MOVPE-grown quantum cascade laser structures studied by Kelvin probe force microscopy

A technique for direct study of the distribution of the applied voltage within a quantum cascade laser (QCL) has been developed. The detailed profile of the potential in the laser claddings and laser core region has been obtained by gradient scanning Kelvin probe force microscopy (KPFM) across the cleaved facets for two mid-infrared quantum cascade laser structures. An InGaAs/InAlAs quantum cascade device with InP claddings demonstrates a linear potential distribution across the laser core region with constant voltage drop across the doped claddings. By contrast, a GaAs/AlGaAs device with AlInP claddings has very uneven potential distribution with more than half of the voltage falling across the claddings and interfaces around the laser core, greatly increasing the overall voltage value necessary to achieve the lasing threshold. Thus, KPFM can be used to highlight design and fabrication flaws of QCLs.