In this paper, we discuss an impact of thin titanium dioxide (TiO2) coatings on refractive index (RI) sensitivity and biofunctionalization of long-period gratings (LPGs). The TiO2 overlays on the LPG surfaces have been obtained using atomic layer deposition (ALD) method. This method allows for a deposition of conformal, thickness-controlled, with well-defined optical properties, and high-RI thin films which are highly desired for optical fiber sensors. It has been found that for LPGs working at a dispersion turning point of higher order cladding modes only tens of nanometers of TiO2 overlay thickness allow to obtain cladding mode transition effect, and thus significant improvement of RI sensitivity. When the TiO2 overlay thickness reaches 70 nm, it is possible to obtain RI sensitivity exceeding 6200 nm/RIU in RI range where label-free sensors operate. Moreover, LPGs with TiO2-enhanced RI sensitivity have shown improved sensitivity to bacteria endotoxin (E. coli B lipopolysaccharide) detection, when TiO2 surface is functionalized with endotoxin binding protein (adhesin) of T4 bacteriophage.

Label-free sensitivity of long-period gratings enhanced by atomic layer deposited TiO 2 nano-overlays

This work presents an application of thin aluminum oxide (Al2O3) films obtained using atomic layer deposition (ALD) for fine tuning the spectral response and refractive-index (RI) sensitivity of long-period gratings (LPGs) induced in optical fibers. The technique allows for an efficient and well controlled deposition at monolayer level (resolution ~ 0.12 nm) of excellent quality nano-films as required for optical sensors. The effect of Al2O3 deposition on the spectral properties of the LPGs is demonstrated experimentally and numerically. We correlated both the increase in Al2O3 thickness and changes in optical properties of the film with the shift of the LPG resonance wavelength and proved that similar films are deposited on fibers and oxidized silicon reference samples in the same process run. Since the thin overlay effectively changes the distribution of the cladding modes and thus also tunes the device’s RI sensitivity, the tuning can be simply realized by varying number of cycles, which is proportional to thickness of the high-refractive-index (n > 1.6 in infrared spectral range) Al2O3 film. The advantage of this approach is the precision in determining the film properties resulting in RI sensitivity of the LPGs. To the best of our knowledge, this is the first time that an ultra-precise method for overlay deposition has been applied on LPGs for RI tuning purposes and the results have been compared with numerical simulations based on LP mode approximation.

https://www.mdpi.com/1424-8220/13/12/16372/pdf

Capability for Fine Tuning of the Refractive Index Sensing Properties of Long-Period Gratings by Atomic Layer Deposited Al2O3 Overlays

We demonstrate localized surface plasmon (LSP)-enhanced near-ultraviolet light-emitting diodes (NUV-LEDs) using silver (Ag) and platinum (Pt) nanoparticles (NPs) The optical output power of NUV-LEDs with metal NPs is higher by 201% for NUV-LEDs with Ag NPs and 579% for NUV-LEDs with Pt NPs at 20 mA than that of NUV-LEDs without metal NPs The time-resolved photoluminescence (TR-PL) spectra shows that the decay times of NUV-LEDs with Ag and Pt NPs are faster than that of NUV-LEDs without metal NPs The TR-PL and absorbance spectra of metal NPs indicate that the spontaneous emission rate is increased by resonance coupling between excitons in the multiple quantum wells and LSPs in the metal NPs

Localized surface plasmon-enhanced near-ultraviolet emission from InGaN/GaN light-emitting diodes using silver and platinum nanoparticles.

Rapid progress in the performance of organic devices has increased the demand for advances in the technology of thin-film permeation barriers and understanding the failure mechanisms of these material systems. Herein, we report the extensive study of mechanical and gas barrier properties of Al2O3/ZnO nanolaminate films prepared on organic substrates by atomic layer deposition (ALD). Nanolaminates of Al2O3/ZnO and single compound films of around 250 nm thickness were deposited on polyethylene terephthalate (PET) foils by ALD at 90 °C using trimethylaluminium (TMA) and diethylzinc (DEZ) as precursors and H2O as the co-reactant. STEM analysis of the nanolaminate structure revealed that steady-state film growth on PET is achieved after about 60 ALD cycles. Uniaxial tensile strain experiments revealed superior fracture and adhesive properties of single ZnO films versus the single Al2O3 film, as well as versus their nanolaminates. The superior mechanical performance of ZnO was linked to the absence of a roughly 500 to 900 nm thick sub-surface growth observed for single Al2O3 films as well as for the nanolaminates starting with an Al2O3 initial layer on PET. In contrast, the gas permeability of the nanolaminate coatings on PET was measured to be 9.4 × 10−3 O2 cm3 m−2 day−1. This is an order of magnitude less than their constituting single oxides, which opens prospects for their applications as gas barrier layers for organic electronics and food and drug packaging industries. Direct interdependency between the gas barrier and the mechanical properties was not established enabling independent tailoring of these properties for mechanically rigid and impermeable thin film coatings.

https://hal.umontpellier.fr/hal-02076308/document

Fracture Mechanics and Oxygen Gas Barrier Properties of Al2O3/ZnO Nanolaminates on PET Deposited by Atomic Layer Deposition

A study on the plasma-enhanced atomic layer deposition of amorphous inorganic oxides SiO2 and Al2O3 on polypropylene (PP) was carried out with respect to growth taking place at the interface of the polymer substrate and the thin film employing in situ quartz-crystal microbalance (QCM) experiments. A model layer of spin-coated PP (scPP) was deposited on QCM crystals prior to depositions to allow a transfer of findings from QCM studies to industrially applied PP foil. The influence of precursor choice (trimethylaluminum (TMA) vs [3-(dimethylamino)propyl]-dimethyl aluminum (DMAD)) and of plasma pretreatment on the monitored QCM response was investigated. Furthermore, dyads of SiO2/Al2O3, using different Al precursors for the Al2O3 thin-film deposition, were investigated regarding their barrier performance. Although the growth of SiO2 and Al2O3 from TMA on scPP is significantly hindered if no oxygen plasma pretreatment is applied to the scPP prior to depositions, the DMAD process was found to yield comparable...

PEALD of SiO2 and Al2O3 Thin Films on Polypropylene: Investigations of the Film Growth at the Interface, Stress, and Gas Barrier Properties of Dyads.

Mechanical and electrical properties of Al2O3 films are compared for plasma-assisted atomic layer deposition (ALD) and thermal ALD on two substrates, GaAs and Si, of different thermal expansion coefficient. Films with stable chemical structure and mechanical residual stress could be produced by both techniques without inducing any damage to sensitive multiquantum-well structures. However, the as-deposited residual stress in the plasma ALD Al2O3 films is lower and decreases, while that in the thermal ALD films increases with the deposition temperature. Moreover, the stress hysteresis observed upon thermal cycles is much lower for the plasma ALD films compared to that for the thermal ALD films. The biaxial elastic modulus (BEM or stiffness parameter) increases with the deposition temperature for both ALD films, being higher for the plasma ALD than that for the thermal ALD at a given temperature. The higher BEM is reflected in better electrical properties of the films. Thus, the leakage current of metal–oxide–semiconductor capacitors with the plasma ALD-Al2O3 film is three orders of magnitude lower and the breakdown voltage 20% higher than that of the capacitors with the thermal ALD film.

Mechanical and electrical properties of plasma and thermal atomic layer deposited Al2O3 films on GaAs and Si

We report on the PAMBE-growth of high-quality InN layers with In polarity and on the influence of growth parameters such as Indium-to-nitrogen flux ratio as well as substrate temperature on the electronic transport characteristics of these layers. The flux ratio was changed between 0.94 to 1.18 near stoichiometric conditions and the growth temperature was varied between 400 and 485 °C as measured by calibrated pyrometer temperature. The InN films were grown on metal-organic vapour deposition (MOCVD) GaN templates 3″ in diameter with InN layer thicknesses of 700 nm as determined by spectroscopic ellipsometry (SE). Under In-rich and near stoichiometric growth conditions a minimum surface root mean square roughness of 0.6 nm was achieved. High resolution X-ray diffraction show purely hexagonal phase growth and no evidence for the presence of cubic inclusions. InN layers grown under optimized growth conditions exhibit a narrow (FWHM 20 meV) near band edge low temperature photoluminescence (PL) spectrum with a PL peak energy of 0.67 eV, indicating an InN band gap energy of ≈ 0.7 eV, consistent with the dielectric function spectrum derived from SE measurements. Room temperature van-der-Pauw Hall measurements reveal high electron mobilities up to 1910 cm2/Vs with mean bulk electron concentrations as low as 5 x 1017 /cm3. Furthermore, efficient surface emission of THz radiation has been demonstrated. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transport characteristics of indium nitride (InN) films grown by plasma assisted molecular beam epitaxy (PAMBE)

Two wavelength series (373-435 nm) of GaInN/GaN triple quantum well LED-structures, one on sapphire and one on GaN-templates, as well as a quantum well thickness series were grown by MOVPE. In order to study luminescence efficiency limiting effects the LED-structures were characterized by electroluminescence- and by temperature and excitation power dependent photoluminescence spectroscopy. The electroluminescence output and the internal quantum efficiency are peaked near 400 nm in both wavelength series, but output and efficiency are significantly enhanced for growth on templates. The photoluminescence data indicate that below 400 nm the efficiency, apart from insufficient vertical carrier confinement, is limited by poor localization within the quantum film plane while above 400 nm piezo-electric fields appear to be an important efficiency limiting effect. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Near‐UV to violet LEDs – Wavelength dependence of efficiency limiting processes

Temperature and excitation power dependent photoluminescence spectroscopy on GaInN UV-violet LED structures allows to separate the influence of the quantum confined stark effect (QCSE) from carrier localization in In-induced band tail states as well as to determine the internal quantum efficiency (IQE). A LED series with different quantum well widths (1-3.7 nm) was grown by MOVPE on sapphire and low defect density (LDD) GaN templates. The photoluminescence data reveal, that the QCSE cannot be neglected even for low In-content (<10%) ultra violet LED structures. From temperature and laser power density dependent photoluminescence, the internal QE is evaluated. The data shows, that upon growth on LDD substrates, the internal QE is considerably increased for short wavelength (375-400 nm) LEDs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Well width dependent luminescence characteristics of UV‐violet emitting GaInN QW LED structures

Three series of GaInN/GaN light-emitting diodes (LEDs) emitting at 400 nm with well widths varying between 3 and 18 nm were grown on sapphire, ultra-low dislocation density GaN templates, and free-standing GaN substrates. Photoluminescence and pulsed electroluminescence were systematically investigated. Photoluminescence spectroscopy showed a noteworthy increase in radiative efficiency for GaInN/GaN double-heterostructures or wide wells on low dislocation density substrates. The electroluminescence properties significantly differ for wide well LEDs on sapphire and on low defect density substrates but are comparable for LEDs containing thin single quantum wells. Increasing well width leads to decreasing quantum efficiency at all currents for LEDs on sapphire. The highest overall efficiency is obtained for LEDs with 11 nm and 18 nm thick DHs on GaN templates and free-standing GaN substrates, respectively, at current densities above 200 A/cm2. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Crenguta-Columbina Leancu

Papers

Mechanical and electrical properties of plasma and thermal atomic layer deposited Al2O3 films on GaAs and Si

Transport characteristics of indium nitride (InN) films grown by plasma assisted molecular beam epitaxy (PAMBE)

Near‐UV to violet LEDs – Wavelength dependence of efficiency limiting processes

Well width dependent luminescence characteristics of UV‐violet emitting GaInN QW LED structures

Influence of substrate dislocation density and quantum well width on the quantum efficiency of violet-emitting GaInN/GaN light-emitting diodes