arXiv subjects
Publications and source records attributed to H. Ehrenreich.
The techniques used previously to calculate the three-fold thermal conductivity reduction due to phonon dispersion in GaAs/AlAs superlattices (SLs) are applied to HgTe/CdTe SLs. The reduction factor is approximately the same, indicating that this SL may be applicable both as a photodetector and a thermoelectric cooler.
The experimentally observed order-of-magnitude reduction in the thermal conductivity along the growth axis of (GaAs)_n/(AlAs)_n (or n x n) superlattices is investigated theoretically for (2x2), (3x3) and (6x6) structures using an accurate model of the lattice dynamics. The modification of the phonon dispersion relation due to the superlattice geometry leads to flattening of the phonon branches and hence to lower phonon velocities. This effect is shown to account for a factor-of-three reduction in the thermal conductivity with respect to bulk GaAs along the growth direction; the remainder is attributable to a reduction in the phonon lifetime. The dispersion-related reduction is relatively insensitive to temperature (100 < T < 300K) and n. The phonon lifetime reduction is largest for the (2x2) structures and consistent with greater interface scattering. The thermal conductivity reduction is shown to be appreciably more sensitive to GaAs/AlAs force constant differences than to those associated with molecular masses.
The effective transport coefficients and figure of merit ZT for anisotropic systems are derived from a macroscopic formalism. The full tensorial structure of the transport coefficients and the effect of the sample boundaries are included. Induced transverse fields develop which can be larger than the applied fields and which reduce the effective transport coefficients. A microscopic model relevant for multi-valleyed materials is introduced which utilizes the effective-mass and relaxation-time approximations. The thermopower and Lorentz number are independent of the tensorial structure of the transport coefficients in this case and are therefore isotropic. ZT is also isotropic for vanishing lattice thermal conductivity κ_\ell. For non-vanishing but sufficiently isotropic κ_\ell, ZT is maximal along the direction of highest electrical conductivity σ. Numerical calculations suggest that maximal ZT generally occurs along the principal direction with the largest σ/κ_\ell. An explicit bound on ZT is derived. Several results for specific systems are obtained: (1) Bulk n-type Bi_2Te_3 exhibits easily observable induced transverse fields and anisotropic ZTs. (2) Increased anisotropy in HgTe/ Hg_{1-x}Cd_xTe superlattices (SLs) is associated with larger induced fields. (3) The valley degeneracy is split and the bulk masses modified in isolated Bi_2Te_3 quantum wells, resulting in optimal ZTs for wells grown along the trigonal direction. (4) Non-parabolic dispersion in SLs has little effect on the thermopower at the carrier concentrations which maximize ZT.
A new mechanism for exciton lasing in ZnSe/ZnCdSe quantum wells is proposed. Lasing, occurring below the lowest exciton line, may be associated with a BCS-like condensed (coherent) exciton state. This state is most stable at low temperatures for densities in the transition region separating the exciton Bose gas and the coherent exciton state. Calculations show the gain region to lie below the exciton line and to be separated from the absorption regime by a transparency region of width, for example, about 80 meV for a 90 Angstrom ZnSe/Zn_(0.75)Cd_(0.25)Se quantum well. Experimental observation of the transparency region using differential spectroscopy would confirm this picture.