Electronic and atomic structure of complex defects in Al- and Ga-highly doped ZnO films

Abstract

Artículo de publicación ISIPoint defects in Ga- and Al-doped ZnO thin films are studied by means of first principles electronic structure calculations. Candidate defects are identified to explain recently observed differences in electrical and spectroscopic behavior of both systems. Substitutional doping in Ga-ZnO explain the metallic behavior of the electrical properties. Complexes of interstitial oxygen with substitutional Ga can behave as acceptor and cause partial compensation, as well as gap states below the conduction band minimum as observed in photoemission experiments. Zn vacancies can also act as compensating acceptors. On the other hand, the semiconducting behavior of Al-ZnO and the small variation in the optical gap compared with pure ZnO, can be explained by almost complete compensation between acceptor Zn vacancies and substitutional Al donors. Interstitial Al can also be donor levels and can be the origin of the small band observed in photoemission experiments below the Fermi level. Combinations of substitutional Al with interstitial oxygen can act simultaneously as compensating acceptor and generator of the mentioned photoemission band. The theoretical calculations have been done using density functional theory (DFT) within the generalized gradient approximation with on-site Coulomb interaction. In selected cases, DFT calculations with semilocal-exact exchange hybrid functionals have been performed. Results explain photoelectron spectra of Ga-ZnO and Al-ZnO at the corresponding doping levels

Monitoring the characteristic properties of Ga-doped ZnO by Raman spectroscopy and atomic scale calculations

“…Thus, based on the total energy calculations, substitutional doping of Zn by Ga atoms is highly probable. In addition, previous studies revealed that substitutional doping of Ga atom in the wurtzite ZnO is more likely to occur [40,41]. Therefore, in the following we only consider the substitutional doping.…”

High performance GZO/p-Si heterojunction diodes fabricated by reactive co-sputtering of Zn and GaAs through the control of GZO layer thickness

“…71 It is also known that nonstoichiometric Ga x O y phases can be formed during the sputtering of Ga 2 O 3 lms, leading to a wide range of oxygen substoichiometry 72 and there is also a possibility of forming ZnGa 2 O 4 phase. 73,74 Another possibility is the strong compensation due to the formation of acceptor complexes, such as, (Ga Zn + O i ) and (Al Zn + O i ), [75][76][77] respectively reported in Ga and Al doped ZnO lms, grown under O-rich conditions. A recent X-ray absorption study of reactively sputtered GZO lms from our group 47 has shown that the formation of (Ga Zn + O i ) acceptor complex depends critically on the availability oxygen during sputtering.…”

Effect of Al and Ga codoping on the morphological, electronic, and optical properties of ZnO transparent conductive thin films prepared by spray pyrolysis technique

“…The Al atomic size (0.054 nm) is much smaller than that of Zn (0.074 nm), while the Ga atomic size (0.062 nm) and that of Zn atom are close. The Al atoms can be located in either substitutional or octahedral interstitial sites, while the Ga atoms can only be located in substitutional sites [14][15][16]. For high doping concentration, excessive dopants may reside in the interstitial sites or form oxides, which causes a reduction of crystallinity [13,29].…”

“…However, the atomic substitution of Al for Zn remains difficult for high doping concentrations due to the difference in ionic radiuses and coordination preference [14]. Reportedly, Al atoms, whose ionic radius is 0.54 Å, can be found in both substitutional and interstitial sites [15]. Ga has an ionic radius of 0.62 Å, which is close to the Zn ionic radius of 0.74 Å [13].…”