Reflectance and Photomodulated Reflectance Studies of Cavity Mode and Excitonic Transitions in an InGaAs/GaAs/AlAs/AlGaAs VCSEL Structure

Angle dependent photomodulated reflectance (PR) and conventional reflectance (R) studies have been performed on an InGaAs/GaAs/AlAs/AlGaAs vertical-cavity surface-emitting laser (VCSEL) structure at room temperature. The PR spectra show three prominent signals: one from the ground-state excitonic transition in the InGaAs quantum well (QW) embedded deep in the structure; Franz-Keldysh oscillations (FKO) from the GaAs layers; and a PR signal attributed to the cavity mode, whereas in the R spectra only the cavity mode is clearly visible. It is found that the ground-state QW transition is not in resonance with the cavity mode. The period of the FKOs shows that the built-in electric field is about 18 kV/cm. The energy positions of the excitonic features are not dependent on the angle of incidence, in contrast to that of the cavity mode, whose angular dependence can be fitted with a very simple model. The full R spectrum of the VCSEL structure has also been calculated using a Jones matrix formalism which not only predicts the actual angular dependence of the cavity mode, but also permits a determination of the layer thicknesses in the VCSEL structure by fitting to the experimental R spectrum. Excellent fits are achieved using only two adjustable parameters; the Ga and Al growth rates, which are found to be 5.3% less, and 2.9% greater, than nominal, respectively. Modelling of the measured PR spectrum of an etched sample shows that the In composition of the QW is 5% higher than the nominal value.