Method for forming GaN semiconductor single crystal...

Details Method for forming GaN semiconductor single crystal... Method for forming GaN semiconductor single crystal...

1997-12-05

2001-01-23

Bowers, Charles (Department: 2813)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

Compound semiconductor

C438S047000, C438S481000, C438S493000, C117S952000, C257S013000, C257S076000, C257S103000, C372S043010, C372S044010

Reexamination Certificate

active

06177292

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to a method for forming a single crystal GaN substrate, and more particularly, to a method for forming a single crystal GaN semiconductor substrate, which allows fast homoepitaxial growth of GaN group materials, and formation of a bulk single crystal GaN substrate, which has a very high quality almost free from crystalline defects, of a size required for growth of thin film in fabrication of an optical device; and a GaN diode fabricated with the single crystal substrate.
2. Discussion of the Related Art
In a conventional growth of GaN which is used in fabrication of blue optical device, a sapphire(Al
2
O
3
) substrate has been mostly used. However, the significant lattice mismatch (13.8%) and difference(25.5%) of thermal expansion coefficients between gallium nitride and sapphire cause difficulty in growing a good quality thin film. Even if a buffer layer disclosed in JP H4-170390 is employed in the growth of the thin film, the concentration of crystal defects occurring in the grown thin film is 10
9
-10
11
cm
−2
, which should be significantly reduced for practical laser diode applications, even though they may not cause any significant problems in LED applications. Although many efforts have been made for reducing the lattice mismatch employing silicon carbide(SiC) or spinnel as the substrate, which have lattice mismatches less than sapphire, the concentration of internal crystal defects in the grown thin film caused by the lattice mismatch could not be reduced significantly. Moreover, the difference of cleavage directions of GaN and sapphire in using a sapphire substrate causes difficulty in the application of a general cleaving method to fabrication of a laser cavity and also to the formation of electrodes on the back side of the sapphire substrate, which is an insulator. This places a limitation in the fabrication of a LED because the electrode formation process becomes complicated.
The requirement for employing a GaN substrate to overcome the aforementioned problems and to grow a large sized bulk single crystal GaN at room temperature equilibrium by an existing crystal growth method has been impossible because GaN has a melting point of over 2400° C. while nitrogen N
2
has an equilibrium vapor pressure of about 100 Kbar at 1,100° C. and 10,000 Kbar at 1,500° C. Recently, I. Gzegory et al. in “J. Phys. Chem. Solids, 56,656(1995)” discloses a solution method conducted at a high temperature and high pressure state of 1,300-1,600° C. and 8-17 Kbar to obtain a thin single crystal plate of a several millimeter size having a good quality with crystal defect concentration of about 100/cm
2
, which is still not satisfactory for use as a substrate. Detchprhom et al. in “J. Crystal Growth, 123,384(1993)” discloses a method in which a thick GaN layer is grown by HVPE (Halide Vapor Phase Epitaxy) on a ZnO buffer layer which has a comparatively little lattice mismatch of 2.2% with GaN formed on a sapphire substrate and the ZnO buffer layer is removed to separate bulk GaN single crystal layer from the sapphire substrate, intending to use the bulk GaN single crystal layer as a substrate. However, this method has a limitation in obtaining a good quality substrate of a required size due to the unstable chemical etching of the ZnO layer. T. Okada et al. in “J. J. Applied Physics, 35(5), 1637, 1966” discloses a sublimation method in which powdered GaN is subjected to sublimation into a GaN substrate under an ambient of nitrogen and ammonia, which was found not satisfactory. And, recently, R. J. Molnar et al. in “MRS Symp. Proc. Vol. 423,221, 1996” discloses a method in which about 50 &mgr;m thick GaN is grown on a sapphire substrate to form a homoepitaxy. However, as this method also requires a form of heterepitaxy, this method has a limitation in reducing the crystalline defect concentration in a GaN film.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a method for forming a single crystal GaN substrate and a GaN diode with the substrate that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method for forming a good quality bulk GaN group single crystal substrate of an area required for growth of a thin film in fabrication of an LED using homoepitaxy.
Another object of the present invention is to provide a method for forming a GaN single crystal substrate, which can be processed within a short time.
Another object of the present invention is to provide a method for forming a GaN single crystal substrate which has not only a low crystalline defect concentration, but also excellent reproducibility.
A Further object of the present invention is to provide a blue laser diode where possible crystal defects caused by lattice mismatch and thermal expansion coefficient are avoided by using an n
+
-GaN substrate grown by HVPE; and a method for fabricating the same.
A still further object of the present invention is to provide a GaN light emitting diode from which crystal defects caused by a lattice mismatch and a thermal expansion coefficient difference between a substrate and a light emitting device on the substrate are removed; and a method for fabricating the same.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the method for forming a GaN single crystal substrate includes the steps of (1) providing and pre-treating an oxide substrate, (2) growing a primary GaN layer on the pre-treated oxide substrate, (3) polishing the oxide substrate having the primary GaN layer grown thereon to remove a portion of the oxide substrate, (4) growing GaN again on the primary GaN layer after the step (3) to form a secondary GaN layer, (5) polishing the oxide substrate again to remove the remaining portion of the oxide substrate entirely, (6) growing a tertiary GaN layer on the primary and secondary GaN layers having the oxide substrate removed completely therefrom to form a bulk of a GaN semiconductor single crystal, and (7) polishing the GaN semiconductor single crystal to form a mirror like polished GaN substrate.
In other aspect of the present invention, there is provided a method for forming a GaN single crystal substrate including the steps of (1) providing an oxide substrate and pre-treating the oxide substrate in the reactor, (2) growing GaN on the pre-treated oxide substrate, to form a primary GaN layer, (3) cooling down the oxide substrate and the primary GaN layer, to separate the primary GaN layer from the oxide substrate, (4) growing GaN in a high growth rate with the separated GaN layer used as a substrate to form a secondary GaN layer, and forming a GaN bulk single crystal of a predetermined thickness from the secondary GaN layer, and (5) polishing the GaN bulk single crystal to form a mirrorlike polished GaN single crystal substrate.
In another aspect of the present invention, there is provided a GaN light emitting diode, including n
+
-type GaN substrate obtained by homoeptaxial growth with a halide vapor phase epitaxy and silicon doping, an n-type clad layer of Ga
x
Al
1-x
n (0≦x≦1) grown on the n
+
-type GaN substrate, an active layer of In
x
Ga
1-x
N (0≦x≦1) grown on the n-type clad layer and doped with magnesium, a p-type clad layer of an Ga
x
Al
1-x
N layer(0≦x≦1) grown on the active layer, and an n-type ohmic contact electrode and a p-type ohmic contact electrode on an under side of the substrate and on the p-type clad layer, respectively.
In a fu

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