Deep-UV Transparent Conducting Oxide La-Doped SrSnO₃ with a High Figure of Merit

Perovskite stannate SrSnO₃ (SSO) is attracting attention as ultraviolet transparent conducting oxides (UV TCOs) due to its ultrawide band gap and high conductivity. Here, we investigate in detail the thickness-dependent electrical, structural, and optical properties of sequentially strain-relaxed La-doped SrSnO₃ (SLSO) epitaxial thin films. We find that the SLSO films grow as an orthorhombic Pnma phase with a ^- a ^- c ⁺ in the c ⁺ direction under the tensile strain. With the strain relaxation, as the films become thicker, vertical grain boundaries are created and the orthorhombic phase becomes reoriented to all three possible orientations. Simultaneously, the conductance starts to deviate from the linear behavior with increasing film thickness. Through the analysis of thickness fringes in optical transmittance, we found that a 120 nm thick nominally 4% La-doped SrSnO₃ film has a figure of merit (φ_TC = 2.65 × 10^-3 Ω^-1) at λ = 300 nm in the deep-UV region, which is the highest value among the well-known candidates for UV TCOs reported to date.