High aspect ratio etching of GaSb/AlGaAsSb for photonic crystals
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High aspect ratio etching of GaSb/AlGaAsSb for photonic crystals Nilsen, Tron Arne 1, 2 ; Martinez, Anthony 2 ; Bugge, Renato 1 ; Moscho, Aaron 2 ;Lester, Luke F2 and Fimland, Bjørn-Ove 1 1 Department of Electronics and Telecommunications, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway. 2 Center For High Technology Materials, University of New Mexico, Albuquerque, NM, USA. ABSTRACT Photonic crystal structures defined by interferometric lithography were etched into GaSb and AlGaAsSb with 90% Al content using Inductively Coupled Plasma (ICP) Reactive Ion Etching (RIE) with BCl3 and BCl3/Ar gas mixture. Effects of DC bias, hole diameter, etch time and gas composition, on the etch rate of GaSb were investigated. Hardened photoresist (PR) was used as an etch mask for the experiments. INTRODUCTION Emitters in the 1.7-2.4 μm (mid-IR) spectral range are useful for a broad range of applications such as tunable diode laser absorption spectroscopy, free space optical communications and medical surgery. GaSb-based emitters are well suited for emission in the mid-IR range and we are interested in using photonic crystals (PC) in these materials. PCs are useful for enhancing light extraction from LEDs, PC defect lasers and PC distributed feedback lasers. As the periodicity of any given PC structure is directly proportional to the wavelength the PC is designed for, working in the mid-IR would also be beneficial to the study of photonic crystals themselves as the critical dimensions would be larger than at near IR or visible and thus the PCs will be easier to fabricate. The structure of a typical GaSb based light emitter consists of high Al content (80-90%) AlGaAsSb upper and lower cladding layers with a core consisting of low Al (around 25%) AlGaAsSb spacer and high Ga content GaInAsSb quantum wells (QW). To have a uniform etch when the PC pattern is etched into or through the core, there should be no selectivity between the high and low Al content AlGaAsSb. It is assumed that the selectivity between GaSb and high AlGaAsSb forms the upper bound of the selectivity between the high and low Al content AlGaAsSb. The cladding layers of such a structure are usually 1-2 μm and the core between 0.4 and 1 μm. We wish to examine the feasibility of manufacturing a photonic crystal with an inplane TE band gap in this structure. As a first step we whish to characterize the etch of photonic crystal features in these structures in order to determine what structures are possible to manufacture. There has been some previous work done on dry etching of these materials [1,2,3], but there is little on the etching of the features required for PCs [4]. PCs with different lattice constants and hole diameters are experimented with in order to give us flexibility in the future design of PCs. Sidewall profiles are paid close attention to as non-vertical sidewalls can increase out-of-plane losses and makes realistic simulations harder. We begin this paper by examining the effect of changing DC bias on the GaSb etch
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