Materials Science Forum Vols. 717-720

Abstract: In all heteroepitaxial systems the interface between substrate and layer is a crucial point. In this work SEM and TEM studies on the interface between silicon substrate and cubic silicon carbide (3C-SiC) layers obtained by chemical vapor deposition (CVD) are presented. A clear connection between process parameters, like the design of substrate cleaning, and the heating ramp, and resulting defect structures at the substrate-layer interface could be found. Whereas the process step of etching in hot hydrogen for oxide removal is crucial for avoiding the generation of closed voids of type 2, the design of the temperature ramp up to growth temperature during carbonization influences the interface roughness. Here a fast ramp helps to obtain a flat interface.

Abstract: We discuss the results of electrically detected magnetic resonance (EDMR) spectroscopy on SiC-SiO₂ interfaces interacting with hydrogen and nitrogen. Using EDMR, three types of 4H-SiC MOSFETs, which were prepared by dry oxidation (“Dry” sample), post hydrogen anneal (“Hydrogen” sample), and post nitridation anneal (“Nitrogen” sample), were examined in the temperature range of 4–300 K. These samples revealed several different results from the earlier ESR (electron spin resonance) and EDMR studies on SiC-SiO₂ interfaces. The most significant finding was the high-density doping of nitrogen into the channel region after the post nitridation anneal. The incorporated nitrogen donors were observed as the “Nh” EDMR signal at 4–20 K. Roles of these nitrogen donors are discussed in correlation with the electrical properties of SiC MOSFETs.

Abstract: We utilize electrically detected magnetic resonance (EDMR) via spin dependent recombination (SDR) to provide a definitive identification of an interface/near interface defect present in a wide variety of 4H SiC/SiO2 metal oxide semiconducting field effect transistors (MOSFETs).

Abstract: N-channel MOSFETs were manufactured on p-type and on p-implanted, n-type 4H-SiC substrates. The electron mobility in the inversion channel was measured to be correlated with the structural and chemical properties determined by transmission electron microscopy. With regard to what was previously discussed in the literature, interfacial layer formation and carbon distribution across the SiC/SiO2 interface were considered in relation with the measured Hall electron mobility.

Abstract: Carrier generation characteristics in n-type substrate SiC MOS capacitors induced by sub-bandgap energy light are reported. The generation rate is high enough to create an inversion layer in ~20 minutes with monochromatic light (front side illumination) of energy 2.1 eV in 4H-SiC for electric fields smaller than 1 MV/cm. Generation and recovery results strongly indicate involvement of a metastable defect whose efficiency as a generation center increases under hole-rich and decreases under electron-rich conditions. The generation dependence on bias history and light energy shows the defect to have properties consistent with the metastable silicon vacancy / carbon vacancy-antisite complex (VSi / Vc - CSi).

Abstract: The defects at the interface and in the oxide have been considered as the sources of mobility degradation at the SiC/SiO₂ interface as in the case of Si/SiO₂ system. By examining available experimental and theoretical results and performing new calculations, we show that thermal oxidation creates immobile carbon di-interstitial defects inside the semiconductor substrate, which are a major cause of the poor mobility in SiC/SiO₂ structures.

Abstract: We present transition layer electron mobility versus field curves for several 4H-SiC/SiO2 structures, simulated by a newly developed Monte Carlo simulator that uses density of states calculated by density functional theory (DFT). Our calculations show that among all structures, abrupt SiC/SiO2 has the highest transition layer mobility.

Abstract: Large group-I elements such as sodium, rubidium and cesium have recently been incorporated in the gate oxide of SiC power MOSFETs. In the case of sodium incorporation, enhanced field effect mobilities have been definitively observed. Based on density functional calculations, we find large group-I elements serve as a shallow impurities near the interface. The enhanced mobility, observed in the case of sodium, can be explained in terms of an impurity band model.

Abstract: We compare the effect of hydrogen, nitrogen, and phosphorous passivation on total near interface trap density and mobility of 4H(0001)-SiC/SiO2 structure. The results show that nitrogen and phosphorous passivation decrease total near interface trap density by pushing the energy levels of interface traps away from the conduction band. The density of states (DOS), including interface states (Dit), are calculated for several 4H(0001)-SiC/SiO2 structures using density functional theory (DFT).

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.