Band Gap Engineering and Room-Temperature Ferromagnetism by Oxygen Vacancies in SrSnO₃ Epitaxial Films

Abstract

Perovskite SrSnO (SSO) thin films were epitaxially grown on LaAlO (001) substrates by pulsed laser deposition at various oxygen pressures. X-ray diffraction was carried out to characterize the microstructure of the films, and the results showed that the unit-cell volume of the films increased gradually with lowering the growth oxygen pressures while remaining the perovskite structure. X-ray photoelectron spectroscopy results indicated that oxygen vacancies (OVs) existed in SSO thin films. Optical property measurements showed that all samples have a transmittance of more than 75% in the visible and near-infrared wavelength region. Furthermore, the band gaps of SSO films were found to increase from 4.56 to 5.21 eV with the decrease of deposition oxygen pressures calculated by linear fitting absorption edges of optical transmittance. In order to further ascertain the effect of OVs on band gaps of SSO films, the as-deposited 10 Pa film was annealed at 10 Pa oxygen pressures and the band gap was found to increase by more than 1 eV. Density functional theory was used to explain the effects of OVs on band gaps and the ferromagnetism of SSO films, and the results suggested that an impurity energy level of OVs appeared near the Fermi level, causing the widening of the band gaps, which is consistent with our experimental results. Meanwhile, the room-temperature ferromagnetism was observed in the SSO films, and saturation magnetization increased gradually from 4.46 to 7.69 emu/cm with decreasing the growth oxygen pressures.

Electrical and Optical Properties of Nb-doped SrSnO3 Epitaxial Films Deposited by Pulsed Laser Deposition

“…It is noticed that more oxygen is beneficial for the nucleation, crystallization, and growth of SSNO films. Similar phenomenon has been reported in oxygen-deficient SSO films and ZnO films [ 35 , 36 ]. Figure 1 d and e depict the SSNO films in group B oriented along (002) and (101) reflections without other phases.…”

“…The calculated values of the three parameters for these deposited samples are presented in Table 1 . The variation in lattice constants with deposition oxygen pressure has also been observed in other oxygen-deficient perovskite films [ 29 , 35 ] and it can be ascribed to the existence of the oxygen vacancies. In fact, there exists strong Coulomb repulsion between A and B cations (Sr and Sn or Nb in this case), and this interaction will be enhanced by a high density of the positively charged oxygen vacancies [ 29 , 37 ].…”

“…As the low energy levels of conduction band were filled up by the conduction electrons, only the photons with higher energies can be absorbed, leading to an enlarged band gap [ 63 ]. For the samples deposited at lower oxygen pressures, the further increment in band gap is due to the generation of considerable oxygen vacancies in this film [ 35 ].…”

High electrical conducting deep-ultraviolet-transparent oxide semiconductor La-doped SrSnO3 exceeding ∼3000 S cm−1

“…Although conventional transparent oxide semiconductors such as Al-doped ZnO and Sn-doped In 2 O 3 (ITO) are opaque in DUV region (wavelength λ < 300 nm) due to their small bandgaps (E g ~3.2 eV), DUV transparent oxide semiconductors are transparent in DUV region. Among several DUV transparent oxide semiconductors, La-doped SrSnO 3 (E g ~4.6 eV [3][4][5][6] , LSSO hereafter) would be the most promising material because LSSO can be grown heteroepitaxially on the single crystalline substrates such as MgO 7 , SrTiO 3 8 , and KTaO 3 9 . Further, the electrical conductivity of LSSO films (~1000 S cm −1 ) 10 is larger than well-known DUV transparent oxide semiconductors such as β-Ga 2 O 3 (E g ~4.9 eV, ~1 S cm −1 ) 11,12 , α-Ga 2 O 3 (E g ~5.3 eV, ~0.3 S cm −1 ) 13 , electron-doped calcium aluminate (C12A7:e − , ~4 eV, ~800 S cm −1 ) [14][15][16] , and recently developed Al:Mg 0.43 Zn 0.57 O (E g ~4.2 eV, ~400 S cm −1 ) 17 .…”

Wide-Range Band-Gap Tuning and High Electrical Conductivity in La- and Pb-Doped SrSnO3 Epitaxial Films

“…Importantly, one of the crucial factors is to improve the absorption of the solar radiation, which corresponds closely to the lower and more ideal band gap of photovoltaic materials . However, many oxide materials have wide band gaps beyond the visible-light range (1.7–3.1 eV) and greatly limit its applications. , In addition, band offsets are very important factors in the field effect transistor (FET), and the large conduction band offset at the gate/channel interface may lead to high mobility of the FET. , To modify the band gap, many strategies have been adopted, including doping, alloying, straining, etc. ,− Doping was considered as an effective route and can tailor flexibly the band gaps due to the formation of localized states. Furthermore, doping can considerably optimize the microstructure, electronic structure, and physical properties of semiconductors. ,− …”