The effects of ion bombardment during deposition upon the properties of hydrogenated amorphous silicon-germanium thin films and photovoltaic devices
Ring, Matthew (2004) The effects of ion bombardment during deposition upon the properties of hydrogenated amorphous silicon-germanium thin films and photovoltaic devices. PhD thesis, Iowa State University.
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Ion bombardment is inherent in the growth of amorphous materials by conventional PECVD methods, such as electron cyclotron resonance (ECR) of radio-frequency (rf) discharge. In these methods plasma ions are necessary to decompose the source gases; however these ions also impinge upon the growing film surface, imparting their energy to the material. In conventional deposition techniques it is difficult to isolate the effects of the ions so a novel approach is taken in this research where an ECR ion source is attached to a “hot-wire” deposition reactor. This unique reaction system allows researchers to vary the ion bombardment density and energy to study the effects of the ion bombardment on the resulting material. Throughout this research, HW-ECR materials outperformed the HWCVD materials deposited at identical substrate temperatures for photovoltaic applications. The addition of ion bombardment saw a substantial decrease in Urbach energy, hydrogen microstructure, and a corresponding increase in photoconductivity and photosensitivity, regardless of band-gap. During the experiments, the ECR microwave power applied to the reactor was adjusted and showed that as band-gap decreased; less ion energy was required to show improvements in material quality. In addition, a lower filament temperature was required as band gap decreased to maintain a high photoconductivity. The first ever solar cell devices having intrinsic layers deposited by HW-ECR were deposited in this work, and show that this deposition method can produce solar cells with performance on par with current PECVD and HWCVD materials. In addition, these devices are deposited at much higher growth rates, reducing fabrication time dramatically. The standard growth model fails to address the role of ion bombardment; however J. Robertson of Cambridge University has proposed a model for hydrogen ions’ role in the removal of excess hydrogen. The results presented here support that model, and in addition neutral ions, such as helium, are shown to improve the material. Helium ions are postulated to aide mostly in the surface diffusion of reactive radicals during deposition, as helium is a non-reactive chemical species.
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