Morphological Design of Semiconductors During Molecular-Beam Epitaxy and Pulsed-Laser Deposition

Abstract

Low-dimensional semiconductors exhibiting quantum confinement are promising for enhanced efficiency in energy-conversion devices, including photovoltaics, thermoelectrics, and light-emission devices. Ideal arrays of semiconductor nanostructures enable the formation of well-defined minibands that enhance light absorption. Nanostructure arrays are often achieved using extrinsic patterning methods which introduce lattice defects. Processes for the spontaneous formation and ordering of both QDs and NWs have emerged, but these are insufficiently mature for full integration into device manufacturing. Indeed, for molecular beam epitaxy (MBE) of InAs QD arrays, the influence of local surface curvature on semiconductor QD positioning after the nucleation stage is not well understood. On the other hand, for pulsed-laser deposition (PLD) of In2O3:Sn, the relative roles of vapor-solid (VS) and vapor-liquid-solid (VLS) growth modes during the morphological transitions resulting in semiconductor NW formation remain controversial. In the first part of this thesis, we quantify the aligned positioning of MBE-grown InAs/(Al)GaAs QDs and the effect of local surface curvature on the evolution of the InAs layer. In the second part of this thesis, we investigate the role of plasma expansion on the VS-VLS growth mode transition during PLD of In2O3:Sn.

The influence of surface curvature on the placement of InAs quantum dots (QDs) on (Al)GaAs surfaces is examined using an experimental-computational approach. We simulate QD deposition with a 2D phase field model, which describes the time evolution of the InAs layer driven by a chemical potential gradient. Both atomically flat and mounded surfaces, which contain elongated corrugations generated via a surface instability, are employed as templates for the subsequent deposition of InAs QDs. For flat surfaces, simulations result in QD densities comparable to experimental observations following MBE deposition. For mounded surfaces, the simulations reveal QDs preferentially positioned in regions of positive curvature (valleys), e.g., at along the side of surface mounds, consistent with the anisotropic QD placement observed experimentally. This substrate curvature-driven direction of QD placement offers a method for immediate in-plane QD crystals and is extendable to a wide range of groups III-V compounds systems.

Complex oxides such as ITO are widely utilized as transparent conductors in a variety of functional devices. Using PLD, both high transparency and high conductivity In2O3:Sn has been achieved without annealing, using instead selected gas species and pressures. However, the relative roles of VS and VLS growth modes during morphological transitions remains controversial. Here, we report on PLD of In2O3:Sn in an inert-gas environment, identifying the role of plasma plume expansion in the selection of VLS vs. VS growth. For the lowest N2 pressure, indium-tin droplet formation, followed by self-catalyzed VLS growth, is observed. With increasing N2 pressure, a transition from VLS to VS growth is apparent. We hypothesize that oxygen scattering at the lowest N2 pressure induces a metal-rich plume which leads to metal droplet formation, followed by VLS growth. As the N2 pressure is increased, the plasma-plume and its metal-rich core are compressed, resulting in a transition to VS growth. This tunable compression of the plasma-plume offers a route to morphological design of a wide range of functional complex oxide devices with tunable optical and electronic performance.