Modulation of Optical and Electronical Properties for Transparent Oxide Semiconductor ASnO3 by the A-Site Ion Substitution

Abstract

Abstract Body: Transparent oxide semiconductors (TOSs) that transmit deep ultraviolet (DUV, 200−300 nm in wavelength) are considered to be preferable as the active material for next generation optoelectronics such as biosensor.[1,2] Among many TOSs, La-doped ASnO3 (A = Ba, Sr and Ca) are considered as a promising candidate f DUV-TOS because of their rather high electrical conductivity and large bandgap (Eg). The optical Eg of electron-doped BaSnO3 and SrSnO3 are ~3.1 eV[3] and ~4.6 eV[4], respectively. Based on this trend, if Ca, an even smaller alkaline element, occupies the A-site, further widening of the Eg value can be expected. However, the effect of A-site substitution on the optical and electrical properties of ASnO3 has not been clarified in detail due to the lack of a systematic study. Here we show that the optical and electrical properties of ASnO3 films can be tuned systematically by changing the average size of A-site ions.[5] We used lattice parameter as a bridge to build the relationship between the A-site substitution and structure factors, and clarified the effect of lattice parameter on the optical and electrical properties. The lattice parameter almost linearly increased from 3.95 to 4.14 Å with increasing A-site ionic radius from 1.34 Å (Ca2+) to 1.61 Å (Ba2+). The optical bandgap (Eg opt) of the resultant films gradually decreases from ~4.6 to ~3.6 eV with increasing lattice parameter with a small positive bowing, while the electrical conductivity gradually increased from ~100 to ~103 S cm−1 due to gradual increases in both the carrier concentration and mobility. These results indicate that larger A-site ion substitutions reduce the Eg opt and the Fermi energy exceeds the mobility edge (degenerate semiconductor). On the other hand, the carrier generation efficiency increases when the Eg opt decreases. The carrier relaxation time increases when the mobility edge decreases. Thus, the conductivity increases when larger ion is substituted in the A-site. Meanwhile, La-doped SrSnO3 exhibited a great balance of wide bandgap and high electrical conductivity, which makes it a suitable ASnO3 system for advanced optoelectronic applications. The present results are essentially important for designing ASnO3 based DUV-TFTs.