Fishing – trapping – and vermin destroying
Patent
1987-08-05
1989-05-09
Hearn, Brian E.
Fishing, trapping, and vermin destroying
148DIG65, 148DIG72DIG.110, 148DIG169, 148DIG160, 156612, 437110, 437126, 437949, H01L 21203
Patent
active
048290220
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for forming thin III-V compound semiconductor films and more particularly a method for forming not only thin III-V compound films in which each atomic layer has a flat grown surface but also multilayer thin film structures in which each atomic layer has a flat surface and a heterojunction interface at which elements of compounds are sharply distributed.
BACKGROUND ART
Compound semiconductors have been noted as the most suitable semiconductors for obtaining semiconductor devices which can satisfy recent demands for super high-speed operation and high performance. In some compound semiconductors, semiconductor mixed crystals can be synthesized which passes the zinc-blende crystal structure, and it is possible to obtain heterojunctions at which lattice matching between various dissimilar semiconductor crystal is achieved. Therefore, by utilizing such heterojunctions, devices such as ultra-high-speed electronic devices utilizing modulated doping, various optical devices such as laser diodes and novel electronic devices utilizing the superlattice structure have been developed and some of them have been already used in practice.
Furthermore, novel devices utilizing the new physical properties of such heterojunction structures or superlattice structures and especially compound semiconductor devices typically represented by lasers and HEMT (high electron mobility transistors) have been recently extensively studied at high speed. Such elements comprise in general a substrate and a thin film which is formed on the substrate and have a complicated structure consisting of many layers of dissimilar compounds. And it is well known to those skilled in the art that the performance of these elements is greatly influenced by the structure of the thin film, especially the sharp distribution of composition or the sharp distribution of doping concentration at the interface between the adjacent layers.
It follows, therefore, that the most fundamental technique for fabricating the heterojunction structures or the superlattice structures is to grow the layers each in a desired thickness of different compounds in such a way that the interface between the adjacent layers can be defined as sharply as possible, without causing any crystal defect.
One process for growing a thin III-V compound semiconductor film over the surface of a substrate is molecular beam epitaxy (MBE).
In the case of growing a III-V compound crystal by the conventional MBE method, at least one element of Group III and at least one element of Group V of the periodic table are simultaneously supplied on the surface of a substrate which disposed in a vacuum chamber and heated. (Chang, L. L. et al.; J. Vac. Sci, Technol., Vol. 10, P. 11, 1973). In this case, in order to prevent the escape of the Group V element having a high vapor pressure from the grown layer, a large amount of the Group V element is normally supplied on the surface of the growing layer. As a result, the Group III element immediately combines with the Group V element on the growing surface so that III-V molecules are formed. In order to obtain a flat grown surface in each atomic layer, the molecules thus obtained must be sufficiently diffused over the grown surface. However, the diffusion coefficient of such molecules is extremely low as reported by Neave, J. H. et al. (Appl. Phys. Lett. Vol. 47, P. 100, 1985), so that small hills and small valleys which differ in height by a few atomic layers always exist on the grown surface as shown in FIGS. 1A-1C. In FIG. 1, reference numeral 1 represents a substrate; 2, Ga atoms; and 3, As atoms. The Ga-As molecules thus formed on the substrate 1 are gradually increased in number in the order of steps shown in FIGS. 1A, 1B and 1C, but as shown in FIG. 1B, the second Ga-As molecular layer is formed before the first Ga-As molecular layer has completely covered the surface of the substrate 1. Since the mobility of Ga-As molecules is low, the second and third Ga-As layers are formed before the second Ga
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Horikoshi Yoshiji
Kobayashi Naoki
Sugiura Hideo
Bunch William D.
Hearn Brian E.
Nippon Telegraph and Telephone Corporation
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