Open Access
Add to BibSonomyAbstract
We demonstrate single CdTe microwire field-effect transistors (FETs) that are highly sensitive to ultraviolet (UV) light. Dense CdTe microwires were catalytically grown using a close-spaced sublimation system. Structural, morphological and transport properties in conjunction with the optoelectronic properties were systemically investigated. CdTe microwire FETs exhibited p-type behaviors with field-effect mobilities up to 1.1 × 10−3 cm2 V−1 s−1. Optoelectronic properties of our CdTe microwire FETs were studied under dark and UV-illumination conditions, where photoresponse was highly dependent on the back-gate bias conditions. Our CdTe microwire FET-based photodetectors are promising for high-performance micro-optoelectronic applications.
© 2014 Optical Society of America
1. Introduction
One-dimensional nano/microwires have attracted significant attention due to their potential applications in nano-/micro-optoelectronics, including solar cells, light-emitting diodes, biosensors, and photodetectors [1–4]. In particular, photodetectors are widely used in inter-chip optical communications and imaging. Specifically, highly sensitive nano/microwire-based photodetectors have been intensively investigated due to their advantageous characteristics that include high surface-area-to-volume ratio, superior crystallinity, high charge collection efficiency, and ease of integration with the advanced silicon microelectronics [5, 6]. For instance, Bae et al. demonstrated a Si nanowire-based photodetector with indium tin oxide films as top gate electrode [7]. In addition, Bugallo et al. reported p-i-n junction GaN nanowire-based UV photodetectors, which were grown using plasma-assisted molecular beam epitaxy [8].
CdTe is one of the II-VI compound semiconductor materials that have the optimal band-gap (~1.5 eV) for the solar spectrum with high absorption efficiency (> 5 × 105 cm−1). To date, CdTe thin film-based solar cells have been extensively studied and commercialized [9, 10]. In addition, excellent optical and optoelectronic properties of CdTe make CdTe nano/microwires promising for photodetector and photovoltaic applications. Xie et al. and Kum et al. reported photodetectors based on synthesized CdTe nanoribbons [11, 12]. Park et al. fabricated CdTe microwire-based UV photodetectors by precisely aligning CdTe microwires using dielectrophoretic force [13]. Recently, ITO/ZnO/CdS/CdTe nanowire-based solar cells with maximum efficiency of 2.49% were reported by using core-shell structures [14]. In this work, a large quantity of CdTe microwires were selectively grown using a simple Au-catalyzed close-spaced sublimation (CSS) method and formed into the back-gated FETs for UV light detection. Structural and morphological properties were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) techniques. The electrical and optoelectronic properties of single CdTe microwire-based photodetectors, such as current-voltage (I-V) characteristics, carrier mobility, and photoresponse, were also investigated.
2. Experimental details
Figure 1 shows the schematic diagram of the overall fabrication processes to demonstrate back-gated single CdTe microwire FETs. Firstly, an oval-shaped Au pattern (thickness of 5 nm) for catalyzed vapor-liquid-solid (VLS) growth of the microwires was deposited on sapphire substrates by using an electron-beam evaporator (Fig. 1(a)). Rapid thermal annealing (RTA) was conducted at 800 °C for 60 sec under N2 ambient to prepare the Au nanoparticles (Fig. 1(b)). Then, the prepared substrate was loaded in the CSS chamber, where the surface with Au nanoparticles faced the CdTe powder (99.999%, Alfa Aesar) with 3 mm spacing. High-purity CdTe powder was put into a SiC-coated graphite holder. CdTe microwires were selectively grown in the Au catalyst region, as shown in Fig. 1(c). The temperatures of the substrate and CdTe source were held at 520 and 580 °C, respectively, under N2 ambient conditions. The pressure in the CSS chamber was ~1 Torr during CdTe microwire growth. The details of the CSS growth of CdTe microwires were previously reported [15].
Fig. 1 Schematic of CdTe microwire-based FET fabrication process: (a) deposition of oval-shaped Au pattern, (b) formation of Au nanoparticles by RTA process, (c) growth of CdTe microwires using the CSS method, and (d) back-gated CdTe FET fabricated by conventional photolithography process.
Figure 1(d) shows the schematic of the back-gated FET based on single CdTe microwire. In our experiments, three FETs using a single CdTe microwire were prepared. After CdTe microwires were mechanically separated from the sapphire substrate, they were dispersed in an isopropyl alcohol solution, followed by dropping the dispersed solution on a SiO2 (300 nm) / p-type Si substrate. The back side of the SiO2 / p-Si substrate was pre-deposited by the Ti/Au (20 nm/80 nm) electrodes using an electron-beam evaporator. Standard photolithography processes were employed to define the source and drain electrodes on a single CdTe microwire, followed by the Ti/Au (40 nm/160 nm) metallizations by using electron-beam evaporation technique. RTA was performed at 400 °C for 60 sec under N2 ambient conditions to improve the contact resistance. SEM (S-4300, Hitachi) equipped with EDX was used to characterize the CdTe microwires. The crystallinity of the CdTe microwires was investigated by using XRD analysis (copper target, 2-theta mode, DMAX-2500, Rigaku). Electrical measurements were conducted by using a semiconductor parameter analyzer (4155C, Agilent) connected to the probestation. The photosensitivity of our FETs was characterized under the different gate bias conditions using a light source of UV lamp (UVItec LTD., 15 W) with the emission wavelength of 365 nm.
3. Results and discussion
Figure 2(a) and the inset of Fig. 2(b) show the as-grown CdTe microwires on a sapphire substrate. CdTe was selectively grown on an Au-catalyzed region, which indicates that the Au catalyst is essential for the growth of CdTe microwires. Tapered structure of our microwires is attributed to the catalyst diffusion and the sidewall growth during VLS growth [16]. The XRD results of the as-grown CdTe are shown in Fig. 2(b). Cubic zinc blende structure with the preferential (111) orientation was observed. Full width at half maximum of the preferential (111) peak is 0.345, which indicates high crystallinity of our CdTe. Figure 2(c) shows SEM image of the fabricated CdTe microwire-based photodetector, where the channel length and the diameter of our CdTe microwire at the center are approximately 21 μm and 6 μm, respectively. The composition of single CdTe microwire was investigated using EDX, as shown in Fig. 2(d). The atomic ratio of Cd and Te is close to 1, which is consistent with other studies [11, 12].
Fig. 2 (a) SEM image and (b) XRD results of as-grown CdTe microwires, (inset) SEM image of CdTe microwires, (c) SEM image of single CdTe microwire FET, and (d) EDX data from CdTe microwire shown in inset SEM image.
Drain-source current-voltage characteristics (IDS-VDS) at different gate bias (VGS) conditions were obtained (Fig. 3(a)).Non-Ohmic behaviors were observed between the annealed Ti/Au electrodes and the CdTe microwire, which may originate from high electron affinity of CdTe (~4.5 eV). Formation of good Ohmic contacts to p-CdTe has been an issue in CdTe solar cells [17]. Figures 3(a) and 3(b) show that IDS increases (decreases) when VGS becomes more negative (positive), which indicates that our CdTe microwires are p-type due to Cd vacancies [18]. The field-effect mobility (μfe) can be estimated using the following equation,
where gm, L, W, and C are the transconductance, channel length, channel width and oxide capacitance, respectively [19]. C was calculated using the infinite plate model for a cylinder. The highest μfe in our experiments was 1.1 × 10−3 cm2 V−1 s−1, which is lower than the hole mobility of CdTe thin films [20]. This can be explained by surface scattering, and trapping at grain boundaries because low field-effect mobility has been reported in nano/microwires [12, 21, 22].Fig. 3 (a) IDS-VDS at VGS ranging from −30 V to + 30 V and (b) IDS–VGS of CdTe microwire FET at VDS = + 5 V.; (inset) optical microscope image of CdTe microwire FET.
Figure 4 represents the current-time characteristics of the single CdTe microwire FET at VDS = + 20 V and VGS = ± 20 V, acquired by alternatingly switching the UV light source on (60 sec) and off (60 sec). Photogenerated carriers increased the currents when the CdTe microwire was exposed to UV light. Our CdTe microwire-based photodetector shows good reproducibility with fast response and recovery. Figures 4(a) and 4(b) show the time-dependent current change with different gate voltages. The average response times, defined as the time required to reach from the baseline to 90% of the saturation current, were ~6.2 sec and ~5.8 sec for VGS = + 20 V and −20 V, respectively. Slower average decay times, ~13.3 sec and ~8.3 sec for VGS = + 20 V and −20 V, were also obtained. It has been generally known that there is a trade-off between the response/recovery speed and the response gain. Our results of the response and decay times are comparable to those of CdTe nanoribbon-based photodetectors [12, 13]. Photoresponse (R) under alternating conditions of dark and UV illumination was calculated using the following equation,
where IDS(t) is the drain-source current at a time of t sec. IDS(0) is the averaged baseline drain-source current under dark condition. The photoresponse at VGS = + 20 V is two-fold of magnitude higher than that at VGS = −20 V, which can be explained by the channel modulated by the back-gate. The averaged dark currents were ~0.3 nA at VGS = + 20 V and ~1.04 nA at VGS = −20 V, respectively (Figs. 4(c) and 4(d)). High photoresponse with rapid response and recovery as well as good reproducibility of our CdTe microwire FETs make them a promising candidate for UV photodetectors.Fig. 4 Time-resolved photocurrents of a single CdTe microwire FET at (a) VGS = + 20 V and (b) VGS = −20 V. Time-resolved photoresponse (R) of a single CdTe microwire FET at (c) VGS = + 20 V and (d) VGS = −20 V.
4. Conclusion
CSS-grown single CdTe microwires were used to fabricate photosensitive FETs. High quality CdTe microwires, which were confirmed by XRD and SEM/EDX analysis, exhibited p-type behaviors with relatively low field-effect mobilities (reaching 1.1 × 10−3 cm2 V−1 s−1). Fast response/recovery characteristics to UV light were obtained from our CdTe microwire FETs. The photoresponse under the positive back-gate bias (VGS = + 20 V) condition was two-fold of magnitude higher than that obtained under the negative back-gate bias (VGS = −20 V) condition due to the channel modulation. Our single CdTe microwire FETs can be a strong candidate for UV photodetectors.
Acknowledgments
This research was supported by a Korea University Grant, Basic Science Research Program (2012R1A1A2042761) and Radiation Technology R&D program (2013M2A2A6043608) through the National Research Foundation of Korea (NRF) funded by the Korea Government Ministry of Science, ICT & Future Planning.
References and links
1. M. Law, L. E. Greene, J. C. Johnson, R. Saykally, and P. Yang, “Nanowire dye-sensitized solar cells,” Nat. Mater. 4(6), 455–459 (2005). [CrossRef] [PubMed]
2. H. Park, B.-J. Kim, and J. Kim, “Electroluminescence from InGaN/GaN multi-quantum-wells nanorods light-emitting diodes positioned by non-uniform electric fields,” Opt. Express 20(23), 25249–25254 (2012). [CrossRef] [PubMed]
3. M.-W. Shao, Y.-Y. Shan, N.-B. Wong, and S.-T. Lee, “Silicon nanowire sensors for bioanalytical applications: Glucose and hydrogen peroxide detection,” Adv. Funct. Mater. 15(9), 1478–1482 (2005). [CrossRef]
4. Q. Yang, X. Guo, W. Wang, Y. Zhang, S. Xu, D. H. Lien, and Z. L. Wang, “Enhancing sensitivity of a single ZnO micro-/nanowire photodetector by piezo-phototronic effect,” ACS Nano 4(10), 6285–6291 (2010). [CrossRef] [PubMed]
5. N. S. Ramgir, Y. Yang, and M. Zacharias, “Nanowire-based sensors,” Small 6(16), 1705–1722 (2010). [CrossRef] [PubMed]
6. E. C. Garnett, M. L. Brongersma, Y. Cui, and M. D. McGehee, “Nanowire solar cells,” Annu. Rev. Mater. Res. 41(1), 269–295 (2011). [CrossRef]
7. J. Bae, H. Kim, X.-M. Zhang, C. H. Dang, Y. Zhang, Y. J. Choi, A. Nurmikko, and Z. L. Wang, “Si nanowire metal-insulator-semiconductor photodetectors as efficient light harvesters,” Nanotechnology 21(9), 095502 (2010). [CrossRef] [PubMed]
8. A. Bugallo, M. Tchernycheva, G. Jacopin, L. Rigutti, F. H. Julien, S.-T. Chou, Y.-T. Lin, P.-H. Tseng, and L.-W. Tu, “Visible-blind photodetector based on p-i-n junction GaN nanowire ensembles,” Nanotechnology 21(31), 315201 (2010). [CrossRef] [PubMed]
9. P. Sinha, “Life cycle materials and water management for CdTe photovoltaics,” Sol. Energy Mater. Sol. Cells 119, 271–275 (2013). [CrossRef]
10. M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, “Solar cell efficiency tables (version 43),” Prog. Photovolt. Res. Appl. 22(1), 1–9 (2014). [CrossRef]
11. X. Xie, S.-Y. Kwok, Z. Lu, Y. Liu, Y. Cao, L. Luo, J. A. Zapien, I. Bello, C.-S. Lee, S.-T. Lee, and W. Zhang, “Visible-NIR photodetectors based on CdTe nanoribbons,” Nanoscale 4(9), 2914–2919 (2012). [CrossRef] [PubMed]
12. M. C. Kum, H. Jung, N. Chartuprayoon, W. Chen, A. Mulchandani, and N. V. Myung, “Tuning electrical and optoelectronic properties of single cadmium telluride nanoribbon,” J. Phys. Chem. C 116(16), 9202–9208 (2012). [CrossRef]
13. H. Park, G. Yang, S. Chun, D. Kim, and J. Kim, “CdTe microwire-based ultraviolet photodetectors aligned by a non-uniform electric field,” Appl. Phys. Lett. 103(5), 051906 (2013). [CrossRef]
14. B. L. Williams, A. A. Taylor, B. G. Mendis, L. Phillips, L. Bowen, J. D. Major, and K. Durose, “Core-shell ITO/ZnO/CdS/CdTe nanowire solar cells,” Appl. Phys. Lett. 104(5), 053907 (2014). [CrossRef]
15. G. Yang, Y. Jung, S. Chun, D. Kim, and J. Kim, “Catalytic growth of CdTe nanowires by closed space sublimation method,” Thin Solid Films 546, 375–378 (2013). [CrossRef]
16. K. Nagashima, T. Yanagida, K. Oka, H. Tanaka, and T. Kawai, “Mechanism and control of sidewall growth and catalyst diffusion on oxide nanowire vapor-liquid-solid growth,” Appl. Phys. Lett. 93(15), 153103 (2008). [CrossRef]
17. K. Durose, P. R. Edwards, and D. P. Halliday, “Materials aspects of CdTe/CdS solar cells,” J. Cryst. Growth 197(3), 733–742 (1999). [CrossRef]
18. A. McEvoy, T. Markvart, and L. Castañer, Solar Cells: Materials, Manufacture and Operation, in CdTe Thin-Film PV Modules 2nd ed. (Elsevier, 2013).
19. D. K. Schroder, Semiconductor Material and Device Characterization 2nd ed. (Wiley, 1998).
20. M. Takahashi, K. Uosaki, H. Kita, and S. Yamaguchi, “Resistivity, carrier concentration, and carrier mobility of electrochemically deposited CdTe films,” J. Appl. Phys. 60(6), 2046–2049 (1986). [CrossRef]
21. E. Ramayya, D. Vasileska, S. M. Goodnick, and I. Knezevic, “Electron transport in Si nanowire,” J. Phys. Conf. Ser. 38, 126–129 (2006). [CrossRef]
22. A. K. Buin, A. Verma, A. Svizhenko, and M. P. Anantram, “Significant enhancement of hole mobility in [110] silicon nanowires compared to electrons and bulk silicon,” Nano Lett. 8(2), 760–765 (2008). [CrossRef] [PubMed]
