CARRIER CAPTURE AND TRANSPORT WITHIN TUNNEL INJECTION LASERS: A QUANTUM TRANSPORT ANALYSIS

Abstract

Hot electron distributions within the active region of quantum well lasers lead to gain suppression, reduced quantum efficiency, and increased diffusion capacitance, greater low-frequency roll-off and high-frequency chirp. Recently, "tunnel injection lasers" have been developed to minimize electron heating within the active quantum well region by direct injection of cool electrons from the separate confinement region into the lasing subband(s) through a tunneling barrier. Tunnel injection lasers, however, also present a rich physics of transport and scattering, and a correspondingly rich set of challenges to simulation and device optimization. In this work, some of the fundamental physics of carrier capture and transport that should be addressed for optimization of such lasers is elucidated using Schrödinger Equation Monte Carlo (SEMC) based quantum transport simulation. In the process, qualitative limitations of the Golden-Rule of scattering in this application are pointed out by comparison. Specifically, a Golden-Rule-based analysis of the carrier injection into the active region of the ideal tunnel injection laser would suggest approximately uniform injection of electrons among the nominally degenerate quantum well states from the separate confinement region states. However, such an analysis ignores (via a random-phase approximation among the final states) the basic real-space transport requirement that injected carriers still must pass through the wells sequentially, coherently or otherwise, with an associated attenuation of the injected current into each subsequent well due to electron-hole recombination in the prior well. Transport among the wells then can be either thermionic, or, of theoretically increasing importance for low temperature carriers, via tunneling. Coherent resonant tunneling between wells, however, is sensitive to the potential drops between wells that split the energies of the lasing subbands and (further) localozes the electron states to individual wells. In this work such transport issues are elucidated using Schrödinger Equation Monte Carlo (SEMC) based quantum transport simulation.