Ultrafast electron tunnelling in a reverse-biased, high-efficiency quantum well laser structure

We have performed a detailed series of photoconductivity (PC) and photoluminescence (PL) measurements on a reverse-biased GaAs/AlGaAs single quantum well graded index separate confinement heterostructure laser. The PC was performed, as a function of bias, at room temperature on a high-speed ridge waveguide structure. The PL was performed at low temperatures (20, 70, and, 150 K) on a ring mesa sample as a function of bias. The measurements show that this device behaves as an extremely efficient high-speed photodetector with an internal quantum efficiency of 100% and a FWHM response time of 35 ps. The data is fitted using a simple model based on electron recombination in the quantum well or escape out of the well. The escape occurs by one of three possible routes: direct tunnelling out of the lower electron level, thermally assisted tunnelling through the upper electron level, or thermionic emission over the barrier. Each of these three terms is calculated theoretically. A comparison of theory and experiment leads us to the conclusion that the theories explaining thermal emission of carriers from a quantum well underestimate the lifetimes.