Materials Science Forum Vols. 740-742

Abstract: This work reports on the morphological, structural and electrical effects of a nitrous oxide (N₂O) ambient post-oxidation annealing (POA) of the SiO₂/4H-SiC interface. In particular, a conventional electrical characterization of MOS capacitors showed that nitrous oxide POA reduces the presence of both fixed oxide charge and the density of interface states. A local atomically flat interface was observed by transmission electron microscopy with only a moderate step bunching observed at a macroscopic scale. A novel nanoscale characterization approach via scanning spreading resistance microscopy resolved local electrical changes induced at the SiC surface exposed to N₂O POA. This result subsequently revealed additional insight into the mechanism for the improved device performance subjected to N₂O POA treatment.

Abstract: This work reports on the morphological, structural and electrical effects of a nitrous oxide (N₂O) ambient post-oxidation annealing (POA) of the SiO₂/4H-SiC interface. In particular, a conventional electrical characterization of MOS capacitors showed that nitrous oxide POA reduces the presence of both fixed oxide charge and the density of interface states. A local atomically flat interface was observed by transmission electron microscopy with only a moderate step bunching observed at a macroscopic scale. A novel nanoscale characterization approach via scanning spreading resistance microscopy resolved local electrical changes induced at the SiC surface exposed to N₂O POA. This result subsequently revealed additional insight into the mechanism for the improved device performance subjected to N₂O POA treatment.

Abstract: We investigated the effects of the post-oxidation annealing (POA) atmosphere on the electrical properties and interfacial roughness of SiO₂ deposited on a 4H-SiC (0001) face and SiC. POA in ammonia (NH₃) gave MOS capacitors with a lower interface trap density and n-channel MOSFETs with higher field-effect mobility than POA in nitrous oxide (N₂O) or nitrogen (N₂). In contrast, POA in N₂O gave a lower interface trap density than POA in N₂, but it gave the lowest field-effect mobility of all the samples. Cross-sectional TEM observations revealed that N₂O POA gave a higher interfacial roughness than NH₃ POA. We thus considered that N₂O POA degraded the inversion-layer mobility due to increased roughness scattering.

Abstract: Effects of combination of NO and POCl₃ annealing on electrical properties and their stability of 4H-SiC MOS capacitors and MOSFETs were investigated. Channel mobility of MOSFETs processed with both NO and POCl₃ annealing did not exceed that of POCl₃ annealed MOSFETs. As for the stability of flat-band voltage and threshold voltage using a constant field stress test, the combined annealed sample indicated very stable characteristics compared with single annealed devices with NO or POCl₃. The reason for obtaining stable electrical properties is discussed based on nitridation and phosphorization effects at the interface.

Abstract: In this article, an influence of nitrogen implantation dosage on SiC MOS structure is analyzed using wide range of nitrogen implantation dose (between ~1013 – 1016). Authors analyzed electrical and material properties of investigated samples using C-V, I-V measurements, Raman spectroscopy, and XPS profiling. It has been shown that surface state trap density is directly connected to implantation damage and thus implantation conditions. Using research results a trap origin at given energy can be concluded.

Abstract: In the present study, applying Al₂O₃ capping layer as suppressing layer of oxygen diffusion to SiC-MOS structures, we investigated the effect of Al₂O₃ layer as suppression of reoxidation at SiO₂/SiC interface and decrease of Dit. We evaluated MOS capacitors which have SiO₂/SiC and Al₂O₃/SiO₂/SiC structure on 4H-SiC (0001) Si-face epitaxial wafers after post-oxidation anneal process (N₂O:N₂=1:9 [slm]) at temperatures ranging from 1000 °C to 1300 °C for 30min. The Al₂O₃/SiO₂/SiC structure of reoxidation thickness, surface roughness and Dit are smaller than those of SiO₂/SiC structure. These results show that the suppression of reoxidation during POA is important to improve the SiO₂/SiC interfacial qualities.

Abstract: We report on the harmful impact of ultraviolet (UV) light irradiation on thermally grown SiO2/4H-SiC(0001) structures and its use in subsequent thermal annealing for improving electrical properties of SiC-MOS devices. As we previously reported [1], significant UV-induced damage, such as positive flatband voltage shift and hysteresis in capacitance-voltage curves as well as increased interface state density, was observed for SiC-MOS devices with thermally grown oxides. Interestingly, the subsequent annealing of damaged SiO2/SiC samples resulted in superior electrical properties to those for untreated (fresh) devices. These findings imply that UV irradiation of the SiO2/SiC structure is effective for eliciting pre-existing carbon-related defects and transforming them into a simple configuration that can be easily passivated by thermal treatment.

Abstract: Time-dependent dielectric breakdown (TDDB) measurement of MOS capacitors on an n-type 4 ° off-axis 4H-SiC(0001) wafer free from step-bunching showed specific breakdown in the Weibull distribution plots. By observing the as-grown SiC-epi wafer surface, two kinds of epitaxial surface defect, Trapezoid-shape and Bar-shape defects, were confirmed with confocal microscope. Charge to breakdown (Qbd) of MOS capacitors including an upstream line of these defects is almost the same value as that of a Wear-out breakdown region. On the other hand, the gate oxide breakdown of MOS capacitors occurred at a downstream line. It has revealed that specific part of these defects causes degradation of oxide reliability. Cross-sectional TEM images of MOS structure show that gate oxide thickness of MOS capacitor is non-uniform on the downstream line. Moreover, AFM observation of as-grown and oxidized SiC-epitaxial surfaces indicated that surface roughness of downstream line becomes 3-4 times larger than the as-grown one by oxidation process.

Abstract: We investigate the strong passivation of shallow interface traps located near the SiC conduction band after enhanced oxidation of Si-face 4H-SiC in the presence of sodium. We find that removing the sodium ions present at the SiO2/SiC interface since oxidation by way of bias stress or annealing does not lead to a significant increase in the density of interface traps. The presence of sodium ions at the SiO2/SiC interface is therefore not responsible for the passivation of such interface traps in oxides formed by sodium enhanced oxidation.

Abstract: The effect of the n-type 4H-SiC (0001) oxidation in wet O2 at temperature of 1175 °C followed by low temperature annealing in N2O at temperature of 800°C for 2 or 4 hours followed by high temperature annealing in nitrogen ambient on nitrogen distribution in silicon dioxide was investigated. It was shown that the oxidation and annealing have a strong impact on the behavior of electrical parameters of MOS capacitors using the oxides as gate dielectric what is probably an effect of nitrogen incorporation. The explanation of the observed electrical properties is included.