Quantum barriers with a polarization self-screening effect for GaN-based VCSELs to increase the electron-hole stimulated recombination and output performance

Hole injection is one of the fundamental limitations that affect the lasing power for GaN-based vertical-cavity surface-emitting lasers (VCSELs). In this report, a GaN-based VCSEL with a composition gradient quantum barrier (CGQB) structure is proposed and investigated. The designed In x Ga 1-x N quantum barrier has a linear gradient level of InN composition along the [0001] orientation, which is effective in reducing the energy band barrier height for holes. Furthermore, the polarization-induced bulk charges that are generated in the proposed quantum barriers can reduce the electric field magnitude in quantum wells, which is known as the polarization self-screening effect. Therefore, the hole injection and the electron-hole stimulated recombination rate can be both enhanced. We also find that although the hole injection can be enhanced and the polarization induced electric field in the quantum wells can be reduced, an increased gradient level of the InN composition for the polarization self-screened quantum barriers is not always favored. The reduced quantum barrier height will redistribute the energy subbands and make peak gain not coupled with the cavity resonance wavelength, which will decrease the lasing power. Hence, to avoid the substantial variation of the subbands, we suggest that the polarization self-screened active region shall possess properly thick quantum wells for maximizing the lasing power. Moreover, the optimized active region design can increase the 3dB frequency.