NANO LETTERS
Nanotwinning in Silicon Nanocrystals Produced by Ion Implantation
2004 Vol. 4, No. 10 2041-2045
Y. Q. Wang, R. Smirani, and G. G. Ross* INRS-EMT, 1650, BouleVard Lionel-Boulet, Varennes, Que´ bec, Canada J3X 1S2 Received July 30, 2004; Revised Manuscript Received August 25, 2004
ABSTRACT Si+
Si nanocrystals (Si nc) were formed by the implantation of into a SiO2 film on (100) Si, followed by high-temperature annealing. The microstructure of the Si nc produced by a high-dose (3 × 1017 cm-2) implantation has been extensively investigated using high-resolution transmission electron microscopy (HRTEM). For most of the Si nc (∼90%) with diameters larger than 6 nm, their configurations are characterized by the existence of nanotwins. The twinning structures include single twins, double twins, and multiple twins. The other planar defects such as stacking faults are also observed in some nanoparticles. However, in the Si nc smaller than 5 nm, no evident microstructural defects are observed. Possible reasons that no evident microstructural defects are found in the smaller Si nc are discussed. The microstructural defects inside the Si nc have a great influence on the optical properties.
Si nanocrystals (Si nc) embedded in a SiO2 matrix have attracted much interest as a candidate system to act as an efficient light emitter. In recent years, a lot of progress has been made in both the preparation1-7 and characterization7-9 of the Si nc. However, the mechanism underlying the light emission from this system is still unclear. A key point for a full understanding of the optical properties of the Si nc is to study their microstructure and surrounding environments. It is believed that the defects, especially the interfacial defects10 around the Si nc/SiO2, play an important role in the light emission from this system. However, the microstructural defects inside the Si nc or around the interface have not been observed using transmission electron microscopy (TEM) until now. In this letter, we report the observation of the nanotwinning configurations in most of the Si nc (∼90%) with diameters larger than 6 nm using high-resolution transmission electron microscopy (HRTEM). Other planar defects such as stacking faults (SFs) are also observed in some Si nc. However, in the Si nc with diameters smaller than 5 nm, no evident microstructural defects are observed. Possible reasons that no microstructural defects are found in the smaller Si nc are also discussed. The microstructural defects have a great influence on the optical properties. A 1-µm-thick film of amorphous SiO2 was produced by the thermal oxidation of a (100) Si substrate at high temperature (∼1100 °C under oxygen flow). The SiO2 layer was implanted at room temperature with 100-keV Si+ (implantation dose of 3 × 1017 cm-2 resulting in a local Si concentration in excess of ∼2.4 × 1022 Si/cm3), followed by high-temperature annealing at 1100 °C for 1 h under an * Corresponding author. E-mail:
[email protected]. 10.1021/nl048764q CCC: $27.50 Published on Web 09/09/2004
© 2004 American Chemical Society
atmosphere of nitrogen (N2). After the annealing, hydrogenation (hydrogen passivation) was carried out by heating to 500 °C for 1 h in a forming gas of 5% H2 + 95% N2. The specimens for TEM examination were prepared in a crosssectional orientation ([011] zone axis for the Si substrate) using conventional techniques of mechanical polishing and ion thinning. Dark-field examination was carried out on a Philips CM30 microscope operating at 300 kV. HRTEM was performed using a JEOL JEM 2010F scanning transmission electron microscope (STEM) operating at 200 kV and equipped with a Gatan imaging filter (GIF). A dark-field TEM examination11 shows that the Si nc range from 2 to 22 nm in diameter and that there are three dominant groups with diameters of about 3, 6, and 12 nm, respectively. The microstructure of the Si nc was extensively investigated by HRTEM. Before the HRTEM examination of the Si nc, the Si substrate was first tilted to the [011] zone axis. Our HRTEM observations indicate that nanotwinning configurations exist in most of the Si nc (∼90%) with diameters larger than 6 nm. The twinning structures include single twins, double twins, and multiple twins. To give a reliable prevalence distribution of different defect structures, we recorded more than 300 nanocrystals and carried out a statistical analysis of the different twinning structures among the overall population of the Si nc. The Si nc with singletwin and double-twin structures have similar volume fractions (each about 25% of the total Si nc); Si nc with multiple (triple and 5-fold) twins are relatively rare (around 15% of the total Si nc in volume); Si nc with nanotwins and SFs (about 20% of the total Si nc in volume) are also observed. The other types of nanotwins have a volume fraction of around 5% of the total Si nc. However, for the Si nc with diameters of
Figure 2. HRTEM image of a typical Si nanoparticle with a double-twin structure.
Figure 1. (a) HRTEM image of a typical Si nanoparticle with a single-twin structure. (b) Schematic diagram (1:1) for the projected shape and facets of the Si nanoparticle.
less than 5 nm, HRTEM observations indicate that there are no evident microstructural defects such as twins and SFs. Figure 1a shows an example of a Si nanoparticle (around 12 nm in diameter) with a single-twin structure, where the nanoparticle is oriented along [011]. The twinning elements are given in Figure 1a, and the twin boundary is pointed out by a black arrow labeled with TB. It can be clearly seen from Figure 1a that the projected shape of this nanoparticle (along the [011] direction) is not circular but more hexagonal. This nanoparticle consists of flat {111} and (100) crystal facets. The schematic diagram (1:1) of the projected shape and facets of this particle is shown in Figure 1b. Besides the single-twin structure, the double-twin configuration is also frequently observed in the Si nc. An example is shown in Figure 2, where the twin planes are {111} planes and the 2042
twin boundaries are indicated by black arrows labeled with TB1 and TB2. The diameter of this nanocrystal is around 10 nm, and the particle is slightly elongated. The projected shape of this nanoparticle is elliptical along the [011] direction without evident facets. The nanotwin (II) connecting nanocrystals I and III is composed of only five atomic planes. From our HRTEM observation, the most interesting finding is that two major projected shapes (along [011] direction) are commonly observed for the Si nc with diameters larger than 6 nm: hexagonlike (Figure 1a) and ellipselike (Figure 2). HRTEM results suggest that the projected shape of the nanoparticles is very dependent on the twinning structure. For the Si nc with a single-twin structure, a projected shape of a hexagon with {111} and (100) facets is preferably observed. This can be explained from the energy optimization of the surfaces for the Si nc. In the cubic close-packed Si crystal, the {111} planes are the faces with the lowest surface energy (γ(111) ) 1.23 J/m2) and the {001} planes are the second lowest energy faces (γ(001) ) 1.1γ(111)),12 so it is understandable that these planes form the facets of nanocrystals. However, for the Si nc with a double-twin structure (Figure 2), no favorable facets are observed. HRTEM observations also show that nanotwins and SFs can coexist in some Si nc. Figure 3a shows a [011] zone axis HRTEM image of a typical nanoparticle with nanotwins and SFs. The diameter of this nanoparticle is around 11 nm. The projected shape of this nanocrystal (along the [011] direction) is more or less hexagonal but irregular somewhere around the interface of the nanocrystal/oxide. The nanotwins are located near the left half of this nanoparticle (Figure 3a). A closer examination of Figure 3a shows that there are also two SFs passing through the whole nanoparticle. The intrinsic (with displacement of R ) -1/3〈111〉 ) and extrinsic (with Nano Lett., Vol. 4, No. 10, 2004
Figure 4. HRTEM image of a typical Si nanoparticle with multiple twins.
Figure 3. (a) [011] zone axis HRTEM image of a typical Si nanoparticle with nanotwins and SFs. (b) Close-up of the nanotwins near the left-hand side of the Si nanoparticle.
displacement of R ) -1/3〈111〉 ) SFs are pointed out by the white arrows labeled with in-SF and ex-SF, respectively. For clarity, the close-ups of the nanotwins are shown in Figure 3b. The twin boundaries are indicated by white arrows labeled with T1 and T2. The twin planes are {111} planes, and the angle between T1 and T2 is around 70°. It should also be noted that the angle between the stacking faults and twin plane T1 is around 70°, whereas the stacking faults are parallel to twin plane T2. In Figure 3b, it can be also seen that the lattice is slightly distorted around the interface of the Si nanocrystal/SiO2. The straight dashed lines (Figure 3b) indicate the nanotwins (with only three atomic planes) caused by the extrinsic stacking fault. Because of the coexistence of the nanotwins and SFs, the projected shape of the nanoparticle becomes irregular (with a ledge indicated by the dashed lines in Figure 3a) somewhere around the Nano Lett., Vol. 4, No. 10, 2004
interface of the nanocrytal/SiO2 near the left-hand side of the particle. HRTEM observation also shows that some nanoparticles have multiple-twin structures such as triple twinning and 5-fold twinning. The HRTEM image of a typical Si nanoparticle with multiple twins is shown in Figure 4, where the nanocrystal is oriented along [011]. In the right half of the nanoparticle, there is a triple twin, and in the left half of this nanoparticle, there is a 5-fold twin. For the triple twin, the twin boundaries are indicated by three white arrows, and each twin variant in the triple twin is marked with a number (I, II, or III). After triple twinning, a ∑27 boundary is created, and the lattice is very much distorted. For the 5-fold twin, the twin boundaries are indicated by five white arrows, and each twin variant in the 5-fold twin is marked with a number (1 to 5). For a cubic structure such as Si or diamond, it is well known that the {111} twinning angle between two adjacent twin variants in equilibrium is 70.53°.13-15 After 5-fold twinning, there is still a mismatch angle of 360° (70.53° × 5) ) 7.35 between the first and the fifth variant. Because of the small size of the nanocrystals, this mismatch angle can be accommodated by lattice distortions and other lattice defects. The coexistence of triple and 5-fold twins in such a small nanoparticle (around 8 nm) is very interesting. The formation mechanism of these multiple twins is not clear and needs further study. TEM characterization (both dark-field and HRTEM) was carried out on four samples with different implantation doses of 6 × 1016, 8 × 1016, 1 × 1017, and 3 × 1017/cm2, respectively. All of the TEM samples were prepared using the same techniques. HRTEM observations show that the defects such as twins and stacking faults exist only in the sample with an implantation dose of 3 × 1017/cm2, not in the samples with lower implantation doses. Therefore, we can exclude the possibility that the sample preparation process produces the 2043
Figure 6. PL spectra obtained from the samples with implantation doses of 8 × 1016 and 3 × 1017 Si+/cm2. Figure 5. Typical HRTEM image of the Si nc with diameters less than 5 nm, showing that there are no evident microstructural defects inside the Si nc.
observed defects. When we recorded the HRTEM images, the exposure time for every image is very short (only 0.5 s). This can reduce the effect of electron irradiation and the possibility of the decomposition of SiO2. In addition, the Si nc are stable under the electron beam (200 kV), and there is no structural change after several minutes. Therefore, the observed defects are intrinsic, not induced by the electron beam. Another interesting finding is that no evident microstructural defects are observed in the Si nc with diameters less than 5 nm. Figure 5 shows a typical HRTEM image of two nanocrystals smaller than 5 nm in which no evident defects can be found. It can also be seen from Figure 5 that the shape of the nanocrystals is nearly spherical. The results are consistent with those obtained by other groups.21 The possible reasons are described as follows. When the Si nc are smaller, the residual strain arising from volume contraction during crystallization is smaller, and it can be more easily adjusted by the elasticity. In addition, because the defects have higher energy and are less stable, they can easily glide through the smaller nanoparticle and disappear. The surfaces of the nanoparticles can also be responsible for the defect-free microstructure in the smaller nanoparticles. For the smaller Si nc, they have a larger surface area-to-volume ratio and a higher total energy. The higher-energy surface in the smaller Si nc is not compatible with the high-energy defects. Twinning is one of the most common planar defects in nanocrystals, and it is frequently observed in fcc-structured metallic nanocrystals.16-18 As a major microstructural characteristic, it is expected to have important effects on physical (particularly the electronic and mechanical) properties of the metallic nanocrystals. Here we have found that several 2044
different types of twinning structures exist in the Si nc (larger than 6 nm) produced by high-dose Si implantation and annealing. It should be noted that some twinning structures can also be regarded as a coalescence of two or several small Si nc. The coalescence of the small Si nc by twinning may play an important role in the growth of the large Si nc. The coalescence of small particles by twinning was also reported in FePt nanocrystals.19,20 A detailed investigation of the growth mechanism for the Si nc is still under way. Similar to twinning defects in the metallic nanocrystals, the heavily twinned microstructure in the Si nc larger than 6 nm is also expected to have important effects on the optical properties of this sample. To correlate the microstructure with the optical properties of the Si nc, we carried out a systematic photoluminescence (PL) measurement. It has been found that for an ion energy of 100 keV the PL intensity increases with the implantation dose until doses of ∼8 × 1016 to 1 × 1017/ cm2 are reached, and then the PL intensity slowly decreases with the increasing ion dose. This indicates that the microstructural defects could begin to appear in the samples with implantation doses larger than 1 × 1017/cm2, which is consistent with our HRTEM observations. The PL spectra from the samples with implantation doses of 8 × 1016 and 3 × 1017/cm2 are presented in Figure 6. As shown in Figure 6, an intense PL response is obtained from the sample with an implantation dose of 8 × 1016 Si+/cm2, and the PL intensity from the sample studied here has dropped by a factor of 5 compared with that of the PL spectrum from the sample with implantation dose of 8 × 1016/cm2. The size of the Si nc in the sample with an implantation dose of 8 × 1016/cm2 is comparatively homogeneous, and the average diameter of the Si nc is around 3 nm.11 In addition, HRTEM observations of the sample with an implantation dose of 8 × 1016/cm2 also confirm that there are no evident microstructural defects in the Si nc, similar to those smaller than 5 nm in the sample investigated in this paper. Therefore, the observed microstructural defects are thought to be mainly responsible for the PL intensity reduction. Nano Lett., Vol. 4, No. 10, 2004
In summary, for most of the Si nc (∼90%) with diameters larger than 6 nm, their configurations are characterized by the existence of nanotwins. The twinning structures include single twins, double twins, and multiple twins. Other planar defects such as SFs are also observed in some nanocrystals. However, no evident microstructural defects are observed in the Si nc with diameters less than 5 nm. The elasticity of the smaller nanoparticles, the higher-energy state of defects, and the surfaces of the smaller particles are supposed to be responsible for the defect-free microstructure in the Si nc with diameters less than 5 nm. These microstructural defects are mainly responsible for the intensity reduction of the PL spectrum obtained from this sample. Acknowledgment. This work has been supported by NanoQue´bec and the Natural Science and Engineering Research Council of Canada (NSERC). HRTEM experiments were conducted using the TEM facility at Brockhouse Institute for Materials Research, McMaster University. References (1) Shimizu-Iwayama, T.; Fujita, K.; Nakao, S.; Saitoh, K.; Fujita, T.; Itoh, N. J. Appl. Phys. 1994, 75, 7779. (2) Zhu, J. G.; White, C. W.; Budai, J. D.; Withrow, S. P.; Chen, Y. J. Appl. Phys. 1995, 78, 4386. (3) Min, K. S.; Shcheglov, K. V.; Yang, C. M.; Atwater, H. A.; Brongersma, M. L.; Polman, A. Appl. Phys. Lett. 1996, 69, 2033.
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