Physics of intraband quantum dot optoelectronic devices

摘要

In last two decades, semiconductor nanostructures, such as quantum wells, wires and dots, have been recognised as sources and detectors of radiation in the mid- and far-infrared region of the spectrum. Much of a success has been obtained with quantum well based intraband devices, such as quantum cascade lasers and quantum well infrared photodetectors. However due to longer carrier lifetimes in quantum dots, it is expected that optoelectronic devices based on intraband transitions in self-assembled quantum dots would have superior performance to their quantum well counterparts. In order to fully exploit this prospect, appropriate theoretical models describing electronic, optical and transport properties of the active region of these devices need to be developed, which was the subject of this thesis. It was shown how symmetry of the dot shape can be exploited to efficiently calculate the energy levels within the framework of the multiband envelope function method. The implementation of the method in the plane wave representation of the Hamiltonian eigenvalue problem and the results of its application to square based pyramidal InAs/GaAs quantum dots and hexagonal III-nitride quantum dots were given. A semiclassical model of intraband carrier dynamics in quantum dots was then developed and applied to design an optically pumped long wavelength mid-infrared laser based on intersublevel transitions in InAs/GaAs quantum dots. Two orders of magnitude lower pumping flux was predicted than in similar quantum well based devices. Next, simulations of the optical absorption spectrum in the existing quantum dot infrared photodetector structures were performed. A special emphasis was put into quantum dots-in-a-well structures and explanation of the effect of well width on the detection wavelength. A theory of transport in quantum dot infrared photodetectors starting from the energy levels and wavefunctions obtained by solving the envelope Hamiltonian, yielding as output the device characteristics such as dark current and responsivity, was then developed. The comparison with experimental data available in the literature was made, yielding a good agreement. Finally, the theory of electron transport through arrays of closely stacked quantum dots, where coherent and polaronic effects become important, therefore requiring the treatment within the formalism of the nonequilibrium Green's functions, rather than the semiclassical approach, was presented. A design of a structure promising to act as a terahertz quantum dot cascade laser was given.