JP2007201377A - Light emitting device and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light emitting device consisting of a group III nitride system compound semiconductor which solves such a problem that there arises a stress distortion in a light emitting layer, according to a difference or the like of a lattice constant of each layer in the light emitting device constituted by pinching the light emitting layer by an n-cladding layer and a p-cladding layer and consequently there occurs a piezoelectric field, there is also some possibility that an internal quantum effect is reduced by this piezoelectric field, and there occurs a wavelength shift (Stark effect); and to provide a method of manufacturing the light emitting device. <P>SOLUTION: The method of manufacturing the light emitting device comprises the steps of relaxing the stress distortion arising in the light emitting layer by giving a stress relaxing function to the p-cladding layer; and adopting at least one of the following manners in order to give the stress relaxing function to the p-cladding layer, (1) to carry out a thick film of the p-cladding layer, (2) to carry out a low-temperature growth of the p-cladding layer, and (3) to select a carrier gas at the time of growing the p-cladding layer. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a light emitting device made of a group III nitride compound semiconductor and a method for manufacturing the same.

In a light emitting device having a structure in which a light emitting layer 1 composed of an InGaN layer is sandwiched between an n clad layer 3 and a p clad layer 5, a group III nitride constituting the light emitting layer is theoretically shown in FIG. The emission wavelength is determined according to the band gap energy based on the composition of the compound semiconductor.
However, when forming a light-emitting element, multiple Group III nitride compound semiconductor layers with different compositions are stacked, so stress strain occurs in the light-emitting layer based on the difference in their lattice constants, which generates a piezoelectric field. Let As shown in FIG. 1B, it is known that electrons and holes are spatially separated and the light emission efficiency is reduced due to the influence of the piezoelectric field (Quantum Confined Stark Effect). )).

See Patent Documents 1 to 3 as documents related to the present case.
JP 2000-261106 A JP 2005-57308 A JP 2005-85932 A

Such a Stark effect appears more conspicuously in a green light emitting device having a large In composition in the light emitting layer, and lowers its internal quantum efficiency.
When electrons and holes are spatially separated by the Stark effect, when carriers are accumulated in the light emitting layer by current injection, a large wavelength shift is caused by the screening effect, and the emission wavelength may shift to the longer wavelength side. is there.

The present invention has been made to solve the above-described problems, and is configured as follows.
That is, a light-emitting element comprising a group III nitride compound semiconductor having a p-cladding layer between the light-emitting layer and the p-contact layer, wherein the p-cladding layer has a stress relaxation function.

According to the light emitting device configured as described above, since the p clad layer has a stress relaxation function, the stress applied to the light emitting layer is reduced. As a result, the piezoelectric field is reduced and the spatial separation of electrons and holes is reduced.
FIG. 2 shows a state in which the stress applied to the light emitting layer is relaxed by the present invention.
When spatial separation is suppressed or eliminated in this way, the internal quantum efficiency is improved, and when the In spatial decomposition effect is small, the wavelength shift is also reduced or substantially eliminated.

Hereafter, each element of the said invention is demonstrated in detail.
The light emitting layer, the p contact layer, and the cladding layer are made of a group III nitride compound semiconductor. Here, in general, a group III nitride compound semiconductor is a general formula of Al _X Ga _Y In _1-XY N (0 ≦ X ≦ 1, 0 ≦ Y ≦ 1, 0 ≦ X + Y ≦ 1). A so-called binary system of AlN, GaN and InN, Al _x Ga _1-x N, Al _x In _1-x N and Ga _x In _1-x N (in the above, 0 <x <1) Includes so-called ternary systems. Part of group III elements may be substituted with boron (B), thallium (Tl), etc., and part of nitrogen (N) may also be phosphorus (P), arsenic (As), antimony (Sb), bismuth. It can be replaced with (Bi) or the like. Moreover, the light emitting layer may contain an arbitrary dopant.
The effect of the present invention is remarkable when the light-emitting layer contains InGaN, the p-contact layer is made of GaN, and the p-cladding layer is made of a single layer of AlGaN or a superlattice structure in which AlGaN and GaN or InGaN are stacked. It is.

The light emitting layer may include a quantum well structure (multiple quantum well structure or single quantum well structure), and the structure of the light emitting element may be a single hetero type, a double hetero type or a homojunction type. Good.
The p-contact layer is doped with a p-type impurity such as Mg, and when formed by MOCVD, it is epitaxially grown to a thickness of about 100 nm at a temperature of usually 1000 ° C. or higher using hydrogen gas as a carrier. As a result, a good crystal structure is obtained in the p contact layer.

For the p-clad layer, conventionally, the same growth conditions as those for the p-contact layer have been executed. As a result, a good crystal structure was obtained also in the p-cladding layer. On the other hand, the crystal structure of the n-clad layer and the n-contact layer opposite to the p-clad layer as viewed from the light emitting layer was also good. That is, a relatively soft light-emitting layer because InGaN is contained between a p-type layer (p-clad layer and p-contact layer) and a n-type layer (n-clad layer and n-contact layer) that have a good crystal structure and is relatively hard. Is sandwiched. As a result, a stress strain based on the difference in lattice constant between the p-type layer and the n-type layer is generated in the light emitting layer.
On the other hand, according to the present invention, since the p-clad layer has a stress relaxation function, stress based on the difference in lattice constant between the p-type layer and the n-type layer is relaxed and absorbed by the p-clad layer. , It does not reach the light emitting layer or its influence becomes small. Therefore, the Stark effect is relaxed, the internal quantum efficiency is improved, and the wavelength shift can be suppressed.

As a measure for imparting a stress relaxation function to the p-cladding layer, at least one of the following measures (1) to (3) can be employed.
(1) Thicken the p-cladding layer.
It was confirmed that by increasing the thickness of the p-cladding layer, the internal quantum efficiency can be improved and the wavelength shift can be suppressed.
FIG. 3 shows the relationship between the thickness of the p-cladding layer and VF in the light-emitting element.
The results in FIG. 3 are obtained by measuring changes in VF when the thickness of the p-cladding layer is changed in a light-emitting element having the following configuration. From the results of FIG. 3, it can be seen that VF improves as the thickness of the p-cladding layer increases.
Layer: Composition p contact layer: p-GaN: Mg 100 nm
p-clad layer: AlGaN: Mg / InGaN: Mg Adjust the number of pairs (25-52 nm)
MQW light emitting layer: InGaN / GaN
n-clad layer: n-InGaN / GaN: Si 10 pairs (50 nm)
n contact layer: n-GaN: Si (4 μm)
Buffer layer: AlN
Substrate: Sapphire

  FIG. 4 similarly shows the relationship between the thickness of the p-cladding layer and the output (luminous intensity) of the light-emitting element. From the result of FIG. 4, it can be seen that the output improves as the p-cladding layer becomes thicker. According to the study by the present inventors, even if the p-cladding layer is made thicker than 50 nm, the increase in luminous intensity reaches a peak or decreases. This is presumably because the damage to the light emitting layer increases as the growth time increases.

FIG. 5 similarly shows the relationship between the thickness of the p-cladding layer and the dominant wavelength difference at low current (1 mA) and at the rated current (20 mA). From the result of FIG. 5, the dominant wavelength difference becomes smaller as the p-cladding layer becomes thicker. This suggests that the influence of the piezoelectric field applied to the light emitting layer is reduced.
From the above, it can be seen that the thickness of the p-cladding layer is preferably thick but not more than 50 nm. According to the study by the present inventors, it was preferable that the p-cladding layer be 40 to 50 nm in the light emitting device.

(2) The p-clad layer is grown at a low temperature.
In forming the group III nitride compound semiconductor light emitting device exemplified above, the growth conditions are selected so that the semiconductor layers except the buffer layer have good crystallinity. In general, since the p-cladding layer and the p-contact layer have similar layer compositions, for example, the growth temperature when executing the MOCVD method is often 1000 ° C. or more for both layers. As a result, a p-clad layer and a p-contact layer with good crystallinity were obtained.
In the present invention, the growth temperature of the p-clad layer is set lower than the growth temperature of the p-contact layer. When the growth temperature of the p-clad layer is lowered, the crystallinity of the p-clad layer is lowered. Thereby, since the rigidity of the p-cladding layer is lowered, the stress can be relaxed by the p-cladding layer, and the Stark effect can be reduced. More specifically, when the growth temperature of the p-cladding layer is set to 800 ° C. to 950 ° C., the Stark effect can be reduced while ensuring a sufficient output for the light emitting element.
At this time, the thickness of the p-cladding layer is preferably 40 to 50 nm.

(3) Selection of nitrogen gas as carrier gas Hydrogen gas is generally used as a carrier gas for growing AlGaN or GaN by MOCVD, and the use of nitrogen gas lowers its crystallinity.
In the present invention, nitrogen gas is employed as a main carrier gas when the AlGaN and GaN layers constituting the p-cladding layer are grown by the MOCVD method. Here, the main carrier gas refers to the gas component that is the largest among all the gas components of the carrier gas.
When nitrogen is used as a carrier gas for forming the p-clad layer, the crystallinity of the p-clad layer is lowered. Thereby, since the rigidity of the p-cladding layer is lowered, the stress can be relaxed by the p-cladding layer, and the Stark effect can be reduced. More specifically, when 50% by volume or more of the carrier gas for growing the p-cladding layer is nitrogen gas, the Stark effect can be reduced while ensuring a sufficient output for the light emitting element.
At this time, the thickness of the p-cladding layer is preferably 40 to 50 nm, and the growth temperature is preferably 800 to 950 ° C.

The specifications of the light emitting device of the example are as follows.

Layer: Composition p contact layer: p-GaN: Mg 100 nm
p-clad layer: p-AlGaN / InGaN: Mg 10 pair (50 nm)
MQW light emitting layer: InGaN / GaN 4-6 pairs (18 nm)
Third n-clad layer: n-InGaN / GaN: Si 10 pair (50 nm)
Second n-clad layer: n-GaN: Si (30 nm)
First n-clad layer: i-GaN (300 nm)
n contact layer: n-GaN: Si (4 μm)
Buffer layer: AlN
Substrate: Sapphire

An n-type layer is formed on the substrate via a buffer layer. Here, sapphire was used as the substrate, but the substrate is not limited to this, and sapphire, spinel, silicon, silicon carbide, zinc oxide, gallium phosphide, gallium arsenide, magnesium oxide, manganese oxide, and group III nitride A physical compound semiconductor single crystal or the like can be used. Further, the buffer layer is formed by MOCVD using AlN, but the present invention is not limited to this, and GaN, InN, AlGaN, InGaN, AlInGaN, etc. can be used as the material, and molecular beam crystal growth is used as the manufacturing method. A method (MBE method), a halide vapor phase epitaxy method (HVPE method), a sputtering method, an ion plating method, an electron shower method, or the like can be used. When a group III nitride compound semiconductor is used as the substrate, the buffer layer can be omitted.
Further, the substrate and the buffer layer can be removed as necessary after the semiconductor element is formed.

The n-type layer is formed by sequentially laminating an n contact layer, a first n clad layer, a second n clad layer, and a third n clad layer.
Here, GaN is exemplified as the n-type layer, but AlGaN, InGaN, or AlInGaN can be used.
Further, Si is exemplified as the n-type impurity doped in the n-type layer, but Ge, Se, Te, C, or the like can also be used as the n-type impurity.

The light emitting layer may include a quantum well structure (multiple quantum well structure or single quantum well structure), and the structure of the light emitting element may be a single hetero type, a double hetero type or a homojunction type. Good.
The light emitting layer can also include a group III nitride compound semiconductor layer with a wide band gap doped with an acceptor such as magnesium on the p-type layer side. This is for effectively preventing electrons injected into the light emitting layer from diffusing into the p-type layer.

A p-cladding layer is formed on the light emitting layer. In this example, a superlattice structure composed of 5 to 10 pairs of AlGaN (2 nm) / GaN (2.5 nm) was used. As a result, the thickness of the p-clad layer was 25 to 50 nm.
The p-clad layer was grown by MOCVD at 840 ° C., and nitrogen was adopted as the carrier gas at that time.

The growth temperature of the p-contact layer similarly formed by MOCVD on the p-cladding layer is 1000 ° C., and the carrier gas is hydrogen.
As the p-type impurity, Zn, Be, Ca, Sr, and Ba can be used in addition to Mg.
In the light emitting device having the above structure, each group III nitride compound semiconductor layer is formed by performing MOCVD. In addition, it can also be formed by methods such as molecular beam crystal growth (MBE), halide vapor phase epitaxy (HVPE), sputtering, ion plating, and electron shower.

A part of the p-type layer, the light emitting layer, the n-cladding layer, and the n-contact layer is etched, and an n-electrode is formed on the n-contact layer by vapor deposition. The n electrode is composed of two layers of Al and V.
A thin-film translucent electrode containing gold is laminated on the entire surface of the p-contact layer. A p-electrode containing gold is formed on the translucent electrode by vapor deposition.
After forming each semiconductor layer and each electrode by the above process, a separation process of each chip is performed.

  When a current is applied to the light emitting element of the embodiment configured as described above, green light emission (wavelength: 520 nm) is obtained. There is almost no difference between the dominant wavelength when a low current (1 mA) is applied to the light emitting element and the dominant wavelength when a rated current (20 mA) is applied. This is probably because the p-cladding layer functions as a stress relaxation layer, and as a result, the influence of the piezoelectric field is reduced.

  The present invention is not limited to the description of the embodiments and examples of the invention described above. Various modifications may be included in the present invention as long as those skilled in the art can easily conceive without departing from the description of the scope of claims.

It is a schematic diagram of the band gap energy of a conventional light emitting device. It is a schematic diagram of the band gap energy of the light emitting device of the present invention. It is a graph which shows the relationship between the film thickness of the p clad layer in the light emitting element of the test example of this invention, and VF. It is a graph which similarly shows the relationship between the film thickness of a p clad layer, and an output. It is a graph which similarly shows the relationship between the film thickness of a p clad layer, and the dominant wavelength difference at the time of a low electric current (1 mA) and a rated current (20 mA).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light emitting layer 3 n clad layer 5 p clad layer

Claims (7)

A light emitting device comprising a group III nitride compound semiconductor having a p clad layer between a light emitting layer and a p contact layer, wherein the p clad layer has a stress relaxation function. The light emitting layer includes an InGaN layer, the p contact layer is made of GaN, and the p cladding layer is made of a single layer of AlGaN or a superlattice structure in which AlGaN and GaN or InGaN are stacked. 2. The light emitting device according to 1. The light-emitting element according to claim 1, wherein the p-cladding layer has a thickness of 40 to 50 nm. The light emitting layer, the p contact layer, and the p clad layer are formed by MOCVD, and the p clad layer is formed at a lower growth temperature than the p contact layer. The light emitting element as described in. 3. The light emitting device according to claim 2, wherein the light emitting layer, the p contact layer, and the p clad layer are formed by MOCVD, and a main carrier gas when forming the p clad layer is nitrogen gas. 6. The light emitting device according to claim 5, wherein the p-clad layer has a thickness of 40 to 50 nm and / or the p-clad layer is formed at a lower growth temperature than the p-contact layer. . In a method for manufacturing a light-emitting element made of a group III nitride compound semiconductor having a p-cladding layer between a light-emitting layer and a p-contact layer by MOCVD,
The main carrier gas of the p-clad layer is nitrogen gas, and / or the p-clad layer is grown at a growth temperature lower than that of the p-contact layer, and the p-clad layer has a stress relaxation function. Manufacturing method of light emitting element.

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