Self-assembled growth of MnSi~1.7 nanowires with a single orientation and a large aspect ratio on Si(110) surfaces
© Zou et al.; licensee Springer. 2013
Received: 21 December 2012
Accepted: 16 January 2013
Published: 22 January 2013
MnSi~1.7 nanowires (NWs) with a single orientation and a large aspect ratio have been formed on a Si(110) surface with the molecular beam epitaxy method by a delicate control of growth parameters, such as temperature, deposition rate, and deposition time. Scanning tunneling microscopy (STM) was employed to study the influence of these parameters on the growth of NWs. The supply of free Si atoms per unit time during the silicide reaction plays a critical role in the growth kinetics of the NWs. High growth temperature and low deposition rate are favorable for the formation of NWs with a large aspect ratio. The orientation relationship between the NWs and the reconstruction rows of the Si(110) surface suggests that the NWs grow along the direction of the silicon substrate. High-resolution STM and backscattered electron scanning electron microscopy images indicate that the NWs are composed of MnSi~1.7.
Self-assembled nanowires (NWs) of metal silicides have received much attention recently for their potential applications as electrical interconnects on a scale that cannot be attained with conventional lithographic methods [1–4]. In addition, such structures are expected to display novel physical properties related to the structural anisotropy and quantum confinement effects and could be used as active elements for the new generation of electronic, optoelectronic, magnetic, and thermoelectric devices [5–7]. In the past decade, it has been reported that NWs of rare-earth silicides such as ScSi2, ErSi2[8, 9], DySi2[2, 10, 11], GdSi2[12, 13], and HoSi2[14, 15] and 3d transition metal silicides such as FeSi2, CoSi2, NiSi2, and TiSi2[17–19] can be formed on silicon substrates by the molecular beam epitaxy method. While the NW shape of rare-earth silicides is thought to result from an anisotropic lattice mismatch that is small (<1%) in length direction and large (>5%) in width direction of the NW, the NW shape of FeSi2, CoSi2, and NiSi2 results from an ‘endotaxial’ growth mechanism which involves the growth of silicide into the Si substrate [1, 3].
Very recently, we have reported that MnSi~1.7 NWs can also be grown on the Si substrates with reactive epitaxy method at temperatures above approximately 500°C [20–22]. The growth mechanism of the NWs was considered to be anisotropic lattice mismatch between the silicide and the Si substrates. The growth direction of the NWs is confined along Si<110>, resulting in the NWs orienting with the long axis along one direction (Si), two orthogonal directions (Si and ), and three directions (Si, , and ) on the Si(110), (001), and (111) surfaces, respectively. However, for scientific investigation as well as device applications, it would be highly expected to grow NWs with a single orientation because the NWs grown in this mode would never cross and have larger length. Parallel NW arrays can be used as nanomechanical devices , and using parallel NWs, the anisotropic electronic structure of silicide NWs can be investigated by angle-resolved photoelectron spectroscopy . On the other hand, the Si(110) surface is currently attracting renewed interests because of its unusual properties such as high hole mobility, unique surface reactivity, and strong structural anisotropy. The Si(110) surface has a potential use in fabricating vertical double-gate metal oxide semiconductor field effect transistors that enable much higher integration . Although the formation of MnSi~1.7 NWs with sole orientation on Si(110) was demonstrated in our previous works , a detailed investigation on how the growth parameters affect the growth of MnSi~1.7 NWs on Si(110), which is of key importance for a comprehensive understanding of the growth kinetics and thus the controllable growth of the NWs, is still lacking. In this paper, we examine in detail, mainly using scanning tunneling microscopy (STM), the influence of growth temperature, deposition rate, and deposition time on the formation of MnSi~1.7 NWs on the Si(110) surface.
The experiments were performed in an ultra-high vacuum molecular beam epitaxy-STM system (Multiprobe XP, Omicron, Taunusstein, Germany) with a base pressure of less than 5.0 × 10−11 mbar. Substrates used for the deposition were cut from a phosphorus-doped, n-type Si(110) wafer with resistivity of approximately 0.01 Ω cm and have a size of 12 × 2.5 × 0.3 mm3. Atomically clean Si(110)-16 × 2 surfaces were prepared by degassing the substrates at about 600°C for 12 h, followed by flashing to 1,200°C and annealing at 600°C for 10 min. Mn was deposited on the Si(110)-16 × 2 surfaces by heating Mn lumps (purity 99.999%) in a Mo crucible with electron bombardment. The Mn flux was monitored by an internal ion collector mounted near the evaporation source. The deposition rate was controlled from approximately 0.01 to 0.5 ML/min (1 ML = 1 metal atom per 1 × 1 surface mesh = 4.78 × 1014 Mn atoms/cm2) . During deposition, the substrates were heated by radiation from a tungsten filament located at the back of the sample holder. The temperature was set from 450°C to 600°C and measured using a thermocouple. An electrochemically etched tungsten tip was used for scanning. All STM images were recorded at room temperature (RT) with a bias voltage of 2 to 3 V and a tunneling current of 0.1 to 0.2 nA. A backscattered electron scanning electron microscope (BE-SEM) (Nova NanoSEM 230, FEI, Hillsboro, OR, USA) was used to ex situ observe the elemental distribution of the samples on a large scale.
Results and discussion
Effects of growth parameters on the formation of NWs
After surveying the flashed Si(110) surface by STM, we evaporate Mn atoms onto the surface at different substrate temperatures in the range of RT to 600°C, while the deposition rate and time are kept at approximately 0.02 ML/min and 50 min, respectively. We find that only clusters or irregular three-dimensional (3D) islands are formed on the Si(110) surface when the temperature is lower than approximately 475°C. At approximately 475°C, elongated silicide islands begin to form on the surface. With further increasing temperature, the elongated islands grow rapidly in the length direction and remain almost invariant in the width direction, forming a NW-like shape. Meantime, the number density of the NWs is also increased significantly, while that of the 3D islands is decreased. Figure 1b is a typical STM image of the Si(110) surface after deposition at 585°C. It can be seen that straight and parallel NWs with a large aspect (length/width) ratio were formed on the surface. The NWs are about 600 to 1,370-nm long, approximately 18-nm wide, and 2.5-nm high, and their aspect ratios are in the range of approximately 33 to 76.
Average dimensions and number density of the NWs and 3D islands grown at different deposition rates
Deposition rate (ML/min)
Length of NWs (nm)
Width of NWs (nm)
Height of NWs (nm)
Density of NWs (number/μm2)
Size of 3D islands (nm)
Height of 3D islands (nm)
Density of 3D islands (number/μm2)
Average dimensions and number density of the NWs and 3D islands grown at different deposition times
Deposition time (min)
Length of NWs (nm)
Width of NWs (nm)
Height of NWs (nm)
Density of NWs (number/μm2)
Size of 3D islands (nm)
Height of 3D islands (nm)
Density of 3D islands (number/μm2)
As suggested in our previous studies, the formation mechanism of the Mn silicide NWs can be attributed to the anisotropic lattice mismatch between the Mn silicide and the Si(110) substrate [20, 21]. In the width direction of NWs (i.e., Si direction), the lattice mismatch has a relatively large value, and the adatoms are not easily attached to the two long edges of the NWs because of the high strain energy, leading to the limited growth along this direction. However, with extension of deposition time, more Mn atoms are supplied, and this will introduce dislocations in the NWs [9, 27, 28], resulting in the fragmentation of NWs and, finally, the reduction in their lengths. Meanwhile, the dislocations can relax the high strain along the width direction of NWs and thus make the adatoms attach to the wire edges more easily, leading to the increase in the wire width and height. The ‘A’-type change of the NWs shown in Figure 5c,d can be considered as a result induced by the dislocations. On the other hand, the appearance of ‘B’-type change of the NWs at a deposition time of 25 min (Figure 5c) indicates that the growth of NWs at this stage undergoes Ostwald ripening. Compared to large 3D islands, NWs have a large surface-to-volume ratio and thus a high chemical potential. According to the Gibbs-Thomson principle, the atoms would dissolve from thin NWs, diffuse over the surface, and finally attach to the large 3D islands, making the 3D islands become larger and the NWs become thinner until they disappear.
Chemical composition of the NWs
The formation of Mn silicides on a Si substrate can be considered as a diffusion-determined chemical reaction between Mn and Si atoms . The Si atoms that take part in the silicide reaction mainly come from the surface step edges or surface defects because the Si atoms at these places have less Si-Si bonds. In the above paragraphs, we mentioned that it is relatively easy to grow NWs with a large aspect ratio at a high temperature or a low Mn deposition rate. This fact indicates that the supply of sufficient free Si atoms per unit time plays an important role in the formation of NWs because more Si atoms can be detached from the substrate step edges at a high temperature, and the Mn atoms can encounter more Si atoms in the unit time at a low deposition rate. On the contrary, at a relatively low growth temperature or a high deposition rate, the supply of free Si atoms in the unit time is not sufficient and the formation of more 3D islands (Mn silicides rich in manganese) is the result. The Mn-Si binary alloy phase diagram shows that MnSi~1.7 is the only Si-rich silicide phase, and this phase is favored for high concentrations (≥50 at.%) of Si mixed with Mn at temperatures between approximately 400°C and 1,144°C . Therefore, the Si-rich environment for the NW formation implies that the NWs are likely to be MnSi~1.7.
In summary, the influence of growth conditions such as growth temperature, deposition rate, and deposition time on the formation of MnSi~1.7 NWs on a Si(110) surface has been investigated by STM. High growth temperature and low Mn deposition rate are found to be favorable for the formation of NWs with a large aspect ratio, indicating that the supply of free Si atoms per unit time plays a crucial role in the growth of the NWs. The NWs orient solely with the long axis along the Si direction. The I-V curves measured on top of the NWs, and the BE-SEM image reveal that the NWs consist of MnSi~1.7. The growth of the parallel MnSi~1.7 NWs on the Si substrate provides an opportunity for the study of electronic properties of NWs and the fabrication of nanoelectronic devices with novel functions.
This work was supported by the National Natural Science Foundation of China under grant no. 61176017 and the Innovation Program of Shanghai Municipal Education Commission under grant no. 12ZZ025.
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