Cu4O3-based all metal oxides for transparent photodetectors

Sensors and Actuators A 253 (2017) 35–40

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Cu4 O3 -based all metal oxides for transparent photodetectors Hong-Sik Kim a , Melvin David Kumar b , Wang-Hee Park a , Malkeshkumar Patel a , Joondong Kim a,∗ a Photoelectric and Energy Device Application Lab (PEDAL) and Department of Electrical Engineering, Incheon National University, Incheon 406772, Republic of Korea b Department of Physics, Aditanar College of Arts and Science, Tamil Nadu, 628216, India

a r t i c l e

i n f o

Article history: Received 28 August 2016 Received in revised form 8 November 2016 Accepted 16 November 2016 Available online 17 November 2016 Keywords: All-oxide devices Cu4 O3 Transparent photodetectors NiO Hole transporting layer

a b s t r a c t All-oxide photodetectors based on Cu4 O3 were fabricated using DC and RF magnetron sputtering. A quality paramelaconite Cu4 O3 was formed by using large-scale available sputtering method and identified by X-ray diffraction. In order to establish a transparent junction, p-type Cu4 O3 was deposited onto an n-type ZnO. The indium-tin-oxide (ITO) layer was served as an electron transporting layer. The general device has a structure of Cu4 O3 /ZnO/ITO to show overall high-transmittance for broad wavelengths with a peak transmittance over 72%. To enhance the photodetector performances, a functional NiO layer was applied as a hole transporting layer, which actively controls the carrier movements, resulting in a quick photoresponse of 33 ms. We demonstrated the paramelaconite Cu4 O3 as a transparent entity and provide the route for effective designs for transparent photoelectric applications. © 2016 Elsevier B.V. All rights reserved.

1. Introduction It becomes inevitable in modern technology to design any advanced optoelectronic devices without having a transparent conductive oxide. Previously, TCOs were used only for the purpose of anti-reflection and ohmic contacts. And, in some cases, they made a rectifying junction with the compatible metals in Schottky diodes [1–5]. However, in recent years, entire device is fabricated using TCOs which work as an efficient light absorbers, hole transport layer, electron transport layer and transparent electrodes [6–10]. Such devices in which conventional components are replaced by suitable TCOs are called as all oxide photo-electronic devices. This would be the future technology in economic energy sectors. The advantages of using TCOs cover the non-toxicity, earth abundant, cost effective and stability under practical atmospheric conditions. These characters make them ideal candidates in large scale optoelectronic industries. Based on the optical properties, most of the TCOs are suitable for device fabrication. However, most of the TCO materials may have drawbacks in their electronic properties like excited states of short lifespan, low conductivity and low carrier concentration which prevent their service in active sensors and solar cells [9]. However, the well-known p-type materials such as oxides of copper (CuO, Cu2 O, Cu4 O3 ), NiO and n-type materials such

∗ Corresponding author. E-mail address: [email protected] (J. Kim). http://dx.doi.org/10.1016/j.sna.2016.11.020 0924-4247/© 2016 Elsevier B.V. All rights reserved.

as indium tin oxide (ITO), fluorine doped tin oxide (FTO), ZnO, SnO2 , TiO2 exhibit a better tradeoff between their optical and electrical properties which enhance the performance of the photoelectric devices [11–17]. Generally, copper exhibits three oxide phases namely CuO (tenorite or cupric phase), Cu2 O (cuprite phase), and Cu4 O3 (paramelaconite phase). Among them, the CuO is widely used phase because of its suitable band gap of 1.55 eV which is close to Shockley–Queisser limit [8]. However, CuO has few drawbacks like low carrier concentration and high series resistance which limit the efficiency of the devices [18]. Also, temperature of more than 450 ◦ C is required for the formation of CuO [19,20]. The Cu2 O phase has been employed by few researchers as an absorber [21,22]. It has some unique features such as higher absorption coefficient, high carrier mobility and long diffusion length [23–26]. But, the solar cells and sensors based on Cu2 O did not yield an expected efficiency in the visible region due to its large band gap [25,27,28]. After careful analysis of these results, we have chosen Cu4 O3 phase as electron absorber in our device due to the following reasons. The Cu4 O3 (Cu(I)2Cu(II)2O3) is an intermediate phase between CuO and Cu2 O and contains an equal number of Cu(II) and Cu(I) ions which are having contrast properties [29]. Considering the oxidation number of +1.5, Cu4 O3 is good as a potential catalyst for oxidation [30]. The optical band gap of Cu4 O3 is 1.75 eV which is little higher than that of CuO and more suitable for photodetectors operating at visible wavelengths. Moreover, it has higher absorption co-efficient of 5 × 104 /cm, carrier concentration of 1019 cm−3 and conductivity

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of 5 × 10−2 S/cm [31]. In fact, little information is available in the literature concerning Cu4 O3 based photo-electronic devices. These advantageous features had drawn our attention towards Cu4 O3 . In this work, we have realized transparent photodetectors by using all metal oxide materials. The transparent photodetector was formed by the rectifying junction formation between Cu4 O3 (paramelaconite) and ZnO. To control the carrier movement effectively, p-type NiO was employed as hole transporting layer (HTL) and n-type ITO served as electron transport layer (ETL). We have demonstrated two all-oxide photodetectors in order to explain the influence of HTL (NiO) on Cu4 O3 active layer to enhance the photoresponse performances at zero bias condition. Both the prepared devices produced the photocurrent in the range of few nA when there is no external bias. The physical transport of electrons under zero bias condition is achieved due to the tunneling of electrons through asymmetric Schottky barriers. 2. Experimental procedure The glass substrates were cleaned by acetone, methanol and DI water for 5 min each under ultra-sonication process. The substrates and sputtering targets were loaded into the DC sputtering system (SNTEK, Korea). Then, the DC power of 300 W was applied to a 4inch ITO target under Ar/O2 (10:0.1) for about 10 min to deposit ITO layer of 200 nm thickness. Then, the glass substrates with ITO coating were undergone rapid thermal process (RTP) at 500 ◦ C for 10 min in order to get highly ordered ITO layer which serves as ETL. Followed by that, RF power density of 3.58 W cm−2 was given to a 4-inch ZnO target (99.999%) to deposit ZnO layer of 100 nm thickness under Ar (50 sccm) atmosphere for 15 min at room temperature (RT). In order to make a rectifying junction, Cu4 O3 layer was deposited over ZnO by applying DC power of 100 W to a 4-inch Cu target for 10 min under Ar/O2 (3:1) atmosphere at RT. To form a HTL, 4-inch Ni target (99.999%) was exposed to the DC power of 50 W for 10 min to deposit a thin NiO layer (∼30 nm) under Ar/O2 (10:1) at RT. All the layers were deposited under RT condition other than the bottom ITO which was heated up to 500 ◦ C during the deposition process. The reference sample has structures of NiO/Cu4 O3 /ZnO/ITO. In order to investigate the HTL, a comparison sample was also prepared without NiO layer to have Cu4 O3 /ZnO/ITO structures. The prepared NiO/Cu4 O3 /ZnO/ITO and Cu4 O3 /ZnO/ITO heterojunction photodetectors were characterized using the following techniques. The crystal structure and lattice plane orientation were analyzed using X-ray diffraction (XRD, Rigaku, D/Max 2500) with Cu K␣ radiation in ␪–2␪ scan mode. The scanning electron microscope (SEM) images of the surface and cross-sections of the prepared devices were recorded by JEOLJSM-7001F. Transmittance profiles of samples were studied using an UV–vis spectrophotometer (Shimadzu, UV-2600). The electrical properties were analyzed by a source meter unit (Keithley 2400). Mott−Schottky analyses were employed to obtained energy band diagram of NiO/Cu4 O3 /ZnO/ITO and Cu4 O3 /ZnO/ITO heterojunction photodetectors by using the potentiostat/galvanostat (ZIVE SP1, WonA Tech, Korea). The carrier collection performances of the Cu4 O3 -templated photodetector were studied using a quantum efficiency measurement system (McScience-K3100) coupled with a monochromatic (Oriel Cornerstone 130 1/8 m Monochromator) source measurement unit (2440, Keithley) and a lock-in amplifier (K102, McScience). 3. Results and discussions Among the oxides of Cu, the cuprite phase (Cu2 O) is easily formed one at room temperature. Increasing the temperature of Cu2 O more than 450 ◦ C results in reduction of oxygen and then

forming CuO phase. Also, Cu2 O can be converted into Cu4 O3 by increasing the oxygen content during deposition. Due to the structural tuning nature, sometimes Cu exhibited mixture of two phases at certain deposition conditions. Therefore, it is important to confirm the formation of Cu4 O3 phase and its degree of crystallinity in the fabricated devices. The high resolution XRD profile of specially prepared Cu4 O3 film is shown in Fig. 1(a). The characteristic peaks appeared at 36◦ and 30.5◦ corresponding to (004) and (200) planes, respectively to confirm the paramelaconite phase with tetragonal structure (COD id: 9000603). The other peaks at 59◦ , 64.35◦ and 75.7◦ were assigned for (224), (026) and (008) planes, respectively to indicate the polycrystalline nature of the prepared Cu4 O3 . In the XRD spectrum of Cu4 O3 , the absence of CuO and Cu2 O peaks inform that the prepared sample is chemically pure. The surface and cross-sectional views of NiO/Cu4 O3 /ZnO/ITO photodetector were recorded using FESEM as shown in Fig. 1(b) and (c), respectively. It is observed that particles were clustered into small grains that are uniformly distributed all over the surface. The inhomogeneity in the grain size is aroused as a result of polycrystalline nature of Cu4 O3 . In Fig. 1(c), columnar growth of ITO, ZnO and Cu4 O3 layers are observed. The top NiO layer is formed on the Cu4 O3 layer was observed. The boundary lines were drawn at the interfaces of ITO/ZnO and ZnO/Cu4 O3 and Cu4 O3 /NiO so as to distinguish each layer and also to indicate the non-perplexed state at the interfaces. The optical transmittance profile portraits the light utilization capacity of the fabricated devices and summarized in Table 1. Fig. 2 shows the transmittance profiles of NiO/Cu4 O3 /ZnO/ITO and Cu4 O3 /ZnO/ITO photodetectors for the wavelengths (␭s) ranged in 220–1400 nm to give the averaged transmittance value of 43.965% for Cu4 O3 /ZnO/ITO structure and 46.161% for NiO/Cu4 O3 /ZnO/ITO structure, respectively. This is quite interesting to produce the enhanced transmittance by using the NiO layer, which is mainly attributed to the improved transmittance at long wavelength region. A peak transmittance value of 72.7% was observed at ␭ = 860 nm for Cu4 O3 /ZnO/ITO structure; while, the transmittance peak is moved to ␭ = 1042 nm and also maximum transmittance of ≈ 72% was maintained in the spectral range of ␭ = 950 to 1042 nm in NiO/Cu4 O3 /ZnO/ITO structure. This is attributed to the higher band gap value of NiO (3.6 eV) than that of ZnO (3.37 eV). The relatively low transmittance in the visible region is attributed to the band gap value of Cu4 O3 (1.75 eV) which absorbs the visible wavelengths to make electronic transitions. Meanwhile, The NiO layer also affects the visible range transmittance spectrum. For the middle wavelengths (500 nm < ␭ <800 nm), the NiO/Cu4 O3 /ZnO/ITO structure has a lower transmittance of 51.321%, compared to 53.56% of the Cu4 O3 /ZnO/ITO structure. The averaged transmittance values were significantly changed for wavelengths, which demonstrate the light-modulation property of the NiO layer. For the short wavelength region, the large band gap materials such as ITO, ZnO and NiO absorb UV radiations which results in meager transmittance at UV region. The energy equivalent to these absorbed wavelengths is used to passage the free charge carriers thereby producing photocurrent, out of which, incident wavelengths could be easily detected. Most of the important electrical parameters such as built-in voltage (Vbi ), space charge region (SCR), rectifying ratio, leakage current etc., could be analyzed from I–V studies. We have recorded the I–V characteristics under dark condition for both the devices as shown in Fig. 3(a) which revealed the distinct rectifying nature of the detectors. Since the SCR is formed between Cu4 O3 and ZnO, the Vbi is almost same (≈0.6 V) for both the detectors. When the applied voltage exceeds Vbi , the forward current is rapidly increased in Cu4 O3 /ZnO/ITO detector, whereas it is gradually increasing in NiO/Cu4 O3 /ZnO/ITO heterojunctions. To understand the reason, it is important to visualize the elec-

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Fig. 1. (a) XRD profile of paramelaconite (Cu4 O3 ) phase. FESEM images for (b) surface (c) cross-section of NiO/Cu4 O3 /ZnO/ITO structures. (d) Schematics of NiO/Cu4 O3 /ZnO/ITO photodetector.

Table 1 Transmittance profiles of Cu4 O3 /ZnO/ITO and NiO/Cu4 O3 /ZnO/ITO structures.

Peak transmittance Averaged transmittance for broad wavelengths (220 nm < ␭ < 1400 nm) Averaged transmittance for short wavelengths (220 nm < ␭ < 500 nm) Averaged transmittance for middle wavelengths (500 nm < ␭ < 800 nm) Averaged transmittance for long wavelengths (800 nm < ␭ < 1400 nm)

tron and hole transports with respect to the applied voltage. Therefore, the band alignment diagrams of sandwiched structures were drawn using SCAPS software and the same is presented in Fig. 3(b) and (c). After contact, the band alignment has formed the cascaded energy levels for conduction band (Ec ) and valence band (Ev ) to have Ec (NiO) > Ec (Cu4 O3 ) > Ec (ZnO) > Ec (ITO) and Ev (NiO) > Ev (Cu4 O3 ) > Ev (ZnO) > Ev (ITO), respectively. Also, each interface exhibits type-II band alignment where valence band energy levels of lower band gap semiconductor are kept in between the energy levels of higher band gap semiconductor and the conduction band of lower band gap material is seen above the conduction band of higher band gap semiconductor. Such step-down energy levels drive the charge carriers very quickly through various layers of the photodetectors which results in fast response to the incident light. From Fig. 3(b) and (c), it is observed that NiO

Cu4 O3 /ZnO/ITO

NiO/Cu4 O3 /ZnO/ITO

72.7%at ␭ = 860 nm 43.965% 32.107% 53.56% 55.469%

72.867%at ␭ = 1042 nm 46.161% 30.267% 51.321% 61.540%

layer is acting as an electron blocking layer and ZnO is acting as a hole blocking layer. In Cu4 O3 /ZnO/ITO detector, the electrons are speedily drifted from Cu4 O3 /ZnO interface to ITO. This causes high forward current above Vbi . Meanwhile, the holes (here, majority carriers) are tunneled into the external circuit due to the absence of NiO layer. This may cause a serious problem of increasing recombination loss. As a result of this, the reverse saturation current (IRS ) or noise current which is correlated to leakage current density in Cu4 O3 /ZnO/ITO device was found to be higher as 48 nA than 29 nA of NiO/Cu4 O3 /ZnO/ITO device in which, holes are drifted from Cu4 O3 to NiO. Due to the regulatory system for both holes and electrons movement in NiO/Cu4 O3 /ZnO/ITO device, noise current has been reduced. Increasing noise current decreases the detectivity of a photodetector [32]. Rectification ratio is calculated from the ratio of forward biased current to reverse biased current. At 1.5 V,

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Fig. 2. Transmittance profile of NiO/Cu4 O3 /ZnO/ITO and Cu4 O3 /ZnO/ITO photodetectors. Table 2 Photoresponse ratio of Cu4 O3 /ZnO/ITO and NiO/Cu4 O3 /ZnO/ITO structures. Photodetectors

NiO/Cu4 O3 /ZnO/ITO Cu4 O3 /ZnO/ITO

Photoresponse ratio for ␭= 450–455 nm

515–520 nm

620–625 nm

546 1049

353 278

324 302

the calculated rectification ratios for both NiO/Cu4 O3 /ZnO/ITO and Cu4 O3 /ZnO/ITO detectors were 2.45 and 12.10 respectively. The rectification ratio is the measure of ability of a junction in terms of collecting and separating the charge carriers [33]. Therefore, Cu4 O3 /ZnO/ITO photodetector collects and separates more number of electrons and holes when compared to NiO inserted detector. However, swift response is an ultimate factor for an efficient photodetector. The prepared NiO/Cu4 O3 /ZnO/ITO and Cu4 O3 /ZnO/ITO photodetectors were exposed to the wavelengths of 450 nm, 515 nm and 620 nm in order to measure the photoresponses. The targeted wavelengths were produced using blue, green and red color light emitting diodes (LEDs) respectively. The photocurrent values were measured at zero bias by real time ON-OFF of the LEDs as shown in Fig. 4(a)–(c). The importance and influence of NiO layer is evidently realized from the photoresponse measurements. In the NiO/Cu4 O3 /ZnO/ITO detector, electrons and holes are swiftly drifted in opposite directions via ETL and HTL respectively to increase the photocurrent. Though, the optical and electrical properties of Cu4 O3 /ZnO/ITO device were competitive, its photoresponse was inferior due to the absence of HTL. The photoresponse ratio was calculated from the ratio of on and off current values (Ion /Ioff ) for both the prepared devices as tabulated in Table 2. The NiO/Cu4 O3 /ZnO/ITO detector showed the dominated photoresponses at all the chosen wavelengths other than the short wavelength range of 400–450 nm. The photons of having energies corresponding to these wavelengths are completely absorbed by NiO layer due to its large band gap. The response time was measured by estimating rise time ( r ) and fall time ( f ) for the fabricated photodetectors as shown in Fig. 4(e) & (f). The rise time is defined as the time taken by the device to increase its photocurrent value from 10% to 90% of the peak value. The fall time is just opposite (ie., 90% to 10%) [32]. The rise and fall times were calculated as 33 ms and 89 ms for NiO/Cu4 O3 /ZnO/ITO photodetector and 38 ms and 109 ms for Cu4 O3 /ZnO/ITO photodetector respectively. Hence, the absence of HTL in Cu4 O3 /ZnO/ITO device leads to have comparatively large

Fig. 3. (a) I–V characteristics of NiO/Cu4 O3 /ZnO/ITO and Cu4 O3 /ZnO/ITO photodetectors under dark condition. Energy band diagrams of (b) NiO/Cu4 O3 /ZnO/ITO and (c) Cu4 O3 /ZnO/ITO photodetectors.

reverse saturation current which increased the response time. The fall time is usually higher than the rise time due to the presence of deeper traps and other defect states which take more time to release the charge carriers and thus slowed down the speed of fall time [32]. Hence, the proposed NiO/Cu4 O3 /ZnO/ITO photodetector exhibited an appreciative response time, that is, it could respond

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Fig. 4. Photoresponses of prepared detectors at (a) ␭ = 450 nm (b) ␭ = 520 nm (c) ␭ = 620 nm. Response speed in terms of rise and fall times of (d) NiO/Cu4 O3 /ZnO/ITO and (e) Cu4 O3 /ZnO/ITO photodetectors.

to the incident light within 33 ms. To the best of our knowledge, this is the first report on paramelaconite (Cu4 O3 ) based all oxide photodetector which yields better photoresponse at visible wavelengths.

Foundation (NRF) of Korea by the Ministry of Education (NRF2015R1D1A1A01059165) and Korea Research Fellowship Program through the NRF by the Ministry of Science, ICT and Future Planning (NRF-2015H1D3A1066311). H. Kim, M. D. Kumar, and W. Park equally contributed to this work.

4. Conclusions NiO/Cu4 O3 /ZnO/ITO and Cu4 O3 /ZnO/ITO photodetectors were fabricated to demonstrate the role of paramelaconite (Cu4 O3 ) and impact of hole transporting NiO layer in all-oxide photodetectors. p-type Cu4 O3 and n-type ZnO are joined together to form a rectifying junction at the interface and they were sandwiched between a top HTL and a bottom ETL to increase the speed of carrier transport. Visible and UV radiations were absorbed by both the devices, as a result, transmittance of the devices was lower at this wavelength range. In Cu4 O3 /ZnO/ITO photodetector, as there is no proper channel for hole movement, the IRS is increased thus increasing recombination loss. The band alignments of oxide materials showed a staircase like structure which is an additional benefit for a high speed photodetector. The speed of the response to the incident light was quantified from rise and fall times. When exposed to incident light, the NiO/Cu4 O3 /ZnO/ITO photodetector raises the photocurrent within a short time of 33 ms and get relaxed by 89 ms. The fast response paramelaconite (Cu4 O3 ) based all-oxide photodetectors would begin a new chapter in the energy sectors. Acknowledgements The authors acknowledge the financial support of the Korea Institute of Energy Technology Evaluation and Planning by the Ministry of Knowledge Economy (KETEP-20133030011000), Basic Science Research Program through the National Research

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Biographies

Hong-Sik Kim is currently a Senior Researcher in Photoelectric and Energy Device Application Lab (PEDAL), Incheon National University while he is pursuing a PhD program at Sungkyunkwan University. He has worked in industry for 5 years for LEDs and solar cells. His research interests cover nanostructured and flexible semiconductors for photoelectric device applications.

Melvin David Kumar received his M.Sc in Physics (2004) from Pope’s College, India and he continued his research in analyzing quantum well structures to obtain his PhD (2013) from Karunya University, India in the field of quantum confined structures for optoelectronic applications. He is now serving as a professor at department of Physics in the Aditanar College of Arts and Science, Tamilnadu, India. In addition, he taught Physics to postgraduate and undergraduate students in Karunya University, India. He was a post-doctoral researcher in the research lab (PEDAL) of Prof. Joondong Kim at Incheon National University (INU), Korea. His current research interests are to fabricate photovoltaics and photoelectric sensors with ITO nanowires and semiconductor quantum dots.

Wang-Hee park received his B.S. in electrical engineering (2016) from Incheon National University. He is pursuing his M.S. degree at Energy Device Application Lab (PEDAL), Incheon National University. He has research topics on functional semiconductor designs and transparent photoelectric devices, including photodetectors and solar cells.

Malkeshkumar Patel is currently a postdoctoral researcher in Photoelectric and Energy Device Application Lab (PEDAL), Incheon National University. He earned his PhD in Photovoltaic Science and Engineering from School of Solar Energy, Pandit Deendayal Petroleum University (PDPU) for development and studies of Cu2 ZnSnS4 and SnS materials for solar energy application. Prior to doctoral program, he worked as an optics design engineer in Jekson Vision and Sahajanand Laser Technology Ltd. for 6 years. His research interests include nanostructured semiconductor materials for visible light transparent photoelectric devices and their application for solar energy conversions.

Joondong Kim is a professor in the Department of Electrical Engineering at Incheon National University (INU) in Korea and a principal researcher at in Photoelectric and Energy Device Application Lab (PEDAL). He majored in Electrical Engineering and earned his PhD in 2006 from the University at Buffalo, State University of New York, Buffalo, NY, USA; he received his MS in 2001 from Rensselaer Polytechnic Institute, Troy, NY, USA. Before joining INU, he was a professor at Kunsan National University. He also served as a senior researcher at the Korea Institute of Machinery and Materials. His research covers the synthesis of functional materials and the design of highperforming devices, including photoelectric sensors and photovoltaics.