STRUCTURAL AND ELECTRON-TRANSPORT PROPERTIES OF CDS GROWN BY MOLECULAR-BEAM EPITAXY

Epitaxial films of CdS were grown by the molecular beam epitaxy method on single-crystal spinel substrates and on epitaxial films of gold and aluminium. Epitaxial films with a wurtzite structure were grown on the spinel substrates at a substrate deposition temperature of 420 °C and with film growth rates of 0.3-15 μm h-1. The resistivity of the undoped films was approximately 105 Ω cm and it exhibited an exponential decrease with temperature with an activation energy of 1.6 eV. Extrinsic n-type doping was carried out with indium, and films with donor concentrations of 1018 cm-3 were produced with Hall mobilities of 65 cm2 V-1 s-1. Epitaxial films of CdS were also grown on epitaxial gold and aluminium films (grown on mica) at a substrate deposition temperature of approximately 300 °C. The films grown on gold had a wurtzite structure and were found to be n type from measurements of the thermo-e.m.f. Films grown on aluminium were of a cubic or sphalerite structure with a (111) orientation and the measured thermo-e.m.f. indicated that they were p type. This result is thought to be due to an inversion layer within the CdS which arises from a thin layer of Al2O3 sandwiched between the epitaxial aluminium and the CdS. The junction characteristics of the Al/Al2O3/CdS structures were measured and the forward-bias current was found to be proportional to exp(qv/nkT) where n = 2.2; the thickness of the depletion region was approximately 0.3 μm from capacitance-voltage measurements. The polarity of the bias voltage for forward conduction indicated that a p-n junction may exist within the CdS film. © 1979.