Papers by Author: Kazutoshi Kojima

Abstract: In this study, 4H-SiC bonded substrates (bonded-SiC) with an average resistivity of 2.4–31.5 mΩ·cm were prepared, and attention has been directed toward the relationship between the resistivity of bonded-SiC and the contact resistance at the backside where metal Ti/Ni was applied. A circular transmission line model (cTLM) was used to accurately measure the backside contact resistance. A linear correlation was found between and the resistivity of bonded-SiCs at room temperature (RT). This result indicates the existence of a threshold resistivity at which the specific contact resistance in the range of 2.2 × 10⁻⁶ to 1.5 × 10⁻⁵ Ω·cm² can be achieved without contact annealing; it also indicates that the temperature dependence of between 17.4 and 34.4 mΩ·cm is eliminated. This phenomenon can occur because is dominated by tunneling current above the nitrogen concentration at the threshold resistivity, which is driven by the high nitrogen concentration and sufficient carrier activation in the polycrystalline portion (polycrystalline layer) of bonded-SiCs. These are important properties resulting from a polycrystalline layer with a 3C structure in bonded-SiC.

Abstract: A unique hybrid structure of the 4H-SiC bonded substrate offers advantages not achievable with conventional 4H-SiC bulk substrates, such as a reduction in on-state resistance and the suppression of forward bias degradation in power devices. This study focuses on the contact resistance between the polycrystalline layer and the backside metal (Ni/Ti) of 4H-SiC bonded substrates, along with its temperature dependence. The results indicate that the bonded substrates exhibit low backside specific contact resistance (SCR) , even without annealing, and this resistance remains stable at elevated temperatures. Furthermore, power devices utilizing bonded substrates demonstrated reduced on-state resistance, as evaluated using Schottky barrier diodes (SBDs). Specifically, 4H-SiC bonded substrates without contact annealing lowered the forward voltage by 13.4% at room temperature (RT) compared to 4H-SiC bulk substrates with contact annealing. These findings suggest that 4H-SiC bonded substrates simplify the backside contact process compared to 4H-SiC bulk substrates, offering significant benefits in reducing on-state resistance in SiC power devices.

Abstract: In this study, we investigated the generation of trap centers through hydrogen implantation to understand its role in the suppression of forward bias degradation in 4H-silicon carbide (4H-SiC) bonded substrates. During the production of bonded substrates, hydrogen implantation is used for layer splitting. Transmission electron microscopy (TEM) observations revealed that the basal plane dislocation (BPD) in the bonded substrate did not extend into the Shockley-type stacking fault (SSF) and remained stable in the transferred layer below the epitaxial interface even under high forward current stress. Additionally, carrier lifetime, measured using microwave photoconductivity decay (μ-PCD), was considerably reduced by hydrogen implantation. Annealing at 1700°C reduced the implanted hydrogen to levels below the detection limit of secondary ion mass spectrometry (SIMS), yet the carrier lifetime remained short. Deep level transient spectroscopy (DLTS) revealed that, after annealing at 1700°C following hydrogen implantation, the concentration of the Z_1/2 center increased by more than two orders of magnitude compared to pre-implantation levels. Trap centers, including the Z_1/2 center, are believed to help prevent forward bias degradation in the bonded substrates by inhibiting the expansion of SSFs in the transferred layer.

Abstract: A novel substrate of 4H-SiC bonded substrate is expected to solve issues such as decreasing the on-resistance, which has attracted much attention. Therefore, several studies have been conducted on the use of bonded substrates. In this study, we fabricated a DMOSFET on a bonded substrate and compared its static and dynamic characteristics with those on a single-crystal substrate. Consequently, the on-resistance of the DMOSFET fabricated on a bonded substrate was lower than that on a single-crystal substrate owing to the low resistivity of the polycrystalline substrate. Also, reverse recovery loss of the DMOSFET fabricated on a bonded substrate was lower than that on single-crystal substrate at high temperature due to low carrier lifetime in a drift layer. Additionally, we observed that the DMOSFET fabricated on a bonded substrate did not generate bipolar degradation despite the application of a forward-current stress of over 1500 A cm^-2. According to these results, we expected that the carrier lifetime in both drift layer and transfer layer was decreased on a bonded substrate.

Abstract: Analysis of forward bias degradation reduction of 4H-Silicon Carbide (4H-SiC) PiN diodes on bonded substrates was performed. In the analysis, cathodoluminescence (CL), photoluminescence imaging (PL imaging), and transmission electron microscope (TEM) were used. Under high forward bias stress, the Shockley-type stacking fault (SSF) does not expand into the transferred layer of the bonded substrate, while in the monocrystalline substrate, the SSF expands below the epilayer/substrate interface. The basal plane dislocation (BPD) within the transferred layer does not expand to the SSF. The transferred layer has the effect of suppressing the expansion of SSFs. This effect can be caused by hydrogen implantation for wafer splitting to produce bonded SiC substrates.

Abstract: The advantage in reducing forward bias degradation of bipolar 4H-SiC devices using a 4H-SiC bonded substrate is demonstrated. To evaluate the differences in forward bias degradation between a 4H-SiC bonded substrate and a commercially available 4H-SiC bulk substrate, a forward current stress test and subsequent photoluminescence (PL) imaging of PiN diodes fabricated on both the substrates were performed. Unlike the bulk substrate, the bonded substrate maintained a low ΔVf and the variation among the measured diodes was extremely small even after applying the highest current density of 1500 A/cm². The investigated number of bar-shaped SSFs within the electrically stressed diodes with more than 1000 A/cm² revealed the possibility that the BPDs existing at deep positions below the epilayer/substrate interface were drastically reduced in the 4H-SiC bonded substrate.

Abstract: We evaluated stacking faults expanding by body diode current stress in the SiC Semi-SJ MOSFET for the first time. It was found that body diode degradation of the SJ MOSFETs tends to be smaller than that of conventional Non-SJ MOSFETs. Detailed crystal evaluations revealed that the stacking faults did not expand into the SJ structure. It is assumed that the expansion stops due to low carrier densities. The result suggests that the SJ device has a high potential as a device for suppressing the body diode degradation.

Abstract: We demonstrated optically detected magnetic resonance (ODMR) measurements using three-dimensional (3D) arrayed silicon vacancies (V_Sis) in in-plane SiC pn diodes. Proton beam writing successfully created 3D arrayed V_Sis using different ion (proton) energies. The results of PL mapping analysis indicate that the features of luminescent spot such as size and depth can be estimated by a Monte Carlo simulation (SRIM). This suggests that diagnosis at any locations in SiC devices can be realized using V_Si quantum sensors. Luminescent spots with different depth ranging 4-60 μm showed similar ODMR spectra including its contrast, which means that a similar sensor sensitivity is expected. The results suggest that 3D arrayed V_Si can act as quantum sensor elements with uniform sensitivity in SiC devices.

Abstract: Spin defects of which states can be manipulated in Silicon Carbide (SiC) have drawn considerable attention because of their applications to quantum technologies. The single negatively-charged pairs of V_Si and nitrogen atom (N) on an adjacent C site (N_CV_Si^- center) in SiC is suitable for them. This paper reports the formation of N_CV_Si^- centers on 4H-SiC epilayers with different nitrogen concentrations using light/heavy ion irradiation and subsequent thermal annealing. The formation of N_CV_Si^- centers is characterized by the near infrared photoluminescence (PL) spectroscopy. It is shown that the PL intensity from N_CV_Si^- centers depends on the N concentration and the ion irradiation conditions. The PL intensity increases monotonically with increasing the N concentration when the N concentration is above 2.6×10¹⁶ cm^-3, whereas no linear correlation between them does not appear below that N concentration. Although the PL intensity increases with increasing defects induced by ion irradiation, the PL quenching due to neighboring residual defects appear at above the areal vacancy concentration of 10¹⁷ vac/cm² and the broad Raman scattering spectra originated from vibration modes of amorphized regions hinder the PL from N_CV_Si^- centers at above 10¹⁸ vac/cm². The formation mechanism and the charge state stability of N_CV_Si^- centers are discussed based on the obtained results.

Abstract: We measure the temperature-dependent resistivity (ρ(T)) for thick heavily Al- and Ncodoped p-type 4H-SiC samples grown by chemical vapor deposition (CVD), physical vapor transport (PVT), and solution growth (SG), and investigate their conduction mechanisms. For samples with an Al concentration (CAl) of 3.5×10¹⁹ to 1×10²⁰ cm^-3 grown by CVD, PVT, and SG, the conduction mechanisms at high and low temperatures are band and nearest-neighbor hopping (NNH) conduction, respectively. In the range C_Al of 1×10¹⁹ to 3.5×10¹⁹ cm^-3, on the other hand, an anomalous conduction, referred to as X conduction here, is observed between the band and NNH conduction regions for the samples grown by CVD and PVT, but not those grown by SG. One of the differences between the samples grown by CVD and PVT and those grown by SG is the off-orientation toward [11-20] of the (0001) 4H-SiC substrate. We discuss the reason for the appearance of X conduction, which appears to be consistent with dopant-concentration inhomogeneity model.