Growth, Surface Structure and Morphology of Semiconductor Nano-Structures

Abstract

Self-assembled growth of semiconductor quantum dots has perspective applications in optoelectronic devices and a detailed understanding of their growth mechanisms is of high practical importance. Within this thesis work, the controlled formation and structural characterization of self-organized semiconductor nano-structures are discussed with special focus on their surface properties. Two types of semiconducting nano-structures have been investigated namely as Si?Ge based as well as GaN based quantum dot structures. In addition, GaN growth on Si(111) for integration of nitride semiconductors into Si technology has also been studied.Self-organized nano-patterning of Ga/Si(111) surfaces after submonolayer Ga deposition has been studied by STM, LEEM and XPEEM. The surface defects of the Si(111)-7Ã 7 surface (steps and the domain boundaries) play an important role in the initial Ga nucleation. Post-deposition annealing of Ga/Si(111) surface or elevated temperature Ga deposition on Si(111) leads to a 2D phase separation and surface nano-patterning. Varying the amount of Ga coverages, structures of the Ga/Si(111) nano-patterns can be controlled. Subsequent Ge deposition on partially root(3)Ã root(3)-Ga covered nano-patterned substrate leads to a selective nucleation of 3D Ge islands and thus to the formation of an array of highly aligned Ge dots. The formation of the nano-patterns can be explained in terms of surface stress and surface free energy.The structure, morphology and stoichiometry of silicon nitride films prepared by plasma assisted reactive growth on the Si(111) surface at elevated temperatures have been examined using LEED, STM and ESCA microscopy. Low temperature nitridation results in smooth silicon nitride films of poor crystallinity, whereas nitridation at high temperatures leads to the formation of crystalline Si3N4 layers but with a rough surface morphology. To facilitate the wurtzite GaN growth on Si(111), various silicon nitride inter-layers have been used as a buffer and a significant improvement of the subsequently grown GaN films morphology is obtained for Si(111) nitridation at higher temperatures.In-situ cleaning and surface defects characterization of MOVPE grown GaN(0001)/sapphire templates and subsequent homo-epitaxial growth of thin GaN layer using MBE have been studied using XPS and STM and RHEED. A combination of initial plasma assisted active nitrogen cleaning at low temperature followed by a short duration high temperature cleaning leads to an efficient removal of surface oxide while maintaining a smooth surface morphology. For a smooth template surface, a layer-by-layer growth morphology of GaN has been achieved for Ga-rich growth condition.MOVPE grown self-organized InGaN nano-islands on GaN(0001) templates have been investigated using STM. The morphology and density of these InGaN nano-islands can be controlled by the choice of proper MOVPE growth conditions such as temperature, InGaN deposit, growth rate and III-V flux ratio. With decreasing growth temperatures, the InGaN island size is reduced and the homogeneity of the InGaN surface is improved. Homogeneous nucleation of small InGaN islands of high densities (10^12/cm^2), suitable for three-dimensional quantum confinement is found for growth at 600°C at a low In partial pressure. However, these nano-islands do not show quantum-dot like emission after overgrowth. Instead, two-dimensional (2D) InGaN islands grown at 700°C show a sharp photoluminescence (PL) emission lines, indicative of single dot emission, after the GaN capping process.