Phase Contrast X-Ray Imaging of Defects Transformation in Silicon Carbide (SiC)

Phase contrast imaging (PCI) based on using synchrotron X-rays is an ideal method to visualize internal microstructure of various materials, in particular, of semiconductor materials. PCI spatial resolution has been greatly improved by the advent of modern synchrotron radiation (SR) sources with small angular divergences and high spatial coherences.

Silicon carbide (SiC) is a promising semiconductor material for high-power, high-frequency, and high-temperature electronics. In spite of significant progress, silicon carbide crystals, in particular, grown by sublimation growth technologies¹, still contain various structural defects, such as dislocations, micropipes, inclusions, etc. PCI using synchrotron X-rays is a powerful tool to understand such defects, specifically permitting area mapping of hollow defects or 3-dimensional (3D) distribution of defects and their evolution.

For defects studies in SiC, we have applied PCI method based on using white beam SR that provides high time resolution, large imaging area, and simplified experimental setup². We have investigated various types of defects in SiC, such as micropipes, inclusions, and pores, and suggested several models to explain their formation mechanisms³. Furthermore we developed a computer simulation to understand phase contrast images of defects and precisely analyzed defect size such as cross-section of micropipes^4-7.

Defects Evolution During SiC Growth

Defects evolution during SiC growth was revealed using the white SR beam phase contrast imaging⁸. Typical experimental setup is illustrated in Fig. 1(a). Six on-axis wafers were consecutively cut off from a 6H‑SiC boule, named the wafer nearest to the seed “wafer I”, the next one “wafer II”, and so on. The orientation of each wafer was identified by Bragg diffraction. Defects evolution from wafer to wafer was investigated on the same area of interest, marked as “field of view” (blue dot) in Fig. 1(b). In initial growth stage, foreign polytype inclusions (FPIs) are generated near the seed, as confirmed by photoluminescence (Fig. 1(b)). FPIs induce massive generation of full-core dislocations and dislocated micropipes, and subsequently attract them. As a result, slit-type pores are formed at the inclusion boundaries, as seen in the phase contrast image of pores in Fig. 1(b). In the intermediate stage, the inclusions stop to grow and are overgrown by the matrix. Then the pore density significantly reduces, presumably transformed into new micropipes (Fig. 1(b)). In later stage, the micropipe density decreases by partial annihilation and healing mechanism.

Fig. 1. (a) Schematic of phase contrast imaging setup using whitebeam SR. (b) Diagram displaying the defects evolution during the growth of 6H-SiC boule. The same area of interest ("field of view"; blue dot) on each wafer was investigated. Polytype inclusions (photoluminescence image) on the wafer I provoke pore formation, as observed in the PCI image. The pores transform into new micropipes (PCI image) in intermediate growth stage. In later stage, the micropipes density reduces by mutual interaction.

Micropipes Reactions in SiC Crystals

Micropipes grow up by the propagation of the crystal growth front and come into reactions with each other as well as with foreign polytype inclusions and pores. The reactions between micropipes are classified by contact-free ^3,7 and contact ^3,9-11 reactions. Contact-free reaction occurs when one micropipe emits a full-core dislocation while another micropipe accepts it. We have experimentally documented various contact reactions such as ramification of a dislocated micropipe into two smaller ones ^3,9, bundling and merging leading to the generation of new micropipes or the annihilation of initial ones ^3,10, interaction of micropipes with foreign polytype inclusions followed by agglomeration and coalescence of micropipes into pores ^3,11.

Control of Defects Behavior for the Improvement of SiC Growth

It is noteworthy that any defect reaction described above can be positively utilized because it leads to defect reduction after all, for instance, by healing micropipes and/or cleaning corresponding crystal areas from micropipes. Understanding of the reaction mechanisms or the factors affecting to the reaction is therefore critical, in particular, to control defect behavior and thus improve SiC growth.

White SR Beam Phase Contrast Imaging in Materials Science

The efficiency of white SR beam PCI was demonstrated for studying structural defects in SiC as an example. The white SR beam PCI that has a positive effect in enhancing the contrast for objects with small cross-section has a negative effect as well. In the far-field approximation offering a strong edge-enhancement, the phase contrast image provides a universal shape but with no detailed information on the object ⁶. Therefore we developed a numerical simulation method that enabled us to accurately measure defects sizes from the object images, as a result confirming transformation behaviors of micropipes. This approach could be applied to the investigations of phase objects with small cross-sections in various materials, including soft materials.

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