Quantum Dot Laser

Self-organised InGaAs / GaAs Quantum Dot Lasers

Debabrata Bhattacharyya, A. C. Bryce, John H. Marsh and Clivia M. Sotomayor-Torres

The aim of this project is the realization of new generation of injection lasers, based on self-organised quantum dots (QDs), that have superior properties like higher differential gain, lower threshold current density (Jth), and higher characteristic temperature (To) as compared to quantum well (QW) lasers. The ultimate limit is the fabrication of quantum dot laser diode having-function like gain spectrum ,i.e., discrete level nature of conventional gas and solid state laser. In such a laser, detuning effect of optical gain vanishes and consequently, simultaneous improvement of modulation bandwidth (fr) and linewidth enhancement factor () are expected. Quantum dot lasers, therefore, exhibit suitable characteristics for optical modulator with low chirping effect.

Currently, the self-organisation of nanoscale three-dimensional coherent strained islands following Stranski-Krastanov growth mechanism is considered as the most promising way of in-situ QDs fabrication. The ordered arrays so formed may result in distributed feedback and in stabilization of single-mode lasing. In addition, intrinsically buried QDs spatially localize carriers and prevent them from recombining non-radiatively at resonator facets. Overheating of facets at high power operation may thus be avoided. A real challenge lies in the optimization of growth parameters to achieve a dense and uniform array of QDs, identical in size and shape.

Quantum Dot Sketch

MOCVD-grown thin (9Å) In0.5Ga0.5As layer on the GaAs (100) substrate results in two types of QDs : Pyramid-like with a square base of about 200 x 200Å, along the [010] and [001] directions and a height of nearly 80Å; and Disk-like with characteristic size of about 300 to 500Å and a height of about 15 to 30Å. The preliminary results indicate high characteristic temperature (To = 500K) which significantly exceeds the theoretical limit of 285K for QW lasers in the temperature range, T not greater than 200K.

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