Nitride-based semiconductor element and method of forming...

Details Nitride-based semiconductor element and method of forming... Nitride-based semiconductor element and method of forming...

2002-05-06

2004-04-13

Mai, Son L. (Department: 2818)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

C438S036000, C438S037000, C438S045000, C438S046000, C438S047000

Reexamination Certificate

active

06720196

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nitride-based semiconductor element and a method of forming a nitride-based semiconductor, and more specifically, it relates to a nitride-based semiconductor element having a group III-V nitride-based semiconductor (hereinafter referred to as a nitride-based semiconductor) such as GaN (gallium nitride), AlN (aluminum nitride), InN (indium nitride), BN (boron nitride) or TlN (thallium nitride) or a mixed crystal thereof and a compound semiconductor layer consisting of a group III-V nitride-based semiconductor such as a mixed crystal containing at least one element of As, P and Sb and a method of forming a nitride-based semiconductor.
2. Description of the Prior Art
A semiconductor element utilizing a GaN-based compound semiconductor is actively developed as a semiconductor element applied to a semiconductor light-emitting device such as a light-emitting diode or an electronic element such as a transistor. In order to fabricate such a GaN-based semiconductor element, a GaN-based semiconductor layer is epitaxially grown on a substrate consisting of sapphire, SiC, Si, GaAs or the like, since a substrate consisting of GaN is hard to fabricate.
However, the substrate of sapphire or the like and GaN have different lattice constants. Therefore, threading dislocations (lattice defects) vertically extending from the substrate are present in the GaN-based semiconductor layer grown on the substrate of sapphire or the like. The dislocation density of these lattice defects is about 10
9
cm
−2
. Such dislocations in the GaN-based semiconductor layer deteriorate the element characteristics of the semiconductor element and reduce the reliability thereof.
In order to reduce the number of dislocations in the aforementioned GaN-based semiconductor layer, therefore, epitaxial lateral overgrowth (ELO) is proposed in general. This epitaxial lateral overgrowth is disclosed in Journal of Oyo Denshi Bussei Bunkakai (1998) pp. 53 to 58 and pp. 210 to 215, for example.
In a method of forming a GaN-based semiconductor layer employing the aforementioned epitaxial lateral overgrowth, selective growth masks are first formed on prescribed regions of an underlayer. A GaN-based semiconductor layer is epitaxially laterally overgrown from exposed portions of the underlayer. In this case, facets of the GaN-based semiconductor having a triangular section are grown upward from the exposed portions of the underlayer, and thereafter epitaxially laterally grown on the selective growth masks. Thus, the facets coalesce with each other on the selective growth masks, to form a continuous film. Thus, a flat GaN-based semiconductor layer is formed on the underlayer and the selective growth masks. Crystal defects of the underlayer only partially propagate to the GaN-based semiconductor layer obtained by such epitaxial lateral overgrowth, and hence the dislocation density can be reduced to about 10
7
cm
−2
.
Further, a method of reducing the number of dislocations in a GaN-based semiconductor layer through a dislocation loop effect of quantum dots is developed in general. This method is disclosed in Jpn. J. Appl. Phys. Vol. 39 (2000), L831-834, for example. In the aforementioned conventional method utilizing the dislocation loop effect of the quantum dots, dislocations of an underlayer are trapped in the quantum dots in a looped manner, to be only partially propagated to a GaN-based semiconductor layer. Thus, a GaN-based semiconductor layer having a small number of dislocations can be formed.
However, the aforementioned methods of reducing the number of dislocations in the nitride-based semiconductor by epitaxial lateral overgrowth and the quantum dots respectively have the following problems:
In the aforementioned method of reducing the dislocation density of the nitride-based semiconductor through the epitaxial lateral overgrowth, laterally grown layers (facets) for forming the nitride-based semiconductor layer coalesce (bond) with each other on central portions of the selective growth masks, and hence portions having relatively high dislocation density are disadvantageously formed above the central portions of the selective growth masks (above bonding portions of the facets). Further, portions having relatively high dislocation density are disadvantageously formed above the central portions of openings of the selective growth masks (around the tops of the facets) due to relatively high dislocation density around the tops of the facets.
In order to solve the aforementioned problems, a method of repeating epitaxial lateral overgrowth thereby reducing the number of dislocations is proposed in Jpn. J. Appl. Phys. Vol. 39 (2000) L647-650, for example. In this conventional proposed method, a first GaN-based semiconductor layer is formed on selective growth masks provided on an underlayer by first epitaxial lateral overgrowth, followed by formation of selective growth mask layers on the first GaN-based semiconductor layer. A second GaN-based semiconductor layer is formed on the first GaN-based semiconductor layer by second selective epitaxial lateral overgrowth, so that the second GaN-based semiconductor layer is further reduced in dislocation density as compared with the first GaN-based semiconductor layer. Such epitaxial lateral overgrowth is so repeated that a GaN-based semiconductor layer having a smaller number of dislocations can be formed.
In the aforementioned conventional proposed method, however, epitaxial lateral overgrowth must be repeated and hence the step of forming the GaN-based semiconductor layer is disadvantageously complicated.
In the aforementioned conventional proposed method, further, a plurality of GaN-based semiconductor layers must be formed by repeating epitaxial lateral overgrowth. Therefore, the thickness of the wafer is so increased that the wafer is disadvantageously warped. Thus, the number of failures is increased due to the warped wafer in later steps, to disadvantageously reduce the yield as a result.
In the aforementioned method of reducing the number of dislocations in the GaN-based semiconductor layer by the dislocation loop effect of the quantum dots, the dislocation density can be reduced to only about 10
8
cm
−2
.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of forming a nitride-based semiconductor capable of forming a nitride-based semiconductor layer having low dislocation density with a small thickness.
Another object of the present invention is to provide a nitride-based semiconductor element having excellent element characteristics, including a nitride-based semiconductor layer having low dislocation density with a small thickness.
A method of forming a nitride-based semiconductor according to a first aspect of the present invention comprises steps of laterally growing a nitride-based semiconductor layer on the upper surface of an underlayer and forming quantum dots on the laterally grown nitride-based semiconductor layer.
In the method of forming a nitride-based semiconductor according to the first aspect, the nitride-based semiconductor is laterally grown on the upper surface of the underlayer and the quantum dots are formed on the laterally grown nitride-based semiconductor layer, whereby the number of dislocations reduced by the lateral growth can be further reduced by a dislocation loop effect by the quantum dots. Thus, a nitride-based semiconductor layer having lower dislocation density can be formed as compared with a case of reducing the number of dislocations only by lateral growth. Consequently, a high-quality nitride-based semiconductor having a small number of dislocations can be formed. The number of dislocations can be sufficiently reduced by single lateral growth due to the effects of reducing the number of dislocations by the lateral growth and the quantum dots, whereby the lateral growth may not be repeated for attaining a sufficient effect of reducing the number of dislocations. Thus, the thickness of the nitride-based se

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