|M.Sc Student||Nikolay Akopian|
|Subject||Optical Spectroscopy of Two-Dimensional Electron Gas in|
Nitride Compound Heterostructures
|Department||Department of Physics||Supervisor||Full Professor Gershoni David|
In this work we report on a study of optical and transport properties of molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) grown AlGaN/GaN single heterostructures. The two-dimensional electron gas confined at the interface of these heterostructures is now understood to arise due to the strong spontaneous and piezoelectric polarization fields present there. The AlGaN/GaN system is unique in that extremely high two-dimensional electron concentrations >1013 cm-2 are generated without modulation doping. These heterostructures are considered to be promising systems for high power, high temperature electronic devices. In addition, the combination of high breakdown fields and high current densities make AlGaN/GaN heterostructures very attractive for high power radio frequency transistors.
Our optical studies reveal differences in the photoluminescence spctra, due to the two-dimensional electron gas of the MBE and MOCVD grown samples. These differences are related to hole localization at the different layers (AlGaN or GaN, respectively) in the heterostructures. A strong enhancement of the photoluminescence intensity around the electron Fermi energy is observed at the low temperature spectra of the MBE grown Al17Ga83N/GaN sample, which is due to multiple electron-hole scattering processes between states at the vicinity of the Fermi energy (“Fermi edge singularity”).
Our magnetotransport measurements allowed us to calculate the energy levels and the concentration of the two-dimensional electron gas in the samples, which are in good agreement with our self-consistent Schrödinger and Poisson calculations. We show that the two-dimensional electron gas in MOCVD grown Al0.30Ga0.70N/GaN sample is confined in two main domains at the interface, which gives two close frequencies in the Shubnikov-de Hass oscillations.