# O-Band and C/L-Band III-V Quantum Dot Lasers Monolithically Grown on Ge and Si Substrate
## 1 Introduction  
Over the past few years, III-V-based photonic materials and devices have attracted much attention in the silicon photonics research fields [1; 2; 3; 4; 5; 6; 7; 8]. For the large-scale integration of complex optoelectronic circuits, on-chip light source could potentially achieve a higher integration density with a compact size and provide a better performance due to its economic advantages and energy efficiency [4; 5; 9; 10; 11; 12]. Due to the rapid growth of cloud-based application and data centers, the demand for high capacity and compact optical links is significantly increasing. Despite the tremendous progress towards silicon photonic integration with building blocks including modulators, detectors, waveguides, and multiplexers/de-multiplexers, an efficient, reliable and electrically-pumped laser on a silicon substrate has been the major challenge [7; 8; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. Advances have been made in group-IV-based light emitters widely used in integrated circuits, such as Si Raman lasers, germanium-on-Si lasers and GeSn-on-Si lasers [23; 24; 25; 26]; however, due to the indirect bandgap of group IV materials, they do not emit light efficiently. Therefore, the integration of other light-emitting materials is highly demanded for siliconphotonic integrated circuits. Due to the inherent limitations of Group IV materials for gain, modulation and detection at near-infrared wavelengths, in the foreseeable future, by combining the existing silicon photonics techniques with the outstanding optical properties of III-V materials, the hybridization of group III-V and IV materials would be very promising to boost the performances of photonics integration [9; 27]. The direct epitaxial growth of III-V materials on Si substrates is considered as one of the most promising methods to provide Si-based lasers for future photonics integration. Furthermore, after being extensively investigated for decades, silicon photonics is going beyond a technology that is purely communication-oriented and has evolved into a versatile integration platform, which also has great potential in various application fields but not limited to sensing, signal processing, quantum sciences, microwave engineering, imaging, spectroscopy, and high-performance computing [28; 29; 30; 31; 32; 33]. The monolithic integration of III-V materials on an Si platform have been investigated for decades [34; 35]. Unfortunately, monolithic III-V-on-Si integration poses critical issues stemming from the large material dissimilarity between III-V and group IV materials, including the formation of antiphase boundaries (APBs), threading dislocations (TDs) and the creation of thermal cracks. When polar III-V materials grow on non-polar Si substrates, anti-phase boundaries form; these boundaries are electrically charged planar defects and act as non-radiative recombination centers for optoelectronic devices and electronics devices, leading to the performance degradation. The 4% (7.5%) lattice mismatch between GaAs (InP) and Si makes direct growth of high-quality GaAs (InP) on Si very difficult, resulting in high dislocation densities on the order of (>10\({}^{9}\)-10\({}^{10}\) cm\({}^{-2}\)). In addition, the difference in the coefficient of thermal expansion between III-V and IV materials could promote the accumulation of thermal stress, which then leads to thermal micro-cracks in the epilayer while the wafer is cooled down from the growth temperature to room temperature (RT). There have been many researches carried out on the monolithic growth of InAs/GaAs quantum-dot (QD) lasers on Si substrates in the last few years, including techniques such as offcut Si (001) substrates, Ge/Si virtual substrates, hydrogen annealing process, intermediate GaP buffer, and V-grooved Si substrates [36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51]. However, most of the above techniques suffer from high defect density and thermal mismatch-induced material degradation.