Investigations on preferentially oriented Al-doped ZnO films developed using rf magnetron sputtering

Preferentially oriented Al-doped ZnO (AZO) films of thickness 0.5–4 µm are prepared using rf magnetron sputtering. The structural, optical and electrical properties of the films deposited on glass substrates are analyzed using X-ray diffraction (XRD), field effect scanning electron microscope (FESEM), energy dispersive X-ray (EDX) analyses, Raman spectroscopy, UV–visible (UV–Vis) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and four-point probe measurements. The observed deposition rate is 16 ± 0.6 nm/min. EDX results confirm an Al content of (2.6 ± 0.3) % in the films. XRD results show that the deposited films are crystalline and are preferentially oriented along (002) plane with their c-axis perpendicular to the substrate plane. The average crystallite size (22–39 nm) increases with film thickness. FESEM micrographs confirm that the surface morphology of the films is rough and shows irregular hills and valleys like patterns due to grain overlapping. Raman spectra show A₁ (LO) and A₁ (TO) modes of wurtzite ZnO and three prominent anomalous modes 273, 510 and 577 cm⁻¹ which are the characteristics of doped ZnO. FTIR results confirm the presence of Zn–O and Al–O stretching modes in the films. Optical transmittance of the films at 550 nm decreases from 77 to 25% with the increase in film thickness. Their band gap also decreases from 3.39 eV to 2.53 eV. The resistivity of the films gradually reduces beyond a thickness of 1 µm to 1.42 × 10⁻⁴ Ωcm. The obtained resistivity values are comparable to that of In-doped SnO₂ (ITO).