Atomistic Calculations of Dopant Binding Energies in ZnGeP 2
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ABSTRACT Atomistic model has been applied to study various cation dopants, namely Cu, Ag, B, Al, Ga and In in ZnGeP 2 . The pairwise interatomic potential terms representing the interaction of dopants with the host lattice ions are derived using first principle methods. Defect calculations based on Mott-Littleton methodology predict small binding energies for Cu and Ag substituting Zn in the lattice which are in agreement with the available experimental data. The group III dopants (i.e. B, Al, Ga and In) at the Ge site are predicted to have large binding energies for a hole except B which shows a distinct behavior. This may be due to large mismatch in atomic sizes of B and Ge. At the Zn site, the calculated binding energies of the group III dopants place donor levels in the middle of the band gap. INTRODUCTION Zinc germanium phosphide (ZnGeP 2 ) is the NLO material for mid IR lasers allowing 12 high-power tunability
in the spectral region of 2-5 .tm. - An absorption band around 1-2 gnm is, however, found to affect the usefulness of this material. This band is attributed to 3 photoionization of a deep native acceptor center associated with the zinc vacancy. -5 Experimental efforts involving selective doping of the material are underway to reduce the concentration of this acceptor-complex in the lattice. Earlier work on the properties of dopants in ZnGeP 2 have been limited to few photoluminescence and Hall effect studies. Averkieva et al. have reported 6 photoluminescence spectra and hole concentrations for crystals melt doped with Cu, Ga, In, Se, Fe, Cd and Si. The room temperature hole concentration for these samples were reported to vary from a low of 2.8xl0 10 /cm 3 for Si, nominally the value seen in undoped crystals, to a high of 1017/cm 3 for In. Based on a photoluminescence study 7 , it is reported that Cu diffusion can increase the hole concentration by a factor of 10,000 in ZnGeP 2 by a proper choice of the diffusion regime. Grigoreva et al., however, have reported 8 Hall effect data on samples doped with Au, Cu, Se, Ga and In and stated that the first two dopants were inactive and the rest were acceptors. The binding energy of Se was reported to be 0.40 eV and that of Ga to be 0.30 eV. In a more recent review paper, Rud 9 has indicated that Au, Cu, Ga, In, Se and Pt are acceptors with activation energies in eV of respectively 0.50, 0.30, 0.06 for heavy doping, 0.03 for heavy doping and =0.4 for light doping, 0.40 and 0.50. These results will be compared with our calculated values later in the paper.
525 Mat. Res. Soc. Symp. Proc. Vol. 484 0 1998 Materials Research Society
We have initiated an extensive theoretical study 5 on ZnGeP 2 and, in this paper, we will report the results of atomistic calculations involving cation dopants, namely Cu, Ag, B, Al, Ga and In. The choice of dopants is expected to indicate the ion-size effect on the binding energies for the groups I and III dopants which lie left and right columns of the host Zn ion in the periodic table. Our approach is based on the pair-wise descri
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