Electric field effect on thermopower in cylindrical microwires

V. Kantser1, Z. Dashevsky2, H. Scherrer3, D. Meglei1, M. Dantu1

1LISES Institute of Applied Physics ASM, Chisinau MD-2028, R.Moldova

2Ben-Gurion University of the Negev, Beer-Sheva, 84105 Israel

3Laboratoire de Physique des Matériaux, Ecole des Mines, Nancy, 54042 France

The electric field effect (EFE) is an important approach to engineer the electronic properties of solid state structures. In the present paper the EFE in a coaxial cylindrical capacitor configuration Gate-Dielectric-Semiconductor (or Semimetal) was used to study the transport and thermoelectric properties of semiconductor (Bi2Te3) and semimetallic (Bi) glass coated microwires and to clarify the possibilities of their improvement by EFE.

In the first part of the paper a model calculation of EFE is developed for a semiconductor and semimetallic cylindrical microwires with degenerate carrier gas. A self-consistent, semiclassical theory is used to study the capacitance and static screening capability. Due to large value of the dielectric constant and strong dependence of the chemical potential on the carrier concentration in Bi and Bi2Te3 materials an enhanced value of the screening length was establish to occur in cylindrical microwires. The contribution of the surface states induced by wire glass coating in the EFE and wire screening properties have been considered. The relative changes of wire conductance and thermopower is introduced through the radial space dependence of the conductivity and Seebeck coefficient induced by the carrier concentration redistribution under EFE. The last was determined by solving the Poisson equation in cylindrical geometry with corresponding boundary conditions. The carrier mobility was assumed to be the same as in the bulk materials. The relative resistance D R/R0 and thermopower D S/S0 have been calculated in dependence of the gate voltage Ug and the wire diameter.

The second part of the paper includes the results of experimental investigations of EFE of conductance and thermopower in cylindrical Bi and Bi2Te3 microwires coated with glass and fabricated by Ulitovsky method. The gate electrode was prepared by coating the surface of glass with Ag thin film. Owing to the coaxial configuration the highest electric field value about 107 V/cm on the wire surface was achieved.

The EFE of the transport properties of Bi2Te3-microwires ranging in diameter from 2 m m to 20 m m are characterised by the following features:

From our theoretical and experimental results we conclude that EFE in coaxial cylindrical wire configuration offers an effective route to engineer and to control thermoelectric properties of the materials and to design new thermoelectric devices. In particular at the paper end we also discuss a realistic design of coaxial wire microthermocouple, which consists of two thermoelectric coaxial Bi2Te3 microwires isolated by glass coating.