OPTICAL SPECTROSCOPY AND THEORETICAL MODELLING OF CARRIER DYNAMICS IN GROUP-IV ALLOY QUANTUM DOTS
Master of Science
Electrical & Computer Engineering
Dr. Umit Ozgur
In recent years, Ge1−xSnx alloy quantum dots (QDs) have attracted significant interest due to their potential applications in photodetectors and light emitting devices in visible to mid IR spectral range and compatibility with silicon based platforms. While bulk Ge is an indirect bandgap semiconductor (0.66 eV), direct transitions can be made possible by incorporation of α-Sn at concentrations of ~10%, which however lowers the bandgap. Utilizing quantum confinement by reducing the size to below the Bohr radius also promotes direct transitions and more importantly increases the fundamental transition energies in GeSn alloy QDs, making them suitable for a variety of optoelectronic applications. The emission energy of the GeSn alloy QDs can be tuned (1.31eV to 2.0 eV) by changing the size as well as varying the Sn composition. Incorporation of α-Sn is also predicted to increase the transition oscillator strengths in both GeSn bulk and GeSn alloy QDs, and therefore, enhance radiative recombination rates. Colloidal synthesis, which is used here to realize the GeSn QDs, provide the added advantage of being a facile, low cost approach to production of alloy QDs that can be passivated with different types of ligands via solution processing to enhance emission and change emission wavelength.
Variation of size and alloying with Sn impacts carrier dynamics by changing the bright and dark exciton splitting energy and recombination pathways in the colloidal GeSn QD system. As a result, the ultra-small QDs (size equal to or less than 2nm) exhibit a distinct characteristic of blue shift in peak photoluminescence with increasing temperature. Larger size QDs (~4nm) on the other hand show little to no shift in PL peak emission with temperature. In this thesis, temperature dependent steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements were performed on ultrasmall (~2nm) and larger size (~4nm) QDs with varying Sn content (up to 6%) to reveal the underlying size and composition dependent physical properties that govern the carrier dynamics of the system. A rate equation model was developed considering the temperature dependences of excitonic levels, trap state densities, carrier transfer between excitonic and trap states, and radiative and non-radiative recombination to fit the integrated PL intensity and peak PL energy dependences on temperature.
The model shows that there is an increase in surface trap state density with decrease in QD size and increase in Sn content. Smaller QDs show larger changes in excitonic levels with temperature which causes a steady increase in PL peak emission for ultra-small QDs from 15K to 300K. There is also activation of bright excitons which dominate higher temperature PL peak emission while lower temperature PL peak has significant contribution from trap states and dark excitons. These dependences explain the observed steady increase in PL peak emission for ultra-small QDs and the drastic reduction of PL decay time from μs to ns range indicating gradual activation of bright excitons with increasing temperature from 15 to 300 K. On the other hand, for larger QD’s the exciton splitting is smaller and there is little change in the excitonic levels with temperature causing the PL peak emission to show negligible change with change in temperature. The model also shows that, the non-radiative channels activate at lower energies when there is significant increase in Sn content for QD’s with comparable sizes and exciton binding energy stays more or less independent of Sn composition.
Although GeSn QD system can span the spectrum from the visible to near-IR region, to extend the spectral coverage to shorter wavelengths (UV) alloying with Si can be employed. This thesis also explores GeSiSn alloy QD system using PL and TRPL studies. Introducing Si changes excitonic dynamics in the GeSiSn system with the decay time staying in ns range for the whole temperature range (15-300 K). Extension of the rate equation modelling is used to determine the luminescence mechanisms of the system and the temperature independent decay time is understood to be due to significant lowering of the dark-bright exciton splitting energy with Si incorporation. These investigations into changes in optical properties, carrier relaxation and recombination processes with change in size and composition is crucial to designing future efficient optoelectronic devices based on abundant group IV elements.
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