Refractory semiconductor of boron phosphide
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Refractory semiconductor of boron phosphide Y. Kumashiro Department of Materials Science and Chemical Engineering, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama 240, Japan (Received 29 May 1990; accepted 15 July 1990)
The single crystal growth of boron phosphide (BP) by employing the high pressure flux method and chemical vapor deposition (CVD) process is described together with characterization of the prepared BP and its electrical, thermal, semiconducting, and electrochemical properties. BP single crystals prepared by the high pressure flux method contain copper used as the flux, but they are promising for photocathode materials. BP single crystalline wafers prepared by the CVD process using Si wafer substrate contained autodoped silicon with the concentration of 1018-1020 atoms • cm"3, depending on the growth temperature and the substrate plane. The Si atoms which act as acceptors are incorporated at phosphorus sites in BP. The lattice constants determined by the Bond method explain the conduction type of BP. Some electronic transport properties such as donor and acceptor levels and lattice scattering process before and after thermal neutron experiments are clarified. The thermal conduction is limited by three-phonon processes. The formation of defects by thermal neutron irradiation and that of structural disorder by ion-irradiation are mentioned. Schottky diodes consisting of rc-BP and Sb or n-BP and Au, which are denoted as n-BP-Sb and -Au, respectively, show excellent characteristics, and their barrier heights are independent of metals and two-thirds of energy bandgap, expected from the surface-state model. Finally, recent results on thermoelectric properties of sintered specimens are mentioned.
I. INTRODUCTION
Boron phosphide (BP) is a III-V compound semiconductor with zinc-blende structure, and it can exist as n- and /"-type materials. The constituent atoms of BP are the light elements, and especially boron belongs to the first law of group III of the periodic table, those with small inner shells. The characteristics of boron phosphide are summarized as follows: (1) BP forms a tetragonal bonded structure with a small lattice constant and exhibits strong covalent bonding with small ionicity. (2) BP is mechanically strong and has an elastic constant as large as /3-SiC. (3) BP has a high melting point, high Debye temperature, and high thermal conductivity, resulting in high thermal and chemical stabilities. (4) BP has a wide band gap with indirect transition. (5) The unharmonicity of the lattice is large in comparison with other conventional semiconductors. However, BP has been considered to have few outstanding features in comparison with other III-V compound semiconductors for three basic reasons. First, it is difficult to prepare well-characterized single crystals because of high melting pointsx (> 3000 °C) and high decomposition pressures 1 (~10 5 atm at 2500 °C). Second, the material handling process is complicated due to its refractory hardness and brittleness. Third, it has lower
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