hv photonics Communication Development of Micron Sized Photonic Devices Based on Deep GaN Etching Karim Dogheche 1, Bandar Alshehri 2, Galles Patriache 3 and Elhadj Dogheche 1,* 1 Institute of Electronics, Université Polytechnique Hauts de France, Microelectronics & Nanotechnology IEMN CNRS UMR 8520, 59313 Valenciennes, France; [email protected] 2 TMO Transformation Management Office, Riyad 11564, Saudi Arabia; [email protected] 3 Centre for Nanoscience and Nanotechnology (C2N), CNRS, UMR 9001, 91460 Marcoussis, France; [email protected] * Correspondence: [email protected] Abstract: In order to design and development efficient III-nitride based optoelectronic devices, technological processes require a major effort. We propose here a detailed review focussing on the etching procedure as a key step for enabling high date rate performances. In our reported research activity, dry etching of an InGaN/GaN heterogeneous structure was investigated by using an inductively coupled plasma reactive ion etching (ICP-RIE). We considered different combinations of etch mask (Ni, SiO2, resist), focussing on the optimization of the deep etching process. A GaN mesa process with an etching depth up to 6 µm was performed in Cl2/Ar-based plasmas using ICP reactors for LEDs dimen sions ranging from 5 to 150 µm2. Our strategy was directed toward the mesa formation for vertical-type diode applications, where etch depths are relatively large. Etch characteristics were studied as a function of ICP parameters (RF power, chamber pressure, fixed total flow rate). Surface morphology, etch rates and sidewall profiles observed into InGaN/GaN structures were compared under different types of etching masks. For deep etching up to few microns into the GaN template, we state that a Ni or SiO2 mask is more suitable to obtain a good selectivity and Citation: Dogheche, K.; Alshehri, B.; vertical etch profiles. The optimized etch rate was about 200nm/min under moderate ICP conditions. Patriache, G.; Dogheche, E. We applied these conditions for the fabrication of micro/nano LEDs dedicated to LiFi applications. Development of Micron Sized Photonic Devices Based on Deep Keywords: InGaN/GaN; ICP; deep etching; mesa; photonic devices GaN Etching. Photonics 2021, 8, 68. https://doi.org/10.3390/ photonics8030068 1. Introduction Received: 27 December 2020 Accepted: 27 February 2021 GaN and related alloys are among the most attractive materials, due to their excel- Published: 2 March 2021 lent characteristics as well as large bandgap, high breakdown electrical field and high electron saturation velocity, for applications such light emitting diodes (LEDs), lasers, pho- Publisher’s Note: MDPI stays neutral todetectors and high power electronics [1–6]. Mature material associated with optimized with regard to jurisdictional claims in technological processes is the key issue for the near future development of active/passive published maps and institutional affil- RF-optoelectronic components. Dry etching processes dedicated to ternary structures (such iations. as InxGa1−xN and GaN) are of particular importance, since such devices generally include heterostructures. Hence, the fabrication of GaN-based optoelectronic devices depends on dry etching either partially or totally. Typically, the etch process should yield high etch rates, minimal surface roughening, good reproducibility and a high degree of anisotropy. All of these properties can be achieved with inductively coupled plasma (ICP) etching Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Sidewall and etched surface morphology are critical parameters in the formation of mesas This article is an open access article or vertical devices with high ratio aspect. In the actual context of photonic devices, require- distributed under the terms and ments for GaN etching are fixed in the order of few hundred nanometers [7–9], whereas conditions of the Creative Commons this is completely different for those expected for vertical power devices [10–12]. Deep Attribution (CC BY) license (https:// etching of GaN is presently needed for new applications such as photonic, power and creativecommons.org/licenses/by/ microelectromechanical system devices for device isolation, waveguide or vertical channel 4.0/). Photonics 2021, 8, 68. https://doi.org/10.3390/photonics8030068 https://www.mdpi.com/journal/photonics Photonics 2021, 8, x FOR PEER REVIEW 2 of 7 Photonics 2021, 8, 68 2 of 7 and microelectromechanical system devices for device isolation, waveguide or vertical channel formation [13–15]. Low etch depth has been successfully employed for InGaN/GaN devices to create photonics crystal and LED mesas with, in general, etch depths below 1um,formation while deep [13–15 etch]. Low has etchbeen depth mainly has focused been successfully on GaN materials employed and for not InGaN/GaN de- actual devices. Hencevices to more create work photonics is required crystal to and apply LED and mesas optimize with, deep in general, etch to etch create depths below 1um, functional deviceswhile [16–18] deep The etch approach has been mainlyis to etch focused the epilayer on GaN from materials p-type and GaN not to actual sap- devices. Hence phire to form individualmore work or is interconnected required to apply microchips and optimize for a deepmicro-LED etch to createarray [19]). functional Our devices [16–18] purpose is to haveThe a approach chip-to-chip is to isolation etch the and epilayer to improve from p-type the electrical GaN to performance sapphire to formby individual or reducing the metalinterconnected surface and microchips global capa forcitance a micro-LED induced array by [19metallization]). Our purpose contacts is to have a chip-to- chip isolation and to improve the electrical performance by reducing the metal surface and (pads, connection line) on n-GaN. The deep etching process was performed to define in- global capacitance induced by metallization contacts (pads, connection line) on n-GaN. The dependent mesa. Different configurations of etching masks were defined. Photoresist deep etching process was performed to define independent mesa. Different configurations (AZ9260), metal (Ni) and silicon dioxide (SiO2) were used as etching masks. The objective of etching masks were defined. Photoresist (AZ9260), metal (Ni) and silicon dioxide (SiO ) was to assess surface morphology, mesa edges and the etching sidewall quality. Several 2 were used as etching masks. The objective was to assess surface morphology, mesa edges dimensions of etched mesas ranging from 5 × 5 to 150 × 150 µm² were evaluated as a func- and the etching sidewall quality. Several dimensions of etched mesas ranging from 5 × 5 to tion of etching profile. The effect of different masks on the etched mesa are discussed for 150 × 150 µm2 were evaluated as a function of etching profile. The effect of different masks depths varying from hundreds of nanometers up to a few micrometers. We also illustrated on the etched mesa are discussed for depths varying from hundreds of nanometers up to the effects of etched parameters on the resulting characteristics, such as etch rate, mask a few micrometers. We also illustrated the effects of etched parameters on the resulting erosion and, as a consequence, surface roughness. characteristics, such as etch rate, mask erosion and, as a consequence, surface roughness. 2. Experimental2. Details Experimental Details In our investigation,In our III-Nitride investigation, layers III-Nitride were grown layers by were NovaGaN grown Ltd by NovaGaN(Lausanne, Ltd. (Lausanne, Switzerland), usingSwitzerland), a metalorganic using achemical metalorganic vapour chemical deposition vapour system deposition (MOCVD) system on (MOCVD) c- on c-face face sapphire substrates.sapphire substrates.The epilayer The struct epilayerure consisted structure of consisted 2 µm thick of undoped 2 µm thick GaN, undoped 1 GaN, 1 µm µm thick n-GaN,thick ten n-GaN,periods tenof 10 periods nm thick of 10 GaN/3 nm thick nm GaN/3thick InGaN nm thick MQW InGaN and MQW100 nm and 100 nm thick thick p-GaN (as p-GaNshown in (as Figure shown 1a,b). in Figure Cross-1sectionsa,b). Cross-sections and plan-views and of plan-views the samples of were the samples were investigated by investigatedhigh-angle annular by high-angle dark fiel annulard scanning dark transmission field scanning electron transmission microscopy electron microscopy (HAADF-STEM).(HAADF-STEM). Samples were prepared Samples by were tilting prepared along bythe tilting (1100) along and (1120) the (1100) axes andon (1120) axes on thin foils by Focusedthin foils Ion Beam by Focused (FIB) in Ion order Beam to (FIB)see the in dislocations, order to see the chemical dislocations, analysis chemical and analysis and stacking faults. Pinchingstacking faults.zones generate Pinching thre zonesading generate dislocations threading (yellow dislocations arrows) (yellowassociated arrows) associated with plastic relaxationwith plastic mechanism. relaxation They mechanism. also lead to They variation also lead of the to variationtop layer ofthickness the top layer thickness (p-GaN layer) above(p-GaN these layer) zones. above Our these invest zones.igations Our revealed investigations that InGaN/GaN revealed that samples InGaN/GaN samples had threading dislocationhad threading densities dislocation in the range densities of 3–5 in the× 10 range8 cm−2. of Dislocations 3–5 × 108 cmwere−2 .cre- Dislocations were ated during growthcreated due during to the lattice growth mismatch due to the between