(Invited, Digital Presentation) Interface Charge Trapping and Scattering in SiC MOSFET Channels

The channel conductivity of SiC MOSFETs is severely limited by charge trapping at interface states and mobility limiting scattering processes. In this talk, the status of interface state passivation for both n- and p-channel 4H-SiC MOSFETs will be presented. First, results from capacitance-based methods to extract interface state density (Dit) across the entire 4H-SiC band gap for thermally grown nitrided silicon dioxide gate dielectrics will be presented, emphasizing the critical importance of the SiO2(N)/SiC interface composition and nitrogen bonding. Next, the scattering mechanisms for channel electrons and holes in these devices will be analyzed and contrasted by Hall measurements. [1] The transport will be characterized as a function of temperature and substrate-bias within an effective transverse electric field model. The final part of the talk will focus on latest development on deposited dielectrics for 4H-SiC. Deposited dielectrics are an attractive alternative to thermal oxidation as it minimizes the release of carbon and the perturbation of surface crystal structure and opens the possibility of ‘high-k’ dielectrics with higher permittivity than SiO2. Herein, the key importance of 4H-SiC surface control prior to dielectric deposition will highlighted using results obtained for atomic layer deposition (ALD) of Aluminum Oxide (Al2O3) [2]. To this end, a systematic variation of SiC surface termination with processes involving oxygen, hydrogen and nitrogen will be presented. The composition of the deposited films and the interfacial bonding will be characterized for various deposition processes and correleated with electrical measurements . It will be demonstrated that surface nitridation followed by H2 annealing prior to ALD results in a factor of 2 higher channel mobility than typical NO annealed thermal SiO2. This is due to the formation of stable sub-nm SiON layers that passivates the surface and enables the formation of high-quality interfaces between SiC and deposited dielectric. In addition, the results indicate that clean Si-H terminated surfaces formed by hydrogen etching and annealing, prior to dielectric deposition leads to uniform nucleation of Al2O3. The reduction of Dit is also accompanied by improvements in oxide charge trapping and bias instability, but dielectric leakage current is significantly higher for Al2O3 compared to SiO2 due to bulk trap-assisted tunneling in Al2O3. The reliability is a significant challenge that need to be overcome to exploit the enhanced mobilities and make deposited dielectrics competitive with traditional nitrided SiO2. Acknowledgments: The authors acknowledge support from the US Army Research Laboratory (grant ARMY-W911NF-18-2-0160) and US Department of Energy (subcontracted to National Renewable Energy Laboratory, grant NREL-AHL-9-92632-01). The authors also acknowledge the support Dr. Leonard C. Feldman (Auburn, Rutgers) and Mr. Hengfei Gu (Rutgers) for help with XPS measurements. The authors also deeply thank Ms. T Isaacs-Smith, Ms. Lu Wang (Auburn), Dr. K. Ramadoss and Dr. D. Morisette (Purdue Univ.) for useful discussions and help with fabrication. [1] S. Das et al., J. Appl. Phys. 130, 225701, (2021). [2] I. U. Jayawardhena et al., J. Appl. Phys. 129, 75702, (2021) Figure 1