Adventures of III-V Semiconductor Surfaces
Sammanfattning: Tailoring the surface composition and morphology of materials to enable new electronic devices is important for a wide range of applications such as quantum computing or spintronics. A fundamental understanding of the changes induced in the surface during different process steps can help to establish new synthesis routes as well as devices. This thesis focuses on the manipulation of III-V semiconductor compounds, in particularthe surfaces of InAs and GaAs crystals.By implementing lithographically defined metal islands on the InAs surface, we push the boundaries of substrate annealing temperatures inhibiting the formation of In droplets locally. The employed Pd layer acts as a sink for freely diffusing In atoms above the congruent melting temperature. Here, As atoms go into gas phase at a higher rate compared to In due to the difference in vapor pressure. This lateral control over the concentration of In on the surface was investigated via scanning electron, atomic force as well as X-ray photoemission electron microscopy (XPEEM), and opens new pathways for epitaxy and the synthesis of heterostructures. Furthermore, theoretical studies have shown that the implementation of Bi atoms into the lattice of III-V compound semiconductors can facilitate band gap reduction and increased spin-orbit coupling desirable for fabricating of topological insulators. Particularly, the interaction of group III elements with Bi has attracted great interest. However, manufacturing these diluted Bismides is not trivial, since most approaches like molecular beam epitaxy, synthesis from the melt or metal organic vapor deposition suffer from limited and inhomogeneous Bi incorporation into the crystal.By following the approach of depositing Bi atoms onto a III-V sample and subsequent annealing, this thesis aims to synthesize and characterize heterostructures displaying III-V bulk properties and a surface made of III-As-Bi compounds. Different sample preparation routes were explored focusing on GaAs and InAs substrates with zinc blende (ZB) and wurtzite (WZ) crystal structure. The latter is only achievable in low-dimensional materials and will be employed in the form of InAs nanosheets. Part of this study focusses on the investigation of Bi-induced structural and chemical changes in the surface of the III-V compounds by utilizing surface sensitive techniques such as scanning tunneling microscopy, X-ray photoemission spectroscopy, low energy electron diffraction and XPEEM. Our results show that the mechanism of Bi incorporation is highly dependent on the underlying crystal structure, as well as process parameters such as time and substrate temperature. Additionally, first band structure measurements of InAs WZ crystal nanosheets collected via averaging angleresolved photoemission spectroscopy (ARPES) are presented. In contrast to other ZB crystal facets, a 2D electron gas (2DEG) is already detected after removing the native oxide and diminished after Bi deposition. We attribute the origin of the 2DEG to unique step and edge morphologies found on the WZ nanosheets. The thesis concludes with an ARPES study on InAs(111)B substrates presenting new electronic states inside the band gap based on the interaction of Bi and As atoms.
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