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n-ZnO heterojunction ultraviolet photodetectors prepared on flexible substrates
Surface & Coatings Technology 362 (2019) 57–61
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Transparent p-NiO/n-ZnO heterojunction ultraviolet photodetectors prepared on flexible substrates ⁎
T ⁎
Jian Huanga, , Bing Lia, Yan Hua, Xinyu Zhoua, Zilong Zhanga, Yuncheng Maa, Ke Tanga, , Linjun Wanga, Yicheng Lub a b
School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China Department of Electrical and Computer Engineering, Rutgers University, Piscataway, NJ 08854, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO Heterojunction Detector NiO PET
Transparent B and Ga co-doped ZnO (BGZO)/n-ZnO/p-NiO heterojunctions were prepared on flexible polyethylene terephthalate (PET) substrates by RF magnetron sputtering at room temperature. The heterojunction shows high transmittance above 75% in the visible region, high flexibility and a good rectifying behavior. Compared with the heterojunction without BGZO layer, the performance of the device with BGZO layer is significantly improved. The turn-on voltage and the ideality factor of the heterojunction with BGZO layer is 1.2 and 2.3, respectively. The n-BGZO/n-ZnO/p-NiO heterojunction ultraviolet (UV) detectors show high UV sensitivity and fast time response.
1. Introduction
2. Experimental
ZnO is a typical n-type wide band gap (3.37 eV) semiconductor in room temperature. The combination of unique properties such as direct band gap, high exciton binding energy, high transparency, high mobility, low cost and room-temperature processing makes ZnO a very promising material in application of next generation low-cost electronic and optoelectronic devices (especially invisible and flexible devices) [1–4]. At present, the realization of ZnO-based homojunctions remains a challenging task due to the difficulty in obtaining stable p-type ZnO. As an alternative, significant efforts have been made to develop ZnO-based heterojunctions [5–10]. NiO is a promising p-type semiconductor at room temperature with wide band gap (3.6–4.0 eV), visible light transparency and chemical stability. It is a suitable material to form heterojunctions with n-ZnO [11–14]. With the explosive development of intelligent wearable electronic devices, flexible photoelectric detectors have become a hot research field [15,16]. Thin polyethylene terephthalate (PET) substrate has been applied in many electric device for its attractive properties, such as free of bend, environmentally-friendly, low product cost and high transmittance [17–19]. In this work, a transparent p-NiO/n-ZnO p-n heterojunction ultraviolet photodetector was fabricated on flexible PET substrate. The crystalline structural, morphology, optical and electrical properties of the device were investigated in detail.
The PET substrates were ultrasonically cleaned by acetone, methanol and deionized water respectively before sputtering, and dried in highly pure N2 (99.99%) afterwards. NiO films with thickness of about 350 nm were prepared on PET substrates by RF magnetron sputtering with a cylindrical NiO ceramic target (purity 99.99%). Pure O2 (purity 99.999%) was used as sputtering gas for deposition of NiO films. The RF power was 100 W. Then, intrinsically ZnO films with thickness of about 300 nm were deposited on NiO films by RF magnetron sputtering with a ZnO ceramic target (purity 99.999%). Argon (Ar) (purity 99.999%) was used as sputtering gas. The RF power was 150 W. Finally, ~100 nm thick B and Ga co-doped ZnO (BGZO) films with high conductivity and transparency were prepared on ZnO films by using RF magnetron sputtering with a BGZO ceramic target (97 wt% ZnO, 2.5 wt% Ga2O3 and 0.5 wt% B2O3, purity 99.99%). For deposition of all films, the pressure of sputtering was 6 mTorr and the substrate was not heated intentionally. Prior to the formal sputtering, the sputtering chamber was evacuated to a pressure below 5 × 10−7 Torr and the target was pre-sputtered for 5 min to remove impurities on the target surface. Au contacts (~100 nm of thickness) were deposited on BGZO and NiO films by electron-beam evaporation to fabricate n-BGZO/n-ZnO/pNiO structure heterojunction UV photodetectors. The schematic of the device was shown in Fig. 1.
⁎
Corresponding authors. E-mail addresses: [email protected] (J. Huang), [email protected] (K. Tang).
https://doi.org/10.1016/j.surfcoat.2019.01.099 Received 11 August 2018; Received in revised form 22 January 2019; Accepted 27 January 2019 Available online 28 January 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 362 (2019) 57–61
J. Huang et al.
Fig. 1. Schematic of BGZO/ZnO/NiO structure heterojunction photodetectors.
Fig. 3. The AFM images of (a) NiO films and (b) BGZO/ZnO/NiO films grown on PET substrates.
spectral response of the detector were analyzed by semiconductor characterization system (4200A-SCS, Keithley) and a Xenon Lamp optical system with monochromator. All the measurements were performed at room temperature. 3. Results and discussion The XRD pattern of BGZO/ZnO/NiO heterojunction deposited on PET substrate is shown in Fig. 2(a). From the figure, it can be observed that there is a strong diffraction peak at about 34.5° corresponding to the (002) peak of hexagonal wurtzite structure ZnO [20]. The full width at half-maximum (FWHM) of the ZnO (002) diffraction peak is 0.32°. A very weaker diffraction peak near 36.9° is the (111) diffraction peak of NiO. It is obvious that the NiO film is not very crystalline. There are also two weak and broad peaks located at about 46.9° and 53.7°, which is introduced by the substrate of PET. Fig. 2(b) shows the cross-section SEM image of BGZO/ZnO/NiO structure. The thickness of BGZO, ZnO and NiO layers is about 100 nm, 300 nm and 350 nm, respectively. All the films in BGZO/ZnO/NiO structure are uniform and dense. The atomic force microscopy (AFM) surface morphology of NiO films and BGZO/ZnO/NiO films is shown in Fig. 3(a) and (b), respectively. According to the AFM results, the films are compact, uniform and very smooth with root mean square (RMS) roughness of about 11.24 nm and 5.63 nm for the NiO film and the BGZO/ZnO/NiO composite film respectively. The Hall effect measurement at room temperature shows that the prepared NiO film is a p-type semiconductor with resistivity of about 2.9 Ω·cm and Hall mobility of about 2.7 cm2/V·s. ZnO films and BGZO films are n-type conductive. The resistivity of the ZnO film and BGZO
Fig. 2. (a) The XRD pattern and (b) the cross-section SEM image of BGZO/ZnO/ NiO heterojunction deposited on PET substrate.
The crystalline structure of films was measured by the X-ray diffraction instrument (XRD 3KW D/MAX-2200V). The optical properties of the samples were characterized by the UV–Vis spectrophotometer (Shimadzu UV-2501). The morphology of films surface and cross-section images were observed by atomic force microscopy (AFM) and scanning electron microscope (SEM JEOL JSM-7500F). The electrical properties of the films were measured by Hall effect measurements (Accent optical HL5500PC). The I–V characteristics, time response and 58
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Fig. 4. (a) The transmittance and (b) absorption spectra for the ZnO film, NiO film and the BGZO/ZnO/NiO heterojunction, (c) the plot of (αhv)2 versus hv for the ZnO film and NiO film and (d) the photograph of the heterojunction sample.
film is 1.3 Ω·cm and 9.2 × 10−4 Ω·cm, respectively. Fig. 4(a) and (b) shows the transmittance and absorption spectra for the ZnO film, NiO film and the BGZO/ZnO/NiO heterojunction. The transmittance of the NiO film in the visible light range is about 80%, and the transmittance of the ZnO film is over 90%. The transmittance of the BGZO/ZnO/NiO heterojunction device is also over 70%, which indicates that the prepared device has better visible light transparency. The absorption edge of the NiO film is located at about 320 nm and at about 380 nm for the ZnO film. From the Fig. 4, the optical band gap (Eg) of NiO and ZnO films can also be estimated by using Tauc model [21]:
(αhν )2 = A(hν − Eg )
been introduced in Ref. [5]). The ρc is 3.7 × 10−5 Ω·cm2 and 5.3 × 10−3 Ω·cm2 for Au/BGZO and Au/NiO contacts, respectively. The results indicate that a very good ohmic contact is formed between the Au electrodes and the films with very low contact resistivity. Fig. 5(b) shows the I–V curve of BGZO/ZnO/NiO heterojunctions in the dark. For comparison, a ZnO/NiO heterojunction without BGZO layers was also characterized. The illustration in Fig. 5(b) is the semilog I–V curves of the heterojunctions. From the figure, both heterojunctions show good rectification characteristics. The forward current (IF) and reverse current (IR) ratio at bias voltage of 4 V is about 1.3 × 104 and 3.6 × 103 for BGZO/ZnO/NiO and ZnO/NiO heterojunctions, respectively. The turn-on voltage is about 1.2 V and 1.3 V for BGZO/ZnO/NiO and ZnO/NiO heterojunctions, respectively. The BGZO/ZnO/NiO heterojunction has a lower turn-on voltage, a higher IF/IR and a larger IF value by the introduction of a BGZO conductive layer. The possible reason is that the higher carrier collection capability and smaller contact resistance of the BGZO/ZnO/NiO heterojunction (the ρc of Au/ZnO is about 6.1 × 10−2 Ω·cm2, much larger than that of Au/BGZO) [22]. From the Fig. 5(b), the ideality factor (n) of the heterojunction can also be estimated by the equation [5]:
(1)
where hν is the photon energy, α is the absorption coefficient and A is a constant. The value of the optical band gap is acquire by extend the linear part to hν-axis as shown in Fig. 4(c). The Eg of ZnO films and NiO films is about 3.26 eV and 3.88 eV at room temperature, respectively. Fig. 4(d) shows the photograph of the BGZO/ZnO/NiO heterojunction sample. The size of the sample is 2 cm × 2 cm. The heterojunction is prepared on a flexible PET substrate and can be bent greatly. Fig. 5(a) shows the current-voltage (I–V) plots of Au electrodes on the NiO film and Au electrodes on the BGZO film. It can be seen from the linear I–V curves that the Au electrodes form a good ohmic contact with both NiO and BGZO film. In order to further characterize the contact characteristics between the Au electrode and the film, the contact resistivity (ρc) of Au/NiO and Au/BGZO was calculated by using transmission line method (TLM) (the specific principles of TLM have
n=
q ∂V kT ∂ (ln I )
(2)
where T, k, and q is the absolute temperature, Boltzmann's constant and the electronic charge, respectively. The value of n is about 2.3 and 2.7 for BGZO/ZnO/NiO and ZnO/NiO heterojunctions, respectively. The high ideality factor of both heterojunctions in this work may be due to the presence of an interfacial barrier between the highly crystalline ZnO 59
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Fig. 5. (a) The I–V plots of Au electrodes on the NiO film and BGZO film, and (b) the I–V curves of BGZO/ZnO/NiO and ZnO/NiO heterojunctions in the dark. The illustration is the semilog I–V curves of the heterojunctions.
Fig. 6. (a) Time response of the BGZO/ZnO/NiO heterojunction UV detector with different reverse bias voltage and (b) the fall time and rise time of the detector at −2 V bias voltage.
and partially amorphous NiO layers [23]. The time response of the BGZO/ZnO/NiO heterojunction detectors with different reverse bias voltage with and without UV illumination (365 nm) at room temperature is shown in Fig. 6(a). Whether the bias voltage is −1 V or −2 V, the detector responds significantly to the UV light and the UV sensitivity (photo-current/dark-current) is above 20. As the reverse bias increases, the detector shows higher UV photocurrent due to the wider depletion layer and enhanced carrier collection of the device [24]. The rise time and fall time are defined as the time that varies between 10% and 90% of the maximum photo-current. As indicated in Fig. 6(b), the rise time and fall time is about 160 ms and 370 ms, respectively. The fall time is much longer than the rise time which may be due to the release of carriers trapped by oxygen defectsrelated traps in the oxide film is slow after the UV light is turned off [25].
rectifying behavior including higher IF/IR value about 1.3 × 104 and lower turn-on voltage of 1.2 V. The rise time and the fall time of the BGZO/ZnO/NiO heterojunction UV detectors is about 160 ms and 370 ms, respectively. Acknowledgments This work was funded by Science and Technology Commission of Shanghai Municipality (No. 16010500500). References [1] A. Chrissanthopoulos, S. Baskoutas, N. Bouropoulos, V. Dracopoulos, P. Poulopoulos, S.N. Yannopoulos, Synthesis and characterization of ZnO/NiO p–n heterojunctions: ZnO nanorods grown on NiO thin film by thermal evaporation, Photonics Nanostruct. Fundam. Appl. 9 (2011) 132–139. [2] K. Wang, Y. Vygranenko, A. Nathan, Fabrication and characterization of NiO/ZnO/ ITO p–i–n heterostructure, Thin Solid Films 516 (2008) 1640–1643. [3] R.K. Gupta, K. Ghosh, P.K. Kahol, Fabrication and characterization of NiO/ZnO p–n junctions by pulsed laser deposition, Physica 41 (2009) 617–620. [4] M. Sultan, S. Mumtaz, A. Ali, M.Y. Khan, T. Iqbal, Band alignment and optical response of facile grown NiO/ZnO nano-heterojunctions, Superlattice. Microst. 112 (2017) 210–217. [5] Y.X. Lu, J. Huang, B. Li, K. Tang, Y.C. Ma, M. Cao, L. Wang, L.J. Wang, A boron and gallium co-doped ZnO intermediate layer for ZnO/Si heterojunction diodes, Appl. Surf. Sci. 428 (2018) 61–65. [6] J. Huang, L.J. Wang, K. Tang, J.J. Zhang, Y.B. Xia, X.G. Lu, The fabrication and photoresponse of ZnO/diamond film heterojunction diode, Appl. Surf. Sci. 258 (2012) 2010–2013. [7] H. Zheng, Z.X. Mei, Z.Q. Zeng, Y.Z. Liu, L.W. Guo, J.F. Jia, Q.K. Xue, Z. Zhang,
4. Conclusions In this work, transparent n-BGZO/n-ZnO/p-NiO heterojunctions were fabricated on PET substrates. The band gap of ZnO and NiO films is about 3.26 eV and 3.88 eV, respectively. The transmittance of the BGZO/ZnO/NiO heterojunction is over 70% in the visible range. The device can be bent greatly. Very small contact resistivity value of 3.7 × 10−5 Ω·cm2 and 5.3 × 10−3 Ω·cm2 was obtained for Au/BGZO and Au/NiO contacts, respectively. By introducing BGZO layers with high conductivity, the BGZO/ZnO/NiO heterojunction shows better 60
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[8]
[9]
[10]
[11]
[12]
[13]
[14] [15]
[16]
X.L. Du, Fabrication and characterization of high quality n-ZnO/p-GaN heterojunction light emission diodes, Thin Solid Films 520 (2011) 445–447. W. Tian, T. Zhai, C. Zhang, S. Li, X. Wang, K. Tsukagoshi, D. Golberg, Y. Bando, Low-cost fully transparent ultraviolet photodetectors based on electrospun ZnOSnO2 heterojunction nanofibers, Adv. Mater. 25 (2013) 4625–4630. D.Y. Guo, H.Z. Shi, Y.P. Qian, M. Lv, P.G. Li, Y.L. Su, Q. Liu, K. Chen, S.L. Wang, C. Cui, C.R. Li, W.H. Tang, Fabrication of β-Ga2O3/ZnO heterojunction for solarblind deep ultraviolet photodetection, Semicond. Sci. Technol. 32 (2017) 03LT01. A. Kathalingam, D. Vikraman, H.S. Kim, J.P. Hui, Facile fabrication of n-ZnO nanorods/p-Cu2O heterojunction and its photodiode property, Opt. Mater. 66 (2017) 122–130. Y. Zhao, H. Wang, C. Wu, W.C. Li, F.B. Gao, G.G. Wu, B.L. Zhang, G.T. Du, Study on the electro luminescence properties of diodes based on n-ZnO/p-NiO/p-Si heterojunction, Opt. Commun. 336 (2015) 1–4. Anas A. Ahmed, M. Devarajan, N. Afzal, Fabrication and characterization of high performance MSM UV photodetector based on NiO film, Sensors Actuators A 262 (2017) 78–86. T.F. Chen, A.J. Wang, B.Y. Shang, Z.L. Wu, Y.L. Li, Y.S. Wang, Property modulation of NiO films grown by radio frequency magnetron sputtering, J. Alloys Compd. 643 (2015) 167–173. T. Zhang, L. Li, J.P. Ao, Temperature-dependent electrical transport characteristics of a NiO/GaN heterojunction diode, Surf. Interfaces 5 (2016) 15–18. H.S. Gill, B. Elshahat, A. Kokil, L. Li, R. Mosurkal, P. Zygmanski, E. Sajo, J. Kumar, Flexible perovskite based X-ray detectors for dose monitoring in medical imaging applications, Phys. Med. 5 (2018) 20–23. M. Patel, S.H. Parka, J. Kim, Optical, electrical and photoresponse data of flexible
[17] [18]
[19]
[20] [21] [22]
[23] [24]
[25]
61
and high-performing NiO/ZnO ultraviolet photodetector, Data Brief 17 (2018) 520–525. S.U. Park, J.H. Koh, Low temperature rf-sputtered in and Al co-doped ZnO thin films deposited on flexible PET substrate, Ceram. Int. 40 (2014) 10021–10025. B.G. Kim, J.Y. Kim, S.J. Lee, J.H. Park, D.G. Lim, M.G. Park, Structural, electrical and optical properties of Ga-doped ZnO films on PET substrate, Appl. Surf. Sci. 257 (2010) 1063–1067. T.T. Guo, G.B. Dong, F.Y. Gao, Y. Xiao, Q. Chen, X.G. Diao, High performance ZnO:Al films deposited on PET substrates using facing target sputtering, Appl. Surf. Sci. 282 (2013) 467–471. S. Takada, Relation between optical property and crystallinity of ZnO thin films prepared by RF magnetron sputtering, J. Appl. Phys. 73 (1993) 4739–4742. K.C. Park, D.Y. Ma, K.H. Kim, The physical properties of Al-doped zinc oxide films prepared by RF magnetron sputtering, Thin Solid Films 305 (1997) 201–209. H. Zhou, J. Mei, P.B. Gui, P. Tao, Z.H. Song, H. Wang, G.J. Fang, The investigation of Al-doped ZnO as an electron transporting layer for visible-blind ultraviolet photodetector based on n-ZnO nanorods/p-Si heterojunction, Mater. Sci. Semicond. Process. 38 (2015) 67–71. R.K. Gupta, K. Ghosh, P.K. Kahol, Fabrication and characterization of NiO/ZnO p-n junctions by pulsed laser deposition, Phys. E. 41 (2009) 617–620. L.J. Mandalapu, Z. Yang, F.X. Xiu, D.T. Zhao, J.L. Liu, Homojunction photodiodes based on Sb-doped p-type ZnO for ultraviolet detection, Appl. Phys. Lett. 88 (2006) 092103. L.Q. Qin, D.L. Shao, C. Shing, S. Sawye, Wavelength selective p-GaN/ZnO colloidal nanoparticle heterojunction photodiode, Appl. Phys. Lett. 102 (2013) 071106.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Transparent p-NiO/n-ZnO heterojunction ultraviolet photodetectors prepared on flexible substrates ⁎
T ⁎
Jian Huanga, , Bing Lia, Yan Hua, Xinyu Zhoua, Zilong Zhanga, Yuncheng Maa, Ke Tanga, , Linjun Wanga, Yicheng Lub a b
School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China Department of Electrical and Computer Engineering, Rutgers University, Piscataway, NJ 08854, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO Heterojunction Detector NiO PET
Transparent B and Ga co-doped ZnO (BGZO)/n-ZnO/p-NiO heterojunctions were prepared on flexible polyethylene terephthalate (PET) substrates by RF magnetron sputtering at room temperature. The heterojunction shows high transmittance above 75% in the visible region, high flexibility and a good rectifying behavior. Compared with the heterojunction without BGZO layer, the performance of the device with BGZO layer is significantly improved. The turn-on voltage and the ideality factor of the heterojunction with BGZO layer is 1.2 and 2.3, respectively. The n-BGZO/n-ZnO/p-NiO heterojunction ultraviolet (UV) detectors show high UV sensitivity and fast time response.
1. Introduction
2. Experimental
ZnO is a typical n-type wide band gap (3.37 eV) semiconductor in room temperature. The combination of unique properties such as direct band gap, high exciton binding energy, high transparency, high mobility, low cost and room-temperature processing makes ZnO a very promising material in application of next generation low-cost electronic and optoelectronic devices (especially invisible and flexible devices) [1–4]. At present, the realization of ZnO-based homojunctions remains a challenging task due to the difficulty in obtaining stable p-type ZnO. As an alternative, significant efforts have been made to develop ZnO-based heterojunctions [5–10]. NiO is a promising p-type semiconductor at room temperature with wide band gap (3.6–4.0 eV), visible light transparency and chemical stability. It is a suitable material to form heterojunctions with n-ZnO [11–14]. With the explosive development of intelligent wearable electronic devices, flexible photoelectric detectors have become a hot research field [15,16]. Thin polyethylene terephthalate (PET) substrate has been applied in many electric device for its attractive properties, such as free of bend, environmentally-friendly, low product cost and high transmittance [17–19]. In this work, a transparent p-NiO/n-ZnO p-n heterojunction ultraviolet photodetector was fabricated on flexible PET substrate. The crystalline structural, morphology, optical and electrical properties of the device were investigated in detail.
The PET substrates were ultrasonically cleaned by acetone, methanol and deionized water respectively before sputtering, and dried in highly pure N2 (99.99%) afterwards. NiO films with thickness of about 350 nm were prepared on PET substrates by RF magnetron sputtering with a cylindrical NiO ceramic target (purity 99.99%). Pure O2 (purity 99.999%) was used as sputtering gas for deposition of NiO films. The RF power was 100 W. Then, intrinsically ZnO films with thickness of about 300 nm were deposited on NiO films by RF magnetron sputtering with a ZnO ceramic target (purity 99.999%). Argon (Ar) (purity 99.999%) was used as sputtering gas. The RF power was 150 W. Finally, ~100 nm thick B and Ga co-doped ZnO (BGZO) films with high conductivity and transparency were prepared on ZnO films by using RF magnetron sputtering with a BGZO ceramic target (97 wt% ZnO, 2.5 wt% Ga2O3 and 0.5 wt% B2O3, purity 99.99%). For deposition of all films, the pressure of sputtering was 6 mTorr and the substrate was not heated intentionally. Prior to the formal sputtering, the sputtering chamber was evacuated to a pressure below 5 × 10−7 Torr and the target was pre-sputtered for 5 min to remove impurities on the target surface. Au contacts (~100 nm of thickness) were deposited on BGZO and NiO films by electron-beam evaporation to fabricate n-BGZO/n-ZnO/pNiO structure heterojunction UV photodetectors. The schematic of the device was shown in Fig. 1.
⁎
Corresponding authors. E-mail addresses: [email protected] (J. Huang), [email protected] (K. Tang).
https://doi.org/10.1016/j.surfcoat.2019.01.099 Received 11 August 2018; Received in revised form 22 January 2019; Accepted 27 January 2019 Available online 28 January 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 362 (2019) 57–61
J. Huang et al.
Fig. 1. Schematic of BGZO/ZnO/NiO structure heterojunction photodetectors.
Fig. 3. The AFM images of (a) NiO films and (b) BGZO/ZnO/NiO films grown on PET substrates.
spectral response of the detector were analyzed by semiconductor characterization system (4200A-SCS, Keithley) and a Xenon Lamp optical system with monochromator. All the measurements were performed at room temperature. 3. Results and discussion The XRD pattern of BGZO/ZnO/NiO heterojunction deposited on PET substrate is shown in Fig. 2(a). From the figure, it can be observed that there is a strong diffraction peak at about 34.5° corresponding to the (002) peak of hexagonal wurtzite structure ZnO [20]. The full width at half-maximum (FWHM) of the ZnO (002) diffraction peak is 0.32°. A very weaker diffraction peak near 36.9° is the (111) diffraction peak of NiO. It is obvious that the NiO film is not very crystalline. There are also two weak and broad peaks located at about 46.9° and 53.7°, which is introduced by the substrate of PET. Fig. 2(b) shows the cross-section SEM image of BGZO/ZnO/NiO structure. The thickness of BGZO, ZnO and NiO layers is about 100 nm, 300 nm and 350 nm, respectively. All the films in BGZO/ZnO/NiO structure are uniform and dense. The atomic force microscopy (AFM) surface morphology of NiO films and BGZO/ZnO/NiO films is shown in Fig. 3(a) and (b), respectively. According to the AFM results, the films are compact, uniform and very smooth with root mean square (RMS) roughness of about 11.24 nm and 5.63 nm for the NiO film and the BGZO/ZnO/NiO composite film respectively. The Hall effect measurement at room temperature shows that the prepared NiO film is a p-type semiconductor with resistivity of about 2.9 Ω·cm and Hall mobility of about 2.7 cm2/V·s. ZnO films and BGZO films are n-type conductive. The resistivity of the ZnO film and BGZO
Fig. 2. (a) The XRD pattern and (b) the cross-section SEM image of BGZO/ZnO/ NiO heterojunction deposited on PET substrate.
The crystalline structure of films was measured by the X-ray diffraction instrument (XRD 3KW D/MAX-2200V). The optical properties of the samples were characterized by the UV–Vis spectrophotometer (Shimadzu UV-2501). The morphology of films surface and cross-section images were observed by atomic force microscopy (AFM) and scanning electron microscope (SEM JEOL JSM-7500F). The electrical properties of the films were measured by Hall effect measurements (Accent optical HL5500PC). The I–V characteristics, time response and 58
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Fig. 4. (a) The transmittance and (b) absorption spectra for the ZnO film, NiO film and the BGZO/ZnO/NiO heterojunction, (c) the plot of (αhv)2 versus hv for the ZnO film and NiO film and (d) the photograph of the heterojunction sample.
film is 1.3 Ω·cm and 9.2 × 10−4 Ω·cm, respectively. Fig. 4(a) and (b) shows the transmittance and absorption spectra for the ZnO film, NiO film and the BGZO/ZnO/NiO heterojunction. The transmittance of the NiO film in the visible light range is about 80%, and the transmittance of the ZnO film is over 90%. The transmittance of the BGZO/ZnO/NiO heterojunction device is also over 70%, which indicates that the prepared device has better visible light transparency. The absorption edge of the NiO film is located at about 320 nm and at about 380 nm for the ZnO film. From the Fig. 4, the optical band gap (Eg) of NiO and ZnO films can also be estimated by using Tauc model [21]:
(αhν )2 = A(hν − Eg )
been introduced in Ref. [5]). The ρc is 3.7 × 10−5 Ω·cm2 and 5.3 × 10−3 Ω·cm2 for Au/BGZO and Au/NiO contacts, respectively. The results indicate that a very good ohmic contact is formed between the Au electrodes and the films with very low contact resistivity. Fig. 5(b) shows the I–V curve of BGZO/ZnO/NiO heterojunctions in the dark. For comparison, a ZnO/NiO heterojunction without BGZO layers was also characterized. The illustration in Fig. 5(b) is the semilog I–V curves of the heterojunctions. From the figure, both heterojunctions show good rectification characteristics. The forward current (IF) and reverse current (IR) ratio at bias voltage of 4 V is about 1.3 × 104 and 3.6 × 103 for BGZO/ZnO/NiO and ZnO/NiO heterojunctions, respectively. The turn-on voltage is about 1.2 V and 1.3 V for BGZO/ZnO/NiO and ZnO/NiO heterojunctions, respectively. The BGZO/ZnO/NiO heterojunction has a lower turn-on voltage, a higher IF/IR and a larger IF value by the introduction of a BGZO conductive layer. The possible reason is that the higher carrier collection capability and smaller contact resistance of the BGZO/ZnO/NiO heterojunction (the ρc of Au/ZnO is about 6.1 × 10−2 Ω·cm2, much larger than that of Au/BGZO) [22]. From the Fig. 5(b), the ideality factor (n) of the heterojunction can also be estimated by the equation [5]:
(1)
where hν is the photon energy, α is the absorption coefficient and A is a constant. The value of the optical band gap is acquire by extend the linear part to hν-axis as shown in Fig. 4(c). The Eg of ZnO films and NiO films is about 3.26 eV and 3.88 eV at room temperature, respectively. Fig. 4(d) shows the photograph of the BGZO/ZnO/NiO heterojunction sample. The size of the sample is 2 cm × 2 cm. The heterojunction is prepared on a flexible PET substrate and can be bent greatly. Fig. 5(a) shows the current-voltage (I–V) plots of Au electrodes on the NiO film and Au electrodes on the BGZO film. It can be seen from the linear I–V curves that the Au electrodes form a good ohmic contact with both NiO and BGZO film. In order to further characterize the contact characteristics between the Au electrode and the film, the contact resistivity (ρc) of Au/NiO and Au/BGZO was calculated by using transmission line method (TLM) (the specific principles of TLM have
n=
q ∂V kT ∂ (ln I )
(2)
where T, k, and q is the absolute temperature, Boltzmann's constant and the electronic charge, respectively. The value of n is about 2.3 and 2.7 for BGZO/ZnO/NiO and ZnO/NiO heterojunctions, respectively. The high ideality factor of both heterojunctions in this work may be due to the presence of an interfacial barrier between the highly crystalline ZnO 59
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Fig. 5. (a) The I–V plots of Au electrodes on the NiO film and BGZO film, and (b) the I–V curves of BGZO/ZnO/NiO and ZnO/NiO heterojunctions in the dark. The illustration is the semilog I–V curves of the heterojunctions.
Fig. 6. (a) Time response of the BGZO/ZnO/NiO heterojunction UV detector with different reverse bias voltage and (b) the fall time and rise time of the detector at −2 V bias voltage.
and partially amorphous NiO layers [23]. The time response of the BGZO/ZnO/NiO heterojunction detectors with different reverse bias voltage with and without UV illumination (365 nm) at room temperature is shown in Fig. 6(a). Whether the bias voltage is −1 V or −2 V, the detector responds significantly to the UV light and the UV sensitivity (photo-current/dark-current) is above 20. As the reverse bias increases, the detector shows higher UV photocurrent due to the wider depletion layer and enhanced carrier collection of the device [24]. The rise time and fall time are defined as the time that varies between 10% and 90% of the maximum photo-current. As indicated in Fig. 6(b), the rise time and fall time is about 160 ms and 370 ms, respectively. The fall time is much longer than the rise time which may be due to the release of carriers trapped by oxygen defectsrelated traps in the oxide film is slow after the UV light is turned off [25].
rectifying behavior including higher IF/IR value about 1.3 × 104 and lower turn-on voltage of 1.2 V. The rise time and the fall time of the BGZO/ZnO/NiO heterojunction UV detectors is about 160 ms and 370 ms, respectively. Acknowledgments This work was funded by Science and Technology Commission of Shanghai Municipality (No. 16010500500). References [1] A. Chrissanthopoulos, S. Baskoutas, N. Bouropoulos, V. Dracopoulos, P. Poulopoulos, S.N. Yannopoulos, Synthesis and characterization of ZnO/NiO p–n heterojunctions: ZnO nanorods grown on NiO thin film by thermal evaporation, Photonics Nanostruct. Fundam. Appl. 9 (2011) 132–139. [2] K. Wang, Y. Vygranenko, A. Nathan, Fabrication and characterization of NiO/ZnO/ ITO p–i–n heterostructure, Thin Solid Films 516 (2008) 1640–1643. [3] R.K. Gupta, K. Ghosh, P.K. Kahol, Fabrication and characterization of NiO/ZnO p–n junctions by pulsed laser deposition, Physica 41 (2009) 617–620. [4] M. Sultan, S. Mumtaz, A. Ali, M.Y. Khan, T. Iqbal, Band alignment and optical response of facile grown NiO/ZnO nano-heterojunctions, Superlattice. Microst. 112 (2017) 210–217. [5] Y.X. Lu, J. Huang, B. Li, K. Tang, Y.C. Ma, M. Cao, L. Wang, L.J. Wang, A boron and gallium co-doped ZnO intermediate layer for ZnO/Si heterojunction diodes, Appl. Surf. Sci. 428 (2018) 61–65. [6] J. Huang, L.J. Wang, K. Tang, J.J. Zhang, Y.B. Xia, X.G. Lu, The fabrication and photoresponse of ZnO/diamond film heterojunction diode, Appl. Surf. Sci. 258 (2012) 2010–2013. [7] H. Zheng, Z.X. Mei, Z.Q. Zeng, Y.Z. Liu, L.W. Guo, J.F. Jia, Q.K. Xue, Z. Zhang,
4. Conclusions In this work, transparent n-BGZO/n-ZnO/p-NiO heterojunctions were fabricated on PET substrates. The band gap of ZnO and NiO films is about 3.26 eV and 3.88 eV, respectively. The transmittance of the BGZO/ZnO/NiO heterojunction is over 70% in the visible range. The device can be bent greatly. Very small contact resistivity value of 3.7 × 10−5 Ω·cm2 and 5.3 × 10−3 Ω·cm2 was obtained for Au/BGZO and Au/NiO contacts, respectively. By introducing BGZO layers with high conductivity, the BGZO/ZnO/NiO heterojunction shows better 60
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[8]
[9]
[10]
[11]
[12]
[13]
[14] [15]
[16]
X.L. Du, Fabrication and characterization of high quality n-ZnO/p-GaN heterojunction light emission diodes, Thin Solid Films 520 (2011) 445–447. W. Tian, T. Zhai, C. Zhang, S. Li, X. Wang, K. Tsukagoshi, D. Golberg, Y. Bando, Low-cost fully transparent ultraviolet photodetectors based on electrospun ZnOSnO2 heterojunction nanofibers, Adv. Mater. 25 (2013) 4625–4630. D.Y. Guo, H.Z. Shi, Y.P. Qian, M. Lv, P.G. Li, Y.L. Su, Q. Liu, K. Chen, S.L. Wang, C. Cui, C.R. Li, W.H. Tang, Fabrication of β-Ga2O3/ZnO heterojunction for solarblind deep ultraviolet photodetection, Semicond. Sci. Technol. 32 (2017) 03LT01. A. Kathalingam, D. Vikraman, H.S. Kim, J.P. Hui, Facile fabrication of n-ZnO nanorods/p-Cu2O heterojunction and its photodiode property, Opt. Mater. 66 (2017) 122–130. Y. Zhao, H. Wang, C. Wu, W.C. Li, F.B. Gao, G.G. Wu, B.L. Zhang, G.T. Du, Study on the electro luminescence properties of diodes based on n-ZnO/p-NiO/p-Si heterojunction, Opt. Commun. 336 (2015) 1–4. Anas A. Ahmed, M. Devarajan, N. Afzal, Fabrication and characterization of high performance MSM UV photodetector based on NiO film, Sensors Actuators A 262 (2017) 78–86. T.F. Chen, A.J. Wang, B.Y. Shang, Z.L. Wu, Y.L. Li, Y.S. Wang, Property modulation of NiO films grown by radio frequency magnetron sputtering, J. Alloys Compd. 643 (2015) 167–173. T. Zhang, L. Li, J.P. Ao, Temperature-dependent electrical transport characteristics of a NiO/GaN heterojunction diode, Surf. Interfaces 5 (2016) 15–18. H.S. Gill, B. Elshahat, A. Kokil, L. Li, R. Mosurkal, P. Zygmanski, E. Sajo, J. Kumar, Flexible perovskite based X-ray detectors for dose monitoring in medical imaging applications, Phys. Med. 5 (2018) 20–23. M. Patel, S.H. Parka, J. Kim, Optical, electrical and photoresponse data of flexible
[17] [18]
[19]
[20] [21] [22]
[23] [24]
[25]
61
and high-performing NiO/ZnO ultraviolet photodetector, Data Brief 17 (2018) 520–525. S.U. Park, J.H. Koh, Low temperature rf-sputtered in and Al co-doped ZnO thin films deposited on flexible PET substrate, Ceram. Int. 40 (2014) 10021–10025. B.G. Kim, J.Y. Kim, S.J. Lee, J.H. Park, D.G. Lim, M.G. Park, Structural, electrical and optical properties of Ga-doped ZnO films on PET substrate, Appl. Surf. Sci. 257 (2010) 1063–1067. T.T. Guo, G.B. Dong, F.Y. Gao, Y. Xiao, Q. Chen, X.G. Diao, High performance ZnO:Al films deposited on PET substrates using facing target sputtering, Appl. Surf. Sci. 282 (2013) 467–471. S. Takada, Relation between optical property and crystallinity of ZnO thin films prepared by RF magnetron sputtering, J. Appl. Phys. 73 (1993) 4739–4742. K.C. Park, D.Y. Ma, K.H. Kim, The physical properties of Al-doped zinc oxide films prepared by RF magnetron sputtering, Thin Solid Films 305 (1997) 201–209. H. Zhou, J. Mei, P.B. Gui, P. Tao, Z.H. Song, H. Wang, G.J. Fang, The investigation of Al-doped ZnO as an electron transporting layer for visible-blind ultraviolet photodetector based on n-ZnO nanorods/p-Si heterojunction, Mater. Sci. Semicond. Process. 38 (2015) 67–71. R.K. Gupta, K. Ghosh, P.K. Kahol, Fabrication and characterization of NiO/ZnO p-n junctions by pulsed laser deposition, Phys. E. 41 (2009) 617–620. L.J. Mandalapu, Z. Yang, F.X. Xiu, D.T. Zhao, J.L. Liu, Homojunction photodiodes based on Sb-doped p-type ZnO for ultraviolet detection, Appl. Phys. Lett. 88 (2006) 092103. L.Q. Qin, D.L. Shao, C. Shing, S. Sawye, Wavelength selective p-GaN/ZnO colloidal nanoparticle heterojunction photodiode, Appl. Phys. Lett. 102 (2013) 071106.