Voltage-dependent responsivity of ZnO Schottky UV photodetectors with different electrode spacings

Accepted Manuscript Title: Voltage-dependent responsivity of ZnO Schottky UV photodetectors with different electrode spacings Authors: Xuan Zhou, Dayong Jiang, Xiaojiang Yang, Yuhan Duan, Wei Zhang, Man Zhao, Qingcheng Liang, Shang Gao, Jianhua Hou, Tao Zheng PII: DOI: Reference:

S0924-4247(18)30878-1 https://doi.org/10.1016/j.sna.2018.09.032 SNA 11007

To appear in:

Sensors and Actuators A

Received date: Revised date: Accepted date:

23-5-2018 11-9-2018 11-9-2018

Please cite this article as: Zhou X, Jiang D, Yang X, Duan Y, Zhang W, Zhao M, Liang Q, Gao S, Hou J, Zheng T, Voltage-dependent responsivity of ZnO Schottky UV photodetectors with different electrode spacings, Sensors and amp; Actuators: A. Physical (2018), https://doi.org/10.1016/j.sna.2018.09.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Voltage-dependent responsivity of ZnO Schottky UV photodetectors with different electrode spacings Xuan Zhoua, Dayong Jiang a,*, Xiaojiang Yanga, Yuhan Duana,b, Wei Zhanga, Man Zhaoa, Qingcheng Lianga, Shang Gaoa, Jianhua Houa, Tao Zhenga aSchool

of Materials Science and Engineering, Changchun University of Science and

bResearch

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Technology, Changchun 130022, China

Center for Space Optical Engineering, Harbin Institute of Technology,

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Harbin 150001, China Highlights 

• ZnO MSM UV photodetector has been prepared by RF magnetron sputtering with

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different electrode spacings.

• Both of the spectral responses show a saturation behavior dependence on the applied

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bias voltage.

• The rise and fall ratios of responsivity in the narrower spacing are approximately three times faster than that in the wider one.

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• The origin has been interpreted considering the depletion width and electron-hole

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pairs separation.

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Abstract

Metal-semiconductor-metal ultraviolet photodetector with different electrode

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spacing is fabricated on ZnO film, prepared by radio frequency magnetron sputtering

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technique on a quartz substrate. The responsivity of the ZnO photodetector increased with decreasing electrode spacing for the same bias. Meanwhile, both of the spectral responses show a saturation behavior due to the gain effect dependence on the applied

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bias voltage. Additionally, more attractive thing is that the rise and fall ratios of responsivity in the narrower spacing are approximately three times faster than that in the wider one. A physical mechanism primary focused on depletion width and electron-

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Corresponding author: Tel.: +86 431 85583017; fax: +86 431 85583015 E-mail address: [email protected] (D.Y. Jiang)

hole pairs is given to explain the above results. It is demonstrated that a straightforward way to enhance the responsivity of ZnO photodetectors for application is with different electrode spacings.

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Keywords: ZnO; MSM; electrode spacing; responsivity; rise and fall ratios

1. Introduction In recent years, ultraviolet (UV) photodetectors (PDs) have received much attention owing to their commercial and military applications [1-3]. Because of the wide band gap of ∼3.37 eV, radiation hardness, low cost, and environmentally friendly, ZnO

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is emerging as a promising candidate in photodetectors for the UV spectral range [4-6]. Compared with the commonly reported photodetectors [7, 8], the metal-semiconductormetal (MSM) PDs [9-11] have the simplest fabrication progresses, intrinsic high speed,

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low capacitance and compatibility with field effect transistors technology, which make MSM PDs more suitable for practical application.

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Although MSM PDs are not expected to exhibit any internal gain, usually this is

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not the case. The presence of gain mechanism has been widely observed in

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GaN/MgZnO based UV Schottky detectors [12-14], as well as in MSM structures [15-

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17]. Several theories such as photoconductive gain [18], trapping of carriers at metalsemiconductor interface [19] and image force reduce effect [20] have been employed

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to explain an interesting phenomenon, the responsivity shown saturation behavior firstly, and then decrease at high bias voltage. Nevertheless, as the key parameter to

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determine the responsivity of a photodetector, the effects of different depletion widths

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among different electrode structures on responsivities have rather limited researches. More significantly, however, the different rise and fall ratios of responsivity dependence on bias voltage applied to the depletion region at different spacing

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electrodes are unexplained so far. In this paper, we report the MSM Schottky photodetectors with different electrode

spacing (3 and 8 μm) fabricated on high quality ZnO film, which are grown on the quartz substrate (SiO2) by radio frequency (RF) magnetron sputtering method. We 1

interpret the reason for bias voltage-dependent responsivity with the emphasis on the depletion width and gain to account for the significantly higher rise and fall ratios in the narrower electrode than that in the wider one.

2. Experimental

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First, the SiO2 substrate was sequentially cleaned with acetone, ethanol, and de-

ionized water for 30 minutes, blown dry with N2 gas. Then, the ZnO thin film was

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grown on quartz substrate by using the RF magnetron sputtering technique, with a total

pressure of 0.5 Pa, a sputtering power of 150 W and an O2/Ar flow ratio of 10:40 for 3

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hours. Based on the prepared ZnO film, Au was sputtered on to serve as the metal

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contact. Finally, two types of Au interdigitated electrodes were obtained by

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conventional UV exposure and wet etching to fabricated MSM structural UV detector.

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The widths of the Au fingers were 5 μm and the length were 500 μm, with the interval

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spacing of 3 and 8 μm, respectively. The schematic of the PDs is shown in the Fig. 1. The phase of the ZnO film was characterized using RigakuUltima VI X-ray

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diffractometer (XRD) with Cu Kα radiation (λ = ∼1.543 Å) at 40 kV and 20 mA. The

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morphologies of the products were observed by Japan Electronics JSM-6701F Cold Field Emission Scanning Electron Microscope for Testing and Analysis. The

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absorption, transmittance and reflection spectra were measured by PerkinElmer Lambda 950 UV/vis Spectrometer from 200-850 nm. An Agilent B1500 semiconductor parameter analysis meter with 16442A Test Fixture is used for the current-voltage (I-

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V) characteristics of the ZnO PDs in the dark. The ZolixDR800-CUST Spectrometer at room temperature surveyed the spectral response of the sample. 3. Results and Discussion

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The inset of Fig. 1 shows scanning electron microscopy (SEM) image of the crosssectional morphology of the ZnO film deposited for 3hr. Due to the irregularity of

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artificial cross section, we can know the thickness of the film is about 340.3 nm. Fig. 2

shows the typical XRD pattern of ZnO film. There are (002) and (004) diffraction peaks located at 34.4° and 72.6° with no other peaks can be observed. Furthermore, the lattice

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constant c of nanocrystal, a key parameter to the film quality, can be calculated by the

𝜆

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following equation:

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(1)

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𝑐 = 2 sin 𝜃 √3(𝑎/𝑐)2 (ℎ2 + ℎ𝑘 + 𝑘 2 ) + 𝑙 2

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Where λ and θ is X-ray wavelength (λ=1.54184 Å) and Bragg angle, respectively. And h, k, l are Miller exponents, Thus the value of c can be obtained as 5.21, which is close

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to the standard value [21]. Both of them proved that the excellent preferred c-axis

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orientation hexagonal film has a high quality. The absorption and transmittance spectra of the ZnO film are shown in the Fig. 3. The inset is the reflectance spectrum. A sharp absorption edge located at ~378 nm, a lower reflectance in the UV region and a high

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transmittance (more than 80%) in the long wavelength region illustrate that the film has potential application in UV sensitive PDs. The current–voltage (I–V) characteristics of the photodetectors in dark are shown in Fig. 4. The nonlinear I-V curves indicate that the classic Schottky contact has been 3

obtained. Dark current is one of the main sources for noise in the photodetectors. In order to detect small signals from strong backgrounds, the dark current must be minimized [22]. The device exhibits a relative low dark current of about 5.56 pA at 10V bias, which is helpful for enhancing the photodetector's signal-to-noise (S/N) ratio.

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What’s more, it can be seen from the inset that the dark current increases as the electrode spacing decreases, which may be caused by the lower resistance of ZnO and

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wider depletion width promote the movement of holes and electrons. The specific reasons will be explained below.

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Fig. 5 (a), (c) shows the photo-response spectra of the PDs as a function of the

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incident light wavelength at a series bias voltage (15-60V). The single sharp

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responsivity peaks primarily generated in the ZnO films located at 365 nm are observed.

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And the UV-to-visible rejection ratio (365 versus 500 nm) of the photodetector could

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be more than four orders of magnitude high, indicating a high degree of visible blindness. Meanwhile, an interesting phenomenon occurred. Both of the photo-

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response curves first increased gradually; hit a peak, then further increasing applied

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bias led to a drop of the responsivity. This significant bias dependence can be explained by the gain effect of the responsivity. Calculated by the equation [19]: η𝑒𝑥𝑡 = (Iph ⁄q) ∙ (ℎ𝜈 ⁄𝑃𝑖𝑛𝑐 )

(2)

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where ηext is the external quantum efficiency, Iph is the average photocurrent, q is the electronic charge, h is Planck’s constant, ν is the frequency of light, Pinc is the incident light power. The representative external quantum efficiencies (EQEs) of the two ZnO MSM PDs, measured at the biases of 15, 45 (50), and 60 V are shown in Fig. 5 (b) and 4

(d), respectively. It is noteworthy that the ηext of the device increases by 5-6 times at the peak, which indicating that an internal gain exists within the photodetector. In general, the decrease of the responsivity gain will occur as the voltage increase, the enhancement of the responsivity is mainly due to the sweep effect of the applied bias [14, 23].

enough [15, 24]. As a result, there will be a decline in the responsivity.

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Moreover, the responsivity gain will suffer big recession when the applied bias is strong

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To reveal the nature of this phenomenon, a couple of peak responsivity curves of two electrode spacing versus bias voltage are plotted in Fig. 6 (a). Fig. 6 (b) expresses

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the slopes of two curves in order to intuitively compare the rise and fall ratios. The

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slope values and discrete distribution statistics show an attractive tendency that the rise

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and fall ratios of responsivity in narrower spacing are about three times higher

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obviously than that in wider spacing. This appearance can be interpreted by the

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collective effect of depletion width and gain below. In MSM structure, the length of semiconductor (ZnO) would be narrower as the

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electrode spacing decreases, so the resistance of ZnO will be reduced at the same time,

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which will result in an enhanced bias voltage act on the depletion region when the applied bias voltages are constant. The depletion region as shown in Fig. 7 can be

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expressed as:

W = [2𝜀0 𝜀1 (𝜑0 + 𝑉)/𝑞𝑁𝐷 ]1/2

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where ε0 is the absolute dielectric constant, ε1 is the relative dielectric constant, q is the electron charge, φ0 is the built-in potential, V is the applied bias voltage and ND is the donor concentration (about 1016 cm-3). The expression is given for the voltage5

dependent depletion layer width. Additionally, deduced from the formula: E = V/L

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where V is the bias voltage applied on the depletion regions, L is the length of semiconductor. The electrode with narrower spacing owns a larger electric intensity in

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the depletion layer. As a result, when the same bias voltage applied on the device, according to these

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two equations, the narrower spacing electrode will possess a wider depletion width, in which any photo-generated carriers would be swept out by the higher electric-field in

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this region and drift to the metal electrodes [25]. Therefore, the larger active receiving

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area, stronger separation ability and larger amount of photo-generated carriers should

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half of narrower spacing electrode.

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be responsible for the larger dark current and remarkably faster growth ratio in the first

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As for the falling part, under the same parameter of voltage, more bias voltage will

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be applied to the depletion layer at the shorter spacing electrode, so the electric field here will increase faster. In this condition, more holes will be separated in the depletion

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region, which leads to a faster responsivity gain reduction at sufficiently high bias voltage [26]. Consequently, the overall rise and fall ratios of responsivity in narrower

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electrode spacing will be faster obviously than that in the wider one. 4. Conclusion In summary, high performance ZnO based MSM structured UV photodetector has been fabricated using the RF magnetron sputtering method with different electrode 6

spacing. The responsivity of photodetector will increase with decreasing the electrode spacing when the applied bias voltages are constant. Both of the responsivity of ZnO PDs show a saturation behavior and a decrease at high bias voltage due to the gain of responsivity. More importantly, the rise and drop ratios of responsivity in narrower

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electrode are three times faster than that in the wider electrode, which is attribute to the collective effects of depletion width and electron-hole pairs separation. It’s

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demonstrated that reducing electrode spacing is a promising route to improve the performance of ZnO based photodetectors.

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Acknowledgments

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My name is Xuan Zhou. I received a bachelor’s degree from Shandong

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Agricultural University, majored in Applied Chemistry. I am now a Ph.D. candidate

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in School of Materials Science and Engineering, Changchun University of Science

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and Technology. My research direction is wide bandgap semiconductor optoelectronic

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functional materials, mainly the modification and application of ultraviolet detectors.

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This work is supported by the National Natural Science Foundation of China (Grant

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No. 61774023), Scientific and Technological “13th Five-Year Plan” Project of Jilin Provincial Department of Education (Grant No. JJKH20170609KJ), the National Key Research and Development Program of China (Grant No. 2016YFB0303805), Scientific and Technological Development Project of Jilin Province, China (Grant No. 20150311086YY), the National Key

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Research and Development Program of China (Grant No. 2016YFB0303805), Postdoctoral Advanced Programs of Jilin Province (2014), Postdoctoral Fund of Changchun University of

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Science and Technology.

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Figure captions:

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by hole sweep-out effect, Electron Devices Meeting. 1985 International IEEE. 31 (1985) 471-

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Fig. 1 Schematic of a ZnO PD with different electrode spacing (3 and 8 μm). The inset is SEM image of the cross-sectional morphology of the ZnO film.

Fig. 2 XRD spectrum of the ZnO film.

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Fig. 3 UV-visible absorption and transmission spectra of the ZnO film. The inset is reflectance

spectrum of the ZnO film. Fig. 4 I-V characteristics of the ZnO PDs as a function of the bias voltage under dark conditions. The inset is an enlarged view between 10-20V 11

Fig. 5 (a) (c) Photoresponse spectra of the ZnO PDs with different electrode spacing 3μm and 8μm in the range of 15-60 V. (b) (d) Spectral responses and external quantum efficiencies (EQEs) of the two ZnO MSM PDs, measured at the biases of 15, 45, and 60 V, respectively.

Fig. 6 (a) Peak response as a function of the bias voltage in the ZnO PDs. (b) A comparison diagram

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of the slope values of two curves. The inset is the discrete distribution statistics of two slopes

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in the rising stage

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Fig. 7 Schematic illustrating the depletion regions in the MSM structure

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