Effect of deposition conditions on properties of a-(Si,Ge): H films and devices using ECR-plasma deposition
Date
1998
Authors
Haroon, Sohail
Major Professor
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Dalal, Vikram L.
Committee Member
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Abstract
The work presented here explores the relationship between electronic properties of hydrogenated amorphous silicon-germanium (a-Si,Ge:H) materials and devices and their fabrication technique. The films and devices were grown using a remote, low pressure, Electron Cyclotron Resonance (ECR) technique. In this technique, a beam of hydrogen or helium ions produced by an ECR plasma is directed towards the substrate, where they impart energy to the surface of the growing films. The hydrogen atoms, being reactive, also etch the growing film, thereby changing the localized bonding configurations. The films were grown at pressure between 5 and 15 mT. A lower pressure resulted in increased ion energy and ion flux density incident on the surface.
Optical properties such as absorption, reflection and transmission were measured. Tauc gap (bandgap) for the materials was estimated from the absorption coefficient ([alpha]) vs energy measurement. Subgap absorption down to a value of [alpha subscript sub]=l/cm and Urbach energy (E[subscript ur]) of valence band tails were measured using a two-beam photoconductivity technique. It was found that both [alpha subscript sub] and E[subscript ur] values were lower, indicating higher quality material, for films produced at lower pressure. It was also found that Tauc gap itself was a sensitive function of pressure in the discharge when H was used in the plasma. This was an unexpected result, which indicates that ion bombardment is needed to achieve more homogeneous films. The electronic properties measured included photo and dark conductivity, activation energy and subgap absorption.
It was found that the ratio of photo and dark current was higher for films prepared at lower pressure, for materials with similar Tauc gap, again indicating better quality material. Standard substrate type p-I-n devices were fabricated on stainless steel substrate. The properties of the devices also depended critically on the growth conditions. Lower pressure resulted in better devices. The mobility-lifetime product ([Mu Tau]) of holes was measured using quantum efficiency-voltage technique. A simplified field calculation model was used to analyze the data. It was found that [Mu Tau] of holes was higher in devices produced at lower pressure, a result consistent with the results obtained on the film properties.
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thesis