Published on 03 April 2014. Downloaded by State University of New York at Stony Brook on 27/10/2014 09:29:25.
PCCP View Article Online
PAPER
Cite this: Phys. Chem. Chem. Phys., 2014, 16, 9525
View Journal | View Issue
Self-powered ultraviolet photodetectors based on selectively grown ZnO nanowire arrays with thermal tuning performance Zhiming Bai,†a Xiang Chen,†a Xiaoqin Yan,a Xin Zheng,a Zhuo Kanga and Yue Zhang*ab A self-powered Schottky-type ultraviolet photodetector with Al–Pt interdigitated electrodes has been fabricated based on selectively grown ZnO nanowire arrays. At zero bias, the fabricated photodetector exhibited high sensitivity and excellent selectivity to UV light illumination with a fast response time of 81 ms. By tuning the Schottky barrier height through the thermally induced variation of the interface chemisorbed oxygen, an ultrahigh sensitivity of 3.1 104 was achieved at 340 K without an external power source, which was 82%
Received 1st March 2014, Accepted 2nd April 2014
higher than that obtained at room temperature. According to the thermionic emission–diffusion theory and
DOI: 10.1039/c4cp00892h
system temperatures were calculated, which agreed well with the experimental data. This work demon-
the solar cell theory, the changes in the photocurrent of the photodetector at zero bias with various strates a promising approach to modulating the performance of a self-powered photodetector by heating
www.rsc.org/pccp
and provides theoretical support for studying the thermal effect on the future photoelectric device.
Introduction Due to a wide-direct bandgap of 3.37 eV and a high exciton binding energy of 60 meV, ZnO is a semiconducting material of growing interest in optoelectronic devices in the ultraviolet (UV) spectral range.1–4 In recent years, a tremendous amount of research has been devoted to UV photodetection using ZnO nanowires (NWs) because of the massive surface-to-volume ratio, high surface state density and excellent electrical transport property.5–8 Nowadays, most of the traditional photodetectors (PDs) need an external electric field to drive the photogenerated carriers to generate photocurrent. A novel self-powered PD based on the photovoltaic effect can operate at zero bias without consuming external power, which is highly desirable to meet the demands of the low-carbon age. In terms of the charge separation features of the interface, the self-powered PDs have three structure types: photoelectrochemical type,9,10 p–n junction type11,12 and Schottky junction type.13,14 Compared to the photoelectrochemical PDs, the Schottky junction devices have a faster response time owing to the higher carrier mobility in the semiconductor than in aqueous solution. In addition, the Schottky junction ones have advantages over the p–n junction ones, a
State Key Laboratory for Advanced Metals and Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China. E-mail: [email protected]; Tel: +86-010-62334725 b Key Laboratory of New Energy Materials and Technologies, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China † These authors contributed equally to this work.
This journal is © the Owner Societies 2014
both in the simple preparation process and in high frequency application.14 So far, great effort has been made to improve the photosensing performance of the self-powered PDs. Yang et al. reported that the performance of the PD dramatically increased as the Sb-doping concentration increased in the ZnO nanobelt.15 Lin et al. have observed that the strain-induced piezopotential could tune the energy band profile at the heterointerface and increase/ decrease the barrier height, consequently modulating the performance of the self-powered devices.16 To the best of our knowledge, however, temperature dependent characteristics of the self-powered UV PDs have rarely been reported thus far. The ambient temperature is the key factor that must be taken into account in the application of the self-powered PDs in long-term unattended harsh environment monitoring, such as deep-space exploration. In this work, we demonstrate the fabrication and investigation of the self-powered UV PDs based on ZnO NWs selectively grown on the gaps of the interdigitated (IDT) electrodes. It was observed that under zero bias the photoresponse properties of the fabricated UV PDs exhibited evident dependence on the temperature. The possible physical mechanism behind such dependence was proposed and discussed in detail.
Experimental details PD fabrication A 350 nm-thick ZnO seed layer was firstly deposited on the silicon-on-insulator substrate using the RF magnetron sputter technique at room temperature. The as-grown ZnO film was
Phys. Chem. Chem. Phys., 2014, 16, 9525--9529 | 9525
View Article Online
Published on 03 April 2014. Downloaded by State University of New York at Stony Brook on 27/10/2014 09:29:25.
Paper
PCCP
annealed at 700 1C for 30 min in air. A comb-like Al film of thickness 200 nm was deposited onto the ZnO film by lithography and the DC magnetron sputter technique to serve as an Ohmic contact. Then, the corresponding comb-like Pt film (200 nm) was coated on the gaps between the fingers of the Al electrode by overlaying and the DC sputter technique to act as a Schottky contact. The fingers of the Al–Pt IDT electrodes were 10 mm wide and 100 mm long, with a spacing of 3 mm. There were 50 pairs of fingers and the total active area was 43 000 mm2. Finally, the ZnO nanowire arrays were synthesized using a hydrothermal method detailed in a previous report.17 Characterization and measurements The morphology of the selectively grown ZnO NWs was characterized by field emission scanning electron microscopy (FESEM, LEO1530). The luminescence properties of the ZnO NWs were investigated by room temperature photoluminescence (PL) spectroscopy (Jobin-Yvon, HR 800, 325 nm He–Cd laser). The photoelectrical properties of the fabricated PD were measured using a semiconductor characterization system (Keithley 4200-SCS). The light source used in this work was a 355 nm laser with a power of 20 mW.
Results and discussion A schematic diagram of the selectively grown ZnO nanowire metal–semiconductor–metal (MSM) PD is shown in Fig. 1(a). Fig. 1(b) shows the SEM image of the selectively grown ZnO NW MSM PD and it can be seen that the ZnO NWs are selectively grown on the gaps between the fingers of the Al–Pt IDT electrodes. The sectional and top SEM images of the ZnO NWs are depicted in Fig. 1(c) and the inset. The average diameter and length of the well-aligned ZnO NWs are about 200 nm and 3 mm, respectively. The photoluminescence (PL) spectrum excited by a 325 nm He–Cd laser was measured to show the optical properties and crystallinity of the ZnO NWs (Fig. 1(d)).
Fig. 1 (a) Schematic diagram and (b) SEM image of the selectively grown ZnO NW MSM PD. (c) Cross-sectional SEM image of the ZnO NWs. Inset shows the top image of the ZnO NWs. (d) PL spectrum of the ZnO NWs.
9526 | Phys. Chem. Chem. Phys., 2014, 16, 9525--9529
Fig. 2 (a) I–V curve of the self-powered ZnO NW PDs measured in the dark. (b) Photocurrent–time curve of the PD upon 355 nm light illumination being turned on and off at zero bias. (c) Photocurrent as a function of light intensity. (d) Spectral responsivity of the device and the corresponding absorption spectra of ZnO NW arrays.
The sharp 376 nm peak corresponds to the band gap of ZnO (3.37 eV) and the weak emission in the visible region is related to oxygen vacancies and surface states,18 which indicates the NWs have excellent crystallinity. The dark rectifying property of the MSM PD is shown in Fig. 2(a). The asymmetric I–V characteristics indicate the formation of a Pt–ZnO Schottky junction. According to the thermionic emission theory,19 the Schottky barrier height (SBH) was estimated to be 0.846 eV. This value is higher than the reported one (0.613 eV)20 because there may be more surface states in the interface between the Pt electrode and the ZnO film. Fig. 2(b) shows the dynamic photoresponse of the ZnO NW PD when the 355 nm laser with a power of 23 mW cm2 is on and off at zero bias. The photocurrent was increased by 1.7 104 times upon UV illumination. The rise time (the time taken by the PD to reach 63% of the maximum photocurrent from the dark current) and recovery time (the time taken to reach 37% of the maximum photocurrent) are 81 ms and 95 ms, respectively. The response speed is much faster than the ZnO NW based photoconductive PDs, which generally have a long reset time in the second scale.21 To understand the observed phenomena, the photoresponse mechanism of the photoconductive PDs is compared with that of the self-powered PDs. Upon UV illumination, electron–hole pairs are generated in the ZnO NWs and a portion of the holes are captured by the oxygen anions absorbed on the surface of the NWs. For the traditional photoconductive PDs, the excessive photoexcited electrons and residual photoexcited holes drift along the NWs in opposing directions driven by the external electric field, which increases the photoconductance of the PD. For the fabricated self-powered PDs, the Pt Schottky contacts induce a strong built-in electric field in the ZnO side. The internal electric field sweeps the photoexcited electrons to the n-ZnO side and the photoexcited holes to the Pt side, which results in a large photocurrent without the external power
This journal is © the Owner Societies 2014
View Article Online
Published on 03 April 2014. Downloaded by State University of New York at Stony Brook on 27/10/2014 09:29:25.
PCCP
Paper
supply. Therefore, the photocurrent is dominated by the drifting of the majority carriers in the space charge region and the diffusion of the minority carriers outside the depletion layer,22 which is very different from the longtime photoelectrical response of the ZnO NWs related to the adsorption and desorption of oxygen. Further experiments were performed to show the dependence of photocurrent on the light intensity. The relationship between the photocurrent and the light intensity (P) can be expressed by a simple power law I = AP y
(1)
where A is a constant for a particular wavelength. It can be seen that the photocurrent is almost linearly dependent on the illumination power, which demonstrates that the carrier generation rate is constant.23 In order to get insight into the selectivity of the self-powered PD for the wavelength of exciting light, the spectral responsivity was measured in the wavelength range of 330–700 nm under zero bias (see Fig. 2(d)). Notably, the device shows an evident response to the UV light and has poor performance in the long wavelength region. The UV reponsivity peak centered at 365 nm can be observed, corresponding to the absorption peark of the ZnO NW arrays. The peak responsivity is about 1.82 mA W1, to the best of our knowledge, which is much larger than the reported result (about 4 103 mA W1) for the ZnO NW based Schottky type self-powered PDs15 and comparable with the ZnO NW based p–n type self-powered PDs.3,24 The sensing performance for the self-powered PD at various temperatures in ambient air is shown in Fig. 3(a). At a fixed optical power (23 mW cm2), the sensitivity increases first and then decreases with the increase of temperature, and reaches a maximum value (3.1 104) when the temperature is 340 K (see Fig. 3(b)), which is 82% higher than that obtained at room temperature. The changes in temperature have little effect on the response speed. To explore the physical mechanism behind the thermal tuning performance of the device and changes in the SBH as temperatures rose were investigated. Fig. 4(a) depicts the I–V curves of the PD measured at temperatures from 290 K to 450 K in the dark. According to the thermionic emission–diffusion theory, the difference between the SBH (Dfsb) at the two ends of
Fig. 4 (a) I–V curves of the self-powered ZnO NW PDs measured at different temperatures in the dark. (b) The DSBH versus temperature plotted at zero bias. (c) Schematic energy band diagram of the PD upon heating.
the fabricated device can be estimated using the following equation25 I DA Dfsb (2) ¼ ln Iþ A kT where I and I+ are the currents at reverse and forward 2 V bias, respectively. A**, k and T are the effective Richardson constant, the Boltzmann constant and ambient temperature, respectively. The value of DA**/A** is very small and can be neglected. The changes of DSBH extracted from the I–V curves as a function of temperature are plotted in Fig. 4(b). It is clear that DSBH has the same trend as photocurrent with the increase of temperature, implying that the Schottky contact dominates the photoresponse of the PD at zero bias. The DSBH first increases to a peak value at 340 K, due to the increase of absorbed negatively charged oxygen at the Schottky contact. As shown in Fig. 4(c), the black line illustrates the initial band structure and the red line depicts the band structure of the PD after heating. It is well known that there are abundant oxygen vacancies in the outside surface layer of the n-ZnO film, which can act as adsorption sites. The adsorpted oxygen molecules capture the free electrons (O2(g) + e - O2(ad.)), thereby inducing interface band bending (Vb), which can be formulated as26 Vb ¼
Fig. 3 (a) Photocurrent–time curves of the self-powered ZnO NW PDs measured at different temperatures under zero bias. (b) The measured photocurrent and the corresponding simulated photocurrent as a function of temperature.
This journal is © the Owner Societies 2014
eNI2 2er e0 N0
(3)
where NI, er, e0 and N0 are the interface state density, relative dielectric constant of the ZnO, the vacuum dielectric constant and the electron density, respectively. At low temperature, the absorption quantity of oxygen increases with temperature,27 resulting in a larger interface state density, and then a higher interface band bending (see Fig. 4(c)). Therefore, when the temperature is below the turning point temperature (340 K), the DSBH is proportional to the temperature, which is ascribed to interface oxygen adsorption. As the temperature is increased successively, the DSBH gradually drops due to higher energies of the electrons at higher temperature. When the system
Phys. Chem. Chem. Phys., 2014, 16, 9525--9529 | 9527
View Article Online
Published on 03 April 2014. Downloaded by State University of New York at Stony Brook on 27/10/2014 09:29:25.
Paper
PCCP
temperature rises to 450 K, the DSBH dives to a negative value. Meanwhile it is noticed that the dark current starts falling above 390 K (Fig. 4(a)). These may be because an alumina layer is formed at the interface of the Al electrode/ZnO film. Owing to a smaller electron affinity (2.6 eV)28 of alumina than ZnO (4.2 eV),29 the photoexcited electrons can hardly get through the contact barrier between the Al–alumina layer and the ZnO film, so no photocurrent was observed at 450 K. In order to obtain a further understanding of the thermal effect on the sensing properties, the photocurrent (Iph) at zero bias was calculated by the changes in DSBH, based on the solar cell theory. In an ideal n-type semiconductor/metal Schottky diode, the SBH is the sum of the built-in electric field (Vbi) and the difference between the conduction band and the Fermi level (DEc) (as shown in Fig. 4(c)). Dfsb = Vbi + DEc
(4)
Upon illumination, the maximum photovoltage (Voc) of the Schottky diode is the difference between the work function of the metal and the work function of the semiconductor, that is, Vbi. According to eqn (4), the open-circuit voltage of a Schottky barrier can be expressed as30 kT Iph ln þ1 (5) Voc ¼ Dfsb DEc ¼ q Io where Io is the dark saturation current opposing the photoinduced current density. By assuming that Io and DEc are independent of temperature for small variations, the photocurrent (Iph0 ) of the PD upon heating can be determined by 0 hq i Iph 0 Dfsb Dfsb ¼ exp kT Iph
(6)
The results that are estimated by the DSBH at each temperature are plotted in Fig. 2(d). It is evident that the simulated results are in accordance with the experimental results measured below 390 K, which can provide reliable evidence for evaluating the thermal effect on the photoresponse of the self-powered PDs. Because the insulative alumina layer hinders the photogenerated electron transfer, the measured data are smaller than the simulated data above 390 K. In this case, eqn (6) is no longer applicable.
Conclusions In summary, we fabricated a self-powered UV PD with the selectively grown ZnO NW arrays, which showed rapid rise/ reset time of 81/95 ms and a high sensitivity of 1.7 104 to UV light at zero bias. Due to the interface band bending caused by the adsorbed oxygen molecules, the enhanced SBH increased the sensitivity to 3.1 104 at 340 K. The photocurrents of the PD measured at different temperatures were consistent with the simulated results, using the thermionic emission–diffusion theory and the solar cell theory. The zero-energy self-powered UV PDs can continuously operate in the unwatched environment, implying that they have promising applications in early
9528 | Phys. Chem. Chem. Phys., 2014, 16, 9525--9529
warning systems, space communication and harsh environment monitoring.
Acknowledgements This work was supported by the National Major Research Program of China (2013CB932601), the Major Project of International Cooperation and Exchanges (2012DFA50990), the Program of Introducing Talents of Discipline to Universities, NSFC (51232001, 51172022, 51372023), the Research Fund of Co-construction Program from Beijing Municipal Commission of Education, the Fundamental Research Funds for the Central Universities, the Program for Changjiang Scholars and Innovative Research Team in University.
Notes and references 1 Y. Zhang, X. Yan, Y. Yang, Y. Huang, Q. Liao and J. Qi, Adv. Mater., 2012, 24, 4647–4655. 2 X.-M. Zhang, M.-Y. Lu, Y. Zhang, L.-J. Chen and Z. L. Wang, Adv. Mater., 2009, 21, 2767–2770. 3 Y. Q. Bie, Z. M. Liao, H. Z. Zhang, G. R. Li, Y. Ye, Y. B. Zhou, J. Xu, Z. X. Qin, L. Dai and D. P. Yu, Adv. Mater., 2011, 23, 649–653. 4 D. Shao, M. Yu, H. Sun, T. Hu, J. Lian and S. Sawyer, Nanoscale, 2013, 5, 3664–3667. 5 F. Zhang, S. Niu, W. Guo, G. Zhu, Y. Liu, X. Zhang and Z. L. Wang, ACS Nano, 2013, 7, 4537–4544. 6 C. Soci, A. Zhang, B. Xiang, S. A. Dayeh, D. P. R. Aplin, J. Park, X. Y. Bao, Y. H. Lo and D. Wang, Nano Lett., 2007, 7, 1003–1009. 7 Z. Bai, X. Yan, X. Chen, H. Liu, Y. Shen and Y. Zhang, Curr. Appl. Phys., 2013, 13, 165–169. 8 D. Shao, M. Yu, J. Lian and S. Sawyer, Appl. Phys. Lett., 2013, 102, 021107. 9 X. Li, C. Gao, H. Duan, B. Lu, X. Pan and E. Xie, Nano Energy, 2012, 1, 640–645. 10 X. Li, C. Gao, H. Duan, B. Lu, Y. Wang, L. Chen, Z. Zhang, X. Pan and E. Xie, Small, 2013, 9, 2005–2011. 11 S. M. Hatch, J. Briscoe and S. Dunn, Adv. Mater., 2013, 25, 867–871. 12 Z. Bai, X. Yan, X. Chen, Y. Cui, P. Lin, Y. Shen and Y. Zhang, RSC Adv., 2013, 3, 17682. 13 Z. Zhan, L. Zheng, Y. Pan, G. Sun and L. Li, J. Mater. Chem., 2012, 22, 2589–2595. 14 Z. Gao, W. Jin, Y. Zhou, Y. Dai, B. Yu, C. Liu, W. Xu, Y. Li, H. Peng, Z. Liu and L. Dai, Nanoscale, 2013, 5, 5576–5581. 15 Y. Yang, W. Guo, J. Qi, J. Zhao and Y. Zhang, Appl. Phys. Lett., 2010, 97, 223113. 16 P. Lin, X. Yan, Z. Zhang, Y. Shen, Y. Zhao, Z. Bai and Y. Zhang, ACS Appl. Mater. Interfaces, 2013, 5, 3671–3676. 17 W. Lin, X. Yan, X. Zhang, Z. Qin, Z. Zhang, Z. Bai, Y. Lei and Y. Zhang, Solid State Commun., 2011, 151, 1860–1863. 18 X. T. Zhang, Y. C. Liu, J. Y. Zhang, Y. M. Lu, D. Z. Shen, X. W. Fan and X. G. Kong, J. Cryst. Growth, 2003, 254, 80–85.
This journal is © the Owner Societies 2014
View Article Online
Published on 03 April 2014. Downloaded by State University of New York at Stony Brook on 27/10/2014 09:29:25.
PCCP
19 S. M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981, pp. 160–171. 20 K. Ip, Y. W. Heo, K. H. Baik, D. P. Norton, S. J. Pearton, S. Kim, J. R. LaRoche and F. Ren, Appl. Phys. Lett., 2004, 84, 2835. 21 L. Hu, J. Yan, M. Liao, H. Xiang, X. Gong, L. Zhang and X. Fang, Adv. Mater., 2012, 24, 2305–2309. 22 J. Liu, Photonic Devices, Cambridge University Press, New York, 2005, pp. 968–972. 23 Y. Dong, B. Tian, T. J. Kempa and C. M. Lieber, Nano Lett., 2009, 9, 2183–2187. 24 S. M. Hatch, J. Briscoe, A. Sapelkin, W. P. Gillin, J. B. Gilchrist, M. P. Ryan, S. Heutz and S. Dunn, J. Appl. Phys., 2013, 113, 204501.
This journal is © the Owner Societies 2014
Paper
25 J. Zhou, Y. Gu, P. Fei, W. Mai, Y. Gao, R. Yang, G. Bao and Z. L. Wang, Nano Lett., 2008, 8, 3035–3040. 26 Z. Zhang and J. T. Yates, Jr., Chem. Rev., 2012, 112, 5520–5551. 27 T. Y. Wei, P. H. Yeh, S. Y. Lu and Z. L. Wang, J. Am. Chem. Soc., 2009, 131, 17690–17695. 28 M. L. Huang, Y. C. Chang, C. H. Chang, T. D. Lin, J. Kwo, T. B. Wu and M. Hong, Appl. Phys. Lett., 2006, 89, 012903. 29 H. Kobayashi, H. Mori, T. Ishida and Y. Nakato, J. Appl. Phys., 1995, 77, 1301. 30 P. V. Kamat, K. Tvrdy, D. R. Baker and J. G. Radich, Chem. Rev., 2010, 110, 6664–6688.
Phys. Chem. Chem. Phys., 2014, 16, 9525--9529 | 9529
PCCP View Article Online
PAPER
Cite this: Phys. Chem. Chem. Phys., 2014, 16, 9525
View Journal | View Issue
Self-powered ultraviolet photodetectors based on selectively grown ZnO nanowire arrays with thermal tuning performance Zhiming Bai,†a Xiang Chen,†a Xiaoqin Yan,a Xin Zheng,a Zhuo Kanga and Yue Zhang*ab A self-powered Schottky-type ultraviolet photodetector with Al–Pt interdigitated electrodes has been fabricated based on selectively grown ZnO nanowire arrays. At zero bias, the fabricated photodetector exhibited high sensitivity and excellent selectivity to UV light illumination with a fast response time of 81 ms. By tuning the Schottky barrier height through the thermally induced variation of the interface chemisorbed oxygen, an ultrahigh sensitivity of 3.1 104 was achieved at 340 K without an external power source, which was 82%
Received 1st March 2014, Accepted 2nd April 2014
higher than that obtained at room temperature. According to the thermionic emission–diffusion theory and
DOI: 10.1039/c4cp00892h
system temperatures were calculated, which agreed well with the experimental data. This work demon-
the solar cell theory, the changes in the photocurrent of the photodetector at zero bias with various strates a promising approach to modulating the performance of a self-powered photodetector by heating
www.rsc.org/pccp
and provides theoretical support for studying the thermal effect on the future photoelectric device.
Introduction Due to a wide-direct bandgap of 3.37 eV and a high exciton binding energy of 60 meV, ZnO is a semiconducting material of growing interest in optoelectronic devices in the ultraviolet (UV) spectral range.1–4 In recent years, a tremendous amount of research has been devoted to UV photodetection using ZnO nanowires (NWs) because of the massive surface-to-volume ratio, high surface state density and excellent electrical transport property.5–8 Nowadays, most of the traditional photodetectors (PDs) need an external electric field to drive the photogenerated carriers to generate photocurrent. A novel self-powered PD based on the photovoltaic effect can operate at zero bias without consuming external power, which is highly desirable to meet the demands of the low-carbon age. In terms of the charge separation features of the interface, the self-powered PDs have three structure types: photoelectrochemical type,9,10 p–n junction type11,12 and Schottky junction type.13,14 Compared to the photoelectrochemical PDs, the Schottky junction devices have a faster response time owing to the higher carrier mobility in the semiconductor than in aqueous solution. In addition, the Schottky junction ones have advantages over the p–n junction ones, a
State Key Laboratory for Advanced Metals and Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China. E-mail: [email protected]; Tel: +86-010-62334725 b Key Laboratory of New Energy Materials and Technologies, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China † These authors contributed equally to this work.
This journal is © the Owner Societies 2014
both in the simple preparation process and in high frequency application.14 So far, great effort has been made to improve the photosensing performance of the self-powered PDs. Yang et al. reported that the performance of the PD dramatically increased as the Sb-doping concentration increased in the ZnO nanobelt.15 Lin et al. have observed that the strain-induced piezopotential could tune the energy band profile at the heterointerface and increase/ decrease the barrier height, consequently modulating the performance of the self-powered devices.16 To the best of our knowledge, however, temperature dependent characteristics of the self-powered UV PDs have rarely been reported thus far. The ambient temperature is the key factor that must be taken into account in the application of the self-powered PDs in long-term unattended harsh environment monitoring, such as deep-space exploration. In this work, we demonstrate the fabrication and investigation of the self-powered UV PDs based on ZnO NWs selectively grown on the gaps of the interdigitated (IDT) electrodes. It was observed that under zero bias the photoresponse properties of the fabricated UV PDs exhibited evident dependence on the temperature. The possible physical mechanism behind such dependence was proposed and discussed in detail.
Experimental details PD fabrication A 350 nm-thick ZnO seed layer was firstly deposited on the silicon-on-insulator substrate using the RF magnetron sputter technique at room temperature. The as-grown ZnO film was
Phys. Chem. Chem. Phys., 2014, 16, 9525--9529 | 9525
View Article Online
Published on 03 April 2014. Downloaded by State University of New York at Stony Brook on 27/10/2014 09:29:25.
Paper
PCCP
annealed at 700 1C for 30 min in air. A comb-like Al film of thickness 200 nm was deposited onto the ZnO film by lithography and the DC magnetron sputter technique to serve as an Ohmic contact. Then, the corresponding comb-like Pt film (200 nm) was coated on the gaps between the fingers of the Al electrode by overlaying and the DC sputter technique to act as a Schottky contact. The fingers of the Al–Pt IDT electrodes were 10 mm wide and 100 mm long, with a spacing of 3 mm. There were 50 pairs of fingers and the total active area was 43 000 mm2. Finally, the ZnO nanowire arrays were synthesized using a hydrothermal method detailed in a previous report.17 Characterization and measurements The morphology of the selectively grown ZnO NWs was characterized by field emission scanning electron microscopy (FESEM, LEO1530). The luminescence properties of the ZnO NWs were investigated by room temperature photoluminescence (PL) spectroscopy (Jobin-Yvon, HR 800, 325 nm He–Cd laser). The photoelectrical properties of the fabricated PD were measured using a semiconductor characterization system (Keithley 4200-SCS). The light source used in this work was a 355 nm laser with a power of 20 mW.
Results and discussion A schematic diagram of the selectively grown ZnO nanowire metal–semiconductor–metal (MSM) PD is shown in Fig. 1(a). Fig. 1(b) shows the SEM image of the selectively grown ZnO NW MSM PD and it can be seen that the ZnO NWs are selectively grown on the gaps between the fingers of the Al–Pt IDT electrodes. The sectional and top SEM images of the ZnO NWs are depicted in Fig. 1(c) and the inset. The average diameter and length of the well-aligned ZnO NWs are about 200 nm and 3 mm, respectively. The photoluminescence (PL) spectrum excited by a 325 nm He–Cd laser was measured to show the optical properties and crystallinity of the ZnO NWs (Fig. 1(d)).
Fig. 1 (a) Schematic diagram and (b) SEM image of the selectively grown ZnO NW MSM PD. (c) Cross-sectional SEM image of the ZnO NWs. Inset shows the top image of the ZnO NWs. (d) PL spectrum of the ZnO NWs.
9526 | Phys. Chem. Chem. Phys., 2014, 16, 9525--9529
Fig. 2 (a) I–V curve of the self-powered ZnO NW PDs measured in the dark. (b) Photocurrent–time curve of the PD upon 355 nm light illumination being turned on and off at zero bias. (c) Photocurrent as a function of light intensity. (d) Spectral responsivity of the device and the corresponding absorption spectra of ZnO NW arrays.
The sharp 376 nm peak corresponds to the band gap of ZnO (3.37 eV) and the weak emission in the visible region is related to oxygen vacancies and surface states,18 which indicates the NWs have excellent crystallinity. The dark rectifying property of the MSM PD is shown in Fig. 2(a). The asymmetric I–V characteristics indicate the formation of a Pt–ZnO Schottky junction. According to the thermionic emission theory,19 the Schottky barrier height (SBH) was estimated to be 0.846 eV. This value is higher than the reported one (0.613 eV)20 because there may be more surface states in the interface between the Pt electrode and the ZnO film. Fig. 2(b) shows the dynamic photoresponse of the ZnO NW PD when the 355 nm laser with a power of 23 mW cm2 is on and off at zero bias. The photocurrent was increased by 1.7 104 times upon UV illumination. The rise time (the time taken by the PD to reach 63% of the maximum photocurrent from the dark current) and recovery time (the time taken to reach 37% of the maximum photocurrent) are 81 ms and 95 ms, respectively. The response speed is much faster than the ZnO NW based photoconductive PDs, which generally have a long reset time in the second scale.21 To understand the observed phenomena, the photoresponse mechanism of the photoconductive PDs is compared with that of the self-powered PDs. Upon UV illumination, electron–hole pairs are generated in the ZnO NWs and a portion of the holes are captured by the oxygen anions absorbed on the surface of the NWs. For the traditional photoconductive PDs, the excessive photoexcited electrons and residual photoexcited holes drift along the NWs in opposing directions driven by the external electric field, which increases the photoconductance of the PD. For the fabricated self-powered PDs, the Pt Schottky contacts induce a strong built-in electric field in the ZnO side. The internal electric field sweeps the photoexcited electrons to the n-ZnO side and the photoexcited holes to the Pt side, which results in a large photocurrent without the external power
This journal is © the Owner Societies 2014
View Article Online
Published on 03 April 2014. Downloaded by State University of New York at Stony Brook on 27/10/2014 09:29:25.
PCCP
Paper
supply. Therefore, the photocurrent is dominated by the drifting of the majority carriers in the space charge region and the diffusion of the minority carriers outside the depletion layer,22 which is very different from the longtime photoelectrical response of the ZnO NWs related to the adsorption and desorption of oxygen. Further experiments were performed to show the dependence of photocurrent on the light intensity. The relationship between the photocurrent and the light intensity (P) can be expressed by a simple power law I = AP y
(1)
where A is a constant for a particular wavelength. It can be seen that the photocurrent is almost linearly dependent on the illumination power, which demonstrates that the carrier generation rate is constant.23 In order to get insight into the selectivity of the self-powered PD for the wavelength of exciting light, the spectral responsivity was measured in the wavelength range of 330–700 nm under zero bias (see Fig. 2(d)). Notably, the device shows an evident response to the UV light and has poor performance in the long wavelength region. The UV reponsivity peak centered at 365 nm can be observed, corresponding to the absorption peark of the ZnO NW arrays. The peak responsivity is about 1.82 mA W1, to the best of our knowledge, which is much larger than the reported result (about 4 103 mA W1) for the ZnO NW based Schottky type self-powered PDs15 and comparable with the ZnO NW based p–n type self-powered PDs.3,24 The sensing performance for the self-powered PD at various temperatures in ambient air is shown in Fig. 3(a). At a fixed optical power (23 mW cm2), the sensitivity increases first and then decreases with the increase of temperature, and reaches a maximum value (3.1 104) when the temperature is 340 K (see Fig. 3(b)), which is 82% higher than that obtained at room temperature. The changes in temperature have little effect on the response speed. To explore the physical mechanism behind the thermal tuning performance of the device and changes in the SBH as temperatures rose were investigated. Fig. 4(a) depicts the I–V curves of the PD measured at temperatures from 290 K to 450 K in the dark. According to the thermionic emission–diffusion theory, the difference between the SBH (Dfsb) at the two ends of
Fig. 4 (a) I–V curves of the self-powered ZnO NW PDs measured at different temperatures in the dark. (b) The DSBH versus temperature plotted at zero bias. (c) Schematic energy band diagram of the PD upon heating.
the fabricated device can be estimated using the following equation25 I DA Dfsb (2) ¼ ln Iþ A kT where I and I+ are the currents at reverse and forward 2 V bias, respectively. A**, k and T are the effective Richardson constant, the Boltzmann constant and ambient temperature, respectively. The value of DA**/A** is very small and can be neglected. The changes of DSBH extracted from the I–V curves as a function of temperature are plotted in Fig. 4(b). It is clear that DSBH has the same trend as photocurrent with the increase of temperature, implying that the Schottky contact dominates the photoresponse of the PD at zero bias. The DSBH first increases to a peak value at 340 K, due to the increase of absorbed negatively charged oxygen at the Schottky contact. As shown in Fig. 4(c), the black line illustrates the initial band structure and the red line depicts the band structure of the PD after heating. It is well known that there are abundant oxygen vacancies in the outside surface layer of the n-ZnO film, which can act as adsorption sites. The adsorpted oxygen molecules capture the free electrons (O2(g) + e - O2(ad.)), thereby inducing interface band bending (Vb), which can be formulated as26 Vb ¼
Fig. 3 (a) Photocurrent–time curves of the self-powered ZnO NW PDs measured at different temperatures under zero bias. (b) The measured photocurrent and the corresponding simulated photocurrent as a function of temperature.
This journal is © the Owner Societies 2014
eNI2 2er e0 N0
(3)
where NI, er, e0 and N0 are the interface state density, relative dielectric constant of the ZnO, the vacuum dielectric constant and the electron density, respectively. At low temperature, the absorption quantity of oxygen increases with temperature,27 resulting in a larger interface state density, and then a higher interface band bending (see Fig. 4(c)). Therefore, when the temperature is below the turning point temperature (340 K), the DSBH is proportional to the temperature, which is ascribed to interface oxygen adsorption. As the temperature is increased successively, the DSBH gradually drops due to higher energies of the electrons at higher temperature. When the system
Phys. Chem. Chem. Phys., 2014, 16, 9525--9529 | 9527
View Article Online
Published on 03 April 2014. Downloaded by State University of New York at Stony Brook on 27/10/2014 09:29:25.
Paper
PCCP
temperature rises to 450 K, the DSBH dives to a negative value. Meanwhile it is noticed that the dark current starts falling above 390 K (Fig. 4(a)). These may be because an alumina layer is formed at the interface of the Al electrode/ZnO film. Owing to a smaller electron affinity (2.6 eV)28 of alumina than ZnO (4.2 eV),29 the photoexcited electrons can hardly get through the contact barrier between the Al–alumina layer and the ZnO film, so no photocurrent was observed at 450 K. In order to obtain a further understanding of the thermal effect on the sensing properties, the photocurrent (Iph) at zero bias was calculated by the changes in DSBH, based on the solar cell theory. In an ideal n-type semiconductor/metal Schottky diode, the SBH is the sum of the built-in electric field (Vbi) and the difference between the conduction band and the Fermi level (DEc) (as shown in Fig. 4(c)). Dfsb = Vbi + DEc
(4)
Upon illumination, the maximum photovoltage (Voc) of the Schottky diode is the difference between the work function of the metal and the work function of the semiconductor, that is, Vbi. According to eqn (4), the open-circuit voltage of a Schottky barrier can be expressed as30 kT Iph ln þ1 (5) Voc ¼ Dfsb DEc ¼ q Io where Io is the dark saturation current opposing the photoinduced current density. By assuming that Io and DEc are independent of temperature for small variations, the photocurrent (Iph0 ) of the PD upon heating can be determined by 0 hq i Iph 0 Dfsb Dfsb ¼ exp kT Iph
(6)
The results that are estimated by the DSBH at each temperature are plotted in Fig. 2(d). It is evident that the simulated results are in accordance with the experimental results measured below 390 K, which can provide reliable evidence for evaluating the thermal effect on the photoresponse of the self-powered PDs. Because the insulative alumina layer hinders the photogenerated electron transfer, the measured data are smaller than the simulated data above 390 K. In this case, eqn (6) is no longer applicable.
Conclusions In summary, we fabricated a self-powered UV PD with the selectively grown ZnO NW arrays, which showed rapid rise/ reset time of 81/95 ms and a high sensitivity of 1.7 104 to UV light at zero bias. Due to the interface band bending caused by the adsorbed oxygen molecules, the enhanced SBH increased the sensitivity to 3.1 104 at 340 K. The photocurrents of the PD measured at different temperatures were consistent with the simulated results, using the thermionic emission–diffusion theory and the solar cell theory. The zero-energy self-powered UV PDs can continuously operate in the unwatched environment, implying that they have promising applications in early
9528 | Phys. Chem. Chem. Phys., 2014, 16, 9525--9529
warning systems, space communication and harsh environment monitoring.
Acknowledgements This work was supported by the National Major Research Program of China (2013CB932601), the Major Project of International Cooperation and Exchanges (2012DFA50990), the Program of Introducing Talents of Discipline to Universities, NSFC (51232001, 51172022, 51372023), the Research Fund of Co-construction Program from Beijing Municipal Commission of Education, the Fundamental Research Funds for the Central Universities, the Program for Changjiang Scholars and Innovative Research Team in University.
Notes and references 1 Y. Zhang, X. Yan, Y. Yang, Y. Huang, Q. Liao and J. Qi, Adv. Mater., 2012, 24, 4647–4655. 2 X.-M. Zhang, M.-Y. Lu, Y. Zhang, L.-J. Chen and Z. L. Wang, Adv. Mater., 2009, 21, 2767–2770. 3 Y. Q. Bie, Z. M. Liao, H. Z. Zhang, G. R. Li, Y. Ye, Y. B. Zhou, J. Xu, Z. X. Qin, L. Dai and D. P. Yu, Adv. Mater., 2011, 23, 649–653. 4 D. Shao, M. Yu, H. Sun, T. Hu, J. Lian and S. Sawyer, Nanoscale, 2013, 5, 3664–3667. 5 F. Zhang, S. Niu, W. Guo, G. Zhu, Y. Liu, X. Zhang and Z. L. Wang, ACS Nano, 2013, 7, 4537–4544. 6 C. Soci, A. Zhang, B. Xiang, S. A. Dayeh, D. P. R. Aplin, J. Park, X. Y. Bao, Y. H. Lo and D. Wang, Nano Lett., 2007, 7, 1003–1009. 7 Z. Bai, X. Yan, X. Chen, H. Liu, Y. Shen and Y. Zhang, Curr. Appl. Phys., 2013, 13, 165–169. 8 D. Shao, M. Yu, J. Lian and S. Sawyer, Appl. Phys. Lett., 2013, 102, 021107. 9 X. Li, C. Gao, H. Duan, B. Lu, X. Pan and E. Xie, Nano Energy, 2012, 1, 640–645. 10 X. Li, C. Gao, H. Duan, B. Lu, Y. Wang, L. Chen, Z. Zhang, X. Pan and E. Xie, Small, 2013, 9, 2005–2011. 11 S. M. Hatch, J. Briscoe and S. Dunn, Adv. Mater., 2013, 25, 867–871. 12 Z. Bai, X. Yan, X. Chen, Y. Cui, P. Lin, Y. Shen and Y. Zhang, RSC Adv., 2013, 3, 17682. 13 Z. Zhan, L. Zheng, Y. Pan, G. Sun and L. Li, J. Mater. Chem., 2012, 22, 2589–2595. 14 Z. Gao, W. Jin, Y. Zhou, Y. Dai, B. Yu, C. Liu, W. Xu, Y. Li, H. Peng, Z. Liu and L. Dai, Nanoscale, 2013, 5, 5576–5581. 15 Y. Yang, W. Guo, J. Qi, J. Zhao and Y. Zhang, Appl. Phys. Lett., 2010, 97, 223113. 16 P. Lin, X. Yan, Z. Zhang, Y. Shen, Y. Zhao, Z. Bai and Y. Zhang, ACS Appl. Mater. Interfaces, 2013, 5, 3671–3676. 17 W. Lin, X. Yan, X. Zhang, Z. Qin, Z. Zhang, Z. Bai, Y. Lei and Y. Zhang, Solid State Commun., 2011, 151, 1860–1863. 18 X. T. Zhang, Y. C. Liu, J. Y. Zhang, Y. M. Lu, D. Z. Shen, X. W. Fan and X. G. Kong, J. Cryst. Growth, 2003, 254, 80–85.
This journal is © the Owner Societies 2014
View Article Online
Published on 03 April 2014. Downloaded by State University of New York at Stony Brook on 27/10/2014 09:29:25.
PCCP
19 S. M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981, pp. 160–171. 20 K. Ip, Y. W. Heo, K. H. Baik, D. P. Norton, S. J. Pearton, S. Kim, J. R. LaRoche and F. Ren, Appl. Phys. Lett., 2004, 84, 2835. 21 L. Hu, J. Yan, M. Liao, H. Xiang, X. Gong, L. Zhang and X. Fang, Adv. Mater., 2012, 24, 2305–2309. 22 J. Liu, Photonic Devices, Cambridge University Press, New York, 2005, pp. 968–972. 23 Y. Dong, B. Tian, T. J. Kempa and C. M. Lieber, Nano Lett., 2009, 9, 2183–2187. 24 S. M. Hatch, J. Briscoe, A. Sapelkin, W. P. Gillin, J. B. Gilchrist, M. P. Ryan, S. Heutz and S. Dunn, J. Appl. Phys., 2013, 113, 204501.
This journal is © the Owner Societies 2014
Paper
25 J. Zhou, Y. Gu, P. Fei, W. Mai, Y. Gao, R. Yang, G. Bao and Z. L. Wang, Nano Lett., 2008, 8, 3035–3040. 26 Z. Zhang and J. T. Yates, Jr., Chem. Rev., 2012, 112, 5520–5551. 27 T. Y. Wei, P. H. Yeh, S. Y. Lu and Z. L. Wang, J. Am. Chem. Soc., 2009, 131, 17690–17695. 28 M. L. Huang, Y. C. Chang, C. H. Chang, T. D. Lin, J. Kwo, T. B. Wu and M. Hong, Appl. Phys. Lett., 2006, 89, 012903. 29 H. Kobayashi, H. Mori, T. Ishida and Y. Nakato, J. Appl. Phys., 1995, 77, 1301. 30 P. V. Kamat, K. Tvrdy, D. R. Baker and J. G. Radich, Chem. Rev., 2010, 110, 6664–6688.
Phys. Chem. Chem. Phys., 2014, 16, 9525--9529 | 9529
