Ultraviolet photodetectors based on ZnO sheets: The effect of sheet size on photoresponse properties

Applied Surface Science 258 (2012) 5405–5411

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Ultraviolet photodetectors based on ZnO sheets: The effect of sheet size on photoresponse properties Abbas Ghasempour Ardakani a , Meysam Pazoki a , Seyed Mohammad Mahdavi a,b,∗ , Ali Reza Bahrampour a,∗∗ , Nima Taghavinia a,b a b

Department of Physics, Sharif University of Technology, Tehran, Iran Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 8 September 2011 Received in revised form 10 January 2012 Accepted 6 February 2012 Available online 15 February 2012 Keywords: ZnO sheets UV photodetector Electrodeposition Response time

a b s t r a c t In this work, ultraviolet photodetectors based on electrodeposited ZnO sheet thin films were fabricated on a glass substrate. Before electrodeposition, a thin buffer layer of ZnO was deposited on the glass by pulsed laser deposition method. This layer not only acted as a nucleation site for ZnO sheet growth, but also made it possible to use cheap glass substrate instead of conventional fluorine-doped tin oxide (FTO) substrate. Our results showed that photoresponse properties of the photodetectors strongly depend on the sheet sizes. The smaller sheets exhibited enhanced photosensitivity, shortened fall times and decreased gain compared to larger ones. We showed that photodetectors based on ZnO sheets have a faster response than ones based on polycrystalline films. It was also shown that even less response time could be obtained by using comb-like electrodes instead of two-electrode. © 2012 Elsevier B.V. All rights reserved.

1. Introduction UV photodetectors have an important role in industrial and commercial applications. They can be used in flame detection, space communications, ozone monitoring, and water purification [1–3]. In recent years, wide band gap semiconductors such as GaN, ZnSe and ZnO have been used to produce high performance photodetectors which are sensitive in the UV region. Among them ZnO has attracted more attention because of its large and direct band gap (3.34 eV), high exciton binding energy (60 meV), non-toxicity, high transparency and high radiation hardness [4]. It has been investigated as a good candidate to use in LEDs, laser diodes, UV photodetectors and gas sensors [5–21]. From literature it is known that it is not simple to make p-type ZnO through doping and consequently it is difficult to have ZnO p–n junctions with high quality [9,10]. Therefore, metal–semiconductor–metal (MSM) structures are more appropriate to use in ZnO-based UV detectors. In the past decade, many studies have been done on photoconductive properties of ZnO thin films in MSM structure [10–12]. On the other hand, the focus has been on the ZnO nanostructures, in recent years and it was shown, using ZnO nanostructures can improve the responsivity

∗ Corresponding author at: Department of Physics, Sharif University of Technology, Tehran, Iran. ∗∗ Corresponding author. E-mail addresses: [email protected] (S.M. Mahdavi), [email protected] (A.R. Bahrampour). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.024

of ZnO-based detectors mainly caused by the following reasons: (1) Large surface to volume ratio and deep surface trap states increase the life time of photo-generated carriers. (2) Dimension reduction decreases the transit time of photo-generated carriers [13,19]. So far, studies regarding photodetectors based on two-dimensional (2D) ZnO network-like nanostructures including nanowalls and nanosheets have been less orthodox compared to the number of reports regarding photodetectors based on 1D ZnO nanostructures [13–19]. Due to high surface to volume ratio of these 2D structures, they can be a good candidate to use in UV photodetectors. Some of physical properties of these sheets have been studied such as optical and photoluminescence properties [22] and field emission [23]. In this paper, we use the ZnO sheet thin film to fabricate UV photodetectors. The ZnO sheets are prepared using different methods such as: electrochemical deposition [24–29], vapor–liquid–solid method (VLS) [30,31] and chemical solution route [23]. Among them, electrochemical deposition has been widely adapted for the growth of ZnO sheets due to the advantage of high growth rate, large area, low cost and sheet size controllability. In previous publications, ZnO sheets were grown on a conductive substrate such as (indium tin oxide) ITO, Zn and (fluorine-doped tin oxide) FTO [24–29]. Here, the sheets were deposited on a buffer layer of ZnO by using electrochemical method. It should be noted that ZnO sheets, deposited on the conducting substrates, cannot be used as UV detectors, because the conductivity of underlying substrate leads to short circuiting of the device under UV illumination. Therefore using this buffer layer instead of

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conductive substrate makes it possible to use the sheets in MSM structures and yet decreases the cost of fabrication. In a further step, transient photoresponse and current–voltage of ZnO sheet thin films was measured. We compared photodetecting properties of these UV photodetectors with various sheet sizes and studied the effect of ZnO sheet size on the photoreponsivity and fall time of the fabricated photodetectors. We have shown that the photoresponse properties of fabricated devices are controllable by changing the sheet size. We also fabricated a comb-like electrode patterned UV photodetector using standard photolithography method which showed faster photoresponse. 2. Experimental details 2.1. Device fabrication using shadow mask ZnO sheets were grown on a ZnO buffer layer by electrodeposition method. A 200 nm thick ZnO buffer layer was deposited on a glass substrate using a KrF laser ( = 248 nm, pulse repetition rate of 10 Hz and pulse duration of 20 ns) at room temperature. A diskshaped target was obtained by uniaxial pressing of highly pure ZnO powder (99.99%) and sintering it at 1100 ◦ C for 5 h. The PLD chamber was initially evacuated to 2 × 10−5 Torr. The background O2 pressure was 100 mTorr and the target-substrate distance was kept at 4 cm. The synthesized ZnO film was used as a working electrode in a conventional three-electrode electrochemical cell, while a FTO glass was used as a counter electrode. The electrolyte was an aqueous solution of 0.1 M KCl acted as a supporting electrolyte and 0.05 M zinc nitrate hexa hydrate (Zn (NO3 )·6H2 O) as a Zn2+ precursor. Electrodeposition was performed at 70 ◦ C under constant potential (−1.1 V versus saturated calomel electrode). After deposition, in order to improve the crystallinity of the films, samples were annealed at 450 ◦ C for 1 h in O2 ambient. UV detectors based on MSM structure were fabricated via synthesized ZnO sheets. As an ohmic contact, Al films with thickness of 100 nm were thermally evaporated on ZnO sheet thin film using a shadow mask (Fig. 1). 2.2. Fabrication of device with comb-like electrodes A 100 nm-thick chromium film was patterned on a glass substrate in a comb-like structure by conventional photolithography process using Shipley 1800 photoresist. We designed an etching mask so that the fingers of comb-like pattern were 60 ␮m wide and 4 mm long with a spacing of 60 ␮m. Afterwards, a 200 nm thick ZnO compact layer was deposited onto the comb-like pattern by PLD and ZnO sheets were grown onto it by electrochemical method as we explained in part 2.1. The growth time was 20 min. Scanning electron microscopy (SEM, HITACHI-S4160) and Xray diffraction (XRD, PHILIPS X’PERT) were used to characterize structural properties of the ZnO sheets. Measurements of the current–voltage characteristics were performed by a PalmSens electrochemical sensor interface under dark and steady UV illumination (365 nm wavelength and 0.5 mW/cm2 intensity). 3. Results and discussion A schematic of fabricated UV photodetector is shown in Fig. 1(a) and (b) shows the top view SEM image of grown sheets with two Al electrodes (MSM photodetector). The distance between the two electrodes is around 500 ␮m. In order to get ZnO sheets with different sizes, three samples with different electrodeposition times of 1,1.5 and 2 h were fabricated and named sample a (sa), sample b (sb) and sample c (sc) respectively. Fig. 2(a)–(c) shows the SEM images of sa, sb and

Fig. 1. (a) A schematic of the cross-sectional view of a ZnO photodetector based on sheets. (b) Top view SEM image of grown sheets with two Al electrodes (MSM photodetector).

sc. As can be seen in Fig. 2(a)–(c), the sheet sizes depend on the growth time and increasing the electrodeposition time leads to larger sheets of ZnO. Increasing the sheet size with growth time in electrodeposition was previously reported [26]. Therefore, the size of ZnO sheets can be made controllable by adjusting the electrodeposition time. The average length and thickness of ZnO sheets in Fig. 2(a)–(c) are 600 nm and 50 nm for sa, 3 ␮m and 300 nm for sb, and 6 ␮m and 500 nm for sc. The three samples have the same electrode spacing. Longer growth time of samples sa–sc in comparison with conventional electrodeposition of ZnO sheets on the conducting substrate is due to high resistance of ZnO buffer layer [24–29]. It should be noted that the resistance of ZnO sheet is three order of magnitudes less than ZnO buffer layer and the resistance of ZnO buffer layer is increased after annealing in O2 ambient[32], so the underlying ZnO buffer layer is considered as an insulator layer with respect to ZnO sheet thin film. Fig. 3 shows the X-ray diffraction (XRD) pattern of the grown sheet thin film. It is clear that the sample is indexed to typical wurtzite-type ZnO. All sharp peaks demonstrate the good crystals of the obtained sheets. Fig. 4(a)–(c) shows the typical I–V characteristics of sa, sb and sc, measured at room temperature under dark and UV illumination, respectively. The linearity of the curves indicates an ohmic contact between ZnO sheets and Al electrodes. The dark current values at 5 V bias for sa, sb and sc are 0.5 nA, 20 nA and 2500 nA, respectively. It is found that the dark current is enhanced with increasing the size. This can be due to two reasons: First, a sample with smaller sheets has higher surface-to-volume ratio which increases the surface trap states. It is known that surface states absorb oxygen molecules and

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Fig. 3. The XRD pattern of ZnO sheets.

the remaining free electrons have a longer life time which increases the photocurrent. The gain of a MSM detector is given by [34]: G=

 t

(1)

where  is the hole life time and  t is the transit time of electrons to reach electrodes [34]. If we assume the quantum efficiency to be 1, the gain can be alternatively obtained by the following relation: G=

Iph Popt

×

h q

(2)

where Iph is the maximum photocurrent and Popt is the incident power. The relation between responsivity and gain is as follow: R=G

Iph q = Popt h

(3)

Also, photosensitivity of a photodetector is defined as the ratio of the photocurrent to the dark current as follows: Photosensitivity =

Fig. 2. SEM images of ZnO sheets: (a) sample (sa), (b) sample (sb), and (c) sample (sc).

under dark conditions, free electrons are captured by these surface states and form O2 − species [12,13,17,19,33]. Therefore, the reduction in free electron density in the films with smaller sheets is more than films with larger ones. Second, when the size of sheets is reduced, the electrons will pass through larger number of sheets between two electrodes which corresponds to the higher resistance against electrical current. For these two reasons, thin films based on smaller sheets show lower dark current. Upon UV illumination, photo-generated holes are trapped by O2 − species. In the absence of trapped holes (reduction of electron–hole recombination rate),

Iph

(4)

Id

where Id is the dark current. The gain value, responsivity and photosensitivity at applied bias of 5 V for three samples are given in Table 1. It was observed that the gain and responsivity of the photodetectors are enhanced with an increase in sheet size, while photosensitivity decreases at the same time. The enhancement of the gain and responsivity are due to higher photocurrent in samples with larger sheets. By reducing the sheet size, both the dark current and photocurrent decrease (as can be seen in Fig. 4 and Table 1), but the reduction of dark current is more than photocurrent. It leads to the enhancement of sensitivity of smaller sheets in accordance with relation (4). Time dependent on/off photoconduction measurements were also carried out at 5 V applied bias (Fig. 5). Measured time characteristics are summarized in Table 2. When UV illumination is switched on, the current increases and reaches a saturated value Table 1 Values of measured photoconductive properties of three samples with two electrodes. Sample

sheet size (nm)

Gain

Responsivity (A/w)

Sensitivity

Dark current (nA)

sa sb sc

600 3000 6000

1.78 18.93 61.35

0.522 5.56 18.04

2 × 104 7 × 103 188

0.5 20 2500

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Fig. 5. Time dependent photoresponse at a bias voltage of 5 V: (a) sample (sa), (b) sample (sb), and (c) sample (sc). Fig. 4. Typical I–V characteristics of dark current and photocurrent under UV illumination through sheets of ZnO at room temperature: (a) sample (sa), (b) sample (sb), and (c) sample (sc).

Table 2 Values of measured time characteristics for three samples with two electrodes. Sample

Fall time to 80% (s)

Fall time to 40% (s)

sa sb sc

4 12.5 42

26.5 87 361

which subsequently decays by turning the UV off. The rise time of photocurrent to saturated value is less than fall time as reported in previous works related to ZnO nanostructure based photodetctors [16,33]. As the UV lamp was turned off, the photocurrent decreased from the maximum value and returned to 80% of its maximum within about 4, 12.5 and 42 s for sa, sb and sc, respectively. The fall times of the photocurrent to 40% of its maximum are given in Table 2. At the dark, surface states absorb oxygen molecules and free electrons are captured by these surface states, forming O2 − species.

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Fig. 6. (a) A schematic of ZnO sheet photodetector with comb-like electrode. (b) Top view SEM image of the ZnO sheets that are grown on the chromium patterned electrodes when there is no buffer layer. (c) Top view SEM image of ZnO sheets based photodetectors with comb-like electrode in the presence of the ZnO buffer layer. (d) A partially enlarged photograph of the sample in (c).

This leads to the reduction of electron density and low dark current of the films. After UV illumination, photo-generated holes diffuse to the surface and will trap by O2 − species. Consequently with the absence of trapped holes, electron–hole recombination rate is reduced and the remaining free electrons have a longer life time which increases the photocurrent and causes the photoresponse of the device. Surface to volume ratio and density of surface traps in the smaller sheets is more than larger sheets. So we expect the hole life time (corresponding to the time of electron diffusion to the trapped holes at the surface) of the sa to be shorter than that of sc. Also, the resistance of smaller sheet thin films is more than larger sheet thin films (Confirmed by dark currents in Fig. 4) and as a result electron transit time of the sa is higher than that of sc. Consequently, larger sheet based photodetectors with longer hole life time and shorter electron transit time show higher gains compared to smaller sheet based photodetectors (see Eq. (1)). It seems that surface recombination is the main mechanism of electron–hole recombination. When the UV lamp is turned off, we expect shorter electron–hole lifetime in sa, due to its higher surface to volume ratio and higher density of trapped holes. Consequently fabricated UV photodetectors consisting of smaller sheets exhibited shorter fall time compared to larger sheet photodetectors.

Fig. 6(a), (c) and (d) shows a schematic and top view SEM image of fabricated UV photodetector with a comb-like electrode. The average length and thickness of ZnO sheets were about 16 ␮m and 300 nm, respectively. As shown in Fig. 6(b), if we do not use any ZnO buffer layer, the growth of ZnO sheets only occurred on the chromium patterned electrodes and there are no sheets in the electrode spacing. Therefore, adjacent electrodes cannot connect to each other and this system does not show any photoconductivity. In the presence of the buffer layer, the ZnO sheets grow on it and these sheets cover the spacing between electrodes. In this case, as shown in Fig. 6(c), the sheets are distributed on the whole surface of comb-like electrode. The measured I–V characteristic of this sample (at room temperature) is shown in Fig. 7(a). The ohmic behavior of ZnO Chromium contact can be seen in the I–V plots. The gain value, responsivity and photosensitivity at applied bias of 4 V for this sample are given in Table 3. Fig. 7(b) also shows the transient response of the measured current with switching on and off UV illumination. From Fig. 7, we can obtain some time characteristics of the device which are given in Table 3. The rise and fall time in this sample were less than sa, sb and sc (as can be seen in Fig. 5(a)–(c) and Fig. 7(b)). Decreasing the spacing between electrodes can reduce the rise and fall times. This reduction is due to enhancement of the electron collection efficiency in

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Table 3 Measured photoconductive properties of sample with comb-like electrode. Gain

Responsivity (A/w)

Sensitivity

Dark current (nA)

Fall time to 80% (s)

Fall time to 40% (s)

53.4

15.7

108.77

5725

0.6

35.4

the comb-like electrodes. In the case of comb-like electrodes, there are some conductive fingers which are in parallel connected to each other in the device, these fingers are electron collectors to the external circuit. Also, since finger spacing is smaller than electrode spacing of previous samples, the overall path of electron transport to the electrodes is reduced. In addition using the lithographic electrodes reduced the growth time of ZnO sheets. It was found that dark current of this sample was more than that of sa, sb and sc and this might be related to shorter distance between electrodes that are connected to each other in parallel. Therefore our results show that reducing the electrode spacing and size of ZnO sheets can result in fast response of ZnO sheet based photodetectors. We compared our fabricated ZnO sheets based photodetector with ones based on polycrystalline thin films which were fabricated by PLD and sputtering methods [33,35]. It is found that our fabricated photodetectors with comb-like electrode indicate faster response (rise and fall times to 40% of saturated current are 33 and 35.4 s respectively) compared to ZnO polycrystalline films grown by PLD (rise and fall times to 70% of saturated current are 90 and 900 s respectively) [33] and sputtering (fall time to 70% of saturated current is 600 s) [35]. Sensitivity and gain of our devices

were better (108, see Table 3) than those of nanowire-based photodetectors (sensitivity was about 2) [17]. Our results show that it is possible to obtain faster sheet based UV photodetectors with reducing the size of the sheets and the spacing between electrodes. Also we could manage to control the photoresponse properties of ZnO sheet UV photodetectors by changing the size of the sheets. 4. Conclusions In conclusion, for the first time UV photodetectors were fabricated based on ZnO sheet thin films by electrodeposition method. Reducing the sheet size exhibited an enhanced sensitivity and shortened fall time compared to larger sheets. The origin of these behaviors is related to the enhancement of surface to volume ratio with reducing the sheet size. Furthermore, increasing the sheet size enhances the gain and responsivity of the UV photodetectors. Also, ZnO sheet based UV photodetector with comb-like electrodes was fabricated and it indicated faster responses in comparison with two electrode ZnO sheet based photodetectors. Therefore reducing the electrode spacing and size of ZnO sheets can result in faster ZnO sheet based photodetectors. The response time of device, reported here is better than that of polycrystalline ZnO based photodetector which fabricated by PLD and sputtering. Our device shows higher gain compared to nanowire-based photodetectors. Acknowledgments The authors would like to thank the vice presidency of research and technology of Sharif University of Technology for their support for this work. Also, we are grateful of Mr. Mahmoud Samadpour for his useful discussion. References

Fig. 7. (a) Typical I–V characteristics under dark and UV illumination through sheets of ZnO with comb-like electrode at room temperature. (b) Time dependent photoresponse at a bias voltage of 4 V of the same sample.

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