## 2 Materials and Methods  
In-rich InGaAs/GaAsP QWs were grown on n-type GaAs (001) substrates misoriented by 0\({}^{\circ}\), 2\({}^{\circ}\), and 15\({}^{\circ}\) toward (111) using a MOCVD AIXTRON 200/4 reactor under identical growth conditions. The following reactants were used: trimethylindium (TMIn), trimethyl-gallium (TMGa), arsine (AsH\({}_{3}\)), and phosphine (PH\({}_{3}\)). Purified H\({}_{2}\) was used as a carrier gas. The growth pressure and temperature of the MOCVD system were set to 100 mbar and 630 \({}^{\circ}\)C, respectively, a growth rate of 0.5 nm/s during the growth and V/III ratios of 80 were employed. The structure of an InGaAs/GaAsP QW is depicted in Figure 1. For all three samples, the substrate surface was deoxidized via thermal desorption at 680 \({}^{\circ}\)C for 5 min under an AsH\({}_{3}\) atmosphere. A 300 nm GaAs buffer layer was subsequently grown on the 2-inch oriented GaAs substrate using TMGa and AsH\({}_{3}\) with a growth time of \(\sim\)600 s. A 5 nm GaAsP barrier was subsequently grown, followed by the growth of a 7 nm InGaAs QW. Finally, a 5 nm GaAsP barrier with a PH\({}_{3}\):AsH\({}_{3}\) ratio of 3.5:1 in the vapor phase was employed as the capping layer.  
To achieve lasing at near-infrared wavelength in this study, the In and phosphorous compositions were fixed at 38% and 20% in the InGaAs QWs and GaAsP barriers, respectively; the thicknesses of the QWs and barriers were estimated to be 7 and 5 nm, respectively. Lattice constants are known to change with composition, and the effects of the composition of In on lattice constants can be expressed as follows:  
\[a_{\text{InGaAs}}=a_{\text{GaAs}}+(a_{\text{InAs}}-a_{\text{GaAs}})\cdot x. \tag{1}\]  
The strain in the layers can be defined as follows:  
\[\varepsilon=\frac{\Delta a}{a_{sub}} \tag{2}\]  
where \(\Delta a\) is the difference between the lattice constants of the two materials and \(a_{sub}\) is the lattice constant of the substrate. The lattice constant of In\({}_{0.38}\)GaAs is obtained as 5.807 A using Equation (1) and the compressive strain of the In\({}_{0.38}\)Ga\({}_{0.62}\)As layer is evaluated to be 27,224 ppm (Equation (2)). Tensile strain GaAsP barrier layers were employed to compensate for the compressive strain in the InGaAs QW layer, and then the defect density was reduced. The average strain is defined as [7]  
\[(T_{\text{InGaAs}}\times\varepsilon_{\text{InGaAs}}+T_{\text{GaAs}P}\times \varepsilon_{\text{GaAs}P})/(T_{\text{InGaAs}}+T_{\text{GaAs}P}). \tag{3}\]  
where \(T\) and \(\varepsilon\) represent the thickness and strain of each layer, respectively. \(\varepsilon\) is positive if the layer has compressive strain and is negative for tensile strain. The accumulated strain in the InGaAs/GaAsP QW was 7006 ppm, which indicated that it was not fully compensated and residual strain was likely present in the material.  
To investigate the crystal quality, and strain mismatch of the epitaxial layers, the samples were identified by X-ray diffraction (XRD, Bruker D8). A Raman spectrometer (RENISHAW InVia) was employed to study the structural disorder and residual strain in the epitaxial layers of the three samples. Atomic force microscopy (AFM, Bruker MultiMode 8) was used to conduct morphological analysis and determine the RMS surface roughness. The optical characteristics of the InGaAs/GaAsP QW were investigated using temperature-dependent photoluminescence spectroscopy (ARS 8200 cryogenic system) with a 488 nm continuous-wave laser.  
Figure 1: Schematic of the InGaAs/GaAsP quantum well.