Nano letters adapted from our previous report. 25 Initially, boron-doped of 0.6 V in forward bias. These results are consistent wi p-type SiNWs were grown for 15 min at a calibrated growth previously studies of straight SiNWs with axial p-n junctions te of 0. 7 um/min. The reactor was evacuated for ca. 15 s, and In addition, measurements made on devices with two contacts then growth was continued using phosphine dopant to create (Supporting Information Figure S2)showed that no an n-type nanowire segme p-n junction metal/ SiNW Schottky barriers were present and allowed the 30 s, followed by a second cycle of reactor evacuation and estimation of the dopant concentrations. Specifically, the dopant continued growth using phosphine for ca. 15 min. Scanning concentrations of the p-arm and n-arm were estimated to be electron microscopy(SEM)images of the SINWs prepared in ca. 9 x 108 and 9 x 109cm-,respectively this way( Figure la)showed that the majority(90%)of the In a planar p-n diode device, the p-n junction is mostly buried beneath the surface and thus can only be partially gated with a top gate electrode. In contrast, the axial design of our kinked nanowires fully exposes the nanoscale p-n junction to external potential and enables a much more effective gate schematic band diagram change of the nanowire diode when a gate potential is applied at the p-n junction. The heavily doped p-and n-arms are not affected by the gate and the Fermi energy is pinned along the nanowire. When a negative potential applied, the electron energy levels in both the conduction band b and the valence band are raised(blue dashed lines in Figure 1c) As a result, the p-depletion region becomes more conductive while the n-depletion region less conductive. In the case of applying a positive potential, the opposite occurs. In order to estimate the overall gate response of our device, we assume that (1) the carrier concentration distribution in the depletion region is linear,(2)the depletion region can be approximated as a number of small segments, each of which can be treated as a V(volt) field-effect transistor with uniform doping, (3)the gate coupling doped is ideal, and(4)the width of the depletion region is the same the abrupt junction. In addition, the mobility along the nanowire nplify the calculation withou affecting the physics. It follows that the resistanc of the p- n junction can be expressed as△R∝△vn(NA∥N2 (ND)/ND](see Supporting Information), where AV is the change of the gate potential, and NA and Np are the dopant Figure 1. Design and controlled synthesis of kinked p-n nanowires. concentration of the p- and n-arms, respectively. When NA and (a)Representative SEM image of a kinked p-n SiNW with 120 tip Np are equal or comparable, the p-n junction will behave as an angle Scale bar, I Am Inset: Schematic of a kinked p-n nanowire ambipolar FET. However, when NA ND, the p-n junction will ith 120%tip angle. The blue and red lines designate the p-doped and doped arms, respectively.(b) Current vs voltage (I-v)data function as a p-type FET, and similarly, when NA > Np, the p-n recorded from a representative kinked p-n nanowire device. Inset Inction will function as an n-type. In our design, the doping SEM image of the device structure. Scale bar, 2 um.(c)Schematic level of the p-arm is ten times lower than the n-arm, thus the band diagram(black curves)and band diagram change of kinked p-n device is predicted to behave as a p-type FET nowires under gate potential. The blue and red dashed lines Tip-modulated scanning gate microscopy(tmSGM)was designate band diagram under negative and positive gate potentials used to identify directly the gate response and length-scale of respectively. Eo Ev, and Ep mark the position of the conduction band the sensitive regions in kinked p-n nanowire devices. Briefly, valence band, and Fermi energy, respectively conductive atomic force microscopy(AFM)tip was us 9 local gate to modulate the conductance of the kinked p kinked nanowires have a 120 angle between the two arms, nanowire junction. The conductance change was phase-locked which is consistent with our previous results that the abrupt to the vibration of the tip to enhance the spatial resolution, evacuation/resumption of feeding gases during the growth n-type SiNWs introduces a 1200 kink in high-yield. In addition, imposed over the topological image of the device. Representa- analysis of images showed that a small fraction (<10%)of the tive data(Figure 2a)show several key features. First, only kinked nanowires exhibited a 60 angle( Supporting Informa- the region close to the kink where the p-n junction was tion Figure S1), indicating that the switching between p-an synthetically defined showed clear gate response. Second, the orientation )albeit at a much lower yield. Here we focus on the than the n-depletion region(inset traces, Figure 2a).This result is consistent with our theoretical estimate using the calculated To assess the overall electrical characteristics of the kinked dopant concentration of the arms and implies that the device p-n nanowires, contacts(Cr/Pd/Cr 1.5/120/60 nm) were behavior is similar to a p-type FET. Third, the length of the defined on both arms by electron-beam lithography(EBL) and p-depletion region, which defines the spatial resolution of the metallization(inset, Figure 1b). Typical current versus device, was estimated from the full width at half-maximum ltage(I-v) data(figure 1b) show clear rectification with no( whm) of the conductance line profiles along the nanowire axis measurable current in reverse bias and an onset for current flow (inset traces, Figure 2a) and found to be 210 nm. while the 1712 dxdoloro/0.1021/l300256 rINgno Lett.2012,12,1711-1716adapted from our previous report.5,25 Initially, boron-doped p-type SiNWs were grown for 15 min at a calibrated growth rate of 0.7 μm/min. The reactor was evacuated for ca. 15 s, and then growth was continued using phosphine dopant to create an n-type nanowire segment (forming the p−n junction) for 30 s, followed by a second cycle of reactor evacuation and continued growth using phosphine for ca. 15 min. Scanning electron microscopy (SEM) images of the SiNWs prepared in this way (Figure 1a) showed that the majority (>90%) of the kinked nanowires have a 120° angle between the two arms, which is consistent with our previous results that the abrupt evacuation/resumption of feeding gases during the growth of n-type SiNWs introduces a 120° kink in high-yield.5 In addition, analysis of images showed that a small fraction (<10%) of the kinked nanowires exhibited a 60° angle (Supporting Informa￾tion Figure S1), indicating that the switching between p- and n-dopant could also introduce a similar kink (with both in cis orientation2 ) albeit at a much lower yield. Here we focus on the 120° kinked SiNWs. To assess the overall electrical characteristics of the kinked p−n nanowires, contacts (Cr/Pd/Cr 1.5/120/60 nm) were defined on both arms by electron-beam lithography (EBL) and metallization2,26 (inset, Figure 1b). Typical current versus voltage (I−V) data (Figure 1b) show clear rectification with no measurable current in reverse bias and an onset for current flow of 0.6 V in forward bias. These results are consistent with previously studies of straight SiNWs with axial p−n junctions.4 In addition, measurements made on devices with two contacts per arm (Supporting Information Figure S2) showed that no metal/SiNW Schottky barriers were present and allowed the estimation of the dopant concentrations. Specifically, the dopant concentrations of the p-arm and n-arm were estimated to be ca. 9 × 1018 and 9 × 1019 cm−3 , respectively.26 In a planar p−n diode device, the p−n junction is mostly buried beneath the surface and thus can only be partially gated with a top gate electrode.28 In contrast, the axial design of our kinked nanowires fully exposes the nanoscale p−n junction to external potential and enables a much more effective gate modulation of the transport behavior. Figure 1c illustrates a schematic band diagram change of the nanowire diode when a gate potential is applied at the p−n junction. The heavily doped p- and n- arms are not affected by the gate and the Fermi energy is pinned along the nanowire. When a negative potential is applied, the electron energy levels in both the conduction band and the valence band are raised (blue dashed lines in Figure 1c). As a result, the p-depletion region becomes more conductive, while the n-depletion region less conductive. In the case of applying a positive potential, the opposite occurs. In order to estimate the overall gate response of our device, we assume that (1) the carrier concentration distribution in the depletion region is linear, (2) the depletion region can be approximated as a number of small segments, each of which can be treated as a field-effect transistor with uniform doping, (3) the gate coupling is ideal, and (4) the width of the depletion region is the same as the abrupt junction. In addition, the mobility along the nanowire is treated as uniform, to simplify the calculation without affecting the physics. It follows that the resistance change (ΔR) of the p−n junction can be expressed as ΔR ∝ ΔV[ln(NA)/NA 2 − ln(ND)/ND 2 ] (see Supporting Information), where ΔV is the change of the gate potential, and NA and ND are the dopant concentration of the p- and n-arms, respectively. When NA and ND are equal or comparable, the p−n junction will behave as an ambipolar FET. However, when NA < ND, the p−n junction will function as a p-type FET, and similarly, when NA > ND, the p−n junction will function as an n-type. In our design, the doping level of the p-arm is ten times lower than the n-arm, thus the device is predicted to behave as a p-type FET. Tip-modulated scanning gate microscopy (tmSGM)29 was used to identify directly the gate response and length-scale of the sensitive regions in kinked p−n nanowire devices. Briefly, a conductive atomic force microscopy (AFM) tip was used as a local gate to modulate the conductance of the kinked p−n nanowire junction.30 The conductance change was phase-locked to the vibration of the tip to enhance the spatial resolution,29 and the conductance map at different tip biases was super￾imposed over the topological image of the device. Representa￾tive data (Figure 2a) show several key features. First, only the region close to the kink where the p−n junction was synthetically defined showed clear gate response. Second, the p-depletion region gave 3−5 fold larger conductance change than the n-depletion region (inset traces, Figure 2a). This result is consistent with our theoretical estimate using the calculated dopant concentration of the arms and implies that the device behavior is similar to a p-type FET. Third, the length of the p-depletion region, which defines the spatial resolution of the device, was estimated from the full width at half-maximum (fwhm) of the conductance line profiles along the nanowire axis (inset traces, Figure 2a) and found to be 210 nm.30 While the Figure 1. Design and controlled synthesis of kinked p−n nanowires. (a) Representative SEM image of a kinked p−n SiNW with 120° tip angle. Scale bar, 1 μm. Inset: Schematic of a kinked p−n nanowire with 120° tip angle. The blue and red lines designate the p-doped and n-doped arms, respectively. (b) Current vs voltage (I−V) data recorded from a representative kinked p−n nanowire device. Inset: SEM image of the device structure. Scale bar, 2 μm. (c) Schematic band diagram (black curves) and band diagram change of kinked p−n nanowires under gate potential. The blue and red dashed lines designate band diagram under negative and positive gate potentials respectively. EC, EV, and EF mark the position of the conduction band, valence band, and Fermi energy, respectively. Nano Letters Letter 1712 dx.doi.org/10.1021/nl300256r | Nano Lett. 2012, 12, 1711−1716