Study Of SiO₂/Si₃N₄Distributed Bragg Reflectors For Ultraviolet Optoelectronic Applications

As a necessary part of VCSEL, RCLED and RCE PD, distributed Bragg reflectors (DBRs) have recently attracted much attention. In order to get higher internal quantum efficiency and less absorption, an efficient solution consists in inserting two DBRs in these devices to build up a Fabry-Perot cavity. DBRs can indeed be used to improve the light extraction efficiency and to obtain a narrower and more directional emission. In this thesis, SiO2/Si3N4DBR has been studied systematically and main achievements include:1. The deposition rates of SiO2and Si3N4increase with the deposition time, and the film quality is very sensitive to growth conditions. The quality of SiO2deposited with standard conditions is not satisfied, however, it is improved remarkably after the growth conditions are optimized. Further studies indicate that there are significant amount of oxygen atoms not only in the surface but also the entire Si3N4film. The oxygen atoms under the surface of Si3N4film are due to the contaminated cavity during pumping to vacuum condition, and the oxygen in the surface is mainly due to the surface oxidation.2. A SiO2/Si3N4DBR is designed based on the TMM, which was grown by PECVD on sapphire (0001). Typical transmission spectra show that the absorption of SiO and Si3N4should be considered when the edge of stopband is less than300nm. The peak wavelength of reflectivity spectra blue shifts with the increasing number of DBR pairs. The13-pair DBR provides a58nm wide stopband centered at333nm with the maximum reflectivity higher than99%, as the refractive index ratio of Si3N4to SiO2is relatively high. It is proved by the SEM and AFM measurements that the variations of thickness and roughness of Si3N4layer with respect to SiO2layer during growth contribute to the blue shift of peak wavelength. The XRR measurements indicate that the interfacial degradation of the samples has little effect on the maximum reflectivity. The calculated results, which use the quasi-three layers model instead of ideal two layers model, fit the measurements well.3. The studies of double DBRs have been carried out. The interface of the two DBRs must be well determined or there will be a reflectivity decrease when its thickness is almost as much as double layers.The reflectivity spectra shows that the reflectivity of double DBRs is not linear superposition of two DBRs, as the interference between the top and bottom DBR is evident. However, the maximum reflectivity at the stopband of double DBRs is higher than single DBR with same pairs, and this indicates that the reflectivity of one DBR of double DBRs could be enhanced by the other DBR. How such interference influences the reflectivity spectra of double DBRs is not clear right now. Detailed work is needed in the future.