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High photosensitivity and low dark current of photoconductive semiconductor switch based on ZnO single nanobelt
Solid-State Electronics 55 (2011) 49–53
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
High photosensitivity and low dark current of photoconductive semiconductor switch based on ZnO single nanobelt Bo Yuan a, Xue Jun Zheng a,b,⇑, Yi Qiang Chen a, Bo Yang a, Tong Zhang c a
Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan, Hunan 411105, China Key Laboratory of Low Dimensional Materials and Application Technology, Xiangtan University, Ministry of Education, Xiangtan, Hunan 411105, China c State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, Jilin 130012, China b
a r t i c l e
i n f o
Article history: Received 21 May 2010 Received in revised form 13 August 2010 Accepted 5 September 2010 Available online 25 September 2010 The review of this paper was arranged by Prof. E. Calleja Keywords: Semiconductor switch ZnO single NB Photoconductive UV light
a b s t r a c t Photoconductive semiconductor switch (PCSS) based on ZnO single nanobelt (NB) was fabricated by controlling the concentration of mixed solution and using the probe technique, and applied into a test circuit to control the circuit state. The current–voltage characteristics and voltage spectra were investigated by system source meter and oscillograph, and the results show that the PCSS is of high photosensitivity of 104, low dark current of 103 lA, low power consumption of 2.45 lW, typical rise time 0.12 s, and decay time 0.15 s. Within the wavelength range of 280–340 nm, the shorter the wavelength is, the higher the voltage response is. The test circuit state conversion between ‘‘1” and ‘‘0” is obviously corresponding to UV illumination ‘‘on” and ‘‘off”. The high photosensitivity and low dark current of PCSS can be reasonably explained by using the view point of light absorption and oxygen chemisorption mechanism. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Low-dimensional nanostructures, such as nanobelts (NBs) [1], nanowires (NWs) [2], and thin films [3], have been extensively studied in recent years for the potential applications in nanoscale devices. Numerous researches have been made in electronic and optoelectronic devices based on ZnO nanostructures [4,5]. The photoconductive characteristics of ZnO NW [6] and CdS single NB [7] suggest that they are good candidates for optical switching devices. Photoconductive semiconductor switch (PCSS) based on bulk materials have been widely investigated for many high voltage applications [8]. Although a few of the current–voltage (I–V) characteristics at the voltage less than 1 V were reported for an individual ZnO NW [6,9], CdS single NB [7], SnO2 single NW [10] and ZnO NBs film [11], relatively few researches for PCSS based on nanostructures to control logic circuit is available in the literatures. Due to the high surface to volume ratio of ZnO NB, it has excellent photoconductive characteristic [11], and it may be good candidate for PCSS to use in memory storage or logic circuit, where the performance critically relies on the binary switching [6].
⇑ Corresponding author at: Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan, Hunan 411105, China. Tel.: +86 731 58293648; fax: +86 731 58298119. E-mail address: [email protected] (X.J. Zheng). 0038-1101/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2010.09.002
In this paper, ZnO single NB was assembled on planar interdigital electrodes to fabricate the PCSS by controlling the concentration of mixed solution and using the probe technique. In order to choose the suitable wavelength to measure the characteristics of PCSS, the wavelength range of sensitive light was obtained from the absorption spectrum of ZnO NBs. Keithley system source meter was used to measure the I–V characteristic curves, in which the dark current Idark, photocurrent Iph, photosensitivity S and the resistance ratio of off-state to on-state Roff/Ron can be determined. The load resistance voltage spectra obtained via oscillograph were used to understand the switch function of ‘‘on” and ‘‘off” states corresponding to the circuit states ‘‘1” and ‘‘0”. It is worth mentioning that this research may provide useful guideline for the future integration of PCSS based on single semiconductor NB. 2. Experimental details ZnO wool like NBs were synthesized through thermal evaporation of ZnO powders without the presence of catalyst [1]. Phase identification and crystalline orientation of ZnO NBs were investigated by X-ray powder diffraction (XRD) (Rigaku, D/Max 2400, Japan), and it is given in Fig. 1a. All diffraction peaks are indexed according to the standard diffraction pattern data of ZnO powder compiled in JCPDS card [12]. It indicates that the NBs are hexagonal wurtzite structure and there is no characteristic peak of impurities. The absorption spectrum was obtained in the wavelength range
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B. Yuan et al. / Solid-State Electronics 55 (2011) 49–53
Fig. 1. XRD pattern (a) and UV–visible absorption spectrum (the inset shows the micrograph) (b) of ZnO wool like NBs.
Fig. 2. (a) FE-SEM image of PCSS based on ZnO single NB (the inset is schematic diagram), and (b) the three-dimensional morphology image of ZnO single NB (the inset shows the section analysis).
Fig. 3. The schematic diagram of experimental setup: (a) optical part, and (b) electric part for measurement on switching properties.
from 200 to 800 nm by ultraviolet (UV)-visible spectrophotometer (Shimadzu, UV-1700, Japan), and it is described as Fig. 1b. The absorption edge is 337 nm, indicating a characteristic of preferred absorption for UV light with the wavelength less than the band gap absorption edge, therefore we chose the UV-lights with the wavelength range from 280 to 340 nm to measure I–V characteristics and voltage spectra, in order to determine the certain wavelength corresponding to a maximum photocurrent as a control condition
of PCSS. The micrograph of ZnO NBs was recorded by a field emission scanning electron microscopy (FE-SEM) (JEOL, JSM-6700F, Japan), and it is given as the inset of Fig. 1b. Obviously, the length of ZnO NBs is in the range of several tens to several hundreds of micrometers. The fabrication process of PCSS based on ZnO single NB is described as follows. Pt interdigital electrodes were subsequently patterned on the prepared SiO2/Si substrate by radio frequency
B. Yuan et al. / Solid-State Electronics 55 (2011) 49–53
magnetron sputtering and photolithography. ZnO NBs were dispersed in acetone by ultrasonication, and the solution including ZnO NBs was dropped on the electrodes. After drying at 120 °C for 35 min in vacuum drying oven, acetone was volatilized, and ZnO NBs were assembled onto electrodes as photosensitive material for the PCSS. By controlling the concentration of mixed solution and using the probe technique, ZnO single NB was setup on the interdigital electrodes. FE-SEM image of the PCSS is given in Fig. 2a, and the schematic diagram is shown in the inset. To analyze the shape and size of the single NB, the three-dimensional morphology was obtained by the nanoindenter (Hysitron Triboindenter, Hysitron Inc., USA) with a standard Berkovich tip (see Fig. 2b). The inset is the section analysis of three-dimensional morphology for ZnO single NB, and the thickness H and width W are about 120 and 600 nm. In order to test switching properties, a measure setup as Fig. 3 was built up and it is composed of the optical and electrical parts. In Fig. 3a, UV-lights with the different wavelengths were obtained by the white light passed through a grating monochromator (WDG 30, Beijing Optical Instrument Factory, China), and the UV-light power density was adjusted by the focus of converging lens. In Fig. 3b, for the ‘‘a–b–b0 –a0 ” circuit I–V characteristic curves were measured by Keithley system source meter (2601, Keithley, USA) to determine the dark current and photocurrents, and for ‘‘a–c– c0 –a0 ” circuit the voltage spectra were recorded by an oscillograph (TDS3012B, Tektronix, USA) in order to test the conversions between the circuit states ‘‘1” and ‘‘0” corresponding to UV illumination ‘‘on” and ‘‘off”.
3. Results and discussions I–V curves were measured in darkness and under illumination of UV light with different wavelengths of 280, 300, 320, and 340 nm at power density 35 lW/cm2, and they are given in Fig. 4a. The PCSS shows highly insulating in darkness because of the low dark current in the range of 1010–109 A, indicating that the PCSS has an excellent ‘‘off-state”. The photocurrents under illumination of UV light with different wavelengths of 280, 300, 320, and 340 nm are four orders of magnitude larger than the dark current, and it indicates that PCSS based on ZnO single NB is of a more effective response to UV light. Generally, the switch off-state resistance to on-state resistance ratio is used to describe the capability of PCSS to control the circuit [13,14]. The NB device’s resistance is contained by the resistance of Schottky (or Ohmic) contacts and NB itself, and it is measured by using four-terminal electrical measurement in pervious paper [15]. Analogous to the definition of nanorod device’s resistance [16], the total resistance of PCSS based on
51
ZnO single NB is obtained by the sum of the resistance of the Schottky contacts and of the NB itself. At 20 V bias, the switch off-state resistance Roff and the on-state resistance Ron of PCSS based on ZnO single NB are 12,800 MX in darkness and 257 k X under illumination of UV light with wavelength of 280 nm, therefore the Roff/Ron is 4.98 104 for the conversion from darkness to UV irradiation. Comparing with PCSS based on ZnS single NB 1.47 104 [13] at the same 20 V bias, the Roff/Ron of PCSS based on ZnO single NB is larger than that for ZnS single NB, and it indicates that PCSS based on ZnO single NB is more capable than the latter in controlling the circuit. The photosensitivity S would be simply the photocurrent to dark current ratio [3], the ratio of sheet resistance R in the dark to that under illumination [17], and the photoconductivity to dark conductivity ratio [18], and they are defined as S = [(Iph Idark)/ Idark] 100, S = [(Rd Ri)/Ri], and S = [(rph rdark)/rdark], respectively. In our investigation, the photocurrent Iph and the dark current Idark can be directly obtained from I–V curves therefore we choose the definition S = [(Iph Idark)/Idark] 100 to discuss the photosensitivity. Under the bias voltage of 1 V, the photocurrent Iph, dark current Idark and S are 2.45 lA, 4.56 104 lA and 5.37 103 for PCSS based on ZnO single NB. Comparing with the optical switch based on individual ZnO NW, Iph and S are larger than 2.1 lA and 12.13, meanwhile Idark is much smaller than 0.16 lA [9]. Under the bias voltage of 5 V, the Iph, Idark, S are respectively 9.42 lA, 0.8 103 lA, 1.15 104 for PCSS based on ZnO single NB and 66.3 lA, 1.1 103 lA, 7.37 104 for PCSS based on ZnO NBs film [11]. Because of NBs interlacement and hang in the NBs film, it is of the larger surface to volume ratio and larger ratio of coverage area to total area toward light incidence than those of the single NB. Therefore the Iph and S of PCSS based on ZnO single NB are smaller than those of PCSS based on ZnO NBs film, while the Idark are comparative for two kinds of PCSS. The power consumption of PCSS based on ZnO single NB (2.45 lW) is smaller than those of PCSS based on ZnS single NB (108 lW) [13] and ZnO NBs film (17.1 lA) [11] at 1 V bias, indicating the PCSS based on ZnO single NB has potential application in integrated circuit. The voltage spectra of PCSS were measured via the oscillograph, and they are given as Fig. 4b. After turning on the UV light with the different wavelengths, the voltages on load resistor of the test circuit abruptly increase during the ‘‘off–on” process. The steady voltages under illumination indicate the closed test circuit in ‘‘1” state corresponding to the ‘‘on-state” of PCSS. After turning off the UV light, the voltages abruptly decrease from the above steady voltages to the original dark level during the ‘‘on–off” process. The steady low voltages in darkness indicate the open test circuits in ‘‘0” state corresponding to the ‘‘off-state” of PCSS. The rise and decay time of PCSS based on ZnO single NB are 0.12 and 0.15 s, and
Fig. 4. IV curves of PCSS based on ZnO NB (a) and the voltage spectra of the test circuit (b).
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B. Yuan et al. / Solid-State Electronics 55 (2011) 49–53
they are much shorter than both 1 s for ZnO single NW UV optical switches [6], 1 and 3 s for CdS NBs photoconductors [7] and 3.5 and 20 s for ZnO NBs film [11]. Analogous to the on–off current ratio [19], the on–off voltage ratio is defined as the voltage ratios under UV illumination and in darkness, in order to estimate the control capability of PCSS. For PCSS under illumination of UV light with the wavelength range from 280 to 340 nm, the shorter the wavelength is, the higher the on–off voltage ratio is, indicating that the PCSS under UV light with the shortest wavelength has most effective control capability to the test circuit. It is obvious that the circuit state conversion between ‘‘0” and ‘‘1” can be reversibly controlled by the PCSS, and many hours of continuous operation were observed with no degradation in performance. The results imply that PCSS based on ZnO single NB may have promising optoelectronic application as nanoscale photosensors because of the high photosensitivity, low dark current, low consumption, fast response time and good reproducibility. Previous experimental results have shown that oxygen chemisorption mechanism plays a central role in regulating the conductance for ZnO NWs [20], ZnS NBs [21] and CdS Nanoribbons [22] under either oxygen-containing or oxygen-free conditions. In our investigation, the dark conductivity under ‘‘off-state” and the photoconductivity under ‘‘on-state” for ZnO single NB are regulated by oxygen chemisorption and oxygen desorption. The switch conversions of PCSS based on ZnO single NB can be explained by oxygen chemisorption mechanism, and they are interpreted as follows. The energy band schematic diagram of ZnO single NB is shown in Fig. 5. In darkness the oxygen molecules ‘‘O2 ðgÞ” absorbed by the exposed surface of ZnO single NB capture electrons ‘‘e ” from the conduction band to form negatively charged oxygen ions ‘‘O 2 ðadÞ”:
O2 ðgÞ þ e ! O2 ðadÞ
ð1Þ
This process leads to band bending upwards near the surface, as shown in Fig. 5a. After sufficient absorption of oxygen, the increase of depletion width causes a few free electrons ‘‘e ” left in the interior of ZnO single NB to result in the low conductivity state, which corresponds to ‘‘off-state” described as Fig. 4a and the dark level voltage ‘‘0” state in the test circuit shown in Fig. 4b. When ZnO single NB is illuminated by UV light with energy higher than the band gap (see Fig. 5b), electron–hole pairs are genþ erated. Photon-generated holes ‘‘h ” in valence band migrate along the bending band to recombine with the ‘‘O 2 ðadÞ” and discharge the ‘‘O2 ðgÞ” into air and the electrons into the conduction band [7]: þ
O2 ðadÞ þ h ! O2 ðgÞ
ð2Þ
Thus, the released electrons markedly increase the density of electrons in conduction and decrease the degree of band bending
upwards. After sufficient UV-light illumination, photon-generated þ holes ‘‘h ” continuously migrate to recombine with the ‘‘O 2 ðadÞ” to discharge ‘‘O2 ðgÞ” and ‘‘e ”, and consequently the ‘‘O 2 ðadÞ” will be decreased. When there is equilibrium between the generation of electron–hole pairs and the desorption of oxygen molecules ‘‘O2 ðgÞ” near the NB surface, the massive photon-generated electrons ‘‘e ” keep a steady high conductivity state corresponding to the on-state in Fig. 4a and the test circuit ‘‘1” state in Fig. 4b. From the view point of light absorption, the contribution of the high photocurrent can be discussed by using the ratio of coverage area to total area toward light incidence. When the UV light with larger intensity is illuminated on the PCSS, more photo electron– hole pairs due to the high energy absorbed on NB surface will result in the high photocurrent corresponding to the enhancement on photosensitivity. Obviously, the high photocurrent can be reasonably explained by the high ratio of coverage area to total area toward light incidence, however it is difficult to explain the low dark current in darkness. According to oxygen chemisorption mechanism, the low dark current and high photosensitivity of PCSS based on ZnO single NB may attribute to the high surface to volume ratio. One-dimensional nanostructures, such as NBs and NWs, have higher surface to volume ratio than those of film and bulk material [23,24]. If the surface is large for the same volume, more electron–hole pairs photoexcited on the large illuminated area of the photosensitive materials lead to higher photocurrent under UV illumination [21], while more oxygen adsorbed on the surface of the ZnO single NB result in lower carrier density and higher the depletion width in darkness [7]. Due to the higher surface to volume ratio, ZnO single NB can bring more effective oxygen adsorption than the counterparts of film and bulk [8,13]. In a word, the advantages of nanostructures are of the high surface to volume ratio and high coverage area to total area ratio compared to the corresponding thin film or bulk, therefore the devices based on nanostructures are of the novel performance generally. According to the definition of photosensitivity [3], higher photocurrent and lower dark current lead to higher photosensitivity for ZnO single NB comparing with the counterparts of film and bulk. 4. Conclusions In summary, PCSS based on ZnO single NB was fabricated on planar interdigital electrodes, and UV optoelectronic performance was explored. The PCSS is of high photosensitivity of 104, low dark current of 103 lA, low power consumption of 2.45 lW, typical rise time 0.12 s, and decay time 0.15 s. Within the wavelength range of 280–340 nm, the shorter the wavelength is, the higher the voltage response is. High photosensitivity and low dark current of PCSS based on ZnO single NB can be reasonably explained by high surface to volume ratio and high coverage area to total area ratio. As UV light turns on and off, PCSS based on ZnO single NB can be reversibly conversion between ‘‘1” and ‘‘0”. The results suggest that ZnO single NB is a good candidate as photosensitive materials for PCSS, which can be further applied in nanoscale photosensor. Acknowledgements
Fig. 5. Energy band schematic diagram of oxygen chemisorption mechanism: (a) off-state, (b) on-state.
This work was supported by NNSF of China (50872117, 10672139, and 10825209), Changjiang Scholar Incentive Program ([2009]17), Project of Hunan’s Prestigious Fu-rong Scholar Award ([2007]362), Natural Science Foundation of Hunan Province for Innovation Group (09JJ7004), Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, and Postgraduate Innovation Foundation of Hunan Province (cx2009B128).
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Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
High photosensitivity and low dark current of photoconductive semiconductor switch based on ZnO single nanobelt Bo Yuan a, Xue Jun Zheng a,b,⇑, Yi Qiang Chen a, Bo Yang a, Tong Zhang c a
Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan, Hunan 411105, China Key Laboratory of Low Dimensional Materials and Application Technology, Xiangtan University, Ministry of Education, Xiangtan, Hunan 411105, China c State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, Jilin 130012, China b
a r t i c l e
i n f o
Article history: Received 21 May 2010 Received in revised form 13 August 2010 Accepted 5 September 2010 Available online 25 September 2010 The review of this paper was arranged by Prof. E. Calleja Keywords: Semiconductor switch ZnO single NB Photoconductive UV light
a b s t r a c t Photoconductive semiconductor switch (PCSS) based on ZnO single nanobelt (NB) was fabricated by controlling the concentration of mixed solution and using the probe technique, and applied into a test circuit to control the circuit state. The current–voltage characteristics and voltage spectra were investigated by system source meter and oscillograph, and the results show that the PCSS is of high photosensitivity of 104, low dark current of 103 lA, low power consumption of 2.45 lW, typical rise time 0.12 s, and decay time 0.15 s. Within the wavelength range of 280–340 nm, the shorter the wavelength is, the higher the voltage response is. The test circuit state conversion between ‘‘1” and ‘‘0” is obviously corresponding to UV illumination ‘‘on” and ‘‘off”. The high photosensitivity and low dark current of PCSS can be reasonably explained by using the view point of light absorption and oxygen chemisorption mechanism. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Low-dimensional nanostructures, such as nanobelts (NBs) [1], nanowires (NWs) [2], and thin films [3], have been extensively studied in recent years for the potential applications in nanoscale devices. Numerous researches have been made in electronic and optoelectronic devices based on ZnO nanostructures [4,5]. The photoconductive characteristics of ZnO NW [6] and CdS single NB [7] suggest that they are good candidates for optical switching devices. Photoconductive semiconductor switch (PCSS) based on bulk materials have been widely investigated for many high voltage applications [8]. Although a few of the current–voltage (I–V) characteristics at the voltage less than 1 V were reported for an individual ZnO NW [6,9], CdS single NB [7], SnO2 single NW [10] and ZnO NBs film [11], relatively few researches for PCSS based on nanostructures to control logic circuit is available in the literatures. Due to the high surface to volume ratio of ZnO NB, it has excellent photoconductive characteristic [11], and it may be good candidate for PCSS to use in memory storage or logic circuit, where the performance critically relies on the binary switching [6].
⇑ Corresponding author at: Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan, Hunan 411105, China. Tel.: +86 731 58293648; fax: +86 731 58298119. E-mail address: [email protected] (X.J. Zheng). 0038-1101/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2010.09.002
In this paper, ZnO single NB was assembled on planar interdigital electrodes to fabricate the PCSS by controlling the concentration of mixed solution and using the probe technique. In order to choose the suitable wavelength to measure the characteristics of PCSS, the wavelength range of sensitive light was obtained from the absorption spectrum of ZnO NBs. Keithley system source meter was used to measure the I–V characteristic curves, in which the dark current Idark, photocurrent Iph, photosensitivity S and the resistance ratio of off-state to on-state Roff/Ron can be determined. The load resistance voltage spectra obtained via oscillograph were used to understand the switch function of ‘‘on” and ‘‘off” states corresponding to the circuit states ‘‘1” and ‘‘0”. It is worth mentioning that this research may provide useful guideline for the future integration of PCSS based on single semiconductor NB. 2. Experimental details ZnO wool like NBs were synthesized through thermal evaporation of ZnO powders without the presence of catalyst [1]. Phase identification and crystalline orientation of ZnO NBs were investigated by X-ray powder diffraction (XRD) (Rigaku, D/Max 2400, Japan), and it is given in Fig. 1a. All diffraction peaks are indexed according to the standard diffraction pattern data of ZnO powder compiled in JCPDS card [12]. It indicates that the NBs are hexagonal wurtzite structure and there is no characteristic peak of impurities. The absorption spectrum was obtained in the wavelength range
50
B. Yuan et al. / Solid-State Electronics 55 (2011) 49–53
Fig. 1. XRD pattern (a) and UV–visible absorption spectrum (the inset shows the micrograph) (b) of ZnO wool like NBs.
Fig. 2. (a) FE-SEM image of PCSS based on ZnO single NB (the inset is schematic diagram), and (b) the three-dimensional morphology image of ZnO single NB (the inset shows the section analysis).
Fig. 3. The schematic diagram of experimental setup: (a) optical part, and (b) electric part for measurement on switching properties.
from 200 to 800 nm by ultraviolet (UV)-visible spectrophotometer (Shimadzu, UV-1700, Japan), and it is described as Fig. 1b. The absorption edge is 337 nm, indicating a characteristic of preferred absorption for UV light with the wavelength less than the band gap absorption edge, therefore we chose the UV-lights with the wavelength range from 280 to 340 nm to measure I–V characteristics and voltage spectra, in order to determine the certain wavelength corresponding to a maximum photocurrent as a control condition
of PCSS. The micrograph of ZnO NBs was recorded by a field emission scanning electron microscopy (FE-SEM) (JEOL, JSM-6700F, Japan), and it is given as the inset of Fig. 1b. Obviously, the length of ZnO NBs is in the range of several tens to several hundreds of micrometers. The fabrication process of PCSS based on ZnO single NB is described as follows. Pt interdigital electrodes were subsequently patterned on the prepared SiO2/Si substrate by radio frequency
B. Yuan et al. / Solid-State Electronics 55 (2011) 49–53
magnetron sputtering and photolithography. ZnO NBs were dispersed in acetone by ultrasonication, and the solution including ZnO NBs was dropped on the electrodes. After drying at 120 °C for 35 min in vacuum drying oven, acetone was volatilized, and ZnO NBs were assembled onto electrodes as photosensitive material for the PCSS. By controlling the concentration of mixed solution and using the probe technique, ZnO single NB was setup on the interdigital electrodes. FE-SEM image of the PCSS is given in Fig. 2a, and the schematic diagram is shown in the inset. To analyze the shape and size of the single NB, the three-dimensional morphology was obtained by the nanoindenter (Hysitron Triboindenter, Hysitron Inc., USA) with a standard Berkovich tip (see Fig. 2b). The inset is the section analysis of three-dimensional morphology for ZnO single NB, and the thickness H and width W are about 120 and 600 nm. In order to test switching properties, a measure setup as Fig. 3 was built up and it is composed of the optical and electrical parts. In Fig. 3a, UV-lights with the different wavelengths were obtained by the white light passed through a grating monochromator (WDG 30, Beijing Optical Instrument Factory, China), and the UV-light power density was adjusted by the focus of converging lens. In Fig. 3b, for the ‘‘a–b–b0 –a0 ” circuit I–V characteristic curves were measured by Keithley system source meter (2601, Keithley, USA) to determine the dark current and photocurrents, and for ‘‘a–c– c0 –a0 ” circuit the voltage spectra were recorded by an oscillograph (TDS3012B, Tektronix, USA) in order to test the conversions between the circuit states ‘‘1” and ‘‘0” corresponding to UV illumination ‘‘on” and ‘‘off”.
3. Results and discussions I–V curves were measured in darkness and under illumination of UV light with different wavelengths of 280, 300, 320, and 340 nm at power density 35 lW/cm2, and they are given in Fig. 4a. The PCSS shows highly insulating in darkness because of the low dark current in the range of 1010–109 A, indicating that the PCSS has an excellent ‘‘off-state”. The photocurrents under illumination of UV light with different wavelengths of 280, 300, 320, and 340 nm are four orders of magnitude larger than the dark current, and it indicates that PCSS based on ZnO single NB is of a more effective response to UV light. Generally, the switch off-state resistance to on-state resistance ratio is used to describe the capability of PCSS to control the circuit [13,14]. The NB device’s resistance is contained by the resistance of Schottky (or Ohmic) contacts and NB itself, and it is measured by using four-terminal electrical measurement in pervious paper [15]. Analogous to the definition of nanorod device’s resistance [16], the total resistance of PCSS based on
51
ZnO single NB is obtained by the sum of the resistance of the Schottky contacts and of the NB itself. At 20 V bias, the switch off-state resistance Roff and the on-state resistance Ron of PCSS based on ZnO single NB are 12,800 MX in darkness and 257 k X under illumination of UV light with wavelength of 280 nm, therefore the Roff/Ron is 4.98 104 for the conversion from darkness to UV irradiation. Comparing with PCSS based on ZnS single NB 1.47 104 [13] at the same 20 V bias, the Roff/Ron of PCSS based on ZnO single NB is larger than that for ZnS single NB, and it indicates that PCSS based on ZnO single NB is more capable than the latter in controlling the circuit. The photosensitivity S would be simply the photocurrent to dark current ratio [3], the ratio of sheet resistance R in the dark to that under illumination [17], and the photoconductivity to dark conductivity ratio [18], and they are defined as S = [(Iph Idark)/ Idark] 100, S = [(Rd Ri)/Ri], and S = [(rph rdark)/rdark], respectively. In our investigation, the photocurrent Iph and the dark current Idark can be directly obtained from I–V curves therefore we choose the definition S = [(Iph Idark)/Idark] 100 to discuss the photosensitivity. Under the bias voltage of 1 V, the photocurrent Iph, dark current Idark and S are 2.45 lA, 4.56 104 lA and 5.37 103 for PCSS based on ZnO single NB. Comparing with the optical switch based on individual ZnO NW, Iph and S are larger than 2.1 lA and 12.13, meanwhile Idark is much smaller than 0.16 lA [9]. Under the bias voltage of 5 V, the Iph, Idark, S are respectively 9.42 lA, 0.8 103 lA, 1.15 104 for PCSS based on ZnO single NB and 66.3 lA, 1.1 103 lA, 7.37 104 for PCSS based on ZnO NBs film [11]. Because of NBs interlacement and hang in the NBs film, it is of the larger surface to volume ratio and larger ratio of coverage area to total area toward light incidence than those of the single NB. Therefore the Iph and S of PCSS based on ZnO single NB are smaller than those of PCSS based on ZnO NBs film, while the Idark are comparative for two kinds of PCSS. The power consumption of PCSS based on ZnO single NB (2.45 lW) is smaller than those of PCSS based on ZnS single NB (108 lW) [13] and ZnO NBs film (17.1 lA) [11] at 1 V bias, indicating the PCSS based on ZnO single NB has potential application in integrated circuit. The voltage spectra of PCSS were measured via the oscillograph, and they are given as Fig. 4b. After turning on the UV light with the different wavelengths, the voltages on load resistor of the test circuit abruptly increase during the ‘‘off–on” process. The steady voltages under illumination indicate the closed test circuit in ‘‘1” state corresponding to the ‘‘on-state” of PCSS. After turning off the UV light, the voltages abruptly decrease from the above steady voltages to the original dark level during the ‘‘on–off” process. The steady low voltages in darkness indicate the open test circuits in ‘‘0” state corresponding to the ‘‘off-state” of PCSS. The rise and decay time of PCSS based on ZnO single NB are 0.12 and 0.15 s, and
Fig. 4. IV curves of PCSS based on ZnO NB (a) and the voltage spectra of the test circuit (b).
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B. Yuan et al. / Solid-State Electronics 55 (2011) 49–53
they are much shorter than both 1 s for ZnO single NW UV optical switches [6], 1 and 3 s for CdS NBs photoconductors [7] and 3.5 and 20 s for ZnO NBs film [11]. Analogous to the on–off current ratio [19], the on–off voltage ratio is defined as the voltage ratios under UV illumination and in darkness, in order to estimate the control capability of PCSS. For PCSS under illumination of UV light with the wavelength range from 280 to 340 nm, the shorter the wavelength is, the higher the on–off voltage ratio is, indicating that the PCSS under UV light with the shortest wavelength has most effective control capability to the test circuit. It is obvious that the circuit state conversion between ‘‘0” and ‘‘1” can be reversibly controlled by the PCSS, and many hours of continuous operation were observed with no degradation in performance. The results imply that PCSS based on ZnO single NB may have promising optoelectronic application as nanoscale photosensors because of the high photosensitivity, low dark current, low consumption, fast response time and good reproducibility. Previous experimental results have shown that oxygen chemisorption mechanism plays a central role in regulating the conductance for ZnO NWs [20], ZnS NBs [21] and CdS Nanoribbons [22] under either oxygen-containing or oxygen-free conditions. In our investigation, the dark conductivity under ‘‘off-state” and the photoconductivity under ‘‘on-state” for ZnO single NB are regulated by oxygen chemisorption and oxygen desorption. The switch conversions of PCSS based on ZnO single NB can be explained by oxygen chemisorption mechanism, and they are interpreted as follows. The energy band schematic diagram of ZnO single NB is shown in Fig. 5. In darkness the oxygen molecules ‘‘O2 ðgÞ” absorbed by the exposed surface of ZnO single NB capture electrons ‘‘e ” from the conduction band to form negatively charged oxygen ions ‘‘O 2 ðadÞ”:
O2 ðgÞ þ e ! O2 ðadÞ
ð1Þ
This process leads to band bending upwards near the surface, as shown in Fig. 5a. After sufficient absorption of oxygen, the increase of depletion width causes a few free electrons ‘‘e ” left in the interior of ZnO single NB to result in the low conductivity state, which corresponds to ‘‘off-state” described as Fig. 4a and the dark level voltage ‘‘0” state in the test circuit shown in Fig. 4b. When ZnO single NB is illuminated by UV light with energy higher than the band gap (see Fig. 5b), electron–hole pairs are genþ erated. Photon-generated holes ‘‘h ” in valence band migrate along the bending band to recombine with the ‘‘O 2 ðadÞ” and discharge the ‘‘O2 ðgÞ” into air and the electrons into the conduction band [7]: þ
O2 ðadÞ þ h ! O2 ðgÞ
ð2Þ
Thus, the released electrons markedly increase the density of electrons in conduction and decrease the degree of band bending
upwards. After sufficient UV-light illumination, photon-generated þ holes ‘‘h ” continuously migrate to recombine with the ‘‘O 2 ðadÞ” to discharge ‘‘O2 ðgÞ” and ‘‘e ”, and consequently the ‘‘O 2 ðadÞ” will be decreased. When there is equilibrium between the generation of electron–hole pairs and the desorption of oxygen molecules ‘‘O2 ðgÞ” near the NB surface, the massive photon-generated electrons ‘‘e ” keep a steady high conductivity state corresponding to the on-state in Fig. 4a and the test circuit ‘‘1” state in Fig. 4b. From the view point of light absorption, the contribution of the high photocurrent can be discussed by using the ratio of coverage area to total area toward light incidence. When the UV light with larger intensity is illuminated on the PCSS, more photo electron– hole pairs due to the high energy absorbed on NB surface will result in the high photocurrent corresponding to the enhancement on photosensitivity. Obviously, the high photocurrent can be reasonably explained by the high ratio of coverage area to total area toward light incidence, however it is difficult to explain the low dark current in darkness. According to oxygen chemisorption mechanism, the low dark current and high photosensitivity of PCSS based on ZnO single NB may attribute to the high surface to volume ratio. One-dimensional nanostructures, such as NBs and NWs, have higher surface to volume ratio than those of film and bulk material [23,24]. If the surface is large for the same volume, more electron–hole pairs photoexcited on the large illuminated area of the photosensitive materials lead to higher photocurrent under UV illumination [21], while more oxygen adsorbed on the surface of the ZnO single NB result in lower carrier density and higher the depletion width in darkness [7]. Due to the higher surface to volume ratio, ZnO single NB can bring more effective oxygen adsorption than the counterparts of film and bulk [8,13]. In a word, the advantages of nanostructures are of the high surface to volume ratio and high coverage area to total area ratio compared to the corresponding thin film or bulk, therefore the devices based on nanostructures are of the novel performance generally. According to the definition of photosensitivity [3], higher photocurrent and lower dark current lead to higher photosensitivity for ZnO single NB comparing with the counterparts of film and bulk. 4. Conclusions In summary, PCSS based on ZnO single NB was fabricated on planar interdigital electrodes, and UV optoelectronic performance was explored. The PCSS is of high photosensitivity of 104, low dark current of 103 lA, low power consumption of 2.45 lW, typical rise time 0.12 s, and decay time 0.15 s. Within the wavelength range of 280–340 nm, the shorter the wavelength is, the higher the voltage response is. High photosensitivity and low dark current of PCSS based on ZnO single NB can be reasonably explained by high surface to volume ratio and high coverage area to total area ratio. As UV light turns on and off, PCSS based on ZnO single NB can be reversibly conversion between ‘‘1” and ‘‘0”. The results suggest that ZnO single NB is a good candidate as photosensitive materials for PCSS, which can be further applied in nanoscale photosensor. Acknowledgements
Fig. 5. Energy band schematic diagram of oxygen chemisorption mechanism: (a) off-state, (b) on-state.
This work was supported by NNSF of China (50872117, 10672139, and 10825209), Changjiang Scholar Incentive Program ([2009]17), Project of Hunan’s Prestigious Fu-rong Scholar Award ([2007]362), Natural Science Foundation of Hunan Province for Innovation Group (09JJ7004), Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, and Postgraduate Innovation Foundation of Hunan Province (cx2009B128).
B. Yuan et al. / Solid-State Electronics 55 (2011) 49–53
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