Materials Science Forum Vol. 806

Abstract: This work presents the 10 MeV protons irradiation effects on 4H-SiC MOSFETs at different fluences. MOSFETs main electrical parameters, such as the channel mobility (µ_EFF), threshold voltage (V_TH), transconductance (g_m) and subthreshold current, were analyzed using the time bias stress instability (BSI) technique. Applying this method allowed us to study the effect of carriers interaction with generated interface traps, whether in the bulk or at the interface. Improvements, such as V_TH stabilization in time and a significant increase of the µ_EFF at high fluencies, have been noticed. We assume that this behavior is connected with the atomic diffusion from the SiO₂/SiC interface, towards the epilayer during proton irradiation. These atoms, in majority Nitrogen, may create other bonds by occupying various vacancies coming from Silicon and Carbon’s dangling bond. Therefore, by enhancing the passivated Carbon atoms number, we show that high irradiation proton could be a way to improve the SiO₂/SiC interface quality.

Abstract: In this work, electrical properties of lateral n-channel MOSFETs implanted with differentnitrogen doses in the channel region were measured by Hall-effect technique at 300K. A mobility improvement with increasing nitrogen implantation doses is observed. Interface trap density (Dit) was determined from the experimentally measured Hall carrier density. Our results show a high Dit near and within the conduction band that does not change significantly when the nitrogen implantation dose is increased, despite observed mobility improvement.

Abstract: The electrical properties of metal-oxide-semiconductor (MOS) devices fabricated using dry oxidation on phosphorus-implanted n-type 4H-SiC (0001) epilayers have been investigated. MOS structures were compared in terms of interface traps and reliability with reference sample which was produced by dry oxidation under the same conditions. The notably lower interface traps density measured in MOS capacitor with phosphorus concentration exceeding 10¹⁸ cm^-3 at the SiO₂/SiC interface was attributed to interface traps passivation by incorporated phosphorus ions.

Abstract: The NO (nitric oxide) passivation process for 4H-SiC MOSFETs (silicon carbide metal-oxide-semiconductor filed effect transistors) effectively reduces the interface trap density and increases the inversion channel mobility from less that 10 to around 35cm²/V.s, only 5% of the bulk mobility. Recent results on the phosphorous passivation of the SiO₂/4H-SiC interface have shown that it improves the mobility to about 90 cm²/V.s. Phosphorous passivation converts oxide (SiO₂) into phosphosilicate glass (PSG) which is a polar material and results in device instabilities under abias-temperature stress (BTS) measurements. To limit the polarization effect, a new thin PSG process has been developed. The interface trap density of 4H-SiC-MOS capacitors using this process is as low as 3x10¹¹cm^-2 eV^-1. BTS results on MOSFETs have shown that the thin PSG devices are as stable as NO passivated devices with mobility around 80 cm²/V.s.

Abstract: In this paper a comparative study of the impact of N₂O and POCl₃ annealing on the SiO₂/SiC system is presented, combining nanoscale electrical characterization of SiC surface doping by scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM) to the conventional capacitance-voltage (C-V) and current-voltage (I-V) measurements on MOS-based devices. A significant reduction of the interface states density (from 1.8×10¹² to 5.7×10¹¹ cm^-2eV^-1) and, correspondingly, an increase in the carrier mobility (from 19 to 108 cm²V^-1s^-1) was found moving from N₂O to POCl₃ annealing. Furthermore, SSRM measurements on bare p⁺-type SiC regions selectively exposed to N₂O and POCl₃ at high temperature provided the direct demonstration of the incorporation of N or P-related donors in the SiC surface, leading to a partial compensation of substrate acceptors during N₂O treatment and to an overcompensation during POCl₃ annealing. Finally, cross-sectional SCM profiles performed on epitaxial n-doped 4H-SiC with 45 nm SiO₂ (subjected to post deposition annealing in the two ambients) allowed to quantify the active donors concentrations associated to P or N incorporation under the gate oxide, showing almost a factor of ten higher doping (4.5×10¹⁸cm^-3 vs 5×10¹⁷cm^-3) in the case of P related donors.

Abstract: Silicon carbide based devices have the potential to surpass silicon technology in high power, high frequency and high temperature applications. 4H-SiC MOS transistors currently suffer from a low channel mobility due to a high density of traps near the oxide/SiC interface. In this work, oxides have been grown on the Si face of 4H-SiC using oxygen flow rates ranging from 2.5 l/min to 0.05 l/min. Capacitance-voltage measurements on MOS capacitors revealed approximately a fourfold reduction in the interface trap density and a 25% increase in oxide thickness by reducing the flow rate from 2.5 l/min to 0.05 l/min.