1. Field of the Invention
The present invention relates to a semiconductor substrate and a method for preparing the same. More specifically, it relates to a semiconductor substrate suitable for dielectric isolation, or an electronic device or an integrated circuit formed in a monocrystalline semiconductor layer on an insulator, and a method for preparing the semiconductor substrate.
2. Related Background Art
It is widely known that the formation of a monocrystalline semiconductor layer on an insulator is accomplished by a silicon on insulator (SOI) technique. A device formed by the utilization of the SOI technique has various advantages which cannot be achieved by a bulk Si substrate for use in the manufacture of a usual Si integrated circuit, and for this reason, many researches have been conducted. That is, by utilizing the SOI technique, the following advantages can be obtained:
1. dielectric isolation is easy, and high integration is possible, PA1 2. excellent radiation resistance can be obtained, PA1 3. floating capacity can be reduced, and speed up is possible, PA1 4. a well step can be omitted, PA1 5. latch up can be prevented, and PA1 6. a complete depletion type field effect transistor can be obtained by the formation of a thin film. PA1 (1) After the surface oxidation of a Si monocrystal substrate, a window is formed to partially expose the Si substrate, and epitaxial growth is then carried out in a lateral direction, using the exposed portion as a seed, to form a Si monocrystal layer on SiO.sub.2 (in this case, a Si layer is simultaneously deposited on SiO.sub.2). PA1 (2) The Si monocrystal substrate itself is used as an active layer, and SiO.sub.2 is then formed under the same (in this procedure, no Si layer is deposited). PA1 (3) After the epitaxial growth of Si on the Si monocrystal substrate, dielectric isolation is carried out (this method is accompanied by the deposition of the Si layer).
In order to realize the above-mentioned many advantages based on device characteristics, research has focused on made about formation methods of the SOI structure over the last several decades. The content of the research is summarized in the following literature.
Special Issue: "Single-crystal silicon on non-single-crystal insulators"; edited by G. W. Cullen, Journal of Crystal Growth, Volume 63, No. 3, pp. 429-590 (1983).
From old times, SOS (silicon on sapphire) has been known which can be formed by the hetero-epitaxy of Si on a monocrystal sapphire substrate by the use of CVD (chemical vapor deposition). This has succeeded for the present as the best SOI technique. In this case, however, many crystal defects are formed owing to lattice mismatch in the interface between the Si layer and an underlying sapphire substrate. However, the Si layer tends to be contaminated with alumina coming from the sapphire substrate. Above all, the substrate of SOS is expensive and the development of area enlargement is slow, which prevents the expansion of its application. In relatively recent years, it has been attempted to realize the SOI structure without using the sapphire substrate. Attempt can be classified into the following three categories.
However, Si is an indirect transition type semiconductor, and in order to form an optical device such as a light emitting element, a monocrystal layer of the III-V group, particularly GaAs or the like of a direct transition type is required.
A method for preparing the monocrystal thin film of the III-V group comprises first forming a nucleus for the lattice information of the crystal or a stripe-like pattern from SiN or the like, and then growing it in accordance with MBE (molecular beam epitaxy) or MOCVD (metal organic CVD). However, the monocrystal region is limited to a region of at most several .mu.m, and since a crystal direction is unsettled, it is difficult to apply the monocrystal thin film to semiconductor to provide still more integrated circuits.
In the case that a compound semiconductor such as GaAs is epitaxially grown on the Si substrate, the grown film is poor in crystallinity because of the difference between the lattice constant and the thermal expansion coefficient, and therefore it is very difficult to apply the grown film to devices.