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Fabrication of Nanocone Arrays in Solution Synthesized ZnO Thin Films for Broad-band Anti- reflection
KTH, School of Information and Communication Technology (ICT).
2015 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Antireflection coatings are very important elements in several optoelectronic devices, such as solar cells and photo-detectors. In the recent years, various nanostructured dielectric materials have been investigated as surface coatings for their broad-band and omni-directional anti-reflection properties. In general, direct surface structuring of optoelectronic devices often implies additional processing steps such as electronic passivation and in some situations, e.g. thin film crystalline Si or III-V solar cells, the demands are more stringent. Some of these issues can be solved by providing appropriate nanostructuring in the surface coating. ZnO is one of the suitable materials for antireflection coatings due to its transparency in visible light spectral region and relatively high refractive index (n~1.9). In addition, Al doped ZnO (AZO) is increasingly employed as transparent conducting contacts in several optoelectronic devices.

This master thesis work investigates methods to obtain relatively thick (~100 nm -1

µm) ZnO films on Si by solution chemistry and develops processes for nanostructuring of ZnO films using colloidal lithography (CL) and reactive ion beam etching (RIBE). Guidelines for the ZnO nanocones/pillar array geometry for low reflectivity in the visible light spectral range were obtained by electromagnetic simulations, and total reflectivity measurements of the fabricated ZnO nanostructures on Si were used for validation. Sputtered ZnO films and solution synthesized ZnO nanowires are also studied for comparison.

Two approaches have been applied for obtaining the ZnO layers. The first approach uses infilling dense assemblies of hydrothermally grown randomly oriented ZnO nanowires with the ZnO seed solution. The second approach is based on drop-casting of the ZnO seed solution on a plane Silicon (Si) substrate followed by annealing at relatively low temperatures (200-400 oC). Only the latter approach

showed a homogeneous, relatively thick (~300 nm) polycrystalline ZnO layer; and by multiple deposition layers up to ~1 µm was easily obtained. The ZnO layers are characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD) and spectrophotometry. XRD results confirm the growth of poly-crystalline ZnO. Transmission measurements performed on the grown ZnO layers, on glass, confirm high transparency in the visible-NIR spectral region, similar to sputtered ZnO films.

Lumerical’s finite difference time domain (FDTD) tool was used to simulate the reflectivity spectra of ZnO nanostructure arrays (hexagonal lattice) with different geometrical parameters such as the period, nanopillar dimensions and shape. The results show that ZnO nanocone arrays have the lowest reflection. Hexagonal arrays of ZnO nanocones/frusta with a period of 500 nm and with nanocone height, bottom and top radii of 600 nm, 250 nm and 30 nm, respectively, can reduce the surface reflection to ~6% in the wavelength range of 400-1000 nm.

Conical frustum ZnO nanostructured arrays were fabricated by a combination of colloidal lithography (CL) and reactive ion beam etching (RIBE). A spin-coating technique was used to obtain a self-assembled silica (SiO2) colloidal particle

(diameter: 0.5 µm) monolayer (~0.1 x 0.1 mm2). The silica (SiO2) particle self-assembly still has to be improved to obtain larger area monolayer coverage. Size reduction of the SiO2 particles was performed by RIBE using CHF3/Ar chemistry to obtain conical frustum structures with the desired top and bottom radius dimensions. A new ZnO etching recipe has been developed in the RIBE system based on CH4/H2/Ar chemistry. The etch rate of the fabricated solution based ZnO layers was found to be ~51 nm/min. In comparison, sputtered films showed much lower etch rate

~30 nm/min. The higher etch rates for the solution synthesized films is partially attributed to porosity. Experimentally measured reflectivities of the different ZnO nanostructure coatings on Si show that nanocone arrays have relatively low reflectivity in the visible spectral range (~8%), and the obtained results are comparable to the simulated data. Future work should examine the angular dependence of reflectivity, and different materials to engineer the refractive index profiles of the nanostructured coatings for optimal anti-reflection.

Place, publisher, year, edition, pages
2015. , 62 p.
Series
TRITA-ICT-EX, 2015:249
Keyword [en]
solution synthesis, ZnO, anti-reflection, FDTD simulation, nanostructures, nanocones, reactive ion beam etching, colloidal lithography
National Category
Nano Technology
Identifiers
URN: urn:nbn:se:kth:diva-210319OAI: oai:DiVA.org:kth-210319DiVA: diva2:1118262
Subject / course
Microelectronics and Applied Physics
Educational program
Master of Science - Nanotechnology
Examiners
Available from: 2017-06-30 Created: 2017-06-30 Last updated: 2017-06-30Bibliographically approved

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