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Student Number 88521019
Author Wei-Sheng Liu(╝B║ű¬@)
Author's Email Address s8521019@cc.ncu.edu.tw
Statistics This thesis had been viewed 2395 times. Download 869 times.
Department Electrical Engineering
Year 2005
Semester 2
Degree Ph.D.
Type of Document Doctoral Dissertation
Language English
Title Long-Wavelength InAs/GaAs Quantum
Dot Heterostructures and Lasers
Date of Defense 2006-07-03
Page Count 125
Keyword
  • Long-Wavelength Quantum Dot Heterostructure
  • Quantum Dot Laser
  • Quantum Dots
  • Abstract This dissertation is devoted to growing and characterizing high quality long-wavelength InAs quantum dot heterostructures and lasers by an ultra-high vacuum molecular beam epitaxy system.
    Since the growth of QD active layers is the key factor to the development of QD lasers, it is most important to study the epitaxy growth of a high quality QD active layer. First, we focus on the modification of the QD growth recipe, including growth rate, temperature, nominal thickness and growth interruption. In the optimization of the QD growth recipe for fine optical properties, the importance of dot-height uniformity to the optical properties is disclosed through the employment of atomic force microscopy and photoluminescence measurement. Long-wavelength QD lasers with different dot-height uniformity are also fabricated to demonstrate the significance of dot-height uniformity in achieving high performance QD lasers. The results reveal that a ridge-type QD laser with high dot-height uniformity shows better characteristics than its counterpart, and can be operated at 1328 nm under room temperature. The threshold current density of this as-cleaved laser is 250 A/cm2 , which is comparable to the reported results obtained with high-reflectivity facet coating and similar ridge sizes, indicating the potential of the QD laser in this work for optical-fiber communication.
    In order to further improve the characteristics of the long-wavelength QD laser, we systemically study and clarify the mechanisms of elongated QD emission wavelength within different matrices. It is found that the thickness of the InGaAs overgrown layer would alter the wavelength-extension mechanisms and that strain-reducing effect is more dominant in extending the emission wavelength of QDs when the overgrown layer is thin. We also overgrow an InAlAs layer on the InAs QDs to act as a strain-reducing layer (SRL). Since the InAlAs layer suppresses indium segregation and increases potential barrier height in InAs/GaAs band diagram, uniform dot size and large state separation results are obtained by QDs with an InAlAs overgrown layer. The mechanisms of red-shifted wavelength discussed here could be constructive in realizing long-wavelength QD LDs with high characteristic temperature when operated above room temperature.
    After optimizing the QD growth parameters and studying the mechanisms of elongated QD emission wavelength, multi-stack QDs are grown for increasing the modal gain of QD lasers. However, the surface stress caused by lattice mismatch between the InAs and (In)GaAs overgrown layers often results in defects that are detrimental for optical devices. The surface stress would also lead to the formation of pinhole-like defects in the growth of multi-stack QDs. These not only impede the formation of uniform QDs, but also deteriorate the crystal as well as the optical quality of the multi-stack QDs. Growth interruptions during GaAs spacer layer formation and thermal annealing after the GaAs growth are employed to lead to indium-flush behavior and increase the diffusion length of GaAs adatoms for a smooth GaAs surface, and multi-stack quantum dot structures without pinhole-like defects are thus obtained. Based on the investigation, the demonstration of high performance 1.3 um quantum dot lasers with a 5-stack QD active region proves the effectiveness of the novel method in eliminating the pinhole-like defects.
    Additionally, the employment of an InGaAs strain-reducing layer is now the most favorable approach for long-wavelength (1.3 um) QD lasers. However, further extending the emission wavelength to 1.5 um always induces misfit dislocations and degrades the radiative efficiency of the QD structure because of lattice mismatch. Besides, since the InGaAs strain-reducing layer acts as a transit channel for facilitating the carriersíŽ thermal escape from InAs/GaAs QD structure, the thermal stability of this kind of laser at elevated temperatures is therefore far from expected. For improving the characteristics of QDs, we employ InGaAsSb or InAlAsSb instead of typical InGaAs or InAlAs strain-reducing layers, and find it can actually improve the band structure of QD active layer. In this investigation, the influence of Sb incorporation in SRL on QD optical properties and thermal stability is comprehensively studied. Enhanced emission intensity, thermal stability and extended emission wavelength of InAs/GaAs QDs is shown by the use of an Sb-contained strain-reducing layer. The superior optical characteristics of this novel structure make it a promising candidate for high-performance QD optoelectronic devices.
    Table of Content ŻÎĄň║Kşn          II
    şP┴┬             IV
    Abstract           V
    List of Figure CaptionsVIII
    List of Table CaptionsXIII
    Chapter 1 Introduction
    1.1 Quantum Dot Formation1
    1.2 Quantum Dot Laser4
    1.3 Outline of this Dissertation10
    Chapter 2 Effects of Growth Parameters on the Dot-Height Uniformity and the Performance of 1.3 um InAs Quantum Dot Lasers
    2.1 Sample Design of Quantum Dots with Different Dot-Height Uniformity14
    2.2 Experimental and Simulation Results of Dot-Height Uniformity19
    2.2.1 Experimental Results of Quantum Dot Samples         19
    2.2.2 Characteristics of Quantum Dot Lasers              31
    2.2.3 Simulation Results of the Influence of Dot-Height Uniformity on the  Optical Properties of QDs                       39
    2.3. Conclusion                            44
    Chapter 3 Optical Properties of Long-Wavelength InAs Quantum Dots with InAlAs/InGaAs Composite Matrix
    3.1 Sample Design within Different Matrix46
    3.2 Study of Matrix Dependent Optical Properties51
    3.2.1. Mechanisms of wavelength extension of quantum dots51
    3.2.2. Increased dot uniformity and state separation of QDs with InAlAs overgrown layer53
    3.3 Conclusion58
    Chapter 4 Pinhole-Like Defects in Multi-Stack 1.3 um InAs Quantum Dot Lasers
    4.1 Sample Preparation and Characterization60
    4.2 Origin and Elimination of Pinhole-Like Defects63
    4.3 Conclusion79
    Chapter 5 Enhanced Thermal Stability and Emission Intensity of InAs Quantum Dots Covered by InGaAsSb Strain-Reducing Layer
    5.1 Sample Design for Studying the Effect of InGaAsSb Capping Layer on Dots81
    5.2 Optical Properties of InGaAsSb Capped Quantum Dots86
    5.3 Conclusion96
    Chapter 6 Enhanced Optical Properties of InAs Quantum Dots Covered by an InAlAsSb Strain-Reducing Layer
    6.1 Sample Design for Studying the Effect of an InAlAsSb Capping      Layer on Dots98
    6.2 Optical Properties of InAlAsSb Capped Quantum Dots100
    6.3 Conclusions108
    Chapter 7 Summaries and Future Direction
    7.1 Summaries109
    7.2 Future Direction111
    Reference 112
    List of Publications120
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