⇦Chinese Physics Letters, 2017, Vol. 34, No. 1, Article code 012801 H-ion Irradiation-induced Annealing in He-ion Implanted 4H-SiC * Yi Han(韩驿)1,2, Bing-Sheng Li(李炳生)1**, Zhi-Guang Wang(王志光)1**, Jin-Xin Peng(彭金鑫)3, Jian-Rong Sun(孙建荣)1, Kong-Fang Wei(魏孔芳)1, Cun-Feng Yao(姚存峰)1, Ning Gao(高宁)1, Xing Gao(高星)1, Li-Long Pang(庞立龙)1, Ya-Bin Zhu(朱亚滨)1, Tie-Long Shen(申铁龙)1, Hai-Long Chang(常海龙)1, Ming-Huan Cui(崔明焕)1, Peng Luo(骆鹏)1, Yan-Bin Sheng(盛彦斌)1, Hong-Peng Zhang(张宏鹏)1, Xue-Song Fang(方雪松)1,2, Si-Xiang Zhao(赵四祥)1, Jin Jin(金锦)1, Yu-Xuan Huang(黄玉璇)1, Chao Liu(刘超)1,2, Dong Wang(王栋)1,2, Wen-Hao He(何文豪)1,2, Tian-Yu Deng(邓天虞)1,2, Peng-Fei Tai(台鹏飞)1, Zhi-Wei Ma(马志伟)1 Affiliations 1Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000 2University of Chinese Academy of Sciences, Beijing 100049 3School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000 Received 20 September 2016 *Supported by the National Natural Science Foundation of China under Grant Nos 11005130, 11475229 and 91026002, and the Strategic Priority Research Program of Chinese Academy of Sciences under Grant No XDA03010301.
**Corresponding author. Email: b.s.li@impcas.ac.cn; zhgwang@impcas.ac.cn Citation Text: Han Y, Li B S, Wang Z G, Peng J X and Sun J R et al 2017 Chin. Phys. Lett. 34 012801 Abstract Radiation-induced defect annealing in He$^{+}$ ion-implanted 4H-SiC via H$^{+}$ ion irradiation is investigated by Raman spectroscopy. There are 4H-SiC wafers irradiated with 230 keV He$^{+}$ ions with fluences ranging from $5.0\times10^{15}$ cm$^{-2}$ to $2.0\times10^{16}$ cm$^{-2}$ at room temperature. The post-implantation samples are irradiated by 260 keV H$^{+}$ ions at a fluence of $5.0\times10^{15}$ cm$^{-2}$ at room temperature. The intensities of Raman lines decrease after He implantation, while they increase after H irradiation. The experimental results present that the magnitude of Raman line increment is related to the concentration of pre-existing defects formed by He implantation. A strong new peak located near 966 cm$^{-1}$, which is assigned to 3C-SiC LO (${\it \Gamma}$) phonon, is found in the He-implanted sample with a fluence of $5.0\times10^{15}$ cm$^{-2}$ followed by H irradiation. However, for the He-implanted sample with a fluence of $2.0\times10^{16}$ cm$^{-2}$ followed by H irradiation, no 3C-SiC phonon vibrations are found. The detailed reason for H irradiation-induced phase transformation in the He-implanted 4H-SiC is discussed. DOI:10.1088/0256-307X/34/1/012801 PACS:28.41.Qb, 61.80.Jh, 61.82.Fk, 78.30.-j © 2017 Chinese Physics Society Article Text Silicon carbide (SiC) is an important nuclear material due to its high melting temperature, corrosion resistance, high thermal conductivity, low thermal expansion and low neutron reaction cross section. Therefore, SiC materials are very suitable for fusion, accelerator driven systems and advanced fission reactors.[1,2] As for the main candidate materials of the project of the China Initiative Accelerator Driven System,[3] SiC materials have been extensively studied on displacement cascades, defect accumulation and amorphization. Usually, irradiation damage induces material hardening, embrittlement, swelling, and so on, thus it is important to reduce the irradiation-induced damage. Thermal annealing is an ordinary method to reduce irradiation damage. Once the amorphous phase in SiC is formed, the annealing temperature over 2000 K is required to fully recrystallize the amorphous phase. Alternatively, the strong electronic energy loss ($>$1.4 keVnm$^{-1}$) or the nuclear energy loss due to other ion irradiation can cause pre-existing defect annealing in SiC.[4-6] It has been reported that the 3C-SiC samples were implanted with 100 keV Fe ions at room temperature with fluences of 0.36 and 0.72 dpa, and then irradiated with 0.87 GeV Pb ions at room temperature, the recrystallization process induced by electronic energy loss ranging from 10 to 33 keVnm$^{-1}$.[4] Zhang et al. reported that some light ions (C, O, Si and Ni) with energy range of several MeV to tens of MeV could also anneal SiC pre-existing defects and restore the structural order due to the electronic energy loss ($>$1.4 keV nm$^{-1}$).[5] Heera et al. reported the amorphous layer formed by 200 keV Ge$^{+}$ ion implantation into 6H-SiC and subsequent 300 keV Si$^{+}$ ion irradiation. They found ion-beam-induced random nucleation of crystalline grains at 773 K and ion-beam-induced epitaxial crystallization at 1323 K.[6] In addition, the same temperature of 1323 K, the recrystallization of 6H-SiC is substantially improved by Si$^{+}$ ion irradiation compared with that of thermal annealing. In the strong radiation environments, dense H and He atoms are produced by (n, p) and (n, $\alpha$) reactions, respectively. Energetic H and He ions would induce lattice defects in SiC. Therefore, synergistic effects of H and He irradiation into SiC are important for their nuclear energy applications. The present work shows that the nuclear energy loss of H ions causes pre-existing defect annealing in He-implanted SiC. This finding is very important to facilitate the design of radiation-tolerant materials for advanced nuclear energy systems. We use Raman scattering to investigate pre-existing defect annealing in 6H-SiC, because it is non-destructive and it can detect some complementary aspects of the transformation between the crystalline and amorphous state. Raman spectroscopy is an excellent spectroscopic tool to give additional details on the composition of damaged layers and shows the appearance of chemical reordering. In addition, Raman spectra is very sensitive to the initial formation stage of lattice defects, which are difficult to detect by transmission electron microscopy.[7,8]
cpl-34-1-012801-fig1.png
Fig. 1. (a) Depth distribution of He concentration and displacement damage in SiC implanted with 230 keV He$^{+}$ ions to a fluence of 1.$\times$10$^{16}$ ions/cm$^{2}$, according to SRIM-2008. (b) The TRIM result of the samples irradiated with 230 keV H$^{+}$ ions to a fluence of $5.0\times10^{15}$ cm$^{-2}$. Here $(dE/dx)_{\rm elec}$ denotes the electronic energy loss, and $(dE/dx)_{\rm nucl}$ denotes the nuclear energy loss.
The n-type (0001)$_{\rm Si}$ 4H-SiC single crystals were supplied by the MTI company. The 4H-SiC wafers were irradiated with 230 keV He$^{+}$ ions with fluences of $5.0\times10^{15}$ cm$^{-2}$ and $2.0\times10^{16}$ cm$^{-2}$ at room temperature, respectively. The post-implantation samples were irradiated by 260 keV H$^{+}$ ions at a fluence of $5.0\times10^{15}$ cm$^{-2}$ at room temperature. The experiment was taken in a terminal of the 320 kV high-voltage experimental platform equipped with an electron cyclotron resonance (ECR) ion source in the Institute of Modern Physics, Lanzhou. The beam current was limited to 4 μA to avoid the specimen heating effect. The stopping and range of ions in matter (SRIM) calculations (Fig. 1) use threshold displacement energies of 20 and 35 eV for C and Si, respectively.[9,10] After He implantation, the maximum displacement damage peak is at 730 nm from the surface, the corresponding damage peak is about 0.4 dpa, and the He ion concentration peak is 0.7% at about 790 nm from the surface. Figure 2 shows the transport of ions in matter (TRIM) result of the samples irradiated with 260 keV H$^{+}$ ions to a fluence of $5.0\times10^{15}$ cm$^{-2}$. Here $(dE/dx)_{\rm elec}$ denotes the electronic energy loss, and $(dE/dx)_{\rm nucl}$ denotes the nuclear energy loss. With increasing the depth along the ion range, the electronic energy loss initially increases slowly, and finally decreases sharply. However, the nuclear energy loss increases along the ion range and then obtains the maximum at the end of the ion range.[11] As can be seen, the projected range of He$^{+}$ ions is approximately 800 nm, where $(dE/dx)_{\rm elec}$ and $(dE/dx)_{\rm nucl}$ are 0.156 keV/nm and 0.00015 keV/nm, respectively. We can regard that in the H$^{+}$ ion irradiation, the electron energy loss is three orders of magnitude larger than the nuclear energy loss at the depth of the He-induced damage layer.
cpl-34-1-012801-fig2.png
Fig. 2. Raman spectra measured with a quasi backscattering geometry for 4H-SiC polytype.
Raman scattering and confocal Raman spectra were recorded at room temperature in a backscattering geometry on the analysis of the test platform of Institute of Modern Physics, Chinese Academy of Sciences, using the Horiba Jobin–Yvon Labram HR 800UV laser confocal Raman microspectroscopy. The microscope stage could be adjusted with an accuracy of 0.5 along the optical axis. The 532 nm line of an argon ion laser was focused on a spot, collected through a 100 objective with a numerical aperture value analyzing the different Si–Si and Si–C related modes, the Raman spectra ranges from 100 to 1500 cm$^{-1}$. Figure 2 shows the whole Raman spectra of 4H-SiC polytype. For 4H-SiC, the E$_{2}$ (TA), A$_{1}$ (LA), E$_{2}$ (TO), E$_{1}$ (TO) and A$_{1}$ (LO) bands are observed at 196/204, 610, 776, 797 and 996 cm$^{-1}$, respectively, which are very close to some existing values.[12-14] Figure 3(a) shows the Raman spectra of 4H-SiC before and after implantation with He ions to fluences of 1.0$\times$10$^{15}$, $5.0\times10^{15}$, $8.0\times10^{15}$, $1.0\times10^{16}$ and $2.0\times10^{16}$ cm$^{-2}$. It should be noted that the characteristic peaks of the samples after He implantation do not occur with lateral shift, which indicates that the He implantation-induced stress is not enough to cause the offset of Raman peaks. The Raman spectra of $1.0\times10^{15}$ cm$^{-2}$ ions implanted sample have no significant change compared with that of the as-grown sample. With increasing the fluence, the width of A$_{1}$ (LO) peak decreases obviously. Because phonon lifetime is inversely proportional to the peak width, He implantation reduces carrier concentration profiles, resulting in decreasing the scattering probability and increasing the phonon lifetime.[13,14] When fluence increases to $8.0\times10^{15}$ cm$^{-2}$, there are three new Raman scattering peaks at 200, 540 and 660 cm$^{-1}$, corresponding to crystalline Si (TA), crystalline Si (TO) and disordered SiC Raman spectra, respectively. It is well known that pre-existing chemical bonds break to form new Si–Si and C–C bonds in the disordered SiC. In addition, the E$_{2}$ (TA), A$_{1}$ (LA), and E$_{1}$ (TO) characteristic peaks of Raman spectra completely disappear. The intensities of Raman spectra decrease due to the increase of the absorption of Raman scattering.[15]
cpl-34-1-012801-fig3.png
Fig. 3. (a) Raman spectra of He-implanted 4H-SiC to fluences of $1.0\times10^{15}$ cm$^{-2}$ (2), $5.0\times10^{15}$ cm$^{-2}$ (3), $8.0\times10^{15}$ cm$^{-2}$ (4), $1.0\times10^{16}$ cm$^{-2}$ (5) and $2.0\times10^{16}$ cm$^{-2}$ (6) at room temperature, compared with the as-grown sample (1). (b) The relative Raman intensity of He-implanted 4H-SiC at room temperature is plotted versus the fluence of E$_{2}$ (TO).
It is a usual method to calculate the SiC total disorder based on the Raman peaks relative strength. In this work, the area of E$_{2}$ (TO) Raman peak is used to calculate the relative disorder intensity. Figure 3(b) shows that the disorder of the sample increases with the fluence. The damage accumulation versus implantation fluence can be divided into two stages (0–0.24 dpa and 0.24–0.8 dpa). In the first stage (0–0.24 dpa), the irradiation area mainly consists of point defects and small clusters, corresponding to the low damage level. At the second stage (0.24–0.8 dpa), the damage level rapidly increases with the fluence. The relative Raman intensity equals to 1 at 0.32 dpa, corresponding to the dimorphous phase formed in the damage layer. Figure 4 shows that the intensities of the E$_{2}$ (TA), A$_{1}$ (LO), E$_{2}$ (TO), E$_{1}$ (TO) and A$_{1}$ (LO) peaks increase after H irradiation, indicating pre-existing defect recovery after H irradiation. The Raman intensity increment is related to the pre-existing damage level. In detail, the Raman intensity increment is more significant at a lower initial damage level, as shown in Fig. 4. Other data proves the damage recovery, that a strong new peak appears nearly at 966 cm$^{-1}$, which is assigned to 3C-SiC LO (${\it \Gamma}$) presented in Fig. 4. It is well known that the 3C-SiC structure nucleates in amorphous layers of hexagonal SiC by thermal annealing and by ion-beam induced epitaxial recrystallization. The present experimental results demonstrate that H irradiation plays a role the same as thermal annealing. The pre-existing defect recovery upon H irradiation is suggested to originate from ion beam induced epitaxial crystallization. Heera et al. investigated ion beam induced recrystallization of amorphous layers in 6H-SiC.[6] They used 300 keV Si$^{+}$ irradiation with a fluence of $1.0\times10^{17}$ cm$^{-2}$ at 773 K and 1323 K. The 6H-SiC epitaxially aligned with the substrate and polycrystalline materials containing 3C-SiC structure after Si$^{+}$ ion irradiation at 773 K. However, the necessary ion fluence of $1.0\times10^{17}$ cm$^{-2}$ is too high for practical application. In the present work, we use 260 keV H$^{+}$ ion irradiation to a fluence of $5.0\times10^{15}$ cm$^{-2}$ at room temperature. Though almost no effects of H irradiation on an amorphous layer are formed in He-implanted 6H-SiC, the significant recrystallization of He-implanted 4H-SiC with a relative damage of 0.55 occurs after H irradiation. This is very useful for practical applications. For example, in the nuclear radiation environments, dense energetic H and He atoms are produced simultaneously. Therefore, the He implantation-induced defects can be annealed out upon H irradiation immediately, indicating that the amorphous phase has never been formed under such a condition. The present experimental results demonstrate a strong reduction of the damage production to extend SiC material lifetime in nuclear reactors.
cpl-34-1-012801-fig4.png
Fig. 4. Raman spectra of He-implanted 4H-SiC to fluences of $5.0\times10^{15}$ cm$^{-2}$ (3) and $2.0\times10^{16}$ cm$^{-2}$ (5) at room temperature, post-He implantation, H irradiation to a fluence of $5.0\times10^{15}$ cm$^{-2}$ at room temperature for the low fluence He-implanted sample (2) and the high fluence He-implanted sample (4), compared with the as-grown sample (1).
A clear effect of defect annealing is observed in 4H-SiC implanted with He$^{+}$ ions and subsequently irradiated with H$^{+}$ ions to a fluence of $5.0\times10^{15}$ cm$^{-2}$ at room temperature. The pre-existing defect recovery is related to the initial damage level. The significant defect recovery is observed in the He-implanted 4H-SiC with a fluence of $5.0\times10^{15}$ cm$^{-2}$, corresponding to a relative damage of 0.55. Our findings have significant implications for SiC materials in extreme radiation environments, where expected combined energetic H and He atoms may lead to a strong reduction of damage production allowing the preservation of the physical properties of SiC materials. We would like to warmly thank staff and teachers in the 320 kV high-voltage platform and the instruments and equipments sharing platform in Institute of Modern Physics for their assistance in the ion irradiation experiment. In addition, we would like to appreciate Xu Lijun for Raman spectroscopy measurement. References Influence of irradiation spectrum and implanted ions on the amorphization of ceramicsCritical issues and current status of SiC/SiC composites for fusionCombined experimental and computational study of the recrystallization process induced by electronic interactions of swift heavy ions with silicon carbide crystalsIonization-induced annealing of pre-existing defects in silicon carbideAmorphization and recrystallization of 6H-SiC by ion-beam irradiationDisplacement energy surface in 3C and 6H SiCComparison of threshold displacement energies in β-SiC determined by classical potentials and ab initio calculationsOptical spectroscopy study of damage induced in 4H-SiC by swift heavy ion irradiationRaman spectroscopy study of heavy-ion-irradiated α-SiCDeep ultraviolet Raman scattering characterization of ion-implanted SiC crystals
[1] Zinkle S J and Snead L L 1996 Nucl. Instrum. Methods Phys. Res. Sect. B 116 92
[2] Hasegawa A, Kohyama A, Jonec R H, Snead L L, Riccardi B and Fenici P 2000 J. Nucl. Mater. 283 128
[3]Fu S N, Chen H S, Chi Y L, Fang S X, Ma L, Pan W M, Tang J Y, Zhang C, He Y and Zhao H W 2011 15th International Conference on RF Superconductivity (Chicago, USA 25–29 July 2011) p 977
[4] Debelle A, Backman M, Thomé L, Weber W J, Toulemonde M, Mylonas S, Boulle A, Pakarinen O H, Juslin N, Djurabekova F, Nordlund K, Garrido F and Chaussende D 2012 Phys. Rev. B 86 100102
[5] Zhang Y W, Sachan R, Pakarinen O H, Chisholm M F, Liu P, Xue H Z and Weber W J 2015 Nat. Commun. 6 8049
[6] Heera V, Stoemenso J, Koler R and Skorupa W 1995 J. Appl. Phys. 77 2999
[7]Zhang H H, Zhang C H, Li B S, Zhou L H, Yang Y T and Fu Y Z 2009 Acta Phys. Sin. 58 3302 (in Chinese)
[8]Xu C L, Zhang C H, Li B S et al 2011 Nucl. Phys. Rev. 28 209 (in Chinese)
[9] Devanathan R and Weber W J 2000 J. Nucl. Mater. 278 258
[10] Lucas G and Pizzagalli L 2005 Nucl. Instrum. Methods Phys. Res. Sect. B 229 359
[11] Sorieul S, Kerbiriou X, Costantini J M, Gosmain L, Calas G and Trautmann C 2012 J. Phys.: Condens. Matter 24 125801
[12] Sorieul S, Costantini J M, Gosmain L, Thomé L and Grob J J 2006 J. Phys.: Condens. Matter 18 5235
[13] Nakashima S, Mitani T, Senzaki J, Okumura H and Yamamoto T 2005 J. Appl. Phys. 97 123507
[14]Wang S G, Chai J R and Wang D 2015 J. Northwest Univ. (Nat. Sci. Ed.) 45 229 (in Chinese)
[15]Du Y Y, Li B S, Wang Z G, Sun J R, Yao C F, Chang H L, Pang L L, Zhu Y B, Cui M H, Zhang H P, Li Y F, Wang J, Zhu H P, Song P and Wang D 2014 Acta Phys. Sin. 63 216101 (in Chinese)