Guided and deterministic self organization of quantum dots

This thesis presents the fabrication and characterization of InGaAs and InAs QDs formed by self-organized anisotropic strain engineering of InGaAs/GaAs SL templates on planar GaAs (311) B substrates, and guided and directed self organization of the SL template formation through artificial patterning of the substrates for creation of complex QD architectures and lateral positioning by MBE. Detailed studies of SL template evolution as a function of growth and annealing temperatures for growth of ordered InGaAs QD arrays and InAs QD molecules, and transition of the QD molecules into single QDs through optimization of growth temperature, InAs amount, and annealing, analyzed in-situ by RHEED, were demonstrated on planar substrates. The QD arrays, molecules and single QDs have been grown on patterned substrates, following the optimization of the growth conditions for well-ordered, mesoscopic InGaAs QD arrays, and isolated InAs QD molecules and single QDs, in order to create complex architectures of QDs and accurate position control of QDs depending on the pattern design. These QDs exhibit excellent optical quality revealed by temperature dependent macro-PL spectra with strong emission up to room temperature and ultra-sharp peaks in micro-PL at low temperature from individual QDs, which is required for future quantum functional devices. Semiconductor QDs have been intensively investigated in recent decades since they possess full size quantization bearing the opportunity for development of novel device applications and/or increase of device performance for applications such as QD-lasers and/or single photon source/detectors for quantum communication. The properties such as electron-hole interactions, spins of electron/holes, light particles (photons), and lattice vibrations (phonons) can be manipulated in single QDs with sizes comparable or smaller than the de Broglie wavelength of charge carriers in bulk material. For producing such QDs with high opto-electronic quality, needed for device fabrication, different growth approaches have been suggested. Molecular beam epitaxy of In(Ga)As on GaAs substrates grown in the SK mode is the most extensively studied technique for self organization of QDs and QWRs. Based on the lattice mismatch between the substrate and QD material, the ordering, density, and structural properties of QDs have been improved by vertical stacking of strained layers providing lower mismatch regions where the QDs nucleate. Moreover, 1-D QD arrays have been formed by self-organized anisotropic strain engineering of InGaAs/GaAs SL templates on planar and patterned GaAs (100) substrates, which has been extended to laterally ordered and well-isolated 2-D QD molecules on planar GaAs (311)B substrates providing the basis for our study. The stacking and annealing in each InGaAs/GaAs SL template period produces a strain modulated surface with 2-D nodes due to lateral and vertical strain field coupling which serves as a template for QD positioning when the number of SL template periods is increased to ten. The InGaAs QD arrays and InAs QD molecules locate preferentially on top of the nodes due to strain-field recognition. The study of the growth- and annealingtemperature dependent SL template evolution provides a systematic understanding of the SL template development as well as QD ordering on top. The size and the density of the QD molecules are controlled by changing the InAs growth temperature followed by annealing. The QD molecules are transformed into single QDs at high InAs and SL template growth temperatures, whose structural quality is enhanced by annealing and reducing the QD layer thickness, analyzed in situ by RHEED during growth. The study of guided self-organized anisotropic strain engineering for formation of complex laterally ordered InGaAs QD arrays and InAs QD groups is demonstrated on shallow-patterned GaAs (311) B substrates. On stripe-patterned substrates the well-ordered, spot-like arrangement of ordered QD molecules on planar, unpatterned substrates is transformed into a zigzag arrangement of periodic stripes which become straight, well ordered, and connected over macroscopic distances on zigzag mesapatterned substrates. In addition, laterally ordered complex InGaAs and InAs QD architectures are created on deep etched artificial patterns where the ordering is based on self-organized strain engineering of InGaAs/GaAs SL templates on stripe, zigzag, and round hole patterns with faceted mesa sidewalls guiding the self organization. The formation of slow-growing facets on deep-patterned substrates produces QD-free mesa sidewalls, while InGaAs QD arrays and InAs QD molecules and single QDs form on the GaAs (311) B top and bottom planes with arrangements modified only close to the sidewalls. Moreover, the QD array and single QD ordering as well as anisotropic self organization due to the slow and fast growing sidewalls is clearly shown for deep etched round holes. As a result, highly ordered complex QD architectures with QD –free and –rich areas along the pattern sidewalls and mesa- top and bottom regions are created on macro- and microscopic scales on shallow- and deep-patterned GaAs (311) B substrates, respectively. The patterns of round holes, zigzags, and stripes with medium depth of 100 nm direct the self-organized anisotropic strain engineering of InGaAs/GaAs superlattice templates on GaAs (311) B substrates for absolute InAs QD position control. Beyond the complex, lateral QD ordering over large and local areas demonstrated precedingly, deterministic self organization is introduced due to stepped and faceted sidewall formation, where rows of densely packed QD molecules and single QDs develop along the pattern sidewalls and corners. The QD molecules and single QDs in the neighborhood are spatially locked to the QD rows, hence, pattern sidewalls and corners, directing the natural lateral ordering with unchanged periodicities. Moreover, the rotation of the pattern orientation counter clockwise by 90º shows that these arrangements strongly depend on the orientation of the sidewalls, which reveals also that the Ga and In diffusion is anisotropic in the [-233] direction. This extends the concept of guided self organization to deterministic self organization with absolute position control of the QDs without one-to-one pattern definition. Finally, photoluminescence studies of capped and uncapped, ordered QD molecules, and single QDs on patterned GaAs (311)B substrates have shown excellent optical quality. Temperature dependent macro-PL analysis of InAs QD molecules on shallow- and deep- etched substrates revealed clear influences of the pattern designs on the PL intensities and emission energies. The micro-PL spectra at low temperature of capped and uncapped single QDs exhibit distinct emission lines which are broadened for uncapped QDs revealing strong interaction with surface states. Ultra-sharp peaks from capped single QDs on patterned GaAs (311) B substrates are observed at low temperature by high resolution micro-PL. These findings highlight the potential of guided and deterministic self-organized anisotropic strain engineering for the realization of future quantum functional devices requiring ordered and position controlled QD arrays with high structural and optical quality