Focused Ion Beam Micromachining of GaN Photonic Devices
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ABSTRACT Ga+ and Au+ focused ion beams (FIB) are used to micromachine GaN films. The GaN micromachining has been studied at energies from 30-90 keV, incident angle from 0-30°, and number of repetitive scans from 10 to 50 scans. Trenches milled in GaN have vertical and smooth side-walls and very smooth bottoms. The micromachining rate was found to be fairly independent of ion dose, ranging from 0.4 to 0.6 µm 3/nC for Ga+ and 1 to 2 µm 3/nC for Au+. This translates into an effective yield of of 6-7 atoms/ion for Ga+ and 21-26 atoms/ion for Au+. This represents the highest direct FIB removal yield reported to date. We have also investigated the micromachining of GaN substrate material: c-face sapphire. Using FIB Ga+, sapphire has an effective yield of ~2-2.5 atoms/ion, or approximately 1/3 of the GaN sputtering yield. For the materials investigated, we found the sputtering yield to be inversely proportional to the strength of the material chemical bond. We also describe the application of the FIB µmachining technique to the fabrication of small period Distributed Bragg Reflector (DBR) mirrors for a short cavity GaN laser structure. INTRODUCTION GaN and its alloys are of great interest for visible and UV light emitting devices because of their applications for displays, scanners, printers, optical disks, etc. Recently, commercialized 1 laser diodes have been announced using GaN-based structure grown on sapphire and SiC substrate. Most of the research and development to date is focused on GaN grown on sapphire substrates because of good crystal quality. However, there still remain some fabrication issues concerning sample preparation and processing. In particular, suitable cavity and high reflectivity mirror facets are hard to obtain by conventional processing procedures due to the large 2 misalignment between sapphire and GaN-based materials. Currently, facets are formed by either 3
the cleaving method (which cannot provide lower roughness on the sidewalls), or by dry plasma 4
etching (which generates high ion-induced damage and smooth etched sidewalls). Therefore, it is important to find a simple and efficient processing technique in order to fabricate low mirror loss and mode selection of laser diodes. Conventional edge-emitting semiconductor lasers fabricated by cleaving result in the low mirror reflectivity in the range of ~0.3-0.4. Even though there are distributed feedback (DFB) or 5,6 distributed Bragg reflector (DBR) in the ends of the laser cavity, the overall mirror reflectivity 7,8,9
still remains low. Recently several research groups reported an alternative method to increase the mirror reflectivity of semiconductor lasers by introducing a short (but deep) stack of DBR grating consisting of multilayers of semiconductor materials and air. This is attractive since
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