טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentDivinsky Alon
SubjectOptical Transitions in Mixed Type I - Type II
Quantum Wells under a Magnetic Field
DepartmentDepartment of Physics
Supervisors Professor Emeritus Elisha Cohen
Professor Emeritus Arza Ron


Abstract

The objective of this work is to find the relation between the density () of a two dimensional electron gas (2DEG)created by photoexcitation, in a GaAs/AlAs mixed type I - type II quantum well (MTQW) structure and the intensity of the exciting light. The MTQW structure enables to carry out a spatial separation between electrons and holes, i.e. electrons and holes are excited in the narrow well of the MTQW, but because of specific properties of this structure, electrons transfer the wide well of the MTQW in a very high rate as compared to holes, thus a stable 2DEG is achieved.


All the performed experiments were of photoluminescence type. The varying parameters were magnetic field and excitation intensity.

In order to determine the relation between and the excitation intensity from the photoluminescence spectra two methods were used, depending on the density of the 2DEG. In the

wide barrier mixed type I - type II quantum well (MTQW) a higher 2DEG density, as compared to the narrow barrier MTQW, was achieved (). In the wide barrier MTQW sample the 2DEG density was calculated by measuring the magnetic field values at which the electron Landau levels quench, and how many Landau levels are in the optical spectrum. This method, known as Landau levels freezing, was applied to a close to ideal system, namely, a modulation doped QW (MDQW) doped with a known 2DEG density, in order to confirm its reliability. The calculated density using this method, was very close to the 2DEG density of  the MDQW that was measured by Hall conductivity. The last fact

gave the validity for applying the method to the wide barrier MTQW. The measured densities show a qualitative agreement with the expected behavior, i.e. the density increase along with the excitation intensity. In the narrow barrier MTQW the 2DEG density was lower () than in the wide barrier MTQW and the MDQW samples, as a consequence the optical spectra had a completely different character, namely, it was dominated by exciton and trion recombination lines.