The constituent layers of a semiconductor quantum well system were studied systematically and sequentially using various spectroscopic and optoelectronic techniques such as photoluminescence, photoreflectance, photovoltage, electroreflectance, or electroluminescence. Complete single quantum well samples with various well parameters were then studied with the appropriate technique. The purpose of the study was to find how the energy levels (the bound states as well as the continuum states) of the quantum well structure were influenced by the surroundings (such as the cap layer, the buffer or the substrate) and the well parameters. To that effect, the well width and the alloy composition (x) of an In$\sb{\rm x}$Ga$\sb{\rm 1-x}$As/GaAs single quantum well were varied, and the results obtained with samples having different cap thicknesses (the other parameters being kept the same) were compared. A theory predicting the influence on the energy levels of the cap layer thickness was developed, and experimental evidence was found in modulated reflectance measurements which showed spectral oscillations for energies above the barrier. The behavior is explained by deriving the wavefunction of the carriers with energies larger than the barrier (continuum states) taking into account the finite size of the cap layer. It was also found in the sequential study that in luminescence, depending on the excitation used, the substrate can contribute to the response obtained with the complete device. The influence of an external perturbation such as an electric field was also studied. Electroreflectance spectra showed that the bound states were Stark shifted, while the peaks of the cap-related oscillations of the continuum states were shifted linearly in energy with the electric field. Study of a multiple quantum well under the same conditions showed that the cap-related oscillations of the continuum states were strengthened by the presence of the additional wells. The theory also predicts that the first well(s), closer to the surface, will be more energy selective in capturing carriers in the continuum. This may result in different capture efficiencies for the various wells of a multiple quantum well structure.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/7516 |
| Date | January 1992 |
| Creators | Fafard, Simon. |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
| Format | 194 p. |
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