# 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
### C/L-Band InAs Quantum Dots on Ge Substrate  
Recently, we achieved monolithic growth of III-V on Ge miscut substrate by only using a 250-nm-thick GaAs buffer layer through the formation of a double atomic layer which could reduce the TDs [68]. In addition, as previously stated, most of the work so far has referred to the 1.3 \(\upmu\)m wavelength at the O-band telecom window. Few work regarding the emission at the C/L-band telecom window has been reported to date [50; 64]. Long wavelength III-V light sources on Si substrates at 1550 nm or C/L-band have become strongly demanded, since most of the Si-based photonic passive and active devices are based on applications at the C-band window. Furthermore, QD lasers emitting at 1550 nm benefit from the low loss transmission in mid/long-haul communications. To extend the emission wavelength from O-band to C/L-band for incoming silicon photonic integration, the step-graded epitaxial growth method was developed. The InAs/In\({}_{0.25}\)Ga\({}_{0.75}\)As QD structure was grown by solid-source MBE on Ge (001) substrate with 2\({}^{\circ}\) offcut toward [110] direction. The formation of a double atomic layer at the Ge surface is essential for the subsequent III-V material growth, because the APDs would form while growing polar III-V materials on a non-polar germanium substrate. The cross-sectional scanning transmission electron microscopy (STEM) of the complete epitaxial III-V quantum dot structure is shown in Figure 1a. The growth direction of the structure is shown as a white arrow in Figure 1a. Firstly, before epitaxial growth, the Ge substrate is de-oxidized at 450 \({}^{\circ}\)C for 15 min. Then an ultra-thin 60 nm layer of Ge buffer is deposited on the Ge substrate followed by an in-situ annealing to create the double atomic layer of Ge for the subsequent III-V material growth. APD free surface can be obtained then by migration-enhanced epitaxy (MEE) of the GaAs nucleation layer and two-step GaAs growth of 20 nm and 230 nm at 450 \({}^{\circ}\)C and 560 \({}^{\circ}\)C, respectively. Therefore, for photonics integration, only an ultra-thin GaAs buffer layer of 250 nm thick which is APD free was deposited, in comparison with the work stated previously, in which at least a 1-\(\upmu\)m-thick buffer layer for III-V QD structure growth is required [68]. Furthermore, as reported in our previous work [68; 69], with the step-graded epitaxial growth method, the InGaAs metamorphic buffer layer with the thickness of 700 nm is grown on top of the GaAs buffer layer. To suppress the propagation of misfit dislocations, the low growth temperature of 380 \({}^{\circ}\)C was used for the InGaAs metamorphic layer. Here, the InGaAs metamorphic buffer consists of two layers: a 200 nm step-graded InGaAs layer from In\({}_{0.09}\)Ga\({}_{0.91}\)As to In\({}_{0.13}\)Ga\({}_{0.87}\)As, followed by a 200 nm In\({}_{0.13}\)Ga\({}_{0.87}\)As and a 200 nm step-graded InGaAs from In\({}_{0.13}\)Ga\({}_{0.87}\)As to In\({}_{0.25}\)Ga\({}_{0.75}\)As,followed by a 100 nm In\({}_{0.25}\)Ga\({}_{0.75}\)As layer at 380 \({}^{\circ}\)C and 500 \({}^{\circ}\)C, respectively. Then, three periods of InAs QD layer grown on the In\({}_{0.25}\)Ga\({}_{0.75}\)As buffer layer form the active region. Each InAs QD layer consists of a 2.8 monolayer of InAs. They are capped by a 4 nm In\({}_{0.25}\)Ga\({}_{0.75}\)As spacer layer grown at an optimum temperature of 500 \({}^{\circ}\)C. Lastly, the surface InAs QDs are deposited with the same growth condition as the buried InAs QD layer for subsequent AFM characterization. More details of heterostructure growth can be found in [PERSON] et al. [68]. As shown in Figure 1, the epitaxial structure is characterized using scanning transmission electron microscopy (STEM) on a focused ion beam (FIB) fabrication cross-sectional lamella. It is clear in Figure 1a that due to the usage of a 250-nm-thick APD-free ultra-thin GaAs buffer layer, there is no apparent defect propagation from the GaAs/Ge interface and InGaAs metamorphic buffer to the active layer.