Semiconductor device and method of fabricating the same

Details Semiconductor device and method of fabricating the same Semiconductor device and method of fabricating the same

2000-02-23

2002-07-16

Smith, Matthew (Department: 2825)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

C438S020000, C438S022000, C438S047000, C257S009000, C257S010000, C257S011000, C257S013000, C257S021000, C372S007000, C372S043010, C372S044010, C372S045013

Reexamination Certificate

active

06420197

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device comprising a nitride compound semiconductor layer and a method of fabricating the same.
A nitride compound semiconductor such as GaN, InN, or AlN is a material suitable for use in a blue semiconductor laser device or in a transistor operable at a high speed even at a high temperature.
There has conventionally been known a technique for the crystal growth of a nitride compound semiconductor on a Si (silicon) substrate (A. Watanabe et al., Journal of Crystal Growth volume 128 (1993) pp. 391-396).
As a first conventional embodiment, a laser diode comprising a nitride compound semiconductor layer formed on a silicon substrate will be described with reference to FIG.
6
.
As shown in
FIG. 6
, an AlN layer
11
as a buffer layer, a GaN layer
12
as a first contact layer, a first clad layer
13
made of n-type AlGaN, an active layer
14
made of undoped GaInN, a second clad layer
15
made of p-type AlGaN, and a second contact layer
16
made of p-type GaN are stacked successively in layers on a silicon substrate
10
. The AlN layer
11
is formed by growing an AlN crystal on the silicon substrate
10
. The GaN layer
12
is formed by growing a GaN crystal on the AlN layer
11
at a temperature of 1050° C. and doped with an impurity such as Si, Ge, or Se to have the n-type conductivity. To form the GaN layer
12
, metal organic vapor phase epitaxial growth (hereinafter referred to as MOVPE) is used.
A p-type electrode
18
made of an Ni—Au alloy on the second contact layer
16
with a current restricting layer
17
having an opening of
17
a
interposed therebetween, while an n-type electrode
19
is formed on a back surface of the silicon substrate
10
.
In the first conventional embodiment, a tensile strain is applied from the silicon substrate
10
to the GaN layer
12
and an internal stress is produced in the GaN layer
12
in response to the tensile strain when the temperature of the silicon substrate
10
is lowered from the crystal growth temperature of 1050° C. to a room temperature after the formation of the GaN layer
12
. This is because the thermal expansion coefficient (2.55×10
−6
/K) of Si is lower than the thermal expansion coefficient (5.59×10
−6
/K) of GaN. The internal stress produced in the GaN layer
12
increases disadvantageously to form a crack (crevice) in the GaN layer
12
. Thus, the method for the crystal growth of a nitride compound semiconductor on a silicon substrate is not practical.
Therefore, a technique for the crystal growth of a nitride compound semiconductor on a sapphire substrate has been used instead (U.S. Pat. No. 5,777,350).
As a second conventional embodiment, a laser diode comprising a nitride compound semiconductor layer formed on a sapphire substrate will be described with reference to FIG.
7
.
As shown in
FIG. 7
, an AlN layer
21
as a buffer layer, a GaN layer
22
as a first contact layer, a first clad layer
23
made of n-type AlGaN, an active layer
24
made of undoped GaInN, a second clad layer
25
made of p-type AlGaN, and a second contact layer
26
made of p-type GaN are stacked successively in layers on a sapphire substrate
20
. The AlN layer
21
is formed by growing an AlN crystal on the sapphire substrate
20
. The GaN layer
22
is formed by growing a GaN crystal on the AlN layer
21
by using MOVPE at a temperature of 1050° C. The GaN layer
22
is doped with an impurity such as Si, Ge, or Se to have the n-type conductivity. It is to be noted that a device structure composed of the GaN layer
22
as the first contact layer, the first clad layer
23
, the active layer
24
, the second clad layer
25
, and the second contact layer
26
has been partially removed by dry etching till the GaN layer
22
is etched halfway.
A p-type electrode
28
made of an Ni—Au alloy is formed on the second contact layer
26
with a current restricting layer
27
having an opening
27
a
interposed therebetween, while an n-type electrode
29
made of an Ni—Au alloy is formed in a space corresponding to the etched portion of the GaN layer
22
, i.e., the first contact layer.
According to the second conventional embodiment, a crack is less likely to occur in the GaN layer
22
than in the first conventional embodiment since the difference between the thermal expansion coefficient (7.5×10
−6
/K) of sapphire (Al
2
O
3
) and the thermal expansion coefficient of GaN is smaller than the difference between the thermal expansion coefficient of Si and that of GaN.
In the second conventional embodiment, however, a compression strain is applied from the sapphire substrate
20
to the GaN layer
22
and an internal stress is produced in the GaN layer
22
in response to the compression strain when the temperature of the sapphire substrate
20
is lowered from the crystal growth temperature of 1050° C. to a room temperature after the formation of the GaN layer
22
, since the thermal expansion coefficient of sapphire is higher than that of GaN. This prevents an improvement in the crystalline characteristics of the GaN layer
22
and causes the first problem that it is difficult to reduce an operating current for the laser diode.
The second conventional embodiment also presents the second problem that it is difficult to fabricate a laser diode having a smooth reflecting mirror surface since it is difficult to cleave the sapphire substrate
20
.
To solve the second problem, there has been proposed a technique for forming a semiconductor substrate composed of a thick-film nitride compound semiconductor layer which has been formed by the crystal growth of a nitride compound semiconductor on a sapphire substrate and separated therefrom (Japanese Unexamined Patent Publication No. HEI 7-165498).
As a third conventional embodiment, a method of forming a laser diode by using a semiconductor substrate composed of a nitride compound semiconductor layer will be described with reference to FIGS.
8
(
a
) to (
d
).
First, as shown in FIG.
8
(
a
), an AlN layer
31
as a buffer layer is formed by growing an AlN crystal on a sapphire substrate
30
.
Next, as shown in FIG.
8
(
b
), a GaN layer
32
as a compound semiconductor layer is formed by growing a GaN crystal on the AlN layer
31
at a temperature of 1050° C.
Next, as shown in FIG.
8
(
c
), the AlN layer
31
and the GaN layer
32
are separated from the sapphire substrate
30
by removing the sapphire substrate
30
by polishing, whereby a semiconductor substrate
33
composed of the AlN layer
31
and the GaN layer
32
is formed.
Next, as shown in FIG.
8
(
d
), a first contact layer
34
made of n-type GaN, a first clad layer
35
made of n-type AlGaN, an active layer
36
made of undoped GaInN, a second clad layer
37
made of p-type AlGaN, and a second contact layer
38
made of p-type GaN are formed successively on the semiconductor substrate
33
. Thereafter, a p-type electrode is formed on the second contact layer
38
with a current restricting layer interposed therebetween and an n-type electrode is formed on a back surface of the semiconductor substrate
33
, though they are not depicted, whereby the laser diode is completed. According to the third conventional embodiment, a laser diode having a smooth reflecting mirror surface can be fabricated since the semiconductor substrate
33
is cleaved easily.
However, the third conventional embodiment has the problem that the crystalline characteristics of the GaN layer
32
composing the semiconductor substrate
33
cannot be improved due to the difference between the thermal expansion coefficient of sapphire and that of GaN, similarly to the second conventional embodiment. The third conventional embodiment also has the problem that the crystalline characteristics of the GaN layer are further degraded as the thickness of the GaN layer
32
, i.e., the thickness of the semiconductor substrate
33
is increased.
SUMMARY OF THE INVENTION
In view of the foregoing, it is therefore an object of the present invention to improve the crystall

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