Improving the p-Type Conductivity and Transparency of Pure Phase SnO by Ga and Na Doping

Tin monoxide (SnO) has attracted much attention as a p-type transparent conducting oxide (TCO) with high hole mobility, which was attributed to its relatively delocalized valence band. Nominally undoped SnO can achieve a free hole concentration of >10¹⁸ cm^-3 due to the low formation energy of the Sn vacancy acceptor. Although several calculations showed that many acceptors have low formation energies and were considered efficient acceptors in SnO, very few experimental works have been reported. In this work we explore the electrical and optical properties of Ga- and Na-doped SnO thin films synthesized by room temperature magnetron sputtering. We find that all as-grown films are amorphous and insulating but become polycrystalline with a tetragonal structure after post-growth rapid thermal annealing (RTA) at temperatures >400 °C. While the resistivity ρ of pure phase SnO is slightly lowered from ∼0.9 ω·cm for undoped to ∼0.2 ω·cm with 1.2% Ga doping, a much lower ρ of ∼0.01 ω·cm with a high hole concentration of 4-5 × 10¹⁹ cm^-3 and a high mobility of >10 cm²/(V·s) can be achieved with ∼2.3-2.8% Na doping. However, the visible transparency T_vis of these highly p-type SnO:Na is only ∼50%, lower than the ∼65% of SnO:Ga films. This is likely due to the presence of excess Sn in SnO:Na films. Detailed variable temperature Hall measurements reveal that the acceptor ionization energies for the substitutional Ga and Na (Ga_Sn and Na_Ga) to be 52 ± 3 and 20 ± 5 meV, respectively. Moreover, all films have a wide band gap of ∼2.7-2.9 eV. We believe that with further optimization in the growth and annealing process, SnO:Na films with ρ < 10^-2 ω·cm and T_vis> 65% can be realized. These results demonstrate that Ga and Na are effective acceptors in SnO, where Na acceptor has a lower formation as well as ionization energies and, hence, can achieve a higher free hole concentration and lower resistivity. Therefore, with appropriate doping, SnO is a high-performance p-type transparent oxide that has technological potential for the advancement of transparent optoelectronics.