Thermoelectric Properties of Films and Monocrystalline Whiskers of Tellurium
Tellurium is an interesting semiconductor for thermoelectric applications because its thermopower is high (500 muV/K). Its figure of merit is low in the bulk because it has high thermal conductivity. However, in confined geometries, the phonon conductivity decreases due to phonon scattering. Therefore we have investigated the thermoelectric properties of Te films and whiskers. We report a study of the thermoelectric properties of Te films obtained by vacuum-condensation method on glass, polyamide, mica at various temperatures of condensation. The crystallographic structure of the films is characterized by the distribution function of the grain major axis. It is shown that the films condensed at 433 K possess the highest mobility. Te whiskers were grown from the paragas phase and they were single crystals in the form of hexahedral cylinder having cavities inside. When deposited on cold substrate, the cylinders had no cavities. The direction of the whisker crystal growth was always [0001] with respect to the whiskers length. It is shown that in films with the thickness of ~1 mum, the room temperature thermopower S is 330 muV/K and increases with temperature increasing (S = 450 muV/K at 450 K). The resistivity decreases with the temperature growth, as a result the power factor alpha2sigma increases by almost a factor of two. The 77 K electric conductivity of Te whiskers is 2-to-3 order-of-magnitude higher than in bulk crystals, presumably due to high structural perfection of whisker crystals. The thermopower in the range of 300 K is S = 400-750 muV/K
Structural and Photoconductivity Properties of Tellurium/PMMA Films
Owing to the very brittle nature of tellurium powder, nanoscopic grains with an average size of 4.8 ± 0.8 nm were produced by dry vibration milling technique using a mixer/mill apparatus. A novel material was obtained by binding the nanosized tellurium grains with poly(methyl methacrylate) (PMMA) polymer. The morphology, elemental composition, and structural and optical properties of Te/PMMA films were investigated. The prepared material was composed of hexagonal tellurium and α-phase of tellurium oxide. The electrical properties of the films were studied, for different electrode contact configurations, in dark condition and under white light illumination varying the optical power density from 2 to 170 mW/cm2 and turning the light on and off cyclically. Data analysis shows that the photoconductivity of the film with sandwich contact configuration is a linear function of the light power density and increases more than 2 orders of magnitude as compared to the photoresponse of the film with coplanar contact configuration.
Field-Induced Conductivity Changes in Germanium
Variations in the conductance of a germanium filament due to an electric field normal to the surface were measured for fields up to ${10}^{5}$ volts/cm. Observation of a minimum in the conductance curve makes it possible to calculate charge in surface states as a function of surface potential. Information of this sort obtained with several gaseous ambients is compared with the Brattain-Bardeen observations on contact potential. Measurements as a function of temperature down to 170\ifmmode^\circ\else\textdegree\fi{}K give information about properties of "fast" surface states. Phase shift loops observed at frequencies around a kilocycle are interpreted in terms of minority carrier lifetime. Phase shift loops at low temperatures are believed to be due to traps.