Direct Observation of Energy Distribution of Interface States at SiO2/4H-SiC Interface
Authors: Yamashita Yoshiyuki , Hasunuma Ryu , Nagata Takahiro , Chikyow Toyohiro 
Published: February 2020

For the interface states at the SiO2/SiC interface, the energy distribution of interface states in the whole-gap region of SiC cannot be obtained from electrical method such as capacitance-voltage and conductance-voltage methods because deep interface states cannot respond to the electrical signal due to their extremely large time constants. Thus interface states at the SiO2/SiC interface in the SiC whole-gap region have not been obtained even though the observation of the energy distribution of interface states is indispensable for further improvement of the electrical properties. We have recently developed a spectroscopic method to observe the energy distribution of interface states based on photoelectron spectroscopic measurements under bias voltage (PES-BA) [1,2]. In the present study, we have investigated the energy distribution of interface states at the SiO2/4H-SiC interface using PES-BA.
A Pt(10nm)/SiO2(8nm)/4H-SiC(0001) was used in the present study. The PES-BA measurements were performed using BL15XU at SPring-8. For PES-BA measurements, the incident photon energy was 5.95 keV, while the total energy resolution was set to 240 meV.
Applying a positive bias voltage of 2.0V to SiC with respect to the Pt overlayer shifted the Si 1s substrate peak toward a higher binding energy by 0.61 eV. On the other hand, applying a -2.4 V voltage shifted the Si 1s peak toward a lower binding energy by 0.68 eV. These bias-induced shifts were completely reversible; that is, the shift diminished upon removing the bias voltage. Therefore, these shifts were not due to a bias-induced chemical reaction, but were caused by the accumulation or release of charge in the electronic states by the bias. Thus, by analyzing the these shifts as a function of bias voltage, we obtained the energy distribution of the interface states for the SiO2/4H-SiC(0001) interface. Note that the principle for the determination of interface states was described in our previous papers [1-3]
For the observed interface states, uniform interface states in the whole gap existed while high density interface states were present near the CBM. By referencing previous theoretical calculations, the observed interface states were assigned. The sharp interface states observed below CBM may be attributed to Si2-C-O states at the interface. For the uniform interface states, two dimensional graphitic carbon and/or chemical disorder may be responsible for the uniform interface states because of the spectrum shape.
The synchrotron measurements were performed under the approval of NIMS Beamline Station (Proposal No. 2013B4600 and No. 2014B4602). This study is collaboration with Prof. R. Hasunuma at Tsukuba University
References: [1] Y. Yamashita, H. Yoshikawa, T. Chikyow, and K. Kobayashi, J. Appl. Phys. 113, 16370 (2013). [2]Y. Yamashita, K. Ohmori, S. Ueda, H. Yoshikawa, T. Chikyow, and K. Kobayashi, e-J. Surf. Sci. Nanotech. 8, 81 (2010). [3] Y. Yamashita, A. Asano, Y. Nishioka, and H. Kobayashi, Phys. Rev. B59, 15872 (1999).