JP2011187862A - Method of manufacturing group iii nitride semiconductor light emitting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve luminous efficiency by reducing carrier losses in a cap layer in a light emitting layer of an MQW structure. <P>SOLUTION: The light emitting layer 13 is of the MQW structure, in which a well layer 131 consisting of InGaN, a cap layer 132 consisting of AlGaN, and a barrier layer 133 consisting of AlGaN are repeatedly stacked 3-5 times in order. The well layer 131 and the cap layer 132 are grown at same temperature. The barrier layer 133 is grown at a temperature higher than the well layer 131 and the cap layer 132. An Al composition ratio of the cap layer 132 is set to be larger than 0, and smaller than or equal to the Al composition ratio of the barrier layer 133. The thickness of the cap layer 132 is set to be 1-8 molecular layer. The luminous efficiency can be improved by constituting the cap layer 132 in this way. Since the dependence on thickness of the cap layer 132 in the luminous efficiency is reduced, reproducibility, productivity, and yield can also be improved. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a group III nitride semiconductor light emitting device, and particularly has a feature in a method for forming an MQW layer.

  As a light emitting layer of a group III nitride semiconductor light emitting device, an MQW structure in which a well layer made of InGaN and a barrier layer made of AlGaN are repeatedly stacked has been conventionally known. In order to obtain good crystallinity in such a light emitting layer, the growth temperature of the barrier layer needs to be higher than the growth temperature of the well layer. However, when the temperature is raised during the formation of the barrier layer, the In in the well layer is detached and the crystallinity is deteriorated.

  Therefore, a structure in which a cap layer made of GaN formed at the same growth temperature as that of the well layer is provided between the well layer and the barrier layer to prevent the separation of In from the well layer due to the temperature rise during the formation of the barrier layer. Has been proposed (for example, Patent Document 1). In this Patent Document 1, although the relationship between the thickness of the barrier layer and the light emission efficiency is studied, there is only a description that the thickness of the cap layer is 2 nm. The relationship has not been studied. In addition, there is only a description that GaN is used for the material of the cap layer, and other group III nitride semiconductor materials are not studied.

  Patent Document 2 discloses that an intermediate layer made of AlGaN having a thickness of 2 nm or less is provided between the well layer and the barrier layer, and the band gap of the intermediate layer is set to be equal to or larger than the band gap of the barrier layer. This indicates that the crystallinity of the barrier layer is improved and the light emission output is improved.

JP2007-2011146 JP-A-11-330552

  However, if a cap layer with a smaller band gap than the barrier layer is provided between the well layer and the barrier layer, carriers are recombined in the cap layer or trapped in the impurity level before the carriers fall into the well layer. In some cases, the carrier injection efficiency is lowered and the light emission efficiency is lowered. This is particularly noticeable when the drive current density is high.

  Therefore, the inventors examined whether the light emission efficiency could be improved by the thickness of the cap layer made of GaN, but the light emission efficiency strongly depends on the thickness of the cap layer, and the thickness needs to be accurately controlled. Therefore, it was found that there are problems in terms of reproducibility, productivity, and yield.

  Accordingly, an object of the present invention is to suppress a decrease in light emission efficiency due to a cap layer between a well layer and a barrier layer in a method for manufacturing a group III nitride semiconductor light emitting device having a light emitting layer having an MQW structure, and reproducibility, It is to improve productivity and yield.

  A well layer made of a group III nitride semiconductor containing In, a cap layer made of a group III nitride semiconductor having a larger band gap than the well layer, and a barrier layer made of a group III nitride semiconductor having a larger band gap than the well layer, A light-emitting layer having an MQW structure that is repeatedly formed in order, the growth temperature of the well layer and the cap layer are equal, and the growth temperature of the barrier layer is higher than the growth temperature of the well layer and the cap layer. A manufacturing method of a group III nitride semiconductor light emitting device, characterized in that the barrier layer is made of AlGaN and the cap layer is made of AlGaN having an Al composition ratio larger than 0 and less than or equal to the Al composition ratio of the barrier layer. It is.

Here, the group III nitride semiconductor is a compound semiconductor represented by the general formula Al _x Ga _y In _z N (x + y + z = 1, 0 ≦ x, y, z ≦ 1), and includes Al, Ga, and In. It also includes those in which part is substituted with other group 13 elements B and Tl, and part of N is substituted with other group 15 elements P, As, Sb, and Bi. Usually, Si is used for n-type impurities, and Mg is used for p-type impurities.

  The Al composition ratio of the cap layer and the barrier layer is desirably equal. “Equal” here does not mean strictly equal, but the difference between the Al composition ratio of the cap layer and the Al composition ratio of the barrier layer may be in the range of −3 to + 3%. By setting the Al composition ratio of the cap layer in this way, the band gap of the cap layer and the band gap of the barrier layer are equalized, and carriers are not recombined in the cap layer, so that the light emission efficiency is further improved. be able to. Further, the Al composition ratio of the cap layer and the barrier layer is preferably 5 to 40%. If the Al composition ratio is less than 5%, the band gap difference from the well layer is too small, making it difficult to function as a cap layer or a barrier layer. It is not desirable because it decreases.

  The thickness of the cap layer is desirably 1 to 8 molecular layers. If the thickness is within this range, the thickness dependency of the light emission efficiency is further reduced, so that precise control of the thickness is not required, and reproducibility, productivity, and yield can be further improved.

  When the well layer is made of InGaN, the In composition ratio is desirably larger than 0 and not larger than 0.35. Moreover, the barrier layer and the cap layer may be a single layer or a multilayer.

  The number of repetitions of the well layer, the cap layer, and the barrier layer is desirably 3 to 10 times. This is because if the number is less than 3 times, the effect of improving the light emission efficiency due to the MQW structure is not sufficient, and if the number is more than 10 times, the light emission efficiency is lowered.

  The growth temperature of the well layer and the cap layer is desirably 700 to 900 ° C. The growth temperature of the barrier layer is desirably 850 to 1100 ° C. The thickness of the well layer is desirably 1 to 4 nm. The thickness of the barrier layer is preferably 1 to 6 nm.

  Further, the growth temperatures of the well layer and the cap layer need not be exactly the same, and may be a temperature difference that does not cause the well layer In to be separated during the formation of the cap layer.

  A second invention is a method for producing a group III nitride semiconductor light emitting device according to the first invention, wherein the cap layer is made of AlGaN equal to the Al composition ratio of the barrier layer.

  A third invention is a method for producing a group III nitride semiconductor light-emitting device according to the first or second invention, wherein the cap layer has a thickness of 1 to 8 molecular layers.

  The monomolecular layer is ½ of the lattice constant of the c-axis of the group III nitride semiconductor, and in the case of GaN, it is about 2.59Å.

  When the cap layer is formed as in the first invention, the light emission efficiency can be improved and the thickness dependency of the light emission efficiency is sufficiently reduced as compared with the case where GaN is used as the cap layer. Can be made. As a result, it is not necessary to strictly control the thickness, and reproducibility, productivity, and yield can be improved.

  Further, as in the second aspect of the invention, by making the Al composition ratio of the cap layer equal to the Al composition ratio of the barrier layer, the band gap of the cap layer and the band gap of the barrier layer become equal. The recombination will not occur and the luminous efficiency can be further improved.

  Moreover, if the thickness of the cap layer is 1 to 8 molecular layers as in the third aspect of the invention, the dependency of the luminous efficiency on the thickness of the cap layer is further reduced within this thickness range. , Productivity and yield can be further improved.

FIG. 3 shows a configuration of a light-emitting element 1 of Example 1. The figure which showed the structure of the light emitting layer. FIG. 6 shows a manufacturing process of the light-emitting element 1. The graph which showed the relationship between the thickness of the cap layer 132, and light output. The graph which showed the relationship between Al composition ratio of the cap layer 132, and optical output.

  Specific examples of the present invention will be described below, but the present invention is not limited to the examples.

  FIG. 1 is a diagram illustrating a configuration of the light-emitting element 1 according to the first embodiment. The light-emitting element 1 includes an n-type contact layer 11 made of n-GaN on an sapphire substrate 10, an n-type cladding layer 12 having a superlattice structure in which i-GaN and i-InGaN are alternately formed, and an MQW structure. A light emitting layer 13, a p-type cladding layer 14 having a superlattice structure in which p-InGaN and p-AlGaN are alternately and repeatedly formed, and a p-type contact layer 15 made of p-GaN are sequentially stacked. A structure in which a p-electrode 16 is formed on the n-type contact layer 11 exposed by etching a part from the surface side of the p-type contact layer 15 to a depth reaching the n-type contact layer 11. It is. An ESD layer made of i-GaN and n-GaN may be provided between the n-type contact layer 11 and the n-type cladding layer 12 to enhance the electrostatic withstand voltage characteristics.

  In addition to the sapphire substrate 10, SiC, Si, ZnO, spinel, GaN, or the like can be used as the substrate. Further, the substrate may be subjected to uneven processing such as stripes or dots.

  As shown in FIG. 2, the light emitting layer 13 has an MQW structure in which a well layer 131 made of InGaN, a cap layer 132 made of AlGaN, and a barrier layer 133 made of AlGaN are sequentially stacked three to five times. The Al composition ratio of the cap layer 132 is greater than 0 and less than or equal to the Al composition ratio of the barrier layer 133. The thickness of the well layer 131 is 1 to 4 nm, and the thickness of the barrier layer 133 is 1 to 6 nm. The cap layer 132 has a thickness of 1 to 8 molecular layers. One molecular layer is ½ of the c-axis lattice constant of the group III nitride semiconductor, and in the case of GaN, it is about 2.59Å.

  The Al composition ratio of the cap layer 132 is desirably equal to the Al composition ratio of the barrier layer 133. The band gap of the cap layer 132 is equal to the band gap of the barrier layer 133, so that recombination of carriers in the cap layer 132 is eliminated, and the light emission efficiency can be improved.

  When the p-electrode 16 is a face-up type, for example, an ITO electrode formed on almost the entire surface of the p-type contact layer 15 and a wiring electrode made of Ni / Au formed in a wiring shape on the ITO electrode; In the case of a flip chip type, for example, an electrode made of a highly reflective metal material such as Ag or Rh. The n electrode 18 is, for example, Ti / Al.

Next, a method for manufacturing a light emitting element will be described with reference to FIG. First, an n-type contact layer 11 and an n-type cladding layer 12 are sequentially formed on a sapphire substrate 10 by MOCVD via a buffer layer (not shown) (FIG. 3A). Hydrogen and nitrogen are used as the carrier gas, ammonia is used as the nitrogen source, TMG (trimethylgallium) is used as the Ga source, TMA (trimethylaluminum) is used as the Al source, and SiH ₄ (silane) is used as the dopant gas.

  Next, a step of forming a well layer 131 made of InGaN having a growth temperature of 750 to 800 ° C. and a thickness of 1 to 4 nm (In composition ratio of 0.05 to 0.15%), the well layer on the well layer 131 The step of forming a cap layer 132 made of AlGaN having a thickness of 1 to 8 molecular layers while maintaining the growth temperature at the time of forming 131, from AlGaN having a growth temperature of 850 to 950 ° C. and a thickness of 1 to 6 nm on the cap layer 132 Thus, the barrier layer 133 whose Al composition ratio is equal to or higher than the Al composition ratio of the cap layer 132 (that is, the barrier layer 133 having an Al composition ratio such that the Al composition ratio of the cap layer 132 is equal to or lower than the Al composition ratio of the barrier layer 133) The light emitting layer 13 having the MQW structure is formed on the n-type clad layer 12 by repeating the three steps of forming the step 3 to 5 times in order (FIG. 3B). All of the well layer 131, the cap layer 132, and the barrier layer 133 are formed by MOCVD, TMI (trimethylindium) is used as the In source, and the carrier gas and other source gases are the n-type contact layer 11 and the n-type contact layer. This is similar to the formation of the cladding layer 12. The cap layer 132 is provided to prevent the separation of In from the well layer 131 when the barrier layer 133 is formed by raising the temperature from 750 to 800 ° C. to 850 to 950 ° C. Since the cap layer 132 is AlGaN and grows at a low temperature, its crystallinity is lower than the case where GaN is used as the cap layer 132, but the thickness is very thin as 1 to 8 molecular layers. The low of does not affect the luminous efficiency.

Next, a p-type cladding layer 14 and a p-type contact layer 15 are sequentially formed on the light emitting layer 13 (FIG. 3C). TMI (trimethylindium) is used as the In source, and Cp ₂ Mg (biscyclopentadienylmagnesium) is used as the p-type dopant source. The carrier gas and other source gases are the same as in forming the n-type contact layer 11 and the like.

  Next, after activating Mg by heat treatment, dry etching is performed from the surface side of the p-type contact layer 15 to form a groove reaching the n-type contact layer 11. Then, a p-electrode 17 is formed on the p-type contact layer 15, and an n-electrode 18 is formed on the n-type contact layer 11 exposed on the groove bottom surface by dry etching. Thus, the light-emitting element 1 shown in FIG. 1 is manufactured.

  FIG. 4 is a graph showing the relationship between the thickness of the cap layer 132 and the light output. The Al composition ratio of the barrier layer 133 was 12%, and the Al composition ratio of the cap layer 132 was 0% (that is, GaN), 12%, and 18%. Hereinafter, the case of Al composition ratio 0% is referred to as Comparative Example 1, the case of 18% is referred to as Comparative Example 2, and the case of 12% is referred to as Example 1. The horizontal axis is the thickness of the cap layer 132, and the unit is a single molecular layer. The light output on the vertical axis is a relative value with 1 when the cap layer 132 is GaN and the thickness is 12 molecular layers. As shown in FIG. 4, in the case of Comparative Example 1 in which the cap layer 132 is made of GaN, the light output decreases almost linearly as the thickness increases, and the dependency of the light output on the thickness of the cap layer 132 is strong. I understand. On the other hand, in the case of Example 1 in which the Al composition ratio is 12% and Comparative Example 2 in which the Al composition ratio is 18%, the light output is little with respect to the change in the thickness of the cap layer 132 between 2 to 8 molecular layers. In the case of Comparative Example 2, the light output is substantially constant at about 1.02, and in the case of Example 1, the light output is substantially constant at about 1.05. Yes. From this, it is speculated that the use of AlGaN as the cap layer 132 reduces the dependency of the light emission efficiency on the thickness of the cap layer 132. However, if the cap layer 132 is thicker than the 8-molecular layer, it is considered that the luminous efficiency is lowered due to the poor crystallinity of AlGaN and the influence of lattice distortion. Therefore, it is presumed that it is in the range of the thickness of 1 to 8 molecular layers that the dependency of the luminous efficiency on the thickness of the cap layer 132 is reduced and the light output becomes almost constant.

  FIG. 5 is a graph showing the relationship between the Al composition ratio of the cap layer 132 and the light output. The Al composition ratio of the barrier layer 133 was 12%, and the thickness of the cap layer 132 was 4 molecular layers, 8 molecular layers, and 12 molecular layers. Similarly to the case of FIG. 4, the optical output on the vertical axis is a relative value of 1 when the cap layer 132 is made of GaN and the thickness thereof is 12 molecular layers. As shown in FIG. 5, when the cap layer 132 has a thickness of four molecular layers, the light output is almost constant at about 1.05 until the Al composition ratio is 12%. Output has dropped. Further, when the thickness of the cap layer 132 is 8 molecular layers, the light output increases as the Al composition ratio increases up to 12%, but the light output decreases when the Al composition ratio exceeds 12%. You can see that From this, it can be inferred that the luminous efficiency decreases when the Al composition ratio of the cap layer 132 exceeds the Al composition ratio of the barrier layer 133. This is presumably because the strain on the well layer 131 and the barrier layer 133 increases as the Al composition ratio increases.

  4 and 5 above, the Al composition ratio of the cap layer 132 made of AlGaN is set to a range larger than 0 and less than or equal to the Al composition ratio of the barrier layer 133, and the thickness of the cap layer 132 is 1 to 8 molecular layers. Thus, it can be seen that the light emission efficiency of the light-emitting element 1 can be improved, and that the dependency of the light emission efficiency on the thickness of the cap layer 132 can be reduced. As a result of reducing the thickness dependency, it is not necessary to strictly control the thickness of the cap layer 132, so that reproducibility, productivity, and yield can be improved.

  In each of Examples 1 and 2, the cap layer and the barrier layer are single layers, but multiple layers may be used.

  In addition, since the present invention is characterized by the manufacturing process of the light emitting layer of the group III nitride semiconductor light emitting device, the structure of the group III nitride semiconductor light emitting device other than the light emitting layer and the manufacturing process thereof have been conventionally Various known structures and manufacturing processes can be employed. For example, the present invention can also be applied to a light-emitting element having a structure in which a conductive material is used as a substrate and electrodes are provided on the upper and lower sides to conduct in a direction perpendicular to the substrate.

  The group III nitride semiconductor light-emitting device obtained by the present invention can be used for lighting devices and the like.

10: sapphire substrate 11: n-type contact layer 12: n-type cladding layer 13, 23: light emitting layer 14: p-type cladding layer 15: p-type contact layer 16: p-electrode 17: n-electrode 131: well layer 132: cap layer 133: Barrier layer

Claims (3)

A well layer made of a group III nitride semiconductor containing In, a cap layer made of a group III nitride semiconductor having a larger band gap than the well layer, and a barrier layer made of a group III nitride semiconductor having a larger band gap than the well layer , And the well layer and the cap layer have the same growth temperature, and the growth temperature of the barrier layer is higher than the growth temperature of the well layer and the cap layer. In the method for manufacturing a group nitride semiconductor light emitting device,
The barrier layer is AlGaN,
The cap layer is AlGaN having an Al composition ratio larger than 0 and less than or equal to the Al composition ratio of the barrier layer.
A method for producing a Group III nitride semiconductor light-emitting device.   2. The method for manufacturing a group III nitride semiconductor light emitting device according to claim 1, wherein the cap layer is made of AlGaN equal to an Al composition ratio of the barrier layer.   3. The method for producing a group III nitride semiconductor light-emitting device according to claim 1, wherein the cap layer has a thickness of 1 to 8 molecular layers.

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