Yusuke Miyata

Strain Dependent Electronic Structure and Band Offset Tuning at Heterointerfaces of ASnO 3 (A=Ca, Sr, and Ba) and SrTiO 3

The valence band (VB) electronic structure and VB alignments at heterointerfaces of strained epitaxial stannate ASnO3 (A=Ca, Sr, and Ba) thin films are characterized using in situ X-ray and ultraviolet photoelectron spectroscopies, with band gaps evaluated using spectroscopic ellipsometry. Scanning transmission electron microscopy with geometric phase analysis is used to resolve strain at atomic resolution. The VB electronic structure is strain state dependent in a manner that correlated with a directional change in Sn-O bond lengths with strain. However, VB offsets are found not to vary significantly with strain, which resulted in ascribing most of the difference in band alignment, due to a change in the band gaps with strain, to the conduction band edge. Our results reveal significant strain tuning of conduction band offsets using epitaxial buffer layers, with strain-induced offset differences as large as 0.6 eV possible for SrSnO3. Such large conduction band offset tunability through elastic strain control may provide a pathway to minimize the loss of charge confinement in 2-dimensional electron gases and enhance the performance of photoelectrochemical stannate-based devices.

Low-voltage operation of Si-based ferroelectric field effect transistors using organic ferroelectrics, poly(vinylidene fluoride–trifluoroethylene), as a gate dielectric

Si-based metal–ferroelectric–semiconductor (MFS) capacitors have been fabricated using poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)] as a ferroelectric gate. The pinhole-free P(VDF–TrFE) thin films with high resistivity were able to be prepared by spin-coating directly onto hydrogen-terminated Si. The capacitance–voltage (C–V) characteristics of the ferroelectric gate field effect transistor (FeFET) using this MFS structure clearly show butterfly-shaped hysteresis originating from the ferroelectricity, indicating carrier modulation on the Si surface at gate voltages below 2 V. The drain current–gate voltage (I D–V G) characteristics also show counterclockwise hysteresis at gate voltages below 5 V. This is the first report on the low-voltage operation of a Si-based FeFET using P(VDF–TrFE) as a gate dielectric. This organic gate FeFET without any insulator layer at the ferroelectric/Si interface should be one of the promising devices for overcoming the critical issues of the FeFET, such as depolarization field and a decrease in the gate voltage.

Large enhancement of positive magnetoresistance by Ce doping in Si epitaxial thin films

We report the investigation into a large enhancement of the magnetoresistance (MR) by Ce doping in Si epitaxial thin films at room temperature. The positive MR is proportional to the square of the magnetic field at low magnetic fields below 5 T, while it increases linearly with regards to the strength of the magnetic field above 5 T. Based on the experimental finding that the change in the donor level corresponds to that of the MR ratio as a function of Ce concentration, the electronic state turns out to be influenced by Ce doping and strongly correlate the magnetotransport characteristics. It is concluded that this MR effect appears via the Lorentz force effect on the carrier motion, which is enhanced by the random scattering potential distribution arising from the Ce doping.

Interface energetics and atomic structure of epitaxial La1−xSrxCoO3 on Nb:SrTiO3

The energetics at oxide semiconductor/La1−xSrxCoO3 heterojunctions, including the respective alignment of the valence and conduction bands, govern charge transfer and have to be determined for the design of future La1−xSrxCoO3-based devices. In this letter, the electronic and atomic structures of epitaxial La1−xSrxCoO3 on Nb-doped strontium titanate are revealed by scanning transmission electron microscopy, electron energy loss spectroscopy, and in situ x-ray and ultra violet photoelectron spectroscopies. For LaCoO3, a valence band (VB) offset of 2.8 ± 0.1 eV is deduced. The large offset is attributed to the orbital contributions of the Co 3d states to the VB maximum of the LaCoO3 thin films, with no evidence of interface dipole contributions. The sensitivity of the valence band orbital character to spin state ordering and oxygen vacancies is assessed using density functional theory.