ZnO heterojunctions

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Optical and electrical characterization of CuO/ZnO heterojunctions R. Yatskiv , S. Tiagulskyi , J. Grym , J. Vanis , N. Basinova , ˇ P. Horak , A. Torrisi , G. Ceccio , J. Vacik , M. Vrnata PII: DOI: Reference:

S0040-6090(19)30683-2 https://doi.org/10.1016/j.tsf.2019.137656 TSF 137656

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Thin Solid Films

Received date: Revised date: Accepted date:

19 June 2019 15 October 2019 21 October 2019

Please cite this article as: R. Yatskiv , S. Tiagulskyi , J. Grym , J. Vanis , N. Basinova , P. Horak , ˇ A. Torrisi , G. Ceccio , J. Vacik , M. Vrnata , Optical and electrical characterization of CuO/ZnO heterojunctions, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137656

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Highlights    

CuO/ZnO heterojunctions are based on solution-grown ZnO nanorods; Optoelectrical properties of the CuO/ZnO junctions are investigated; Charge transport is controlled by the depletion region at the CuO/ZnO junction; The CuO/ZnO heterostructures enable to detect hydrogen at room-temperature;

Optical and electrical characterization of CuO/ZnO heterojunctions R. Yatskiv1* S. Tiagulskyi1, J. Grym1, J. Vanis1, N. Basinova1, P. Horak2, A. Torrisi2, G. Ceccio2, J. Vacik2, M. Vrňata3 1

Institute of Photonics and Electronics, CAS; Chaberska 57, 18251; Prague; Czech Republic. Nuclear Physics Institute, CAS; 250 68 Rez; Czech Republic. 3 Department of Physics and Measurements, Faculty of Chemical Engineering, University of Chemistry and Technology Prague; 166 28 Prague 6; Czech Republic. * Corresponding author: [email protected] 2

Abstract CuO/ZnO p-n heterojunctions are fabricated on ZnO nanorod arrays by sputtering of metallic Cu thin films and by their subsequent thermal annealing at 400°C. Structural, morphological, and optical properties of both copper oxide nanocrystalline films and zinc oxide nanorod arrays are discussed with the emphasis on the electrical junction properties investigated by current-voltage and impedance spectroscopy measurements. Electrical characteristics of these junctions are sensitive to gas mixtures with a low hydrogen concentration and show fast response and recovery time. The copper oxide/zinc oxide heterojunctions are shown to be more efficient to hydrogen detection at room temperature in comparison with the resistivity sensors based on zinc or copper oxides. Keywords Zinc oxide, Copper oxide, Hybrid heterojunction, Hydrogen sensor, Impedance spectroscopy. 1. Introduction Zinc oxide has been widely investigated as a promising material for light emitting devices, UV photodetectors, piezoelectric generators, chemical and biological sensors, and solar cells due to its excellent physical, chemical, and electronic properties such as: wide band gap, large exciton binding energy, optical transparency in the visible region, piezoelectricity, radiation hardness, electrically active surfaces, and biological compatibility [1]. Nevertheless, the lack of p-type electrical conductivity in ZnO emphasizes the importance of the study of hybrid heterojunctions [2-4]. Among different p-type materials, copper oxide is a good candidate to create p-n heterojunctions with ZnO with a broad potential in different optoelectronic devices [5-7]. The main advantage of copper oxide is nontoxicity, low market price, and high absorption coefficient. Moreover, copper oxide can be prepared by a variety of methods, including thermal oxidation, magnetron sputtering, chemical bath deposition, pulsed laser deposition, vacuum arc plasma evaporation, atomic layer deposition, electro deposition, ion sputtering, wet-chemical oxidation process, and spray pyrolysis [8-17]. Copper oxide has two stable polymorphs: cuprous oxide (Cu2O) and cupric oxide (CuO), both of them are of p-type conductivity in nature with the band gap ~ 2.1eV and ~1.4eV, respectively [18] The combination of p-type copper oxide and n-type zinc oxide appears promising for the implementation in photovoltaic devices, where copper oxide is employed as an absorbing layer and the wide band gap zinc oxide as a window layer [5]. Nevertheless, up to date the reported highest efficiency (η≈4%) [5], is far from the theoretically predicted efficiency limit (η≈20%) [19]. Another application area is gas sensing; however, metal oxide sensors suffer from poor selectivity. The selectivity can be enhanced by the fabrication of p-n junctions between different metal oxides [6]. Copper oxide/zinc oxide junctions have been successfully used for the detection of various types of gases, such as CO, H2, volatile organic compounds, H2S [5,6,20-23], with the working temperature in the range of 150-400°C. To improve the efficiency and performance of both photovolatic devices and gas sensors, it is crucial to understand how the quality of the interface affects charge transport in such junctions. While plenty of studies have been devoted to applications, detailed investigations of charge transport in copper oxide/zinc oxide heterojunctions are scarce [24-28].

In this work we present a cost-effective technology for the preparation of copper oxide/zinc oxide heterojunctions, which can be used for the fabrication of highly sensitive room temperature hydrogen sensors. Current-voltage (I-V) measurements and impedance spectroscopy are applied to investigate the copper oxide/zinc oxide interface with the aim to shed light on their electrical transport and sensing properties. 2. Experimental Vertically-oriented ZnO nanorod arrays were grown on seed layers by the chemical bath deposition method. Detailed information about the preparation of the seed layer and about the growth of ZnO nanorods was given in our previous papers [29,30]. A copper film was deposited on top of the ZnO nanorods by ion beam sputtering from a copper target (99.99 %, MaTecK.com), which was sputtered by argon ions (of purity 99.999 %) with the beam energy of 25 kV and the beam current of 400 A. The copper thin film was subsequently oxidized in a quartz furnace open to the air at 400 °C for 7 h. The DC electrical properties of the hybrid heterojunctions were studied by the measurement of their I-V characteristics using a source measure unit Keithley 237. The impedance spectra were collected using Impedance Analyzer Keysight E4990A. The measuring system for gas sensing consisted of: (a) a throughflow gas cell; (b) a sensor holder; (c) a current/voltage source measure unit; (d) mass flow controllers; and (e) of a data acquisition system. The photoluminescence (PL) spectra were measured with a set-up comprising a He–Cd laser (325 nm) as an excitation source, a grating monochromator Jobin Yvon THR 1000, and a GaAs photomultiplier detection system. Morphological and structural properties of both the copper oxide film and the zinc oxide nanorods were investigated by scanning electron microscopy (SEM), and time-of-flight secondary ion mass spectrometry (TOF-SIMS). SEM images were collected with Tescan Lyra 3 GM using the in-beam SE detector. The TOF-SIMS spectra were measured using the Ga+ ion beam with the energy of 30 keV and current of 1000 pA. 3. Result and discussion The ZnO nanorods grow preferentially along the c-axis with the length of 900 nm and average diameter of 70 nm (Fig. 1a). Copper oxide forms a compact nanocrystalline film with the average crystallite size of 180 nm and the thickness of approximately 150 nm (Fig. 1b). 0

0

100

100

Z [frames]

0.06 0.10 200

200

300

0.04

300 0.05

400

63

Cu

400

64

Zn

X [px]

Fig. 1 Top-view and cross-sectional SEM images of: (a) ZnO Fig. 2 Depth distribution of the 64Zn and 63 nanorods; (b) ZnO/CuO heterojunction. Cu ion isotopes in the ZnO/CuO heterojunction. The color bar represents the number of counts/TOF-extraction.

The time-of-flight SIMS was used to measure the depth distribution of 63Cu and 64Zn ion isotopes in the CuO/ZnO heterojunction (Fig. 2). The depth profile shows that the CuO film is deposited uniquely on the top of the ZnO nanorod array. Fig. 3 shows room temperature PL of the ZnO nanorods and of the CuO thin film. The PL spectra from the ZnO nanorods are representative of ZnO nanostructures grown from solutions with the near band edge emission in the UV region and a broad band emission in the visible region [31]. The PL spectra from the CuO film show a broad, asymmetric excitonic band with the maximum at 430 nm. The peak position is blue-shifted in comparison with the bulk CuO, which can be explained by quantum confinement effect that arises from the nanocrystals forming the CuO thin film [32, 33].

1.2

Normalized PL intensity

325 nm, RT 1.0

CuO ZnO NRs

0.8 0.6 0.4 0.2 0.0 400

500 600 Wavelength (nm)

700

800

Fig. 3 Room temperature photoluminescence spectra for CuO nanocrystalline film and ZnO nanorods.

A non-alloyed ohmic contact on the top of ZnO nanorods was created by thermal evaporation of a 100nm film of Al covered with a 50 nm film of Au to prevent surface oxidation. To form ohmic contact on p type CuO, Au was used for its low reactivity and large work function. Ohmic behavior of the Au/CuO/Au contacts as well as of the Al-ZnO-Al contacts was confirmed by the measurement of I-V characteristics (inset in Fig. 4a). A representative room temperature I-V characteristic of the CuO/ZnO heterojunction is presented in Fig. 4a. The forward current is approximately one order of magnitude higher than the reverse current at 10 V, which confirms the formation of a p-n junction. The turn-on voltage of this junction is ~2 V, which is in agreement with the expected barrier height at the copper oxide /zinc oxide interface [24, 34, 35]. The I-V curves were ( ) fitted by using standard diode equation . The high ideality factor~10) can be attributed to the high density of defects at the interface between the ZnO nanorods and the nanocrystalline CuO film.

10-5

CuO/ZnO

10-7

10-8

30µ 20µ 10µ 0 -10µ -20µ -30µ

Al-ZnO-Al Au-CuO-Au

Current (A)

Current (A)

10-6

-10

10-9

-10

-5 0 5 Voltage (V)

(a)

10

-5

0 5 10 Voltage (V) Fig. 4 (a) A representative semi-logarithmic I-V characteristic of the CuO/ZnO heterojunction. The inset shows I-V characteristics of the ohmic contacts on both the CuO nanocrystalline film and on the ZnO nanorods; (b) A schematic diagram of the p-n junction (Type I: Au/CuO; Type II: Al/ZnO; Type III: CuO/ZnO). To deeper understand the charge transport mechanism in CuO/ZnO heterojunctions the I-V curves were drawn in log-log coordinates. The I-V curves can be divided into three distinct regions (Fig. 5). At low forward voltage V<0.4 V (region I), the current transport follows linear behavior (I~V). Such behaviour is attributed to thermally generated carrier tunneling [36]. In the region II, the current increases exponentially, I~exp (aV), and can be explained by recombination-tunneling mechanism, which is often observed in heterojunctions of wide band gap semiconductors [37]. In the region III (V>5 V), the current follows a square-law dependence, I~V2,which is related to the space-charge-limited current conduction [38].

10

(b)

(a)

(a)

(b)

5

III I~U2

I~U

10-6

RT

-0.1V

(b)- air

(b)

(a)

II

-7

I~exp(aU)

Response, S (%)

Current (A)

10

(a)-1000ppm H2

6

CuO/ZnO Al/ZnO/Al Au/CuO/Au

-5

4 3 2 1

10-8 I I~U

10-9 0.01

0.1 1 Voltage (V)

0 Type III

10

Fig. 5 The log-log of the current-voltage under forward bias of the CuO/ZnO heterojunction and ohmic contacts on both the CuO nanocrystalline film and on the ZnO nanorods.

0

200

400

600

Type II 800

Type I 1000

1200

1400

1600

Time (s)

Fig. 6 Transient response of the: (type I) resistivity sensor based on CuO; (type II) resistivity sensor based on ZnO; (type III) copper oxide/zinc oxide p-n heterojunction sensor at 25°C and -0,1V.

Three different structures were tested for their sensitivity to hydrogen in the cell with a through-flow gas system. The structures are designated as follows: type I - a resistivity sensor based on CuO nanocrystalline

film with ohmic contacts; type II - a resistivity sensor based on ZnO nanorods with ohmic contacts; and type III - a sensor based on p-n heterojunction between copper oxide and zinc oxide. For all sensor types the current-transient characteristics were measured at identical conditions (temperature, humidity, flow rate, applied bias). Fig. 6 shows the sensor response to 1000 ppm of hydrogen at room temperature for the three different sensor types. The sensor response was calculated by the following equation {( )⁄ } , where and are the saturation current under hydrogen and air, respectively. No response to 1000 ppm of hydrogen was observed for the type I structure. The type II structure showed sensor response S~ 2% at room temperature. The response of the type III structure was two times higher than that of the type II structure. The current measured after exposure to hydrogen does not recover to the initial level for neither the type II structure nor for the type III structure. Such behavior can be explained by the presence of adsorbed gas molecules, which were not removed from the surface of the ZnO nanorods for the type II structure and from the copper oxide/zinc oxide interface for the type III structure. The exponential fitting of the current transient characteristics by equation was used to compare the response ( ) and recovery ( ) times. The response/recovery times were calculated as 55/2s for the type II structure and 92/2s for type III structure, respectively. Moderate reduction in the response time after the first cycle supports the claim that not all hydrogen molecules are desorbed from the zinc oxide nanorod surface or from the copper oxide/zinc oxide interface. The sensor parameters obtained in our experiment were measured at room temperature, while similar structures were tested at temperatures above 250°C with no detection at room temperature [22, 39, 40]. The sensing mechanism in the resistivity-based sensor (type II, Fig. 7a) can be explained as follows. Under exposure to ambient air, the oxygen species from the air are adsorbed on the surface of the ZnO nanorods. The absorbed oxygen molecules on the surface extract electrons from the conduction band, which results in the formation of the electron depletion region with reduced electron concentration. For homogeneous not fully depleted nanorods with ohmic contact under bias V, the current is given by equation

where

is

the electron mobility, n is the free electron density, L is the nanorod length, and S is the non-depleted crosssectional area of the nanorod. Under exposure to hydrogen, the hydrogen molecules react with adsorbed oxygen species and form H2O molecules while the released electron contributes to the increase of nondepleted cross-sectional area resulting in the current increase (

. In the p-n junction sensor

(type III, Fig. 7b) the same effect occurs at the interface between CuO and zinc ZnO. The electrons in ZnO and holes in CuO diffuse in opposite direction until the Fermi levels reach a balance. As a result, a depletion layer is formed at the interface between CuO and ZnO. When H 2 molecules are introduced to the CuO/ZnO heterojunction, they react with adsorbed oxygen species and form H2O molecules while the released electron lead to the decrease of the depletion layer width. The current flow in the p-n junction is dominantly controlled by the variation of the depletion region at the interface, which is largely sensitive to gas species and results in significantly higher current change in comparison with the resistivity-based sensor. As a result, a higher sensitivity response is obtained for the type III sensor structure.

Fig. 7 Schematic of the sensing mechanism of ZnO nanorods (a) and of the CuO/ZnO heterojunction (b) sensor in air and H2. Impedance spectroscopy studies were carried out using a Keysight E4990A impedance analyzer in the frequency range of 20 Hz to 10 MHz with the amplitude of the ac signal of 10 mV. The impedance spectra were measured for bare ZnO and CuO structures and for CuO-ZnO structures in the same configuration of metal contacts as in the I-V measurements. Fig. 8a shows the relationship between the real and imaginary part of the complex impedance (Nyquist diagrams) of Au/CuO/Au, Al/ZnO/Al and Au/CuO/ZnO/Al structures at room temperature. Equivalent electrical circuits were applied to fit the experimental spectra. For the type I and II structures a conventional R/RC equivalent circuit was employed (Fig. 8b, top), where the parallel (R gbCgb) circuit is associated with the interface phenomena while Rc is associated with the bulk conductivity and ohmic metal contacts. For the bare CuO structure, the parallel RC network is ascribed to the depletion layers at the grain boundaries of the polycrystalline CuO film while for the bare ZnO structure it is ascribed to the surface depletion layer of rod-to-rod interfaces in the nanorods arrays [41]. In this study, we fit the capacitance of the depletion regions using the constant phase element (CPE) to account for the surface inhomogeneity resulting from the roughness and interfacial states. The CPE element represents a capacitive element with a certain frequency dispersion. The electrical impedance of CPE can be expressed as

When n is 1, the CPE is an ideal capacitor with the capacitance equal to Q. As the homogeneity of the interface decreases, ϕ is expected to deviate further from 1 [42]. The modeling was performed using the EIS spectrum analyzer software [43]. The fitted parameters of equivalent circuits are collected in Table 1. Regarding the obtained values, we consider the contribution of ZnO to impedance spectra of CuO-ZnO as negligible. Therefore, the impedance spectra of CuO-ZnO structure (type III) were modeled by a series connection of two parallel RC circuits (Fig. 8 b, bottom) attributed to the interface phenomena of the CuO layer (R gbCgb) and to the depletion region of p-CuO/n-ZnO interface (RjCj). The Rj and Cj parameters are ascribed to the shunt resistance and depletion layer capacitance of the p-n junction [42].

5

Au-CuO-Au (Type I) Al-ZnO-Al (Type II) CuO-ZnO (Type III)

(a) 5 4

3

-Im(Z) (105 Ohm)

-Im(Z) (105 Ohm)

4

2

1

3 2 1 2 4 Re(Z) (105 Ohm)

0 0.0

0.5

1.0

1.5

2.0 2.5 3.0 3.5 Re(Z) (105 Ohm)

4.0

4.5

5.0

Figure 8. (a) Impedance spectra of Au/CuO/Au, Al/ZnO/Al and Au/CuO/ZnO/Al structures. In the inset, the symbols depict experimental data while the solid lines depict modelled curves. (b) Equivalent electrical circuits employed for the modeling.

Table 1. The parameters of equivalent electrical circuit extracted from the fitting of the impedance spectra of Au/CuO/Au, Al/ZnO/Al and Au/CuO/ZnO/Al structures. Rgb, Ohm

Cgb, F

Rj, Ohm

(n) 1.4∙10-9

Au/CuO/Au 4.5∙10

Rc, Ohm

(n) -

-

5

(type I)

≈103 (0.7) 1∙10-9

Al/ZnO/Al 1.1∙10

-

-

5

(type II)

≈103 (0.7) 9∙10-9

Au/CuO/ZnO/Al 4.5∙10 (type III)

Cj, F

5

2∙10-9 14.6∙10

(0.6)

6

≈103 (0.95)

The impedance spectra have significant bias dependence (Fig. 9). Applied modeling shows that the second semicircle of the Nyquist plots shrinks with the applied bias mainly due to the decrease of Rj by an order of magnitude when the DC bias varies from 0 V to 1 V. R j blocks the current flow through the p-n junction under zero DC bias. On the forward DC bias Rj drops significantly as the bias voltage increases. It confirms our suggestion that the (RjCj) circuit is associated with the p-CuO/n-ZnO interface. To conclude, using the impedance spectroscopy we demonstrated that the electrical transport and, consequently, the sensing properties of CuO ZnO structures are mainly defined by the high resistance depletion layer at the interface between the CuO layer and the ZnO nanorods.

(b)

0V

Rj (Ohm)

Re(Z) (Ohm)

3x10

6

1.5x107

(a)

0 mV 200 mV 400 mV 600 mV 800 mV 1V

4x106

2x106

1.0x107

5.0x106

1x106

1V

0 0

1x106 -Im(Z) (Ohm)

2x106

0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Figure 9. (a) The impedance spectra of Au/CuO/ZnO/Al structure measured at different forward biases. (b) The biasdependent resistance of the CuO/ZnO junction. 4. Conclusions We demonstrated that rectifying copper oxide/zinc oxide heterojunctions can be prepared by simple, scalable, and cost-effective methods. A thin layer of copper evaporated on solution-grown vertically-oriented zinc oxide nanorods was oxidized in the air. The optical properties of copper oxide nanocrystalline films were investigated by photoluminescence spectroscopy. The broad luminescence band with the maximum at 430 nm was explained by the quantum confinement effect. Three different structures were tested for their sensitivity to hydrogen: (a) a resistivity sensor based on CuO nanocrystalline film with ohmic contacts; (b) a resistivity sensor based on ZnO nanorods with ohmic contacts; and (c) a sensor based on a p-n heterojunction between copper oxide and zinc oxide. The heterojunction-based sensors were shown to have higher sensitivity than their resistivity-based counterparts. This was explained by the fact that the electrical conductivity of the heterojunction device is dominantly controlled by the depletion layer at the interface between copper oxide and zinc oxide, and that the depletion layer is largely sensitive to the adsorbed gas species. Acknowledgements This work was supported by the Czech Science Foundation project 19-02804S. References [1] U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.J. Cho, H. Morkoc, A comprehensive review of ZnO materials and devices, J Appl Phys, 98 (2005) 041301. [2] A. Janotti, C.G. Van de Walle, Fundamentals of zinc oxide as a semiconductor, Rep Prog Phys, 72 (2009) 126501. [3] D.C. Look, B. Claflin, P-type doping and devices based on ZnO, physica status solidi (b), 241 (2004) 624630. [4] J.G. Reynolds, C.L. Reynolds, Progress in ZnO Acceptor Doping: What Is the Best Strategy?, Advances in Condensed Matter Physics, 2014 (2014) 15. [5] T. K. S. Wong, S. Zhuk, S. Masudy-Panah, and G. K. Dalapati, Materials 9 (4) (2016). T.K.S. Wong, S. Zhuk, S. Masudy-Panah, G.K. Dalapati, Current Status and Future Prospects of Copper Oxide Heterojunction Solar Cells, Materials, 9 (2016) 271. [6] D.R. Miller, S.A. Akbar, P.A. Morris, Nanoscale metal oxide-based heterojunctions for gas sensing: A review, Sensors and Actuators B: Chemical, 204 (2014) 250-272. [7] X. Jiang, T. Herricks, Y. Xia, CuO Nanowires Can Be Synthesized by Heating Copper Substrates in Air, Nano Letters, 2 (2002) 1333-1338.

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