Temperature dependence of plasma-reflection spectra of bismuth-antimony crystals

The temperature dependence of plasma-reflection spectra of Bi0.93Sb0.07 Sin gle crystals doped with tin acceptor impurity was studied in the temperatur e range of T = 4-300 K. A nonmonotonic shift of the plasma frequency wp wit h increasing temperature and a substantial increase in the reflection coeff icient R in the high-frequency region of the spectrum for the frequencies o mega >> omega (p) were observed. It is shown that the observed effects are caused by passage of the chemical-potential level through the direct energy gap E-gL at the L point of the Brillouin zone. It is in this case that the concentration of free charge carriers and the plasma frequency are found t o be the lowest. In addition, the reflection spectrum cannot be described b y the Drude dielectric function, because, in the situation under considerat ion, (h) over bar omega (p) approximate to E-gL and direct band-to-band tra nsitions contribute significantly to the interaction of electromagnetic rad iation with crystal. Calculation of dielectric function in terms of a model which accounts for the contributions of the plasma of free charge carriers acid band-to-band transitions to this function makes it possible to satisf actorily describe the experimental reflection spectra and also to determine a number of parameters that specify the interaction of electromagnetic rad iation with charge carriers in Bi1-xSbx crystals. (C) 2001 MAIK "Nauka/Inte rperiodica".