Tzer-En Nee

Gd2O3/GaN metal-oxide-semiconductor field-effect transistor

Gd2O3 has been deposited epitaxially on GaN using elemental Gd and an electron cyclotron resonance oxygen plasma in a gas-source molecular beam epitaxy system. Cross-sectional transmission electron microscopy shows a high concentration of dislocations which arise from the large lattice mismatch between the two materials. GaN metal-oxide-semiconductor field-effect transistors (MOSFETs) fabricated using a dielectric stack of single crystal Gd2O3 and amorphous SiO2 show modulation at gate voltages up to 7 V and are operational at source drain voltages up to 80 V. This work represents demonstrations of single crystal growth of Gd2O3 on GaN and of a GaN MOSFET using Gd2O3 in the gate dielectric.

Matrix dependence of strain-induced wavelength shift in self-assembled InAs quantum-dot heterostructures

We report on the matrix-dependent strain effect in self-assembled InAs quantum-dot heterostructures using photoluminescence measurements. A series of samples were prepared to examine the effect of quantum dot position with respect to the so-called strain-reducing layer (SRL). Since the SRL reduces the residual hydrostatic strain in the quantum dots, long emission wavelength of 1.34 μm is observed for the InAs quantum dots with an In0.16Ga0.84As SRL. The dependence of the emission wavelength on the thickness of the cap layer on SRL also indicates the importance of the role of matrix in the strain relaxation process of the dots. Using In0.16Al0.84As instead of In0.16Ga0.84As as the SRL, a blueshift in wavelength is observed because the elastic stiffness of In0.16Al0.84As is higher than that of In0.16Ga0.84As and less strain is removed from the dots with In0.16Al0.84As SRL.

GaN electronics for high power, high temperature applications

A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers, GaN/AlGaN heterojunction bipolar transistors and GaN metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.

Effects of thermal annealing on the luminescence and structural properties of high indium-content InGaN/GaN quantum wells

Postgrowth thermal annealing was applied to investigate the optical and structural properties of InxGa1−xN/GaN multiple quantum wells with high InN mole fraction. Thermal annealing at 900 °C results in a twentyfold increase of the integrated photoluminescence intensity. Photoluminescence emission is also improved from a broad band for the as-grown sample to two dominant peaks for the annealed sample. Cross-sectional transmission electron microscopy shows the existence of quantum dot-like islands in the wells for the as-grown sample but these islands are significantly reduced after thermal annealing at 900 °C.

Formation of self-organized In0.5Ga0.5As quantum dots on GaAs by molecular beam epitaxy

Abstract Self-organized In 0.5 Ga 0.5 As quantum dots have been successfully grown on vicinal GaAs substrates by molecular beam epitaxy. The density of the quantum dots can be changed by nucleating the dots under different As overpressure. Substrate tilt angle of 15° results in much larger dot size and density than that of 4° due to the closely spaced step edges on the surface. Through investigations of the dots grown on In 0.1 Ga 0.9 As buffer, the strain energy of the buffer layer is also found to be an important factor that affects the size and density of the quantum dots.

Effect of column III vacancy on arsenic precipitation in low-temperature grown III–V arsenides

Separately grown p-type, intrinsic, and n-type GaAs at low temperatures as well as a combined p-i-n structure have been used to study the formation of As precipitates upon annealing at 800 °C. For the separate structures, least precipitates have been noticed in the n-type material. In contrast, the highest density of precipitates appears in the n region for the p-i-n structure. In addition, an obvious band depleted of precipitates, exists in the intrinsic region near the n-i interface. A general vacancy model, including Fermi level effect and crystal bonding strength (thermodynamic factor), has been developed to explain the current results as well as to predict As precipitation in various low temperature grown III–V heterostructures.

Forward turn-on and reverse blocking characteristics of GaN Schottky and p-i-n rectifiers

Abstract High voltage GaN Schottky and p-i-n rectifiers have been fabricated on heteroepitaxial layers. The Schottky diodes have reverse blocking voltages around 500 V for vertical devices employing undoped, conducting GaN, whereas these voltages are >3000 V for lateral devices employing resistive GaN. The forward turn-on voltages are ⩾3.5 V for the Schottky rectifiers, with ideality factors of 1.5–2. The dominant current transport mechanism is Shockley–Read–Hall recombination. The p-i-n rectifiers fabricated with conducting i layers also have reverse blocking voltages of ∼500 V, but the forward turn-on voltages are typically ∼5 V. A comparison with the state-of-the-art SiC Schottky and p-i-n rectifiers is also given.

Carrier dynamics study of the temperature- and excitation-dependent photoluminescence of InAs/GaAs quantum dots

We investigate the effects that the carrier dynamics have on the temperature and excitation intensity dependence of the photoluminescence (PL) of self-assembled InAs∕GaAs quantum dot heterostructures having different size uniformities. We propose a rate equation model that takes into account the dot size distribution, the random population of density of states, and all of the important mechanisms of carrier dynamics, including radiative and nonradiative recombinations, thermal escaping and relaxing, and state filling effects. We used this model to simulate the PL spectra obtained from our samples; the results agree well with the measured data. We discuss in detail our quantitative calculations of the corresponding mechanisms of the thermal redistribution and state filling effects. These mathematical analyses provide distinct explanations for the phenomena we observed in the temperature- and incident power-dependent PL spectra of samples having different size uniformities.

Excitation Density and Temperature Dependent Photoluminescence of InGaAs Self-Assembled Quantum Dots

In this paper, we shall report on the excitation density and temperature dependent photoluminescence produced by discrete energy levels from InGaAs self-assembled quantum dots. While increasing the photoexcitation density, five peaks originating from discrete energy levels of quantum dot and wetting layer are observed. By deconvoluting these spectra using multiple Gaussian fit, the intensity of each state is saturated following its degeneracy. We describe the lateral confinement of quantum dots using a parabolic potential model. The saturated values are in good agreement with the degeneracy of this potential type. From the temperature dependent photoluminescence, we observed the thermally activated quenching of each state. Our results suggest that the wetting layer acts as a barrier to the carrier thermallization processes offering a two dimensional path for inter-dot coupling.

Sidegating effect improvement of GaAs metal–semiconductor field effect transistor by multiquantum barrier structure

The sidegating effect in a GaAs metal–semiconductor field effect transistor is significantly suppressed by the use of a multiquantum barrier (MQB) structure as a buffer layer. The enhancement of potential barrier height of the MQB buffer layer is confirmed from the comparison with a heterostructure buffer layer device.