First principle calculations in II-VI semiconductors : intrinsic vacancy defects for laser cooling

• Main Author: Liu, Yang
• Other Authors: School of Physical and Mathematical Sciences
• Format: Final Year Project
• Language: English
• Published: 2014
• Subjects: DRNTU::Science::Physics::Optics and light
• Online Access: http://hdl.handle.net/10356/59935
• Institution: Nanyang Technological University

This thesis presents a number of results for II-VI semiconductors intended to be used in the laser cooling based on density functional theory. The emphasis was put on the intrinsic defects as they often introduce rich luminescent properties in semiconductor and such understanding is crucial towards the exciton dynamics and laser cooling. Our group has demonstrated a net cooling effect of 40 kelvin from room temperature for Cadmium sulfide nanobelts and donor-acceptor pairs originated from vacancy defects coupled with phonons plays a vital role in the up-conversion processes. A clear understanding of the defect-related excitons is particularly important for design and optimization of future cooling devices. After a brief introduction to the density functional theory, the repeated calculation for CdS is presented to confirm the shallow acceptors and donors are indeed coming from intrinsic vacancy defects. Compared to interstitials and anti-sites usually corresponding to high energy configurations, vacancies are more common and generate shallow defect levels so I restrict the calculation for vacancy type only for the II-VI family. Some experimental measurements including photoluminescent spectrum on CdSe was done as well. The extensive first principles study of structures and vacancy defects in II-VI semiconductors suggest Zinc telluride to be the potential candidate for cooling. Since the basic Born-Oppenheimer approximation in first principle calculation assumed atoms are fixed and the electron-phonon coupling can not be modeled, experimental verification shall be done in the future.