No abstract

In this study, 4H-SiC stripe-shaped trenches preformed on an n + substrate were filled by adding HCl to the chemical vapor deposition process at relatively high pressures. HCl was found capable of counterbalancing the deposition on the mesa top by strong etching, and it thus enabled quasiselective epitaxial growth across the whole extents of the trenches, where the epilayer preferentially grows from the trench bottom. Using the established technique, the 1-µm-wide 4H-SiC trenches, with an aspect ratio of 5, which is the highest aspect ratio to date, were completely filled at a growth rate above 0.5 µm/h and acquired a flat end surface.

A trench filling epitaxial growth technique using hot-wall chemical vapor deposition with HCl gas has been developed for SiC super-junction (SJ) device fabrication. 2-6 kV class SJ devices require p/n column structures with depths of over 10 µm. However, rapid trench closure before the trench backfilling process is complete makes these structures difficult to realize. Stripe trenches that were intentionally inclined within +2°on a surface plane towards the ½11 20 direction were formed on an off-angled wafer, and the effects of trench direction misalignment from the offdirection were investigated. Slight trench direction misalignment was found to affect the tilt angle of the mesa top epi-layer strongly. Tilted growth on the mesa top reduced the filling rate at the trench bottom and caused void formation. When a wafer with high orientation-flat accuracy relative to the ½11 20 direction was used, 25-µm-deep trench backfilling was successfully demonstrated.

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 (CAl) 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 CAl between 1x1019 and 4x1019 cm-3, an unexpected conduction appeared between the regions of the band and NNH conductions.

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.