Shunsuke Akasaka

Observation of the fractional quantum Hall effect in an oxide

The quantum Hall effect arises from the cyclotron motion of charge carriers in two-dimensional systems. However, the ground states related to the integer and fractional quantum Hall effect, respectively, are of entirely different origin. The former can be explained within a single-particle picture; the latter arises from electron correlation effects governed by Coulomb interaction. The prerequisite for the observation of these effects is extremely smooth interfaces of the thin film layers to which the charge carriers are confined. So far, experimental observations of such quantum transport phenomena have been limited to a few material systems based on silicon, III-V compounds and graphene. In ionic materials, the correlation between electrons is expected to be more pronounced than in the conventional heterostructures, owing to a large effective mass of charge carriers. Here we report the observation of the fractional quantum Hall effect in MgZnO/ZnO heterostructures grown by molecular-beam epitaxy, in which the electron mobility exceeds 180,000 cm(2) V(-1) s(-1). Fractional states such as ν = 4/3, 5/3 and 8/3 clearly emerge, and the appearance of the ν = 2/5 state is indicated. The present study represents a technological advance in oxide electronics that provides opportunities to explore strongly correlated phenomena in quantum transport of dilute carriers.

Nitrogen doped MgxZn1−xO/ZnO single heterostructure ultraviolet light-emitting diodes on ZnO substrates

We have grown nitrogen-doped MgxZn1−xO:N films on Zn-polar ZnO single crystal substrates by molecular beam epitaxy. As N-sources, we employed NO-plasma or NH3 gas itself. As x increased, optimum growth temperature window for smooth film morphology shifted to higher temperatures, while maintaining high N-concentration (∼1×1019 cm−3). The heterosructures of MgxZn1−xO:N (0.1≤x≤0.4)/ZnO were fabricated into light emitting diodes of 500-μm-diameter. We observed ultraviolet near-band-edge emission (λ∼382 nm) with an output power of 0.1 μW for a NO-plasma-doped LED and 70 μW for a NH3-doped one at a bias current of 30 mA.

High Electron Mobility Exceeding 104 cm2 V-1 s-1 in MgxZn1-xO/ZnO Single Heterostructures Grown by Molecular Beam Epitaxy

Nominally undoped MgxZn1-xO/ZnO (x = 0.05 and 0.08) single heterostructures were prepared on Zn-polar ZnO substrates by using plasma assisted molecular beam epitaxy (MBE). The samples showed a metallic conductivity below 50 K and a mobility exceeding 104 cm2 V-1 s-1 at 0.5 K. We observed quantum Hall effect accompanying Shubnikov–de Haas oscillations, in which zero-resistance states were clearly seen above 5 T. Rotation experiments in magnetic field suggest strong two-dimensional carrier confinement at low temperatures. The results indicate that the MBE grown films have much higher quality than the previously reported samples grown by pulsed laser deposition.

Plasma-assisted Molecular Beam Epitaxy of High Optical Quality MgZnO Films on Zn-polar ZnO Substrates

The excellent structural and optical properties of pseudomorphic MgxZn1-xO films (0≤x≤0.39) are reported in this work. The MgxZn1-xO films were grown on Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy. Those MgxZn1-xO films for which x≤0.18 exhibited atomically flat surfaces, and the typical full-width-at-half-maximum (FWHM) value of the (0002) X-ray diffraction ω-rocking curves for these films was 35 arcsec. The FWHM values were less than 100 meV for the near-band-edge photoluminescence (PL) at 300 K. We observed PL lifetimes of the order of ns, and the longest fast-decay component reached 3.5 ns for the Mg0.12Zn0.88O alloy.

Electronic‐Field Control of Two‐Dimensional Electrons in Polymer‐Gated–Oxide Semiconductor Heterostructures

2010 WILEY-VCH Verlag Gm The creation of functional interfaces by combining different classes of materials is one of the emerging interests in condensed matter science. This trend will become increasingly important for developing novel processing techniques or device concepts to realize new applications having vastly superior performance and much lower production costs than achievable with current devices. A big challenge lies beyond well-established semiconductor devices exemplified by silicon metal– oxide–semiconductor field-effect transistors (MOSFETs) and high-electron-mobility transistors based on modulation-doped III-V heterostructures. In this regard, recently there have been remarkable advances in the growth of abrupt interfaces between metal oxides, for example, demonstrating high-mobility 2D electron gas (2DEG) at LaAlO3/SrTiO3 and MgxZn1–xO/ ZnO. In both interfaces, a strong built-in potential arises from the polarization mismatch, resulting in the formation of the 2DEG. A sophisticated field-effect control method has recently been developed for the former by using a scanning atomic force microscope (AFM) tip as a nanometer-scale gate, whose principle relies on the retention of charge trapping at the surface states. Metal–semiconductor FET (MESFET) and conventional MOSFETstructures can in principle be used for these oxides, yet a number of nontrivial issues remain for realizing practical applications. The notion of a Schottky barrier is a purely electrostatic effect arising from surface space charges in semiconductors, but its energy height tends to remain intact in covalent bonding semiconductors due to the presence of a surface pinning state (i.e., the Bardeen limit). Oxide semiconductors, on the other hand, inherently accord with the Schottky model (i.e., Schottky limit). In reality, however, the quality of the Schottky contacts on oxides is highly sensitive to the processes used for chemical cleaning of the surface and metallization with noble metals such as gold, silver, and platinum. Instead of addressing the subtle issues of conventional processes, we chose a totally different approach. The experiments reported here are based on a simple spin-coating process to make a MESFET structure with unconventional materials, in which a 2DEG channel is tunable from a normally-on state to a dilute off state. The MESFET devices, which are composed of wide-bandgap MgxZn1–xO/ZnO heterostructures and a conducting polymer, poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), exhibited excellent performance including a 2D electron mobility in excess of 20 000 cmV 1 s 1 at 2 K. The 2D quantum transport inMgxZn1–xO/ZnO heterostructures was successfully modulated by an electrostatic field-effect via the polymer–oxide interface. The chemical structure of PEDOT:PSS is shown in the inset of Figure 1a. A polythiophene backbone of PEDOT molecules (broken line) forms a hole-conduction pathway and PSS molecules act as acceptors. The PEDOT:PSS thin film has a resistivity as low as several milliohm centimeters, a large work function of 5.0 eV, suitable for making a Schottky contact with n-type semiconductors, and high optical transparency in a wavelength range from 250 to 800 nm (Fig. 1a). By combining this transparent polymer Schottky contact with ZnO-based materials, high-performance visible-blind photodiodes have been demonstrated. The schematic device structure used in this study is shown in Figure 1b. Three heterostructures having different Mg contents (x) and thicknesses (d) of the MgxZn1–xO layer were prepared, as listed in the inset table in Figure 1b. We fabricated two individual devices located close to one another for magnetotransport and current/capacitance–voltage measurements, respectively (Fig. 1c). The devices showed remarkable stability in their electrical properties during measurements at temperatures from 300 to 2K, repeated intermittently over the course of months. Figure 1d shows typical current–voltage (I–V) curves for sample A at 300 and 2K. A similarly large rectification ratio of 10 and ideality factor (n) near unity at 300 K were obtained for all devices, indicating a homogeneous Schottky contact over a wide area. Figure 1e shows the gate voltage (VG) dependencies of capacitance (C, black curve) and longitudinal resistivity (rxx, orange curve) measured for sample B at 2 K, indicating consistent properties along the carrier depletion direction. As VG was swept

Direct correlation between the internal quantum efficiency and photoluminescence lifetime in undoped ZnO epilayers grown on Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy

The equivalent internal quantum efficiency (ηinteq) at 300K of the near-band-edge excitonic photoluminescence (PL) peak in ZnO epilayers grown by plasma-assisted molecular beam epitaxy on Zn-polar ZnO substrates was directly correlated with the PL lifetime (τPL) for the first time. This relation seems to be universal for O-polar ZnO films grown by other methods. Present homoepitaxial ZnO epilayers grown above 800°C exhibited atomically flat surfaces, and the best full-width-at-half-maximum value of (0002) ZnO x-ray diffraction ω-rocking curves was 17.6arcsec. The high-temperature growth also led to a long τPL of 1.2ns at 300K. As a result, a record high ηinteq value (9.6%) was eventually obtained under an excitation density of 5W∕cm2 (He–Cd, 325.0nm). The homoepitaxial Zn-polar ZnO films grown by molecular beam epitaxy are coming to be used for p-n junction devices.

MgxZn1-xO-Based Schottky Photodiode for Highly Color-Selective Ultraviolet Light Detection

Spectral response of Schottky photodiodes consisting of a MgxZn1-xO (x≤0.43) thin film and a transparent conducting polymer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), was characterized under a zero-bias condition at room temperature. The cut-off wavelength in each photodiode was systematically tuned by Mg content (x), while its high quantum efficiency was kept near unity. The steepness in the photo-response around the cut-off wavelength was maintained for higher x samples, implying a small alloy broadening effect in this system.

Improvement of Electron Mobility above 100,000 cm2 V-1 s-1 in MgxZn1-xO/ZnO Heterostructures

We discuss the electron mobility (µ) of a two-dimensional electron gas (2DEG) confined at the MgxZn1-xO/ZnO heterointerface grown by molecular-beam epitaxy. With increasing x from 0.05 to 0.2, the electron density (n) was enhanced and µ was suppressed due to interface roughness or alloy disorder scattering. By the optimization of growth conditions, in particular growth rate, ionized impurity scattering in the ZnO channel could be reduced significantly. With tuning n by a gate voltage on top-gated Hall-bar devices, the peak µ at 2 K was enhanced to 130,000 cm2 V-1 s-1 at n = 3×1011 cm-2.

MgxZn1-xO Films with a Low Residual Donor Concentration (<1015 cm-3) Grown by Molecular Beam Epitaxy

We have grown undoped MgxZn1-xO films on Zn-polar ZnO substrates by using a plasma-assisted molecular beam epitaxy technique. The residual donor concentration (ND) was evaluated by capacitance–voltage measurements of the Schottky junctions formed using poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) films as electrodes. Increasing the Zn/O flux ratio during the growth reduced the value of ND to reach (2–7)×1014 cm-3 for MgxZn1-xO films with MgO molar fractions x ranging from 0 to 0.39. These MgxZn1-xO films with low residual ND values would be suitable host materials for p-type doping.

Optimization of the Growth Conditions for Molecular Beam Epitaxy of MgxZn1-xO (0≤x≤0.12) Films on Zn-Polar ZnO Substrates

We report on optimization of the growth conditions for MgxZn1-xO (x=0,0.04,0.05,0.12) thin films grown on c-plane Zn-polar ZnO single crystal substrates by using plasma-assisted molecular beam epitaxy (PAMBE). A normal vector to the ZnO substrate surfaces was angled at 0.5±0.1° off from the [0001] c-axis toward the [1100] direction, leading to a stable step-and-terrace structure. A growth temperature (Tg) higher than 800 °C led to the ZnO films presenting the first excited state luminescence of A-free excitons in photoluminescence (PL) spectra at 12 K. A Tg higher than 800 °C enhanced optical attributes of a MgxZn1-xO film. The longest PL lifetime of fast-decay components reached 3.5 ns in time-resolved PL measurement for an Mg0.12Zn0.88O film grown at 900 °C, indicating a concentration of nonradiative recombination centers is substantially eliminated compared to the previously reported PL lifetime of 60 ps for an Mg0.11Zn0.89O film grown by pulsed laser deposition.