Phase-controlled synthesis of SnOx thin films by atomic layer deposition and post-treatment

Abstract

Tin oxide (SnOx) is a promising oxide semiconductor due to the distinct properties of n-type SnO2 and p-type SnO based on its stoichiometry. However, the stoichiometry control of SnOx remains challenging due to the thermodynamic instability of SnO. In the study, we focus on establishing the controllable stoichiometry of SnOx via atomic layer deposition (ALD) and subsequent treatment. The controllable synthesis of SnO2 and SnO is investigated by multiple analyses involving the chemical composition, crystal structure, and band structure. The ALD SnOx is composed mostly of Sn4+-O bonds with intrinsic oxygen vacancies and is transformed into crystalline SnO2 phase via post-annealing. The refractive index (similar to 1.8) and optical bandgap energy (similar to 3.6 eV) of ALD SnOx correspond to those of SnO2. Post-deposition treatment with H-2 plasma enables the effective transformation of SnO2 into SnO due to the easy penetration of H+ ion into the film and de-bonding of Sn-O via ion bombardment. The transformed SnO exhibits a significant amount of Sn2+-O bonds with a refractive index of 2.8 and optical bandgap energy of similar to 2.9 eV. Specifically, the transformed SnO exhibits promise as an oxide semiconductor because it exhibits excellent stability with respect to re-oxidation into SnO2 or further reduction into Sn metal. The present study advances practical applications that require a stable p-n junction through n-type SnO2 and p-type SnO in various forms of device architectures.