Dobri Simeonov
Over the past few decades, III-V nitrides have attracted much attention due to the possibility to realize high efficiency optoelectronic devices covering all the UV and visible part of the light spectrum. However, the device fabrication has gone much faster than the understanding of the physical phenomena occurring in these materials. In this context, we propose a system for prototypical studies of low dimensional excitons in III-nitride materials based on single polar Stranski-Krastanow (SK) GaN/AlN quantum dots. Here, the demonstration of single dot spectroscopy is performed in three stages dealing with the issues related to instrumentation, sample design and optical measurements, successively. First, a state of the art microphotoluminescence setup operating in the UV light spectrum has been built. The system exhibits diffraction limited sub micron excitation laser spot size and spatial resolution. The performances of the developed setup have been successfully tested through the characterization of III-nitride heterostructures and devices. The second stage is dedicated to the sample development and preparation. Special emphasis was given to the SK growth mode transition and the parameters influencing the QD formation. It has been seen that when using standard growth conditions the achievement of samples suitable for single dot spectroscopy, namely small QDs with low areal density, is quite difficult. We show that this problem can been circumvented by promoting the material kinetics over the sample surface. As a result of the allowed material redistribution, the final sample exhibits AlN step edges covered by numerous large dots that leave place to a low density small quantum dots found over the AlN terraces. In-depth optical spectroscopic studies of single polar GaN/AlN quantum dots are carried out by means of low-temperature micro-photoluminescence. Luminescence linewidths as low as 590 µeV are obtained allowing a thorough characterization of the QD electronic properties. The measured linewidths at low temperature still exhibit a large inhomogeneous broadening, which is studied in order to demonstrate the decreasing effect of the quantum confined Stark effect with the dot size reduction. Biexciton emission is observed for a wide range of dot size. It is shown that the binding energy ( Ebxx ) exhibits two regimes. The main one is governed by the dot height through the quantum confined Stark effect leading to a variation of Ebxx from + 10 meV for the smallest dots down to -11 meV for the largest ones. A secondary variation of opposite sign is demonstrated for dots having the same height but different lateral size. Finally, our most recent studies allow reporting the first observation of excited levels in any III-nitride quantum dot system. The lowest energy excited states are found to originate from the lateral confinement of heavy holes. In addition, when two excitons are present in the dots the permutations of the determined excitonic states give rise to three biexciton states characterized by four luminescence transitions.
EPFL2009
