Residual stress and defect content in magnetron sputtered ZnO films grown on unheated glass substrates

Abstract

Structural, vibrational, photoluminescence and photo electron spectroscopic properties of rf magnetron sputtered ZnO films grown on unheated glass substrates are discussed. Variations in the c-axis orientation, average crystallite size, residual stress, and presence of disoriented grains, with varying oxygen content (40%–100%) during film growth are found to correlate with variation in the E2(high) and A1(LO) modes of the Raman spectra. Room temperature photoluminescence (RTPL) spectra exhibit a characteristic splitting in the near-band-edge emission (NBE) with three emission peaks (around 355, 386 and 395 nm) under an excitation wavelength of 290 nm. The blue emission (443–455 nm) is significantly controlled by oxygen vacancies and seen in films grown under 80% oxygen content. Quenching of NBE emission, for films prepared under oxygen rich ambient, is caused by the increased non-radiative recombination centers owing to reduced size of ZnO nano-crystallites. The shifts in the optical band gap of films grown under different oxygen content supplement the observed changes in the nano-crystallite size and luminescence properties. X-ray photoelectron spectroscopy confirms the fractional changes in the oxygen vacancy content in films deposited under different oxygen content in the sputtering gas.