Surface Nanostructured Al-AlN Composite Thin Films with Excellent

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Surface Nanostructured Al-AlN Composite Thin Films with Excellent Broadband Antireflection Properties Fabricated by Limited Reactive Sputtering Qian Yuan, Joachim Döll, Henry Romanus, Hongmei Wang, Heike Bartsch, Arne Albrecht, Martin Hoffmann, Peter Schaaf, and Dong Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00302 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018

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ACS Applied Nano Materials

Surface Nanostructured Al-AlN Composite Thin Films with Excellent Broadband Antireflection Properties Fabricated by Limited Reactive Sputtering Qian Yuan,† Joachim Döll,‡ Henry Romanus,‡ Hongmei Wang,† Heike Bartsch,§ Arne Albrecht,‡ Martin Hoffmann,ǁ Peter Schaaf,† and Dong Wang*† †

FG Werkstoffe der Elektrotechnik, Institut für Werkstofftechnik und Institut für Mikro- und Nanotechnologien MacroNano®, TU Ilmenau, Gustav-Kirchhoff-Str. 5, 98693 Ilmenau, Germany ‡ Zentrum für Mikro- und Nanotechnologien, TU Ilmenau, Gustav-Kirchhoff-Str. 7, 98693 Ilmenau, Germany § FG Elektroniktechnologie, Institut für Mikro- und Nanotechnologien MacroNano®, TU Ilmenau, Gustav-Kirchhoff-Str. 1, 98693 Ilmenau, Germany ǁ Lehrstuhl für Mikrosystemtechnik, Ruhr-Universität Bochum, Universitätsstr. 150, 44801, Bochum, Germany * Email: [email protected], Tel.: +49-3677-69-3170. Fax: +49-3677-69-3171 Abstract Al-AlN composite thin films with surface nanostructures possessing excellent broadband antireflection property, can be simply fabricated by using a simple sputtering-based method, namely limited reactive sputtering at the elevated temperature. A mixture of Ar and N2 gases with limited gas flow of N2 is used. During the process, the limited amount of the N ions and radicals can only consume a part of the sputtered Al atoms for the formation of AlN, so that the Al-AlN composite thin film can be finally deposited on the substrate. Comparing to the traditional co-sputtering for composite deposition which works necessarily with two or more targets simultaneously, here only single Al target is required. When the substrate temperature is higher than 100 °C, surface nanostructures can be evolved due to the different diffusion kinetics of Al and AlN. The total reflection was reduced drastically (< 10%) in the spectral range 300 – 2000 nm. The Al-AlN composite thin films exhibit also a good electric conductivity. Keywords Antireflection property, broadband, reactive sputtering, nanostructure, Al-AlN, composite 1. Introduction Reduction of optical reflection from surfaces is interesting and very important for many applications, including optical components,1 solar cells,2-4 and light-emitting diodes (LED).5 For dielectric antireflection (AR) coatings which are used for increasing transmittance and reducing Fresnel reflection losses, the theoretical base is to lower the difference of refractive index between the incident medium and adjacent layer.6 Typically, dielectric AR coating can be classified into step-index coating and graded index coating depends on its layer structure.7 The former reduces the reflection by destructive interference at interfaces. It could be single layer or multilayer, but the layer itself is homogeneous. Alternatively, nanostructured surface can act as a region with graded index of refraction between air and the dielectric material, possessing a prosperous reflectance reduction in a broader spectral bandwidth and omnidirectional incidence.8 Such nanostructured surface was actually inspired by the moth eye. Moth can see well in the dark due to its remarkable natural antireflective surface covering on the eyes.9 This antireflective surface consists of about millions of periodical protuberances with 300 nm in space and 200 nm in height,10 which are smaller than the wavelength of visible and infrared light. The laterally structured sub-wavelength surfaces act ACS Paragon Plus Environment

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as a graded transition between the air and medium, leading to eliminate the reflection to a large extent. The nanostructured surface can be obtained by both top-down and bottom-up approaches in labs. Aydin et al. have realized the AR nanostructure in the zinc germanium phosphide substrate by using interference lithography and reactive ion etching (RIE).8 The tailored 3D nanostructure with controlled density and shape could be also achieved by using glancing angle deposition (GLAD).11 It was found that the oblique deposited film density decrease with the increased incidence angle.12 Kennedy et al. have deposited graded-index SiO2 by varying the incident angle while rapidly rotating the substrate throughout the process, so that the porosity, as a function of layer thickness, was accurately controlled.7 Such graded-index films with hierarchical nanostructure possess a good antireflection property. In addition, surface nanostructures are often designed and fabricated in non-dielectric material or material system containing non-dielectric material to reduce the reflection and increase the absorption. For instance, silicon wafer surface can be processed in nanotip structure with large reduced reflection property by using plasma etching.1315 Ziegler et al. have fabricated hierarchically 3D silver-silica hybrid nanostructures by using metastable state assisted atomic layer deposition (MS-ALD).14 The metastable silver oxide forms and its decomposition repeat cyclically during the MS-ALD process, leading to cyclic side reaction of silica growth and the consequent formation of the hierarchical nanostructures with highly enhanced absorption and reduced reflection. Magnetron sputtering, as a simple and low-cost method for large-scale production of thin films, is often used to fabricate the dielectric multilayer AR coatings.15-17 Typically, the multilayer stack model consists of a metal base as the substrate, several absorbing layers (cermets), and an antireflection layer on the top.18-22 Zhao and Wäckelgård have fabricated such a multilayer stack, which consists of the absorbing Al/AlN bi-layers with a graded feature and an antireflection layer.23 The metal content in the cermet film can be controlled by the applied nitrogen flow and finally optimized around the maximum refractive index. Recently, Wang et al. have deposited a nanostructured antireflection AlN layer onto a double cermet absorber layer with a DC reactive magnetron sputtering system.24 Interestingly, the silkworm cocoon-like nanostructure was achieved by the optimization of process parameters, which strongly reduces the reflection and scattering of light because of the high amount of nanopores which uniformly distributed at the surface.27 Furthermore, the optimized process parameters in reactive sputtering system play an important role in the realization of AlN films with specific properties.25-30 Zang et al. have deposited AlN films with a preferred orientation via DC reactive sputtering, whose transmittivity has enhanced up to 90% in the visible and near-infrared region.26 In their case, a desired crystallization quality of the films was obtained at a higher sputtering temperature and the flow ratio between nitrogen and argon is 1:3, resulting in an improved texture of the film. Taurino et al. have tuned the film crystallographic orientation of AlN from (002) to (101) by increasing the total pressure in the sputtering system.27 Similarly, this structural switch could also be achieved by increasing the N2 percentage in the total gas flow.28 In this paper, we have used a simple approach based on magnetron sputtering to fabricate the Al-AlN composite thin films with nanostructured surface and with excellent antireflective properties. The Al-AlN composite thin films with submicron thickness are nontransparent due to large amount of the non-dielectric component Al. The approach is based on limited reactive sputtering at the elevated temperature. During the process, a mixture of Ar and N2 gases is used, and the gas flow of N2 is very limited in contrast to Ar. A part of the sputtered Al atoms reacts with N ions and radicals in the plasma and thus formed AlN, while the unreacted Al atoms are directly deposited on the substrate in the same time. Al-AlN composite thin film with flat surface can be fabricated by the single-target reactive sputtering at room temperature, while surface nanostructures can be formed at the elevated temperature (100 – 400 °C) due to ACS Paragon Plus Environment

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different diffusion kinetics between Al and AlN, leading to a dramatic reduction (< 10%) of the average total reflection in the spectral range 300 – 2000 nm. 2. Experimental Section Al-AlN composite thin films were deposited on 4 inch (111)-oriented silicon wafers by DC pulsed reactive magnetron sputtering (CS400ES, von Ardenne) in a gas mixture of Ar (flow rate of 46 sccm) and N2 (flow rate of 5 sccm). Prior to the deposition, the Al target was sputter-conditioned for 5 minutes in Ar plasma to remove the residual contamination on the target surface. The magnetron discharge is pulsed in 300 kHz. The substrate temperature for the deposition has been set for 21 °C, 100 °C, 200 °C, 300 °C and 400 °C, respectively, and the same deposition time (1000 s) and the same sputtering power (700 W) were adopted for all the samples. The samples were cooled down to the room temperature in the vacuum to avoid oxidation of the Al component, before they were taken out of the deposition chamber. In order to investigate the evolution of the surface morphology, another series of samples have been fabricated with varied sputtering time from 30 to 1300 s at temperature of 300 °C. All the other parameters (Ar flow rate: 46 sccm, N2 flow rate: 5 sccm, and power: 700 W) were kept as the same. Scanning electron microscopy (SEM S4800 Hitachi) was used to characterize the surface morphology. The surface roughness was measured by laser scanning microscopy (LSM, LEXT 4100 Olympus). The crystal structure of the films was investigated by X-ray diffraction (XRD, Siemens D5000). EDX (energy dispersive X-ray spectroscopy, thermoFischer scientific) was used for elemental analysis and chemical characterization. UVVis-NIR spectrometry (Cary 5000 Varian) was used to measure the. The electrical resistivity has also been measured by using 4 points measurements (FOUR DIMENSIONS INC., Model 280PI). All the characterized samples were measured directly after the deposition process.

3 Results and Discussions 3.1 Influence of the substrate temperatures In order to investigate the influence of the substrate temperature on the evolved morphology, Al-AlN composite thin films were fabricated at different temperatures, and all the other parameters were kept as the same, as described in the experimental section. Figure 1 (a-e) shows the morphology of the Al-AlN composite thin films deposited at different substrate temperatures. Figure 1 (f-j) shows the corresponding cross-sectional SEM images. It is very clear that surface nanostructures were evolved when the temperature was higher than 100 °C. In order to study the morphology quantitatively, surface roughness was measured by using Laser Scanning Microscopy (LSM). The LSM images are shown in Figure S1 (Supporting Information). Figure 2 shows the plot of surface roughness as a function of the substrate temperature. It can be seen that a maximal roughness can be reached in the sample fabricated at 300 °C. Figure S2 (Supporting Information) shows the plot of Al-AlN composite film thickness as a function of the deposition temperature. The thickness varies from 600 nm to 1.1 µm upon the temperature. The film thickness was measured from the highest point of the film.

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Figure 1. SEM images at tilt of 20° (a-e) of the deposited Al-AlN films at different substrate temperatures: (a) 21 °C, (b) 100 °C, (c) 200 °C, (d) 300 °C, (e) 400 °C, and (f-j) the corresponding cross-sectional images. Inserts from (a) to (e) are the photographs of the Si wafers with deposited Al-AlN thin films at different temperatures, respectively.

Figure 2. Plot of surface roughness as a function of deposition temperatures.

The crystalline structure of the deposited films has been studied with XRD, and Figure 3 shows the X-ray diffraction patterns. 3 groups of peaks from the hcp AlN, fcc AlN and fcc Al are visible in the XRD pattern. The peak intensities of fcc Al for the samples fabricated at the elevated temperatures are clearly higher than those for the sample fabricated at room temperature (21 °C). Both (100) peak of hcp AlN and (111) peak of fcc AlN are clearly visible. In order to estimate the preferred orientation of Al component quantitatively, the texture coefficient was calculated from Eq.1:31, 32

RTC ∑

/

/

100% [1]

In Eq. 1, I corresponds to the diffraction intensities of the (hkl) lines measured in the diffraction pattern and I are the corresponding intensities of an Al powder sample with random orientation. The texture variation depending on different substrate temperatures is shown in Figure S3 (Supporting Information), and there is no strong texture of Al evolved for all samples.


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Figure 3. XRD patterns of the deposited Al-AlN composite thin films at different temperature.

Figure 4. Amount in atomic percentage of Al and AlN in the deposited Al-AlN composite thin films at different temperatures. EDX (energy dispersive X-ray spectroscopy) was used to determine the element composition, and Figure S4 (Supporting Information) show EDX spectra of the composite thin films deposited at different temperatures. Furthermore, the stoichiometric composition of AlN in the deposited thin films was calculated, and Figure 4 shows the plot of the composition of both Al and AlN components as a function of the deposition temperature. All deposited films consist of nearly 50 at% Al and 50 at% AlN. The deposition temperature has almost no influence on the composition. It can be imagined that during the sputtering process, Al atoms have been sputtered out of the surface of the target by Ar ions. A part of the sputtered Al atoms reacted with N ions and radicals in the plasma, and AlN was formed and deposited on the substrate. However, in contrast to the Ar, the flow rate of N2 was very limited. This means that the amount of the N ions and radicals in the plasma were not enough to consume all sputtered Al atoms to form AlN, and still a part of unreacted Al atoms were deposited on the ACS Paragon Plus Environment


substrate at the same time. The ratio of flow rate between Ar and N2 is actually very important for the composition. Only AlN has been deposited even with the flow rate ratio of 5 (Ar:N2 = 5:1).22 In this work, higher flow rate ratio (46:5) was chosen for the formation of Al-AlN composite. Figure 5 shows the total reflection spectra of the Al-AlN composite thin films deposited at different temperatures. The diffuse reflection spectra are shown in Figure S5 (Supporting Information). The reflectance of the thin film deposited at room temperature (21 °C), which has a flat surface and smaller surface roughness, is quite large as around 60% in the wavelength range from 300 nm to 2000 nm. Clearly reduced reflectance (< 10%) can be observed for all the other samples fabricated at elevated temperatures (≥ 100 °C), which have the surface nanostructures and larger surface roughness. It is obvious, that the formed surface nanostructures are responsible for the reduced reflectance. Scattering or even multiple scattering of light is favored on the nanostructured surface, and the reflectance diminishes significantly.

Figure 5. Reflectance spectra of the Al-AlN composite thin films fabricated at different deposition temperatures.

Besides, one additional sample has been fabricated on glass substrate at 300 °C for the measurements of the transmittance and the electrical resistivity. The formed Al-AlN composite thin film is non-transparent, and this is reasonable because around 50% of the AlAlN composite thin film (thickness over 600 nm) is the non-dielectric material, Al. Very thin Al films can show the transparency, only when the film thickness is smaller than 30 nm. So the Al-AlN composite thin films here cannot be used as the AR coating for the enhancement of the transmittance. The measured electrical resistivity is about 0.015 Ω·cm. This means the Al-AlN composite thin films with reduced reflection property possess also a good electrical conductivity, and it is clear that the good electrical conductivity is resulted from the Al component. 3.2 Investigation of evolution of the surface nanostructure The reflectance is highly reduced due to the formation of the surface nanostructure and so it is important to understand the evolution of the surface nanostructure. For that purpose, samples have been fabricated with different deposition time at the temperature of 300 °C. All the other ACS Paragon Plus Environment

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parameters (flow rates of Ar and N2 and power) were kept as the same as described in the experimental section. Figure 6 shows the morphology of the films deposited after 30 to 1300 s. Figure S6 (Supporting Information) shows the corresponding cross-sectional images. Figure S7 (Supporting Information) shows Al-AlN film thickness with the increase of deposition time. Thin film with protruded islands is evolved at first, then the islands grow up with time, and finally surface nanostructure with knoll regions and valley regions is formed.

Figure 6. SEM images at tilt of 20° (a-h) of the deposited Al-AlN composite thin films by different deposition time: (a) 30 s, (b) 180 s, (c) 360 s, (d) 720 s and (e) 1300 s. Inserts are the photographs of the Si wafers with deposited Al-AlN thin films with different deposition time, respectively.

For the sample deposited after 30 s, only weak Al Peaks can be identified in the XRD pattern, and for the other samples, 3 groups of peaks from the hcp AlN, fcc AlN and fcc Al are visible (Figure S8, Supporting Information). The peak intensities increase with increasing deposition time. There is also no strong texture evolution of the component Al by increasing deposition time (Figure S9, Supporting Information). For the evaluation of the texture coefficient, the data from the sample after 30 s deposition is not included due to the weak signals. EDX analysis has been performed (Figure S10, Supporting Information), and the stoichiometric composition of AlN has been calculated, as shown in Figure 7a. It is interesting to note that the amount of Al increases and the amount of AlN decreases upon deposition time. This could be resulted from the depletion of N2 by the reactive sputtering with increasing deposition time. The data of the composition in Figure 4 and Figure 7a are averaged values over a large area of about 2052 µm2. In order to obtain the information about the local composition in the knoll regions and valley regions of the nanostructures, point and shoot mode EDX analysis has been also performed at least in 5 positions (Figure S11, Supporting Information), and local composition is plotted as a function of deposition time in Figure 7b. In the knoll regions, there is a clear fluctuation but generally both components of Al and AlN remain almost the same in halves upon deposition time. However, in the valley regions, the amount of Al increase and the amount of AlN decrease clearly upon the deposition time. Combining the EDX results and the SEM results, it can be seen that at the beginning of the deposition, reactively formed AlN is clearly dominant and AlN protruded islands are formed. Then the component Al becomes dominated, but it is seemed that the Al prefers to ACS Paragon Plus Environment


agglomerate in the island or knoll regions. Finally, the surface nanostructures are evolved. By comparison, only flat AlN thin films could be obtained with dominated reactive sputtering by smaller flow rate ratio (Ar:N2 = 5:1) at 300 °C.22

Figure 7. (a) Amount in atomic percentage of Al and AlN in the deposited Al-AlN films by different deposition time; (b) amount in atomic percentage of local composition by different deposition time.

Figure 8. (a) Cross sectional TEM image of the film deposited after 1300 seconds at 300 °C; and (b) cross sectional TEM image of a region of the film with well-identified Al/AlN interfaces or Al grain boundaries which are marked by the dashed lines. Figure 8a shows the cross sectional TEM image of the Al-AlN composite film deposited after 1300 seconds at 300 °C. The formed surface nanostructures can be clearly seen in the cross sectional view. Figure 8b shows the cross sectional TEM image of a region of the film with well identified Al/AlN interfaces and/or Al grain boundaries. A FFT image of this region is shown in Figure S12 (Supporting Information), and fcc (110) oriented Al, hcp (110) oriented AlN and hcp (100) oriented AlN have been identified. This means the probability of the existence of Al/AlN interfaces in this investigated region is very high. In the Al-AlN composite thin films fabricated at the elevated temperatures, Al clusters are embedded in the AlN. This could also probably explain why no strong Al texture was evolved (Fig. S3, Supporting Information). Usually, thin Al films exhibit strong (111) texture because (111) plane has the smallest surface energy. But in the case of Al clusters embed in AlN, a weak ACS Paragon Plus Environment

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texture could be evolved due to the compensation of strain energy, because the (100) plane has the smallest strain energy. It is clear here, that the interaction of the concurrent deposition of both Al and AlN is very important for the formation of the surface nanostructures. However, the surface nanostructures can be formed only at the elevated temperature, indicating that the active diffusion of Al is important and the different diffusion kinetics between Al and AlN could play an important role here. The melting point of Al and AlN is 660 and 2200 °C, respectively, and the difference is significantly large. At the elevated temperatures (100 – 400 °C), atom diffusion is clearly activated for Al but not for AlN, while there is no active atom or molecule diffusion for both Al and AlN at room temperature (21 °C). Al and AlN should be relatively well mixed in the deposited film at room temperature, which has a flat surface (Fig. 1a and f). However, during the deposition at the elevated temperatures, Al atoms will agglomerate into clusters via atom diffusion, which can be confirmed by the identified Al/AlN interface in Fig. 8b. The clustering and diffusion of Al atoms will disturb the further ongoing sputtering process, leading to the large surface roughness and even nanostructured surface (Fig. 1b-e). 4. Conclusion Al-AlN composite thin films with surface nanostructures can be fabricated by using limited reactive sputtering at the elevated temperature. Such thin films are electrically conductive and possess excellent antireflective properties. The evolution of the surface nanostructures relies on two important conditions: (1) concurrent deposition of both AlN and Al on the substrate, and (2) elevated deposition temperature (100 – 400 °C) at which atom diffusion of Al is well activated and the molecule diffusion of AlN is not. This simple method for the deposition of Al-AlN composite thin films with excellent antireflective properties can be very useful and interesting for many applications. Acknowledgment The authors are grateful to Mrs. Birgitt Hartmann, Mrs. Ilona Marquardt and Mrs. Gabriele Harnisch for their help with sample preparation. This work is funded by Deutsche Forschungsgemeinschaft (DFG, grant SCHA 632/20). Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Additional information on the laser scanning micrographs, texture evolution, EDX spectra, diffuse reflectance spectra, cross-section SEM images, thickness evolution, XRD pattern and FFT image. Reference: 1. Lohmuller, T.; Helgert, M.; Sundermann, M.; Brunner, R.; Spatz, J. P. Biomimetic Interfaces for High-Performance Optics in the Deep-UV Light Range. Nano Lett. 2008, 8, 1429-33. 2. Chhajed, S.; Schubert, M. F.; Kim, J. K.; Schubert, E. F. Nanostructured Multilayer Graded-Index Antireflection Coating for Si Solar Cells with Broadband and Omnidirectional Characteristics. Appl. Phys. Lett. 2008, 93, 251108. 3. Bouhafs, D.; Moussi, A.; Chikouche, A.; Ruiz, J. M. Design and Simulation of Antireflection Coating Systems for Optoelectronic Devices: Application to Silicon Solar Cells. Sol. Energ. Mat. Sol. C. 1998, 52, 79-93. ACS Paragon Plus Environment


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21. Shen, Y.; Shi, Y. Y.; Wang, F. C. High-Temperature Optical Properties and Stability of AlxOy-AlNx-Al Solar Selective Absorbing Surface Prepared by DC Magnetron Reactive Sputtering. Sol. Energ. Mat. Sol. C. 2003, 77, 393-403. 22. Wang, C. B.; Shi, J.; Geng, Z. R.; Ling, X. M. Polychromic Al-AlN Cermet Solar Absorber Coating with High Absorption Efficiency and Excellent Durability. Sol. Energ. Mat. Sol. C. 2016, 144, 14-22.23. Zhao, S. X.; Wackelgard, E. The Optical Properties of Sputtered Composite of Al-AlN. Sol. Energ. Mat. Sol. C. 2006, 90, 1861-1874. 24. Wang, C. B.; Cheng, W.; Ma, P. J.; Xia, R. B.; Ling, X. M. High Performance Al-AlN Solar Spectrally Selective Coatings with a Self-Assembled Nanostructure AlN AntiReflective Layer. J. Mater. Chem. A 2017, 5, 2852-2860. 25. Engelmark, F.; Fucntes, G.; Katardjiev, I. V.; Harsta, A.; Smith, U.; Berg, S. Synthesis of Highly Oriented Piezoelectric AlN Films by Reactive Sputter Deposition. J. Vac. Sci. Technol. A 2000, 18, 1609-1612. 26. Zang, Y.; Li, L. B.; Ren, Z. Q.; Cao, L.; Zhang, Y. Characterization of AlN Thin Film Prepared by Reactive Sputtering. Surf. Interface Anal. 2016, 48, 1029-1032. 27. Taurino, A.; Signore, M. A.; Catalano, M.; Kim, M. J. (101) and (002) Oriented AlN Thin Films Deposited by Sputtering. Mater. Lett. 2017, 200, 18-20. 28. Iqbal, A.; Walker, G.; Iacopi, A.; Mohd-Yasin, F. Controlled Sputtering of AlN (002) and (101) Crystal Orientations on Epitaxial 3C-SiC-on-Si (100) Substrate. J. Cryst. Growth 2016, 440, 76-80. 29. Ke, G. S.; Tao, Y.; Lu, Y. S.; Bian, Y. B.; Zhu, T.; Guo, H. B.; Chen, Y. G. Highly CAxis Oriented AlN Film Grown by Unbalanced Magnetron Reactive Sputtering and its Electrical Properties. J. Alloy Compd. 2015, 646, 446-453. 30. Liao, B. H.; Lee, C. C. Antireflection Coatings for Deep Ultraviolet Optics Deposited by Magnetron Sputtering from Al Targets. Opt. Express 2011, 19, 7507-7512. 31. Spanou, S.; Pavlatou, E. A.; Spyrellis, N. Ni/nano-TiO2 Composite Electrodeposits: Textural and Structural Modifications. Electrochim. Acta 2009, 54, 2547-2555. 32. Camargo, M. K.; Schmidt, U.; Grieseler, R.; Wilke, M.; Bund, A. Electrodeposition of Zn-TiO2 Dispersion Coatings: Study of Particle Incorporation in Chloride and Sulfate Baths. J. Electrochem. Soc. 2014, 161, D168-D175.

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Surface Nanostructured Al-AlN Composite Thin Films with Excellent

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