Deep-UV transparent conducting oxide La-doped SrSnO3 with a high figure of merit

Abstract

Perovskite stannate SrSnO3 (SSO) is attracting attention as ultraviolet transparent conducting oxide (UV TCO) due to its ultra-wide 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 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 the film thickness. Through the analyses of thickness fringes in optical transmittance we found that 120 nm thick nominally 4% La-doped SrSnO3 film has a figure of merit (φTC = 2.65×10-3 Ω-1) at λ = 300 nm in the deep-UV region, which is the highestvalue among the well-known candidates for UV TCOs reported to date.