Fabrication of p-type conductivity in SnO2 thin films through Ga doping

P-type transparent tin oxide (SnO2) based semiconductor thin films were deposited onto alkali-free glass substrates by a sol–gel spin-coating method using gallium (Ga) as acceptor dopant. In this study, we investigated the influence of Ga doping concentration ([Ga]/[Sn] + [Ga] = 0%, 5%, 10%, 15%, and 20%) on the structural, optical and electrical properties of SnO2 thin films. XRD analysis results showed that dried Ga-doped SnO2 (SnO2:Ga) sol–gel films annealed in oxygen ambient at 520 °C for 1 h exhibited only the tetragonal rutile phase. The average optical transmittance of as-prepared thin film samples was higher than 87.0% in the visible light region; the optical band gap energy slightly decreased from 3.92 eV to 3.83 eV with increases in Ga doping content. Hall effect measurement showed that the nature of conductivity of SnO2:Ga thin films changed from n-type to p-type when the Ga doping level was 10%, and when it was at 15%, Ga-doped SnO2 thin films exhibited the highest mean hole concentration of 1.70 × 1018 cm−3. Furthermore, a transparent p-SnO2:Ga (Ga doping level of 15%)/n-ZnO:Al (Al doping level of 2%) heterojunction was fabricated on alkali-free glass. The I–V curve measurement for the p–n heterojunction diode showed a typical rectifying characteristic with a forward turn-on voltage of 0.65 V.