Joel T. AsubarUniversity of Fukui · Faculty of Engineering
Joel T. Asubar
PhD
About
98
Publications
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Introduction
Joel T. Asubar currently works at the Department of Electrical and Electronics Engineering, University of Fukui. Joel does research in Solid State Physics, Electronic Device Physics, Materials Science and Condensed Matter Physics.
Additional affiliations
November 2014 - present
April 2010 - November 2015
September 2009 - March 2010
Publications
Publications (98)
Uniform thickness Al2O3 thin films have been deposited by eco-friendly mist chemical vapor deposition (CVD). The obtained Al2O3 film have optical band gap value of more than 6.5 eV and refractive index of 1.64 at 633 nm (≈ 2.0 eV). The combination of capacitance–voltage (C–V) fitting method with non-linear least-squares algorithm, frequency dispers...
We have compared the electrical performance of a proposed normally-off GaN-based MIS-HEMTs employing ultrathin AlGaN barrier layer in the channel with those of conventional recessed-gate structure MIS-HEMTs. The proposed device exhibited much less density of interface states extracted from the measured capacitance-voltage characteristics, suggestin...
We report on highly improved performance of Al2O3/AlGaN/GaN MIS-HEMTs using V/Al/Mo/Au metal stack as ohmic electrodes. Transfer length method (TLM) test structures using V/Al/Mo/Au metal stack annealed at temperature of 660 °C revealed highly linear current-voltage curves and smooth surface morphology. Compared with reference MIS-HEMTs using Ti/Al...
We report on the impact of the 3-nm-thick ex-situ AlGaN regrown layer prior to insulator deposition on the interfacial properties of Al2O3/AlGaN/GaN metal-insulator-semiconductor (MIS) structures. MIS-capacitors (MIScaps) with regrown AlGaN layer exhibited anomalously excessive threshold voltage shift compared to reference sample without regrown Al...
In this paper, a technique to develop small signal circuit (SSC) models of AlGaN/GaN high electron mobility transistors (HEMTs) using dependent current sources is presented. In this technique, experimentally measured broadband Y-parameters of AlGaN/GaN HEMTs are mathematically modeled as a sum of pole-residue terms. By representing each pole-residu...
Conventional lumped small signal circuit (SSC) models of AlGaN/GaN metal oxide semiconductor high electron mobility transistors (MOS-HEMTs) are derived from the low frequency portion of the experimentally measured Y-parameters. Consequently, these models cannot faithfully capture the high frequency behavior of the device. In this work, modified SSC...
Body weight fluctuation in an unrestricted free-living adult was investigated by taking daily measurements for a decade. It was found from the time series analyses that the Ornstein–Uhlenbeck process defined as the solution of the Langevin equation, is the most dominant factor affecting the body weight fluctuation. This process was shown to reasona...
Using numerical integration of the Boltzmann transport equation, depth distributions of recoil-implanted Ga and N atoms in Mg-implanted GaN are studied. Mg implantation into GaN is found to produce significant nonuniform stoichiometric distribution as a consequence of the recoil process, as compared to other III–V materials of GaP, GaAs and GaSb. O...
Traditional lumped small signal equivalent circuit models of AlGaN/GaN metal oxide semiconductor high electron mobility transistors (MOS-HEMTs) are made up of constant valued circuit elements. Such models are unable to capture the high frequency behavior (above 20 GHz) of the device. In this work, a modified small signal equivalent circuit model of...
Gallium nitride (GaN) is one of the front-runner materials among the so-called wide bandgap semiconductors that can provide devices having high breakdown voltages and are capable of performing efficiently even at high temperatures. The wide bandgap, however, naturally leads to a high density of surface states on bare GaN-based devices or interface...
We report on the fabrication and characterization of AlGaN/GaN metal-insulator-semiconductor (MIS) capacitors and high-electron-mobility transistors (MIS-HEMTs) using a 5 nm thick Al2O3 dielectric deposited by cost-effective and environmental-friendly mist chemical vapor deposition (mist-CVD) technique. Practically hysteresis-free capacitance–volta...
We report on an Al2O3/AlGaN/GaN metal-insulatorsemiconductor high-electron-mobility transistor (MIS-HEMT) with recessed-gate structure and regrown AlGaN barrier. After analyzing the possibility of obtaining high threshold voltage (Vth) within the framework of Tapajna and Kuzmik model from preliminary experiments using MIS-diode structures, we fabri...
This paper reports the first successful fabrication of enhancement (E)‐mode AlGaN/GaN vertical trench metal‐insulator‐semiconductor (MIS) high‐electron‐mobility transistors (HEMTs) using n+‐GaN/p‐GaN/n−‐GaN epi‐structures on free‐standing n+ substrates. A trench with smooth semi‐polar planes (sidewalls) with angles of 45 ˚ and 135 ˚ from the c‐plan...
We conducted X-ray photoelectron spectroscopy (XPS) and first-principles calculations based on density functional theory (DFT) to confirm the presence of Ga 2 O sub-oxide in high-pressure water vapor annealed AlGaN surface. We note that the Ga 3d XPS peak broadens and shifts towards higher binding energies, which suggests surface oxide formation. D...
The ferromagnetic transition in (Zn,Sn,Mn)As 2 thin films is explained in terms of magnetic percolation in a Mn-As clustering network. We first studied the relationship between the spatial distribution of Mn-As clusters and the Curie temperature (T C ). The local atomic structure was reconstructed from datasets of atomic positions in (Zn,Sn,Mn)As 2...
Electron concentration in highly resistive GaN substrates with intentional iron (Fe) dopants as well as unintentionally incorporated silicon (Si) and carbon (C) dopants has been investigated. Si, C, and Fe atomic concentrations were 2 × 10¹⁷ 1 × 10¹⁶ and 1 × 10¹⁹ respectively as measured by secondary ion mass spectroscopy (SIMS). Temperature depend...
The relationship between Mn concentration and Curie temperature (TC) is studied for Mn-doped ZnSnAs2 ferromagnetic semiconductors, epitaxially grown on InP substrates by molecular beam epitaxy. In the ferromagnetic phase, Mn distributions in a (Zn,Mn,Sn)As2 thin film with 7.2 cation percent (cat. %) Mn are investigated using three-dimensional atom...
We investigated the impact of rounded electrode corners on the breakdown characteristics of AlGaN/GaN high-electron mobility transistors. For standard reference devices, catastrophic breakdown occurred predominantly near the sharp electrode corners. By introducing a rounded-electrode architecture, premature breakdown at the corners was mitigated. M...
An analytical technique is proposed to reveal the relaxation time distribution of dynamic charge events using the current noise spectrum of a transistor, by applying an inverse integral transformation to the McWhorter model. In the proposed method, the continuous relaxation-time distribution function G(t) can be analytically derived from the noise...
We report on the correlation between the electroluminescence and current collapse of AlGaN/GaN high-electron-mobility transistors (HEMTs). Standard passivated devices suffering from severe current collapse exhibited high-intensity whitish electroluminescence confined near the drain contact. In contrast, devices with reduced current collapse resulti...
We present an analytical expression of sub-threshold swing (SS) in AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs), and describe dependences of SS on structural and relevant material parameters of the MIS-HEMTs. In this work, we derived two different equations where interface states reside at the dielectric/Al...
This letter describes DC characteristics of AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) with Al2O3 deposited by atomic layer deposition (ALD) as gate dielectric. Comparison was made for the samples deposited using ozone (O-3) or water as oxidant. The effect of pretreatment, where O-3 was solely supplied pr...
Interface characterization was carried out on Al2O3/GaN structures using epitaxial n-GaN layers grown on free-standing GaN substrates with relatively low dislocation density (<3 × 10⁶ cm⁻²). The Al2O3 layer was prepared by atomic layer deposition. The as-deposited metal-oxide-semiconductor (MOS) sample showed a significant frequency dispersion and...
Recent years have witnessed GaN-based devices delivering their promise of unprecedented power and frequency levels and demonstrating their capability as an able replacement for Si-based devices. High-electron-mobility transistors (HEMTs), a key representative architecture of GaN-based devices, are well-suited for high-power and high frequency devic...
In this paper, we give an overview of the recent progress in GaN-based high-electron-mobility transistors (HEMTs) developed for mainstream acceptance in the power electronics field. The comprehensive investigation of AlGaN/GaN HEMTs fabricated on a free-standing semi-insulating GaN substrate reveals that an extracted effective lateral breakdown fie...
This paper describes breakdown characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) with multi-finger gate patterns. We studied the spatial profile of electroluminescence (EL) from AlGaN/GaN HEMTs under high drain and near pinch-off gate bias. As a result, different EL emission profiles and breakdown characteristics were observed...
The purpose of this work is to investigate the effect of ohmic electrode processing on the breakdown voltage of AlGaN/GaN HEMTs. The impact of ultrasonic cleaning condition during the lift-off process on metal edge definition was investigated. It was verified that the shape of the ohmic electrode was indeed crucial for ensuring high breakdown volta...
We report on the highly reduced current collapse in AlGaN/GaN high-electron-mobility transistors (HEMTs) by combined application of pre-passivation oxygen (O2) plasma treatment and gate field plate (FP) structures schemes. Four different devices were fabricated in this work: (1) conventional HEMT as reference device, (2) field-plated HEMT, (3) O2 p...
The structure of a ZnSnAs2thin film epitaxially grown on an InP substrate was evaluated using x-ray fluorescence holography. The reconstructed three-dimensional atomic images clearly show that the crystal structure of the ZnSnAs2thin film is mainly of the sphalerite type, in contrast to the bulk form. A large disordering of the As layers is observe...
We studied the effects of pre-passivation oxygen plasma treatment of the AlGaN surface on the current collapse of AlGaN/GaN high-electronmobility transistors (HEMTs). Oxygen plasma-treated devices generally exhibited significantly less dynamic on-resistance (Ron) compared with untreated control devices. We also extended our investigation to HEMTs w...
We have demonstrated for the first time a remarkable reduction of current collapse in AlGaN/GaN high-electron-mobility transistors (HEMTs) by high-pressure water vapor annealing (HPWVA). The device subjected to HPWVA exhibited considerably low dynamic ON-resistance ( ), suggesting highly improved performance of these devices. Analyses of the result...
This paper describes significantly suppressed current collapse in AlGaN/GaN HEMTs with a 3-dimensional field plate (3DFP) structure. 3DFP consists of a gate field plate deposited on a passivated SiN film and equally-spaced multiple grooves formed by dry-etching of AlGaN/GaN layers, and hence the effective field plate area has been efficiently incre...
In this paper, the effect of thermal expansion in ohmic metals during ohmic annealing was studied to understand the mechanism of n-type ohmic formation in AlGaN/GaN heterostructures. We measured ohmic contact resistance as a function of annealing temperature for metal stacks composed of Cu/Al/Mo/Au, Ni/Al/Mo/Au, and Ti/Al/Mo/Au. Specific contact re...
In this paper we have investigated the relationship between off-state breakdown voltage and gate-to-drain distance (Lgd) for AlGaN/GaN HEMTs fabricated on a free-standing GaN substrate. The off-state breakdown voltage exhibited a linear increase up to Lgd of around 80 µm but saturated at about 4000 V when Lgd > 80 µm. Therefore, we proposed that wh...
This paper describes electrical characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) with a GaN cap layer. The maximum drain current was kept constant at around 0.4 A/mm for a GaN cap layer thickness up to 5 nm. It was found that the degree of current collapse was gradually improved when the GaN cap thickness was increased from 0...
We have investigated the effects of the inductively coupled plasma (ICP) etching of AlGaN surface on the resulting interface properties of the Al2O3/AlGaN/GaN structures. The experimentally measured capacitance–voltage (C–V) characteristics were compared with those calculated taking into account the interface states density at the Al2O3/AlGaN inter...
A novel method is presented for obtaining the distribution function of relaxation times from power spectrum . It is derived using McWhorter model and its inverse Stieltjes transform. Unlike the pre-assumed conventional distribution, the extracted has a peak whose width increases as the slope of the power spectrum α decreases. The peak position dete...
This paper presents a systematic characterization of electronic states at insulators/(Al)GaN interfaces, particularly focusing on insulator/AlGaN/GaN structures. First, we review important results reported for GaN metal–insulator–semiconductor (MIS) structures. SiO2 is an attractive material for MIS transistor applications due to its large bandgap...
Dramatic reduction of thermal resistance was achieved in AlGaN/GaN Multi-Mesa-Channel (MMC) high electron mobility transistors (HEMTs) on sapphire substrates. Compared with the conventional planar device, the MMC HEMT exhibits much less negative slope of the ID-VDS curves at high VDS regime, indicating less self-heating. Using a method proposed by...
Using a dual-gate transistor structure, we have evaluated the effect of off-bias-stress spatially induced surface charging at the AlGaN/GaN surface. Pulsed voltage stress on the drain access region led only to increase in the on-resistance, whereas the stress on the source region additionally decreased the saturation drain current. In addition, sig...
We present further experimental results that demonstrate the superior current stability of multi-mesa-channel (MMC) AlGaN/GaN HEMTs over their conventional planar counterparts. Pulsed ID-VDS measurements revealed almost overlapping ID-VDS curves for the MMC device even with increasing drain-to-source quiescent bias VDSq with gate-to-source quiescen...
In this present work, we give an overview of significant results for the past several years of our work with II-IV-V2 ternary ZnSnAs2 that we believe is one of the promising ferromagnetic semiconductor host prospects. The procedures for the molecular beam epitaxy (MBE) preparation of stoichiometric un-doped ZnSnAs2 epitaxial films using an optimum...
We develop and characterize multi-mesa-channel (MMC) AlGaN/GaN high-electron mobility transistors (HEMTs), in which a periodic trench structure is fabricated only under the gate electrode. A surrounding-field effect in the MMC structure results in a shallower threshold voltage and a smaller subthreshold slope than those of the standard planar-type...
We have directly grown zinc-blende (zb)-type MnAs thin films on InP (001) substrates without the aid of any buffer layer using molecular beam epitaxy (MBE). From the High-resolution X-ray diffraction (XRD) data, assuming face-centered cubic (fcc) MnAs structure, the average lattice constants values were calculated to be 6.068 and 6.060 Å for growth...
We investigated the growth and magnetic properties of ternary ZnSnAs2 thin films doped with a various degrees of Mn content. It was confirmed that Mn-doped ZnSnAs2 thin films are pseudomorphically grown on nearly lattice-matched InP(001) substrates. Magnetization measurements on Mn-doped ZnSnAs2 thin films revealed a ferromagnetic transition temper...
We investigated the growth and magnetic properties of ternary ZnSnAs2 thin films doped with a various degrees of Mn content. It was confirmed that Mn-doped ZnSnAs2 thin films are pseudomorphically grown on nearly lattice-matched InP(001) substrates. Magnetization measurements on Mn-doped ZnSnAs2 thin films revealed a ferromagnetic transition temper...
Un-doped II-IV-V2 ZnSnAs2 thin films have been grown epitaxially on semi-insulating InP(001) substrates by molecular beam epitaxy using a substrate temperature of Ts=300°C. In-situ reflection high-energy electron diffraction observations during the growth revealed streaky patterns indicating an atomically flat surface. After verification of the res...
Since the ferromagnetism in ternary chalcopyrite semiconductor CdGeP2:Mn was first reported by Medvedkin et al., several ferromagnetic ternary compounds such as II-IV-P2 and II-IV-As2 doped with Mn have been exploited from the viewpoint of experimental and theoretical aspects. Almost of these compounds have been synthesized in forms of bulk II-IV-V...
ZnSnAs2 epitaxial films of various thicknesses have been grown on InP(001) substrates by molecular beam epitaxy using different growth times. No manifestations of epitaxial layer lattice relaxation such as broadening of the full width at half maximum of high-resolution x-ray diffraction rocking curves, reduction of the ratio of the diffraction peak...
We formed a ~23-nm-thick MnAs thin film on an epi-ready semi-insulating GaAs(1 11) substrate by molecular beam epitaxy at a growth temperature Ts of 250°C. Streaky in-situ reflection high-energy electron diffraction patterns were obtained during the entire growth process indicating layer-by-layer growth. High-resolution X-ray diffraction revealed t...
We have prepared Mn-doped ZnSnAs2 thin films with varying Mn doping content (2.1%, 2.7%, and 5.0%) using molecular beam epitaxy, by changing the Mn-to-Sn beam equivalent pressure ratio during growth. All the samples were grown on InP(001) substrates using the optimum substrate temperature Ts=300°C previously reported. As a reference, an un-doped Zn...
X-ray fluorescence holographic study on a room-temperature ferromagnetic semiconductor film of ZnSnAs2:Mn was performed using a strong X-ray beam of third generation synchrotron radiation of SPring-8. The real space reconstructions of the environments around Mn atoms were successfully visualized from the observed holograms despite the very small am...
We present for the first time the temperature dependence of resistivity, anomalous Hall effect, and extraordinary magnetoresistance (MR) in 6.5% Mn-doped ZnSnAs2 epitaxial film prepared by molecular beam epitaxy (MBE) on InP(001) substrates. The magnetic field dependence of magnetization (M--H curve) show clear hysteresis loops at 300 K for magneti...
X-ray fluorescence holographic study on a room-temperature ferromagnetic semiconductor film of ZnSnAs2:Mn was performed using a strong X-ray beam of third generation synchrotron radiation of SPring-8. The real space reconstructions of the environments around Mn atoms were successfully visualized from the observed holograms despite the very small am...