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. 2020 Jan 8;5(3):1457–1462. doi: 10.1021/acsomega.9b03028

Athermal Crystal Defect Dynamics in Si Revealed by Cryo-High-Voltage Electron Microscopy

Kazuhisa Sato †,‡,*, Hidehiro Yasuda †,‡,*
PMCID: PMC6990428  PMID: 32010818

Abstract

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Low-temperature crystal defect dynamics in Si has been studied by a newly developed cryo-high-voltage electron microscopy. The planar {113} defects of self-interstitial atoms were introduced at 94 K by 1 MeV electron irradiation with damage higher than 0.42 displacements per atom (dpa), unlike past findings. The defects once grew and then shrunk during the observation. We show that the nucleation and the dissociation dynamics of the {113} defects can be attributed to an athermal process, which is deduced from anomalously fast diffusion of self-interstitial atoms at a low temperature.

Introduction

High-voltage transmission electron microscopy (HVEM) is a powerful tool to characterize interior structures of a micrometer-thick specimen with a high spatial resolution. To date, a plenty of useful findings regarding materials physics such as crystal defects, phase transition, and mechanical properties have been obtained by HVEM.1−3 Recently, installation of a high-performance direct electron detection (DED) CMOS camera has significantly improved the performance of the HVEM. For example, structural dynamics of nanoparticles has been revealed in sub-millisecond to a millisecond time scale,4,5 which could not be detected by a conventional CCD camera so far. As a result, in addition to the high spatial resolution and high penetration power,6,7 the latest HVEM opens up a new horizon for in situ characterization of materials’ structure–property relationships. Recent developments of the HVEM are not limited to the aforementioned detection system; we have successfully installed a cryogenic specimen stage to our HVEM (cryo-HVEM). In general, cryo-EM (typically, 200–300 kV-class TEM) has been used to observe biological samples and soft materials that are sensitive to electron radiation damage.8−12 It features an imaging system capable of observing images with a low electron dose together with a dedicated specimen stage cooled by liquid nitrogen. In our newly developed cryo-HVEM, up to 12 specimens are stored in the inner storage that is always cooled with liquid nitrogen, and a selected specimen is transferred to the observation position in the pole piece gap by the autoloader system.

Radiation damage in silicon (Si) induced by MeV electron irradiation has been studied for over 40 years. The main conclusion is that in most cases, point defects and their assemblies such as {113} defects are introduced by MeV electron irradiation above room temperature, especially at 673–823 K,13−16 and amorphization can only be realized under limited conditions at low temperatures with a high electron dose.17,18 Here, we report on the structural properties of a Si single crystal at a low temperature obtained using the novel cryo-HVEM. Mechanical stability of our new cryo-HVEM makes the atomic resolution observation possible at liquid nitrogen temperature. By low-dose observation using a DED camera, we observed formation of planar {113} defects of self-interstitials even at a low temperature.

Results

Figure 1a shows a high-resolution TEM (HREM) image of Si obtained at 94 K with beam incidence in the [110]Si. The observation was performed at an area about 500 nm from the specimen edge (“edge” means the boundary between the specimen and a hole opened by the Ar ion milling). As seen, Si(110) dumbbells, 136 pm in distance, are observed as lattice images. The corresponding fast Fourier transform (FFT) pattern is shown in Figure 1b. The unusual diffuse contrast from the top left to the bottom right of the image is due to drift correction (<100 pm) and image integration. It is worth noting that 008 reflection seen in the FFT pattern, which corresponds to 67 pm in real space indicates the mechanical stability of the cryo-stage kept at 94 K. Also, 4̅40 reflection (96 pm) is seen in the orthogonal direction. Figure 1c shows a selected area electron diffraction (SAED) pattern obtained from an area ∼600 nm in diameter. Note that the pattern is free from a halo ring of amorphous SiO2: a surface-damaged layer by Ar ion milling is presumed to be extremely thin.

Figure 1.

Figure 1

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(a) Lattice image of Si obtained at 94 K. The beam incidence is in the [110]Si. (b) FFT pattern processed from the image shown in panel (a). (c) SAED pattern obtained with the beam incidence of [110]Si.

Figure 2 shows a representative HREM image of Si including lattice defects observed with beam incidence in the [110]Si. The corresponding FFT pattern is shown in the inset. During the observation with a dose rate of 12.4 e/pixel s (550 e/Å2s) for a couple of minutes, such defect contrast suddenly appeared. It was found that the defect is the planar {113} defects of self-interstitials elongated in the <332> direction. The {113} defect was firstly found in electron-irradiated Ge,19 and then the atomic structure of the defect formed in Si was revealed in detail by TEM.16,20,21 The appearance of the {113} defects in Si at 94 K is curious since this type of defects is known to appear by MeV electron irradiation at temperatures above room temperature.13−16

Figure 2.

Figure 2

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HREM image of Si including the planar {113} defects of self-interstitials observed at 94 K with beam incidence in the [110]Si. The corresponding FFT pattern is shown in the inset.

Figure 3 shows a series of HREM images obtained in situ with a dose rate of 18 e/pixel s (800 e/Å2s). The observation was performed at a thick area about 5 μm from the specimen edge. Thickness of the observed area is presumed to be at most 700 nm (this was estimated by our preliminary work using electron energy-loss spectroscopy). In Figure 3a (2 min after the start of observation), there is no defect contrast in the observed area. After 13 min observation (received damage of 0.42 displacements per atom (dpa)), a fine defect ∼3 nm in length appeared as shown in the circle (Figure 3b). It grew as the irradiation proceeds and reached 6 nm in length after 15 min irradiation (Figure 3c, 0.48 dpa) and kept its size until 21 min irradiation (Figure 3d, 0.67 dpa). After 23 min irradiation (Figure 3e, 0.74 dpa), the defect shrunk, and a new defect as shown by an arrow appeared alternatively in the immediate vicinity. The appearance of new defects in a short time (<2 min) indicates that the clustering of self-interstitials of Si is a very fast event even at 94 K. Both these defects seen in Figure 3e were maintained after 27 min irradiation (0.87 dpa, not shown), while they disappeared after 35 min irradiation (Figure 3f, 1.14 dpa).

Figure 3.

Figure 3

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Series of HREM images obtained in situ with a dose rate of 18 e/pixel s (800 e/Å2s = 8.0 × 1022 e/m2 s) at 94 K. (a) 9.6 × 1024 e/m2 (0.06 dpa), (b) 6.2 × 1025 e/m2 (0.42 dpa), (c) 7.2 × 1025 e/m2 (0.48 dpa), (d) 1.0 × 1026 e/m2 (0.67 dpa), (e) 1.1 × 1026 e/m2 (0.74 dpa), and (f) 1.7 × 1026 e/m2 (1.14 dpa).

We also investigated the defects’ motion under prolonged irradiation at 94 K. After obtaining the image shown in Figure 3f (35 min irradiation, 1.14 dpa), we found formation of another defect in the field of view. Figure 4a shows an HREM image of the above newly formed {113} defect observed after 40 min irradiation (5 min irradiation after the formation, total damage: 1.27 dpa). As seen, the defect gradually shrunk with irradiation (Figure 4b–d), while it once regrew after 60 min irradiation (Figure 4e, 1.95 dpa), and it shrunk again after 67 min (Figure 4f, 2.15 dpa). As shown in Figures 3 and 4, it was found that the {113} defects were generated and then disappeared at about 30 min intervals during the 1 MeV electron irradiation at 94 K. These results imply that dynamics of the planar {113} defects of self-interstitials can be attributed to an athermal process.

Figure 4.

Figure 4

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Series of HREM images obtained in situ with a dose rate of 18 e/pixel s (800 e/Å2s = 8.0 × 1022 e/m2 s) at 94 K. (a) 1.9 × 1026 e/m2 (1.27 dpa), (b) 2.5 × 1026 e/m2 (1.68 dpa), (c) 2.6 × 1026 e/m2 (1.74 dpa), (d) 2.7 × 1026 e/m2 (1.81 dpa), (e) 2.9 × 1026 e/m2 (1.95 dpa), and (f) 3.2 × 1026 e/m2 (2.15 dpa).

Figure 5a,b shows irradiation time dependence of the length of {113} defects in the <332> direction extracted from the results obtained in Figures 3 and 4, respectively. Although the defocus was fine-tuned so that sharp defect contrast can be obtained, the measured length is the apparent value since the size and shape of a planar {113} defect in the [110] direction (electron propagation direction) are unknown. The error bars were estimated from the spread of the defect contrast in the direction perpendicular to the defect plane. As seen, defects are formed and grown, and after maintaining a certain size for a while, they tend to disappear by further irradiation. We deduced the diffusivity of self-interstitials to be 3.8–7.5 × 10–20 m2/s for the grown ones and 4.3–6.4 × 10–20 m2/s for the shrunk ones. According to the literature,22 the obtained diffusivity corresponds to a value expected for self-diffusion of self-interstitials at 1250 K (a frequency factor of 0.53 m2/s and an activation energy of 4.75 eV were used for estimation). Thus, diffusion of self-interstitials is significantly enhanced under 1 MeV electron irradiation at 94 K. This result indicates that the defects’ motion is not a thermally activated process under the present experimental conditions.

Figure 5.

Figure 5

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Irradiation time dependence of the length of {113} defects in the <332> direction. Panels (a) and (b) are extracted from the results obtained in Figures 3 and 4, respectively.

Discussion

Formation of {113} defects by MeV electron irradiation has been extensively studied in the temperature range above room temperature, especially at 673–823 K.13,14,16 Vanhellemont et al. reported that rate of the {113} defects formation under 2 MeV electron irradiation is reduced at room temperature compared to that of at 573 K.23 Also note that the {113} defects are formed at relatively high temperatures, while voids are introduced at lower temperatures as reported in the literature.24 The experimentally determined border of the formation temperature between {113} defects and voids is in the temperature range between 100 and 200 K.24 In this study, we found that the {113} defects are also introduced by 1 MeV electron irradiation at 94 K. The irradiation temperature of 94 K is just below the lower limit of the aforementioned border. Electron dose required for the defect formation was in the order of 1025 e/m2, which is comparable to those reported on most of the previous studies (1025–1026 e/m2). It has been reported that increasing the oxygen content in Si reduces the formation rate of the {113} defects,23 while increasing the partial pressure of oxygen in the atmosphere enhances the defect formation.25 Oxygen acts as a sink for vacancies, forming V–O pairs, which eventually leads to recombination with interstitials.23,26 In this study, low-energy ion milling effectively reduced surface oxide and a possible damaged layer, as evidenced by the fact that obvious halo rings of amorphous SiO2 was not observed in the SAED pattern (Figure 1c) as well as the FFT patterns of HREM images (Figure 1b, inset of Figures 2 and 3a); the specimen surface will be coated by thin native oxide. The specimen chamber of the cryo-HVEM is evacuated by turbo-molecular pumps and sputter-ion pumps and maintains a vacuum of approximately 1 × 10–5 Pa during the observation. Furthermore, since the cryogenic specimen stage is cooled with liquid nitrogen all the time, residual H2O vapor will be eliminated by adsorption. Thus, electron irradiation was performed in a clean condition excluding contamination sources as much as possible.

Similar phenomena, the formation and disappearance of {113} defects, as observed in this study, were previously found by 1 MeV irradiation at room temperature.15 The motion of the defects was reported at the electron dose of 0.6–1.3 × 1026 e/m2, and these values are almost equivalent to the dose used in this study. As we have shown in the present study, the {113} defect formation, usually induced at higher temperatures, also occurs at a low temperature. Such a low-temperature structural dynamics indicates an athermal process as the formation mechanism of the {113} defect, and it also explains the dissociation of the defects during the 1 MeV electron irradiation at 94 K. Since the dynamics is not a thermally activated process, {113} defects are presumed to be able to grow and shrink at a very high speed as observed in this study. It should be noted that the {113} plane of the diamond structure consists of 12 equivalent planes, and when a {113} defect satisfies the edge-on condition with respect to the incident electron beam, the linear defect contrast can be clearly observed, as shown in Figures 2–4. It is presumed that the disappearance of the defects under 1 MeV electron irradiation is due to migration of interstitial atoms to an equivalent plane, which does not satisfy the edge-on condition, or annihilation at the specimen surface. Shrinkage of the defects can also be explained by the purification of the irradiated area by the absorption of pre-existing impurities into interstitial clusters.27 Since {113} defects are not formed in a thin region near the specimen edge,16 observation in a thick region is essential for such defect dynamics characterization. The veiled phenomenon was hence detected by the experiment simultaneously satisfying the following three conditions using a sufficiently thick specimen: low temperatures (<100 K), high-energy electron irradiation (1 MeV), and atomic resolution. Thus, we have demonstrated that the novel cryo-HVEM has a high potential in a research of irradiation-induced defects and their dynamics.

Regarding lattice defects characterization of a thick specimen, the authors recently demonstrated that bright-field scanning transmission electron microscopy (BF-STEM) mode in the HVEM shows extreme penetration power.6 In fact, it has been demonstrated that dislocations in Si can be observed even at a thickness of 14 μm using the 1 MV-STEM.28,29 In this study, we focused on HREM imaging using the cryo-HVEM, while in the future, STEM in cryo-HVEM has potential to reveal crystal defects in a thicker specimen that can be regarded as a bulk.

Conclusions

We have studied lattice defects dynamics in Si at 94 K using a newly developed cryo-HVEM operating at 1 MV. Lattice images resolved Si(110) dumbbells of 136 pm distance, and the FFT pattern of the image showed the 008 reflection corresponding to 67 pm in real space. We found that the planar {113} defects of self-interstitial atoms, usually formed at temperatures above room temperature, appear at 94 K by 1 MeV electron irradiation with a damage higher than 0.42 dpa. The defects once grew and then shrunk during the observation. The diffusivity of self-interstitials at 94 K during 1 MeV electron irradiation was deduced to 10–20 m2/s in order, which corresponds to a value of self-diffusion of self-interstitials at 1250 K. High-diffusivity at a low temperature indicates that the {113} defect dynamics can be attributed to an athermal process instead of a thermally activated process. We show that novel cryo-HVEM has a high potential in the research of irradiation-induced defects and their dynamics.

Methods

TEM specimens of a Si single crystal were prepared in the following manner. First, a small piece of Si(110) wafer (Cz, p-type, 8–12 Ω cm) approximately 2 × 2 × 0.5 mm3 in size was glued on a Mo single hole grid 3 mm in diameter. Then, the crystal was thinned by mechanical polishing, and finally, a small hole was made at the center of the specimen by low-energy Ar ion milling. The prepared specimen was cooled using liquid nitrogen and then transferred to the specimen stage of the cryo-HVEM. We confirmed that the area of interest of the sample was not broken or damaged by immersion in liquid nitrogen. Microstructures of the specimens prepared as above and loaded on the cryogenic stage were characterized by JEOL JEM-1000EES cryo-HVEM installed in Osaka University operating at 1 MV equipped with a LaB6 cathode. The spherical aberration coefficient (Cs) and the chromatic aberration coefficient (Cc) of the cryo-HVEM are 4.1 mm and 5.1 mm, respectively. The stage temperature was kept at 94 K during the TEM observation. Electron imaging as well as electron irradiation were performed with a beam incidence in the [110]Si direction. HREM images were recorded using a DED camera (Gatan K2 summit, 4k × 4k). HREM images were obtained with a dose rate of 5.5 × 1022–8.0 × 1022 e/m2 s (550–800 e/Å2s). Note that the K2 summit camera can be operated at low electron doses, and it accepts at most 20 e/pixel s (890 e/Å2s). The cross section (σ) of atomic displacements of Si was estimated to be 67.1 barns (67.1 × 10–28 m2) under 1 MeV electron irradiation based on the McKinley–Feshbach formula30 for the collision between an electron of charge e and a nucleus of charge Ze:

graphic file with name ao9b03028_m001.jpg 1

where Z is the atomic number, m is the rest mass of electron, c is the velocity of light, v is the velocity of electron with β = v/c and γ = (1 – β2)−1/2, Tm is the maximum recoil energy, and Td is the displacement threshold energy (Td = 13 eV31 and Tm = 155 eV for Si under 1 MeV electron irradiation). The size of each HREM image is 3710 × 3390 pixels with a pixel size of 0.015 nm. Each image was taken at an exposure time of 0.8 s, and a total of 10 images were integrated with drift correction. SAED patterns were recorded using a charge-coupled device (CCD) camera (Gatan Orius SC200, 2k × 2k).

Acknowledgments

The authors wish to thank Mr. A. Ohsaki, Mr. S. Takakuwa, and all the members of JEOL Ltd. involved in construction of the JEM-1000EES cryo-HVEM. K.S. acknowledges Dr. J. Yamasaki of Osaka University for invaluable comments and Ms. Y. Hiraizumi of Osaka University for technical assistance in sample preparation. This study was partially supported by the JSPS KAKENHI grant no. JP17H02746.

Author Contributions

All authors contributed to the discussion and writing of the manuscript. The final version of the manuscript was approved by all authors.

The authors declare no competing financial interest.

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