High Carrier Concentration Improved N-Type SiGe/GaP Alloys
- PDF / 305,879 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 81 Downloads / 184 Views
HIGH CARRIER CONCENTRATION IMPROVED N-TYPE SiGe/GaP ALLOYS A. Nancy Scoville%, Clara Bajgar*, Jan Vandersande**, and Jean-Pierre Fleurial** *Thermo Electron Technologies Corporation, 85 First Ave., Waltham, MA02254 "*Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109
ABSTRACT SiGe alloys have been fabricated with the hiqghest carrier concentrations achieved to date. Values in excess of 4.0 x 160 have been observed. The thermoelectric behavior of these samples has been characterized, both in the as-pressed state and as a function of 3 anneal time and temperature. Figuresof-merit between 0.85 and 0.90 x 10- K- have been reproducibly achieved. Further improvements will be achieved by reducing the carrier concentration.
INTRODUCTION Silicon-Germanium (SiGe) alloys have been successfully used for well over a decade in thermoelectric devices for space applications. Most of the early research work [1] was done during the early 1960's on standard SiGe, doped with P for n-type conduction and with B for p-type conduction. Though the material conversion efficiency was low, it was used for example, in the Voyager missions and is now powering the Galileo and Ulysses missions. Several years ago, serious consideration was given to improving the n-type material by multi-doping with GaP. The indication was that both the zoneleveled and hot-pressed P-doped materials were limited by carrier concentration only (due to the solubility limits of P) and work was started by several groups. Recently, several n-type SiGe/GaP samples were found to have power factor and figure of merit (Z) values about 20 to 30% greater than those obtained for zone-leveled or hot-pressed P-doped SiGe materials [2]. These particular samples had undergone high temperature annealing treatments between 1200 and 1275C at the Jet Propulsion Laboratory (JPL). These improvements were subsequently confirmed independently at Thermo Electron Technologies Corporation (TTC) using similar processes at a temperature of up to 1320C [3]. The improvement in the figure of merit Z was found to be mainly due to an increased power factor while thermal conductivity values remained virtually unchanged. These improved n-type samples were fabricated by hot-pressing using 2 mole percent of GaP added to Si8 Ge~o material. The high temperature annealing treatments resulted in some Ga being lost as Ga203 lowering the Ga/P ratio to less than a value of 1. Unfortunately, the heat treatment results were erratic and as a result the improved samples were not reproducible. Thermodynamic modeling resulted in an understanding of the cause of the observed improvement. The presence of Ga (as Ga-P ion pairing) increases the solubility of P beyond normal levels and hence leads to an increased carrier concentration. This understanding combined with more systematic heat treatments resulted in several additional improved samples [4] but improvement was still not quite attainable at will. In an effort to more thoroughly understand the factors resulting in these improved samples a systemat
Data Loading...
