In this study, on one hand, we present our theoretical analysis of III-N photonic waveguides at visible spectral wavelength. Due to the n-type conductivity and high dislocation density inside GaN, free carrier absorption contributes significant amount of loss when waveguide scale is on order of microns. Moreover, at high power density operation, two photon absorption loss dominates in GaN waveguide. However, for AlN waveguide, the optical loss is still contributed from sidewall scattering. The theoretical analysis may guide the design of III-N waveguide for wide range of applications.
On the other hand, we developed a fabrication process for GaN and AlN waveguide on sapphire substrate and did a comprehensive study on the process to minimize the optical loss. Samples in this work are all grown by metal-organic chemical vapor deposition (MOCVD) Different types of masks for this process (Photoresist, Cr/SiO2, Cr/SiNx, Cr, Ni) are investigated and our results show that Cr/SiO2 and Cr hardmasks are good for GaN/AlN waveguide fabrication when using electron beam lithography (EBL). Inductively coupled plasma (ICP) etching parameters are optimized to minimize sidewall roughness. Scanning electron microscope (SEM) is used to justify the surface morphology for different etching recipes. 500/70W ICP/Platen power at 5mT pressure with 30/8/5 sccm Cl2/BCl3/Ar2 etching chemistry is found to be good for GaN etching, while for AlN, 400/180W ICP/Platen power at 2mT pressure with 30/8/5 sccm Cl2/BCl3/Ar2 etching chemistry is found to be good for etching. The low pressure and high platen power for AlN etching recipe is to enhance the ion directionality in order to break chemical bonds efficiently.
To minimize sidewall roughness, Tetramethylammonium hydroxide (TMAH) wet etching post treatment is implemented. SEM images show that the chemical wet etching process behaves totally different for low temperture (buffer layer) and high temperture grown III-N layers. SEM imags also indicate that wet etching has different impact on straight and curved sidwalls.
To remove mask, buffered HF and diluted HCl are used for SiO2 and Cl hardmasks, respectively, while for photoresis, the mask is removed by O2 plamsa and acetone. The HF is found to cause surface erosion on AlN which is also observed in sputtered AlN. Edge polishing is performed to increase coupling efficiency after the whole process.
Using the process developed in this research, low loss GaN waveguides are fabricated at the height of 1.5 um and width varies from 1.2 to 2 um. Testing is performed using a Ti:S laser at 800 nm and 720 nm. Our characterization shows that 70% of the waveguide exhibits optical loss <10dB/cm, 20% of waveguides show low loss <2dB/cm. The lowest loss obtained in this work is less than 1dB/cm, which is the lowest loss to the best of our knowledge on GaN waveguide. We will also discuss the pathway to improve the fabrication process. We hope our works bring valuable information to the society.