Growth and anisotropic transport properties of self-assembled InAs nanostructures in InP

Self-assembled InAs nanostructures in InP, comprising quantum wells, quantum wires, and quantum dots, are studied in terms of their formation and properties. In particular, the structural, optical, and anisotropic transport properties of the nanostructures are investigated. The focus is a comprehending exploration of the anisotropic in-plane transport in large ensembles of laterally coupled InAs nanostructures. The self-assembled Stranski-Krastanov growth of InAs nanostructures is studied by gas-source molecular beam epitaxy on both nominally oriented and vicinal InP(001). I demonstrate that the off-cut direction of vicinal substrates — largely independent of growth conditions — determines the morphology of nanostructures, i.e. quantum dot, quantum wire, or twodimensional growth; whereas, on nominally oriented substrates, the morphology is very strongly dependent on the growth conditions. The optical properties of InAs nanostructures embedded in InP are investigated, and their principal applicability for optical devices, particularly operating in the technologically important 1.55 μm wavelength range, is shown. Optical polarization of the interband transitions arising from the nanostructure type is demonstrated by photoluminescence and transmission spectroscopy. The experimentally convenient four-contact van der Pauw Hall measurement of rectangularly shaped semiconductors, usually applied to isotropic systems, is extended to yield the anisotropic transport properties. This extended van der Pauw method is greatly used for the transport measurements. Temperature dependent transport measurements are performed in large ensembles of laterally closely spaced nanostructures. The transport of quantum wire-, quantum dashand quantum dot containing samples is highly anisotropic with the principal axes of conductivity aligned to the <110> directions. The direction of higher mobility is [1̄10], which is parallel to the direction of the quantum wires. In extreme cases, the anisotropies exceed 30 for electrons, and 100 for holes. The extreme anisotropy for holes is due to diffusive transport through extended states in the [1̄10] , and hopping transport through laterally localized states in the [110] direction, within the same sample. The data is discussed in terms of two-dimensional carrier systems with anisotropic scattering, and in terms of coupled 1D or 0D structures: In the anisotropic-2D context, the principal mobilities at low temperature are dominated by interface roughness scattering in the [110] direction, and by remote impurity scattering in the [1̄10] direction. In the coupled nanostructure context, I demonstrate that the transport anisotropy results from directionally anisotropic tunnel coupling between adjacent nanostructures rather than from the nanostructure shape anisotropy. The contribution to conductivity resulting from weak localization — a quantum mechanical effect based on electron self-interference — is studied in InAs quantum wells, coupled quantum wires, and coupled quantum dots. It is demonstrated that the weak-localization correction shows a directional anisotropy equal to the anisotropy of the total conductivity, which is typical for anisotropic 2-dimensional systems. This result suggests that even for a conductivity anisotropy ratio as high as 38, the coupling between the quantum wires plays a more important role than the quasi-one-dimensional nature of the electron transport in describing the quantum mechanical contributions due to weak localization. A novel 5-terminal electronic switching device based on gate-controlled transport anisotropy is proposed. The gate-control of the transport anisotropy in modulation-doped, self-organized InAs quantum wires embedded in InP is demonstrated. These quantum wires are shown to be a candidate for implementation of the device.