Liu, Zizhuo; (2022) High-performance III-V quantum structures and devices grown on Si substrates. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

III-V material laser monolithically grown on silicon (Si) substrate is urgently required to achieve low-cost and high-yield Si photonics. Due to the material dissimilarity between III-V component and Si, however, several challenges, such as dislocations and antiphase domains, remain to be solved during the epitaxial growth. In this regard, quantum dot (QD) laser diodes have been demonstrated with impressive characteristics of temperature insensitive, low power consumption and defects tolerance, and thus QD material is regards as an ideal material for laser directly grown on Si substrate. In this thesis, both QD laser diodes with 1.3 µm wavelength and quantum dot cascade laser with mid-infrared wavelength have been investigated. To understand the unique advantages of QD material, the comparison of QD and quantum well (QW) materials and devices grown on Si substrate is carried out in chapter 3. Based on identical fabrication and growth conditions, Si-based QW devices are unable to operate at room temperature, while the room-temperature Si-based QD is obtained with threshold current density of 160 A/cm2 and single-facet output power of >100 mW under continuous wave (c.w.) injection current driving. Besides, Si-based QD laser also shows remarkable temperature stability which the c.w. operation temperature reaches 66 ℃. The results point out that QD material has great potential in monolithic growth of III-V on Si for silicon photonics. Then, a novel approach of all-MBE grown QD laser on Si substrate is reported in chapter 4, with the optimization of buffer layer. The all-MBE grown QD laser on on-axis Si substrate with maximum operation temperature of 130 oC is achieved by utilizing thin Germanium (Ge) buffer. The mid-infrared silicon photonics has wide applications and market, but the lack of Si-based mid-infrared laser is a subsistent problem. Because the bandgap of conventional QW and QD materials is impossible to match the wavelength in mid-infrared range (3 µm to 20 µm), the Si-based quantum cascade laser (QCL) devices is regarded as an effective method to meet the requirement. Therefore, the high-performance QCL is firstly explored in chapter 5, and then, several methods in fabrication process are researched to enhance the performance for QCL devices. After the optimization of structure design and development of fabrication process, the InP-based QCL shows impressive properties with 600 mW emission power and over 100℃ operation temperature under c.w. mode. Following the previous work on Si-based QD laser, the quantum dot cascade laser (QDCL) is expected as a suitable solution for Si-based QCL devices. With the continuous improvement in structure design, the QDCL with multilayer QDs shows comparable performance, compared with conventional QCL devices. It is noted that the QDCL generates both TE and TM modes output, which is a breakthrough towards surface emitting QCL because the common QW-based QCL has only-TM emission in principle. Finally, the Si-based QCL is attempted with different structure design based on the pervious results.

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