Method of forming nitride based semiconductor layer

Details Method of forming nitride based semiconductor layer Method of forming nitride based semiconductor layer

1999-07-27

2001-11-20

Bowers, Charles (Department: 2813)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

Compound semiconductor

C117S952000

Reexamination Certificate

active

06319742

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having a compound semiconductor layer composed of BN (boron nitride), GaN (gallium nitride), AlN (aluminum nitride) or InN (indium nitride) or a group III-V nitride compound semiconductor (hereinafter referred to as a nitride based semiconductor) which is their mixed crystal and a method of fabricating the same, and a method of forming a nitride based semiconductor layer.
2. Description of the Background Art
In recent years, a GaN based light emitting semiconductor device (a light emitting semiconductor device based on GaN) has been put to practical use as a light emitting semiconductor device such as a light emitting diode which emits light in blue or violet or a semiconductor laser device. In fabricating the GaN based light emitting semiconductor device, there exists no substrate composed of GaN. Therefore, each layer is epitaxially grown on an insulating substrate composed of sapphire (Al
2
O
3
) or the like.
FIG. 18
is a cross-sectional view showing the structure of a conventional GaN based light emitting diode. A light emitting diode shown in
FIG. 18
is disclosed in Nikkei Micro Device, February, 1994, pp. 92 to 93.
In
FIG. 18
, a GaN buffer layer
62
, an n-GaN layer
63
, an n-AlGaN cladding layer
64
, an InGaN active layer
65
, a p-AlGaN cladding layer
66
, and a p-GaN layer
67
are formed in this order on a sapphire substrate
61
. A partial region from the p-GaN layer
67
to the n-GaN layer
63
is removed by etching. A p electrode
68
is formed on the top surface of the p-GaN layer
67
, and an n electrode
69
is formed on the exposed top surface of the n-GaN layer
63
. Such a structure of the light emitting diode is referred to as a lateral structure.
The light emitting diode shown in
FIG. 18
has a pn junction having a double hetero structure in which the InGaN active layer
65
is interposed between the n-AlGaN cladding layer
64
and the p-AlGaN cladding layer
66
, and can emit light in blue.
In a conventional GaN based light emitting semiconductor device as shown in
FIG. 18
, however, dislocations of around 10
9
/cm
2
generally exist in a GaN based semiconductor crystal which is grown on a sapphire substrate depending on the difference in lattice constant between GaN and the sapphire substrate. Such dislocations are propagated from the surface of the sapphire substrate to a GaN based semiconductor layer. In the light emitting semiconductor device composed of the GaN based semiconductor layer on the sapphire substrate, device characteristics and reliability are degraded due to the dislocations.
As a method of solving the problem of the degradation of the device characteristics and the reliability due to the dislocations, epitaxial lateral overgrowth has been proposed. The epitaxial lateral overgrowth is reported in “Proceedings of The Second International Conference on Nitride Semiconductors”, Oct. 27-31, 1997, Tokushima, Japan, pp.444-446.
FIG. 19
is a schematic sectional view showing the steps for explaining the conventional epitaxial lateral overgrowth.
As shown in FIG.
19
(
a
), an AlGaN buffer layer
82
is grown on a sapphire substrate
81
, and a GaN layer
83
is formed on the AlGaN buffer layer
82
. Dislocations
91
extending in the vertical direction exist in the GaN layer
83
. Striped SiO
2
films
90
are formed on the GaN layer
83
.
As shown in FIG.
19
(
b
), a GaN layer
84
is regrown on the GaN layer
83
exposed between the striped SiO
2
films
90
. Also in this case, the dislocations
91
extend in the vertical direction in the regrown GaN layer
84
.
As shown in FIG.
19
(
c
), when the GaN layer
84
is further grown, the GaN layer
84
is also grown in the lateral direction. Accordingly, the GaN layer
84
is also formed on the SiO
2
films
90
. No dislocations exist in the GaN layer
84
on the SiO
2
films
90
.
As shown in FIG.
19
(
d
), when the GaN layer
84
is further grown, the GaN film
84
is formed on the SiO
2
films
90
and on the GaN layer
83
between the SiO
2
films
90
.
When the epitaxial lateral overgrowth is used, a GaN crystal of high quality having no dislocations can be formed on the SiO
2
films
90
.
In a region where the SiO
2
films
90
do not exist, however, the dislocations
91
from the underlying GaN layer
83
extend to the surface of the regrown GaN layer
84
. Accordingly, the dislocations still exist on the surface of the GaN layer
84
. When the light emitting semiconductor device is fabricated, therefore, a light emitting region must be limited to a region on the SiO
2
films. Therefore, it is impossible to increase the size of the light emitting region.
When the area of the SiO
2
film is increased in order to increase the area of the GaN layer of high quality, the surface of the GaN layer which is grown in the lateral direction cannot be flattened.
In the conventional GaN based light emitting diode shown in
FIG. 18
, the sapphire substrate
61
is an insulating substrate. Therefore, the n electrode
69
cannot be provided on the reverse surface of the sapphire substrate
61
, and must be provided on the exposed surface of the n-GaN layer
63
. Therefore, a current path between the p electrode
68
and the n electrode
69
is longer, so that an operation voltage is higher, as compared with those in a case where the n electrode is provided on the reverse surface of a conductive substrate.
Furthermore, when a GaN based semiconductor laser device is fabricated, it is difficult to form cavity facets by a cleavage method as in a semiconductor laser device for emitting red light or infrared light using a GaAs substrate.
FIG. 20
is a diagram showing the relationship between the crystal orientations of a sapphire substrate and a GaN based semiconductor layer. In
FIG. 20
, an arrow by a solid line indicates the crystal orientation of the sapphire substrate, and an arrow by a broken line indicates the crystal orientation of the GaN based semiconductor layer.
As shown in
FIG. 20
, the a-axis and the b-axis of the GaN based semiconductor layer formed on the sapphire substrate are shifted 30° away from the a-axis and the baxis of the sapphire substrate.
FIG. 21
is a schematic perspective view of a semiconductor laser device composed of a GaN based semiconductor layer formed on a sapphire substrate.
In
FIG. 21
, a GaN based semiconductor layer
70
is formed on a (0001) plane of a sapphire substrate
61
. A striped current injection region
71
is parallel to a <11{overscore (2)}0> direction of the GaN based semiconductor layer
70
. In this case, a {1 {overscore (1)}00} plane of the GaN based semiconductor layer
70
is inclined at 30° to a {1{overscore (1)}00} plane of the sapphire substrate
61
. Both the sapphire substrate
61
and the GaN based semiconductor layer
70
are easily cleaved along the {1{overscore (1)}00} plane.
The respective cleavage directions of the sapphire substrate
61
and the GaN based semiconductor layer
70
thus deviate. When a GaN based semiconductor laser device is fabricated, therefore, it is difficult to form cavity facets by a cleavage method, as in the semiconductor laser device for emitting red light or infrared light which is formed on the GaAs substrate. In this case, the necessity of forming the cavity facets by etching is brought about. When the cavity facets are formed by etching, however, it is impossible to reduce an operation current of the semiconductor laser device because it is difficult to form facets perpendicular to the substrate.
On the other hand, various reports and proposals are made with respect to methods of controlling a transverse mode of the GaN based semiconductor laser device. Almost all of the methods of controlling the transverse mode include two types, i.e., a ridge waveguided structure and a self-aligned structure which are employed by the conventional semiconductor laser device for emitting red light or infrared light.
Since the GaN based semiconductor layer is chemically

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