In-cascade ionization effects on defect production in 3C silicon carbide*

ABSTRACT Understanding how energy deposited in electronic and atomic subsystems may affect defect dynamics is a long-standing fundamental challenge in materials research. The coupling of displacement cascades and in-cascade ionization-induced annealing are investigated in silicon carbide (SiC). A delayed damage accumulation under ion irradiation is revealed with a linear dependence as a function of both increasing ionization and increasing ratio of electronic to nuclear energy deposition. An in-cascade healing mechanism is suggested with a low threshold value of electronic energy loss (∼1.0 keV nm−1). The in-cascade ionization effects must be considered in predicting radiation performance of SiC. GRAPHICAL ABSTRACT IMPACT STATEMENT A considerable impact of ionization-induced in-cascade defect annealing is presented. A surprisingly low threshold of electronic energy loss is revealed.


Introduction
A long-standing fundamental challenge in materials research is to understand how energy is dissipated in electronic and atomic subsystems in irradiated materials, and how related non-equilibrium processes may affect defect dynamics and microstructure evolution [1]. SiC is an important semiconductor material [2][3][4]. Besides its excellent mechanical properties [5,6], SiC shows remarkable thermal stability and mechanical strength at elevated temperatures [7]. Its wide band gap, high break down voltage and excellent thermal conductivity make SiC an ideal candidate for electronic applications in harsh environments [8][9][10]. SiC, especially its 3C polytype, is also considered as an important nuclear material. A relatively small neutron displacement cross-section [11] and a low critical temperature for amorphization [12] make SiC a promising structural material, such as a coating layer in TRISO (tristructural-isotropic) fuel particles [13]. For both electronic and nuclear applications, radiation damage from ion implantation processes or due to nuclear collisions are inevitable. Intense energy transfer in collisions leads to atomic displacements and accumulation of defects [14,15], amorphization [16], volume swelling [17] and changes in other physical properties. Moreover, ionization due to electronic energy deposition to the electronic subsystem occurs simultaneously. The ionization effects are, however, less understood, due to the inelastic interactions, non-equilibrium processes and materialdependent behavior.
Some early works on ionizing ion irradiation have focused on swift heavy ions (SHIs) [18][19][20]. For the SHIs, the ion energies are more than a few hundred MeV, and electronic stopping powers are more than 20 keV nm −1 [18,19,21]. Defect annealing and damage recovery are attributed to melting within the thermal spikes and epitaxial recrystallization. Recent work has drawn attention to ionization effects at a lower energy regime [1]. Ionizing irradiation with ions in the intermediate energy regime ( ∼ tens of MeV and electronic stopping of 5-10 keV nm −1 ) has been shown to anneal preexisting radiation-induced defects. This healing mechanism could allow tailoring of operating conditions and materials design to promote the repair of the damage caused by radiation exposure in nuclear reactors and space environments, and therefore increase the lifetime of materials. This ionization-induced defect annealing can be utilized to heal defects created by doping during fabrication of advanced electronic materials. However, unlike SHIs, for which the ionization due to electronic stopping is dominant, in the intermediate or lower energy regime, the electronic and nuclear stopping powers can be comparable and vary as a function of ion velocity. Most experimental investigations in this regime have focused on damage accumulation at the damage maximum [14,[22][23][24], where ionization effects are mainly ignored.
In this work, ionization effects are studied using MeV Si ion irradiations in single crystal 3C-SiC. The general concept is derived from the basic nature of ion-solid interactions for MeV heavy ions, for which the electronic energy loss and the ratio between electronic and nuclear stopping powers (r Se/Sn ) decrease as the ions penetrate from the surface into the bulk. By comparing the behavior of damage accumulation at different depths as a function of the local dose, the role of ionization effects is revealed. Advancing the understanding of in-cascade ionization effects on defect accumulation in SiC will promote improved predictive modeling of the response of materials to energetic ions used in fabricating electronic devices and to the high radiation environments associated with space exploration and nuclear power generation.

Experimental details
Single crystal SiC wafers used in this study were obtained from NOVA Electronic Materials. The (001) oriented 3C-SiC films were grown on a silicon substrate. The thickness of the SiC film is 3.8 μm, and the strain due to lattice mismatch is negligible in our region of interest (surface to 1.5 μm depth). Ion irradiations and Rutherford backscattering spectrometry in channeling geometry (RBS/C) were performed at the UT-ORNL Ion Beam Materials Laboratory (IBML) [25] using a 3 MV Tandem accelerator and ion beam analysis capabilities. Two groups of Si irradiations were performed: (1) 1.5 MeV Si + ion irradiation at 7°off the surface normal direction to avoid channeling effects in the single crystals and (2) 5.0 MeV Si 2+ ion irradiation at 60°off the surface normal to create a shallow damage profile that is comparable with the damage profile resulting from the 1.5 MeV Si ion irradiation. The charge states of the Si ions will not affect the ion-solid interactions because the ions reach chargestate equilibration within an extremely short time in the solid target [26] and the energy losses are independent of the initial change state [27]. Ion fluences from 9.0 × 10 13 to 1.2 × 10 16 cm −2 were employed to create damage profiles from slightly disordered to fully amorphous. The ion fluxes were 1.4 × 10 12 and 3.8 × 10 12 cm −2 s −1 for the 5.0 and 1.5 MeV ion irradiations, respectively. RBS/C measurements employing 3.5 MeV He ions were performed in situ to obtain the damage profile for each ion fluence. All the ion irradiations and RBS/C measurements were at room temperature in a high vacuum better than 2.0 × 10 −7 Torr. During the ion irradiations and RBS/C measurements, a temperature increase due to beam heating was negligible ( < 10 K). The depth profiles of local displacement dose (displacements per atom or dpa) were predicted using the SRIM 2008 code [28] in full cascade mode with a density of 3.21 g cm −3 . The threshold displacement energies used were 35 eV for Si and 20 eV for C in SRIM predictions [29].

Results
The impact of ionization effects on defect production and damage accumulation during the simultaneous deposition of irradiation energy to atomic nuclei and electrons in SiC are investigated using 5.0 and 1.5 MeV Si ions (Table 1). Energy deposition to the atomic (dE/dx nucl ) and electronic (dE/dx ele ) subsystems and damage profiles are predicted by the SRIM code, where the electronic and nuclear stopping powers are less than 4.0 and 0.40 keV nm −1 , respectively. The coupled effects are investigated with a relatively low-density energy deposition, but high r Se/Sn .
The results in Figure 1 show the SRIM-predicted displacement damage (right axis) and ionization energy (left axis) deposited from both ions and energetic recoils, as well as the electronic and nuclear stopping powers (left axis). For the 5.0 MeV irradiation at 60°, the energy deposition shown in Figure 1(a) is projected perpendicular to the surface; however, the actual stopping powers in the SiC target deposit energy along the ion penetration path. Since the ion path length (60°off the surface normal) is twice as long as the projected ion range, half of the predicted energy loss values (per unit depth) in Figure  1 were included as the actual stopping powers shown in Table 1. It is known that the surface may act as a defect sink, but the active range is not expected to be more than Table 1. SRIM-predicted energy deposition. 5.0 MeV Si at 60°1.5 MeV Si at 7°D , since the path length is twice the predicted project range perpendicular to the surface, as shown in Figure 1(a). a few tens of nanometers at room temperature [30,31]. For both irradiation conditions, the displacement damage peaks are located close to a depth of 990 nm. The damage accumulation behavior under both irradiation conditions should be comparable, with similar effects, if any, of the surface. The disorder profiles resulting from 5.0 to 1.5 MeV Si ion irradiations at different fluences are calculated by an iterative procedure [32] and shown in Figure 1(c,d), respectively. The damage accumulation as a function of local dose is plotted at different depths with different r Se/Sn and summarized in Figure 2(a-d). Ionization has its maximum at the surface (Figure 1(a,b)), which is 7.9 and 2.3 keV nm −1 for 5.0 and 1.5 MeV Si ions, respectively. The energy of the ions decreases along their path length, with dE/dx ele gradually decreasing with depth and dE/dx nucl increasing until the damage peak region at ∼ 990 nm. As a result, r Se/Sn decreases considerably from ∼ 87 and 17 to 2.0 and 0.85 for 5.0 and 1.5 MeV Si ions (Table 2), respectively. Inspection of the experimentally determined radiation damage profiles shown in Figure  1(c,d) reveals a similar shape as the SRIM-predicted displacement damage for the low-fluence samples before the completely amorphous state is achieved. With further irradiation, the disorder level increases in width. Under both Si irradiation conditions, complete amorphization  The disorder level is given at 0.10, 0.15 and 0.20 dpa. The disorder at 200 and 400 nm relative to the peak disorder at each given dose are shown as percentage for easy comparison. The corresponding curves are shown in Figure 2(b, c).
(disorder level = 1.0), as determined at the damage peak, occurs at an ion fluence of ∼ 1.1 × 10 15 cm −2 ( ∼ 0.6 dpa), as shown in Figure 1(c,d) and more clearly in Figure 2(a). In the damage peak region, the ion energy deposition to the atomic subsystem is comparable to (5.0 MeV Si), or higher (1.5 MeV Si) than that of, the electronic subsystem (Table 1). Since the energy deposition processes in the damage peak region are similar under both irradiation conditions, a similar damage response, as demonstrated in Figure 2 To better predict SiC performance under irradiation and evaluate the effects of ionization, defect evolution and damage accumulation under different energy deposition conditions (different dE/dx ele , dE/dx nucl and r Se/Sn ) are examined. The effect of varying r Se/Sn is determined through analyzing the disorder level at different depths. As shown in Table 1, electronic stopping is the primary process for energy deposition at the intermediate region from 200 to 600 nm. For these large r Se/Sn values, energy loss is dominated by ionization processes. With increasing depth, nuclear energy loss increases and the r Se/Sn values become relatively small, as the atomic displacement processes become more dominant. The damage accumulation behavior at different depths, or for different values of r Se/Sn , is shown in Figure 2(b,c) as a function of local dose for the 5.0 and 1.5 MeV Si ion irradiation, respectively. For clarity purposes, the corresponding values of dE/dx ele (keV nm −1 ) and r Se/Sn at each depth are shown. The result determined from the damage peak region, with r Se/Sn ≤ 2.0, is also included as a reference and shown as a dashed line. The role of ionization can be evaluated by comparing the trend of damage accumulation.
Evaluation of the damage accumulation at the damage peak region ( ∼ 990 nm) with respect to the intermediate depths from 200 to 500 nm is shown in Figure  2(b,c), and a much lower disorder level is observed at the shallower depths. Under the 5.0 MeV Si ion irradiation at depths of 200 and 400 nm, where r Se/Sn is 87 and 43, respectively, the disorder level is only about 53-54% or 70-74% (Table 2) relative to the damage peak region. Similar damage reduction is observed under the 1.5 MeV Si irradiation at depths of 200 and 400 nm, where r Se/Sn is 17 and 10, respectively, and the disorder level is 69-71% and 78-80%, respectively, relative to the peak region. It is worth noting that lower relative disorder is observed under the 5.0 MeV Si irradiation, compared to the 1.5 MeV results. This reduction is attributed to a more effective in-cascade ionization-induced annealing under irradiation with 5.0 MeV Si ions. As shown in Table 1 Additional evidence for in-cascade ionization-induced defect annihilation is the significant increase of the amorphization dose, shown in Figure 2(d), for 1.5 MeV Si irradiation. The critical dose for amorphization is determined to be > 1.2, 1.0 and 0.7 dpa at 200, 600 and 950 nm, respectively. Delayed damage accumulation is also demonstrated in the results shown in Figure  2(b,c), where all the trend lines at different depths are delayed when compared with the disorder accumulation at the damage peak. The delay in damage accumulation is attributed to more effective defect annihilation, resulting from both simultaneous in-cascade recovery (coupled energy deposition in both subsystems) and from ionization-induced recovery of previously created irradiation-induced defects that occurs continuous along the ion path for electronic energy loss above the threshold of 1.4 keV nm −1 determined previously [1]. For the 1.5 MeV Si irradiation, the trend of shifting to higher dose to reach the same level of disorder at shallower depths clearly indicates more effective defect annealing, resulting from the increase of r Se/Sn from ∼ 0.85 to 17 and the decrease of energy deposition to the atomic subsystem (0.426-0.108 keV nm −1 ), as indicated in Table 1. While ionization-induced annealing of pre-existing damage is active above 1.4 keV nm −1 [1], ionization-induced annealing process is also observed at 400, 500 and 600 nm, as shown in Figure 2(c,d), where the electronic energy deposition is 1.41, 1.21 and 1.04 keV nm −1 , respectively. This suggests that incascade ionization-induced annealing has a lower electronic energy loss threshold at ∼ 1.0 keV nm −1 , less than the threshold of 1.4 keV nm −1 determined in a separate effects study [1].
In-cascade ionization-induced defect annihilation is further demonstrated in Figure 3 from the increasing critical dose for amorphization with the increasing ratio of electronic to nuclear energy loss. Under Si irradiations at room temperature (T RT ), the thermally induced recovery is insignificant, the critical dose, D, for amorphization has a general expression that [33,34]: where D 0 is the amorphization dose at 0 K or at a temperature where flux effects are negligible, E irr is the activation energy for the irradiation-induced recovery, σ r and σ d are the irradiation-induced recovery cross-section and the damage cross-section, respectively. For the 1.5 MeV Si irradiation, the critical dose, D, can be estimated from the disorder profiles at different depths. To reduce the analysis error, the critical dose D is estimated at a disorder level of 0.97. Based on Equation (1), 1/D should be linearly proportional to σ r /σ d , and as shown in Figure  3, 1/D exhibits a linear dependence on the ratio of ionization to dpa rate. These results suggest that ionizationinduced recovery and displacement damage production through elastic collisions are the competing mechanisms responsible for the observed delayed damage accumulation shown in Figure 2.

Discussion
Normally when materials are bombarded with energetic particles, such as ions or neutrons, the materials are damaged at the atomic scale. The accumulation of radiation defects results in structure and property degradation. Recent work of Zhang et al. [1] has demonstrated that pre-existing defects in SiC can be healed by exposure to a beam of highly energized charged particles, due to the energy transferred to the target electrons that causes a highly localized thermal spike that promotes healing. The present work has further revealed that in-cascade ionization effects are also effective in suppressing or delaying damage accumulation. The experimentally determined threshold for the in-cascade ionization-induced annealing is an electronic energy loss of ∼ 1.0 keV nm −1 , which is slightly less than the threshold of 1.4 keV nm −1 determined from the previous separate effects study [1]. The exposure of SiC to energetic ions, even with energies of a few MeV, can cause local heating along the ion path and thermally instability of the potential energy landscape within the cascades. Such non-equilibrium processes can result in a significant reduction in the formation of stable defects from cascade events, as well as structural disorder at the microscopic and atomic scales. Both ionization-induced healing of pre-existing defects from our previous study [1] and suppression of the defect production within cascade events in this work draw attention to the fact that energy transferred to electrons in SiC by energetic ions via ionization can effectively heal defects and restore structural order. Because SiC is a critical material for nuclear applications and power electronics for space applications, ionization-induced selfhealing may contribute to increased radiation tolerance in extreme radiation environments.

Conclusion
In-cascade defect, production and annealing have been studied in single crystal 3C-SiC using MeV Si ion irradiations. This work has demonstrated that, in the MeV energy regime, damage accumulation is effectively delayed as the ratio of electronic to nuclear stopping, r Se/Sn , increases. The critical dose for amorphization increases as a function of ionization to dpa rate. For in-cascade annealing, a threshold value of ionization is estimated to be surprisingly low at ∼ 1.0 keV nm −1 , which is in the region of interest for routinely utilized ion implantation doping and ion irradiation processes. Such a significant healing mechanism must be considered in evaluating the response of SiC to ion implantation processes for the electronic devices and to the high radiation environments in space exploration and nuclear applications.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division [DE-AC05-00OR22725]. Ion beam work was performed at the Ion Beam Materials Laboratory located on the campus of the University of Tennessee, Knoxville.