Stimulated emission and lasing in III-V nitride heterostructures

Stimulated emission, lasing, and related properties of III-V nitride heterostructures are studied. A strain-dependent semi-empirical tight-binding model is developed, using the valence force-field model of Keating, to predict the atomic positions in the strained wurtzite crystal lattice. Predicted deformation potentials and strain-induced exciton splitting are shown to closely match data in the literature. The spectral properties of the edge luminescence from GaN-AlGaN heterostructures is investigated. The existence of stimulated emission is demonstrated and a measurement of the optical gain spectra is reported. In addition, the light emission properties of GaN-AlGaN separate confinement heterostructures is studied. The measured luminescence properties are improved for active region designs with fewer, thicker wells. An analysis of the trend is presented demonstrating recombination at the well-barrier interface as a significant factor. The results also indicate that the quantum wells experience compressive strain from the lattice mismatch with the AlGaN cladding layers. Further experimental results demonstrate that the commonly observed surface stimulated emission is related to in-plane optical gain, and is observed most commonly in samples with rough surface morphology. Photopumping results from GaN-AlGaN laser platelets are presented and discussed. Laser oscillation in GaN-AlGaN separate confinement heterostructures is demonstrated in which the optical cavity is formed unintentionally by parallel cracks in the epilayer. The observed laser modes are broad and shift to shorter wavelengths with increasing pump intensity. An analysis is presented revealing that mode shifting resulting from carrier-induced refractive index changes restricts the observation of laser modes to short optical cavities. GaN-AlGaN Bragg reflectors are investigated through reflectivity modeling and characterization. A transfer-matrix model is developed with an empirical relation for the refractive indices and predictions of the model are compared with data in the literature. Experimental results are then presented and compared with the predictions of the model. The design and characterization of GaN-AlGaN vertical cavity surface emitting lasers is studied. Luminescence spectra are presented from two devices which demonstrate sharp, highly-polarized, regularly spaced modes for pump intensities above a threshold. The spacing of the laser modes is shown to match the mode spacing predicted by the reflectivity model