ARTICLE pubs.acs.org/JPCC
Effect of Defects on the Behavior of ZnO Nanoparticle FETs Anthony J. Morfa,*,† Nicholas Kirkwood,† Matthias Karg,† Th. B. Singh,‡ and Paul Mulvaney† † ‡
School of Chemistry & Bio21 Institute, University of Melbourne, Parkville, 3010, Victoria, Australia CSIRO Division of Materials Science and Engineering, Clayton, 3169, Victoria, Australia
bS Supporting Information ABSTRACT: The effects of ZnO crystal defects and the ubiquitous defect fluorescence on the electronic properties of nanocrystal thin-films were determined. Films were prepared from particles prepared in DMSO with controllable defect fluorescence. Particles were determined to range in size from 5 to 12 nm on average, with little bearing on the electronic properties. Thin-film electron mobilities were found to decrease from 0.04 cm2 V1 s1 to 0.008 cm2 V1 s1 with decreasing defect fluorescence, indicating crystal defects are pivotal to highmobility ZnO nanoparticle films. The threshold voltage of ZnO nanoparticle FET devices was found to decrease from 120 to 40 V while the resistivity increased 100-fold with decreasing defect fluorescence. These results are found to be in excellent agreement with theory and greatly improve our understanding of ZnO nanoparticle conduction.
anoparticle films of zinc oxide (ZnO) have been used in a plethora of thin-film devices including transistors,13 light emitting diodes,4,5 and photovoltaic cells.68 Large electron mobilities have been measured in ZnO,9,10 and have been found to greatly depend on thermal annealing and film density.2 While the results of Sun and Sirringhaus1 demonstrated that ZnO nanocrystals can be used in solution processed electronics, the important relationship between nanocrystal conduction and crystalline defects has yet to be addressed. The presence of a broad green fluorescence has been seen in ZnO since the earliest investigations of its optical properties,11 and there is strong experimental1215 and theoretical9,16 work to suggest it is the result of oxygen or zinc vacancies in the crystalline structure. Importantly, oxygen vacancies have been shown to contribute to conduction in both colloidal and polycrystalline ZnO system.1618 Vanheusden et al.15 demonstrated that the intensity of the green emission is proportional to the concentration of oxygen vacancies, while Leiter et al.13 provided a compelling mechanism to explain this fluorescence in terms of recombination of doubly ionized oxygen vacancies with conduction electrons. This mechanism is consistent with the fact that the trap emission is blue-shifted in quantized ZnO nanocrystals, an observation that strongly suggests recombination involves a free charge carrier.19 Based on quantum size effects, Wood et al. likewise concluded the trap emission resulted from recombination of trapped holes with conduction electrons. For the purpose of the present work, we assume the ZnO nanoparticle visible fluorescence is related to oxygen vacancies. The effect of vacancies on the electron mobility and threshold voltage of thin-film metaloxide field-effect transistors is determined. Films are prepared from nanoparticles synthesized in
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r 2011 American Chemical Society
DMSO and ripened in solution to reduce crystalline defects. In this way, the defect fluorescence can be decreased while the average particle size is found to increase only slightly. Over a wide range of defect fluorescence intensities, the mobility and threshold voltage are found to increase with increasing defect fluorescence. Conversely, the resistivity through the film is found to decrease with increasing defect fluorescence. These results will be explained using a theory proposed by J. Seto,20 and are found to be in excellent agreement. Nanoparticles of ZnO were prepared using a similar synthesis as previously reported,2 with chemicals obtained from SigmaAldrich. Zinc acetate dihydrate was dissolved in 50 mL of DMSO at 40 °C to a solute concentration of 58 mM. To the zinc acetateDMSO solution, seven 400 μL aliquots of 25% tetramethyl ammonium hydroxide (in methanol) were added over 120 s with vigorous mixing. Before heating, unreacted precursor materials were removed by precipitation of the ZnO nanoparticles with acetone. The solution of precipitated particles was separated by centrifugation at 2500 rpm followed by removal of the supernatant. The ZnO nanoparticles were redispersed in DMSO. Solutions of clean ZnO were heated to 80, 100, 120, and 140 °C under the flow of nitrogen. Prior to heating, the solution was degassed under vacuum and purged with nitrogen three times using a Schlenk line. The desired temperature was reached within 10 min and was maintained for 60 min, before the solution was rapidly cooled in a room-temperature water bath. Received: January 8, 2011 Revised: February 24, 2011 Published: April 06, 2011 8312
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Figure 1. Absorbance spectra of ZnO solutions (in ethanol) prepared in DMSO at various temperatures (A), where the λexc value indicates the excitation wavelength for the fluorescence measurements (B). The inset of part B shows the exponential decrease of the integrated fluorescence versus preparation temperature.
Absorbance and fluorescence measurements were made on the cooled samples using an Agilent 8453 UVvisible absorbance spectrometer and a Horiba Fluorolog fluorimeter. The DMSO solutions were diluted with ethanol in a quartz cuvette until the absorbance of the solution was 0.1 at 300 nm. Parts A and B of Figure 1 show the measured absorbance and fluorescence spectra of the prepared ZnO, respectively. The exciton band of ZnO is seen to red-shift with increasing preparation temperature, indicating particle growth. Transmission electron micrographs were taken of particles grown over the temperature range of this experiment and the average particle size was found to increase from 5 to 12 nm (see Supporting Information). Figure 1B shows the fluorescence of the same solutions shown in Figure 1A, where the solution was excited at 300 nm. The fluorescence was found to decrease dramatically over this temperature range (the integrated fluorescence (PLint) is plotted in the inset). The decrease in the vacancy fluorescence intensity
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Figure 2. Electrical impedance data from ITO/ZnO/Au sandwich structures. Bode plots (A) show the frequency dependence of ZnO prepared in DMSO and ethanol, with the lack of low frequency component in DMSO samples marked by an arrow. Resistivity (described in text) from modeled data shown in part A plotted versus preparation temperature in solution (B).
indicates that oxygen vacancy density is effectively reduced by this method.14,15 Electronic impedance and thin-film transistor measurements were made on thin-films of ZnO nanoparticles.2 Onto appropriate substrates, concentrated ZnO solutions in DMF were spun at 1500 rpm for 180 s. ZnO solutions were concentrated by precipitating 2 mL of solution with a 50:50 blend of acetone and hexanes. The solution was then centrifuged at 2500 rpm for 180 s before pouring off the supernatant. The nanoparticles were then resuspended in 100 μL of DMF. This solution was then immediately placed on to the appropriate substrate and spun. The samples were then heated to 175 °C to evaporate the DMF. Samples were approximately 100 ( 5 nm thick for impedance measurements and 120 ( 10 nm for FET measurements (as determined by profilometry). 8313
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Figure 3. Electronic measurements and calculated properties of ZnO thin-film transistors. Transfer curves (A) at several drain voltages for a device (layout shown in the inset) prepared from ZnO prepared at 140 °C in DMSO. Output curves (B) for the same samples as in part A are shown for various gate voltages. Determined mobility (C) and threshold voltages (C, inset) compiled versus defect fluorescence of inks. Simple schematic (D) of the energy levels involved for high and low defect densities.
Figure 2A shows the electrical impedance Nyquist plot of two ZnO samples between ITO and gold electrodes. The major difference between samples prepared in ethanol and DMSO is clearly seen at lower frequencies, where surface states dominate the conductivity.21 The lack of this feature indicates that DMSO is, like mercapto-butane,2 passivating surface states. Figure 2B shows the change in the pseudoresistivity of the ZnO films determined from the measured impedance profiles. As is clear from Figure 2A, a Randles cell can be used to model the electrical impedance of the film and the parallel resistance determined. By normalizing this modeled resistance (Rp) to thickness (d) and device area (A) the resistivity (F) was estimated (see eq 1) and found to decrease dramatically with increasing defect fluorescence. F ¼ Rp 3 A=d
ð1Þ
Metal oxide thin-film field effect transistors were fabricated from the same solutions to determine how mobility varies with the concentration of oxygen vacancies. The inset in Figure 3A shows the device layout, which is similar to previous work.2 As is clearly seen in Figure 3A, the transfer curves vary quadratically with gate voltage, while the output curves (Figure 3B) saturate at approximately 20 V. These results demonstrate typical thin-film
FET operation. The electron mobility for films prepared from each of the solutions is plotted versus PLmax1 from Figure 1B (Figure 3C), while the threshold voltage is plotted versus the square root of PLmax (inset Figure 3C). These results demonstrate that dramatic control of threshold voltage is achievable without varying the dielectric layer. The measured mobility increases with increasing defect fluorescence. Vanheusden et al.15 found that the defect fluorescence increases proportionally to the number of vacancies in the sample, so these results are contrary to classical semiconductor principles.22 These results are not unprecedented however; similar results have been seen in polycrystalline ZnO1618 and silicon20 systems. In those systems, it was shown that when the number of defects in the grain was much higher than the number of defects at the grain boundary, the injection barrier and depletion layer width at the surface of the grain were reduced (see Figure 3D).20 It was then determined that the mobility would follow eq 2,20 where j and β are arbitrary constants.15 β μeff µ j exp ð2Þ PLmax The dashed line in Figure 3C is a fit to the experimentally determined mobilities using eq 2 and shows a remarkably good fit. 8314
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The Journal of Physical Chemistry C Although great care is typically taken to passivate surface states in nanoparticle thin-film electronics, these results indicate defects in the nanoparticle can also have a beneficial impact on thin-film mobility. Further, these results demonstrate that a simple solution preparation can be used to prepare ZnO transistors or electronics with tunable optical and electronic properties.
’ ASSOCIATED CONTENT
bS
Supporting Information. TEM micrographs of ZnO particles. This material is available free of charge via the Internet at http://pubs.acs.org.
’ ACKNOWLEDGMENT The authors thank the ARC for support under grant DP109485. MK acknowledges the Alexander von Humboldt foundation for a Feodor Lynen research fellowship. AJM would like to acknowledge the NSL for the B€uchner Fellowship.
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