3.1. Controlled growth of atomic-scale Si layer with huge strain in the nano-heterostructure NiSi/Si/NiSi through point contact reaction between nano-wires of Si and Ni and reactive epitaxial growth[47,48]
As the end of the roadmap approaches for very-large-scale-integration of transistors on Si wafers, nanoscale transistor devices
[(Fig._10)TD$FIG]
Fig. 10. Overview of NiSi formation within Si nanowires by point contact reaction. (a) A TEM image of Si and Ni nanowires dispersed on a Si3N4membrane. (b–f) Sequence of in situ TEM images depicting the growth of a bamboo-type grain of NiSi within a Si nanowire at 700 8C. Inset in (b) selected area diffraction pattern showing the[1–10]zone axis of Si. Inset in (f) selected area diffraction pattern showing the[1–12]zone axis of NiSi. The time of the image capture is given in the rectangular box at the upper-right corner.
The first two numbers are in unit of seconds and the following two smaller numbers are in unit of 1/100 s. (g) A schematic illustration of NiSi growth within a Si nanowire. (h) A schematic illustration of growth of a NiSi/Si/NiSi heterostructure where the length of the Si section is controlled at the atomic scale[48].
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based on Si nanowires are expected to have significant potential and wide interest. To realize this potential, nanoscale device elements such as ohmic contacts and gates on Si nanowires must be developed[49]. The formation of these device elements requires a systematic study of chemical reactions in the nanoscale. The point contact reactions between Si nanowires and Ni nanowires were investigated[47,48].
Ni and Si were selected for the study because the monosilicide NiSi is one of the three silicides which have the lowest resistivity for applications in shallow junction devices [50]. In addition, single crystal NiSi nanowires can be scaled to ultrasmall dimension without degradation of electrical conductivity and exhibit remarkably high maximum transport current [42]. Si nanowires were prepared on a Si wafer by the VLS method with nano-Au dots as nucleation sites for single-crystal Si nanowires with a [1 1 1] growth direction [51,52]. Polycrystalline Ni nanowires were synthesized via the anodic aluminum oxidation (AAO) method and stored in isoproponal[53]. The Ni nanowires and Si nanowires have diameters ranged from 10 nm to 40 nm and lengths of a few microns. The Si and Ni nanowires were stored in solutions. To prepare point contact samples, droplets of both solutions are put on Si grids having a square opening covered with a window of glassy Si3N4 films, as shown in Fig. 10(a). The thickness of the glassy film is about 20 nm so that it is transparent to the electron beam in the microscope and does not interfere with the images of Si and Ni nanowires. The samples were dried under light bulbs.
Fig. 10(b)–(f) shows a time-lapsed series of in situ TEM images capturing the growth of a bamboo-type NiSi grain within a straight Si nanowire at 700 8C. Based on the high-resolution TEM images and selected area diffraction patterns, we determined the structure of the material to be single-crystal NiSi.Fig. 10(g) is a schematic diagram depicting the growth of NiSi, in which the Ni atoms dissolve and diffuse interstitially in Si[54]and stop at the ends of the Si nanowire, thereby nucleating growth of NiSi to form a NiSi/
Si/NiSi heterostructure. In point contact reaction, growth of single crystal NiSi started from both ends of the Si nanowire with an activation energy of 1.25 eV/atom rather than from the point contact with an activation energy of 1.7 eV/atom. This is attributed to the ease of nucleation and fast diffusion of Ni atoms in Si towards the ends.
Fig. 11shows the linear growth behavior of the NiSi nanowire in the Si nanowire of 20 nm in diameter over the temperatures ranged from 500 8C to 650 8C. The activation energy of the epitaxial growth was determined to be 1.25 eV/atom, compared to the activation energy of interstitial diffusion of Ni in Si of about 0.47 eV/atom[55], indicating that the growth may be interfacial-reaction-controlled.
To identify the phase and structure of the NiSi nanowire, its HRTEM image is provided in Fig. 12(a). The inset is the corresponding diffraction pattern with a zone axis of [1 1 3], which verifies that the wire is a single crystal of NiSi. Composi-tional analysis has also confirmed that it is NiSi.Fig. 12(b) is a high-resolution TEM image showing an epitaxial interface which is atomically sharp between Si and NiSi within a Si nanowire.
Fig. 12(a) and (b) demonstrates that point contact reaction enables the formation of nano-NiSi in perfect single crystal and atomically flat NiSi/Si interface without misfit dislocations, which is impor-tant for electrical contact properties.
The atomistic mechanism of the epitaxial growth involving a moving interface is of interest. Reactive epitaxial growth is unlike molecular beam epitaxy by deposition, in which atomic layers are built on the substrate surface and film growth requires no breaking of the substrate bonds. In the present case, the covalent Si–Si bonds must be broken and transformed into metallic Ni–Si bonds in order for NiSi to grow and have a moving interface.
Although the thermal energy at 700 8C is sufficient to break the covalent Si–Si bonds, the interstitial Ni atoms are crucial in the bond-breaking process[56].
Fig. 10(g) and (h) depicts the growth of NiSi from both ends of a Si nanowire. It leads to the formation of a NiSi/Si/NiSi nano-heterostructure as shown inFig. 13(a) and (b). The darker regions at the two ends are NiSi and the lighter region in the middle is Si. If annealing is stopped before the entire nanowire transforms into NiSi, as inFig. 13(c), a nano-heterostructure is formed as shown in Fig. 13(a) and (b). The nanoscale epitaxial growth rate of single-crystal NiSi has been measured using high resolution lattice imaging videos. Based on the growth rates, the remaining length of the Si region between the two NiSi regions can be controlled as illustrated inFig. 10(h). Two Ni nanowires can be patterned or deposited with a given spacing over a Si wire, and then the reaction can be utilized to bring the two NiSi grains as close as possible. At 500 8C, the reaction rate can be controlled down to atomic scale. InFig. 13(d)–(g), a set of lattice images of the nano-heterostructure of NiSi/Si/NiSi with 10, 8.1, 5 and 2 nm lengths of Si is shown. By measuring the length of the Si region and counting the number of (1 1 1) lattice planes within the region, the strain can be determined and it is found that the Si is highly compressed.
Fig. 13(h) shows the relationship between the compressive strain and the length of the Si region in the nano-heterostructure NiSi/Si/
NiSi at room temperature. In the process of NiSi formation, the diffusion of Ni atoms into Si lattice could have led to volume expansion. However, due to the confinement of the oxide on the Si nanowire, NiSi coming from both sides compressed the middle Si
[(Fig._11)TD$FIG]
Fig. 11. Kinetic analysis of the NiSi epitaxial growth within a Si nanowire of 20 nm in diameter. (a) Plot of the NiSi nanowire length versus reaction time at various temperatures, illustrating a linear growth rate. The lines are drawn as guides. (b) Arrhenius plot of the NiSi epitaxial growth, from which the activation energy was determined[48].
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region, resulting in compressive stress on Si in the length direction. As a result, the strain increased when the length of the Si region decreased. The strain can be controlled because the length of the Si region can be controlled. Since one-dimensional heterostructures may have potential applications in nano-electronic devices[57], the strain will affect carrier mobility in the Si region.
To sum up, nano-heterostructures of NiSi/Si/NiSi, in which the length of the Si region can be controlled down to 2 nm, have been produced using in situ point contact reaction between Si and Ni nanowires in an UHV-TEM. The Si region was found to be highly strained, more than 12%. It is proposed that the reaction is assisted by interstitial diffusion of Ni atoms within the Si nanowire and is limited by the rate of dissolution of Ni into Si at the point contact interface. The rate of incorporation of Ni atoms to support the growth of NiSi has been measured to be 7 104s per Ni atom.
Based on the rate, the consumption of Si can be controlled and, in turn, the dimensions of the nano-heterostructure down to less than 2 nm, thereby far exceeding the limit of conventional patterning process. The controlled huge strain in the controlled atomic scale Si region, potential gate of Si-nanowire-based transistors, is expected to significantly impact the performance of electronic devices.
3.2. Repeating events of nucleation in epitaxial growth of nano-CoSi2
and NiSi in nanowires of Si[58,59]
Nanostructures in Si nanowires have been studied for basic components in electronic and optoelectronics devices, especially biosensors[60–62]. Well-defined nanoscale building blocks such as ohmic contacts and gates on Si nanowires must be developed in order to be assembled into functional electronic structures [42,49,63]. It requires a systematic study of chemical reactions
in the nanoscale to form these circuit components. When a Si nanowire touches a metal nanowire, a point contact is formed and the chemical reaction between them starts from the point of contact. The point contact reactions between Si nanowires and Ni and Co nanowires or nanodots were investigated by in situ HRTEM.
The metallic NiSi and CoSi2may serve as the source-drain contacts to the Si in the heterostructure; it is the first step to produce a nanoscale field-effect transistor.
The formation of CoSi and CoSi2in Si nanowires was observed at 700 8C and 800 8C, respectively, by point contact reactions between nanodots of Co and nanowires of Si by in situ HRTEM.
The CoSi2 has undergone an axial epitaxial growth in the Si nanowire and a stepwise growth mode was found, as shown in Fig. 14(a)–(d). We observed that the stepwise growth occurs repeatedly in the form of an atomic step sweeping across the CoSi2/Si interface, as shown in Fig. 14(e). It appears that the growth of a new step or a new silicide layer requires an independent event of nucleation. We are able to resolve the nucleation stage and the growth stage of each layer of the epitaxial growth in video images. In the nucleation stage, the incubation period is measured to be much longer than the period needed to grow the layer across the silicide/Si interface.
So the epitaxial growth consists of a repeating nucleation and a rapid stepwise growth across the epitaxial interface. This is a general behavior of epitaxial growth in nanowires. The nucle-ation of silicides in Si nanowires is supply limited and source-limited reaction by point contact reaction rather than diffusion-controlled or interfacial diffusion-controlled reaction. Besides, the axial heterostructure of CoSi2/Si/CoSi2with sharp epitaxial interfaces has been obtained, which are promising as high performance transistors based on intrinsic Si nanowires.
The HRTEM videos show that the overall axial growth rate of the Ni and Co silicide layers is linear. However, the linear curve can be
[(Fig._12)TD$FIG]
Fig. 12. (a) HRTEM image of a NiSi nanowire. The inset is the corresponding diffraction pattern with a [1 1 3] zone axis. (b) HRTEM image of an epitaxial interface between NiSi and Si within a Si nanowire. (c) High angle annular dark-field image of the NiSi/Si/NiSi heterostructure. The bright image is NiSi and the dark image is Si[47].
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decomposed into many stair-steps with the step height equal to an atomic layer thickness and the step width equal to the incubation time of nucleating a new step. After a step is nucleated, it propagates very rapidly across the Si/silicide interface. The overall reaction rate is limited by the incubation time of nucleation, so it is a nucleation-limited reaction, neither diffusion-limited nor interfacial-reaction-limited.
Repeating events of homogeneous nucleation in epitaxial growth of CoSi2and NiSi silicides in Si nanowires by using in situ HRTEM was observed as shown inFig. 15. The growth of every
single atomic layer requires nucleation. Heterogeneous nucle-ation is prevented because of non-microreversibility between the oxide/Si and oxide/silicide interfaces so homogeneous nucleation occurs. The incubation time of homogeneous nucleation is determined to be 3 s and 6 s for NiSi and CoSi2, respectively.
Moreover, the calculated and the measured nucleation rates are in good agreement. Using Zeldovich factor to estimate the number of molecules in the critical nucleus; it is about 10 and reasonable.
A very high supersaturation is found for the homogeneous nucleation.
[(Fig._13)TD$FIG]
Fig. 13. In situ TEM images showing the formation of NiSi/Si/NiSi heterostructures within a Si nanowire, and compressive strain in the Si region. (a and b) In situ TEM images of the NiSi/Si/NiSi heterostructure. The bright area is Si and the dark area is NiSi. (d–g) HRTEM images of NiSi/Si/NiSi heterostructures. The bright and dark portions of the lattice images correspond to Si and NiSi, respectively. (h) Plot of the compressive strain versus the length of the Si region in the nano-heterostructure NiSi/Si/NiSi[48].
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