Depletion Mode MOSFET Using La-Doped BaSnO₃ as a Channel Material

The high room-temperature mobility that can be achieved in BaSnO₃ has created significant excitement for its use as channel material in all-perovskite-based transistor devices such as ferroelectric field effect transistor (FET). Here, we report on the first demonstration of <i>n</i>-type depletion-mode FET using hybrid molecular beam epitaxy grown La-doped BaSnO₃ as a channel material. The devices utilize a heterostructure metal-oxide semiconductor FET (MOSFET) design that includes an epitaxial SrTiO₃ barrier layer capped with a thin layer of HfO₂ used as a gate dielectric. A field-effect mobility of ∼70 cm² V^–1 s^–1, a record high transconductance value of >2mS/mm at room temperature, and the on/off ratio exceeding 10⁷ at 77 K were obtained. Using temperature- and frequency-dependent transport measurements, we quantify the impact of the conduction band offset at the BaSnO₃/SrTiO₃ interface as well as bulk and interface traps on device characteristics

Observation of Electrically-Inactive Interstitials in Nb-Doped SrTiO₃

Despite rapid recent progress, controlled dopant incorporation and attainment of high mobility in thin films of the prototypical complex oxide semiconductor SrTiO₃ remain problematic. Here, analytical scanning transmission electron microscopy is used to study the local atomic and electronic structure of Nb-doped SrTiO₃ both in ideally substitutionally doped bulk single crystals and epitaxial thin films. The films are deposited under conditions that would yield highly stoichiometric <i>undoped</i> SrTiO₃, but are nevertheless insulating. The Nb incorporation in such films was found to be highly inhomogeneous on nanoscopic length-scales, with large quantities of what we deduce to be interstitial Nb. Electron energy loss spectroscopy reveals changes in the electronic density of states in Nb-doped SrTiO₃ films compared to undoped SrTiO₃, but without the clear shift in the Fermi edge seen in bulk single crystal Nb-doped SrTiO₃. Analysis of atomic-resolution annular dark-field images allows us to conclude that the interstitial Nb is in the Nb⁰ state, confirming that it is electrically inactive. We argue that this approach should enable future work establishing the vitally needed relationships between synthesis/processing conditions and electronic properties of Nb-doped SrTiO₃ thin films

Roles of Point Defects in Thermal Transport in Perovskite Barium Stannate

Perovskite barium stannate (BaSnO₃) is a promising candidate that can be used as transparent conducting oxide in optoelectronic devices and as the channel material in high-mobility oxide electronics. In this work, we calculate the lattice thermal conductivity and investigate the impact of point defects on thermal transport in BaSnO₃ based on the phonon Boltzmann transport equations with interatomic force constants from first-principles calculations. In pristine BaSnO₃, we find the contribution of acoustic phonons to thermal transport accounts for 54% at 300 K and the rest is attributed to the lower-frequency (27.5–50 THz) optical modes with relatively high group velocity. We show oxygen vacancies and impurities can cause noticeable reduction in thermal conductivity of BaSnO₃, but the corresponding mechanisms differ in terms of scattering rates on different phonon modes. The thermal conductivity reduction due to oxygen vacancies at 300 K is mainly caused by the increased scattering of the acoustic phonons and the low-frequency optical phonons. Lanthanum and potassium impurities mainly increase the scattering of acoustic phonons, but antimony impurity lowers the thermal conductivity by increasing the scattering rate of dominant phonons, including both the acoustic modes and the low-frequency optical modes. The results and findings facilitate us to better understand the thermal transport mechanisms of perovskite oxides with an emphasis on the impact of oxygen vacancies and impurities on the thermal properties of BaSnO₃

A New Line Defect in NdTiO₃ Perovskite

Perovskite oxides form an eclectic class of materials owing to their structural flexibility in accommodating cations of different sizes and valences. They host well-known point and planar defects, but so far no line defect has been identified other than dislocations. Using analytical scanning transmission electron microscopy (STEM) and ab initio calculations, we have detected and characterized the atomic and electronic structures of a novel line defect in NdTiO₃ perovskite. It appears in STEM images as a perovskite cell rotated by 45°. It consists of self-organized Ti–O vacancy lines replaced by Nd columns surrounding a central Ti–O octahedral chain containing Ti⁴⁺ ions, as opposed to Ti³⁺ in the host. The distinct Ti valence in this line defect introduces the possibility of engineering exotic conducting properties in a single preferred direction and tailoring novel desirable functionalities in this Mott insulator

A New Line Defect in NdTiO₃ Perovskite

Perovskite oxides form an eclectic class of materials owing to their structural flexibility in accommodating cations of different sizes and valences. They host well-known point and planar defects, but so far no line defect has been identified other than dislocations. Using analytical scanning transmission electron microscopy (STEM) and ab initio calculations, we have detected and characterized the atomic and electronic structures of a novel line defect in NdTiO₃ perovskite. It appears in STEM images as a perovskite cell rotated by 45°. It consists of self-organized Ti–O vacancy lines replaced by Nd columns surrounding a central Ti–O octahedral chain containing Ti⁴⁺ ions, as opposed to Ti³⁺ in the host. The distinct Ti valence in this line defect introduces the possibility of engineering exotic conducting properties in a single preferred direction and tailoring novel desirable functionalities in this Mott insulator