Study of enhanced phosphorus activity in n -type Si 80 Ge 20 as a function of the doping process
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Study of enhanced phosphorus activity in n-type Si80 Ge20 as a function of the doping process S. H. Han and B. A. Cook Ames Laboratory, Iowa State University, Ames, Iowa 50011-3020 (Received 4 April 1994; accepted 22 September 1995)
The electrical activity of phosphorus in Si80 Ge20 alloys prepared by two nonconventional doping processes has been investigated over the temperature range 25–1250 ±C. Both a solid state (mechanical alloying) and gaseous phase doping processes were found to extend the electrical activity of phosphorus in Si80 Ge20 alloys beyond the reported maximum equilibrium value s2.1 3 1020ycm3 d to 2.5 –2.9 3 1020ycm3 within the temperature range 900 1200 ±C. It is likely that this extended electrical activity of phosphorous is associated with a high density of defects. The enhanced electrical activity of phosphorus enabled Si80 Ge20 alloys to have 300 to 1000 ±C integrated average electrical power factors in the range 30.1–35.7 mWycm-±C2 .
I. INTRODUCTION
Silicon-germanium (Si–Ge) alloys have been used a high temperature thermoelectric materials over the past 30 years. Early research work focused on P-doped Si –Ge alloys for the n-leg and B-doped Si –Ge for the p-leg. However, for n-type Si–Ge, the phosphorus solubility is too low to produce the high carrier concentrations needed for an optimum figure of merit (Z).1 Several years ago, it was reported that gallium phosphide (GaP) enhances phosphorus solubility beyond the maximum equilibrium limit in Si –Ge alloys.2,3 Many researchers have used GaP-doped Si –Ge alloys to obtain high carrier concentrations by zone leveling, liquid phase epitaxy (LPE),4,5 and mechanical alloying (MA).6 MA is a high energy ball milling process, which was originally developed to produce complex oxide dispersion-strengthened (ODS) alloys.7,8 Over the past ten years, MA has been widely used to produce amorphization,9 nanophase structures,10 and metastable alloys.11 Recently, it was reported that MA can be used to produce high temperature or high pressure metastable phases of the rare earth sulfides at room temperature.12,13 In this study, we found that both mechanical alloying and gaseous state diffusion can extend the electrical activity of phosphorus beyond the maximum reported equilibrium value in Si80 Ge20 alloys, resulting in a high integrated average power factor between 300 and 1000 ±C. The results of this study are reported below. II. EXPERIMENTAL PROCEDURES
Phosphorus doping in Si80 Ge20 alloys was performed by two different techniques, i.e., mechanical alloying (MA) and gaseous phase diffusion. In the MA study, phosphorus (Cerac), silicon (Hemlock), and gemanium J. Mater. Res., Vol. 11, No. 1, Jan 1996
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(Cabot) powders were placed in a hardened steel vial and sealed in a helium-filled glovebox. These powders were mechanically alloyed for 6 h in a Spex 8000 mixer/mill using three 12.70 mm diameter and three 6.35 mm diameter hardened steel balls. The weight rat
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