Measurement of Quasi-Fermi Levels in Quantum Well Lasers

Heterojunction Quantum Well Lasers were tested under forward and reverse bias, by Scanning Electron Microscopy (SEM) working in Differential Voltage Contrast(DVC) mode. DVC utilizes the impact of electrical potential across a device on the emission of secondary electrons. DVC consists of storing and subtracting two digitized images of the tested device under zero bias and in an operational regime. Calibration of the resultant image provides for quantitative measurements of the potential across the device. The question of whether the DVC technique measures the electron affinity of a material or its work function or the thermodynamic potential has been addressed in recent papers, however questions remained regarding discrepancies between the expected Quasi Fermi Energy (QFE) and the measured values. The experimental part of our work concentrated on taking (QFE) profiles by DVC across a single quantum well laser operating in inverse population, threshold, and high power emission modes of photoemission. The intensity was measured by a calibrated photodetector aligned with the laser in the SEM chamber. The cleaved and yet operational lasers have as a central part a layer of In0.2Ga0.8As 600nm thick between two undoped layers of Al0.3Ga0.7As 0.1 μm thick each with cladding layer of n-type Al0.6Ga0.4As on one side and a similar p-type layer on the other. The shape of QFE indicated that eletrons and photons contribute to total energy over the intrinsic area. This QFE profiling across the laser reveals areas of electron and photon confinement.