JP2002043695A - Light emitting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such a problem that a conventional nitride-based light emitting element has an insufficient service life because an Al included in its active layer applies a large stress to an active layer and the stress produces cracking. SOLUTION: This light emitting element is provided with an n-Al1-x1GAx1N (0.05<x1<0.3) clad layer (film thickness of d1 μm), an active layer, and a p- Al1-x2Gax2N (0.05<x2<0.3) clad layer (film thickness of d2 μm). The film thickness and Ga composition thereof are set to a range of 2<(x1×d1)/(x2×d2)>=10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting device using a nitride semiconductor.

[0002]

2. Description of the Related Art A semiconductor laser device that emits light in the blue to ultraviolet region has been experimentally manufactured from a nitride-based semiconductor material represented by GaN, InN, AlN, and a mixed crystal semiconductor thereof. FIG. 5 shows Japanese-Journal-of-Applied-Physics No. 38 pages L184-L186 (Masaru KURAMOTO et al. J.
pn. J. Appl. Phys. vol. 38 (199
9) pp. L184-L186), wavelength 4
FIG. 3 is a diagram showing a nitride semiconductor laser device oscillating at 05 nm. The present semiconductor laser device is an n-GaN substrate 200
N-Al _0.07 Ga _0.93 N lower cladding layer 201 (thickness 1 μm), n-GaN lower guide layer 202 (thickness 0.1 μm), In _0.2 Ga _0.8 N (thickness 3 nm) / In _0.05 Ga _0.95 N (5 nm thick) -3 quantum well active layer 203, p-Al _0.19 Ga _0.81 N cap layer 204 (20 nm thick), p-GaN upper guide layer 205 (0.1 μm thick) , P-Al _0.07 Ga _0.93 N upper cladding layer 206 (0.5 μm in thickness) and a p-GaN contact layer 208 (0.05 μm in thickness) are sequentially laminated. A p-electrode 209 is formed on the upper side, and an n-electrode 210 is formed below the n-GaN substrate 200. Further, an SiO ₂ film 207 for performing current confinement is formed on the p-Al _0.07 Ga _0.93 N upper cladding layer 206. Further, a mirror end face 211 is formed on a plane parallel to the paper surface by a cleavage method. This laser device has a waveguide structure in which an active layer and a guide layer are sandwiched between cladding layers. Light emitted from the active layer is confined in this waveguide structure, and the mirror end face is subjected to laser resonance. It functions as a mirror and generates a laser oscillation operation.

[0003]

However, when a laser device was manufactured with a conventionally used structure, APC driving (changing the driving current (Iop) so as to keep the light output constant) 5 mW (light output) At a temperature of 50.degree. C., the life of the device was as short as several hours. here,
The life is defined as the time until the Iop at that time increases by 20% with respect to the Iop at the start of the life test at an optical output of 5 mW and 20 ° C. by APC driving. In addition, the life of the 5 mW APC drive shown here is reduced to the life of a low output, 30 m
The life at W, APC drive and 20 ° C. is defined as the life at high output. Normally, 30 mW, 100 m
A life of 00 hours is required.

[0004]

A light emitting device according to the present invention comprises a substrate made of a nitride semiconductor and an Al formed sequentially on the substrate.
_{The first made of 1-x1} Ga _x1 N (0.05 <x1 <0.3)
Cladding layer (film thickness d1 μm), active layer, Al _1-x2 G
a second cladding layer (thickness d2 μm) made of a _x2 N (0.05 <x2 <0.3), wherein the first cladding layer and the second cladding layer have a thickness of , 2 <(x1 × d1) / (x2 × d2) ≦ 10.

Further, the thickness of the second cladding layer is set in a range of 0.2 ≦ d2 ≦ 0.4.

[0006] Note that the nitride semiconductor in the present specification, nitrogen (N) represents the group III-V compound semiconductor consisting mainly of V group _{elements, XN 1-st As s P} t
(0 ≦ s, 0 ≦ t <0.5, 0 ≦ s + t <0.5, X:
(At least one of Al, Ga, In, and Tl)
And a compound semiconductor represented by In this specification, Al _1−x1 Ga _x1 N (0 <x1 <1) or Al
_1-x2 Ga _x2 N (0 <x2 <1) includes the case where a small amount (3% or less of the number of atoms) of another element which does not affect the effect of the present invention is added.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In general, as a method for growing a nitride semiconductor crystal, a metal organic chemical vapor deposition (hereinafter referred to as MOC) method is used.
VD method), molecular beam epitaxy method (MBE method), and hydride vapor phase epitaxy method (HVPE method), and any crystal growth method may be used. Hereinafter, examples of a nitride semiconductor laser and a nitride semiconductor light emitting diode manufactured using a GaN substrate as a substrate and using a MOCVD method as a growth method will be described. The substrate may be a substrate which is composed of a nitride semiconductor, Al _x G
a _y In _z N _1-xyz substrate may be used. In particular, in the case of a nitride semiconductor laser, in order to make the vertical and transverse modes unimodal,
A layer having a lower refractive index than the clad layer needs to be in contact with the outside of the clad layer, and it is best to use an AlGaN substrate.

In the present invention, the thicknesses of the n-AlGaN cladding layer and the p-AlGaN cladding layer are indicated by d1 and d2, respectively, and the Al mixed crystal ratio is indicated by y1 and y2, respectively. Here, each clad layer has a multilayer structure (super lattice structure)
The calculation method of d1, d2, y1, and y2 in the case of is described. For example, consider a clad layer having a multilayer structure as shown in FIG. In FIG. 3, reference numeral 401 denotes an n-GaN contact layer, and from 402 to 407, 402 denotes n-GaN.
-Al _0.1 Ga _0.9 N, 403 is n-GaN, 404 is n
It is an n-type clad layer having a multilayer structure alternately laminated with -Al _0.1 Ga _0.9 N. 408 is an n-GaN guide layer. In this case, d1 is the thickness of the cladding layer. Even when the thickness of each layer is different, d1 = d
2 + d3 + d4 +... + D7. Here, an n-type cladding layer is shown as an example, but the same applies to a p-type cladding layer. The film thickness is measured using a means such as a TEM (transmission electron microscope).

Further, when the clad layer has a multilayer structure, the average mixed crystal ratio is obtained as a weighted average as follows. In the case of a multilayer structure as shown in FIG. 3, the average mixed crystal ratio y is represented by y = (y1 × d1 + y2 × d2 +...) / (D1 + d2 +...) (1)

[0010] Mixed crystal of each of the p and n cladding layers described in the claims.
The ratios x1 and x2 are as follows when the cladding layer has a multilayer structure.
The average mixed crystal ratio y shown in the equation (1) is obtained. (Embodiment 1) In Embodiment 1, a nitride semiconductor
A method for forming a photodiode element will be described. FIG.
Indicates a nitride semiconductor light emitting diode structure, and C represents
Surface (0001 surface) n-type GaN substrate 100, n-type GaN
Buffer layer 101, n-type Al_x1Ga _1-x1N clad layer 1
02, active layer 103, p-type Al_x2Ga_1-x2N cladding layer
104, p-type GaN contact layer 105, n-type electrode 10
6, p-type electrode 107, SiO _TwoConsists of 108
You. The method of manufacturing the nitride semiconductor light emitting diode shown in FIG.
The method will be described.

First, a seed substrate (for example, surf
Layer of GaN on the substrate
Peel off the sapphire substrate with thickness 400μm, size
2 inch φ C-plane (0001) n-type GaN substrate 100
Was prepared. The n-type polarity of the n-type GaN substrate is
And the concentration of the Si is 2
× 10¹⁸cm^-3Met. Further, the n-type GaN substrate
About 8 × 10 in¹⁶cm ^-3Of chlorine (Cl)
ing. Next, the n-type GaN substrate was placed in a MOCVD apparatus.
100, and n-type GaN at a growth temperature of 1050 ° C.
The buffer layer 101 was formed to 1 μm. This n-type GaN substrate
The buffer layer is used to peel the n-type GaN substrate from the seed substrate.
Of surface distortion of n-type GaN substrate caused by
This layer is provided for the purpose of improving logistics and surface irregularities (flattening).
It doesn't matter. n-type GaN buffer layer 101
After the formation, the n-type Al_x1Ga_1-x1N layer
102 was formed. At this time, x1 was 0.1.

Next, the temperature of the substrate is lowered to about 700 ° C. to 800 ° C., and an InGaN well layer having a thickness of 2 nm and a
An active layer (multiple quantum well layer) 103 in which three InGaN barrier layers are stacked three times is grown. At that time, SiH ₄ may or may not be supplied. Note that SiH ₄ is
The supply was continued up to the step of forming the n-type Al _x1 Ga _1-x1 N layer 102.

Next, the substrate temperature is raised again to 1050 ° C., and a 0.4 μm thick p-type Al _x2 Ga _{1 -x2N} layer 104 is formed.
Grow. At this time, x2 = 0.1. afterwards,
A 0.1 μm-thick p-type GaN contact layer 105 was grown. The active layer 103 of the present embodiment has a multi-quantum well structure having three periods, but may have another periodic structure or a single quantum well structure having only a well layer. The active layer may be composed of In _x1 Ga _1-x1 N, and the In composition or the thickness of the well layer may be changed according to a desired emission wavelength.

Usually, when the active layer is composed of multiple quantum wells and the emission wavelength is 370 nm or longer, the well layer is formed of In.
The barrier layer is composed of at least GaN
Or it must be a nitride semiconductor containing Al.
At least either the well layer or the barrier layer must be doped with a polar impurity. The polar impurities of the well layer or the barrier layer in the active layer are:
Any of Si, Ge, O, C, Zn, Be, and Mg is preferable.

It is preferable that the p-type impurity concentration of the p-type GaN contact layer 105 is increased toward the position where the p-type electrode 107 is formed.

This reduces the contact resistance due to the formation of the p-type electrode. Further, in order to remove residual hydrogen in the p-layer which prevents activation of Mg which is a p-type impurity, a trace amount of oxygen may be mixed during growth of the p-type layer. Further, the layer thickness of the p-type GaN contact layer 105 is 0.1 μm.
The thickness of the p-type contact layer does not affect the life of the device even if the thickness is larger than 0.1 μm.
Light leaks into the contact layer, causing ripples in the radiation pattern and disturbing. Therefore, the thickness of the p-type contact layer is 0.1 μm.
m or less is good. After the growth of the p-type GaN contact layer 105 in this manner, the temperature in the reactor of the MOCVD apparatus was decreased at 60 ° C./min by changing to a total nitrogen carrier gas and NH ₃ . When the substrate temperature reaches 850 ° C., NH 3
The supply of ₃ was stopped, and after waiting at the substrate temperature for 5 minutes, the temperature was lowered to room temperature. Holding temperature of the above substrate ぱ 65
The temperature was preferably between 0 ° C. and 900 ° C., and the waiting time was preferably 3 minutes or more and 15 minutes or less. Further, the reaching speed of the temperature drop is preferably 30 ° C./min or more. As a result of evaluating the thus-grown film by Raman measurement, it was found that the growth film had already exhibited p-type characteristics after growth without performing the conventionally used p-type annealing. Further, the contact resistance due to the formation of the p-type electrode was also reduced. SIMS
(Secondion mass spector)
As a result of the measurement, the residual hydrogen concentration was 3 × 10 ¹⁸ cm ⁻³ or less near the outermost surface of the p-type GaN contact layer 105. According to experiments by the inventors, after forming a grown film, N
When the substrate temperature was lowered to room temperature in an H ₃ atmosphere, the residual hydrogen concentration near the outermost surface of the grown film was higher than the NH ₃ concentration after the growth was completed because the residual hydrogen concentration was higher near the outermost surface of the grown film.
Atmosphere may be the cause. This residual hydrogen is p
It is known that it prevents activation of Mg, which is a type impurity. The residual hydrogen concentration is preferably 1 × 10 ¹⁹ cm ⁻³ or less.

After the growth of the p-type GaN contact layer 105, the carrier gas is replaced with N ₂ , the supply of NH ₃ is stopped, and the growth temperature is maintained for a predetermined time.
The p-type was promoted, the residual hydrogen concentration near the outermost surface of the grown film was reduced, and the contact resistance was reduced. Also, as a method of further reducing the contact resistance due to the formation of the p-type electrode, it is preferable to remove the vicinity of the outermost surface of the growth film (the outermost surface of the p-type layer) by etching and form the p-type electrode on the removed surface. The layer thickness for removing the outermost surface of the grown film (the outermost surface of the p-type layer) is 10 n
m is preferable, and there is no particular upper limit. However, it is preferable that the residual hydrogen concentration in the vicinity of the removal surface be 1 × 10 ¹⁹ cm ⁻³ or less.

Next, SiO ₂ 108 is deposited on the p-type GaN contact layer 105. After that, SiO ₂ is removed as a stripe having a width of 3 μm by photolithography and etching. Next, as shown in FIG.
And on _{SiO 2 108, Pd (10nm)} / Mo (10
After patterning the p-type electrode 107 by lithography in the order of nm) / Au (150 nm), annealing was performed in an N ₂ atmosphere while introducing a small amount of oxygen. As a result, the resistance of the contact resistance was reduced by the formation of the p-type electrode.

Next, a description will be given of chip division of an epiwafer on which the above-described nitride semiconductor light emitting diode element is formed. The process procedure until chip division is shown. First, the GaN substrate side of the epi-wafer is ground by a grinder,
The thickness of the GaN substrate doped with chlorine is set to 150 μm. Next, on the polished GaN substrate side, Ti (30 n
m) / Al (200 nm) n-type electrode is formed by vapor deposition on the entire GaN substrate surface. Here, a scribe line is formed by scribing with a diamond stylus on the epi film side (the surface opposite to the GaN substrate surface) of the epi wafer. The scribing direction is <1-100> for the nitride semiconductor. The scribing portion is about 1-2 mm from the periphery of the substrate. The element is divided by the next braking. In this breaking, a breaking blade is applied from the GaN substrate surface side (opposite to the scribed surface) to break the wafer so as to match the scribe line previously inserted. In this manner, the elements can be divided into bars. Next, this bar is changed to <11-20>
In the direction, a scribe line is made on the epi film side at about 1-2 mm from the periphery. Furthermore, a breaking blade is applied from the GaN substrate side (the surface opposite to the scribed surface) so as to coincide with the scribe line inserted first, and the chip is divided into one chip unit by breaking.

The LD chip thus obtained is mounted on a stem using In, and is driven at 50 ° C. by APC.
Life test at 5 mW and 30 mW Figure 2
The result was as shown in the figure. The emission wavelength of each element is 400
nm. A 20-sample life test was performed for each point, and 10 samples were subjected to an APC-driven high-output life test at 30 mW, and the remaining 10 samples were subjected to a 5-mW, APC-driven low-output test. The symbol shown in FIG.
W, and a life test of APC drive of 5 mW of 90% or more had a life of 10,000 hours or more.
A sample having a life of at least 10,000 hours had a life of at least 10%, and an X indicates a lot that did not satisfy the above conditions in any of the life tests of 30 mW and 5 mW. The experimental result shown in FIG. 2 this time is a case where x1 = x2. As shown in FIG. 2, the thickness of the p-cladding layer is not less than 0.2 μm and not more than 0.4 μm, and (x1 × n-cladding layer thickness) / (x2
× p cladding layer thickness) should be greater than 2.0 and 10 or less, and preferably (x1 × n cladding layer thickness) / (x2 × p cladding layer thickness) should be 2.5 or more and 9 or less. I found it.

When the thickness of the p-cladding layer is smaller than 0.2 μm, effective carrier confinement and light confinement cannot be performed, and laser oscillation does not occur. 0.4μ
When it is more than m, many dislocations are generated in the p-cladding layer due to the influence of the strain, and the dislocations multiply during the energization and reach the active layer to shorten the life of the laser.

Also, (x1 × n clad layer thickness) / (x2
(Xp clad layer thickness) of 2.0 or less, an extremely large compressive strain is applied to the active layer sandwiched between the two clad layers. Due to excessive strain applied to the active layer, threading dislocations multiply and the life of the device is shortened.
If (x1 × n clad layer thickness) / (x2 × p clad layer thickness) is larger than 10, the strain applied to the n clad itself increases, causing cracks and significantly shortening the element life. (Embodiment 2) Growth procedure, composition of film to be grown, film thickness,
The process such as the temperature was the same as that of the first embodiment, and the experiment was performed by changing only the Al composition of the n-cladding layer and the p-cladding layer. In this embodiment, a 2.0 μm thick n-type A
x1 = 0.15 in the l _x1 Ga _1-x1 N layer;
X2 = 0.1 for a 3 μm thick p-type Al _x2 Ga _1-x2 N layer
It was 5.

Under the same conditions as in Embodiment 1, 30 mW,
A life test was performed in an APC drive of 5 mW. Even when the composition of Al is changed, the condition that 90% or more of the elements have a life of 10,000 hours or more is that the thickness of the p-cladding layer is 0.2 μm to 0.4 μm and (x1 × n
It is necessary that (cladding layer thickness) / (x2 × p cladding layer thickness) is larger than 2.0 and 10 or less, and preferably (x1 × n cladding layer thickness) / (x2 × p cladding layer thickness).
Is required to be 2.5 or more. (Embodiment 3) Growth procedure, composition of film to be grown, film thickness,
The process such as the temperature was the same as that of the first embodiment, and the experiment was performed by changing only the Al composition of the n-cladding and the p-cladding. In this embodiment, the n-type Al _{x1 having a} thickness of 0.7 μm is used.
X1 = 0.09 in the Ga _1-x1 N layer, and 0.25
x2 = 0.07 in the p-type Al _x2 Ga _1-x2 N layer having a thickness of μm.

Under the same conditions as in Embodiment 1, 30 mW,
A life test was performed in an APC drive of 5 mW. FIG. 4 shows the experimental results. Even when the Al composition is changed between the n-cladding layer and the p-cladding layer, the condition that 90% or more of the devices have a lifetime of 10,000 hours or more is that the p-cladding layer has a thickness of 0.2 μm or more. 4 μm or less, (x1 × n
It is necessary that (cladding layer thickness) / (x2 × p cladding layer thickness) is larger than 2.0 and 10 or less, and preferably (x1 × n cladding layer thickness) / (x2 × p cladding layer thickness).
Is required to be 2.5 or more. (Embodiment 4) Growth procedure, composition of film to be grown, film thickness,
The processes such as the temperature are the same as those in the first embodiment, and three periods, a 2 nm-thick InGaN well layer and a 4 nm-thick InGaN well layer are used.
Active layer composed of GaN barrier layer (multiple quantum well layer)
As is added only to the 103 InGaN well layer. At this time, the emission wavelength of the device was 400 nm.

Under the same conditions as in Embodiment 1, 30 mW,
A life test was performed in an APC drive of 5 mW. Even when As is added to the well layer of the active layer, the condition that 90% or more of the elements have a lifetime of 10,000 hours or more is that the p-cladding layer has a thickness of 0.2 μm to 0.4 μm.
(X1 × n clad layer thickness) / (x2 × p clad layer thickness)
Must be greater than 2.0 and 10 or less;
Preferably, (x1 × n clad layer thickness) / (x2 × p clad layer thickness) needs to be 2.5 or more. (Embodiment 5) Growth procedure, composition of film to be grown, film thickness,
The processes such as the temperature are the same as those in the first embodiment, and three periods, a 2 nm-thick InGaN well layer and a 4 nm-thick InGaN well layer are used.
Active layer composed of GaN barrier layer (multiple quantum well layer)
As is added to both the InGaN well layer 103 and the InGaN barrier layer 103. At this time, the emission wavelength was 400 nm.

Under the same conditions as in Embodiment 1, 30 mW,
A life test was performed in an APC drive of 5 mW. Even when As is added to the well layer and the barrier layer of the active layer, 9
The condition that 0% or more of the elements have a life of 10,000 hours or more is that the thickness of the p-cladding layer is 0.2 μm or more and 0.4 μm or less, and (x1 × n-cladding layer thickness) / (x2 × p-cladding layer thickness). Must be greater than 2.0 and 10 or less, and preferably (x1 × n cladding layer thickness) / (x2 ×
(p-cladding layer thickness) of 2.5 or more was found to be necessary. (Embodiment 6) Growth procedure, composition of film to be grown, film thickness,
The processes such as the temperature are the same as those in the first embodiment, and three periods, a 2 nm-thick InGaN well layer and a 4 nm-thick InGaN well layer are used.
Active layer composed of GaN barrier layer (multiple quantum well layer)
P is added only to the 103 InGaN well layer. At this time, the emission wavelength of the device was 400 nm.

Under the same conditions as in Embodiment 1, 30 mW,
A life test was performed in an APC drive of 5 mW. Even when P is added to the well layer of the active layer, the condition that 90% or more of the elements have a lifetime of 10,000 hours or more is that the p-cladding layer has a thickness of 0.2 μm or more and 0.4 μm or less. x
It is necessary that (1 × n cladding layer thickness) / (x2 × p cladding layer thickness) is greater than 2.0 and 10 or less, and preferably (x1 × n cladding layer thickness) / (x2 × p cladding layer thickness). ) Was required to be 2.5 or more. (Embodiment 7) Growth procedure, composition of film to be grown, film thickness,
The processes such as the temperature are the same as those in the first embodiment, and three periods, a 2 nm-thick InGaN well layer and a 4 nm-thick InGaN well layer are used.
Active layer composed of GaN barrier layer (multiple quantum well layer)
P is added to both the InGaN well layer 103 and the InGaN barrier layer 103. At this time, the emission wavelength of the device was 400 nm.
Met.

Under the same conditions as in Embodiment 1, 30 mW,
A life test was performed in an APC drive of 5 mW. Even when P is added to the well layer and the barrier layer of the active layer, 90
% Of the device has a lifetime of 10,000 hours or more, when the thickness of the p-cladding layer is 0.2 μm or more and 0.4 μm or more.
In the following, (x1 × n cladding layer thickness) / (x2 × p cladding layer thickness) needs to be greater than 2.0 and 10 or less, and preferably (x1 × n cladding layer thickness) / (x2
× p cladding layer thickness) of 2.5 or more was required.

In Embodiment 1-8 described above, the emission wavelength of the device was 400 nm, but it was 370 to 370.
When the life test was performed in the range of 440 nm, the same result as in the first embodiment was obtained. In the above embodiment, L
Although the description has been made with respect to D, the same result as that of Embodiment 1 was obtained with respect to the light output intensity of the LED.

[0030]

According to the present invention, a substrate made of a nitride semiconductor and Al1-x1Gax1N (0.05 <x
1 <0.3) first cladding layer (film thickness d1 μm)
, An active layer, and Al1-x2Gax2N (0.05 <x
2 <0.3) second cladding layer (film thickness d2 μm)
Wherein the first cladding layer and the second cladding layer have a thickness of 2 <(x1 × d1) / (x2
× d2) ≦ 10: 25, and by controlling the distortion and the like applied to the device, the light emitting device of the present invention has a 3
In the life tests of the APC drive of 0 mW and 5 mW, 90% or more of the samples could maintain a long life of 10,000 hours or more.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a structure of a light emitting device of the present invention.

FIG. 2 is a graph of a life test result.

FIG. 3 is a schematic diagram of an element structure.

FIG. 4 is a graph of a life test result.

FIG. 5 is a cross-sectional view illustrating a structure of a light emitting element of a conventional example.

[Explanation of symbols]

Reference Signs List 100 n-type GaN substrate 101 n-type GaN buffer layer 102 n-type Al _x1 Ga _1-x1 N clad layer 103 active layer 104 p-type Al _x2 Ga _1-x2 N clad layer 105 p-type GaN contact layer 106 n-type electrode 107 p Type electrode 108 SiO ₂ 401 n-GaN contact layer 402, 404, 406, n-Al _0.1 Ga _0.9 N 403, 405, 407 n-GaN 408 n-GaN guide layer

Claims (2)

[Claims] 1. A substrate comprising a nitride semiconductor, and a substrate
Al formed sequentially _1-x1Ga_x1N (0.05 <x1 <
0.3) (thickness d1 μm),
Active layer and Al_1-x2Ga_x2N (0.05 <x2 <0.
3) a second cladding layer (film thickness d2 μm)
Light-emitting device, comprising: the first cladding layer and the second cladding layer.
A light emitting device, wherein the thickness of the lad layer is set in a range of 2 <(x1 × d1) / (x2 × d2) ≦ 10. 2. The light emitting device according to claim 1, wherein the thickness of the second cladding layer is set in a range of 0.2 ≦ d2 ≦ 0.4.