Fabrication and Characterization of InGaAs/GaAs Quantum Structures on (111)B GaAs Substrates
|關鍵字:||分子束磊晶;量子點;砷化鎵;砷化銦鎵;量子井;Molecular Beam Epitaxy;Quantum dots;GaAs;InGaAs;Quantum well|
In this dissertation, a simple in situ method is used to fabricate high-quality InGaAs/GaAs quantum dots on (111)B GaAs substrates. The formation of quantum dots was not due to strain relaxation, but due to the growth characteristics on (111)B GaAs under the 2’2 surface reconstruction. The formation of the quantum dots was verified by AFM images and the shift of photoluminescence. Excellent PL result was obtained with a linewidth of 7.7 meV at 8 K, which is the narrowest linewidth reported for quantum dots. We have also tried various growth condition to get the proper range of growth parameter for good quantum dots. Besides, we have used magneto-optical measurement in high magnetic fields to investigate InGaAs/GaAs quantum well and quantum dots grown on (111)B GaAs substrates. Photoluminescence were measured under magnetic fields up to 45 T in both Faraday and Voigt configurations. The exciton binding energies can be deduced from the high field diamagnetic shifts and the values are 2.8 meV for quantum well and 5.5 meV for quantum dots. In addition, the extents of carrier wave functions are also obtained from the low field diamagnetic shifts. The ECWFs in growth and lateral directions are both smaller for quantum dots as compared with the quantum well, showing stronger confinement for quantum dots. This is consistent with the result of high-field measurement. Comparing the in plane ECWFs of InGaAs quantum dots with various In composition, we found that it is smaller for higher In composition, showing stronger lateral confinement. While for quantum wells of various In composition, the in plane ECWFs are comparable. In addition, we have studied the carrier dynamic of the InGaAs/GaAs strained quantum wells located at different positions on (111)B GaAs substrates, together with the references of (100) samples grown side by side with the (111)B samples, by the measurements of the high resolution time-resolved photoluminescence. Longer lifetime of the PL peaks was found from the quantum wells deep inside the samples, as compared with that from the quantum wells near surface. Besides, we have also measured the time-resolved PL of the quantum well and quantum dots. The slower decay time of quantum dots is another proof of the increase in the growth direction when islands form in our quantum dots fabrication method. Finally, we have studied time-resolved photoluminescence of InxGa1-xAs/GaAs quantum wells grown on (111)B and (100) substrates. Magnetic fields up to 6 T in Faraday configuration were used during measurements. Spin-flip process was found to play an important role in the exciton relaxation in (111)B strained QWs because of the strong piezoelectric fields. The spin-flip time from the dark exciton level to the bright exciton level increases with the magnetic field. This is the first time that the spin-flip process under strong piezoelectric fields was studied.
|Appears in Collections:||Thesis|