טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentBerkowicz Eyal
SubjectOptical Properties of Nitride Quantum Structures
DepartmentDepartment of Physics
Supervisors Professor David Gershoni
Professor Emeritus Gad Bahir


Abstract

Considerable progress in group-III nitrides (GaN,AlN,InN) growth technology, have recently led to the demonstration of new devices operating in the Ultra-Violet spectral range. Therefore, the electronic and optical properties of this material system have become a subject of intensive research effort. Although a wealth of optical studies on GaN based quantum wells have been reported, the results in the literature are sometimes controversial, and there is an obvious need for a consistent model, which quantitatively describes the experimental observations.


This thesis presents both experimental and theoretical study of GaN based quantum wells. Experimentally, we applied photoluminescence, photoluminescence excitation and time-resolved optical spectroscopy for studying a set of  InxGa1-x­N/GaN periodic structures, which had been characterized by high-resolution x-ray diffraction including x-ray mapping in reciprocal space.


We found that the energy differences between the absorption edge and the photoluminescence peak (Stokes-shift) increase with the InxGa1-x­N/GaN  layer thickness, and that the photoluminescence decay time drastically increase with the layer thickness and with the sample temperature. We were able to quite accurately determine the radiative and non-radiative decay times of excitons in these structures by measuring the temperature-dependence of the photoluminescence decay time, the integrated photoluminescence intensity and the photoluminescence intensity immediately after a picosecond excitation pulse. The intrinsic radiative lifetimes, which are inversely proportional to the exciton oscillator strengths, were then calculated from the temperature dependence of the radiative lifetimes.


These experimental findings were analyzed using an eight-band k·P model, which quantitatively explains both the Stokes-shifts and the intrinsic radiative lifetimes. Their strong dependence on the quantum well width was found to be due to a large (~1 MVolt/cm) lattice-mismatch strain-induced piezoelectric field along the growth axis. The model was also used for calculating expected mid infrared absorption due to optical transitions of carriers between their respective subbands in these heterostructures. An extensive experimental effort to measure this absorption is described in some detail.


Theoretical calculations of the binding energies and the oscillator strengths of excitons in these quantum structures are also presented. These calculations were used for an improved quantitative comparison with the measured data.