A STUDY OF INSTABILITY IN CdSe THIN FILMS AND TFTs

Abstract

Electrical instability of operation of the thin film transistor is the major problem preventing the development of active thin film micro- circuits. A study of various instabilities in CdSe thin films and TFTs is presented in this thesis. The CdSe semiconductor has been considered both as a separate film layer, and as the major layer in the CdSe TFT. Comparison of instabilities in the thin semiconductor films and in the TFT has been made to separate and identify individual instability- causing mechanisms in the TFT. In particular, slow negative drift is studied in depth in CdSe films, and results related to a similar effect in the TFT. A model of slow trapping of conduction electrons in the semiconductor is des- cribed in terms of adsorption of oxygen at the film surface. An Elovich type charge-limited adsorption relation is found to fit observed changes in CdSe film conductivity with oxygen adsorption. A picture of the conduction process in polycrystalline CdSe is presented as involving grain boundary potential barriers. It is shown that the n-type majority carrier conduction exhibited by CdSe thin films is due to selenium donor vacancies, and that trapping by oxygen adsorption involves the exhaustion of a shallow donor energy level to a deeper level associated with selenium vacancies. Factors affecting thin film CdSe properties during the deposition process are shown to greatly change electrical characteristics and instability observations. Conclusions are made of the importance of these in reduction of instabilities in the TFT. Suggestions for the application of these conclusions, and for further research into the problem of TFT instability are made with the hope of eventually realizing the fabrication of a workable TFT.