Abstract
Similar to graphene, zero band gap limits the application of silicene in nanoelectronics despite of its high carrier mobility. By using firstprinciples calculations, we reveal that a band gap is opened in silicene nanomesh (SNM) when the width W of the wall between the neighboring holes is even. The size of the band gap increases with the reduced W and has a simple relation with the ratio of the removed Si atom and the total Si atom numbers of silicene. Quantum transport simulation reveals that the sub10 nm singlegated SNM field effect transistors show excellent performance at zero temperature but such a performance is greatly degraded at room temperature.
Introduction
Silicene, silicon analog of graphene, is predicted to possess a Diracconeshaped energy band^{1} and ultrahigh carrier mobility^{2} and thus has a potential application in highperformance nanoelectronics. Recently silicene has been successfully grown on Ag^{3,4,5,6,7}, ZrB_{2}^{8}, Ir^{9} and MoS_{2}^{10} substrates. However, zero band gap of pristine silicene limits its application as a logic element in electronic devices directly. It is critical to open the band gap of silicene without degrading its carrier mobility. Several methods have been proposed to open the band gap of silicene from firstprinciples calculations, such as vertical electric field^{2,11,12}, surface adsorption^{13,14}, or semihydrogenation^{15}. Nonetheless, an experimentally approachable vertical electric field can only open a gap below 0.1 eV in silicene, evidently smaller than the minimum band gap requirement (0.4 eV) for traditional field effect transistors (FETs)^{2}. Metal atom adsorption is able to induce a larger band gap up to 0.66 eV in silicene^{13,14,16}, but a large supply voltage (V_{dd}) of about 1.7 ~ 30 V is required^{13,14}. It is highly desirable to design a new silicene FET with a high on/off ratio under a low supply voltage.
Computing technology requires a channel length of FET smaller than 10 nm in next decades. However, bulkSi FET will not perform at sub10 nm channel length because of its shortchannel effects^{17}. In order to enable continued FET scaling, one can modify Si device structure or use an alternative channel geometry/material. Up to now, 8 ~ 10 nm advanced Si FETs (including Si nanowire with gateallroundconfiguration^{18}, doublegated FinFET^{19} and extremely thin Si on insulator (ETSOI)^{20}) and 9 nm carbon nanotube (CNT) FETs^{21} have been fabricated. It is interesting to examine whether silicene FETs are competitive with these existing advanced Si FETs and CNTFETs.
In this Article, we propose a novel method to open the band gap of silicene, namely, fabrication of silicene nanomesh (SNM). Theoretical calculations^{22,23,24,25,26,27,28,29,30,31,32} have shown that the band gap of graphene can be opened by making a periodic array of holes (namely nanomesh) and the size of the gap depends on the structural parameters of graphene nanomesh (GNM). Experimentally, a transport gap has been observed in GNM FETs with a triangular array of hexagonal holes^{33,34,35,36,37} and the on/off current ratio can reach 100 ^{38}, which is one order of magnitude larger than that of pristine graphene FETs. Based on density functional theory (DFT) calculations, a band gap is opened up in SNM, whose size strongly depends on the width W of the wall between the neighboring holes and has a maximum value of 0.68 eV. Subsequently, we simulate the transport properties of the sub10 nm SNM FETs based on the nonequilibrium Green's function (NEGF). The simulated SNM FETs show excellent device performance with an on/off ratio up to 10^{4} at a supply voltage of 0.5 V. When phonon scattering is considered, the performance is greatly degraded with an on/off ratio down to 100.
Results and Discussion
Geometry and electronic structure
The SNM model is built by digging a simple triangular array of hexagonal holes in a silicene sheet, as shown in Fig. 1(a). The edge of the holes has zigzag shape and the edge Si atoms of the holes are passivated by hydrogen atoms. Each type of SNM is designated by the notation [R, W], where the R index reflects the radius of the hole calculated by N_{removed} = 6R^{2} (N_{removed} is the number of the removed Si atoms from one lattice cell) and the W index is the width of the wall between the nearestneighboring holes. Fig. 1(a) shows an example of [R, W] = [1, 4] SNM.
After relaxation, the unit cell size of the SNM structures is nearly unchangeable compared with the corresponding silicene supercell. However, constructing periodic holes will slightly affect the buckling distance (Δ) of the edge silicon atoms, which is larger than that of pristine silicene (0.46 Å) and increases with the increase of W given the same R = 1 (Fig. S1(a)). For the silicon atoms far away from the holes, the Δ tends to be the value of pristine silicene, as shown in Fig. S1(b).
To investigate the stability of SNMs, the cohesive energy E_{coh} and Gibbs free energy δG, are calculated. The peratom cohesive energy E_{coh} is calculated according to the follow equation:
where E(Si) and E(H) are the energies of the free silicon and hydrogen atoms, respectively, E(SNM) the total energy of a SNM in one supercell, N_{Si} and N_{H} the numbers of silicon and hydrogen atoms in a SNM supercell, respectively. The calculated E_{coh} for [R = 1, W] SNM is positive (3.64 ~ 4.71 eV) and increases with the increasing W as shown in Fig. 2a. We define the peratom Gibbs free energy (δG) of formation for SNMs as
where n_{Si} and n_{H} are the mole fraction of Si and H atoms, respectively, for a given structure and μ_{Si} and μ_{H} are the peratom chemical potentials of Si and H, respectively, at a given state. We chose μ_{Si} and μ_{H} as the binding energies per atom of bulk Si and H_{2} molecule, respectively. As given in Fig. 2b, the calculated δG values for W = 1 ~ 3 are 0.012 ~ 0.075 eV and therefore a small amount of additional energy is required to make these reaction processes happen. However, SNMs have a negative δG when W > 3, indicating a higher thermodynamical stability relative to their elemental reservoirs. Based on the width dependence of E_{coh} and δG, the stability of SNMs monotonically increases with the increasing W given the same R = 1. To further study its stability, a molecule dynamic simulation of the [R = 1, W = 2] SNM is performed at temperature of 1000 K. As shown in the Supplementary Movie S1 we added, the structure is well kept, suggesting SNMs are stable enough against the high temperature.
The calculations done by Liu et al.^{26} show that the band gap of graphene hexagonal nanomeshes is only opened when W is even. While W is odd, the GNMs behave semimetallically like pristine graphene. Our calculations show that SNMs have similar properties. The band structures of [1, W] SNMs with W = 1 ~ 10 are provided in Fig. 1(b) and 1(c). Similar to pristine silicene, the SNMs with odd W show semimetallic behavior, with a pair of linear bands crossing at the K point (Fig. 1(b)). By contrast, a direct band gap (E_{g}) is opened at the Г point when W is even (Fig. 1(c)). The band structures of [2, W] SNMs with even W are provided in Fig. S2 and the band gaps are also opened in them. The opened band gap when W is even monotonically decreases with the increasing W given the same R. A maximum band gap of about 0.68 eV is observed in both R = 1 and 2 cases (Fig. 1(d)).
Pedersen et al.^{25} found that the band gap E_{g} in GNM is determined by the relation
where N_{total} and N_{removed} are the numbers of the total Si atoms before digging the holes and the removed hole atoms in a unit cell, respectively and g is a fitting factor. For GNM, one has g = 25 eV. Fig. 1(e) shows the band gap of SNM against N^{1/2}_{removed}/N_{total}. The linear relation remains with g = 7.246 eV, which is much smaller than that for GNM. Therefore, given the same N_{total} and N_{removed}, i.e. with the same notation [R, W], the band gap in SNM is much smaller than that in GNM. The band gap opening in GNM is proved not directly caused by quantum confinement as in graphene nanoribbions (GNRs); instead, it has a geometric symmetry origin^{32,37}. When the two reciprocal lattice vectors of a GNM overlap with Dirac points of the pristine graphene, degeneracy at the Dirac points is lifted and a sizable band gap appears; otherwise, it's semimetal like graphene. SNM shares similar mechanism of band gap opening with GNM. In [R, W] SNMs, when W is even its two reciprocal lattice vectors overlap with Dirac points of the pristine silicene, i.e. the K and K' points of pristine silicene are folded into the Г points of SNM. Due to the intervalley scattering, a band gap is opened in SNMs when W is even.
can be explained if we approximately treat SNM as a periodic potential perturbation U(r) to pristine silicene. If K − K′ = G, where G is one reciprocal lattice vector of the SNM supercell, there is interaction between the two degenerate Dirac points. As a result, a band gap is opened and is expressed in terms of degeneracy perturbation theory as,
where S is the square of the supercell of SNM, and are the Bloch function of A sublattice at the K and K′ points, with periodic part of and , respectively. The external periodical potential U(r) induced by the hole is assumed to be rather localized. Given the same hole of different SNMs, is approximately independent of the size of the supercell and we therefore have due to N_{total} ∝ S.
Fig. 3 presents the effective mass (m*) of the conduction band bottom of SNMs along the Γ → K (m_{e}^{ΓK}) and Γ → M (m_{e}^{ΓM}) directions as a function of W. The effective mass is calculated by using the formula:
The effective mass m* monotonically decreases from 0.093 to 0.022 m_{0} for R = 1 and from 0.151 to 0.034 m_{0} for R = 2 with the increasing W because of the reduced band gap, where m_{0} is the free electron mass. m_{e}^{ΓK} is approximately equal to m_{e}^{ΓM} except for W = 2 case. At the same W, the m* values with R = 2 are slightly larger than their respective m* values with R = 1.
Transport properties of SNM FETs
The schematic model of a singlegated FET based on the [1, 2] SNM is presented in Fig. 4(a). The electrodes are composed of semiinfinite silicene. To avoid the interaction between SNM and SiO_{2} dielectric, a hBN buffer layer is inserted between the SNM and SiO_{2} substrate^{2} and the thickness of SiO_{2} dielectric plus hBN buffer region is d_{i} = 11 Å. To start with, we calculated the transmission spectrum of a 6.5 nmgatelength SNM FET by using the DFT method with singleζ (SZ) basis set to benchmark our SE extended Hückel result (Fig. S3). The transmission spectra calculated between the two methods are similar, except that the size of the transmission gap generated by the SE method (0.9 eV) is a litter larger than that by the DFT method (0.7 eV) and the transmission coefficients generated by the SE method near the Fermi level are unsmooth and generally larger than those by the DFT method. The larger transmission gap generated by the SE method can cause the decrease of on/off ratio compared with that by the DFT method given the same gate voltage window. The on/off ratio may be further slightly decreased when using the SE method due to the relative larger conductance in the offstate contributed by the larger transmission coefficient near E_{f}. However these won't affect much the results. Then we focus on the transport properties of the SNM FET with a larger gate length L_{gate} = 9.1 nm. The conductance in SNMs can be modulated by applying a gate voltage to the channel and an on/off switch is expected.
The transmission spectra of the 9.1 nmgatelength SNM FET at V_{g} = 0 and 0.5 V with V_{bias} = 0.2 V are presented in Fig. 4(b). When V_{g} = 0 V, there is a transport gap of 0.9 eV centered at the Fermi level (E_{f}). The transmission coefficient nearly vanishes within the bias window, indicating an off state. By applying a positive gate voltage, the transport gap can be shifted towards low energy direction. At V_{g} = 0.5 V, relatively large transmission coefficients are moved inside the bias window. According to Eq. (6) in the Method section, the drain current is calculated and then normalized by the channel width to obtain the current density I_{ds} (Fig. 4(c)). Clear on/off current modulation is achieved. If we set V_{dd} = V_{on}−V_{off} = 0.5 V and V_{g} = 0.5 V is chosen as the onstate, the on/off ratio can reach 5.1 × 10^{4}, which is about three orders of magnitude larger than the maximum on/off ratios obtained in dualgated silicene FET^{2} and already meets the requirement of 10^{4} ~ 10^{7} for the highspeed logic applications. The subthreshold swing (SS, here is defined as dV_{gate}/d(logI)) is 68 mV/dec, which approaches the 60 mV/dec switching limit of the classical transistors. To provide an insight into the switch capability, we investigate the transmission eigenchannels of the offstate (V_{g} = 0 V) and onstate (V_{g} = 0.5 V) at E = 0.05 eV and k = (0, 0), as shown in Fig. 4(d). The transmission eigenvalue of the offstate is merely 6.71 × 10^{−7} and the corresponding incoming wave function is obviously scattered and unable to reach to the other lead. On the contrary, the transmission eigenvalue of the onstate is 0.78; as a result, the scattering is weak and the most of the incoming wave is able to reach to the other lead.
To determine the scaling effect of the gate length L_{gate} on the device performance, we calculate the transfer characteristics of the SNM FET with different gate lengths (3.8 ~ 9.1 nm) at a fixed bias voltage of V_{bias} = 0.2 V as shown in Fig. 5. The maximum current I_{max} is insensitive to L_{gate}. By contrast, the minimum current I_{min} increases with the decreasing L_{gate}. Such a scaling behavior is attributed to the increasing offstate leakage current with the decreased L_{gate}. Therefore, the maximum and minimum current ratio I_{max}/I_{min} decreases significantly from 5.8 × 10^{5} at L_{gate} = 9.1 nm to 1.9 × 10^{2} at L_{gate} = 3.8 nm (Fig. 6(a)). The on/off current ratio I_{on}/I_{off} (the gate voltage window is limited to a supply voltage) is a more important parameter than I_{max}/I_{min} to characterize switching effect of an electronic device. We limit the gate voltage window to 0.5 V and show the L_{gate} dependent I_{on}/I_{off} in Fig. 6(b). It also monotonously decreases from 5.1 × 10^{4} at L_{gate} = 9.1 nm to 17 at L_{gate} = 3.8 nm.
The subthreshold swing SS = dV_{gate}/d(logI) is another important parameter of FET and determines how effectively the transistor can be turned off by changing the gate voltage. The SS of the SNM FETs monotonously increases from to 68 to 336 mV/dec when L_{gate} scales down from 9.1 to 3.8 nm. Transconductance g_{m} is another important parameter to characterize switching effect of an electronic device, which is computed from g_{m} = ∂I_{ds}/∂V_{g}. The g_{m} value decreases from 555 μS/μm at L_{gate} = 3.8 nm to 351 μS/μm at L_{gate} = 9.1 nm (Fig. 6(d)). Another key parameter the intrinsic gate capacitance C_{g} is calculated in Fig. 6(e). C_{g} is defined as C_{g} = ∂Q_{ch}/∂V_{g}, where Q_{ch} is the total charge of the channel. The relationship between the C_{g} and L_{gate} is the following equation^{39}: C_{g} = ε_{0}ε_{r}W_{gate}L_{gate}/t_{ox}, where ε_{0} and ε_{r} are the dielectric constant of vacuum and the relative dielectric constant of the gate dielectric, W_{gate} is the width of the gate and t_{ox} is the thickness of the gate dielectric. As shown in Fig. 6(e), C_{g} indeed increases almost linearly with L_{gate} from 159 aF/μm at L_{gate} = 3.8 nm to 258 aF/μm at L_{gate} = 9.1 nm. According to the charge control model at low bias, , since C_{g} ∝ L_{gate}, if we assume the mobility is a constant, the transconductance g_{m} tends to vary inversely with L_{gate} and our results is consistent with this tendency.
Fig. 6(f) shows L_{gate} dependence of charge carrier transit time τ based on the calculated C_{g} and g_{m}, i.e. τ = C_{g}/g_{m}. τ increases from 0.29 to 0.73 ps when L_{gate} increases from 3.8 to 9.1 nm. The intrinsic cutoff frequency f_{T} indicates how fast the channel current is modulated by the gate and is described as f_{T} = 1/(2πτ)^{40,41,42}. f_{T} decreases monotonically with L_{gate} from 557 GHz at L_{gate} = 3.8 nm to 217 GHz at L_{gate} = 9.1 nm (Fig. 6(g)), which is much smaller compare with that in the sub10 nm graphene FETs (4 ~ 22 THZ)^{43}. The drift velocity of a transistor can be derived by v_{drift} = L_{gate}/τ. As shown in Fig. 6(h), v_{drift} is insensitive to the gate length and is 12.4 × 10^{5} ~ 13.3 × 10^{5} cm/s when L_{gate} = 3.8 ~ 9.1 nm.
Future FET technologies will require operation at voltages at or below 0.5 V to reduce power consumption. To compare the SNM FETs with the Si based and CNT transistors at a supply voltage V_{dd} = 0.5 V, we summarize the critical performance parameters of the sub10 nm SNM (9.1 and 7.8 nm), advance Si and CNT FETs at V_{bias} = 0.5 V in Table 1. The 9.1 nm SNM FET carries an onstate current of 464 μA/μm, which is larger than those (41 ~ 300 μA/μm) of the 8 ~ 10 nm advanced Si devices but slightly smaller than that (630 μA/μm) of the 9 nm CNT device. The on/off current ratio of the 9.1 nm SNM FET is 7.4 × 10^{3}, which is a little smaller than those (1 × 10^{4}) of the 10 nm Si nanowire, 8 nm ETSOI and 9 nm CNT devices but larger than that (1 × 10^{3}) of 10 nm Si Fin device. The SS value (82 mV/dec) of the 9.1 nm SNM FET is slightly smaller than those (83 ~ 125 mV/dec) of the 10 nm Si nanowire and Si Fin, 8 nm ETSOI and 9 nm CNT devices. Taking the three criterions together, the 9.1 nm SNM FET is competitive with the sub10 nm advanced Si devices but is inferior to the 9 nm CNT device.
Adding the total area of the gates is an effective way to strengthen the gates' control over the channel, the gate control ability of a FET is expected to be improved by using a dual gate configuration. The transfer characteristic of the 9.1 nm dualgated SNM FET at V_{bias} = 0.5 V is provided in Fig. 7(a) to compare with that of the singlegated one with the same L_{gate} and improved gate control is apparent. The performance parameters of the 9.1 nm dualgated SNM FET are generally better than those of the singlegated counterpart as listed in Table 1. The SS is reduce by 8 meV/dec, the large on/off current ratio is increased by a factor of 2.7 and the onstate current is increased by a factor of 1.6. The 9.1 nm dualgated SNM FET delivers an onstate current of 870 μA/μm, which is larger than those (41 ~ 630 μA/μm) of the 8 ~ 10 nm advanced Si devices and 9 nm CNT device. The on/off current ratio of the 9.1 nm dualgated SNM FET is 1.2 × 10^{4}, which is comparable with those of the 10 nm Si nanowire, 8 nm ETSOI and 9 nm CNT devices and one order of magnitude larger than that of the 10 nm Si Fin device. The SS (74 mV/dec) of the 9.1 nm dualgated SNM is smaller than those (83 ~ 125 mV/dec) of the 8 ~ 10 nm advanced Si devices and 9 nm CNT device. Taking the three criterions together, the 9.1 nm dualgated SNM has a better performance than the sub10 nm advanced Si devices and 9 nm CNT device. The excellent performance of the SNM FET is attributed to the depressed short channel effects due to their extremely small thickness and fewer traps on semiconductordielectric interface due to the smooth interface (Fig. 8).
The output characteristics for the 9.1 nm SNM FET at different gate voltages are shown in Fig. 7(b). The sourcedrain ballistic current increases with the applied bias voltage and no current saturation is observed until V_{bias} = 0.7 V. The current of the dualgated SNM FET is much larger than that of the singlegated SNM FET at the same V_{bias} under V_{g} = 0.5 V, indicating an improved gate controlling.
The transfer and output characteristics of the 7.8 nm SNM FET are provided in Fig. S4. Although the performance of the 7.8 nm singlegated SNM FET is inferior to the sub10 nm advanced Si devices and the 9 nm CNT device, the 7.8 nm dualgated SNM FET is sufficiently improved: The SS is reduce by 4 meV/dec to 68 mV/dec, the large on/off current ratio is increased by a factor of 2.7 to 8.9 × 10^{3} and the onstate current is increased by a factor of 3 to 607 μA/μm at a supply voltage of 0.5 V (Table 1). Consequently, the 7.8 nm dualgated SNM FET has a better performance than the sub10 nm advanced Si devices and is competitive with the 9 nm CNT device.
It is interesting to examine whether the SNM FETs can meet the requirements for the highperformance FETs from the 2013 edition of the International Technology Roadmap for Semiconductors (ITRS)^{44}. The required gate lengths of HP logic of 2022 and 2023 are 8.9 nm and 8.0 nm and supply voltages are 0.72 and 0.71 V, respectively. The transfer characteristic of the 9.1 nm dualgated SNM FET at V_{bias} = 0.72 V is provided in Fig. 6(a). The 9.1 nm dualgated SNM FET, whose gate length is approximately meet the requirement of HP logic of 2022 (8.9 nm), carries an onstate current of 3122 μA/μm at a supply voltage of V_{dd} = 0.72 V and greatly satisfies the requirement of I_{on} = 1350 μA/μm for the HP logic of ITRS of 2022. As shown in Fig. S4(a), the calculated onstate current (1963 μA/μm) of the 7.8 nm dualgated SNM FET at a supply voltage of V_{dd} = 0.71 V also meets the requirement (I_{on} = 1330 μA/μm) of the HP logic of ITRS of 2023. Unfortunately, the on/off current ratios of the 9.1 and 7.8 nm dualgated SNM FETs are only 1.8 × 10^{3} and 1.2 × 10^{3}, respectively. Both of them cannot meet the requirement of HP logic of ITRS (1.33 × 10^{4} in 2022 and 1.35 × 10^{4} in 2022).
We perform a molecular dynamics (MD) simulation of the channel region in a 9.1 nm singlegated SNM FET at room temperature to check how the transport properties change as phonon effect is partially included (only elastic scattering is considered) in the device. Compared with the transmission spectra of 9.1 nm singlegated SNM FET at V_{bias} = 0.2 V without considering the phonon scattering, the transport gap is increased from 0.9 eV to 1.5 eV and the transmission coefficients of both the conduction and valence bands are greatly depressed at 300 K after phonon scattering effect is included, as shown in Fig. 9. The offstate current at V_{g} = 0 V isn't affected much. Whereas the onstate current is decreased significantly to 3.9 × 10^{2} μA/μm and the on/off current ratio is decreased to 39 at V_{bias} = 0.2 V when the gate bias window is fixed at 0.5 V. When V_{bias} = 0.5 V, the onstate current is decreased to 4.0 μA/μm and the on/off current ratio is decreased to 100. Therefore, SNM FETs still works at room temperature, but its performance is greatly degraded. Phonon scattering plays an import role on accurate assessment of SNM FETs even at a shortgate length below 10 nm.
In summary, a band gap is opened in SNM when the width W of the wall between the neighboring holes is even from the firstprinciples calculations. The size of the band gap increases with the reduced W and is proportional to the ratio of the removed Si atom and the total Si atom numbers of silicene. We simulate the transport of the FETs with a sub10 nm SNM channel based on quantum transport theory and find that the sub10 nm SNM FETs have an excellent performance at zero temperature, characterized by a large onstate current up to 870 μA/μm, a large on/off current ratio up to 1.2 × 10^{4} and a small subthreshold swing low to 68 mV/dec at a supply voltage of 0.5 V. However the performance is greatly degraded when phonon scattering effect is included.
Methods
The geometry optimizations and the band structure calculations are performed using the double numerical basis set plus polarization (DNP), implemented in the DMol^{3} package^{45}. We chose the generalized gradient approximation (GGA)^{46} of the PerdewBurkeErnzerhof (PBE) form to the exchangecorrelation functional^{47}. Both the atomic positions and lattice constant are relaxed without any symmetry constraints until the maximum force is smaller than 0.01 eV/Å. A 16 × 16 × 1 MonkhorstPack kpoints grid^{48} is used in the first Brillouin zone sampling. A vacuum space of 20 Å normal to silicene plane is used to avoid spurious interaction between periodic images. To examine the thermal stability of SNMs, ab initio MD simulation within the NVT ensemble is carried out using the DMol^{3} package at 1000 K and the process lasts for more than 1.0 ps with a time step of 1.0 fs.
A singlegated twoprobe model is built to simulate the transport of SNM and the pristine silicene is used as source and drain electrodes for simplicity. Transport properties are calculated by the semiempirical (SE) extended Hückel method coupled with NEGF formalism implemented in the Atomistix Tool Kit (ATK) 11.2 package^{49,50,51}. The Hoffman basis is used and the temperature is set at 300 K. The kpoint meshes of the electrodes and central region are set to 1 × 50 × 50 and 1 × 50 × 1, respectively. The current is calculated with the LandauerBűttiker formula^{52}:
where T(E, V_{g}, V_{bias}) is the transmission probability at a given gate voltage V_{g} and bias voltage V_{bias}, f_{L/R} the FermiDirac distribution function for the left(L)/right(R) electrode and μ_{L}/μ_{R} the electrochemical potential of the L/R electrode.
To include the phonon effect in the calculation of transport properties, ab initio MD simulation of the central region of the device within the NVT ensemble is performed by using the Dmol^{3} package at 300 K and the process lasts for 3.0 ps with the electrode extension parts constrained. The time step is 1.5 fs. Then different configurations of the central region are built into twoprobe models after every 400 MD steps and their transport properties are evaluated and finally averaged over 5 configurations using a NEGF approach implemented in the ATK package.
During the production of this paper we would like to add that we are aware that silicene FET operating at room temperature has been fabricated recently^{53}, corroborating theoretical expectations regarding its ambipolar Dirac charge transport.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (Nos. 11274016, 51072007 and 61471301), the National Basic Research Program of China (Nos. 2013CB932604 and 2012CB619304), National Foundation for Fostering Talents of Basic Science (Nos. J1030310/J1103205), Program for New Century Excellent Talents in University of MOE of China and the Special Fund of Education Department of Shaanxi Province, China (Grant No. 2013JK0635).
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J.L. conceived the idea. F.P. and Y.W. did the transport simulations. K.J. performed the electronic calculations. The data analyses were performed by F.P., Y.W., K.J., Z.N. and J.M. J.Z., R.Q., J.S., J.Y. and C.C. helped discussing. This manuscript was written by F.P., Y.W. and J.L. All authors reviewed this manuscript.
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Pan, F., Wang, Y., Jiang, K. et al. Silicene nanomesh. Sci Rep 5, 9075 (2015). https://doi.org/10.1038/srep09075
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