Growth of ZnTe by metalorganic vapor phase epitaxy: Surface adsorption reactions, precursor stoichiometry effects, and optical studies

Abstract

The metalorganic vapor phase epitaxy growth of ZnTe by di-isopropyl-telluride and di-methyl-zinc (Me2Zn) precursors is investigated by studying the epilayer growth rate as a function of both growth temperature and precursor transport rates. The ZnTe growth is a thermally activated process involving the heterogeneous pyrolysis of both Zn and Te alkyls onto the ZnTe surface. The growth rate dependence on growth conditions is explained in terms of surface adsorption-desorption reactions, assuming that the incorporation of Zn and Te atoms into ZnTe takes place through their selective adsorption on different surface lattice sites. There is also evidence that the occurrence of a competitive species for the surface adsorption of Zn atoms, which is identified as the CH3⋅ (methyl) radical, is produced by the pyrolysis of Me2Zn. Photoluminescence (PL) and absorption measurements performed on ZnTe allow to identify two new donor-acceptor pair (DAP) bands, originated from the recombination of a Ga donor with two acceptor centers, whose ionization energies are 56 meV for the higher energy band and around 140–150 meV for the lower energy one. Hall measurements show that the 56 meV acceptor is responsible of the p-type conductivity of the layers. The nature of the impurities originating such PL features is discussed with the support of secondary ion mass spectrometry. It is shown that Ga, Si, and C are dominant impurities in the layers, whereas Cu does not occur in our ZnTe. Unintentional C doping occurs in ZnTe as a consequence of the strong methyl and iso-propyl radical surface adsorption. We show that C is incorporated as an acceptor in ZnTe, originating the DAP bands observed in the PL spectra. Within this view, the 56 meV ionization energy acceptor is tentatively assigned to substitutional C atoms on Te lattice sites.