Enhancing the p-type conductivity of pure phase SnO via stoichiometry control and annealing

While n-type transparent conducting oxides (TCOs) have been well developed and widely used in optoelectronic devices, the availability of high-performance p-type TCOs is still scarce which in turn hinders the further advancement of oxide based transparent optoelectronic devices. Unlike the most widely used p-type NiO, Tin monoxide (SnO) has a relatively delocalized valence band, which leads to a high hole mobility of >1 cm²/Vs. In this work, we enhance the p-type conductivity of SnO thin films by tunning the oxide stoichiometry through adjusting the growth conditions and post-growth rapid thermal annealing (RTA) treatment. Specifically, we synthesized nominally undoped Sn-rich SnO (SnO_1-δ) thin films by co-sputtering an SnO target and a metallic Sn target and control the film stoichiometry by varying the sputter power of the Sn target so that SnO_1-δ films with excess Sn (δ) varying from 0 to ∼5% was incorporated. We find that the addition of dilute amount of excess Sn is an effective approach to regulate the film stoichiometry so that a low p-type resistivity can be achieved after post-growth RTA. All as-grown films are amorphous and after RTA exhibit a polycrystalline tetragonal structure, with p-type resistivity decreases with increasing δ. However, metallic β-Sn clusters are formed for films with δ≳ 0.05. The presence of these β-Sn clusters results in a degraded optical transmittance by ∼10% in the visible range. With δ∼0.03 in the as-grown film, a p-type pure phase SnO film with a low resistivity of ∼0.5 Ω−cm, and a decent visible transparency of ∼60% is achieved after 300 °C RTA in N₂. This p-type SnO has a wide band gap of 2.8 eV with a high valence band maximum (VBM) located in the range of 4.6–4.8 eV below the vacuum level, making it suitable for many device applications, particularly as hole transport layers in optoelectronic devices.