Yasuo Cho1,Kohei Yamasue1
Tohoku University1
Yasuo Cho1,Kohei Yamasue1
Tohoku University1
Wide-bandgap semiconductors such as SiC, GaN, and diamond are promising materials for next-generation power devices. However, the performance of MOS devices using wide-bandgap semiconductors has not yet reached the level expected from their excellent material properties. For example, the on-resistance of 2-3 kV-class SiC MOSFETs is limited by low MOS channel mobility. Channel mobility is dominated by the quality of the dielectric-semiconductor interface such as SiO<sub>2</sub>/SiC. To investigate the cause of low channel mobility from a microscopic perspective, we have proposed the application of scanning nonlinear dielectric microscopy (SNDM) to study dielectric-semiconductor interfaces. SNDM is a near-field microwave scanning probe microscopy method that is exceptionally sensitive to the change in capacitance between the conductive tip and the sample [1]. SNDM can detect the change in microscopic MOS capacitance when measuring a semiconductor wafer with gate dielectric top layer. In particular, time-resolved SNDM (tr-SNDM), an extension method of SNDM, has recently permitted the detection of rapid changes in local capacitance more accurately than conventional SNDM [2]. Tr-SNDM has therefore enabled local DLTS (deep level transient spectroscopy) and local CV (capacitance-voltage) profiling, which give nanoscale spatial resolution to DLTS and CV profiling [3].<br/>Here we present a local CV profiling method based on tr-SNDM and application to the SiO<sub>2</sub>/SiC interface [3]. We show how microscopic CV profiles are obtained by tr-SNDM and their nanoscale spatial fluctuations are analyzed in a real-space. Spatial fluctuations of local CV profiles reflect the potential fluctuations at the interface, or surface potential fluctuations. We apply our method to a SiO<sub>2</sub>/SiC interface treated by NO-POA (post-oxidation-annealing). We extract feature values from the local CV profiles and their derivative (d<i>C</i>/d<i>V</i>-<i>V</i>) profiles to quantitively investigate the fluctuations of local CV profiles. By extracting the voltages at the infection points of local CV and d<i>C</i>/d<i>V</i>-<i>V</i> profiles, we can characterize the fluctuations of local CV profiles in depletion, accumulation, and intermediate regions. We found that the spatial fluctuations of local CV profiles are reduced by NO-POA (post-oxidation-annealing) treatment, as expected, but still larger than the thermal energy at room temperature (~26 meV) even in deep depletion region. This indicates the density fluctuations of fixed charges including the carriers trapped at the deep levels are high enough to create significant potential fluctuations at the interface. In addition, we found that fluctuations become higher as the interface are accumulated by the dominant carriers. This suggests that dominant carriers trapped to acceptor-type interface defects further increase the spatial non-uniformity of the interface. It is known that surface potential fluctuations are strongly related to the carrier transport properties of the channel. Therefore, our results suggest that high surface potential fluctuations may be the cause of low channel mobility in SiC MOSFETs. Our method can also be applied to other dielectric-semiconductor interfaces such as Al<sub>2</sub>O<sub>3</sub>/diamond and Al<sub>2</sub>O<sub>3</sub>/GaN and will give microscopic insights on these interfaces.<br/><br/>References:<br/>[1] Y. Cho, Scanning Nonlinear Dielectric Microscopy: Investigation of Ferroelectric, Dielectric, and Semiconductor Materials and Devices. Elsevier, ISBN 978-0-08-102803-2 (2020).<br/>[2] Y. Yamagishi and Y. Cho, Appl. Phys. Lett. 111, 163103 (2017).<br/>[3] K. Yamasue and Y. Cho, Microelectron. Reliab. 135, 114588 (2022).