Modeling of the Properties of Dopants in the NLO Semiconductor CdGeAs 2
- PDF / 387,479 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 32 Downloads / 194 Views
**
ABSTRACT The results of a shell-model study on CdGeAs 2 doped with Cu, Ag, B, Al, Ga, and In are presented here. The pairwise interatomic potential terms representing the interaction of dopants with the host lattice ions are derived using first principle methods while empirical fitting methods are used for the host-lattice potentials. Defect calculations based on Mott-Littleton methodology predict small binding energies for Cu and Ag substituting Cd 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 placing acceptor levels in the middle of the band gap. INTRODUCTION Cadmium germanium arsenide (CdGeAs 2 ) is an ideal candidate material for nonlinear optical (NLO) applications since it has the highest nonlinear optical coefficient, 236 pm/V, known for a phase matchable compound semiconductor along with a wide transparency region extending across the infrared from 2.4 to 18 Atm.1-4 Recently, single crystals of CdGeAs2 have been used for frequency doubling (i.e., for second harmonic generation) of carbon dioxide laser emission lines from 9.3 to 10.6 jim. However, a major problem still persists regarding the transparency of these single crystals which is found to be limited by the photo-ionization of native acceptors in the lattice. The experimental measurements employing the Hall effect 5-6 , radiation damage techniques 7 , EPR 8 and thermal admittance spectroscopy 9 have established the undoped Bridgman grown crystals to be p-type with at least two native acceptor complexes. There is an acceptor level with an activation energy of 0.11-0.13, and a second deeper level with an activation energy of 0.346 eV in CdGeAs 2 . To reduce the concentration of these native acceptor-complexes in as-grown material, experimental efforts are underway which involve selective doping of the material by appropriate cations or anions. It is expected that alio-valent dopants would reduce the concentration of native acceptors via charge compensation, thereby increasing the transparency of the material. It is to be noted here that previous work on the properties of dopants in CdGeAs 2 was limited to the Hall effect studies. Bairamov et al. have reported 10 that Cu and Ga act as acceptors with the activation energy of 0.06 and 0.12 eV respectively while In and Te act as donors with a small activation energy of 0.01 eV in heavily doped-CdGeAs 2 . In this paper, we will present the results of a theoretical study on CdGeAs 2 doped with 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 Cd ion in the periodic table. Note that the present study is the
471 Mat. Res. Soc. Symp. Proc. Vol. 607 C 2000 Materials Research Society
extension of our earlier work on native defects 11 and electronic structure 12 of undoped CdGeAs 2 . Our theoretical method is based on the
Data Loading...
