Long-Wavelength InAs/GaAs Quantum Dot Heterostructures and Lasers

• Main Authors: Wei-Sheng Liu, 劉維昇
• Other Authors: Jen-Inn Chyi
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/77535364471787375731

博士 === 國立中央大學 === 電機工程研究所 === 94 === 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 carriers’ 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.