Epitaxial Growth of Icosahedral Boride Semiconductors for Novel Energy Conversion Devices

Abstract

The chemical vapor deposition and properties of the boron-rich semiconductors B12As2 and B12P2 on 6H-SiC(0001) and silicon substrates were investigated. Crystalline, stoichiometric films were deposited between 1200 C and 1500 C using two types of reactants, hydrides (B2H6 and AsH3) for B12As2 and halides (BBr3 and PBr3) for B12P2. 6H-SiC proved to be the better substrate for B12As2 heteroepitaxy, in terms of the residual impurity concentrations. Films on Si substrates suffered from high concentrations of Si (up to 4at.%); in contrast, the Si and C concentrations in the B12As2 films deposited on 6H-SiC at 1300 C were at or below the detection limits of secondary ion mass spectrometry (SIMS). The deposition temperature was significant as films deposited at 1450 C contained high residual C and Si concentrations (>1020 cm-3), probably due to the decomposition of the substrate. The hydrogen concentration in all B12As2 films was relatively high, with a minimum concentration of 3x1019 cm-3 in undoped B12As2. SIMS measurements showed that the hydrogen concentration was directly proportional to and tracked the Si concentration, reaching values as high as 3 x 1020 cm-3. The structural properties of the B12As2 films were characterized by x-ray diffraction and transmission electron microscopy. The FWHMmore » of typical high resolution x-ray rocking curves for the (333) peaks of the B12As2 films were 800 arcsec. The films are under tensile strain due the higher coefficient of thermal expansion for B12As2 than SiC. Rotational twins were present in B12As2 films deposited on (0001) oriented 6H-SiC substrates, as revealed by cross-sectional TEM and x-ray diffraction pole figures. While the c-plane 6H-SiC has six-fold rotational symmetry, rhombohedral B12As2 has only 3-fold symmetry (along its (111) axis), thus it randomly nucleates with two different in-plane orientations. The electrical properties of undoped and silicon-doped B12As2 deposited on semi-insulating 6H-SiC substrates were characterized by Hall effect measurements. The resistivity of p-type B12As2 films on semi-insulating 6H-SiC(0001) substrates was controllably varied over nearly four orders of magnitude by changing the concentrations of silicon into the films, incorporated by adding silane during deposition. The electrical properties of the B12As2 suffered from low hole mobilities, typically less than 3 cm2/V's. This was possibly a consequence of structural defects in the films. The resistivity of as-deposited undoped and silicon-doped B12As2 films decreased by two or more orders of magnitude after annealing at temperatures above 600 C in argon. This unexpected but reproducible effect of annealing on the resistivity of the semiconductor warrants further investigation. The properties of palladium, platinum, and chromium/platinum electrical contacts to B12As2 were tested at Pennsylvania State University. The Pd and Pt contacts exhibited nonlinear I-V characteristics and severe agglomeration upon annealing, but the Cr/Pt contacts were ohmic and remained smooth even after they were annealed at 750 C. The specific contact resistance of the Cr/Pt contacts dropped four orders of magnitude after samples were annealed in Ar for 30 s at 750 C. This reduction in specific contact resistance was linked to a simultaneous drop in the resistivity of B12As2 upon annealing. In subsequent experiments, a low specific contact resistance was also achieved when Cr/Pt was deposited on B12As2 films that were annealed prior to metallization instead of afterwards.« less