Grating overgrowth and defect structures in distributed-feedback-buried heterostructure laser diodes

Distributed-feedback (DFB)-buried heterostructure lasers incorporating a substrate grating require epitaxy of waveguide layers over the corrugated grating surfaces. Unlike epitaxy on planar (100) substrate, the corrugated substrate surface contains undesirable crystal facets which lead to an uncontrollable variation in local composition during epitaxy, and thereby results in strong localized misfit stresses. These localized misfit stresses further affect the subsequent growth of high quality layers which constitute the active device structures. To address the general issue of epitaxy on a corrugated surface, a thermodynamic analysis is performed for surface mass transport on a grating surface at normal growth temperature to show that grating wash-out is a thermodynamically favorable process. However, growth on a perfectly preserved grating is undesirable due to the composition shift on groove facets. A systematic study of substrate orientation dependent composition variation of quaternary layers indicates a composition shift generally toward In and P rich directions for orientations from (100)-(111). Conditions for a strain free waveguide layer growth are demonstrated. The commonly observed imperfections associated with a grating overgrowth are summarized, and their effects on device reliability are discussed.