Formation Energy study of Cd impurities in Sapphire

G. N. DARRIBA; M. RENTERÍA; H. M. PETRILLI; L. C. V. ASSALI

San Miguel de Tucumán

Congreso; Sólidos 2011; 2011

The Electric-Field Gradient (EFG) at 111Cd impurity site in a-Al2O3 has been extensively studied by means of the TDPAC technique (using the 111In/111Cd and 111mCd /111Cd ion-implanted PAC probes) and, due to the excellent quality of the experimental spectra and to the presence of a very well-defined quadrupole frequency with a very small distribution, the authors assigned the hyperfine interaction as generated by 111Cd probes located at the substitutional Al site [1,2]. Indeed, a single well-defined hyperfine interaction just only proves that the tracers are all occupying the same crystalline site, either substitutional or interstitial. On the other hand, Rutherford Backscattering/Channeling spectroscopy (RBS/C) experiments have shown that In impurities implanted in a-Al2O3 occupy interstitial sites [3]. In order to elucidate this controversy we performed a DFT study obtaining the EFG at Cd impurities located at cationic and interstitial sites in the semiconductor a-Al2O3 using two state-of-the-art ab initio all-electron methods: the Full-Potential Augmented Plane Wave plus local orbitals (FP-APW+lo) and the Projector Augmented Wave (PAW). For this study we obtained the same EFG for substitutional Cd in a negatively ionized charge state and for Cd at interstitial sites in a neutral charge state, and therefore we could not decide the impurity site localization. In this work we present a detailed study of formation energies of Cd impurities in a-Al2O3 at both substitutional and interstitial sites and for different charge state of the impurity. We found that for each scenario (substitutional or interstitial) the lower energy corresponds with the charge state that reproduces the experimental EFG. Finally, we could resolve this indetermination showing that 111Cd impurities occupying the substitutional Al site in the a-Al2O3 host, in the negative ionized charge state, have the lowest formation energy, and should be the correct scenario.