Article pubs.acs.org/JPCC
Drastic Change in Electrical Properties of Electrodeposited ZnO: Systematic Study by Hall Effect Measurements Tsutomu Shinagawa,*,† Masaya Chigane,† Kuniaki Murase,‡ and Masanobu Izaki§ †
Electronic Materials Research Division, Osaka Municipal Technical Research Institute, Osaka 536-8553, Japan Department of Material Science and Engineering, Kyoto University, Kyoto 606-8501, Japan § Department of Mechanical Engineering, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan ‡
ABSTRACT: We report a systematic study of the electrical properties of ZnO films prepared by galvanostatic electrodeposition from an aqueous zinc nitrate solution. The structural characterization of the ZnO films electrodeposited at various applied current densities and deposition temperatures are carried out with XRD and FESEM, and the electrical properties including resistivity, carrier density, and carrier mobility are determined by Hall effect measurements at room temperature. The Hall effect measurements reveal that the resistivity changes drastically by about 5 and 6 orders of magnitude with the applied current density (0.1−2.0 mA cm−2) and the deposition temperature (60−80 °C), respectively. The variation in the resistivity is mainly attributed to carrier density not mobility and shows a good correlation with the degree of c-axis orientation of the ZnO films. Annealing effect on the electrical properties of the ZnO films is also examined similarly, revealing that the resistivity increases by ∼50 times with increasing temperature (150−500 °C) accompanied by the formation of nanosized holes. (FTO), as-deposited films cannot be subjected to Hall effect measurements. As an alternative method, I−V curves and Mott−Schottky plot have been measured to evaluate the series resistance and carrier density of electrodeposited ZnO, respectively.32−34 These methods, however, are indirect and provide only a part of the electrical properties. In the present study, we have performed a systematic study to determine the electrical properties of electrodeposited ZnO using Hall effect measurements. The transfer of ZnO from FTO substrates to insulative epoxy resin (>1013 Ω cm) without damage enabled us to conduct Hall effect measurements. Furthermore, the variation of the electrical properties has been discussed from the viewpoint of the crystallinity and surface morphology of the electrodeposited ZnO.
1. INTRODUCTION Zinc oxide (ZnO) is an n-type oxide semiconductor with a wide band gap of 3.3 eV and a large free exciton binding energy of 69 meV.1 Since the development of ZnO electrodeposition techniques by Izaki et al.2 and Lincot et al.3 in 1996, a wide range of studies of ZnO using the electrodeposition have been reported due to significant advantages over vacuum deposition techniques.4 The electrodeposition technique offers (i) ease of thickness and morphology control, (ii) ease of coating on complicated shapes or micropatterns, and (iii) ease of largescale production with low cost. So far the electrodeposition of ZnO has mainly focused on morphology control,5−9 such as nanowires, nanorods, and nanotubes, epitaxial growth on single crystalline substrates,10−12 and optical properties such as photoluminescence including room-temperature UV emission.13−16 Recently, electrodeposited ZnO has especially attracted increasing attention as an electron transporting material in solar cells17−24 and light-emitting diodes.25−29 Although electrical properties, including resistivity, carrier density, and carrier mobility, of the electrodeposited ZnO are quite important to improve the performance of these devices, little has been reported so far. Hall effect measurements using the van der Pauw method are one of the most reliable methods to determine the electrical properties of semiconductor films.30,31 It is, however, necessary for the accurate measurement to deposit the film on insulative substrates such as glass and alumina. Since the electrodeposition naturally uses conductive substrates such as stainless steel and F-doped SnO2-coated glass © 2012 American Chemical Society
2. EXPERIMENTAL SECTION All aqueous solutions were prepared using reagent-grade chemicals and deionized (DI) water purified by a Milli-RX12 Plus system. F-doped SnO2-coated glass (FTO, Asahi glass, 10 Ω/□) was used as a substrate. Linear sweep voltammetry (LSV) was carried out with an automatic polarization system (Hokuto Denko HSV-100) using a Zn-bar counter electrode, Ag/AgCl reference electrode, and ZnO-coated FTO working electrode. The LSV was started from steady rest potential to negative potential in an aqueous solution containing 75 mM Received: May 15, 2012 Revised: June 13, 2012 Published: July 11, 2012 15925
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(M = mol dm−3) Zn(NO3)2 and 0.1 M NaNO3 at a sweep rate of 5 mV s−1. Cathodic electrodeposition of ZnO was performed galvanostatically in aqueous solutions containing 75 mM Zn(NO3)2 and 0.1 M NaNO3 by applying various current densities (0.1−2.0 mA cm−2) at different bath temperatures (60−80 °C). The total electrical charge was fixed at 2.0 C cm−2, and a Zn bar and FTO substrate were used as a counter electrode and working electrode, respectively. A part of the electrodeposited ZnO samples was annealed at 150, 350, and 500 °C for 30 min in air. Structural and morphological characterization was performed with an X-ray diffractometer using Cu Kα radiation (XRD, Rigaku RINT2500) and a field-emission scanning electron microscope (FESEM, JEOL JSM6700F). Electrical properties of the ZnO obtained were evaluated with a Hall effect measurement system (Toyo Technica, Resitest8310) using the van der Pauw method at room temperature. Samples for the Hall effect measurements were prepared using a transfer method reported by Miyake et al.;35 a ZnO film (8 × 8 mm in size and ∼1.6 μm in thickness) electrodeposited on FTO was transferred to an epoxy resin pellet (Agilent Technologies Torr seal, >1013 Ω cm). Four gold probes were attached to the corner of the sample mounted on a holder, and dc current was applied along a side and a diagonal of the square ZnO film for the resistivity and Hall effect measurements, respectively. During the Hall effect measurements, a magnetic field of 0.41 T was applied perpendicular to the ZnO surface.
Figure 1. LSV curve in an aqueous solution containing Zn(NO3)2 and NaNO3 using a ZnO-coated FTO electrode, Zn-bar counter electrode, and Ag/AgCl reference electrode at a sweep rate of 5.0 mV s−1. White circles indicate the applied current density employed in this study for the ZnO electrodeposition.
atures of 60, 70, and 80 °C were selected because deposition temperatures above 50 °C are required to obtain crystallized ZnO.37,38 Therefore, four additional ZnO samples were prepared at bath temperatures of 60 and 80 °C at two different current densities of 0.4 and 1.6 mA cm−2. 3.1. Crystal Morphology and Structure of Electrodeposited ZnO. Figure 2 shows top-view and cross-sectional FESEM images and normalized XRD patterns of the ZnO films electrodeposited on FTO substrates at the various current densities and deposition temperatures. All the ZnO films ∼1.6 μm in thickness were a hexagonal wurtzite structure. For the current density dependence (see the column of 70 °C), while well-hexagonal faceted (001) surfaces were observed in the topview FESEM images at current densities below 0.7 mA cm−2, rounded faces were appeared above 1.6 mA cm−2. In the corresponding XRD patterns, a preferred 002 (c-axis) growth orientation was observed over the column of 70 °C, while the other ZnO peaks such as 103 diffraction as well as FTO peaks begin to appear obviously at higher current densities. The degree of preferred c-axis orientation for the ZnO films was estimated quantitatively by calculating Harris’s texture coefficient, Tc.39,40 The Tc is defined as
3. RESULTS AND DISCUSSION A plausible reaction mechanism of the cathodic ZnO electrodeposition from Zn(NO3)2 aqueous solutions has been proposed as follows:36 NO3− + H 2O + 2e− → NO2− + 2OH−
(1)
Zn 2 + + 2OH− → Zn(OH)2
(2)
Zn(OH)2 → ZnO + H 2O
(3)
A local-pH increase in the vicinity of a substrate is caused by reduction reaction of nitrate ion, followed by the formation of amorphous Zn(OH)2 and dehydration to form crystalline ZnO. According to this mechanism, the formation rate of Zn(OH)2 is affected by applied current density and Zn2+ concentration, [Zn2+], and the crystallization accompanied by dehydration may depend on the deposition temperature (bath temperature). In this paper, [Zn2+] was fixed at 75 mM, and the applied current density and deposition temperature were changed to examine systematically the electrical properties of electrodeposited ZnO. In order to determine the range of the applied current density, the electrochemical behavior of ZnO electrodeposition from a 75 mM Zn(NO3)2−0.1 M NaNO3 solution was investigated by linear sweep voltammetry (LSV) using a ZnO-coated FTO as a working electrode. The measurement was started from rest potential to negative potential at a sweep rate of 5 mV s−1. Cathodic current due to reduction of NO3− was emerged at a potential of −0.6 V vs Ag/AgCl and rapidly increased at around −0.85 V as shown in Figure 1. Within the 0 range of −0.6 to −1.0 V (E(Zn 2+ /Zn) = −0.96 V vs Ag/AgCl), we chose eight points, corresponding to current densities of 0.1, 0.2, 0.4, 0.7, 1.0, 1.6, 1.8, and 2.0 mA cm−2, and electrodeposited ZnO galvanostatically on FTO at an electrical charge of 2.0 C cm−2 at a deposition temperature of 70 °C. For the deposition temperature dependence, three different temper-
Tc(hkl) = n
Im(hkl)/I0(hkl) n ∑1 Im(hkl)/I0(hkl)
(4)
where Im(hkl) is the relative intensity of the measured peak corresponding to the hkl diffraction, I0(hkl) is the relative intensity of the same diffraction in the standard powder sample (JCPDS no. 36-1451), and n is the total number of diffraction peaks considered in the evaluation. The Tc can take values from 0 to n, and samples having a high and random orientation exhibit ∼n and ∼1, respectively. In the present case, we chose seven diffractions (n = 7) of 100, 002, 101, 102, 110, 103, and 112, and the calculated results for Tc(002) are plotted in Figure 3a. While Tc(002) deceases as the current density increases, all the values are higher than 3.5. Especially for ZnO electrodeposited at below 0.7 mA cm−2, high values ∼6 are represented. These results indicate that crystallinity decreases with increasing applied current density, which can be explained qualitatively by the deposition mechanism described above. Since the deposition temperature is fixed at 70 °C, a relative reaction rate for Zn(OH) 2 formation (reaction 2) vs dehydration (reaction 3) will increase with increasing applied current density, causing a rise in amorphous Zn(OH)2 in the crystal growth process. Although the amorphous Zn(OH)2 will 15926
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Figure 2. Top-view and cross-sectional FESEM images and XRD patterns of ZnO films galvanostatically electrodeposited on FTO substrates from 75 mM Zn(NO3)2−0.1 M NaNO3 aqueous solutions at various current densities and deposition temperatures. The XRD patterns are normalized by their maximum values. The symbols ○ and ● indicate 002 and the other diffractions assigned to ZnO, respectively. Inset right below is the JCPDS(#36-1451) data of wurtzite ZnO.
For the deposition temperature dependence (60−80 °C), considerable change in the crystal morphology was observed by just 20 °C (see rows of 0.4 and 1.6 mA cm−2 in Figure 2); both crystalline facets and a preferred c-axis orientation are
be eventually transformed to ZnO, it brings random morphology.37 The reciprocal Tc plotted in the inset is discussed later. 15927
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Figure 4. Schematic preparation procedure of ZnO/epoxy pellets for Hall effect measurement. Cross-sectional FESEM image of a pellet is also shown.
Figure 3. Variation of texture coefficient Tc(002) calculated from the XRD patterns in Figure 2 as a function of (a) current density at 70 °C and (b) deposition temperature at (□, △) 0.4 and (■, ▲) 1.6 mA cm−2. Each inset shows the variation of 1/Tc.
dramatically improved with increasing temperature. The ZnO films electrodeposited at 60 °C have an almost random orientation irrespective of the current density, and their surfaces consist of crystal edges composed of ZnO (001) and (100) faces. In contrast, electrodeposition at 80 °C gave highly c-axis oriented ZnO with a well-faceted (001) surface. The Tc(002) value calculated in the same way as above was also changed considerably from ∼0.5 to ∼6.6 as shown in Figure 3b. These changes observed is probably attributed to the promotion of dehydration (reaction 3) and subsequent rearrangement in crystal by elevated temperature. Furthermore, less difference in deposition temperature dependence between 0.4 and 1.6 mA cm−2 suggests that deposition temperature is much more dominant on the resulting ZnO crystallinity than applied current density. 3.2. Electrical Properties of Electrodeposited ZnO. For Hall effect measurement, ZnO films (8 × 8 mm in size) electrodeposited on FTO were transferred to epoxy resin pellets as shown in Figure 4. No clacks and no trace of FTO were detected in the transferred ZnO by FESEM and X-ray photoelectron spectroscopy analysis, respectively. The Hall effect measurement was carried out at room temperature in the dark. Figures 5a and 5b show variation in electrical properties (resistivity, carrier density, and mobility) of the ZnO films electrodeposited galvanostatically at various applied current densities at a constant bath temperature of 70 °C. Resistivity ρ was almost constant around 30−40 Ω cm at current densities ranging from 0.1 to 0.7 mA cm−2 and then increased rapidly up to ∼1 MΩ cm above 1.8 mA cm−2, showing a drastic change by about 5 orders of magnitude. Figure 5b shows the
Figure 5. Variation of electrical properties (resistivity, carrier density, and mobility) of electrodeposited ZnO films as a function of (a, b) current density at 70 °C and (c, d) deposition temperature at (○, □, △) 0.4 and (●, ■, ▲) 1.6 mA cm−2.
corresponding carrier density n and mobility μ dependence on the current density; note that n and μ values for the ZnO electrodeposited above 1.6 mA cm−2 were not obtained due to the high resistivity over the measuring system performance. The relationship among these values is as follows: 1 1 ρ= = σ qnμ (5) where σ and q are electrical conductivity and the elementary charge of 1.6 × 10−19 C, respectively. As shown in Figure 5b, carrier density decreased monotonically from 4 × 1017 to 3 × 1015 cm−3 with increasing current density, and mobility has a peak of about 11 cm2 V−1 s−1 at 0.7 mA cm−2. In general, the carrier generation of nondoped ZnO is based on native defects such as oxygen vacancy and interstitial zinc: 15928
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OxO → V•O + e′ + 1/2O2 and/or ZnO → Zn•1 + e′ +1/2O2.41 Carrier density for a sputtered polycrystal ZnO film has been reported to be as high as ∼1019, probably because oxygen vacancy is generated easily in a vacuum condition.42 On the other hand, carrier mobility is mainly affected by impurity scattering and grain-boundary scattering; the impurity includes the native defects and dopant atoms. For instance, mobilities of single-crystal bulk and the sputtered polycrystal ZnO were reported to be ∼250 and ∼20 cm2 V−1 s−1, respectively.42,43 The initial increase in mobility is due to the decrease in carrier density, and the subsequent decrease is probably attributed to an increase of grain boundaries as expected from the corresponding FESEM images (Figure 2). Thus, the variation in resistivity is mainly due to a change in carrier density that decreases with increasing current density. Since the carrier generation and carrier mobility are generally affected by the ZnO crystallinity, there may be some relation between resistivity and crystallinity. Interestingly, comparing Figure 5a with the inset in Figure 3a reveals that the variation curve of resistivity is in good agreement with that of the reciprocal Tc(002), indicating that the resistivity increased with decreasing the degree of c-axis orientation. In order to examine the dependence of electrical properties on deposition temperature, four additional ZnO films were prepared at different bath temperatures of 60 and 80 °C at current densities of 0.4 and 1.6 mA cm−2. As shown in Figure 5c, resistivity of the ZnO electrodeposited at 1.6 mA cm−2 decreased rapidly from 20 M to 12 Ω cm as the temperature increases, showing a considerable change by about 6 orders of magnitude, which is larger by 2 orders of magnitude than at 0.4 mA cm−2. Again one can find a good correlation between the resistivity variation and the reciprocal Tc(002) (see Figure 5c and the inset in Figure 3b). Figure 5d shows the corresponding variations in carrier density and mobility; these values for ZnO deposited at 60 °C at 1.6 mA cm−2 were not obtained due to the high resistivity over the measuring system performance. The ZnO films electrodeposited at 0.4 mA cm−2 exhibited changes by about 4 and 1 order of magnitude in the carrier density and mobility, respectively, suggesting that the carrier density has a dominant influence on the resistivity change. 3.3. Effect of Annealing on Electrical Properties. The effects of annealing on the structural morphology and electrical properties of electrodeposited ZnO were examined in a similar manner. For the evaluation, ZnO films were electrodeposited at 70 °C at a current density of 0.4 mA cm−2, and then were annealed in air at different temperatures of 150, 350, and 500 °C for 1 h. XRD patterns and calculated Tc(002) show that the resulting ZnO films remain high c-axis orientation also after the annealing, and no significant changes in peak position and peak width were recognized (Figure 6). Figure 7 shows the surface and cross-sectional FESEM images of the annealed ZnO films. Whereas no significant changes in crystal shape and thickness were observed in all samples, a large number of holes 10−100 nm in diameter was recognized for the ZnO annealed above 350 °C. Since the holes are formed uniformly across the thickness, the formation of something like a precursor may occur in the process of crystal growth, which evaporates leaving holes at annealing temperatures ranging from 150 to 350 °C. Pauporté et al.44 have reported similar thermal formation of holes at 400 °C in ZnO electrodeposited from a ZnCl2 solution and considered Cl-incorporated ZnO and/or Cl-based impurities as a precursor. Since the chloride medium is not employed in the present study, the main component of a hole
Figure 6. XRD patterns of electrodeposited ZnO films (0.4 mA cm−2, 70 °C) before and after annealing at 150, 350, and 500 °C in air for 1 h. Inset shows a variation of Tc(002).
Figure 7. Top-view and cross-sectional FESEM images of electrodeposited ZnO films (0.4 mA cm−2, 70 °C) after annealing at (a, b) 150, (c, d) 350, and (e, f) 500 °C in air for 1 h.
precursor may be amorphous hydroxides, which can generate evaporable water by dehydration. Figure 8 shows the electrical properties of the annealed ZnO films. The resistivity was once decreased one-fifth that of the asgrown ZnO by annealing at 150 °C and then increased by 2 orders of magnitude with increasing anneal temperature. Figure 8b reveals that the variation in the resistivity depends on carrier density not on mobility. The initial rise in the carrier density at 150 °C is probably due to relaxation of lattice strain and/or evaporation of remaining moisture in the film, and the subsequent drop above 350 °C is attributed to the reaction of native defects, such as oxygen vacancy and interstitial zinc, with oxygen in the air. A number of holes generated by annealing may act as a carrier scattering factor because the 15929
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REFERENCES
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Figure 8. Variation of electrical properties (resistivity, carrier density, and mobility) of electrodeposited ZnO films as a function of anneal temperature.
mobility is almost constant even though the carrier density decreases.
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CONCLUSIONS In this paper, the electrical properties (resistivity, carrier density, and carrier mobility) of ZnO films electrodeposited at various applied current densities (0.1−2.0 mA cm−2) and deposition temperatures (60−80 °C) from 75 mM Zn(NO3)2 aqueous solutions are examined systematically by Hall effect measurements. The crystal structure of the ZnO films obtained was characterized with XRD and FESEM, and the degree of caxis orientation, Tc(002), was calculated. The crystal morphology and Tc(002) values varied significantly with current density and deposition temperature. The Hall effect measurements revealed that the resistivity was increased dramatically with increasing current density and decreasing deposition temperature, overall ranging from 2.6 Ω cm (0.4 mA cm−2, 80 °C) to 20 MΩ cm (1.6 mA cm−2, 60 °C), and had a good correlation with the 1/Tc(002) value. The variation of the resistivity depended mainly on the carrier density (overall range, 2 × 1014−6 × 1018 cm−3) not on the mobility (overall range, 0.5− 12 cm2 V−1 s−1). The dependences of electrical properties and crystal structure on anneal temperature (150−500 °C) were also examined for the ZnO films electrodeposited at 0.4 mA cm−2 at 70 °C. Except for the formation of a number of holes 10−100 nm diameter in ZnO annealed above 350 °C, no significant changes in crystal structure were recognized. The resistivity increased with increasing annealing temperature due to the disappearance of native defects such as oxygen vacancy and interstitial zinc. Thus, we demonstrated here that the electrodeposition of ZnO offers the facile and wide-range control of the resistivity, which is expected to develop new applications as well as enhance devise performance.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by a grant-in-aid from the Ministry of Education, Culture, Sports, Science and Technology of Japan. 15930
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(35) Miyake, M.; Murase, K.; Hirato, T.; Awakura, Y. J. Electrochem. Soc. 2003, 150, C413. (36) Izaki, M.; Omi, T. J. Electrochem. Soc. 1996, 143, 53. (37) Goux, A.; Pauporté, T.; Chivot, J.; Lincot, D. Electrochim. Acta 2005, 50, 2239−2248. (38) Peulon, S.; Canava, B.; Lincot, D. Electrodeposition of zinc oxide f rom oxygenated aqueous solutions. Formation mechanism and film properties. The Electrochemical Society: New Jersey, 1998; Vol. 97, p 172. (39) Jones, M. I.; McColl, I. R.; Grant, D. M. Surf. Coat. Technol. 2000, 132, 143−151. (40) Korotkov, R.; Ricou, P.; Farran, A. Thin Solid Films 2006, 502, 79−87. (41) Bonasewicz, P.; Hirschwald, W.; Neumann, G. Phys. Status Solidi A 1986, 97, 593−599. (42) Minami, T. MRS Bull. 2000, 25, 38−44. (43) Look, D.; Hemsky, J.; Sizelove, J. Phys. Rev. Lett. 1999, 82, 2552−2555. (44) Lupan, O.; Pauporté, T.; Chow, L.; Viana, B.; Pellé, F.; Ono, L.; Roldan Cuenya, B.; Heinrich, H. Appl. Surf. Sci. 2010, 256, 1895− 1907.
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