Showing papers by "Tsunenobu Kimoto published in 2019"
TL;DR: In this article, homoepitaxial GaN p-n junction diodes with a negative beveled-mesa termination were investigated using TCAD simulation, and the devices were designed using currently available GaN growth techniques.
Abstract: We report on homoepitaxial GaN p-n junction diodes with a negative beveled-mesa termination. The electric field distribution in a beveled-mesa was investigated using TCAD simulation, and the devices were designed using currently available GaN growth techniques. Shallow-angle (ca. 10°) negative bevel GaN p-n junction diodes were fabricated with various Mg acceptor concentrations in the p-layers. The suppression of electric field crowding and improvement of the breakdown voltage were observed, as the Mg concentration was decreased. The parallel-plane breakdown field of 2.86 MV/cm was obtained for a device with the breakdown voltage of 425 V.
64 citations
TL;DR: In this paper, the authors demonstrate normally-off 400 °C operation of n-channel and p-channel junction field effect transistors (JFETs) fabricated by an ion implantation into a common high-purity semi-insulating silicon carbide (SiC) substrate.
Abstract: We demonstrate normally-off 400 °C operation of n-channel and p-channel junction field-effect transistors (JFETs) fabricated by an ion implantation into a common high-purity semi-insulating silicon carbide (SiC) substrate. The side-gate structure proposed in this letter has good controllability of threshold voltage. The present results assure the potential of JFET-based complementary logic integrated circuits (ICs) operational at high temperature.
29 citations
TL;DR: In this article, the authors studied deep levels in quartz-free hydride-vapor-phase epitaxy (QF-HVPE)-grown homoepitaxial n-type GaN layers within which three electron and eight hole traps were detected.
Abstract: We studied deep levels in quartz-free hydride-vapor-phase epitaxy (QF-HVPE)-grown homoepitaxial n-type GaN layers within which three electron and eight hole traps were detected. The dominant electron and hole traps observed in the QF-HVPE-grown GaN layers were E3 (EC − 0.60 eV) and H1 (EV + 0.87 eV), respectively. We found that the E3 trap density of QF-HVPE-grown GaN (∼1014 cm−3) was comparable with that of MOVPE-grown GaN layers, whereas the H1 trap density of QF-HVPE-grown GaN (∼1014 cm−3) was much smaller than that of an MOVPE-grown GaN layer with a low-residual-carbon growth condition. A detailed analysis of the QF-HVPE-grown GaN layers revealed that the H1 trap density is almost equal to the carbon impurity concentration and other impurities that compensate the Si donors besides the carbon impurity were hardly detected in the QF-HVPE-grown GaN layers.
28 citations
TL;DR: Avalanche multiplication characteristics of GaN p-n junction diodes with double-side-depleted shallow bevel termination, which exhibit nearly ideal avalanche breakdown, were investigated by photomultiplication measurements using subbandgap light as mentioned in this paper.
Abstract: Avalanche multiplication characteristics of GaN p-n junction diodes (PNDs) with double-side-depleted shallow bevel termination, which exhibit nearly ideal avalanche breakdown, were investigated by photomultiplication measurements using sub-bandgap light. In GaN PNDs under reverse bias conditions, optical absorption induced by the Franz-Keldysh (FK) effect is observed, resulting in a predictable photocurrent. The avalanche multiplication factors were extracted as a ratio of the measured values to the calculated FK-induced photocurrent. In addition, the temperature dependences of the avalanche multiplications were also investigated.
26 citations
TL;DR: In this article, the authors investigated the expansion/contraction behavior of single Shockley-type stacking faults (1SSFs) in 4H-SiC crystals by using an electronic energy model.
Abstract: The expansion/contraction behavior of single Shockley-type stacking faults (1SSFs) in 4H-SiC crystals is investigated by using an electronic energy model. The model takes into account several factors that were not considered in the previous models and provides a guideline to understand the 1SSF behavior. The authors calculated the threshold excess carrier density, which is the critical excess carrier density for the expansion/contraction of a 1SSF, for two models: with and without carrier recombination at a 1SSF. In the case of the model without carrier recombination, the obtained threshold excess carrier density at room temperature was at least 1 × 10 17 c m − 3. On the other hand, the threshold excess carrier density at room temperature given by the model with carrier recombination was in the range of 2 × 10 14 to 2 × 10 16 c m − 3, which shows good agreement with the previous experimental results quantitatively. The authors also discuss the temperature-, doping-concentration-, and conduction-type-dependences of the threshold excess carrier density. The calculated doping-concentration- and conduction-type-dependences of the threshold excess carrier density imply that the 1SSF expansion easily occurs in the heavily-doped crystals, and n-type 4H-SiC is slightly less tolerant against bipolar degradation than the p-type one.
24 citations
TL;DR: In this article, the minimum specific on-resistance of 4H-SiC{0001} unipolar devices as a function of the breakdown voltage was updated based on latest studies on intrinsic physical properties, such as impact ionization coefficients.
Abstract: The minimum specific on-resistance of 4H-SiC{0001} unipolar devices as a function of the breakdown voltage was updated based on latest studies on intrinsic physical properties, such as impact ionization coefficients. Both punch-through (PT) and nonpunch-through (NPT) structures were considered, because a PT structure generally gives a lower on-resistance at a given breakdown voltage. The minimum specific on-resistance of 1 kV 4H-SiC devices can be as low as 0.20 mΩ cm2 at room temperature. An analytical expression for the relationship between the specific on-resistance and breakdown voltage is given.
24 citations
01 Dec 2019
TL;DR: In this article, a method to extract impact ionization coefficients of electrons and holes using above and sub-bandgap illuminations for a p−/n+ junction diode was proposed.
Abstract: We propose a novel method to extract impact ionization coefficients of electrons and holes using above-and sub-bandgap illuminations for a p−/n+ junction diode. For above-bandgap illumination, the light is absorbed near p-GaN surface. Then, generated minority carriers diffuse and reach the edge of the depletion layer, resulting in an electron-injected photocurrent. On the other hand, for sub-bandgap illumination, the light is selectively absorbed near the p-n junction interface by the Franz-Keldysh effect, resulting in a hole-injected photocurrent. The electron- and hole-initiated multiplication factors are obtained as the ratios of the measured photocurrents to the calculated unmultiplicated photocurrents. By analyzing the electron- and hole-initiated multiplication factors, the impact ionization coefficients of electrons and holes in GaN are extracted separately.
23 citations
TL;DR: In this article, the temperature dependence of impact ionization coefficients of electrons and holes along 4H-SiC photodiodes was determined in a wide temperature range from 156 K to 561 K.
Abstract: The temperature dependence of impact ionization coefficients of electrons and holes along 4H-SiC 〈0001〉 was determined in a wide temperature range from 156 K to 561 K. Photomultiplication measurements were conducted using several types of 4H-SiC photodiodes (PDs) with different structures under high vacuum conditions. The hole ionization coefficient showed a negative temperature dependence, which can be attributed to enhanced phonon scattering at elevated temperatures. Temperature dependence of the electron ionization coefficient was small, and this result may be ascribed to the simultaneous effects of enhanced phonon scattering and shrinkage of minigaps in the conduction band. Accuracy of the obtained results was verified by comparing the calculated breakdown voltage with the experimental breakdown voltage of various diodes.
22 citations
TL;DR: In this article, the authors analyzed forward currentvoltage (I-V) characteristics in GaN-on-GaN p-n+ junction diodes with mesa-isolation structure and obtained an SRH lifetime of 46 ps at 298 K.
Abstract: The Shockley–Read-Hall (SRH) lifetime in homoepitaxial p-GaN (N a = 1 × 1017 cm−3) is investigated by analyzing forward current–voltage (I–V) characteristics in GaN-on-GaN p–n+ junction diodes with mesa-isolation structure. The ideality factor around 2 due to recombination current was obtained in the 1.8–2.7 V window, which is different from the characteristic of a p+-n− junction involving considerable diffusion current. The recombination current was proportional to the junction area, indicating that the recombination current is a bulk component, not a mesa-surface component. Analyzing the recombination current with consideration of the SRH recombination rate in the depletion layer, we obtained an SRH lifetime of 46 ps at 298 K. The temperature dependence of the I–V characteristics was also investigated and the SRH lifetimes were extracted in the range of 223–573 K. The SRH lifetime in homoepitaxial p-GaN followed the empirical power law of = 1.2 × 10−16 × T 2.25 (s).
17 citations
TL;DR: In this article, annealing in low-oxygen-partial-pressure (low-p$_{\rm O2}$) ambient is reported to reduce the interface state density at a SiC (0001)/SiO$(2) interface near the conduction band edge of SiC.
Abstract: We report that annealing in low-oxygen-partial-pressure (low-p$_{\rm O2}$) ambient is effective in reducing the interface state density (D$_{\rm IT}$) at a SiC (0001)/SiO$_{\rm 2}$ interface near the conduction band edge (E$_{\rm C}$) of SiC. The D$_{\rm IT}$ value at E$_{\rm C}$$-$0.2 eV estimated by a high (1 MHz)-low method is 6.2$\times$10$^{12}$ eV$^{-1}$cm$^{-2}$ in as-oxidized sample, which is reduced to 2.4${\times}$10$^{12}$ eV$^{-1}$cm$^{-2}$ by subsequent annealing in O$_{\rm 2}$ (0.001%) at 1500${}^\circ$C, without interface nitridation. Although annealing in pure Ar induces leakage current in the oxide, low-p$_{\rm O2}$ annealing (p$_{\rm O2}$ = 0.001 - 0.1 %) does not degrade the oxide dielectric property (breakdown field ~ 10.4 MVcm$^{-1}$).
12 citations
TL;DR: In this paper, annealing in low-oxygen-partial-pressure (low-p O2) ambient is reported to reduce the interface state density (D IT) at a SiC (0001)/SiO2 interface near the conduction band edge (E C) of SiC.
Abstract: We report that annealing in low-oxygen-partial-pressure (low-p O2) ambient is effective in reducing the interface state density (D IT) at a SiC (0001)/SiO2 interface near the conduction band edge (E C) of SiC. The D IT value at E C − 0.2 eV estimated by a high (1 MHz)-low method is 6.2 × 1012 eV−1 cm−2 in as-oxidized sample, which is reduced to 2.4 × 1012 eV−1 cm−2 by subsequent annealing in O2 (0.001%) at 1500 °C, without interface nitridation. Although annealing in pure Ar induces leakage current in the oxide, low-p O2 annealing does not degrade the oxide dielectric property (breakdown field ~10.4 MV cm−1).
01 Jan 2019
TL;DR: In this article, the crystal structure, electronic band structure, and physical properties of silicon carbide (SiC) are introduced and the impacts of these defects on performance and reliability of SiC power devices are described.
Abstract: This chapter introduces the crystal structure, electronic band structure, and physical properties of silicon carbide (SiC). Physical properties are critical parameters for accurate device simulation. Major features in physical properties of SiC (4H-SiC) include wide bandgap, reasonably high electron mobility, high critical electric field strength (low impact ionization coefficients), and high thermal conductivity. In addition to the intrinsic physical properties, extended defects (dislocations, stacking faults, and other types of defects) present in SiC epitaxial wafers are briefly reviewed. The impacts of these defects on performance and reliability of SiC power devices are described. Major deep levels observed in SiC epitaxial layers and carrier lifetimes are also explained.
TL;DR: In this paper, a SiC n-and p-channel junction field effect transistors with vertical channels were fabricated by direct ion implantation into a high-purity semi-insulating 4H-SiC substrate in order to further develop the path towards complementary JFET integrated circuits for applications in harsh environments.
Abstract: Silicon carbide (SiC) n-and p-channel junction field effect transistors (JFETs) with vertical channels were fabricated by direct ion implantation into a high-purity semi-insulating 4H-SiC substrate in order to further develop the path towards complementary JFET integrated circuits for applications in harsh environments. Compared with the conventional structure (lateral channel), the proposed structure is suitable for integration and inherently has a high transconductance owing to the double-gate configuration. The threshold voltage (Vth) can be controlled by mask design, while Vth in the conventional structure is solely determined by the ion implantation conditions. We demonstrate the transistor operation of the vertical-channel n-and p-channel JFETs fully fabricated by ion implantation.
TL;DR: In this article, the effects of post-deposition annealing (PDA) on the threshold voltage of AlGaN/GaN MOS heterojunction field effect transistors with atomic-layer-deposited Al2O3 using H2O vapor or oxygen plasma as the oxidant.
Abstract: In this paper, we investigated the effects of the post-deposition annealing (PDA) on the threshold voltage (V th) of AlGaN/GaN MOS heterojunction field-effect transistors with atomic-layer-deposited Al2O3 using H2O vapor or oxygen plasma as the oxidant. By PDA, the V th shifts positively with the V th variations depending on the oxidants. The capacitance–voltage measurements reveal that the V th variation is attributed to the differences in the initial fixed charge density in the Al2O3 or/and the Al2O3/AlGaN for both oxidants.
TL;DR: In this paper, SiC bipolar junction transistors (BJTs) were fabricated based on the design criterion proposed in our previous study, which quantitatively proved the importance of decreasing a base spreading resistance.
Abstract: SiC bipolar junction transistors (BJTs) were fabricated based on the design criterion proposed in our previous study, which quantitatively proved the importance of decreasing a base spreading resistance. To reduce the base spreading resistance, Al+-implantation was performed in the parasitic base region. No negative influences due to the implantation damage on the current gain were confirmed when the implantation is performed sufficiently apart from the emitter mesa sidewall, the distance of which is longer than ${3}~\mu \text{m}$ . Since the fabricated BJTs satisfied the design criterion, clear conductivity modulation was achieved, resulting in a reduced collector-resistance, that is, 50% of the unipolar resistance. In addition, we experimentally demonstrated that the conductivity modulation in SiC BJTs could be enhanced by decreasing the base spreading resistance.
TL;DR: In this paper, the authors determined the interface state density in the vicinity of the conduction band edge in silicon carbide (SiC) metal-oxide-semiconductor (MOS) structures by reproducing the experimental currentvoltage characteristics of MOS field effect transistors with numerical calculations.
Abstract: We determined the interface state density ($D_{\rm it}$) distributions in the vicinity of the conduction band edge in silicon carbide (SiC) metal-oxide-semiconductor (MOS) structures by reproducing the experimental current-voltage characteristics of MOS field effect transistors (MOSFETs) with numerical calculations. In the calculation, potential distributions and energy sub-bands in the inversion layer were calculated by solving Poisson and Schrodinger equations, respectively. We demonstrate that gate characteristics of the MOSFETs are well described by considering that the interface states are caused by fluctuation of free-electron density of states in a two-dimensional system. The $D_{\rm it}$ distributions are almost uniquely determined by the oxide formation process (as oxidation or interface nitridation) and independent of the acceptor concentration ($3\times10^{15}-1\times10^{18}\ {\rm cm^{-3}}$).
19 May 2019
TL;DR: In this article, the authors investigated the valanche multiplication characteristics in GaN p-n junction diodes under high reverse bias conditions and showed that the photocurrent induced by the Franz-Keldysh effect can be well reproduced by the theoretical calculations of the optical absorption, and their avalanche multiplications were observed.
Abstract: A valanche multiplication characteristics in GaN p-n junction diodes (PNDs) under high reverse bias conditions were investigated. The GaN-on-GaN PNDs with double-side-depleted shallow bevel termination, which showed low reverse leakage current and excellent avalanche capability, were used for the measurements. Under sub-bandgap light illumination, the photocurrents induced by Franz-Keldysh (FK) effect, which can be well reproduced by the theoretical calculations of the optical absorption, and their avalanche multiplications were observed. The multiplication factors were extracted as the ratios of the experimental photocurrents to the calculated FK-induced photocurrent. Under an assumption of equal impact ionization coefficients of electrons and holes, the electric-field dependence of an impact ionization coefficient in GaN were estimated.
TL;DR: In this paper, the authors investigated the influence of vacuum annealing on the interface properties of silicon carbide (SiC) metal-oxide-semiconductor (MOS) structures.
Abstract: We investigated the influence of vacuum annealing on the interface properties of silicon carbide (SiC) metal–oxide–semiconductor (MOS) structures. For as-oxidized and nitric oxide (NO)-annealed samples, the interface state density (D it) near the conduction band edge (E C) of SiC did not increase by subsequent vacuum annealing. For phosphoryl chloride (POCl3)-annealed samples, in contrast, D it at E C − 0.2 eV increased from 1.3 × 1010 to 2.2 × 1012 cm−2 eV−1 by vacuum annealing, and the channel mobility of MOS field effect transistors (MOSFETs) decreased from 109 to 44 cm2 V−1 s−1. The mechanism of the observed increase in D it was discussed based on the results of the secondary ion mass spectrometry measurement.
TL;DR: In this paper, the influence of vacuum annealing on interface properties of silicon carbide (SiC) metal-oxide-semiconductor (MOS) structures was investigated.
Abstract: We investigated the influence of vacuum annealing on interface properties of silicon carbide (SiC) metal-oxide-semiconductor (MOS) structures. For as-oxidized and nitric oxide (NO)-annealed samples, the interface state density ($D_{\rm it}$) near the conduction band edge ($E_{\rm C}$) of SiC did not increase by subsequent vacuum annealing. For phosphoryl chloride (POCl$_3$)-annealed samples, in contrast, $D_{\rm it}$ at $E_{\rm C}-0.2$ eV increased from $1.3\times10^{10}$ to $2.2\times10^{12}$ cm$^{-2}$eV$^{-1}$ by the vacuum annealing, and the channel mobility of MOS field effect transistors (MOSFETs) decreased from 109 to 44 cm$^2$V$^{-1}$s$^{-1}$. Mechanism of the observed increase in $D_{\rm it}$ was discussed based on the results of secondary ion mass spectrometry measurement.
TL;DR: In this paper, the authors investigated the effect of forming on bipolar resistive switching (RS) characteristics in Pt/TaOx/Ta2O5/Pt cells and found that the forming resulted in a transition from an initial state to a particular high resistance state (HRS).
Abstract: Impacts of a forming process on bipolar resistive switching (RS) characteristics in Pt/TaOx/Ta2O5/Pt cells were investigated. We found that the forming resulted in a transition from an initial state to a particular high resistance state (HRS) in most of the Pt/TaOx/Ta2O5/Pt cells. Evaluation of electrical characteristics after the transition to the particular HRS revealed that two modes of bipolar RS with the conventional polarity based on valence change mechanism and with the opposite polarity could be selectively obtained by adjusting the magnitude of the applied voltage. Moreover, the cell resistance decreased gradually during set processes in the bipolar RS with the opposite polarity.