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Modelling nanoscale processes at GaAs(001) surfaces

Rodriguez Hannikainen, Kennet 2020. Modelling nanoscale processes at GaAs(001) surfaces. PhD Thesis, Cardiff University.
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GaAs(001) is one of the most important semiconductor surfaces in today's technology. Since the invention of molecular beam epitaxy in the late 1960s, this surface has been the substrate to complex heterostructures and diverse nanostructures that have found and continue to find novel applications in optoelectronic devices. For this reason, the fundamental science of the GaAs(001) surface has been studied intensively over the past few decades, from the diverse reconstructions it exhibits to the mechanisms of epitaxial growth occurring on it. For the most part these studies have been documented by ex-situ characterization techniques such as atomic force microscopy (for morphological analysis) or scanning tunneling microscopy (for atomistic analysis). During the early years of MBE, investigation of the ongoing kinetic surface processes during growth was essentially limited to the use of in-situ reflection high energy electron diffraction, which can only give structural information averaged-out across the surface. The design and assembly of a hybrid system simultaneously a low energy electron microscope and a molecular beam epitaxy reactor (by Prof David Jesson) has been a major breakthrough in investigating the fundamental physical processes occurring on GaAs(001) surfaces. In fact, it is the only instrument in the world capable of imaging III-As surfaces (with III = Ga or In) in real time and under realistic growth conditions. Over the past few years, the unique capabilities of this system have led to numerous experimental findings regarding fundamental aspects of the GaAs(001) surface which were previously inaccessible to conventional molecular beam epitaxy systems. Together with theoretical modelling, this has enlightened our understanding of this important surface. However, much still remains unknown regarding GaAs(001) in relation to growth. Therefore, making use of this unique experimental system and developing theoretical models, this thesis presents novel contributions to our understanding of the GaAs(001) surface, in particular regarding surface phase stability. Having this been team work, the author of this thesis has specialised in producing theoretical models and computations to help understand the experimental observations. The first piece of work presented in this thesis is a novel form of surface phase coexistence on GaAs(001), occuring when the surface is heated under vacuum above 580 °C (a typical temperature for epitaxial growth on GaAs(001)). Under these conditions, Langmuir evaporation of the crystal becomes important and due to this, we directly imaged the spontaneous formation of metastable surface phase domains on GaAs(001) corresponding to a (6×6) periodicity. These metastable phases exist for some time before spontaneously transforming back to the thermodynamically stable parent phase (the well-known c(8×2) reconstruction), producing a dynamic phase coexistence between the two phases. Monte Carlo simulations were used to identify the key kinetic processes and investigate the interplay between phase metastability and evolving surface morphology. This is used to explain the measured temperature dependence of the time-averaged (6×6) coverage. Next we present another piece of work in which we are able to map the surface phase diagram of GaAs(001) by combining droplet epitaxy with low energy electron microscopy imaging techniques. Upon subjecting Ga droplets on the GaAs(001) surface to an As flux we observe a sequence of well-defined surface phases with distance to the droplet edge. Using a simple model which links the spatial coordinates of phase boundaries to the free energy, we are able to interpret these phase patterns produced during droplet epitaxy. Based on the observed sequential order of the phases away from the droplet, it is possible to obtain important new information on surface phase stability. This is used to augment existing T=0 K phase diagrams generated by density functional theory calculations. We also establish the existence of a (3×6) phase, and confirm, that the controversial (6×6) phase is thermodynamically stable over a narrow range of conditions. The last piece of work presented here is one that is purely theoretical. Since the mid 2000s a technique called local droplet etching has been used to produce nanohole templates on different III-As surfaces. It consists of high-temperature annealing (T∼550 °C → 600 °C) of nanoscale group III liquid droplets on these surfaces under low As flux. Its main application is the production of strain-free quantum dots by subsequent filling of these holes by means of epitaxial growth. The main advantage of this etching technique versus ex-situ etching is the absence of defects and impurities as it is carried under epitaxial conditions and does not include foreign elements. Despite this technique having been widely used, the physical mechanism of droplet etching is not yet well understood. Here we present a model for the case of Ga droplet etching of GaAs which, considering the different kinetic mechanisms of mass transport, produces dynamic simulations of nanohole formation and is able to answer the major open questions regarding this technique.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Physics and Astronomy
Subjects: Q Science > QC Physics
Uncontrolled Keywords: GaAs(001), crystal growth modelling, LEEM, surface phases
Funders: Cardiff University, College of Physical Sciences
Date of First Compliant Deposit: 8 October 2020
Last Modified: 25 May 2021 01:32

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