Oxygen Effects in Mechanically Alloyed Si 80 Ge 20 Doped with GaP and P
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OXYGEN EFFECTS IN MECHANICALLY ALLOYED Si80 Ge20 DOPED WITH GaP AND P
B. A. COOK, J. L. HARRINGA, AND B. J. BEAUDRY Ames Laboratory, Iowa State University, Ames, IA 50010
ABSTRACT A neutron activation study was performed to follow the total oxygen content during the preparation sequence of mechanically alloyed Si-20 at.% Ge n-type alloys using both elemental powders and chunk starting materials. The Si-20 at. % Ge alloys were doped with 1.6 at. % GaP and 3.4.at. % P and the total oxygen concentration was measured in the starting materials, after six hours of mechanical alloying in a helium environment, after hot pressing, and after a short II00*C soak in fused silica ampoules. The alloys containing high oxygen levels showed low carrier mobility and low thermal conductivity whereas those containing low oxygen showed high mobility and thermal conductivity. The microstructure, as observed by optical metallography and SEM, was found to differ greatly with oxygen content as the low oxygen alloys showed relatively large, well defined grains and the high oxygen alloys showed evidence of poor sintering and limited grain growth.
INTRODUCTION In order to improve the performance of thermoelectric materials such as Si-Ge, it is important to understand the role of neutral impurities which are an inevitable consequence of handling and processing the raw materials. Central to this is the role of oxygen, since it is ubiquitous and highly reactive. While there is an exhaustive amount of information available on the role of oxygen in silicon up to the solubility limit, little has been published on the effects of oxygen in silicon-germanium alloys especially with regard to the thermoelectric properties. Bhandari and Rowe (1980) [1] mention that oxygen may be responsible for some degradation of electrical properties observed in hot pressed Si-Ce relative to zone-leveled alloys. More recent studies by Vining et. al. [2] on hot pressed, sintered Si 8 0 Ge 20 prepared by a variety of comminution techniques show that total oxygen contents can range from 0.3 at.% to 6.7 at.%, as measured by neutron activation. A further recent TEM characterization of Si-Ge alloys used in the MOD-RTG program performed by Owusu-Sekyere, Jesser, and Rosi [3] showed that these alloys contained 30 to 400 mu SiO0 2 crystallites at a density of 800 cM- . 8 3 Since oxygen solubility in silicon is approximately 101 cm(0.002 at. %) near the liquidus [4], virtually all excess oxygen is expected to exist as a second phase compound. Doremus [5] showed that in an oxidized silicon-germanium mixture the silicon is oxidized, but not the germanium. The objective of this study is to evaluate the effect of these second phase oxides on the fundamental thermoelectric properties of mobility and lattice thermal conductivity.
Mat. Res. Soc. Symp. Proc. Vol. 234. @1991Materials Research Society
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EXPERIMENTAL Si 0 . 747 Ge 0 . 1 87 (GaP)0.0 16 PO.034 alloys were prepared by the mechanical The details of this alloying (MA) of the elemental constituents and GaP. high energy
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