Cathodoluminescence Studies of Quantum Structures and III-V nitrides

Abstract: Characterization of low-dimensional semiconductor structures is a challenging task. The thesis is based on experiments, using cathodoluminescence (CL) as a tool for characterization. The high spatial resolution and the possibility of spectroscopy of small structures make CL one of the most powerful techniques for these types of investigations. QDs were fabricated of InP on GaInP and InAs on GaAs by the Stranski-Krastanow (SK) method. CL was used to determine the spatial distribution of luminescence from individual quantum dots (QDs), and from QD ensembles. The size and position of QDs, produced by SK method, can be manipulated by growth conditions and by patterns on the surface. By introducing growth interrupt before capping of dots the kinetics of the QD formation could be investigated. QDs form during a long interrupt and a rough two dimensional quantum well form with no interrupt. These two extremes and intermediate stages were investigated. In particular CL was used to observe luminescence from single dots. Trenches in circular patterns made it possible to investigate how the luminescence from the QDs was effected by different crystallographic orientations of trenches. A maskless technique utilizing a focused ion-beam has been used to fabricate quantum wires (QWRs). The focused ion-beam creates shallow grooves and subsequent growth of InP will produce ridges. By growing a few monolayers of InGaAs on these ridges QWRs were formed and CL was used to determine spatial distribution, and energy variations in the luminescence from these. In III-V nitrides, GaN QDs on AlGaN epitaxial layers with low Al content, can be fabricated by a recently developed method. Both the luminescence and structural properties of QD were determined. This is the first report in literature of single dot luminescence from GaN dots. Alloy fluctuations in AlxGa1-xN epitaxial films, appearing on the micrometer scale, were investigated with a resolution not achieved by other methods. Further, growth artifacts displaying intense luminescence, were investigated. These are related to vertical structures, similar to vertical quantum wires recently reported in AlGaAs films grown on patterned substrates.