows.
  • the value of the peak gain is improved by about 1.36 times compared to the comparative example, and a peak gain close to that assumed when average carrier injection is assumed is obtained. .
  • the gain peak wavelength of each well layer can be set while improving the non-uniformity of carriers to some extent, so that the transition wavelength of each well layer can be set as compared with the first and second embodiments.
  • the range can be narrowed. Therefore, even in crystal growth, all layers can be grown under conditions close to optimum conditions, and there is an advantage in manufacturing.
  • a more stable strain compensation quantum well structure can be fabricated using AlGaN having a relatively large aluminum composition for the barrier layer, it becomes easier to cope with longer emission wavelengths.
  • the case where the indium composition of each quantum well layer is changed is taken as an example.
  • the structure in which the layer thickness of each quantum well layer is changed or of course, the present invention can also be applied to a structure in which both are combined to design a wavelength.
  • Embodiment 4 Furthermore, a fourth embodiment of the semiconductor laser device according to the present invention will be described.
  • the fourth embodiment is different from the first to third embodiments in the structure of the quantum well active layer, the details of which are shown in FIG.
  • the quantum well active layer includes a three-period quantum well, and is composed of three InGaN quantum well layers 421a, 421b, and 421c and four barrier layers 422a, 422b, 422c, and 422d sandwiching them.
  • Each layer of the barrier is made of AlGaN having an aluminum composition of 12%.
  • the layer thickness is 15 nm in the barrier layer 422a closest to the p-type cladding layer and the barrier layer 422d closest to the n-type cladding layer, and is sandwiched between the quantum wells.
  • the thickness is 4 nm (barrier layer 422b) and 8 nm (barrier layer 422c) in order from the side closer to the p-type cladding layer.
  • the carrier nonuniformity is improved to some extent, and each layer has substantially the same gain peak wavelength according to the improved nonuniform carrier density distribution. It is characterized by the design of quantum wells. That is, each layer of the InGaN quantum well has the same indium composition as 30%, and the layer thickness is determined so that each layer has substantially the same gain peak wavelength according to the injected carriers in operation.
  • the aluminum composition of the AlGaN barrier layer is constant, and the barrier layer sandwiching the quantum well layer is designed to be thinner as the barrier layer is closer to the p-type cladding layer.
  • the barrier layer thickness is less than this value, the hole escape probability due to the tunnel effect slightly increases, and the quantum well in which the hole escaped from the barrier layer by thermionic emission is adjacent to the n-type cladding layer side.
  • the hole transport efficiency is improved. Therefore, according to the present invention, it becomes possible to suppress the effect of enhancing the carrier nonuniformity due to the deepest quantum level of the hole in the quantum well layer closest to the p-type cladding layer.
  • the thicknesses of the barrier layers 422a and 422d are designed to be slightly thick in order to improve the strain stabilization effect.
  • the carrier density ratio of each well layer is expected to be about 1: 2.5: 4.5, and the well layer 421c close to the n-type cladding layer 102 is expected.
  • the well layer 421a close to the p-type cladding layer 106 is the largest.
  • each quantum well layer is 1.5 nm for the quantum well layer 421a, 2.0 nm for the quantum well layer 421b so that each layer has substantially the same gain peak wavelength according to the injected carriers during the improved operation,
  • the quantum well layer 421c is set to 2.6 nm.
  • FIG. 17 shows the gain spectrum of the entire active layer when current injection of about 2 kA / cm 2 is performed in this embodiment.
  • carrier non-uniformity occurs in the quantum well of the same thickness (comparative structure: dotted line)
  • the gain spectrum of (broken line) is also shown.
  • the peak gain value is improved by 1.25 times compared to the comparative structure, and a slightly higher peak gain is obtained than when average carrier injection is assumed. It has been.
  • the case where the layer thickness of each quantum well layer is changed is taken as an example.
  • the structure in which the indium composition of each quantum well layer is changed or
  • the present invention can also be applied to a structure in which both are combined to design a wavelength.
  • the semiconductor laser device on the n-type GaN substrate is taken as an example, but a semiconductor laser device on a substrate other than the GaN substrate such as a sapphire substrate or a silicon substrate may be used.
  • the ridge stripe type semiconductor laser structure has been described.
  • an inner stripe type semiconductor laser device, a surface emitting laser element, or the like, an element using an InGaN quantum well as an active layer and its waveguide gain If it is applied to any structure, the effect can be obtained.
  • the present invention can be applied to, for example, a nitride semiconductor laser including first and second cladding layers and a quantum well active layer provided between the first and second cladding layers and a manufacturing method thereof.

Abstract

Provided is a nitride semiconductor laser wherein a high gain is generated by multiplexing a quantum well even when an active layer having a high indium composition is used for lengthening the wavelength, and laser oscillation characteristics are excellent with a long wavelength. A method for manufacturing such nitride semiconductor laser is also provided. The nitride semiconductor laser is provided with a first clad layer (102) and a second clad layer (106); and a quantum well active layer (104) arranged between the first clad layer (102) and the second clad layer (106). The quantum well active layer (104) has a plurality of quantum wells composed of a plurality of quantum well layers (121) and barrier layers (122) which sandwich the quantum well layers. Each quantum well layer (121) contains In, Ga and N, and the gain peak wavelength of the emission wavelength of each quantum well layer (121) is controlled corresponding to a carrier density in each quantum well layer (121) when the nitride semiconductor laser is operated.

Description

窒化物半導体レーザ及びその製造方法Nitride semiconductor laser and manufacturing method thereof
　本発明は、窒化物半導体レーザ及びその製造方法に関する。 The present invention relates to a nitride semiconductor laser and a method for manufacturing the same.
　窒化ガリウム（ＧａＮ）をベースとするＩＩＩ－Ｖ窒化物系半導体発光素子のうち、青紫色で発光する半導体レーザは、次世代の高密度光ディスク光源として実用化されている。今後、更に市場も拡大していくものと期待されている。一方、この材料系では、インジウムガリウム窒素（ＩｎＧａＮ）活性層のインジウム組成を制御することにより、青色～赤色にかけての可視波長領域での発光も可能である。そのため、高輝度発光ダイオード等も開発されている。近年では、高性能ディスプレイ等への応用に向けて可視光で発振する半導体レーザへの要求も高まっている。 Of the III-V nitride semiconductor light emitting devices based on gallium nitride (GaN), semiconductor lasers emitting blue-violet light have been put into practical use as next-generation high-density optical disk light sources. The market is expected to expand further in the future. On the other hand, this material system can emit light in the visible wavelength region from blue to red by controlling the indium composition of the indium gallium nitrogen (InGaN) active layer. Therefore, high-intensity light emitting diodes have been developed. In recent years, there is an increasing demand for semiconductor lasers that oscillate with visible light for application to high-performance displays and the like.
　ＧａＮ基板上に作製された一般的なリッジストライプ型の窒化物系半導体レーザ素子において、ディスプレイ用途の可視領域での発光を得るには、原理的にはＩｎＧａＮ量子井戸活性層のインジウム組成を変更すればよい。例えば、波長約４５０ｎｍの純青色帯で発光させるためには、井戸層のインジウム組成を約０．２程度以上に上げる必要がある。緑色帯で発光させるためには、更に高いインジウム組成が必要となる。ここで、ＩｎＧａＮ量子井戸を活性層とするレーザ素子では、インジウム組成の高濃度化により、以下の問題が生じる。 In general, in order to obtain light emission in the visible region for display applications in a general ridge stripe nitride semiconductor laser device fabricated on a GaN substrate, the indium composition of the InGaN quantum well active layer should be changed. That's fine. For example, in order to emit light in a pure blue band having a wavelength of about 450 nm, it is necessary to increase the indium composition of the well layer to about 0.2 or more. In order to emit light in the green band, a higher indium composition is required. Here, in the laser element using the InGaN quantum well as the active layer, the following problems occur due to the high concentration of the indium composition.
　第１に、ＩｎＮとＧａＮでは格子定数差が約１１％と大きいことから、インジウム組成の増加と共に圧縮歪が増大し、歪臨界膜厚が小さくなる。そのため、良好な結晶の活性層を作製することが困難となる。ある波長以上では、発光層として機能する層厚を得ることさえ困難になる。 First, since the lattice constant difference between InN and GaN is as large as about 11%, the compressive strain increases as the indium composition increases, and the strain critical film thickness decreases. Therefore, it becomes difficult to produce an active layer having a favorable crystal. Above a certain wavelength, it is even difficult to obtain a layer thickness that functions as a light emitting layer.
　第２に、通常用いられるｃ面基板上のＩｎＧａＮ材料では、圧縮歪に起因するピエゾ電界及び分極により量子井戸層に内部電界が生じ、量子井戸内の電子とホールの波動関数の空間的分離が生じる。インジウム組成の増加によって圧縮歪が増大すると、このピエゾ電界が非常に大きくなる。そのため、波動関数の空間的分離が顕著になり、光学遷移確率の低下が大きな問題となる。 Secondly, in the commonly used InGaN material on the c-plane substrate, an internal electric field is generated in the quantum well layer due to the piezoelectric field and polarization caused by compressive strain, and the spatial separation of the wave function of electrons and holes in the quantum well is caused. Arise. When the compressive strain increases due to an increase in indium composition, this piezo electric field becomes very large. For this reason, the spatial separation of the wave function becomes remarkable, and the reduction of the optical transition probability becomes a serious problem.
　一般に、発光素子において光学遷移確率が低下すると、キャリアの発光再結合寿命が大きくなって非発光再結合の影響を受けやすくなるため、発光効率が低下する。発光ダイオードでは、良好な結晶品質により非発光再結合確率を極力小さくすれば、このような発光効率の低下はある程度抑えられる。一方、半導体レーザでは、発光効率低下を抑制しても、光学遷移確率減少による活性層利得の低下は避けられず、発振閾値の増大を招く。 Generally, when the optical transition probability is lowered in a light emitting element, the light emission recombination lifetime of the carrier is increased and the light emission element is easily affected by non-light emission recombination, so that the light emission efficiency is lowered. In the light emitting diode, if the probability of non-radiative recombination is made as small as possible due to good crystal quality, such a decrease in light emission efficiency can be suppressed to some extent. On the other hand, in a semiconductor laser, even if the reduction in light emission efficiency is suppressed, a decrease in active layer gain due to a decrease in optical transition probability is unavoidable, resulting in an increase in oscillation threshold.
　このような利得低下を補うためには、量子井戸層を多重化して活性層全体のモード利得を増加させることが有効であるが、前述の強圧縮歪の問題により多重量子井戸構造は作製が困難である。このような強歪による臨界膜厚低下の問題を解決するには、逆の引張歪を有する材料を用いて歪補償構造とすることが有効である。ＩｎＧａＮ材料系でも、ＧａＮよりも小さい格子定数を有するＡｌＧａＮをバリア層として用いることにより、長波長域で発光するＩｎＧａＮの結晶品質を改善できる。 In order to compensate for such a decrease in gain, it is effective to increase the mode gain of the entire active layer by multiplexing the quantum well layers, but it is difficult to produce a multiple quantum well structure due to the above-mentioned problem of strong compressive strain. It is. In order to solve the problem of the critical film thickness reduction due to such strong strain, it is effective to use a material having the reverse tensile strain as a strain compensation structure. Even in an InGaN material system, the crystal quality of InGaN that emits light in a long wavelength region can be improved by using AlGaN having a lattice constant smaller than that of GaN as a barrier layer.
　しかしながら、ＩｎＧａＮ系量子井戸において井戸数を多重化すると、別の問題が生じる。特許文献１～３などに記載されているように、この材料系では、ＧａＡｓ系やＩｎＰ系と比較して、ホールの有効質量が大きく、井戸層とバリア層との間の価電子帯バンド不連続エネルギーΔＥｖが大きい。そのため、多重量子井戸活性層を用いた場合、各量子井戸層間の注入キャリアが不均一になる。一般に、キャリアの不均一が生じると、各井戸層での利得が異なるため、電流－利得特性が悪化して閾値が高くなる。また、高注入井戸層でのキャリアオーバーフローによりスロープ効率の低下等が生じる。 However, when the number of wells is multiplexed in an InGaN quantum well, another problem arises. As described in Patent Documents 1 to 3, etc., this material system has a larger effective mass of holes than the GaAs system and InP system, and the valence band band gap between the well layer and the barrier layer is large. The continuous energy ΔEv is large. Therefore, when a multiple quantum well active layer is used, the injected carriers between the quantum well layers become non-uniform. In general, when carrier non-uniformity occurs, the gain in each well layer is different, so that the current-gain characteristics deteriorate and the threshold value increases. Further, the slope efficiency is lowered due to the carrier overflow in the high injection well layer.
　一方、ＩｎＧａＮ材料系では、キャリア密度に対する利得ピーク波長の短波長化が他の材料系に比べて大きい。２つの理由が考えられる。第１に、前述の内部電界の影響により、低キャリア密度では量子シュタルク閉じ込め効果により量子準位間の遷移エネルギーが小さくなっている。それが、注入キャリア密度の増加と共に電界が遮蔽され、遷移エネルギーが短波化していくからである。 On the other hand, in the InGaN material system, the gain peak wavelength with respect to the carrier density is shortened compared to other material systems. There are two possible reasons. First, due to the influence of the internal electric field described above, the transition energy between quantum levels is reduced due to the quantum Stark confinement effect at a low carrier density. This is because the electric field is shielded as the injected carrier density increases, and the transition energy becomes shorter.
　第２に、ＩｎＮとＧａＮでは原子間距離の差が大きいため、ＩｎＧａＮ材料系では相分離が起こりやすい。そのため、インジウム組成が高くなると、井戸層面内でインジウムの組成分布が生じるからである。この場合、量子井戸の状態密度は低エネルギー側に裾を引くような形となり、注入キャリア密度と共に利得ピーク波長及び吸収端波長が連続的に大きく短波シフトする。これらの複合作用により、ＩｎＧａＮ材料系では、他の材料系では見られない程、キャリア注入に伴う顕著なブルーシフトが生じる。 Second, because the difference in interatomic distance between InN and GaN is large, phase separation is likely to occur in the InGaN material system. For this reason, when the indium composition becomes high, an indium composition distribution occurs in the well layer plane. In this case, the state density of the quantum well has a shape with a tail toward the low energy side, and the gain peak wavelength and the absorption edge wavelength are continuously shifted by a short wave together with the injected carrier density. These combined actions cause a significant blue shift associated with carrier injection in InGaN material systems that is not seen in other material systems.
　このように、注入キャリア密度に対する利得・吸収スペクトルの短波シフトが大きい場合、注入キャリアの井戸層間不均一が生じると、注入の低い井戸層での利得の低下のみならず、各井戸層間での利得ピーク波長の不一致によってピーク幅の拡大や、更には注入の高い井戸層での利得ピーク波長において吸収が生じることによる活性層全体としてのピーク利得低下が発生し、井戸層を多重化して活性層全体の利得を増加させる効果は全く得られなくなってしまう。 In this way, when the shortwave shift of the gain / absorption spectrum with respect to the injected carrier density is large, non-uniformity of the injected carrier between the well layers causes not only a decrease in gain in the well layer with low injection but also a gain between the well layers. The peak width is increased due to the mismatch of peak wavelengths, and further, the peak gain is lowered as a whole active layer due to the absorption at the gain peak wavelength in the well-implanted well layer. The effect of increasing the gain cannot be obtained at all.
　ＩｎＧａＮ井戸層のインジウム組成を増加させたり、バリアにＡｌを含むＡｌＧａＮ層などを用いる場合には、更に価電子帯バンド不連続エネルギーが増大し、キャリア不均一の影響は一層大きくなる。よって、青色帯以上の波長で必要とされるようなインジウム組成が約０．２以上の領域では、ＩｎＧａＮ井戸層とＡｌＧａＮバリア層とを用いた量子井戸の多重化に多大な困難が伴う。 When the indium composition of the InGaN well layer is increased or an AlGaN layer containing Al is used for the barrier, the valence band discontinuous energy is further increased, and the influence of the carrier nonuniformity is further increased. Therefore, in a region where the indium composition as required at a wavelength of the blue band or more is about 0.2 or more, it is very difficult to multiplex quantum wells using InGaN well layers and AlGaN barrier layers.
　特許文献２では、キャリア不均一を低減するため、バリア層にｐ型ドーピングを施している。しかし、窒化物系結晶の一般的なｐ型ドーパントであるＭｇをバリア層の一部のみにドーピングするのは、現在の結晶成長技術では極めて困難である。また、Ｍｇはレーザ動作中に容易に拡散し得るため、発光層である量子井戸層のごく近傍にＭｇドープを施すのは信頼性の観点からも現実的ではない。 In Patent Document 2, p-type doping is applied to the barrier layer in order to reduce carrier nonuniformity. However, it is very difficult to dope only a part of the barrier layer with Mg, which is a general p-type dopant of nitride-based crystals, with the current crystal growth technology. Further, since Mg can easily diffuse during laser operation, it is not practical from the viewpoint of reliability to apply Mg doping in the very vicinity of the quantum well layer as the light emitting layer.
　特許文献３には、キャリア不均一による影響を低減するために、量子井戸層数を制限する、あるいはバリア層厚を制限するといった手法が提案されている。しかし、前述したようにインジウム組成の高い量子井戸では利得低下を補うために井戸層の多層化が必須であること、またバリア層厚を薄くしてもキャリア不均一の改善には限界があることなどから、インジウム組成が高い量子井戸活性層では十分な効果が得られない。なお、他の関連技術として、特許文献４及び５を挙げることができる。 Patent Document 3 proposes a technique of limiting the number of quantum well layers or limiting the barrier layer thickness in order to reduce the influence of carrier nonuniformity. However, as described above, a quantum well with a high indium composition must have a multilayered well layer to compensate for a decrease in gain, and there is a limit to improving carrier nonuniformity even if the barrier layer thickness is reduced. Thus, a sufficient effect cannot be obtained in a quantum well active layer having a high indium composition. In addition, patent documents 4 and 5 can be mentioned as another related technique.
特開平１１－３４０５８０号公報JP 11-340580 A 特開平８－１１１５５８号公報JP-A-8-111558 特開平１０－２６１８３８号公報Japanese Patent Laid-Open No. 10-261838 特開２００７－１２３８７８号公報JP 2007-123878 A 特開平１０－２８４７９５号公報Japanese Patent Laid-Open No. 10-284795
　本発明は、上記を鑑みなされたものであり、長波長化のためにインジウム組成の高い活性層を用いた場合でも、量子井戸を多重化して高い利得を発生させ、長波長でのレーザ発振特性が良好である窒化物半導体レーザ及びその製造方法を提供することを目的としている。 The present invention has been made in view of the above, and even when an active layer having a high indium composition is used to increase the wavelength, a quantum well is multiplexed to generate a high gain, and laser oscillation characteristics at a long wavelength An object of the present invention is to provide a nitride semiconductor laser having a good resistance and a method for manufacturing the same.
　本発明に係る窒化物半導体レーザは、
　第１及び第２のクラッド層と、
　前記第１及び第２のクラッド層の間に設けられた量子井戸活性層とを備え、
　前記量子井戸活性層は、複数の量子井戸層とこれを挟むバリア層とからなる複数の量子井戸を有し、
　各前記量子井戸層はＩｎ、Ｇａ及びＮを含み、かつ、当該窒化物半導体レーザ動作時の各前記量子井戸層内のキャリア密度に応じて、各当該量子井戸層の発光波長の利得ピーク波長が制御されていることを特徴とするものである。 The nitride semiconductor laser according to the present invention is
First and second cladding layers;
A quantum well active layer provided between the first and second cladding layers,
The quantum well active layer has a plurality of quantum wells composed of a plurality of quantum well layers and a barrier layer sandwiching the quantum well layers,
Each quantum well layer contains In, Ga, and N, and the gain peak wavelength of the emission wavelength of each quantum well layer depends on the carrier density in each quantum well layer during operation of the nitride semiconductor laser. It is characterized by being controlled.
　本発明に係る窒化物半導体レーザの製造方法は、
　第１及び第２クラッド層を形成し、
　前記第１及び第２クラッド層の間に設けられた量子井戸活性層を形成し、
　前記量子井戸活性層は、複数の量子井戸層とこれを挟むバリア層とからなる複数の量子井戸を有し、
　各前記量子井戸層はＩｎ、Ｇａ及びＮを含み、かつ、当該窒化物半導体レーザ動作時の各前記量子井戸層内のキャリア密度に応じて、各当該量子井戸層の発光波長の利得ピーク波長が制御されていることを特徴とするものである。 A method for manufacturing a nitride semiconductor laser according to the present invention includes:
Forming first and second cladding layers;
Forming a quantum well active layer provided between the first and second cladding layers;
The quantum well active layer has a plurality of quantum wells composed of a plurality of quantum well layers and a barrier layer sandwiching the quantum well layers,
Each quantum well layer contains In, Ga, and N, and the gain peak wavelength of the emission wavelength of each quantum well layer depends on the carrier density in each quantum well layer during operation of the nitride semiconductor laser. It is characterized by being controlled.
　本発明によれば、長波長化のためにインジウム組成の高い活性層を用いた場合でも、量子井戸を多重化して高い利得を発生させ、長波長でのレーザ発振特性が良好である窒化物半導体レーザ及びその製造方法を提供することができる。 According to the present invention, even when an active layer having a high indium composition is used to increase the wavelength, a nitride semiconductor that has a good laser oscillation characteristic at a long wavelength by generating a high gain by multiplexing quantum wells A laser and a manufacturing method thereof can be provided.
第１の実施の形態に係る半導体レーザ装置の断面図である。1 is a cross-sectional view of a semiconductor laser device according to a first embodiment. 図１の半導体レーザ装置の活性層の断面図である。It is sectional drawing of the active layer of the semiconductor laser apparatus of FIG. 第１の実施の形態に係る半導体レーザ装置の製造工程を示す模式図である。It is a schematic diagram which shows the manufacturing process of the semiconductor laser apparatus which concerns on 1st Embodiment. 第１の実施の形態に係る半導体レーザ装置の製造工程を示す模式図である。It is a schematic diagram which shows the manufacturing process of the semiconductor laser apparatus which concerns on 1st Embodiment. 第１の実施の形態に係る半導体レーザ装置の製造工程を示す模式図である。It is a schematic diagram which shows the manufacturing process of the semiconductor laser apparatus which concerns on 1st Embodiment. 比較例に係る窒化物系半導体レーザ装置の活性層の断面図である。It is sectional drawing of the active layer of the nitride-type semiconductor laser apparatus which concerns on a comparative example. 比較例に係る注入キャリア密度と利得ピーク波長との関係を示す図である。It is a figure which shows the relationship between the injection | pouring carrier density and gain peak wavelength which concern on a comparative example. 比較例に係る各井戸層の利得スペクトルを示す図である。It is a figure which shows the gain spectrum of each well layer which concerns on a comparative example. 比較例に係る活性層全体の利得スペクトルを示す図である。It is a figure which shows the gain spectrum of the whole active layer which concerns on a comparative example. 第１の実施の形態に係る各井戸層の利得スペクトルを示す図である。It is a figure which shows the gain spectrum of each well layer which concerns on 1st Embodiment. 第１の実施の形態に係る各井戸層の注入キャリア密度と利得ピーク波長との関係を示す図である。It is a figure which shows the relationship between the injection | pouring carrier density of each well layer and gain peak wavelength which concern on 1st Embodiment. 第１の実施の形態に係る活性層全体の利得スペクトルを示す図である。It is a figure which shows the gain spectrum of the whole active layer which concerns on 1st Embodiment. 第２の実施の形態に係る半導体レーザ装置の活性層の断面図である。It is sectional drawing of the active layer of the semiconductor laser apparatus concerning 2nd Embodiment. 第２の実施の形態に係る各井戸層の注入キャリア密度と利得ピーク波長との関係を示す図である。It is a figure which shows the relationship between the injection | pouring carrier density of each well layer and gain peak wavelength which concern on 2nd Embodiment. 第２の実施の形態に係る活性層全体の利得スペクトルを示す図である。It is a figure which shows the gain spectrum of the whole active layer concerning 2nd Embodiment. 第３の実施の形態に係る半導体レーザ装置の活性層の断面図である。It is sectional drawing of the active layer of the semiconductor laser apparatus concerning 3rd Embodiment. 第３の実施の形態に係る活性層全体の利得スペクトルを示す図である。It is a figure which shows the gain spectrum of the whole active layer concerning 3rd Embodiment. 第４の実施の形態に係る半導体レーザ装置の活性層の断面図である。It is sectional drawing of the active layer of the semiconductor laser apparatus concerning 4th Embodiment. 第４の実施の形態に係る活性層全体の利得スペクトルを示す図である。It is a figure which shows the gain spectrum of the whole active layer concerning 4th Embodiment.
　以下、本発明を適用した具体的な実施の形態について、図面を参照しながら詳細に説明する。ただし、本発明が以下の実施の形態に限定される訳ではない。また、説明を明確にするため、以下の記載及び図面は、適宜、簡略化されている。 Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings. However, the present invention is not limited to the following embodiment. In addition, for clarity of explanation, the following description and drawings are simplified as appropriate.
実施の形態１
　図１は、本発明の第１の実施の形態に係る窒化物半導体レーザ装置を示す断面図である。本実施の形態に係る窒化物半導体レーザ装置は、リッジストライプ型である。この半導体レーザ装置は、ｎ型ＧａＮ基板１０１、基板１０１上に設けられたｎ型ＡｌＧａＮクラッド層１０２（第１クラッド層）、ｎ型ＡｌＧａＮクラッド層１０２上に順に設けられたＩｎＧａＮ下側光導波路層１０３、ＩｎＧａＮ／ＡｌＧａＮ量子井戸活性層１０４、ＩｎＧａＮ上側光導波路層１０５、ＩｎＧａＮ上側光導波路層１０５上に設けられ、電流狭窄部を構成するｐ型ＡｌＧａＮクラッド層１０６（第２クラッド層）を備えている。 Embodiment 1
FIG. 1 is a sectional view showing a nitride semiconductor laser device according to the first embodiment of the present invention. The nitride semiconductor laser device according to the present embodiment is a ridge stripe type. This semiconductor laser device includes an n-type GaN substrate 101, an n-type AlGaN cladding layer 102 (first cladding layer) provided on the substrate 101, and an InGaN lower optical waveguide layer sequentially provided on the n-type AlGaN cladding layer 102. 103, an InGaN / AlGaN quantum well active layer 104, an InGaN upper optical waveguide layer 105, and a p-type AlGaN cladding layer 106 (second cladding layer) provided on the InGaN upper optical waveguide layer 105 and constituting a current confinement portion. Yes.
　ｐ型ＡｌＧａＮクラッド層１０６とその上に設けられたｐ型ＧａＮコンタクト層１０７とはリッジストライプ形状に加工されており、このリッジストライプが電流狭窄部として機能する。また、このリッジストライプは、水平方向の屈折率導波機構としても機能する。リッジストライプの幅は、例えば２．０μｍである。また、コンタクト層１０７上にｐ側電極１０８が、ＧａＮ基板１０１の下部にｎ側電極１０９が設けられている。また、図１の半導体レーザ装置の表面は、リッジストライプ又は電極１０８が設けられた部分を除いて絶縁膜１１０で覆われている。 The p-type AlGaN cladding layer 106 and the p-type GaN contact layer 107 provided thereon are processed into a ridge stripe shape, and this ridge stripe functions as a current confinement portion. The ridge stripe also functions as a horizontal refractive index waveguide mechanism. The width of the ridge stripe is 2.0 μm, for example. A p-side electrode 108 is provided on the contact layer 107, and an n-side electrode 109 is provided below the GaN substrate 101. Further, the surface of the semiconductor laser device of FIG. 1 is covered with an insulating film 110 except for the portion where the ridge stripe or electrode 108 is provided.
　ＩｎＧａＮ／ＡｌＧａＮ量子井戸活性層１０４の詳細を図２に示す。ここで、量子井戸活性層１０４は３周期の量子井戸を含み、３層のＩｎＧａＮ量子井戸層１２１ａ，１２１ｂ，１２１ｃとこれを挟む４層のＡｌＧａＮバリア層１２２から構成されている。 Details of the InGaN / AlGaN quantum well active layer 104 are shown in FIG. Here, the quantum well active layer 104 includes a three-period quantum well, and includes three InGaN quantum well layers 121a, 121b, and 121c and four AlGaN barrier layers 122 sandwiching the InGaN quantum well layers 121a, 121b, and 121c.
　この半導体レーザ装置は、約５１０ｎｍで発振するように設計されているため、量子井戸層１２１ａ，１２１ｂ，１２１ｃのインジウム組成を高くする必要がある。そこで、量子井戸層の圧縮歪を安定化させるために、引張歪を有するＡｌＧａＮをバリア層１２２として用いている。 Since this semiconductor laser device is designed to oscillate at about 510 nm, it is necessary to increase the indium composition of the quantum well layers 121a, 121b, and 121c. Therefore, AlGaN having tensile strain is used as the barrier layer 122 in order to stabilize the compressive strain of the quantum well layer.
　バリア層１２２の各層は、いずれもアルミニウム組成が５％のＡｌＧａＮであり、層厚は１０ｎｍである。ＩｎＧａＮ量子井戸の各層１２１ａ，１２１ｂ，１２１ｃは、層厚がいずれも２．６ｎｍと同一であり、レーザ動作時の注入キャリアに応じて各層がほぼ同じ利得ピーク波長を有するようにその組成が定められていることを特徴としている。 Each layer of the barrier layer 122 is AlGaN with an aluminum composition of 5%, and the layer thickness is 10 nm. The layers 121a, 121b, 121c of the InGaN quantum well are all the same in thickness as 2.6 nm, and the composition thereof is determined so that each layer has substantially the same gain peak wavelength according to the injected carriers during laser operation. It is characterized by having.
　本実施の形態では、ｐ型クラッド層１０６に近い側の量子井戸層１２１からそれぞれ、３５％（量子井戸層１２１ａ）、３０％（量子井戸層１２１ｂ）、２５％（量子井戸層１２１ｃ）のインジウム組成を有するＩｎＧａＮとなっている。これらの量子井戸層１２１ａ，１２１ｂ，１２１ｃにおける基底準位間の遷移波長は、それぞれ５０５ｎｍ、４８０ｎｍ、４５６ｎｍとなっている。即ち、ｐ型クラッド層１０６に近い量子井戸層１２１程、バンドギャップが小さくなっている。 In the present embodiment, 35% (quantum well layer 121a), 30% (quantum well layer 121b), and 25% (quantum well layer 121c) of indium from the quantum well layer 121 on the side close to the p-type cladding layer 106, respectively. It is InGaN having a composition. The transition wavelengths between the ground levels in these quantum well layers 121a, 121b, and 121c are 505 nm, 480 nm, and 456 nm, respectively. That is, the band gap is smaller as the quantum well layer 121 is closer to the p-type cladding layer 106.
　ここでの基底準位間の遷移波長とは、量子井戸に内部電界がかかっていないと仮定した場合の、電子及びホールの基底準位間のエネルギーに対応するものとする。内部電界の影響が小さい場合には、一般的な低励起フォトルミネッセンスピーク波長に対応するものである。一方、インジウム組成が高い等内部電界の影響が大きい場合には、そのインジウム組成と層厚から遷移波長が求められる。 Here, the transition wavelength between the ground levels corresponds to the energy between the ground levels of electrons and holes when it is assumed that an internal electric field is not applied to the quantum well. When the influence of the internal electric field is small, it corresponds to a general low excitation photoluminescence peak wavelength. On the other hand, when the influence of the internal electric field is large, such as a high indium composition, the transition wavelength is obtained from the indium composition and the layer thickness.
　図１に示した窒化物系半導体レーザ装置の製造方法につき、図３Ａ～図３Ｃを用いて説明する。まず、ｎ型ＧａＮ基板１０１上に、有機金属気相成長法（ＭＯＶＰＥ法）等を用いて、上記基板１０１上に、ｎ型クラッド層１０２、下側光導波路層１０３、ＩｎＧａＮ／ＧａＮ量子井戸活性層１０４、上側光導波路層１０５、ｐ型クラッド層１０６、コンタクト層１０７を順次積層する（工程１、図３Ａ）。 A method for manufacturing the nitride-based semiconductor laser device shown in FIG. 1 will be described with reference to FIGS. 3A to 3C. First, an n-type cladding layer 102, a lower optical waveguide layer 103, an InGaN / GaN quantum well activity are formed on the n-type GaN substrate 101 by using a metal organic chemical vapor deposition method (MOVPE method) or the like. The layer 104, the upper optical waveguide layer 105, the p-type cladding layer 106, and the contact layer 107 are sequentially stacked (step 1, FIG. 3A).
　次に、通常のフォトリソグラフィー工程を用いて幅２μｍ程度のストライプ状のエッチングマスクを形成し、塩素系ガスを用いたドライエッチングにより、コンタクト層１０７及びｐ型クラッド層１０６の途中までエッチングを行う。これにより、幅２μｍ程度のリッジストライプが形成される。リッジ幅の値、及びｐ型クラッド層１０６のエッチング深さは、本発明とは直接関係しないが、半導体レーザ装置の水平横モード特性を始め、電流－光出力特性、電流－電圧特性に影響するので、要求されるデバイス特性等を考慮して、最適な値を選択する。（工程２、図３Ｂ）。 Next, a stripe-shaped etching mask having a width of about 2 μm is formed using a normal photolithography process, and etching is performed partway through the contact layer 107 and the p-type cladding layer 106 by dry etching using a chlorine-based gas. As a result, a ridge stripe having a width of about 2 μm is formed. The value of the ridge width and the etching depth of the p-type cladding layer 106 are not directly related to the present invention, but affect the current-light output characteristics and current-voltage characteristics including the horizontal transverse mode characteristics of the semiconductor laser device. Therefore, an optimum value is selected in consideration of required device characteristics and the like. (Step 2, FIG. 3B).
　次に、素子全体にＣＶＤ法などを用いて、酸化シリコン膜等の絶縁膜１１０を形成する。そして、通常のフォトリソグラフィー工程を用いて、ｐ側電極１０８形成部の絶縁膜１１０を除去する。その後、チタン及び金を蒸着し、適当な条件で加熱してアロイ処理を行うことにより、ｐ側電極１０８を形成する。また、基板１０１の裏面にもチタン及び金を蒸着し、適当な条件で加熱してアロイ処理を行うことにより、ｎ側電極１０９を形成する。最後に、劈開によりレーザミラー端面を形成する（工程３、図３Ｃ）。 Next, an insulating film 110 such as a silicon oxide film is formed on the entire element by using a CVD method or the like. Then, the insulating film 110 in the p-side electrode 108 formation portion is removed using a normal photolithography process. Thereafter, titanium and gold are vapor-deposited, and heated under appropriate conditions to perform an alloy process, thereby forming the p-side electrode 108. Further, titanium and gold are vapor-deposited on the back surface of the substrate 101, and heated under appropriate conditions to perform an alloy process, thereby forming the n-side electrode 109. Finally, a laser mirror end face is formed by cleavage (step 3, FIG. 3C).
　ここで、本発明によって得られる利得改善効果について、比較例との対比により図を用いて説明する。まず、比較例の構造による窒化物半導体レーザ装置の層構造における利得低下について説明する。比較例と本発明とは、量子井戸活性層の構造が異なっている。 Here, the gain improvement effect obtained by the present invention will be described with reference to a comparative example. First, the gain reduction in the layer structure of the nitride semiconductor laser device having the structure of the comparative example will be described. The structure of the quantum well active layer is different between the comparative example and the present invention.
　図４に、比較例の量子井戸活性層構造の詳細を示す。ここでは、量子井戸活性層は、３周期の量子井戸を含み、３層のＩｎＧａＮ量子井戸層２１とこれを挟む４層のバリア層２２から構成されている。各量子井戸層２１は、いずれも同一の層厚・インジウム組成であり、層厚が２．６ｎｍ、インジウム組成が３０％のＩｎＧａＮからなる。また、各バリア層２２は、いずれも層厚は１０ｎｍであり、量子井戸層２１の圧縮歪を補償するためにアルミニウム組成が５％のＡｌＧａＮからなる。これらの量子井戸層２１における基底準位間の遷移波長は約４８０ｎｍとなっている。 FIG. 4 shows details of the quantum well active layer structure of the comparative example. Here, the quantum well active layer includes three periods of quantum wells, and is composed of three InGaN quantum well layers 21 and four barrier layers 22 sandwiching them. Each quantum well layer 21 has the same layer thickness and indium composition, and is made of InGaN having a layer thickness of 2.6 nm and an indium composition of 30%. Each barrier layer 22 has a thickness of 10 nm and is made of AlGaN with an aluminum composition of 5% in order to compensate for the compressive strain of the quantum well layer 21. The transition wavelength between the ground levels in these quantum well layers 21 is about 480 nm.
　このような多重量子井戸構造では、量子井戸層２１とバリア層２２との価電子帯バンド不連続エネルギーは凡そ４５０ｍｅＶと非常に大きいためホールの閉じ込めが大きく、順バイアス時のホール輸送が制限され、ｐ型クラッド層に近い井戸層ほど高いホール濃度となる。 In such a multi-quantum well structure, the valence band discontinuity energy between the quantum well layer 21 and the barrier layer 22 is as large as about 450 meV, so that confinement of holes is large, and hole transport during forward bias is limited, The well layer closer to the p-type cladding layer has a higher hole concentration.
　一方、電子濃度についてはホールよりも有効質量が小さいため、クーロン力によってホールと同様の濃度分布となり、即ち、量子井戸間の注入キャリアが不均一な状況となっている。例えば、注入電流密度が約２ｋＡ／ｃｍ２程度の場合、各井戸層のキャリア密度比はおよそ１：６：２５となっており、ｎ型クラッド層に近い井戸層で最も小さく、ｐ型クラッド層に近い井戸層で最も大きくなっている。 On the other hand, since the effective mass of the electron concentration is smaller than that of the hole, the concentration distribution is the same as that of the hole due to the Coulomb force, that is, the injected carriers between the quantum wells are not uniform. For example, when the injection current density is about 2 kA / cm 2 , the carrier density ratio of each well layer is about 1: 6: 25, which is the smallest in the well layer close to the n-type cladding layer, and the p-type cladding layer It is the largest in the well layer near.
　この場合の、不均一注入による利得への影響について説明する。図５は、ここで用いられている量子井戸の利得ピーク波長と注入キャリア密度との関係を示したものである。量子井戸層のインジウム組成が３０％と高いため、組成揺らぎに起因する状態密度分布の影響と、キャリア密度増加に伴う内部電界遮蔽効果により、キャリア密度と共に利得ピーク波長は大きくブルーシフトしている。 In this case, the effect of non-uniform injection on the gain will be described. FIG. 5 shows the relationship between the gain peak wavelength of the quantum well used here and the injected carrier density. Since the indium composition of the quantum well layer is as high as 30%, the gain peak wavelength is largely blue-shifted with the carrier density due to the influence of the state density distribution due to the composition fluctuation and the internal electric field shielding effect accompanying the increase in the carrier density.
　このような状況でキャリア不均一が生じている場合の３つの各井戸層２１の利得スペクトルを図６に示す。このように、注入キャリア密度の異なる各量子井戸層の利得ピーク波長は、各々約５４５ｎｍ、５２０ｎｍ、４９５ｎｍと大きく異なっており、ｐ型クラッド層に近い井戸層で高い利得が得られている波長帯において、ｎ型クラッド層に近い井戸層では吸収となっていることが分かる。 FIG. 6 shows gain spectra of the three well layers 21 when carrier nonuniformity occurs in such a situation. As described above, the gain peak wavelengths of the quantum well layers having different injected carrier densities are greatly different from about 545 nm, 520 nm, and 495 nm, respectively, and a wavelength band in which a high gain is obtained in the well layer close to the p-type cladding layer. It can be seen that the well layer near the n-type cladding layer is absorbing.
　これらの各井戸層のスペクトルに各々の光閉じ込め係数をかけて足し合わせた活性層全体の利得スペクトルを図７の実線に示す。図７には、比較のため平均的な注入キャリア密度を仮定した場合の利得スペクトルを破線で示している。これに比べてピーク利得が約５０％に大きく下がっている。即ち、レーザ発振に必要な利得を得るための活性層全体への注入キャリア即ち閾値電流密度が大きく増加してしまうことが分かる。 The solid line in FIG. 7 shows the gain spectrum of the entire active layer obtained by adding the respective optical confinement coefficients to the spectrum of each well layer. In FIG. 7, for comparison, a gain spectrum when an average injected carrier density is assumed is indicated by a broken line. Compared to this, the peak gain is greatly reduced to about 50%. That is, it can be seen that the injected carriers, that is, the threshold current density in the entire active layer for obtaining a gain necessary for laser oscillation greatly increases.
　次に、本発明の第１の実施の形態の場合について説明する。本発明の量子井戸活性層１０４でも、量子井戸層１２１とバリア層１２２との価電子帯バンド不連続エネルギーは同様に大きいため、比較例の場合と同様にキャリアの不均一が起こっている。但し、各量子井戸層１２１とそのｎ側のバリア層１２２との間の価電子帯バンド不連続エネルギーが変化しているため、キャリア密度分布は若干変化している。即ち、注入電流密度が約２ｋＡ／ｃｍ２程度の場合、各井戸層のキャリア密度比は概ね１：５：２５程度となっており、ｎ型クラッド層１０２に近い井戸層１２１ｃで最も小さく、ｐ型クラッド層１０６に近い井戸層１２１ａで最も大きくなっている。 Next, the case of the first embodiment of the present invention will be described. Also in the quantum well active layer 104 of the present invention, since the valence band discontinuous energy of the quantum well layer 121 and the barrier layer 122 is similarly large, carrier nonuniformity occurs as in the comparative example. However, since the valence band discontinuity energy between each quantum well layer 121 and the n-side barrier layer 122 changes, the carrier density distribution slightly changes. That is, when the injection current density is about 2 kA / cm 2 , the carrier density ratio of each well layer is about 1: 5: 25, which is the smallest in the well layer 121c close to the n-type cladding layer 102, p The well layer 121a closest to the mold cladding layer 106 is the largest.
　この場合の各量子井戸層の利得スペクトルを図６と同様に図８に示す。図８を見ると、各井戸層のピーク利得値は図６の比較例の場合と殆ど変わらず、注入キャリアの不均一の影響を受けている。しかしながら、３つの井戸層の利得ピーク波長はほぼ一致しており、比較例のように各々がこれを相殺し合うことがない状況となっていることが分かる。 FIG. 8 shows the gain spectrum of each quantum well layer in this case, as in FIG. Referring to FIG. 8, the peak gain value of each well layer is almost the same as that of the comparative example of FIG. 6, and is affected by nonuniformity of injected carriers. However, it can be seen that the gain peak wavelengths of the three well layers are substantially the same, and each of them does not cancel each other as in the comparative example.
　図９は、この各量子井戸層における、キャリア密度と利得ピーク波長との関係を示している。本発明では、図９の点線で示したように、各井戸層のキャリア密度に応じて概ね等しい利得ピーク波長を有するようにその組成が定められたものであり、ここでは矢印で示した注入キャリアに応じてほぼ５２０ｎｍに等しいピーク波長を持っている。 FIG. 9 shows the relationship between the carrier density and the gain peak wavelength in each quantum well layer. In the present invention, as indicated by the dotted line in FIG. 9, the composition is determined so as to have a gain peak wavelength substantially equal to the carrier density of each well layer. Here, the injected carriers indicated by arrows And has a peak wavelength approximately equal to 520 nm.
　これらの各井戸層のスペクトルに各々の光閉じ込め係数をかけて足し合わせた活性層全体の利得スペクトルを図１０に示す。図１０には、比較のため、同一組成の量子井戸に平均的なキャリア注入がなされた場合を仮定した利得スペクトル（破線）も示している。本発明では、これと比較して遜色のないピーク利得が得られている。本発明では、比較例（点線）のピーク利得値に比べ約１．９倍である。 FIG. 10 shows the gain spectrum of the whole active layer obtained by adding the respective optical confinement coefficients to the spectrum of each well layer. For comparison, FIG. 10 also shows a gain spectrum (broken line) on the assumption that average carrier injection is performed in quantum wells having the same composition. In the present invention, a peak gain comparable to this is obtained. In the present invention, it is about 1.9 times the peak gain value of the comparative example (dotted line).
　本発明では、比較例に比べ必要な閾値利得に要する電流密度を低減できる。さらに、閾値キャリア密度の低減によって、量子井戸の平均組成が同一であっても閾値における利得ピーク波長を長波化することができる。よって、発振波長と歪量の観点から見てもその効果が大きいと言える。 In the present invention, the current density required for the required threshold gain can be reduced as compared with the comparative example. Furthermore, by reducing the threshold carrier density, the gain peak wavelength at the threshold can be lengthened even if the average composition of the quantum wells is the same. Therefore, it can be said that the effect is great from the viewpoint of the oscillation wavelength and the distortion amount.
　以上の説明では、キャリア不均一の度合いを各場合に応じて見積もって設計を行ったが、これはレーザ素子の動作レベルや量子井戸層の構造・特性にも依存するものであり、その状況に応じて設計をすることが望ましい。 In the above description, the degree of carrier non-uniformity was estimated and designed in each case, but this depends on the operating level of the laser element and the structure and characteristics of the quantum well layer. It is desirable to design accordingly.
　キャリア密度と利得ピーク波長との関係は、ある程度実験的に見積もることが可能である。但し、一般には、キャリアの不均一度合いの数値を実験的に見積もることは困難である。そこで、デバイスシミュレータ等を用いて数値計算を行うか、或いは、本発明のようにある程度遷移波長を変化させた多重量子井戸構造を実験的に作製し、この利得特性を測定するなどの手法を用いて、活性層構造の最適化を行うことが最も望ましい。 The relationship between carrier density and gain peak wavelength can be estimated experimentally to some extent. However, in general, it is difficult to experimentally estimate the numerical value of the degree of carrier nonuniformity. Therefore, numerical calculation is performed using a device simulator or the like, or a method of experimentally producing a multiple quantum well structure in which the transition wavelength is changed to some extent as in the present invention and measuring the gain characteristic is used. It is most desirable to optimize the active layer structure.
　キャリア不均一の度合いを見積もるための目安の一つとして、ホールからの熱電子放出（Thermoionic Emission）確率が挙げられる。ホールの量子井戸間輸送は、主に熱電子放出、バリア層内での拡散、隣接する量子井戸への捕獲プロセス、又は隣接する量子井戸へのトンネル効果、によって支配される。そのため、これらによるホールの脱出確率が、量子井戸内での発光又は非発光再結合確率よりも大きくなれば、各量子井戸間の擬フェルミ準位は概ね一致し、均一なキャリア注入になると考えられる。 One of the criteria for estimating the degree of carrier non-uniformity is the probability of thermionic emission from holes. The transport of holes between quantum wells is mainly governed by thermionic emission, diffusion in the barrier layer, trapping process to adjacent quantum wells, or tunneling to adjacent quantum wells. Therefore, if the hole escape probability due to these becomes larger than the light emission or non-light emission recombination probability in the quantum well, the quasi-Fermi levels between the quantum wells are almost the same, and uniform carrier injection is considered. .
　これらのうち、窒化物半導体においてキャリア不均一を起こす最大の要因は、熱電子放出確率の低下であると考えられる。これは、価電子帯バンド不連続エネルギーが大きいこと、及びホールの有効質量が大きいことにより生じる。よって、ある構造において、最適な本発明の量子井戸設計が得られていれば、新たな構造に対し、熱電子放出確率がどの程度変化するかを求め、これによって本発明の遷移波長変化範囲をある程度決定することができる。 Of these, the greatest cause of carrier non-uniformity in the nitride semiconductor is considered to be a decrease in the probability of thermal electron emission. This is caused by a large valence band discontinuity energy and a large effective mass of holes. Therefore, if an optimum quantum well design of the present invention is obtained in a certain structure, the degree to which the thermionic emission probability changes for a new structure is determined, and thereby the transition wavelength variation range of the present invention is determined. It can be determined to some extent.
　但し、キャリアの不均一の度合いが正確に見積もれない場合であっても、ある程度キャリアの不均一が起こっている可能性が高い場合には、不均一度合いに応じた利得ピーク波長が正確にわからなくとも、不均一性の傾向に沿って本発明のように遷移波長に傾斜を設けた多重量子井戸構造を用いればよい。これにより、各量子井戸間の利得の相殺効果を低減し、活性層全体の利得低下を抑制することが可能であることは自明である。また、結晶成長等の素子作製上の制約により利得ピーク波長を必ずしも一致するように設計できない場合も同様に、キャリア不均一の傾向に沿った組成傾斜を設けて利得低下を抑制する効果を得ることが可能である。 However, even if the degree of carrier non-uniformity cannot be accurately estimated, if there is a high possibility that carrier non-uniformity has occurred to some extent, the gain peak wavelength corresponding to the degree of non-uniformity cannot be accurately determined. In any case, a multi-quantum well structure in which the transition wavelength is inclined as in the present invention along the tendency of non-uniformity may be used. As a result, it is obvious that the effect of canceling the gain between the quantum wells can be reduced and the gain reduction of the entire active layer can be suppressed. Similarly, when the device cannot be designed to match the gain peak wavelength due to restrictions on device fabrication such as crystal growth, it is possible to obtain the effect of suppressing gain reduction by providing a composition gradient along the tendency of carrier nonuniformity. Is possible.
　このように、キャリアの不均一度合いが明確でない場合の遷移波長の設定では、想定の範囲内ではその幅を広く取ることが望ましい。その理由は、仮にｐ型クラッド層１０６に近い量子井戸に対し、ｎ型クラッド層１０２に近い量子井戸の利得ピーク波長が短波になり過ぎたとしても、この井戸はｐ型クラッド層１０６に近い量子井戸の利得ピーク波長に対して透明になるから、吸収となって利得を低下させる心配はないからである。但し、この場合も、利得ピーク波長差が大きくなり過ぎた場合には、活性層全体のピーク利得を上げる効果は小さくなってしまうから、なるべく近いピーク波長を持つように設計することが望ましい。 As described above, when setting the transition wavelength when the degree of nonuniformity of the carrier is not clear, it is desirable to have a wide range within the assumed range. The reason is that even if the gain peak wavelength of the quantum well close to the n-type cladding layer 102 becomes too short compared to the quantum well close to the p-type cladding layer 106, this well is close to the quantum well close to the p-type cladding layer 106. This is because it becomes transparent with respect to the gain peak wavelength of the well, so there is no fear of absorption and lowering of the gain. However, in this case as well, if the gain peak wavelength difference becomes too large, the effect of increasing the peak gain of the entire active layer is reduced, so it is desirable to design the peak wavelength as close as possible.
　また、以上の説明では、３周期の量子井戸における例を採用したが、量子井戸の周期数についてはこれに限定されることはなく、２層であってもよい。また、インジウム組成が高い長波長の活性層のように、１周期あたりの利得が小さいような場合には、更に量子井戸層を多層化してその効果を最大限に得るために本発明を適用することは特に有効である。 In the above description, an example of a three-period quantum well is adopted. However, the number of periods of the quantum well is not limited to this and may be two layers. In addition, when the gain per period is small, such as a long wavelength active layer with a high indium composition, the present invention is applied in order to obtain the maximum effect by further multilayering the quantum well layer. This is particularly effective.
　また、以上の説明では、量子井戸のインジウム組成が３０％と高いが、量子井戸の組成及び発光波長域に関してもこれに限定されることなく、４０５ｎｍ帯、或いは４５０ｎｍ帯のレーザ等にも適用が可能である。なぜなら、本発明の課題である、キャリア不均一と、キャリア注入に伴う利得ピークの短波化の複合効果は、波長帯によってその影響の大小はあるものの、一般の窒化物半導体レーザに存在するからである。 In the above description, the indium composition of the quantum well is as high as 30%, but the composition of the quantum well and the emission wavelength range are not limited to this, and the present invention can be applied to lasers in the 405 nm band or 450 nm band. Is possible. This is because the combined effect of the carrier non-uniformity and the shortening of the gain peak due to carrier injection, which is the subject of the present invention, is present in a general nitride semiconductor laser, although its influence depends on the wavelength band. is there.
　利得ピーク波長のブルーシフトは、内部電界とキャリア注入に伴う遮蔽効果、及び面内或いは層内の組成揺らぎによる状態密度の裾引きに起因するものと考えられる。内部電界は歪量と共に増大し、井戸層厚が大きいほど量子井戸層両端での電位差が大きくなるからＩｎ組成が大きいほど、或いは井戸層厚が厚いほどブルーシフトは顕著になる。また、組成揺らぎはＩｎ組成が高いほど、或いは井戸層厚が厚いほど揺らぎの大きさが大きくなる傾向がある。 The blue shift of the gain peak wavelength is considered to be caused by the shielding effect accompanying the internal electric field and carrier injection, and the tailing of the state density due to the composition fluctuation in the plane or in the layer. The internal electric field increases with the amount of strain, and the greater the well layer thickness, the greater the potential difference across the quantum well layer. Therefore, the blue shift becomes more pronounced as the In composition increases or the well layer thickness increases. Further, the composition fluctuation tends to increase as the In composition increases or the well layer thickness increases.
　よって、窒化物半導体レーザのうちでも、量子井戸層のＩｎ組成が高いほど、或いは井戸層厚が厚い構造ほど、ブルーシフトは大きくなり、本発明の適用効果が大きい。具体的には、ＩｎＧａＮ量子井戸層が、インジウム組成２５％以上である場合、インジウム組成が１５％以上で層厚が２．５ｎｍ以上の場合、層厚が５ｎｍ以上の場合には、本発明の適用効果が特に有効に得られる。すなわち、量子井戸層がＡｌｙ１Ｉｎｘ１Ｇａ１－ｘ１－ｙ１Ｎ（０≦ｙ１＜１，０＜ｘ１＜１）からなり、ｘ１が０．２５以上である場合、ｘ１が０．１５以上であり、かつ、各井戸層厚が２．５ｎｍ以上である場合、層厚が５ｎｍ以上の場合には、本発明の適用効果が特に有効に得られる。また、ここに示したような、インジウム組成が高く、層厚が厚い量子井戸では、価電子帯バンド不連続によるキャリア不均一の影響も更に高くなるため、本発明の適用効果が更に高くなる。 Therefore, among nitride semiconductor lasers, the higher the In composition of the quantum well layer or the thicker the well layer, the greater the blue shift, and the greater the effect of applying the present invention. Specifically, when the InGaN quantum well layer has an indium composition of 25% or more, the indium composition is 15% or more and the layer thickness is 2.5 nm or more, or the layer thickness is 5 nm or more, The application effect can be obtained particularly effectively. That is, when the quantum well layer is made of Al y1 In x1 Ga 1-x1-y1 N (0 ≦ y1 <1, 0 <x1 <1) and x1 is 0.25 or more, x1 is 0.15 or more. In addition, when the thickness of each well layer is 2.5 nm or more, and when the layer thickness is 5 nm or more, the effect of applying the present invention is particularly effectively obtained. Further, in the quantum well having a high indium composition and a large layer thickness as shown here, the influence of the carrier nonuniformity due to the valence band discontinuity is further increased, so that the application effect of the present invention is further enhanced.
　また、以上の説明では、ＩｎＧａＮ／ＡｌＧａＮ量子井戸構造の例を採用したが、バリア層の材料としてはＡｌＧａＮに限定されることなく、ＧａＮでもよく、また最も良好な発光特性が得られるとされる、井戸層よりも小さいインジウム組成を有するＩｎＧａＮであっても適用が可能である。 In the above description, the example of the InGaN / AlGaN quantum well structure is adopted, but the material of the barrier layer is not limited to AlGaN, and GaN may be used, and the best light emission characteristics can be obtained. Even InGaN having an indium composition smaller than that of the well layer can be applied.
　上記の実施の形態のように、バリア層１２４に引張歪を有するＡｌＧａＮを用いることは、ＩｎＧａＮ量子井戸の圧縮歪を安定化する上で非常に重要である。しかし、バリア層１２４にバンドギャップの高いＡｌＧａＮを用いることによって価電子帯のバンド不連続エネルギーは増加してしまうため、よりキャリアの不均一性が増強されてしまうこととなる。よって、ＩｎＧａＮ量子井戸構造のうちでも、バリア層１２４にＡｌＧａＮを用いた構造において、本発明の適用効果が特に有効に得られる。 As in the above embodiment, the use of AlGaN having tensile strain for the barrier layer 124 is very important in stabilizing the compressive strain of the InGaN quantum well. However, the use of AlGaN having a high band gap for the barrier layer 124 increases the band discontinuity energy in the valence band, thereby further enhancing carrier nonuniformity. Therefore, the application effect of the present invention can be obtained particularly effectively in a structure using AlGaN for the barrier layer 124 among InGaN quantum well structures.
　また、上記では、窒化物半導体においてホールの不均一注入が生じる場合の本発明の効果を説明したが、キャリア密度に伴う利得ピークシフトが大きく、ホール又は電子の量子井戸間不均一が顕著に生じるような材料系が存在すれば、その場合においても本発明と同様の構造を設計することが可能である。もし、キャリアの不均一性が電子の輸送効果により律速されるものであれば、ここで説明した場合とは逆に、ｎ型クラッド層に近い量子井戸層のキャリア密度が高くなるような分布となるから、各量子井戸の遷移波長はｎ型クラッド層に近い量子井戸層において最も長波長になるように設計すれば良い。 In the above description, the effect of the present invention in the case where non-uniform injection of holes occurs in a nitride semiconductor has been described. However, the gain peak shift accompanying the carrier density is large, and non-uniformity between holes or electrons between quantum wells occurs remarkably. If such a material system exists, a structure similar to that of the present invention can be designed even in that case. If the carrier non-uniformity is rate-determined by the electron transport effect, contrary to the case described here, the distribution is such that the carrier density of the quantum well layer close to the n-type cladding layer increases. Therefore, the transition wavelength of each quantum well may be designed to be the longest wavelength in the quantum well layer close to the n-type cladding layer.
　以上、示したように、図１の本発明による半導体レーザ装置では、ＩｎＧａＮ量子井戸活性層の発光波長の長波化、即ちインジウム組成の増加に伴って顕著となる、多重量子井戸活性層の各井戸層間のキャリア不均一と、キャリア注入に伴う利得ピーク波長シフトの複合効果による、利得低下の影響を最小限に抑制することが可能となる。 As described above, in the semiconductor laser device according to the present invention shown in FIG. 1, each well of the multi-quantum well active layer that becomes conspicuous as the emission wavelength of the InGaN quantum well active layer becomes longer, that is, the indium composition increases. It is possible to minimize the influence of gain reduction due to the combined effect of carrier non-uniformity between layers and gain peak wavelength shift accompanying carrier injection.
実施の形態２
　次に、図１１を用いて本発明による半導体レーザ装置の第２の実施の形態を説明する。本実施の形態２において、実施の形態１と異なる部分は量子井戸活性層の構造であり、この詳細を図１１に示す。 Embodiment 2
Next, a second embodiment of the semiconductor laser device according to the present invention will be described with reference to FIG. The second embodiment is different from the first embodiment in the structure of the quantum well active layer, the details of which are shown in FIG.
　ここで、量子井戸活性層は４周期の量子井戸を含み、４層のＩｎＧａＮ量子井戸層２２１ａ，２２１ｂ，２２１ｃ，２２１ｄとこれを挟む５層のバリア層２２２から構成されている。バリア層はいずれもアルミニウム組成が５％のＡｌＧａＮであり、層厚は１０ｎｍである。ＩｎＧａＮ量子井戸の各層は、インジウム組成が３０％と同一のＩｎＧａＮから成っており、レーザ動作時の注入キャリアに応じて各層がほぼ同じ利得ピーク波長を有するようにその層厚が定められていることを特徴としている。 Here, the quantum well active layer includes four periods of quantum wells, and includes four layers of InGaN quantum well layers 221a, 221b, 221c, and 221d and five barrier layers 222 sandwiching the layers. All the barrier layers are made of AlGaN having an aluminum composition of 5%, and the layer thickness is 10 nm. Each layer of the InGaN quantum well is made of InGaN having the same indium composition as 30%, and the layer thickness is determined so that each layer has substantially the same gain peak wavelength according to the injected carrier during laser operation. It is characterized by.
　本実施の形態では、各量子井戸層の層厚は、２２１ａが３．３ｎｍ、２２１ｂが２．６ｎｍ、２２１ｃが２．０ｎｍ、２２１ｄが１．５ｎｍと設定されている。また、これらの量子井戸における基底準位間の遷移波長は、２２１ａが４８８ｎｍ、２２１ｂが４８０ｎｍ、２２１ｃが４６８ｎｍ、２２１ｄが４５４ｎｍとなっている。 In the present embodiment, the thickness of each quantum well layer is set to 3.3 nm for 221a, 2.6 nm for 221b, 2.0 nm for 221c, and 1.5 nm for 221d. The transition wavelengths between the ground levels in these quantum wells are 488 nm for 221a, 480 nm for 221b, 468 nm for 221c, and 454 nm for 221d.
　図１２は、各量子井戸層における、シートキャリア密度と利得ピーク波長との関係を示している。ここでは、各井戸層のキャリア密度に応じて概ね約５１０ｎｍに利得ピーク波長を有するようにその層厚が定められたものである。 FIG. 12 shows the relationship between the sheet carrier density and the gain peak wavelength in each quantum well layer. Here, the layer thickness is determined so as to have a gain peak wavelength of about 510 nm in accordance with the carrier density of each well layer.
　本実施の形態において、約２ｋＡ／ｃｍ２の電流注入を行った場合の、活性層全体の利得スペクトルを図１３に示す。比較のため、同一層厚の量子井戸においてキャリアの不均一が生じている場合（比較構造：点線）、及び同一層厚の量子井戸において平均的なキャリア注入が実現したと仮定した場合（破線）の利得スペクトルを合わせて示している。図１３より明らかなように、本実施の形態では、比較構造と比較してピーク利得の値は１．２５倍に改善し、平均的なキャリア注入を仮定した場合に近いピーク利得が得られている。 FIG. 13 shows the gain spectrum of the entire active layer when current injection of about 2 kA / cm 2 is performed in this embodiment. For comparison, when carrier nonuniformity occurs in quantum wells of the same layer thickness (comparison structure: dotted line), and when it is assumed that average carrier injection is realized in quantum wells of the same layer thickness (dashed line) The gain spectrum is also shown. As is clear from FIG. 13, in this embodiment, the peak gain value is improved by 1.25 times compared to the comparative structure, and a peak gain close to that assumed when average carrier injection is assumed is obtained. Yes.
　本実施の形態では、実施の形態１に比べ結晶作製上の利点がある。ＩｎＧａＮ結晶のＭＯＶＰＥ成長におけるインジウム取込みはその成長温度に大きく依存するため、最適成長温度は狙うインジウム組成ごとに異なっている。そのため、実施の形態１では、活性層成長中に成長温度を変更するか、或いは一定温度で成長した場合には最適成長温度からずれた温度で一部の量子井戸層を成長させなければならず、活性層全体で良質な結晶を得るには高度な技術が必要である。しかし、本実施の形態では、各井戸層のインジウム組成は同一であるから、成長温度は最適に保ったまま、成長時間を変化させればよい。 This embodiment has an advantage in crystal production as compared with the first embodiment. Since indium uptake in MOVPE growth of InGaN crystals depends greatly on the growth temperature, the optimum growth temperature differs depending on the target indium composition. Therefore, in the first embodiment, some of the quantum well layers must be grown at a temperature deviating from the optimum growth temperature when the growth temperature is changed during the growth of the active layer, or when growing at a constant temperature. In order to obtain high-quality crystals in the entire active layer, advanced techniques are required. However, in this embodiment, since the indium composition of each well layer is the same, the growth time may be changed while the growth temperature is kept optimal.
実施の形態３
　更に、本発明による半導体レーザ装置の第３の実施の形態を説明する。本実施の形態３において、実施の形態１及び２と異なる部分は量子井戸活性層の構造であり、この詳細を図１４に示す。 Embodiment 3
Furthermore, a third embodiment of the semiconductor laser device according to the present invention will be described. The third embodiment is different from the first and second embodiments in the structure of the quantum well active layer, and details thereof are shown in FIG.
　ここで、量子井戸活性層は４周期の量子井戸を含み、４層のＩｎＧａＮ量子井戸層３２１ａ，３２１ｂ，３２１ｃ，３２１ｄとこれを挟む５層のバリア層３２２ａ，３２２ｂ，３２２ｃ，３２２ｄ，３２２ｅから構成されている。 Here, the quantum well active layer includes four-period quantum wells, and is composed of four InGaN quantum well layers 321a, 321b, 321c, and 321d and five barrier layers 322a, 322b, 322c, 322d, and 322e sandwiching them. Has been.
　バリアの各層は層厚が１０ｎｍのＡｌＧａＮより成っており、そのアルミニウム組成は、最もｐ型クラッド層に近いバリア層３２２ａと、最もｎ型クラッド層に近いバリア層３２２ｅにおいて１２％、量子井戸層に挟まれるバリア層３２２ｂ，３２２ｃ，３２２ｄでは、各々６％、８％、１０％となっている。 Each layer of the barrier is made of AlGaN having a thickness of 10 nm. The aluminum composition is 12% in the barrier layer 322a closest to the p-type cladding layer and the barrier layer 322e closest to the n-type cladding layer, and the quantum well layer. The barrier layers 322b, 322c, and 322d that are sandwiched are 6%, 8%, and 10%, respectively.
　ＩｎＧａＮ量子井戸の各層３２１ａ，３２１ｂ，３２１ｃ，３２１ｄは、いずれも２．６ｎｍと同一の層厚を有し、動作時の注入キャリアに応じて各層がほぼ同じ利得ピーク波長を有するようにそのインジウム組成が定められていることを特徴としている。 Each of the layers 321a, 321b, 321c, and 321d of the InGaN quantum well has the same layer thickness as 2.6 nm, and its indium composition so that each layer has substantially the same gain peak wavelength according to the injected carriers during operation. Is characterized by the fact that
　本実施の形態では、ＩｎＧａＮ量子井戸層の圧縮歪に耐えるように、引張歪であるＡｌＧａＮバリア層の組成を実施の形態１よりも大きく設計しており、活性層全体の歪量が低減される構造となっている。このようなバンドギャップの大きいバリア層を用いて、実施の形態１のような量子井戸構造に適用すると、ｐ型クラッド層に最も近い量子井戸では価電子帯バンド不連続エネルギーが非常に大きくなる。そのため、キャリアの不均一性が更に増強され、ｐ型導電層から最も遠い、ｎ型クラッド層に近い量子井戸層では、反転分布に必要なキャリア注入に至らない恐れがある。この場合、最もｎ型クラッド層に近い量子井戸層は吸収層としてのみ機能することとなり、多層化の効果が得られなくなる。 In the present embodiment, the composition of the AlGaN barrier layer, which is tensile strain, is designed to be larger than that of the first embodiment so as to withstand the compressive strain of the InGaN quantum well layer, and the strain amount of the entire active layer is reduced. It has a structure. When such a barrier layer having a large band gap is used and applied to the quantum well structure as in the first embodiment, the valence band band discontinuity energy becomes very large in the quantum well closest to the p-type cladding layer. Therefore, the carrier non-uniformity is further enhanced, and there is a possibility that carrier injection necessary for the inversion distribution does not occur in the quantum well layer farthest from the p-type conductive layer and close to the n-type cladding layer. In this case, the quantum well layer closest to the n-type cladding layer functions only as an absorption layer, and the multilayer effect cannot be obtained.
　そこで、本実施の形態では、各バリア層のアルミニウム組成をその位置に応じて変化させ、キャリアの不均一性を改善した。ここで用いたアルミニウム組成の範囲では、キャリアの不均一性を解消する程の効果はない。しかし、井戸層からのホールの脱出確率は価電子帯のバンド不連続エネルギーに大きく依存するため、キャリアの不均一性を若干改善することは可能である。その結果、注入電流密度が約２ｋＡ／ｃｍ２程度の場合、各井戸層のキャリア密度比は概ね１：３：５：１０となると予想され、ｎ型クラッド層１０２に近い量子井戸層３２１ｄで最も小さく、ｐ型クラッド層１０６に近い量子井戸層３２１ａで最も大きくなっている。 Therefore, in the present embodiment, the aluminum composition of each barrier layer is changed according to the position to improve carrier nonuniformity. In the range of the aluminum composition used here, there is no effect to eliminate the carrier non-uniformity. However, since the escape probability of holes from the well layer greatly depends on the band discontinuity energy of the valence band, it is possible to slightly improve the carrier nonuniformity. As a result, when the injection current density is about 2 kA / cm 2 , the carrier density ratio of each well layer is expected to be about 1: 3: 5: 10, which is the highest in the quantum well layer 321d close to the n-type cladding layer 102. The quantum well layer 321a which is small and close to the p-type cladding layer 106 is the largest.
　各量子井戸層のインジウム組成は、改善された動作時の注入キャリアに応じて各層がほぼ同じ利得ピーク波長を有するように、ｐ型クラッド層１０６に近い側の井戸層３２１ａからそれぞれ、３２％（量子井戸層３２１ａ）、３０％（量子井戸層３２１ｂ）、２８％（量子井戸層３２１ｃ）、２６％（量子井戸層３２１ｄ）と設定されており、これらの量子井戸における基底準位間の遷移波長はそれぞれ４８９ｎｍ、４８０ｎｍ、４７０ｎｍ、４６１ｎｍとなっている。この場合、量子井戸層のインジウム組成が変更されると価電子帯バンド不連続エネルギーも変化するため、注入キャリアの不均一性も変化するのでこれについても考慮して遷移波長を決定している。 The indium composition of each quantum well layer is 32% (from the well layer 321a on the side close to the p-type cladding layer 106 so that each layer has substantially the same gain peak wavelength in accordance with the improved injected carriers during operation. Quantum well layer 321a), 30% (quantum well layer 321b), 28% (quantum well layer 321c) and 26% (quantum well layer 321d) are set, and the transition wavelength between the ground levels in these quantum wells Are 489 nm, 480 nm, 470 nm, and 461 nm, respectively. In this case, since the valence band discontinuity energy changes when the indium composition of the quantum well layer is changed, the nonuniformity of the injected carriers also changes. Therefore, the transition wavelength is determined in consideration of this.
　本実施の形態において、約２ｋＡ／ｃｍ２の電流注入を行った場合の、活性層全体の利得スペクトルを図１５に示す。比較のため、同一インジウム組成の量子井戸においてキャリアの不均一が生じている場合（比較例）、及び同一インジウム組成の量子井戸において平均的なキャリア注入が実現したと仮定した場合の利得スペクトルを合わせて示している。図１５より明らかなように、本実施の形態では、比較例に比べピーク利得の値は約１．３６倍に改善し、平均的なキャリア注入を仮定した場合に近いピーク利得が得られている。 FIG. 15 shows a gain spectrum of the entire active layer when current injection of about 2 kA / cm 2 is performed in the present embodiment. For comparison, gain spectra are combined for cases where carrier inhomogeneity occurs in quantum wells with the same indium composition (comparative example) and when average carrier injection is realized in quantum wells with the same indium composition. It shows. As is clear from FIG. 15, in this embodiment, the value of the peak gain is improved by about 1.36 times compared to the comparative example, and a peak gain close to that assumed when average carrier injection is assumed is obtained. .
　本実施の形態では、キャリアの不均一性をある程度改善しつつ、各井戸層の利得ピーク波長を設定することができるので、実施の形態１及び２と比較して、各井戸層の遷移波長の範囲を狭めることが可能となる。そのため、結晶成長でも全層を最適条件に近い状況で成長させることができ、作製上の利点がある。また、バリア層に、アルミニウム組成の比較的大きいＡｌＧａＮを用いて、より安定した歪補償量子井戸構造を作製することもできるため、より長波長の発光波長へも対応しやすくなる。 In this embodiment, the gain peak wavelength of each well layer can be set while improving the non-uniformity of carriers to some extent, so that the transition wavelength of each well layer can be set as compared with the first and second embodiments. The range can be narrowed. Therefore, even in crystal growth, all layers can be grown under conditions close to optimum conditions, and there is an advantage in manufacturing. In addition, since a more stable strain compensation quantum well structure can be fabricated using AlGaN having a relatively large aluminum composition for the barrier layer, it becomes easier to cope with longer emission wavelengths.
　なお、本実施の形態では、各量子井戸層のインジウム組成を変化させた場合を例に取ったが、実施の形態２のように、各量子井戸層の層厚を変化させた構造や、或いはその双方を組み合わせて波長設計を行った構造にももちろん適用可能である。 In the present embodiment, the case where the indium composition of each quantum well layer is changed is taken as an example. However, as in the second embodiment, the structure in which the layer thickness of each quantum well layer is changed, or Of course, the present invention can also be applied to a structure in which both are combined to design a wavelength.
実施の形態４
　更に、本発明による半導体レーザ装置の第４の実施の形態を説明する。本実施の形態４において、実施の形態１～３と異なる部分は量子井戸活性層の構造であり、この詳細を図１６に示す。 Embodiment 4
Furthermore, a fourth embodiment of the semiconductor laser device according to the present invention will be described. The fourth embodiment is different from the first to third embodiments in the structure of the quantum well active layer, the details of which are shown in FIG.
　ここで、量子井戸活性層は３周期の量子井戸を含み、３層のＩｎＧａＮ量子井戸層４２１ａ，４２１ｂ，４２１ｃとこれを挟む４層のバリア層４２２ａ，４２２ｂ，４２２ｃ，４２２ｄから構成されている。 Here, the quantum well active layer includes a three-period quantum well, and is composed of three InGaN quantum well layers 421a, 421b, and 421c and four barrier layers 422a, 422b, 422c, and 422d sandwiching them.
　バリアの各層はアルミニウム組成が１２％のＡｌＧａＮより成っており、その層厚は、最もｐ型クラッド層に近いバリア層４２２ａと、最もｎ型クラッド層に近いバリア層４２２ｄにおいて１５ｎｍ、量子井戸に挟まれるバリア層では、ｐ型クラッド層に近い方から順に４ｎｍ（バリア層４２２ｂ）、８ｎｍ（バリア層４２２ｃ）、となっている。 Each layer of the barrier is made of AlGaN having an aluminum composition of 12%. The layer thickness is 15 nm in the barrier layer 422a closest to the p-type cladding layer and the barrier layer 422d closest to the n-type cladding layer, and is sandwiched between the quantum wells. In the barrier layer, the thickness is 4 nm (barrier layer 422b) and 8 nm (barrier layer 422c) in order from the side closer to the p-type cladding layer.
　本実施の形態では、実施の形態３と同様に、キャリアの不均一性をある程度改善し、かつ改善された不均一なキャリア密度分布に応じて各層がほぼ同じ利得ピーク波長を有するように、各量子井戸が設計されていることを特徴としている。即ち、ＩｎＧａＮ量子井戸の各層は、いずれも３０％と同一のインジウム組成を有し、動作時の注入キャリアに応じて各層がほぼ同じ利得ピーク波長を有するようにその層厚が定められている。 In the present embodiment, as in the third embodiment, the carrier nonuniformity is improved to some extent, and each layer has substantially the same gain peak wavelength according to the improved nonuniform carrier density distribution. It is characterized by the design of quantum wells. That is, each layer of the InGaN quantum well has the same indium composition as 30%, and the layer thickness is determined so that each layer has substantially the same gain peak wavelength according to the injected carriers in operation.
　本実施の形態では、ＡｌＧａＮバリア層のアルミニウム組成は一定とし、量子井戸層を挟むバリア層については、ｐ型クラッド層に近いバリア層ほど層厚が薄く設計されている。バリア層厚がこの程度の値以下となると、トンネル効果によるホールの脱出確率が若干ではあるが上昇すること、及び熱電子放出によってバリア層を脱出したホールがｎ型クラッド層側に隣接する量子井戸層に捕獲される確率が上昇することによって、ホールの輸送効率が改善する。よって、本発明により、ｐ型クラッド層に最も近い量子井戸層のホールの量子準位が最も深くなってしまうことによる、キャリア不均一性増強の効果を抑制することが可能になる。また、ここでは量子井戸間のバリア層４２２ｂ，４２２ｃが薄くなっているため、歪安定化効果を向上させるためにバリア層４２２ａ，４２２ｄの厚さをやや厚く設計してある。 In the present embodiment, the aluminum composition of the AlGaN barrier layer is constant, and the barrier layer sandwiching the quantum well layer is designed to be thinner as the barrier layer is closer to the p-type cladding layer. When the barrier layer thickness is less than this value, the hole escape probability due to the tunnel effect slightly increases, and the quantum well in which the hole escaped from the barrier layer by thermionic emission is adjacent to the n-type cladding layer side. By increasing the probability of being trapped in the layer, the hole transport efficiency is improved. Therefore, according to the present invention, it becomes possible to suppress the effect of enhancing the carrier nonuniformity due to the deepest quantum level of the hole in the quantum well layer closest to the p-type cladding layer. Further, since the barrier layers 422b and 422c between the quantum wells are thin here, the thicknesses of the barrier layers 422a and 422d are designed to be slightly thick in order to improve the strain stabilization effect.
　本構造において注入電流密度が約２ｋＡ／ｃｍ２程度の場合、各井戸層のキャリア密度比は概ね１：２．５：４．５程度となると予想され、ｎ型クラッド層１０２に近い井戸層４２１ｃで最も小さく、ｐ型クラッド層１０６に近い井戸層４２１ａで最も大きくなっている。 In this structure, when the injection current density is about 2 kA / cm 2 , the carrier density ratio of each well layer is expected to be about 1: 2.5: 4.5, and the well layer 421c close to the n-type cladding layer 102 is expected. The well layer 421a close to the p-type cladding layer 106 is the largest.
　各量子井戸層の層厚は、改善された動作時の注入キャリアに応じて各層がほぼ同じ利得ピーク波長を有するように、量子井戸層４２１ａが１．５ｎｍ、量子井戸層４２１ｂが２．０ｎｍ、量子井戸層４２１ｃが２．６ｎｍと設定されている。 The layer thickness of each quantum well layer is 1.5 nm for the quantum well layer 421a, 2.0 nm for the quantum well layer 421b so that each layer has substantially the same gain peak wavelength according to the injected carriers during the improved operation, The quantum well layer 421c is set to 2.6 nm.
　本実施の形態において、約２ｋＡ／ｃｍ２の電流注入を行った場合の、活性層全体の利得スペクトルを図１７に示す。比較のため、同一層厚の量子井戸の量子井戸においてキャリアの不均一が生じている場合（比較構造：点線）、及び同一層厚の量子井戸において平均的なキャリア注入が実現したと仮定した場合（破線）の利得スペクトルを合わせて示している。図１７より明らかなように、本実施の形態では、比較構造と比較してピーク利得の値は１．２５倍に改善し、平均的なキャリア注入を仮定した場合よりも若干高いピーク利得が得られている。 FIG. 17 shows the gain spectrum of the entire active layer when current injection of about 2 kA / cm 2 is performed in this embodiment. For comparison, when carrier non-uniformity occurs in the quantum well of the same thickness (comparative structure: dotted line), and when it is assumed that average carrier injection is realized in the quantum well of the same thickness The gain spectrum of (broken line) is also shown. As is clear from FIG. 17, in this embodiment, the peak gain value is improved by 1.25 times compared to the comparative structure, and a slightly higher peak gain is obtained than when average carrier injection is assumed. It has been.
　なお、本実施の形態では、各量子井戸層の層厚を変化させた場合を例に取ったが、実施の形態１のように、各量子井戸層のインジウム組成を変化させた構造や、或いはその双方を組み合わせて波長設計を行った構造にももちろん適用可能である。 In this embodiment, the case where the layer thickness of each quantum well layer is changed is taken as an example. However, as in Embodiment 1, the structure in which the indium composition of each quantum well layer is changed, or Of course, the present invention can also be applied to a structure in which both are combined to design a wavelength.
　以上、実施の形態１～４の説明を行った。しかし、本発明は、これら実施の形態に具体的に示した構成、方法に限定されるものではなく、発明の趣旨に沿うものであれば種々のバリエーションが考えられる。例えば、上記の実施の形態では、ｎ型ＧａＮ基板上の半導体レーザ装置を例に取ったが、サファイア基板、シリコン基板等ＧａＮ基板以外の基板上の半導体レーザ素子でも良い。また、上記の実施の形態では、リッジストライプ型の半導体レーザ構造について説明したが、インナーストライプ型の半導体レーザ装置や、面発光レーザ素子等、ＩｎＧａＮ量子井戸を活性層としてその導波路利得を用いる素子ならば、どのような構造に適用してもその効果は得られる。 The first to fourth embodiments have been described above. However, the present invention is not limited to the configurations and methods specifically shown in these embodiments, and various variations are conceivable as long as they are within the spirit of the invention. For example, in the above embodiment, the semiconductor laser device on the n-type GaN substrate is taken as an example, but a semiconductor laser device on a substrate other than the GaN substrate such as a sapphire substrate or a silicon substrate may be used. In the above embodiment, the ridge stripe type semiconductor laser structure has been described. However, an inner stripe type semiconductor laser device, a surface emitting laser element, or the like, an element using an InGaN quantum well as an active layer and its waveguide gain. If it is applied to any structure, the effect can be obtained.
　この出願は、２００８年５月１４日に出願された日本出願特願２００８－１２６６７３を基礎とする優先権を主張し、その開示の全てをここに取り込む。 This application claims priority based on Japanese Patent Application No. 2008-126673 filed on May 14, 2008, the entire disclosure of which is incorporated herein.
　本発明は、例えば、第１及び第２クラッド層と、第１及び第２クラッド層の間に設けられた量子井戸活性層とを備えた窒化物半導体レーザ及びその製造方法に適応できる。 The present invention can be applied to, for example, a nitride semiconductor laser including first and second cladding layers and a quantum well active layer provided between the first and second cladding layers and a manufacturing method thereof.
１０１　ｎ型基板
１０２　ｎ型クラッド層
１０３　下側光導波路層
１０４　多重量子井戸活性層
１０５　上側光導波路層
１０６　ｐ型クラッド層
１０７　コンタクト層
１０８　ｐ側電極
１０９　ｎ側電極
１２１、２２１、３２１、４２１　量子井戸層
１２２、２２２、３２２、４２２　バリア層 101 n-type substrate 102 n-type cladding layer 103 lower optical waveguide layer 104 multiple quantum well active layer 105 upper optical waveguide layer 106 p-type cladding layer 107 contact layer 108 p-side electrode 109 n-side electrodes 121, 221, 321 and 421 quantum Well layer 122, 222, 322, 422 Barrier layer

Claims (12)

  1. 　第１及び第２クラッド層と、
    　前記第１及び第２クラッド層の間に設けられた量子井戸活性層とを備え、
    　前記量子井戸活性層は、複数の量子井戸層とこれを挟むバリア層とからなる複数の量子井戸を有し、
    　各前記量子井戸層はＩｎ、Ｇａ及びＮを含み、かつ、当該窒化物半導体レーザ動作時の各前記量子井戸層内のキャリア密度に応じて、各当該量子井戸層の発光波長の利得ピーク波長が制御されていることを特徴とする窒化物半導体レーザ。     First and second cladding layers;
    A quantum well active layer provided between the first and second cladding layers,
    The quantum well active layer has a plurality of quantum wells composed of a plurality of quantum well layers and a barrier layer sandwiching the quantum well layers,
    Each quantum well layer contains In, Ga, and N, and the gain peak wavelength of the emission wavelength of each quantum well layer depends on the carrier density in each quantum well layer during operation of the nitride semiconductor laser. A nitride semiconductor laser characterized by being controlled.
  2. 　前記第１クラッド層の導電型がｎ型であり、前記第２クラッド層の導電型がｐ型であり、各前記量子井戸層の発光波長は、前記第２クラッド層に近い程長波長であることを特徴とする請求項１に記載の窒化物半導体レーザ。 The conductivity type of the first cladding layer is n-type, the conductivity type of the second cladding layer is p-type, and the emission wavelength of each quantum well layer is longer as it is closer to the second cladding layer. The nitride semiconductor laser according to claim 1.
  3. 　各前記バリア層はＡｌ、Ｇａ、及びＮを含むことを特徴とする請求項１又は２に記載の窒化物半導体レーザ。 3. The nitride semiconductor laser according to claim 1, wherein each of the barrier layers contains Al, Ga, and N. 4.
  4. 　各前記量子井戸層は、前記第２クラッド層に近い程バンドギャップが小さいことを特徴とする請求項２又は３に記載の窒化物半導体レーザ。 4. The nitride semiconductor laser according to claim 2, wherein each quantum well layer has a smaller band gap as it is closer to the second cladding layer. 5.
  5. 　各前記量子井戸層の前記バンドギャップの相違は、Ｉｎ組成の相違に基づくことを特徴とする請求項４に記載の窒化物半導体レーザ。 The nitride semiconductor laser according to claim 4, wherein the difference in the band gap of each quantum well layer is based on a difference in In composition.
  6. 　各前記量子井戸層は、前記第２クラッド層に近い程層厚が薄いことを特徴とする請求項２又は３に記載の窒化物半導体レーザ。 4. The nitride semiconductor laser according to claim 2, wherein each quantum well layer has a thinner layer thickness as it is closer to the second cladding layer. 5.
  7. 　前記量子井戸層が３層以上であり、前記量子井戸層に挟まれた各前記バリア層は、前記第２クラッド層に近い程、バンドギャップが小さいことを特徴とする請求項２～６のいずれか一項に記載の窒化物半導体レーザ。 The number of the quantum well layers is three or more, and each of the barrier layers sandwiched between the quantum well layers has a band gap that is closer to the second cladding layer. The nitride semiconductor laser according to claim 1.
  8. 　各前記バリア層の前記バンドギャップの相違は、Ａｌ組成の相違に基づくことを特徴とする請求項７に記載の窒化物半導体レーザ。 The nitride semiconductor laser according to claim 7, wherein the difference in the band gap of each barrier layer is based on a difference in Al composition.
  9. 　前記量子井戸層が３層以上であり、前記量子井戸層に挟まれた各前記バリア層は、前記第２クラッド層に近い程、層厚が薄いことを特徴とする請求項２～６のいずれか一項に記載の窒化物半導体レーザ。 7. The quantum well layer according to claim 2, wherein the number of the quantum well layers is three or more, and each of the barrier layers sandwiched between the quantum well layers is thinner as being closer to the second cladding layer. The nitride semiconductor laser according to claim 1.
  10. 　前記量子井戸層とバリア層とが、各々Ａｌｙ１Ｉｎｘ１Ｇａ１－ｘ１－ｙ１ＮとＡｌｙ２Ｉｎｘ２Ｇａ１－ｘ２－ｙ２Ｎ（０≦ｙ１，ｙ２＜１，０＜ｘ１，ｘ２＜１）とからなり、ｘ１が０．２５以上であることを特徴とする請求項１～９のいずれか一項に記載の窒化物半導体レーザ。 The quantum well layer and the barrier layer have Al y1 In x1 Ga 1-x1-y1 N and Al y2 In x2 Ga 1-x2-y2 N (0 ≦ y1, y2 <1, 0 <x1, x2 <1 The nitride semiconductor laser according to any one of claims 1 to 9, wherein x1 is 0.25 or more.
  11. 　前記量子井戸層とバリア層とが、各々Ａｌｙ１Ｉｎｘ１Ｇａ１－ｘ１－ｙ１ＮとＡｌｙ２Ｉｎｘ２Ｇａ１－ｘ２－ｙ２Ｎ（０≦ｙ１，ｙ２＜１，０＜ｘ１，ｘ２＜１）とからなり、ｘ１が０．１５以上であり、かつ、各井戸層厚が２．５ｎｍ以上であることを特徴とする請求項１～９のいずれか一項に記載の窒化物半導体レーザ。 The quantum well layer and the barrier layer have Al y1 In x1 Ga 1-x1-y1 N and Al y2 In x2 Ga 1-x2-y2 N (0 ≦ y1, y2 <1, 0 <x1, x2 <1 10. The nitride semiconductor laser according to claim 1, wherein x1 is 0.15 or more and each well layer thickness is 2.5 nm or more.
  12. 　第１及び第２クラッド層を形成し、
    　前記第１及び第２クラッド層の間に設けられた量子井戸活性層を形成し、
    　前記量子井戸活性層は、複数の量子井戸層とこれを挟むバリア層とからなる複数の量子井戸を有し、
    　各前記量子井戸層はＩｎ、Ｇａ及びＮを含み、かつ、当該窒化物半導体レーザ動作時の各前記量子井戸層内のキャリア密度に応じて、各当該量子井戸層の発光波長の利得ピーク波長が制御されていることを特徴とする窒化物半導体レーザの製造方法。     Forming first and second cladding layers;
    Forming a quantum well active layer provided between the first and second cladding layers;
    The quantum well active layer has a plurality of quantum wells composed of a plurality of quantum well layers and a barrier layer sandwiching the quantum well layers,
    Each quantum well layer contains In, Ga, and N, and the gain peak wavelength of the emission wavelength of each quantum well layer depends on the carrier density in each quantum well layer during operation of the nitride semiconductor laser. A method of manufacturing a nitride semiconductor laser, wherein the method is controlled.
PCT/JP2009/057168 2008-05-14 2009-04-08 Nitride semiconductor laser and method for manufacturing the same WO2009139239A1 (en)

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