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Growth and Structural Characterization of SiGe Nanorings J. H. He,*,†,| C. Y. Chen,† C. H. Ho,† C. W. Wang,‡ M. J. Chen,§ and L. J. Chen‡ Institute of Photonics and Optoelectronics, National Taiwan UniVersity, Taipei, 106 Taiwan, ROC, Department of Electrical Engineering, National Taiwan UniVersity, Taipei, 106 Taiwan, ROC, Department of Materials Science and Engineering, National Tsing Hua UniVersity, Hsinchu, 300 Taiwan, ROC, and Department of Materials Science and Engineering, National Taiwan UniVersity, Taipei, 106 Taiwan, ROC ReceiVed: August 24, 2009; ReVised Manuscript ReceiVed: February 24, 2010
Self-assembled single-crystalline Si1-xGex nanorings (NRs), as small as 10 nm with narrow distributions of height and diameter, have been fabricated without any specific equipment. Compared to other approaches for nanofabricating the ring-like structures, the smallest Si1-xGex NRs with the highest density were obtained by the mediation of Au nanodots. The process promises the availability of Si1-xGex NRs with a wide range of Ge concentration and size, and can serve as a useful platform for the fundamental understanding and future practical applications of NR devices. Introduction Currently nanorings (NRs), the artificial nanoscale clusters, are attractive because there is a great deal of interest in nanostructures from theoretical, experimental, and device perspectives. The interesting properties include the following: persistent currents in metallic1 or superconducting rings,2 tunable optical resonance in metal rings,3 novel magnetooptical behaviors in semiconductor rings,4 and optical emission from a single charged NR.5 Various ring-like nanostructures made of semiconductors, metals, and other materials have been fabricated.6–20 In the past, NRs have been fabricated by using various complex methods, such as electron beam techniques,1,6 nanosphere lithography,7 porous alumina template methods,8 particle imprinted template methods,9 and nanomechanical architecture driven by strain of the Si/Ge layer.10 Moreover, NRs exhibit novel physical and chemical properties resulting from quantum size effects. For example, with the presence of Coulomb correlation, the Aharonov-Bohm effect of excitons exists in a finite but small width of NR. When the width of the ring becomes large, the not-simply connected geometry of NR is destroyed, leading to the suppression of the Aharonov-Bohm effect.20 Accordingly, NRs need to be scaled down to exhibit the size effects. The feasible ultrasmall NR fabrication is demanded for the application of electronic and optoelectronic devices at the nanoscale. Au has been widely used to fabricate the various nanostructures. For example, nanorods/nanobelts have been synthesized via a vapor-liquid-solid process by using Au as a catalyst.21,22 Recently, the growth of self-assembled crystalline Si and Si0.8Ge0.2 NRsmediatedbyAunanodots(NDs)wasdemonstrated.19,23 In situ ultra-high-vacuum transmission electron microscopy (TEM) revealed that the formation of Si NRs involves the mediation of Au NDs and the evaporation of Au-Si eutectic liquid droplets at high temperatures.19 It has been reported that * To whom correspondence should be addressed. E-mail: jhhe@ cc.ee.ntu.edu.tw. † Institute of Photonics and Optoelectronics, National Taiwan University. | Department of Electrical Engineering, National Taiwan University. ‡ Department of Materials Science and Engineering, National Tsing Hua University. § Department of Materials Science and Engineering, National Taiwan University.
SiGe NRs were fabricated with Si-capped Ge NDs, using ultrahigh-vacuum chemical vapor deposition (UHV-CVD)13 or molecular beam epitaxy (MBE).24 X-ray scattering methods demonstrated that SiGe NRs obtained by MBE are composed of a Ge-rich core surrounded by Si-rich ridges, indicating that a substantial material redistribution occurs during the shape transformation from NDs to NRs.25 Therefore, it is difficult to obtain uniform composition of Si1-xGex NRs with use of Sicapped Ge NDs by UHV-CVD and MBE. In this study, we demonstrate a novel self-assembly technique for the fabrication of large-scale, uniform, and ultrasmall Si1-xGex NRs with precise control of composition and sizes without any specific equipment. The morphology and microstructure of the NRs were examined by using atomic force microscopy (AFM), scanning electron microscopy (SEM), and TEM. The formation process of NRs involves the mediation of Au NDs and the evaporation of the Au-Si-Ge eutectic liquid. The process promises the availability of a wide range of Ge concentrations and sizes of Si1-xGex NRs, and can serve as an useful platform for the fundamental understanding and future practical applications of NRs. Experimental Section Single-crystal phosphorus-doped (001)Si wafers were used in the present study. The 700-nm-thick Si0.8Ge0.2 (or Si0.7Ge0.3) and 200-nm-thick low-temperature Si (LT-Si) buffer layers were grown on (001)Si wafers by UHV-CVD. The LT-Si layers effectively suppress the propagation of threading dislocations. More experimental details on the LT-Si buffer layers were shown elsewhere.26 Pure SiH4 and 5% GeH4 diluted in He were used as precursors in UHV-CVD. Si, Si0.8Ge0.2/Si, and Si0.7Ge0.3/ Si wafers were chemically cleaned by a standard RCA cleaning process. The 1- or 2-nm-thick Au ultrathin films were then deposited onto Si, Si0.8Ge0.2/Si, and Si0.7Ge0.3/Si substrates at room temperature in an electron beam evaporation system. The chamber pressure was lower than 5 × 10-6 Torr during the deposition of Au ultrathin films. For the formation of Au NDs, rapid thermal annealing was employed in a high-purity N2 atmosphere at 400 °C for 30 s. Finally, the samples were placed into a tube furnace. After the tube was evacuated for hours to purge O2 from the system by a rotary pump to a pressure of 1
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× 10-3 Torr, the samples were heated to 1050 °C at a rate of 5 deg/min and held at 1050 °C for 2 h with a carrier gas of Ar + 5% H2 flowing through the tube for the formation of NRs. The morphology of the NRs was examined with an AFM (D3100, Digital Instruments) in tapping mode. SEM observation was conducted with a JEOL JSM-6500 SEM operating at 15 kV with a nominal point-to-point resolution of 1.5 nm. Highresolution TEM (HRTEM) and cross-sectional TEM (XTEM) images were obtained with a JEOL JEM-3000F field-emission TEM operating at 300 kV with a nominal point-to-point
He et al. resolution of 0.17 nm. An energy dispersive spectrometer (EDS) attached to the TEM was utilized to characterize the composition of Si1-xGex alloys in the form of NRs and thin films. For the EDS analysis, the electron beam can be focused down to a diameter of 1.5 nm. The accuracy of quantitative EDS analysis is about (2.5% for major elements (Z > 11). The XTEM micrographs were all taken along the [110] zone axis of singlecrystal Si. The PL measurements were performed in a liquid helium bath cryostat at 13 K, using a diode-pumped solid state laser (photon energy ) ∼2.33 eV) as an excitation source.
Figure 1. AFM images of NRs mediated by 2-nm-thick Au ND films on (a) Si, (b) Si0.8Ge0.2, and (c) Si0.7Ge0.3 substrates. The histogram shows the corresponding distributions of the height and the ridge thickness of NRs extracted from AFM images.
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Figure 2. (a) An AFM image of NRs mediated by 1-nm-thick Au films on the Si0.7Ge0.3 substrate; the corresponding distributions of (b) the heights and (c) the ridge thicknesses of NRs extracted from AFM images.
Results and Discussion AFM analysis indicates that the ring-shaped nanostructures were formed on 2-nm-thick Au/Si samples after annealing at 1050 °C for 2 h, as shown in Figure 1a. NRs have a uniform distribution of diameter, height, and ridge thickness with means of 24.9 ( 4.0, 1.4 ( 0.3, and 8.3 ( 1.4 nm, respectively. The diameter is defined as the length of the line crossing the center of an NR between the highest ridges from a cross-sectional AFM image. Similar ring-like nanostructures on various Si1-xGex substrates by the mediation of Au NDs can be obtained as well. For example, the formation of NRs on Si0.8Ge0.2/Si and the Si0.7Ge0.3/Si substrates by the mediation of 2-nm-thick Au ultrathin films could be achieved, as shown in Figure 1b,c. The right columns in Figure 1 show the uniform distributions of diameter and ridge thickness of NRs. In addition, the size of the NRs can be readily controlled by varying the thickness of Au films as the mediation. Figure 2a shows a typical AFM image of NRs mediated by 1-nm-thick Au films on the Si0.7Ge0.3 substrates. Compared with NRs mediated by 2-nm-thick Au films on the Si0.7Ge0.3 substrates in Figure 1c, the NRs mediated by 1-nm-thick Au films on the Si0.7Ge0.3 substrates with smaller diameter and higher density than that by 2-nm-thick Au films were obtained because of the smaller size and higher density of Au NDs. Parts b and c of Figure 2 indicate the uniform distributions of diameter (0.85 ( 0.3 nm) and ridge thickness (4.8 ( 1.6 nm) of the NRs mediated by 1-nm-thick Au films on the Si0.7Ge0.3 substrates. Obtained by the AFM measurements, density, diameter, height, and ridge thickness of NRs on the different substrates by the mediation of Au ND films with different thicknesses are listed in Table 1. The density of Si0.7Ge0.3 NRs (9.3 × 1010 cm-2) mediated by 1-nm-thick Au films is much higher than that by 2-nm-thick Au films (3.2 × 1010 cm-2). Compared with the previous studies for nanofabrication of ring-like structures by other methods,6–17 these structures are the smallest single-crystalline Si1-xGex NRs with
Figure 3. (a) An XTEM image of a 2-nm-thick Au/Si0.7Ge0.3 sample annealed at 1050 °C for 2 h with the linescan of EDS, (b) the corresponding EDS spectrum, and (c) an HRTEM image of NR from the dashed frame in part a.
the highest density achieved to date. The process is demonstrated to be effective in controlling the size and the compositions of NRs without any specific equipment. It is clear that similar density and size of NRs on different substrates can be obtained by the mediation of Au films with the same thickness. Slightly different density and size distribution between Si, Si0.8Ge0.2, and Si0.7Ge0.3 NRs may result from the slight differences in the size and density of Au NDs as the mediation on Si, Si0.8Ge0.2, and Si0.7Ge0.3 substrates. The deposition of Au on the Si1-xGex substrate should follow Volmer-Weber-type kinetics.27 In the Volmer-Weber mode, separated three-dimensional Au NDs are formed on the Si1-xGex substrate since the interaction between Au atoms is greater than that between the adjacent Au atoms and Si (and/or Ge) atoms on Si1-xGex substrates. Island growth (φ > 0) requires that γB < γ* + γA cos φ, whereas layer growth (φ ) 0) requires that γB > γ* + γA cos φ, where φ is wetting angle, γB is the surface energy of Si1-xGex substrates, γA is the surface energy of Au NDs, and γ* is the interface energy of Au NDs and Si1-xGex the substrates. As a result, slight differences in the size and the density of Au NDs on Si1-xGex substrates, controlled by not only the surface energy of Si1-xGex substrate but also the interface energy of Au NDs and Si1-xGex substrates, lead to the growth of Si1-xGex NRs with slightly different sizes and densities. However, it still remains as an open question about how the variation of Ge composition in the Au-Si1-xGex system influences the parameters γB, γ*, and φ.
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TABLE 1: The Density, Diameter, and Height of NRs on the Different Substrates with Different Thicknesses of Au Films NRs Si Si0.8Ge0.2 Si0.7Ge0.3 Si Si0.8Ge0.2 Si0.7Ge0.3
thickness of Au (nm)
density (cm-2)
diameter (nm)
thickness of ridge (nm)
height (nm)
2 2 2 1 1 1
4.3 × 10 3.7 × 1010 3.2 × 1010 1.1 × 1011 1.0 × 1011 9.3 × 1010
24.9 ( 4.0 29.8 ( 4.5 30.8 ( 4.8 10.1 ( 2.0 12.2 ( 2.4 13.8 ( 2.8
8.3 ( 1.4 8.5 ( 1.3 10.4 ( 3.2 4.1 ( 1.1 4.2 ( 1.5 4.8 ( 1.6
1.4 ( 0.3 1.5 ( 0.1 1.8 ( 0.4 0.79 ( 0.2 0.79 ( 0.2 0.85 ( 0.3
10
To examine the NRs on the substrates, XTEM and the line profile of EDS analysis have were conducted. An example is a 2-nm-thick Au film/Si0.7Ge0.3 substrate annealed at 1050 °C for 2 h as shown in Figure 3a,b. The dashed square in Figure 3a indicates the location of NRs. For the EDS analysis of NRs, the electron beam can be converged as small as 1.5 nm in size at the center of NRs. The results of the EDS spectrum show that NRs have the same composition (Si:Ge ) 7:3) with the Si0.7Ge0.3 thin films on the Si substrate, as shown in Figure 3b. The concentration of Au is found to be at an undetectable level after mediating the formation of NRs. The signal of Cu results from the Cu grids used in TEM characterization. As shown in the line profile of EDS analysis in Figure 3a, the whole concentration distribution increasing from the surface to the substrates results from the thickness effect at the edge of crosssectional samples for TEM observation. An HRTEM image indicates that NRs are epitaxially grown on the Si0.7Ge0.3 thin films, as shown in Figure 3c. It is worthwhile to mention that the composition of SiGe NRs induced by strain relief in the UHV-CVD is found to be different from that of the substrates.13 In contrast, NRs formed by the mediation of Au NDs exhibit the same composition as the substrates, which means that the composition of NRs can be precisely controlled by selecting the Si/Ge ratio of substrates.
Figure 4. (a) An XTEM image of a 2-nm-thick Au/Si0.7Ge0.3 sample annealed at 1050 °C for a few minutes with the linescan of EDS, (b) a magnified image from ridge regions of NRs, and (c) a magnified image from Au NDs. The left inset in part a is a corresponding SEM image. The right inset in part a is the corresponding EDS spectrum from ND regions.
To elucidate the formation mechanism of the Si1-xGex NRs, the evolution of surface morphology was investigated. After annealing at 1050 °C for a few minutes, the density of Au NDs decreased drastically and Si0.7Ge0.3 NRs appeared on the Si0.7Ge0.3 substrates. XTEM and EDS analyses for a Au/Si0.7Ge0.3 sample annealed at 1050 °C for a few minutes have been conducted. A shrunken Au ND remaining at the center of an NR was observed in Figure 4a. The SEM image in the left inset of Figure 4a shows the corresponding morphology as well. The EDS spectrum as shown in the right inset of Figure 4a indicates that an ND embedded in a Si0.7Ge0.3 NR is composed of Au. An HRTEM image shown in Figure 4b indicates that the protrusions of Si0.7Ge0.3 ring-shaped nanostructures from the Si0.7Ge0.3 thin films are epitaxially oriented and have the same crystal structure with Si0.7Ge0.3 substrates. In contrast to the protrusions, it can be confirmed again that a residual ND remaining in the center of the NRs shown in an HRTEM image of Figure 4c is a single crystal Au. Since both the Si-Au and Ge-Au systems have a similar eutectic system (i.e., the eutectic temperatures of Au-Si and Au-Ge are 363 ( 3 and 361 °C, respectively28), the formation mechanism of SixGe1-x NR is expected to be similar to the formation mechanism of Si NRs, which has been previously observed with use of in situ TEM.19 The proposed formation mechanism of SiGe NRs is illustrated schematically in Figure 5. Liquid eutectic Au-Si-Ge alloy droplets are expected to form on the surface of SiGe substrates
Figure 5. Schematic of Si1-xGex NRs growth by Au-mediation method: (a) deposition of Au on the Si-based substrate, (b) Si (or Ge) atoms diffusion into Au NDs upon annealing, (c) formation of Si-Au (or Si-Ge-Au) eutectic alloy droplets during annealing, (d) evaporation of Si-Au (or Si-Ge-Au) eutectic alloy droplets during annealing, (e) shrinkage of Si-Au (or Si-Ge-Au) eutectic alloy droplets during annealing, and (f) formation of Au-free Si1-xGex NRs.
Structural Characterization of SiGe Nanorings at high temperature. The single-crystal ring-like morphology is governed by the rapid diffusion of Si and Ge. With the help of annealing, diffusing Si and Ge atoms migrate rapidly to the liquid-solid interface upon annealing. The migration of Si and Ge atoms on the surface promotes the protrusions of Si-Ge ring-shaped nanostructures from the Si-Ge substrate surrounding Au-Si-Ge droplets. In the meantime, the severe evaporation of eutectic Au-Si-Ge droplets at the centers of NRs leads to the shrinkage and eventual disappearance of Au-Si-Ge droplets due to high vapor pressure at high temperature. Finally, the NRs free of Au are formed. It should be noted that the surface mobilities of Si and Ge on the SiGe alloy are temperature dependent based on first-principles calculations of surface diffusion barriers.29 At room temperature, the diffusivity of Ge is much higher than that of Si due to the low adatom diffusion barrier of Ge. However, the Ge/Si surface mobility ratio could be decreased drastically to 1 or less on the strain-relaxed Si1-xGex alloy at high temperature.29 The NRs with the same composition as the underlying Si-Ge film imply that Si and Ge have similar diffusivity at an annealing temperature of 1050 °C, showing the strong temperature dependence of the surface mobilities ratio of Si and Ge in the strain-relaxed Si1-xGex alloy. In addition, Au indiffusion in the Si1-xGex alloy dominated by the Kick-Out mechanism governed by Si and Ge self-interstitials should play a role in the formation of NRs.30,31 Conclusion In summary, size- and composition-controlled single-crystal Si1-xGex NRs which are the smallest to date were fabricated by appropriate thermal annealing with the mediation of Au NDs instead of complicated deposition, etching, and lithography. The fabrication technique can apply to different substrates that could form the eutectic alloy liquid with Au metals. The availability of a wide range of concentrations and size of Si1-xGex NRs also serves as a useful platform for the fundamental understanding and future practical applications of NR devices. Acknowledgment. The research was supported by the National Science Council Grant Nos. NSC 96-2112-M-002-038MY3 and NSC 96-2622-M-002-002-CC3, and Aim for Top University Project from the Ministry of Education. References and Notes (1) Jariwala, E. M. Q.; Mohanty, P.; Ketchen, M. B.; Webb, R. A. Phys. ReV. Lett. 2001, 86, 1594. (2) Matveev, K. A.; Larkin, A. I.; Glazman, L. I. Phys. ReV. Lett. 2002, 89, 096802.
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