#### Special topics

SPECIAL TOPIC — Ion beam modification of materials and applications

 Select Optical spectroscopy study of damage evolution in 6H-SiC by H$_{2}^{ + }$ implantation Yong Wang(王勇), Qing Liao(廖庆), Ming Liu(刘茗), Peng-Fei Zheng(郑鹏飞), Xinyu Gao(高新宇), Zheng Jia(贾政), Shuai Xu(徐帅), and Bing-Sheng Li(李炳生) Chin. Phys. B, 2021, 30 (5): 056106.   DOI: 10.1088/1674-1056/abe9a7 Abstract （268）   HTML （1）    PDF （1911KB）（93）       Lattice defects induced by ion implantation into SiC have been widely investigated in the decades by various techniques. One of the non-destructive techniques suitable to study the lattice defects in SiC is the optical characterization. In this work, confocal Raman scattering spectroscopy and photoluminescence spectrum have been used to study the effects of 134-keV H$_{2}^{ + }$ implantation and thermal treatment in the microstructure of 6H-SiC single crystal. The radiation-induced changes in the microstructure were assessed by integrating Raman-scattering peaks intensity and considering the asymmetry of Raman-scattering peaks. The integrated intensities of Raman scattering spectroscopy and photoluminescence spectrum decrease with increasing the fluence. The recovery of the optical intensities depends on the combination of the implantation temperature and the annealing temperature with the thermal treatment from 700 ℃ to 1100 ℃. The different characterizations of Raman scattering spectroscopy and photoluminescence spectrum are compared and discussed in this study.
 Select Structure and luminescence of a-plane GaN on r-plane sapphire substrate modified by Si implantation Lijie Huang(黄黎杰), Lin Li(李琳), Zhen Shang(尚震), Mao Wang(王茂), Junjie Kang(康俊杰), Wei Luo(罗巍), Zhiwen Liang(梁智文), Slawomir Prucnal, Ulrich Kentsch, Yanda Ji(吉彦达), Fabi Zhang(张法碧), Qi Wang(王琦), Ye Yuan(袁冶), Qian Sun(孙钱), Shengqiang Zhou(周生强), and Xinqiang Wang(王新强) Chin. Phys. B, 2021, 30 (5): 056104.   DOI: 10.1088/1674-1056/abd76a Abstract （273）   HTML （1）    PDF （1426KB）（74）       We show the structural and optical properties of non-polar a-plane GaN epitaxial films modified by Si ion implantation. Upon gradually raising Si fluences from 5×1013 cm-2 to 5×1015 cm-2, the n-type dopant concentration gradually increases from 4.6×1018 cm-2 to 4.5×1020 cm-2, while the generated vacancy density accordingly raises from 3.7×1013 cm-2 to 3.8×1015 cm-2. Moreover, despite that the implantation enhances structural disorder, the epitaxial structure of the implanted region is still well preserved which is confirmed by Rutherford backscattering channeling spectrometry measurements. The monotonical uniaxial lattice expansion along the a direction (out-of-plane direction) is observed as a function of fluences till 1×1015 cm-2, which ceases at the overdose of 5×1015 cm-2 due to the partial amorphization in the surface region. Upon raising irradiation dose, a yellow emission in the as-grown sample is gradually quenched, probably due to the irradiation-induced generation of non-radiative recombination centers.
 Select Evolution of ion-irradiated point defect concentration by cluster dynamics simulation Shuaishuai Feng(冯帅帅), Shasha Lv(吕沙沙), Liang Chen(陈良), and Zhengcao Li(李正操) Chin. Phys. B, 2021, 30 (5): 056105.   DOI: 10.1088/1674-1056/abf102 Abstract （241）   HTML （0）    PDF （633KB）（81）       The relationship between ions irradiation and the induced microstructures (point defects, dislocations, clusters, etc.) could be better analyzed and explained by simulation. The mean field rate theory and cluster dynamics are used to simulate the effect of implanted Fe on the point defects concentration quantitatively. It is found that the depth distribution of point defect concentration is relatively gentle than that of damage calculated by SRIM software. Specifically, the damage rate and point defect concentration increase by 1.5 times and 0.6 times from depth of 120 nm to 825 nm, respectively. With the consideration of implanted Fe ions, which effectively act as interstitial atoms at the depth of high ion implantation rate, the vacancy concentration Cv decreases significantly after reaching the peak value, while the interstitial atom concentration Ci increases significantly after decline of the previous stage. At the peak depth of ion implantation, Cv dropped by 86%, and Ci increased by 6.2 times. Therefore, the implanted ions should be considered into the point defects concentration under high dose of heavy ion irradiation, which may help predict the concentration distribution of defect clusters, further analyzing the evolution behavior of solute precipitation.
 Select Effect of tellurium (Te4+) irradiation on microstructure and associated irradiation-induced hardening Hefei Huang(黄鹤飞), Jizhao Liu(刘继召), Guanhong Lei(雷冠虹), Ondrej Muránsky, Tao Wei, and Mihail Ionescu Chin. Phys. B, 2021, 30 (5): 056108.   DOI: 10.1088/1674-1056/abf039 Abstract （224）   HTML （0）    PDF （3151KB）（64）       The GH3535 alloy samples were irradiated using 15-MeV Te4+ ions at 650 °C to a dose of 0.5, 3.0, 10, and 20 dpa, respectively. The Te atoms distribution and microstructure evolution were examined by electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). The nano-indenter was then used to measure the nano-hardness changes of samples before and after irradiation. TEM results showed the formation of dislocation loops in the irradiated samples. Their mean diameters increase with the increase of irradiation dose and tends to be saturated when irradiation dose exceeds 10 dpa. The ratio of yield strength increments calculated by dispersed barrier hardening (DBH) model is basically consistent with that of nano-hardness increments measured by nano-indenter. In addition, the relationship between the nano-hardness increments and dpa for the GH3535 alloy irradiated by Te ions has been revealed in the study.
 Select Corrosion behavior of high-level waste container materials Ti and Ti-Pd alloy under long-term gamma irradiation in Beishan groundwater Qianglin Wei(魏强林), Yuhong Li(李玉红), Yanliang Huang(黄彦良), Dongyan Yang(杨冬燕), Bo Yang(杨波), and Yibao Liu(刘义保) Chin. Phys. B, 2021, 30 (5): 056109.   DOI: 10.1088/1674-1056/abf03b Abstract （246）   HTML （0）    PDF （3435KB）（75）       Titanium and titanium-palladium alloys are important potential materials for nuclear waste container, which will endure both intense γ-irradiation and groundwater erosion. Therefore, it is very important to investigate the corrosion behavior of the container materials. In this research, the cumulative dose effect of TA8-1 type titanium-palladium alloy (TA8-1) and TA2-type pure titanium (TA2) under γ-irradiation was studied based on the geological disposal of nuclear wastes. The irradiation experiments were performed at room temperature using 60Co gamma sources with a 5.0-kGy·h-1 intensity for 40, 80 or 160 days, respectively. The pH value and conductivity of Beishan groundwater were investigated. The results showed that the pH value changed from alkaline (8.22) to acidic (2.46 for TA8-1 and 2.44 for TA2), while the un-irradiated solution remained alkaline (8.17 for TA8-1 and 8.20 for TA2) after 160 days. With the increase of irradiation dose, the conductivity increases rapidly and then tends to become stable, which indicates that the titanium dioxide corrosion layer formed on the surface of the sample surface effectively prevents further corrosion. Meanwhile, XRD and SEM-EDS analysis results show that the main components of corrosion products are TiO2 and TiO. The titanium on the surface of the sample is oxidized, resulting in slight uneven local corrosion. The results show that TA8-1 and TA2 are suitable to be used as candidate materials for high-level waste (HLW) disposal containers due to their excellent performance under long-term and high-dose irradiation corrosion.
 Select Degradation of β-Ga2O3 Schottky barrier diode under swift heavy ion irradiation Wen-Si Ai(艾文思), Jie Liu(刘杰), Qian Feng(冯倩), Peng-Fei Zhai(翟鹏飞), Pei-Pei Hu(胡培培), Jian Zeng(曾健), Sheng-Xia Zhang(张胜霞), Zong-Zhen Li(李宗臻), Li Liu(刘丽), Xiao-Yu Yan(闫晓宇), and You-Mei Sun(孙友梅) Chin. Phys. B, 2021, 30 (5): 056110.   DOI: 10.1088/1674-1056/abf107 Abstract （252）   HTML （0）    PDF （1008KB）（87）       The electrical characteristics and microstructures of β-Ga2O3 Schottky barrier diode (SBD) devices irradiated with swift heavy ions (2096 MeV Ta ions) have been studied. It was found that β-Ga2O3 SBD devices showed the reliability degradation after irradiation, including turn-on voltage Von, on-resistance Ron, ideality factor n, and the reverse leakage current density Jr. In addition, the carrier concentration of the drift layer was decreased significantly and the calculated carrier removal rates were 5×106-1.3×107 cm-1. Latent tracks induced by swift heavy ions were observed visually in the whole β-Ga2O3 matrix. Furthermore, crystal structure of tracks was amorphized completely. The latent tracks induced by Ta ions bombardments were found to be the reason for the decrease in carrier mobility and carrier concentration. Eventually, these defects caused the degradation of electrical characteristics of the devices. In terms of the carrier removal rates, the β-Ga2O3 SBD devices were more sensitive to swift heavy ions irradiation than SiC and GaN devices.
 Select Influence of temperature and alloying elements on the threshold displacement energies in concentrated Ni-Fe-Cr alloys Shijun Zhao(赵仕俊) Chin. Phys. B, 2021, 30 (5): 056111.   DOI: 10.1088/1674-1056/abf10d Abstract （215）   HTML （0）    PDF （3382KB）（76）       Concentrated solid-solution alloys (CSAs) have demonstrated promising irradiation resistance depending on their compositions. Under irradiation, various defects can be produced. One of the most important parameters characterizing the defect production and the resulting defect number is the threshold displacement energies (Ed). In this work, we report the results of Ed values in a series of Ni-Fe-Cr concentrated solid solution alloys through molecular dynamics (MD) simulations. Based on several different empirical potentials, we show that the differences in the Ed values and its angular dependence are mainly due to the stiffness of the potential in the intermediate regime. The influences of different alloying elements and temperatures on Ed values in different CSAs are further evaluated by calculating the defect production probabilities. Our results suggest a limited influence of alloying elements and temperature on Ed values in concentrated alloys. Finally, we discuss the relationship between the primary damage and Ed values in different alloys. Overall, this work presents a thorough study on the Ed values in concentrated alloys, including the influence of empirical potentials, their angular dependence, temperature dependence, and effects on primary defect production.
 Select Cathodic shift of onset potential on TiO2 nanorod arrays with significantly enhanced visible light photoactivity via nitrogen/cobalt co-implantation Xianyin Song(宋先印), Hongtao Zhou(周洪涛), and Changzhong Jiang(蒋昌忠) Chin. Phys. B, 2021, 30 (5): 058505.   DOI: 10.1088/1674-1056/abee07 Abstract （248）   HTML （0）    PDF （1195KB）（76）       Despite anionic doping has been widely implemented to increase the visible light activity of TiO$_{2}$, it often gives rise to a dramatical anodic shift in current onset potential. Herein, we show an effective method to achieve the huge cathodic shift of TiO$_{2}$ photoanode with significantly enhanced visible light photo-electrochemical activity by nitrogen/cobalt co-implantation. The nitrogen/cobalt co-doped TiO$_{2}$ nanorod arrays (N/Co-TiO$_{2}$) exhibit a cathodic shift of 350 mV in onset potential relative to only nitrogen-doped TiO$_{2}$ (N-TiO$_{2}$). Moreover, the visible-light ($\lambda >420$ nm) photocurrent density of N/Co-TiO$_{2}$ reaches 0.46 mA/cm$^{2}$, far exceeding 0.07 mA/cm$^{2}$ in N-TiO$_{2}$ at 1.23 V $versus$ reversible hydrogen electrode (RHE). Systematic characterization studies demonstrate that the enhanced photo-electrochemical performance can be attributed to the surface synergic sputtering of high-energy nitrogen/cobalt ions.
 Select Evolution of helium bubbles in nickel-based alloy by post-implantation annealing Rui Zhu(朱睿), Qin Zhou(周钦), Li Shi(史力), Li-Bin Sun(孙立斌), Xin-Xin Wu(吴莘馨), Sha-Sha Lv(吕沙沙), and Zheng-Cao Li(李正操) Chin. Phys. B, 2021, 30 (8): 086102.   DOI: 10.1088/1674-1056/ac0783 Abstract （220）   HTML （0）    PDF （3497KB）（59）       Nickel-based alloys have been considered as candidate structural materials used in generation IV nuclear reactors serving at high temperatures. In the present study, alloy 617 was irradiated with 180-keV helium ions to a fluence of 3.6×1017 ions/cm2 at room temperature. Throughout the cross-section transmission electron microscopy (TEM) image, numerous over-pressurized helium bubbles in spherical shape are observed with the actual concentration profile a little deeper than the SRIM predicted result. Post-implantation annealing was conducted at 700 ℃ for 2 h to investigate the bubble evolution. The long-range migration of helium bubbles occurred during the annealing process, which makes the bubbles of the peak region transform into a faceted shape as well. Then the coarsening mechanism of helium bubbles at different depths is discussed and related to the migration and coalescence (MC) mechanism. With the diffusion of nickel atoms slowed down by the alloy elements, the migration and coalescence of bubbles are suppressed in alloy 617, leading to a better helium irradiation resistance.
 Select Microstructure evolution of T91 steel after heavy ion irradiation at 550 ℃ Ligang Song(宋力刚), Bo Huang(黄波), Jianghua Li(李江华), Xianfeng Ma(马显锋), Yang Li(李阳), Zehua Fang(方泽华), Min Liu(刘敏), Jishen Jiang(蒋季伸), and Yanying Hu(胡琰莹) Chin. Phys. B, 2021, 30 (8): 086103.   DOI: 10.1088/1674-1056/ac0a64 Abstract （338）   HTML （1）    PDF （7682KB）（87）       Fe-Cr ferritic/martensitic (F/M) steels have been proposed as one of the candidate materials for the Generation IV nuclear technologies. In this study, a widely-used ferritic/martensitic steel, T91 steel, was irradiated by 196-MeV Kr+ ions at 550 ℃. To reveal the irradiation mechanism, the microstructure evolution of irradiated T91 steel was studied in details by transmission electron microscope (TEM). With increasing dose, the defects gradually changed from black dots to dislocation loops, and further to form dislocation walls near grain boundaries due to the production of a large number of dislocations. When many dislocation loops of primary a0/2<111> type with high migration interacted with other defects or carbon atoms, it led to the production of dislocation segments and other dislocation loops of a0<100> type. Lots of defects accumulated near grain boundaries in the irradiated area, especially in the high-dose area. The grain boundaries of martensite laths acted as important sinks of irradiation defects in T91. Elevated temperature facilitated the migration of defects, leading to the accumulation of defects near the grain boundaries of martensite laths.
 Select Mechanism of defect evolution in H+ and He+ implanted InP Ren-Jie Liu(刘仁杰), Jia-Jie Lin(林家杰), N Daghbouj, Jia-Liang Sun(孙嘉良), Tian-Gui You(游天桂), Peng Gao(高鹏), Nie-Feng Sun(孙聂枫), and Min Liao(廖敏) Chin. Phys. B, 2021, 30 (8): 086104.   DOI: 10.1088/1674-1056/abf640 Abstract （196）   HTML （0）    PDF （1486KB）（78）       The defect evolution in InP with the 75 keV H+ and 115 keV He+ implantation at room temperature after subsequent annealing has been investigated in detail. With the same ion implantation fluence, the He+ implantation caused much broader damage distribution accompanied by much higher out-of-plane strain with respect to the H+ implanted InP. After annealing, the H+ implanted InP did not show any blistering or exfoliation on the surface even at the high fluence and the H2 molecules were stored in the heterogeneously oriented platelet defects. However, the He molecules were stored into the large bubbles which relaxed toward the free surface, creating blisters at the high fluence.
 Select Ion track-based nanowire arrays with gradient and programmable diameters towards rational light management Ran Huang(黄冉), Jiaming Zhang(张家明), Fangfang Xu(徐芳芳), Jie Liu(刘杰), Huijun Yao(姚会军), Yonghui Chen(陈永辉), and Jinglai Duan(段敬来) Chin. Phys. B, 2021, 30 (8): 086105.   DOI: 10.1088/1674-1056/ac078a Abstract （269）   HTML （2）    PDF （1192KB）（86）       Integrating nanowires with nonuniform diameter and random spatial distribution into an array can afford unconventional and additional means for modulating optical response. However, experimental realization of such a nanowire array is quite challenging. In this work, we propose a new fabrication strategy which takes advantage of ion track technology, via sequential swift heavy ion irradiation and ion track etching. Based on this strategy, we unprecedentedly realize nanowire arrays, using gold as an example, with gradient and programmable diameters in a controlled manner. We further demonstrate that such nanowire arrays can support broadband, tunable, and enhanced plasmonic responses. We believe that our new type of nanowire arrays will find great potential in applications such as light management and optoelectronic devices.