Impurity-free quantum well intermixing for high-power laser diodes

Abstract

The demand for ever higher powers and efficiencies from semiconductor lasers, continues. State-of-the-art high power lasers require not only sophisticated designs but also complex fabrication technologies to push the boundaries. A major obstacle to ever higher powers is catastrophic optical mirror damage that occurs at the mirrors of the cavity. Among several approaches to increase the threshold for damage, local manipulation of the band gap near the mirrors stands out, as it eliminates reabsorption. The structure of modern lasers employing quantum wells surrounded by large band gap and low index claddings gives the opportunity in intermix the quantum well and increase the effective band gap close to cavity edges during fabrication. The research presented in this thesis reports the results of Impurity-Free Vacancy Disordering (IFVD) of GaAs quantum wells in high power laser diode structures that leads to blue shifting of the effective band gap. In contrast with previous work, this study concentrates on actual large optical cavity (LOC) high power laser diode structures where the waveguide and cladding layers are thick. Using selective area QWI can be extremely beneficial in terms of enhancing catastrophic optical mirror damage (COMD) threshold, spatial mode instability, propagation losses and overheating which are the main limitations to fabricate HPLDs. In the course of the fabrication of HPLDs, the last and most problematic step is to manage QWI. IFVD was realized by capping the crystal surface with a sputtered dielectric layer of SiO2 to enhance intermixing and thermally evaporated SrF2 to prevent intermixing for selected parts of the laser cavity. Disordering the layers takes place by diffusion of Ga atoms from GaAs QW into sputtered SiO2 layer during rapid thermal annealing (RTA), leaving Ga vacancies in QW. It allows the Ga vacancy defects free to move AlxGa1 the photoluminescence peak. Relative composition in the layers that make up the laser structure was measured with X-ray photoelectron spectroscopy in conjunction with depth proling. A blue shift of 65 nm (154 meV) was achieved, in parallel with both Ga and Al diffusion in the laser structure.