Significant Structural Distortion in the Surface Region of 4H-SiC Induced By Thermal Oxidation and Recovered By Ar Annealing
Authors: Kita Koji , Hatmanto Adhi Dwi 
Published: February 2020

4H-SiC has attractive properties for power devices of high voltage applications, however, the performance of MOSFETs is often severely limited by the existence of electrically active defects at SiO2/SiC interface. The formation of interface states is often attributed to the carbon-related byproducts of thermal oxidation, however as suggested by theoretical study, it may also partly originate from the non-uniformity in electrical structure of 4H-SiC, due to the oxidation-induced local strains at the 4H-SiC surface [1]. In this study we systematically investigated the lattice distortions introduction in 4H-SiC surface region after various oxidation processes and the recovery of that directly by using surface sensitive in-plane x-ray diffractometry (XRD).
4º off-axis 4H-SiC (0001) wafers covered with 5 μm-thick n-type epitaxial layers were employed in this study. The wafers were cleaned by HF solution, then thermal oxidation process in dry oxygen at 1300ºC for various oxidation time were performed. For some of the samples, the thermally grown SiO2 layer was removed by chemical etching in HF solution, followed by annealing in Ar at 1150ºC or 1300ºC for various annealing time. The shallow incident angles less than or slightly more than the critical angle were used for in-plane XRD measurements to limit the x-ray penetration depth which can be widely tuned simply by changing the incident angle.
In the in-plane XRD pattern, a peak corresponding to the diffraction of (1-100) planes was detected. The interplanar spacing was characterized for the result obtained with different incident angles. Before thermal oxidation, as expected, no significant peak position shift was observed even by changing the incident angle, since the lattice constant should be uniformly distributed in the epitaxial layer. In contrast, the peak position of oxidized 4H-SiC sample shifted to the lower angle for small incident angle measurement and gradually shifted back by increasing the incident angle [2]. In addition, when we changed the thermal oxide thicknesses by performing dry oxidation for different time, the shift of the peak position became larger for the longer oxidation time, and reaches to as large as +0.4% distortion for >40nm-thick oxide case [2]. The x-ray penetration depth where such a significant shift of the lattice constant was observed, was estimated to be only several hundreds of nanometers from the SiC surface, by a simple approximation of the penetration depth taking account of the critical angle and the x-ray absorption coefficient of 4H-SiC. These results clearly indicate that the dry oxidation process induces a significant lattice distortion locally in the surface region of 4H-SiC wafer.
We also found that the lattice distortion of SiC surface remained even after the complete removal of SiO2 thin film by HF etching, though it was partially reduced. Thus the detected local strain in the surface region of SiC is distinguishable from an intrinsic strain due to the thermal expansion coefficient difference and atomic density difference of Si between SiC and SiO2 [3]. A further recovery of the lattice constant in the surface region was observed by Ar annealing. It should be noted that the relaxation occurs gradually by extending the Ar annealing time to a few hours even at 1300ºC, which is equivalent to the oxidation temperature in this study. This suggests a thermally-activated process inside SiC is required for the relaxation of the surface distortion. Actually a higher recovery rate was observed for 1300ºC than that for 1150ºC annealing.
From these results we speculate that the observed surface distortion of 4H-SiC is caused by oxidation-induced defects which are accumulated only in the surface region and induce a significant strain locally in the surface region. Then the gradual relaxation by Ar annealing would be corresponding to the processes of decomposition of the defects followed by migration and desorption of the ejected atoms from the defects. Even though the relationship between the observed surface strain and the electrical structure of SiC surface is not clarified yet, such significant strain should have a certain impact on the electrical properties of SiO2/SiC interfaces.
References
[1] K. Shiraishi et al., 2014 IEDM Technical Digest, 538 (2014).
[2] A. D. Hatmanto and K. Kita, Appl. Phys. Express 11, 011201 (2018).
[3] X. Li et al., Appl. Phys. Lett. 110, 141604 (2017).