In electronic materials, defects and disorders play an important role to determine the characteristics, efficiency, and lifetime of the system. This talk presents computational approaches to study fundamental mechanisms about how defects and chemical disorders change the properties of semiconductors and ferroelectric materials.

First, effects of dislocations on optical properties in semiconductors are discussed. Optical properties of semiconductors have been utilized in a wide range of applications including light-emitting diodes, lasers, and solar cells. In heterostructures of semiconductors, dislocation formations are induced by mismatches of lattice constants. Dislocations generate the electrostatic potential and deformation potential associated with inhomogeneous strain fields resulting in inhomogeneous spatial distributions of electrons and holes. The Schrödinger equation is solved using the real-space finite element formulation to calculate the optical responses. This method is useful for inhomogeneous systems such as dislocations, grain boundaries, and quantum dots.

Second, atomic-based continuum modeling is presented to study effects of chemical disorders in ferroelectrics. Ferroelectric materials are used for a number of applications including ferroelectric memory, high density energy storage, and medical imaging. Most common ferroelectric compound, Lead-Zirconate-Titanate (PZT, PbZr1-xTixO3), exhibits very high dielectric/piezoelectric constants and low hysteresis near Morphotropic Phase Boundary (MPB, about x=0.5). To understand fundamental mechanisms near MPB, details of microscopic and macroscopic behaviors are to be fully understood and coupled properly. Effective energy, which provides connections between atomic behaviors and continuum models, is developed. Microscopic knowledge is studied with Density Functional Theory, and Monte Carlo simulations are performed to explain the macroscopic behaviors near MPB at a finite temperature.