Thermally Diffused Al:ZnO Thin Films for Broadband Transparent

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Thermally Diffused Al:ZnO Thin Films for Broadband Transparent Conductor Chong Tong, Ju-Hyung Yun, Yen-Jen Chen, Dengxin Ji, Qiaoqiang Gan, and Wayne Anderson ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11285 • Publication Date (Web): 25 Jan 2016 Downloaded from http://pubs.acs.org on January 27, 2016

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ACS Applied Materials & Interfaces

Thermally Diffused Al:ZnO Thin Films for Broadband Transparent Conductor Chong Tong§, Juhyung Yunǂ, Yen-Jen Chen§, Dengxin Ji§, Qiaoqiang Gan§,*, and Wayne A. Anderson§,†. §

Department of Electrical Engineering, The State University of New York at Buffalo, Buffalo, New

York 14260-1920, USA. ǂ Department of Electrical Engineering, Incheon National University, Yeonsu Incheon, 406772, Korea

Abstract: Here we report an approach to realize highly transparent low resistance Al doped ZnO (AZO) films for broadband transparent conductor. Thin Al films are deposited on ZnO surfaces, followed by thermal diffusion processes, introducing the Al doping into ZnO thin films. By utilizing the interdiffusion of Al, Zn and O, the chemical state of Al on the surfaces can be converted to a fully oxidized state, resulting in a low sheet resistance of 6.2 Ω/sq and an excellent transparency (i.e. 96.5% at 550 nm and higher than 85% up to 2500 nm), which is superior compared with some previously reported values for indium tin oxide (ITO), solution processed AZO, and many transparent conducting materials using novel nanostructures. Such AZO films are also applied as transparent conducting layers for AZO/Si heterojunction solar cells, demonstrating their applications in optoelectronic devices.

Keywords: Al doped ZnO, transparent conductor, broadband, inter-diffusion, chemical state change, heterojunction solar cells

*E-mail: [email protected]; †E-mail: [email protected]

ACS Paragon Plus Environment

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1. Introduction The need for highly transparent low resistance conductor is essential for flat-panel display, touch-panel screen and optoelectronic applications.1 Particularly, solar panels installed on the ground or large area rooftops generally consume large amounts of transparent conducting materials. It is also reported that the predicted market for transparent conducting materials is almost $1 Billion in 2016.2 Therefore, it is important to develop large area and cost-effective transparent conductor materials. Transparent conducting oxides (TCOs) such as indium tin oxide (ITO) are commercially available, which have been widely used on displays,3 light-emitting diodes and thin film solar cells.4-8 However, the limited supply of indium and the increasing demand from the rapidly expanding display and photovoltaic market increased the cost of ITO drastically, which is a severe limitation for the development of lower cost and larger scale applications.9 In addition, their poor mechanical stability can lead to device failure when ITO-coated films are bent on flexible substrates.10 Thus, various indium-free transparent conducting materials, such as nanopatterned metal array structures,9 nanowire networks,11-13 carbon nanotubes,14-16 and conducting oxides (e.g. Ga:ZnO, Al:ZnO and In:ZnO),17-19 have been extensively investigated as alternatives to ITO films. Among these investigations, Al doped ZnO (AZO) received significant attention because of its nontoxicity, low cost, material abundance, excellent electrical/optical properties and compatibility for flexible electronic applications.20,

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Many doping methods for AZO thin films have been

reported, such as magnetron sputtering deposition with a mixture target of ZnO and Al2O3, 22 dual source co-sputtering process with separated ZnO and Al targets,23 ion implantation, etc.24 However, the mixture target with a fixed Al concentration limited the doping tunability. For the co-sputtering method, the dopants are introduced during the thin film growth process. Thus, the growth conditions of the ZnO crystal can be


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affected, resulting in potential morphology change or generation of internal defects.2527

Doping by ion implantation after the ZnO deposition may introduce damages to the

ZnO crystal and bring additional changes.28, 29 To reveal the exact doping effects, a separate doping process is desired, which only introduces minor influences to the thin film growth, so that the doping-dependent changes of thin film properties can be identified.30 In this article, we report a thermal diffusion approach using metallic Al thin films as diffusion sources to achieve broadband transparent AZO thin films with very low resistance. This method allows a flexible doping tunability. Moreover, the doping process is independent of the thin film growth, thus the doping effects on thin film properties can be investigated separately. The doping profiles, crystal structure, chemical, optical and electrical properties of these AZO films were studied. We also implemented the AZO film as a conducting window layer in an AZO/n-Si heterojunction (HJ) solar cell. This study provides a new possible design for improved quality of TCO thin films in optoelectronic applications.

2. Results and discussion 2.1 Improved optical and electrical properties ZnO thin films were first deposited on glass and n-Si substrates by RF magnetron sputtering, followed by a 5-15 nm-thick Al layer deposited on the top. The AZO films with different Al doping (depending on the top Al layer thickness and diffusion temperature) were then formed via the thermal diffusion process with tube furnace annealing in N2 gas flow under the temperature of 200, 400 and 600 oC (details in the Experimental Section). We varied the thickness of Al thin films because it is an


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important entity to study the influence of top metal layer on the properties of the produced AZO films. The influence of the top Al layer thickness and the diffusion temperature on the AZO optical properties can easily be observed by comparing the transmittance of AZO samples treated under different diffusion temperatures, as shown in Figure 1(a). One can see that the “UB” logo is clearly visible through sample #1, i.e, the undoped ZnO thin film on a glass substrate. After deposition of 10nm-thick (sample #2) and 15-nm-thick (sample #3) Al layers on top of the ZnO films and processed at 400 oC for thermal diffusion, these AZO films show a significant decrease of transparency due to the optically thick Al layers. However, the AZO sample changes back to a highly transparent film with a higher diffusion temperature at 600 oC (i.e. sample #4, with a 15-nm-thick Al film on top and treated under 600 oC) due to the Al chemical state change (as will be verified in Section 2.4). The optical transmittance of these AZO films treated at 600 oC with different top Al film thicknesses compared with undoped ZnO are shown in Figure 1(b). Compared with sample #1 (i.e. the as-deposited undoped ZnO), these AZO films obtained under the diffusion temperature of 600 oC show comparable or even higher transmittance in the wavelength regions of 500-1100 nm. The remaining question is whether this highly transparent AZO film has a low sheet resistance (Rs), which is essential for transparent conductor applications. Figure 1(c) shows the measured transmittance at λ = 550 nm versus Rs for the undoped ZnO and the thermally diffused AZO thin films. For clear comparison, some of previous reported outstanding results based on graphene,31 carbon nanotube (CNT),32 ITO

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and silver nanowire (AgNW) based transparent conductors1, 34 were also marked in this figure. One can see that the best results among our AZO thin films (i.e. 10~25 nm-thick Al layer with diffusion temperature of 600 oC) are comparable or even better


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than those previous reported results. To determine the optimum thermal diffusion condition for the AZO thin films, the sheet resistance (Rs) and transmittance (T) at the wavelength of 550 nm were used to calculate the figure of merit ΦTC, as defined by Haacke:35 ΦTC = T10/Rs

(1)

Table 1 summarizes the sheet resistance, transmittance and the calculated ΦTC of the best AZO thin film (e.g. diffusion condition: 15-nm-thick Al layer and diffusion temperature of 600 oC) compared with the undoped ZnO thin film, solution processed AZO film, typical ITO film and other reported outstanding results.1, 2, 31-34, 36, 37 The as-deposited undoped ZnO film exhibited an acceptable transmittance of 88.5%, and a sheet resistance of ~85000 Ω/sq. Our best sample showed a low sheet resistance of 6.2 Ω/sq, an improved transmittance of 96.5%, and a figure of merit value of 112.3 × 103

Ω-1, which is better than other reported results listed in Table 1. The optical constant

characterization for this sample also confirmed the low extinction coefficient (k) values in the corresponding wavelength range (Supporting Information, Fig. S1). Remarkably, the transmittance of this AZO sample in the long wavelength range of 1200 – 2500 nm is higher than 85% as shown in the inset of Fig. 1(b). This broadband transparency is generally higher and broader than previous reported results,1, 31, 38 including ITO films and solution processed high doping AZO films.2 This unique broadband-transparency is very promising to fill the gap of immature mid-infrared transparent conducting materials, which is currently under active investigation.39 To better understand the material related mechanisms for the superior optical and electrical properties of these thermally diffused AZO films, we then investigate the doping profiles, crystal structure, chemical, optical and electrical properties of these AZO films in the following sections.


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2.2 Thin film morphology and diffusion depth profiles The scanning electron microscopy (SEM) image of the as-deposited ZnO thin film is shown in Fig. 2(a). Petal-like surface structures and clear crystal grains can be observed. The surface morphology of the thermally diffused AZO thin film (i.e. Al: 15 nm – 600 oC) is also shown in Fig. 2(b). Obvious surface morphology change can be observed due to the Al layer deposition and the thermal diffusion processes. From the cross-section image shown in Fig. 2(c), one can see that the bottom ZnO layer is about 220 nm thick with a ~15-nm-thick thin film on the top from the Al deposition. To further reveal the elemental distribution of Al, Zn and O throughout the thermally diffused AZO films, we then employ the secondary ion mass spectrometry (SIMS) measurements to characterize these AZO films (sample Al: 15 nm – 400 oC and Al: 15 nm – 600 oC). Depending on the diffusion temperature, different diffusion profiles of the Al element can be observed. As shown in Fig. 3(a), at the diffusion temperature of 400 oC, the Al element diffused into the ZnO layer by ~40 nm. The top 15 nm region is still dominant by Al only, explaining the low transparency observed in Fig. 1(a). In contrast, the Al diffusion depth for the 600 oC sample is ~100 nm, as shown in Fig. 3(b). Also, much higher element concentrations of Zn and O can be observed in the top 15-nm region, indicating that Zn and O inter-diffused into the top Al layer at the higher diffusion temperature. This inter-diffusion process changes the chemical state of Al, improving the transparency of the AZO film. More detailed chemical analysis of the top layer will be discussed in Section 2.4. In addition, uniform distributions of Zn and O are observed in the bottom layers (i.e. the right-hand side of the dashed line) for both samples, indicating uniform stoichiometry throughout the bottom ZnO thin films.


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2.3 Structural characteristics To investigate the crystalline structural properties of these ZnO and AZO films, we employ X-ray diffraction (XRD) measurement to characterize the as-deposited ZnO and thermally diffused AZO films, as shown in Fig. 4. To separately identify the effects of Al doping and thermal treatment, undoped ZnO samples with no Al layers on top but subjected to the same annealing processes are also shown. For all samples, the angular diffraction peak was observed at 2θ of ~34.4o. This is very close to the angular peak position of pure ZnO powder with (002) orientation (i.e. 2θ = 34.47o), indicating that all these films exhibit preferential orientation of (002) with c-axis perpendicular to the substrate surface.40, 41 The slight deviation of the diffraction peak position of the as-deposited ZnO film (i.e. 2θ = 34.38o, as shown by the upper panel of Fig. 4) from the ZnO powder value is mainly due to a uniform state of stress with tensile components parallel to the c-axis.42 The stress in sputtered ZnO films has been found to be associated with the effect of impacting atoms during the sputtering process and interstitial Zn atoms in the ZnO crystal.43 After post-annealing processes at 400 oC and 600 oC, the peak angles of the ZnO films increase to 34.40o and 34.51o, respectively, as shown in the central panels of Figs. 4(a) and (b).This indicates that the tensile stress was released during the thermal annealing process, providing sufficient energy for atoms of ZnO films to rearrange.43 In addition, for AZO samples after the thermal diffusion processes at 400 oC and 600 oC, the peak positions of the (002) plane shifted to larger angles of 34.42o and 34.58o, respectively, as shown in the lower panels of Figs. 4(a) and (b). Because the ionic radii of Zn+2 and Al+3 are 72 pm and 53 pm, respectively, the lengths of the c-axis are expected to be smaller if the Al atoms replace Zn sites in the AZO crystal.41 Thus, this up-shift of (002) peaks for the thermally diffused AZO films indicates that the lengths of c-axis are shortened and a moderate Al diffusion into the ZnO layers may release the residual stress of AZO


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films by substituting the Zn sites with in-diffused Al atoms.41, 44 Compared with those thermally annealed ZnO samples (see central panels of Figs. 4(a) and (b)), the peak shift of the AZO sample treated at a diffusion temperature of 600 o

C (see the lower panel of Fig. 4(b))is larger than that of the AZO sample treated at

400 oC (see the lower panel of Fig. 4(a)), suggesting that more Al atoms are indiffused into ZnO at the higher temperature, which is consistent with the previous SIMS measurement results shown in Fig 3. The average crystal grain size can be estimated from the full-width at half-maximum (FWHM) of the (002) peak using the Scherrer equation:45 D = (0.9 λ) / (B cosθ)

(2)

where D is the average crystal grain size, λ is the x-ray wavelength, θ is the Bragg diffraction angle and B is the FWHM of θ. The calculated average grain sizes of the as-deposited pure ZnO, annealed ZnO at 400 oC and 600 oC are 30.3, 33.5 and 39.5 nm, respectively. These results indicate that the grain size of ZnO films increases with the higher annealing temperature due to the merging process introduced by thermal treatment.45 However, too high annealing temperatures (> 600 oC) are not desired due to the thin film degradation introduced by strong compressive stress.43, 46 The AZO sample with the diffusion temperature of 600 oC (i.e. Al : 15nm – 600 oC, the best sample) shows the largest grain size of 41.6 nm with the best crystal quality, confirming that the AZO film quality can be improved by releasing the thin film stress through moderate Al doping.

2.4 Chemical state on the surface Transparency is an important feature for transparent conductor applications. Since the thermally diffused top Al layers would influence the optical transmittance of AZO films, X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical state


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change of the top Al layers and interfaces. Figs. 5(a) and 5(b) show the XPS data of Al 2p3/2 for the top 15-nm-thick layers on the AZO films obtained at diffusion temperatures of 400 oC and 600 oC, respectively. For the AZO sample treated at400 o

C (i.e. Al: 15 nm – 400 oC), the Al 2p3/2 exhibits a double-peak feature as shown by

the Gaussian-resolved curve in Fig. 5(a), indicating the existence of a two-component Al chemical state on the surface. The strong intensity component with a binding energy of 74.20 ± 0.10 eV [see AlO (400 oC) in Fig. 5(a)], slightly shifted towards a lower binding energy compared to the 74.60 eV peak position of fully oxidized state, indicating that this is an oxidized Al state with an oxygen-deficient matrix.47, 48 The component centered at 70.90 ± 0.10 eV [see Alm (400 oC) in Fig. 5(a)] is a characteristic of the metallic Al state.49, 50 To investigate the depth profile of the two state components, the variation of peak intensities of metallic Al and oxidized Al as a function of etching time is shown in Fig. 5(c). At the surface of the AZO sample processed at 400 oC, the intensity of Alm increases and the signal of AlO decreases sharply with the increasing etching time. This result indicates that the top metallic Al layer is only partially oxidized and the oxidized component should be attributed to the Al thin film oxidation with oxygen in the air environment. However, for the AZO sample processed at 600 oC (i.e. Al: 15 nm – 600 oC), only one peak could be identified at a higher position of 74.60 ± 0.10 eV as shown in Fig. 5(b), suggesting that the top metallic Al layer was fully converted into an oxidized state 47, 48. One can also see from Fig. 5(c) that AlO peak intensity even increases with the increasing etching time, indicating that this complete Al oxidation is attributed to the interdiffusion process of Zn, O from the bottom ZnO layer. This observation is also consistent with the SIMS measurement shown in Fig. 3(b). Thus, we believe that under the diffusion temperature of 600 oC, the metallic Al in the top layer can be fully


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changed to the oxidized state due to the inter-diffusion of Al, Zn and O, and therefore significantly improving the optical transmittance and electrical conductivity of the AZO film.

2.5 Prototype transparent conducting layers for solar cell devices We employ the AZO thin film as the conducting window layer for an AZO/n-Si heterojunction (HJ) solar cell, as illustrated in Fig. 6(a). In this case, the AZO layer serves as a transparent conductor and simultaneously provides an isotype heterojunction with the n-Si substrate. One can see from Fig. 6(b) that the heterojunction device provides a clear rectifying behavior under the dark condition, and gives short-circuit current density and open-circuit voltage of 17.3 mA/cm2 and 250 mV, respectively, under the illumination power of ~87 mW/cm2 from a solar simulator. The current flow at the negative voltage region in the dark condition represents the leakage current across the cell. As shown in Fig.6(c), the leakage current of our AZO/n-Si HJ cell is ~3 × 10-3 mA/cm2. The highly conductive AZO thin film has a graded doping profile with a relatively resistive region near the junction, which can reduce the leakage current. The ideality factor (n) was extracted by fitting the slope of the linear region of the forward bias in the dark J-V (i.e. current density versus voltage) curve. The calculated n value of 1.27 indicates the formation of a high quality junction between the AZO window layer and the Si substrate without serious recombination problems.23, 51 Light absorption in the device produces photogenerated carriers. The built-in electric field in the depletion region of the isotype heterojunction separates the carriers and transports electrons and holes to the Si side and AZO side, respectively.23, 44 The ultimate solar cell performance can be improved further by optimizing the light absorption and inserting well designed buffer layers at the junction.23, 52, 53 The results conclude that these thermally diffused AZO thin films


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could lead to enhanced optical and electrical properties of transparent conductors for photoelectric applications.

3. Conclusion We propose a thermal diffusion approach, utilizing the inter-diffusion process, to realize high performance AZO films for broadband transparent conductors. By depositing a thin Al film on top of ZnO and then performing thermal diffusion treatment, the sheet resistance of the AZO film can be decreased significantly due to the Al doping. Also, the metallic Al top layer can be fully converted to an oxidized state, due to the inter-diffusion process of Al, Zn and O. As a result, the transparency of the AZO films is improved without sacrificing the low sheet resistance (e.g. sheet resistance of 6.2 Ω/sq and a transmittance of 96.5% at 550 nm). Our best sample shows a figure of merit value of 112 × 10-3 Ω-1, which is better than that of commercially-available ITO films,31,

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solution processed AZO films,2 and

comparable to the previous reported results of TCEs based on novel nanostructure materials. Remarkably, these AZO samples show a broadband transparency (> 85%) up to 2500 nm, indicating the potential to realize mid-infrared transparent electrodes. We also applied this AZO film as a transparent conductor window layer for an AZO/Si HJ solar cell, which demonstrated its potential applications for optoelectronic devices.

EXPRIMENTAL SECTION Sample preparation


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ZnO thin films with a thickness of ~220 nm were deposited on glass or n-Si (100) substrates by RF magnetron sputtering at the substrate temperature of 500 oC, using a 4-inch ZnO (99.99% purity) target. The substrates were kept rotating at 11 rpm to maintain uniform growth. The working pressure was kept at 2 mTorr in pure Ar ambient and the RF power was kept at 300 W. The deposition rate and thin film thickness were calibrated by using ellipsometer and scanning electron microscopy (SEM). After ZnO deposition, 5-15 nm-thick Al layers were deposited on top of these samples using thermal evaporation (Denton Vacuum, DV-502). Then, the thermal diffusion treatment was performed via tube furnace annealing in N2 gas flow under the temperature of 200-600 oC. Thin film characterization The morphologies of our samples were characterized by SEM. The aluminum doping depth profile was obtained by secondary ion mass spectrometry (SIMS) measurement. A primary beam of Bi+ ions was rastered over a 150 × 150 µm2 surface area, and the secondary ions were collected from the central region of the crater. The crater depth was estimated from SEM images, and a constant erosion rate was assumed to convert the sputtering time to the sample depth. The AZO crystal structures were analyzed by X-ray diffraction (XRD) measurement. And the X-ray photoelectron spectroscopy (XPS) characterizations were performed to examine the chemical state changes of Al on top surfaces of AZO layers. Samples were etched by Ar+ bombardment with an etching rate of ~1 nm/min for 5 min to remove possible surface contaminations. The position of the C1s peak was employed as the reference (with a binding energy of ~284.6 eV). The optical transmittance spectra were measured using a modified Fourier transform infrared spectroscopy (FTIR, Vertex 70 with Al-based optics covering visible spectrum) in the wavelength range from 500 to


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2500 nm. All recorded transmittance spectra were normalized with spectra through bare glass substrates. The average sheet resistance values were measured using fourpoint probe method (Alessi) with multiple measurements at different areas. Solar cell fabrication For the AZO/n-Si heterojunction (HJ) solar cell fabrication, n-Si substrates were cleaned using acetone, methanol and deionized (DI) water. The native oxide on the substrate was removed by buffer HF solution. After AZO deposition, 100 nm-thick Al back contacts were thermally deposited. Silver-dot front contacts were made on the AZO layer using silver paste. All solar cell devices were characterized at room temperature. The photocurrent was analyzed under the solar simulator with ~87 mW/cm2 illumination condition.


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Figures:

Fig. 1 (a) Photographs of undoped ZnO and AZO thin films with different thermal diffusion conditions on glass substrates and their corresponding description (in the table). (b) Optical transmittance of AZO thin films achieved at the diffusion temperature of 600 oC with different top Al thicknesses. (c) Optical transmittance of AZO films at λ = 550 nm versus their sheet resistance. The star points represent the outstanding results reported in previous literatures based on graphene,31 carbon nanotube (CNT),32 ITO,1, 33 and silver nanowire (AgNW).1, 34


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Fig.2 SEM images of (a) as-deposited undoped ZnO, and (b-c) AZO sample obtained with a 15-nm-thick Al layer diffused at 600 oC (i.e. Al: 15 nm – 600 oC)


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Fig. 3 Depth profiles of AZO films treated at diffusion temperature of (a) 400 oC and (b) 600 oC, respectively, characterized using SIMS.


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Fig. 4 XRD patterns of as-deposited undoped ZnO thin films (upper panel), compared with annealed ZnO films (central panel), and thermal diffused AZO films (lower panel) treated at different temperatures of (a) 400 oC and (b) 600 oC.


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Fig. 5 Surface XPS data of Al 2p3/2 and its Gaussian-resolved components for AZO films treated at diffusion temperature of (a) 400 oC and (b) 600 oC, respectively. (c) Peak intensities of metallic Al (Alm) and oxidized Al (Alo) states of the surface layers of both samples as the function of etching time.


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Fig. 6 (a) Schematic illustration of an AZO/n-Si heterojunction solar cell. (b) Photo current and dark current density – voltage (J-V) characteristics of the device. (c) The dark J-V characteristic in semi-log scale.


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Tables:

Table.1 Comparison of the Electrical and Optical properties


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website. Experimental results of optical constants characterization for the produced AZO thin films via spectroscopic ellipsometry.

AUTHOR INFORMATION Corresponding Authors * Qiaoqiang Gan: [email protected] †

Wayne A. Anderson: [email protected]

Authors Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS C. Tong, J. Yun, Y. Chen and W. A. Anderson gratefully acknowledge the support from the U.S. Air Force Office of Scientific Research (FA95501010154) with Dr. Kitt Reinhardt and Dr. James Hwang as supervisors. D. Ji and Q. Gan acknowledge funding support from National Science Foundation (grant no. ECCS1507312 and ECCS1425648).


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Thermally Diffused Al:ZnO Thin Films for Broadband Transparent

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