ZnO multilayer transparent conductive electrode

Solar Energy Materials and Solar Cells 169 (2017) 122–131

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High-performance radiation stable ZnO/Ag/ZnO multilayer transparent conductive electrode ⁎

Vikas Sharmaa, Parmod Kumarb, Ashish Kumarb, Surbhia, K. Asokanb, , K. Sachdeva,c, a b c

MARK

⁎

Department of Physics, Malaviya National Institute of Technology, Jaipur 302017, India Materials Science Division, Inter-University Accelerator Centre, New Delhi 110067, India Materials Research Center, Malaviya National Institute of Technology, Jaipur 302017, India

A R T I C L E I N F O

A B S T R A C T

Keywords: Transparent conductive oxide Multilayer Ion irradiation Hall effect measurements X-ray photoelectron spectroscopy (XPS)

The present study reports the fabrication and performance of swift heavy ion (SHI) irradiated transparent conducting electrode (TCE) having oxide-metal-oxide multilayer structure; ZnO/Ag/ZnO (ZAZ) deposited at room temperature and exhibiting electrical and optical properties comparable to that of commercial ITO. Pristine and SHI irradiated films show a good stability of electrical parameters; sheet resistance, carrier concentration and mobility of charge carriers under temperature variation from 80 to 340 K. This shows that these films are stable and hence suitable for application in this temperature range. The sputter deposited ZAZ multilayer structure has sheet resistance 24 Ω/□ and average transparency 80% in the visible spectrum. The obtained values of sheet resistance and average transmittance after SHI irradiation of 100 MeV Ag ions with fluence 5E12 ions/cm2 are 27 Ω/□ and ~70%. Ion-induced sputtering of metal ions which get embedded in oxide layer creates centres for the scattering of light that leads to loss of ~10% transparency. The crystalline ZAZ structure shows smooth surface morphology and chemical stability before and after heavy ions treatment. The change in sheet resistance of the multilayer structure in the temperature range 80–340 K (δRS) is ~3.1 Ω/□ for pristine and 3.14 Ω/□ for SHI irradiated sample at a fluence of 5E12 ions/cm2. X-ray photoelectron spectroscopy (XPS) studies of both pristine and irradiated samples reveal that the middle layer is in metallic form and maintains its continuity even after ion irradiation.

1. Introduction Transparent conducting electrodes (TCE) are key components in a variety of optoelectronic devices and systems, including organic and inorganic solar cells, light-emitting diodes (LEDs) and flat panel or liquid crystal displays [1,2]. TCEs are expected to have low sheet resistance and high transparency in the visible spectrum. ITO, the mostly used material till now is facing feasibility problem due to its high cost, rarity, high-temperature synthesis process and poor mechanical properties [3]. The proposed alternatives are carbon nanotubes [4], graphene [5], conducting polymers [6], thin metallic films [7], metallic nanowires [8] and metallic grids [9]. However, these materials have limited applicability due to certain limitations. CNTs have low conductivity; graphene has many issues like difficulty of large area production, polymers are known for high resistance, metallic nanowires have a surface which is rough and metallic grids have smaller contact surface area with the active layer. These factors are very crucial for the performance of the device. So, the prime requirement is to find out an alternative TCE which has high transparency in the visible wavelength ⁎

region and excellent electrical properties at room temperature. Among metal oxides, ZnO is a prime candidate due to its wide band gap, better conductivity and high transparency of light in the visible wavelength region [10]. Native defects like oxygen vacancy (Vo), zinc interstitial (Zi) are present in ZnO film, affecting the optoelectronic properties of film [11]. A continuous ultrathin Ag layer shows high electrical conductivity (at room temperature) and small optical losses (i.e. less reflection and absorption of visible wavelength region in thin film form) in the visible range [12]. However, the bare metal film suffers oxidation issues which in turn degrade the electrical properties and stability in an open chemical environment. Therefore, it is a challenge to develop a low-cost novel transparent electrode containing excellent optical and electrical properties along with good chemical stability at room temperature. Recently, dielectric/metal/dielectric (DMD) multilayer structure has attracted much attention as a transparent electrode showing a combination of required properties viz. good optical transparency and high electrical conductivity, even when these layers are grown at room temperature [13–15]. In DMD structure, the sandwiched metal layer is

Corresponding authors. E-mail addresses: [email protected] (K. Asokan), [email protected] (K. Sachdev).

http://dx.doi.org/10.1016/j.solmat.2017.05.009 Received 26 February 2017; Received in revised form 19 April 2017; Accepted 7 May 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.

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deposition was 1×10−3 mbar for ZnO layers and 1.5×10−2 mbar for Ag layer. The RF power used was 100 W and 15 W for DC during sputtering. The flow of argon (Ar) was kept 15 sccm for the entire process and rate of deposition obtained for ZnO and Ag films were 3 Å/ sec. and 7 Å/sec, respectively. The target to substrate distance was 100 mm and the deposition was done at room temperature (300 K) without external substrate heating. The prepared trilayer (ZnO/Ag/ ZnO) structure, (hereafter referred as ZAZ) were irradiated with 100 MeV Ag7+ ions of different fluences ranging from 5E11 to 5E12 ions/cm2. ZAZ multilayers were then investigated for electrical properties using ECOPIA-5000 low-temperature Hall Measurement system in the temperature range from 80 to 350 K. The optical transmittance of ZAZ multilayers was measured using UV–Vis spectrophotometer without integrating sphere LAMBDA 750 (Perkin Elmer) with a bare glass substrate as a reference in the spectral range 300–800 nm at room temperature. The crystalline properties of as-grown ZAZ and irradiated ZAZ multilayers were determined using X-pert Pro Pan Analytical X-ray diffractometer (XRD) with CuKα radiation. Nova Nano FESEM 450 and atomic force microscope (NanoscopeIIIa) were used to examine the surface features of the pristine and SHI irradiated ZAZ multilayer films). Thickness evaluation was done using Rutherford backscattering (RBS) technique with 2 MeV He+ ion beams at Inter University Accelerator Centre, New Delhi (India). Photoluminescence spectra were observed with 325 nm laser to study the defect behaviour of multilayer films. Detailed investigations on chemical state of elements and interface stability in the multilayer structures were carried out using X-ray photoelectron spectroscopy (XPS, Omicron ESCA) measurements. Monochromatic Al Kα (1486.7 eV) source with a mean radius of 124 mm with monochromatic X-ray resolution of 0.6 eV was used at 3×10−10 mbar pressure in the chamber.

mainly responsible for the charge transport phenomena, and transmittance is enhanced by the dielectric layer having a high refractive index. In addition, the work function of these DMD electrodes could be tuned by selecting the dielectric material as per the requirement to be used as a cathode, anode or an intermediate electrode [16,17]. Due to these advantages, many studies based on DMD structured transparent conducting electrodes (TCE) such as ITO/Ag/ITO, GZO/Ag/GZO, ZTO/Ag/ ZTO, ITO/Au/ITO, and ZnO/Au/ZnO have been carried out [18–22]. Kim et al. have used ZAZ multilayer structure on PEN substrates for flexible electronic applications. However, flexible substrates are always problematic for ion irradiation studies [23]. Chiu et al. have used ebeam technique for the growth of TiO2/Ag/SiO2 for TCO applications [24]. However, stoichiometric and uniform growth production (which are the prime requirement for industrial applications) is not possible with ion assisted e-beam deposition. In this context, sputtering is widely preferred technique for uniformity in the films and large area production. In order to realise high transparency and conductivity in the DMD multilayer electrode, it is necessary to deposit a continuous metal layer sandwiched between the two dielectric layers. The growth of ultrathin metal layer deposited on bottom dielectric layer follows the island or Volmer-Weber growth mode, normally observed by many systems of metals when grown on insulators. In this mode, nucleation of small clusters take place directly on the surface of the substrate; these clusters then grow into islands which further coalesce to form continuous thin films. Sahu et al., have optimized the Ag thickness in ZAZ multilayer structure and found that 8 nm thin sandwiched metal layer exhibits superior optical and electrical properties [25]. Other researchers have also claimed that Ag film becomes continuous for a thickness of 6–12 nm [26–28] and provides excellent optical and electrical properties. Based on these observations, it is inferred that thickness and structure (i.e. morphology and continuity) of the embedded metal layer plays a major role in determining the final properties of the DMD structure. Several methods have been attempted to enhance the electrical conductivity and transmittance of pure ZnO thin film [29] and ZnO/ Ag/ZnO (ZAZ) DMD structure [30,31]. The cost of thin film can be estimated with base materials and required amount (thickness) accordingly. Compared to ITO, ZnO based electrodes are expected to be cheaper and cost effective. Moreover, the fabrication process for ITO needs high temperature treatment compared to ZAZ which is done at room temperature. However, to the best of our knowledge, the stability of these electrodes under temperature variation and in radiation environment have not been reported; an essential requirement to test the performance from a practical point of view. The ZAZ based electrodes may also have potential applications in space electronics as an alternative of ITO based electrode. It is well known that swift heavy ion (SHI) irradiations can drastically modify the structural, optical and electrical properties of the materials [32]. There are only few reports to study the response of standard TCE materials under SHI irradiation. Singh et al. have found the rapid degradation in the optical properties (transmittance) of ITO and FTO films with SHI irradiation [33]. These films were investigated for stability under SHI irradiation of Ag ions with different fluences to understand their behaviour in a dense radiation environment. This motivated us to investigate the electrical behaviour of these films under variation of temperature from LN2 to room temperature.

3. Results and discussion 3.1. X-ray diffraction Fig. 1 shows the XRD patterns of pristine and SHI irradiated ZAZ multilayer structure. It is evident that the pristine layered structure consists of only (002) diffraction peak corresponding to the c-axis oriented hexagonal crystal structure of ZnO (JCPDS (36-1451)). The growth of the film in preferred direction supports the stoichiometric nature of ZnO. The calculated crystallite size using Scherer's formula (0.89λ/βCosθ) is ~7.2 nm for pristine multilayer structure. XRD patterns of 100 MeV Ag7+irradiated layered structure with ion fluences (5E11to 5E12 ions/cm2) consist of two peaks; one corresponding to hexagonal ZnO while the other peak is related to the face-centred cubic (FCC) Ag (111) corresponding to JCPDS (04-0783). The crystallite size is showing an increase for ZnO and Ag up to fluence value of 1E12 ions/

2. Experimental details The dielectric layer of ZnO (40 nm) was deposited on glass (Sodalime glass) substrates at room temperature by radio frequency (RF) magnetron sputtering using 99.99% pure target. Ag layer was then deposited on ZnO dielectric layer by DC sputtering using 99.99% silver target. Finally, ZnO (40 nm) top layer was deposited on the Ag/ZnO/ glass structure again by RF magnetron sputtering technique. The base pressure of the chamber was 5×10−6 mbar and pressure during

Fig. 1. X-ray diffraction pattern of pristine and SHI irradiated ZAZ stacked multilayer structure for different fluences.

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curves and found to be ZnO (40 ± 5 nm)/Ag (9 ± 1 nm)/ZnO (40 ± 5 nm). This suggests the formation of ZnO with pure Ag in the sandwich layer with diffusion occurring at the interfaces. During the sputtering process, deposition of high energetic atoms leads to diffusion of few Ag atoms in ZnO layers. Through RBS, it is seen that SHI irradiation causes an increase in diffusion layer. This indicates that high energetic ions transfer their energy to the host multilayer leading to an increase in mixing or diffusion in multilayer [38].

Table 1 Various parameters calculated for ZnO and Ag from XRD pattern. Samples details

Pristine 5E11 1E12 5E12

ZnO Parameters

Ag Parameters

Crystallite size (nm)

Lattice Parameter (Å)

Stress

Crystallite size (nm)

Lattice Parameter (Å)

7.2 15.4 16.5 12.9

5.261 5.235 5.234 5.217

−4.792 −2.483 −2.475 −0.9414

– 12.32 16.48 9.19

– 4.049 4.049 4.051

3.3. Morphological investigations Pristine and SHI irradiated ZAZ multilayer structure show smooth morphology of the top surface. Regularly arranged circular particles are seen in Fig. 3. The pristine layered structure (Fig. 3a) consists of welldefined continuous particles of nearly equal size completely covering the surface which is smooth in nature. On irradiation, these welldefined continuous particles rearrange themselves and exhibit a change in morphology. XRD measurements confirm grain growth for lower fluence values. Narrow voids are seen to be created by the passage of ions along the path of the ion beam for the highest fluence as shown by SEM image. It can be argued on the basis of existing literature that two different mechanisms (either surface diffusion or sputtering) occur on irradiation [34,39]. When the energy of incident ions is not sufficient to excite the atoms from the surface, then the atoms diffuse causing surface diffusion. On the other hand, electronic sputtering takes place (atoms can leave the surface) for irradiation by energetic ion possessing higher energy than surface binding energy and usually occurs at higher fluences. Due to sputtering, atoms are knocked out from the surface resulting in the decrease in crystallite size (at 5E12 ions/cm2). At higher fluence value, the overlap of ion tracks promotes disorder in the lattice [40]. Fig. 4 illustrates AFM images for pristine and irradiated multilayer thin films. The images confirm the formation of continuous and smooth surface for all samples. These images of films for different fluences demonstrate nearly same morphology of grains. The calculated roughness of pristine multilayer is ~2.97 nm, which continuously decreases with ion fluences and reaches a minimum value of ~2.48 nm for the highest fluence. The high-energy ions with heavy mass act as a cutting tool for the hikes and irregularities at the surface of the deposited thin film and hence irradiation leads to reduction in surface roughness with ion fluences. The height and size of grains present on the top surface of layered structure change with irradiation. All images show that there is no large or continuous cluster formation at the surface of top ZnO layer deposited on the Ag layer. Formation of smooth films with root mean square roughness (RMS) value of ≈2.5 nm, (close to ITO films ≈2–3 nm) is quite important for TCEs because high surface roughness of an electrode allows direct current to flow between cathode and

cm2 (see Table 1). The increase in the crystallite size for both ZnO and Ag is attributed to the annealing effect induced by the ion beam irradiation and may be explained using thermal spike model [34,35]. Energetic ions deposit a significant amount of heat energy through collisions in a small volume of material, thereby increasing the temperature. This energy is distributed among the electrons and consequently to lattice atoms through the electron-lattice coupling. Deposition of such high energy raises the lattice temperature appreciably along the ion tracks within a very short duration of time (∼10−14 s) and quenches rapidly. The rise and fall of temperature within a short interval of time generates pressure waves which in turn modifies the crystal structure [34]. At lower fluence, the energy of incoming ions imparted to the system releases strain among the grains that cause improvement in crystallinity. The incoming swift heavy ions transfer sufficient energy to the grains that lead to the modification of morphology of grains. For the highest fluence, the reduction in grain size is associated with the fragmentation produced by strain. SHI irradiation induced modifications in crystallinity have been reported by various groups earlier [35]. It is also observed that peak corresponding to ZnO (002) shifts systematically towards lower angles. The shift in diffraction peaks lead to relaxation in residual tensile strain [10] and decrease in lattice parameter, estimated using the relation c=λ/sin θ [36]. When highly energetic ions pass through the material, a large number of defects are generated due to the dominance of electronic energy loss. These defects, in turn, induce stress in the system and might be responsible for the decrease in lattice constant.

3.2. Rutherford backscattering analysis Fig. 2 shows the RBS spectra, and RUMP simulated profiles of ZAZ multilayer structure for pristine and SHI irradiated films [37]. Distinct elemental peaks associated with Zn, O and Ag ions are observed. Thicknesses of various layers were calculated with the help of the fitted

Fig. 2. RBS spectra (experimental and fitted) of pristine and SHI irradiated ZAZ stacked multilayer structure for different fluences.

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Fig. 3. SEM images of pristine and SHI irradiated ZAZ multilayer structure with different ion fluences.

structure. Elemental mapping is shown in Fig. 5(c-f) which shows a uniform distribution of elements Zn, Ag, and O in the films. It indicates homogeneity among layers of the multilayer structure which is an essential feature for TCE application [44,45].

anode [41]. This evolution of morphology (with slight roughness and continuous growth) of a multilayer structure with SHI irradiation favours the use of the present layered structure as a transparent electrode for optoelectronic devices in radiation environment [42]. Multilayer ZAZ films deposited on TEM grids were used to analyse the microstructure. These images show a layered deposition with distribution of different particle sizes. High-resolution images from TEM were analysed by digital micrograph software to identify crystallographic planes of top ZnO layer which are (002) and (101) as indicated in Fig. 5(a) [43]. The SAED pattern exhibits different diffraction planes of ZnO and Ag reflected as concentric rings confirming the polycrystalline structure of ZnO and Ag in the multilayer

3.4. X-ray photoelectron spectroscopy XPS measurements were carried out on ZAZ multilayer structure to analyse the chemical states of the elements. Depth profiling using etching was done for analysis of the three layers. XPS results confirm the presence of Zn, O and Ag ions at respective layer positions and the effect of SHI irradiation.

Fig. 4. AFM images of pristine and SHI irradiated ZAZ multilayer structure.

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Fig. 5. (a) Pristine ZAZ multilayer structure, (b) HRTEM image (c) SAED pattern. Elemental image mappings for the (c) total, (d) Ag, (e) Zn and(f) O.

the oxygen vacancies or the oxygen-deficient regions within the ZnO matrix. Fans et al. have de-convoluted the O 1 s peak into two main components as the OI and OII of O2- ions. The higher binding energy peak (OI) at 531.6 eV is associated with oxygen vacancy and OII is related to six nearest neighbor ions. Similarly, Heo et al. have attributed the higher binding energy peak to the oxygen vacancies. They have reported that there is significant change in the oxygen vacancies by varying the oxygen partial pressure [51,52]. An extra peak is observed at 529.0 eV after irradiation, due to formation of Ag2O [53]. The intensity of Ag2O peak is decreased with increased ion fluence due to the mixing of Ag in ZnO sandwich layer. Furthermore, the intensity of peak towards higher binding energy side decreases with SHI irradiation as compared to pure multilayer structure. This indicates that oxygen vacancies which in turn are responsible for the modification of optical properties (such as % transmittance, refractive index, etc.) change as the ion fluence increases [44,22,54]. The spectra for the embedded Ag layer were obtained after etching of top ZnO layer and shown in Fig. 6(d). Two peaks centred at 367 and 372.9 eV are observed in the spectra attributed to Ag 3d5/2 and Ag 3d3/ 2, respectively. A remarkable shift in peak positions of Ag 3d towards the lower binding energies as compared to bulk Ag (Ag3d5/2 at 368.2 eV; Ag 3d3/2 at 374.2 eV) is observed for thin film. The shift is due to transfer of electrons from metallic Ag to ZnO crystal as a result of band bending at the interface to match the Fermi levels. The metal atoms come into contact with the semiconductor oxide at the interface to form ZnO-Ag heterostructures. When Ag layer (work function=4.26 eV) comes in contact with ZnO layer (work function=5.3 eV), electron transfer takes place from Ag to ZnO at the interface of ZnO-Ag. This transfer of electrons results in producing higher charge state in Ag. The monovalent Ag has lower binding energy as compared to zerovalent Ag leading to a shift in binding energies of Ag 3d5/2 and Ag 3d3/2 [55–57].

Fig. 6(a) represents the survey scan of the top layer of the multilayer after cleaning by Ar sputtering. It consists of peaks corresponding to Zn, O and surface C. The binding energy scale was calibrated by considering C 1s peak at 284.6 eV [46]. The curve shows that the trilayer maintains its stability even after SHI irradiation as there is no Ag metal on the top from the middle layer. The smooth and homogenous dielectric layer is retained over the continuous mid metal layer ensuring the stability of the multilayer structure. Elemental analysis for Zn 2p and O 1s edges performed on the top surface of multilayer are shown in Fig. 6(b-c). The peaks corresponding to Zn 2p3/2 and Zn 2p1/2 shown in Fig. 6(b), are present at ~1022.1, and 1045.2 eV respectively. The binding energy difference of Zn 2p3/ 2and Zn 2p1/2 is found to be ~23.1 eV which is in accordance with the standard reference value reported for binding energy of Zn in ZnO [47,48]. It can be inferred from the peak positions and their difference values that Zn ions are present in Zn2+ chemical state on the top surface of the multilayer. A very small shift occurs in the two Zn peak positions after SHI irradiation, exhibiting stability of Zn chemical state. The shift in Zn 2p3/2 towards low binding energy indicates an increase of metallic Zn in the specimen on irradiation giving it a degenerate behaviour [10]. Fig. 6(c) shows the shape and position of the peak corresponding to O 1s in pristine and irradiated thin films from the short scan taken at the top surface. The asymmetric O1s profile for pristine thin film is deconvoluted into two symmetrical peaks at 530.0 and 531. 7 eV. This indicates that there are two different kinds of O species present in this film. The lower energy side peak at ~530 eV is associated with the O2ions surrounded by Zn2+ ions in the hexagonal wurtzite structure of ZnO [49,50]. It is argued that the intensity of this peak accounts the number of oxygen atoms present in the fully oxidized stoichiometric host ZnO lattice. On the other hand, higher energy side peak centred at ~531.7 eV can be attributed to the presence of either oxygen vacancies or hydroxyl group/absorbed oxygen. Based on the existing literature and current findings, the higher energy peak might be associated with 126

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Fig. 6. (a). XPS results of pristine and irradiated ZAZ multilayer structure at the top surface (survey scan). (b). XPS results of pristine and irradiated ZAZ multilayer structure of the top surface (selected region for Zn). (c). XPS results of O 1s of the pristine and irradiated ZAZ multilayer structure at the top surface. (d). XPS result of pristine and irradiated ZAZ at middle layer (elemental scan for Ag 3d XPS).

duct Number: 636908, CAS Number: 50926-11-9) by Sigma-Aldrich has a value of optical transparency of ~80% (with Sheet Resistance Rs 30–70 Ω/sq and refractive index 1.517). Hence these films show comparable results of ITO films and enhanced performance in terms of variation in transmittance value as compared to results given for TCO; ITO, ITO/Au/ITO, and ITO/Cu/ITO by Lee et al. [58] and Au-

3.5. Optical properties The optical measurements of pristine and irradiated stacked multilayer done using UV–Vis spectrophotometer are presented in Fig. 7(a) and exhibit average transmittance between ~70–80% for all the films in the visible spectral range. Commercially available ITO slide ((Pro-

Fig. 7. Pristine and SHI irradiated ZAZ multilayer structure (a) Transmittance spectra and (b) optical band gap from Tauc's plot.

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Fig. 8. Electrical properties of pristine and SHI irradiated ZAZ with temperature variation.

recovers and becomes ~82%. One factor responsible for this increase in transparency at highest fluence could be the least value of surface roughness giving a highly smooth surface leading to less absorption [62,63]. Tauc's formula has been used to obtain optical band gap of multilayer by plotting (αhν)2 v/s energy (hν) as shown in Fig. 7(b) [64]. The pristine multilayer has lesser bandgap (~3.27 eV) than the bulk or single layer ZnO. A subtle change is observed in the band gap of the multilayer on irradiation. Normally irradiation leads to decrease in band gap improving the electrical properties of the TCE. The bandgap determined using Tauc's plot is found to vary from 3.27 to 3.24 eV on irradiation up to 1E12 ion/cm2 fluence due to the increase in grain size. While at highest ion fluence, the band gap exhibits a reverse trend. The band gap increases to 3.28 eV for 5E12 ion/cm2 irradiated multilayer structure. Such behaviour is understood on the basis of the fact that for lower fluences, the annealing takes place (based on thermal spike model) while more of sputtering occurs for higher fluences which may produce more defect states. It is to be noted that variation in the bandgap is not significant. Defect and electronic transition induced photoluminescence (PL) spectra in ZnO have been reported earlier. Here, the PL spectra consist of an intense peak in the UV range and a broad peak ~525 nm in the visible spectrum (Supplementary Material, Fig. S1). The UV emission represents the near band emission (NBE), free charge recombination and band to band transition. The visible (yellow and green) emissions are due to O and Zn vacancies or deep level transitions, and the Zn interstitials generate violet emission. Other optical constants such as refractive index and extinction coefficient

added F-doped SnO2 by Chew et al. [59]. It is desirable that the electrode should show the least variation in its optical properties when used in a radiation environment and hence these films are suitable. Fig. 7(a) shows the transmittance spectra of pristine and irradiated films for visible spectrum 300–800 nm. In the multilayer structure, if the material possesses high refractive index as compared to the substrate, the transmittance of short wavelength radiation increases. Secondly, the screening effect of the bound electrons results in an improvement in transmittance with an increase in energy region. Also, the free charge carrier absorption is significant at low energy region that leads to low transmittance [60,61]. Our heterostructure is showing the same trend (low value at higher wavelength) for the optical transmittance in the visible wavelength. The previous study on heterostructures show that transmittance of Ag layer itself is lesser than it's transmission when it is in combination with metal- oxide layer due to reduction in the reflectance from metal layer [13,38,42]. The pristine ZAZ film shows a high transparency of ~90% which decreases with increasing wavelength. The transparency values fall for the irradiated multilayer samples (for fluence ranging from 5E11 to 5E12 ion/cm2). Substrate effect is excluded from transparency measurements. The irradiated samples show a maximum average transmittance of ~80% over the visible light spectrum. Initially, the transmission decreases on irradiation as sputtering of metal ions from the metal layer and their subsequent deposition in the metal oxide layer creates scattering centres for light. The defects generated in the oxide layer by ion irradiation are also increasing the absorption that results in the reduction of transparency. At highest fluence, the transparency again 128

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have been calculated. A small variation is seen in the values of these optical constants after irradiation (Supplementary Material, Figs. S2 and S3). It can be concluded that the multilayer shows a very good stability and a limited degradation in the radiation environment thus enhancing the suitability and applicability in optoelectronic devices which are most likely to bear such conditions [18,65]. 3.6. Electrical properties One of the main requirements for a transparent electrode for optoelectronic devices is high electrical conductivity with adequate charge carrier number and mobility. In the quest for development of an alternative transparent electrode, it is a prime concern to get the electrical properties comparable to the commercially available ITO electrode. The complete optoelectronic device must be stable under application in a wide temperature range and harsh radiation conditions. The films were tested for electrical properties using Hall measurement technique with electrical resistivity measurements done in Van der Pauw configuration. Fig. 8(a-d) shows the change in sheet resistance, carrier concentration, and charge mobility as a function of the temperature (80–350 K). The values of room temperature resistivity and sheet resistance shown by pristine multilayer were 2.1×10−4 Ω cm and 24 Ω/sq., respectively. These are quite good when compared to the commercial ITO slide (Product Number: 636916, CAS Number: 5092611-9) from Aldrich and other reports (Sahu et al.) [25]. Lee et al. [23] has reported better value of resistivity (31.2×10−5 Ω cm) for ZAZ films. [21]. The conductivity of the multilayer is primarily due to the metal interlayer sandwiched between the two-dielectric metal oxide layers forming a DMD structure and it is essential that the metal film should have a critical thickness so as to be continuous to provide a pathway for electrical conduction. The overall resistance of the multilayer can be derived using a coplanar configuration. The total resistance of this coplanar configuration is generally given by a parallel combination of resistance as: 1/ Rtotal=1/Rmetal+2 /Roxide with Roxide > 1000Rmetal, so the Rtotal≈Rmetal [13,38]. Hence the conductivity comes entirely from the embedded metal film and gives a very low value of resistivity as compared to single layer TCO. The values obtained for room temperature resistivity and sheet resistance values for stacked layered structures indicate that the inner metal layer is continuous for these multilayers. It has been reported that the electrical properties mainly conductivity depends on the sandwiched metal layer [58]. However, the role of oxide layer is also important to explain the other parameters like mobility and carrier concentration of charges. In the ZAZ structure, where two different work function materials Ag (4.4 eV) and ZnO (5.3 eV) come into contact, electrons are transferred from Ag to ZnO to achieve Fermi level alignment at interface as shown in Fig. 9. The behaviour of metal oxide is not purely semiconducting due to diffusion of metal into the semiconductor at the time of deposition of the multilyer and sputtering with high energetic ions later. This structure creates the metal-semiconductor heterojunction with an accumulation layer at the interface resulting in an electron flow between Ag and ZnO. According to Schottky theory, high charge carrier concentration is found in ZAZ structure [25,66]. In degenerate semiconductors, both resistivity and sheet resistance increases with the temperature. An increased concentration of metallic dopant induces metallic behaviour in the semiconductor. Numerous studies have reported similar behaviour of degenerate semiconductors e.g. n-type, Sb-doped SnO2/Ag [67], ZnO doped with Ga [68], B-doped ZnO films [69], Sb-doped p-type ZnO [70]. In the present study, the behaviour of variation of sheet resistance with temperature is due to doping effect (Ag atoms into ZnO). The mobility of charge carriers in ZAZ structure is given by the mobility of that in ZnO and is simultaneously a function of thickness of silver layer. Many scattering mechanisms are considered to explain the mobility behaviour including interface, grain boundary, lattice vibra-

Fig. 9. Symmetric of energy band structure of Ag and ZnO in ZAZ structure (I) before contact and (II) after contact.

tions and ionized impurity scattering [66]. In a degenerate (n≥1019 cm-3) multilayer structure, the charge carriers (n) and mobility (µ) are independent of temperature and scattering from ionized impurities and interface are the main mechanism for the scattering of charge carriers. Generally, less variation in µ and decrease in charge carriers with an increase in temperature suggest the absence of thermally activated conductance and less grain boundary scattering [10]. In our specimen, at low temperature in pristine and irradiated samples, the µ and n do not vary with temperature but follow the trend due to ionized impurity scattering, and at high temperature, there is no systematic behaviour [14]. Hence, it can be concluded that samples show both the degenerate and non-degenerate behaviour implying the role of ionized impurity and interface/ grain boundary scatterings. However, these heterostructures cannot be explained with a single mechanism. Therefore, we have to consider the combined effect of these mechanisms to understand the electrical properties in stacked multilayers. With irradiation, trapping of carriers takes places in defects leading to decrease in charge carrier with temperature (T) in the multilayer structure. The complete mechanism for the stacked multilayer structure is not clear till date due to critical thickness, interfaces involved, doping and charge transfer for Fermi level alignment etc. Fig. 10 shows the variation in electrical properties of ZAZ samples for different fluences of SHI irradiation. There is an increase in the sheet resistance and decrease in carrier concentration with increase in fluence. Mobility is also found to increase for higher fluences. Defects viz vacancies, interstitials are incorporated inherently in the pristine film during deposition. When these films are irradiated intentionally with heavy energetic ions, it creates point and complex defects sites in the ZnO lattice which act as charge trap centres [10]. Due to trapping of charge carriers at defects sites, n is reduced in the multilayer. Decrease in the number of charge carriers due to ion beam irradiation might increase the resistance. Further, the reduction in total number of charge carriers might decrease the effective collision time resulting in increased drift velocity and hence mobility. The increase in crystallite size also supports the enhancement of µ in the multilayer structure. The difference in room temperature sheet resistance (δRs≈3 Ω/sq.) between pristine and sample irradiated with the highest fluence is still very less and negligible compared to the variation in other wide band

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Fig. 10. Electrical properties of pristine and SHI irradiated ZAZ at room temperature.

gap materials irradiated with similar ion fluences [71]. The calculated figure of merit for ZAZ specimen shows small variation with irradiation (Supplementary Material, Fig. S4). 4. Conclusion A multilayered ZAZ structure was fabricated on a glass substrate using RF-DC sputtering method. These films were investigated for its morphological and chemical stability as TCO under SHI irradiation. XRD studies show crystalline structure of the multilayer even after irradiation. An important observation to be highlighted here is that the films are not amorphized for irradiation conditions used in the present investigation. AFM and SEM measurements inferred the growth of smooth surface having very low roughness. This particular trait makes them useful for growing further layers on top for device fabrication. XPS study indicates metallic Ag layer and irradiation generated defect embedded metal oxide layer after SHI irradiation. It is observed that electrical properties do not change much in the temperature range (80–350 K) in terms of sheet resistance, mobility and carrier concentration. The results of the current investigation show that it is possible to tune the electrical and optical properties of these multilayers using irradiation. However, as no drastic change is seen in the electrical and optical properties of the multilayer TCE under irradiation. It can be concluded that one can use such ZnO-based hybrid thin films in radiation harsh environment for various optoelectronic applications. Acknowledgement The authors acknowledge the support of Inter-University Accelerator Centre (IUAC), New Delhi (India), Materials Research Center (MRC), Malaviya National Institute of Technology Jaipur (India), and Wide Band Gap Semiconductor Laboratory, IIT-New Delhi (India) for providing the experimental facilities for this investigation. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2017.05.009. References [1] K. Ellmer, Past achievements and future challenges in the development of optically transparent electrodes, Nat. Photonics 6 (2012) 809–817. [2] T. Minami, Transparent conducting oxide semiconductors for transparent electrodes, Semicond. Sci. Technol. 20 (2005) S35. [3] A. Kumar, C. Zhou, The race to replace tin-doped indium oxide: which material will

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