The effects of low pressure helium ion bombardment on hydrogenated amorphous silicon

Date
2008-01-01
Authors
Beckman, Michael
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Vikram L. Dalal
Gary Tuttle
Mani Mina
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Electrical and Computer Engineering
Abstract

The benefits of low pressure Plasma Enhanced Chemical Vapor Deposition (PECVD) using helium as the dilutant gas were investigated in a variety of conditions to identify the techniques feasibility as a low band gap material yielding deposition method. Films and photovoltaic devices with intrinsic layers processed at lower pressures with helium dilution are thought to possess improved characteristics with lower hydrogen content and lowered optical band gaps. When films are grown at lower pressures in the presence of helium the optical band gap tends to decrease. Amorphous silicon a-Si:H generally has a band gap around 1.75eV. This work intends to decrease this band gap as far as possible.;Films grown under these conditions exhibited reasonable growth rates considering the conditions and yielded very good photo and dark conductivity. It was expected that dilution with helium at low pressure would decrease hydrogen content by increasing ion bombardment. This was confirmed by FTIR results that indicated hydrogen content of 7-9%. Both films and devices were fabricated that achieved optical band gaps around 1.62-1.65eV. Devices exhibited Urbach energies that were typically lower than 50meV indicating a good quality amorphous structure.

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