Abstract

Current confinement in vertical-cavity surface-emitting lasers (VCSELs) is nowadays realized most commonly with buried Al_xO_y oxide apertures very close to the active region [1]. A novel oxide-free and regrowth-free approach uses the generated spontaneous light in the laser resonator in association with an epitaxially integrated phototransistor (PT) for an optical definition of the current path through the device [2]. Initially, the PT, which is configured as an optical switch, acts as an isolating barrier layer. Only leakage current generates some spontaneous emission in the active zone, which is partly absorbed in a thin InGaAs quantum well (α-QW) embedded between the base and the collector layers. The resulting photocurrent takes the role of a base current and turns parts of the PT layers conductive after exceeding a certain threshold base current, depending on the PT’s current gain and the layer structure of the VCSEL. Hence, the current–voltage curve shows a negative differential resistance (NDR) region when the PT opens and the resistance decreases. Once opened, the current aperture remains stable in diameter, thus lasing starts after reaching the threshold current density. Without an oxide layer, potentially problematic strain near the active zone is reduced. Moreover, the manufacture is less complex. Ultimately, the PT-VCSEL is intended to exhibit improved reliability and efficiency compared to present oxidized VCSELs.

© 2017 IEEE