Precipitation of boron from silicon–germanium alloy and its effect in the thermoelectric transport properties

Abstract

The time dependences of the carrier concentration, electrical resistivity, and Seebeck coefficient of boron doped Si63.5Ge36.5 alloy are investigated a t heat treatment temperatures of 1000, 1110, and 1200 K for periods of up to 2000 h. The carrier concentration decreases with time of heat treatment due to the precipitation of dopant and this is reflected in an increase in the electrical resistivity and Seebeck coefficient. Good agreement is obtained between the Lifshitz-Slyozov precipitation model and the experimental data. The activation energies of phosphorus and boron in the alloy are estimated to be rz 30 kcal/mol and = 56 kcal/mol, respectively. It is concluded that although the diffusion coefficient of boron in silicon-germanium alloys over the temperature range of device operation is substantially lower than that of phosphorus, precipitation of boron does occur as a function of time. The rate of precipitation maximises in the temperature range lo00 to 1200 K and is sufficiently fast to significantly affect the transport properties of the material and hence the thermoelectric conversion efficiency of a device, based upon silicon-germanium technology, during its period of operation.Die Zeitabhiingigkeiten der Ladungstriigerkonzentration, des elektrischen Widerstands und des Seebeck-Koeffizienten von Bor-dotierten Si63,6Ge36,5-Legierungen werden nach Temperaturbehandlungen von 1000, 110 und 1200 K fiir Zeiten bis 2000 h untersucht. Die Ladungstriigerkonzentration nimmt mit der Temperungsdauer infolge der Ausfiillung von Doponden ab, was sich im Anwachsen des elektrischen Widerstands und des Seebeck-Koeffizienten widerspiegelt. Zwischen dem Prazipitationsmodell von Lifshitz-Slyozov und den experimentellen Werten wird gute vbereinstimmung erhalten. Die Aktivierungsenergien von Phosphor und Bor in der Legierung werden zu = 30 kcal/Mol bzw. = 56 kcal/Mol berechnet. Es wird angenommen, daB, obwohl der Diffusionskoeffizient von Bor in der Silizium-Germanium-Legierung wesentlich niedriger als der von Phosphor ist, eine Ausfiillung von Bor in Abhiingigkeit von der Zeit stattfindet. Die Priizipitationsgeschwindigkeit nimmt im Temperaturbereich von lo00 bis 1200 K geniigend schnell zu und beeinflul3t damit die Transporteigenschaften des Materials und die thermoelektrische Konversionseffiziens eines Bauelements, das auf Silizium-Germanium-Technologie basiert, wiihrend der Betriebsdauer wesentlich. 1) King Edward VII Avenue, Cardiff CFl 3NU, Great Britain. 16.

Modeling study of thermoelectric SiGe nanocomposites

“…Unlike previous reports for bulk p-type Si 0.8 Ge 0.2 , 37 the nanocomposite does exhibit a change in hole concentration at elevated temperature, reducing from around 2.6 ϫ 10 20 cm −3 at room temperature to 2.0ϫ 10 20 cm −3 at 1300 K. This is somewhat unexpected as carrier concentration changes in p-type Si 0.8 Ge 0.2 had previously been observed only on the time scale of thousands of hours. 37 The explanation for this effect is similar to that for n-type dopant precipitation. At room temperature, the material is supersaturated with B but it must be raised to a higher temperature of about 1000 K to reject B from the lattice.…”

“…As discussed by many authors, 28,29,[37][38][39] Si 1−x Ge x alloys used for thermoelectrics, which are usually doped with P ͑n type͒ or B ͑p type͒, are often doped beyond the solubility point for the dopant, causing the carrier concentration to vary with temperature as dopants precipitate out at lower temperatures and become reactivated at higher temperature. These processes can change the carrier concentration by a factor of 2 or more over the entire temperature range, significantly affecting the observed transport properties.…”

Thermocyclic stability of candidate Seebeck coefficient standard reference materials at high temperature

“…For the p-type materials, the Seebeck coefficient has been measured as stable under isothermal annealing for 39 210 h below 875 K (Si 75 Ge 25 ), 12 for 1500 h below 925 K (Si 78 Ge 22 ), 13 and for 100 h at 1000 K (Si 63.5 Ge 36.5 ). 10 We note that dopant precipitation is largely reversible by quenching after a brief 1300 K isothermal anneal. 14 However, the 1300 K anneal imposes an impractical burden for a commercial reference material.…”

“…For the silicon-germanium alloys, the solid solubilities of the dopants exhibit temperature dependent retrograde characteristics. 10 When the concentration of dopant in the solid solution exceeds the solubility limit at a specific temperature, the dopant can precipitate out of solution. The kinetics of precipitation in silicon-germanium alloys can be described using the diffusion-limited Lifshitz and Slyozov dopant precipitation model.…”

“…Precipitation effects have been investigated by a number of researchers for both n-(phosphorus-doped) and p-type (borondoped) silicon-germanium alloys. 10,[12][13][14][15] Since the diffusion coefficient for phosphorus in Ge is larger than that of boron (%100 times at 1000 K), 10 and the activation energy smaller, n-type dopants precipitate at significantly lower annealing temperatures. For the p-type materials, the Seebeck coefficient has been measured as stable under isothermal annealing for 39 210 h below 875 K (Si 75 Ge 25 ), 12 for 1500 h below 925 K (Si 78 Ge 22 ), 13 and for 100 h at 1000 K (Si 63.5 Ge 36.5 ).…”