טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentTisch Ulrike
SubjectEpitaxial Growth and Optical Properties of GaAsN
DepartmentDepartment of Electrical Engineering
Supervisors Professor Emeritus Yosef Salzman
Professor Emeritus Eliezer Finkman


Abstract

The unusual optical properties of GaAsN arouse fundamental and applied research interest. As and N atoms differ greatly in size and electronegativity. This results in a large miscibility gap and a strong perturbation of the electronic band structure, whose nature is yet subject of theoretical controversy. Prior to this study there has been a lack of reliable experimental data, due to varying material quality as well as inconsistent and unsuitable experimental methods used for optical characterization. We have developed growth capabilities for high quality As-rich GaAsN layers on GaAs substrates by metal organic vapor phase epitaxy and studied the growth process. The most striking property of GaAsN, a large anomalous reduction of the fundamental bandgap, was studied by combining transmission, reflection and photoluminescence spectroscopy conducted at low and room temperature. The use of a large number of samples with N content ranging from N-doping to N-supersaturation enabled us to identify four regions of N incorporation with distinctly different bandgap reduction. An empirical expression for the composition dependence of the bandgap was proposed for practical applications. The electronic band structure was further analyzed by studying two weakly perturbed optical transitions above the fundamental band gap, E1 and E1+D1, by variable angle spectroscopic ellipsometry. Evidence was found for a small N-induced splitting of E1+D1. The phonon modes of GaAsN were studied by Raman spectroscopy. A very unusual temperature evolution of the dominant Raman line was observed, indicating N-induced coupling of longitudinal optic phonon modes and electrons in undoped GaAsN.