Crystal growth of photovoltaic polycrystalline Si1-xGex by die-casting growth

ABSTRACTCoin-shaped multicrystalline Si1-xGex crystals were grown using a Brigdman method combined with die-casting growth. Si1-xGex alloy is known as a candidate material for producing Auger generation, which creates more than one electron/hole pair per absorbed photon. Since Si1-xGex alloy shows a complete series of solid solutions, precipitating crystals with a certain composition of silicon or germanium by conventional selective growth methods is burdensome. Using die-casting combined with Bridgman growth brought about Si1-xGex precipitation in a form completely different from that predicted by the Si-Ge phase diagram. By combining this growth with subsequent heat treatment of the precipitated Si1-xGex sample, Si1-xGex (x= 0.5 ± 3 %) could be obtained. Indirect band-gap energy was estimated by measuring room-temperature optical absorption coefficient of the grown samples.

Optical and Structural Changes of Fe Implanted Sapphire

Single crystalline colorless α-Al2O3 samples were implanted with several fluences of Fe+ ions in the range of 1×1016 to 5×1017 Fe+ cm-2 at room temperature. Optical absorption and luminescence measurements were carried out before and after annealing in reducing and oxidizing atmospheres. The structural changes were studied with RBS/channeling and x-ray diffraction. After implantation, the damage induces a brownish coloration in the samples for fluences below 2×1017 Fe+ cm-2. The optical spectra are characterized by an absorption band centered at 200 nm. This band is strongly reduced after annealing at 1100 °C in reducing atmosphere and a new well-defined band develops around 350 nm. This new band shifts to lower values with the implanted fluence, which is an indication of its correlation with the dimensions of the iron precipitates formed in the implanted region. The existence of these precipitates was confirmed by x-ray diffraction and TEM. The samples implanted with fluences above 1×1016 Fe+ cm-2 annealed in oxidizing atmosphere display different optical absorption spectra, with respect to those annealed in reducing atmosphere, characterized by an increase in the intensity of the peak at 200 nm. Luminescence measurements show the presence of F and F+ centers in the samples. The existence of these defects can be explained by the need of charge compensation and strain release due to the formation of mixed iron oxide or metallic precipitates.