Droplet epitaxy of GaAs studied by in situ surface electron microscopy
2017-02-06T06:12:03Z (GMT) by
Droplet epitaxy is a recently developed variant of molecular beam epitaxy (MBE) which is used to form compound semiconductor quantum structures. This usually involves the deposition of group III material liquid droplets on a substrate followed by crystallisation of the droplets under As flux. The electronic properties of quantum structures depend sensitively on the size and shape of nanoscale rings formed in the crystalisation process and so it is important to understand how to control the formation of these features for potential device applications. A significant difficulty in studying quantum structure formation is the presence of the large As flux which limits real-time studies using conventional techniques such as scanning tunnelling microscopy (STM). The first aim of this thesis is therefore to develop a III-V low energy electron microscope (LEEM) system for in situ, real time imaging of droplet epitaxy. The thesis begins with the development of the III-V LEEM system and its application to study Langmuir evaporation of GaAs (Chapters 2 and 3). We find the III-V LEEM can achieve in situ, real time observation of droplet nucleation, evolution, motion and coalescence. This establishes the feasibility of studying droplet epitaxy using III-V LEEM. As a prelude to studying droplet epitaxy, in Chapter 4 we consider the thermodynamics of Ga droplet formation during Langmuir evaporation of GaAs (001). The congruent evaporation temperature Tc plays a critical role in this process. Below Tc, Ga and As evaporate from the surface at equal rates, preserving substrate stoichiometry. However, above Tc, As evaporates more rapidly than Ga leaving behind Ga-rich droplets on the surface. At Tc, the droplets are stable and neither shrink nor grow, which provides an accurate measure of Tc. In Chapter 5 we apply this condition for droplet stability to experimentally measure Tc in the presence of As flux using III-V LEEM. This dependence is explained by modifying the thermodynamic model for evaporation to incorporate As flux. This work provides a method of controlling congruent evaporation which is important for MBE growth, droplet epitaxy, surface preparation and modifying droplet motion. The creation of droplets above Tc during Langmuir evaporation provides a potential means of self-assembling droplet arrays for subsequent crystallisation under As flux. It is therefore important to understand how the droplet size distribution evolves with time during this process. This is considered in Chapters 6 and 7 where we apply real-time surface electron microscopy to make movies of how droplet arrays evolve. Surprisingly, new Ga droplets are seen to form in regions cleared by the coalescence of larger droplets. A simple Monte Carlo model incorporating daughter droplet generation by coalescence is used to reproduce and explain the major features of our experimental droplet size distributions. In Chapter 8 we study droplet epitaxy of GaAs in real-time using surface electron microscopy which provides new insights into the dynamics of Ga droplet crystallisation under As flux. The resulting movies are used as the basis of a theoretical model for quantum ring formation which can qualitatively explain the origin of quantum features observed under a variety of experimental conditions. The model predicts that Ga adatom diffusion, under differing conditions of temperature and As flux, chiefly controls the quantum structure morphology. Local droplet etching (LDE) has received significant attention over recent years as a means of fabricating nanoscale holes in semiconductor surfaces. The technique offers the significant advantage that it avoids the need for lithographic processes and can be applied to a wide range of materials. In Chapter 9 we utilise III-V LEEM to study the time-evolution of Ga droplet etching of GaAs under As flux. A theoretical model of the etching process is developed from the movies which requires a minimum number of assumptions and is simply based on the liquid droplet maintaining a composition close to its equilibrium liquidus value. Conclusions and further work is considered in Chapter 10.