Reactively evaporated photoconductive PbO: Phase transformations induced by water vapor

Abstract

Lead oxide films about 45 μm thick and 80% dense were grown on water-cooled anodized aluminum substrates by the reactive evaporation of pure lead in a low pressure oxygen atmosphere. X-ray diffraction analysis showed that the as-deposited films were a mixture of free lead and orthorhombic β-PbO, the high temperature polymorph. The exposure of these two-phase films to room temperature air (or oxygen) with high partial pressures of water vapor greatly enhanced the kinetics of oxidation as well as the transformation of orthorhombic PbO to tetragonal PbO, the low temperature polymorph. The films were converted to essentially pure tetragonal PbO by a 175 °C anneal in air followed by a 435 °C anneal in nitrogen. With the air anneal only, photoelectronic characteristics of the films were highly variable and non-reproducible. Improved photoresponse and reduced sample-to-sample variation were obtained after the nitrogen anneal. Doubly annealed films exhibited room temperature dark resistivities as high as 2 × 10 14 ω cm and photocurrent quantum efficiencies approaching unity at a field strength of 2 × 10 4 V cm −1 and when illuminated with 45 μW cm −2 of 608 nm (volume absorbed) light. The concentrations of basic lead carbonates, hydrocerussite 2PbCO 3·Pb(OH) 2 and plumbonacrite 6PbCO 3·3Pb(OH) 2·PbO, detected in the films at every stage of processing, were decreased by the high temperature nitrogen anneal.