Design And Fabrication Of UV Band-pass Filters Based On SiO₂/Si₃N₄ Dielectric Distributed Bragg Reflectors

In recent years, ultraviolet(UV) photodetectors(PDs) have been attached much attention for their potential applications to solar UV monitoring, flame sensors, space communication and missile alarms. Due to the band edge absorption of semiconductors, some traditional kinds of PDs, such as photoconductors, Schottky photodiodes and p-i-n junction photodiodes, usually present photo-detection on long wavelength edge, which becomes a bottle-neck for responsivity enhancement in specific wavelength.Recently, a concept of resonant cavity enhanced (RCE) was suggested on AlGaN-based UV PDs to overcome above shortcomings. The RCE PD structure is consisting of a thin active layer inside a Fabry-Perot cavity with one top DBR mirror and one bottom DBR. Based on the sandwich structure, a single absorption layer actually serves many times in generating photo-carriers, so that the responsivity is enhanced dramatically at the resonant wavelength, therefore, high quantum efficiency can be achieved.In contrast, the dielectric distributed Bragg reflectors (DDBR), consisted by different oxide or nitride dielectric materials, can achieve very high reflectivity-99% with fewer pairs compared to nitride-based DBRs because of the greater difference between their refractive index. Secondly, the larger band gap of dielectric materials, the self-absorption problem becomes less serious. Moreover, the SiO2/Si3N4 DBR can be deposited by plasma-enhanced chemical vapor deposition(PECVD) or sputtering, which is more cost-efficient than MOCVD growth of nitride-based DBRs. The PECVD technique can also control the precise layer thickness and clear interfaces between multi-layers.In this article, wede signed series of band-pass filters composed of tandem SiO2/Si3N4 DDBRs which owns a wide and flexible passband by controlling the structural parameters. A number of methods were employed to analyze the causes for degradation of these filters. The main conclusions are listed as follows:1. The refractive index, deposition rate, thickness and many other important parameters of grown films were measured. All samples were deposited by PECVD under standard condition. The results show that grown SiO2 and Si3N4 films have high quality which is actually the basis of manufacturing DBR and filters.2. We designed and fabricated two series of band-pass filters. One series of filters with different central wavelengths of passband ranging from 310nm to 370nm, and the other one with varied passband width ranging from 30nm to 45nm.The generated ripples originating from the interference effects between the coupled top and bottom DDBRs is believed to be one reason for filters’ degradation of optical performance.3. All the filters exhibited smooth surface and good periodicity, as revealed by AFM and SEM measurements. Roughness is considered to be one reason for filters’ degradation.The formed silicon oxynitride layer between SiO2 and Si3N4 sub-layer as well as oxidation of Si3N4 layer has been proved to be one influence factor for reflectivity and width of stop bands by XPS analysis. Thus, a new model of 3-layer unit structure has been introduced which can give a better fitting compared with that of 2-layer unit structure.