Papers by Author: Sadafumi Yoshida

Abstract: The temperature dependencies of the resistivity and Hall coefficient for heavily Al-doped 4H-SiC epilayers with Al concentration (C_Al) higher than 2×10¹⁹ cm^-3 were investigated. The signs of measured Hall coefficients (R_H) change from positive to negative at low temperatures. For the epilayers with C_Al < 3×10¹⁹ cm^-3 the sign inversion occurred in the hopping conduction region, which was reported to be explicable using the model for amorphous semiconductors. For the epilayers with C_Al > 3×10¹⁹ cm^-3, on the other hand, the sign inversion occurred in the band conduction region, which is a striking feature, because the movement of free holes in the valence band should make R_H positive. The sign-inversion temperature increased with increasing C_Al, while the dominant-conduction-mechanism-change temperature was almost independent of C_Al.

Abstract: By inspecting the CVD growth parameters, such as the flow rates of HCl and H₂ carrier gases, the pressure and the C/Si ratio, the trench filling in a high-rate mode with a high growth rate on the bottom and a relatively low growth rate on the mesa top was carried out. 4H-SiC trenches with the depths of 48 and 55 μm have been completely filled at the rates of 6.2 and 5.5 μm/h, respectively.

Abstract: By mapping the source and HCl flow rates dependent growth rates, the evolving trend of a quasi-selective epitaxial growth (quasi-SEG) that growing very thin epilayer on mesa top and ensuring an extremely low risk of voids defect generation was firstly figured out on a 5-μm 4H-SiC trench. Then, basing on the acquired knowledge, a 25-μm 4H-SiC trench with an aspect ratio up to ~10 was completely filled in the quasi-SEG mode.

Abstract: The conduction mechanism in heavily Al-doped or heavily Al-and N-codoped p-type 4H-SiC epilayers was investigated. In both the singly-doped and codoped samples with an Al concentration (C_Al) between 4x10¹⁹ and 2x10²⁰ cm^-3, band and nearest-neighbor hopping (NNH) conductions appeared in high and low temperature ranges, respectively. The codoping of N donors makes the NNH conduction dominant at temperatures higher than in the singly-doped samples. In both the singly-doped and codoped samples with C_Al between 1x10¹⁹ and 4x10¹⁹ cm^-3, an unexpected conduction appeared between the regions of the band and NNH conductions.

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: We have developed the computer simulation including cluster effect and Schwoebel effect and investigated the conditions generating GSB using the simulation. We have demonstrated that the simulation developed can reproduce GSB. We have found for the occurence of GSB that there exists a threshold value of the surplus flux rate of Si-or C-source gases not contributing to growth, which depends on the flux rate of each source gas, namely the boundary between with and without GSB. It is noted that this boundary does not depend on the off-angle of substrates. We have also found the mechanism for explaining the occurrence of wavy surface morphplogy.

Abstract: Trapezoid-shape (T-S) defects on epilayer surfaces, which include two kinds of the giant step bunching (GSB), are one of killer defects for MOSFETs. We have investigated the generation mechanism of the two GSBs using "step kinetics simulator" we developed. The simulator has reproduced the behavior of the GSBs. Based on results from the simulation, we have discussed the generation mechanism of the two GSBs.

Abstract: The defect evolution on 90 μm-thick heavily Al-doped 4H-SiC epilayers with Al doping level higher than 10²⁰ cm^-3 was studied by tracing back to initial growth stage to monitor major dislocations and their propagations in each growth stage. Results from X-ray topography and KOH etching demonstrate that all existing dislocations on the surface of 90 μm-thick epilayer can be identified as the defects originating from substrate. In other words, there seems no new dislocation generated after a long-term growth. Nevertheless, a high density of misfit dislocation was found appearing near the substrate/epilayer interface for epilayer with Al doping level of 3.5×10²⁰ cm^-3, while misfit dislocation cannot be seen on epilayer with Al doping level of 1.5×10²⁰ cm^-3.

Abstract: By using hot-wall CVD method, thick heavily Al-doped 4H-SiC epilayers (~90 μm) were grown on 3-inches 4H-SiC wafers. Around the solubility limit, the incorporation behaviors of Al into 4H-SiC were investigated by varying the growth conditions. Among the samples having smooth surfaces, the maximum Al dopants concentration of 3.5×10²⁰ cm^-3 and the minimum resistivity of 16.5 mΩcm were achieved. The results of Hall-effect measurement demonstrate that, along with the increase of Al doping level, the activation ratio of Al dopants gradually increases from several percent up to 100% where the Al dopants concentration is 1.5×10²⁰ cm^-3.

Abstract: We found that the ‘Si and C emission model’ that we proposed as an oxidation model of SiC could not reproduce the initial oxide growth rates of SiC at sub-atmospheric pressures. The comparison between calculated and observed growth rates suggests that the oxide growth on the oxide surface is enhanced in the initial oxidation stage and thus our oxidation model is inaccurate in the description on the surface oxidation. Accordingly, we reconsidered the parameters on surface oxidation and, as a result, found that a much enlarged oxygen concentration on the oxide surface is necessary for solving the discrepancy between calculated and observed growth rates.