Anisotropic Defect Distribution in 4H-SiC Induced by He+ Irradiation: Impact of Stress on Defect Distribution *
1. Abstract
Hexagonal crystal α-SiC undergoes anisotropic expansion due to irradiation, compromising its mechanical properties. Despite this, the underlying physical mechanisms and microstructural processes are not yet fully understood. This study introduces conditions of anisotropic expansion in 4H-SiC using area-selected He+ radiation. The radiation allows constrained transverse free expansion along the surface normal. Internal defect distributions were examined using Transmission Electron Microscopy (TEM) and advanced scanning TEM. This distribution was compared with that in 4H-SiC irradiated with non-area-selected He+ and electron-irradiated 4H-SiC TEM foils. Anisotropic defect distribution is observed in the He+-irradiated 4H-SiC areas, where interstitial defects preferentially redistribute along the surface normal ([0004]), while negative volume defects (such as vacancies and/or carbon antisite defects) predominantly occupy transverse directions ([11-20] and [10-10]). Such anisotropy in defect distribution is significantly lower in samples irradiated non-selectively with He+ and electrons. The stress states of the three specimens were measured and analyzed. In the He+ irradiated areas of 4H-SiC, compressive stress was introduced transversely ([10-10] and [11-20]) with minimal stress along the surface normal direction ([0004]); these stress conditions were introduced at the onset of ion irradiation. Compressive stress may suppress the formation of transverse interstitial defects, enhancing the anisotropy of defect distribution in SiC.
2. He+ Irradiated SiC Substrate Experiments and Observations
4H-SiC single-crystal n-type substrates (0001)(from PAM-XIAMEN) sized 10×10×0.33mm^3 were irradiated at room temperature using 100keV He+ ions to fluences of 1×10^15 and 5×10^16cm^−2. During the irradiation, the flux was maintained at 6.2×10^12 He·cm^−2·s^−1 using a beam raster scan to achieve uniform irradiation conditions across the area.
For comparative purposes, both selected-area and non-selected-area irradiations were performed. During the selected-area irradiation, part of the sample was covered with a mask featuring an 8mm diameter hole to clearly distinguish between irradiated and non-irradiated regions. Non-selected-area irradiation was also prepared without the use of a mask at room temperature, and at a flux of 5×10^16 cm^−2. Damage and helium distribution in SiC due to He+ ions were calculated using SRIM2013 in full cascade mode. The density and threshold displacement energies for the C and Si sublattices used in the simulations were 3.21 g·cm^−3, and 21 and 35 eV respectively. Simulations predicted a total penetration depth of approximately 600nm and peak damage estimated at about 450nm deep, with a dose of about 4.2 dpa (displacements per atom), and a fluence of 5×10^16 cm^−2. Furthermore, a peak helium concentration of about 2.95% was observed at a depth of around 470nm.
Post-irradiation, cross-sectional thin foils for TEM examination were prepared using a Ga-focused ion beam system (JEOL, JEM-90320FIB) from the irradiated areas. The ion acceleration voltage was 30kV, and the SiC wafers were thinned down to a final thickness of about 100nm. To minimize damage to the TEM samples from the Ga ions during the FIB process, they were then polished using a GentleMill with lower energy Ar ions. The TEM samples were polished on both sides with a 1.5kV beam at an 8º angle for 40 minutes, followed by polishing at a 15º angle for 30 minutes with a 0.5kV beam. The microstructural features of the irradiated 4H-SiC were observed using TEM (JEOL, JEM-2000FX) at an operating voltage of 200 kV. The average size and number density of defects were calculated based on weak-beam dark-field TEM images, with 3-5 images used for each calculation. The TEM images for damage counting were all taken at the same magnification and then adjusted to the same background contrast and brightness using Gatan Digital Micrograph. The BSD was then marked in Adobe Photoshop based on the contrast difference with the background, and automatic counting could be performed using the software MAC-View Version.4. The average size of the BSD was recorded by Heywood diameter. The thickness of the observed area was measured using electron energy loss spectroscopy (EELS) with a Cs-corrected scanning transmission electron microscope (FEI, Titan G2 60-300). High-resolution TEM (HR-TEM) analysis, high-angle annular dark-field (HAADF) and annular bright-field (ABF) scanning transmission electron microscopy (STEM), as well as core-loss EELS studies, were also conducted by the Cs-corrected STEM.
Along with strain, the elastic stress and elastic coefficients in the irradiated areas of 4H-SiC were also determined using EBSD and Crosscourt3 software. EBSD patterns were obtained using a field emission scanning electron microscope (JEOL JSM–7001FA) equipped with an EBSD detector, operating at an acceleration voltage of 20kV, a sample tilt of 70°, and a scan size and step length of 20×20μm^2 and 0.1μm, respectively.
Electron irradiation was performed on thin-foil 4H-SiC samples using a multi-beam ultra-high voltage electron microscope (multi-beam HVEM, JEOL, JEM-ARM1300). TEM samples for electron irradiation were prepared using FIB from the unirradiated regions of the selected-area He+ irradiated 4H-SiC samples; prior to electron irradiation, the TEM samples were annealed for 30 minutes at 600°C in the multi-beam HVEM to eliminate any potential internal stresses. Electron irradiation was conducted at room temperature with an acceleration voltage of 1.25MV, and the irradiated area diameter was approximately 2μm. The electron flux was about 1.2×10^24 e·m^−2·s^−1, and the total irradiation time was 1 hour. During the irradiation, the electron beam was controlled to be parallel to the [11-20] orientation. After electron irradiation, the defect distribution in the electron-irradiated thin-foil 4H-SiC samples was also characterized using a 200kV TEM (JEOL, JEM-2000FX).
Fig. 1 Size distribution of BSD in different directions: (a) 4H-SiC substrates irradiated with selected ions, (b) 4H SiC wafers irradiated with non selected ions, and (c) SiC thin film TEM irradiated with electrons; Error bars represent standard deviation
Fig. 2 Stress distribution in the selected area of 4H SiC irradiated with He+ of a fluence of 5 × 10^16 cm^−2: (a) SEM image and (b) – (d) corresponding stress components in the (b) X, (c) Y, and (d) Z directions
Fig. 3 Stress distribution in the selected area of 4H-SiC irradiated with He+ at a fluence of 1 × 10^15 cm^−2: (a) SEM image and (b) – (d) corresponding stress components in the (b) X, (c) Y, and (d) Z directions
Fig. 4 TEM images of BSD distribution in the center of electron irradiation zone of 4H SiC: (a, b) g=0004 and (c, d) g=10-10; where (a, c) bright field image and (b, d) weak beam dark field image, g/3g
3. Conclusion
Using transmission electron microscopy technology, the defect distribution of He+irradiated 4H-SiC under the anisotropic expansion caused by irradiation was studied, and the anisotropy of defect distribution was observed. Interstitial defects are preferentially redistributed to the free expansion direction (Z direction, [0004] direction), while negative volume defects are mainly located in the strain expansion direction (X and Y directions, [11-20] and [10-10] directions). The anisotropy of this defect distribution is significantly greater than that in 4H SiC irradiated by He+ in the unselected region and 4H SiC in the thin foil irradiated by electrons. Compressive stress is introduced in the transverse direction (X and Y directions, [10-10] and [11-20] directions), while in the selected region of 4H SiC irradiated with He+, due to the constraint on lateral expansion, it is almost not introduced in the surface normal direction (Z direction, [0004] direction), and these compressive stresses are introduced at the beginning of ion irradiation. It is speculated that the compressive stress introduced during the irradiation process will suppress the formation of interstitial defects and enhance the anisotropic defect distribution in the 4H-SiC selection area under ion irradiation.
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