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

Tin oxide (SnO _x ) is a promising oxide semiconductor due to the distinct properties of n-type SnO ₂ and p-type SnO based on its stoichiometry. However, the stoichiometry control of SnO _x remains challenging due to the thermodynamic instability of SnO. In the study, we focus on establishing the controllable stoichiometry of SnO _x via atomic layer deposition (ALD) and subsequent treatment. The controllable synthesis of SnO ₂ and SnO is investigated by multiple analyses involving the chemical composition, crystal structure, and band structure. The ALD SnO _x is composed mostly of Sn ⁴⁺ –]O bonds with intrinsic oxygen vacancies and is transformed into crystalline SnO ₂ phase via post-annealing. The refractive index (~1.8) and optical bandgap energy (~3.6 eV) of ALD SnO _x correspond to those of SnO ₂ . Post-deposition treatment with H ₂ plasma enables the effective transformation of SnO ₂ 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 Sn ²⁺ –]O bonds with a refractive index of 2.8 and optical bandgap energy of ~2.9 eV. Specifically, the transformed SnO exhibits promise as an oxide semiconductor because it exhibits excellent stability with respect to re-oxidation into SnO ₂ or further reduction into Sn metal. The present study advances practical applications that require a stable p-n junction through n-type SnO ₂ and p-type SnO in various forms of device architectures.