Multilayer MgZnO/ZnO thin films for UV photodetectors Journal of ...

Journal of Alloys and Compounds 764 (2018) 724e729

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Multilayer MgZnO/ZnO thin films for UV photodetectors Vijay S. Rana a, Jeevitesh K. Rajput a, Trilok K. Pathak a, b, L.P. Purohit a, * a b

Semiconductor Research Lab, Department of Physics, Gurukula Kangri University, Haridwar, India Department of Physics, TKCOE, Teerthanker Mahaveer University, Moradabad, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 April 2018 Received in revised form 11 June 2018 Accepted 13 June 2018 Available online 14 June 2018

MgyZn1-yO/ZnO thin films have been deposited on soda lime glass substrates by using solegel spin coating technique with varying Mg contents (y ¼ 2, 4, 6, 8 at. %). X-ray diffraction (XRD) studies indicate that films exhibit the wurtzite phase with a preferential c-axis (002) orientation. The scanning electron micrographs revealed that at low doping level round and oval shaped microstructure were formed and on increasing Mg content nanoflower and nanoplate morphology were found. The transmittance of the thin films was measured in the wavelength range 300 nme800 nm and the bandgap increased from 3.25 eV to 3.29 eV with increasing Mg content. The I-V measurements were performed in dark and illumination conditions show Ohmic behaviour. MgyZn1-yO/ZnO (y ¼ 2 at. %) thin films show high stability and fast switching UV photoresponse behaviour. The highest responsivity of 0.16 A/W was obtained at 3.2 mW/cm2exposure of UV light (365 nm) at 5 V bias voltage. © 2018 Published by Elsevier B.V.

Keywords: MgZnO/ZnO Sol-gel method I-V characteristics Photodetection

1. Introduction Ultraviolet (UV) photodetectors (PDs) based wide bandgap semiconductors have been stabilised and used in different potential applications such as UV-photography, chemical agent sensing, UVastronomy and flame detection [1,2]. For the preparation of UV photodetectors many aspects were approached, such as p-n junction [3,4], Schottky junction, metal-semiconductor-metal (MSM) [5], and their photo detection performance was explored extensively [6]. ZnO is wide band gap (3.37 eV) semiconductor and large excitonic energy (60 meV) [7] which is useful for optoelectronic applications such as UV detectors [8], LEDs [9], Laser [10]. The wavelength of emission or detection can be modulated by alloying ZnO with a higher bandgap material [11,12]. The bandgap of zinc oxide can be tuned by alloying it with group II and III elements, e.g. Be, Mg, Cd [13,14]. Among of these materials, magnesium (Mg) doped ZnO thin films have been a research focus due to similar ionic radii of Mg2þ and Zn2þ, and their excellent optoelectronic properties of the parent structure of ZnO. Because of the high solid solubility of MgO in ZnO; the lattice constants remain almost unaffected even after Mg incorporation. As a result, MZO thin film exhibits tunable electrical and optical properties and it has been established as a promising active layer material in optoelectronic

* Corresponding author. E-mail address: profl[email protected] (L.P. Purohit). https://doi.org/10.1016/j.jallcom.2018.06.139 0925-8388/© 2018 Published by Elsevier B.V.

applications, for example, ultraviolet photodetectors [12,15,18]. Moreover, the band gap of ZnO can be tuned from 3.37 to 7.8 eV by composition with varying magnesium content [16]. Mg is incorporated into ZnO lattice and it has tended to reduce the interstitial oxygen vacancies and electron density [17,18]. In addition, effect of oxygen pressure compared with pure ZnO and MgZnO films, multilayer films show a better UV performance [21]. Among the previous MgxZn1xO/ZnO UV photodetectors, the bottom ZnO layer used as a buffer layer to improve the quality of the top MgZnO layer [22,23]. Some researcher focused the different Mg composition on the characteristics of MgxZn1xO/ZnO heterostructure and MgxZn1xO/ZnO quantum well photodetectors [24,25]. Although, multilayer films GaN/AlxGa1xN have been used in various PDs, but for growing the GaN and AlxGa1xN materials high temperature technology is required, such as molecular beam epitaxy and metal organic chemical vapor deposition systems [26e28]. Nevertheless, the fundamental PDs based on double-layer films have not attracted considerable attention that it deserves. So far, ZnO/MgZnO and MgZnO/ZnO double-layer films had been prepared by the magnetron sputtering method [24,29e31]. High quality ZnO films have been synthesised by ultrahigh vacuum systems, such as pulsed laser deposition (PLD) [32,33], molecular beam epitaxy (MBE) [34] and RF sputtering [35]. However, these processes are not suitable for synthesising large area films at a low cost. The sol - gel method has been used to synthesize various kinds of oxide materials from liquid chemical sources, which is known as a simple and economical technique to deposit large area oxide thin

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films [36,37]. Al-Hardan et al. [19] worked on Ag/ZnO/p-Si/Ag heterojunction and their optoelectronic characteristics under different UV wavelength illumination and reported responsivity 0.60 A/W. Shewale et al. [20] have studied photoelectric properties of Mg0.2Zn0.8O thin films and reported responsivity 0.22 A/W. There are very few reports published on MgZnO/ZnO multilayer thin films grown by solgel method for UV photo detection application. In this article, ZnO thin films were grown on glass substrates and then MgyZn1-yO (y ¼ 2 to 8 at. %) thin films were deposited on ZnO thin film by solegel method. All prepared samples were characterized by X-ray diffraction (XRD) and field emission-scanning electron microscopy (FE-SEM). After making the contacts of silver paste, the electrical and photoelectrical properties of all thin films samples were analyzed using IeV measurement, in dark and illumination conditions as ambient UV-light (365 nm). 2. Experimental Zinc acetate dihydrate [Zn(CH3COO)2$2H2O] (Alfa Asear) and magnesium acetate tetrahydrate [Mg(CH3COO)2$4H2O] (Alfa Asear) were used as the source of zinc and magnesium respectively. 2methoxy ethanol [CH3eOeCH2eCH2eOH, 99.5% purity] (Alfa Aesar) and monoethanolamine [C2H7NO] (Alfa Aesar) were used as solvent and stabilizer reagent respectively. Firstly, the precursor solution of ZnO 0.5 mol/l concentration was prepared by dissolving zinc acetate dehydrate in 2-methoxy ethanol at room temperature, then the monoethanolamine was added to this solution as a sol stabilizer and magnetic stirrer for 60 min. The transparent and homogenous solution was obtained and then filtered by Whatman's grade GF/A glass microfiber filter papers. The ZnO thin films were coated with the prepared solution on glass substrates using spin coater at the rotating speed of 2500 rpm for 30 s. A glass substrate was cleaned and degreased ultrasonically by keeping it for 10 min and then in acetone. After each coating, the as deposited films were dried at 200 C in the air for 10 min in a furnace on a hot plate to evaporate the solvent and remove organic residuals. The procedures for coating to drying were repeated ten times in order to obtain the desired thickness of film. The films were then inserted into a furnace and annealed in air at 450 C for 60 min. Moreover, the MgyZn1-yO (x ¼ 2, 4, 6 and 8 at.%, namely MZ-1, MZ-2, MZ-3 and MZ-4) thin films were also deposited on top of the ZnO film. After making contacts with silver paste the samples were annealed at 70 C for 1 h. The structural properties were studied using X-ray diffraction (XRD) (Philips X'pert Pro-diffractometer) with Cu Ka radiation. The surface morphology of the thin films were investigated using a scanning electron microscopy (SEM, EVO-40 ZEISS). The Optical transmittance spectra were collected using a UVeVis-IR spectrophotometer (Shimadzu UV-3600, Japan). An ultraviolet LED bulb with a wavelength of 365 nm and an intensity of 3.2 mW/cm2 was used as a UV light source. Electrical measurements were carried out using a Keithley 4200-Semiconductor Characterization System. 3. Results and discussion 3.1. X-ray diffraction analysis The phase identification and crystallinity of MgyZn1-yO/ZnO thin films were analysed by XRD in the scan range of 25 to 60 . XRD patterns of the MgyZn1-yO/ZnO double-layer thin films with varying Mg content from (2e8 at. %) are shown in Fig. 1. From, XRD pattern exhibits a strong dominated orientation at (002) plane of all thin films samples. This analysis revealed that all the films, having a hexagonal wurtzite crystal structure of JCPDS card no. 36-1451. No

Fig. 1. XRD patterns of the MgyZn1-yO/ZnO thin films (y ¼ 2, 4, 6, 8 at. % namely MZ-1, MZ-2, MZ-3 and MZ-4 respectively).

other peaks were obtained for any impurities corresponding to either magnesium, zinc or their complex oxide. This suggests that, all the Mg2þ ions would substitute into ZnO lattice without any defects [31]. Moreover, the diffraction peak hexagonal (002) position of the MgyZn1-yO/ZnO thin films shifted towards the higher angle side was attributed to the decrease in lattice spacing with increasing Mg content because Mg2þ ionic radius (0.057 nm) is less than Zn2þ ionic radius (0.060 nm) [24,46]. Similar result has also been obtained by Kim et al. [24] for MgZnO/ZnO heterostructures deposited by co-sputtering. The peak intensity ratio defined texture of crystal, which define domination of crystallite orientation in the films. The lattice parameters ‘c’ and ‘a’ shown in Table 1 were calculated corresponding to (002) and (100) peaks by using Eq. (1).

l2 1 4 2 2 h þ 2 ¼ þ hk þ k d2 3a2 c

(1)

where, d and hkl indicates plane spacing and miller indices, respectively [37]. In this work no structural changes were observed, all the thin film have wurtzite crystal structure with little variation in lattice parameters and size. The crystal size for the films was calculated by using Debye Scherer's formula,

D¼

0:94l bcosq

(2)

where, q is the Bragg diffraction angle, b is the broadening of the diffraction line measured at half of its maximum intensity (FWHM) in the XRD pattern and l is the X-ray wavelength (1.5418 Å). From Fig. 1, it has been observed that the values of the FWHM of the diffraction peaks decreased with increasing Mg contents, indicating that an increase in Mg content leads to increase in the crystalline quality of the MgZnO/ZnO thin films [24]. 3.2. Surface morphology The surface micrographs for MZ-1, MZ-2, MZ-3 and MZ-4 are shown in Fig. 2 respectively. The magnification for all samples MZ-

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Table 1 The structural and optical parameters of MgyZn1-y/ZnO thin films. MgyZn1-yO/ZnO Samples

Peak position (2q)

FWHM (b)

Lattice Parameter (a) (Å)

Lattice Parameter (c) (Å)

Crystallite size (nm)

Optical Band Gap (eV)

(MZ-1) (MZ-2) (MZ-3) (MZ-4)

34.87 34.95 34.98 34.94

0.2565 0.2437 0.2412 0.2384

3.204 3.197 3.194 3.198

5.141 5.130 5.126 5.131

32.46 34.17 34.52 34.93

3.30 3.33 3.34 3.36

Fig. 2. FE-SEM image of MgyZn1-yO/ZnO thin films (with y ¼ 2, 4, 6, 8 at.%).


1, MZ-2, MZ-3 and MZ-4 carried out at 25k and 50k respectively. It has been found that MZ-1 sample have round and oval shaped nanocrystalline surface morphology. Further increasing Mg content in ZnO other samples (MZ-2, MZ-3 and MZ-4) nanoflower and nanoplate like morphology were obtained. The nanosheets were connected to each other in the form of network-shaped nanosheet film. The nanosheets are mostly joined together, so they do not appear as regular hexagonal shape. The results show the successful synthesis of nanoflower and nanoplate like morphology of increasing Mg content on ZnO. At low doping in ZnO, the high surface energy of the polar (0001) plane lead to faster crystal growth in c-axis direction of ZnO nanostructure favouring the formation of round and oval shape structure along that direction. On the other hand, after increasing Mg content on ZnO with incorporation of Mgþ2 in ZnO crystal lattice, the adsorption of hydroxide anion increased on the polar plane (0001) due to higher q/ r ¼ 2/0.57 of Mgþ2 relative to q/r ¼ 2/0.60 of Znþ2, which leads to partial reduction of crystal growth in c-axis [50]. Or it might be due to the OH ions present in the growth medium for the formation of nanoflower and nanoplate [47]. Similar result was also obtained by Zamiri et al. [50] for Er doped ZnO nanoplates. It might be due to the OH ions present in the growth medium for the formation of nanoflower and nanoplate. Uniformity and roughness of the surfaces increased with the incorporation of Mg [28,38]. It is evident that surface morphology of film is dependent on concentration of solution prepared. This fact is in consistent with the results found in our XRD results. 3.3. Optical study The optical properties were analysed for MgyZn1-yO (y ¼ 2 to 8 at. %) thin films deposited on glass substrates without ZnO layer. The transmission spectra of MgZnO thin films were recorded in the wavelength range 200e800 nm as shown in Fig. 3 (a). Average transparency was found 80% in the wavelength range 400 nme800 nm for all thin films and transparency increased as the Mg contents was increased. With varying Mg content (y) increase in the transmittance may be due to increasing grain size [48]. The optical band gap of the MgZnO thin films are shown in Table 1 and calculated with Tauc's plot method (Eq. (3)) using transmittances by using the following Eq.

ðahyÞ

1=n

¼ A hy Eg

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band transition [35]. The band gap energies can be estimated by extrapolation of the linear portion of an (ahn)2 vs (hn) plot which is shown in Fig. 4 (b). Mia et al. [48] have done similar work on MgZnO thin films and reported effect of doping on optical band gap which increases as doping level increases. The bandgap increased from 3.25 eV to 3.29 eV as doping level increases. The increase in bandgap of MgZnO thin films, may be due to the defects introduced after Mg2þ substitutes for Zn2þ, and due to the difference in electronegativity and ionic radius between Zn and Mg [24], or by the fact that MgO (7.7 eV) has a wider band-gap than ZnO (3.20 eV) [7,16,49]. 3.4. Electrical properties and UV sensing measurements Fig. 4 shows the I-V characteristics of all samples in dark and UV illumination light (365 nm) at room temperature. The inset in Fig. 4 (a) shows a schematic diagram of sample prepared for I-V characteristics. The linear IeV curves were obtained between two Ag contacts both in the dark and photo-illuminated, demonstrating the Ohmic behaviour by Ag electrodes [29]. The linear IeV characteristics result is the proportionate linear correlations between the collected carriers and external electrical field, according to the simple model of photo-generation and collection [39e41]. The photocurrent were recorded under UV illumination power density i. e 3.2 mW/cm2. The measured dark and photo currents were approximately 429, 294, 250, 233 mA and 530, 365, 251, 236 mA at 4 V bias voltage corresponding to MZ-1, MZ-2, MZ-3 and MZ-4 samples, respectively. Thus, from the observed results, the dark as well as photo current decreases with increasing doping level and the sample MZ-1 exhibits highest photocurrent than other samples. The contrast ratio (CR) (the ratio of the photo Ip and dark current ID) were approximately 1.23, 1.24, 1.0 and 1.01 at 4 V bias voltage corresponding to MZ-1, MZ-2, MZ-3 and MZ-4 samples, respectively. The high photocurrent obtained on a MZ-1 sample compared to other samples may be due to its high crystallinity or grain size. Because of the higher crystallinity the film has more photogenerated electrons [42]. The decrease in photocurrent is attributed to grain boundaries and crystallinity of lattice. The responsivity (R) can be calculated as follows;

R¼

Iph Popt

(4)

(3)

where, Eg is the bandgap corresponding to a particular transition occurring in the film, A is the band edge constant, y is the transition frequency, and the exponent ‘n’ characterizes the nature of the

where, Iph is the photocurrent and Popt is the incident optical power at a specific wavelength. The device was illuminated with constant UV illumination of 3.2 mW/cm2. The measured responsivities at 5 V bias voltage were about 0.165, 0.114, 0.078 and 0.074 A/W,

Fig. 3. Optical parameters of MgyZn1-yO thin films (y ¼ 2, 4, 6, 8 at. %) (a) Transmittance (b) Tauc's plot for bandgap calculation.

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Fig. 4. IeV characteristics of the of MgyZn1-yO/ZnO thin films (y ¼ 2, 4, 6, 8 at. %) (a) dark (b) UV light (365 nm) (c) MgyZn1-yO/ZnO thin films (y ¼ 2 at.%) under dark and UV light (365 nm) (d)MgyZn1-yO/ZnO thin films (y ¼ 2 at.%) growth and decay of the photo response.

corresponding to MZ-1, MZ-2, MZ-3 and MZ-4 samples, respectively. Shewale et al. [20] worked on Mg0.05Zn0.95O thin films and reported responsivity 0.22 A/W. The responsivity of most commercial UV photodetectors lies in the range of 0.1e0.2 A/W [42]. Photoconductive response is considered as a main parameter for a photodetector. Therefore, to check the stability of the UV detection performance of the samples, switching on and off the UV light periodically at regular time intervals of 25 s shown in Fig. 4(d). The illumination of UV light on the samples, the photocurrent increases rapidly, tend to saturate and then decreases exponentially to its initial value after switching off the UV light. Therefore, the amplitude of obtained photoresponse curves remains almost stable under the UV illumination switching, suggesting their good stability. The mechanism for UV light detection can be explained from the above results. When the high-energy photons (hy>Eg) of UV light (365 nm) exposes on the MZ-1 sample, the photo-generated carriers were formed as electrons in the conduction band and as holes in the valence band due to band-to-band excitation [43,44]. The produced holes (hþ) then combine with negatively charged adsorbed oxygen species O2 to release the oxygen molecules from the surface. This causes an increase in the free carrier concentration and thereby increases the photocurrent during photo illumination. Further, when the UV illumination is turned-off, both the recombination of the electrone hole and re-adsorption of atmospheric oxygen molecules on the film surface decreases the carrier concentration, which results in the photocurrent decay [45]. Nevertheless, these transient photoresponse results confirmed the fast response and recovery property of MZ-1 sample. 4. Conclusion In this study MgyZn1-yO/ZnO thin films were grown on glass substrates by using solegel spin coating technique with varying Mg contents (y ¼ 2, 4, 6, 8 at. %). X-ray diffraction pattern shows the

wurtzite structure with a preferential c-axis (002) orientation. At the low doping level round and oval shaped microstructure were observed and on increasing Mg contents nanoflower and nanoplate like morphology were obtained. It is observed that the optical band gap of MgZnO films increases in the range of 3.25e3.29 eV with varying Mg content from 2 to 8 at. %. Thin films show high UV detection performance with higher responsivity of 0.165 A/W upon 3.2 mW/cm2 after illumination UV light (365 nm) at 4 V bias voltage. The observed results revealed that MgyZn1-yO/ZnO (y ¼ 2 at.%) thin films have a good nanocrystal structure and highest responsivity with a narrow band gap. These films may be used as an optical sensor in optoelectronic applications. Acknowledgments The authors acknowledge the Department of Science and Technology (DST), Govt. of India for support under FIST program. References [1] Z. Zhang, H. Wenckstern, M. Schmidt, M. Grundmann, Wavelength selective metal-semiconductor-metal photodetectors based on (Mg,Zn)O heterostructures, Appl. Phys. Lett. 99 (2011), 083502. [2] S.J. Pearton, F. Ren, Y.L. Wang, B.H. Chu, K.H. Chen, C.Y. Chang, W. Lim, J. Lin, D.P. Norton, Recent advances in wide bandgap semiconductor biological and gas sensors, Prog. Mater. Sci. 55 (2010) 1e59. [3] S.K. Sharma, S.P. Singh, D.Y. Kim, Fabrication of the heterojunction diode from Y-doped ZnO thin films on p-Si substrates by sol-gel method, Solid State Commun. 270 (2018) 124e129. [4] Y.H. Leung, Z.B. He, L.B. Luo, C.H.A. Tsang, N.B. Wong, W.J. Zhang, S.T. Lee, ZnO nanowires array p-n homojunction and its application as a visible-blind ultraviolet photodetector, Appl. Phys. Lett. 96 (2010), 053102. [5] M.R. Alenezi, A.S. Alshammari, T.H. Alzanki, P. Jarowski, S.J. Henley, S.R.P. Silva, ZnO nano disk based UV detectors with printed electrodes, Langmuir 30 (2014) 3913e3921. [6] Z. Alaie, S.M. Nejad, M.H. Yousefi, Recent advances in ultraviolet photo detectors, Mater. Sci. Semicond. Process. 29 (2015) 16e55. [7] T.K. Pathak, V. Kumar, J. Prakash, L.P. Purohit, H.C. Swart, R.E. Kroon,


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