Hisashi Kanie

Polarization-induced two-dimensional electron gases in ZnMgO/ZnO heterostructures

Both the formation mechanism and the origin of the two-dimensional electron gas (2DEG) in ZnMgO/ZnO heterostructures have been investigated. The 2DEG in the heterostructures was confirmed to originate from polarization-induced charge and was found to be dominant for transport at low temperatures as well as room temperature (RT) by transport measurements. The origin of 2DEG was concluded to be the surface of the ZnMgO layer based on both capacitance-voltage measurements and the dependence of the carrier concentration on the ZnMgO layer thickness. The largest sheet carrier concentration was 1.1×1013 cm−2 and the highest mobility for the heterostructure was obtained for a Mg composition of 0.61 at RT.

Strong excitonic transition of Zn1−xMgxO alloy

A strong excitonic optical transition in a Zn1−xMgxO alloy grown by radical source molecular beam epitaxy was observed using both optical reflectivity measurements and photoluminescence (PL) measurements. Clear and strong reflectance peaks at room temperature (RT) were observed from 3.42eV (x=0.05)to4.62eV (x=0.61) from ZnMgO layers at RT. Distinct clear PL spectra at RT were also observed for energies up to 4.06eV (x=0.44). The peak intensity of the reflected signal increased for x values up to x∼0.2 simultaneously with an increase in PL intensity; however, a Stokes shift between the reflectance peak and the PL peak was not observed for x values below 0.2. These facts suggest that the oscillator strength of ZnMgO is enhanced by alloying, and the underlying mechanism is discussed. Furthermore, we demonstrate that the strong reflectance properties even at RT provide an easy method to determine the Mg composition of a thin ZnMgO layer in a ZnMgO∕ZnO heterostructure.

Band profiles of ZnMgO/ZnO heterostructures confirmed by Kelvin probe force microscopy

The band profiles of ZnMgO/ZnO heterostructures were confirmed through surface potential measurements by Kelvin probe force microscopy. A simple model for the band profile was proposed and the various band parameters were evaluated experimentally and theoretically based on the band model. The band profile was calculated and validated with experimental results using the Schrodinger–Poisson equation. The energy level of the ZnMgO surface donor state, which serves as the source of the two-dimensional electron gas in ZnMgO/ZnO heterostructures, was estimated from the band parameters; nearly identical energy levels around 0.8 eV were obtained for Zn1−xMgxO layers with Mg compositions x ranging from 0.12 to 0.42 and the corresponding charge densities were estimated to be 8×1012 cm−2.

Study of oxide-silicon interface state generation and annihilation by rapid thermal processing

To investigate the mechanism of interface state generation and annihilation by rapid thermal processing (RTP), the interface state density Dit and the mechanical stress between silicon and oxide layers were measured using a surface charge analyzer and the curvature of the wafers, respectively. After RTP, Dit increases up to 750 °C and then decreases above 800 °C. The stress in the oxidized wafer increases linearly with temperature up to 700 °C; however, above this temperature, it saturates owing to the viscosity of the oxide. From the RTP temperature dependence of the relaxation time of the interface state annihilation, the activation energy is obtained as 1.36 eV. The same value is estimated assuming the Arrhenius-type relaxation process from the reported viscosity data. The decrement in Dit by RTP at high temperatures is attributed to the viscous flow in the oxide layer.

Demonstration of doubly rotated X-cut quartz plate oscillators with a slot vibrating in length extensional mode

This paper describes the finite element method analysis of (XYlt) -35°/-1° rotated X-cut quartz oscillators vibrating in the length-extensional mode and the experimental data of the sample processed monolithically by the wet etching. The oscillator consists of the resonator with a slot and an integral mounting system and has the first order temperature frequency coefficient α = -0.6 ×10^-6 /°C, the second order temperature frequency coefficient β = -3.5 ×10^-8 /°C², and the parameters of the electrical equivalent circuit in the air: the series inductance L₁ = 2.70 H, the capacitance ratio r = 192, the series resistance R₁ = 1080 Ω, and Q = 19000.

Zero temperature coefficient characteristic of four-corner-truncated square quartz resonator supported at four nodal points in Lamé mode

This paper describes analytical and experimental study of a zero-temperature-coefficient square Lamé-mode quartz resonator supported at four nodal points. Comparison of vibration mode shapes of the resonators with and without four supporting beams in the modal analysis using finite element analysis indicated that several contour modes vibrating at large amplitude in the corners with frequency lower than that of the Lamé mode get a boost in frequency by the constraint at the corners and resulted in spurious modes to the Lamé mode by temperature shift. Comparison of mode shapes of these unwanted contour vibrating modes with and without truncation at the corners of the square quartz plate indicated another boost in frequency caused by the reduction in mass where the mode vibrates at large amplitude. Four-corner truncated square quartz resonators supported by four beams were designed and fabricated by wet etching process. Measured data of the fabricated resonators made of LQ1T and LQ2T cut quartz plates indicated a zero temperature coefficient and electrical equivalent circuit parameters comparable to those of a fundamental mode Lamé resonators with two point support.

InGaN Growth with Indium Content Controlled by GaN Growth Plane

Highly spatially resolved cathodoluminescence (HRCL) imaging and electron backscattering pattern (EBSP) study of InGaN microcrystals were achieved. InGaN grown by the nitridation of a mixture of GaN microcrystals and indium sulfide powders with ammonia at 1000°C had crystal habit with twelve prismatic planes. The prismatic planes were grouped into two groups: the first group is pentagonal shaped planes yielding blue CL (420nm) and rectangular planes without luminescence. EBSP showed that the pentagonal shaped planes are indexed to the {11-20} a-plane and the rectangular plane to the {1-100} m-plane. HRCL imaging showed InGaN accumulated at the V pits on the basal plane. The obevation indicated that the crystal qualty of InGaN and the indium contents incorpated were controlled by the indices of GaN growth planes on which InGaN grew.