The structural and electron transport properties of CdS grown by molecular beam epitaxy

Abstract

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 10 5 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 10 18 cm -3 were produced with Hall mobilities of 65 cm 2 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 Al 2O 3 sandwiched between the epitaxial aluminium and the CdS. The junction characteristics of the Al/Al 2O 3/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.