Single-crystalline ZnTe nanowires were prepared by a simple vapor transport and deposition method. Photodetectors of individual ZnTe nanowires were fabricated to study photoconductivity of the nanowires. It was observed the nanowire photodetectors show the highest visible-light photoconductive gains among all reported photodetectors based on 1D nanostructure semiconductors, including CdS, CdSe, ZnSe, etc. The high photosensitivity and relatively fast response speed are attributable to the high crystal quality of the ZnTe nanowires. These results reveal that such single-crystalline ZnTe nanowires are excellent candidates for optoelectronic applications.
© 2011 OSA
One-dimensional (1D) nanostructures have been studied extensively for their novel properties and potential applications in nanoscaled devices [1,2], such as ultrasensitive nanosensors for detecting a wide range of chemicals, gases, and biomedical species [3–6]. Photodetector is another important application of these 1D nanostructures [7–10].
Zinc Telluride (ZnTe), an important II-VI semiconductor with a wide and direct band gap of ~2.26 eV, has been considered as a prospective material for optoelectronic devices and green light-emitting-diodes (LEDs) [11–14]. As compared to Si and GaAs, ZnTe is more sensitive to green/UV-light. While there has been some research on the synthesis of 1D ZnTe nanostructures [15–17], it is noted that so far few works have been carried out to study their photoelectric properties [18,19]. The reported literature on 1D ZnTe nanostructures only provided limit information under white light or green LED illumination. For the potential importance of this material, it is desirable to provide a more comprehensive picture on the photoelectrical performance of its 1D nanowires, including operation stability, reproducibility, on-off ratio, response time etc.
In this work, ZnTe nanowires were synthesized by a hydrogen-assisted vapor transport and deposition method . ZnTe nanowires can be prepared in a large scale by this simple and low-cost method without using the complex and high-cost metal-organic chemical vapor deposition (MOCVD) method . Individual ZnTe-nanowire photodectors were then fabricated and characterized. The detectors showed good potential for application as green/UV-light-sensitive photodetector and optoelectronic switch. In particular, the devices revealed high photoconductive gains and a relatively fast time response. The ZnTe nanowire photodetector showed the best sensitivity at a wavelength of 500 nm in the visible range of 400-700 nm, and performances of the present ZnTe nanowire detectors surpass those of the reported photodetectors based on 1D nanomaterials [10,18,19].
2. Experimental setup
The ZnTe nanowires were synthesized in a quartz tube placed in a three zone horizontal tube furnace. Si substrates were sequentially cleaned with acetone, and deionized water. A thin layer (5 nm) of gold was deposited onto the cleaned Si substrates by sputtering. ZnTe (99.99%) powder was placed at the center of the quartz tube. Au-coated silicon substrate was placed at about 15-20 cm downstream from the ZnTe powder. After the quartz tube was pumped down to a pressure of 2 × 10−2 mbar, high purity argon pre-mixed with 5% hydrogen was fed at a rate of 50 sccm (standard cubic centimeters per minute) into the quartz tube. Temperature of the middle section of the furnace was then ramped up at a rate of 25 °C/minute to 860 °C and kept at this temperature for 1-2 hours. Temperature of the Si substrate in the downstream was kept at about 600 °C. The pressure inside the quartz tube was maintained at 100 mbar throughout the whole heating process. The system was then cooled down naturally to room temperature under the same gas flow and pressure. Deposits were collected from the Au-coated silicon substrate.
The as-prepared samples were investigated with a scanning electron microscope (SEM, Philips XL 30 FEG) attached with an energy-dispersive X-ray spectrometer (EDX). In addition, the as-prepared products were characterized with an X-ray diffractometer (XRD, Siemens D-500) and transmission electron microscopes (TEM, Philips CM20, or CM200 FEG, operated at 200 kV). Room temperature photoluminescence (PL) and Raman measurements were performed in a Renishaw 2000 spectrometer. X-ray photoemission spectroscopy (XPS) measurements were carried out using a monochromatic Al Kα source (1486.6 eV) to determine composition of the deposition product. The energy resolution of the analyzer was 0.36 eV for the XPS measurement. For optoelectrical characterization, ZnTe nanowires were dispersed in alcohol and spread onto a SiO2 (500 nm)/p+-Si wafer. Photolithography and successive Ti (2 nm)/Au (60 nm) deposition were performed to pattern electrodes onto individual ZnTe nanowires. Two-probe method was applied to measure the I-V characteristics of the nanowire device. An illumination system combining a Xenon lamp (450 W) and a monochromator was employed to provide monochromatic light, which was focused and guided onto the nanowires perpendicularly. A mechanical chopper was employed to turn the illumination on and off. All measurements were performed in air and at room temperature.
3. Results and discussion
Figure 1(a) depicts a typical XRD pattern of the deposition product. All diffraction peaks, can be indexed to a zinc-blende type ZnTe structure (Joint Committee for Powder Diffraction Studies, JCPDS, Card: No. 15-0746). No peaks of ZnO, TeO2, or other impurities can be detected. Figures 1(b)-(d) show SEM images of the as-synthesized nanostructures under different magnifications. Figures 1(c) and 1(d) clearly shows that the product consists of a high density of wire-like structures. A low-magnification SEM image in Fig. 1(b) reveals that the nanowires are several to tens of micrometers in length. The high-magnification SEM images in Figs. 1(c) and 1(d) show that the nanowires have diameters of about 60-400 nm.
Figure 2(a) shows a TEM image of a typical nanowire. Selected area electron diffraction (SAED; inset in Fig. 2(a)) and high-resolution TEM (HRTEM, Fig. 2(b)) show that the nanowires are single-crystals of a cubic structure with their length along the  direction. The marked interplanar d-spacings of ca.0.35 nm correspond to the (111) lattice planes of a zinc-blende ZnTe. An EDX spectrum (Fig. 2(c)) of the wires shows clear peaks from Zn and Te. The Cu signal originates from the TEM copper grid. Figure 2(d) depicts a Raman spectrum of the ZnTe nanowires. The two Raman peaks centered at 176 and 206 cm−1 are observed, which can be respectively attributed to the transverse optic (TO) and longitudinal optic (LO) phonon modes of ZnTe . The Raman peaks confirm that ZnTe nanostructures possess high crystal quality and are chemically pure, which is consistent with the XRD results (Fig. 1(a)). The sample was further characterized with XPS (Figs. 2e and 2f). The binding energy of the 1021.81 eV peak matches well to that of Zn 2p3. In Fig. 2(f), the dominant peaks are assigned to Te 3d with binding energies of 573.9 eV and 584.2 eV. Additional peaks are found at binding energy at 577.3 eV, which is caused by chemical shifts for both spin orbit couplings of the 3d electrons . This is consistent with the reported literature results and further confirms that the nanostructures consist of pure ZnTe.
Figures 3(a) and 3(b) respectively show a schematic diagram of the device configuration for the photocurrent measurement of a single nanowire device and a representative image of a nanowire device. Ti/Au (2 nm/60 nm) interdigitated electrodes with ~3 μm separation were deposited on the nanowires dispersed on a SiO2/Si substrate. The uncovered part of the nanowire was exposed to the light. The measurement was performed using a two-probe method under ambient conditions with a monochromatic illumination onto the nanowire. The nanowire in Fig. 3(c) and 3(d) has a typical length and width of ~88 μm and ~330 nm, respectively. The comparative I-V characteristics of a ZnTe-nanowire photodetector, which was illustrated with light of 500 nm and under dark conditions, are shown in Fig. 3(e). It shows that the I-V curves are nearly linear with a small contact barrier. Comparing with the dark current, the photocurrent increases by ~100% when the device is illuminated with light of an energy above the threshold excitation energy, Eg, (~2.26 eV, ~548 nm), such as 500-nm visible light (Fig. 3(e)). We also measured room temperature photoluminescence (PL) spectrum of the nanowires, as shown in Fig. 3(f). At room temperature, a strong emission peaked at ~546 nm dominates the PL spectrum. According to the peak position, it is the near band-edge emission of ZnTe. The spectral response of the ZnTe nanowire is also depicted in Fig. 3(f). It can be seen that the sensitivity of the ZnTe device is rather low for the wavelength longer than 600 nm. When the wavelength is shorter than 550 nm, the sensitivity reaches a maximum at ~500 nm. The response spectrum is directly related to the energy band structure of ZnTe nanowires, and the spectral character of the joint density of states (JDOS) is reflected by the photocurrent measurement. Thus it can be concluded that the enhancement of the photoconductive sensitivity is due to the electron-hole pairs excited by the incident light with energy larger than the band gap; i.e., only photon with enough energy is able to induce a significant increase in conductance. Photon with a smaller energy does not have enough energy to excite electrons from the valence band to the conduction band and thus contributes little to the photocurrent. The slight increase of photosensitivity in the long wavelength side is possibly due to transition of carriers from defect states in the band gap to the conduction band . The drop of sensitivity on the shorter wavelength side, which was also observed in the CdS nanowire devices, is attributed to the enhanced absorption of high-energy photons at or near the surface region of the semiconductor . The electron-hole pairs generated near the suface region typically have a lifetime shorter than those in the bulk; thus they contribute less to the photoconductance . The sensitivity of the ZnTe nanowires in the illumination range of green light to ultraviolet (UV) light indicates that the developed photodetector is not only valuable under UV-light illumination but is also applicable for green-light-sensitive photodetectors (below 548 nm).
Figure 4(a) shows I-V curves of the ZnTe nanowire device measured in the dark and under different illumination intensities at an excitation wavelength of 500 nm. It can be seen that the ZnTe nanowire was highly sensitive to the 500 nm green light. The photocurrents in log scale at a bias voltage of 10 V under different light intensities are given in Fig. 4(b). The photocurrent can be fitted with a simple power law: I = APθ, where A is a constant for a certain wavelength, and the exponent θ determines the response of photocurrent to light intensity . Fitting the equation to the experimental data gives the following relationship: I~0.86P 0.23. Fractional power dependence is believed to be related to carrier traps in the nanowires. Because ZnTe is a p-type semiconductor, as previously demonstrated be a field-effect transistor made with a ZnTe nanowire , we assume that the carrier traps are mainly distributed near the HOMO level. As the light intensity increase, the quasi-Fermi level will shift towards the HOMO and an increasing number of traps are converted into recombination centers. Usually, θ ranges between 0.5 and 1, as observed in other II-VI semiconductors, such as CdS and ZnO nanowires/-ribbons [9,24]. The small θ for the ZnTe nanowire may imply the existence of abundant trap states in the nanowire.
Figure 5(a) illustrates the current response of a ZnTe-nanowire photodetector upon 500-nm-light illumination measured for the light-on and light-off conditions at a 10 V bias voltage. The enlarged portions of a 198-216 s range and a 236-254 s range correspond to light-on to light-off and light-off to light-on transitions, respectively, and are shown in Figs. 5(b) and 5 (c). These indicate that both rise and decay times (~1.3 s) are relatively fast. The present photodetector also shows a very good photocurrent reproducibility and stability. As one of the key factors for sensor performance, a slow time response usually limits the scope of application. The time responses both on rise and decay acquired from most of the reported 1D-structure-based photodetectors typically ranged from seconds to several tens of minutes, or even several hours, which possibly resulted from different experimental conditions and device geometries used [25–28]. It has been reported that the response-time of individual metal-oxide nanowire photodetector can be enhanced by various ways, such as improving the electron mobility of the nanomaterial, increasing the width of the photoactive area, or decrease the distance between electrical contacts . The relatively fast response-time in the present ZnTe nanowire photodetector is probably due to its high chemical purity, and good crystallinity.
The spectral responsivity (R λ) measurements the input-output gain of a detector system, which is defined as R λ = ΔI/P; where P is the light power irradiated on an individual nanowire and ΔI = I photocurrent – I dark current. External quantum efficiency (EQE) or quantum efficiency (QE) is a quantity defined for a photosensitive device such as a photographic film or a charge-coupled device (CCD). It relates to the percentage of photons hitting a photoreactive surface that will produce the electron-hole pairs. Theoretically, R λ can be expressed in terms of the photon energy E ph (E ph = hc/λ), the photoconductive gain g, and the quantum efficiency η = ηi(1-R ref)(1-e-αd) (where η i is the intrinsic quantum efficiency, R ref the reflectivity, α the absorption coefficient, and d the nanowire diameter), R λ = qηg/E ph, where q is the electron charge. Assuming the intrinsic quantum efficiency is unity, the photoconducitive gain (g) at 10 V is calculated to be 15396 . The R λ of the present ZnTe nanowire biased at 10 V is ~2165 AW−1 for a photoexcitation at 500 nm. The quantum efficicency is around 34.9%. The values of photoconductive gain (g) are much larger than the reported values of the ZnTe nanowire (8640, Ref. 19) synthesized by metal-organic chemical vapor deposition (MOCVD) under green LED illumination. It is reported that the CdS single nanoribbon exhibited a gain of ~100 and for the CdSe nanowires and ZnSe nanowires, g<1000 and g = ~500, repectively [9,26,31]. As summarized in Table 1 , the present large photoconductive gain value is not only the highest ones among all ZnTe-nanostructure photodetectors, but also prevail over that of other reported visible-light-sensitive photodectors based on any 1D semiconductor nanostructure (such as CdS, CdSe, ZnSe etc.) [9,19,26,31].
Trapping at surface states drastically affects the transport and photoconduction properties of nanowires due to the high surface-to-volume ratio. We believe that O2 is primarily responsible for the current increase in air. Upon illumination with photon energy larger than the semiconductor band gap (E g), electron-hole pairs are photogenerated and electrons are readily trapped at the surface, leaving behind unpaired holes, which increase the conductivity under an applied electric field. Understandably, the absorbed gas molecules on the nanowire surface can strongly affect the transport properties by inducing charge transfer between the nanowire and the gas molecules. It has been previously shown the similar trapping mechanism in governing the photoconduction in nanowires . In our cases, O2 adsorption may increase the hole concentration in the nanowire by capturing electrons and forming O- ions on the nanowire surface. The unpaired holes are either collected at the electrode or recombine with electrons. Because ZnTe nanowires are p-type , the enhanced conductance of the nanowire can be attributed to the formation of a hole accumulation layer near the nanowire surface. This trapping mechanism through oxygen adsorption in nanowire demonstrates the high density of trap states usually found in nanowire because of dangling bonds at the surface and thus enhances the nanowire photoconductivity. The explanation for such high conductive gain in our case is that photogenerated electrons are rapidly trapped while holes remain mobile for an extended period until they finally are either collected or recombine with trapped electrons.
The conductive gain (g) can be also expressed as g = τ/t tran, where τ is the carrier lifetime and t tran the transit time between the electrode. The spectral responsivity (R λ) depends on the applied electric field, which primarily depends on the field through the carrier transit time, t tran ~l/μE, where l is the intercontact separation, E the applied electric field, and μ the carrier mobility. Using this expression for the transit time, R λ = cE, with the constant c = qητμ/lE ph. In our experiment, c is deduced to be ~6.49 × 10−2 cmV−2. Assuming the carrier mobility lies in the range 1 × 10−3 to 4 × 10−3 cm2V−1s−1 at room temperature , the carrier lifetime is calculated to be τ ~0.138 to 0.553 s. Li et al. have cited carrier mobility of bulk ZnTe crystals for the calculations in ZnTe nanowires . However, based on our previous results, the carrier mobility of ZnTe nanowires is much smaller than that of bulk ZnTe crystals . Therefore, in our case, the value of carrier lifetime is calculated to be much larger compared with the results in the ref 19. According to the expression g = τ/t tran, the corresponding transit time is estimated to be t tran ~8.9 × 10−6 to 3.6 × 10−5 s. Equation g = τ/t tran shows that a larger lifetime τ (lower response speed) would lead to a larger gain; thus further improvement of the response speed would adversely reduce the gain. Consequently, a unique advantage of ZnTe nanowire-based photodetectors is that both large conductive gain and acceptable response speed can be achieved through reduction of electrode separation or smaller t tran.
In summary, we developed a simple to prepare single-crystalline ZnTe nanowires in large scale via the vapor transport and deposition technique. Photoconductive properties of individual ZnTe nanowires in green/UV-light-sensitive photodetectors have been investigated in detail. The ZnTe nanowire photodetector shows a best sensitivity at wavelength of 500 nm and a photoconductive gain of 15396 which surpasses those of reported photodetectors based on 1D nanomaterials for visible wavelength. The ZnTe nanowire photodetector also exhibit a relatively fast response speed of 1.3 s. The unique properties are attributed to the high crystal quality of the fabricated ZnTe nanowires.
This work was financially supported by the Research Grants Council of HKSAR (No. CityU 101910).
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