NATURE Vol 449 18 October 20 LETTERS inductance of 132 uS, higher than that of the core(3 uS); the cal- contributions from tunnelling and avalanche mechanisms". Overall, culated shell resistivity is within a factor of two of that measured these results indicate that tunnelling or leakage currents are more for single-crystal n-type silicon nanowire prepared with a similar significant in the p-n diode", and that the diode quality factor and SiH4: PH, ratio. The highly conductive n-shell will reduce or elim- breakdown behaviour are readily controlled during nanowire growth curves recorded from different core-shell contact geometries show die introduction of the i-layer as in planar structures. 9 les were characterized under air mass 1.5 global (AM 1.5G)illu fying behaviour, and demonstrate that the p-i-n coaxial silicon mination. FV data recorded from one of the better devices(Fig 3 nanowires behave as well-defined diodes. The reproducibility of the yields an open-circuit voltage Vo of 0. 260 V, a short-circuit current further demonstrated by defining more complex 'ANDand OR' output Pmax for the silicon nanowire device at 1-sun(see Methods)is diode logic gates using single p-i-n coaxial silicon nanowires 72 pW. Notably, these values were constant for measurements made over a seven-month period, thus demonstrating excellent The core/shell silicon nanowire diodes were further characterized stability of our nanowire photovoltaic elements. In addition, F-V by analysing data recorded with and without the i-layer asa function of data recorded using contacts to the n-shell that were 5.9 um(n1) temperature Fits to In(D-V data recorded in forward bias from p-i-n and 13.3 um(n2) from the p-core contact( Fig. 3b)exhibited essen- and p-n coaxial structures(Fig 2d)are linear, and yield diode ideality tially the same photovoltaic response, thus indicating that the n-shell factors Nof 1.96 and 4.52, respectively(see Methods). The N-values is equipotential with radial carrier separation occurring uniformly show that introduction of the i-layer yields much better quality diodes. along the entire length of the core/shell silicon nanowire device. Reverse bias measurements from p-i-n and p-n diodes(Fig. 2e)also Measurements of Isc as a function of the p-i-n coaxial silicon nano- show markedly different behaviour: the p-i-n diode breaks down at wire(Fig. 3c) length show linear scaling with values of 1 nA silicon much larger reverse-bias voltage(approximately -7V)than the p-n nanowire readily achieved for lengths of 10 um(1-sun), whereas diode(approximately -1 V) for all temperatures studied. In addition, Voc is essentially independent of length. The linear scaling of Isc with the reverse-bias breakdown voltage of the p-n diode increases with silicon nanowire length suggests that photogenerated carriers ar decreasing temperature, which is consistent with a Zener(tunnelling) collected uniformly along the length of these radial nanostructures, breakdown mechanism, whereas the breakdown voltage of the and that scattering of light by the metal contacts does not make a p-i-n structures exhibits little temperature dependence, suggesting major contribution to the observed photocurrent. Figure 3 Characterization of the p-i-t nowire photovoltaic device. a, Dark -V curves. b, Light I-V curves for two different n-shell contact locations. Inset, optical 0.0 microscopy image of the device. Scale bar, 5 um. Device-length-dependent Isc and Jsc(upper Inset, light-intensity-dependent Isc and Voc plots. e, Temperature-dependent measurement. The levice was illuminated at 0. 6-sun to reduce 0.1000.1 ple heating, which may cause temperature blue circle correspond to Frll, Voc and Isct evice was characterized a,d and e. The p-i-n coaxial silicon nanowire using conditions as in Fig. 2. Intensity (mw cm V 0 s0:2。000。00000a 250 E2007 Nature Publishing Groupconductance of 132 mS, higher than that of the core (3 mS); the cal￾culated shell resistivity is within a factor of two of that measured for single-crystal n-type silicon nanowire prepared with a similar SiH4:PH3 ratio15. The highly conductive n-shell will reduce or elim￾inate potential drop along the shell, thereby enabling uniform radial carrier separation and collection when illuminated8 . Third, I–V curves recorded from different core–shell contact geometries show rectifying behaviour, and demonstrate that the p-i-n coaxial silicon nanowires behave as well-defined diodes. The reproducibility of the selective etching and contact formation to p-cores and n-shells was further demonstrated by defining more complex ‘AND’ and ‘OR’ diode logic gates using single p-i-n coaxial silicon nanowires (Supplementary Fig. 1). The core/shell silicon nanowire diodes were further characterized by analysing data recorded with and without the i-layer as afunction of temperature. Fits to ln(I)–V data recorded in forward bias from p-i-n and p-n coaxial structures (Fig. 2d) are linear, and yield diode ideality factors N of 1.96 and 4.52, respectively (see Methods). The N-values show that introduction of the i-layer yields much better quality diodes. Reverse bias measurements from p-i-n and p-n diodes (Fig. 2e) also show markedly different behaviour: the p-i-n diode breaks down at much larger reverse-bias voltage (approximately 27 V) than the p-n diode (approximately 21 V) for all temperatures studied. In addition, the reverse-bias breakdown voltage of the p-n diode increases with decreasing temperature, which is consistent with a Zener (tunnelling) breakdown mechanism, whereas the breakdown voltage of the p-i-n structures exhibits little temperature dependence, suggesting contributions from tunnelling and avalanche mechanisms16. Overall, these results indicate that tunnelling or leakage currents are more significant in the p-n diode17, and that the diode quality factor and breakdown behaviour are readily controlled during nanowire growth by the introduction of the i-layer as in planar structures18,19. The photovoltaic properties of the p-i-n coaxial silicon nanowire diodes were characterized under air mass 1.5 global (AM 1.5G) illu￾mination. I–V data recorded from one of the better devices (Fig. 3a) yields an open-circuit voltage Voc of 0.260 V, a short-circuit current Isc of 0.503 nA and a fill factor Ffill of 55.0%. The maximum power output Pmax for the silicon nanowire device at 1-sun (see Methods) is ,72 pW. Notably, these values were constant for measurements made over a seven-month period, thus demonstrating excellent stability of our nanowire photovoltaic elements. In addition, I–V data recorded using contacts to the n-shell that were 5.9 mm (n1) and 13.3 mm (n2) from the p-core contact (Fig. 3b) exhibited essen￾tially the same photovoltaic response, thus indicating that the n-shell is equipotential with radial carrier separation occurring uniformly along the entire length of the core/shell silicon nanowire device. Measurements of Isc as a function of the p-i-n coaxial silicon nano￾wire (Fig. 3c) length show linear scaling with values of 1 nA silicon nanowire–1 readily achieved for lengths of 10 mm (1-sun), whereas Voc is essentially independent of length. The linear scaling of Isc with silicon nanowire length suggests that photogenerated carriers are collected uniformly along the length of these radial nanostructures, and that scattering of light by the metal contacts does not make a major contribution to the observed photocurrent. a b –0.1 0.0 0.1 0.2 –0.6 –0.3 0.0 0.3 p-n1 p-n2 I (nA) p n1 n2 100 150 200 250 300 0.0 0.2 0.4 0.6 0.8 I sc Ffill Ffill and Voc (V) and Isc (nA) Temperature (K) e c –0.1 0.0 0.1 0.2 0.3 –0.3 0.0 0.3 I (nA) Vbias (V) Vbias (V) Dark Light d 0 400 800 0.2 0.3 Intensity (mW cm–2) Voc (V) 0 2 4 Isc (nA) 0.1 0.2 0.3 –24 –21 –18 –15 ln (Isc) 5 10 15 20 25 0.0 0.5 1.0 1.5 2.0 2.5 Device length (µm) Isc (nA) 10 20 30 40 Apparent Jsc (mA cm–2) Voc (V) Voc Figure 3 | Characterization of the p-i-n silicon nanowire photovoltaic device. a, Dark and light I–V curves. b, Light I–V curves for two different n-shell contact locations. Inset, optical microscopy image of the device. Scale bar, 5 mm. c, Device-length-dependent Isc and Jsc (upper bound) plots. d, Plot of ln(Isc) versus Voc; each point corresponds to a different light intensity. Inset, light-intensity-dependent Isc and Voc plots. e, Temperature-dependent measurement. The device was illuminated at 0.6-sun to reduce sample heating, which may cause temperature fluctuations. The red triangle, black square and blue circle correspond to Ffill, Voc and Isc, respectively. The same device was characterized in a, d and e. The p-i-n coaxial silicon nanowire was grown using conditions as in Fig. 2. NATURE| Vol 449|18 October 2007 LETTERS 887 ©2007 NaturePublishingGroup