Physics-based modelling of multijunction VCSELs

The growing demand of VCSEL-based 3D imaging system requires the development of high-power AlGaAs-based VCSELs. In this context, the purpose of this Master’s thesis is to contribute to the optimization of efficient multijunction AlGaAs VCSELs, overcoming the power limitations of conventional pin configurations. These designs comprise multiple active regions (ARs), electrically coupled by tunnel junctions (TJs). TJs are reversely biased pn junctions where, due to high doping levels, the conduction band (CB) overlooks the valence band (VB), enabling current flow through band-to-band tunneling (BTBT) [1]. Multistage VCSELs exploit BTBT across TJs to re-promote the recombined electrons in the quantum wells (QWs) back into the CB, making them available for further recombinations. This process, known as carrier recycling, boosts the quantum efficiency and the output power [2,3]. The thesis relies on a computer-aided design (CAD) approach, as an alternative to time-consuming prototyping campaigns. The CAD tool implemented is the in-house multiphysics and multiscale Vcsel Electro-opto-thermal NUmerical Simulator VENUS [4], able to accurately describe quantum transport across TJs by means of the non-equilibrium Green’s function (NEGF) formalism [5]. Three VCSELs are investigated: a single-, two-, and three-stage AlGaAs VCSEL. VENUS’s predictive character is validated through comparative appraisals between simulations and measurements of the test structures, available thanks to the collaboration with the TRUMPF Photonics group. The CAD tool is then exploited to justify the limited output power improvements reported for the two- and three-stage designs of the first testing campaign with respect to the single-stage design. Multijunction designs are expected to increase the quantum efficiency by a factor equal to the number of ARs included in the stack. Both measurements and simulations show that this theoretical goal is not achieved. The quantities extracted from VENUS suggest that the limitations stem from ineffective carrier recycling. In fact, in the initial proposed designs, current across TJs is driven not by BTBT, but rather by holes "leaked" out of the ARs without recombining, and swept by the TJ electric field. Parametric analyses are also carried out for different TJ configurations, to assess how higher TJ conductivity impacts BTBT. Their results prove that the output power limitations of the multijunction designs cannot be traced back to the TJ operation. VENUS is thus exploited to investigate a different solution. A hole-blocking layer (HBL), inserted between TJs and ARs in the two- and three-stage designs, is designed to suppress hole leakage and increase BTBT’s contribution to the current. Simulations report compelling advancements in output power with respect to the original designs, validating the HBL approach. TRUMPF Photonics accepted these design proposals, and demonstrated similar improvements at the experimental level. The results from VENUS show that non-effective BTBT and hole leakage are the main aspects hindering the output power of the initial proposed two- and three-stage designs. Despite slight improvements, the theoretical output power doubling and tripling with respect to the single-stage design are not reached. The insertion of HBLs is demonstrated to be a promising solution to the issue. Nonetheless, future analyses on this approach should focus on the optical and thermal response, as this thesis primarly addresses the carrier transport aspect of the problem.