Showing papers by "Tsunenobu Kimoto published in 2018"

Sources of carrier compensation in metalorganic vapor phase epitaxy-grown homoepitaxial n-type GaN layers with various doping concentrations

Abstract: The source of carrier compensation in metalorganic vapor phase epitaxy (MOVPE)-grown n-type GaN was quantitatively investigated by Hall-effect measurement, deep-level transient spectroscopy, and secondary ion mass spectrometry. These analysis techniques revealed that there were at least three different compensation sources. The carrier compensation for samples with donor concentrations below 5 × 1016 cm−3 can be explained by residual carbon and electron trap E3 (E C − 0.6 eV). For samples with higher donor concentrations, we found a proportional relationship between donor concentration and compensating acceptor concentration, which resulted from a third source of compensation. This is possibly due to the self-compensation effect.

Injected carrier concentration dependence of the expansion of single Shockley-type stacking faults in 4H-SiC PiN diodes

Abstract: We investigated the relationship between the dislocation velocity and the injected carrier concentration on the expansion of single Shockley-type stacking faults by monitoring the electroluminescence from 4H-SiC PiN diodes with various anode Al concentrations. The injected carrier concentration was calculated using a device simulation that took into account the measured accumulated charge in the drift layer during diode turn-off. The dislocation velocity was strongly dependent on the injected hole concentration, which represents the excess carrier concentration. The activation energy of the dislocation velocity was quite small (below 0.001 eV between 310 and 386 K) over a fixed range of hole concentrations. The average threshold hole concentration required for the expansion of bar-shaped single Shockley-type stacking faults at the interface between the buffer layer and the substrate was determined to be 1.6–2.5 × 1016 cm−3 for diodes with a p-type epitaxial anode with various Al concentrations.

Parallel-Plane Breakdown Fields of 2.8-3.5 MV/cm in GaN-on-GaN p-n Junction Diodes with Double-Side-Depleted Shallow Bevel Termination

Abstract: We report homoepitaxial GaN p-n junction diodes with novel beveled-mesa structures. The n-layers and p-layers, the doping concentrations of which are comparable, were prepared. We found that electric field crowding does not occur in the structure using TCAD simulation. The fabricated devices showed the breakdown voltages of 180–480 V, small leakage currents, and excellent avalanche capabilities. The breakdown voltages increased at elevated temperature. At the breakdown, nearly uniform luminescence in the entire p-n junctions was observed in all the devices. These results are strong evidences that the uniform avalanche breakdowns occurred in the devices. We carefully characterized the depletion layer width at the breakdown, and the parallel-plane breakdown electric fields of 2.8-3.5 MV/cm were obtained, which are among the best of the reported non-punch-through GaN vertical devices.

High-Temperature Operation of n- and p-Channel JFETs Fabricated by Ion Implantation Into a High-Purity Semi-Insulating SiC Substrate

Abstract: Silicon carbide (SiC) n-channel and p-channel junction field-effect transistors (JFETs) were fabricated by direct ion implantation into a high-purity semi-insulating 4H-SiC substrate toward complementary-JFET integrated circuit applications under a harsh environment. The fabricated n-JFET and p-JFET on a common substrate without an epitaxial layer show normal transistor operations up to 400 °C. Their electrical characteristics are comparable with theoretical estimations obtained from material parameters of n- and p-type SiC epitaxial layers. The present results assure that not only JFETs but also a variety of devices can be made by direct ion implantation while keeping material properties of epitaxially grown SiC.

Estimation of Threshold Voltage in SiC Short-Channel MOSFETs

Abstract: In this brief, the influence of high-density traps at the SiO2/SiC interface on short-channel effects was investigated, and a model describing channel length dependence of the threshold voltage (i.e., the gate voltage at a given drain current) is proposed. First, we determined the densities of interface states and fixed charge in 4H-SiC n-channel MOSFET by fitting the calculated gate characteristics to the experimental data, and acquired the density of trapped electrons from the obtained results. Then, we calculated the threshold voltage by considering that the majority of the trapped electrons are emitted near the drain end due to formation of a depletion layer. The channel length dependence of the threshold voltage calculated by the proposed model showed a good agreement with the experimental results even when the definition of threshold voltage was changed. The present model taking account of the fixed charge and of the trapped electron charge enables the estimation of the threshold voltage in the 4H-SiC short-channel MOSFETs.

Franz-Keldysh effect in GaN p-n junction diode under high reverse bias voltage

Abstract: Photocurrent induced by sub-bandgap light absorption due to the Franz-Keldysh effect was observed in GaN p-n junction diodes under a high reverse bias voltage. The photocurrent increased with the reverse bias voltage and the increase was found to be more significant as the wavelength approached the absorption edge of GaN. The photocurrent was calculated with consideration of light absorption induced by the Franz-Keldysh effect in the depletion layer. The calculated curves showed excellent agreement with the experimental curves. The photocurrent also increased with an increase in temperature and this could be quantitatively explained by the red-shift of the GaN absorption edge with the increase in temperature.Photocurrent induced by sub-bandgap light absorption due to the Franz-Keldysh effect was observed in GaN p-n junction diodes under a high reverse bias voltage. The photocurrent increased with the reverse bias voltage and the increase was found to be more significant as the wavelength approached the absorption edge of GaN. The photocurrent was calculated with consideration of light absorption induced by the Franz-Keldysh effect in the depletion layer. The calculated curves showed excellent agreement with the experimental curves. The photocurrent also increased with an increase in temperature and this could be quantitatively explained by the red-shift of the GaN absorption edge with the increase in temperature.

Observation of carrier recombination in single Shockley stacking faults and at partial dislocations in 4H-SiC

Abstract: Because the expansion of single Shockley stacking faults (1SSFs) is an important problem for the viability of SiC bipolar devices, there is a need to suppress it during device operation The expansion mechanism, however, is still unclear Therefore, the method to suppress the expansion has never been established An important factor for the expansion could be carrier recombination in 1SSFs because the expansion has only been observed during bipolar operation or light illumination In this study, we characterized carrier recombination by observing the photoluminescence from 1SSFs and partial dislocations (PDs) The luminescence from 1SSFs and PDs showed a fast decay component compared with that from the band edge This result indicates that the carrier recombination in 1SSFs and at PDs was faster than that in regions without 1SSFs in 4H-SiC In addition, because of the slower recombination at Si-core PDs compared with that in 1SSFs and at C-core PDs, the velocity of 1SSF expansion would be limited by the carrier recombination at Si-core PDs The temperature dependence of the decay time implies that the recombination at the Si-core PD was enhanced on increasing the temperature

Theoretical analysis of Hall factor and hole mobility in p-type 4H-SiC considering anisotropic valence band structure

Abstract: The temperature dependencies of hole density and hole mobility of p-type 4H-SiC obtained by Hall effect measurement were theoretically analyzed taking account of its anisotropic valence band structure. The experimental Hall factor, which was derived from the ratio of theoretical hole density to experimental Hall hole density, was reproduced by theoretical Hall factor computed using the valence band structure and relaxation times of scattering mechanisms. The product of the theoretical Hall factor and drift mobility computed by the same transport model agreed well with the experimental Hall mobility. Based on analyses of the results, it was revealed that the temperature dependence of Hall factor can be explained by considering the anisotropic valence band structure along with consideration of anisotropic relaxation times. The contribution of each scattering mechanism was also discussed, and empirical formulas for Hall and drift mobilities are presented.

Accurate method for estimating hole trap concentration in n-type GaN via minority carrier transient spectroscopy

Abstract: We propose an analysis method for the accurate estimation of the hole trap (H1, E V + 0.85 eV) concentration in n-type GaN via minority carrier transient spectroscopy (MCTS). The proposed method considers both the hole occupation during a filling (current injection) period and the quick carrier recombination via the hole traps near the depletion layer edge immediately after a reverse bias is applied. The reverse bias voltage dependence of the MCTS spectrum indicates that an accurate trap concentration, as well as the hole diffusion length and electron capture cross section of the hole trap, can be determined.

Passivation of Surface Recombination at the Si-Face of 4H-SiC by Acidic Solutions

Abstract: We carried out carrier lifetime measurements for 4H-SiC single crystals in aqueous solutions with various pH by the microwave photoconductivity decay method. For both n- and p-type 4H-SiC, carrier lifetimes measured by Si-face excitation were longer in acidic aqueous solutions compared with carrier lifetimes measured in other solutions. On the other hand, for C-face excitation, carrier lifetimes did not depend on immersion solutions. These results indicate that carrier recombination centers at the surface of the Si-face was passivated by hydrogen ions. We also estimated surface recombination velocities Ss by a numerical analysis. S of the Si-face was reduced from 700 cm/s (in Na2SO4 1 M) to 200 cm/s (in H2SO4 1 M) for the n-type 4H-SiC.

27.5 kV 4H-SiC PiN diode with space-modulated JTE and carrier injection control

Abstract: Ultra-high-voltage 4H-SiC PiN diode with a space-modulated junction termination extension and carrier injection control has been investigated. The introduction of a space-modulated region results in a high breakdown voltage of 27.5 kV, that is the highest among the values reported for 20 A class 4H-SiC PiN diodes. The simulated and measured forward characteristics of the 4H-SiC PiN diode with the carrier injection control are also reported. Forward voltage and on-resistance decrease as carrier lifetime increases. The introduction of carrier injection control at the anode and cathode sides results in reduction in carrier concentration. The measured characteristics exhibit good correlation with simulated results. Based on these results, we can confirm the effect of carrier lifetime on electrical characteristics.

Suppression of Punch-Through Current in 3 kV 4H-SiC Reverse-Blocking MOSFET by Using Highly Doped Drift Layer

Abstract: Low on-resistance 4H-SiC reverse-blocking (RB) metal–oxide–semiconductor field-effect transistors (MOSFETs) have been developed by adopting a non-punch-through (NPT) drift layer in order to suppress the punch-through (PT) current under the reverse-blocking condition. The n-type NPT drift layer was 40- ${\mu }\text{m}$ thick doped to $3.7\times 10^{15} $ cm−3. The forward and reverse breakdown voltages of the fabricated NPT RB MOSFET were 3.6 kV and −3.0 kV, respectively. The differential specific on-resistance was 13.5 $\text{m}\Omega \cdot $ cm2 at room temperature, which was 33% lower than that of a 3 kV PT RB MOSFET, demonstrating superiority of the developed NPT RB MOSFET as a high-performance bidirectional switch.

Conductance fluctuation in NiO-based resistive switching memory

Abstract: In nonvolatile resistive memory and neuromorphic computing, the formation and rupture of a conductive filament after the forming process causes a reversible resistance transition between low- and high-resistance states. We confirm herein that conductance fluctuations by sweeping the applied voltage before and after “semi-forming” appear in Pt/NiO/Pt resistive switching cells through an investigation of nonpolar resistive transitions after semi-forming and “second forming.” The increase in conductance owing to conductance fluctuations originates from the modification of oxygen-vacancy densities at grain boundaries in the NiO layer. Moreover, this modification may disappear at 470 K. The influence of a significant Joule heating caused by current through a fat filament created by second forming determines whether the cell conductance after the reset takes on the value of the conductance just after second forming or that of the initial conductance. Thus, Joule heating seems to be the driving force behind the reset in Pt/NiO/Pt cells.In nonvolatile resistive memory and neuromorphic computing, the formation and rupture of a conductive filament after the forming process causes a reversible resistance transition between low- and high-resistance states. We confirm herein that conductance fluctuations by sweeping the applied voltage before and after “semi-forming” appear in Pt/NiO/Pt resistive switching cells through an investigation of nonpolar resistive transitions after semi-forming and “second forming.” The increase in conductance owing to conductance fluctuations originates from the modification of oxygen-vacancy densities at grain boundaries in the NiO layer. Moreover, this modification may disappear at 470 K. The influence of a significant Joule heating caused by current through a fat filament created by second forming determines whether the cell conductance after the reset takes on the value of the conductance just after second forming or that of the initial conductance. Thus, Joule heating seems to be the driving force behind the ...

Determination of Surface Recombination Velocity From Current–Voltage Characteristics in SiC p-n Diodes

Abstract: Surface recombination velocity on mesa sidewalls of SiC p-n diodes with various surface passivation conditions was evaluated from the device-size-dependent preexponential factor of recombination current ( ${J}_{\text {0rec}}$ ) The diodes passivated by SiO2 with postoxidation nitridation were dipped into HF to eliminate a shunt current, which is evoked by the nitrided SiO2 layer and disturbs the analysis of the recombination current For accurate determination of the surface recombination velocity, an effective recombination zone width was precisely derived taking account of the distribution of carrier density in the depletion layer The surface recombination velocity of the diodes without any passivation and with the postoxidation nitridation (NO annealing at 1250 °C for 70 min) was determined as $\text {12}\times \text {10}^{\text {7}}$ and $\text {60}\times \text {10}^{\text {5}}$ cm/s, respectively, which indicates that the postoxidation nitridation can reduce the surface recombination by a factor of about 20 We confirmed that TCAD simulation could reproduce the current–voltage characteristics by utilizing the extracted parameters In addition, an evaluation method was proposed to determine the surface recombination velocity from the high-current region, where diffusion current is dominant

Effects of Parasitic Region in SiC Bipolar Junction Transistors on Forced Current Gain

Abstract: Effects of a parasitic region in SiC BJTs on conductivity modulation and a forced current gain (βF) were investigated by using TCAD simulation with various device structures. By introducing an Al+-implanted region below the base parasitic region, βF can be improved because the implanted region can reduce the base spreading resistance, leading to alleviation of debiasing effect. βF in devices with various parasitic areas, whose base spreading resistances were reduced by the Al+-implantation, were compared. We found that βF can be enhanced by expanding the parasitic area if the base spreading resistance is sufficiently reduced. The higher βF is attributed to an expanded conductivity-modulated region. The collector current spreading in the collector layer and the hole injection from the parasitic region as well as from the intrinsic region can play a role to evoke the conductivity modulation. Thus, the larger parasitic region can expand the conductivity-modulated region, resulting in expansion of an active area and the enhancement of βF.

Phonon-assisted optical absorption due to Franz–Keldysh effect in 4H-SiC p–n junction diode under high reverse bias voltage

Abstract: Photocurrent in a 4H-SiC p–n junction diode under illumination with sub-bandgap light was investigated. Under a high reverse bias condition, the photocurrent significantly increased with an increase in the reverse bias voltage. We calculated the photocurrent taking into consideration the phonon-assisted optical absorption due to the Franz–Keldysh effect. The calculated photocurrent showed good agreement with the experimental results. The photocurrent also increased at elevated temperatures, which could be quantitatively explained by the redshift of the 4H-SiC absorption edge (the shrinkage of the bandgap) and the increase in the phonon occupation number with rising temperature.

Glide velocities of Si-core partial dislocations for double-Shockley stacking fault expansion in heavily nitrogen-doped SiC during high-temperature annealing

Abstract: We investigated the glide velocities of 30° Si-core partial dislocations for the expansion of double-Shockley stacking faults (DSFs) in heavily nitrogen-doped 4H-SiC crystals at high temperatures of approximately 1000 °C. The heavily doped epilayers grown by chemical vapor deposition were successively annealed. The expansion of DSFs in the heavily doped epilayers was tracked by a photoluminescence (PL) imaging technique. From the PL images obtained after each annealing treatment, the glide velocities of the 30° Si-core partial dislocations were estimated. In particular, temperature dependence and nitrogen-concentration dependence of the dislocation velocities were obtained. We also report the influence of the strain energies of the bounding dislocations on the velocities. Based on the experimental results, the quantitative expression of the dislocation glide is discussed.

Effects of TiO2 crystallinity and oxygen composition on forming characteristics in Pt/TiO2/Pt resistive switching cells

Abstract: “Forming” is a stage in resistive switching (RS) devices that occurs before switching and represents an important physical phenomenon in the universal operating mechanism of such devices. Forming in a resistance change material appears to be a kind of dielectric breakdown. In this study, we performed time-dependent forming (TDF) characterization of Pt/TiO2/Pt resistive switching cells with TiO2 layers of different crystallinities and oxygen compositions. We prepared two samples in which the grain boundary density and the density of oxygen vacancies differ, while both the TiO2 layers exhibited the similar crystal structures. Our results reveal that the Weibull slope and variation of time to forming are determined by the deposition method of the Pt bottom electrode (BE) films. Moreover, the initial cell resistance and distribution of the TDF characteristics depend not only on the crystallinity but also on the oxygen composition of the TiO2 layers. The variation of time to forming increases as the distribution of initial resistance is reduced in Pt/NiO/Pt resistive switching cells with different NiO crystallinities. Conversely, the variation of time to forming decreases as the distribution of the initial resistance is reduced in the case of the Pt/TiO2/Pt cells. These results reflect differences in both the grain boundary density (crystallinity) and the density of oxygen vacancies (oxygen composition) of resistance change materials used in the resistive switching cells. The clear difference of crystallinities and oxygen compositions might originate from differences in the oxide deposition mode during reactive sputtering.

Phonon-assisted optical absorption due to the Franz-Keldysh effect in a 4H-SiC p-n junction diode under high reverse bias voltage

Abstract: Photocurrent in a 4H-SiC p–n junction diode under illumination with sub-bandgap light was investigated. Under a high reverse bias condition, the photocurrent significantly increased with an increase in the reverse bias voltage. We calculated the photocurrent taking into consideration the phonon-assisted optical absorption due to the Franz–Keldysh effect. The calculated photocurrent showed good agreement with the experimental results. The photocurrent also increased at elevated temperatures, which could be quantitatively explained by the redshift of the 4H-SiC absorption edge (the shrinkage of the bandgap) and the increase in the phonon occupation number with rising temperature.

Impacts of Finger Numbers on ON-State Characteristics in Multifinger SiC BJTs With Low Base Spreading Resistance

Abstract: Impacts of finger numbers on ON-state characteristics in multifinger 10-kV-class SiC bipolar junction transistors (BJTs), whose base spreading resistance is sufficiently reduced by using aluminum ion implantation, were investigated by performing TCAD simulations. Common-emitter current–voltage characteristics of the BJTs with different base current densities and carrier lifetimes in the collector layer were analyzed. The simulation results exhibited that the BJTs with fewer finger numbers could achieve superior characteristics owing to an expansion of a conductivity-modulated region and to a higher current density per finger. In addition, we showed that BJTs with a punchthrough structure have a potential to achieve superior characteristics suitable for power device applications under a certain condition, where strong conductivity modulation occurs. The presented results indicate that the appropriate finger numbers should be designed for a better performance of the multifinger SiC BJTs.

A comparative study on electrical characteristics of 1-kV pnp and npn SiC bipolar junction transistors

Abstract: We investigate the electrical characteristics of 1-kV pnp SiC bipolar junction transistors (BJTs) and compare them with those of npn SiC BJTs. The base resistance, current gain, and blocking capability are characterized. It is found that the base resistance of pnp SiC BJTs is two orders of magnitude lower than that of npn SiC BJTs. However, the obtained current gains are low below unity in pnp SiC BJTs, whereas npn SiC BJTs exhibit a current gain of 14 without surface passivation. The reason for the poor current gain of pnp SiC BJTs is discussed.

Impacts of energy relaxation process on quasi-ballistic hole transport capability in germanium and silicon nanowires

Abstract: The quasi-ballistic hole transport in germanium and silicon nanowires was theoretically investigated by solving the Boltzmann transport equation taking account of phonon scattering in an atomistic framework. Comparison of quasi-ballistic hole transport capabilities between germanium and silicon nanowires showed that the transmission coefficients for the two materials are similar. Then, the behavior of forward and backward current fluxes was analyzed focusing on the impact of energy relaxation process. The slower energy relaxation of holes in germanium nanowires leads to a longer distance where backscattering enables holes to return to the source. This cancels the benefit of the longer mean free path of holes in germanium nanowires, resulting in similar transmission coefficients in germanium and silicon nanowires.

Characterization of carrier concentration and mobility of GaN bulk substrates by Raman scattering and infrared reflectance spectroscopies

Abstract: n-Type GaN bulk substrates with carrier concentrations (n) of 1016–1020 cm−3 were characterized by Raman scattering (350–5000 cm−1) and infrared reflectance (400–5000 cm−1) spectroscopies. Experimental spectra were fitted with the curves calculated from the dielectric function and carrier concentration and mobility of the GaN bulk. The obtained values agree well with the values from Hall-effect measurements for n of 1016–1019 cm−3 in Raman scattering measurements and for n of 1018–1020 cm−3 in infrared reflectance measurements.

Microscopic mechanism of carbon annihilation upon SiC oxidation due to phosphorus treatment: Density functional calculations combined with ion mass spectrometry

Abstract: We report first-principles static and dynamic calculations that describe the microscopic mechanism of carbon annihilation due to phosphorus treatment upon silicon carbide oxidation. We identify the most stable form of phosphorus in the oxide as being fourfold coordinated with the dangling PO unit and find that the unit attracts carbon ejected from the interface, thus operating as a carbon absorber. This finding provides a possible microscopic reasoning for the annihilation of C-related defects at the interface. The secondary ion mass spectrometry and electrical characterization of interface state density are also performed to corroborate the theoretical finding above.