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Jun 5, 2009 - Electrical and Photoresponse Properties of an Intramolecular p-n Homojunction in Single Phosphorus-Doped ZnO Nanowires. Ping-Jian Li ...
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VOLUME 9, NUMBER 7, JULY 2009  Copyright 2009 by the American Chemical Society

Letters

Electrical and Photoresponse Properties of an Intramolecular p-n Homojunction in Single Phosphorus-Doped ZnO Nanowires Ping-Jian Li,† Zhi-Min Liao,† Xin-Zheng Zhang,† Xue-Jin Zhang,† Hui-Chao Zhu,† Jing-Yun Gao,† K. Laurent,‡ Y. Leprince-Wang,‡ N. Wang,§ and Da-Peng Yu*,† State Key Laboratory for Mesoscopic Physics, and Electron Microscopy Laboratory, School of Physics, Peking UniVersity, Beijing 100871, Peoples’s Republic of China, Laboratoire de Physique des Materiaux DiVises et Interfaces (LPMDI), CNRS-UMR 8108, UniVersite Paris-Est, 77454 Marne la Vallee Cedex 2, France, and Physics Department, Hong Kong UniVersity of Science and Technology, Hong Kong Received November 14, 2008; Revised Manuscript Received May 21, 2009

ABSTRACT The single-crystal n-type and p-type ZnO nanowires (NWs) were synthesized via a chemical vapor deposition method, where phosphorus pentoxide was used as the dopant source. The electrical and photoluminescence studies reveal that phosphorus-doped ZnO NWs (ZnO:P NWs) can be changed from n-type to p-type with increasing P concentration. Furthermore, we report for the first time the formation of an intramolecular p-n homojunction in a single ZnO:P NW. The p-n junction diode has a high on/off current ratio of 2.5 × 103 and a low forward turn-on voltage of ∼1.37 V. Finally, the photoresponse properties of the diode were investigated under UV (325 nm) excitation in air at room temperature. The high photocurrent/dark current ratio (3.2 × 104) reveals that the diode has a potential as extreme sensitive UV photodetectors.

Zinc oxide (ZnO) has been studied as a short-wavelength light-emitting material due to a wide direct band gap of 3.37 * Author to whom correspondence should be addressed. E-mail: [email protected]. † Peking University. ‡ CNRS-UMR 8108, Universite Paris-Est. § Hong Kong University of Science and Technology. 10.1021/nl803443x CCC: $40.75 Published on Web 06/05/2009

 2009 American Chemical Society

eV at room temperature and a large exciton binding energy of 60 meV.1 In recent years, ZnO nanowires (NWs) have attracted a considerable amount of research interest due to the potential applications for nanoscale optoelectronics such as field-effect transistors (FETs),2 ultraviolet (UV) lasers,3 light-emitting diodes,4 and UV photodetectors.5,6 In order to develop nanodevices based on ZnO, complementary doping

(both n-type and p-type doping) is necessary. Nominally undoped ZnO is intrinsically n-type due to the formation of donors such as O vacancies and Zn interstitials. On the other hand, it is known that the fabrication of low resistivity p-type ZnO is difficult due to the self-compensation and low solubility of dopants.7 Although p-type doping in ZnO film or ZnO NWs has been reported,8-12 it still remains controversial. The lack of p-type doping ZnO NWs hinders the progress of functional nanodevices based on ZnO. To the best of our knowledge, the p-n homojunction formed in a single ZnO NW has not been reported thus far. In this letter, the n-type and p-type phosphorus-doped ZnO (ZnO:P) NWs were simultaneously synthesized by a simple chemical vapor deposition (CVD) method using phosphorus pentoxide (P2O5) as the dopant source. The electrical properties and photoluminescence (PL) spectra of single ZnO:P NWs were examined at room temperature. Furthermore, we report for the first time the synthesis of an intramolecular p-n homojunction in a single ZnO NW. The electrical and photoresponse measurements of the p-n junction were performed at room temperature. The ZnO:P NWs were grown through a CVD method in a simple tube furnace. The source materials consisted of ZnO powder, graphite powder with a molar ratio of 1:1 ZnO/C, and P2O5 powder. They were put at the center of tube furnace. A single crystalline (111) Si wafer was set at a fixed distance (8 cm) downstream the source materials as a collecting substrate. A mixture of argon and oxygen (3 sccm) with a total flow rate of 200 sccm was used as a carrier gas. The growth was performed at 1050 °C for 90 min. The growth procedure for undoped ZnO NWs was similar to that of ZnO:P NWs, but a mixture of ZnO and graphite powder only was used as the source. The as-synthesized ZnO:P NWs were characterized using scanning electron microscope (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD) technique. Supporting Information, Figure S1a shows a typical field emission SEM image of as-synthesized ZnO:P NWs on a Si substrate. The nanowires were 100-300 nm in diameter and 50-200 µm in length. Supporting Information, Figure S1b presents a high-resolution TEM image of a ZnO:P NW with SAED pattern. This TEM image confirms that ZnO:P NWs are single crystalline and have a distinct growth direction, which is different from [001] direction of undoped ZnO NWs.13 Similar microstructures and diffraction patterns have always been observed for different ZnO:P NWs. The EDS analysis in the TEM mode (Supporting Information, Figure S1c) shows that there is no P element detected within the detection resolution. To confirm, electron energy loss spectroscopy was also conducted on single ZnO:P NWs, and no signal corresponding to P element was detected. This because the P concentration is very low in the ZnO:P NW, which is beyond the resolution of the detector. The result was consistent with the previous reports for p-type single ZnO nanowires.11,12 The XRD spectrum (Supporting Information, Figure S1d) confirms that the phase of nanowires is ZnO. No ZnP or any other phases are observed. In additional, after 2514

Figure 1. (a) The schematic structure and SEM image of a backgated ZnO:P NW FET. (b) The Ids-Vds plots of the p-type ZnO:P NW FET (Device 1) at different Vg. The radius and length of the nanowire are 95 nm and 6.1 µm, respectively. (c) Ids-Vg plots of Device 1 at Vds ) -1 V. (d) Ids-Vg plots of the n-type ZnO:P NW FET (Device 2) at Vds ) 1 V. The radius and length of the nanowire are 105 nm and 5.3 µm, respectively.

the single ZnO:P NWs were electrically measured, the PL spectrum of every single nanowire was obtained. The PL results are consistent with that for ZnO. So we can confirm the synthesized nanowires are pure ZnO NWs instead of other phases. To confirm the conductivity type of as-synthesized ZnO:P NWs, we studied the electrical properties in single nanowire FETs. ZnO:P NWs were first dispersed in alcohol and then deposited onto a p++ silicon chip covered with 300 nm thick stoichiometric silicon nitride (Si3N4). The p++ silicon layer acted as the back-gate in the FET configuration. Electron beam lithography technique was utilized to define source and drain contacts on the two ends of a single nanowire. The source/drain contacts were formed via magnetron sputtering of Ti (20 nm) and Au (80 nm), followed by lift-off pattern transfer. The channel length is ∼5 µm between the source and drain electrodes. The schematic structure and SEM image of a back-gated ZnO:P NW FET are shown in Figure 1a. The electrical measurements were performed at room temperature using a Keithley 4200 semiconductor characterization system. After investigating the transport properties of 50 single ZnO:P NW FETs, we found 32% of the samples showed p-type conductivity and 68% of the samples showed n-type conductivity. Figure 1b presents the gate-dependent drain-source current (Ids) versus voltage (Vds) curves of a representative single p-type ZnO:P NW FET (Device 1). A quasi linear Ids versus Vds curve under Vg ) 0 V indicate that the wire-electrode contact is ohmic. With negatively increasing gate voltage (Vg: 5 V f -5 V), the conductance of the NW increases, revealing a p-type conductivity of the ZnO:P NW in Device 1.14 To determine the efficiency of the gating behavior, the transfer characteristics (Ids-Vg) were obtained with a fixed drain-source voltage of -1 V, as shown in Figure 1c. The Nano Lett., Vol. 9, No. 7, 2009

Figure 2. The room-temperature PL spectra of (a) a single undoped ZnO NW, (b) a single p-type ZnO:P NW (Device 1), (c) a single n-type ZnO:P NW (Device 2), (d) the PL spectrum of a single p-type ZnO:P NW (Device 1) measured at 10 K.

threshold voltage (Vth) is ∼ 1.0 V for this device, as obtained from a linear extrapolation of the Ids-Vg curve in Figure 1c. The Vth varies from -0.5 to 2 V for 16 measured samples. The hole concentration of the p-type ZnO:P NW can be estimated as p ) CVth/πeLr2,15,16 where e is the electron charge and L is the nanowire channel length (6.1 µm). C is the nanowire capacitance, given by C ) 2πεε0L/ln(2h/r) with ε being the relative dielectric constant of the gate Si3N4 dielectric (7.5), h the thickness of the Si3N4 layer (300 nm), and r the nanowire radius (95 nm). The carrier concentration p is thus estimated to be ∼5.0 × 1016 cm-3. In addition, the carrier mobility of the nanowire can be calculated from µ ) gmL/(2πεε0Vds/ln(2h/r)),15,16 where gm ) dIds/dVg was obtained from the Ids-Vg plot (Figure 1c) to be 1.5 × 10-7A/V at Vds ) -1 V. So we can get the hole mobility of ∼40.4 cm2/V·s. The values of hole concentration and mobility are comparable to the reported results for p-type ZnO films with phosphorus dopants.17,18 Figure 1d presents the room temperature transfer characteristics of a representative single n-type ZnO:P NW FET (Device 2). The Ids versus Vg curve clearly reveals n-type conductivity of the ZnO:P NW in Device 2. From Figure 1d, we can get the threshold voltage of -0.5 V for this device (The Vth varies from -0.3 to -0.8 V for 34 measured samples). The estimated electron concentration and mobility of Device 2 are 2.2 × 1016 cm-3 and 22.2 cm2/V·s, respectively. The electrical studies reveal that p-type and n-type ZnO:P NWs were simultaneously obtained in our synthesized nanowires. In order to further investigate how the P affects the conductivity type of ZnO nanowire, PL measurements were performed at room temperature using a He-Cd laser (325 nm) as the excitation source. The laser beam was focused by a microscope objective normal to the substrate surface down to a spot size of around 2 µm in diameter. The PL spectra were detected using a monochromator with 1800 grooves/mm grating (Renishaw micro-PL System) and a CCD camera. Figure 2a-c presents the PL spectra of a single undoped ZnO NW (n-type), p-type ZnO NW (Device 1), n-type ZnO NW (Device 2), respectively. The room-temperature PL spectrum of a undoped ZnO NW (Figure 2a) shows a very weak near-band-edge (NBE) emission at 3.26 eV in the UV, but a strong and broad visible emission at 2.36 eV, which is attributed to oxygen vacancies (Vo).17 It is well known that Vo are native donor defects which contribute to the n-type conductivity of undoped ZnO NWs.18 Compared with the Nano Lett., Vol. 9, No. 7, 2009

PL spectrum of a single undoped ZnO NW, the single p-type ZnO:P NW of Device 1 has only a very strong NBE emission at 3.23 eV with fwhm ∼80 meV at room temperature, as shown in Figure 2b. It is noted that no green emission associated to Vo was observed in the single p-type ZnO:P NW. This means P-related acceptors compensate donor defects (Vo), and hence the Device 1 shows p-type conductivity and has a hole concentration of 5.0 × 1016 cm-3. Figure 2c shows the room-temperature PL spectrum of the single n-type ZnO:P NW of Device 2. Compared with the PL spectrum of a single undoped ZnO NW, the NBE emission is enhanced and the green emission associated to Vo is weakened for Device 2 (there is a shoulder (∼3.18 eV) in the NBE peak in Figure 2c, which may correspond to the FX-2LO (second longitudinal optical phonon replica of the free exciton) peak which was reported in the room-temperature PL spectrum of ZnO NWs by Voss groups19). Please note that the green emission associated to Vo is weak but visible at 2.36 eV. This means P-related acceptors can only compensate a part of donor defects (Vo). Hence the Device 2 shows n-type conductivity and has a electron concentration of 2.2 × 1016 cm-3, which is significantly lower than the previous reported values for single undoped ZnO NWs (1017-1018 cm-3).20,21 In order to get more information on the energy level and doping mechanism of phosphorus acceptor in ZnO nanowires, we performed low temperature PL measurements. Figure 2d shows the PL spectrum of the single p-type ZnO:P NW (Device 1) at 10 K. It has five peaks at 3.2355, 3.3102, 3.3564, 3.3603, and 3.3741 eV, which can be due to donorto-acceptor pair transition (DAP), free electrons to the acceptor transition (FA), a neutral acceptor bound exciton (A0X), a neutral donor bound exciton (D0X) and a free exciton (FX), respectively.11,12,22,23 The A0X is strongest and has a fwhm of 1.4 meV. The acceptor energy of the phosphorus dopant is estimated from the FA transition at 3.3102 eV PL spectrum of the single p-type ZnO:P NW. The FA energy is given by EFA ) Eg - EA + kT/2,23 where Eg and EA are the band gap and acceptor energies, respectively, and k is Boltzmann’s constant. Since the thermal energy term can be neglected at 10 K, EA is estimated to be 126.9 meV, using EFA ) 3.3102 eV and Eg ) 3.4371 eV.22,23 The shallow acceptor energy level of 126.9 meV shows the phosphorus can act as a desirable acceptor in ZnO nanowires. The above electrical and PL studies reveals that n-type and p-type ZnO:P NWs were simultaneously obtained in our products. On the basis of the recent report that P-late acceptors increase with the P concentration in ZnO:P films,24 we conclude that the P concentration controlled the conductivity type of ZnO:P NWs in our samples. When the concentration of the P-related acceptors is in excess of the concentration of donor defects, ZnO:P NWs exhibit p-type characteristics (Device 1). When the P concentration in ZnO is low, the concentration of P-related acceptors is not high enough to compensate the donor defects, and hence Device 2 shows n-type conductivity. The result means our synthesized materials may have different P concentrations in single ZnO:P NWs. We propose the high growth temperature (1050 2515

conductivity type as similar to single ZnO nanowires. It is more significant that the p-n homojunctions were successfully obtained in these T-junction nanostructures.

Figure 3. (a) Ids-Vg plots of the branch nanowire at Vds ) -1 V. The inset is the SEM image of a T-junction structure with two contact electrodes on the branch (Device 3). (b) The SEM image of a T-junction structure of ZnO:P NWs with four contact electrodes (Device 4). (c) The IDE-Vg plots of the D-E terminal at VDE ) -1 V. The inset is the IDE-VDE plots of the D-E terminal. (d) The IFG-Vg plots of the F-G terminal at VFG ) 1 V. The inset is the IFG-VFG plots of the F-G terminal. (e) The IDF-VDF plots of the p-n homojunction in DF section. The inset is the model of the DF section when the p-n junction and the Schottky junction were formed in the DF section and F region, respectively. (f) The semilog plots of IDF-VDF curves in absolute magnitude of the current of panel e.

°C) and long reaction time (90 min) are major factors that cause the gradual decrease of P concentrations in single ZnO:P NWs with the P source vaporization during the growth process. Furthermore, if the P concentration is gradually decreased along the length of a single ZnO:P NW, we will have an opportunity to obtain the intramolecular p-n homojunction. Because the exact position of the p-n junction was difficult to be confirmed, we have focused on the nanowires with length over 20 µm. In the synthesized materials, we observed the ZnO:P NWs tended to form T-junction nanostructures when their length is more than 20 µm. The representative high-resolution TEM images of the T-junction nanostructure were shown in Supporting Information, Figure S2. In these T-junction nanostructures, some showed p-type conductivity as similar to single ZnO:P NWs. Figure 3a presents the transfer characteristics of the branch on a T-junction structure (Device 3). With negatively increasing gate voltage (Vg: 5 V f -5 V), the conductance of the branch increases, revealing that the branch nanowire is p-type semiconductor material. The result means the P was also doped in the T-junction nanostructures and changed their 2516

Figure 3b presents the SEM image of a T-junction structure of ZnO:P NWs with four contact electrodes (Device 4). As shown in Figure 3b, the four electrodes and the junction point are marked with D, E, F, G, and H, respectively. Figure 3c shows the transfer characteristics (IDE-Vg) of D-E terminal at bias of -1 V (VDE). With negatively increasing gate voltage (Vg: 5 V f -5 V), the conductance of the device increases, revealing that the DH and HE section of Device 4 are p-type semiconductor materials. The inset in Figure 3c shows the IDE versus IDE curve of D-E terminal at room temperature. It is clear that the curve is almost linear, indicating that ohmic contacts were formed between the nanowire and the electrodes (both D and E terminal). Figure 3d shows the transfer characteristics (IFG-Vg) of F-G terminal at bias of 1 V (VFG). With increasing gate voltage (Vg: -5 V f 5 V), the conductance of the device increases, revealing that the FG section is n-type. The inset in Figure 3d shows the IFG versus VFG curve of F-G terminal at room temperature. The current saturation in the positive bias does not reverse when switching the F-terminal and G-terminal electrode, or sweeping VFG from positive to negative. This phenomenon has also been observed in other n-doped FETs,20,21 which was due to the pinch-off effect.14 It is noted that the nonlinear behavior shown in the inset in Figure 3d reveals that Schottky barriers were formed between the nanowire and electrodes. In order to characterize the contact barrier, the IFG -VFG curves were obtained under a vacuum at different temperature, as shown in the Supporting Information, Figure S3a. The conductance of the F-G terminal was observed to increase monotonically as the temperature increased. The semilogarithmic plot of the conductance versus reciprocal temperature (1000/T) (Supporting Information, Figure S3b) agrees well with the thermionic emission model, in which the conductance ∝ exp(- φb/kT). The effective barrier height was extracted from the slope as φb ) 86 meV. The small value means that the Schottky junction cannot originate the rectifying behavior at room temperature. According to the above discussions, we can confirm that the DH section is p-type, FG section is n-type, the D region is ohmic contact, and the F region has a small Schottky barrier height of ∼86 meV, respectively. So the DF section can be considered as the series connection of a p-n junction and a Schottky junction (F terminal). Please be noted that the rectifying direction of the p-n junction is opposite to that of the Schottky junction, as shown in the inset in Figure 3e. Figure 3e shows the IDF-VDF characteristics of D-F terminal, which display an apparently diode rectifying behavior. It is important to note that the rectifying direction of the D-F terminal is consistent with that of the p-n junction, but opposite to that of the small Schottky junction formed at the F terminal. It means the rectifying behavior of the D-F terminal originated from the p-n junction, and the effect of the small Schottky junction (∼86 meV) can be neglected in this device at room temperature. So the transport mechanism of the D-F terminal can be explained using the p-n junction Nano Lett., Vol. 9, No. 7, 2009

model shown in the Supporting Information (Figure S4). At forward bias, the internal potential barrier in the p-n junction is lowered due to the external potential, thus both majority holes and majority electrons can flow through the p-n junction and contribute to the forward current; at reverse bias, the internal potential barrier is increased, thus neither majority holes nor majority electrons can flow through the p-n junction, and the reverse current is only provided by minority carriers. From Figure 3e, the forward turn-on voltage is ∼1.37 V. The value of turn-on voltage is smaller than the band energy of ZnO (3.37 eV), but this phenomenon is consistent with other reports for ZnO p-n homojunctions, such as ∼0.5 V for p-ZnO:N/n-ZnO,25 ∼1 V for p-ZnO:P/n-ZnO,26 ∼1.4 V for p-ZnO:(N, Al)/n-ZnO:Al,27 ∼1.5 V for n-ZnO/p-(Zn, Mg)O:P,28 ∼3 V for n-ZnO/p-ZnO:(N, In) homojunction.29 Although the low turn-on voltage remains under consideration, it seems to be acceptable as qualified by electroluminescence in oxide p-n junctions.25,26 Because of the low turnon voltage (1.37 V), our p-n homojunction diode is very favorable for long-time operation without serious ohmic heating at the contact. The Figure 3f shows the semilog plot of IDF-VDF curves of Figure 3e. At a 3 V reverse bias voltage, the reverse current was only 0.2 nA. At a 3 V forward bias voltage, the forward current was as high as 493 nA. The on/off current ratio is as high as 2.5 × 103. The measured forward current has two distinct regions. The current increased exponentially with the applied voltage for VDF < 0.2 V. The diode ideality factor can be determined by using the usual junction rectification model in this low bias region, I ) I0[exp (eV/ nkT) - 1],14 where I0 is the reverse saturation current, n is an ideality factor, k is Boltzmann’s constant, and T is the absolute temperature. For an ideal p-n junction diode, the n has a value between 1 (diffusion mechanism) and 2 (recombination mechanism). The ideality factor n for our device derived from above equation is 2.28. The high value (n > 2) may result from the contact resistance and is consistent with previous reports.30,31 For high bias voltage (VDF > 0.2 V), the current increase is proportional to the power of voltage, which is generally attributed to a spacecharge-limited current conduction for single-carrier injection behavior observed in other p-n junction diodes.32-34 According to the above electrical studies, we can conclude an intramolecular p-n homojunction was first formed in the single ZnO:P NW (Device 4, DF section). The p-n junction diode has good performance with a high on/off current ratio of 2.5 × 103 and a low forward turn-on voltage of 1.37 V. Moreover, we have also obtained 5 other p-n junction diodes. They exhibit similar rectifying behavior as Device 4, and the highest on/off current ratio can reach a value of 1.9 × 104 (Device 5). Figure 4a shows the semilog plot of IAC-VAC curves of Device 5 (The SEM image and other electrical data were shown in the Supporting Information, Figure S5; the electrical data of 4 other p-n junction were shown in the Supporting Information, Figure S6). At a 3 V reverse bias voltage, the reverse current was only 0.03 nA; at a 3 V forward bias voltage, the forward current was as high as 573 Nano Lett., Vol. 9, No. 7, 2009

Figure 4. (a) The semilog plots of IAC-VAC curves in absolute magnitude of the current of Device 5. (b) Time dependent photocurrent response of the p-n homojunction diode under UV (325 nm) excitation in air at bias of -3 V (VAC)

nA. In order to explore the applications of the p-n homojunction diodes, the photoresponse properties of the intramolecular p-n homojunction in Device 5 were investigated in air at room temperature. Figure 4b shows the photoresponse of the p-n homojunction diode under the UV (325 nm) excitation at reverse bias of 3 V. The dark current was about 0.04 nA (the value is higher than 0.03 nA shown in Figure 4a, which is due to the unshielded device in the photoresponse measurement), and the saturated photocurrent was about 1.3 µA under UV excitation. The photocurrent/dark current ratio can reach a surprising high value of 3.2 × 104, which is superior to other single NWs as UV photodetectors.31,35,36 It means that the photoexcited electron-hole pairs can greatly increase the concentration of minority carriers, which dominate the current through a reverse biased diode. Upon UV excitation, the conductance of the diode was increased three orders within 2 s and to 80% of its saturation value within 53 s. When the UV light is turned off, the conductance was decreased to 1.5% of its saturation value within 300 s and did not recover to the original dark level after 2500 s. The decay process follows an exponential decay function I ) I0e-t/τd with different time constants, τd1 ) 74 s and τd2 ) 1026 s. The slow decay process may be attributed to the carrier trap states in the ZnO:P NWs.37 After the UV light is turned off, the holes (majority carriers) in the p-type section of the p-n junction are captured by trap states, the electrons (minority carriers) remain in the conduction band and contribute to the reverse current of the p-n homojunction diode. The same is true for the n-type section of p-n junction.14,38 The trap states may be originated from P-related defects in ZnO:P NWs.17,39 Although further investigations should be done for improving the photoresponse of the p-n homojunction, the high photocurrent/dark current ratio (3.2 × 104) reveals that the p-n homojunction opens up significant opportunities for the fabrication of photonic and electronic nanodevices. In summary, we demonstrate the growth of single crystal ZnO:P NWs using P2O5 as the dopant source via a simple CVD progress. Electrical measurements reveal that n-type (68%) and p-type (32%) ZnO:P NWs were simultaneously obtained in our products. The p-type ZnO NWs have a hole concentration of ∼5.0 × 1016 cm-3, and a hole mobility of ∼40.4 cm2/V·s. The room-temperature PL spectra show that the p-type ZnO:P NWs have only a strong NBE emission, 2517

and the n-type ZnO:P NWs have a strong NBE emission and a weak green emission associated to Vo. The low temperature PL studies (10 K) reveal that the acceptor energy of the phosphorus dopant is estimated to be 126.9 meV and the phosphorus can act as a desirable acceptor in ZnO nanowires. According to the electrical and PL studies, we conclude that the donors were gradually compensated by P-relate acceptors by increasing the P concentration in ZnO:P NWs. Furthermore, the intramolecular p-n homojunction was formed for the first time in a single ZnO:P NW, revealing that the P concentration was gradually decreased along the length of the single ZnO:P NW in our samples. The p-n junction diode has excellent performance with a high on/off current ratio of 2.5 × 103 and a low forward turn-on voltage of 1.37 V at room temperature. The highest on/off current ratio in our samples can reach a value as high as 1.9 × 104. Finally, the p-n junction diode was used as a high sensitive UV photodetector. The photocurrent/dark current ratio is as high as 3.2 × 104. Considering their small size, we expect ZnO:P NWs and corresponding homojunctions will play an important role in nanoscale electronics and optoelectronics. Acknowledgment. This project was financially supported by the National Natural Science Foundation of China (Grants 90606023, 10574003, 20731160012, 10804002, 10804003), a national 973 Project Grant (No. 2007CB936202, 2009CB623703, MOST) from China’s Ministry of Science and Technology and Hong Kong’s NSFC/RGC Joint Research Scheme (Project No. N HKUST615/06). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Robert, F. Service. Science 1997, 276, 895. (2) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659. (3) Yang, P.; Yan, H.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R.; Choi, H. J. AdV. Funct. Mater. 2002, 12, 323. (4) Liu, C. H.; Zapien, J. A.; Yao, Y.; Meng, Z. M.; Lee, C. S.; Fan, S. S.; Lifshitz, Y.; Lee, S. T. AdV. Mater. (Weinheim, Ger.) 2003, 15, 838. (5) Kind, H.; Yan, H.; Messer, B.; Law, M.; Yang, P. AdV. Mater. (Weinheim, Ger.) 2002, 14, 158. (6) Keem, K.; Kim, H.; Kim, G. T.; Lee, J. S.; Min, B.; Cho, K.; Sung, M. Y.; Kim, S. Appl. Phys. Lett. 2004, 84, 4376. (7) Park, C. H.; Zhang, S. B.; Wei, S. H. Phys. ReV. B 2002, 66, 073202. (8) Bian, J, M.; Li, X. M.; Zhang, C. Y.; Yu, W. D.; Gao, X. D. Appl. Phys. Lett. 2004, 85, 4070. (9) Heo, Y. W.; Park, S. J.; Ip, K.; Pearton, S. J.; Norton, D. P. Appl. Phys. Lett. 2003, 83, 1128. (10) Bang, K.H.; Hwang, D. K.; Park, M. C.; Ko, Y. D.; Yun, I.; Myung, J. M. Appl. Surf. Sci. 2003, 210, 177.

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Nano Lett., Vol. 9, No. 7, 2009