Low carrier concentration (PbSn)Te by molecular beam epitaxy

Abstract

pb0.77.77SN0.23Te was grown by evaporation of the pure elements or of the binary compounds from Knudsen cell sources at l0-10 torr. The growth chamber was provided with ion bombardment, LEED, Auger spectroscopy and quartz crystal deposition moni-toring. A Sn-wetted heating block was used for close control of substrate temperature. On chemomechanically-polished BaF2 (100) substrates, monocrystalline films smooth to tens of Angstroms were obtained at 1 micrometer/hour growth rate over a substrate temperature range from 300 to 420°C; above A20°C, re-evaporation occurred. In growth from the elements, excess Pb remains as droplets on the film surface, but most excess Te re-evaporates. The remaining Te produces Pb vacancies which generate p-carriers; conversely, excess Pb produces n-carriers. A quartz crystal deposition rate monitor operat-ing at the growth temperature was used to tune impinging (Te/ Pb+Sn) ratio by observing the discontinuity in rate vs. (Te/ Pb+Sn) which occurred at the stoichiometric ratio. This technique yielded films with as-grown carrier concentrations in the high l017 p/cm3 range at 77K. Closer stoichiometry control was obtained by growing from PbTe and SnTe sources with the use of a separate Pb source for reduction of inher-ent Te excess. For Pb0.77Sn0.23Te, The metal-rich single-phase boundary was found to cross the stoichiometric composition at 395 + 5°, so that only p-type material was obtainable above this temperature. By using 0.014 atomic fraction excess Pb, it was possible to constrain the film composition to this boundary and thus obtain close control over carrier concen-tration by appropriate adjustment of substrate temperature. Carrier concentrations of 2×l017/cc at 77K were obtained in this manner for both n- and p-type material. Excess Pb coalesced into droplets spaced sparsely enough on the sur-face so as not to interfere with device fabrication. Hall mobilities were about 1×104 cm2 /v⋅sec at 77K. Attempts to reduce carrier concentration below 2×1017 cm-3 resulted in anomalously low Hall voltages, suggesting existence of p and n domains in the films. The origins of this inhomogeneity are not clear at present.