TRANSPORT-PROPERTIES AND BAND-STRUCTURE OF NONSTOICHIOMETRIC CU2-XTE

Transport properties, namely the electrical conductivity sigma, Hall effect coefficient R(H), and thermoelectric power Q, were measured for bulk films (d much greater than 1000 angstrom) as a function of temperature between 90 and 480 K for non-stoichiometric compositions Cu2-xTe in the homogeneity range 0 less-than-or-equal-to x less-than-or-equal-to 0.025 in steps of x = 0.05. Studies of cr and Q show that a phase transition occurs at low temperatures. The transition temperature shifts towards lower temperature as x increases, being 280 K for Cu2Te and reaching 225 K for Cu1.75Te. Hall effect measurements show that, for all compositions, R(H) is independent of temperature which indicates that the samples are highly degenerate p-type semiconductors. The number N(g) of copper vacancies was calculated (N(g) = (0.082-1.62) x 10(21) cm-3) and compared with the number p of free carriers (p = (0.28-5.12) X 10(21) cm-3). These results suggest that all copper vacancies are electrically active. We also found that N(g) and consequently p increase markedly as x increases. The temperature dependence of the Hall mobility indicates that at low temperatures (T < 200 K) optical phonon scattering is predominant, while for higher temperatures (T > 300 K) acoustic phonon and grain boundary scattering are predominant; in between, a phase transition occurs in which the mobility does not depend on temperature. Combining the results of the optical (previously reported) and electrical measurements, the effective mass m*/m0 for the highly degenerate composition was calculated and its dependence on the deviation from stoichiometry is given. It is found that m*/m0 = 0.39 for Cu2Te and increases markedly with increasing x, being 2.956 for Cu1.75Te.