I. Ohkubo

Structure and optical properties of ZnO/Mg0.2Zn0.8O superlattices

ZnO/Mg0.2Zn0.8O superlattices with a band-gap offset of about 0.5 eV were epitaxially grown by laser molecular-beam epitaxy on a sapphire(0001) substrate using a ZnO buffer layer. The superlattice structure with a period ranging from 8 to 18 nm was clearly verified by cross-sectional transmission electron microscopy, Auger depth profile, and x-ray diffraction. As the well layer thickness decreased below 5 nm, the photoluminescence peak and absorption edge in the photoluminescence excitation spectra showed a blueshift, indicating a quantum-size effect.

High mobility thin film transistors with transparent ZnO channels

We have fabricated high performance ZnO thin film transistors (TFTs) using CaHfOx buffer layer between ZnO channel and amorphous silicon?nitride gate insulator. The TFT structure, dimensions, and materials set are identical to those of the commercial amorphous silicon (a-Si) TFTs in active matrix liquid crystal display, except for the channel and buffer layers replacing a-Si. The field effect mobility can be as high as 7 cm2?V-1?s-1 for devices with maximum process temperature of 300?C. The process temperature can be reduced to 150?C without much degrading the performance, showing the possibility of the use of polymer substrate.

Excitonic ultraviolet laser emission at room temperature from naturally made cavity in ZnO nanocrytal thin films

Abstract Hexagonally shaped ZnO nanocrystal thin films were fabricated on sapphire(0001) substrates by laser molecular beam epitaxy. Nanocrystal structure was investigated by atomic force microscopy and transmission electron microscopy. Epitaxial growth of ZnO nanocrystal thin films on sapphire substrates was found to occur in a spiral and grain growth mode. The grain growth mode was interpreted by taking higher order epitaxial relationship of oxygen sublattice units between ZnO and sapphire into account. Nanocrystal size could be tuned from 50 to 200 nm controlling film thickness, growth conditions and stoichiometry of the target. The films having small nanocrystal size of about 50 nm showed excitonic stimulated emission having peak energy of 3.2 eV at room temperature with a very low threshold (24 kW cm−2). Mode transition from excitonic stimulated emission to electron hole plasma appeared above another threshold (50 kW cm−2). Well defined Fabry–Perot cavity mode was observed in the emission spectra measured from side edge of the film. It was concluded that the grain boundaries between nanocrystals serve not only as potential barriers confining excitons but also as cavity mirrors.

Thermal stability of supersaturated MgxZn1−xO alloy films and MgxZn1−xO/ZnO heterointerfaces

We have examined the thermal stability of wurtzite-phase MgxZn1−xO alloy films and ZnO/MgxZn1−xO bilayer films with x exceeding the reported solubility limit of 0.04. When a Mg0.23Zn0.78O film was annealed, the segregation of MgO started at 850 °C and the band gap was reduced to the value of that for an x=0.15 film after annealing at 1000 °C. Mg0.15Zn0.85O films showed no change of the band gap even after annealing at 1000 °C. Therefore, we conclude that the thermodynamic solubility limit of MgO in MgxZn1−xO epitaxial film is about x=0.15. The thermal diffusion of Mg across the MgxZn1−xO/ZnO interface was observed only after annealing above 700 °C. Unlike other II–VI semiconductors, ZnO-based alloy films and heterointerfaces are stable enough for the fabrication of high-crystallinity heterostructures.

Fabrication of alloys and superlattices based on ZnO towards ultraviolet laser

Abstract We have grown epitaxial Mg x Zn 1− x O alloy films and ZnO/Mg x Zn 1− x O ( x =0.20) superlattices on sapphire(0001) substrates by laser molecular beam epitaxy and characterized their structures by X-ray diffraction. Single phase Mg x Zn 1− x O could be obtained up to x =0.33, whereas MgO impurity phase with (111) orientation segregated at x >0.33. The bandgap of Mg x Zn 1− x O was successfully controlled as verified by the photoluminescence peaks shifting 3.36 eV ( x =0) to 3.87 eV ( x =0.33). It was found that the structure of the superlattices was greatly improved by the use of a ZnO buffer layer on sapphire substrate prior to the deposition of superlattice. Small angle X-ray diffraction peaks corresponding to the period of the superlattices ranging from 8 to 18 nm could be clearly observed.

Analysis of the polar direction of GaN film growth by coaxial impact collision ion scattering spectroscopy

Nondestructive determination of the polarity of GaN has been achieved by the use of coaxial impact-collision ion scattering spectroscopy analysis. The polarity of a GaN film with a smooth surface on non-nitrided c-plane sapphire was identified (0001) (Ga face; +c). GaN films with a 20 nm buffer layer on nitrided sapphire had (0001) (N face; −c) polarity and a hexagonal faceted surface. The influence of both the buffer layer and of substrate nitridation on the polarity of wurtzite {0001} GaN films deposited by two-step metal organic chemical vapor deposition (MOCVD) has been investigated. The polarity of the buffer layer on a nitrided sapphire substrate was altered by varying its thickness or the annealing time. It was found that the polarity of the GaN film is determined by the polarity of the annealed buffer layer; MOCVD-GaN films on buffer layers with +c and −c polarity have either +c (smooth surface) or −c (hexagonal facet) polarity, respectively.

In-plane and polar orientations of ZnO thin films grown on atomically flat sapphire

Abstract Epitaxial ZnO thin films were prepared on atomically flat sapphire (0001) substrates at various temperatures by laser molecular beam epitaxy. On the as-polished substrates, the in-plane orientation of ZnO thin films was found to be ZnO [1010]‖sapphire [1120] regardless of the deposition conditions. However, films on atomically flat substrates showed two in-plane orientations, ZnO [1010]‖sapphire [1010] and ZnO [1010]‖sapphire [1120], rotated by 30° for films grown at low (400–450° C) and high (800–835° C) temperatures respectively. Ion scattering spectroscopy revealed that the former films were (0001) oriented with the Zn-face forming the topmost surface, whereas the latter films had the (0001) orientation with the O-face at the film surface. This orientation change is discussed by taking thermodynamic stability and growth kinetics into account. The films grown at very high temperature (835°C) showed superior crystallinity even in comparison with bulk single crystals.

Trap-controlled space-charge-limited current mechanism in resistance switching at Al∕Pr0.7Ca0.3MnO3 interface

Well-defined metal-insulator-metal trilayered structures composed of epitaxial Pr0.7Ca0.3MnO3 insulator layers, epitaxial LaNiO3 bottom electrodes, and Al metal top electrodes were fabricated on LaAlO3 (100) substrates. The I-V characteristics of the trilayer structures show electric-field-induced resistance switching. The resistance switching ratio of the heterostructures was up to 100 when positive and negative pulsed voltages were applied. Detailed I-V analysis indicates the importance of both trap-controlled space-charge-limited current and Poole–Frenkel effect in resistance switching at Al∕Pr0.7Ca0.3MnO3 interfaces.

Thickness-dependent electronic structure of ultrathin SrRuO3 films studied by in situ photoemission spectroscopy

In situ thickness-dependent photoemission spectroscopy (PES) has been performed on SrRuO3 (SRO) layers deposited on SrTiO3 substrates to study the structure-induced evolution of the electronic structure. The PES spectra showing the existence of two critical film thicknesses reveal that a metal-insulator transition occurs at a film thickness of 4–5 monolayers (ML) and the evolution of Ru 4d-derived states around the Fermi level (EF) saturates at about 15 ML. The observed spectral behavior well matches the electric and magnetic properties and thickness-dependent evolution of surface morphology of the ultrathin SRO films. These experimental results suggest the importance of the disorder associated with the unique growth-mode transition in SRO films.

Dielectric and Optical Properties of Epitaxial Rare-Earth Scandate Films and Their Crystallization Behavior

Rare-earth scandates (ReScO3, with Re=Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, i.e., the entire series for which the individual oxides are chemically stable in contact with Si) were deposited in a temperature-gradient pulsed laser deposition system onto LaAlO3 substrates. The crystallization temperature depends monotonically on the Re atomic number and the Goldschmidt tolerance factor, with crystallization temperatures as low as 650°C for LaScO3 and PrScO3. The dielectric constants of the crystalline films K≈30 (determined by microwave microscopy) are significantly larger than those of their amorphous counterparts. In combination with the large observed band gaps (Eg>5.5eV, determined by ellipsometry), these results indicate the potential of these materials as high-K dielectrics for field-effect transistor applications.