Tang, M; (2016) InAs/GaAs Quantum-Dot Light Emitting Sources Monolithically Grown on Silicon Substrates. Doctoral thesis , UCL (University College London).

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Abstract

Si-based light emitting sources are highly demanded for applications in optoelectronic integration circuits. Unfortunately, Si has an indirect bandgap and thus a low efficiency in photon emission. On the other hand, III–V semiconductors have superior optical properties and are considered as strong candidates to achieve efficient light emitting sources on Si platforms via wafer bonding or monolithically epitaxy growth. III–V materials monolithically grown on Si substrate could introduce various types of defects including antiphase domain, threading dislocation, misfit dislocation. These defects must be dealt with satisfactorily in order to fulfill the potential of III–V/Si integration. In this thesis, buffer layers for InAs/GaAs quantum dots (QDs) monolithically grown Si substrate have been investigated. The buffer layer study is mainly focused on the different types of defect filter layers (DFLs). The measurements of atomic force microscopy, photoluminescence and transmission electron microscopy are carried out to investigate the effectiveness of each type of DFLs. The results of lasers and superluminescent diodes (SLDs) have been presented based on the studies of DFLs. In order to improve the performance of InAs/GaAs QDs grown on Si substrates, a GaAs buffer layer and DFLs have been used to reduce the defect density from ~1010 to 106 cm-2 after three sets of DFLs, which consists of strained layer superlattices (SLSs). In the thesis, the optimisation of DFLs has been carried out. Different types of DFLs are investigated in the Chapter 3, including InAs/GaAs QDs, InGaAs submonolayer QDs, InGaAs/GaAs SLSs and InAlAs/GaAs SLSs. DFLs made of InAlAs/GaAs SLSs show the strongest performance, based on the measurements of atomic force microscopy, photoluminescence and transmission electron microscopy. The high performance InAs/GaAs QDs lasers with low threshold current density (194 A/cm2 ) and high operating temperature (85˚C) has been obtained for the samples with optimised DFLs. In addition to III–V/Si lasers, III–V SLDs monolithically grown on silicon substrates would further enrich the silicon photonics toolbox, enabling low-cost, highly scalable, high-functional, and streamlined on-chip light sources. In this thesis, the first InAs/GaAs QD SLDs monolithically grown on a Si substrate have been demonstrated based on the similar growth structure of laser devices. The fabricated two-section InAs/GaAs QD SLD produces a close- 4 to-Gaussian emission spectrum of 114 nm centred at ∼1255 nm wavelength, with a maximum output power of 2.6 mW at room temperature. The optimisation of InGaAs/GaAs SLSs DFLs has been carried out in the Chapter 5. The optimisation includes introducing different growth methods into GaAs spacer layer between each set of DFL, indium composition and GaAs thickness in InGaAs/GaAs SLSs. The optimisation is examined by atomic force microscopy, photoluminescence and transmission electron microscopy. The laser device with optimised InGaAs/GaAs SLSs DFLs has a lower threshold current density, higher operating temperature and characteristic temperature. In conclusion, InAs/GaAs QDs lasers with low threshold current density and the first QDs SLDs monolithically grown on Si substrates have been demonstrated. InAlAs/GaAs SLSs DFLs have been proved that as considerable solution to reduce the threading dislocation density significantly. The optimisations of InGaAs/GaAs SLSs DFLs successfully improve the QDs laser performance which could also be used in III–V/Si monolithically integration. The III–V QDs lasers and SLDs monolithically grown on Si substrate are essential steps for Si photonics integration, which will fill the “holy grail” of opto-electronic integration circuits.

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