Transparent and low resistive nanostructured laser ablated tungsten oxide thin films by nitrogen doping: II. Substrate temperature

Nitrogen incorporated tungsten oxide thin films are deposited onto fused quartz substrates at various substrate temperatures (Ts) in nitrogen ambient (pN2) of 12 Pa by pulsed laser deposition. The structural, optical and electrical properties of the deposited films are found to depend on the substrate temperature/nitrogen doping concentration. Compositional analysis by energy dispersive x-ray spectra confirms the incorporation of nitrogen into the films, with maximum nitrogen incorporation for films deposited at Ts = 973 K. X-ray diffraction analysis reveals an orthorhombic crystalline phase for the WO3 : N films deposited at Ts = 300, 873 and 973 K, with a nanocrystalline structure for the films prepared at intermediate growth temperatures. Morphology investigation of WO3 : N films in relation to substrate temperature/nitrogen doping is done by scanning electron microscopy and atomic force microscopy. Vibrational properties of the WO3 : N films are measured using micro-Raman spectroscopy. Bandgap of WO3 : N films decreases from 3.31 +/- 0.04 to 2.85 +/- 0.03 eV and room temperature resistivity decreases from 1.77 X 103 to 4.3 X 10-2 Omega m with change in substrate temperature/nitrogen content. Analysis of optical and electrical properties reveals that the incorporated nitrogen acts as an electronic dopant in WO3. Narrow bandgap, low dc resistivity at room temperature and average transmittance in the visible range, observed for the films deposited at higher substrate temperatures (873 and 973 K), make very interesting prospects for technological transfer, especially as novel solar cell materials.