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Photovoltaic Response of Transparent Schottky Ultraviolet Detectors Based on Graphene-on-ZnO Hexagonal Rod Arrays Won Woo Lee, Jae Hyung Lee, Su Han Kim, Dong Won Yang, and Won Il Park J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00626 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Photovoltaic Ultraviolet

Response Detectors

of Based

Transparent on

Schottky

Graphene-on-ZnO

Hexagonal Rod Arrays Won Woo Lee†, Jae Hyung Lee†, Su Han Kim†, Dong Won Yang†, and Won Il Park*† †

Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of

Korea. *

Correspondence

and

requests

for

materials

should

be

addressed

to

W.

I.

P.

([email protected]).

ABSTRACT Herein, we report the ultraviolet (UV) photoresponse characteristics of vertically-grown ZnO rod arrays and graphene hybrid detectors. Graphene serves as a UV transparent electrode for efficient light absorption and a Schottky electrode for promoting the separation of charge carriers photogenerated in ZnO. In particular, when the ZnO rods were passivated by polymer encapsulation, the proposed device (Device I) exhibited improved Schottky junction characteristics and photovoltaic response to UV light when compared to the device based on bare ZnO rods (Device II). These improvements, which originated from the reduced photodesorption effect, enable Device I to operate in a zero-bias condition, producing a substantial photovoltaic output current with a photoresponsivity of 5.14 × 10 A/W, which is not achievable using Device II. More importantly, compared to the photoconductive response of Device II operating in bias conditions, the photovoltaic response of Device I yields a higher on/off ratio of ~280 and faster and more reliable cyclic operations.

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INTRODUCTION Ultraviolet (UV) detection tehniques have long been developed for various civil, environmental, and military applications, such as pollution and flame monitoring, chemical and biological sensing, UV astronomy, and early missile plume detection.1-3 More recently, with the increasing concerns of UV radiation exposure due to depletion of the ozone layer, substantial efforts have been devoted to developing portable yet high-sensitive UV detectors, especially in the form of wearable electronics for IoT applications.4, 5 To realize such a device, wide band gap semiconductors (e.g., TiO2, ZnS, CdS, GaN and ZnO) have been exploited as UV reactive elements that selectively generate photo-carriers in response to UV lights via the photoelectric effect.6-12 In particular, due to its direct and wide band gap of 3.37 eV at room temperature, ZnO exhibits an excellent photoresponse to UV and intrinsic visible blindness. These characteristics, together with its superior radiation hardness, make ZnO a promising candidate for UV detectors. UV detectors made with ZnO nanocrystals (e.g., nanorods and nanowires) demonstrated a higher photoresponse efficiency than that of thin film structures, largely considered to be due to the enhanced surface area and crystal quality.13-15 Thus far, high temperature, vapor-phase-grown ZnO nanomaterials have mostly been employed to achieve high performance UV detectors. In particular, by using a monolayer graphene sheet as top Schottky electrode on ZnO, highperformance UV detectors were suggested.16, 17 On the other hand, several previous works have investigated the photoresponse characteristics of hydrothermally-grown ZnO nanorods, given the advantages of this solution-phase process in terms of its low process temperatures and cost, lesshazardous process waste, and compatibility with flexible organic substrates.18-20 However, UV detectors based on those hydrothermally-grown ZnO nanorods displayed relatively low on/off ratios between the photoresponse current and dark current due to their defects, as well as


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degradation in ambient atmospheres. In addition, the detectors suffer from very slow responses since the UV exposure process interacts strongly with the desorption of oxygen molecules via the so-called photodesorption at the ZnO surface. In this study, we propose a vertically-grown UV detector that combines a well-aligned ZnO hexagonal rod (h-rod) array and one-atom-thick graphene sheets. In this structure, the hydrothermally-grown ZnO h-rod arrays are thermally annealed to reduce the surface-state defects, and are then embedded in SU-8 polymer matrix, where only the top surfaces of the ZnO h-rods are exposed so that optically-transparent Schottky junctions are developed at the top of the ZnO h-rods and the graphene interfaces. This structure results in an enhanced photoresponse by minimizing the photocarrier recombination at the depletion zone while also minimizing the optical loss, which is critical for conventional metal electrodes.21-23 In addition, atomically thin graphene sheets covering the ZnO h-rods and polymer composite act as a diffusion barrier for gas molecules and moisture, thus largely suppressing the photodesorption effect and thereby resulting in an enhanced rectifying current-voltage (I–V) characteristic and response speed.21 These characteristic features enable the UV detector to operate in a self-powered mode, with photovoltaic current demonstrating a faster response and higher sensitivity compared to the photoconductive response of the device based on bare ZnO rods operating in a bias mode.



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EXPERIMENTS Vertically-aligned ZnO h-rod arrays. Vertically-aligned ZnO h-rod arrays were obtained through a solution-phase epitaxial growth process by employing ZnO epitaxial seed layers on Al2O3 (0001)/GaN substrates (Fig. 1a). The detailed procedure for this ZnO seed layer growth is described elsewhere.24 To control the position and diameter of the ZnO h-rods, 1.2-µm-thick AZ5214 layers were spin-coated on the ZnO seed layers, in which square arrays of circular holes with diameters and an interspacing of 2 µm and 20 µm, respectively, using conventional photolithography. The hydrothermal growth of ZnO h-rods was performed by placing the holearray-patterned seed layer samples inside an aqueous solution containing equimolar concentrations (0.025 M) of zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma–Aldrich) and hexamethylenetetramine (C6H12N4, Sigma–Aldrich). During ZnO h-rod growth, the solution temperature was maintained in an oven, typically at 70 °C for 20 h.25 Fabrication of graphene-on-ZnO h-rod array structures. To investigate the UV response characteristics, two-types of devices were fabricated using different graphene-on-ZnO h-rod array structures, as schematically shown in Figure 1. Both types of devices exploited chemicalvapor-deposition (CVD)-grown single layer graphene sheets as the top optically-transparent Schottky contact electrodes and ZnO layers as the bottom ohmic contact electrodes.26 The CVD growth of the graphene sheets was performed on a 25 µm-thick catalytic Cu foil (99.999%, Alfa Aesar), as described elsewhere.27 Typically, the graphene exhibited a p-type ambipolar characteristic (Figure S1).28 Fabrication of the first detector (Device I) was started by filling the free space between the ZnO h-rods using SU-8 spin coating. Subsequently the SU-8 layer was gradually etched down using oxygen plasma until only the rod tips (60 nm in length) were exposed. The CVD4 ACS Paragon Plus Environment

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grown graphene was coated with a poly(methyl methacrylate) (PMMA) protecting layer, which was then transferred onto the ZnO h-rod array embedded in the SU-8, followed by removal of the PMMA with acetone solution (Fig. 1b). In the second detector (Device II), a top contact electrode was made by placing a PMMA coated graphene sheet on top of the ZnO h-rod array. A rectangular-shaped SU-8 polymer resist (MicroChem Corp.) with Ag electrodes was made on one side of the substrate. The SU-8 block electrically isolated the top graphene/PMMA sheet from the bottom electrode and supported the graphene/PMMA sheet to keep it from collapsing during electrical probing/wiring (Fig. 1c).29 Characterization. The morphology of the samples were examined by SEM (JEOL JSM-7600F, Tokyo, Japan) and atomic force microscopy (AFM, Park Systems, XE-100), respectively. Electrical and photoelectrical characterization of the devices was carried out by a probe station with a semiconductor parameter analyzer (model HP4145A) and a picoammeter (KEITHLY 6485A).

RESULTS AND DISCUSSION Tilted view scanning electron microscope (SEM) images of the as-grown ZnO h-rod array and the graphene-on-ZnO h-rod array structures of Devices I and II are shown in Fig. 2a and b, respectively. All images show vertically-well aligned ZnO h-rods with a uniform diameter and height of ~2 µm and ~20 µm, respectively. The center-to-center distance was set to 20 µm over 5 × 5 mm2, producing a 250 × 250 array of h-rods in each sample. SEM image and AFM data of Device I show that the ZnO h-rod array was embedded in the SU-8 polymer and only ~200-nm-thick tips of the 2-µm-diameter rods were exposed, over which single-layer graphene was mounted (Fig. 2a and c). Considering the height to width ratio of 0.1, we can expect that the 5 ACS Paragon Plus Environment


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graphene/ZnO interface area of the Device I should be similar to that of the Device II. Importantly, the graphene conformably covered the surface of the SU-8 polymer and ZnO tips without generating noticeable cracks or damage over a large area, demonstrating the mechanical robustness and flexible characteristics of the graphene sheet. In the case of Device II, despite the large spacing between adjacent h-rods (~20 µm), the graphene/PMMA sheet remained tightly suspended over a large area of the ZnO h-rod array (Fig. 2b). In this situation, the underlying graphene layer adhered to the tips of the ZnO h-rods, resulting in robust electrical contact suitable for device operation. It is also noted that both the insulating polymers (PMMA and SU-8) are clearly visible without image distortion from charge build-up, indicating that the graphene sheets provide a highly-conducting pass way to the ground in the SEM chamber. The current density versus voltage (J–V) characteristics of Devices I and II were also investigated. Contrary to our expectations based on the work functions of the two materials, both devices initially exhibited quasi-linear J–V characteristic curves without showing rectifying behavior. Previous cathodoluminescence studies on hydrothermally-grown ZnO rods revealed the rapid near-band edge (NBE) emission quenching on the ZnO top surface, driven by nonradiative carrier recombination at the defective sites.30 For a similar reason, those defects serve to increase carrier recombination at the ZnO/graphene interface and disturb the formation of stable Schottky junctions. Meanwhile, when the devices were made after thermally annealing the ZnO h-rods in an oxygen atmosphere, non-linear and rectifying J–V characteristic behaviors appeared.31, 32 This result is similar to the NBE emission recovery after thermal annealing. Given that the ZnO top surface is terminated with the positively-charged Zn-(0001) polar plane, the generation of such defects after the solution-phase and annihilation by thermal annealing might be related to the adsorption and desorption of charged molecules (such as OH- groups) on the


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surface. It is also noteworthy that more reliable Schottky junctions were formed in Device I with a forward bias turn-on voltage of about ~0.35 V and a small leakage current of −0.7 mA/cm2 at −1 V, which was significantly less than that of Device II (−6 mA/cm2). When the J–V curves were analyzed using thermionic-emission theory, Device I had more ideal Schottky barrier diode characteristics. A typical J–V characteristic of a Schottky junction model, neglecting series and shunt resistance, is described by J = exp

− 1,

where Js is the reverse bias saturation current density, and e, k, and T are the elementary charge, Boltzmann constant, and absolute temperature, respectively. From the slope of ln (J) versus V plot (typically for 0.2 < Vf < 1 V in the inset of Fig. 3a and b, the ideality factors η were estimated to be 1.84 and 3.88 for Devices I and II, respectively. Although no consensus has been reached yet for the origin of the different characteristics between the devices, (i) charge traps at the single layer graphene/PMMA interface and (ii) sufficient oxygen molecular absorption on the bare ZnO h-rod surface can be attributed to the large deviation of Device II from the ideal Schottky barrier diode (Fig. 3b; see also Figure S2). Fig. 3c and d show the changes in the J–V characteristic curves for Devices I and II, respectively, in dark and under exposure to visible-infrared (vis-IR) and UV lights (black, red, and blue lines, respectively). A UV light-emitting diode (LED) with the wavelength of the main peak between ~380-420 nm and a power density of 4 mW/cm2 was used for the UV source, and a solar simulator AM 1.5G (100 mW/cm2, Newport Oriel 92250A-1000w) with a UV filter (cutoff approximately 420 nm) was used for the vis-IR range light source (shown in Figure S3). Commonly, when both Devices I and II were illuminated by UV, the slopes of the J–V curves 7 ACS Paragon Plus Environment


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became greater and the currents at 1 V bias increased by 19.5% and 6.56% for Devices I and II, respectively. In contrast, both devices hardly respond to the UV-filtered AM 1.5G, indicating they are sensitive only to UV light with a photon energy larger than band gap energy of ZnO (3.37 eV). The responsivity and on/off ratio under applied bias of −1 V were estimated to be 8.5 × 103 A/W and ~6 for Device I and 4.9 × 104 A/W and ~4 for Deivce II, respectively. These values are comparable to those of the previously reported graphene-ZnO based devices. Despite the similar photoconductive response to UV light, Devices I and II have different photovoltaic characteristics: Device I exhibits photovoltaic responses with an open-circuit voltage (Voc) of ~0.5 mV and a photoresponsivity of 5.14 × 10-4 A/W (inset of Fig. 3c), while Device II shows a photoconductivity increase without any noticeable shift in the J–V curve near V = 0 V. The enhanced photovoltaic effect in Device I is essentially in agreement with its more ideal Schottky diode behavior. A more detailed discussion of this will be given in the latter part of this paper.

In order to elucidate the photovoltaic and photoconductive behaviors of the devices more clearly, the time-resolved photoresponses of Devices I and II were investigated. As shown in Fig. 4a and b, Device I exhibited significant and rapid increases in the current upon UV light exposure, even at a 0 V applied bias, while it did not respond to the vis-IR light source. When the device was initially exposed to UV irradiation, the absolute value of the current || (i.e., the reverse current) increased suddenly from noise level (

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