J. Phys. Chem. C 2009, 113, 8945–8947
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Optical and Electrical Properties of Ga-Doped ZnO Nanowire Arrays on Conducting Substrates Minjie Zhou,† Haojun Zhu,† Yang Jiao,† Yangyan Rao,† Suikong Hark,† Yang Liu,‡ Lianmao Peng,‡ and Quan Li*,† Department of Physics, The Chinese UniVersity of Hong Kong, Hong Kong, China, and Department of Electronics, Peking UniVersity, Beijing, China ReceiVed: February 4, 2009; ReVised Manuscript ReceiVed: March 19, 2009
Large area Ga-doped ZnO nanowire arrays have been vertically grown on transparent conducting substrate by a simple chemical vapor deposition method. Experimental results reveal the well-aligned array morphology and the uniform Ga concentration in these nanowires with individual nanowires of single crystallinity. In particular, direct I-V measurements performed on a single nanowire on indium-tin oxide (ITO) substrate disclose both the low resistivity of nanowire and its Ohmic contact with the conducting substrate, which characteristics make them promising candidates for various optoelectronic and energy applications including photovoltaic cells. Introduction Compared to other morphologies of ZnO nanostructures, nanowire arrays vertically grown on conducting substrates can provide direct channels for effective charge carrier transporting, and thus they are proposed as promising nanostructured electrodes for a number of optoelectronic and energy applications.1-6 Nevertheless, for practical device realization, two requirements for the properties of these ZnO nanowires must be satisfied. One is low resistivity of individual ZnO nanowires, and the other is Ohmic contact formation between the conducting substrate and nanowires in the array. The undoped ZnO has rather high resistivity, as the carriers are mainly captured by its native defects such as O vacancies,7 with carrier density far from being enough for electrode application. In fact, the existence of native defects could be detrimental to the transport property of ZnO, when they serve as scattering centers for the charge carriers, causing increased material resistivity. Doping of ZnO with group III elements is commonly employed to improve the n-type conductivity in thinfilm ZnO electrodes. Among various choices of dopants, Ga is suggested to be one of the most effective due to its low electronegativity and an ion radius similar to that of Zn.8 In this regard, several groups have recently attempted ZnO nanowire doping with Ga.9-13 In particular, Lee’s group14 has demonstrated improved conductivity of such doped ZnO nanowires based on the electrical measurements of a single ZnO nanowire field-effect transistor (FET) device. Unfortunately, the previously reported Ga-doped ZnO nanowires are grown either as free-standing or on insulating substrates, making it impossible to form Ohmic contact between the nanowire and the substrate. For the same reason, the electrical properties of these Ga-doped nanowires were either estimated from thin film deposited under similar conditions or characterized by making single-nanowire field-effect transistors with additional contact formed between the nanowire and the external electrode. *
[email protected];tel+(852)2609632. † The Chinese University of Hong Kong. ‡ Peking University.
In the present work, Ga-doped ZnO nanowire arrays with well-aligned morphology were grown on indium-tin oxide (ITO) glass by a simple chemical vapor deposition (CVD) method. The existence of Ga in the nanowire was confirmed by secondary ion mass spectroscopy (SIMS). The effect of Ga doping on the optical properties of the nanowires was investigated via photoluminescence, which disclosed the suppression of native defect emission in ZnO nanowires upon Ga doping. The electrical behaviors of individual nanowire on ITO were directly measured with a four-probe system (in the present measurement, only two probes are involved), which reveals the low resistivity of the nanowire and its Ohmic contact with the ITO substrate. Experimental Section Synthesis of ZnO and Ga-Doped Nanowire Arrays. The synthesis was carried out in a vacuum tube furnace with setups described elsewhere.15 A double-tube setting was employed, with a small quartz tube (12 mm × 20 cm) located in the center of the big vacuum tube (40 mm × 70 cm). The premixed source powder was loaded into the closed end of the small tube (at the center of the tube furnace) and a piece of ITO glass substrate was placed at the open end of that tube (upstream of the tube furnace). Both the undoped and Ga-doped nanowire arrays were grown with the same experimental setting, while in the undoped nanowire growth, a powder mixture of ZnO and graphite with a weight ratio of 1:1 were used as the source materials; and extra Ga2O3 powder (with molar ratio 1/20 of ZnO) was added as the Ga source for the doped ZnO nanowire growth. An Ar/O2 mixture (10:1) was used as the carrier gas, and the furnace temperature was maintained at 1100 °C for 20 min. Material Characterization. The general morphology of the products is examined by scanning electron microscopy (SEM Quantum F400). The microstructure and crystallinity of the nanowires were characterized by transmission electron microscopy (TEM; Tecnai G2 FEG) and X-ray diffraction (XRD; Rigaku RU300). The chemical composition of the sample was examined by both X-ray photoelectron spectroscopy (XPS; PHI Quantum 2000) and secondary ion mass spectroscopy (SIMS;
10.1021/jp901025a CCC: $40.75 2009 American Chemical Society Published on Web 04/22/2009
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J. Phys. Chem. C, Vol. 113, No. 20, 2009
Zhou et al.
Figure 2. Room-temperature PL spectra of both undoped and Gadoped ZnO nanowire arrays [inset: low-temperature (8.5 K) PL spectra of Ga-doped ZnO nanowire arrays].
Figure 1. (a) SEM image of Ga-doped ZnO nanowire arrays (inset: HRTEM image taken from a single nanowire). (b) XRD spectrum and (c) SIMS spectrum of Ga-doped ZnO nanowire arrays.
PHI TRIFTIII) with Cs ion as the primary beam source. Both room-temperature and low-temperature (8.5 K) photoluminescence (PL) of the samples are studied by use of the 325 nm line of a HeCd laser. Transport measurements of individual nanowires on ITO are directly carried out on a MM3Ananoprobes system installed in a SEM (FEI XL 30F). Results and Discussion Figure 1a shows a typical SEM image of the Ga-doped ZnO nanowire arrays, whose morphology is similar to that of their undoped counterpart. It can be seen that these nanowires are straight and well aligned on the substrate. Their diameter and length are estimated as ∼250 nm and ∼10 µm, respectively. A typical high-resolution TEM image of the nanowire is shown in the inset of Figure 1a, showing the lattice fringes from the ZnO [100] zone axis. Little defects of either line or plane type are detected. In addition, these nanowires always grow along the ZnO crystalline [002] direction. The excellent crystallinity is further confirmed by the XRD results taken from the same sample. Only two peaks can be observed in the XRD data (Figure 1b), an intense (002) reflection and a weak (004) reflection, suggesting a preferential crystal orientation along [002], which is perpendicular to the substrate surface. Considering the [002] growth direction of the ZnO nanowires, the XRD result is fairly consistent with the excellent vertical alignment of ZnO nanowire arrays on the ITO substrate observed in the SEM image. Similar results are obtained for both undoped and Ga-doped nanowires alike. XPS measurements of both undoped and Ga-doped samples reveal Zn/O ratios close to 1. However, no obvious Ga signal was detected in the Ga-doped sample by XPS, suggesting a Ga concentration low than 1 at. % (considering the sensitivity of the XPS measurement). In order to find out whether Ga had been successfully incorporated in the ZnO nanowires, we then employed SIMS measurement, in which Ga signal (m/z ) 68.93) was indeed detected (Figure 1c), confirming the presence of Ga in the ZnO nanowires. Several tens of different spots on the same sample were analyzed by SIMS, and a similar Zn/Ga ratio was always found, revealing a fairly uniform Ga doping in the ZnO nanowires over the whole substrate area (about 1 cm × 2 cm).
A band-edge emission at ∼380 nm are found in the roomtemperature PL spectra taken from both undoped and Ga-doped ZnO samples (Figure 2), and a small red shift of ∼4 nm is observed in the Ga-doped sample relative to the undoped one. The red shift should result from the band gap narrowing due to Ga doping in ZnO (with Ga contributing to a shallow donor level), as commonly observed in the Ga-doped ZnO materials.14,16,17 This is further confirmed by low-temperature (8.5 K) PL (inset of Figure 2) taken from the Ga-doped sample, in which a dominant peak centered at ∼370 nm is observed, consistent with I8 excitation as a result of the Ga doping in ZnO.18 A shoulder peak is also observed; given its position relative to the dominant peak, it can be ascribed to the exciton recombinations I1-I5.18 The intensity of band-edge emission slightly decreases in the Ga-doped sample as compared to the undoped one, and all the PL spectra are normalized to the maximum emission peak. A significant difference is observed in the intensity ratio of the band-edge emission to the defect emission centered at ∼550 nm, the appearance of which is usually ascribed to the O-related native defect states in ZnO. In fact, such defect is almost completely suppressed in the Ga-doped sample. One should note that different Ga doping levels in ZnO modify the electronic structure of ZnO in different ways. At low doping concentrations (below 1020/cm3 ∼ 0.3%), the substitutional Ga atoms on Zn sites would form a shallow donor level, which increases the free electron density and thus can compensate the positive O vacancy defects, therefore suppressing O vacancy formation in the ZnO lattice.19 This is consistent with our experimental observation of depressed O defect emission in the room-temperature PL of the low doping concentration of Ga (less than 1%). When the doping level becomes high (>1020/cm3), most Ga atoms act as trapping centers of electrons, leading to a decreased ZnO band-edge emission and an increased red luminescence resulting from the compensating defects such as GaZn-Ointerstitial and GaZn-VZn.19 The electrical measurement of single ZnO nanowire-on-ITO has been carried out by use of a four-probe system with two probes involved, that is, one probe pressing on the conducting ITO substrate and another tip on nanowire. A schematic illustrating the measurement setting can be found in the inset of Figure 3. A linear I-V characteristic (Figure 3) is always obtained for all ZnO nanowires measured (undoped and Gadoped alike), suggesting good Ohmic contact between the nanowire arrays and the ITO substrate. We have also estimated the resistivity (F) of the nanowire based on the I-V results (Figure 3) by use of the formula
F ) Rπr2 ⁄ L where R is the resistance obtained from I-V measurement and r and L are the radius and length of the measured nanowire,
Properties of Ga-Doped ZnO Nanowire Arrays
J. Phys. Chem. C, Vol. 113, No. 20, 2009 8947 Acknowledgment. This work was supported by grants from the General Research Fund of Hong Kong SAR under Projects 400207 and 414908, and CUHK Focused Investment Scheme C. References and Notes
Figure 3. Typical I-V curve of single Ga-doped and undoped ZnO nanowire on ITO glass substrate. (Upper left inset) Enlarged I-V curve in the low voltage range; (lower right inset) measurement schematic.
respectively. For tens of Ga-doped ZnO nanowires (randomly chosen) measured, all of them have resistivities on the order of 10-3 Ω · cm. When the possible contact resistance between the W tip and the nanowire is considered, the actual resistivity could be even lower. As a comparison, the resistivities of undoped ZnO nanowires are always in the range of 10-1 Ω · cm (a typical I-V curve of the undoped sample is also shown in Figure 3), consistent with the literature reports on unintentionally doped ZnO nanowires.10,14 Conclusions In conclusion, Ga doping into ZnO nanowire arrays on conducting substrate has been realized by a simple chemical vapor deposition method. Both optical and electrical measurements show properties improvement of the ZnO nanowire as a result of the Ga doping, that is, passivation of O defects and enhancement of conductivity, making these doped ZnO nanostructures promising candidates for nanooptoelectronic and energy applications.
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