Electronic and optical properties of high quality low bandgap amorphous (Ge, Si) alloys
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Electronic and optical properties of high quality low bandgap amorphous (Ge, Si) alloys
S.R. Sheng*, R. Braunstein*, and V.L. Dalal** *Department of Physics and Astronomy, University of California, Los Angeles, California 90024, USA; **Department of Electrical and Computer Engineering, Iowa State University, Ames, Iowa 50011, USA.
ABSTRACT High quality low bandgap a-(Ge,Si):H alloys were prepared using low pressure, reactive ECR plasma deposition with high H dilution and subtle (sub-ppm) B-doping. The charge transport and optical properties of these low bandgap materials as a function of alloy composition have been investigated by employing the microwave photomixing technique and optical absorption spectroscopy. From the measurements of electric field dependence of the drift mobility, we have found strong evidence for the existence of long-range potential fluctuations in a-(Ge,Si):H alloys, and determined the depth and range of the potential fluctuations, and subsequently the charged defect density, as a function of alloy composition. It was found that at ~30% Si in Ge, the photoresponse begins to decrease rapidly with increasing Si content due to the decreases in the mobility and lifetime, and meanwhile, both the charged defect density and the Urbach energy increase significantly. The latter indicates an increase in the compositional disorder. It is the potential fluctuations whose effect can be also enhanced by incorporating Si to the alloy system that result in the deterioration of the electronic properties of a-(Ge,Si):H alloys, similar to the case of the incorporation of Ge at the Si end. This enhanced effect accompanies with an increase in the depth, and a decrease in the range of potential fluctuations, leading to a decrease in the mobility, and consequently in the photoconductivity. Our present results demonstrate that the increased charged scattering centers and compositional disorder upon adding Si to the alloys play an important role in the potential fluctuations.
INTRODUCTION Amorphous (Si,Ge) alloys typically grown using various plasma-CVD techniques have recently attracted considerable interest because of their potential of improving the performance of a-Si based multijunction solar cells. In general, high hydrogen dilution is required to fabricate good films and cells [1,2]. It was found that the quality of the material and the device degrades significantly with alloys having >50% Ge content being rather poor in quality so that the transport parameters at the Ge end have not been successfully determined so far. This deterioration in quality is generally ascribed to poorer microstructure of the materials with high Ge content [3]. Our previous study of charge transport properties of a-SiGe:H alloys at the Si end grown using HWCVD provided strong evidence for enhanced long-range potential fluctuations by adding Ge to the alloy system that results in the deterioration of the opto-electronic properties of A8.4.1
a-SiGe:H.[4] The monotonic increase in the depth and decrease in the range of the potentia
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