Elsevier

Surfaces and Interfaces

Volume 34, November 2022, 102355
Surfaces and Interfaces

SPR based refractive index modulation of nanostructured SiO2 films grown using GLAD assisted RF sputtering technique

https://doi.org/10.1016/j.surfin.2022.102355Get rights and content

Abstract

Low refractive index materials have always gained attention for avoiding optical losses in numerous devices like Light emitting diodes (LEDs), Distributed Bragg Reflectors (DBRs) etc. Glancing angle deposition (GLAD) has proven to be an effective method for depositing porous nanostructures with low refractive index. SiO2 has the lowest refractive index in comparison to most of the materials present in nature and is therefore utilized extensively in the fabrication of optical devices with minimum optical losses. The present work focuses on the SiO2 nanostructured thin films grown using glancing angle (GLAD) configuration in RF Sputtering technique under varying deposition conditions (glancing angle and pressure) at fixed thickness. The deposited porous SiO2 nanostructured layers have well-defined nanorods of feature size smaller than 100 nm. The refractive index corresponding to all the deposited SiO2 nanostructured films came out to be smaller than the corresponding value (1.49) mentioned in the literature for bulk SiO2. The refractive index has been observed to vary in a wide range (1.05 to 1.40). Characterization of the deposited SiO2 nanostructured films was carried out using Fourier Transform Infrared (FTIR) spectroscopy, Energy Dispersive X-Ray (EDS) analysis, and Atomic Force Microscopy (AFM) to assess the qualitative properties of the deposited nanostructured SiO2 films. Surface Plasmon Resonance (SPR) and Ellipsometry studies are performed to determine the refractive index value of the deposited nanostructured SiO2 thin films. A refractive index value as low as ∼ 1.05 at a wavelength of 633 nm is achieved for SiO2 nanostructured thin film deposited at 30 mTorr deposition pressure and 80° glancing angle.

Introduction

Refractive index of any material is considered to be the fundamental quantity in optical sciences, which was first introduced by Issac Newton in 1807. Refractive index has a key role in determining the reflection, refraction and diffraction occurring at the interface of two media having different optical properties [1]. The importance of refractive index for the study of light propagation at the interface of two dielectric media, has attracted attention of research scientists and engineers worldwide [2]. There is a scarcity of materials having low refractive index especially when it comes to optical thin films [2]. To the best of our knowledge, there is no natural existing material with refractive index value in the range of 1.0 – 1.2. The demand of low refractive index materials is pitch high because of their application as Anti Reflection coating in various optical devices such as in Distributed Bragg Reflectors, filters, optical micro-resonators, LEDs, solar cells, photonic crystals, lasers and optical interconnects, to reduce the reflective losses from the interface [1], [2], [3], [4]. Of all the available materials, SiO2 is known to possess low refractive index value of 1.49 in its bulk form as well as good durability and environmental resistance [5]. It is reported by several researchers that the refractive index of a material can be lowered by introducing porosity in it, keeping the size of the pores much smaller as compared to the electromagnetic wavelength under consideration [6]. Porosity can be introduced in SiO2 by depositing its nanostructures as reported in literature [7,8]. The technique mainly employed by researchers to deposit SiO2 nanostructures is oblique angle assisted e-beam evaporation [1,[9], [10], [11]]. Oblique angle deposition is an excellent method for the deposition of low refractive index nanostructured thin films using self-shadowing effect. In this effect, the region falling in the shadow of the nucleus does not receive any further vapor flux during the nucleation of the evaporant on the substrate. As a result, columnar structure is developed since vapor is deposited only on nucleus. In oblique angle deposition, the substrates placed at oblique angle with respect to the target are kept stationery during deposition [12]. However, e-beam evaporation suffers from some disadvantages such as non-uniform evaporation rate, high maintenance and capital costs, large apparatus size, elevated temperatures, difficult to control the deposition parameters such as deposition pressure, film thickness, foaming, scaling or corrosion issues etc. Radio Frequency (RF) Sputtering is a comparatively better technique for the deposition of SiO2 nanostructured thin films. It possesses many advantages such as capability of in-situ substrate cleaning before the film deposition, stoichiometric transfer of target composition in the deposited films, superior film quality and better step coverage, uniformity, higher packing densities and better adhesion, lower temperature depositions, better reproducibility etc.

Glancing Angle Deposition (GLAD) is an advanced version of Oblique angle deposition where the substrate is rotated during deposition to modify the structure of the nanostructures as per requirement. The location of vapor source with respect to the growing columnar structure changes as a consequence of the rotation of substrate. Hence, the growth of columns is in accordance with the apparent change in the location of source. This enables the sculpting of the columnar growth by dynamically rotating the substrate during the deposition. GLAD technique has been exploited by many researchers in recent times for the deposition of controllable columnar nanostructures for applications in various devices. GLAD is a physical vapor deposition technique which provides good control over the porosity, achievable by the surface diffusion and self-shadowing effect [13].

Surface Plasmon Resonance (SPR) is an excellent optical tool to determine the optical constants such as refractive index and dielectric constant of any material. There are two configurations to study prism coupled SPR: Kretschmann configuration and Otto configuration. Otto configuration is advantageous as it can be utilized to study the film deposited on any substrate unlike the case in Kretschmann configuration where the film must be deposited on Gold (Au) coated prism. In Otto configuration, the dielectric thin film having a metal thin film over it, is placed closed enough to the base of the prism so that the evanescent waves generated due to the total internal reflection can interact with the plasma waves generated at the metal-dielectric interface, thereby exciting the Plasmons.

In the current work, details of the fabrication of porous nanostructured SiO2 thin films using GLAD incorporated RF Sputtering technique are reported. The tunability of refractive index of the deposited amorphous SiO2 nanostructures has been determined by varying the pressure of deposition (10 mTorr, 20 mTorr and 30 mTorr) and the glancing angle (70°, 75°, 80° and 85°), keeping the thickness constant (200 nm). Firstly the deposition pressure was varied keeping the thickness and glancing angle constant (at 70°) to achieve the lowest refractive index and then glancing angle was varied from 75° to 85° keeping the deposition pressure constant for attaining further reduction in the refractive index. The structural, elemental and morphological studies of the grown SiO2 nanostructured films are carried out by Fourier Transform Infrared (FTIR) Spectroscopy, Energy Dispersive X-Ray (EDS), and Atomic Force Microscopy (AFM) analysis. Refractive index of the deposited films is calculated with the aid of Surface Plasmon Resonance (SPR) method employing Otto configuration. Also obtained refractive index values from SPR have been verified by Ellipsometry studies. As per our knowledge, this presents the first comprehensive report on refractive modulation of SiO2 nanostructures deposited by GLAD assisted RF sputtering technique (at varying deposition parameters), using Surface Plasmon Resonance technique.

Section snippets

Experimental Details

The SiO2 nanostructured films have been deposited via GLAD assisted RF magnetron sputtering technique. The sputtering technique in glancing angle deposition (GLAD) configuration has been utilized in many studies for the growth of nanostructures. GLAD is an extension to oblique angle deposition technique. In oblique angle deposition, the substrate surface is tilted with respect to the incoming vapor flux (angle of substrate tilt is α). The incoming atoms from the target (source) reach the

FTIR spectroscopy

Fig. 2(a) and (b) show the IR spectra of the prepared amorphous SiO2 nanostructured thin films on Si substrate at a fixed glancing angle of 70° and three different deposition pressures (10 mTorr, 20 mTorr and 30 mTorr) and at fixed deposition pressure 30 mTorr and three different glancing angles of 75°, 80° and 85°, respectively. In both the Fig. 2(a) and (b), the observed peak at 574 cm−1 is assigned to the Si–O vibrational bond. Whereas, peaks at 813 cm−1 and 1067 cm−1 show the shape and

Conclusion

The surface plasmons are excited at SiO2/Au/air interface and the refractive index of the films grown using Glancing angle deposition (GLAD) configuration in RF magnetron sputtering at varying deposition parameters (deposition pressure and glancing angle) were estimated using SPR and verified by Ellipsometry studies. The deposited porous SiO2 nanostructured layers have well defined nanorods of feature size smaller than 100 nm (a lot lower than the visible light wavelength). The grown low

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Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

All persons who have made substantial contributions to the work reported in the manuscript (e.g., technical help, writing and editing assistance, general support), but who do not meet the criteria for authorship, are named in the Acknowledgements and have given us their written permission to be named. If we have not included an Acknowledgements, then that indicates that we have not received substantial contributions from non-authors.

Authors are thankful to Department of Science and Technology

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The author passed away prior to the submission of this paper. This is one of the last works of him.

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