Quantum dot lasers: breakthrough in optoelectronics

Semiconductor heterostructures with self-organized quantum dots (QDs) have experimentally exhibited properties expected for zero-dimensional systems. When used as active layer in the injection lasers, these advantages help to strongly increase material gain and differential gain, to improve temperature stability of the threshold current, and to provide improved dynamic properties. Molecular beam epitaxy (MBE) represents a developed technology well suited for fabrication of self-organized QDs. Optimization of deposition parameters can ensure that the self-organized islands are small (similar to 10 nm), have a similar size and shape and form dense arrays. Saturation material gain is as high as 150000 cm(-1) compared with QW values of about 3000 cm(-1). Maximum differential gain reported for QD lasers approaches 10(-12) cm(2) and exceeds the QW laser values by about three orders of magnitude. Direct observation of relaxation oscillations reveals present cut-off frequencies close to 10 GHz. High internal (>96%) and differential (70%) efficiencies at 300 K are realized. Using the novel concept of electronically-coupled QDs and oxide-defined 10 mu m apertures, CW lasing with J(th) = 180 A/cm(2) is realized in surface-emitting QD lasers (300 Ii). Wall-plug efficiencies are up to 16%. Total currents as low as 68 mu A are measured for 1 mu m apertures. GaAs-based lasers for the 1.3 mu m range with low J(th) (65 A/cm(2)) at room temperature (RT) are realized using InAs/InGaAs/GaAs QDs obtained by activated spinodal decomposition. In stripes the lasing occurs via the QD ground state (J(th) = 90 A/cm(2)) for cavity lengths L > 1 mm (uncoated). Differential efficiency is 55% and internal losses are 1.5 cm(-1). A characteristic temperature near RT is 160 K. 3W CW operation at RT is achieved. The recent progress in lasers based on self-organized MBE QDs already made it possible to fabricate devices with dramatically improved characteristics as compared to recent QW devices for the most important commercial applications. (C) 2000 Elsevier Science S.A. All rights reserved.