Heteroepitaxial Si-, Ge-, and Gaas-On-Insulator Structures on Si Substrates by Use of Fluoride Insulators
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HETEROEPITAXIAL Si-, Ge-, and GaAs-ON-INSULATOR STRUCTURES ON Si SUBSTRATES BY USE OF FLUORIDE INSULATORS T. ASANO, H. ISHIWARA, and S. FURUKAWA Graduate School of Science and Engineering, Tokyo Institute of Technology 4259 Nagatsuda, Midoriku, Yokohama 227, Japan ABSTRACT Heteroepitaxial growth of alkaline earth fluoride films on Si substrates and Si, Ge, and GaAs films on the fluoride/Si structures, is reviewed. Growth of single crystalline fluoride films on Si is first discussed. Then the usefulness of novel heteroepitaxial technologies, the predeposition method and the electron beam irradiation method, is demonstrated in the growth of Si and Ge films on CaF 2 /Si structures. Finally fundamental growth characteristics of GaAs films on CaF 2 /Si structures and annealing effects on the crystallinity of the GaAs films are described. INTRODUCTION Semiconductor-on-insulator structures composed of various semiconductor layers are very attractive for future electron devices such as very high speed ICs, three dimensional ICs, and integrated intelligent sensors. Although the direct growth of high quality semiconductor films such as GaAs films on Si substrates has become possible, the semiconductor-on-insulator structure is more suitable particularly for high speed application because it can reduce parasitic capacitances and the short channel effects. Since the success of growing heteroepitaxial Si/CaF2 /Si structures in 1981[l], there have been many extensive studies on heteroepitaxial growth of alkaline earth fluorides and semiconductors on Si substrates, including Sill-6], Ge[5-9], PbSnTe[lO], and GaAs[ll-12]. Generally in heteroepitaxy, following material properties affect the quality of epitaxial films: 1) Crystal structure and lattice constant. 2) Thermal expansion . 3) Chemical reaction at the interface and related autodoping in epitaxial films. 4) Wettability. The crystal structure of the alkaline earth fluorides(CaF 2 , SrF 2 , and BaF 2 ) is cubic fluorite structure. The lattice constants of CaF 2 , SrF2 , and BaF2 are 0.546, 0.580, and 0.620nm, respectively. So, the lattice constants of the mixed fluorides can be continuously varied in this range. Both of these properties match well to most semiconductors of interest. Thus the fluorides satisfy the first requirement in the above. However the other factors become serious in semiconductor/fluoride systems. For example, l)the thermal expansion coefficients of the fluorides are about 20xlO- 6 /degree at room temperature which is much higher than those of semiconductors such as Si(2.5xlO- 6 /degree) and GaAs(6xlO- 6 /degree), and 2)the fluorides are ionic bonding materials and the surface free energy of the (111) of the fluorides is particularly low. Therefore, growth of high quality semiconductor layers on the fluorides strongly depends on developments of growth processes to supress defects caused by these differences in material properties between the fluorides and semiconductors. Upon these standpoints, we review in this paper recent progress in the research of g
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