Advances in the spectroscopic characterisation of vertical-cavity optoelectronic devices and structures using modulated reflectance

摘要

The vertical-emitting devices, resonant-cavity LEDs (RCLEDs) and vertical-cavity surface-emitting lasers (VCSELs) are key components in a broad range of applications including optical communications. However, the complexity of these multi-layer structures causes significant difficulties in their non-destructive characterisation at the pre-fabrication stage, and they have defied analysis by conventional optical techniques, such as photo-luminescence (PL). Fortunately, a complementary spectroscopy, modulated reflectance (MR), provides a viable alternative. MR is a simple technique in which the reflection spectrum of a semiconductor is periodically externally perturbed - most usefully using a mechanically-chopped laser beam, i.e. photo-modulated reflectance (PR). This yields sharp derivative-like spectra which are replete with features from ground-state, and, in contrast to PL, many other possible higher-energy optical transitions. This detailed information enables the deduction of material parameters crucial to efficient device operation, such as compositions, layer thicknesses, in-built electric fields and band line-ups. PR is truly non-destructive because samples need no special mounting, can be studied in air at room-temperature, and can be full-sized pre-fabrication wafers. At Surrey we have pioneered the application and interpretation of MR to the assessment of VCSELs and RCLEDs, and here we discuss the advances that we have made, which have attracted interest from the growth industry.