IMPACT OF IMPURITIES ON SUPERLATTICE TRANSPORT-COEFFICIENTS IN THE ABSENCE OF A MAGNETIC-FIELD

Using the transfer matrix method, effective mass approximation, and a Kronig-Penney potential, we have calculated the analytical normalized wave functions and the band structure of a superlattice. We have also studied the nonparabolicity effect for the conduction band. This latter effect was negligible for a GaAs-GaAlAs superlattice. We have used these wave functions to calculate the transport coefficients of superlattices. The linear response theory and the relaxation time (electron-impurity collisions) approximation were used. At very low temperatures the parallel (sigma-zz) and perpendicular (sigma-xx) conductivities do not vary with temperature, but increase with the electron concentration. The thermoelectric power (Q(zz), Q(xx)) increases with temperature and decreases with the electron concentration. The thermal conductivities (K(zz), K(xx)) increase with both temperature and electron density. These results are in agreement with the diffusive behavior of the transport coefficients. sigma-zz and K(zz) increase with the width of the miniband. The perpendicular transport coefficients are larger than the parallel ones, verifying the anisotropy of the superlattice.