Site-controlled In As/Ga As(001) Quantum Dots Fabricated By Molecular Beam Epitaxy And Multi-beam Pulsed Laser Interference

In As/Ga As quantum dots(QDs) have been extensively studied because of their unique physical properties and potential applications in high performance optoelectronic devices such as QD lasers, QD infrared photodetectors, single-photon emitters and QD solar cells. Though high quality QDs have been fabricated by self-organized growth methods, it is hard to control the QDs’ nucleation sites, size distribution and density on smooth substrate. In order to exploit the advantageous features of the QD devices, it is desirable to control the lateral position of QDs. Patterned substrates fabricated by different techniques have been used for the site-controlled QD growth. To transfer the lithographically defined patterns to substrate, reactive ion etching techniques are applied in nano fabrication processes. However, due to the energetic reactive ions, damages resulted from structural disruption, stoichiometric modification are unavoidably introduced to substrates, leading to degraded QD quality and device performance. It is important to develop new techniques to produce defect-free and contaminant-free site-controlled QDs.In this thesis, a new technique combining molecular beam epitaxy(MBE) and muti-beam pulsed laser interference was developed to fabricate site-controlled In As/Ga As(001) QDs. In-situ irradiation of four coherent pulsed laser beams was carried out in the growth of QDs. Periodic nanostructures and chemical constituent variation according to interference pattern were achieved in In Ga As intermixing layer with thickness of 3 to 5 monolayer(ML) on sample surface, which resulted in the growth of site-controlled QDs. The in-situ laser-induced surface modification is contaminant-free and almost defect-free, therefore the technique presented in the thesis is promising in the fabrication of high quality site-controlled QDs. In order to achieve the research objective mentioned above, different topics including self-organized In As/Ga As(001) quantum dots grown by MBE, effects of in-situ surface modification on In As/Ga As(001) QD growth by single-beam pulsed laser irradiation, multi-beam laser interference pattern and lithography, site-controlled In As/Ga As(001)QD fabricated by combining MBE and in-situ irradiation of four-beam pulsed laser interference were studied in this thesis.The QD growth conditions should be varied for different samples which base on their application. To determine the dependence of QD density and size on MBE parameters, self-organized In As/Ga As(001) QDs were grown by varing growth conditions including substrate temperature, growth rate and In As coverage. In conditions of high substrate temperature and slow growth rate, the increase of the surface immigration length of adsorbed Indium atoms is in favor of the growth of QDs with low density and large size. In terms of In As coverage, by depositing more In As, both the density and the size of QDs increase, but coalescence of QDs and dislocations in QDs will be caused by over-deposition.In order to have a deep insight into effects and mechanism of in-situ pulsed laser induced surface modification on In As/Ga As system, single-beam pulsed laser irradiation was carried out in MBE growth. Atomic layer removal and nano-holes elongated in the direction were observed on sample surface. The change of surface morphology is due to the atoms desorption resulted from laser induced electronic excitation and intensified by the volatile nature of Indium atoms at high substrate temperature. The in-situ surface modification affects the QD growth significantly. On one hand, the QD nucleation is delayed because of the atomic layer removal on irradiation area. On the other hand, nano-holes formed at high energy intensities are preferential nucleation sites for QD formation, which is attributed to enhanced Indium accumulation for low surface energy. The results suggest that selective QD growth can be acheived by in-situ laser-induced surface modification.The multi-beam laser interference pattern was studied by numerical simulation based on electromagnetic theory. It was concluded that the spatially modulated light field can be calculated by squaring the sum of coherent beams’ complex amplitude. A variety of interference patterns were presented by changing the intensity ratio, incident direction and polarization of laser beams. Periodic nanostructures were fabricated on epi-ready and homo-epitaxial Ga As wafers by making use of pulsed laser interference ablation. Based on photo-thermal mode, temperature distribution on sample surface was calculated by solving the heat diffusion equations, which suggests that the changes of sample surface morphology is resulted from the material removal and molten material transport in the irradiation process. The results of laser interference ablation also indicate that the substrates are inevitabelly damaged in traditional patterning process for site-controlled QD growth.Site-controlled In As/Ga As(001) QDs were fabricated by in-situ irradiation of four-beam pulsed laser interference during MBE growth. After the irradiation, periodic nano-holes and nano-islands in In Ga As intermixing layer were observed on sample surface. Because the beam energy profile is inhomogeneous, the intensity and contrast ratio of interference pattern varies with the location. Nano-holes and nano-islands are formed on low intensity, high contrast and high intensity, low contrast areas respectively. The change of In Ga As intermixing layer morphology is attributed to the atom desorption resulted from laser induced electronic excitation. Because the maximum intensity of interference optical field is 3 times higher than that in single-beam pulsed laser irradiation experiment, the effects of pulsed laser irradiation occurs in top 3 to 5 atomic layer rather than wetting layer surface. Periodic nanostructures and chemical constituent variation created in the irradiation of pulsed laser interference show significant influence on QD growth. On one hand, the nano-islands and the outside areas of nano-holes are Indium-rich, in which fewer In As coverage is needed to achieve 2D-to-3D transition. On the other hand, because of enhanced Indium accumulation, the edges of nanostructures are preferential nucleation sites for QD growth. Ordered QD array is obtained on sample surface when small nano-islands with size of 50 nm to70nm and height of 1ML to 3 ML are merged by adjacent QDs because of mass transport between substrate and QDs during the growth.To the best knowledge of author, there is no report on similar technique applied in site-controlled QD growth. Because laser-induced surface modification is contaminant-free and nearly damage-free, site-controlled quantum dots fabricated in the experiment should have better photoelectric properties compared with those achieved by traditional methods. The study presented in the thesis has potential applications in photoelectric devices based on In As/Ga As QDs.