Epitaxial aluminium-gallium nitride semiconductor substrate

Details Epitaxial aluminium-gallium nitride semiconductor substrate Epitaxial aluminium-gallium nitride semiconductor substrate

2001-09-06

2003-03-18

Tran, Minh Loan (Department: 2826)

Active solid-state devices (e.g., transistors, solid-state diode

Specified wide band gap semiconductor material other than...

C257S022000, C257S103000, C257S192000, C257S197000, C257S613000, C257S615000, C257S190000

Reexamination Certificate

active

06534791

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to semiconductor deposition substrates and more particularly to Group III nitride semiconductor deposition substrates and a method for forming same.
BACKGROUND OF THE INVENTION
In the past Group III nitride semiconductor deposition substrates have been used for laser diodes, modulation doped field effect transistors, ridge wave-guide laser diodes p-n junction type photo-detectors, semiconductor multi-layer reflective films, and sub-band transition devices. Also, high efficiency, short wavelength lasers have been developed using Group III nitride semiconductors containing aluminum (Al), gallium (Ga), indium (In), and nitrogen (N), and having the general structural formula of Al
x
Ga
1−x−y
In
y
N (where x and y are mole fractions). However, the wafer size of the single crystalline Group III nitride semiconductors has been small so far because it has been difficult to grow large scale bulk crystals of these materials. In order to attain an acceptable wafer size, Group III nitride semiconductor deposition substrates have been used, which are formed by the deposition of the Group III nitride semiconductors on top of substrates of materials such as sapphire (Al
2
O
3
), silicon carbide (SiC), spinel (MgO), gallium arsenide (GaAs), or silicon (Si).
However, this solution has not been entirely satisfactory because there are considerable lattice mismatches and differences in coefficients of thermal expansion between the different types of substrates and the Group III nitride semiconductors. For example, the lattice mismatching is 11% to 23%, and the difference in coefficients of thermal expansion is approximately 2*10
−6
K
−1
between a sapphire substrate and a typical Group III nitride semiconductor. Consequently. Group III nitride semiconductor layers, which consist of a thin film of the Group III nitride semiconductor deposited on different types of substrate, have poor crystal quality, as well as poor electrical and optical properties.
There have been numerous attempts to improve the crystal quality of the Group III nitride semiconductor layers deposited or grown on the various substrates. One of the most effective methods has been to obtain a multi-layer deposition substrate by growing several pairs of single crystalline layers and low-temperature-deposited buffer layers of Group III nitride semiconductor on different types of substrates. Here a low-temperature-deposited buffer layer is one which is deposited at a temperature at which single crystals do not grow.
There are a number of different types of low-temperature-deposited buffer layers, such as those in which single crystallization of the buffer layers is carried out to the desired extent prior to the growth of the next single crystal after deposition.
Occasionally, it is necessary to grow aluminum gallium nitride (Al
x
Ga
1−x
N, where x is a mole fraction from zero to one (0≦x≦1)) on underlying Group III nitride semiconductor deposition layers. A number of different techniques have been developed for accomplishing this.
In Japanese Laid-Open Patent Application No. H 4-297023 to Nakamura, a single crystalline layer of a gallium nitride (GaN)-based semiconductor, grown on a buffer layer of gallium aluminum nitride (Ga
x
Al
1−x
N, where x is a mole fraction, 0≦x≦1) at a temperature at which single crystals will not grow on the substrate, yielded a single crystalline layer of a higher quality, GaN-based semiconductor than when a single crystalline layer of a GaN-based semiconductor was grown on an aluminum nitride (AlN) buffer layer. Nakamura goes on to say that when a GaN thin film was grown on a substrate, the advantages of using a GaN buffer layer, rather than an AlN buffer layer, included:
(1) single crystallization that occurred more readily even when the temperature rose because of the lower melting point (therefore, the benefits of the buffer layer were realized even if the buffer layer was made thicker); and
(2) an increase in crystal quality since the material being grown was the same as the material on which it was grown (e.g., when an epitaxial layer of GaN was grown over the GaN buffer layer).
In Japanese Laid-Open Patent Application No. H 9-199759, Akasaki et al. disclosed a technique in which a low-temperature-deposited buffer layer, formed at a temperature at which single crystals will not grow, and a single crystalline layer, formed at a temperature at which single crystals will grow, were alternately built up in three or more pairs on a substrate of a different material. The targeted Group III nitride semiconductor layer was then formed over the top-most single crystal layer at a temperature at which single crystals grow. A working example was disclosed in which an AlN low-temperature-deposited buffer layer (deposition temperature of 400° C. and a thickness of 50 nm) and a GaN single crystalline layer (at a deposition temperature of 1150° C. and a film thickness of 300 nm) was alternately built up in three layers each. The uppermost GaN single crystalline layer in Akasaki was grown to a thickness of 1.5 &mgr;m and was etched with potassium hydroxide (KOH). The etch pit density was measured from a scanning electron micrograph and found to be 4*10
7
cm
−2
, after the deposition of one layer pair on a sapphire substrate and 8*10
5
cm
−2
when three layer pairs were deposited.
In Japanese Laid-Open Patent Application No. H 7-235692, Sato disclosed a technique for improving the crystal quality of a single crystalline layer grown by using a plurality of low-temperature-deposited buffer layers. A working example was disclosed in which AlN low-temperature-deposited buffer layers and AlGaN low-temperature-deposited buffer layers were consecutively deposited on a sapphire substrate, over which a GaN single crystalline layer was grown to a thickness of 4 &mgr;m. The low-temperature-deposited buffer layers were not limited to AlGaN low-temperature-buffer layers and could have been any one of those whose lattice constant was between that of sapphire and GaN.
In Japanese Laid-Open Patent Application No. H 10-256666, Uchida proposed making the deposition temperature of the low-temperature-deposited buffer layers deposited first higher than that of the low-temperature-deposited buffer layers subsequently deposited for improving the crystal quality of a single crystalline layer grown by using a plurality of low-temperature-deposited buffer layers.
In Japanese Laid-Open Patent Application No. H 11-162847, Amano et al. disclosed the basic process of alternately growing multiple layers on the same or different type of substrate of a low-temperature-deposited buffer layer, formed at a temperature at which single crystals will not grow, and a single crystalline layer formed at a temperature at which single crystals will grow.
In Japanese Patent Application No. H 10-313993, Takeuchi et al. disclosed a nitride semiconductor laser element, including a low-temperature-deposited buffer layer containing AlN and a nitride semiconductor single crystalline layer containing AlN and grown directly on the low-temperature-deposited buffer layer as an over 1-&mgr;m-thick cladding layer, which provided a single-peak far field pattern.
In Japanese Patent Application No. H 10-322859, Iwaya et al. disclosed that when a GaN single crystalline layer was grown, cracking occurred by the ninth layer pair if a GaN low-temperature-deposited buffer layer was used, but no cracking occurred even with twelve layer pairs if an AlN low-temperature-deposited buffer layer was used. If an AlN low-temperature-deposited buffer layer was used, the in-plane strain of the GaN single crystalline layer was compressive strain and was nearly constant with respect to the number of layer pairs so it was believed that even more layers could be built up without cracking.
Although there has been a great deal of study in this area, there is a need for improvement of the crystal quality of an AlGaN deposition substrate. In particular AlN single crystals are great

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