Back-surface passivation of polycrystalline CdSe thin-film transistors

Abstract

The electrical characteristics of simple inverted-gate CdSe thin-film transistors (TFTs) were monitored after annealing and back-surface passivation treatments and the results were correlated with chemical analysis of the back interface. A dramatic increase in TFT current was observed after a short vacuum anneal at 320 °C. X-ray photoelectron spectroscopy (XPS) analysis showed that the vacuum anneal removed part of the oxide bonded to Se from the back (top) surface. This was confirmed by temperature-programed desorption data for oxidized CdSe, which showed two distinct selenium oxide species desorbing above 300 °C. XPS analysis showed that evaporated SiO, or SiO2 deposited by microwave plasma-enhanced chemical-vapor deposition, reacted strongly with an oxidized CdSe surface by removing the Se-bound oxygen present after an air anneal. Changes in the width of the Cd 3d5/2 peak suggested that the Cd bound oxygen was also transferred to the SiO during the deposition of the oxide. A reoxidation of the CdSe/SiO interface occurred after annealing in air at 350 °C. In contrast, silicon dioxide deposited by e-beam evaporation did not react as strongly with the native oxide. The interfacial oxide was reduced by annealing in forming gas. A reduced CdSe surface with no Se bonded to oxygen and a narrow Cd 3d5/2 XPS peak was associated with a high density of donors at the interface.