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Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector
Materials Chemistry and Physics xxx (2015) 1e6
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector Yaxue Zhao a, Junjie Qi a, *, Chandan Biswas b, Feng Li a, Kui Zhang a, Xin Li a, Yue Zhang a, c, ** a b c
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China Department of Electrical Engineering, University of California Los Angeles, California 90095, USA Key Laboratory of New Energy Materials and Technologies, University of Science and Technology Beijing, Beijing 100083, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A self-powered Schottky barrier UV photodetector based on 1-D ZnO is fabricated. For the first time we investigate the local irradiation effects of UV detector. Irradiating both the junctions and ZnO can optimize the performance of the device.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 April 2015 Received in revised form 18 August 2015 Accepted 19 September 2015 Available online xxx
A self-powered metal-semiconductor-metal (MSM) UV photodetector was successfully fabricated based on Ag/ZnO/Au structure with asymmetric Schottky barriers. This exhibits excellent performance compared to many previous studies. Very high photo-to-dark current ratio (approximately 105e106) was demonstrated without applying any external bias, and very fast switching time of less than 30 ms was observed during the investigation. Opposite photocurrent direction was generated by irradiating different Schottky diodes in the fabricated photodetector. Furthermore, the device performance was optimized by largely irradiating both the ZnO microwire (MW) junctions. Schottky barrier effect theory and O2 adsorptionedesorption theories were used to investigate the phenomenon. The device has potential applications in self-powered UV detection field and can be used as electrical power source for electronic, optoelectronic and mechanical devices. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Semiconductors Chemical vapour deposition Electrical properties
1. Introduction ZnO material has excellent optical, optoelectronic, piezoelectric, field emission, wave-absorbing, and photocatalytic properties * Corresponding author. ** Corresponding author. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China. E-mail addresses: [email protected] (J. Qi), [email protected] (Y. Zhang).
among atypical II-VI group semiconductors. Recently, ZnO micro/ nano wires are widely studied by numbers of researchers in the applications of light emitting devices [1e3], nanogenerators [4e6], solar cells [7,8] and photodetectors [9e16]. Moreover, ZnO micro/ nano wires are particularly attractive for photodetectors due to its carrier confinement, high surface states and high surface-tovolume ratio. Furthermore, ZnO micro/nano wires can be easily obtained under low temperature with low production cost capabilities [17,18].
http://dx.doi.org/10.1016/j.matchemphys.2015.09.034 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Zhao, et al., Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.034
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Y. Zhao et al. / Materials Chemistry and Physics xxx (2015) 1e6
Previously reported UV photodetectors can be divided into two categories: the pen junction type [19e21] and the Schottky barrier type [22e24]. Schottky barrier based photodetectors are easier to assemble and have faster response time in comparison with the pen junction photodetectors. It has also been demonstrated that the UV sensitivity of the MSM device structure can be greatly improved by replacing the Ohmic contact to the Schottky contact in either sides of the two electrode photodetector system [25]. However, there were no reports about UV local irradiation effects on different junctions of asymmetric Schottky barrier type photodetectors. Low energy consumption of the fabricated devices is among the main research target of the scientific communities, recently. However, most of the previously demonstrated photodetectors still need external voltage source in order to operate. In this paper, the MSM structure based UV photodetectors were fabricated with ZnO MW and two different metals (Ag and Au). UV local irradiation effects of asymmetric Schottky junctions were studied in this investigation. Currentevoltage characteristics of the device under dark and UV illumination were analyzed. IeV characteristics of the device under local UV light irradiation on the junctions were also tested. Moreover, the self-powered properties of the device were investigated under local and global UV irradiation conditions and related mechanism was discussed. 2. Experiment Pure ZnO micro/nano wires were prepared by chemical vapour deposition (CVD) method. Firstly, pure ZnO and C powder was mixed evenly with the molar ratio of 1:1 and put at the bottom of a porcelain boat as evaporation source. A Si wafer was then covered upside down over the mixed powder boat to collect necessary samples. The entire porcelain boat was then loaded in the middle of the horizontal furnace at 950 C temperature under the flowing gas mixture of Ar and O2 (ratio of 80:1sccm)for 15 min. Finally, the boat containing ZnO micro/nano wires was taken out as white samples collected on the Si wafer. The samples were scraped into deionized water by ultra sonication method for further use. 500 nm thin insulating SiO2layer on the Si wafer with a size of 6 mm 6 mm was used as a substrate. Au film was sputtered as electrode covering half of the substrate. 60 nm thick Au film was deposited by3 times DC sputtering with 40 mA current for 45 s. A ZnO MW was put across the junction of Au electrode and insulating silicon, and Ag paste was used to fix the end of ZnO wire which was in contact with the insulating wafer. The other end of the ZnO wire on the Au electrode was fixed by non-conductive polydimethylsiloxane (PDMS). Ag paste was also used to wire bond Au electrode far away from ZnO region. Two electric wires of the device were both led out using Ag paste. Thus, the device was finished by consisting asymmetric Schottky junctions of different barriers (Fig. 1(c)). The scanning electron microscopic (SEM) images of the synthesized ZnO MWs were obtained by a field emission scanning electron microscope (FE-SEM, LEO1530). The Raman spectrum was taken using a Raman system (HR800, Horiba Jobin Yvon) with a laser wavelength of 514 nm at room temperature. The device morphology was captured by an optical microscope. The electrical performances of the devices were tested by using a semiconductor characterization system (Keithley 4200-SCS). A UV lamp (Spectronics-L980683) emitting at 365 nm was used to irradiate the device. 3. Results and discussion The typical SEM image of the synthesized ZnO samples was represented in Fig. 1(a). SEM micrograph clearly shows wide
diameter distribution of the ZnO MW ranges from several tens of nanometers to several micrometers with length distribution in the order of several tens of micrometers. The figure inset shows the FESEM image of a hexagonal prism structured ZnO MW. Raman spectrum of the samples was taken at room temperature in order to characterize ZnO structure. We calibrated the vibration modulus and wave numbers in the Raman spectrum of the ZnO samples that shown in Fig. 1(b) by referring to all the peaks of ZnO single crystal high as Elow , A1(TO), A1(LO), E1(TO) and E1(LO). Two strong peaks at 2 , E2 98 cm1 and 438 cm1 corresponded to the non-polar Elow vibra2 high tion mode and E2 vibration mode owing to the wurtzite phase, while two weak waves of 331 cm1and 584 cm1 accorded with high E2 Elow difference frequency peak and E1(LO) mode was 2 observed respectively. The peak at 520 cm1 was from the Silicon wafer in Raman spectrum. Hence, the results above clearly proved the formation of highly crystalline ZnO micro/nano wires. Fig. 1(c) shows the schematic diagram of the fabricated photodetectors in which asymmetric Schottky barriers were formed with two different electrodes (Ag and Au) on the two ends of the ZnO MW. Fig. 1(d) illustrates the device micrograph taken from an optical microscope. IeV curves in dark and under UV illumination were tested in order to investigate the electrical properties and the asymmetric Schottky diodes as shown in Fig. 2(a). The shape of the IeV curves shows typical back-to-back Schottky characteristics. The photodetector can be considered as two back-to-back Schottky diodes. The voltage mainly drops at the reversely biased Schottky barrier side. Ag electrode was grounded and Au electrode was connected with sweeping voltage for all the experiments during IeV characteristic. Ag/ZnO contact stayed at reversed bias condition and determine the dark current of the device while photodetector kept under positive voltage (0 Ve5 V) condition. Conversely, when the device is under negative voltage (5 V ~0 V), Au/ZnO becomes reversed bias. The dark current shown in Fig. 2(a) under negative voltage is several times larger than positively biased condition. The thermionic emission with barrier lowering is usually the dominant current transport in a Schottky barrier under reversed bias the thermionic emission-diffusion theory (for 3kT=qz70 mV) according to the following equation for a Schottky barrier,
dark
I
e∅dark ¼ A ST exp kT
2
!
eVa exp 1 kT
(1)
Where Idark is the dark current, A* the effective Richardson constant, S the area of the Schottky contact, T the temperature, q the unit electronic charge, k the Boltzmann constant, ∅dark the Schottky Barrier Height (SBH), Va the applied voltage. It can be found that lower SBH results in higher dark current under negative voltage, when A*, S, T, Va are constant. This was because the lower SBH can improve the holes accumulation and capture process resulting in the generation of internal gain. Therefore, Ag/ZnO diode has higher SBH than Au/ZnO diode in the fabricated device. Although the work function of Ag (4.26eV) was lower than the electron affinity of ZnO (4.5 eV) and Ag/ZnO contact type was Ohmic contact type theoretically, Schottky type contact was quite possibly formed between Ag paste and ZnO MW. An interfacial layer (mainly including adsorbed oxygen, hydroxide, moisture, etc) was formed on the surface of Zinc Oxide micro/nanowire due to high surface-to-volume ratio and ambient experimental conditions in air. This had a thickness of one or two monolayers [26,27], thus resulting in the existence of abundant surface states on the ZnO surface. An upward band bending was then generated and a potential barrier was formed on the surface. This was almost independent of the work function of Ag due to the pinning of Fermi
Please cite this article in press as: Y. Zhao, et al., Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.034
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Fig. 1. (a) Low-magnification SEM of the ZnO micro/nano wires are synthesized by CVD method. The inset shows high-magnification SEM of the ZnO MW. (b) The Raman spectrum of ZnO MWs. (c) Schematic diagram of the fabricated asymmetric Schottky barrier type photodetectors. (d) Low-magnification SEM of the fabricated ZnO MW photodetector. The inset shows high-magnification of the Ag/ZnO junction.
Fig. 2. (a) IeV curves of the fabricated photodetector in dark and under UV illumination with a 5 V ~ 5 V sweeping voltage applied. (b) The IeV curves near 0 V bias. (c) The I-t curve at 0 V bias when the device was irradiated under a 365 nm UV light which was turned on and off periodically. Note the on/off period is 30 s. (d) The I-t curve at 0.5 V bias when the device was irradiated under a 365 nm UV light which was turned on and off periodically.
Please cite this article in press as: Y. Zhao, et al., Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.034
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energy. As a conclusion, the barrier height was not only controlled by the difference in the work function value, but also dominated by the surface states or band bending in ZnO itself as pointed out by Bardeen [28] and others [29,30]. When the density of surface states was high enough, the barrier height controlled by the surface states must dominate the IeV rectifying behavior [30,31].The current under UV illumination is significant when photon energy was larger than the band energy of ZnO, thus increasing the carriers concentration resulting in an increase of the device conductance as shown in Fig. 2(a). Secondly, holes discharge effects reduce the charge concentration (Qi) on the Schottky interface this reduces the width of the depletion layer and consequently increases the possibility of electrons to pass through the layer. Thirdly, the SBH can be expressed as:
∅ ¼ qVbi þ Ecf ¼
Q 2i 2Ns εS2
þ Ecf
(2)
where ∅ is the SBH, Vbi the built-in potential in the depletion layer, Ecf the deviation between the conduction and Fermi energy of ZnO, Qi the concentration of charges on the interface of Schottky barrier, Ns the concentration of space charges, 3 the dielectric constant. The reduced concentration Qi makes the SBH decline and hence promotes carriers transit through the SBH more easily. The IeV curves near 0 V bias was magnified and demonstrated in Fig. 2(b) in order to investigate the self-potential generated under UV irradiation. Fig 2(b) indicates the open-circuit voltage (Voc) of 0.021 V and the short-circuit current (Isc) of 82 nA. This means the self-potential was observed as 0.021 V under open circuit condition. Furthermore, the photo detection properties of the fabricated device were investigated thoroughly. The device was placed under a 365 nm UV light which was turned on and off
periodically. The on/off period is about 30 s. The detector showed quite short response time and stable output photocurrent (as shown in Fig. 2(c)) under 0 V bias. Dark current fluctuates was in the range of 3.4 1013 A to 1.1 1012 A. The photocurrent was nearly 120 nA (1.2 107 A) under UV illumination with 0.68 mW/ cm2power density. The sensitivity (photo-to-dark current ratio) was approximately 105 ~ 106, ultra high for a single MW device [32e34].The formation of the Schottky barriers lead to selfpowered performance. I-t performance under a constant voltage bias condition was measured in order to investigate the device performance further. The device dark current was increased to 2.2 mA while corresponding photocurrent was observed around 2.9 mA at 0.5 V bias shown in the I-t curve demonstrated in Fig. 2(d). The sensitivity at 0.5 V bias calculated was 1.32. The sensitivity at 0.5 V bias was much lower than that at 0 V bias due to the presence of particularly low dark current at 0 V bias conditions compare to 0.5 V bias cases. The device was irradiated on the Ag/ZnO junction and Au/ZnO junction separately in order to study UV local irradiation effects in the asymmetric Schottky barrier type photodetector. Fig. 3(a) shows the IeV curves while irradiating the Ag/ZnO junction locally and the corresponding currentetime (I-t) curves at 0 V bias are shown in Fig. 3(b). In comparison, IeV and I-t curves shown in Fig. 3(c) and (d) respectively were obtained by locally irradiated the Au/ZnO junction. Ag/ZnO junction was under reversed bias while applied voltage ranges from 0 Ve5 Vas shown in Fig. 3(a). The difference between dark current curve and photocurrent curve was more significant at 0 Ve5 V bias than that at 5 V ~0 V bias range. This was probably due to the photodetector Schottky junction controlled device current as explained above. Photocurrents of opposite directions measured were about 100 nA (Iill 1 ) (Fig. 3(b)) and 33 nA (Iill 2 ) (Fig. 3(d)), respectively. On the contrary, local
Fig. 3. (a) The IeV curves in dark and when the Ag/ZnO junction of the device was irradiated locally under UV light. The inset shows the schematic diagram when Ag/ZnO junction of the device was irradiated locally. (b) The corresponding I-t curve of Fig. 3(a) at 0 V bias when the light was turned on and off periodically. (c) The IeV curves in dark and when the Au/ZnO junction of the device was irradiated locally under UV light. The inset shows the schematic diagram when Ag/ZnO junction of the device was irradiated locally. (d) The corresponding I-t curve of Fig. 3(c) at 0 V bias when the light was turned on and off periodically.
Please cite this article in press as: Y. Zhao, et al., Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.034
Y. Zhao et al. / Materials Chemistry and Physics xxx (2015) 1e6
irradiation on Au/ZnO junction resulted in reversed bias under applied voltage ranges 0 Ve5 V as shown in Fig. 3(c). The difference between dark current curve and photocurrent curve is more significant at 5 V ~0 V bias compare to 0 Ve5 V bias conditions. This also proved that good Schottky contacts were formed at the two ends of ZnO MW. Fig. 4(a) shows the changes in the photodetector generated current by periodically turned on and off UV light exposure with different light intensities. The dark currents were observed identical without normalization, this proves that the device performance was extraordinary stable and reproducible. The experimental data and fitted curve of the photocurrent-light intensity were shown in Fig. 4(b). Photocurrent and light intensity showed nearelinear relationship. The carrier lifetime is relatively long under weak UV intensity thus the photocurrent was proportional to the illuminated light flux. Fig. 4(c) and (d) shows the enlarged rise and recovery process of the current response under 365 nm UV illumination. The current on/off process follows a biexponential relaxation equation:
t t I ¼ I0 þ C1 exp þ C2 exp t1 t2
(3)
Where t1 and t2 are two time constants. t1 is related to the rapid recombination of photoinduced electrons in the conduction band and holes in the valence band. t1 corresponds to the longer process of desorption and adsorption of oxygen molecules on the surfaces of ZnO MW. The rise time (tr, photocurrent increased from 10%e 90% of the maximum) and recovery time (td, photocurrent dropped from 90% to 10% of the maximum) of the device was estimated to be about 23msunder light on to off conditions and vice versa. Fig. 5 was shown to explain the UV light detection principle of
5
the photodetector. Firstly, photocurrents were generated resulting from the Schottky barrier separation effect. Electron-hole pairs are generated under UV illumination at the junctions of Ag/ZnO and Au/ZnO, this quickly swept away from the space charge region by the built-in electric field on the Schottky barrier interface. Metalsemiconductor interface generates certain directional current under photoexcitation conditions. This was the driving force of the self-powered device. Secondly, the photodetector was fabricated by two back-to-back Schottky junctions generating current of opposite directions under UV illumination. Due to this reasons, the photocurrent of opposite directions were tested when the two junctions were irradiated locally by the UV light. Moreover, the SBH of Ag/ ZnO (∅1 ) was higher than that of Au/ZnO (∅2 ) in the photodetector. This results Schottky barrier separation effect of Ag/ZnO junction more effective and corresponding I1>I2 as shown in Fig. 5(a). So the current direction of the whole device Iill was the same with I1. Schottky barriers provide driving force of the self-powered ill ill ill ill currents. The device results Iill > Iill 1 and I > I2 although I1 and I2 have opposite directions. That was due to the large number of electronehole pair generation at the metal-semiconductor interface and ZnO MW core under global UV illumination which provided more free electrons for the device like a storage. These electrons might do directional movement in the device and corresponding increase in the output current due to the driving force from the Schottky barriers. Moreover, photogenerated holes in the ZnO drift towards the surface of the MW due to the built-in potential gradient effect sand discharge the oxygen anions by capturing electrons on the ZnO surface ½hþ þ O 2 ðadÞ ¼ O2 ðgÞ [35] as shown in Fig. 5(b). Excess photogenerated free electrons were left in the core structure of ZnO wire; this improves the conductivity of the device and corresponding increases in the photocurrent Iill.
Fig. 4. (a) I-t curves of the photodetector recorded at 0 V bias under 365 nm UV illumination at a series of intensities. (b) The experimental data and fitting line of the photocurrentlight intensity. (c) The enlarged rise edges of I-t curve when the UV light was on. (d) The enlarged recovery edges of I-t curve when the UV light was off.
Please cite this article in press as: Y. Zhao, et al., Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.034
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Program of China (No. 2013CB932601), the Major Project of International Cooperation and Exchanges (No. 2012DFA50990), the Program of Introducing Talents of Discipline to Universities, National Natural Science Foundation of China (NSFC) (Nos. 51232001,51172022), the Fundamental Research Funds for Central Universities, the Research Fund of Co-construction Program from Beijing Municipal Commission of Education.
References
Fig. 5. (a) The energy band diagrams of the Ag/ZnO/Au asymmetric Schottky barrier UV photodetector under UV illumination. (b) The photoconductive principle model in the coreeshell structure ZnO MW under UV illumination.
4. Conclusion In conclusion, photodetectors based on ZnO MW with asymmetric Schottky barriers were fabricated and exhibited excellent performances. Metals with different work function such as Ag and Au were in contact with ZnO at the two ends of the ZnO MW to form different Schottky barriers. The photocurrent generated from the device under UV illumination at 0 V bias was about 120 nA, this was very high for a single wire device. The device also had a fast rising and decay time of less than 30 ms. The UV local irradiation effects of self-powered asymmetric Schottky barrier UV photodetector were studied. The possible mechanism for the excellent performance was investigated by the Schottky barrier theory and O2 adsorptionedesorption theories. This unique property of the ZnO MW might expand its applications opportunities in the field of medicine, communication, and environmental monitoring areas. Acknowledgments This work was supported by the National Major Research
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Please cite this article in press as: Y. Zhao, et al., Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.034
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector Yaxue Zhao a, Junjie Qi a, *, Chandan Biswas b, Feng Li a, Kui Zhang a, Xin Li a, Yue Zhang a, c, ** a b c
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China Department of Electrical Engineering, University of California Los Angeles, California 90095, USA Key Laboratory of New Energy Materials and Technologies, University of Science and Technology Beijing, Beijing 100083, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A self-powered Schottky barrier UV photodetector based on 1-D ZnO is fabricated. For the first time we investigate the local irradiation effects of UV detector. Irradiating both the junctions and ZnO can optimize the performance of the device.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 April 2015 Received in revised form 18 August 2015 Accepted 19 September 2015 Available online xxx
A self-powered metal-semiconductor-metal (MSM) UV photodetector was successfully fabricated based on Ag/ZnO/Au structure with asymmetric Schottky barriers. This exhibits excellent performance compared to many previous studies. Very high photo-to-dark current ratio (approximately 105e106) was demonstrated without applying any external bias, and very fast switching time of less than 30 ms was observed during the investigation. Opposite photocurrent direction was generated by irradiating different Schottky diodes in the fabricated photodetector. Furthermore, the device performance was optimized by largely irradiating both the ZnO microwire (MW) junctions. Schottky barrier effect theory and O2 adsorptionedesorption theories were used to investigate the phenomenon. The device has potential applications in self-powered UV detection field and can be used as electrical power source for electronic, optoelectronic and mechanical devices. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Semiconductors Chemical vapour deposition Electrical properties
1. Introduction ZnO material has excellent optical, optoelectronic, piezoelectric, field emission, wave-absorbing, and photocatalytic properties * Corresponding author. ** Corresponding author. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China. E-mail addresses: [email protected] (J. Qi), [email protected] (Y. Zhang).
among atypical II-VI group semiconductors. Recently, ZnO micro/ nano wires are widely studied by numbers of researchers in the applications of light emitting devices [1e3], nanogenerators [4e6], solar cells [7,8] and photodetectors [9e16]. Moreover, ZnO micro/ nano wires are particularly attractive for photodetectors due to its carrier confinement, high surface states and high surface-tovolume ratio. Furthermore, ZnO micro/nano wires can be easily obtained under low temperature with low production cost capabilities [17,18].
http://dx.doi.org/10.1016/j.matchemphys.2015.09.034 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Zhao, et al., Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.034
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Previously reported UV photodetectors can be divided into two categories: the pen junction type [19e21] and the Schottky barrier type [22e24]. Schottky barrier based photodetectors are easier to assemble and have faster response time in comparison with the pen junction photodetectors. It has also been demonstrated that the UV sensitivity of the MSM device structure can be greatly improved by replacing the Ohmic contact to the Schottky contact in either sides of the two electrode photodetector system [25]. However, there were no reports about UV local irradiation effects on different junctions of asymmetric Schottky barrier type photodetectors. Low energy consumption of the fabricated devices is among the main research target of the scientific communities, recently. However, most of the previously demonstrated photodetectors still need external voltage source in order to operate. In this paper, the MSM structure based UV photodetectors were fabricated with ZnO MW and two different metals (Ag and Au). UV local irradiation effects of asymmetric Schottky junctions were studied in this investigation. Currentevoltage characteristics of the device under dark and UV illumination were analyzed. IeV characteristics of the device under local UV light irradiation on the junctions were also tested. Moreover, the self-powered properties of the device were investigated under local and global UV irradiation conditions and related mechanism was discussed. 2. Experiment Pure ZnO micro/nano wires were prepared by chemical vapour deposition (CVD) method. Firstly, pure ZnO and C powder was mixed evenly with the molar ratio of 1:1 and put at the bottom of a porcelain boat as evaporation source. A Si wafer was then covered upside down over the mixed powder boat to collect necessary samples. The entire porcelain boat was then loaded in the middle of the horizontal furnace at 950 C temperature under the flowing gas mixture of Ar and O2 (ratio of 80:1sccm)for 15 min. Finally, the boat containing ZnO micro/nano wires was taken out as white samples collected on the Si wafer. The samples were scraped into deionized water by ultra sonication method for further use. 500 nm thin insulating SiO2layer on the Si wafer with a size of 6 mm 6 mm was used as a substrate. Au film was sputtered as electrode covering half of the substrate. 60 nm thick Au film was deposited by3 times DC sputtering with 40 mA current for 45 s. A ZnO MW was put across the junction of Au electrode and insulating silicon, and Ag paste was used to fix the end of ZnO wire which was in contact with the insulating wafer. The other end of the ZnO wire on the Au electrode was fixed by non-conductive polydimethylsiloxane (PDMS). Ag paste was also used to wire bond Au electrode far away from ZnO region. Two electric wires of the device were both led out using Ag paste. Thus, the device was finished by consisting asymmetric Schottky junctions of different barriers (Fig. 1(c)). The scanning electron microscopic (SEM) images of the synthesized ZnO MWs were obtained by a field emission scanning electron microscope (FE-SEM, LEO1530). The Raman spectrum was taken using a Raman system (HR800, Horiba Jobin Yvon) with a laser wavelength of 514 nm at room temperature. The device morphology was captured by an optical microscope. The electrical performances of the devices were tested by using a semiconductor characterization system (Keithley 4200-SCS). A UV lamp (Spectronics-L980683) emitting at 365 nm was used to irradiate the device. 3. Results and discussion The typical SEM image of the synthesized ZnO samples was represented in Fig. 1(a). SEM micrograph clearly shows wide
diameter distribution of the ZnO MW ranges from several tens of nanometers to several micrometers with length distribution in the order of several tens of micrometers. The figure inset shows the FESEM image of a hexagonal prism structured ZnO MW. Raman spectrum of the samples was taken at room temperature in order to characterize ZnO structure. We calibrated the vibration modulus and wave numbers in the Raman spectrum of the ZnO samples that shown in Fig. 1(b) by referring to all the peaks of ZnO single crystal high as Elow , A1(TO), A1(LO), E1(TO) and E1(LO). Two strong peaks at 2 , E2 98 cm1 and 438 cm1 corresponded to the non-polar Elow vibra2 high tion mode and E2 vibration mode owing to the wurtzite phase, while two weak waves of 331 cm1and 584 cm1 accorded with high E2 Elow difference frequency peak and E1(LO) mode was 2 observed respectively. The peak at 520 cm1 was from the Silicon wafer in Raman spectrum. Hence, the results above clearly proved the formation of highly crystalline ZnO micro/nano wires. Fig. 1(c) shows the schematic diagram of the fabricated photodetectors in which asymmetric Schottky barriers were formed with two different electrodes (Ag and Au) on the two ends of the ZnO MW. Fig. 1(d) illustrates the device micrograph taken from an optical microscope. IeV curves in dark and under UV illumination were tested in order to investigate the electrical properties and the asymmetric Schottky diodes as shown in Fig. 2(a). The shape of the IeV curves shows typical back-to-back Schottky characteristics. The photodetector can be considered as two back-to-back Schottky diodes. The voltage mainly drops at the reversely biased Schottky barrier side. Ag electrode was grounded and Au electrode was connected with sweeping voltage for all the experiments during IeV characteristic. Ag/ZnO contact stayed at reversed bias condition and determine the dark current of the device while photodetector kept under positive voltage (0 Ve5 V) condition. Conversely, when the device is under negative voltage (5 V ~0 V), Au/ZnO becomes reversed bias. The dark current shown in Fig. 2(a) under negative voltage is several times larger than positively biased condition. The thermionic emission with barrier lowering is usually the dominant current transport in a Schottky barrier under reversed bias the thermionic emission-diffusion theory (for 3kT=qz70 mV) according to the following equation for a Schottky barrier,
dark
I
e∅dark ¼ A ST exp kT
2
!
eVa exp 1 kT
(1)
Where Idark is the dark current, A* the effective Richardson constant, S the area of the Schottky contact, T the temperature, q the unit electronic charge, k the Boltzmann constant, ∅dark the Schottky Barrier Height (SBH), Va the applied voltage. It can be found that lower SBH results in higher dark current under negative voltage, when A*, S, T, Va are constant. This was because the lower SBH can improve the holes accumulation and capture process resulting in the generation of internal gain. Therefore, Ag/ZnO diode has higher SBH than Au/ZnO diode in the fabricated device. Although the work function of Ag (4.26eV) was lower than the electron affinity of ZnO (4.5 eV) and Ag/ZnO contact type was Ohmic contact type theoretically, Schottky type contact was quite possibly formed between Ag paste and ZnO MW. An interfacial layer (mainly including adsorbed oxygen, hydroxide, moisture, etc) was formed on the surface of Zinc Oxide micro/nanowire due to high surface-to-volume ratio and ambient experimental conditions in air. This had a thickness of one or two monolayers [26,27], thus resulting in the existence of abundant surface states on the ZnO surface. An upward band bending was then generated and a potential barrier was formed on the surface. This was almost independent of the work function of Ag due to the pinning of Fermi
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Y. Zhao et al. / Materials Chemistry and Physics xxx (2015) 1e6
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Fig. 1. (a) Low-magnification SEM of the ZnO micro/nano wires are synthesized by CVD method. The inset shows high-magnification SEM of the ZnO MW. (b) The Raman spectrum of ZnO MWs. (c) Schematic diagram of the fabricated asymmetric Schottky barrier type photodetectors. (d) Low-magnification SEM of the fabricated ZnO MW photodetector. The inset shows high-magnification of the Ag/ZnO junction.
Fig. 2. (a) IeV curves of the fabricated photodetector in dark and under UV illumination with a 5 V ~ 5 V sweeping voltage applied. (b) The IeV curves near 0 V bias. (c) The I-t curve at 0 V bias when the device was irradiated under a 365 nm UV light which was turned on and off periodically. Note the on/off period is 30 s. (d) The I-t curve at 0.5 V bias when the device was irradiated under a 365 nm UV light which was turned on and off periodically.
Please cite this article in press as: Y. Zhao, et al., Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.034
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energy. As a conclusion, the barrier height was not only controlled by the difference in the work function value, but also dominated by the surface states or band bending in ZnO itself as pointed out by Bardeen [28] and others [29,30]. When the density of surface states was high enough, the barrier height controlled by the surface states must dominate the IeV rectifying behavior [30,31].The current under UV illumination is significant when photon energy was larger than the band energy of ZnO, thus increasing the carriers concentration resulting in an increase of the device conductance as shown in Fig. 2(a). Secondly, holes discharge effects reduce the charge concentration (Qi) on the Schottky interface this reduces the width of the depletion layer and consequently increases the possibility of electrons to pass through the layer. Thirdly, the SBH can be expressed as:
∅ ¼ qVbi þ Ecf ¼
Q 2i 2Ns εS2
þ Ecf
(2)
where ∅ is the SBH, Vbi the built-in potential in the depletion layer, Ecf the deviation between the conduction and Fermi energy of ZnO, Qi the concentration of charges on the interface of Schottky barrier, Ns the concentration of space charges, 3 the dielectric constant. The reduced concentration Qi makes the SBH decline and hence promotes carriers transit through the SBH more easily. The IeV curves near 0 V bias was magnified and demonstrated in Fig. 2(b) in order to investigate the self-potential generated under UV irradiation. Fig 2(b) indicates the open-circuit voltage (Voc) of 0.021 V and the short-circuit current (Isc) of 82 nA. This means the self-potential was observed as 0.021 V under open circuit condition. Furthermore, the photo detection properties of the fabricated device were investigated thoroughly. The device was placed under a 365 nm UV light which was turned on and off
periodically. The on/off period is about 30 s. The detector showed quite short response time and stable output photocurrent (as shown in Fig. 2(c)) under 0 V bias. Dark current fluctuates was in the range of 3.4 1013 A to 1.1 1012 A. The photocurrent was nearly 120 nA (1.2 107 A) under UV illumination with 0.68 mW/ cm2power density. The sensitivity (photo-to-dark current ratio) was approximately 105 ~ 106, ultra high for a single MW device [32e34].The formation of the Schottky barriers lead to selfpowered performance. I-t performance under a constant voltage bias condition was measured in order to investigate the device performance further. The device dark current was increased to 2.2 mA while corresponding photocurrent was observed around 2.9 mA at 0.5 V bias shown in the I-t curve demonstrated in Fig. 2(d). The sensitivity at 0.5 V bias calculated was 1.32. The sensitivity at 0.5 V bias was much lower than that at 0 V bias due to the presence of particularly low dark current at 0 V bias conditions compare to 0.5 V bias cases. The device was irradiated on the Ag/ZnO junction and Au/ZnO junction separately in order to study UV local irradiation effects in the asymmetric Schottky barrier type photodetector. Fig. 3(a) shows the IeV curves while irradiating the Ag/ZnO junction locally and the corresponding currentetime (I-t) curves at 0 V bias are shown in Fig. 3(b). In comparison, IeV and I-t curves shown in Fig. 3(c) and (d) respectively were obtained by locally irradiated the Au/ZnO junction. Ag/ZnO junction was under reversed bias while applied voltage ranges from 0 Ve5 Vas shown in Fig. 3(a). The difference between dark current curve and photocurrent curve was more significant at 0 Ve5 V bias than that at 5 V ~0 V bias range. This was probably due to the photodetector Schottky junction controlled device current as explained above. Photocurrents of opposite directions measured were about 100 nA (Iill 1 ) (Fig. 3(b)) and 33 nA (Iill 2 ) (Fig. 3(d)), respectively. On the contrary, local
Fig. 3. (a) The IeV curves in dark and when the Ag/ZnO junction of the device was irradiated locally under UV light. The inset shows the schematic diagram when Ag/ZnO junction of the device was irradiated locally. (b) The corresponding I-t curve of Fig. 3(a) at 0 V bias when the light was turned on and off periodically. (c) The IeV curves in dark and when the Au/ZnO junction of the device was irradiated locally under UV light. The inset shows the schematic diagram when Ag/ZnO junction of the device was irradiated locally. (d) The corresponding I-t curve of Fig. 3(c) at 0 V bias when the light was turned on and off periodically.
Please cite this article in press as: Y. Zhao, et al., Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.034
Y. Zhao et al. / Materials Chemistry and Physics xxx (2015) 1e6
irradiation on Au/ZnO junction resulted in reversed bias under applied voltage ranges 0 Ve5 V as shown in Fig. 3(c). The difference between dark current curve and photocurrent curve is more significant at 5 V ~0 V bias compare to 0 Ve5 V bias conditions. This also proved that good Schottky contacts were formed at the two ends of ZnO MW. Fig. 4(a) shows the changes in the photodetector generated current by periodically turned on and off UV light exposure with different light intensities. The dark currents were observed identical without normalization, this proves that the device performance was extraordinary stable and reproducible. The experimental data and fitted curve of the photocurrent-light intensity were shown in Fig. 4(b). Photocurrent and light intensity showed nearelinear relationship. The carrier lifetime is relatively long under weak UV intensity thus the photocurrent was proportional to the illuminated light flux. Fig. 4(c) and (d) shows the enlarged rise and recovery process of the current response under 365 nm UV illumination. The current on/off process follows a biexponential relaxation equation:
t t I ¼ I0 þ C1 exp þ C2 exp t1 t2
(3)
Where t1 and t2 are two time constants. t1 is related to the rapid recombination of photoinduced electrons in the conduction band and holes in the valence band. t1 corresponds to the longer process of desorption and adsorption of oxygen molecules on the surfaces of ZnO MW. The rise time (tr, photocurrent increased from 10%e 90% of the maximum) and recovery time (td, photocurrent dropped from 90% to 10% of the maximum) of the device was estimated to be about 23msunder light on to off conditions and vice versa. Fig. 5 was shown to explain the UV light detection principle of
5
the photodetector. Firstly, photocurrents were generated resulting from the Schottky barrier separation effect. Electron-hole pairs are generated under UV illumination at the junctions of Ag/ZnO and Au/ZnO, this quickly swept away from the space charge region by the built-in electric field on the Schottky barrier interface. Metalsemiconductor interface generates certain directional current under photoexcitation conditions. This was the driving force of the self-powered device. Secondly, the photodetector was fabricated by two back-to-back Schottky junctions generating current of opposite directions under UV illumination. Due to this reasons, the photocurrent of opposite directions were tested when the two junctions were irradiated locally by the UV light. Moreover, the SBH of Ag/ ZnO (∅1 ) was higher than that of Au/ZnO (∅2 ) in the photodetector. This results Schottky barrier separation effect of Ag/ZnO junction more effective and corresponding I1>I2 as shown in Fig. 5(a). So the current direction of the whole device Iill was the same with I1. Schottky barriers provide driving force of the self-powered ill ill ill ill currents. The device results Iill > Iill 1 and I > I2 although I1 and I2 have opposite directions. That was due to the large number of electronehole pair generation at the metal-semiconductor interface and ZnO MW core under global UV illumination which provided more free electrons for the device like a storage. These electrons might do directional movement in the device and corresponding increase in the output current due to the driving force from the Schottky barriers. Moreover, photogenerated holes in the ZnO drift towards the surface of the MW due to the built-in potential gradient effect sand discharge the oxygen anions by capturing electrons on the ZnO surface ½hþ þ O 2 ðadÞ ¼ O2 ðgÞ [35] as shown in Fig. 5(b). Excess photogenerated free electrons were left in the core structure of ZnO wire; this improves the conductivity of the device and corresponding increases in the photocurrent Iill.
Fig. 4. (a) I-t curves of the photodetector recorded at 0 V bias under 365 nm UV illumination at a series of intensities. (b) The experimental data and fitting line of the photocurrentlight intensity. (c) The enlarged rise edges of I-t curve when the UV light was on. (d) The enlarged recovery edges of I-t curve when the UV light was off.
Please cite this article in press as: Y. Zhao, et al., Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.034
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Program of China (No. 2013CB932601), the Major Project of International Cooperation and Exchanges (No. 2012DFA50990), the Program of Introducing Talents of Discipline to Universities, National Natural Science Foundation of China (NSFC) (Nos. 51232001,51172022), the Fundamental Research Funds for Central Universities, the Research Fund of Co-construction Program from Beijing Municipal Commission of Education.
References
Fig. 5. (a) The energy band diagrams of the Ag/ZnO/Au asymmetric Schottky barrier UV photodetector under UV illumination. (b) The photoconductive principle model in the coreeshell structure ZnO MW under UV illumination.
4. Conclusion In conclusion, photodetectors based on ZnO MW with asymmetric Schottky barriers were fabricated and exhibited excellent performances. Metals with different work function such as Ag and Au were in contact with ZnO at the two ends of the ZnO MW to form different Schottky barriers. The photocurrent generated from the device under UV illumination at 0 V bias was about 120 nA, this was very high for a single wire device. The device also had a fast rising and decay time of less than 30 ms. The UV local irradiation effects of self-powered asymmetric Schottky barrier UV photodetector were studied. The possible mechanism for the excellent performance was investigated by the Schottky barrier theory and O2 adsorptionedesorption theories. This unique property of the ZnO MW might expand its applications opportunities in the field of medicine, communication, and environmental monitoring areas. Acknowledgments This work was supported by the National Major Research
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Please cite this article in press as: Y. Zhao, et al., Local irradiation effects of one-dimensional ZnO based self-powered asymmetric Schottky barrier UV photodetector, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.09.034