טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentVaxenburg Roman
SubjectElectronic Properties and Auger Processes in Nanoscale
Semiconductors
DepartmentDepartment of Chemistry
Supervisor Professor Efrat Lifshitz
Full Thesis textFull thesis text - English Version


Abstract

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A study of electronic structure and non-radiative recombination dynamics of charge carriers in quantum-confined semiconductor systems is presented. Reduction of the semiconductor dimensions down to nanometric scale induces significant modifications in the material’s physical properties. The flexible size-dependent properties of these nanometric semiconductors can be advantageous in various opto-electronic applications, while at the same time the reduction of size introduces new challenges. Here we cover several topics exploring different aspects of the quantum-confined semiconductor physics.


We investigate the electronic structure of two-component PbSeS/PbSeS narrow-gap nanocrystal heterostructures and its dependence on the chemical composition, size, and structure of the material. We find that in addition to the conventional control over the nanocrystals’ properties by variation of their size, it is also possible to engineer their electronic structure by introducing variable alloy composition. Further, we study the recombination dynamics in wide-gap InGaN/GaN quantum well light-emitting diodes. In these devices, operating at high carrier density regimes, efficient non-radiative recombination channels are capable of reducing the yield of the useful radiative recombination process. One of these energy dissipation channels is the non-radiative Auger recombination. Our calculations show that owing to the strong enhancement of the Auger recombination in quantum-confined systems, the Auger process can be efficient enough to interfere with the radiative recombination in the nitride quantum well materials. We propose a strategy to suppress the rate of the Auger recombination by modifying the potential energy profile of the quantum wells by introducing alloy layers which produces smooth potential profiles. This improves the radiative efficiency of the light-emitting diodes studied. We then extend our study to a broader range of quantum well systems. Namely, we study AlN/GaN quantum well light-emitting diodes grown in polar crystallographic directions and therefore subject to strong electric polarization fields. We show that these polarization fields can strongly enhance the rate of the Auger recombination, while at the same time can reduce the efficiency of the radiative recombination. As in the case of InGaN/GaN quantum wells, we suggest that the Auger recombination in AlN/GaN can be suppressed by a suitable modification of the confining potential by gradual alloying. Finally, we investigate the Auger recombination in CdSe nanocrystals, specifically addressing its dependence on the nanocrystal dimensions. We find that the rate of Auger process is a strongly oscillating function of the nanocrystal size - a fact that is obscured in standard ensemble measurements. We then show that the apparent Auger rate of size-dispersed nanocrystal ensembles can scale differently with nanocrystal size than the true Auger rate in underlying individual nanocrystals.