First Principles Investigation of Entropy and Equilibrium Atomic Structure at the Surface of III-V Semiconductors.

Abstract

The cleaved surfaces of III-V semiconducting compounds can assume a wide variety of surface structures over a range composition. Because III-V alloys, heterostructures, and devices are synthesized epitaxially, under carefully controlled conditions, the ability to reproducibly control surface structure of these materials could enable powerful optimization pathways for III-V applications. Unfortunately, significant challenges prevent accurate prediction of thermodynamic stability and equilibrium properties of multicomponent crystalline surface structures. This dissertation describes rigorous methods that address these challenges and the application of these methods to the surfaces of III-V materials generally, and to the (001) surface of GaAs and its dilute alloys in particular. The methods presented build upon one another, following a progression that begins with systematic enumeration of physically plausible surface structures, proceeds with the use of rigorous energy models and simulation techniques to account for different forms of disorder, and culminates in applications of these techniques to explore alloy ordering and structural disorder of more complex systems.

First, we present a new algorithm to enumerate all possible surface structures that obey observed structural trends, enabling generation of large databases of potential structures that can be explored in detail to determine equilibrium behavior for specific materials systems. We use this method to identify several new low-energy surface structures of the (001)-oriented GaAs surface. For each low-energy structure, we use first-principles calculations to parameterize an effective Hamiltonian that can be used in rigorous finite-temperature simulations, including Monte Carlo. We use results of these simulations to predict the surface reconstruction phase diagram of As-rich GaAs(001). Accounting for the error of our first principles calculations, our phase diagram corroborates the existence of the experimentally observed (4×3) surface reconstruction. We also construct and effective Hamiltonian to study surface alloying with In at the GaAs(001) surface in terms of the complex substitution and adsorption degrees of freedom of the experimentally-observed GaAs (2×4) structure. We compare the finite temperature ordering phenomena of the alloyed (2×4) to detailed analysis of zero-temperature surface phase stability of the alloyed (2×4), along with the alloyed (4×3) and c(4×4) surface phases.