Micro-photoluminescence characterisation of structural disorder in resonant tunneling diodes for THz applications

We investigated the difference between a macro scale PL and μPL (excitation and detection area ≤ 5μm²). Low-temperature micro-photoluminescence (μPL) is used to evaluate structural perfection of high current density InGaAs/AlAs/InP resonant tunnelling diodes (RTD) structure on different length scales. The thin and highly strained quantum wells (QWs) is subject to monolayer fluctuations in well and barrier thickness that can lead to random fluctuations in their band profile. μPL is performed reducing the laser spot size using a common photolithography mask to reach typical RTD mesa size (a few square microns). We observed that for spot size around 1μm² the PL line shape present strong differences on multiple points on the wafer. These variations in the PL is investigated by line-shape fitting and discussed in terms of variations in long-range disorder brought about by strain relaxation processes. We also highlight this μPL as a powerful and cost-effective non-destructive characterization method for RTD structures.