M.Sc Thesis

M.Sc StudentSvidovsky Polina
SubjectOptical Study of Wide GaAs/AlGaAs Quantum Wells Containing
a Two Dimensional Electron Gas
DepartmentDepartment of Physics
Supervisors DR. Boris Ashkinadze
Full Thesis textFull thesis text - English Version


The effects of a two-dimensional electron gas on optical properties of quantum wells are studied. I investigated the photoluminescence (PL) and photoluminescence excitation (PLE) spectra of a wide (400A, 500A and 700A) GaAs/Al0.2Ga0.8As modulation doped quantum wells (MDQWs) containing the 2DEG of variable density and high electron mobility (~ 9·106 cm2/ V·sec). The 2DEG cyclotron resonance absorption (CRA) at the microwave frequency of 36 GHz was measured in order to monitor n2D values. I use the fact that the CR band area is proportional to n2D value. Microwave radiation was also used to study the effect of 2DEG heating on the PL spectra.The PL spectrum  is modified as n2D decreases. The broad 2DEG-hole PL spectrum transforms into asymmetric PL line which is due to a charged exciton (trion) radiative recombination. With further n2D reduction, the trion PL line shifts to higher energy and broadens. Then, the exciton PL line appears. The latter shifts to lower energy with decreased n2D, and the trion PL intensity decreases while the exciton one increases. The energy separation between the trion and exciton lines is of ~1 meV.  At lowest n2D, the width of the exciton PL line is of ~0.2 meV, and it indicates high sample quality. In the PLE spectra, the broad PLE band is observed above the Fermi energy.  As n2D decreases this band transforms into trion and exciton PLE lines. The PLE of the trions shifts to higher energies, while the PLE of the excitons shifts to lower energies with further decreasing n2D .For all the samples the energy separation between trion and exciton lines, the trion binding energy,  is of ~0.7-0.8 meV. This value is higher than theoretical one for free trion in bulk GaAs  (~ 0.22 meV).

The shift of the trion PL and PLE lines doesn't correspond to the theoretically expected shift which equals to the Fermi energy.

Analysis of asymmetric trion PL line shape was performed in a free trion model. This shows that the free trion model disagrees with the trion PL evolution in high quality MDQW.

Based on our experiments we are able to conclude that the trions appear to be localized in the highest quality QWs.  The observed PL and PLE evolution might be explained by an appearance of non-uniform distribution of 2DEG,  that means a percolation mechanism for metal-insulator transition.