JP4162165B2 - Quantum well structure optical semiconductor device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a long wavelength semiconductor laser device. Especially, low threshold current and low drive current characteristics under high temperature environment, used for optical fiber communication so-called FTTH (Fiber To The Home) for high-speed data delivery to homes, and so-called optical interconnection between computers. The present invention relates to a long-wavelength semiconductor laser device that is required to be
[0002]
[Prior art]
Conventionally, an InGaAsP-based long-wavelength semiconductor laser element formed on an InP substrate has been developed as a light source for optical communication systems or optical interconnections using optical fibers. However, in this semiconductor laser device, since the band discontinuity difference (energy gap difference) ΔEc between the active layer and the cladding layer is as small as about 100 meV, electrons can be sufficiently confined in the active layer during high-temperature operation. There was a problem that a so-called carrier overflow occurred. For this reason, only the semiconductor laser element having a temperature characteristic of about 50K was obtained.
[0003]
For this reason, when the semiconductor laser element is used at a severe environmental temperature, a high output of the drive circuit is required due to an increase in operating current at a high temperature. As a result, the circuit configuration becomes very complicated, or a device for cooling the laser element is required, causing problems such as an increase in the size of the optical communication device and an increase in power consumption.
[0004]
In order to improve the above temperature characteristic problem, a semiconductor laser element using InGaAlAs as an active layer on an InP substrate and a semiconductor laser element using InAsP as an active layer on an InP substrate have been developed. A practically sufficient product has not been obtained.
[0005]
On the other hand, in order to improve the temperature characteristics described above, a semiconductor laser element in which a quantum well layer is formed of GaInNAs on a GaAs substrate has been proposed (Japanese Patent Laid-Open No. 8-195522). This semiconductor laser element can oscillate light having a long wavelength band of 1.3 to 1.7 μm and can be expected to have a temperature characteristic of about 150K. In addition, the quantum barrier layer of a quantum well structure is comprised by the multilayer structure of the (Al) GaAs layer or InGaP layer, and a GaAs layer (Examples 4-6 of Unexamined-Japanese-Patent No. 8-195522).
[0006]
Hereinafter, the relationship between the structure and physical properties of the GaInNAs semiconductor laser element will be described in more detail.
[0007]
FIG. 7 shows the relationship between the composition of In and N and the lattice constant or band gap of the GaInNAs-based material. As can be seen from this figure, the GaInNAs mixed crystal obtained by adding nitrogen to the GaInAs material has a lattice constant smaller than that of GaInAs depending on the amount of addition, and gradually approaches the lattice constant of GaAs. (It changes in the direction of lattice matching with GaAs). Further, due to the bowing effect with a large energy gap with respect to nitrogen, the energy level at the band edge of the conduction band changes from A → B → C, and the energy level at the band edge of the valence band changes from D → E → F. As a result, the band gap Eg is reduced.
[0008]
From the above, by using a GaInNAs-based material and appropriately selecting the composition of indium and nitrogen therein, an energy gap smaller than 1.42 eV (λ: 873 μm) of GaAs, that is, 1.3 to It can be seen that a material that oscillates in the long wavelength band of 1.7 μm band and lattice-matches on the GaAs substrate can be obtained.
[0009]
Furthermore, since a GaAs substrate can be used in the case of a GaInNAs-based material, an AlGaAs layer having a larger band gap than the InP cladding layer used in the conventional element structure can be used for the cladding layer. For this reason, it is possible to obtain a large band discontinuity difference and to obtain a laser element having a temperature characteristic of about 150K.
[0010]
FIG. 8 shows an example of the structure of a semiconductor laser device having a single quantum well structure using a GaInNAs-based material (58th JSAP Scientific Lecture Proceedings 2p-ZC-4 October 1997). This semiconductor laser device has an n-type AlGaAs lower cladding layer 704, an n-type GaAs lower optical waveguide layer 705 (thickness, 140 nm), a GaInNAs quantum well layer 706 (thickness, 7 nm), p, A type GaAs upper optical waveguide layer 707 (thickness, 140 nm), a p type AlGaAs upper cladding layer 708, and a p type GaAs cap layer 718 are formed in this order. Note that an n-side electrode 720 and a p-side electrode 722 are provided on the back surface of the substrate 700 and the cap layer 718, respectively.
[0011]
The GaInNAs quantum well layer 706 having a thickness of 7 nm has a strain of 1.0% with respect to the GaAs substrate. With the above structure, when the resonator length is 800 μm, a laser element having a threshold of about 103 mA and an oscillation wavelength of about 1.3 μm can be obtained.
[0012]
[Problems to be solved by the invention]
In the semiconductor laser device disclosed in Japanese Patent Laid-Open No. 8-195522, the quantum well structure constituting the active region uses a GaInNAs layer as the quantum well layer, and an (Al) GaAs layer or an InGaP layer as the quantum barrier layer A multilayer structure of GaAs and GaAs layers is used. However, such a quantum well structure has a problem that sufficiently good device characteristics cannot be obtained.
[0013]
The main cause of the above problem is that it is difficult to form a good heterointerface between a layer containing nitrogen such as a GaInNAs layer and a layer not containing nitrogen such as an (Al) GaAs layer. It seems that there is.
[0014]
A III-V compound semiconductor (hereinafter abbreviated as an N-based semiconductor) material containing nitrogen and a group V element other than nitrogen has high mixing instability, that is, a property of being easily phase separated. In the case of forming a laser element having a GaInNAs quantum well structure, according to the prior art, an N-based semiconductor layer for forming a quantum well layer is formed on a compound semiconductor layer that does not contain nitrogen, which is an (Al) GaAs quantum barrier layer. Alternatively, it is necessary to grow a quantum barrier layer made of (Al) GaAs or the like on an N-based semiconductor. During the growth, particularly at the initial stage of growth, only As is supplied as a group V element when a semiconductor layer not containing nitrogen is formed on the semiconductor layer containing nitrogen due to the mixed instability of the N-based semiconductor. , Localization of nitrogen on the surface facilitates phase separation, making it difficult to form a good heterointerface. Furthermore, since it is difficult to form an N-based semiconductor with a uniform composition on a compound semiconductor that does not contain nitrogen, a transition layer is formed by the transition of the composition in the vicinity of the interface, and an interface with a steep composition is obtained. There is also a problem of not.
[0015]
In addition, the N-based semiconductor material has a high nitrogen dependency with respect to the energy gap in addition to the above-mentioned mixing instability. That is, the energy gap value of the N-based semiconductor material is strongly influenced by the nitrogen concentration. For example, even if the indium mixed crystal ratio changes by 0.01, the energy gap of the N-based semiconductor material changes only by about 0.01 eV and the lattice constant only by about 0.007%. However, the energy gap of the N-based semiconductor changes by about 0.15 eV and the lattice constant changes by about 0.21% only by changing the nitrogen mixed crystal ratio by only 0.01. This is because bowing such as an energy gap with respect to the nitrogen content of the N-based semiconductor is very large. Therefore, when a quantum well structure is formed by a layer containing nitrogen and a layer not containing nitrogen, a quantum well layer having a structure with a sharp energy gap can be obtained unless the nitrogen source is sharply switched. I can't.
[0016]
Furthermore, when the layer containing nitrogen and the layer not containing nitrogen are continuously grown, it is necessary to change the substrate temperature during the growth. More specifically, when a GaInAsN well layer is grown, low temperature growth of about 500 ° C. is necessary to increase the amount of nitrogen taken up. On the other hand, when a Ga (Al) As barrier layer is grown, a high temperature condition of about 700 ° C. is necessary to improve crystallinity. Thus, when switching from the growth of GaInAsN quantum well layers to the growth of Ga (Al) As quantum barrier layers, the substrate temperature must be changed. At this time, an element having a high vapor pressure on the surface of the quantum well layer re-evaporates, or residual impurities such as C, O, or moisture remaining in the growth chamber may be adsorbed on the surface of the quantum well layer. For this reason, the crystallinity of the interface portion of the quantum well layer is deteriorated, and phase separation or clustering is likely to occur due to a change in the surface state of the quantum well layer. As a result, it becomes difficult to form a heterointerface having good crystallinity and steepness.
[0017]
As described above, when a quantum well structure is formed by a layer containing nitrogen and a layer not containing nitrogen, an increase in the ratio of non-radiative recombination at the interface between the quantum well layer and the quantum barrier layer, and the quantum effect By the reduction, the threshold value increases and the slope efficiency decreases. For this reason, as shown in FIG. 9, the slope efficiency of the current-light characteristics at room temperature is a normal real refractive index structure despite the fact that the resonator has a small loss inside, that is, a so-called real refractive index structure. In comparison with the case of (˜0.4 W / A), only a very low value of 0.16 W / A is obtained.
[0018]
Furthermore, despite the fact that the temperature characteristics are good, the drive efficiency during high temperature operation becomes high and the circuit configuration becomes complicated due to poor slope efficiency. As a result of performing a reliability test on the above-described semiconductor laser element, it was found that the element lifetime was only about 5000 hours.
[0019]
As described above, according to the prior art, a semiconductor laser device having good device characteristics due to an increase in threshold and a decrease in slope efficiency due to an increase in non-radiative recombination at the interface and a decrease in the quantum effect. It was not obtained.
[0020]
The present invention has been made in view of the above circumstances, and its object is to improve the crystallinity of the interface between the quantum well layer and the quantum barrier layer, to optimize the band structure, and to further design it. It is an object of the present invention to provide a quantum well structure optical semiconductor device having a high slope efficiency and a low threshold with improved flexibility.
[0021]
[Means for Solving the Problems]
  According to the invention III -V group compound semiconductor laser III A quantum barrier layer formed of two or more group V elements including a group element and nitrogen, III And an active region having a quantum well layer formed of a group V element and two or more group V elements including nitrogen III A V-group compound semiconductor laser between the active region and the cladding layer; III An optical waveguide layer formed of two or more group V elements including a group element and nitrogen is provided, and the optical waveguide layer has an intermediate energy gap between the quantum barrier layer and the cladding layer. An energy gap is provided, and the energy gap of the optical waveguide layer increases from the quantum barrier layer toward the cladding layer, thereby achieving the above object.
  The energy gap of the optical waveguide layer may increase smoothly from the quantum barrier layer toward the cladding layer.
  The energy gap of the optical waveguide layer may increase stepwise from the quantum barrier layer toward the cladding layer.
  Above III The group element may be selected from a group including Al, Ga, In, and Tl, and the group V element may be selected from a group including As, P, and Sb in addition to the nitrogen.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
With reference to FIGS. 1A and 1B, a buried type semiconductor laser device as a first embodiment of a quantum well structure optical semiconductor device according to the present invention will be described.
[0029]
As shown in FIG. 1A, the semiconductor laser device includes a p-type GaAs substrate 100, a p-type GaAs buffer layer 102, and a p-type Al._0.3Ga_0.7As lower cladding layer 104, quantum well structure 106 (active region), n-type Al_0.3Ga_0.7As first upper cladding layer 108, current confinement structure 110, n-type Al_0.3Ga_0.7An As second upper cladding layer 116 and an n-type GaAs cap layer 118 are provided.
[0030]
  The quantum well structure 106 of the semiconductor laser device according to the present invention has an undoped Ga as shown in FIG._0.80In_0.20N_0.022As_0.978Quantum well active layer 106a and undoped Ga_0.98In_0.02N_0.024As_0.976The quantum barrier layer 106b has a three-layer stacked structure. The quantum well active layer 106a has a compressive strain of 1.0% with respect to the GaAs substrate 100,About 6nmWith a thickness of The quantum well active layer 106a has an energy gap of about 0.886 eV and a lattice constant of about 5.710Å. On the other hand, the quantum barrier layer 106b has a tensile strain of 0.25% with respect to the GaAs substrate 100,About 12nmWith a thickness of The quantum barrier layer 106b has an energy gap of about 1.03 eV and a lattice constant of 5.667 Å.
[0031]
The current confinement structure 110 is provided on both sides of the mesa structure 109 constituted by the lower cladding layer 104, the quantum well structure 106, and the first upper cladding layer 108. The current confinement structure 110 includes an n-type InGaP layer 112, a p-type InGaP layer 113, and an n-type InGaP layer 114 that are lattice-matched to the GaAs substrate 100.
[0032]
A p-side electrode 120 is formed on the back surface of the substrate 100, and an n-side electrode 122 is formed on the n-type GaAs cap layer 118.
[0033]
Below, the manufacturing process of said semiconductor laser element is demonstrated.
[0034]
In this manufacturing process, a solid material such as gallium, indium and aluminum is used as a group III element material, and a solid material such as arsenic and ammonia are used as a group V element material. Silicon is used as the n-type impurity material, and beryllium is used as the p-type impurity material.
[0035]
First, the p-type GaAs buffer layer 102 having a thickness of about 0.3 μm and the p-type Al having a thickness of about 1.0 μm are formed on the p-type GaAs substrate 100 by MBE (molecular beam epitaxy) method at a substrate temperature of 650 ° C._0.3Ga_0.7An As lower cladding layer 104 is sequentially formed.
[0036]
Subsequently, in order to prevent arsenic loss, the substrate temperature is lowered to about 500 ° C. which is the optimum growth temperature of the GaInNAs layer while irradiating arsenic flux, and the undoped Ga layer is formed on the lower cladding layer 104._0.80In_0.20N_0.022As_0.978Quantum well active layer 106a and undoped Ga_0.98In_0.02N_0.024As_0.976The quantum barrier structure 106 is formed by stacking the quantum barrier layer 106b for three periods.
[0037]
Thereafter, the substrate temperature was raised to about 650 ° C. while supplying arsenic flux and ammonia, which are raw materials for the group V element, and then n-type Al having a thickness of 1.0 μm._0.3Ga_0.7An As first upper cladding layer 108 and an n-type GaAs protective layer (not shown in FIG. 1A) are formed.
[0038]
Subsequently, a dielectric mask made of a striped SiN film is formed on the n-type GaAs protective layer by a normal photolithography process. Next, in the wet etching process, the lower cladding layer 104, the quantum well structure 106, the first upper cladding layer 108, and the n-type GaAs protective layer are selectively selected so that a part of the surface of the buffer layer 102 is exposed. Etching forms a mesa structure 109 having a width of 1.5 μm.
[0039]
Further, an n-type InGaP layer 112 (lattice-matched to the GaAs substrate 100 is formed on the exposed surface of the buffer layer 102 on both sides of the mesa structure 109 by performing selective growth using a dielectric mask by MOCVD. A thickness of about 0.6 μm), a p-type InGaP layer 113 (thickness, about 1 μm), and an n-type InGaP layer 114 (thickness, about 1 μm) are sequentially formed. The n-type InGaP layer 112, the p-type InGaP layer 113, and the n-type InGaP layer 114 form a current confinement structure 110 that confines light in the lateral direction and confines current.
[0040]
Subsequently, after removing the dielectric mask, an n-type Al is formed on the mesa structure 109 and the current confinement structure 110 by MBE._0.3Ga_0.7An As second upper cladding layer 116 and an n-type GaAs cap layer 118 are sequentially formed.
[0041]
Finally, the p-side electrode 120 and the n-side electrode 122 are formed on the back surface of the substrate 100 and the top surface of the cap layer 118, respectively. Thus, an embedded semiconductor laser element can be obtained.
[0042]
In the above configuration, the lattice constant A of the quantum well active layer 106a_wellIs the lattice constant A of the quantum barrier layer 106b._barrierIt is set larger. Thereby, as shown in FIG. 7, a structure for confining electrons and holes in the quantum well layer can be formed.
[0043]
According to this embodiment, both the quantum well active layer 106a and the quantum barrier layer 106b are formed of a semiconductor containing nitrogen. For this reason, when forming a semiconductor containing nitrogen on a compound semiconductor not containing nitrogen, or when forming semiconductor layers in the reverse order, the interface between these layers (the interface between the quantum well layer and the quantum barrier layer). ) Can be prevented from being deteriorated due to phase separation or clustering. As a result, a heterointerface having good crystallinity and steepness can be formed, and a quantum well structure having a sharp change in energy gap can be obtained.
[0044]
Further, since the constituent elements of the quantum well active layer 106a and the quantum barrier layer 106b are the same, the growth temperature when forming the quantum well structure can be made constant. For this reason, it is not necessary to provide a waiting time for setting the optimum growth temperature when switching from the growth of the quantum well active layer 106a to the growth of the quantum barrier layer 106b, and it is possible to adsorb impurities at those interfaces or to re-evaporate the constituent elements. Deterioration of crystallinity due to can be prevented.
[0045]
FIG. 2 shows the evaluation result obtained by calculating from the satellite peak of the X-ray diffraction pattern by the two-crystal method for the quantum well structure according to the present embodiment. In the structure of the present invention, it can be confirmed that the steepness of the composition at the interface F between the quantum well layer and the quantum barrier layer is improved as compared with the conventional structure. By improving the crystallinity at the interface and the steepness of the composition at the interface, it is possible to reduce the non-light emitting component and improve the light emission efficiency, and the oscillation threshold is about 92 mA, which is reduced by about 10% compared to the conventional one. . According to the present invention, it is possible to obtain a semiconductor laser element with good characteristics such that the slope efficiency shows a value of 0.25 W / A.
[0046]
Further, according to the present invention, a barrier layer having a smaller energy gap can be formed as compared with the above-described conventional example using a Ga (Al) As layer as a quantum barrier layer. As a result, even when the quantum well layer is thickened, the band discontinuity difference between the quantum well layer and the quantum barrier layer can be reduced so that the level of the quantum well level becomes one. As a result, it is possible to suppress the oscillation wavelength jump due to the change of the oscillation level, which occurs when there are many levels in the quantum well.
[0047]
In addition, using an InGaAsN material for forming a quantum well structure has the following advantages over using a GaAsN material. The GaAsN-based material needs to have a higher mixed crystal ratio of nitrogen in order to obtain the same band gap as that of the InGaAsN-based material (see FIG. 7). Due to the high nitrogen mixed crystal ratio, layer separation and clustering are likely to occur, and it is difficult to obtain a good crystal structure. On the other hand, such a problem can be avoided in the case of an InGaAsN-based material. Further, even when it is necessary to form a thick semiconductor film because it does not lattice match with the GaAs substrate, it is desirable to form a quantum well structure using an InGaAsN material rather than a GaAsN material.
[0048]
In the above description, the MBE method is used for forming the semiconductor layer, but other growth methods such as MOCVD (metal organic chemical vapor deposition) may be used.
[0049]
(Second Embodiment)
Hereinafter, an embedded semiconductor laser device, which is a second embodiment of the quantum well structure optical semiconductor device according to the present invention, will be described with reference to FIGS. 3 (a) and 3 (b). The semiconductor laser device of the present embodiment is different from the first embodiment in the configuration of the quantum well structure, and the other elements are the same.
[0050]
As shown in FIG. 3A, the semiconductor laser device includes a p-type GaAs substrate 100, a p-type GaAs buffer layer 102, and a p-type Al._0.3Ga_0.7As lower cladding layer 104, quantum well structure 206, n-type Al_0.3Ga_0.7As first upper cladding layer 108, current confinement structure 110, n-type Al_0.3Ga_0.7An As second upper cladding layer 116 and an n-type GaAs cap layer 118 are provided.
[0051]
As shown in FIG. 3B, the quantum well structure 206 of this embodiment has an undoped Ga._0.90In_0.10N_0.027As_0.973Quantum well active layer 206a and undoped Ga_0.96In_0.04N_0.022As_0.978The quantum barrier layer 206b has a six-cycle stacked structure.
[0052]
  The quantum well active layer 206a has a compressive strain of 0.25% with respect to the GaAs substrate 100,About 7nmWith a thickness of The quantum well active layer 206a has an energy gap of about 0.885 eV and a lattice constant of about 5.667６６. On the other hand, the quantum barrier layer 206b has a tensile strain of 0.14% with respect to the GaAs substrate 100,About 22nmWith a thickness of The quantum barrier layer 206b has an energy gap of about 1.03 eV and a lattice constant of 5.645Å.
[0053]
Below, the manufacturing process of the semiconductor laser device of this embodiment is demonstrated.
[0054]
In the manufacturing process of the present embodiment, a solid material such as gallium, indium and aluminum is used as a group III element material, and arsine (AsH) is used as a group V element material._Three) Or phosphine (PH_Three) And ammonia. Note that hydrogen selenide (H₂Se) is used, and diethyl zinc (DEZ) is used as a raw material for the p-type impurity.
[0055]
First, a p-type GaAs buffer layer 102 having a thickness of about 0.3 μm and a p-type having a thickness of about 1.0 μm are formed on a p-type GaAs substrate 100 at a substrate temperature of 500 ° C. by gas source molecular beam epitaxy (GSMBE). Al_0.3Ga_0.7An As lower cladding layer 104 is sequentially formed. p-type GaAs buffer layer 102 and p-type Al_0.3Ga_0.7The As lower cladding layer 104 is lattice-matched with the p-type GaAs substrate 100.
[0056]
Subsequently, an undoped Ga layer is formed on the lower cladding layer 104._0.90In_0.10N_0.027As_0.973Quantum well active layer 206a and undoped Ga_0.96In_0.04N_0.022As_0.978And a quantum barrier layer 206b. The quantum well structure 206 is formed by stacking the quantum well active layer 206a and the quantum barrier layer 206b for six periods so as to compensate for the distortion stress of the layers. After that, 1.0 μm thick n-type Al_0.3Ga_0.7An As first upper cladding layer 108 is formed.
[0057]
Subsequently, a dielectric mask made of a striped SiN film is formed on the first upper clad layer 108 by a normal photolithography process. In the wet etching process, the lower clad layer 104, the quantum well structure 206, and the first upper clad layer 108 are selectively etched so that a part of the surface of the buffer layer 102 is exposed. A mesa structure 109 is formed.
[0058]
Further, an n-type InGaP layer 112 lattice-matched to the GaAs substrate 100 is formed on the exposed surface of the buffer layer 102 on both sides of the mesa structure 109 by performing selective growth using a dielectric mask by MOCVD. A p-type InGaP layer 113 and an n-type InGaP layer 114 are sequentially formed. The n-type InGaP layer 112, the p-type InGaP layer 113, and the n-type InGaP layer 114 form a current confinement structure 110 that confines light in the lateral direction and confines the current.
[0059]
Subsequently, after removing the dielectric mask, the n-type Al is formed on the mesa structure 109 and the current confinement structure 110 by GSMBE._0.3Ga_0.7An As second upper cladding layer 116 and an n-type GaAs cap layer 118 are sequentially formed.
[0060]
Finally, the p-side electrode 120 and the n-side electrode 122 are formed on the back surface of the substrate 100 and the top surface of the cap layer 118, respectively. Thus, an embedded semiconductor laser element can be obtained.
[0061]
Also in this embodiment, the effect of improving the crystallinity of the interface and the sharpness of the composition of the interface can be obtained by using the GaInAsN layer as the quantum barrier layer, as in the first embodiment. Furthermore, by reducing the strain stress at the interface between the quantum well active layer and the quantum barrier layer, it is possible to improve the problem in the prior art that the crystallinity of the interface due to the instability of the interface state due to the strain stress deteriorates. According to this embodiment, a quantum well structure with good crystallinity can be obtained without causing phase separation. According to this embodiment, the semiconductor laser device can reduce the oscillation threshold by about 15% (about 88 mA), improve the slope efficiency from 0.3 W / A, and improve the reliability from 5000 hours to 20,000 hours compared to the conventional example. Is obtained.
[0062]
(Third embodiment)
Hereinafter, a ridge type semiconductor laser device which is a third embodiment of the quantum well structure optical semiconductor device according to the present invention will be described with reference to FIG. In this embodiment, n-type GaAs is used as the substrate.
[0063]
As shown in FIG. 4, the semiconductor laser device includes an n-type GaAs substrate 300, an n-type GaAs buffer layer 302, and an n-type Al._0.4Ga_0.6As lower cladding layer 304, quantum well structure 306, p-type Al_0.4Ga_0.6As upper cladding layer 308, p-type GaAs cap layer 318, SiO₂And a dielectric layer 310. The dielectric layer 310 is provided on both sides of the ridge structure 309 constituted by the lower cladding layer 304, the quantum well structure 306, the upper cladding layer 308, and the cap layer 318. An n-side electrode 320 is formed on the back surface of the substrate 300, and a p-side electrode 322 is formed on the p-type GaAs cap layer 318 and the dielectric layer 310.
[0064]
  The quantum well structure 306 is made of undoped Ga._0.80In_0.20N_0.022As_0.978Quantum well active layer 306a and undoped Ga_0.98In_0.02N_0.022As_0.978The quantum barrier layer 306b has a three-layer stacked structure. The quantum well active layer 306a has a compressive strain of 1.0% with respect to the GaAs substrate 300,About 6nmWith a thickness of The quantum well active layer 306a has an energy gap of about 0.886 eV and a lattice constant of about 5.710Å. On the other hand, the quantum barrier layer 306b has a tensile strain of 0.25% with respect to the GaAs substrate 300,About 12nmWith a thickness of The quantum barrier layer 106b has an energy gap of about 1.07 eV and a lattice constant of 5.639.
[0065]
Below, the manufacturing process of said semiconductor laser element is demonstrated.
[0066]
In this manufacturing process, the same raw materials as those in the first embodiment are used as the raw materials for the Group III elements such as gallium, indium and aluminum, and the Group V elements such as arsenic and nitrogen.
[0067]
First, an n-type GaAs buffer layer 302 having a thickness of about 0.2 μm is formed on the n-type GaAs substrate 300 at a substrate temperature of 700 ° C. (optimum growth temperature of the GaAs layer) by MBE (molecular beam epitaxy). 0.0μm thick n-type Al_0.4Ga_0.6An As lower cladding layer 304 is sequentially formed.
[0068]
Subsequently, in order to improve the nitrogen incorporation rate into the GaInNAs film, the substrate temperature is lowered to about 500 ° C. in an atmosphere of arsenic flux and ammonia, and an undoped Ga layer is formed on the lower cladding layer 304._0.80In_0.20N_0.022As_0.978Quantum well active layer 306a and undoped Ga_0.98In_0.02N_0.022As_0.978A quantum barrier layer 306b. The quantum well structure 306 is formed by stacking the quantum well active layer 306a and the quantum barrier layer 306b three periods.
[0069]
Thereafter, the substrate temperature is raised to about 700 ° C. in an atmosphere of arsenic flux and ammonia, and then 1.0 μm thick p-type Al._0.4Ga_0.6An As upper cladding layer 308 and a p-type GaAs cap layer 318 are formed.
[0070]
Subsequently, a striped resist pattern having a width of 3 μm is formed on the cap layer 318 by a normal photolithography process. Next, the cap layer 318 and the upper clad layer 308 are selectively etched by a wet etching process to form a ridge structure 309 having a width of 3 μm.
[0071]
Furthermore, SiO 2 is coated so as to cover the upper cladding layer 308 by sputtering.₂A dielectric layer 310 (thickness, about 0.5 μm) is formed.
[0072]
Finally, an n-side electrode 320 and a p-side electrode 322 are formed on the substrate 300 side and the ridge structure 309 side, respectively. Thus, a ridge type semiconductor laser element is obtained.
[0073]
Hereinafter, the supply of raw materials in the manufacturing process of the quantum well structure 306 will be described in more detail with reference to FIG.
[0074]
FIG. 5 shows a shutter opening / closing sequence for controlling the raw material supply of the raw material supply source of the MBE growth apparatus when the quantum well structure 306 is formed. “Ga1 source” and “Ga2 source” indicate supply sources for supplying gallium raw materials at different supply amounts by using molecular beam sources having different intensities. “In1 source” and “In2 source” refer to sources that supply indium raw materials at different supply amounts by using molecular beam sources having different intensities. “As source” and “N source” indicate supply sources for supplying arsenic raw material and ammonia at a constant supply amount, respectively. The supply amount of these supply sources may be set appropriately according to the growth conditions. “ON” (open) and “OFF” (closed) indicate that raw materials are supplied and not supplied, respectively. For example, regarding the supply of gallium, the total supply amount of gallium can be controlled by controlling the opening and closing of the shutters of “Ga1 source” and “Ga2 source”. That is, when both shutters of “Ga1 source” and “Ga2 source” are “open”, the supply amount of gallium is the sum of the supply amount of “Ga1 source” and the supply amount of “Ga2 source”. On the other hand, when only one of “Ga1 source” and “Ga2 source” is “open”, the supply amount of gallium is the supply amount by one of “Ga1 source” and “Ga2 source”. It becomes.
[0075]
As shown in FIG._0.80In_0.20N_0.022As_0.978From the growth of the quantum well active layer 306a, Ga_0.98In_0.02N_0.022As_0.978When switching to the growth of the quantum barrier layer 306b, only the raw material supply ratio of the group III elements gallium and indium is changed, and the raw material supply ratio of the arsenic and nitrogen elements of the group V element is kept constant.
[0076]
Thus, in the formation of the interface portion of the semiconductor layer during the fabrication of the quantum well structure, the band gap, which is an essential problem of the present invention, is made constant by switching the supply amount (concentration) of the nitrogen raw material without switching. And the problem due to high nitrogen concentration dependence on the lattice constant can be avoided. For this reason, the composition of the interface between the quantum well layer and the quantum barrier layer and the steepness of the energy gap are improved. Furthermore, it is possible to suppress the occurrence of phase separation, clustering, or the like, which occurs when the interface state becomes unstable due to changes in the supply amounts of arsenic and nitrogen. By this effect, a quantum well layer / quantum barrier layer structure in which the steepness of the composition and energy gap at the interface is further improved can be obtained.
[0077]
According to the present embodiment, the oscillation threshold is reduced by about 20% (about 80 mA) and the slope efficiency can be improved to 0.4 W / A compared to the conventional element.
[0078]
(Fourth embodiment)
A buried type semiconductor laser device, which is a fourth embodiment of the quantum well structure optical semiconductor device according to the present invention, will be described below with reference to FIGS. 6 (a) and 6 (b). The main difference between this embodiment and the above-mentioned embodiment is that in this embodiment, an optical waveguide layer is provided on at least one of the upper and lower sides of the quantum well structure.
[0079]
As shown in FIG. 6A, the semiconductor laser device includes an n-type GaAs substrate 300, an n-type GaAs buffer layer 302, and an n-type Al._0.4Ga_0.6As lower cladding layer 304 and Ga_aIn_1-aN_bAs_1-bLower optical waveguide layer 405, quantum well structure 406, Ga_cIn_1-cN_dAs_1-dUpper optical waveguide layer 407 and p-type Al_0.4Ga_0.6An As upper cladding layer 308, a p-type GaAs cap layer 318, and a semi-insulating InGaP layer 410 are provided. The semi-insulating InGaP layer 410 is provided on both sides of a mesa structure 409 including a lower cladding layer 304, a lower optical waveguide layer 405, a quantum well structure 306, an upper optical waveguide layer 407, an upper cladding layer 308, and a cap layer 318. It has been. An n-side electrode 320 is formed on the back surface of the substrate 300, and a p-side electrode 322 is formed on the p-type GaAs cap layer 318 and the InGaP layer 410.
[0080]
Ga_aIn_1-aN_bAs_1-bThe gallium composition a of the lower optical waveguide layer 405 gradually decreases from 1.0 to 0.94 from the substrate 300 side in the layer stacking direction. In addition, the condition of a = (0.405-1.11b) / (0.405-0.035b) is satisfied between a and b. As a result, Ga_aIn_1-aN_bAs_1-bWhile the lower optical waveguide layer 405 is lattice-matched to the substrate 300, as shown in FIG. 6B, the energy gap (difference in energy level between the conduction band and the valence band) has a substrate 300 side (FIG. 6). It gradually decreases from the lower side of (b) toward the layer stacking direction.
[0081]
On the other hand, Ga_cIn_1-cN_dAs_1-dThe gallium composition c of the upper optical waveguide layer 407 gradually increases from 0.94 to 1.0 in the layer stacking direction from the substrate 300 side. Note that the condition of c = (0.405−1.11d) / (0.405−0.035d) is satisfied between c and d. As a result, Ga_cIn_1-cN_dAs_1-dThe energy gap of the upper optical waveguide layer 407 gradually increases from the substrate 300 side in the layer stacking direction.
[0082]
  The quantum well structure 406 is composed of undoped Ga._0.85In_0.15N_0.026As_0.974Quantum well active layer 406a and undoped Ga_0.80In_0.20N_0.024As_0.976The quantum barrier layer 406b has a two-layer stacked structure. The quantum well active layer 406a has a compressive strain of 0.5% with respect to the GaAs substrate 300,About 5nmWith a thickness of The quantum well active layer 406a has an energy gap of about 0.855 eV and a lattice constant of about 5.681 Å. On the other hand, the quantum barrier layer 406b is lattice-matched to the GaAs substrate 300,About 12nmWith a thickness of The quantum barrier layer 406b has an energy gap of about 1.03 eV and a lattice constant of 5.632 Å.
[0083]
Below, the manufacturing process of said semiconductor laser element is demonstrated.
[0084]
In this manufacturing process, organometallic gases such as trimethylgallium (TMG), trimethylindium (TMI), and trimethylaluminum (TMA) are used as raw materials for group III elements such as gallium, indium and aluminum, respectively, and arsenic, nitrogen, etc. As raw materials for group V elements, hydrides such as arsine and ammonia are used. Note that hydrogen selenide (H₂Se) is used, and diethyl zinc (DEZ) is used as a raw material for the p-type impurity.
[0085]
First, an n-type GaAs buffer layer 302 having a thickness of about 0.3 μm and an n-type having a thickness of about 1.0 μm are formed on the n-type GaAs substrate 300 at a substrate temperature of 700 ° C. (optimum growth temperature of the GaAs layer) by MOCVD. Type Al_0.4Ga_0.6An As lower cladding layer 304 is sequentially formed.
[0086]
Subsequently, the substrate temperature is lowered to about 500 ° C. in order to improve the nitrogen incorporation rate into the GaInNAs film while adjusting the supply amounts of ammonia and arsine. Thereafter, the Ga layer is formed on the lower cladding layer 304._aIn_1-aN_bAs_1-bThe lower optical waveguide layer 405 is formed to a thickness of 0.3 μm.
[0087]
Next, undoped Ga_0.85In_0.15N_0.026As_0.974Quantum well active layer 406a and undoped Ga_0.90In_0.20N_0.024As_0.976And a quantum barrier layer 406b. A quantum well structure 406 is formed by stacking two periods of the quantum well active layer 406a and the quantum barrier layer 406b.
[0088]
Subsequently, while adjusting the supply amount of ammonia, arsine, and the like, the above Ga well structure 406 is filled with the above Ga._aIn_1-aN_bAs_1-bThe upper optical waveguide layer 407 is formed to a thickness of 0.3 μm.
[0089]
Thereafter, the substrate temperature is raised to about 700 ° C. and 1.0 μm-thick p-type Al is supplied while supplying raw materials such as ammonia and arsine._0.4Ga_0.6An As upper cladding layer 308 and a p-type GaAs cap layer 318 are formed.
[0090]
Subsequently, a stripe-shaped resist pattern having a width of 1.5 μm is formed on the cap layer 318 by a normal photolithography process. Next, using the resist patterning as a mask, reactive ion beam etching (RIBE) using chlorine gas is performed so that a part of the surface of the buffer layer 302 is exposed, so that the lower cladding layer 304, the lower optical waveguide layer 405, the quantum The well structure 406, the upper optical waveguide layer 407, the upper cladding layer 308 and the cap layer 318 are selectively etched to form a mesa structure 409 having a width of 1.5 μm.
[0091]
Further, an InGaP layer 410 that is a light / current confinement layer is formed on the buffer layer 302 on both sides of the mesa structure 409 to a thickness of about 3.1 μm.
[0092]
Finally, the n-side electrode 320 is formed on the back surface of the substrate 300 by vacuum deposition, and the p-side electrode 322 is formed on the top surfaces of the cap layer 318 and the InGaP layer 410. Thus, an embedded semiconductor laser element can be obtained.
[0093]
According to the present embodiment, an optical waveguide layer having an energy gap intermediate between these layers is provided between the cladding layer and the quantum well structure, thereby increasing the optical confinement coefficient of the quantum well layer, The degree of freedom in device design increases.
[0094]
Furthermore, according to the present invention, an interface structure between an N-based semiconductor layer and a nitrogen-free layer having good crystallinity can be formed between the optical waveguide layer and the cladding layer located outside the quantum well structure. Therefore, non-radiative recombination can be prevented.
[0095]
InGaAsN or GaAsN is used as the N-based semiconductor, but InGaAsN is more preferable for the following reason. In the case of GaAsN having a band gap of a wavelength band of 1.1 μm, a high value of 2% is necessary as the nitrogen concentration, and further, there is a tensile strain of about 0.5% with respect to the GaAs substrate. For this reason, when a guide layer having a relatively large thickness is formed, lattice relaxation occurs and a good crystal cannot be obtained. In contrast, an InGaAsN-based material has a band gap in the same wavelength band as GaAsN and is lattice matched to GaAs._0.05Ga_0.95As_0.984N_0.016A semiconductor having a low nitrogen concentration that causes mixing instability can be used.
[0096]
According to this embodiment, a semiconductor laser device having an oscillation threshold value reduced by about 25% (about 75 mA) as compared with the conventional device can be obtained.
[0097]
In the present embodiment, the Ga composition ratio of the optical waveguide layer is gradually changed from 1.0 to 0.94 (and from 0.94 to 1.0). A similar effect can be obtained even if the structure changes in steps of 0.02. In addition, although the clad layer of this embodiment is formed of an AlGaAs clad, the clad layer can be composed of an InGaAlAsN layer containing a small amount of nitrogen and indium, or an InGaAsN layer or InGaAsPN layer containing no aluminum. It is. Even in this case, a laser element is manufactured by forming a band gap structure that smoothly connects the optical waveguide layer and the cladding layer without changing the group III or V group composition element without impairing the crystallinity of the cladding layer. Can do.
[0098]
It goes without saying that the mixed crystal ratio, layer thickness, and the like of the quantum well layer and the quantum barrier layer described in the above embodiments can take various values without departing from the gist of the present invention.
[0099]
In the above embodiment, the quantum well structure and the optical waveguide layer are formed of InGaAsN. However, these layers are made of an N-based semiconductor containing Al or Tl as a group III element and Sb as a group V element. The same effect can be obtained even if it is formed.
[0100]
Further, the growth method, the growth temperature, or the specific configuration of the semiconductor layer is not limited to those described in the above embodiments, and other methods or methods can be used as long as the effects of the present invention can be obtained. Needless to say, a structure may be used.
[0101]
【The invention's effect】
According to the present invention, the following effects can be obtained.
[0102]
Both the quantum well layer and the quantum barrier layer are formed of a group III-V compound semiconductor containing nitrogen and other group V elements, so that mixing instability at the interface between the conventional quantum well layer and the quantum barrier layer can be prevented. Disappear. As a result, a quantum well / quantum barrier layer structure having an interface having a sharp composition and a sharp energy gap can be formed. For this reason, the slope efficiency of the semiconductor laser element can be improved and the threshold value can be lowered.
[0103]
Further, by using a quantum barrier layer made of a III-V group compound semiconductor containing nitrogen and other group V elements, the energy gap can be increased as compared with the conventional example using a Ga (Al) As layer as the quantum barrier layer. A small barrier layer can be formed. Thereby, even when the quantum well layer is thickened, the band discontinuity difference between the quantum well layer and the quantum barrier layer can be reduced so that the quantum well level becomes one. As a result, it is possible to suppress the oscillation wavelength jump due to the change of the oscillation level, which occurs when there are a large number of levels in the quantum well.
[0104]
Furthermore, the lattice constant A of the quantum well layer_wellIs the lattice constant A of the quantum barrier layer_barrierSince it is set larger, a structure for confining electrons and holes in the quantum well can be obtained. Further, by making the strain stresses of the quantum well layer and the quantum barrier layer opposite in direction and equal in magnitude, phase separation at the interface due to strain can be suppressed.
[0105]
Furthermore, by making the supply amount of the group V raw material containing nitrogen constant and forming the quantum well layer and the quantum barrier layer continuously, the occurrence of phase separation or clustering that is likely to occur when switching the supply amount of the group V raw material occurs. Can be suppressed. For this reason, the state of the interface between the quantum well layer and the quantum barrier layer is stable, and the structure of the quantum well layer / quantum barrier layer having the interface having the steepness of the composition and the steepness of the energy gap can be surely formed. .
[0106]
Furthermore, by providing an optical waveguide layer having an energy gap between these layers between the cladding layer and the quantum well structure, the degree of freedom in device design such as an optical confinement factor is increased.
[0107]
When InGaAsN and GaAs, which can be used as an N-based semiconductor forming a quantum well structure, have the same band gap, the concentration of nitrogen that causes mixing instability is small and lattice matching is achieved with the GaAs layer. The use of InGaAsN can achieve the effect of the present invention more sufficiently.
[Brief description of the drawings]
1A and 1B are structural views of a semiconductor laser device according to a first embodiment of the present invention, FIG. 1A is a cross-sectional view, and FIG. 1B is a diagram showing a hand structure of a quantum well structure;
FIG. 2 is a view showing an evaluation result obtained by calculating from data of an X-ray diffraction method for the semiconductor laser element obtained according to the first embodiment of the present invention.
3A and 3B are structural views of a semiconductor laser device according to a second embodiment of the present invention, FIG. 3A is a cross-sectional view, and FIG. 3B is a diagram showing a band structure of a quantum well structure.
FIG. 4 is a structural sectional view of a semiconductor laser device according to a third embodiment of the present invention.
FIG. 5 is a diagram showing an opening / closing sequence of a shutter for controlling material supply of a material supply source of an MBE growth apparatus when a quantum well structure of a semiconductor laser device according to a third embodiment of the present invention is manufactured.
6A and 6B are structural views of a semiconductor laser device according to a fourth embodiment of the present invention, FIG. 6A is a cross-sectional view, and FIG. 6B is a diagram illustrating a hand structure of a quantum well structure.
FIG. 7 is a graph showing the relationship between the composition of In and N and the lattice constant or band gap of a GaInNAs-based material.
FIG. 8 is a sectional view of a conventional GaInNAs strained quantum well structure semiconductor laser device.
FIG. 9 is a graph showing current-light output characteristics of a conventional GaInNAs strained quantum well structure semiconductor laser device.
[Explanation of symbols]
100 p-type GaAs substrate
102 p-type GaAs buffer layer
104 p-type Al_0.3Ga_0.7As lower cladding layer
106, 206, 306, 406 Quantum well structure
106a, 206a, 306a, 406a GaInNAs quantum well layer
106b, 206b, 306b, 406b GaInNAs quantum barrier layer
108 n-type Al_0.3Ga_0.7As first upper cladding layer
109 Mesa structure
110 Current confinement structure
112 n-type InGaP layer
113 p-type InGaP layer
114 n-type InGaP layer
116 n-type Al_0.3Ga_0.7As second upper cladding layer
118 n-type GaAs cap layer
120, 322 p-side electrode
122, 320 n-side electrode
300 n-type GaAs substrate
302 n-type GaAs buffer layer
304 n-type AlGaAs lower cladding layer
308 p-type AlGaAs upper cladding layer
309 Ridge structure
310 Dielectric layer
318 p-type GaAs cap layer
405 GaInNAs lower optical waveguide layer
407 GaInNAs upper optical waveguide layer
409 Mesa structure
410 InGaP layer

Claims (4)

III A quantum barrier layer formed of two or more group V elements including a group element and nitrogen, III And an active region having a quantum well layer formed of a group V element and two or more group V elements including nitrogen III A V group compound semiconductor laser,
Between the active region and the cladding layer, III An optical waveguide layer formed of two or more group V elements including a group element and nitrogen is provided;
The optical waveguide layer has an energy gap intermediate between the energy gap of the quantum barrier layer and the energy gap of the cladding layer;
The energy gap of the optical waveguide layer increases from the quantum barrier layer toward the cladding layer. III -Group V compound semiconductor laser. The energy gap of the optical waveguide layer is increased smoothly from the quantum barrier layer toward the cladding layer. III -Group V compound semiconductor laser. The energy gap of the optical waveguide layer is gradually increased from the quantum barrier layer toward the cladding layer. III -Group V compound semiconductor laser. Said III The group element is selected from the group containing Al, Ga, In, and Tl, and the group V element is selected from the group containing As, P, and Sb in addition to the nitrogen. As described in III -Group V compound semiconductor laser.

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