Wide-gap semiconductor, ZnTe, is an ideal material for pure-green light emitting diodes since it has the energy gap of 2.26 eV at room temperature and a band structure is of a direct optical transition type. It is however difficult to realize p-n junctions because of the well-known compensation effect specific to II-VI materials. To attain the conductivity control for ZnTe, it is necessary to understand the behaviors of impurities and defects in ZnTe.
High quality 80 mm diameter ZnTe single crystals were successfully grown by the vertical gradient freezing (VGF) method without seed crystals [1-2]. The 80 mm diameter crystals with 50mm length are largest in bulk-ZnTe single crystals. The grown direction was nearly <111> or <110> direction. The etch pit density measured after HF solution etching was lower than 10000 cm^-2, which was lower than that of their solution growth method.
In this paper, photoluminescence (PL) measurement was carried out between liquid helium (4.2K) and room temperatures to study point defects in the Indium (In)-doped, Gallium (Ga)-doped and non-doped ZnTe crystals grown by the VGF method. In the PL spectrum of non-doped ZnTe crystal at 4.2K, three distinct kinds of peaks are present at 522.5nm, 532.0nm and 534.2nm. These emissions are assigned to be the deep acceptor-bound exciton emission (I₁^d), the free-to-acceptor emission (FA) and the donor-acceptor pair (DAP) emission, respectively. On the other hand, both FA and DAP emissions are not detected in In- and Ga-doped ZnTe crystals at 4.2K. The Ga donor- and In donor-bound exciton emissions (I₂) are observed at 522.0nm and 521.6nm, respectively. These results suggest that the high-quality non-doped ZnTe can be grown by VGF method, and both Ga and In atoms behaves donor species in the ZnTe crystals, respectively.
[1] K. Sato et al., J. Crystal Growth 197(1998) 413.
[2] T. Asahi et al., J. Crystal Growth 229(2001) 74.

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