No abstract

There has been a significant discrepancy among the reflectivity spectra of wurtzite-type ZnS reported previously. The spectrum measured by Baars with structurally pure wurtzite-type ZnS is different from the other spectra which are very close to that of the cubic phase. In addition, the one by Baars shows a distinct polarization dependence, while the others show a very weak or no polarization dependence. From our reflectivity study on structurally pure wurtzite-type ZnS, we are confident that the discrepancy is caused by the structural difference of the samples. The spectrum measured in this work is in good agreement with that of Baars and appreciably different from that of the cubic phase. The energy positions of peaks in our spectrum are consistent with the values deduced from reflectivity data on ZnS-CdS alloys.Ref lectivity spectra of wurtzite-type ZnS reported previously' ' exhibited both energies and magnitudes very similar to those of the zincblende phase except for the spectrum reported by Baars. ' The spectrum closely related to that of the zinc-blende phase has also been reported recently. ' The spectrum of Eaars is referred to as type I and the others as type II, hereafter. Contrary to these results the data on ZnS-CdS alloys by Drew, Davis, and Leiga' (DDL) have indicated that the spectrum of wurtzite-type crystals does not resemble that of the cubic phase. The spectrum of wurtzite-type Zn», Cdo 0, S is very similar to type I, and that of rotation-twinned-cubic Znp g2Cdp pBS is similar to type II. The alloys Zn"Cd, "S have been reported to have wurtzite structure when x is less than 0.92 and to have rotation-twinned-cubic structure when x is larger than 0.92. ' The purpose of this work is to determine which spectrum of the two types corresponds to wur tz ite -type Z nS.We examined the ref lectivity spectra of structurally pure wurtzite-type ZnS at photon energies from the lowest gap energies to 11 eV at room temperature.Single crystals were grown from the melt using ZnS powder with 10 '-rriol Al per mol ZnS. A spectroscopic analysis showed the Al content to be of the order of 100 ppm in the asgrown crystals. From x-ray patterns of the oscil-