AZO Sandwich Film with Ultralow Optical Loss

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Surfaces, Interfaces, and Applications

A High-quality AZO/Au/AZO Sandwich Film with Ultra-low Optical Loss and Resistivity for Transparent Flexible Electrodes Hua Zhou, Jing Xie, Manfang Mai, Jing Wang, Xiangqian Shen, Shuying Wang, Lihua Zhang, Kim Kisslinger, Hui-Qiong Wang, Jinxing Zhang, Yu Li, Junhong Deng, Shanming Ke, and Xierong Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00685 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Applied Materials & Interfaces

A High-quality AZO/Au/AZO Sandwich Film with Ultra-low Optical Loss and Resistivity for Transparent Flexible Electrodes Hua Zhou,†‡ Jing Xie,† Manfang Mai,§ Jing Wang,♀ Xiangqian Shen,*# Shuying Wang,# Lihua Zhang,ǁ Kim Kisslinger,ǁ Hui-Qiong Wang, ∆ Jinxing Zhang,♀ Yu Li,† Junhong Deng,☼ Shanming Ke*† and Xierong Zeng† †

Shenzhen Key Laboratory of Special Functional Materials, College of Materials Science and

Engineering, Shenzhen University, Shenzhen 518060, China ‡

Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and

Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China §

School of Physics and Optoelectronic Engineering, Foshan University, Foshan 528000, China

#

School of physics and technology, Xinjiang University, Urumqi 830046, China

ǁ

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York

11973, USA ∆

Fujian Provincial Key Laboratory of Semiconductors and Applications, Collaborative

Innovation Center for Optoelectronic Semiconductors and Efficient Devices, Department of Physics, Xiamen University, Xiamen 361005, P. R. China; Xiamen University Malaysia, Sepang 43900, Malaysia ♀

Department of Physics, Beijing Normal University, 100875 Beijing, China

☼

Department of Materials Science and Engnieering, Southern University of Science and

Technology, Shenzhen 518060, China

ACS Paragon Plus Environment

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KEYWORDS: sandwich film, transparent flexible electrodes, mica, interference behavior of light, film heater

ABSTRACT: Transparent flexible electrodes are ever-growing demands for the modern stretchable optoelectronic devices, such as display, solar cell and smart window. Such sandwichfilm-electrodes deposited on polymer substrates are unattainable due to the low quality of the films, inducing the relatively large optical loss and resistivity as well as a difficulty in elucidating the interference behavior of light. In this communication, we report a high-quality AZO/Au/AZO sandwich film with excellent optoelectronic performance, e.g., an average transmittance of about 81.7% (including the substrate contribution) over the visible range, a sheet resistance of 5 Ohm/sq and a figure of merit factor of ~55.1. These values are well ahead of those previously reported for sandwich-film-electrodes. Additionally, the interference behaviors of light modulated by the coat and metal layers have been explored with the employment of transmittance spectra and numerical simulations. In particular, a heater device based on AZO/Au/AZO sandwich film exhibits high performance such as short response time (~5s) and uniform temperature field. This work provides us a deep insight into the improvement of the film quality of the sandwich electrodes and the design of high-performance transparent flexible devices by the application of flexible substrate with an atomically smooth surface.

■ INTRODUCTION In order to meet the rapidly growing market demanding of display technologies, smart windows, solar cells, and wearable devices, the exploration of high-performance transparent


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flexible electrodes is highly desired. However, it is a big challenge to achieve flexible electrodes with high transmittance, low resistivity, excellent mechanical flexibility as well as low production cost.1 Although Indium-tin oxide (ITO) thin film exhibits high transmittance (~90%) and low resistance (~10 Ohm/sq), it intrinsically shows poor mechanical flexibility and limited availability.2 As an alternative, many researchers have devoted great efforts to investigate nanostructure materials, for instance, graphene,3-5 networks of carbon nanotubes,6-8 and metal nanowires.9-11 Nonetheless, there appear some new drawbacks such as low conductivity in carbon-based electrodes, local resistance and poor chemical stability in nano-structure metal electrodes. Recently, an effective strategy of fabricating an oxide/metal/oxide (O/M/O) sandwich structure has been proposed,12-27 which not only shows good chemical stability and uniform resistivity, but also can be manufactured at large-scale with low production cost as well as compatible with ITO production technology. In spite of great potential in the application of transparent flexible electrodes, the quality of O/M/O sandwich thin films still needs to be improved. To date, the undercoat layer of the O/M/O sandwich deposited on silica or glass is normally polycrystalline or even amorphous,19-24 which usually results in the 3D growth mode of the metal layer. If the O/M/O sandwich is deposited on polymer flexible substrates, the quality of the thin films becomes even worse.23-26 That is, it is easy to induce enormous interfaces and grain boundaries in the O/M/O sandwiches, especially on polymer flexible substrates, severely suppressing the optical and electronic properties of the electrodes and deteriorating the stability and efficiency of the devices. Zhang et al.28 and Zou et al.29 proposed novel approaches including chemical doping and interfacial engineering to improve the film quality of the metal layer. However, the achievement of high-quality coating layers for the sandwich structures, especially single crystal films, is still up in the air, which also


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seriously affects the growth mode of the metal layer. Furthermore, the interference behavior of light across the O/M/O sandwich structure is difficult to be elucidated due to the enormous interfaces and grain boundaries. Recently, single crystal Mica has been suggested as a flexible substrate,27,

30-31

which can

replace the conventional flexible substrate because of its excellent optical and mechanical properties, as shown in Figures S1 and S2 in Supporting Information, respectively. In particular, Mica can be obtained by mechanical exfoliation, as demonstrated in Figure 1(a), giving an atomically smoothed surface (see the discussions later), which is in favor of preparation of highquality film. Figure 1(b) shows the corresponding atomic model of mechanical exfoliation. It is well known that the lattice mismatch and crystal symmetry are two important elements for the epitaxial thin-film growth. The in-plane symmetry of the exfoliation (001) plane is six-fold, as illustrated by the inset of Figure 1(b) (corresponding to the amplified image of the blue area from the side view, as marked by the dashed black arrows), showing blue hexagonal area, and hence it is preferred to prepare the films with wurtzite structures. Among the oxide semiconductors, Aldoped ZnO (AZO) is one of the few materials with wurtzite structure (Figure 1(c)). Theoretically, the lattice mismatch between AZO and Mica is about 8.4%, which is acceptable for the Van de Waals epitaxy. Additionally, it is environment friendly and low-cost. Here, the middle layer is ultrathin gold films (Figure 1(d)), which possesses better transmittances over the visible range and anticorrosion properties. Based on these considerations, the mica single crystal is taken as a flexible substrate to prepare high-quality AZO/Au/AZO sandwich films. In this communication, the high-quality films were first obtained via pulsed laser deposition (PLD) method for the high-performance transparent flexible electrodes. Later, the interference behaviors for light crossing over the sandwich structures were explored with the employment of


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transmission spectra as well as numerical simulations via a finite difference time domain (FDTD) algorithm. Finally, it is demonstrated that the optoelectronic performance of the O/M/O sandwich film grown on Mica substrate (an average transmittance of 81.7% over the visible range, a sheet resistance of about 5Ω/sq) is well ahead of the counterpart reported in other works. Besides, the performances of its stretchable electronic devices and heater are standing out compared with other flexible devices.

■ RESULTS AND DISCUSSIONS In-situ reflection high energy electron diffraction (RHEED) patterns (as shown in Figures S3(a) and (b), Supporting Information) and atomic force microscopy (AFM) image (see Figure S2(a), Supporting Information) shows that the surface of flexible Mica substrate is rather flat. Combining with the moderate mismatch between AZO and the substrate for the Van de Waals epitaxy, the growth mode at the initial stage for the AZO films is quasi-2-dimensional, as demonstrated by the RHEED patterns (see Figures S3(c) and (d), Supporting Information). The later RHEED patterns (not shown here) with little variation imply that the growth mode of AZO films remains the same as the initial stage. This quasi-2-dimensional growth mode has a significant effect on the growth of high-quality AZO films. While for the Au growth, it transforms to a 3D island mode, as indicated by the light spots in the RHEED patterns (see the Figures S3(e) and (f), Supporting Information). This feature can be further confirmed by the AFM images as shown in Figure S4(b) in Supporting Information, reflecting rough surface at the initial stage. With the combination of Au islands and the increment of island-thickness, the surface becomes smooth gradually (see Figure S4(b-c), Supporting Information). Similar to the case of Au film, the growth mode of the overcoat layer is 3D island because of the large lattice


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mismatch (see the discussions in S3, Supporting Information) between AZO and Au, as indicated in Figures S3(g,h) and S4(d) in Supporting Information. Surface analysis for the sandwich films implies that the quality of the films deposited on Mica is much higher than that grown on PET (polyethylene terephthalate),23,32 glass,33-34 etc. This point can be further confirmed by the transmission electron microscope (TEM) images from a sample of AZO(75 nm)/Au(12 nm)/AZO(88 nm)/Mica, as shown in Figure 2(a-d). It is clearly seen that the interfacial transitions from Mica to AZO are very sharp, as illustrated by the dashed yellow lines in the low mag TEM image (Figure 2(a)). For AZO/Au, the interface becomes rougher slightly, as labeled by the dashed red line in Figure 2(a), which may be caused by the interface instability due to the coupling between Zn and Au-(111) planes. Experimentally, Utama et. al.35 had demonstrated an incommensurate van der Waals epitaxy for ZnO nanowire on Mica, forming an O-termination at the ZnO/Mica interface. Based on this experimental result, it reasonably concludes that the outmost plane of the AZO film is terminated by Zn-layer, thus forming Au-(111) plane and Zn-plane coupling at the Au/AZO interface. This kind of coupled interface could be unstable due to the charge accumulation from Zn and Au ions, which results in the interface coarsening. For the AZO/Au interface, an O-termination plane can be chosen to couple the Au-(111) plane to form a solid interface. AFM images (Figure S4(b-c), Supporting Information) indicate that the Au film surface with a thickness of about 10 nm has become quite flat via island combinations. Therefore, the AZO/Au interface is rather sharp and flat, as labeled by the white dashed line in Figures 2(a) and (c). Figure 2(b) corresponds to the selected area diffraction pattern (SADP) along [020]Mica or [120]AZO azimuth. These diffraction spots from the AZO films and Mica substrate, as labelled by the yellow and red arrows, respectively, reflect that the interfacial relationships are


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(001)[100][120]undercoat-AZO//(001)[200][020]Mica, in good agreement with the observations of RHEED patterns (see the discussions in S3, Supporting Information). The SADP image of the pure film (see Figure S5, Supporting Information) shows one set of good diffraction spots along the zone of [120]AZO, indicating the single crystal structure of AZO films. According to the highresolution TEM (HRTEM) images (Figure 2(c) and (d)), it clearly suggests that the AZO is a typical high-quality film without any grain boundary. Moreover, the HRTEM images further confirm these smooth interfaces of AZO/Mica and Au/AZO except for the AZO/Au interface with slight roughness, as shown by the yellow, red and white dashed curve lines in Figure 2(c) and (d), respectively. Figure 2(e) and (f) corresponds to the amplified images of the areas as labeled by the red and blue rectangles in Figure 2(d), respectively, showing lattice images of the Au-(1-12) and AZO-(120) planes, as illustrated by the color solid dots (left upper insets). This result suggests that the orientation of Au/AZO interface is (111)[1-12]Au//(001)[120]overcoat-AZO, consistent with the RHEED patterns. Based on the discussions above, the atomic models of the AZO/Au/AZO/Mica interface along [200]Mica and [020]Mica azimuths can be illustrated, as shown in Figure 2(g) and (h), respectively. According to the above structure analysis, it suggests the coat layers (In this paper, coat layers simultaneously include both of undercoat and overcoat layers) and ultrathin metal films deposited on the flexible Mica are single crystal films, which is an advantage compared to the polycrystalline or amorphous films reported in previous work.23-25, 33-34 Based on the high-quality films, the interference behaviors of light in the sandwich structure was later investigated. Theoretically, the transmittance of light crossing over the multiple films can be understood by the generalized Fresnel equation,36 which gives the formula as follows: tj/m≡tj/k/m=

௧ೕ/ೖ ௧ೖ/೘ ௘ ೔ഁೖ ೏ೖ

ଵି௥ೕ/ೖ ௥ೖ/೘ ௘ మ೔ഁೖ ೏ೖ

(1)


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Where tj/m is the transmittance coefficient for a stack starting at layer j and ending in layer m, k represents any intermediate layer; βl=(kl2-q2)1/2 and q=wsin(θ)/c are the perpendicular and parallel components of the wave vector in layer l, respectively. For the structure of our work as shown in Figure 3(a) (real image) and S6(a) (3D atomic model), equation (1) can be rewritten as follows: t0/5=

௧బ/భ ௧భ/ఱ ௘ ೔ഁభ ೏భ

ଵି௥భ/బ ௥భ/ఱ ௘ మ೔ഁభ ೏భ

(2)

Here, layer number 1 is considered as an intermediate layer, t1/5 and r1/5 represent the total reflection and transmission coefficient of the layers from one to five. Equation (2) suggests that the transmittance (corresponding to t) is a function of the light wavelength and films thickness. The largest average transmittance over one range can be obtained by optimizing the thickness of the film. Experimentally, AFM image (Figure S4(c), Supporting Information) indicates that the Au film surface has become continuous and flat when its thickness approaches ~10 nm. Besides, its sheet resistance has become very small (less than 5 Ω/sq, see discussion later). Therefore, the Au film thickness was fixed as 10 nm when optimizing the coat layer thicknesses. FDTD simulations show that the average transmittance over the visible range approaches the maximum value when dundercoat=62 nm and dovercoat=58 nm, respectively, (see Figure S6(b), Supporting Information). Here, dundercoat and dovercoat represent the thickness of undercoat layer and an overcoat layer, respectively. Based on the simulations, the ratio of dovercoat to dundercoat was controlled as 0.93 in the preparation process of each sample, as illustrated by the black dashed line in Figure S6(b) in Supporting Information. The green scattered curve line (Figure 3(b)) shows an evolution of the average transmittance over the visible region with the increasing thickness of the total coat layer. The maximum average transmittance (including the contribution from the substrate) can approach as high as 81.7% at


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the thickness of 126 nm (close to the simulation result: 120 nm), as labeled by the red star. Figure 3(c) displays the transmittances with different total-coat-layer thickness. For the 126-nm sample (green line in Figure 3(c)), the transmittance is nearly close to the substrate (Mica transmittance ≈91%) for the range from 550 nm to 650 nm. Remarkably, there appears a broad peak in almost every transmission spectra. More interestingly, these peaks appear a red shift, as illustrated by the black solid dots in Figure 3(c). This phenomenon suggests the strong interference at the interface in the flexible film. Furthermore, these interference peaks can be modulated by the thickness of overcoat and undercoat layers, as demonstrated by equation (2). When the thickness of total coat layer approaches to about 126 nm, the interference peak shifts to the middle position (at about 600 nm) and becomes strongest. Consequently, the highest average transmittance will appear, which explains the transmittance of the sample with a total thickness of coat layer of about 126 nm is very close to the substrate at the middle region of the visible range. Furthermore, from the angle dependent transmittance (see Figure S7 in supporting information), we can see that the transmittance decreases with the increase of the incident angle. Especially for the incident angle smaller about 30 º, the rate of the decrement of the transmittance seems to increases rapidly. Strong absorption is often accompanied by the high reflection in Au films. Normally, its transmittance decreases monotonically with increasing film-thickness according to BeerLambert's law. However, metal film thickness influences its continuity, and then influences light interference at the interfaces, thus the measured transmittance may deviate from Beer-Lambert's law. As described by the blue scattering curve in Figure 3(b), the average transmittance contrarily increases slightly with the Au thickness increasing from 2 to 10 nm. When its thickness is larger than 12 nm, the transmittances decrease drastically. While its absorptions


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become rather strong, as available in Figure S8 in Supporting Information. Figure 3(d) further shows the transmittances with different Au film-thicknesses in detail. Here, sample-xnm represents a sample with the Au film-thickness of about x nm. Sample-0nm (black line in Figure 3(d)) shows very high transmittance, suggesting the high-quality film for AZO. Clearly, there is a weak peak at 500 nm, as labeled by the blue dashed line in Figure 3(d). This peak originates from the existence of bulk absorption modes, as observed in previous work.37 For sample-2nm, Au is not continuous, consisting of isolated islands (see Figure S4(b), Supporting Information), thus there appears quite weak interference at the AZO/Au/AZO interface. Contrarily, its absorption becomes strong (see Figure S8, Supporting Information) due to the possibility of enormous interfacial states at the AZO/particle-Au/AZO interface and Au particle absorption for incident light. As the ultrathin Au films increase from 4 to 10 nm, the films become continuous gradually while its roughness become smaller, as demonstrated by AFM images (Figure S4(b) and (c), Supporting Information). In this case, light interferences become strong gradually, leading to a broad peak, as labeled by the gray area in Figure 3(d). In order to further explore the interference behavior of light when crossing over the Au films, the FDTD method was used to simulate the optical properties of the tri-layer films. The results show that the transmittances (Figure S9(c), (f) and (i), Supporting Information) firstly increase slightly with the increasing thickness of Au film. When the films become continuous, it decreases drastically. The absorptions also firstly increase gradually. When the Au films become continuous, the variation becomes significant (Figure S9(g) and (j), Supporting Information). These results are well consistent with experimental results (Figure 3(b) and Figure S8, Supporting Information). Simultaneously, the reflections firstly decrease gradually and then start to increase after Au film becoming continuous (Figure S9(e), (h) and (k), Supporting


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Information). If the Au thickness further increases, both absorption and reflection will enhance rapidly, resulting in drastic decrease of transmittances, as shown in Figure 3(c) and Figure S9(f) and(i)) in Supporting Information, respectively. Based on these results, it suggests the Au film morphology changing from non-continuity to continuity can influence the light absorption and reflection behaviors, and consequently modulate the strength of light interference. Figure 4 displays the sheet resistance of the AZO/Au/AZO sandwich films, which were characterized by the four-probe method, as shown by the inset in Figure 4(a). For these films with different thickness of AZO layers and 10-nm-thick Au film, the sheet resistance seems to appear no variation, as illustrated by the scattering curve in Figure 4(a). While it decreases with the increment of Au thickness (Figure 4(b)). These results indicate that the resistances are mainly determined by the Au layers, with little influence by the AZO layers. Figure 4(c) displays the sheet resistance and a change percentage as a function of the bending times at a bendable diameter of about 10mm. It shows that the sheet resistances become slightly larger after bending 1000 times. The increment of sheet resistance originates from a few cracks on the film surface after bending 1000 times, as demonstrated by the optical microscope images in Figure S10 in Supporting Information. Figure 4(d) shows the sheet resistance of the O/M/O film under the condition of different bending diameters. When the bending diameter is larger than about 7 mm, the sheet resistance increases rapidly, indicating that the bearable bending diameter is about 7 mm. In fact, we found that the pure mica almost hardly produces cracks on its surface at the bending diameter of about 7 mm. This result reflects that the cause for increasing the sheet resistance rapidly is originated from the film failure rather than the Mica. The real image (insets in Figure 4(d)) show the corresponding measurement state with convex or concave bending.


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To better compare with other works, the figure of merit (FoM) factors was calculated according to the Haacke theory, which gives the formula of FoM as following: Φ=T10/Rs. Here, T and Rs represent the transmittance at 550 nm and sheet resistance, respectively. Taking the contribution from the substrate into account, the transmittance obtained in this work at 550 nm is equal to about 87.9%, as labeled by the dark dashed line in Figure S11 in Supporting Information. Table 1 shows comparison results based on the evaluation factors of FoM. It is clearly seen that the performance of the sandwich structure film grown on Mica substrate is superior to that grown on PET or glass substrate. In addition, one can see that this performance stands out other works based on this comparison with the similar structures but with different materials. This excellent performance is attributed to the high-quality crystalline film on Mica substrate. While those films for other materials or grown on other substrates are often polycrystalline or amorphous,17,

23-24

which effect on its optoelectronic performance, thus

inducing smaller Haack FoM values. For the similar sandwich structure of AZO/Ag/AZO on Mica, the Haack FoM value of the as-grown film is also smaller than our result, as reported in reference 27. Leading to this result, we believe, maybe originate from the differences of the deposition method and the coat layer thickness. It is well known that heater film is an important aspect for the stretchable electronic devices in the applications of the transparent electrode. In order to show the advantage of the high-quality sandwich films grown on Mica compared with other transparent conductive flexible films, the heater properties from a sample with the best FoM was tested. The dark and red curve lines in Figure 5(a) represent the evolution process of the average and maximum temperature under operations of different powers, showing the response time of about 5s, as indicated by Figure 5(b) (corresponding to the amplified image of the area, as labeled by the blue rectangle in Figure


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5(a)). This value is far shorter than that for the PET,40-41 PI (polyimide),42-43 and PES (polyethersulfone) substrate.44 Besides, just applying a small power (about 5.6w), film heater temperature can reach at approximate 150 °C. The short response time and low application power are attributed to the thinner thickness and better thermal conductivity of the Mica than other commercial substrates. Moreover, it clearly shows that the thermal temperature of the whole sample is rather uniform, as shown by the infrared camera thermal images (upper) in Figure 5(a). More importantly, if the heating films are bent to be a semicircle shape with a radius of about 1cm, the temperature and uniformity of the heater seems to remain the same, as indicated by the infrared camera thermal images captured from the side view (below images in Figure 5(a)) and from the top view (Figure S12, Supporting Information), respectively. The performance of the AZO/Au/AZO/Mica heater is outstanding in comparison with the metal nanowire heater.45 This high performance is attributed to the high-quality film with perfect interfacial structures (see Figure 2) without local resistance. Figure 5(c) is the corresponding heating circle at an operating power of about 5.6w. After cycling 50 times, the temperature of the heater film is almost the same as the initial stage, further indicating an excellent candidate of making film heater based on AZO/Au/AZO sandwich films.

■ CONCLUSIONS In summary, a flat, flexible, and high-quality thin film with high transmission and low sheet resistance are realized by designing the AZO/Au/AZO multiple layer structure on the Mica substrate with an atomically flat surface, as demonstrated by the RHEED patterns as well as AFM and TEM images. By changing the thicknesses of undercoat and overcoat layers, a red shift of the broad peak can be observed due to the light interference at the interface. For Au thickness of about 10 nm, the largest average transmittance (including the contribution from the substrate)


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can approach as high as 81.7% over the wavelength range from 400 to 800 nm and the corresponding sheet resistance is as low as about 5 Ohm/sq. Experimental and simulation results suggest that the Au film morphology evolutions with increasing thickness, which can modulate the strength of the light interferences. The largest FoM at 550 nm is about 55.1, far larger than the reported values in the most literatures. Moreover, the response time of the film heater based on the AZO/Au/AZO tri-layer films is about 5s, and the heater temperature can reach as high as 150 °C at an operating power of about 5.6w. Infrared camera thermal images show that the temperature of the films is uniform, even under the condition of bending with a diameter of ~2cm, suggesting no local resistance. These results indicate that the high-quality film is not only an excellent transparent flexible electrode, but also a promising candidate for the flexible film heater. Furthermore, the largest size of commercial Mica with a cost of about 13.9 $ is larger than 210×300 mm, which is acceptable for some scalable approach and the applications in many fields. Combining with sputtering or evaporation deposition techniques with a low-cost production, probably, this type of sandwich structure film grown on Mica flexible substrate as a transparent flexible electrode can be applied in the further novel devices.

■ EXPERIMENT AND CALCULATION METHODS Film preparation: the tri-layer films were deposited on the flexible Mica substrate by the pulsed laser deposition (PLD). A KrF excimer laser (Lambda Physics LPX 305) with a wavelength of 248 nm and pulse duration of 30 ns delivered an energy of about 220 mJ per pulse. The laser was operated at 5 Hz and was focused through a 50 cm focal length lens onto a rotating target at a 45° angle of incidence. The target-substrate distance was fixed at about 6 cm. The substrate was attached with a stainless steel mask to a substrate holder, which was heated by


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electric resistance wire form TSST. The substrate temperature was monitored with a thermocouple at all times. The growth temperature and pressure of AZO (Au) are ~200°C and 0.5pa (~100 °C and

AZO Sandwich Film with Ultralow Optical Loss

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