JPH08330630A - Light-emitting nitride semiconductor element - Google Patents

Abstract

PURPOSE: To obtain a light-emitting nitride semiconductor element in which the crystallinity of a first p-type clad layer is enhanced by a method wherein a second clad layer composed of a nitride semiconductor containing GaN or In is grown between an active layer and the first p-type clad layer. CONSTITUTION: A buffer layer 2 composed of GaN is grown on the C-plane of a sapphire substrate 1, and an n-type contact layer 3 composed of Si-doped n-type GaN is then grown. Then, an n-type clad layer 4 composed of Si-doped n-type Al0.3 Ga0.7 N is grown. Then, an active layer 5 composed of Si+Zn-doped In0.05 Ga0.95 N is grown, and, in succession, a second p-type clad layer 60 composed of Mg-doped In0.1 Ga0.9 N is grown. Then, a first p-type clad layer 6 composed of Mg-doped p-type Al0.3 Ga0.7 N is grown. The second p-type clad layer on the active layer acts as a buffer layer, and the crystallinity of the first p-type clad layer can be enhanced.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a light emitting diode (LE
D), nitride semiconductors (In _a Al _b Ga _1-ab N, 0 ≦ a, 0 ≦ b, a + b used for laser diode (LD), etc.
The present invention relates to a light emitting device having ≦ 1), and particularly to a nitride semiconductor light emitting device having a double hetero structure.

[0002]

2. Description of the Related Art Nitride semiconductors (In _a Al _b Ga) have been used as materials for light emitting devices such as LEDs and LDs that emit ultraviolet to red light.
_1-ab N, 0 ≦ a, 0 ≦ b, a + b ≦ 1) are known.
Using this semiconductor material, we announced a blue LED with a luminous intensity of 1 cd in November 1993, a blue-green LED with a luminous intensity of 2 cd in April 1994, and a blue LED with a luminous intensity of 2 cd in October 1994. Announced. All of these LEDs have been commercialized and are now in practical use for displays, signals and the like.

FIG. 2 shows a conventional blue color composed of a nitride semiconductor,
The structure of the light emitting chip of a blue-green LED is shown. Basically,
On the substrate 21, a buffer layer 22 made of GaN, an n-type contact layer 23 made of n-type GaN, and an n-type AlGa
An n-type clad layer 24 made of N, an active layer 25 made of n-type InGaN, a p-type clad layer 26 made of p-type AlGaN, and a p-type contact layer 27 made of p-type GaN.
And has a structure in which they are sequentially stacked. N of the active layer 25
Type InGaN has donor impurities such as Si and Ge, and /
Alternatively, the LED element is doped with an acceptor impurity such as Zn or Mg, and the emission wavelength of the LED element is InG of the active layer.
It is possible to change from ultraviolet to red by changing the In composition ratio of aN or changing the type of impurities to be doped in the active layer. At present, an LED having an emission wavelength of 510 nm or less, in which an active layer is simultaneously doped with a donor impurity and an acceptor impurity, has been put into practical use.

[0004]

The conventional LED has a forward current of 20 mA and a light emission output of about 3 mW, which is more than 20 times the output of an LED made of SiC.
However, this LED has a drawback that it has a low electrostatic withstand voltage, and when it is biased in the reverse direction and measured, it has only about 50 to 100 V. If the electrostatic withstand voltage is low, handling the LED in a dry atmosphere easily destroys the element due to static electricity, resulting in poor reliability.

Therefore, the present invention has been made in view of such circumstances, and an object of the present invention is to realize a nitride semiconductor light emitting device having a large electrostatic breakdown voltage, and to realize a nitride semiconductor light emitting device. It is to improve reliability.

[0006]

As a result of various experiments conducted on a conventional nitride semiconductor light emitting device having a double hetero structure, many of the causes are found in the p-type cladding layer grown next to the active layer. That is, the present invention has been accomplished. That is, the nitride semiconductor light emitting device of the present invention has a second p-type nitride semiconductor containing at least indium between the active layer and the p-type clad layer (hereinafter referred to as the first p-type clad layer). The p-type clad layer is formed.

FIG. 1 is a schematic sectional view showing the structure of a light emitting device according to an embodiment of the present invention. This light emitting device comprises a substrate 1, a buffer layer 2, an n-type contact layer 3, an n-type clad layer 4, an active layer 5, a second p-type clad layer 60, and a first p-type layer.
This shows a structure in which the type cladding layer 6 and the p-type contact layer 7 are sequentially stacked.

The substrate 1 includes sapphire (including A-plane, C-plane and R-plane), SiC (including 6H and 4H), ZnO,
A substrate having a lattice mismatch with a nitride semiconductor such as Si or GaAs, or a substrate having a lattice constant close to that of a nitride semiconductor made of an oxide single crystal such as NGO (neodymium gallate) can be used.

The buffer layer 2 is composed of GaN, AlN, GaAl.
It is preferable to grow N or the like in a film thickness of 50 angstrom to 0.1 μm, for example, and it can be formed by growing at a low temperature of 400 ° C. to 600 ° C. according to the MOVPE method.

The n-type contact layer 3 is a layer for forming the negative electrode 8. It is preferable to grow GaN, AlGaN, InAlGaN or the like to have a film thickness of, for example, 1 μm to 10 μm. Among them, by selecting GaN, the negative electrode is formed. A favorable ohmic contact with the material can be obtained. As the material of the negative electrode 8, for example, Al, Au, Ti or the like can be preferably used.

The n-type cladding layer 4 is composed of GaN, AlGaN,
InAlGaN or the like is, for example, 500 Å
It is preferable to grow the film with a thickness of 0.5 μm, and by selecting GaN or AlGaN among them, a layer with good crystallinity can be obtained. It is also possible to omit either the n-type cladding layer 4 or the n-type contact layer 3. If either is omitted, the remaining layer acts as an n-type cladding layer and an n-type contact layer.

The active layer 5 is made of InGaN, InAlGaN, A whose band gap energy is smaller than that of the cladding layer.
Any nitride semiconductor such as lGaN may be used, and in particular, InGaN in which the composition ratio of indium is appropriately changed according to a desired band gap is preferable. Further, the active layer 5 may have a multi-quantum well structure in which thin films are laminated by combining InGaN / GaN, InGaN / InGaN (having different compositions), for example. In the active layer having either a single quantum well structure or a multiple quantum well structure, the active layer may be either n-type or p-type. Luminescence or quantum well level luminescence is obtained, and LED
It is particularly preferable for realizing a device and an LD device. When the active layer has a single quantum well (SQW) structure or a multi-quantum well (MQW: multiquantum well) structure, a light emitting device having a very high output can be obtained. SQ
W and MQW refer to the structure of the active layer in which light emission between quantum levels by non-doped InGaN is obtained, and for example, SQW.
Then, the active layer is composed of a single composition of In _X Ga _1-X N (0 ≦ X <1)
By making the film thickness of In _X Ga _1-X N 100 angstroms or less, more preferably 70 angstroms or less, strong light emission between quantum levels can be obtained. MQW is In _X Ga _{1 -X} N with different composition ratios.
A multilayer film is formed by laminating a plurality of thin films (including X = 0 and X = 1 in this case). Thus, by making the active layer SQW and MQW, light emission between quantum levels is about 365 nm to 660.
Emission up to nm is obtained. As described above, the thickness of the quantum well layer is preferably 70 Å or less. In the multiple quantum well structure, it is desirable that the well layer is made of In _X Ga _{1 -X} N and the barrier layer is also made of In _Y Ga _{1 -Y} N (Y <X, including Y = 0 in this case). Particularly preferably, when the well layer and the barrier layer are formed of InGaN, they can be grown at the same temperature, so that an active layer having good crystallinity can be obtained. When the thickness of the barrier layer is 150 angstroms or less, more preferably 120 angstroms or less, a high-power light emitting device can be obtained. Further, the active layer 5 may be doped with a donor impurity and / or an acceptor impurity. If the crystallinity of the impurity-doped active layer is the same as that of the non-doped one, doping with the donor impurity can further increase the interband emission intensity as compared with the non-doped one. Doping with an acceptor impurity can bring the peak wavelength to a low energy side of about 0.5 eV lower than the peak wavelength of band-to-band emission, but the full width at half maximum becomes wider.
When the acceptor impurity and the donor impurity are doped at the same time, the emission intensity of the active layer doped with only the acceptor impurity can be further increased. Particularly when realizing an active layer doped with an acceptor impurity, it is preferable that the conductivity type of the active layer be n-type by simultaneously doping with a donor impurity such as Si. The active layer 5 can be grown to have a film thickness of, for example, several angstroms to 0.5 μm. However, when the active layer is SQW or MQW, n
It is desirable to form a second n-type clad layer made of n-type nitride semiconductor containing In or n-type GaN between the type clad layer 4 and the active layer 5.

Next, the second p-type cladding layer 60, which is the most characteristic feature of the present invention, is a p-type nitride semiconductor containing at least indium (In _X Al _Y Ga _1-XY N, 0 <X, Y ≦ 0, X +
It must be formed of Y <1) or p-type GaN. Among them, it is particularly preferable to use InGaN or a nitride semiconductor containing no Al such as GaN. Furthermore, it is preferable that the second p-type cladding layer 60 is formed to have a film thickness of 200 angstroms or less, and more preferably 100 angstroms or less. This is because by adjusting the film thickness to 200 angstroms or less, it becomes possible to increase the electrostatic breakdown voltage of the light emitting element while almost maintaining the light emission output of the light emitting element. On the other hand, if the film thickness is thicker than 200 Å, the output of the light emitting element tends to decrease.

The first p-type cladding layer 6 is GaN or AlG.
It is preferable to grow aN, InAlGaN, or the like to a film thickness of, for example, 500 angstrom to 0.5 μm, and by selecting GaN or AlGaN among them, a layer with good crystallinity can be obtained. In addition, the second p-type cladding layer 6
When the composition of 0 and the configuration of the first p-type cladding layer 6 are the same, the composition ratio of the first p-type cladding layer 6 is changed to
The bandgap energy is transferred to the second p-type cladding layer 60.
Same as or larger than.

The p-type contact layer 7 is a layer forming the positive electrode 9, and is, for example, GaN, AlGaN, InAlGaN.
Etc. are preferably grown, and among them, by selecting GaN, preferable ohmic contact with the material of the positive electrode can be obtained. Ni, Au as the positive electrode material
Etc. can be preferably used. Further, either the p-type contact layer 7 or the first p-type cladding layer 6 can be omitted. If either is omitted, the remaining layers act as the first p-type cladding layer and the p-type contact layer.

The light emitting device of the present invention is, for example, MOVPE (metal organic chemical vapor deposition), MBE (molecular beam vapor deposition), H
In _a Al _b Ga _{1 -ab} N (0 ≦ a, 0 ≦ is formed on the substrate by using a vapor phase growth method such as DVPE (hydride vapor phase growth method).
It is obtained by stacking b, a + b ≦ 1) with conductivity types such as n-type and p-type. Although the n-type nitride semiconductor can be obtained in a non-doped state, it can be obtained by introducing a donor impurity such as Si, Ge, and S into the semiconductor layer during crystal growth. The carrier concentration of the n-type layer can be adjusted by adjusting the concentration of these donor impurities. On the other hand, in the p-type nitride semiconductor layer, acceptor impurities such as Mg, Zn, Cd, Ca, Be, and C should be introduced into the semiconductor layer during crystal growth, or after the introduction, annealing should be performed at 400 ° C. or higher. Is obtained by Similarly, the carrier concentration of the p-type layer can be adjusted by adjusting the acceptor impurity concentration. The buffer layer 2 is provided to alleviate the lattice mismatch between the substrate 1 and the nitride semiconductor, but a substrate having a lattice constant close to that of the nitride semiconductor such as SiC or ZnO, or a substrate lattice-matched to the nitride semiconductor is used. In doing so, the buffer layer may not be formed.

[0017]

In the conventional LED, the first p-type clad layer containing Al is grown on the active layer containing In, for example.
On the other hand, in the present invention, a second p-type clad layer made of a nitride semiconductor containing GaN or In is newly grown between the active layer and the first p-type clad layer. With this configuration, the electrostatic breakdown voltage of the light emitting element can be improved. This is because the second p-type clad layer on the active layer acts as a buffer layer to improve the crystallinity of the first p-type clad layer and improve the electrostatic breakdown voltage of the device. Nitride semiconductors have the highest band gap energy, AlN>GaN> In
The crystal itself has a soft property in the order of N. In other words, the crystal itself of the second p-type clad layer made of a nitride semiconductor containing In or GaN is softer than that of the first p-type clad layer whose bandgap energy is larger than that of the second p-type clad layer. . The second p-type clad layer, which is a soft crystal, acts as a buffer layer, so that the crystallinity of the first p-type clad layer grown on the second p-type clad layer is improved and the lattice Since the number of defects is reduced, the electrostatic breakdown voltage of the entire device is improved.

A second p that preferably acts as a buffer layer.
The film thickness of the mold cladding layer is preferably 200 angstroms or less. The electrostatic breakdown voltage tends to improve as the thickness of the second p-type clad layer increases, but if the thickness is too large, many crystal defects occur in the second p-type clad layer itself, resulting in a buffer layer. It tends to be hard to work. When the first p-type clad layer is grown on the second p-type clad layer having many crystal defects, the crystal defects are transmitted to the first p-type clad layer. It becomes difficult for the p-type cladding layer to grow. Therefore, if the thickness of the second p-type cladding layer is too large, the output of the light emitting device tends to decrease. The lower limit of the film thickness of the second p-type cladding layer is not particularly limited, and it may be formed to have a film thickness of several angstroms, which corresponds to, for example, one atomic layer or two atomic layers.

[0019]

EXAMPLES The present invention will be described below based on specific examples. The following example shows a growth method by the MOVPE method.

[First Embodiment] A first embodiment will be described with reference to FIG. First, TMG (trimethylgallium) and NH
With ₃ and the buffer layer 2 of GaN at 500 ° C. in the C-plane of the sapphire substrate 1 was set in the reaction vessel 500
It is grown to a film thickness of angstrom.

Next, the temperature is raised to 1050 ° C. and TMG,
Si-doped n-type GaN using silane gas in addition to NH _3.
The n-type contact layer 23 is made to have a thickness of 4 μm.

Subsequently, TMA (trimethylaluminum) was added to the source gas, and the n-type cladding layer 4 consisting of Si-doped n-type Al0.3Ga0.7N layer was also formed at 0.150 ° C.
Grow with a film thickness of μm.

Next, the temperature is lowered to 800 ° C. and TMG, TM
I (trimethylindium), NH ₃, Silane gas, D
Si + Zn-doped n-type using EZ (diethyl zinc)
The active layer 5 made of In0.05Ga0.95N has a thickness of 0.1 μm.
Grow thick.

Subsequently, at 800 ° C., a second p-type clad made of Mg-doped p-type In0.01Ga0.99N is used using TMG, TMI (trimethylindium), NH ₃ and Cp 2 Mg (cyclopentadienyl magnesium) gas. Layer 60
To 50 angstroms.

Next, the temperature is raised to 1050 ° C. and TMG, T
Using MA, NH ₃ , and Cp2Mg (cyclopentadienylmagnesium), a first p-type cladding layer 6 made of Mg-doped p-type Al0.3Ga0.7N is grown to a thickness of 0.1 μm.

Then, at 1050 ° C., TMG, NH ₃ , Cp 2
Using Mg, a p-type contact layer 7 made of Mg-doped p-type GaN is grown to a film thickness of 0.5 μm.

After the reaction is completed, the temperature is lowered to room temperature, the wafer is taken out of the reaction container, and the wafer is annealed at 700 ° C. to further reduce the resistance of the p-type layer. Next, a mask having a predetermined shape is formed on the surface of the uppermost p-type contact layer 7, and etching is performed until the surface of the n-type contact layer 3 is exposed. After etching, the negative electrode 8 made of Ti and Al and the p-type contact layer 7 are formed on the surface of the n-type contact layer 3.
A positive electrode 9 made of Ni and Au is formed on the surface of. After the electrodes were formed, the wafer was separated into chips of 350 μm square to obtain LED elements. This LED element is If20mA
Vf3.6V, emission peak wavelength 450nm, half-width 7
It showed blue emission of 0 nm, and the emission output was 3 mW.
Further, when the electrostatic withstand voltage was measured by applying a reverse bias to both electrodes of this LED, the device did not break down to 400V.

[Embodiment 2] An LED device was obtained in the same manner as in Embodiment 1 except that the film thickness of the second p-type cladding layer 60 was set to 100 Å. The breakdown voltage had improved to 450V.

[Embodiment 3] An LED device was obtained in the same manner as in Embodiment 1 except that the film thickness of the second p-type cladding layer 60 was set to 200 Å.
The W and electrostatic withstand voltage had improved to 550V.

[Embodiment 4] An LED element was obtained in the same manner as in Embodiment 1 except that the film thickness of the second p-type cladding layer 60 was set to 300 Å, and the electrostatic breakdown voltage was 650V.
However, the light emission output decreased to 1 mW.

[Embodiment 5] An LED device was obtained in the same manner as in Embodiment 1 except that Mg-doped p-type GaN was formed on the second p-type cladding layer 60 to have a film thickness of 10 Å. Same as Example 1, 3 mW, electrostatic withstand voltage is 3
It was 60V.

[Sixth Embodiment] FIG. 3 is a schematic sectional view showing the structure of a light emitting device according to a sixth embodiment. The difference between this light emitting device and the light emitting device of FIG. 1 is that n containing In is added as a new buffer layer between the n-type cladding layer 4 and the active layer 5.
-Type nitride semiconductor or a second n-type composed of n-type GaN
The mold cladding layer 40 is being formed. This second clad layer 40 has a thickness of 10 angstroms or more, 0.
It is desirable to form the first n-type cladding layer 40 containing In and In when the thickness of the second n-type cladding layer 40 and the active layer 5 is 300 angstroms or more. The active layer 5 acts as a buffer layer, and the n-type clad layer 4 and the p-type clad layer 6 can be grown with good crystallinity without cracks. Furthermore, by growing the second n-type cladding layer 40, an active layer in which no impurities are doped can be realized, and the half width and the output can be increased.

The second n-type clad layer 40 acts as a buffer layer between the active layer 5 and the n-type clad layer 4 containing Al and Ga. That is, since the second n-type cladding layer 40 containing In and Ga has a soft crystal property, the lattice constant mismatch between the n-type cladding layer 4 containing Al and Ga and the active layer 5 is caused. It has the function of absorbing the strain caused by the difference in thermal expansion coefficient. Therefore, even if the active layer 5 is SQW or MQW having a thin quantum structure, cracks do not occur in the active layer 5 and the n-type clad layer 4, so that the active layer is elastically deformed even if the active layer has a quantum structure. However, the crystal defects of the active layer are reduced. That is, even when the thickness of the active layer is thin, the crystallinity of the active layer is improved, so that the light emission output is increased. Further, the thinning of the active layer increases the light emission output due to the quantum effect and the exciton effect. In other words, in the conventional light emitting device, the thickness of the single active layer is increased to, for example, 1000 angstroms or more to prevent cracks from forming in the cladding layer and the active layer. However, the active layer is always affected by the difference in coefficient of thermal expansion and strain due to lattice misalignment, and in conventional light emitting devices, the thickness of the active layer exceeds the elastically deformable critical film thickness. It cannot be deformed, many crystal defects are generated in the active layer, and the band-to-band emission does not emit much. By forming the second n-type cladding layer 40, it is possible to dramatically improve the light emission output of the light emitting element in the active layer having the quantum structure.

Specifically, after growing the n-type cladding layer 4 in Example 1, the temperature was lowered to 800 ° C.
A second n-type cladding layer 40 made of Si-doped n-type In0.01Ga0.99N is grown to a thickness of 500 Å using G, TMI (trimethylindium), NH ₃ , and silane gas.

Then, using TMG, TMI and NH ₃ , 80
An active layer 5 having a single quantum well structure and made of non-doped n-type In0.05Ga0.95N is grown to a thickness of 80 Å at 0 ° C. After that, when the second p-type cladding layer 60, the first p-type cladding layer 6 and the p-type contact layer 7 were grown into an LED element in the same manner as in Example 1, the LED element showed an If of 20 mA. Blue emission with Vf3.2V and emission peak wavelength of 400 nm, and emission output of 12 m
It was W. Furthermore, the full width at half maximum of the emission spectrum is 20n
m, and emitted light with very good color purity. The electrostatic breakdown voltage was 400 V as in Example 1.

[Embodiment 7] In Embodiment 6, the active layer 5 is used.
The composition of the non-doped In0.05Ga0.95N is 25 angstroms for the well layer and the non-doped In0.01Ga0.
A barrier layer of 99N is grown to a thickness of 50 Å. This operation was repeated 13 times, and finally well layers were laminated to grow the active layer 6 having a total thickness of 1000 angstroms. After that, when the second p-type cladding layer 60, the first p-type cladding layer 6 and the p-type contact layer 7 were grown to be an LED element in the same manner as in Example 1, this LE was obtained.
The D element showed Vf3.2 V at If20 mA, blue light emission with an emission peak wavelength of 400 nm, and had an emission output of 12 mW. Further, the full width at half maximum of the emission spectrum was 20 nm, and the emission showed very good color purity. The electrostatic breakdown voltage was 500V. This indicates that the device having the active layer having the multiple quantum well structure has a higher electrostatic breakdown voltage than the active layer having the single quantum well structure.

[Embodiment 8] An LED element was obtained in the same manner as in Example 6 except that the thickness of the active layer 5 was set to 500 Å. The thickness of the active layer of this LED element was increased. Although the light emission output was reduced to 3 mW, blue light emission with an emission peak wavelength of 390 nm and a half width of 20 nm was exhibited, and the electrostatic breakdown voltage was 400V.

[Example 9] An LED element was obtained in the same manner as in Example 6 except that the film thickness of the second p-type cladding layer 60 was set to 200 Å. Blue light emission with a half-value width of 20 nm was exhibited, the emission output was improved to 10 mW, and the electrostatic breakdown voltage was improved to 550V.

[0039]

The conventional nitride semiconductor light emitting device is weak in electrostatic withstand voltage, and the device is easily broken by static electricity, especially in a dry environment, and the reliability is poor. However, since the electrostatic withstand voltage of the light emitting element is improved by the present invention, the element is less likely to be easily broken and the reliability is extremely improved.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the structure of a light emitting device according to an embodiment of the invention.

FIG. 2 is a schematic cross-sectional view showing the structure of a conventional light emitting element.

FIG. 3 is a schematic cross-sectional view showing the structure of a light emitting device according to another embodiment of the present invention.

[Explanation of symbols]

 1 ... Substrate 2 ... Buffer layer 3 ... N-type contact layer 4 ... N-type cladding layer 5 ... Active layer 60 ... Second p-type cladding layer 6 ... First p-type cladding layer 7 ... P-type contact layer 8 ... Negative electrode 9 ... Positive electrode

Claims (2)

[Claims] 1. A p-type nitride semiconductor containing at least indium between the active layer and the p-type cladding layer, or p
Type p-type clad layer made of p-type GaN is formed. 2. The film thickness of the second p-type cladding layer is 20.
The nitride semiconductor light emitting device according to claim 1, wherein the thickness is 0 angstrom or less.