Photoelectric properties of ZnSe

Various photoelectric techniques have been exploited in an investigation of the ionization energies of donor and acceptor type defects, and the photoionization cross-sections for electrons and holes from the acceptors in single crystals of zinc selenide doped with indium, gallium or copper. Measurements of d.c. photoconductivity and infra-red quenching were made on high resistivity material, whereas low resistivity samples were fabricated into Schottky diodes for investigation using transient photocapacitance and photocurrent techniques. Interest was focussed on zinc selenide doped with indium or gallium where strong compensation effects occurred, i.e. the resistivity increased with increasing indium (gallium) content. Self-activated acceptor centres with hole ionization energies of 0.59 eV and 0.55 eV were clearly revealed by the above techniques in indium or gallium doped samples. With increasing indium concentration a new acceptor with an ionization energy of 0.41 eV appeared. This may have been responsible for the compensation effect. To ensure that the observations were not affected by the presence of unintentionally incorporated copper impurities, Schottky diodes on samples deliberately doped with copper have also been examined. The dominant copper acceptor level was clearly revealed. It lay ~ 0.67 eV above the valence band.