|Ph.D Student||Levy Moshe|
|Subject||Intersubband Transitions in Semiconductor|
Bragg Confining Structures
|Department||Department of Physics||Supervisor||Professor Emeritus Robert Beserman|
Electronic Bragg mirrors are used to confine carriers at energy levels above the barrier height in asymmetric coupled quantum wells (ACQW) which are composed of a wide quantum well (WQW) weakly coupled to a narrow quantum well (NQW). An electric field inside the quantum structure is created by transferring carriers from the WQW, into the NQW by using intersubband transitions (ISBT). For the ISBT we used a CO2 laser to excite carriers from the WQW into an electronic level which is common to the WQW and NQW. A small fraction of the excited carriers release their energy in the NQW, thus generating electric dipole. Two classes of above the barrier states are resolved by using modulated resonant Raman spectroscopy. The first level is the Bragg state which is highly localized by the Bragg reflector above the ACQW and it is red shifted when a photogenerated local electric field is created in the ACQW region. The second one is a reflector level which extend mainly above the reflectors region, is seen by photoluminescence (PL) and PL excitation measurements. It is less shifted than the resonant level, when the photogenerated local field is applied, which is due to the smaller localization of these states in the ACQWs. We used modulated PL and Raman spectroscopy to resolve the stark shifts of the bound and above the barrier levels as a function of the infrared photoexcitation intensity. Our results indicate that as a result of the photoinduced electric field, the shifts of the above the barrier levels are linked to their degree of localization. These shifts are much stronger than those of the bound states inside the well and a model is proposed to explain the shifts of the continuum and bound levels using perturbation theory. To further increase the localization of the above the barrier states a two mono layers of AlAs barriers were added in each side of the ACQW. In addition, an electrical contacts were realized on the structure and we were able to study the effect of external applied electric fields on the different energy states in the sample. We found that the energy peak of the PL spectra were blue shifted and the line shapes narrowed considerably under the applied bias. These changes are linked to the depletion of the 2 dimensional electron gas (2DEG) which initially occupied the ground state in the WQW. The 2DEG was transferred into the reflectors QWs by the external field. The resonant Raman spectroscopy of this structure revealed an above the barrier state which was resolved only by the Raman spectra of the ACQW phonons and was blue shifted more than the states bound inside the QWs, indicating that this state is highly localized in the ACQW region. The influence of interband photoexcitation intensity on the PL line shape was also investigated in GaAs/AlGaAs modulation-doped asymmetric structure The emission spectra show a broadening and a narrowing of the line widths from the WQW and from the NQW respectively, with increasing laser power. In addition, we observe a fast increase of the emission intensity from the NQW with respect to that from the WQW. These processes are shown to be the result of carrier transfer into the NQW. We propose a charge transfer model based on rate equatins to explain semi-quantitatively our results.