(Invited) Non-Destructive Three-Dimensional Imaging of Extended Defects in 4H-SiC
Authors: Tanuma Ryohei , Kamata Isaho , Tsuchida Hidekazu 
Published: February 2020

Grazing-incidence X-ray topography and photoluminescence (PL) imaging are utilized to understand the nature and behavior of extended defects in 4H-SiC. Although these methods are useful for non-destructive defect analysis, they provide two-dimensional (2D) images with low depth resolution. Advanced three-dimensional (3D) imaging methods are thus expected to be used for characterizing extended defects. In particular, very high-voltage SiC bipolar devices with a thick epilayer require defect imaging as deep as 100-200 μm. We established a 3D imaging technique using an X-ray microbeam and a novel fine slit, which was successfully applied to characterizing threading screw dislocations (TSDs) [1, 2], threading edge dislocations (TEDs), and basal-plane dislocations (BPDs) [3, 4] in 4H-SiC. However, this method requires a large-scale synchrotron-radiation facility.
In this study, we show how second-harmonic generation (SHG) and two-photon-excited photoluminescence (2PPL) imaging techniques are powerful tools for 3D analysis of extended defects in 4H-SiC epilayers [5, 6]. The SHG and 2PPL methods can be performed by a multi-photon microscope in individual laboratories. The SHG method provides clear 3D images of 3C-inclusions because 3C-SiC is SHG-active, but not 4H-SiC host crystal in c-axis incidence (Fig. 1). The 2PPL method yields 3D images; not only of 3C-inclusions but also 8H stacking faults (Fig. 2), and single Shockley stacking faults in the epilayers.
The 2PPL method also achieved 3D imaging of TSDs, TEDs, and BPDs using band-edge emission [7]. Since band-edge emission quenches near defects, these dislocations can be visualized as dark contrasts on a bright background. We obtained 3D images of TSDs and TEDs extending ~200 μm from the surface (Fig. 3). Unlike 2D PL imaging with uniform illumination, the dark-contrast imaging is governed by the diffusion and transport of excess carriers injected from a scanning focal point. A simulation study was conducted to reveal the mechanism of the dark-contrast imaging.
Each of the dislocations provided 2D dark-contrast images stacked depthwise, from which the tilt angles of dislocations were determined. Figure 4 shows the plots of φ versus θ for 89 TSDs in the 4º off epilayer, where θ denotes the angle of the dislocation line from the c-axis, and φ the counter-clockwise angle of (0 0 0 1) projected dislocation lines from the step-flow [1 1 -2 0] direction. Their dislocation lines incline in the step-flow direction. It is also shown that the regions of left- and right-handed (LH and RH) dislocations clearly differ. The plots in Fig. 4 include those of 1c and c+a dislocations. We found that 1c dislocations had very similar tilt angles, whereas those of c+a dislocations were largely spread out. The tilt angles were also investigated for 105 TSDs in the 8º off epilayer, which exhibits larger θ angles　and a smaller φ range than the 4º off-cut case. We examined the tilt angles of more than 300 TEDs and found that the TEDs not only inclined in the step-flow direction, but also the direction of the extra half planes [7].
The results of the tilt-angle analyses cannot be explained by the energy minimization model in bulk materials. We consider that the directions of TEDs and TSDs depend upon the interactions between dislocations and advancing steps on a growing surface.
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Figure 1