X-ray absorption spectroscopy study of Ga-doping in reactively sputtered ZnO films

Abstract

Ga-doped ZnO (GZO) films were grown by reactive co-sputtering of a Zn-GaAs target (3% area coverage of Zn by GaAs) at 375 °C with different partial pressures of oxygen. The resistivity of GZO films increases drastically from ≲10−3 Ω-cm to ~ 0.2 Ω-cm as the O2 in the sputtering atmosphere is changed from 5 % to 6 %. The films grown at 5% or lower O2 have the Ga/Zn ratio of ~ 0.1 and display low resistivity with carrier concentration ~ 1021 cm−3, while the films grown at 6% or higher O2 have lower Ga/Zn ratio of ~ 0.01 and display high resistivity with carrier concentration ~ 1019 cm−3. Extended X-ray absorption fine structure (EXAFS) measurements at Zn and Ga K-edges reveal substitutional incorporation of Ga in low resistivity films. X ray photoelectron spectroscopy (XPS) studies reveal a substantial decrease in oxygen vacancies together with increase in oxygen interstitials, in high resistivity GZO films. EXAFS measurements of the high resistivity films show a substantial increase in Ga-O and Ga-Zn bond distances and Ga-O coordination, indicating incorporation of oxygen interstitials in the vicinity of Ga. The comparison of measured and simulated X–ray absorption near edge spectra at O K-edge, together with XPS and EXAFS results, provides adequate experimental evidence of the presence of acceptor type (GaZn+Oi) defect complex, which cause carrier compensation in high resistivity films.