## 4 Results and Discussion  
In order to predict the electrical and optical properties of semiconductor lasers with various structures, an advanced simulation should be conducted in the device design. However, it always failed to simulate the InGaAsSb/AlGaAsSb LDs, due to the lack of an effective material database of the antimonide material system. Based on the basic parameters of related binary compounds and Vegard's Law, we set up the material database for InGaAsSb/AlGaAsSb LDs and simulate the devices with and without CBL by Pics3d software package. The key material parameters are shown in Table 1.  
The simulated band diagrams and distributions of the carrier concentrations in the InGaAsSb/AlGaAsSb LDs without and with CBL are shown in Figure 3a,b, respectively. It can be seen that electrons and holes meet in the quantum wells and recombine completely. However, a large portion of the electrons from the n-side into the MQW leak into the p-cladding layer. This electron leakage strongly depends on the band offset between the waveguide and the cladding layers. According to Figure 3, the effective barrier height for minority electrons is increased from 0.448 eV to 1.022 eV with the insert of CBL into the InGaAsSb/AlGaAsSb LDs, which results in a remarkable reduction in the concentration of electrons injected into the p-cladding layer. The concentration of electrons leakage is also down from 10\({}^{7}\) cm\({}^{-3}\) to near 0. Meanwhile, the CBL also reduces the barrier height for holes in the p-side from 0.291 eV to 0.112 eV, which can enhance hole injection efficiency. As for the  
Figure 2: Schematic diagram of InGaAsSb/AlGaAsSb 3 QW LD with CBL. N-side, the \(\bigtriangleup\)Ev for minority holes is slightly increased from 0.953 eV to 1.024 eV, which leads to the concentration of holes leakage being reduced from 10\({}^{2}\) cm\({}^{-3}\) to 10\({}^{1.5}\) cm\({}^{-3}\). Figure 4 plots the simulated radiative recombination rates in the active region of the two various structures. It is obviously seen that the radiative recombination rates in the last quantum well are the highest ones. This can be attributed to the highest carrier concentration accumulated in the last QWs next to the \(p\)-type layer. The higher radiative recombination rates in the active region are also observed for the devices with CBL, which proved that the CBL can result in a reduction of the carrier leakage and enhancement of the carriers injection in the active region. The simulated P-I (optical power-current) characteristics for the 0.8-mm-long devices with and without CBL having the stripe width of 90 \(\upmu\)m resonator length are illustrated in Figure 5. It can be found that the threshold current density is reduced from 197 A/cm\({}^{2}\) to 126 A/cm\({}^{2}\) and the differential efficiency (\(\eta_{d}\)) is increased from 0.154 W/A to 0.193 W/A due to the optimization of the epitaxial structure design. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **GaAs** & **InAs** & **AIAs** & **GaSb** & **AISb** & **InSb** \\ \hline a0 (Å) & 5.6523 & 6.0584 & 5.6605 & 6.0959 & 6.136 & 6.479 \\ dielectric-constant & 13.1 & 15.15 & 10.1 & 14.8 & 10.3 & 16.8 \\ electron-mass & 0.067 & 0.026 & 0.15 & 0.039 & 0.14 & 0.0135 \\ affinity & 4.07 & 4.07 & 3.5 & 4.06 & 3.65 & 4.59 \\ c11 & 11.9 & 8.33 & 12.02 & 8.842 & 8.769 & 6.487 \\ c12 & 5.38 & 4.53 & 5.7 & 4.026 & 4.341 & 3.735 \\ Eg (eV) & 1.424 & 0.3595 & 3 & 0.725 & 2.3 & 0.1737 \\ real-index & 3.65 & 3.892 & 3.006 & 3.9731 & 3.29 & 4 \\ electron-mobility & 0.85 & 1 & 0.02 & 0.22 & 0.02 & 5 \\ hole-mobility & 0.004 & 0.036 & 0.279 & 0.1 & 0.04 & 0.07 \\ thermal-kappa & 55 & 2.7 & & 32 & & 18 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Key parameters of related binary compounds in antimonide material database. Figure 3: band diagrams and carrier concentrations distributions in the various InGaAsSb/AlGaAsSb 3 QW LDs at 400 mA (**a**) Without CBL (**b**) With CBL. The grey shadow indicates the bandgap and the blue shadow indicates the location of the carrier blocking layer.