Fundamentally, Light Emitting Diodes (LEDs) are semiconductor light source that emit light when current flows through them. As such, they have found a slew of applications, particularly in lighting, mainly due to their higher luminous efficiency when compared to incandescent or fluorescent technologies. Additionally, LEDs have become cheap to produce; consequently, becoming the lighting technology of choice both indoor and outdoor for private companies and communities in general. However, recent publications have highlighted a fundamental challenge that plagues LEDs, i.e. white LED emission spectrum differs from the incandescent light emission spectrum, with much higher emission in the blue range. This blue wavelength has been proven to tamper with the circadian cycle (biological clock) through the reduction of melatonin hormone production in human beings. The lack of melatonin has serious health implications if left unchecked. Further, the red and near-infrared (IR) emitted from LEDs has been reported to perturb the biological cycle of vegetation and also reduce the mortality of positive phototaxis insects. All said, researchers have proposed LED interference filters that are composed of two materials stacked into many layers for purposes reducing the blue, red and IR from LEDs. However, such a high number of layers implies a high cost added to the difficulty to achieve with good control over a large surface.
In this context, University of Sherbrooke researchers’ team headed by Professor Abdelatif Jaouad and Professor Vincent Aimez from the Laboratory of Nanotechnologies and Nano-systems, Interdisciplinary Institute of Technological Innovations proposed a novel method to fabricate interference filters using Plasma Enhanced Chemical Vapor Deposition (PECVD) to reduce blue and near-infrared wavelengths that are inherent to LED lighting. PECVD materials have proven to be robust, keeping their good quality over time as they are used to passivate most of electronic devices including LEDs, transistors, solar cells – among others, in addition to relatively low cost and high deposition rate. Their work is currently published in the research journal, Optics Express.
The researchers were motivated by the fact that the application of LED filters technology is not possible without reducing the number of layers and without using a low cost technique for thin films deposition. In this view, their research focus was to lower the number of layers by developing a silicon rich silicon nitride material with a high refractive index and a high extinction coefficient in the blue spectral region, with very low absorption in the visible and IR part of the spectrum.
The authors reported that the use of this hybrid absorption/interference concept allowed for a simple fabrication process of the filter composed of only six layers only, even for the targeted complex spectral response. Moreover, the filter response was seen to be uniform and tolerant to incidence angle variation.
In summary, the Canadian research team demonstrated the merging of Si-rich silicon nitride material with silicon oxide to realize an LED interference filter. The proposed PECVD LED interference filters was mainly meant to be coupled to a phosphor-based white LED, however it could still be used with other kind of white LEDs, such as RGB LEDs, with no limitation. Overall, the high potential of PECVD technique for the fabrication of low cost and reproducible interference filters could be used in various applications.
Joffrey Belin, Etienne Grondin, Vincent Aimez and Abdelatif Jaouad. Hybrid absorption interference wide-angle filter using PECVD Si-Rich SixNy and SiOx for LED lighting. Volume 27, Number 9 | 2019 | OPTICS EXPRESS 12519.