EFFECTS OF THERMAL STRAIN ON THE OPTICAL-PROPERTIES OF HETEROEPITAXIAL ZNTE

Optical measurements have been used to study the biaxial tensile strain in heteroepitaxial ZnTe grown by molecular-beam epitaxy on both GaAs and GaSb substrates, and its effect on the low-temperature photoluminescence (PL) spectrum of the material. The observed strain (0.92 X 10(-3) for ZnTe/GaAs and 0.45 X 10(-3) for ZnTe/GaSb) agrees with that expected for differential thermal contraction from the growth temperature to low temperature, based on the difference in thermal expansion coefficients. The amount of strain increases with growth temperature, as expected, but decreases slightly in thin layers on GaAs. The latter effect is due to incomplete relaxation of the large (7.6%) lattice mismatch strain between ZnTe and GaAs, the unrelieved part of which constitutes a biaxial compressive strain. Reflectance and variable temperature PL measurements show that the free exciton splits into heavy-hole (X(hh)) and light-hole (X(lh) components, which both shift to lower energy. The J = 1 (allowed) and J = 2 (forbidden) components of the oxygen isoelectronic center bound exciton are mixed and shifted to lower energy by the strain. Temperature-dependent PL measurements show that the oscillator strengths of the two components are strongly redistributed by the strain. We calculate the strain-induced splittings in the O-bound exciton and in the excitons bound to neutral acceptors and neutral double acceptors, and find good agreement with the experimentally observed peak positions. Further confirmation of our assignments of the strain-split and shifted bound exciton peaks is obtained using magnetospectroscopy in fields up to 12 T. The diamagnetism, g factors, and splitting patterns of the free and bound excitons in the magnetic field are discussed.