Kazuto Koike

Characteristics of a Zn0.7Mg0.3O∕ZnO heterostructure field-effect transistor grown on sapphire substrate by molecular-beam epitaxy

Characterization of a Zn0.7Mg0.3O∕ZnO heterostructure field-effect transistor (HFET) is reported. The HFET was based on a Zn0.7Mg0.3O∕ZnO∕Zn0.7Mg0.3O single quantum well (SQW) grown on an a-plane sapphire substrate by molecular-beam epitaxy, and was fabricated by a conventional photolithography technique combined with dry etching. Room-temperature characteristic of the HFET was a n-channel depletion type with a transconductance of 0.70mS∕mm and a field-effect mobility of 140cm2∕Vs, in good agreement with the electron Hall mobility in SQW of 130cm2∕Vs. The on∕off ratio at VDS=3V was ∼800, which was limited by an insufficiently suppressed leakage current through the bottom Zn0.7Mg0.3O barrier.

ZnO and ZnMgO growth on a-plane sapphire by molecular beam epitaxy

The growth of ZnO and ZnMgO alloy on a-plane sapphire substrates using an Epiquest-MBE system with an RF oxygen plasma cell was studied. Prior to growth, thermal treatment at 680/spl deg/C followed by oxygen plasma irradiation at 500/spl deg/C was performed for the improvement of the surface roughness of sapphire substrates. Then, ZnO layers with thickness of 0.6 /spl mu/m were grown in the range of 400-700/spl deg/C. Two factors were found for obtaining high quality layers. One is the use of a low temperature (250/spl deg/C) ZnO buffer layer and successive thermal treatment at high temperature (750/spl deg/C), which suppressed the formation of twin structure of subsequent layers. The other is the choice of growth temperature. The root mean square (RMS) roughness measured by atomic force microscopy (AFM) was minimized to 0.42 nm when growth temperature was 500/spl deg/C.

Centrosymmetric PbTe∕CdTe quantum dots coherently embedded by epitaxial precipitation

A concept for the fabrication of highly symmetric quantum dots that are coherently embedded in a single crystalline matrix is demonstrated. In this approach, the formation of the quantum dots is induced by a transformation of an epitaxial two dimensional quantum well into an array of isolated precipitates with dimensions of about 25nm. The formation process is driven by the immiscibility of the constituent materials resulting from their different lattice structures. The investigated PbTe∕CdTe heterosystem combines two different cubic lattices with almost identical lattice constants. Therefore, the precipitated quantum dots are almost strain-free and near thermodynamic equilibrium they exhibit the shape of small-rhombo-cubo-octahedrons. The PbTe∕CdTe quantum dots, grown on GaAs substrates, display intense room temperature luminescence at wavelength of around 3.2μm, which makes them auspicious for applications in midinfrared photonic devices.

Implant isolation of ZnO

We study ion-irradiation-induced electrical isolation in n-type single-crystal ZnO epilayers. Emphasis is given to improving the thermal stability of isolation and obtaining a better understanding of the isolation mechanism. Results show that an increase in the dose of 2 MeV 16O ions (up to ∼2 orders of magnitude above the threshold isolation dose) and irradiation temperature (up to 350 °C) has a relatively minor effect on the thermal stability of electrical isolation, which is limited to temperatures of ∼300–400 °C. An analysis of the temperature dependence of sheet resistance suggests that effective levels associated with irradiation-produced defects are rather shallow (<50 meV). For the case of implantation with keV Cr, Fe, or Ni ions, the evolution of sheet resistance with annealing temperature is consistent with defect-induced isolation, with a relatively minor effect of Cr, Fe, or Ni impurities on the thermal stability of isolation. Results also reveal a negligible ion-beam flux effect in the case o...

Electrical isolation of ZnO by ion bombardment

The evolution of sheet resistance of n-type single-crystal wurtzite ZnO epilayers exposed to bombardment with MeV 1H, 7Li, 16O, and 28Si ions at room temperature is studied in situ. We demonstrate that sheet resistance of ZnO can be increased by about 7 orders of magnitude as a result of ion irradiation. Due to extremely efficient dynamic annealing in ZnO, the ion doses needed for isolation of this material are about 2 orders of magnitude larger than corresponding doses in the case of another wide-bandgap semiconductor, GaN. Results also show that the ion doses necessary for electrical isolation close-to-inversely depend on the number of ion-beam-generated atomic displacements. However, in all the cases studied, defect-induced electrical isolation of ZnO is unstable to rapid thermal annealing at temperatures above ∼300 °C.

Piezoelectric Carrier Confinement by Lattice Mismatch at ZnO/Zn0.6Mg0.4O Heterointerface

This paper describes a strong piezoelectric carrier confinement at a ZnO/Zn0.6Mg0.4O heterointerface grown on an a-plane sapphire substrate by molecular beam epitaxy. A ZnO/Zn0.6Mg0.4O double-heterojunction structure was grown without intentional doping and the formation of a deep potential well for electrons was confirmed using cathodoluminescence and transmittance spectra. Photoluminescence spectra at 4.5 K consisted of an intense near-band-edge emission at 3.359 eV and a broad and weak peak on the low-energy side. The results of Hall effect measurement revealed that the conduction is n-type with a high carrier concentration of ?1.2?1013 cm-2. The mobilities are ?170 cm2/V?s at 300 K and ?400 cm2/V?s at 77 K without deterioration at lower temperatures; these values are much higher than those of a thick single-layer ZnO film grown on an a-plane sapphire substrate. We attribute these optical and electrical properties to the formation of two-dimensional electron gas at the ZnO/Zn0.6Mg0.4O heterointerface by the piezoelectric polarization in a strained ZnO well.

Room-temperature operation of a memory-effect AlGaAs/GaAs heterojunction field-effect transistor with self-assembled InAs nanodots

This letter describes the memory effect of an AlGaAs/GaAs heterojunction field-effect transistor that contains InAs nanodots in the barrier layer. The device experiences a shift of threshold gate voltage, as a function of the amount of the electrons trapped in the nanodots. These trapped electrons can be injected by applying a positive gate voltage and be erased by a visible light illumination at negative gate bias. Although the shift of the threshold gate voltage volatilizes with the time after the memory programing operation, a considerable part of the shift is retained even after 100 h at room temperature.

Size control and midinfrared emission of epitaxial PbTe∕CdTe quantum dot precipitates grown by molecular beam epitaxy

Epitaxial quantum dots with symmetric and highly facetted shapes are fabricated by thermal annealing of two-dimensional (2D) PbTe epilayers embedded in a CdTe matrix. By varying the thickness of the initial 2D layers, the dot size can be effectively controlled between 5 and 25nm, and areal densities as high as 3×1011cm−2 can be achieved. The size control allows the tuning of the quantum dot luminescence over a wide spectral range between 2.2 and 3.7μm. As a result, ultrabroadband emission from a multilayered quantum dot stack is demonstrated, which is a precondition for the development of superluminescent diodes operating in the near infrared and midinfrared.

Observation of Blue Shifts in ZnO/ZnMgO Multiple Quantum Well Structures by Ion-implantation Induced Intermixing

Implantation with low-energy (80 keV) oxygen ions and subsequent rapid thermal annealing at 800 °C are used to induce intermixing in a stack of 19 ZnO/Zn0.7Mg0.3O multiple quantum wells grown on sapphire by molecular beam epitaxy. Large blue shifts of more than 300 meV have been observed for doses up to 1 × 1016 cm−2, with no observation of saturation. This process is driven by the creation of defects by implantation which encourage the diffusion of Mg from the barrier layers into the ZnO quantum wells. Although defects are introduced during the implantation process, good recovery of the cathodoluminescence is seen following rapid thermal annealing. The Zn–Mg interdiffusion in this system has also been calculated for the corresponding ion doses, and the diffusion coefficient extracted. This study has significant implications for band gap engineering of ZnO/ZnMgO optoelectronic devices.

Improved Stability of High-Performance ZnO/ZnMgO Hetero-MISFETs

Improved performance and stability was demonstrated for ZnO/ZnMgO hetero-MISFETs. The MIS gate structures that were formed using either a 50-nm-thick Al₂O₃ or HfO₂ gate dielectric layer were examined by observation of the transfer characteristic hysteresis. A significantly reduced hysteresis of less than 0.1 V was obtained for HfO₂ as compared to that for the Al₂O₃ gate dielectric. By reducing the access resistance, the 1-mum gate devices showed improved transconductance values, as high as 54 mS/mm for Al₂O₃ and 71 mS/mm for HfO₂, which are the highest values ever reported for ZnO-based FETs.