Self-aligned surface transfer p-type doping of ZnO for thin-film transistor applications

Abstract

In recent years, wide band gap ZnO films have been extensively investigated for possible application as UV emission sources, gas sensors and transparent thin-film transistors. However, since ZnO typically exhibits n-type behavior, a reliable p-type doping method is required for p-n junction fabrication. Here, we report on a method of effectively doping ZnO thin-films by self-aligned phosphor surface transfer doping. A thin layer of phosphor thermally deposited on the ZnO thin-film surface will transfer holes to the ZnO forming a surface inversion layer, creating a shallow depletion region at the n-type ZnO interface. ZnO films were deposited by sputtering on oxidized p-type silicon substrates followed by SiO2 deposition. Contact regions were defined and transferred to the SiO2 by photolithography and reactive ion etching. Then phosphor was deposited on the exposed ZnO surface by thermal sublimation at 600 oC. After contact metal deposition, rapid thermal annealing (RTA) was performed to allow shallow level diffusion of the dopant. I-V characteristics of the samples showed rectification characteristics indicative of p-n junction formation. Also, transfer characteristics, using substrate as the back gates, showed far reduced leakage current compared with undoped ZnO TFTs. We will further report on the dependence of the ZnO TFT characteristics on dopant deposition temperature and RTA conditions.