Fabrication and Characterization of Si(1-x)Ge(x) Semiconductor Alloy for Sensor Applications

Si_1-xGe_x [Si(1-x)Ge(x)] semiconductor alloys have emerged as materials with many important applications in the electronic industry due to its tunable electronic, optical, and physical properties. It has been studied and analyzed for the fabrication of high-speed micro electronics (e.g., SiGe heterojunction bipolar transistors (HBT) and high electron mobility field effect transistors) and thermo-photovoltaics (e.g., photodetectors, solar cells, thermoelectric power generators and temperature sensors). Other applications of Si_1-xGe_x include tunable neutron and x-ray monochromators and γ- ray [gamma-ray] detectors. In these applications, the Si_1-xGe_x alloy is generally used in the form of epilayers that have to be deposited on a lattice-matched substrate (wafer). Therefore Si_1-xGe_x bulk single crystals with a specific composition (x) are needed for the extraction of such wafers.

Amorphous silicon made by plasma decomposition of silane (SiH₄) usually has high resistivity and good photoconductivity with low optical absorption; however, investigations establish a-Si as a semiconductor of lower electrical conductivity than crystalline Si with a large number of unpaired crystal-surface-like electrons distributed throughout the bulk of a-Si. It has also been shown that films sputtered in an argonhydrogen atmosphere exhibit similar properties1. In addition, a-Si produced by sputtering in a pure argon atmosphere has low resistivity, poor photoconductivity, and high optical absorption below 1.5 eV. By co-sputtering silicon and germanium in a pure argon atmosphere in an ultra-low vacuum (≥ [greater than or equal to] 10₇ [10^7] torr) the semiconductor quality can be greatly enhanced with capabilities of using the Si1-xGex alloy for near infrared (NIR) photosensor applications.

This thesis presents an experimental study for RF sputter deposited thin films of compositionally varying a-SiGe on Si/SiO₂ [silicon/silicon dioxide] wafers of ~200 nm film thickness that are thermally annealed to induce uniform crystallization. The goal is to develop a NIR optical sensor with high efficiency and good thermal stability. Asdeposited and annealed wafers were characterized by using Energy Dispersive X-ray analysis (EDX) for composition analysis, X-ray diffraction (XRD) for microstructural characterization, Raman spectroscopy for structural and compositional analysis, and electrical probe measurements.