FIELD EFFECT CONDUCTANCE MODULATION IN HYDROGENATED AMORPHOUS SILICON

TIEN CHIUN LEE, Purdue University

Abstract

The primary purpose of the research was to develop techniques to improve the quality of amorphous silicon (a-Si), i.e. with a lower density of localized states. Several techniques were investigated, but hydrogenation by ion implanting proved the most promising. This technique permits independent control of amorphous silicon disorder and the hydrogenation level, thereby separating the process of hydrogenation from that of film deposition. Electrical measurement of field effect conductance changes was used as a probing tool to monitor changes in the properties of a-Si before and after hydrogenation. Field effect data was transcribed by a computer program to determine the density of localized states. Amorphous silicon films were prepared by electron beam evaporation of a high purity silicon onto the surface of a thermally oxidized crystalline silicon substrate. The films were deposited at a fixed rate in a high vacuum. Immediately after deposition, some films were subjected to in situ thermal anneal and some films were not. A comparison of the results of these two cases revealed the porous nature of evaporated a-Si. Hydrogen incorporation into a-Si films was performed by ion implantation followed by a low temperature thermal activation of the hydrogen. After hydrogenation, a field effect conductance change of four orders of magnitude was observed on the devices which were not in situ thermally annealed. A comparison before and after hydrogenation demonstrates that almost three orders of magnitude reduction (from about 10('22) to about 10('19)/cm('3)-eV) in the density of localized states near the Fermi level (N(,T)('F)) was achieved. Varying the hydrogen implantation dosage between 1 x 10('16) to 1.5 x 10('17)/cm('2), with all other sample preparation procedures fixed, caused a decrease in N(,T)('F) from 8.6 x 10('20) to 1 x 10('19)/cm('3)-eV. The effect of in situ thermal annealing prior to hydrogen implantation was also investigated. By performing a 400(DEGREES)C anneal for four hours immediately following film deposition the film porosity was greatly reduced. The film was then implanted with hydrogen to a total dose of 1 x 10('17)/cm('2). A field effect conductance change of six orders of magnitude was observed which yielded a N(,T)('F) of 4 x 10('17)/cm('3)-eV, approaching that of high quality glow discharge produced films.

Degree

Ph.D.

Subject Area

Electrical engineering

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