Electrochemistry in Photonic Devices Processing: a Disruptive Approach in GaN Technology

Abstract

Electrochemistry, except the electrodeposition of metal for making contacts, is rarely represented in microelectronics, despite the many possibilities it may offer like “soft” surface treatments (no surface bombardment, low damage), very sensitive methods (femtoampers sensitivity) and its large compatibility with high aspect ratio structures (TSV Cu filling). Nowadays, the More than Moore law compels researchers to look for other solutions than standard microelectronic processes to reach disruptive solutions: electrochemical processes may provide this new pathway. In that context, our study focuses on III-N material, specifically on GaN porosification through electrochemical anodization for photonic applications. After a study to understand GaN electrochemistry, we will see how new photonic device architectures may arise from the controlled porosification of GaN.Before going towards photonic applications, the understanding of the GaN electrochemical system is required. First we show that the variation of the main electrochemical and material parameters like GaN doping level, anodization potential, and electrolyte composition enables the fine-tuning of the porosity level, pore density, pore size and pore morphology. Then nano-indentation, ellipsometry and reflectometry measurements give us access to a good knowledge of the variation of the Young modulus and refractive index versus porosity level and pore morphology. The measured physico-chemical properties, mechanical as well as optical, of porous GaN eventually paves the way for new device developments.In our talk, we will show how electrochemistry requirements have an impact on the design of the final structure. We will focus on two porous GaN structures built from the previous electrochemical systemic study on GaN thin layers: i) a LED on porous GaN Bragg mirror (DBR) and ii) a relaxed InGaN pseudo-substrate for RGB micro-display applications. LED on DBR: Specifically designed epitaxial stack and technology allow the fabrication of a blue GaN/InGaN based LED structure on a DBR made of alternating porous and dense GaN layers. The optimized one-step electrochemical process results in a very efficient DBR with 99,99% reflectance and a large stop band centered on 460-520 nm. The presence of the porous GaN DBR below the LED stack exalts the photoluminescence and the cathodoluminescence signals compared with standard LEDs grown on bulk substrates. These results open a new field of research regarding the capability of tuning GaN refractive index. Fully relaxed InGaN structure for red light emission: Results on partially or totally relaxed InGaN pseudo-substrate will be presented. The InGaN pseudo-substrate relaxation helps indium incorporation in the overgrown quantum wells and enables light emission in long wavelength towards red. The control of the electrochemical and semi-conductor parameters shows that the relaxation of the III-N structure measured by HR-DRX clearly depends on the porosity level and pore morphology. This means that relaxation can be fine-tuned: that will allow an epitaxial regrowth of micro-LED structures compatible with high indium concentration in the quantum wells and possibly RGB micro-LED fabrication for micro-display applications. In that part, we will demonstrate how electrochemistry associated with microelectronic technology brings a new way of controlling relaxation of InGaN on GaN and how the obtained experimental results are promising for the fabrication of high indium content relaxed structure. Through these two examples, we put in evidence how electrochemistry allows inventing solutions unimaginable with the standard microelectronics processes.