Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
2001-01-02
2001-09-18
Chaudhuri, Olik (Department: 2814)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
Reexamination Certificate
active
06291258
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor photonic device, a method for making the same, and a method for forming a ZnO film. In particular, the present invention relates to a semiconductor photonic device using a Group III-V compound, such as GaN, InGaN, GaAlN or InGaAlN and a method for making the same. Also, the present invention relates to a method for making a ZnO film formed on a substrate, such as a Si substrate or a glass substrate.
2. Description of the Related Art
As materials for semiconductor photonic devices, such as light-emitting diodes (LEDs) and semiconductor laser diodes (LDs) which emit blue or ultraviolet light, Group III-V semiconductor compounds represented by the general formula In
x
Ga
y
Al
z
N wherein x+y+z=1, 0≦x≦1, 0≦y≦1, and 0≦z≦1 are known. Since the semiconductor compounds are of direct transition type, they have high luminescent efficiency. Furthermore, since the luminescent wavelength can be controlled by the indium content, they have attracted attention as materials for photonic devices.
Since it is difficult to make a large In
x
Ga
y
Al
z
N single crystal, a so-called heteroepitaxial growth process in which an In
x
Ga
y
Al
z
N film is grown on a substrate composed of a different material is used in the production of the crystal film, and it is typically grown on a C-plane sapphire substrate. Because the C-plane sapphire substrate is expensive and there exists a large lattice mismatch between the C-plane sapphire substrate and the In
x
Ga
y
Al
z
N film (for example, the lattice mismatch rate for GaN ranges to 16.1%), many crystal defects with a defect density of 10
8
/cm
2
to 10
11
/cm
2
are inevitably formed in the grown crystal, and thus a high-quality crystal film having high crystallinity cannot be formed.
In order to solve this problem, a proposed method for obtaining a crystal with decreased defects by reducing lattice mismatch when In
x
Ga
y
Al
z
N is deposited on a C-plane sapphire substrate is to provide a polycrystalline or amorphous AlN buffer layer or a low-temperature-deposited GaN buffer layer on the C-plane sapphire substrate. Since this method can reduce the lattice mismatch not only between the C-plane sapphire substrate and the buffer layer but also between the buffer layer and the In
x
Ga
y
Al
z
N, a crystal film with reduced defects can be formed. The C-plane sapphire substrate used in this method, however, is expensive and since the configuration is complicated, higher production costs are unavoidable.
A SiC substrate has been studied and has small lattice mismatch (for example, the lattice mismatch rate for GaN is 3.5%). The SiC substrate, however, is considerably expensive compared to the C-plane sapphire substrate (its price is approximately ten times the price of the C-plane sapphire substrate).
Accordingly, production of a semiconductor photonic device using an inexpensive Si or glass substrate has been desired. A possible method is depositing a ZnO buffer layer on a Si or glass substrate, and providing a GaN layer on the ZnO buffer layer followed by forming an In
x
Ga
y
Al
z
N semiconductor layer for emitting light on the GaN layer (or providing an In
x
Ga
y
Al
z
N semiconductor layer containing a GaN layer). Because the lattice constant in the a-axis direction (hereinafter referred to as “a-constant”) and the lattice constant in the c-axis direction (herein after referred to as “c-constant”) of the ZnO single crystal are nearly equal to the a-constant and the c-constant, respectively, of GaN, a GaN layer with reduced lattice defects is considered to be formed. The ZnO crystal is hexagonal and the crystal grows so that the c-axis direction is perpendicular to the surface of the Si or glass substrate whereas the a-axis direction is parallel to the surface of the Si or glass substrate.
TABLE 1
Crystal
a-constant
c-constant
GaN
3.1860 Å
5.1780 Å
ZnO
3.24982 Å
5.20661 Å
A device having a ZnO buffer layer provided on a Si substrate has a substrate cost which is approximately one-tenth that of a C-plane sapphire substrate and thus cost reduction can be achieved. Since the Si substrate can have conductivity in contrast to insulating characteristics of the C-plane sapphire, a p-type electrode and a n-type electrode can be provided on the upper face and the lower face and the device configuration can be simplified.
A lattice mismatch rate of 2% is still present between the ZnO buffer layer formed on the Si substrate and GaN layer, as shown in Table 1, although the rate is smaller than that in a combination of GaN and a C-plane sapphire substrate or a SiC substrate. Thus, defects formed by the lattice mismatch still remain.
SUMMARY OF THE INVENTION
The present invention is directed to solve the above-explained technical problems. The semiconductor photonic device comprises: a substrate; a ZnO buffer layer provided on the substrate; and a semiconductor compound provided on the ZnO buffer layer and represented by In
x
Ga
y
Al
z
N wherein x+y+z=1, 0≦x≦1, 0≦y≦1 and 0≦z≦1, wherein the ZnO buffer layer has a lattice constant of about 5.2070Å or more in the c-axis direction.
It is preferable that the ZnO buffer layer has a lattice constant of about 5.21 to 5.28 Å in the c-axis direction and a lattice constant of about 3.24 to 3.17 Å in the a-axis direction.
The method for making a semiconductor photonic device which comprises a semiconductor compound represented by In
x
Ga
y
Al
z
N wherein x+y+z=1, 0≦x≦1, 0≦y≦1 and 0≦z≦1, comprises when the lattice constant in the a-axis direction of the compound semiconductor formed on the Zn buffer layer is smaller than the lattice constant in the a-axis direction of a ZnO single crystal, the lattice constant in the c-axis direction of a ZnO buffer layer is adjusted so as to be larger than the lattice constant in the c-axis direction of the ZnO single crystal, and when the lattice constant in the a-axis direction of the semiconductor compound formed on the Zn buffer layer is larger than the lattice constant in the a-axis direction of the ZnO single crystal, the lattice constant in the c-axis direction of the ZnO buffer layer is adjusted so as to be smaller than the lattice constant in the c-axis direction of the ZnO single crystal, so that the lattice constant in the a-axis direction of the ZnO buffer layer is nearly equal to the lattice constant in the a-axis direction of the semiconductor compound.
The method forms a ZnO film on a substrate in which the lattice constant in the a-axis direction of the ZnO film is controlled by the lattice constant in the c-axis direction of the ZnO film.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
REFERENCES:
patent: 8-139361 (1996-05-01), None
patent: 8-116090 (1996-05-01), None
patent: 10-178202 (1998-06-01), None
patent: 10-173228 (1998-06-01), None
Srivastav et al., Effects of oxygen on the physical parameters of RF sputtered ZnO thin film, 1989, J. Phys D V22, pp1768-1772.*
“Growth and Structure Characterization of GaN Crystal on Polycrystalline ZnO Substrate by ECR-MBE Process”; H. Tochishita, et al.;Journal of Japanese Association of Crystal Growth; vol. 25, No. 3; 1998; p. A38.
“In-plane Structure Characterization of ZnO Nanocrystal Films Using Four-Circles X-ray Diffractometer”; I. Ohkubo, et al.; The Japan Society of Applied Physics; Catalog No. AP971120-01; No. 1; 58thAutumn Meeting; 1997; p. 281.
“Relation between optical property and crystallinity of ZnO thin films prepared by rf magnetron sputtering”; Syuichi Takada; J. Appl. Phys.; 73 (10); May 15, 1993, pp. 4739-42.
Chaudhuri Olik
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Wille Douglas A.
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