Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
2002-04-10
2004-04-20
Wood, Elizabeth D. (Department: 1755)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S089000, C117S090000, C117S092000, C117S103000
Reexamination Certificate
active
06723165
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for fabricating a Group III nitride semiconductor substrate for use in a semiconductor laser which emits light at a shorter wavelength such as blue or purple light and in a transistor operating at a high temperature.
A Group III nitride semiconductor represented by Al
x
Ga
y
In
1-x-y
N (0≦x≦1, 0≦y≦1, 0≦x+y≦1) (hereinafter referred to as a Group III nitride semiconductor) is a material used in an optical device which emits light at a wavelength ranging in color from red to ultraviolet so that the applications thereof to a light emitting device and a light receiving device are expected. Thus far, a Group III nitride semiconductor film of relatively high quality has been formed conventionally by crystal growth on a sapphire substrate.
However, since the Group III nitride semiconductor film and the sapphire substrate do not lattice-match each other, the Group III nitride semiconductor film contains numerous crystal defects so that a device using a Group III nitride semiconductor has degraded properties.
If the Group III nitride semiconductor film formed on the sapphire substrate is used in a semiconductor laser or transistor, all electrodes should be formed on the Group III nitride semiconductor film since the sapphire substrate is a so-called insulating substrate which does not allow the passage of electricity. This has complicated a fabrication process for a device composed of a Group III nitride semiconductor and reduced the production yield thereof.
To increase the fabrication yield and performance of the device using the Group III nitride semiconductor, a group III nitride semiconductor substrate (especially a GaN substrate) having a high quality and a large area has been in growing demand. Under such circumstances, there have been proposed various methods in each of which a Group III nitride semiconductor film is grown on a substrate made of a different type of material (sapphire substrate or the like) and then the substrate made of the different type of material is removed.
For example, there has been known a conventional method in which a sapphire substrate and a GaN film are separated from each other by irradiation with an intense laser beam (Michael K. Kelly et al., Japanese Journal of Applied Physics Vol.38 p.L217-L219, 1999). A description will be given herein below to the conventional method with reference to
FIGS. 11A
to
11
C, which are cross-sectional views illustrating the process steps of the conventional method.
In the step shown in
FIG. 11A
, a GaN layer
102
with a thickness of 200 to 300 &mgr;m is formed on a sapphire substrate
101
having a diameter of 2 inches and a C surface as a principal surface by using hydride vapor phase epitaxy (hereinafter referred to as HVPE).
Next, in the step shown in
FIG. 11B
, the sapphire substrate
101
formed with the GaN layer
102
is retrieved from a HVPE reactor. Then, the lower surface of the GaN layer
102
a
is entirely scanned with a laser beam at a wavelength of 355 nm applied thereto through the sapphire substrate
101
. The arrow in the drawing represents the laser beam. As a result, heat is generated at the portion irradiated with the laser beam to decompose a lower portion of the GaN layer
102
.
Next, in the step shown in
FIG. 11C
, the sapphire substrate
101
and the GaN layer
102
are separated from each other so that an independent GaN substrate
102
a
is obtained.
However, the foregoing method has the following problems.
The respective thermal conductivities of GaAs and InP which are representatives of Group III-V compound semiconductors are 0.54 W/cmK and 0.68 W/cmK, while the thermal conductivity of Si used for a submount for heat dissipation is 1.5 W/cmK.
On the other hand, the thermal conductivity of GaN is 1.3 W/cmK. From a comparison between the thermal conductivity of GaN and the thermal conductivities of the foregoing materials, it will be understood that GaN is a material which readily conducts heat. In accordance with the conventional method which irradiates GaN with the laser beam, the heat generated in the lower portion of the GaN layer
102
through the absorption of the laser beam is likely to be diffused. This causes the problem that an amount of heat required to completely decompose the portion of the GaN layer
102
irradiated with the laser beam in the step shown in
FIG. 11B
is insufficient and the efficiency with which GaN is decomposed is reduced. If the efficiency with which GaN is decomposed is reduced, the sapphire substrate
101
and the GaN layer
102
should be separated from each other by increasing the number of times that the GaN layer
102
is scanned with the laser beam and thereby supplying a sufficient amount of heat to completely decompose the portion of the GaN layer
102
irradiated with the laser beam. Accordingly, the time required to perform the step shown in
FIG. 11B
is increased so that productivity is lowered.
Since GaN and sapphire do not lattice-match each other, the GaN layer
102
contains numerous crystal defects and distortions. As a result, an impact resulting from the release of a stress when GaN is decomposed may cause a fracture in the GaN substrate
102
a
obtained. If the number of scannings is increased, the probability of a fracture occurring in the GaN substrate
102
a
is increased.
Even if the GaN substrate
102
a
undergoes, a crack may remain within the GaN substrate
102
a
. If a device such as a light-emitting diode or a laser diode is fabricated by using a GaN substrate
102
a
having a crack remaining therein, the crack causes a leakage current and reduces the reliability of the device.
SUMMARY OF THE INVENTION
The present invention has been achieved to solve the foregoing problems and it is therefore an object of the present invention to provide a high-quality Group III nitride semiconductor substrate.
A method for fabricating a Group III nitride semiconductor substrate according to the present invention comprises the steps of: (a) preparing a substrate; (b) forming, on the substrate, a first semiconductor layer composed of a Group III nitride semiconductor; (c) forming, on the first semiconductor layer, a heat diffusion suppressing layer lower in thermal conductivity than the first semiconductor layer; (d) forming, on the heat diffusion suppressing layer, a second semiconductor layer composed of a Group III nitride semiconductor; and (e) irradiating the first semiconductor layer through the substrate with a light beam transmitted by the substrate and absorbed by the first semiconductor layer to decompose the first semiconductor layer.
In accordance with the present invention, the heat diffusion suppressing layer lower in thermal conductivity than the first semiconductor layer is formed between the first and second semiconductor layers to suppress the diffusion of heat generated through the absorption of the light beam by the first semiconductor layer. Accordingly, the majority of the generated heat contributes to the decomposition of the first semiconductor layer so that the first semiconductor layer is decomposed efficiently. Even if the number of scannings with the light beam is smaller than in the conventional embodiment, heat required to decompose the first semiconductor layer completely can be supplied in a sufficient amount so that productivity is increased. Since the number of scannings with the light beam is smaller than in the conventional embodiment, the probability of a fracture occurring in the Group III nitride semiconductor substrate separated from the second semiconductor layer can be reduced.
The Group III nitride semiconductor composing the heat diffusion suppressing layer may be lower in thermal conductivity than the Group III nitride semiconductor composing the first semiconductor layer.
The heat diffusion suppressing layer may be composed of a semiconductor represented by In
x
Ga
1-x
N (0<x≦1).
Preferably, the step (c) includes forming the heat diffusion suppressing layer and then forming an opening extending
Ishida Masahiro
Ogawa Masahiro
Takigawa Shinichi
Tamura Satoshi
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
Wood Elizabeth D.
LandOfFree
Method for fabricating Group III nitride semiconductor... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for fabricating Group III nitride semiconductor..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for fabricating Group III nitride semiconductor... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3221807