Strain-Driven Phenomena upon Overgrowth of Quantum Dots: Activated Spinodal Decomposition and Defect Reduction

Abstract

Strain-driven decomposition of an alloy layer is investigated as a means to control the structural and electronic properties of self-organized quantum dots. Coherent InAs/GaAs islands overgrown with an InGaAs alloy layer serve as a model system. Cross-sectional and plan-view transmission electron microscopy, as well as photoluminescence studies, consistently indicate an increase in height and width of the quantum dots with increasing indium content and/or thickness of the alloy layer. The increase in dot size is attributed to the phase separation of the alloy layer driven by the surface strain introduced by the initial InAs islands. The density of large dislocated clusters in the quantum dot samples can be dramatically reduced by special in situ annealing (defect reduction) techniques. A laser based on multiple (up to 10) layers of InAs/GaAs quantum dots grown under optimized conditions with the use of defect reduction techniques show considerably enhanced optical gain and improved performance. Differential efficiency as high as 88% is achieved in the lasers. An emission wavelength of 1280 nm, a threshold current density of 147 A/cm2, a differential efficiency of 80%, and a characteristic temperature of 150 K are realized simultaneously in one device. Wavelength extension up to 1500 nm for InAs quantum dot lasers on GaAs substrates is possible by using metamorphic (plastically relaxed) buffer InGaAs layers. In cases when the threading dislocations are avoided in the active region, high-performance operation with quantum efficiency exceeding 60–70% and pulsed output powers up to 7 W are realized.