LETTERS NATURE Vol 449 18 October 2007 diameters of -300 nm(280-360 nm for other nanowires), which is in nanowires were etched selectively using potassium hydroxide agreement with independent transmission electron microscopy (KOH) solution(see Methods)to expose the p-core in a lithograph- (TEM) and atomic force microscopy measurements. In addition, ically defined region, and then metal contacts were made to the high-resolution TEM images(Fig. lc) confirm that the nanowire p-core and n-shell after a second lithographic patterning step, as shell is indeed polycrystalline. We note that this nanocrystalline shown in the SEM images of Fig. 2b. Dark current-voltage(H shell structure could enhance light absorption in the nanowires curves obtained from devices fabricated in this way(Fig. 2c)exhibit several notable features. first, the linear ev curves from core-core To characterize electrical rt through pl-P2) and shell-shell (n1-n2)configurations indicate that ohmic silicon nanowires, we fabricat netal contacts selectively contacts are made to both core and shell portions of the nanowires. inner p-core and outer n-she a). Briefly, core/shell silicon Second, the FV curve for the shell-shell contact reveals a shell 1-n2 p p-i-n Figure 2 Device fabrication and diode characterization. a, Schematics of of p-i-n and p-n diodes. The ideality factor N can be extrapolated(dashed ond to the lines )from the diode linear diode, 0. 12-0.50 V; P-n diode, p-core, i-shell, n-shell and PECVD-coated SiO2, respectively. Middle, 0. 10-0.60V),which are 1.96. 52forp-j-n andp-n diodes, respectively. To selective etching to expose the p-core. Right, metal contacts deposited on the keep the total diameters of the p-n and p-i-n silicon nanowires approximately p-core and n-shell. b, SEM images corresponding to schematics in a. Scale the same, the p-n silicon nanowire was grown with a 100-nm bars are 100 nm(left), 200 nm(middle)and 1.5 um(right). c, Dark I-v catalyst and an n-shell growth time of 100 min The SiH, /dopants feeding rves of a p-i-n device with contacts on core-core, shell-shell and different ratios for p-n and p-i-n nanowires are the same(silicon:boron, 500: 1; core-shell combinations. Vbias, the applied bias voltage. Inset, optical silicon:phosphorus, 200: 1 ). e, Temperature-dependent I-V measurement of microscope image of the device Scale bar, 5 um. d, Semi-log scale I-V curves the p-n and p-i-n diodes in the reverse bias voltage regime E2007 Nature Publishing Groupdiameters of ,300 nm (280–360 nm for other nanowires), which is in agreement with independent transmission electron microscopy (TEM) and atomic force microscopy measurements. In addition, high-resolution TEM images (Fig. 1c) confirm that the nanowire shell is indeed polycrystalline. We note that this nanocrystalline shell structure could enhance light absorption in the nanowires (see below). To characterize electrical transport through the p-i-n coaxial silicon nanowires, we fabricated metal contacts selectively to the inner p-core and outer n-shell (Fig. 2a). Briefly, core/shell silicon nanowires were etched selectively using potassium hydroxide (KOH) solution (see Methods) to expose the p-core in a lithograph￾ically defined region, and then metal contacts were made to the p-core and n-shell after a second lithographic patterning step, as shown in the SEM images of Fig. 2b. Dark current–voltage (I–V) curves obtained from devices fabricated in this way (Fig. 2c) exhibit several notable features. First, the linear I–V curves from core–core (p1-p2) and shell–shell (n1-n2) configurations indicate that ohmic contacts are made to both core and shell portions of the nanowires. Second, the I–V curve for the shell–shell contact reveals a shell a b e –10 –8 –6 –4 –2 0 –30 –20 –10 0 120 K 180 K 240 K 300 K c –2 –1 0 1 2 –8 –4 0 4 8 n2-p1 n2-p2 I (µA) Vbias (V) Vbias (V) Vbias (V) n1-n2 p1-p2 n1-p1 n1-p2 p1 n1 n2 p2 d 0.0 0.2 0.4 0.6 –25 –20 –15 ln(I) p-n p-i-n p-i-n p-n I (µA) Figure 2 | Device fabrication and diode characterization. a, Schematics of device fabrication. Left, pink, yellow, cyan and green layers correspond to the p-core, i-shell, n-shell and PECVD-coated SiO2, respectively. Middle, selective etching to expose the p-core. Right, metal contacts deposited on the p-core and n-shell. b, SEM images corresponding to schematics in a. Scale bars are 100 nm (left), 200 nm (middle) and 1.5 mm (right). c, Dark I–V curves of a p-i-n device with contacts on core–core, shell–shell and different core–shell combinations. Vbias, the applied bias voltage. Inset, optical microscope image of the device. Scale bar, 5 mm. d, Semi-log scale I–V curves of p-i-n and p-n diodes. The ideality factor N can be extrapolated (dashed lines) from the diode linear regimes (p-i-n diode, 0.12–0.50 V; p-n diode, 0.10–0.60 V),which are 1.96 and 4.52 for p-i-n and p-n diodes, respectively. To keep the total diameters of the p-n and p-i-n silicon nanowires approximately the same, the p-n silicon nanowire was grown with a 100-nm diameter gold catalyst and an n-shell growth time of 100 min. The SiH4/dopants feeding ratios for p-n and p-i-n nanowires are the same (silicon:boron, 500:1; silicon:phosphorus, 200:1). e, Temperature-dependent I–V measurement of the p-n and p-i-n diodes in the reverse bias voltage regime. LETTERS NATURE|Vol 449| 18 October 2007 886 ©2007 NaturePublishingGroup