J. Phys. Chem. C 2009, 113, 16951–16953
16951
CdTe Nanorod Arrays on ITO: From Microstructure to Photoelectrical Property Xina Wang,†,‡ Juan Wang,† Minjie Zhou,† Haojun Zhu,† Hao Wang,*,‡ Xiaodong Cui,§ Xudong Xiao,† and Quan Li*,† Department of Physics, The Chinese UniVersity of Hong Kong, Shatin, New Territory, Hong Kong, China, Faculty of Physics and Electronic Technology, Hubei UniVersity, Wuhan 430062, China, and Department of Physics, The UniVersity of Hong Kong, Hong Kong, China ReceiVed: June 15, 2009; ReVised Manuscript ReceiVed: August 5, 2009
CdTe nanorod arrays on ITO have been demonstrated using a catalyst-free thermal evaporation method. Despite the stacking faults observed in the nanorods, they show intense near band edge emission at ∼1.5 eV with a narrow full width at half-maximum of 79 meV and negligible deep-level emission during the room temperature photoluminescence measurement. The device based on the aligned array-on-ITO configuration demonstrates excellent photoresponse to the visible light, which is ascribed to the large absorption coefficient of the material and also suggests good electronic structure quality of the nanorods. Introduction Being a direct band gap semiconductor with high atomic number and electron density, CdTe has been widely used in applications of photovoltaics, sensors, and detectors.1-3 In particular, its high optical absorption coefficient and its band gap of ∼1.5 eV, matching the preferred range of the solar irradiation spectrum, make CdTe an excellent light harvester for solar energy applications.4 In recent years, it has been suggested that one-dimensional (1D) nanostructures can serve as functional building blocks for optoelectronic and photovoltaic nanodevices.5,6 Compared to their thin film and/or bulk counterparts, a number of nanostructures, such as ZnO nanowires, TiO2 nanotubes, and Si nanowires, are found to have a superior photovoltaic property due to their improved light absorption efficiency and carrier transport.7-9 Some research efforts have been devoted to the growth of CdTe nanowires. For example, it has been found that CdTe nanoparticles can spontaneously organize into nanowires and/ or nanoribbions.10,11 Nevertheless, while considering real device applications using CdTe nanostructures, their ordered assembly into array-on-conducting substrate is highly desired. In this regard, two major methodologies have been developed for aligned CdTe nanowire/nanorod growth to date. One is the restricted electrodeposition of CdTe nanowires in a porous anodized aluminum oxide (AAO) template,12-14 forming the array configuration. Although CdTe nanowires with high aspect ratios have been achieved by filling the AAO template at controlled electrochemical potential, they tend to grow into a polycrystalline structure for individual nanowires.12 Moreover, removal of the AAO template would lead to aggregation and collapse of the array configurations in most cases.14 The other method that can lead to the vertically aligned growth of CdTe nanowires is pulsed laser deposition (PLD).15,16 Using Bi2Te3 or Bi as the catalytic seeds together with further substrate modification, vertically aligned CdTe nanowire arrays have been demonstrated on sapphire substrates. Nevertheless, such a * To whom correspondence should be addressed. E-mail: liquan@ phy.cuhk.edu.hk (Q.L.);
[email protected] (H.W.). † The Chinese University of Hong Kong. ‡ Hubei University. § The University of Hong Kong.
method requires a small lattice mismatch between the substrate and CdTe to realize the lateral overgrowth suppression of the nanorod by selective area epitaxy,15,16 which limits the type of substrate material that can be used. To date, a report on highquality CdTe nanorod arrays on conductive substrate is not available. In this work, it is demonstrated that CdTe nanorod arrays can be achieved on ITO substrate via a simple thermal evaporation method in the absence of any catalyst. The microstructure, optical, and photoelectrical properties were carefully investigated. The device fabricated directly from the nanorod array-on-ITO demonstrates excellent photoresponse to visible light, suggesting their potential application in nanostructured solar energy conversion devices. Experimental Methods The samples were grown in a high-temperature vacuum tube furnace, whose setting can be found elsewhere.17 CdTe powder (5N) was loaded at the center of the alumina vacuum tube. ITO glass with a resistivity of ∼10 Ω/sq has been used as the substrate, which is placed in a one-close-end glass tube, being positioned at the downstream of the tube furnace with its open end oriented to the source material. The growth was carried out at 700 °C for 30 min under a pressure of ∼10-2 mbar. During the growth, the distance between the source material and the ITO substrate is held within the range 25-30 cm, the temperature of the ITO substrate is in the range 300-400 °C, and no carrier gas was used. The crystallinity, microstructure, and optical property of the samples were examined using fieldemission scanning electron microscopy (FE-SEM, FEI Quautum F400), X-ray diffraction (XRD, Ragiku RU300), transmission electron microscopy (TEM, Tecnai 20, FEG), and photoluminescence (PL) and absorption measurements at room temperature. A simple device with ITO/CdTe/Au structure was fabricated by evaporating 50 nm thick Au film on the as-grown sample for the photoresponse measurement using a precision semiconductor parameter analyzer system (Agilent 4156C). Results and Discussion Figure 1a is a SEM image of the as-grown sample on ITO substrate, showing high-density CdTe nanorod arrays vertically
10.1021/jp905577u CCC: $40.75 2009 American Chemical Society Published on Web 09/08/2009
16952
J. Phys. Chem. C, Vol. 113, No. 39, 2009
Wang et al.
Figure 3. (a) Absorption and (b) PL spectra of the CdTe nanorod arrays on ITO substrate.
Figure 1. (a) SEM image and (b) XRD pattern of CdTe nanorod arrays-on-ITO substrate fabricated by the thermal evaporation method. The inset in part a is an enlarged SEM image.
Figure 2. (a) Low-magnification TEM image showing the morphology of a single nanorod. (b and c) Typical HRTEM images taken from different regions of a single CdTe nanorod, with part b showing the dominant crystalline structure of the nanorod. Heavy stacking faults can be found in part c, with some region showing the hexagonal wurzite structure; the arrows in parts a, b, and c represent the nanorod growth direction.
aligned in a large area. These nanorods are highly faceted with irregular hexagonal shape and a mean diameter of around 300 nm (inset of Figure 1a). The XRD of such CdTe nanorod arrays was shown in Figure 1b, with all diffraction peaks being indexed to zinc-blende (ZB) CdTe (JCPDS file No. 65-880). The intense diffraction intensity of the {111} plane indicates the preferential growth direction of these nanorods, i.e., along its [111] crystalline direction. Nevertheless, one shall note that the {0002} diffraction in the hexagonal wurtzite phase overlaps with that of the {111} plane in the ZB structure, so that the existence of the wurtzite phase cannot be excluded. Figure 2a shows the low-magnification TEM image of a single CdTe nanorod. In the high-resolution image taken from the nanorod, we found the coexistence of the cubic (Figure 2b) and the hexagonal (Figure 2c) phase. The nanorod grows along a direction that is normal to the closest packed crystalline planes in both phases (note that the {111} plane in the ZB and the {0002} plane in the wurtzite structure are identical). These are consistent with the XRD results. On the other hand, stacking faults (SFs) are constantly found in the nanorods, possibly
explaining the coexistence of the ZB and the wurtzite phasesthe two of them can switch to each other by introducing one SF.18 Neither catalyst nor substrate surface modification is employed during the thermal evaporation growth of CdTe nanorod arrays. The one-dimensional growth of CdTe mainly results from the high supersaturation of the source material in the vapor phase via a self-catalyst process.19 Indeed, dense nanocrystals with sizes ranging from 40 to 100 nm were observed to form on the ITO surface at the initial growth stage, and the density of the nanocrystal nuclei determines that of the nanorods at a later stage. The formation of the SFs in the CdTe nanorods can be attributed to two factors. One is the low formation energy of both the SFs and lamellar twins.20 This is also the reason why SFs and lamellar twins are commonly observed in the growth of CdTe(111) films on Si(001) and GaAs(100) substrate using various techniques.18 The other factor may be the fast growth rate (as high as 4 µm/h) of the CdTe nanorods in the present case.21 The optical properties of the CdTe nanorod arrays were investigated by both PL and absorption. A dramatic increase in absorption can clearly be found at photon energies higher than 1.5 eV (Figure 3a), suggesting that the nanorod arrays have a strong absorption over the light wavelength shorter than 826 nm. Consistently, a strong near-band-edge (NBE) emission peak occurred at ∼1.52 eV with a full width at half-maximum (fwhm) of only 79 meV (Figure 3b). Both the absorption edge and PL peak position agree well with the reported energy band gap of CdTe.22 In addition, little signal of deep level emissions can be observed in the PL spectrum. Due to the high purity (5N) of the source material used in the present study, deep-level emissions (with the range 1.30-1.45 eV) due to extrinsic impurities are not likely to exist.23 On the other hand, despite the low vacuum condition obtained in the tube furnace, deep level emissions related to oxygen defects (such as OTe)24 are also absent. Together with intense and narrow NBE emission, it suggests a good luminescence property of the CdTe nanorod arrays; i.e., the observed planar defects (SFs in the present case) do not contribute to deep level generation in the material band gap that is observable in the PL measurements. This is consistent with the investigations on the electronic structures of a number of other materials of similar structures, such as wurtzite GaN, AlN, and zinc-blende SiC, revealing that the SFs will not produce localized states in the band gap of these materials.25,26 The sample configuration of an aligned array on ITO (readily serving as a conducting electrode) makes it straightforward for device fabrication. Au film, which was extensively verified to form a good contact with p-CdTe due to its high work function,27 with a thickness of about 50 nm was evaporated onto an unmasked region of the CdTe nanorod arrays with an area of about 0.8 mm2 to form a small ITO/CdTe NRs/Au device. On the basis of such a simple device, the photoresponses of the CdTe nanorod arrays were studied by I-V measurements carried
CdTe Nanorod Arrays on ITO
J. Phys. Chem. C, Vol. 113, No. 39, 2009 16953 Although SFs are constantly observed in the CdTe nanorods, they do not introduce deep levels in the material band gap and thus have little adverse effect on the luminescence property of the sample. The excellent photoresponse of the nanorods and its array-on-ITO configuration make them a promising candidate for applications in solar energy conversion devices.
Figure 4. Typical I-V curves of CdTe nanorod arrays on ITO substrate measured in the dark and under white light illumination using a metal halide lamp as the source. The upper inset shows the high-magnification I-V curve measured in the dark, and the lower inset is the photocurrent response to ON-OFF cycles of visible illumination at a constant bias of 1.5 V.
out under both dark and white light irradiations. Figure 4 shows the I-V characteristics of the device. In the dark, the I-V curve (enlarged plot can be found in the upper inset) shows a typical rectifying behavior with both the forward and reverse current falling in the nA range. In unintentionally doped CdTe (such as the present case), Cd vacancies serve as the main source of acceptors, contributing to the p-type semiconductor characteristic.28 Considering the very small barrier between the Au electrode and the CdTe, the rectifying characteristic then results from the p-n junction between the n-type ITO and the p-type CdTe nanorod arrays. A huge surge in the current was observed when the device was illuminated by white lightsan over 10 times increase in the current magnitude when compared to those in the dark. Such an increase is attributed to the generation of charge carriers by photon excitation. Electrons from the conduction band of CdTe can readily travel to that of ITO, an energetically favorable process, as the conductive band minimum of CdTe is located above that of ITO.29,30 From the photocurrent response to ON-OFF cycles of visible illumination (the lower inset), the appearance of the photocurrent was prompt, and the photocurrent generation was steady during the luminescence on and off cycles. Despite the SFs found in the CdTe nanorods, they do not seem to produce deep defect levels in the band gap, contributing to the excellent optical property of these CdTe nanorod arrays without indication of much undesirable recombination due to defects. It is important to point out that the excellent photoresponse of the CdTe nanorod arrays not only results from the intrinsic high absorption efficiency of CdTe with an optimum band gap that matches the solar irradiation spectrum but also suggests good electronic structure quality of these nanorods. In addition, the vertical array configuration of the nanorods on ITO ensures a direct pathway for the photogenerated charge carriers to travel along the longitudinal direction of nanorods with minimum loss. Conclusions In conclusion, CdTe nanorod arrays on ITO, with excellent optical and photoelectrical properties, have been demonstrated using a simple catalyst-free thermal evaporation method.
Acknowledgment. This work is supported by grants from the GRF of HKSAR under Project No. 414908, CUHK Focused Investment Scheme C, CUHK Group Research Scheme, the National Nature Science Foundation of China (No. 50772032), Research Fund for the Doctoral Program of MOE of China (No. 20060512004), and NSF Creative Team Project of Hubei Province (No. 2007ABC005). References and Notes (1) Visoly-Fisher, I.; Cohen, S. R.; Ruzin, A.; Cahen, D. AdV. Mater. 2004, 16, 879. (2) Mamedova, N. N.; Kotov, N. A.; Rogach, A. L.; Studer, J. Nano Lett. 2001, 1, 281. (3) Kang, J.; Parsai, E. I.; Albin, D.; Karpov, V. G.; Shvydka, D. Appl. Phys. Lett. 2008, 93, 223507. (4) Breeze, A. J. The IEEE international Reliability Physics Symposium, 2008; p 168. (5) Huang, M. H.; Mao, S.; Feick, H.; Wu, Y.; Kind, H.; Webber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (6) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737. (7) Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, E.; Boercker, J. E.; Carter, C. B.; Kortshagen, U. R.; Norris, D. J.; Aydil, E. S. Nano Lett. 2007, 7, 1793. (8) Kuang, D.; Brillet, J.; Chen, P.; Takata, M.; Uchida, S.; Miura, H.; Sumioka, K.; Zakeeruddin, S. M.; Gra¨tzel, M. ACS Nano 2008, 2, 113. (9) Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J. Nature 2007, 449, 885. (10) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (11) Cao, X.; Lan, X.; Guo, Y.; Zhao, C. Cryst. Growth Des. 2008, 8, 575. (12) Kum, M. C.; Yoo, Y. B.; Rheem, Y. W.; Bozhilov, K. N.; Chen, W.; Mulchandani, A.; Myung, N. M. Nanotechnology 2008, 19, 325711. (13) Zhao, A. W.; Meng, G. W.; Zhang, L. D.; Gao, T.; Sun, S. H.; Pang, Y. T. Appl. Phys. A 2003, 76, 537. (14) Xu, D. S.; Guo, Y. G.; Yu, D. P.; Guo, G. L.; Tang, Y. Q.; Y, D. P. J. Mater. Res. 2002, 17, 1711. (15) Neretina, S.; Hughes, R. A.; Britten, J. F.; Sochinskii, N. V.; Preston, J. S.; Mascher, P. Nanotechnology 2007, 18, 275301. (16) Neretina, S.; Hughes, R. A.; Devenyi, G. A.; Sochinskii, N. V.; Preston, J. S.; Mascher, P. Nanotechnology 2008, 19, 185601. (17) Zhou, M. J.; Zhu, H. J.; Jiao, Y.; Rao, Y. Y.; Hark, S. K.; Liu, Y.; Peng, L. M.; Li, Q. J. Phys. Chem. C 2009, 113, 8945. (18) Smith, D. J.; Tsen, S. -C. Y.; Chen, Y. P.; Faurie, J.-P.; Sivananthan, S. Appl. Phys. Lett. 1995, 67, 1591. (19) Wang, N.; Cai, Y.; Zhang, R. Q. Mater. Sci. Eng. 2008, R60, 1. (20) Yan, Y. F.; Al-Jassim, M. M.; Jones, K. M. J. Appl. Phys. 2003, 94, 2976. (21) Raizman, A.; Oron, M.; Cinader, G.; Shtrikman, H. J. Appl. Phys. 1990, 67, 1554. (22) Shokhovets, S.; Ambacher, O.; Gobsch, G. Phys. ReV. B 2007, 76, 125203. (23) Vatavu, S.; Zhao, H.; Caraman, I.; Ga0in, P.; Ferekides, C. Thin Solid Films 2009, 517, 2195. (24) Vatavu, S.; Zhao, H.; Padma, V.; Rudaraju, R.; Morel, D. L.; Ga0in, P.; Caraman, Iu.; Ferekides, C. S. Thin Solid Films 2007, 515, 3107. (25) Ka¨ckell, P.; Furthmu¨ller, J.; Bechstedt, F. Phys. ReV. B 1998, 58, 1326. (26) Stampfl, C.; Van de Walle, C. G. Phys. ReV. B 1998, 57, R15052. (27) Roussillon, Y.; Karpov, V. G.; Shvydka, D.; Drayton, J.; Compaan, A. D. J. Appl. Phys. 2004, 96, 7283. (28) Berding, M. A. Phys. ReV. B 1999, 60, 8943. (29) Krishnakumar, V.; Ramamurthi, K.; Klein, A.; Jaegermann, W. Thin Solid Films 2009, 517, 2558. (30) Wei, S. H.; Zhang, S. B.; Zunger, A. J. Appl. Phys. 2000, 87, 1304.
JP905577U
