Single crystals of 4H-SiC irradiated with 900 keV Si and 21 MeV Ni ions separately and sequentially were studied by Rutherford backscattering spectrometry in channeling geometry, single crystal X-ray diffraction, and Raman scattering. SiC irradiated with 900 keV Si ions to a fluence of 6.3 × 1014 ions/cm2 experiences 7.3% strain over the depth of 650 nm. Strain relaxation from ionization-induced annealing was directly observed due to subsequent irradiation with 21 MeV Ni ions to a fluence of 2 × 1014 ions/cm2. These competitive processes suggest the use of ion irradiation to create a specific strain state in 4H-SiC, particularly in films.
Silicon carbide (SiC) is a wide bandgap semiconductor that has broad applications in various electronic devices.1,2 Due to the different stacking sequence of two-dimensional SiC layers, silicon carbide has many polytypes, in which 3C, 4H, and 6H are the most stable structures. As a high-temperature stable and radiation-resistant material,3,4 SiC is also expected to have a wide range of applications in extreme radiation environments as structural components5,6 and sensors.2,7 As an ideal material for extreme environment applications, the energy deposition, microstructure evolution, defect formation, structural transformation, and mechanical properties of SiC irradiated with ions from light H to heavy U in various energies have been intensively investigated.8–19
Significant experimental efforts have been devoted to study radiation-induced changes in SiC due to energy deposited to the atomic nuclei via scattering collisions. It is well-known that SiC undergoes amorphization at a relatively low dose, i.e., fraction of displacements per atom (dpa), if the irradiation temperature is below around 250 °C. This phase-change has been identified, for instance, by transmission electron microscopy,11,12,20 Raman spectroscopy,9 and Rutherford backscattering spectrometry in channeling (RBS/C).21 It is also known that prior to amorphization, there is a significant lattice swelling (increase in the lattice parameter), as determined by x-ray diffraction (XRD), and the amorphization itself induces a large volume change.11,19 In contrast, electronic energy deposition solely generates a low concentration of point defects.14 An ionization-induced decrease in the remaining damage level in Fe-implanted SiC has been observed and discussed before.22 Recently, it has been demonstrated that ionization with different species of ions can induce defect annealing in SiC, either near or inside a collision cascade,23,24 and RBS measurements suggested a significant annealing effect in 21 MeV irradiated SiC crystals.
In the current study, Raman, RBS/C, and XRD techniques are combined to investigate the ionization-induced effects in damaged SiC. These three techniques are complementary because Raman spectroscopy is a powerful technique, which is sensitive to short-range order, RBS/C allows depth profiling of the disorder close to the surface, and XRD enables sampling the crystal over a larger depth. Moreover, strain engineering is a current hot topic in semiconductor materials,25 and therefore, special attention is paid here to the monitoring of the strain in the irradiated layers.
The (0001) single crystal of 4H-SiC, with a thickness of 0.5 mm, was cut to dimensions of 20 × 15 mm2. Half of the sample surface was first irradiated with 900 keV Si ions to a fluence of 6.3 × 1014 ions/cm2, and then different areas were subsequently irradiated with 21 MeV Ni ions to fluences of either 2 × 1014 ions/cm2 or 1 × 1015 ions/cm2. For comparison, the other half of the sample surface was irradiated with 900 keV Si ions to a fluence of 8 × 1015 ions/cm2 and 21 MeV Ni ions to fluences of 2 × 1014 and 1 × 1015 ions/cm2 in different areas. These 6 irradiated areas with different irradiation histories, along with an unirradiated area, comprise a total of 7 areas for characterization. All irradiations were performed at room temperature.
The Raman spectra were collected using a high-resolution Raman spectrometer (LabRaman HR Evolution) at room temperature with a 532 nm green laser as the activation source. The laser light has a spot size of ∼0.7 μm. The RBS/C measurements were performed using 3.5 MeV He+ ions, with a backscattering angle of 150° between the He beam and the Si detector. The XRD measurements were carried out using a PANalytical X'Pert Pro thin film X-ray diffractometer in the 2θ-θ scan mode with Cu Kα1 X-ray (λ = 1.5406 Å). The strain profiles were derived by simulation of the XRD patterns with program RaDMaX.26
Both Raman and RBS measurements suggest that SiC is amorphized with 900 keV Si ion irradiation to the highest fluence of 8 × 1015 ions/cm2 because the energy deposition is larger than the threshold of the amorphization level.27 We thus focus on the areas irradiated with Si ions to a fluence of 6.3 × 1014 ions/cm2 and subsequently irradiated with 21 MeV Ni ions, as well as areas irradiated with just Ni ions. According to SRIM calculations,28 the energy deposition near the SiC surface is dominated by electronic energy loss for both 900 keV Si and 21 MeV Ni ion irradiation, and the predicted ratios of electronic to nuclear stopping power at the surface are 11 and 133, respectively, for the Si and Ni ions. The detailed energy loss at the sample surface and at the depth of the 900 keV Si damage peak (∼650 nm) is listed in Table I. For the case of Si ion irradiation, the nuclear stopping becomes comparable to the electron stopping at the damage peak. Figure 1 illustrates the disorder in the Si sublattice in irradiated SiC samples measured with RBS/C. After Si ion irradiation to a fluence of 6.3 × 1014 ions/cm2, the relative disorder reaches ∼0.8 at a depth of 650 nm, i.e., full amorphization is not achieved. After subsequent irradiation with 21 MeV Ni ions to fluences of 2 × 1014 ions/cm2 and 1 × 1015 ions/cm2, the degree of disorder decreases to ∼0.5 and 0.2, respectively. This result is clear evidence of disorder annealing upon electronic energy deposition in predamaged SiC.
Iron irradiation condition and stopping powers predicted by the SRIM calculation. The peak damage for 900 keV Si and 21 MeV Ni ion irradiation is at 650 nm and 5.6 μm from the surface (s: surface; p: peak damage at a depth of 650 nm).
| Ions | Energy (MeV) | dE/dxe-s (keV/nm) | dE/dxn-s (keV/nm) | Ratio-s | dE/dxe-p (keV/nm) | dE/dxn-p (keV/nm) | Ratio-p |
|---|---|---|---|---|---|---|---|
| Si | 0.9 | 1.6 | 1.4 × 10−1 | 11 | 0.46 | 4.0 × 10−1 | 1.15 |
| Ni | 21 | 8.1 | 6.9 × 10−2 | 133 | 7.2 | 8.5 × 10−2 | 124 |
| Ions | Energy (MeV) | dE/dxe-s (keV/nm) | dE/dxn-s (keV/nm) | Ratio-s | dE/dxe-p (keV/nm) | dE/dxn-p (keV/nm) | Ratio-p |
|---|---|---|---|---|---|---|---|
| Si | 0.9 | 1.6 | 1.4 × 10−1 | 11 | 0.46 | 4.0 × 10−1 | 1.15 |
| Ni | 21 | 8.1 | 6.9 × 10−2 | 133 | 7.2 | 8.5 × 10−2 | 124 |
The disorder parameter of Si measured with RBS for the Si ion irradiated SiC and sequentially irradiated with 21 MeV at different fluences.
The disorder parameter of Si measured with RBS for the Si ion irradiated SiC and sequentially irradiated with 21 MeV at different fluences.
The irradiation-induced elastic strain was determined by recording the scattering signal in the vicinity of the 0004 Bragg reflections of the irradiated SiC crystals. Three samples were specifically studied: SiC crystal irradiated with 900 keV ions to a fluence of 6.3 × 1014 ions/cm2, one irradiated with 21 MeV Ni ions to a fluence of 2 × 1014 ions/cm2, and a third crystal first irradiated with Si ions and then with Ni ions. Experimental data are plotted in Fig. 2(a). For the three patterns, an intense Bragg peak is observed on the high-angle side; it corresponds to the pristine part of the SiC crystals that is probed by x-rays. Distinct features are visible on the low-angle side of the Bragg peak; they come from the irradiated parts and are a sign of a tensile strain gradient along the surface normal of the samples.18 Precise fitting of these curves with the RaDMaX program allowed determining the strain depth profiles in the three samples. Those are displayed in Fig. 2(b), and the strain profiles of Si or Ni irradiated SiC agree well with the damage profiles from the SRIM calculation [Fig. 2(c)].
(a) The (0004) Bragg peak XRD profiles of SiC at different irradiation conditions measured with symmetric 2θ-θ scan. The dot symbols are observed data and the solid lines represent the simulated XRD patterns; (b) the strain profiles are derived by the simulation; and (c) the calculated displacement per atom profiles of SiC irradiated with Si and Ni ions.
(a) The (0004) Bragg peak XRD profiles of SiC at different irradiation conditions measured with symmetric 2θ-θ scan. The dot symbols are observed data and the solid lines represent the simulated XRD patterns; (b) the strain profiles are derived by the simulation; and (c) the calculated displacement per atom profiles of SiC irradiated with Si and Ni ions.
After irradiation with Si ions, the strain exhibits a peak value of ∼7.3% at a depth of 650 nm. The strain profile is similar to that of Si disorder measured with RBS/C (blue symbols in Fig. 1), but the peak damage with the RBS measurement is ∼20 nm deeper than the maximum strain. Similar behavior was also previously observed in 3C-SiC irradiated with 500 keV Ar ions.19 The strain depth profile in the Ni-irradiated crystal follows the SRIM-predicted dpa profile [Fig. 2(c)], with a peak (2.0%) at 5330 nm and a long tail from the surface. After Si and subsequent Ni irradiations, the strain depth profile is a combination of the two separate irradiations, with a first peak at ∼650 nm, a tail up to ∼1000 nm, and another peak at 5330 nm, the two last components being identical to those observed after the sole Ni irradiation. What is notably different is the strain level in the Si-damaged region: it decreased to 3.7%, i.e., it has decreased by a factor of two compared to that before electronic energy deposition. The strain relaxation after an ionizing process is directly measured with XRD though ionization-induced annealing has been observed before with other techniques.8,22,23,29 The significant ionization induced annealing measured with XRD is in agreement with the RBS/C results.
The ratios of predicted electron and nuclear stopping power for 900 keV Si and 21 MeV Ni ion irradiation at the surface and peak damage in SiC are listed in Table I, and the calculated energy loss of nuclear stopping and ionization in the top layer (1 μm thickness) is shown in Fig. 3. The nuclear stopping of 900 keV Si ion irradiation is one-order of magnitude smaller than the ionization effect at the very surface layer and they become comparable in the maximum damaged region (500–700 nm). The generated strains by Si ions dominate in the maximum damaged zone, which is mainly attributed to the collision (nuclear stopping). In the case of 21 MeV Ni ion irradiation, the energy loss due to nuclear stopping in the predamaged zone is negligible compared with the ionization energy loss. It is generally believed that there is no significant displacement damage through elastic collisions in the top 1 μm where predamage exists. However, cross-sectional Raman measurements suggest that there is also damage in the very top layer in Ni ion irradiated SiC. The ionization energy loss may have two competitive effects: generating displacements and annealing the defects. Once the atomic displacement exceeds a critical value, interstitial and vacancy defects will be generated, and Si-Si and C-C bonds or even clusters of Si and C will form. Using transmission electron microscopy and spectroscopy methods, Si clusters have been experimentally observed in electron irradiated SiC before.30 Our Raman measurements clearly show the existence of Si-Si bonds in the Si or Ni ion irradiated crystals. The formation of Si/C antisite defects is an irreversible process. The energy from ionization is insufficient to anneal the antisite defects, though an annealing effect is clearly observable in Ni ion irradiated SiC from both RBS and XRD measurements.
The energy loss of 900 keV Si ions and 21 MeV Ni ions irradiated SiC in the top 1 μm layer predicted from SRIM calculations.
The energy loss of 900 keV Si ions and 21 MeV Ni ions irradiated SiC in the top 1 μm layer predicted from SRIM calculations.
In summary, lattice strain normal to the surface of ion irradiated SiC is studied by symmetric θ/2θ X-ray diffraction measurements and simulation of the observed XRD profiles. Ionization-induced annealing in the predamaged SiC is directly observed from the strain release by quantitative analysis, which is in good agreement with RBS/C results.
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy of Sciences, Materials Sciences and Engineering Division. H.X. was supported by the University of Tennessee Governor's Chair program for RBS/C.
