JP2007201146A - Light emitting element and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To maintain sufficient crystallinity with little surface roughness and to sufficiently maintain an Ir characteristic while high output is realized by thinning an AlGaN barrier layer in an MQW light emitting layer of a group III nitride compound semiconductor light emitting element. <P>SOLUTION: The barrier layer is set to be a two-layered structure of a first barrier layer formed of AlGaN and a second barrier layer formed of GaN. It is desirable that film thickness of the first barrier is set to be 3 nm or below. The first barrier layer is directly brought into contact with an InGaN well 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 order to ensure high luminous efficiency, the light emitting layer of a group III nitride compound semiconductor light emitting device often has a multiple quantum well structure. In this multiple quantum well structure, a well layer is sandwiched between barrier layers, and electrons and holes are confined in the well layer.
In the multiple quantum well structure, it is known that the configuration of the barrier layer affects the light emission efficiency, and various improvements have been made with respect to the barrier layer.
For example, in Patent Document 1, a well layer made of InGaN is sandwiched between a barrier layer made of GaN and a barrier layer made of AlGaN.
Patent Document 2 proposes a barrier layer having a two-layer structure of InAlN / AlGaN.

JP 2003-60231 A JP 2001-237456 A

As a result of intensive studies on the barrier layer in the multiple quantum well structure, the present inventors have found that the use of a barrier layer made of AlGaN relaxes the strain of the well layer and improves the light emission efficiency. (See Japanese Patent Application No. 2005-154382).
As a result of further studies on the barrier layer made of only AlGaN, the inventors have made the AlGaN barrier layer thinner so that charges and holes are efficiently transferred from the n-type layer and the p-type layer to the entire multiple quantum well layer. It was found that the light emission output can be improved by being supplied well.

  However, since the growth temperature of the AlGaN barrier layer is relatively low (about 850 to 900 ° C.), it is difficult to maintain good crystallinity in the thin AlGaN layer. As a result, if the AlGaN layer is made too thin, electrons and holes cannot be efficiently confined in the well, resulting in a decrease in light emission efficiency. In addition, surface roughness occurs, transition occurs in the crystal, and the Ir characteristics deteriorate.

The present invention has been made to solve the above-described problems, and is defined as follows. That is,
A group III nitride compound semiconductor light emitting device comprising a light emitting layer having a multiple quantum well structure in which barrier layers and well layers are alternately stacked,
The light emitting device, wherein the barrier layer includes a first barrier layer made of AlGaN and a second barrier layer made of a group III nitride compound semiconductor substantially free of aluminum.
According to the light emitting device configured as described above, while achieving high output by thinning the first barrier layer made of AlGaN, it is possible to obtain GaN and the like that can obtain better crystal quality than AlGaN at a low temperature growth temperature. By introducing the second barrier layer made of a group III nitride compound semiconductor not containing aluminum, it is possible to maintain good crystallinity with little surface roughness. In addition, the Ir characteristics can be maintained well.

According to the second aspect of the present invention, with respect to the well layer made of InGaN, the composition of the barrier layer is such that the first barrier layer is made of AlGaN and the second barrier layer is made of GaN. It is preferable.
By adopting such a configuration, it is possible to reliably achieve high output.

According to the third aspect of the present invention, the first barrier layer made of AlGaN is in direct contact with the well layer.
In order to ensure that the first barrier layer made of AlGaN is in contact with the well layer, the uppermost layer of the barrier layer is the AlGaN layer. Since the next well layer is formed on the barrier layer, the AlGaN layer is in direct contact with the well layer at this point.
Note that a cap layer made of GaN is usually formed on the InGaN well layer at the same growth temperature as the InGaN layer. The cap layer is for preventing the InGaN well layer from disappearing when forming the barrier layer. In other words, when forming the barrier layer, all or part of the cap layer is scattered instead of the InGaN well layer. Disappear. However, depending on the growth conditions, the cap layer may remain between the InGaN well layer and the barrier layer. Therefore, if an AlGaN barrier layer is formed on the well layer and further formed thereon, the AlGaN barrier layer may not be in direct contact with the well layer. In this case, the performance of the AlGaN layer is lowered, which is not preferable.

According to the study by the present inventors, the first barrier layer made of AlGaN is preferably 1 to 3 nm as defined as the fourth aspect of the present invention. If the first barrier layer is less than 1 nm, sufficient crystallinity cannot be obtained, so that charges cannot be reliably confined in the well layer. Although it is possible to increase the thickness of the first barrier layer beyond 3 nm, the light emission output is reduced.
The second barrier layer made of GaN is preferably 1 to 3 nm. If the thickness of the second barrier layer is less than 1 nm, sufficient crystallinity cannot be obtained. Although it is possible to increase the thickness of the second barrier layer beyond 3 nm, the light emission output is reduced.

Examples of the present invention and comparative examples will be described below.
FIG. 1 shows a light-emitting element 1 of an example.
The specifications of each layer of the light emitting element 1 are as follows.
Layer: Composition p contact layer 11: p-GaN: Mg 100 nm
p-clad layer 9: p-AlGaN / GaN: Mg 3.5-10.5 pair MQW light emitting layer 7: InGaN / (AlGaN / GaN) 3-8 pairs n-clad layer 6: InGaN / GaN: Si 5-20 pairs ESD layer 5: i-GaN / n-GaN
n contact layer 4: n-GaN: Si
Substrate 2: Sapphire

In the above, sapphire was used for 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, group III nitride A physical compound semiconductor single crystal or the like can be used.
GaN is exemplified as the n-type layer, but AlGaN, InGaN or AlInGaN, and other group III nitride compound semiconductors can be used. Here, the group III nitride compound semiconductor is a quaternary system of Al _X Ga _Y In _1- XYN (0 ≦ X ≦ 1, 0 ≦ Y ≦ 1, 0 ≦ X + Y ≦ 1) as a general formula. 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 (where 0 <x <1). Includes the original system. 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.
In addition to Si, Ge, Se, Te, C, or the like can be used as the n-type impurity doped in the n-type layer.
Similarly, the p-type layer can be formed of a group III nitride compound semiconductor. In addition to Mg, Zn, Be, Ca, Sr, or Ba can be used as the p-type impurity doped in the p-type layer.

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.
Blue light emission (wavelength: 460 nm) can be obtained by applying a current to the light emitting element of the embodiment configured as described above.

In the light emitting device having the above-described configuration, the light emitting layer 7 having a multiple quantum well structure is obtained by growing a well layer made of InGaN to a thickness of 3 nm at a growth temperature (first temperature) of 770 ° C. according to a conventional MOCVD growth method. Thereafter, a cap layer 72 made of GaN is grown to a thickness of 2 nm while maintaining the growth temperature.
Thereafter, the growth temperature was raised to 880 ° C. (second temperature) to form a second barrier layer 73 (3 nm) made of GaN and a first barrier layer 74 (3 nm or less) made of AlGaN. When the barrier layer 74 is formed, all or part of the cap layer 72 disappears from the well layer 71.

FIG. 2 shows a change in output when the thickness of the first barrier layer 74 made of AlGaN is changed in the above configuration. Similarly, FIG. 3 shows changes in Ir when the thickness of the first barrier layer 74 is changed.
From the results of FIGS. 2 and 3, it can be confirmed that although the output is improved when the thickness of the first barrier layer 74 made of AlGaN is 3 nm or less, the Ir characteristic is maintained. This is considered to be due to the fact that, by thinning the AlGaN, charges are efficiently supplied to the entire light emitting layer while maintaining the crystallinity.

FIG. 4 shows a light emitting element 21 of a comparative example. In FIG. 4, the same elements as those in FIG.
In the light emitting element 21 of the comparative example, the barrier layer 81 of the light emitting layer 80 has a single layer structure of AlGaN.
According to the light emitting element 21, the output decreases when the thickness of the AlGaN barrier layer 81 exceeds 4 nm (see FIG. 5). Further, when the film thickness is 4 nm or less, the Ir characteristic is lowered and the reverse current easily flows (see FIG. 6).
From the above comparison results, the barrier layer of the multi-quantum well layer has a two-layer structure of a first barrier layer made of AlGaN and a second barrier layer made of GaN as in the light emitting device of the embodiment of the present invention. Thus, it can be seen that high luminous efficiency can be secured.

In the above description, the barrier layer has a two-layer structure including the first barrier layer and the second barrier layer, but this does not prevent the barrier layer from including the third layer.
Further, all or a part of the barrier layers constituting the multiple quantum well structure may have the structure of the present invention.
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 which shows the structure of the light emitting element of the Example of this invention. It is a graph which similarly shows the relationship between the film thickness of a 1st barrier layer, and an output. It is a graph which similarly shows the relationship between the film thickness of a 1st barrier layer, and Ir. It is a schematic diagram which shows the structure of the light emitting element of the comparative example of this invention. It is a graph which similarly shows the relationship between the film thickness of a 1st barrier layer, and an output. It is a graph which similarly shows the relationship between the film thickness of a 1st barrier layer, and Ir.

Explanation of symbols

1, 21 Light emitting element 7, 80 Light emitting layer 71 Well layer 72 Cap layer 73 Second barrier layer 74 First barrier layer 81 Barrier layer of comparative example

Claims (5)

A group III nitride compound semiconductor light emitting device comprising a light emitting layer having a multiple quantum well structure in which barrier layers and well layers are alternately stacked,
The light emitting device, wherein the barrier layer includes a first barrier layer made of AlGaN and a second barrier layer made of a group III nitride compound semiconductor substantially free of aluminum. 2. The light emitting device according to claim 1, wherein the well layer is made of InGaN, and the barrier layer is made of the first barrier layer made of AlGaN and the second barrier layer made of GaN. The light emitting device according to claim 1, wherein the first barrier layer is in direct contact with the well layer. 4. The light-emitting element according to claim 2, wherein the first barrier layer has a thickness of 1 to 3 nm and the second barrier layer has a thickness of 1 to 3 nm. A well layer forming step of forming a well layer made of InGaN and a cap layer thereon at a first temperature;
A second barrier layer made of GaN is formed on the cap layer at a second temperature higher than the first temperature, and the second barrier layer is maintained on the second barrier layer while maintaining the second temperature. A barrier layer forming step of forming a first barrier layer made of AlGaN,
A method of manufacturing a group III nitride compound semiconductor light emitting device, characterized in that a light emitting layer having a multiple quantum well structure is formed by repeating the well layer forming step and the barrier layer forming step.

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