Low-Dimensional Group III-V Compound Semiconductor Structures

1.1 Epitaxial Growth Epitaxial growth of inorganic materials in the form of single-crystal thin films can be viewed as a special case of crystal growth that is essentially a first-order phase transition exhibited by a wide range of single chemical elements and a variety of compounds including insulator, metals, and semiconductors. Epitaxial growth of thin films can also be viewed as a unique case of those among various deposition processes of thin films. Epitaxial growth of thin films requires a combination of stringent crystal growth parameters that need to be dynamically tuned to establish appropriate growth environment. Everlasting quest to raise the level of perfection of single-crystal thin films by understanding and improving epitaxial growth processes has been a scientific subject that has inspired many scientists in the past. An early attempt of constructing a theory of crystal growth from vapor phase, including many essential aspects of crystal growth, treated surface features (e.g., steps and kinks) on a crystal surface in equilibrium with vapor by obtaining the rate of advancement of monomolecular steps as a function of supersaturation in the vapor and mean concentration of kinks in the steps.1 Influence of dislocations was also analyzed and growth rate of a surface containing dislocations was shown to be proportional to the square of the supersaturation for low supersaturation and to the first power for high supersaturation. Equilibrium structures of steps (e.g., the statistics of kinks in steps) were also studied in terms of surface temperature, binding energy parameters, and crystallographic orientation. Shape and size of a two-dimensional nucleus in unstable equilibrium with a given supersaturation at a given temperature were obtained. Analyses on the temperature dependence of the structure of perfect surfaces (i.e., surfaces free from steps at absolute zero temperature) showed that a perfect surface remains flat until the surface reaches a roughening transition temperature at which the surface undergoes a morphological transition by which the surface becomes rough dramatically.2 With the view more practical, a unified theory was developed to understand the physics of epitaxial thin films and applied to tailor epitaxial growth conditions to obtain perfect epitaxial thin films of uniform 4