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 reflectanc e (R) studies have been performed on an InGaAs/GaAs/AlAs/AlGaAs vertical-ca vity surface-emitting laser (VCSEL) structure at room temperature. The PR s pectra show three prominent signals: one from the ground-state excitonic tr ansition in the InGaAs quantum well (QW) embedded deep in the structure; Fr anz-Keldysh oscillations (FKO) from the GaAs layers; and a PR signal attrib uted to the cavity mode. whereas in the R spectra only the cavity mode is c learly visible. It is found that the ground-state QW transition is not in r esonance 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 o f the cavity mode, whose angular dependence can be fitted with a very simpl e model. The full R spectrum of the VCSEL structure has also been calculate d using a Jones matrix formalism which not only predicts the actual angular dependence of the cavity mode, but also permits a determination of the lay er thicknesses in the VCSEL structure by fitting to the experimental R spec trum. Excellent fits are achieved using only two adjustable parameters; the Ga and Al growth rates, which are found to be 53% less and 2.9% greater, t han nominal, respectively. Modelling of the measured PR spectrum of an etch ed sample shows that the In composition of the QW is 5% higher than the nom inal value.