Electrochemical Route to p-Type Doping of ZnO Nanowires

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Electrochemical Route to p-Type Doping of ZnO Nanowires M. A. Thomas†,‡ and J. B. Cui*,† †

Department of Physics and Astronomy and ‡Department of Applied Science, University of Arkansas at Little Rock, Little Rock, Arkansas 72204

ABSTRACT p-Type ZnO nanowires doped with Ag were successfully obtained by low-temperature electrochemical growth followed by annealing. The p-type conductivity is achievable only under certain growth conditions such that the incorporation of Ag does not significantly affect the quality of the ZnO nanowires. The applied potential during growth plays a key role in achieving p-type conductivity. As the potential decreases below a critical value of -0.65 V (more negative relative to Ag/AgCl), the conductivity changes from n- to p-type in the doped ZnO nanowires. In addition, p-type ZnO nanowires exhibit a band gap reduction and strong acceptor-related photoluminescence, while n-type nanowires show band gap broadening with a strong donor-bound exciton emission. This study sheds light on the rational growth of p-type ZnO nanomaterials by lowcost electrochemical deposition. SECTION Electron Transport, Optical and Electronic Devices, Hard Matter

s a wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV),1,2 ZnO has been extensively studied due to its potential applications in optoelectronic devices such as short-wavelength lightemitting diodes and ultraviolet lasers. However, the lack of a reliable and effective method for p-type doping limits the practical applications of ZnO. Possible problems associated with the difficulty of doping ZnO p-type include low dopant solubility, large dopant ionization energy, and the presence of unwanted donor defects acting as acceptor compensators.3,4 Theoretical calculations suggest that inherently n-type wide band gap oxides, including ZnO, are difficult to dope p-type using growth processes occurring under thermodynamic equilibrium.5 Electrochemical growth of ZnO6-8 usually takes place at temperatures below 100 °C and is constantly under nonequilibrium conditions due to the applied potential, both of which may aid p-type doping from enhanced dopant solubility9-12 and mitigated compensation of native defects during growth.11,12 To our knowledge, however, real success in p-type doping of ZnO using an electrochemical process has not been reported so far. Our recent studies have shown that the relative concentration of acceptors was significantly increased due to the applied potential in electrochemical growth.13 In this study, we achieved for the first time p-type conduction of Ag-doped ZnO nanowires grown by a low-temperature electrochemical process. Effects of the applied potential and annealing on the p-type properties were investigated. Interestingly, the conductivity type of the doped ZnO nanowires was found to correlate with their photoluminescence (PL) at both room and low temperature. This finding may be

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potentially useful for determining the conductivity type of ZnO with nondestructive and noncontact optical measurements, significant especially for nanowires because their electrical properties are challenging to measure using standard semiconductor characterization techniques. Figure 1a shows a scanning electron microscope (SEM) image of an undoped ZnO sample consisting of dense arrays of nanowires with diameters of 100-150 nm. A small change in morphology is observed for the Ag-doped nanowire arrays as shown in Figure 1b and c. Figure 1d is a SEM cross section of the sample shown in Figure 1b, showcasing the vertically oriented growth of the doped nanowires. As the Ag concentration in the growth solution reaches 1% and higher, however, more drastic changes in the morphology of the ZnO occur.14 A high concentration of Ag ions in the growth solution disturbs the regular ZnO nanowire formation, making the growth of high-quality, oriented nanowires very difficult. The Ag content in the doped ZnO nanowires was measured by energy-dispersive X-ray spectroscopy (EDX), which shows that atomic percentages of Ag in the nanowires increase as the Ag concentration in the growth solution is increased. However, highly cathodic potentials (more negative) produce less Ag content in the resulting nanowires (assuming the same Ag concentration in the growth solution). This phenomenon has been observed in electrochemically grown ZnO doped with B15 and In.16 As we found before, the growth of ZnO becomes much faster at more negative potentials.8 However, Ag deposition is a strongly diffusion-limited process due to its low Received Date: February 20, 2010 Accepted Date: March 4, 2010 Published on Web Date: March 11, 2010

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DOI: 10.1021/jz100246e |J. Phys. Chem. Lett. 2010, 1, 1090–1094

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Figure 1. SEM images of (a) undoped ZnO at -1.13 V, (b) 0.1% Ag at -0.72 V, (c) 0.1% Ag at -0.92 V, and (d) a cross section of sample in (b). The Ag concentrations are mol % in the growth solution.

concentration in the growth solution. Since the Ag content in the nanowires depends on the relative deposition rates of ZnO and Ag, the increased growth rate of ZnO at more negative potentials causes a relative decrease of Ag in the nanowires. The Ag dopants in our ZnO nanowires are likely substitutional impurities (AgZn), as suggested by theoretical calculations, which reveal that Ag substitution of Zn is the preferred location under most growth conditions.17-19 The nanowire morphologies of the samples make it difficult to characterize electrical properties with typical solid-state methods. As an alternative approach, photoelectrochemical cell (PEC) measurements involving a semiconductor/electrolyte junction were employed in this work. By monitoring the polarity of the open-circuit voltage change (ΔVoc) between dark and illuminated conditions, the conductivity type of the ZnO nanowires can be determined. The “dark” open-circuit potential is defined as the equilibrium position reached after an explicit charge-transfer process lines up the redox potential of the electrolyte and the Fermi level of the ZnO semiconductor surface. Under illumination by a white light source, new charge carriers are excited in the ZnO sample, resulting in an open-circuit potential shift, ΔVoc. A negative ΔVoc is indicative of n-type material, while p-type material produces a positive ΔVoc. The PEC method has been used previously to determine the conductivity type of various semiconductors such as ZnSe,20 Cu(InGa)(SeS)2,21 and ZnO.22,23 Measurements on n- and p-type Si wafers were also performed in this study prior to being applied to the ZnO nanowires. The Si yielded ΔVoc values of -25 and þ40 mV, respectively, for n-type (5 1014 cm-3) and p-type (2 1015 cm-3) wafers. Figure 2 displays the PEC responses of both undoped and Ag-doped samples as grown and after annealing. Note that the Ag concentrations in the growth solution are labeled in the figure. As can be seen in Figure 2a, both as-grown doped and undoped nanowires have negative PEC responses (n-type conductivity) upon illumination. However, the doped samples tend to show a smaller magnitude of the negative PEC signal,


Figure 2. PEC responses for (a) as-grown and (b) annealed samples. The mol % of Ag in the growth solution is indicated in the figure.

indicating a possible decrease in donors and increase in acceptors in these samples due to Ag doping. After annealing, the undoped ZnO samples still show negative PEC responses with increases in magnitude compared to the corresponding as-grown samples. However, the annealed Ag-doped nanowires display significant changes in their PEC responses depending on their growth conditions. The annealed Agdoped samples grown at potentials below -0.65 V all have positive PEC responses, except those obtained with only 0.01% Ag in the solution, revealing a p-type conductivity of these nanowires. In contrast, the samples grown at potentials higher than -0.65 V are still n-type after annealing. It is worth noting that the p-type properties of the Ag-doped ZnO nanowires are stable and reproducible. The positive PEC responses remain intact after storing the samples in ambient conditions for over 4 months. As far as the Ag concentration in the growth solution is concerned, a reasonable Ag concentration of less than 1% but above 0.01% is necessary to obtain p-type conductivity in the doped ZnO nanowires. Only under these conditions is the normal, high-quality ZnO nanowire growth process still favored while enough Ag dopants are also incorporated into the host material. When the Ag concentration in the growth solution is too high or too low, the resulting materials show n-type conductivity. This observation may be explained as follows. If the Ag concentration is too high, the normal ZnO growth process is disrupted, producing lower-quality material with more defects. As a result, the active acceptor concentration may be low even though the Ag content in the nanowires is high. Our experimental data showed that the doped ZnO nanowires with Ag content less than 1% may exhibit p-type

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Figure 4. Typical photoluminescence spectra at 10 K of annealed n-type and p-type ZnO nanowires doped with Ag. The mol % of Ag in the growth solution is indicated in the figure. The spectra have been normalized to the most intense emission and are vertically offset for clarity.

This trend is consistent for the doped samples in this study, not only those shown in Figure 3b. Such an observation suggests that optical characterization may reveal the conductivity type of doped ZnO nanowires without electrical measurements. The correlation between the conductivity type and NBE peak shift is consistent with doping-induced band gap changes reported in the literature.23-30 Figure 3c shows the data taken from various reports on n- and p-type ZnO thin films. For a given dopant, the band gap changes typically scale with the charge carrier concentration. A widening of the band gap is usually observed for ZnO doped with donors, while p-type ZnO has shown band gap reduction. The underlying mechanism for such changes in n-type ZnO has been attributed to a combination of band gap renormalization24,25 and the Moss-Burstein effect,24-26 that is, the blocking of band edge states. For highly n-type doped ZnO, the Moss-Burstein effect dominates in the band gap widening.24-26 The band gap shrinkage commonly observed in p-type ZnO, on the other hand, has generally been associated with impurity band formation inside the band gap.23,27,29,30 While it is evident in Figure 3b that the PEC responses of our doped samples do not scale perfectly with the corresponding NBE peak shifts, the conductivity types do match consistently with the direction of the peak shifts. These observations may be related to the material quality, for example, the amount of defects in the nanowires. The degradation of the material quality is partially reflected in the PL spectra by the increase of defect bands for the samples obtained with high Ag concentrations in the growth solution. These defects may enhance the charge compensation effect for p-type doping or have negative effects on the hole mobility. A degradation of the material quality may affect hole conduction in terms of mobility much more strongly than electronic transitions involving acceptors in PL.31 The low-temperature PL of the Ag-doped samples is also related to their conductivity type, as shown in Figure 4. Two typical peaks, one associated with donors and the other

Figure 3. (a) NBE peak position shift as a function of applied potential for annealed Ag-doped samples. (b) PEC response as a function of NBE peak shift for annealed Ag-doped samples. (c) Band gap shifts versus carrier concentration for n- and p-type ZnO in the literature. Note that the NBE peak shifts in (a,b) and the band gap shifts in (c) were determined by using the corresponding undoped ZnO values.

properties, while those with much higher Ag content are n-type (see Figure S1 in the Supporting Information). If the Ag concentration in the growth solution is too low, on the other hand, the nanowires do not have enough Ag dopant to facilitate p-type doping. EDX measurements indicated that the Ag content in the nanowires obtained with 0.01% Ag in the growth solution was below the detection limit, and these samples show n-type conductivity even after annealing. In addition to the change in electrical properties, Ag doping affects the PL properties of the ZnO nanowires. One of the prominent changes in the room-temperature PL is a peak position shift of the near-band edge emission (NBE) relative to undoped samples (see Figure S2 in the Supporting Information). Figure 3a shows the NBE peak shift as a function of applied potential for the annealed nanowires. The NBE peak shows a red shift at more negative potentials and a blue shift at less negative potentials, likely due to doping-induced band gap changes. Figure 3b shows the PEC response as a function of the NBE peak shift. It is evident that the nanowires with a NBE peak red shift are p-type, while those possessing blue shifts are n-type.


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associated with acceptors,13 were observed in these samples. A neutral donor-bound exciton (D0X) emission near 3.364 eV32 dominates the low-temperature PL spectra of the n-type ZnO nanowires. Although the p-type ZnO nanowires still show a D0X peak, its intensity is significantly reduced. Instead, a free electron to neutral acceptor (e,A0) transition at 3.323 eV becomes the strongest peak in these p-type samples. Fits of temperature-dependent PL data yielded an activation energy of 117 meV for the acceptor involved in this (e,A0) transition.13 Previous work on Ag-doped ZnO grown by vacuum methods also showed intense acceptor-related PL for p-type samples.33-36 Even though the PL measurements cannot yield information about charge carrier concentrations, we confirm that the relative intensity of emission peaks associated with donors and acceptors qualitatively reflects the conductivity type of the ZnO nanowires. The as-grown nanowires do not exhibit p-type properties, possibly resulting from a native defect compensation effect and inactivation of acceptors. Annealing may cause lattice relaxation and recoordination of the Ag dopants, leading to a desirable condition for p-type doping. Hydrogen has been strongly suggested to be responsible for the inherent n-type conductivity in undoped ZnO both theoretically37 and experimentally.38-40 Considering the fact that our electrochemical process takes place in water at low temperature, hydrogen is likely incorporated into the as-grown ZnO nanowires. However, hydrogen is unstable in ZnO and can typically be annealed out at temperatures above 500 °C.40 Other acceptor compensators like Zn interstitials may also be removed by annealing.41,42 This reduction of donor defects helps increase the acceptor concentration and therefore lead to p-type properties of the Ag-doped ZnO nanowires. Another possibility is that the activation of Ag acceptors takes place at high temperature, enhancing the concentration of holes. In summary, p-type Ag-doped ZnO nanowires were attained by low-temperature electrochemical deposition followed by annealing. Effects of growth conditions and postgrowth annealing on the p-type conductivity were investigated and discussed. A distinct connection between the optical and electrical properties of the doped nanowires was observed, opening up a possibility for determining the conductivity type with noncontact and nondestructive optical measurements. The low-cost, electrochemically doped ZnO nanostructures may have potential applications in electronic and optoelectronic devices.

both energy-dispersive X-ray and X-ray photoelectron spectroscopies, confirming the presence of Ag. Optical properties were measured by temperature -dependent photoluminescence using the 325 nm line of a He-Cd laser and a Jobin Yvon 320 spectrometer with a charge coupled device camera. The samples were cooled in a cryostat. The electrical properties were determined by photoelectrochemical cell measurements in a 0.005 M NaCl electrolyte. The open-circuit voltage between the nanowire arrays and a gold electrode was monitored under both dark and illuminated conditions. A 150 W Xe lamp white light source was used for illumination.

SUPPORTING INFORMATION AVAILABLE Photoelectrochemical cell responses of Ag-doped ZnO nanowires as a function of Ag content in the samples, as well as room-temperature photoluminescence spectra of as grown undoped and Ag-doped ZnO nanowires obtained under various conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Tel: 501-5698962. E-mail: [email protected].

ACKNOWLEDGMENT The authors would like to thank the UALR Nanotechnology Center for use of its SEM facilities.

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EXPERIMENTAL DETAILS The ZnO nanowire arrays were grown on gold-covered silicon substrates using an electrochemical process at 95 °C.8,14 In brief, an aqueous solution with equimolar amounts of zinc nitrate and hexamine was used for undoped ZnO growth, while for Ag-doped samples, 0.01-1 mol % of zinc nitrate was replaced by silver nitrate. All samples were grown potentiostatically in the range of -0.21 to -1.13 V (versus Ag/ AgCl reference) for 1 h. Annealing was performed in 100 Torr of oxygen at 600 °C for 20 min. The samples were characterized structurally by scanning electron microscopy (JEOL JSM7000F at 15 kV). Their composition was quantified with


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Electrochemical Route to p-Type Doping of ZnO Nanowires

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