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Pt structure ultraviolet photodetector
Materials Science and Engineering B 176 (2011) 740–744
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Effects of thermal annealing on the performance of Al/ZnO nanorods/Pt structure ultraviolet photodetector Hai Zhou, Guo-Jia Fang ∗ , Nishuang Liu, Xing-Zhong Zhao Department of Electronic Science and Technology and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 12 October 2010 Received in revised form 25 January 2011 Accepted 13 March 2011 Keywords: ZnO nanorods Ultraviolet Photodetector Schottky
a b s t r a c t ZnO nanorod arrays were fabricated on ZnO coated glass substrate by hydrothermal method. Schottky barrier ultraviolet photodetectors (PDs) were obtained by sputtering Pt electrode and evaporating Al electrode on the top of ZnO nanorod arrays with thermal treatment. It is illustrated that Schottky contacts at the electrode/ZnO NRs interface were formed at the annealing temperature of 300 ◦ C and above. When annealing temperature was up to 300 ◦ C, the performance of the PDs was improved with the great decrease of response and recovery times. At the forward bias of 2 V, the Schottky contact PDs showed the biggest responsivity and the best detectivity at the annealing temperature of 300 ◦ C. For annealing temperature at 300 ◦ C and above, the responsivity decreases with increasing annealing temperature and the ratio of detectivity (D254 * to D546 * ) was calculated as high as 103 for all PDs annealed at 300 ◦ C and above. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the deep ultraviolet (UV) photodetector (PD) has attracted much attention. A one-dimensional (1D) nanomaterial, which takes possession of peculiar characteristics and quantum size effect, has attracted great interests both for fundamental research and potential nano-device applications [1,2]. Among the various nano-structured materials, due to its direct and wide energy bandgap (3.37 eV), ZnO nanorods (NRs) are a promising functional material as potential candidates for short-wavelength optoelectronics applications such as nanoscale lasers [3], lightemitting diodes [4], and UV PDs [5–9]. Although ZnO NRs have many advantages, it is very easy to form Ohmic contacts at the electrode/ZnO NRs interface, which is an obstacle to applications in PDs due to its slow response and recover behaviors. The Schottky barrier plays an important role to improve the performance of the PDs, and many researchers have investigated the Schottky contact between ZnO NRs and metal [10–14], but investigations of the carrier transport mechanism on ZnO NR Schottky PDs by using post-deposition thermal annealing treatment are seldom reported. ZnO film-based metal–insulator–semiconductor Schottky barrier ultraviolet PDs were reported by Ali et al. [12] and showed that the performance of the device was improved with increasing post-deposition annealing temperature up to 250 ◦ C
∗ Corresponding author. E-mail address: [email protected] (G.-J. Fang). 0921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2011.03.003
after metal deposition. For annealing temperature beyond 250 ◦ C, the performance of the device degrades drastically. In this study, to investigate the carrier transport mechanism on ZnO NR Schottky PDs, we introduce a simple route to gain Schottky barrier by deposition of Al and Pt electrodes on the top of hydrothermal prepared n-ZnO nanorods followed by thermal annealing process to form Ohmic or Schottky contacts. The PDs show the biggest responsivity of ∼4.5 A/W at 254 nm at the annealing temperature of 300 ◦ C and the ratio of D254 * to D546 * is calculated as high as 103 for all PDs annealed at 300 ◦ C and above. The attractiveness of this work is the simplicity of the fabrication process, which could easily be scaled up, and our results may pave the way for the application of low-cost ZnO NRs UV PDs. 2. Experimental methods The glass substrates were initially cleaned with acetone in an ultrasonic bath, rinsed with deionized water, and then blown dry with dry N2 . Then, an 80-nm ZnO seed layer was deposited by radio frequency reactive magnetron sputtering from a ZnO target at 100 ◦ C. Then, ZnO NRs were grown on ZnO coated glass substrate by hydrothermal method. The details of the hydrothermal conditions for obtaining ZnO NRs have already been reported elsewhere [9]. In brief, the nutrient solution was an aqueous solution of a 0.05 M zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O) and methenamine (C6 H12 N4 ). The reaction was kept at 100 ◦ C for 2 h. And then, we got the ZnO NRs flat film. After that, formation of Ohmic and Schottky contacts is carried out by a simple mask plate
H. Zhou et al. / Materials Science and Engineering B 176 (2011) 740–744
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Fig. 1. The top (a) and cross-sectional (b) views of the SEM photograph of the as-prepared ZnO nanorods. (c) XRD pattern of as-prepared ZnO NRs. The insert shows Raman spectrum of as-prepared ZnO NRs. (d) PL spectra of ZnO NRs with different annealing temperature.
with evaporating Al and radio frequency reactive magnetron sputtering Pt onto ZnO NRs, respectively. Finally, the devices were annealed in air at the temperature ranging from 200 to 500 ◦ C for 1 h. The morphology was observed by Sirion field emission scanning electron microscopy (SEM) (Philips XL30). The crystal structures of the films were characterized by X-ray diffraction (XRD, Bruker Axs, D8 Advance) using Cu K␣ radiation at 40 KV and 40 mA. Photoluminescence (PL, a spectrometer (ACTON2500) with 320 nm wavelength) and Raman (a Renishaw (RMRe1000) micro-Raman spectrometer with the 514 nm line of an Ar+ laser as the excitation source) were employed. The photosensitivity was performed by using 66984 Xe Arc source (300 W Oriel) and Oriel Cornerstone TM 260 1/4 m Monochromator. The sample was under illumination directly (parallel with the nanorods) and the optical power of light was measured by a UV-enhanced Si detector. All the I–V characteristics were measured by a Keithley 4200 electrometer.
3. Results and discussion In our experiment, the scanning electron microscopy (SEM) images of ZnO nanorods are shown in Fig. 1(a) and (b), which are the top and cross-sectional views of the as-prepared ZnO nanorods on glass substrate, respectively. From the photographs, the asprepared ZnO nanorods grow vertically and closely packed on the ZnO seed layer, the gap between ZnO nanorods is very little, and the average diameter and length of these ZnO nanorods are around 90–150 nm and 1.2 m. XRD pattern of ZnO NRs is shown in Fig. 1(c). From XRD pattern, we can see that the peak is located at 34.4◦ , which reveals that the ZnO NRs has c-axis preferential orientation and displays the highly oriented nature of the nanorod as well as its hexagonal wurtzite phase. Raman spectrum of the as-prepared ZnO NRs is shown in the insert of Fig. 1(c). One characteristic peak is observed around 437 cm−1 , which is assigned to the lattice vibration modes E2 (high) of ZnO and shows that the ZnO NRs are strongly oriented along the c-axis. Here, we can see that the full width at half maximum (FWHM) of the Raman peak is very small with the value of about 9.5 cm−1 , which shows that the crys-
tal perfection of the ZnO NRs structure is very good. Fig. 1(d) shows PL spectra of ZnO NRs with different annealing temperature, which are consisting of a strong UV peak at the wavelength of ∼380 nm, which could be related to the band gap of ZnO. From the PL spectra, the as-prepared ZnO NRs show a weak UV peak and a strong green band in the range of 500–650 nm, which indicates that the as-prepared ZnO NRs have some defects, such as oxygen vacancies. After annealing, the green band almost disappears, which demonstrates that the defects are reduced. Also, we can see that the UV emission increases with increasing annealing temperature, indicating that better crystal quality of ZnO NRs will be obtained at higher annealing temperature. The I–V curves of the Al/ZnO NRs/Pt structure are shown in Fig. 2(a) with the annealing temperature ranging from 200 to 500 ◦ C at dark. From the curves, the plots of I vs. V are straight lines at the annealing temperature below 300 ◦ C, which show that the contacts at the Pt/ZnO NRs and the Al/ZnO NRs interfaces are Ohmic. When the annealing temperature is 300 ◦ C and above, the plots of I vs. V are straight at the reverse biases, but at the forward biases the bent curves are shown with saturated characteristic, which is shown that the Schottky contacts between Pt and ZnO NRs are obtained and the contacts between Al and ZnO NRs are still Ohmic for the annealing temperature at 300 ◦ C or above. Also, with the increase of the annealing temperature, the dark current decreases greatly. Due to high-density carrier in the ZnO NRs fabricated by hydrothermal method, the contacts at the electrode/as-prepared ZnO NRs interface are normally Ohmic and are very hard to form Schottky contacts, even the contacts between as-prepared ZnO NRs and electrodes with high work function metals, such as Au, Ni and Pt. When ZnO NRs are annealed at certain temperature, contacts at the electrode/ZnO NRs interface can form Schottky contacts due to the decrease of the carrier density in the ZnO NRs and the modified interface. Fig. 2(b) shows the I–V curves of the devices annealed at 400 ◦ C, which demonstrates the contact characteristics between two Al electrodes on ZnO NRs at dark, between Al and Pt at dark and Al and Pt under 365 nm UV light, respectively. From the curves, we can see that the contacts between Al and ZnO NRs are good Ohmic
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Fig. 2. (a) The I–V curves of the Al/ZnO NRs/Pt structure with the annealing temperature ranging from 200 to 500 ◦ C at dark. The insert shows a schematic diagram of Al/ZnO NRs/Pt structure. (b) The I–V curves of the devices annealed at 400 ◦ C, which were measured at the contacts between Al and Al at dark, Al and Pt at dark and Al and Pt under 365 nm UV light, respectively.
Fig. 3. The dependences of photocurrents on operating time for PDs annealed at different temperatures under UV light (365 nm) with power density of 16.7 W/cm2 at the bias of 2 V; (a) as-prepared; (b) annealed at 300, 400 and 500 ◦ C.
that higher annealing temperature will result in higher Schottky barrier. The performance of PD is critically dependent on its response and recovery times, which are a primary factor to the application. Herein, the dependence of photocurrent on operating time for the PDs under UV light (365 nm) with power density of 16.7 W/cm2 at the bias of 2 V is investigated by varying annealing temperature. Fig. 3(a) displays the dependence of photocurrent on operating time for the as-prepared PDs and Fig. 3(b) displays the dependence of photocurrent on operating time for the PDs with annealing temperature at 300, 400 and 500 ◦ C. The performance ratings of the PDs are extracted from Fig. 3(a)–(d) and are listed in Table 1. From Table 1, under 365 nm UV illumination, the current of the asprepared device increases from 39.5 A to 53.0 A with 190 s and does not reach saturation. When turn off the UV lamp, the current decreases very slowly and the time is more than 212 s when the photocurrent decreases 80%. The response time is about 35, 30 and 24 s for PDs annealed at 300, 400 and 500 ◦ C, respectively, and the recovery time (the photocurrent decreases 80%) is about 40, 13 and 13 s, respectively. The ratio of photocurrent to dark current (Iph /Id ) is about 0.3 for as-prepared PD. For the PDs annealed at the temperature of 300, 400 and 500 ◦ C, the ratio of Iph /Id is about 2.2, 4.2 and 3.7, respectively. From above, we can see that the performance of PDs improves greatly, especially the response and recovery times
and Schottky contacts are formed between ZnO NRs and Pt. At the bias of 2 V, the current is about 171 nA under dark, and when the sample is illuminated with 365 nm UV light with power density of 16.7 W/cm2, the current is observed to be ∼1.18 A. Under UV illumination, the electron–hole pairs will be generated and resulting in the increase of free carrier density in ZnO. When a bias voltage was applied on the electrode, the photogenerated electron–hole pairs will be separated out of the depletion region and resulting in the increase of the photocurrent with voltage. The ratio of this photocurrent to dark currents is about 7. For Schottky barrier PDs, the actual barrier height at the electrode/semiconductor interface is an important part of the PDs under investigation. The Schottky barrier height can be determined by using forward-biased I–V measurements as in Eq. (1) [12]
q˚ B
I = A∗ AT 2 exp −
KT
exp
qV nKT
−1
(1)
where n is the ideally factor, K is the Boltzmann’s constant, T is the absolute temperature, ФB is the barrier height, A is the Schottky contact area, and A* is the effective Richardson coefficient constant. From Eq. (1), we find that Schottky barrier height ФB at the Pt/ZnO NRs interface is about 0.651, 0.679, 0.707 eV when the annealing temperature is 300, 400, 500 ◦ C, respectively. So we can deduce Table 1 Performance ratings of Al/ZnO NRs/Pt structure photodetectors. Annealing temperature (◦ C)
Dark current (nA)
Photo-current (nA)
As-prepared 300 400 500
3.94 × 104 738.4 173 46.5
1.36 × 104 1621.6 727 172.5
Response time (s) 190 27 21 22
Recover time (s) 212 40 13 13
The ratio of Iph /Id
Barrier height (eV)
0.3 2.2 4.2 3.7
– 0.651 0.679 0.707
H. Zhou et al. / Materials Science and Engineering B 176 (2011) 740–744
Fig. 4. The spectral responsivity curves of Al/ZnO NRs/Pt structure with different post-annealing temperature under the forward biases of 2 V.
decrease very much when the annealing temperature reaches to 300 ◦ C and the Schottky contacts appear. In the dark, oxygen is adsorbed at the surface of the nanorod to form a chemically adsorbed surface state [O2 + e− → O2 − ]. Under UV illumination, electron–hole pairs are generated by light absorption when photon energy exceeds the energy band gap (h > Eg ) [hv → e− + h+ ]. The adsorbed oxygen is photodesorbed from the surface [h+ + O2 − → O2 ], and the photon-desorption of oxygen at the metal electrode/semiconductor interface modifies the density of defects states and reduces the Schottky barrier, resulting in an increase in free carrier density [6,11]. Finally, unpaired electrons in the nanorod migrate to the electrodes at a bias voltage and contribute to the photocurrent. For Ohmic contacts ZnO NRs PDs, the carrier density is very high, and the density of defect states is very hard to be modified by the photon-desorption of oxygen at the metal electrode/semiconductor interface, which results in poor separation of the photoelectron–hole pairs and large ratio of the recombination of electron–hole pairs. So the photocurrent of Ohmic contact ZnO NRs PD is very slow to reach saturation until desorption and readsorption of O2 reach an equilibrium state. Upon turning off the UV light, the more rapid recovery characteristics are attributed to the existence of the Schottky contacts, where oxygen is only required to be readsorbed close to the interface to reduce the current [14]. Above 300 ◦ C, with the increase of annealing temperature, the Schottky barrier of PDs increases a little, the response and recovery times reach to minimum at the annealing temperature of 400 ◦ C, which shows that the Schottky contacts at the metal electrode/semiconductor interface are improved significantly by annealing treatment at 400 ◦ C. The responsivity (R) is an important parameter to reflect the performance of PDs, so the spectral R curves obtained from Al/ZnO NRs/Pt structure with different post-annealing temperature under the forward biases of 2 V are presented in Fig. 4. From the spectrum, we can see that a distinct spectral responsivity peak is formed near 395 nm, which is ascribed to bound-exciton and vacancy-related transitions in ZnO nanostructures [15,16]. Comparing the value of the peak in PL spectra, there is a slight red shift in peak position, which is due to the heat effect of injection current. This is because heat induced by the current will decrease its effective band gap of ZnO. At this wavelength, the responsivity of the device reaches to as high as ∼3.5 A/W and a biggest response is located at 254 nm with the responsivity of ∼4.5 A/W when the annealing temperature is 300 ◦ C, which is comparable with other reports [7,9]. With the increase of annealing temperature, the responsivity of the devices decreases sharply, which is consistent with the decrease of the carrier density of ZnO NRs.
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Fig. 5. The detectivity curves of Al/ZnO NRs/Pt structure at different post-annealing temperature under the forward biases of 2 V.
Another important parameter for PDs is the detectivity (D* ), which is given by the following [17] D∗ =
R (2qJd )1/2
=
Jph 1 Llight (2qJ )1/2 d
(2)
Here, R is the responsivity of the photodiode, Jd is the dark current, Jph is the photocurrent density, and Llight is the light intensity. Detectivity is calculated and plotted in Fig. 5. From the curves of the detectivity of PDs, the Schottky barrier PDs exhibited spectral response mainly in the range from 250 to 400 nm with the detectivity above 1012 Jones (1 Jones = 1 cmHz1/2 /W). At the wavelength above 400 nm, the PDs show little detectivity and the detectivity decreases with the increase of wavelength. The ratio of D254 * to D546 * is calculated as high as 103 when the PDs annealed at 300 ◦ C or above, which is shown that the PDs can work better in UV region with less influence of light with long wavelength. From the curves, higher detectivity for the annealing temperature at 500 ◦ C, we think, may be caused by lower dark current than that for the annealing temperature at 400 ◦ C at the wavelength below 350 nm. With varying annealing temperature, the best detectivity of PDs appears at the annealing temperature of 300 ◦ C. From Eq. (2), we by two factors, one is the R(D* ∝ R) can see that the D* was affected and another is Jd (D∗ ∝ 1/ Jd ). With the increase of annealing temperature, both the R and the Jd decrease, which can be seen from Fig. 4 and Table 1, respectively, but the R decreases more slowly than that of the Jd . So the PDs show the best detectivity at the annealing temperature of 300 ◦ C.
4. Conclusions In conclusion, the performance of Al/ZnO NRs/Pt structure PDs was investigated by using post-deposition heat treatment. The investigation showed that the formation of Schottky contacts at the electrode/ZnO NRs interface was realized at the annealing temperature up to 300 ◦ C. And the performance of the PDs was improved with increasing annealing temperature up to 300 ◦ C with the great decrease of response and recovery times. For annealing temperature above 300 ◦ C, the Schottky contacts PDs show decreasing responsivity with increasing annealing temperature and the biggest responsivity of ∼4.5 A/W located at 254 nm when the annealing temperature is 300 ◦ C at the forward bias of 2 V. The PDs show the best detectivity at the annealing temperature of 300 ◦ C and the ratio of D254 * to D546 * was calculated as high as 103 when the PDs were annealed at 300 ◦ C or above. The results may provide a simple route to gain low-cost and high performance UV PDs.
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H. Zhou et al. / Materials Science and Engineering B 176 (2011) 740–744
Acknowledgements This work was partially supported by the National High Technology Research and Development Program of China (2009AA03Z219), the National Basic Research Program (2011CB933300) of China, the National Natural Science Foundation of China (11074194) and the Natural Science Foundation of Hubei Province (2010CDA016). References [1] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [2] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113. [3] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R. He, H.J. Choi, Adv. Funct. Mater. 12 (2002) 323. [4] O. Lupan, T. Pauporté, B. Viana, Adv. Mater. 22 (2010) 3298–3302. [5] H. Kind, H. Yan, B. Messer, M. Law, P. Yang, Adv. Mater. 14 (2002) 158–160.
[6] C. Soci, A. Zhang, B. Xiang, S.A. Dayeh, D.P.R. Aplin, J. Park, X.Y. Bao, Y.H. Lo, D. Wang, Nano Lett. 7 (2007) 1003–1009. [7] Y.H. Leung, Z.B. He, L.B. Luo, C.H.A. Tsang, N.B. Wong, W.J. Zhang, S.T. Lee, Appl. Phys. Lett. 96 (2010) 053102. [8] O. Harnack, C. Pacholski, H. Weller, A. Yasuda, J.M. Wessels, Nano Lett. 3 (2003) 1097–1101. [9] H.H. Huang, G.J. Fang, X.M. Mo, L.Y. Yuan, H. Zhou, M.J. Wang, H.B. Xiao, X.Z. Zhao, Appl. Phys. Lett. 94 (2009) 063512. [10] W.Y. Chang, C.A. Lin, J.H. He, T.B. Wu, Appl. Phys. Lett. 96 (2010) 242109. [11] J. Zhou, Y. Gu, Y. Hu, W. Mai, P. Yeh, G. Bao, A.K. Sood, D.L. Polla, Z.L. Wang, Appl. Phys. Lett. 94 (2009) 191103. [12] G.M. Ali, P. Chakrabarti, Appl. Phys. Lett. 97 (2010) 031116. [13] S.M. Peng, Y.K. Su, L.W. Ji, C.Z. Wu, W.B. Cheng, W.C. Chao, J. Phys. Chem. C 114 (2010) 3204–3208. [14] Y. Jin, J. Wang, B. Sun, J.C. Blakesley, N.C. Greenham, Nano Lett. 8 (2008) 1649–1653. [15] A. Teke, Ü. Özgür, S. Dogan, X. Gu, H. Morkocü, B. Nemeth, J. Nause, H.O. Everitt, Phys. Rev. B 70 (2004) 195207. [16] B.P. Zhang, N.T. Binh, K. Wakatsuki, Y. Segawa, Y. Kashiwaba, K. Haga, Nanotechnology 15 (2004) 382–388. [17] X. Gong, M. Tong, Y. Xia, W. Cai, J.S. Moon, Y. Cao, G. Yu, C.L. Shieh, B. Nilsson, A.J. Heeger, Science 325 (2009) 25.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Effects of thermal annealing on the performance of Al/ZnO nanorods/Pt structure ultraviolet photodetector Hai Zhou, Guo-Jia Fang ∗ , Nishuang Liu, Xing-Zhong Zhao Department of Electronic Science and Technology and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 12 October 2010 Received in revised form 25 January 2011 Accepted 13 March 2011 Keywords: ZnO nanorods Ultraviolet Photodetector Schottky
a b s t r a c t ZnO nanorod arrays were fabricated on ZnO coated glass substrate by hydrothermal method. Schottky barrier ultraviolet photodetectors (PDs) were obtained by sputtering Pt electrode and evaporating Al electrode on the top of ZnO nanorod arrays with thermal treatment. It is illustrated that Schottky contacts at the electrode/ZnO NRs interface were formed at the annealing temperature of 300 ◦ C and above. When annealing temperature was up to 300 ◦ C, the performance of the PDs was improved with the great decrease of response and recovery times. At the forward bias of 2 V, the Schottky contact PDs showed the biggest responsivity and the best detectivity at the annealing temperature of 300 ◦ C. For annealing temperature at 300 ◦ C and above, the responsivity decreases with increasing annealing temperature and the ratio of detectivity (D254 * to D546 * ) was calculated as high as 103 for all PDs annealed at 300 ◦ C and above. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the deep ultraviolet (UV) photodetector (PD) has attracted much attention. A one-dimensional (1D) nanomaterial, which takes possession of peculiar characteristics and quantum size effect, has attracted great interests both for fundamental research and potential nano-device applications [1,2]. Among the various nano-structured materials, due to its direct and wide energy bandgap (3.37 eV), ZnO nanorods (NRs) are a promising functional material as potential candidates for short-wavelength optoelectronics applications such as nanoscale lasers [3], lightemitting diodes [4], and UV PDs [5–9]. Although ZnO NRs have many advantages, it is very easy to form Ohmic contacts at the electrode/ZnO NRs interface, which is an obstacle to applications in PDs due to its slow response and recover behaviors. The Schottky barrier plays an important role to improve the performance of the PDs, and many researchers have investigated the Schottky contact between ZnO NRs and metal [10–14], but investigations of the carrier transport mechanism on ZnO NR Schottky PDs by using post-deposition thermal annealing treatment are seldom reported. ZnO film-based metal–insulator–semiconductor Schottky barrier ultraviolet PDs were reported by Ali et al. [12] and showed that the performance of the device was improved with increasing post-deposition annealing temperature up to 250 ◦ C
∗ Corresponding author. E-mail address: [email protected] (G.-J. Fang). 0921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2011.03.003
after metal deposition. For annealing temperature beyond 250 ◦ C, the performance of the device degrades drastically. In this study, to investigate the carrier transport mechanism on ZnO NR Schottky PDs, we introduce a simple route to gain Schottky barrier by deposition of Al and Pt electrodes on the top of hydrothermal prepared n-ZnO nanorods followed by thermal annealing process to form Ohmic or Schottky contacts. The PDs show the biggest responsivity of ∼4.5 A/W at 254 nm at the annealing temperature of 300 ◦ C and the ratio of D254 * to D546 * is calculated as high as 103 for all PDs annealed at 300 ◦ C and above. The attractiveness of this work is the simplicity of the fabrication process, which could easily be scaled up, and our results may pave the way for the application of low-cost ZnO NRs UV PDs. 2. Experimental methods The glass substrates were initially cleaned with acetone in an ultrasonic bath, rinsed with deionized water, and then blown dry with dry N2 . Then, an 80-nm ZnO seed layer was deposited by radio frequency reactive magnetron sputtering from a ZnO target at 100 ◦ C. Then, ZnO NRs were grown on ZnO coated glass substrate by hydrothermal method. The details of the hydrothermal conditions for obtaining ZnO NRs have already been reported elsewhere [9]. In brief, the nutrient solution was an aqueous solution of a 0.05 M zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O) and methenamine (C6 H12 N4 ). The reaction was kept at 100 ◦ C for 2 h. And then, we got the ZnO NRs flat film. After that, formation of Ohmic and Schottky contacts is carried out by a simple mask plate
H. Zhou et al. / Materials Science and Engineering B 176 (2011) 740–744
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Fig. 1. The top (a) and cross-sectional (b) views of the SEM photograph of the as-prepared ZnO nanorods. (c) XRD pattern of as-prepared ZnO NRs. The insert shows Raman spectrum of as-prepared ZnO NRs. (d) PL spectra of ZnO NRs with different annealing temperature.
with evaporating Al and radio frequency reactive magnetron sputtering Pt onto ZnO NRs, respectively. Finally, the devices were annealed in air at the temperature ranging from 200 to 500 ◦ C for 1 h. The morphology was observed by Sirion field emission scanning electron microscopy (SEM) (Philips XL30). The crystal structures of the films were characterized by X-ray diffraction (XRD, Bruker Axs, D8 Advance) using Cu K␣ radiation at 40 KV and 40 mA. Photoluminescence (PL, a spectrometer (ACTON2500) with 320 nm wavelength) and Raman (a Renishaw (RMRe1000) micro-Raman spectrometer with the 514 nm line of an Ar+ laser as the excitation source) were employed. The photosensitivity was performed by using 66984 Xe Arc source (300 W Oriel) and Oriel Cornerstone TM 260 1/4 m Monochromator. The sample was under illumination directly (parallel with the nanorods) and the optical power of light was measured by a UV-enhanced Si detector. All the I–V characteristics were measured by a Keithley 4200 electrometer.
3. Results and discussion In our experiment, the scanning electron microscopy (SEM) images of ZnO nanorods are shown in Fig. 1(a) and (b), which are the top and cross-sectional views of the as-prepared ZnO nanorods on glass substrate, respectively. From the photographs, the asprepared ZnO nanorods grow vertically and closely packed on the ZnO seed layer, the gap between ZnO nanorods is very little, and the average diameter and length of these ZnO nanorods are around 90–150 nm and 1.2 m. XRD pattern of ZnO NRs is shown in Fig. 1(c). From XRD pattern, we can see that the peak is located at 34.4◦ , which reveals that the ZnO NRs has c-axis preferential orientation and displays the highly oriented nature of the nanorod as well as its hexagonal wurtzite phase. Raman spectrum of the as-prepared ZnO NRs is shown in the insert of Fig. 1(c). One characteristic peak is observed around 437 cm−1 , which is assigned to the lattice vibration modes E2 (high) of ZnO and shows that the ZnO NRs are strongly oriented along the c-axis. Here, we can see that the full width at half maximum (FWHM) of the Raman peak is very small with the value of about 9.5 cm−1 , which shows that the crys-
tal perfection of the ZnO NRs structure is very good. Fig. 1(d) shows PL spectra of ZnO NRs with different annealing temperature, which are consisting of a strong UV peak at the wavelength of ∼380 nm, which could be related to the band gap of ZnO. From the PL spectra, the as-prepared ZnO NRs show a weak UV peak and a strong green band in the range of 500–650 nm, which indicates that the as-prepared ZnO NRs have some defects, such as oxygen vacancies. After annealing, the green band almost disappears, which demonstrates that the defects are reduced. Also, we can see that the UV emission increases with increasing annealing temperature, indicating that better crystal quality of ZnO NRs will be obtained at higher annealing temperature. The I–V curves of the Al/ZnO NRs/Pt structure are shown in Fig. 2(a) with the annealing temperature ranging from 200 to 500 ◦ C at dark. From the curves, the plots of I vs. V are straight lines at the annealing temperature below 300 ◦ C, which show that the contacts at the Pt/ZnO NRs and the Al/ZnO NRs interfaces are Ohmic. When the annealing temperature is 300 ◦ C and above, the plots of I vs. V are straight at the reverse biases, but at the forward biases the bent curves are shown with saturated characteristic, which is shown that the Schottky contacts between Pt and ZnO NRs are obtained and the contacts between Al and ZnO NRs are still Ohmic for the annealing temperature at 300 ◦ C or above. Also, with the increase of the annealing temperature, the dark current decreases greatly. Due to high-density carrier in the ZnO NRs fabricated by hydrothermal method, the contacts at the electrode/as-prepared ZnO NRs interface are normally Ohmic and are very hard to form Schottky contacts, even the contacts between as-prepared ZnO NRs and electrodes with high work function metals, such as Au, Ni and Pt. When ZnO NRs are annealed at certain temperature, contacts at the electrode/ZnO NRs interface can form Schottky contacts due to the decrease of the carrier density in the ZnO NRs and the modified interface. Fig. 2(b) shows the I–V curves of the devices annealed at 400 ◦ C, which demonstrates the contact characteristics between two Al electrodes on ZnO NRs at dark, between Al and Pt at dark and Al and Pt under 365 nm UV light, respectively. From the curves, we can see that the contacts between Al and ZnO NRs are good Ohmic
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Fig. 2. (a) The I–V curves of the Al/ZnO NRs/Pt structure with the annealing temperature ranging from 200 to 500 ◦ C at dark. The insert shows a schematic diagram of Al/ZnO NRs/Pt structure. (b) The I–V curves of the devices annealed at 400 ◦ C, which were measured at the contacts between Al and Al at dark, Al and Pt at dark and Al and Pt under 365 nm UV light, respectively.
Fig. 3. The dependences of photocurrents on operating time for PDs annealed at different temperatures under UV light (365 nm) with power density of 16.7 W/cm2 at the bias of 2 V; (a) as-prepared; (b) annealed at 300, 400 and 500 ◦ C.
that higher annealing temperature will result in higher Schottky barrier. The performance of PD is critically dependent on its response and recovery times, which are a primary factor to the application. Herein, the dependence of photocurrent on operating time for the PDs under UV light (365 nm) with power density of 16.7 W/cm2 at the bias of 2 V is investigated by varying annealing temperature. Fig. 3(a) displays the dependence of photocurrent on operating time for the as-prepared PDs and Fig. 3(b) displays the dependence of photocurrent on operating time for the PDs with annealing temperature at 300, 400 and 500 ◦ C. The performance ratings of the PDs are extracted from Fig. 3(a)–(d) and are listed in Table 1. From Table 1, under 365 nm UV illumination, the current of the asprepared device increases from 39.5 A to 53.0 A with 190 s and does not reach saturation. When turn off the UV lamp, the current decreases very slowly and the time is more than 212 s when the photocurrent decreases 80%. The response time is about 35, 30 and 24 s for PDs annealed at 300, 400 and 500 ◦ C, respectively, and the recovery time (the photocurrent decreases 80%) is about 40, 13 and 13 s, respectively. The ratio of photocurrent to dark current (Iph /Id ) is about 0.3 for as-prepared PD. For the PDs annealed at the temperature of 300, 400 and 500 ◦ C, the ratio of Iph /Id is about 2.2, 4.2 and 3.7, respectively. From above, we can see that the performance of PDs improves greatly, especially the response and recovery times
and Schottky contacts are formed between ZnO NRs and Pt. At the bias of 2 V, the current is about 171 nA under dark, and when the sample is illuminated with 365 nm UV light with power density of 16.7 W/cm2, the current is observed to be ∼1.18 A. Under UV illumination, the electron–hole pairs will be generated and resulting in the increase of free carrier density in ZnO. When a bias voltage was applied on the electrode, the photogenerated electron–hole pairs will be separated out of the depletion region and resulting in the increase of the photocurrent with voltage. The ratio of this photocurrent to dark currents is about 7. For Schottky barrier PDs, the actual barrier height at the electrode/semiconductor interface is an important part of the PDs under investigation. The Schottky barrier height can be determined by using forward-biased I–V measurements as in Eq. (1) [12]
q˚ B
I = A∗ AT 2 exp −
KT
exp
qV nKT
−1
(1)
where n is the ideally factor, K is the Boltzmann’s constant, T is the absolute temperature, ФB is the barrier height, A is the Schottky contact area, and A* is the effective Richardson coefficient constant. From Eq. (1), we find that Schottky barrier height ФB at the Pt/ZnO NRs interface is about 0.651, 0.679, 0.707 eV when the annealing temperature is 300, 400, 500 ◦ C, respectively. So we can deduce Table 1 Performance ratings of Al/ZnO NRs/Pt structure photodetectors. Annealing temperature (◦ C)
Dark current (nA)
Photo-current (nA)
As-prepared 300 400 500
3.94 × 104 738.4 173 46.5
1.36 × 104 1621.6 727 172.5
Response time (s) 190 27 21 22
Recover time (s) 212 40 13 13
The ratio of Iph /Id
Barrier height (eV)
0.3 2.2 4.2 3.7
– 0.651 0.679 0.707
H. Zhou et al. / Materials Science and Engineering B 176 (2011) 740–744
Fig. 4. The spectral responsivity curves of Al/ZnO NRs/Pt structure with different post-annealing temperature under the forward biases of 2 V.
decrease very much when the annealing temperature reaches to 300 ◦ C and the Schottky contacts appear. In the dark, oxygen is adsorbed at the surface of the nanorod to form a chemically adsorbed surface state [O2 + e− → O2 − ]. Under UV illumination, electron–hole pairs are generated by light absorption when photon energy exceeds the energy band gap (h > Eg ) [hv → e− + h+ ]. The adsorbed oxygen is photodesorbed from the surface [h+ + O2 − → O2 ], and the photon-desorption of oxygen at the metal electrode/semiconductor interface modifies the density of defects states and reduces the Schottky barrier, resulting in an increase in free carrier density [6,11]. Finally, unpaired electrons in the nanorod migrate to the electrodes at a bias voltage and contribute to the photocurrent. For Ohmic contacts ZnO NRs PDs, the carrier density is very high, and the density of defect states is very hard to be modified by the photon-desorption of oxygen at the metal electrode/semiconductor interface, which results in poor separation of the photoelectron–hole pairs and large ratio of the recombination of electron–hole pairs. So the photocurrent of Ohmic contact ZnO NRs PD is very slow to reach saturation until desorption and readsorption of O2 reach an equilibrium state. Upon turning off the UV light, the more rapid recovery characteristics are attributed to the existence of the Schottky contacts, where oxygen is only required to be readsorbed close to the interface to reduce the current [14]. Above 300 ◦ C, with the increase of annealing temperature, the Schottky barrier of PDs increases a little, the response and recovery times reach to minimum at the annealing temperature of 400 ◦ C, which shows that the Schottky contacts at the metal electrode/semiconductor interface are improved significantly by annealing treatment at 400 ◦ C. The responsivity (R) is an important parameter to reflect the performance of PDs, so the spectral R curves obtained from Al/ZnO NRs/Pt structure with different post-annealing temperature under the forward biases of 2 V are presented in Fig. 4. From the spectrum, we can see that a distinct spectral responsivity peak is formed near 395 nm, which is ascribed to bound-exciton and vacancy-related transitions in ZnO nanostructures [15,16]. Comparing the value of the peak in PL spectra, there is a slight red shift in peak position, which is due to the heat effect of injection current. This is because heat induced by the current will decrease its effective band gap of ZnO. At this wavelength, the responsivity of the device reaches to as high as ∼3.5 A/W and a biggest response is located at 254 nm with the responsivity of ∼4.5 A/W when the annealing temperature is 300 ◦ C, which is comparable with other reports [7,9]. With the increase of annealing temperature, the responsivity of the devices decreases sharply, which is consistent with the decrease of the carrier density of ZnO NRs.
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Fig. 5. The detectivity curves of Al/ZnO NRs/Pt structure at different post-annealing temperature under the forward biases of 2 V.
Another important parameter for PDs is the detectivity (D* ), which is given by the following [17] D∗ =
R (2qJd )1/2
=
Jph 1 Llight (2qJ )1/2 d
(2)
Here, R is the responsivity of the photodiode, Jd is the dark current, Jph is the photocurrent density, and Llight is the light intensity. Detectivity is calculated and plotted in Fig. 5. From the curves of the detectivity of PDs, the Schottky barrier PDs exhibited spectral response mainly in the range from 250 to 400 nm with the detectivity above 1012 Jones (1 Jones = 1 cmHz1/2 /W). At the wavelength above 400 nm, the PDs show little detectivity and the detectivity decreases with the increase of wavelength. The ratio of D254 * to D546 * is calculated as high as 103 when the PDs annealed at 300 ◦ C or above, which is shown that the PDs can work better in UV region with less influence of light with long wavelength. From the curves, higher detectivity for the annealing temperature at 500 ◦ C, we think, may be caused by lower dark current than that for the annealing temperature at 400 ◦ C at the wavelength below 350 nm. With varying annealing temperature, the best detectivity of PDs appears at the annealing temperature of 300 ◦ C. From Eq. (2), we by two factors, one is the R(D* ∝ R) can see that the D* was affected and another is Jd (D∗ ∝ 1/ Jd ). With the increase of annealing temperature, both the R and the Jd decrease, which can be seen from Fig. 4 and Table 1, respectively, but the R decreases more slowly than that of the Jd . So the PDs show the best detectivity at the annealing temperature of 300 ◦ C.
4. Conclusions In conclusion, the performance of Al/ZnO NRs/Pt structure PDs was investigated by using post-deposition heat treatment. The investigation showed that the formation of Schottky contacts at the electrode/ZnO NRs interface was realized at the annealing temperature up to 300 ◦ C. And the performance of the PDs was improved with increasing annealing temperature up to 300 ◦ C with the great decrease of response and recovery times. For annealing temperature above 300 ◦ C, the Schottky contacts PDs show decreasing responsivity with increasing annealing temperature and the biggest responsivity of ∼4.5 A/W located at 254 nm when the annealing temperature is 300 ◦ C at the forward bias of 2 V. The PDs show the best detectivity at the annealing temperature of 300 ◦ C and the ratio of D254 * to D546 * was calculated as high as 103 when the PDs were annealed at 300 ◦ C or above. The results may provide a simple route to gain low-cost and high performance UV PDs.
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Acknowledgements This work was partially supported by the National High Technology Research and Development Program of China (2009AA03Z219), the National Basic Research Program (2011CB933300) of China, the National Natural Science Foundation of China (11074194) and the Natural Science Foundation of Hubei Province (2010CDA016). References [1] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [2] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113. [3] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R. He, H.J. Choi, Adv. Funct. Mater. 12 (2002) 323. [4] O. Lupan, T. Pauporté, B. Viana, Adv. Mater. 22 (2010) 3298–3302. [5] H. Kind, H. Yan, B. Messer, M. Law, P. Yang, Adv. Mater. 14 (2002) 158–160.
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