the growth axis was( 110), consistent with previous studies of smaller diameter SiNWs.4. 22 In addition. TEM studies of 18 SiNWs with diameters from 15 to 80 nm(Figure 2c and 200 Supporting Information, Figure SIb) all showed single- crystalline structures with a growth axis of (1 10)indep ndent of diameter, where 13/18 of the sampled SiNWS had diameters between 29 to 80 nm. Interestingly, the diameter 1000 independent growth direction observed for the ultralong SiNWs contrasts previous studies where a cross-over to(111) 50 direction at approximately 20 nm was observed. 45.22 Previously, Schmidt et al. proposed a model based on he free energy, which is influenced by the interplay of the Time(min) liquid-solid interfacial tension with Si surface edge tension, to explain consistently diameter dependent growth in SiNWS Within the context of this model, a larger critical cross-over diameter from(110) to(111)would be observed for an interfacial thickness that increased with crystallization rate We speculate that the faster growth rates used to achieve the ultralong siNWs might lead to an increase of the interfacial thickness parameter, although future studies of critical diameter versus growth rate will be required to clarify this point. Regardless of the detailed origin, we believe these observations suggest that it will be interesting to explore and possibly exploit kinetic effects as a means to controlling growth directions for NWs produced by the nanocluster- catalyzed VLs process The ultralong SiNWs represent potentially attractive build ing blocks for nanoelectronics because it would be possible to define a large nu facilitating integration as shown schematically in Figure 3a. In addition, the fabrication and characterization of multipl devices on a single Nw could provide important information addressing doping and electronic uniformity of these nano structures. To address these issues, we have prepared ultralong boron-doped SiNWs, aligned the NWs on sub Figure 2.(a)Plot of SiNW length versus growth time for Si2H6 strate by shear contact printing, 7and defined arrays of FETs 400°C( black), SiHg at400°c( red) and Sih4450°C(blue) by electron beam lithography. 0a.11.2628 A representative (b)Lattice-resolved TEM image recorded along(111) zone axis of a 18 nm diameter ultralong SiNw; scale bar is 5 nm.(c) optical image(Figure 3b) highlights the large number of Lattice-resolved TEM image of a 78 nm diameter ultralon addressable fets defined on the ultralong sinws. SiNW; scale bar is 5 nm. Inset is a lower magnification image Electrical transport measurements showed that more than of the siNw 90%o devices behaved as good p-type devices. Specifically, source-drain current(p) versus source-drain voltage(VD) times higher than for SiHa at our optimal growth temperature curves at small V were linear, which demonstrates good f 450C(1.0 um/min). Using the temperature-dependent contacts across the Nw. The decrease in /p with increasingly gas phase decomposition rate data for Si2H6 and SiH4, we positive VG(Figure S2)also showed that devices were p-type Si2H6(400C): SiH4(400 and 450C) of ap- depletion mode FETs In addition, we have assessed and ately 170 and 10, respectively. These values are compared quantitatively key transistor characteristics, includ to the experimental ratios of growth rates and thus ing on-state current, Ion, peak transconductance, GM, and uggest that reactant decomposition kinetics is important in threshold voltage, Vuh, as a function of position across the determining the observed SiNW growth rates. single SiNW array. Notably, the lon(Figure 3c), GM(Figure The structural characteristics of the ultralong SiNWs were 3d), and Vuh(Figure 3e) show very reproducible values across also investigated by transmission electron microscopy (TEM). the entire array spanning almost I mm in length with average Lattice-resolved images of an approximately 18 nm diameter +l standard deviation values of 1. 77+0.33 uA, 213+ 61 SiNW(Figure 2b) show that the SinW is a single-crystalline nS, and 6.0+ 1.1 V, respectively. Previous studies of structure despite the fact that growth occurred at rates at least individual nanowire FETsa.y have exhibited larger variations 10 times greater than previous studies. 4.5-7, 9.22 The TEM in key FET properties, with variations in threshold voltage image and two-dimensional Fourier transform analysis(Sup. of 35-135% versus 20% and transconductance of 58-76% porting Information, Figure Sla) further demonstrated that versus 30%. This comparison indicates that the ultralong Nano Lett., Vol. 8. No. 9, 2008times higher than for SiH4 at our optimal growth temperature of 450 °C (1.0 µm/min). Using the temperature-dependent gas phase decomposition rate data for Si2H6 and SiH4, 24 we estimate Si2H6 (400 °C):SiH4 (400 and 450 °C) of ap￾proximately 170 and 10, respectively. These values are similar to the experimental ratios of growth rates and thus suggest that reactant decomposition kinetics is important in determining the observed SiNW growth rates. The structural characteristics of the ultralong SiNWs were also investigated by transmission electron microscopy (TEM). Lattice-resolved images of an approximately 18 nm diameter SiNW (Figure 2b) show that the SiNW is a single-crystalline structure despite the fact that growth occurred at rates at least 10 times greater than previous studies.4,15-17,19,22 The TEM image and two-dimensional Fourier transform analysis (Sup￾porting Information, Figure S1a) further demonstrated that the growth axis was 〈110〉, consistent with previous studies of smaller diameter SiNWs.4,22 In addition, TEM studies of 18 SiNWs with diameters from 15 to 80 nm (Figure 2c and Supporting Information, Figure S1b) all showed single￾crystalline structures with a growth axis of 〈110〉 independent of diameter, where 13/18 of the sampled SiNWs had diameters between 29 to 80 nm. Interestingly, the diameter￾independent growth direction observed for the ultralong SiNWs contrasts previous studies where a cross-over to 〈111〉 direction at approximately 20 nm was observed.4,5,22 Previously, Schmidt et al.5 proposed a model based on the free energy, which is influenced by the interplay of the liquid-solid interfacial tension with Si surface edge tension, to explain consistently diameter dependent growth in SiNWs. Within the context of this model, a larger critical cross-over diameter from 〈110〉 to 〈111〉 would be observed for an interfacial thickness that increased with crystallization rate. We speculate that the faster growth rates used to achieve the ultralong SiNWs might lead to an increase of the interfacial thickness parameter, although future studies of critical diameter versus growth rate will be required to clarify this point. Regardless of the detailed origin, we believe these observations suggest that it will be interesting to explore and possibly exploit kinetic effects as a means to controlling growth directions for NWs produced by the nanocluster￾catalyzed VLS process. The ultralong SiNWs represent potentially attractive build￾ing blocks for nanoelectronics because it would be possible to define a large number of devices on a single NW, thus facilitating integration as shown schematically in Figure 3a. In addition, the fabrication and characterization of multiple devices on a single NW could provide important information addressing doping and electronic uniformity of these nano￾structures. To address these issues, we have prepared ultralong boron-doped SiNWs,25 aligned the NWs on sub￾strate by shear contact printing,27 and defined arrays of FETs by electron beam lithography.10a,11,26,28 A representative optical image (Figure 3b) highlights the large number of addressable FETs defined on the ultralong SiNWs. Electrical transport measurements showed that more than 90% devices behaved as good p-type devices. Specifically, source-drain current (ID) versus source-drain voltage (VD) curves at small VD were linear, which demonstrates good contacts across the NW. The decrease in ID with increasingly positive VG (Figure S2) also showed that devices were p-type depletion mode FETs. In addition, we have assessed and compared quantitatively key transistor characteristics, includ￾ing on-state current, Ion, peak transconductance, GM, and threshold voltage, Vth, as a function of position across the single SiNW array. Notably, the Ion (Figure 3c), GM (Figure 3d), and Vth (Figure 3e) show very reproducible values across the entire array spanning almost 1 mm in length with average (1 standard deviation values of 1.77 ( 0.33 µA, 213 ( 61 nS, and 6.0 ( 1.1 V, respectively. Previous studies of individual nanowire FETs6a,29 have exhibited larger variations in key FET properties, with variations in threshold voltage of 35-135% versus ∼20% and transconductance of 58-76% versus ∼30%. This comparison indicates that the ultralong Figure 2. (a) Plot of SiNW length versus growth time for Si2H6 at 400 °C (black), SiH4 at 400 °C (red) and SiH4 450 °C (blue). (b) Lattice-resolved TEM image recorded along 〈111〉 zone axis of a 18 nm diameter ultralong SiNW; scale bar is 5 nm. (c) Lattice-resolved TEM image of a 78 nm diameter ultralong SiNW; scale bar is 5 nm. Inset is a lower magnification image of the SiNW. 3006 Nano Lett., Vol. 8, No. 9, 2008