Optical Properties Of ?-? Compound Semiconductor Quantum Dot And Quantum Well Hybrid Nanostructures

?-? semiconductor low-dimensional nanomaterials by cooperating quantum dots(QDs)and quantum well(QW)in together to from hybrid structures have attracted great research efforts recently due to their flexibility for bandgap engineering to achieve novel physical properties.Such hybrid nanomaterials have been extensively exploited for optoelectronic devices,including lasers,infrared photo-detectors,opto-electronic modulators,and solar cells.It is of great significance for conducting dedicated studies for these hybrid nanostructures,with the objective to promote our understanding of carrier dynamics and new physics,as well as to enhance the related device performances.In this dissertation,several hybrid nanostructures are investigated,including the coupled QDs-QW(QDW)structure composed by type-I band alignment InAs/GaAs QDs coupling with an InGaAs/GaAs QW,and the QDW as well as QDs-in-Well(DWELL)structures composed by coupling the type-II GaSb QDs with either an type-I InGaAs/GaAs?GaAs/AlGaAs QW or an InP-based InGaAs/InAIAs QW.First,the epitaxial gro,Awth conditions are optimized for QDs and QW structures according to the QDW design.Then,the morphology and composition,in particular the optical properties are carefully studied by various characterization tools for these hybrid structures.The optical features and related physical mechanisms such as carrier dynamics are interpreted according to the experimental observations.The contributions of this dissertation include:1.Strong photoluminescence emissions are obtained to cover the whole communication wavelength range via modification of the InGaAs/InAIAs QW thickness.Strategy is proposed to reduce the imperfect interface effect to achieve high quality QWs.The growth condition for GaAs-based InAs/GaAs QDs,GaSb/GaAs QDs,and InP-based InGaAs/InAlAs QW are optimized,respectively.Uniform InAs and GaSb QDs with suitable areal density are obtained.The experimental results in together with the theoretical analysis indicate that the imperfect interfaces and carrier localization effect are key factors to be overcome for achieving high quality QW.Accordingly,a solution strategy is proposed regarding to different applications for the QW heterostructure.2.For QDW structure of InAs/GaAs QDs coupling with an InGaAs/GaAs QW,it is found that there is a special carrier double resonance tunneling mechanism.In this QDW structure,the InGaAs/GaAs QW collects and provides carriers to the QDs through quantum coupling and carrier tunneling.The carrier lifetime in QW is reduced by an order of magnitude,while QDs have their PL intensity enhances nearly three times but the carrier lifetime almost unchanged.After calculation of the energy levels,we propose that there is a double resonance tunneling phenomenon in this QDW structure,by which faster carrier transfer and the enhancement of carrier injection efficiency are predicted.3.Artificial type-II band alignment QDW structure is implemented by coupling type-II GaSb/GaAs QDs to a type-1 InGaAs/GaAs QW,in which the wetting layer(WL)plays an important role for the holes' dynamics.The direct bandgap and large absorption cross section enable the InGaAs QW to be a layer for electron storage and hole injection,so that holes in the QW are injected into QDs by tunneling.The experiments confirm that WL of GaAs QDs has the ability to rapidly transfer holes from InGaAs QW to QDs.However,the WL exhibits a strong exciton localization effect,which reduce the efficiency for QDs to get holes from QW and WL.Therefore,it is very necessary to optimize the WL to improve the QDW injection structures.4.Optimized QDW and DWELL hybrid structures are experimentally testified by combination of GaSb/AlGaAs QDs and GaAs/AlGaAs QW.As a results,the luminescence from QDs is measured to be stronger than the QW.Through optimization of WL and introduction of wide-bandgap AlGaAs barriers,the capability to inject holes into QDs has been successfully ernhanced in the QDW.Stronger luminescence is observed from GaSb QDs in comparison with that from QW.The DWELL structure of GaSb QDs surrounded by AlGaAs barrier is also prepared and it shows emission even stroiger than the QDW.5.The QDW hybrid structure is explored by combining InP-based GaSb/InALAs QDs and InGaAs/InAlAs QW to enable luminescence wavelength more than 2?m.This QDW structure has a type ? band alignment.It realizes a wide-range of band gap adjustment by tuning the parameters of QDs,QW,and spacers.It is also found that the luminescence intensity of GaSb QDs in this QDW is higher than that from single layer GaSb/InAlAs QDs or InGaAs/InAlAs QW.In summary of the above experimental results,the QDW and DWELL hybrid structures provide more flexibilities on nanomaterial design and fabrication in comparison with common QDs-only or QW-only structures.The dimensions,material choices,and compositions for QDs,QW,spacer as well as barriers are all possible good choices to tailor the hybrid structure's optical properties and carrier dynamics,including carrier population,tunneling,recombination and carrier lifetimes.Therefore,the hybrid structures combining QDs and QW is an optimized choice for implement band-gap engineering to modify and promote physics properties for low-dimensional semiconductor nanomaterials for novel opto-electronic devices.