JPH10242512A - Semiconductor light emitting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the yield and quality of a semiconductor light emitting device and, at the same time, to decrease the threshold current density of the device, by forming an active layer to contain either a barrier layer or a well layer having a different composition or thickness from those of a barrier layer and a well layer as the central part of a super lattice in its terminating section. SOLUTION: In order to solve such a problem that the light output of a central well layer is absorbed by an outside well layer, InGaN/GaN strain relaxing layers 12 and 13 are formed adjacently to the upper end lower surface of an InGaN/GaN multiple quantum well(MQW) active layer 6. It can be considered that the layers 12 and 13 also have super lattice structure and the structures at both ends of the active layer 6 are different from that at the central part. The InGaN/GaN strain relaxing super lattices 12 and 13 work as buffer layers for making the strain applied to the MQW active layer 6 from a GaN guide layer zero.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor light emitting device, and more particularly to a semiconductor light emitting device having a low operating voltage and a low threshold current density.
It is used for an aN-based semiconductor semiconductor light emitting device.

[0002]

2. Description of the Related Art Conventional GaN-based semiconductor light-emitting devices have succeeded in crystal growth of p-type GaN (hereinafter abbreviated as p-GaN) by the addition of Mg, and have been used for visible semiconductor lasers and semiconductor light-emitting diodes in the ultraviolet and blue regions. (Hereinafter, the semiconductor laser is abbreviated as LD and the light emitting diode is abbreviated as LED.)

The main problems of GaN-based LDs and LEDs are that the operating voltage is high, the threshold current density of LD light emission is large, and the luminous efficiency of the active layer does not show its original value. The reason is that in the case of LD, SCH (Separate Confine
ment Heterostructure) type MQW (Multi Quantum We)
ll) A large strain is present in an active layer having a quantum well structure (hereinafter abbreviated as MQW active layer), which degrades the crystal quality of the active layer, and particularly, the first few well layers and the last of the MQW active layer. This is because a large strain is generated in some of the well layers, and light emission and absorption at both ends are shifted to the longer wavelength side as compared with the central part of the active layer.

[0004] The strain applied from the guide layer and the cladding layer is relaxed in the outer well layer, and the strain applied to the central portion of the MQW active layer is reduced. The band gap of the outer well layer having a large strain amount is smaller than that of the central part, and the MQW
The light output from the active layer is absorbed and attenuated, and the threshold current density of the GaN-based LD increases.

[0005] Further, a lattice defect is generated in the outer well layer due to the strain applied from the guide layer and the cladding layer, so that electrons and holes injected into the active region through defect levels caused by the defect are recombined. GaN L
The luminous efficiency of D decreases. Similar problems are seen with GaN-based LEDs having a uniform active layer.

In order to avoid these problems, a conventional M
A method of increasing the number of well layers of the QW active layer to, for example, 20 or more has been adopted. By increasing the number of well layers in this manner, the light absorption effect of the outer well layers and the effect of attenuating the light output due to recombination of electrons and holes can be reduced.

In addition, a hetero interface is formed between the well layer and the barrier layer constituting the MQW active layer, and a certain degree of defect occurs at the hetero interface due to lattice mismatch.
When the number of well layers increases, many interfaces between the well layers and the barrier layers are included in the MQW active layer, and recombination centers are introduced into the MQW active layer due to generation of defects at the interfaces, thereby reducing optical gain. I do.

For this reason, in a GaN-based LD, the MQW
It has been considered that the number of well layers must be increased in order to obtain sufficient gain required for LD light emission in the active layer.
However, if the number of MQW wells is large, the transport of electrons and holes is hindered, and the number of well layers actually contributing to LD emission is limited to about two to three. Therefore, for example, in an LD having a group III-V MQW active layer, the number of well layers at which the highest luminous efficiency is obtained has been generally set to 10 or less.

In recent years, the need for short-wavelength LDs for reading and writing high-density optical disks has been increasing. However, the emission wavelength of currently practiced InGaAlP-based LDs is about 600 nm, which is required for next-generation DVD systems. It is difficult to realize an LD having a wavelength of 400 nm to 430 nm.

It is known that GaN-based LDs and LEDs can be used as a light source not only in the above wavelength range but also in a shorter wavelength. However, GaN-based LDs and LEDs have many technical problems in their manufacturing methods compared to conventional materials. The main ones are GaN and In _x Ga _1-x N (0 ≦
x ≦ 0.3).

InGaN (the suffix indicating the composition is omitted except where particularly necessary) is usually used as an active layer, and GaN is used as a guide layer. If the average composition of In exceeds 10%, the strain at the interface between the active layer and the guide layer increases. At this time, if a temperature rise of about 200 ° C. to 400 ° C. necessary for forming the p-GaN guide layer is performed,
It is experimentally known that the InGaN active layer is deteriorated or destroyed.

Therefore, the average composition of In in the active layer is usually 1
Although it is set to 0% or less, also in this case, the active layer is deteriorated and the yield is reduced. The degradation of the active layer is considered to be caused by the migration or evaporation of In in the active layer, in addition to the lattice mismatch. One of the other causes is that the In composition in the active layer becomes non-uniform. Operation LD, LE
D. It becomes difficult to obtain an LD having a low threshold current density.

On the other hand, if the average composition of In is lowered, a carrier overflow effect in which electrons and holes overflow from the active region contributing to LD light emission tends to occur, and not only carrier confinement but also light confinement deteriorates, so that LD light emission efficiency is reduced. It is not preferable for improvement.

As described above, at present, an active layer having a large In composition required for both carrier confinement and light confinement cannot be used, and the average In composition of the active layers of GaN-based LDs and LEDs is 10% or less ( In composition of well layer 15
%).

By introducing a cap layer between the MQW active layer and the p-type guide layer, there is also a technique for protecting the active layer from being deteriorated during the temperature rise process during the formation of the p-type guide layer and preventing carrier overflow. Although it has been developed, it has not yet completely prevented deterioration.

In a GaN-based LD, the electron density of the electrons and holes constituting the carriers is larger than the hole density. Therefore, the carrier overflow effect in which the electrons pass through the active layer to the p-side electrode is reduced. Al _x Ga ₁ -xN (x
> 0.1), and there was a problem that the operating voltage was increased due to the large Al composition of the cap layer.

[0017]

As described above, the conventional GaN LD has an excessive number of wells in the MQW active layer for the purpose of avoiding the influence of strain in the MQW active layer. There is a disadvantage that the operating voltage and the threshold current density become excessive.

If there is no influence of distortion due to a difference in lattice constant between the inside of the MQW active layer and its surrounding layers, the smaller the number of wells, the higher the efficiency of LD emission. The present invention has been made to solve the above problems, and has been developed in accordance with MQW.
An object of the present invention is to obtain a GaN-based semiconductor light-emitting device that exhibits high luminous efficiency with a small number of wells by removing the influence of strain applied to an active layer.

Conventionally, the lattice mismatch between the active layer of a GaN-based LD or LED and the guide layer adjacent to the active layer is large. Therefore, when the In composition of the active layer is large, the interface between the active layer and the guide layer becomes large. There is a problem in that energy of strain is accumulated, lattice defects are generated in the interface region and the inside of the active layer, and a high-quality LD cannot be obtained and the yield is poor. Also, when the In composition of the active layer is small, there is a problem that the carrier overflow effect easily occurs, and an LD with a low threshold current density cannot be realized.

The present invention has been made in order to solve the above-mentioned problems, and has a structure in which the MQW is provided between the guide layer and the MQW active layer.
By interposing a well layer and a barrier layer having an In composition or thickness different from those of the well layer and the barrier layer of the W active layer, the above two conflicting problems can be solved at the same time, the yield is high, the quality is high, and , GaN-based L with low threshold current density
It is intended to provide D and to be applied to LED.

[0021]

A semiconductor light emitting device according to the present invention comprises a well layer and a barrier in a region adjacent to an MQW active layer and a guide layer or a cladding layer for absorbing the influence of strain caused by a difference in lattice constant. It is characterized by providing a layer. In this manner, uniform LD light emission can be obtained over the entire stripe-shaped active layer without increasing the number of wells in the MQW active layer.

In the semiconductor light emitting device of the present invention, a well layer and a barrier layer having compositions different from those of the MQW active layer are provided in a region adjacent to the MQW active layer and the guide layer or the clad layer,
The average composition of the well layer and the barrier layer is made closer to the adjacent guide layer or clad layer to reduce the strain generated at the interface between the guide layer or clad layer and the MQW active layer. It is characterized in that the carrier overflow effect is prevented by widening the band gap difference between the well layer and the barrier layer, which are closer to the interface.

Specifically, the semiconductor light emitting device of the present invention has an active layer having a superlattice structure in which barrier layers and well layers are alternately stacked, and the active layer is located at a central portion of the superlattice. It is characterized in that at least one of the barrier layer and the well layer, which differs from the barrier layer and the well layer in at least one of the composition and the thickness, is included in the terminal portion.

Preferably, the active layer having the superlattice structure comprises In _x Al _y Ga _{1 -xy} N (0 <x ≦ 1, 0 ≦ y ≦
1, 0 ≦ x + y ≦ 1) and In _z Al _w
Ga _1-zw N (x <z, 0 <z ≦ 1, 0 ≦ z + w ≦ 1)
And a superlattice alternately laminated with well layers made of, and the composition is at least one of x and z.

According to the semiconductor light emitting device of the present invention, the active layer having a superlattice structure has an average composition of a well layer and a barrier layer in a region adjacent to one of a guide layer and a cladding layer as compared with a central portion of the active layer. Includes a laminated region close to any one of the composition of the guide layer and the cladding.

In the semiconductor light emitting device of the present invention, the active layer having the superlattice structure has a band gap between the well layer and the barrier layer in a region adjacent to one of the guide layer and the cladding layer as compared with the central portion of the active layer. Is characterized in that it includes a stacked region in which the difference between the two is increased.

Preferably, in the semiconductor light emitting device according to the present invention, the active layer having a superlattice structure includes a cap layer between at least one of the guide layer and the clad layer.

[0028]

Embodiments of the present invention will be described below in detail with reference to the drawings. First, items common to the embodiments will be specifically described. FIG. 11 shows a cross-sectional structure of a GaN-based LD currently being developed. The GaN LD includes a sapphire substrate 1, an n-GaN contact layer 2,
Ti / Au lower electrode 3, n-AlGaN cladding layer 4,
n-GaN guide layer 5, MQW active layer 6 of In _x Ga _1-x N / GaN (hereinafter referred to as InGaN / GaN MQW)
Active layer), p-GaN guide layer 7, p-AlGa
N clad layer 8, p-GaN contact layer 9, SiO ₂
The film 10 includes a Ni / Au upper electrode 11.

The active layer is usually made of In _x Ga ₁ -xN / GaN
(0 <x <0.3) The MQW active layer has about 20 wells. The AlGaN cladding layer has a large band gap value and can confine carriers and confine optical output to the active layer.

The GaN-based LD shown in FIG. 11 has a multilayer structure constituting the LD formed on a sapphire substrate. In order to reduce the threshold current density of LD light emission, SiO
_{(2) A} stripe-shaped current confinement structure is formed using the insulating film 10. In addition, other current confinement structures such as a buried stripe type can be used.

FIG. 12 is a band structure diagram showing details in the vicinity of the active region of the GaN-based LD. For simplicity, the number of wells is five. Normal double heterojunction type L
From the structure of D, the structure of a GaN-based LD having an MQW active layer as shown in FIG.

However, since there is a difference in lattice constant between the active region and the light guide region surrounding the active region, this limits the structure of the LD having the MQW active layer. FIG. 13 shows the relationship between the composition of the material constituting the GaN-based LD, the band gap, and the lattice constant.

The active layer of the GaN-based LD is made of In _x Ga ₁ -xN
The guide layer is usually formed using GaN, and the cladding layer is formed using Ga _y Al ₁ -yN. When x changes from 1 to 0 in FIG. 13, the point of GaN and InN
The band gap and the lattice constant change along a straight line connecting the point of and the point of GaN and A when the y changes from 1 to 0.
The band gap and the lattice constant change along a straight line connecting the point 1N.

In order to increase the luminous efficiency of the LD, it is necessary to provide a certain band gap difference between the active layer and the waveguide layer and between the waveguide layer and the cladding layer. FIG.
Since the inclination of the straight line connecting GaN and InN with respect to the horizontal axis is larger than the inclination of the straight axis connecting GaN and AlN with the horizontal axis, the difference between the band gaps of the same degree is
It can be seen that the difference in lattice constant between the GaN waveguide layer and the InGaN active layer is larger than the difference in lattice constant between the GaN waveguide layer and the AlGaN cladding. In FIG. 13, as an example, the composition of the In _x Ga ₁ -xN active layer x = 0.
2 shows the degree of lattice mismatch between the active layer and the GaN waveguide layer.

As described above, when attention is paid to the strain applied to the InGaN active layer from the surrounding material, the InGa
The GaN waveguide layer adjacent to the N active layer is large,
On the other hand, what is added by the change in the composition of the AlGaN cladding layer is small. Accordingly, it can be read from FIG. 13 that strain is accumulated at the interface between the MQW active layer and the guide layer due to the lattice mismatch between the InGaN / GaN MQW active layer and the GaN guide layer.

In the above description, the case where the GaN guide layer is formed adjacently above and below the MQW active layer and the AlGaN cladding layer is formed adjacently above and below the MQW active layer has been described. The structure is not necessarily limited to this, and there are various combinations such as a case where the cladding layer is adjacent to the active layer and a case where the guide layer and the cladding layer are adjacent to the uniform active layer.

For example, also in the case where the AlGaN cladding layer is directly adjacent to the MQW active layer, it is clear from FIG. 13 that the lattice mismatch between the two is large and strain accumulates at the interface. In addition, the active layer is not MQW structure,
It can be seen that even with aN, strain is accumulated at the interface with the GaN waveguide layer.

As a method of forming an LD or LED having zero lattice distortion, for example, In _z Al _w Ga _{1 -z w} N (0 ≦ z + w)
There is a method using a quaternary compound such as ≦ 1). At this time, if z and w are controlled, only the band gap can be continuously changed without changing the value of the lattice constant within the range of the triangle in FIG. LD and LED can be designed.

However, actually, two composition parameters z
It is extremely difficult to perform good crystal growth while controlling w and w to optimal conditions, and at present, the material properties of such quaternary compounds have not been sufficiently elucidated.

Therefore, the constituent materials of the GaN-based LD and LED are actually limited to binary and ternary compounds, and the MQW active layer has a structure as shown in FIG. 14 due to the strain caused by the lattice mismatch. . That is, the band structure of the unstrained MQW active layer shown in FIG. 12 is deformed by the strain from the GaN waveguide layer, and the band width of the outer well layer is narrower than that of the central well layer.

FIG. 14 shows that the light emitted from the central well layer is absorbed in the outer well layer whose band gap is reduced by the strain. Further, such a distortion causes a defect level between the bands of the outer well layer, and the light absorption thereof cannot be ignored.

Next, a first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a cross-sectional view showing a structure of a GaN-based LD having an MQW active layer according to the first embodiment of the present invention. As described above, in order to solve the problem that the light output of the central well layer is absorbed by the outer well layer, an InGaN / GaN strain relaxation layer is provided adjacently above and below the InGaN / GaN MQW active layer 6. 12, 13
Was introduced. As shown in FIG. 2, the strain relaxation layer also has a superlattice structure, and it can be considered that the structure at both ends of the MQW active layer is changed to the central part.

Other structures of the LD are the same as those shown in FIG. The InGaN / GaN strain-relaxed superlattice 1
Reference numerals 2 and 13 function as buffer layers for reducing the strain applied to the MQW active layer 6 from the GaN guide layer to zero.

The MQW active layer 6, the strain relaxation layers 12, 13, G
FIG. 2 shows a band structure of the GaN-based LD according to the first embodiment including the aN waveguide layers 5 and 7 and the AlGaN cladding layers 4 and 8. However, FIG. 2 shows a band structure determined only by the composition of the material without considering the distortion effect based on the difference in the lattice constant between the materials.

Here, In serving as a well layer of the MQW active layer 6 is referred to as In.
The composition of _x Ga _1-x N is x = 0.2, in the range of values of x of the conventional typical GaN-based LD as described in FIG. For the barrier layer of the MQW active layer, GaN or In _x Ga ₁ -xN (0 <x <0.05) having a very small In composition was used.

The well layer in the strain relaxation layer is made of In _x Ga ₁ -xN.
(X = 0.05), where the In composition x is 1/4 to 1/1 of the In composition of the well layer of the MQW active layer.
Values up to 3 could be used. The barrier layer in the strain relaxation layer is made of GaN or In _x G having a very small In composition.
a _1-x N (0 <x <0.05) was used. FIG. 2 shows a case where GaN is used as a barrier layer in the strain relaxation layer.

The number of wells in the strain relaxation layer is determined by the composition of the material forming the well layer of the MQW active layer. The greater the ratio of the In composition of the well layer of the strain relaxation layer to the In composition of the well layer of the MQW active layer, the greater the amount of strain, and the greater the number of wells in the strain relaxation layer to reduce the strain to zero. However, a sufficient effect was obtained when the number of wells was in the range of about 2 to 5.

FIG. 3 shows the band structure of the InGaN / GaN MQW active layer, the InGaN / GaN strain relaxation layer, and the GaN waveguide layer of the GaN-based LD according to the first embodiment in consideration of the distortion effect. As described above, if the material of the well layer in the strain relaxation layer is appropriately selected, the band gap of the strain relaxation layer can maintain a value larger than the band gap of the MQW active layer even in consideration of the strain effect. it can.

Also, as shown in FIG. 3, the deep level of the defect generated in the strain relaxation layer due to the strain is located outside the band gap of the MQW active layer. It has been found that the probability of carrier recombination is negligible compared to the probability of direct carrier recombination in the MQW active layer.

Next, a second embodiment of the present invention will be described with reference to FIGS.
An embodiment will be described. As is clear from FIG. 13, the In composition of the well layer is larger than the In composition of the barrier layer. The MQW active layer is usually composed of a repetition of about 3 to 30 well layers and barrier layers.

The guide layer is usually composed of a single layer of GaN, and the cladding layer is composed of Al _x Ga ₁ -xN (0 <x ≦ 0.3). It is known that if the average In composition of the MQW active layer exceeds 10%, distortion occurs at the interface between the MQW active layer and the guide layer, and a high quality MQW active layer cannot be obtained.

As shown in FIG. 12, usually InGaN / G
In the aN · MQW active layer, the composition and thickness of the well layer and the barrier layer are kept constant, so that the band gap of the well layer and the band gap of the barrier layer are kept constant in the active layer.

FIG. 6 shows the GaN LD having the structure shown in FIG.
3 schematically shows the In composition of a well layer, the In composition of a barrier layer, the average In composition of an active layer, and the band gap in the QW active layer and its vicinity. In the second embodiment, it will be described using the _{In x Ga 1-x N /} In y Ga MQW active layer made of _1-y N (hereinafter referred to as InGaN · MQW active layer).

If the In composition of the well layer is 20%, the thickness is 2 nm, the In composition of the barrier layer is 5%, and the thickness is 4 nm, the average composition of In at this time is about 10%. As described above, if the average composition of In is close to or more than 10%, InG
The strain generated between the aN · MQW active layer and the GaN guide layer became excessive, and it was difficult to obtain a high-quality GaN-based LD.

In order to solve this problem, in the second embodiment, a GaN-based LD having a sectional structure shown in FIG. 4 was prototyped. The difference from the conventional structure of FIG. 11 is that In _x Ga _{1 -x}
An MQW active layer (hereinafter referred to as In) of N / In _y Ga _1-y N
Between the GaN MQW active layer 6 and the GaN guide layers 5 and 7, In _z Ga _{1 -z} N / has a lower In composition of the barrier layer than the barrier layer of the InGaN MQW active layer 6.
A composition modulation layer (hereinafter referred to as InGaN) composed of In _w Ga _1-w N
This is characterized in that the average In composition near the interface between the InGaN MQW active layer and the GaN waveguide layer is reduced as a whole.

The InGaN MQW active layer and G on both sides thereof
FIG. 5 shows a band structure of the InGaN composition modulation layer and the like formed between the aN waveguide layer. As described above, in the second embodiment, the periodicity of the MQW active layer is basically maintained, including the InGaN MQW active layer and the InGaN composition modulation layers on both sides thereof, and the composition of the barrier layer is simply reduced. Since the amplitude of the energy band is modulated by the change, the layer introduced between the InGaN MQW active layer and the GaN waveguide layer is referred to as an InGaN composition modulation layer.

Next, the results of a simulation performed on the threshold voltage and the threshold current density of the GaN-based LD according to the second embodiment will be described. In this simulation, the In composition of the barrier layer in the InGaN composition modulation layer was 0%, that is, this barrier layer was GaN.

However, it is desirable from the viewpoint of obtaining a high-quality LD multilayer structure not to cause a sudden change in the material properties of the well layer and the barrier layer.
A barrier layer to which n is added by about several percent can be used. However, as compared with the In composition of the barrier layer near the center of the InGaN MQW active layer, the In composition of the barrier layer in the composition modulation layer was significantly reduced.

FIG. 5 is a schematic diagram showing the band structure of the active layer 6, the composition modulation layers 14, 15, the guide layers 5, 7, and the cladding layers 4, 8 of the GaN-based LD shown in FIG. FIG. 5 shows that the difference in band gap between the well layer and the barrier layer in the InGaN composition modulation layers at both ends of the InGaN MQW active layer is larger than that near the center of the MQW active layer.

FIG. 6 shows the relationship between the band structure and the average In composition by extracting a region near the active layer 6 of FIG. As described above, a GaN-based LD in which an InGaN composition modulation layer is introduced at both ends of an InGaN MQW active layer,
As described in the first embodiment, InGaN MQW
In addition to the effect of alleviating the influence of strain at the interface between the active layer and the GaN guide layer, the LD device has the following advantages in characteristics.

The operating voltage of the GaN-based LD and InGaN
FIG. 7 shows the relationship with the number of wells in the MQW active layer. The solid line in the drawing is a case where the InGaN composition modulation layer according to the second embodiment is provided, and the broken line is a simulation result of a conventional GaN-based LD having no composition modulation layer shown for comparison. In this simulation, p
The carrier density of both the mold guide layer and the p-type cladding layer is 1 ×
The calculation was performed at 10 ¹⁶ cm ^-3 . The parameters given to each curve showing the simulation results are the same as those of the InG
(In composition of well layer / I of barrier layer)
n composition) is shown as, for example, (20% / 5%).

As described above, the barrier layer of the InGaN composition modulation layer is GaN, and the In composition of the well layer is InGaN.
-The In composition of the well layer in the MQW active layer is made equal. The thickness of the well layer used in the simulation was 2 nm,
The thickness of the barrier layer is 4 nm. When a composition modulation layer is included, the number of MQW wells on the horizontal axis in FIG. 8 is obtained by adding the number of wells included in the composition modulation layer to the number of wells of the MQW active layer. The number is the number of wells of the MQW active layer itself.

As shown in FIG. 7, the operating voltage of a GaN-based LD having an InGaN MQW active layer increases as the number of wells increases. Furthermore, the value of the same parameter 20% / 5
%, G including the composition modulation layer indicated by the solid line
The operating voltage of the aN LD increases as the number of MQW wells increases.
It can be seen that the operating voltage is lower than that of the conventional GaN-based LD that does not include the composition modulation layer.

The reason for the lower operating voltage is that InGaN
In the LD of the present invention including the InGaN composition modulation layer between the MQW active layer and the GaN guide layer, the LD shown in FIG. 6 is provided by the presence of the barrier layer of the composition modulation layer at the interface between the active layer and the guide layer. This is because the effective hetero barrier becomes gentle at the interface.

FIG. 8 shows the dependence of the threshold current density on the number of MQW wells. p-GaN guide layer, p-AlGaN
The calculation was performed assuming that the carrier density of each of the cladding layers was 1 × 10 ¹⁶ cm ⁻³ . The other simulation conditions and the meaning of the parameters assigned to each curve are the same as those in FIG.

As shown in FIG. 8, the threshold current density of a GaN-based LD having an MQW active layer also increases as the number of MQW wells increases. However, when the In composition ratio of the well layer / barrier layer is 20% / 5%, the GaN-based LD including the composition modulation layer has a threshold current density of at most 30 as compared with the conventional GaN-based LD not including the composition modulation layer. It can be seen that the percentage also decreases.

Accordingly, a barrier layer having a low In composition is provided at both ends of the InGaN MQW active layer, and the value of the band gap of the barrier layer acting as a composition modulation layer is set to MQW.
It can be seen that if the width is larger than the barrier layer in the active layer, the overflow effect of carriers is suppressed and the threshold current density is greatly reduced.

Comparing the simulation results of the conventional MQW active layer not including the composition modulation layer shown by the broken lines in FIGS. 7 and 8, it can be seen that the operating voltage and operating current at the same number of wells with respect to the change in the In composition ratio. The magnitude relationship with the density is reversed, and it can be seen that in the conventional GaN-based LD, the reduction of the operating voltage and the reduction of the threshold current density were mutually contradictory subjects.

On the other hand, the MQW active layer according to the second embodiment including the composition modulation layer is characterized in that the operating voltage and the threshold current density can be simultaneously reduced. That is, the most practically important problem of operating at a low voltage and a low threshold current density, which could not be solved as a contradictory request in the past, is the MQW activity including the composition modulation layer described in the second embodiment. It turned out that this was achieved only by layers.

Next, a third embodiment of the present invention will be described with reference to FIG. Conventionally, in order to lower the threshold current density, it was necessary to insert an AlGaN cap layer between the active layer and the p-type guide layer. However, the AlGaN cap layer is effective in reducing the threshold current density. However, it is disadvantageous for the purpose of reducing the operating voltage.

The Al _x Ga ₁ -xN cap layer is made of InGaN
The results obtained by simulating the dependence of the operating voltage of the GaN-based LD on the number of MQW wells in the case where the Al composition x of the cap layer is changed while being interposed between the MQW active layer and the p-GaN guide layer. As shown in FIG. The structure of the GaN-based LD used in this simulation is that the active layer is InGaN MQW, and when x> 0, an AlGa1N cap layer is formed between the InGaN MQW active layer 6 and the p-GaN guide layer 7. Is different from the GaN-based LD shown in FIG.

The main part of the LD used in the simulation is an n-GaN contact layer 2 (3 × 10 ¹⁸ cm ⁻³ ,
0.1 μm thick), n-AlGaN cladding layer 4 (1 ×
10 ¹⁸ cm ⁻³ , thickness 0.3 μm), undoped i-G
aN guide layer 5 (thickness 0.1 μm), In _0.15 Ga _0.85
N (2.5 nm) / In _0.05 Ga _0.95 N (5 nm) · M
QW active layer 6, p-type or undoped i-type Al _x Ga _1-x
N cap layer (1 × 10 ¹⁶ cm ^{−3 for} p-type, thickness 20)
nm), p-type or undoped i-type GaN guide layer 7 (p
In the case of a mold, 1 × 10 ¹⁶ cm ⁻³ , thickness 0.1 μm), pA
l _0.15 Ga _0.85 N cladding layer 8 (1 × 10 ¹⁶ cm ⁻³ , thickness 0.3 μm), p-GaN contact layer 9 (1 × 10 ^16
17 cm ^-3 and a thickness of 0.1 μm).

As shown in FIG. 9, when the cap layer is added, the operating voltage increases as compared with the case where no cap layer is provided. It can be seen that when the Al composition x of the cap layer is increased, the operating voltage is increased because the barrier height of the cap layer is increased. Although the difference between the case where the cap layer and the guide layer are conductive and the case where the cap layer and the guide layer are undoped i-type are small, the operation voltage can be slightly lowered when the cap layer and the guide layer are conductive.

GaN-based LD in Third Embodiment
Suppresses an increase in operating voltage by a synergistic effect of the effect of reducing the threshold current density of the composition modulation layer described in the second embodiment and the effect of reducing the threshold current density of the cap layer. GaN-based LD with low power consumption
It is trying to get.

That is, when manufacturing the GaN-based LD including the composition modulation layer of the second embodiment, if the cap layer is further used between the composition modulation layer and the waveguide layer adjacent thereto, the threshold A, which was conventionally required to reduce the current density
Al _x Ga _1-x N having a large composition (0.1 ≦ x ≦ 0.
2) Since it is not necessary to use a cap layer and Al _x Ga ₁ -xN (x ≦ 0.05) having a small Al composition can be used, it is extremely effective in reducing the operating voltage from the simulation result of FIG. You can see that. At this time, as a function of the cap layer, it goes without saying that the threshold current density is further reduced as compared with the case without the cap layer.

Next, a fourth embodiment of the present invention will be described with reference to FIG. The GaN-based LD according to the fourth embodiment combines the function of the strain relaxation layer described in the first embodiment with the function of the composition modulation layer described in the second embodiment. It seeks to demonstrate the benefits of both.

That is, as shown in FIG.
Forming an InGaN composition modulation layer adjacent to both sides of the MQW active layer, and further forming an InGaN / Ga
An N strain relaxation layer is formed, and a GaN waveguide layer is formed adjacent to both ends thereof.

In this way, the strain based on the lattice constant difference between the GaN waveguide layer and the MQW active layer made of InGaN is generated in the MQW active layer by the action of the strain relaxation layer and the composition modulation layer. It can be set to almost zero, and when the strain is relaxed, the change in the defect level and the band gap generated in the strain relaxing layer are all within the range outside the band gap of the well layer of the MQW active layer. Excess absorption can be ruled out.

As a function of the composition modulation layer made of InGaN, in addition to helping to further reduce the strain, the carrier overflow effect can be suppressed, and the threshold current density of the GaN-based LD can be greatly reduced. Needless to say, in order to further reduce the threshold current density without increasing the operating voltage, it is effective to combine the third embodiment with the solid cap layer.

The present invention is not limited to the above embodiment. In the above embodiment, both the strain relaxation layer and the composition modulation layer are formed in the MQW structure, and the strain relaxation layer mainly has the In composition of the well layer smaller than that of the MQW active layer. Mainly I of barrier layer
n composition is made smaller than the barrier layer of the MQW active layer,
The function is achieved by controlling the value of each band gap.

As a function of the composition modulation layer, the composition and thickness of the barrier layer and the well layer were adjusted, and the average composition of these layers was brought closer to the adjacent guide layer and cladding layer. However, MQW or SQW (Single Quantum) with one well
In the well structure, particularly in a region where the thickness of the well layer is extremely small, the effective band gap value of the well layer can be changed not only by the composition of the well layer but also by the thickness of the well layer alone.

Therefore, the same functions as those of the strain relaxation layer and the composition modulation layer can be achieved by changing not only the composition of the layer constituting the MQW but also the thickness. At this time, it is needless to say that the function can be optimized by changing both the thickness and the In composition.

In the above embodiment, the active layer,
As the structure of the strain relaxation layer and composition modulation layer, well layer / barrier layer _{In x Ga 1-x N /} GaN (0 <x ≦ 1) or In
_x Ga _1-x N / In _y Ga _1-y N (0 <x ≦ 1, x>
y, 0 <has been described is a y ≦ 1,0 ≦ x + y ≦ 1) made of _{MQW, In x Al y Ga 1} -xy N
_{/ In z Al w Ga 1-} zw N (0 <x ≦ 1,0 ≦ y ≦
1, 0 ≦ x + y ≦ 1 and x> z, 0 <z ≦ 1, 0 ≦ w ≦
The same function can be obtained by controlling the composition of In also in the case of using MQW composed of 1, 0 ≦ z + w ≦ 1).

The present invention also relates to a GaN having an MQW active layer.
Although the system LD has been described, it is not necessarily limited to the GaN LD. The present invention can be applied to any light emitting device made of a compound semiconductor having an MQW active layer. Further, for example, the strain relaxation layer of the present invention is not limited to being formed adjacent to the MQW active layer.
The same function can be exerted on a conventional GaN-based LED having an active layer having a uniform composition. In addition, various modifications can be made without departing from the spirit of the present invention.

[0085]

As described above, according to the semiconductor light emitting device of the present invention, the lattice mismatch between the active layer and the guide layer or between the active layer and the clad layer is reduced, the distortion at the interface is reduced, and the low voltage is obtained. In addition, a high-quality GaN-based semiconductor light emitting device that operates at a low threshold current density can be manufactured with a high yield. In particular, when the average In composition of the active layer is 10% or more, the semiconductor light emitting device of the present invention exhibits excellent effects.

In a region close to the interface between the active layer and the guide layer, the difference in band gap between the well layer and the barrier layer in the active layer is widened to increase the efficiency of carrier confinement and prevent the carrier overflow effect. As a result, the threshold current density of laser emission can be reduced.

Further, since the difference between the average composition of the active layer and the composition of the guide layer is reduced in a region near the interface, the effective hetero barrier generated at the interface is reduced, and the operating voltage is maintained while suppressing the effect of carrier overflow. Can be reduced. Further, there is an effect of minimizing an increase in operating voltage due to the addition of the cap layer.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a structure of a semiconductor light emitting device according to a first embodiment of the present invention.

FIG. 2 is a band structure diagram of the semiconductor light emitting device according to the first embodiment of the present invention, in which distortion effects of the MQW active layer, the guide layer, and the cladding layer are ignored.

FIG. 3 is a band structure diagram of the semiconductor light emitting device according to the first embodiment of the present invention, taking into account the distortion effect of the MQW active layer, the guide layer, and the cladding layer.

FIG. 4 is a sectional view showing a structure of a semiconductor light emitting device according to a second embodiment of the present invention.

FIG. 5 is a band structure diagram of an MQW active layer, a guide layer, and a clad layer of the semiconductor light emitting device according to the second embodiment of the present invention.

FIG. 6 is a diagram showing a relationship between an average In composition of an MQW active layer and a band structure of a semiconductor light emitting device according to a second embodiment of the present invention.

FIG. 7 is a diagram comparing the dependence of the operating voltage of a semiconductor light emitting device according to a second embodiment of the present invention on the number of MQW wells with a conventional semiconductor light emitting device.

FIG. 8 is a diagram comparing the dependency of the threshold current density of a semiconductor light emitting device according to a second embodiment of the present invention on the number of MQW wells with a conventional semiconductor light emitting device.

FIG. 9 is a diagram showing a change in the number of MQW wells of an operating voltage with respect to an Al composition of an AlGaN cap layer according to a third embodiment of the present invention.

FIG. 10 is a band structure diagram of a region near an MQW active layer of a semiconductor light emitting device according to a fourth embodiment of the present invention.

FIG. 11 is a cross-sectional view of a conventional semiconductor light emitting device having an MQW active layer.

FIG. 12 is a band structure diagram of a region near an MQW active layer of a conventional semiconductor light emitting device.

FIG. 13 shows InN, GaN,
The figure which shows the relationship between the lattice constant and band gap of AlN mixed crystal.

FIG. 14 is a band structure diagram showing the influence of strain in a region adjacent to a GaN guide layer of an MQW active layer in a conventional semiconductor light emitting device.

FIG. 15 is a band structure diagram showing characteristics of an MQW active layer of a conventional semiconductor light emitting device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Sapphire substrate 2 ... n-GaN contact layer 3 ... Ti / Au lower electrode 4 ... n-AlGaN cladding layer 5 ... n-GaN guide layer 6 ... InGaN / GaN, MQW active layer 7 ... p-GaN guide layer 8 ... p-AlGaN cladding layer 9 ... p-GaN contact layer 10 ... SiO ₂ film 11 ... Ni / Au or Pt / Au upper electrode 12, 13 ... InGaN / GaN strain reducing layer 14, 15 ... InGaN composition modulation layer

Claims (5)

[Claims] 1. A semiconductor light emitting device having an active layer composed of a superlattice in which barrier layers and well layers are alternately stacked, wherein the active layer composed of the superlattice is a barrier layer and a well in a central portion of the superlattice. A semiconductor light emitting device characterized in that at least one of a barrier layer and a well layer, which differs in at least one of composition and thickness from a layer, is included in a termination portion. 2. An active layer comprising the superlattice, wherein the active layer is made of In _x A
_{l y Ga 1-xy N (} 0 <x ≦ 1,0 ≦ y ≦ 1,0 ≦ x +
y ≦ 1) and In _z Al _w Ga _{1 -zwN}
(X <z, 0 <z ≦ 1, 0 ≦ w ≦ 1, 0 ≦ z + w ≦ 1)
And a superlattice alternately laminated with well layers composed of at least one of x and z.
The semiconductor light emitting device according to claim 1, wherein: 3. A semiconductor light emitting device having an active layer composed of a superlattice in which a guide layer, a clad layer, and a barrier layer and a well layer are alternately stacked, wherein the active layer composed of the superlattice is the guide layer and the active layer. A region adjacent to any one of the cladding layers includes a stacked region in which the average composition of the well layer and the barrier layer is closer to any one of the composition of the guide layer and the cladding layer as compared with the central portion of the active layer. A semiconductor light emitting device characterized by the above-mentioned. 4. A semiconductor light emitting device having an active layer composed of a superlattice in which a guide layer, a clad layer, and a barrier layer and a well layer are alternately stacked, wherein the active layer composed of the superlattice is the guide layer and the active layer. A semiconductor light emitting device comprising: a region adjacent to any one of the cladding layers, a laminated region in which a difference in band gap between the well layer and the barrier layer is widened as compared with a central portion of the active layer. 5. The method according to claim 1, wherein the active layer having the superlattice structure includes a cap layer between any one of the guide layer and the clad layer. 4. The semiconductor light emitting device according to any one of the above.