# O-Band and C/L-Band III-V Quantum Dot Lasers Monolithically Grown on Ge and Si Substrate
## 2 O-Band and C/L-Band III-V Quantum Dot Devices via III-V/IV Hybrid Growth
### O-Band and C/L-Band InAs/GaAs Quantum Dots on (111)-Faceted Si Hollow Substrate
By reducing the period of the U shape structures in the initial Si (001) substrate, we may achieve a (111)-faceted sawtooth structure by depositing a thinner Si buffer layer. The entire epi-structure for GaAs buffer growth on Si is schematically shown in Figure 2c. Figure 2: (**a**) Cross-sectional scanning electron microscope image of U-shape Si (001) substrate, (**b**) (111)-faceted-sawtooth structure epitaxial growth on U-shape Si (001) hollow substrate, and (**c**) schematic diagram of epitaxial growth of (111)-faceted hollow structure on U-shape Si (001) substrate. A 10 nm AlAs nucleation layer was grown prior to the buffer layer growth on the (111)-faceted sawtooth hollow Si (001) substrate, and it significantly reduced the anti-phase boundary density at the III-V/Si interface due to the higher bonding energy of AlAs [39; 46]. The 600-nm thick GaAs buffer layers were then grown using a two-step method to flatten the sawtooth structure-induced geometric unevenness. An APD-free surface can be obtained then by migration-enhanced epitaxy (MEE) of the GaAs nucleation layer and two-step GaAs growth of 40 nm and 560 nm at 380 \({}^{\circ}\)C and 580 \({}^{\circ}\)C, respectively. In\({}_{0.15}\)Ga\({}_{0.85}\)As/InAs quantum wells as dislocation filter layers (DFLs) were grown on a flattened surface with 200-nm GaAs spacing layers in-between to effectively reduce the dislocation density, followed by active QD layer growth. The separation GaAs layer was grown at 480 \({}^{\circ}\)C, and the DFLs consisted of five periods of 10 nm In\({}_{0.15}\)Ga\({}_{0.85}\)As/10 nm GaAs. To further reduce the threading dislocation density (TDD), an additional two periods of In\({}_{0.15}\)Al\({}_{0.85}\)As/GaAs DFLs were utilized, which consisted of five-periods of 10 nm In\({}_{0.15}\)Al\({}_{0.85}\)As/10 nm GaAs strained layers separated by 200 nm GaAs. Finally, five periods of GaAs/AlGaAs superlattices (SLs) separated by a 50 nm GaAs spacing layer were deposited at 580 \({}^{\circ}\)C to achieve a smooth GaAs surface for the flattening process. Each SL has 10 periods of 2 nm GaAs/ 2 nm Al\({}_{0.6}\)Ga\({}_{0.4}\)As alternating structures, followed by a 200 nm GaAs grown at 560 \({}^{\circ}\)C [69]. For O-band light emission, a standard five-layer InAs/GaAs dot-in-a-well (DWELL) structure was grown on this GaAs/Si substrate. Each DWELL layer consisted of a 3.1 monolayer InAs QD layer sandwiched by a 2-nm In0.14 Ga0.86 As wetting layer and a 6-nm In0.14 AlO86 As capping layer. The DWELLs were separated by a 50-nm GaAs spacing layer, which was grown at 560 \({}^{\circ}\)C. For the C/L-band emission of InAs QDs, step-graded metamorphic InGaAs buffer layers were introduced as previously presented [68]. Thermal cycle annealing was utilized to further improve the flatness and quality of InGaAs layers. The active region consists of three periods of InAs QD layers grown at 450 \({}^{\circ}\)C. Each of the InAs QDs layers consists of 3.1 monolayer InAs capped by a 4-nm In\({}_{0.3}\)Ga\({}_{0.7}\)As layer and separated by a 45-nm In\({}_{0.3}\)Ga\({}_{0.7}\)As spacing layer grown at 500 \({}^{\circ}\)C. Further details of InAs/GaAs QD growth can be found in References [68; 69]. Cross-sectional TEM measurements were carried out to characterize the crystalline quality of the active regions. Figure 3a,b shows defection-free and high-quality DWELL structures for both samples, with insets illustrating the high resolution TEM images of individual InAs QDs. As presented above, a step-graded InGaAs metamorphic buffer was inserted to obtain InAs/InGaAs QDs with an emission wavelength extended to 1550 nm on a GaAs/Si (001) substrate. In addition, as shown in the insets, the size of QD increases for the C/L-band active region. Figure 3: (**a**) Cross-sectional TEM image of a five-layer InAs QDs grown on the sawtooth hollow structured Si (001) substrate for O-band emission. Inset: High resolution TEM image of an InAs/GaAs QD. (**b**) Cross-sectional TEM image of three-layer InAs QDs on the InGaAs/GaAs/Si (001) substrate for C/L-band emission.