Investigation of the SiO2-SiC Interface Using Low-Energy Muon-Spin-Rotation Spectroscopy

Using positive muons as local probes implanted at low energy enables gathering information about the material of interest with nanometer-depth resolution (low-energy muon-spin-rotation spectroscopy, LE mu SR). In this work, we leverage the capabilities of LE mu SR to perform a detailed investigation of the widely studied yet poorly understood SiO2-SiC interface. Thermally oxidized samples are investigated before and after annealing in nitric oxide (NO) and argon (Ar) environment. Thermal oxidation is found to result in structural changes both in the SiC crystal close to the interface and at the interface itself, which severely degrade the transport properties of charge carriers. Annealing in NO environment is known to passivate the defects leading to a reduction of the density of interface traps (Dit); LE mu SR further reveals that the NO annealing results in a thin layer of high carrier concentration in SiC, extending to more than 50 nm depending on the annealing conditions. From our measurements, we see indications of Si vacancy (VSi) formation in SiC after thermal oxidation. Following NO annealing, nitrogen occupies the VSi sites, leading to the well-documented reduction in Dit and, at the same time, creating a charge-carrier-rich region near the interface. The LE mu SR technique sheds light on the near-interface region in the SiO2-SiC system, which is challenging to access using other techniques. By comparing the LE mu SR data from a sample with known doping density, we perform a high-resolution quantification of the free carrier concentration near the interface after NO annealing and discuss the origin of the observed near-surface variations. Finally, the depletion of carriers in a MOS capacitor in the region exactly below the interface is demonstrated using LE mu SR. The NO-annealed sample shows the narrowest depletion region, likely due to the reduced density of interface traps and the charge-carrier-rich region near the interface. Our findings demonstrate the many benefits of LE mu SR to study critical regions of semiconductor devices that have been inaccessible with other techniques, while simultaneously retaining both nanoscale-depth resolution and a nondestructive approach.