Kosuke Matsuzaki

Epitaxial growth of high mobility Cu2O thin films and application to p-channel thin film transistor

Cu2O epitaxial films were grown for high mobility p-channel oxide thin-film transistors (TFTs). The use of a (110) MgO surface and fine tuning of a growth condition produced single phase epitaxial films with hole Hall mobilities ∼90 cm2 V−1 s−1 comparable to those of single crystals (∼100 cm2 V−1 s−1). TFTs using the epitaxial film channels exhibited p-channel operation although the field-effect mobilities and the on-to-off current ratio were not yet satisfactory (∼0.26 cm2 V−1 s−1 and ∼6, respectively).

Field-induced current modulation in epitaxial film of deep-ultraviolet transparent oxide semiconductor Ga2O3

Epitaxial films of a deep-ultraviolet transparent oxide semiconductor, Ga2O3, were fabricated on α-Al2O3 (0001) substrates by pulsed laser deposition. Four-axes x-ray diffraction measurements revealed that the tin-doped Ga2O3 films have a crystal structure different from any known polymorphs of Ga2O3. Its crystal lattice was determined to be an orthorhombic. Top gate field-effect transistor structures were fabricated using the Ga2O3 epitaxial films for n-channels. The channel conductance was modulated by an order of magnitude by gate voltage at room temperature with an estimated field-effect mobility of 5×10−2cm2(Vs)−1.

Strong evidence for d-electron spin transport at room temperature at a LaAlO3/SrTiO3 interface

A d-orbital electron has an anisotropic electron orbital and is a source of magnetism. The realization of a two-dimensional electron gas (2DEG) embedded at a LaAlO3/SrTiO3 interface surprised researchers in materials and physical sciences because the 2DEG consists of 3d-electrons of Ti with extraordinarily large carrier mobility, even in the insulating oxide heterostructure. To date, a wide variety of physical phenomena, such as ferromagnetism and the quantum Hall effect, have been discovered in this 2DEG system, demonstrating the ability of d-electron 2DEG systems to provide a material platform for the study of interesting physics. However, because of both ferromagnetism and the Rashba field, long-range spin transport and the exploitation of spintronics functions have been believed difficult to implement in d-electron 2DEG systems. Here, we report the experimental demonstration of room-temperature spin transport in a d-electron-based 2DEG at a LaAlO3/SrTiO3 interface, where the spin relaxation length is about 300 nm. Our finding, which counters the conventional understandings of d-electron 2DEGs, highlights the spin-functionality of conductive oxide systems and opens the field of d-electron spintronics.

Controlled bipolar doping in Cu3N (100) thin films

We have fabricated insulating, p- and n-type Cu3N(100) films on SrTiO3(100) by plasma assisted molecular beam epitaxy. By controlling the Cu/N flux rate, p-type doping with 1018–1020 cm−3 in Cu-poor condition and n-type doping with 1019–1020 cm−3 in N-poor condition were obtained without introducing foreign species. Together with formation of insulating Cu3N films with an optical absorption coefficient of ∼105 cm−1 in the photon energy above ∼2.2 eV and an estimated indirect bandgap of ∼1.3 eV, the bipolar doping in Cu3N films would be promising for solar energy conversion applications.

Fabrication of Flat MgO(111) Films on Al2O3(0001) Substrates by Pulsed Laser Deposition

We have found that epitaxial MgO(111) thin films grow under a wide range of deposition conditions (substrate temperatures of 400–800 °C, oxygen partial pressures of 10-4–100 Pa) on α-Al2O3(0001) substrates by pulsed laser deposition (PLD), despite the strongly divergent electrostatic potential of MgO(111). The surfaces of the resulting thin films show step-and-terrace structures reflecting the surface of α-Al2O3(0001) with a small root-mean-square roughness of 0.62 nm even at a film thickness of 80 nm. These results present the possibility of fabricating various artificial oxide structures using flat MgO(111) films produced by a conventional PLD technique.

Structural study of Fe3O4(111) thin films with bulk like magnetic and magnetotransport behaviour

Post-annealing of Fe3O4 films in a CO/CO2 atmosphere results in a significant improvement in magnetic and magnetotransport properties with values close to the single crystal bulk of Ms ∼ 480 emu/cm3 and negative magnetoresistance of 0.05% in a field of 1 T. By using atomic resolution Z-contrast transmission electron microscopy, we show that improved magnetic properties in the annealed films are due to improved structural ordering as a result of the annealing process.

Tuning of Surface Roughness and Lattice Constant in MgO(111)/Al2O3(0001) Grown by Laser Energy Controlled Pulsed Laser Deposition

We have studied the effect of variable laser energy on pulsed laser deposition (PLD) grown MgO(111) films on Al2O3(0001) substrates by X-ray diffraction and reflectivity measurements, atomic force microscopy and reflection high energy electron diffraction. In spite of the polar instability of MgO(111) surface, the surface roughness drastically decreases from ~2 to ~0.5 nm as the laser energy increases from ~50 to ~150 mJ. The in-plane lattice constant is larger than the bulk value at the smaller laser energy while it is smaller than the bulk value at the larger laser energy. This indicates that the balance between the electrostatic energy due to the polar structure, which favors a larger in-plane lattice constant, and the interface epitaxial strain, which favors a smaller in-plane lattice constant in MgO(111)/Al2O3(0001), can be tuned by varying the laser energy in PLD.

Magnetotransport Properties across Verwey Transition in Fe3O4(111) Epitaxial Thin Films

We have studied the magnetoresistance (MR) of high-quality Fe3O4 thin films across the Verwey transition temperature (TV). Different from unsaturated MR observed in many other Fe3O4 films, MR of the present film at a high magnetic field is almost independent of the field, consistent with the fully saturated magnetization. The MR at low field strongly depends on temperature and orientation around TV: While out-of-plane MR sharply decreases from ~+7 to ~-5%, in-plane MR abruptly increases from ~-6 to ~+3% on cooling across TV.

Atomic and electronic structure of twin growth defects in magnetite

We report the existence of a stable twin defect in Fe3O4 thin films. By using aberration corrected scanning transmission electron microscopy and spectroscopy the atomic structure of the twin boundary has been determined. The boundary is confined to the (111) growth plane and it is non-stoichiometric due to a missing Fe octahedral plane. By first principles calculations we show that the local atomic structural configuration of the twin boundary does not change the nature of the superexchange interactions between the two Fe sublattices across the twin grain boundary. Besides decreasing the half-metallic band gap at the boundary the altered atomic stacking at the boundary does not change the overall ferromagnetic (FM) coupling between the grains.

Examination of interfacial charge transfer in photocatalysis using patterned CuO thin film deposited on TiO2

We examined the interfacial charge transfer effect on photocatalysts using a patterned CuO thin film deposited on a rutile TiO2 (110) substrate. Photocatalytic activity was visualized based on the formation of metal Ag particles resulting from the photoreduction of Ag+ ions under visible-light illumination. Ag particles were selectively deposited near the edge of CuO film for several nanometer thick CuO film, indicating that interfacial excitation from the valence band maximum of TiO2 to the conduction band minimum of CuO plays a key role in efficient photocatalytic activity of CuO nanocluster-grafted TiO2 systems with visible-light sensitivity.