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Large-scale high aspect ratio Al-doped ZnO nanopillars arrays as anisotropic metamaterials

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Abstract

High aspect ratio free-standing Al-doped ZnO (AZO) nanopillars and nanotubes were fabricated using a combination of advanced reactive ion etching and atomic layer deposition (ALD) techniques. Prior to the pillar and tube fabrication, AZO layers were grown on flat silicon and glass substrates with different Al concentrations at 150-250 °C. For each temperature and Al concentration the ALD growth behavior, crystalline structure, physical, electrical and optical properties were investigated. It was found that AZO films deposited at 250 °C exhibit the most pronounced plasmonic behavior with the highest plasma frequency. During pillar fabrication, AZO conformally passivates the silicon template, which is characteristic of typical ALD growth conditions. The last step of fabrication is heavily dependent on the selective chemistry of the SF6 plasma. It was shown that silicon between AZO structures can be selectively removed with no observable influence on the ALD deposited coatings. The prepared free-standing AZO structures were characterized using Fourier transform infrared spectroscopy (FTIR). The restoration of the effective permittivities of the structures reveals that their anisotropy significantly deviates from the effective medium approximation (EMA) prognoses. It suggests that the permittivity of the AZO in tightly confined nanopillars is very different from that of flat AZO films.

© 2017 Optical Society of America

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Figures (12)

Fig. 1
Fig. 1 ALD deposition conditions. a) Growth per cycle of Al2O3 ZnO and AZO for each temperature regime: 150 °C, 200 °C and 250 °C. b) Schematic drawing of deposited AZO layers and concept illustration of doping “D” number.
Fig. 2
Fig. 2 XPS investigation of AZO layers. a) The typical survey scan of AZO/ZnO. b) High-resolution scan for Al2p region for AZO/ZnO samples prepared at 250 °C. c) Measured Al concentration in AZO films prepared at three temperatures: 150 °C, 200 °C and 250 °C. d) Deviation of the measured Al at. % concentration from the theoretical estimation in case of 250 °C deposition temperature.
Fig. 3
Fig. 3 Morphology inspection. a) SEM and b) AFM images (500 nm × 500 nm) of flat AZO samples prepared at 250 °C.
Fig. 4
Fig. 4 XRD analysis. a) Diffraction patterns of all AZO/ZnO samples prepared at 250°C shown in logarithmic scale. b) Typical area around (002) and (101) ZnO crystal orientations with applied Lorentzian fit function. c) ZnO lattice dimensions constants a and c as a function of Al at. % concentration (samples deposited at 250 °C). d) Estimated grains size as a function of Al concentration in the samples (samples corresponding to all three deposition temperatures 150°C, 200°C and 250°C).
Fig. 5
Fig. 5 The resistivity of all of the prepared AZO/ZnO samples as a) function of Al at. % concentration and b) resistivity vs. grains size for AZO samples.
Fig. 6
Fig. 6 Optical properties of the AZO/ZnO thin films prepared at 250 °C. a) Real ε1 and imaginary ε2 parts of permittivity. b) Transmission spectra with a small absorption edge shift (shown in the inset). c) Absorption coefficient. d) The plot of (αhν)2 versus photon energy (Tauc plot). e) Optical bandgap as a function of Al concentration.
Fig. 7
Fig. 7 Schematics of the fabrication flow. a) Home-made SOI substrates. b) Deep-UV lithography. Resist spin coating, baking, exposure and developing. c) DRIE etching, fabrication of initial Si template. d) ALD deposition of D25 AZO at 250 °C. Partial deposition will lead to fabrication of tubes, while complete filling will create full pillars. e) Removal of the top AZO layer by Ar+ sputtering. f) Silicon host removal using conventional RIE process.
Fig. 8
Fig. 8 Bird-eye-view SEM images of the prepared structures: a) AZO pillars and b) AZO tubes. The insets show an enlarged view of the metamaterials.
Fig. 9
Fig. 9 TEM inspection. a) High-resolution TEM image of the produced pillars. b) SAED pattern of AZO D25 pillar. c) and d) are dark field images: c) is the image at low magnification, d) are enlarge images of the same area with different positions of the object aperture. e) EDX elemental mapping and HAADF imaging of AZO nanopillar.
Fig. 10
Fig. 10 a) Measured and fitted reflectance spectra for the DSP Si substrate together with b) its retrieved permittivity. c) Measured and fitted reflectance spectra for the 100 nm thick D25 AZO film together with d) its retrieved permittivity.
Fig. 11
Fig. 11 Measured and fitted reflectance spectra for (a) Air/AZO ordinary, (b) Air/AZO extraordinary, (c) Si/AZO ordinary and (d) Si/AZO extraordinary cases.
Fig. 12
Fig. 12 Fitted real and imaginary part of effective ordinary (solid line) and extraordinary (dashed line) permittivities, εo and εe, for (a,c) Air/AZO and (b,d) Si/AZO structures, as well as effective permittivities calculated by effective medium approximation. The inset is the scanning electron microscope image of the measured Air/AZO and Si/AZO structures, respectively. The scale bars in both insets are 1 μm.

Tables (6)

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Table 1 Recipe for one AZO macrocycle.

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Table 2 Screened plasma frequencies from 250 °C AZO thin films retrieved from extrapolated ε1(λ) functions.

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Table 3 DRIE parameters for Si template fabrication.

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Table 4 Retrieved dielectric function parameters for the Si substrate.

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Table 5 Retrieved dielectric function parameters for 100nm AZO film.

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Table 6 Retrieved dielectric function parameters for Air/AZO and Si/AZO pillar metamaterials. (units for plasma frequencies, damping and Lorentzian resonance frequencies are all in THz)

Equations (9)

Equations on this page are rendered with MathJax. Learn more.

nλ=2dsinθ
1 d 2 = 4 3 ( h 2 +hk+ k 2 a 2 )+ l 2 c 2 ,
D= Kλ β INT cosθ , where β INT = β observed INT β istrumental INT .
r p r s =tan( Ψ ) e iΔ ,
αln( T )
( αhν ) 2 ( hν E g ).
ε( ω )= ε ( 1 ω p 2 ω 2 +iωγ )+ j S j ω f,j 2 ω f,j 2 ω 2 iω Γ j
ε o EMA = ( 1+f ) ε m ε d +( 1f ) ε d 2 ( 1+f ) ε d +( 1f ) ε m
ε e EMA =f ε m +( 1f ) ε d
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