Papers by Author: Kazutoshi Kojima

Abstract: Trench-filling epitaxial growth of 4H-SiC was analyzed based on a simulation model for continuous fluid approximation including the Gibbs-Thomson effect. With the use of the radii of curvature at the top and bottom of the trenches, the proposed model well reproduced the measured dependence of the growth rate on the trench pitch (L) in the case of narrow (L ≤ 6.0 μm) trenches.

Abstract: The surface quality of epitaxial layers grown on 2° offcut substrates was improved. These substrates require a lower growth temperature and a lower C/Si ratio than their 4° offcut counterparts to suppress macro step bunching. Surface morphology, triangular defect density, and doping uniformity presented a trade-off relationship with respect to growth parameters. The implementation of a low C/Si ratio buffer layer led to a balance between surface defect density, which reached a minimum of 0.2 cm⁻², and good doping uniformity on an equivalent wafer size (150 mm). An evaluation of metal–oxide–semiconductor capacitors and Schottky barrier diodes fabricated on 2° offcut epitaxial layers showed that the quality of these epitaxial layers was satisfactory for application in devices.

Abstract: The new indicators, effective gate oxide thickness t_c and effective gate electrode area D, and their combination are applied for a new analysis method of Fowler-Nordheim (F-N) tunneling characteristics in MOS capacitor having oxide thickness fluctuation. This method considering the conduction properties of F-N tunneling characteristics correlates its characteristics to the oxide reliability. These indicators quantified with the influence of the oxide thickness fluctuation can provide the net values of the electric field and the current density on the gate oxide. This new analysis method will lead to reducing the evaluation time for the reliability assessment.

Abstract: The effect of H₂ carrier gas on the growth rate during the trench filling using CVD epitaxial growth was investigated in a wide pressure range (10∼38 kPa). It is found that, in the entire pressure range, reducing H₂ flow rate can increase the filling rate (the growth rate inside trench) and the filling efficiency (the thickness ratio between epilayer on trench bottom and mesa top), which means a high productivity and a low risk of void defects. The filling rate and efficiency of ∼1.5 μm/h and ∼18 respectively was achieved at 38 kPa.

Abstract: In this study, 4H–SiC inversion layers were experimentally evaluated by Hall and split C–V measurements, and scattering mechanisms related to gate oxide nitridation were analyzed. Three typical samples with different crystal plane directions and gate oxidation conditions were prepared, and their total trap density and Hall mobility were compared. Based on the temperature dependence of the Hall mobility, we found that scattering mechanisms differed for each sample. The sample C-face oxynitride which had a high nitrogen density at the metal–oxide–semiconductor (MOS) interface, showed a similar temperature dependency to that of ionized impurity scattering. This result suggests that high-density nitrogen acts as donors that supply free carriers and cause ionized impurity scattering, just like in a bulk crystal. In addition, the sample C-face wet has lowest influence of the Coulomb scattering because of the lowest temperature dependence of Hall mobility and the lowest total trap density.

Abstract: Surface defects with scratch-like appearances are often observed locally on 4H-SiC wafers after epitaxial growth. We evaluated such damaged layer which is the cause of local step bunching using Mirror Projection Microscope (MPJ). As a result, MPJ can be detected l damaged layer which could not be detected using Synchrotron X-ray topography, even if these defects are extremely flat surface, no morphology, damaged layer is used to exist on the subsurface region. Thus, MPJ can be detected dislocation loops on the subsurface region of damage, it is effective to elucidate damaged layer of polishing process, MPJ is to be one of the candidates for inspection techniques of the damaged layer at substrate surface.

Abstract: Thick multi-layer 4H-SiC epitaxial growth was investigated for very high-voltage Si-face p-channel insulated gate bipolar transistors (p-IGBTs). The multi-layer included n⁺ buffer, p⁺ field stop, and thick p^- drift layers. Two processes were employed to enhance the carrier lifetime of the p^- drift layer: carbon ion implantation/annealing and hydrogen annealing, and the enhanced carrier lifetime was confirmed by the open-circuit voltage decay measurement. Using the grown thick multi-layer 4H-SiC, simple pin diodes were fabricated instead of p-IGBTs to demonstrate efficient conductivity modulation in the thick p^- drift layer. While the on-state voltage was high at room temperature, it decreased significantly at elevated temperatures, and attained 3.5 V at 100 A/cm² at 200°C for the diode with the carrier lifetime enhancement processes, indicating sufficient conductivity modulation.

Abstract: Homoepitaxial layers with different growth pit density were grown on 4H-SiC Si-face substrates by changing C/Si ratio, and the influence of the growth pit density on Schottky barrier diodes and metal-oxide-semiconductor capacitors were investigated. Even though there were many growth pits on the epi-layer, growth pit density did not affect the leakage current of Schottky barrier diodes and lifetime of constant current time dependent dielectric breakdown. By analyzing the growth pit shape, the aspect ratio of the growth pit was considered to be the key factor to the leakage current of the Schottky barrier diodes and the lifetime of metal-oxide-semiconductor capacitors.

Abstract: The critical electric field (E_cr) of the gate oxide in 4H-Silicon Carbide (SiC) MOSFETs was measured under inversion bias conditions with ion irradiation. The Linear Energy Transfer (LET) dependence of the E_cr at which the gate oxide breakdown occurred in these MOSFETs was evaluated. The linear relationship between the E_cr^-1 and LET was observed for SiC MOSFETs. The slope of the LET-1/E_cr for SiC MOSFETs is almost the same that of the LET-1/E_cr lines for SiC MOS capacitors. The V_ds dependence of E_cr was also evaluated. The correlation between the direction of electric field of drain-source region and direction of ion incidence affects to instability of E_cr.

Abstract: On 4H-SiC Si-face substrates after H₂ etching, the defect with “line” feature parallel to a step as “bunched-step line” was observed. Using X-ray topography and KOH etching, we confirmed that the bunched-step line originated from basal plane dislocation (BPD). Use of the substrate with the lowest BPD density will be effective to reduce bunched-step line that would affect oxide layer reliability on an epitaxial layer. However, more detail investigation needs to classify the BPD that would become a starting point of bunched-step line.