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n-ZnO junctions with tunable electrical properties
Accepted Manuscript Room temperature fabrication of transparent p-NiO/n-ZnO junctions with tunable electrical properties Y.B. Wang, X.H. Wei, L. Chang, D.G. Xu, B. Dai, J.F. Pierson, Y. Wang PII:
S0042-207X(17)31681-0
DOI:
10.1016/j.vacuum.2018.01.015
Reference:
VAC 7764
To appear in:
Vacuum
Received Date: 24 November 2017 Revised Date:
8 January 2018
Accepted Date: 8 January 2018
Please cite this article as: Wang YB, Wei XH, Chang L, Xu DG, Dai B, Pierson JF, Wang Y, Room temperature fabrication of transparent p-NiO/n-ZnO junctions with tunable electrical properties, Vacuum (2018), doi: 10.1016/j.vacuum.2018.01.015. 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.
ACCEPTED MANUSCRIPT
Room temperature fabrication of transparent p-NiO/n-ZnO junctions with tunable electrical properties Y.B. Wang1, X.H. Wei1, L. Chang2, D.G. Xu3, B. Dai1, J.F. Pierson4, Y. Wang1,* State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials,
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Southwest University of Science and Technology, Mianyang 621010, China. China National Machinery Industry Corporation, Beijing 100080, China.
3
Institute of Materials, China Academy of Engineering Physics, Jiangyou 621907, China.
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Institut Jean Lamour, UMR 7198-CNRS, Université de Lorraine, Nancy F-54011, France.
*
Corresponding author: Y. Wang
Tel: +86-816-2419492
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Email: [email protected]
Fax: +86-816-2419492
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Abstract
Transparent p-n heterojunctions composed of p-NiO and n-ZnO thin films have
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been fabricated on indium-tin-oxide (ITO)-coated glass substrates at room temperature by magnetron sputtering. Various oxygen flow rates have been employed
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to the NiO thin films, yielding the tunable resistivity of NiO layers. These p-n junctions exhibit clear rectifying current-voltage characteristics. Moreover, their electrical properties can be effectively tuned by the oxygen flow rate to synthesize NiO layers in these junctions. NiO layer with closely perfect stoichiometry and quite high resistivity produces better performance in these p-n junctions, including the small threshold voltage and ideality factor, as well as high rectifying ratio. The 1
ACCEPTED MANUSCRIPT evolution tendency of threshold voltage as a function of the resistivity of NiO is qualitatively discussed.
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Keywords: p-n junction; Transparent oxide thin films; Sputtering; Tunable properties
1. Introduction
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Transparent electronic devices, like p-n junction and field effect transistor using
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transparent oxide semiconductors (TOSs), have attracted considerable attention due to their great potential application in display technology, ultraviolet optoelectronics and energy conversion [1–5]. The intrinsic properties of TOSs, including optical transparency in the visible range, high defect toleration and easy fabrication, are
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beneficial to such transparent devices with peculiar advantages over traditional semiconductor devices.
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Since the p-n junction is the key unit of electronics, it is of great interest to study the optical and electrical properties of transparent p-n junction based on TOSs. So far,
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ZnO is one of the most widely utilized n-type TOSs to assemble the junction, due to its elemental abundance, large direct band gap and versatile fabrication routes [2,4,6– 9]. On the other hand, several p-type TOSs, such as Cu+-based oxides (like Cu2O and CuCrO2) [6,9], ZnRh2O4 [8] and NiO[2–4], have been chosen to form the junction. Among these p-type TOSs, NiO is a promising one with optical band gap of 3.4 eV [11], simply rock-salt cubic structure and tunablely electrical properties [12]. Besides, 2
ACCEPTED MANUSCRIPT only one stable phase in binary Ni-O system allows the easy fabrication of single phase NiO thin films [11], which is superior to that in Cu+-based oxides [13,14]. Taking the binary Cu-O system as an example, the existence of three different phases
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(Cu2O, Cu4O3 and CuO) with smaller optical band gaps (2.5 eV, 1.37 eV and 1.44 eV, respectively) [15,16], may reduce the optical transmittance and raise the difficulty to control the phase purity in p-type TOSs [13,17]. However, the transparent
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p-NiO/n-ZnO junctions still suffer from high threshold voltage (VON) above 1.5 eV
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[2,9,10,18], which will restrict their applications in low-power electronics. In this work, transparent p-NiO/n-ZnO heterojunctions have been fabricated on indium-tin-oxide (ITO)-coated glass substrates at room temperature by large-scale available magnetron sputtering. Different oxygen flow rates have been employed to
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grow the NiO thin films, yielding the tunable resistivity of NiO layers. The electrical performances of these p-n junctions, especially the threshold voltage, can be
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effectively tuned by the oxygen flow rate to synthesize NiO layers.
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2. Experimental details
Commercial ITO-coated glass (resistivity: 5 × 10-4 Ω · cm) and ordinary glass (microscopy slides) were used as the substrates. These substrates were cleaned by the sequence of ethanol, acetone and distilled water for five minutes, respectively. Followingly, they were dried by nitrogen gas flow. After cleaning the substrates, ZnO (70 nm) and NiO (100 nm) layers were deposited sequentially by reactive pulsed-DC
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ACCEPTED MANUSCRIPT magnetron sputtering in an Ar-O2 reactive mixture. Metallic Zn and Ni targets (50 mm diameter and 2 mm thick with a purity of 99.99 %) were used, respectively. A pulsed-DC supply (Pinnacle + Advanced Energy) with the current of 0.12 A and 0.2
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A was used to sputter the Zn and Ni targets, respectively. The frequency and the off-time during these depositions were 50 kHz and 4 µs, respectively. After the deposition of ZnO, the chamber was open and special metal mask was applied on
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ZnO layer in order to make sure that NiO will not cover on the edges of ZnO. The
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interfacial contamination between p-NiO and n-ZnO layers may mainly come from nickel hydroxide [7]. For the deposition of NiO layer, Ar flow rate was kept at 100 sccm and O2 flow rate varied from 10 to 16 sccm. Finally, Ag point electrodes with the thickness of 200 nm and diameter of 300 µm were deposited on the top of NiO.
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Thus, the p-n junction has the configuration of Ag/p-NiO/n-ZnO/ITO/glass, as shown in Fig. 4(a). During all the depositing, no intentional heating was applied to the substrates, and the deposition temperature was close to room temperature. This means
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all of the depositions in this work can be easily applied for flexible substrates. Phase
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structure of thin films on glass substrates were checked by X-ray diffraction (XRD, Smart Lab Rigaku with Cu Kα1 radiation (λ = 0.154 nm)). The optical transmittance of single layer ZnO and NiO thin films, as well as p-n junction without Ag points were measured by an UV-Vis-NIR spectrometer (Solidspec-3700). The thickness of different layers was checked by a profilometry (Bruker DektakXT) via the deposition of single layer on glass substrates. The resistivity of single layer ZnO and NiO thin
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ACCEPTED MANUSCRIPT films was measured by four-point probe. The stoichiometries of NiO thin films deposited with various oxygen flow rates were studied by a X-ray photoemission spectroscopy (XPS, Thermo ESCALAB 250XI). The electrical characteristics of p-n
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junctions were characterized by a source meter (Keithley 2400).
3. Results and discussion
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The phase structure of ZnO and NiO has been checked by depositing single layers
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on glass substrates. Fig. 1(a) shows the X-ray diffractogram of ZnO thin film, demonstrating the single phase structure with <001> preferred growth orientation. The X-ray diffractograms of NiO thin films deposited with different oxygen flow rates (10, 12, 14 and 16 sccm) are present in Fig. 1(b). Within this range of oxygen
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flow rates, no metallic nickel is evidenced by XRD, indicating that all the NiO thin films are single-phase. Additionally, a gradual shift of diffraction peaks towards lower angle value is observed when increasing the oxygen flow rates from 10 to 16 sccm,
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which is in accordance with the reported results of Seo, S et al [19].
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The optical band gap (Eg) of single layer ZnO and NiO thin films have been determined. Due to the direct band gap characteristic of ZnO, a Tauc fitting of (αE)2 vs. E gives the Eg of 3.25 eV (see Fig. 2(a)). Such Eg is consistent with other reported values [20]. Similarly, the Tauc plots with the exponent value of 2 in NiO thin films produce the Eg in the range of 3.40 - 3.45 eV, agreeing with the reported one of 3.4 eV [11]. These results demonstrate that both ZnO and NiO thin films grown in this work
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ACCEPTED MANUSCRIPT are wide band semiconductors. The optical transmittance of p-n junctions before the deposition of Ag electrodes is presented in Fig. 2(c). Increasing the oxygen flow rate to grow NiO layers from 10 up to 16 sccm, the average transmittance of these
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junctions in visible region (400 - 760 nm) has a tendency to be reduced from 82% (10 sccm), 82% (12 sccm), 78% (14 sccm) to 70% (16 sccm). The photographs of p-NiO/n-ZnO heterojunction on ITO-coated glass substrate and the blank ITO-coated
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glass substrate are shown in Fig. 2(d), demonstrating the good transparency of device.
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Figure 3 shows the room temperature resistivity of NiO thin films deposited on glass substrates with various oxygen flow rates (10, 12, 14 and 16 sccm). Increasing the oxygen flow rate from 10 to 12 sccm, the average resistivity goes up from 1908 to 4313.0 Ω cm. Continuing to increase the oxygen flow rate to 14 and 16 sccm, the
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resistivity falls down to 261.6 and 4.2 Ω cm, respectively. As seen in Fig. 3, the room temperature resistivity firstly rises up to a high value, and then falls down to a low when increasing the oxygen flow rates. Such a tendency has also been observed by
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Park et al. [12]. Besides, the stoichiometries of NiO thin films deposited with the
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oxygen flow rates of 10, 12 and 16 sccm are studied by XPS, giving the chemical compositions of NiO0.945, NiO0.987 and NiO1.138, respectively. This indicates that non-stoichiometric NiO thin films (NiO0.945 and NiO1.138) possess lower resistivity. Whereas, the composition (NiO0.987) close to the perfect stoichiometry yields the quite high resistivity. A schematic device structure of Ag/p-NiO/n-ZnO/ITO/glass is shown in Fig. 4(a).
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ACCEPTED MANUSCRIPT The current-voltage (I-V) characteristics of these devices with different oxygen flow rates (10, 12, 14 and 16 sccm) to synthesize p-NiO layer are plotted in Fig. 4(b). The good electrical rectifying properties are clearly evident in all these junctions due to
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the asymmetric I-V curves between forward and reverse bias. For comparison, symmetrical and linear I-V dependence of Ag/ZnO/ITO and Ag/NiO/ITO devices are presented in Fig. 4(c), indicating that the rectifying performances in Fig. 4(b) is really
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due to the formation of p-n junction. The detailed analyses on the I-V curves of p-n
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junction are performed. Taking the NiO layer deposited with 12 sccm O2 as an example, the linear fitting of the I-V curve with forward bias shows the threshold voltage (Von) of ~0.95 V (see Fig. 4(b)). Such a threshold voltage is much smaller than other reported ones in p-NiO/n-ZnO junctions [2,9,10,18,21,22], indicating potential
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applications in low-power electronics. The forward mode of the p-NiO/n-ZnO junction has also been characterized by the ideality factor (n).The n in such p-NiO/n-ZnO junction with the NiO layer deposited at 12 sccm O2 is calculated to be
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3.7, lower than those in previously reported transparent p-n junctions [21,23,24]. In
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addition, such a junction shows a rectifying ratio (r) as high as 3976 at + 2 V, larger than the reported ones [21,24]. In a word, the junction with NiO layer deposited at 12 sccm O2 exhibits the good overall performance. The threshold voltage, rectifying ratio and ideality factor of p-NiO/n-ZnO junctions
with various O2 flow rates to synthesize NiO layer are list in Table I. It is seen that the p-n junction with closely perfectly stoichiometric NiO layer grown at 12 sccm O2 has
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ACCEPTED MANUSCRIPT better performances, including smaller threshold voltage and ideality factor, as well as high rectifying ratio. Moreover, it is found that the threshold voltage as a function of oxygen flow rate exhibits a first decreases and a following raise, which could be
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qualitatively understood from the electrical properties of NiO. The threshold voltage of a junction is theoretically equal to the difference of work function (W) between pand n-type semiconductors (WNiO - WZnO), without the consideration of interfacial
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effects [25]. Since all the ZnO layers in these junctions are fabricated by the same
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conditions, it is believed that the work function of ZnO is fixed. However, the work function of NiO thin films correlate with the energy difference (∆ ) between Fermi level and valence band maximum (VBM) with the following equation ∆ = χ+Εg-W
(1)
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where χ is the electron affinity. Since the oxygen flow rate has quite a little influence on the χ [26] and band gap of NiO thin films (see Fig. 2(b)), it is speculated that larger work function corresponds to smaller ∆ . Since the hole effective mass in NiO
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is quite large [27], the mobility of these NiO thin films deposited at room temperature
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in this work could be extremely low. Thus, tunable resistivity by oxygen flow rate (see Fig. 3) may be mainly due to the variation of hole concentration. Due to the inverse relationship between the hole concentration and ∆ , the threshold voltage of these p-n junctions is finally correlated with the resistivity of NiO thin films. Larger resistivity of NiO means the higher ∆ , yielding smaller W. Consequently, the larger resistivity of NiO will give rise to smaller threshold voltage in the p-n junction, as the
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ACCEPTED MANUSCRIPT work function of ZnO is fixed. Checking the evolution tendencies in the resistivity (see Fig. 3) and threshold voltage (see Table I) as a function of oxygen flow rate, respectively, it seems to follow the assumption mentioned above. Here, it is worth
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noting that the interfacial states at the p-n junctions are not taken into account. Since NiO is a typically small-polaron transport material and the carriers mainly come from the Ni vacancies [28], it is believed that NiO thin film with high resistivity
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deposited at 12 sccm oxygen flow may have less density of defects. This assumption
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coincides with its closely perfect stoichiometry determined by XPS. Such a low density of defects may be responsible for the small ideality factor and high rectifying ratio in the p-n junctions. However, it is very difficult to detail these two characteristics, as they are much more complicated and correlated with other in the
4. Conclusion
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electrical characteristics of junctions.
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In summary, we have fabricated the transparent p-NiO/n-ZnO heterojunctions on
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ITO-coated glass substrates at room temperature by magnetron sputtering. NiO layers have been deposited at different oxygen flow rates, which gives the tunable resistivity. Typical rectifying current-voltage characteristics have been observed in these p-n junctions, and their electrical performances can be tuned by the oxygen flow rate to synthesize NiO layers. The junction consisting of a closely perfectly stoichiometric
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ACCEPTED MANUSCRIPT NiO layer with quite high resistivity deposited at 12 sccm oxygen flow rate exhibits better electrical properties.
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Acknowledgements This work is supported by the National Natural Science Foundation of China
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(51702267), the Science and Technology Development Foundation of China Academy of Engineering Physics (xk201701), the funding of Southwest University of
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Science and Technology (16zx7165), Longshan Academic Talent Research Supporting Program (17LZX535) and Program for Young Science and Technology Innovation Team of Sichuan Province (2017TD0020).
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X-ray diffractograms of ZnO (a) and NiO (b) thin films deposited on glass
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Fig. 1.
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Figure caption
Fig. 2
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substrates.
(a) Plot of (αhν)2 vs. hν for ZnO thin film. (b) Plot of (αhν)2 vs. hν for NiO
thin films deposited at different oxygen flow rates (10, 12, 14 and 16 sccm). (c)
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Optical transmittance of p-NiO/n-ZnO junctions with different oxygen flow rates to grow NiO layers. (d) Photographs of blank ITO-coated glass substrate and
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p-NiO/n-ZnO heterojunction on ITO-coated glass substrate.
Fig. 3
Room temperature resistivity of NiO thin films deposited on glass substrates
with various oxygen flow rates.
Fig. 4
(a) Schematic device structure of Ag/p-NiO/n-ZnO/ITO/glass. (b) I-V curves
of transparent Ag/p-NiO/n-ZnO/ITO devices with various O2 flow rates (10, 12, 14 16
ACCEPTED MANUSCRIPT and 16 sccm) to synthesize NiO layers. (c) I-V curves of Ag/p-NiO/ITO and
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Ag/n-ZnO/ITO devices.
Table I The threshold voltage, rectifying ratio and ideality factor of p-NiO/n-ZnO junctions with various O2 flow rates to synthesize NiO layers.
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O2 flow rate to synthesize Von (V) r @ + 2 V n NiO layer in the
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p-n junction (sccm) 10 12
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16
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14
17
1.37
446
5.5
0.95
3976
3.7
1.60
1310
6.6
1.63
353
4.4
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Highlights 1. Transparent p-NiO/n-ZnO heterojunctions have been fabricated at room temperature.
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2. The properties of junctions can be tuned by the oxygen flow rate to synthesize NiO layer.
3. The threshold voltage of 0.95 V has been attained in the junction with a high
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resistivity NiO.
S0042-207X(17)31681-0
DOI:
10.1016/j.vacuum.2018.01.015
Reference:
VAC 7764
To appear in:
Vacuum
Received Date: 24 November 2017 Revised Date:
8 January 2018
Accepted Date: 8 January 2018
Please cite this article as: Wang YB, Wei XH, Chang L, Xu DG, Dai B, Pierson JF, Wang Y, Room temperature fabrication of transparent p-NiO/n-ZnO junctions with tunable electrical properties, Vacuum (2018), doi: 10.1016/j.vacuum.2018.01.015. 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.
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Room temperature fabrication of transparent p-NiO/n-ZnO junctions with tunable electrical properties Y.B. Wang1, X.H. Wei1, L. Chang2, D.G. Xu3, B. Dai1, J.F. Pierson4, Y. Wang1,* State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials,
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1
Southwest University of Science and Technology, Mianyang 621010, China. China National Machinery Industry Corporation, Beijing 100080, China.
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Institute of Materials, China Academy of Engineering Physics, Jiangyou 621907, China.
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Institut Jean Lamour, UMR 7198-CNRS, Université de Lorraine, Nancy F-54011, France.
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Corresponding author: Y. Wang
Tel: +86-816-2419492
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Email: [email protected]
Fax: +86-816-2419492
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Abstract
Transparent p-n heterojunctions composed of p-NiO and n-ZnO thin films have
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been fabricated on indium-tin-oxide (ITO)-coated glass substrates at room temperature by magnetron sputtering. Various oxygen flow rates have been employed
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to the NiO thin films, yielding the tunable resistivity of NiO layers. These p-n junctions exhibit clear rectifying current-voltage characteristics. Moreover, their electrical properties can be effectively tuned by the oxygen flow rate to synthesize NiO layers in these junctions. NiO layer with closely perfect stoichiometry and quite high resistivity produces better performance in these p-n junctions, including the small threshold voltage and ideality factor, as well as high rectifying ratio. The 1
ACCEPTED MANUSCRIPT evolution tendency of threshold voltage as a function of the resistivity of NiO is qualitatively discussed.
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Keywords: p-n junction; Transparent oxide thin films; Sputtering; Tunable properties
1. Introduction
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Transparent electronic devices, like p-n junction and field effect transistor using
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transparent oxide semiconductors (TOSs), have attracted considerable attention due to their great potential application in display technology, ultraviolet optoelectronics and energy conversion [1–5]. The intrinsic properties of TOSs, including optical transparency in the visible range, high defect toleration and easy fabrication, are
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beneficial to such transparent devices with peculiar advantages over traditional semiconductor devices.
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Since the p-n junction is the key unit of electronics, it is of great interest to study the optical and electrical properties of transparent p-n junction based on TOSs. So far,
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ZnO is one of the most widely utilized n-type TOSs to assemble the junction, due to its elemental abundance, large direct band gap and versatile fabrication routes [2,4,6– 9]. On the other hand, several p-type TOSs, such as Cu+-based oxides (like Cu2O and CuCrO2) [6,9], ZnRh2O4 [8] and NiO[2–4], have been chosen to form the junction. Among these p-type TOSs, NiO is a promising one with optical band gap of 3.4 eV [11], simply rock-salt cubic structure and tunablely electrical properties [12]. Besides, 2
ACCEPTED MANUSCRIPT only one stable phase in binary Ni-O system allows the easy fabrication of single phase NiO thin films [11], which is superior to that in Cu+-based oxides [13,14]. Taking the binary Cu-O system as an example, the existence of three different phases
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(Cu2O, Cu4O3 and CuO) with smaller optical band gaps (2.5 eV, 1.37 eV and 1.44 eV, respectively) [15,16], may reduce the optical transmittance and raise the difficulty to control the phase purity in p-type TOSs [13,17]. However, the transparent
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p-NiO/n-ZnO junctions still suffer from high threshold voltage (VON) above 1.5 eV
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[2,9,10,18], which will restrict their applications in low-power electronics. In this work, transparent p-NiO/n-ZnO heterojunctions have been fabricated on indium-tin-oxide (ITO)-coated glass substrates at room temperature by large-scale available magnetron sputtering. Different oxygen flow rates have been employed to
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grow the NiO thin films, yielding the tunable resistivity of NiO layers. The electrical performances of these p-n junctions, especially the threshold voltage, can be
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effectively tuned by the oxygen flow rate to synthesize NiO layers.
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2. Experimental details
Commercial ITO-coated glass (resistivity: 5 × 10-4 Ω · cm) and ordinary glass (microscopy slides) were used as the substrates. These substrates were cleaned by the sequence of ethanol, acetone and distilled water for five minutes, respectively. Followingly, they were dried by nitrogen gas flow. After cleaning the substrates, ZnO (70 nm) and NiO (100 nm) layers were deposited sequentially by reactive pulsed-DC
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ACCEPTED MANUSCRIPT magnetron sputtering in an Ar-O2 reactive mixture. Metallic Zn and Ni targets (50 mm diameter and 2 mm thick with a purity of 99.99 %) were used, respectively. A pulsed-DC supply (Pinnacle + Advanced Energy) with the current of 0.12 A and 0.2
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A was used to sputter the Zn and Ni targets, respectively. The frequency and the off-time during these depositions were 50 kHz and 4 µs, respectively. After the deposition of ZnO, the chamber was open and special metal mask was applied on
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ZnO layer in order to make sure that NiO will not cover on the edges of ZnO. The
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interfacial contamination between p-NiO and n-ZnO layers may mainly come from nickel hydroxide [7]. For the deposition of NiO layer, Ar flow rate was kept at 100 sccm and O2 flow rate varied from 10 to 16 sccm. Finally, Ag point electrodes with the thickness of 200 nm and diameter of 300 µm were deposited on the top of NiO.
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Thus, the p-n junction has the configuration of Ag/p-NiO/n-ZnO/ITO/glass, as shown in Fig. 4(a). During all the depositing, no intentional heating was applied to the substrates, and the deposition temperature was close to room temperature. This means
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all of the depositions in this work can be easily applied for flexible substrates. Phase
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structure of thin films on glass substrates were checked by X-ray diffraction (XRD, Smart Lab Rigaku with Cu Kα1 radiation (λ = 0.154 nm)). The optical transmittance of single layer ZnO and NiO thin films, as well as p-n junction without Ag points were measured by an UV-Vis-NIR spectrometer (Solidspec-3700). The thickness of different layers was checked by a profilometry (Bruker DektakXT) via the deposition of single layer on glass substrates. The resistivity of single layer ZnO and NiO thin
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ACCEPTED MANUSCRIPT films was measured by four-point probe. The stoichiometries of NiO thin films deposited with various oxygen flow rates were studied by a X-ray photoemission spectroscopy (XPS, Thermo ESCALAB 250XI). The electrical characteristics of p-n
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junctions were characterized by a source meter (Keithley 2400).
3. Results and discussion
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The phase structure of ZnO and NiO has been checked by depositing single layers
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on glass substrates. Fig. 1(a) shows the X-ray diffractogram of ZnO thin film, demonstrating the single phase structure with <001> preferred growth orientation. The X-ray diffractograms of NiO thin films deposited with different oxygen flow rates (10, 12, 14 and 16 sccm) are present in Fig. 1(b). Within this range of oxygen
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flow rates, no metallic nickel is evidenced by XRD, indicating that all the NiO thin films are single-phase. Additionally, a gradual shift of diffraction peaks towards lower angle value is observed when increasing the oxygen flow rates from 10 to 16 sccm,
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which is in accordance with the reported results of Seo, S et al [19].
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The optical band gap (Eg) of single layer ZnO and NiO thin films have been determined. Due to the direct band gap characteristic of ZnO, a Tauc fitting of (αE)2 vs. E gives the Eg of 3.25 eV (see Fig. 2(a)). Such Eg is consistent with other reported values [20]. Similarly, the Tauc plots with the exponent value of 2 in NiO thin films produce the Eg in the range of 3.40 - 3.45 eV, agreeing with the reported one of 3.4 eV [11]. These results demonstrate that both ZnO and NiO thin films grown in this work
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ACCEPTED MANUSCRIPT are wide band semiconductors. The optical transmittance of p-n junctions before the deposition of Ag electrodes is presented in Fig. 2(c). Increasing the oxygen flow rate to grow NiO layers from 10 up to 16 sccm, the average transmittance of these
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junctions in visible region (400 - 760 nm) has a tendency to be reduced from 82% (10 sccm), 82% (12 sccm), 78% (14 sccm) to 70% (16 sccm). The photographs of p-NiO/n-ZnO heterojunction on ITO-coated glass substrate and the blank ITO-coated
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glass substrate are shown in Fig. 2(d), demonstrating the good transparency of device.
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Figure 3 shows the room temperature resistivity of NiO thin films deposited on glass substrates with various oxygen flow rates (10, 12, 14 and 16 sccm). Increasing the oxygen flow rate from 10 to 12 sccm, the average resistivity goes up from 1908 to 4313.0 Ω cm. Continuing to increase the oxygen flow rate to 14 and 16 sccm, the
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resistivity falls down to 261.6 and 4.2 Ω cm, respectively. As seen in Fig. 3, the room temperature resistivity firstly rises up to a high value, and then falls down to a low when increasing the oxygen flow rates. Such a tendency has also been observed by
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Park et al. [12]. Besides, the stoichiometries of NiO thin films deposited with the
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oxygen flow rates of 10, 12 and 16 sccm are studied by XPS, giving the chemical compositions of NiO0.945, NiO0.987 and NiO1.138, respectively. This indicates that non-stoichiometric NiO thin films (NiO0.945 and NiO1.138) possess lower resistivity. Whereas, the composition (NiO0.987) close to the perfect stoichiometry yields the quite high resistivity. A schematic device structure of Ag/p-NiO/n-ZnO/ITO/glass is shown in Fig. 4(a).
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ACCEPTED MANUSCRIPT The current-voltage (I-V) characteristics of these devices with different oxygen flow rates (10, 12, 14 and 16 sccm) to synthesize p-NiO layer are plotted in Fig. 4(b). The good electrical rectifying properties are clearly evident in all these junctions due to
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the asymmetric I-V curves between forward and reverse bias. For comparison, symmetrical and linear I-V dependence of Ag/ZnO/ITO and Ag/NiO/ITO devices are presented in Fig. 4(c), indicating that the rectifying performances in Fig. 4(b) is really
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due to the formation of p-n junction. The detailed analyses on the I-V curves of p-n
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junction are performed. Taking the NiO layer deposited with 12 sccm O2 as an example, the linear fitting of the I-V curve with forward bias shows the threshold voltage (Von) of ~0.95 V (see Fig. 4(b)). Such a threshold voltage is much smaller than other reported ones in p-NiO/n-ZnO junctions [2,9,10,18,21,22], indicating potential
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applications in low-power electronics. The forward mode of the p-NiO/n-ZnO junction has also been characterized by the ideality factor (n).The n in such p-NiO/n-ZnO junction with the NiO layer deposited at 12 sccm O2 is calculated to be
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3.7, lower than those in previously reported transparent p-n junctions [21,23,24]. In
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addition, such a junction shows a rectifying ratio (r) as high as 3976 at + 2 V, larger than the reported ones [21,24]. In a word, the junction with NiO layer deposited at 12 sccm O2 exhibits the good overall performance. The threshold voltage, rectifying ratio and ideality factor of p-NiO/n-ZnO junctions
with various O2 flow rates to synthesize NiO layer are list in Table I. It is seen that the p-n junction with closely perfectly stoichiometric NiO layer grown at 12 sccm O2 has
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ACCEPTED MANUSCRIPT better performances, including smaller threshold voltage and ideality factor, as well as high rectifying ratio. Moreover, it is found that the threshold voltage as a function of oxygen flow rate exhibits a first decreases and a following raise, which could be
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qualitatively understood from the electrical properties of NiO. The threshold voltage of a junction is theoretically equal to the difference of work function (W) between pand n-type semiconductors (WNiO - WZnO), without the consideration of interfacial
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effects [25]. Since all the ZnO layers in these junctions are fabricated by the same
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conditions, it is believed that the work function of ZnO is fixed. However, the work function of NiO thin films correlate with the energy difference (∆ ) between Fermi level and valence band maximum (VBM) with the following equation ∆ = χ+Εg-W
(1)
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where χ is the electron affinity. Since the oxygen flow rate has quite a little influence on the χ [26] and band gap of NiO thin films (see Fig. 2(b)), it is speculated that larger work function corresponds to smaller ∆ . Since the hole effective mass in NiO
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is quite large [27], the mobility of these NiO thin films deposited at room temperature
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in this work could be extremely low. Thus, tunable resistivity by oxygen flow rate (see Fig. 3) may be mainly due to the variation of hole concentration. Due to the inverse relationship between the hole concentration and ∆ , the threshold voltage of these p-n junctions is finally correlated with the resistivity of NiO thin films. Larger resistivity of NiO means the higher ∆ , yielding smaller W. Consequently, the larger resistivity of NiO will give rise to smaller threshold voltage in the p-n junction, as the
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ACCEPTED MANUSCRIPT work function of ZnO is fixed. Checking the evolution tendencies in the resistivity (see Fig. 3) and threshold voltage (see Table I) as a function of oxygen flow rate, respectively, it seems to follow the assumption mentioned above. Here, it is worth
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noting that the interfacial states at the p-n junctions are not taken into account. Since NiO is a typically small-polaron transport material and the carriers mainly come from the Ni vacancies [28], it is believed that NiO thin film with high resistivity
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deposited at 12 sccm oxygen flow may have less density of defects. This assumption
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coincides with its closely perfect stoichiometry determined by XPS. Such a low density of defects may be responsible for the small ideality factor and high rectifying ratio in the p-n junctions. However, it is very difficult to detail these two characteristics, as they are much more complicated and correlated with other in the
4. Conclusion
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electrical characteristics of junctions.
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In summary, we have fabricated the transparent p-NiO/n-ZnO heterojunctions on
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ITO-coated glass substrates at room temperature by magnetron sputtering. NiO layers have been deposited at different oxygen flow rates, which gives the tunable resistivity. Typical rectifying current-voltage characteristics have been observed in these p-n junctions, and their electrical performances can be tuned by the oxygen flow rate to synthesize NiO layers. The junction consisting of a closely perfectly stoichiometric
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ACCEPTED MANUSCRIPT NiO layer with quite high resistivity deposited at 12 sccm oxygen flow rate exhibits better electrical properties.
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Acknowledgements This work is supported by the National Natural Science Foundation of China
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(51702267), the Science and Technology Development Foundation of China Academy of Engineering Physics (xk201701), the funding of Southwest University of
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Science and Technology (16zx7165), Longshan Academic Talent Research Supporting Program (17LZX535) and Program for Young Science and Technology Innovation Team of Sichuan Province (2017TD0020).
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X-ray diffractograms of ZnO (a) and NiO (b) thin films deposited on glass
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Fig. 1.
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Figure caption
Fig. 2
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substrates.
(a) Plot of (αhν)2 vs. hν for ZnO thin film. (b) Plot of (αhν)2 vs. hν for NiO
thin films deposited at different oxygen flow rates (10, 12, 14 and 16 sccm). (c)
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Optical transmittance of p-NiO/n-ZnO junctions with different oxygen flow rates to grow NiO layers. (d) Photographs of blank ITO-coated glass substrate and
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p-NiO/n-ZnO heterojunction on ITO-coated glass substrate.
Fig. 3
Room temperature resistivity of NiO thin films deposited on glass substrates
with various oxygen flow rates.
Fig. 4
(a) Schematic device structure of Ag/p-NiO/n-ZnO/ITO/glass. (b) I-V curves
of transparent Ag/p-NiO/n-ZnO/ITO devices with various O2 flow rates (10, 12, 14 16
ACCEPTED MANUSCRIPT and 16 sccm) to synthesize NiO layers. (c) I-V curves of Ag/p-NiO/ITO and
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Ag/n-ZnO/ITO devices.
Table I The threshold voltage, rectifying ratio and ideality factor of p-NiO/n-ZnO junctions with various O2 flow rates to synthesize NiO layers.
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O2 flow rate to synthesize Von (V) r @ + 2 V n NiO layer in the
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p-n junction (sccm) 10 12
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16
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14
17
1.37
446
5.5
0.95
3976
3.7
1.60
1310
6.6
1.63
353
4.4
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Highlights 1. Transparent p-NiO/n-ZnO heterojunctions have been fabricated at room temperature.
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2. The properties of junctions can be tuned by the oxygen flow rate to synthesize NiO layer.
3. The threshold voltage of 0.95 V has been attained in the junction with a high
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resistivity NiO.