Electronic Materials Letters, Vol. 9, No. 3 (2013), pp. 273-277 DOI: 10.1007/s13391-013-2176-5

Control of Oxygen Vacancy Concentration in ZnO Nanowires Containing Sulfur as a Reducing Agent Keumyoung Seo, Misook Suh, and Sanghyun Ju* Department of Physics, Kyonggi University, Suwon, Gyeonggi-Do, 443-760, Korea (received date: 28 September 2012 / accepted date: 21 January 2013 / published date: 10 May 2013) Light illumination influences the electrical characteristics and stability of oxide nanowire transistors. In this study, transistor characteristics of oxygen-vacancy-rich ZnO nanowires under illumination were investigated. In order to control the oxygen vacancies on the surface of ZnO, sulfur was used as a reducing agent during nanowire growth. Unlike pure nanowires, ZnO nanowires with sulfur as a reducing agent exhibited a dramatically enhanced green emission peak at around 520 nm in the photoluminescence spectrum, which is primarily generated under oxygen-deficient ambient conditions. The threshold voltage of a nanowire transistor using ZnO with sulfur showed no significant change under illumination. In contrast, the threshold voltage of pure ZnO shifted significantly in the negative direction under illumination. This phenomenon may arise from the fact that light illumination on the channel region of ZnO reduced with sulfur cannot generate additional oxygen vacancies on the nanowire surface because oxygen vacancies were created almost to the saturation point during nanowire growth. Keywords: oxygen vacancy, ZnO, nanowire, sulfur, reducing agent

1. INTRODUCTION The electrical, chemical, and optical characteristics of nanowires with one-dimensional nanoscale structures can be controlled by controlling their growth condition or through surface treatment or doping.[1-3] Studies have been conducted on the growth of various materials in the form of nanowires.[4-7] In particular, some studies have attempted to apply n-type semiconducting ZnO nanowires to electrical, optical, and energy devices and sensors.[8-11] Some researchers have identified the change in structural and optical characteristics of ZnO nanowires using varied growth conditions.[12,13] Others have attempted to control the electrical and optical characteristics of ZnO nanowires using doping materials such as Sn, Ni, Ga, and In.[14-17] Furthermore, studies have been conducted on the change in crystal structure and optical characteristics by sulfur doping in ZnO nanowires.[18-21] In these studies, the photoluminescence (PL) spectrum showed that near-band-edge emission produced a slight blue shift and that green emission was relatively enhanced by sulfur doping of ZnO nanowires. However, there has been no study on the change in electrical characteristics of sulfur-doped ZnO nanowires. In this study, sulfur was used as a reducing agent to understand the origin of the difference in optical and electrical characteristics of ZnO nanowires with sulfur and those of *Corresponding author: [email protected] ©KIM and Springer

pure nanowires. Moreover, the differences in the electrical characteristics of the two materials were investigated on the basis of their different PL characteristics. In particular, how the increase in oxygen vacancy concentration on the ZnO surface due to sulfur doping influenced the electrical characteristics of nanowire transistors under light illumination was investigated.

2. EXPERIMENTAL PROCEDURE Pure ZnO nanowires and ZnO nanowires with sulfur as a reducing agent were grown by chemical vapor deposition on a SiO2/Si substrate coated with 30-nm gold particles as a catalyst. For the growth of ZnO nanowires with sulfur, mixed ZnO and ZnS powders in a weight ratio of 5 : 1 were used. Pure ZnO (99.999%, Sigma-Aldrich) and ZnO + ZnS (99.99%, Sigma-Aldrich) powders were placed 16 cm apart from the substrates in different horizontal quartz tube chambers. The carrier gas (Ar + 5% oxygen) was passed through the tube at a flow rate of 100 sccm (standard cubic centimeters per minute). The temperature of the powder source zone was set to 1050°C, and that of the substrate zone was set to 700°C. Bottom-gate nanowire transistors were fabricated with pure ZnO nanowires and ZnO nanowires with sulfur as semiconducting channel materials. A 30-nm-thick Al2O3 layer, used as a gate dielectric, was deposited on ITO gate electrodes by atomic layer deposition. Subsequently, the pure ZnO nanowires and ZnO nanowires with sulfur were

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dispersed on two different substrates. Al electrodes (100 nm) were DC-sputter-deposited as source-drain electrodes. For these two nanowire transistors, the current-voltage (I-V) characteristics were measured by a semiconductor device analyzer (Agilent B1500A).

3. RESULTS AND DISCUSSION Figure 1(a) shows a field-emission scanning electron microscopy (FE-SEM) image of the pure ZnO nanowires and the ZnO nanowires with sulfur as a reducing agent. The FE-SEM image showed that there were no significant differences between these two nanowire materials. As shown in the figure, the pure ZnO nanowire has a diameter of ~30 nm and a length of ~10 µm and the ZnO nanowire with sulfur has a diameter of ~30 nm and a length of ~5 µm. Energy dispersive spectroscopy (EDS) was conducted together with FE-SEM and exhibited no sulfur peak for the ZnO nanowire with sulfur (The EDS data has not been specified in this paper). Figure 1(b) shows the x-ray diffraction (XRD) pattern of the pure ZnO and the ZnO nanowires with sulfur. The XRD result showed that both nanowires had the same

Fig. 1. FE-SEM images of (a) pure ZnO nanowires (left) and ZnO nanowires with sulfur as a reducing agent (right). The scale bar = 1 µm. (b) XRD analysis of pure ZnO nanowires and ZnO nanowires with sulfur as a reducing agent.

hexagonal structure (lattice constants a = b = 3.241 Å and c = 5.187 Å, JCPDS 79-0205) with the main diffraction peaks of (100), (002), (101), and (110). Traces of sulfur phases were not observed in the ZnO nanowires with sulfur. Previous XRD studies also found that ZnO nanowires with sulfur showed a hexagonal wurtzite structure without sulfurrelated or ZnS-related peaks.[19,20] In order to investigate whether sulfur was doped into the ZnO nanowires or not, x-ray photoelectron spectroscopy (XPS) was performed. The resolution and detection limit of the XPS system (Thermo, K-Alpha) is 0.5 eV and ~0.1 at. %, respectively. Figure 2 shows the XPS survey scan spectra of the ZnO nanowires with sulfur as a reducing agent. The peaks at 1021.58 eV and 1044.58 eV in the Zn 2p spectrum correspond to the doublet of Zn 2p3/2 and Zn 2p1/2, respectively (Fig. 2(a)). The atomic concentration between Zn and O in pure ZnO nanowire was 40.15 at. % and 37.26 at. % respectively. In the contrast, the atomic concentration between Zn and O in ZnO nanowire with sulfur as a reducing agent was 19.38 at. % and 9.01 at. % respectively. The result

Fig. 2. XPS spectra of ZnO nanowires with sulfur as a reducing agent. Fine spectra of (a) Zn 2p and (b) S 2p.

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showed that sulfur could derive the oxygen-vacancy-rich condition in ZnO nanowires. In order to confirm the existence of sulfur in ZnO, sulfur peaks were examined in the range of 158 - 175 eV. (Fig. 2(b)) However, the S 2p peak at 162 - 170 eV[22,23] was not observed. It was thus confirmed that sulfur was not doped into the ZnO nanowire but mainly acted as a reducing agent during nanowire synthesis, resulting in the generation of more oxygen vacancies. The generation of oxygen vacancies, which is related to the presence of abundant defects in ZnO nanowires, resulted from redox reactions due to the competition between Zn and sulfur for oxidation and from the formation of sulfur dioxide (SO2) gas during nanowire growth. The generation of oxygen vacancies using sulfur as a reducing agent was also confirmed from the PL characteristics. A He-Cd laser (Kimon, 1 K, Japan) with a wavelength of 325 nm and a power of 50 mW was utilized as an excitation source for PL measurement. Photoluminescence was dispersed

Fig. 3. Room temperature PL spectra of (a) pure ZnO nanowires and (b) ZnO nanowires with sulfur as a reducing agent.

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by a spectrometer (f = 0.5 m, Acton Research Co., Spectrograph 500i, USA) in conjunction with an intensified CCD (PIMAX3, Princeton Instrument Co., IRY1024, USA). Figure 3 shows the normalized PL spectra of the pure ZnO nanowires and the ZnO nanowires with sulfur. The PL spectrum of the pure ZnO shows two luminescence peaks: a sharp and dominant ultraviolet (UV) peak at 378 nm and a broad green emission peak around 505 nm (Fig. 3(a)). On the other hand, the ZnO nanowires with sulfur show a relatively weak UV emission at 378 nm and a strong and dominant broad green emission peak at 522 nm (Fig. 3(b)). Note that the UV emission peak corresponding to the near-band-edge transition originates from the recombination of free excitons through exciton-exciton collisions in direct band gap of ZnO.[24,25] The green emission peak around 500 nm corresponds to singly ionized oxygen vacancies in ZnO.[24-27] The ratio of the green peak intensity to the blue peak intensity was enhanced from 0.2 (pure ZnO nanowire) to 12.5 (ZnO nanowire with sulfur). As shown in the figure, the green emission intensity was dramatically enhanced after sulfur addition, which indicates the increase of oxygen vacancy concentration in ZnO nanowires. Next, the I-V characteristics of ZnO nanowires were analyzed to understand the effect of sulfur. Figure 4 shows

Fig. 4. Schematic diagram of a nanowire transistor under light emission on the channel region. The inset shows FE-SEM images of representative channel regions of nanowire transistors of pure ZnO and ZnO with sulfur as a reducing agent.

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Fig. 5. (a) Ids-Vgs characteristics of representative pure ZnO nanowire transistors under light illumination. Vd = 0.5 V. (b) Ids-Vgs characteristics of a representative ZnO nanowire transistors composed of ZnO with sulfur as a reducing agent under light illumination. Vd = 0.5 V.

the schematic diagram of a bottom-gate nanowire transistor under light illumination and the FE-SEM image of the nanowire channel region. Nanowire channel regions were illuminated by a light source (470 nm, 36,000 lux) and the change in the I-V characteristics were measured before, during and after light emission. Figure 5 shows the characteristics of the drain-current versus gate-source voltage (IdsVgs) of the nanowire transistors of pure ZnO and ZnO with sulfur as a reducing agent. Figure 5(a) shows the Ids-Vgs characteristics of the nanowire transistors of pure ZnO under light illumination. The Ids-Vgs characteristics of the pure ZnO nanowire transistor before illumination are shown as a black line in Fig. 5(a). The threshold voltage (Vth, Vgs at Ids = 1 nA) was ~4.58 V, the subthreshold slope (SS, the difference between Vgs at Ids = 10 nA and Vgs at Ids = 1 nA) was ~0.29 V/dec, and the on-current (Ion, Ids at Vgs = Vth + 3 V) was ~0.14 µA. The curve with solid circles denotes the measurement results for the nanowire transistor measured under light illumination.

The Ids-Vgs measurement result obtained under light illumination (indicated by a curve with solid circles) showed a Vth of ~2.24 V, exhibiting a considerable negative shift of ~2.34 V from the original value. Furthermore, the SS almost doubled to 0.57 V/dec and Ion slightly decreased to ~0.11 µA. However, the Ids-Vgs re-measurement results performed after turning off light illumination (shown as a curve with empty circles), yielded a Vth of 3.64 V. Thus, Vth was not completely restored to its original value of ~4.58 V immediately after turning off the light illumination. Further, 30 s after the light illumination was turned off, Vth was ~4.24 V, SS was ~0.26 V/dec, and Ion was 0.15 µA (not shown in the figure), which indicates that these parameters were nearly restored to their original levels. Figure 5(b) shows the results of the same experiments on nanowire transistors of ZnO with sulfur as a reducing agent. The initial state of the nanowire transistors using ZnO with sulfur is shown as a black line. In this state, the Vth, SS, and Ion were −3.16 V, ~0.34 V/dec, and ~0.49 µA, respectively. For Ids-Vgs curves under light illumination (shown as a curve with solid circles), Vth was −2.78 V, positively shifting slightly by 0.38 V; SS was 0.34 V/dec; and Ion was ~0.47 µA. After the light illumination was turned off (result shown as a curve with empty circles), Vth was −2.98 V, SS was 0.36 V/dec, and Ion was 0.48 µA, showing a recovering trend. From the results, it was found that the pure ZnO nanowire transistor was relatively sensitive to light emission, exhibiting some parameters change under light illumination. It was clearly observed that Vth negatively shifted by about 2.34 V, and SS changed from 0.29 to 0.57 V/dec. On the other hand, the nanowire transistor composed of ZnO with sulfur as a reducing agent was less sensitive to light illumination. Vth exhibited a slightly positive shift of about 0.38 V and SS remained as 0.34 V/dec. Thus, light illumination caused no significant change in the parameters in the nanowire transistor composed of ZnO with sulfur. A comprehensive explanation of the photostability of ZnO nanowire with sulfur as a reducing agent is not currently available but a plausible explanation is possible with other studies investigating the relation between the photostability and the nanowire characteristics. It is assumed that the pure ZnO nanowire channel exhibits a change in Vth, SS, and Ion when exposed to an external light source as light creates additional oxygen vacancies on the surface of nanowires.[28,29] With light illumination on an oxide nanowire channel, photogenerated electron-hole pairs are induced on the nanowire surface. Holes, created by the pairs separated from the surface, are captured as donor-like traps in the interface between the gate dielectric and the nanowire or react with oxygen ions in the nanowire.[30,31] The donor-like traps or oxygen vacancies created during this process induce a characteristic change in nanowire transistors. On the other hand, in the case of a ZnO nanowire with sulfur as a reducing agent, the sulfur oxidizes to form SO2 during

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nanowire growth and removes oxygen, thus creating more oxygen vacancies almost to the saturation point in ZnO. Therefore, the additional creation of oxygen vacancies is restricted under light illumination.

4. CONCLUSIONS In summary, oxygen-vacancy-rich ZnO nanowires were grown through chemical vapor deposition with mixed ZnO and ZnS powder and with sulfur as a reducing agent. XRD patterns of grown ZnO nanowires with sulfur as a reducing agent matched the hexagonal wurtzite ZnO crystal structure. XPS results showed that sulfur was not doped into the nanowire. Meanwhile, a relatively weak near-band-edge emission peak and enhanced green emission peak were observed in PL spectra at ~378 nm and ~520 nm, respectively. The green emission peak in ZnO nanowires is known to be generated from the recombination of electrons and holes trapped in singly ionized oxygen vacancies and found primarily under oxygen-deficient ambient conditions. It can be inferred that the generation of abundant oxygen vacancies on the ZnO nanowire surface resulted from the competition between ZnO and SO2 for oxidation with vaporized Zn and sulfide as source materials and from the creation of oxygendeficient ambient conditions during ZnO nanowire growth. With light illumination, nanowire transistors fabricated using ZnO nanowires with sulfur as a reducing agent exhibited more stable Vth characteristics as compared to nanowire transistors fabricated from pure ZnO nanowires. This result reveals that an enhanced emission intensity related to oxygen vacancies affects the I-V and optical characteristics of the nanowires.

ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A2A2A01013734, 2011-0023219 and 2012K001317).

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