Papers by Keyword: MOS

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: Video quality assessment can be gotten by combination the distortion in the space area and the time area. As we all known that edge information is one of the most important features in image quality estimation. Based on the edge model in the perceived image quality estimation, we used it in the space and time area in the video, and get the edge information distorted model in space area and time area of the video. Using multilinear regression to combine the two models, we can get the video quality assessment model based on edge information. The proposed model only uses the edge information, and the consumption in both areas is small. After compared with other methods given by video quality estimation group, it’s found that our method is convenient and good at the 50 Hz video sequence of low bit rate(768kb/s -4.5 Mb/s).

Abstract: The causes of extrinsic failures in time-dependent dielectric breakdown characteristics of gate oxide on C-face of 4H-SiC are examined by comparing breakdown points of tested gate oxides with the images of X-ray topography and those of differential interference contrast microscopy. We have concluded as follows: (1) surface morphological defects that originate from threading screw dislocations degrade reliability of gate oxides. (2) These surface defects are not necessarily found on every wafer. (3) Crystallographic defects are not killer defects of MOSFET per se.

Abstract: We have evaluated the reliability of POCl3-annealed oxides on 4H-SiC using time-zero dielectric breakdown (TZDB), constant current time-dependent dielectric breakdown (CC-TDDB), and high-frequency capacitance-voltage (C-V) measurements after electron injection. The POCl3 annealing does not deteriorate oxide breakdown field very much, still keeping an average value of larger than 9 MV/cm. However, the electron injection into POCl3-annealed oxide brings negative charges easily. From the C-V measurements, the POCl3-annealed capacitors were found to indicate a large positive flatband voltage shift after electron injection. Phosphorus atoms in the oxide may be related to the trapping site of injected electrons. The distribution and density of phosphorus in the oxide should be optimized to realize highly reliable 4H-SiC MOSFETs with high performance.

Abstract: A nitride layer was formed on a SiC surface by direct nitridation in pure N2 or in NH3 diluted with N2. The SiO2 layer was deposited by the thermal decomposition of tetraethylorthosilicate (TEOS) on the nitride layer to form an MIS diode. The XPS analysis showed that the nitride layer was oxidized during the deposition process of SiO2. The direct nitridation was effective to reduce the interface state density between the insulating layer and 4H-SiC

Abstract: Since power devices such as DMOSFETs will operate at high temperature where mobile ion effects are enhanced, identifying their presence is a key reliability issue for power electronics applications. We have detected the presence of mobile ion contamination in some SiC MOS device sample sets and correlated those results with observed high temperature bias instability. The differing behaviors of these devices to bias stressing as a function of temperature suggests that in some cases mobile ion drift may be counteracting the typical charge trapping effect. Triangular voltage sweep (TVS) data indicates the presence of roughly 1-4x10¹² cm^-2 mobile ions in samples where the bias instability significantly decreased with higher temperature, while samples with a nearly flat or positive-trending response showed overall lower ion contaminations of roughly 6-9x10¹¹ cm^-2. These results, although preliminary, support the theory that mobile ion contamination is the cause of negative bias instability at elevated temperatures in SiC MOS.

Abstract: SiC based capacitive devices have the potential to operate in high temperature, chemically corrosive environments provided that the electrical integrity of the gate oxide and metallization can be maintained in these environments. We report on the performance of large area, up to 8 x 10^-3 cm², field-effect capacitive sensors fabricated on both the 4H and 6H polytypes at 600°C. Large area capacitors improve the signal/noise (S/N) ratio which is proportional to the slope of the capacitance-voltage characteristic. At 600 °C we obtain a S/N ~ 20. The device response is independent of polytype, either 4H or 6H-SiC. These results demonstrate the reliability of our field-effect structure, operating as a simple potentiometer at high temperature.

Abstract: The impact of device concepts of Si insulated gate bipolar transistors (IGBTs) such as injection-enhanced IGBT (IEGT), high-conductivity IGBT (HiGT), and Si-limit IGBT on the performance of SiC IGBTs is examined. We first show that the forward characteristics of the original type of planer SiC IGBTs are much worse than those of SiC PiN diodes, even if the carrier lifetime is improved. Next, we show that the forward characteristics of SiC IEGTs and SiC HiGTs are comparable to those of SiC PiN diodes. Thus, device concepts of Si IGBTs are effective in improving the device performance of SiC IGBTs. Finally, it is shown that a SiC-limit IGBT can be realized when the mesa width is less than 0.5 μm.

Abstract: The reliability of gate oxides is a fundamental issue for realizing SiC MOSFETs. Many reports said that crystal defects shorten the lifetime of the gate oxide. And, epi defects, the basal plane dislocations and threading screw dislocations (TSD) are considered killer defects. However, because of the high TSD density of commercial SiC wafers, the exact relationship between other kinds of dislocations with lifetime has not been revealed. On the other hand, RAF wafers that we developed have low TSD density, so it is easy to evaluate the relationship between other kinds of dislocations and lifetime. By using RAF wafers, in this study, we clarified the relationship between the lifetime of the gate oxide and crystal defects. We fabricated MOS diodes and measured their lifetimes by TDDB (Time Dependent Dielectric Breakdown) measurement. The breakdown points were defined by the photo-emission method. Finally, we classified the defects by TEM (Transmission Electron Microscopy). As the results, it was clarified that threading edge dislocation (TED) decreases the lifetime as does TSD, which earlier reports said. The lifetime of the gate oxide area, in which a TED is included, was shorter by one order of magnitude than a wear-out breakdown. And, the TSD was two orders.