Effect of substrate surface pretreatment and annealing treatment on morphology, structure, optical and electrical properties of sputtered ZnO films

Abstract

ZnO films were deposited on glass substrates at room temperature by RF magnetron sputtering method. Prior to the deposition, the glass substrates were pretreated with Al target in Ar or Ar/O2 ambient by DC magnetron sputtering. After pretreatment, the glass substrate are still very transparent and no metallic luster. AFM shows that RMS has a very slight change. XPS shows that no Al–O bonds exist in ZnO films. Cauchy fitting model shows that no Al or Al2O3 thin layer is formed between thin film and glass substrate. However, the pretreatment forms some isolated Al or Al2O3 grains and changes the micro-structure of substrate slightly, which lead to different lattice, stress and properties of ZnO films. Interestingly, combining the pretreatment and annealing treatment, the sum of 2θ or lattice constant c between as-grown and annealed samples is equal. The average transmission of all samples is over 85% in the wavelength range of 400–1000nm. The band gap energies of all samples are in range of 3.23–3.26eV. According to PL, three main blue emission peaks located at about 437–438, 444 and 456nm were observed in the S1 and S2 with the pretreatment. However, the wavelength range of the blue peaks of S3 without pretreatment was observed to narrow down, and peaks are located at about 444, 450 and 456nm. The lowest resistivity and highest carrier concentration of S1 are 5.29∗10−5Ωcm and 3.9805∗1022cm−3 respectively. The conductivity of ZnO films can be converted from n-type to p-type by different substrate surface treatment. It is noteworthy that with the unit cell elongation along c-axis, compressive stress and mass difference between as-grown and annealed films increases, while refractive index and PL intensity decreases. The lattice constant c and compressive stress are mainly effect factors of thermal stability and optical properties of ZnO film. The binding energy (BE) of Zn2p3/2 and Zn2p1/2 of as-grown and annealed S1 are 1019.4 and 1019.3, and 1042.5 and 1042.4eV, respectively. The BEs of O1s of as-grown and annealed S1 are relatively low, 529.2 and 528.9eV, respectively.