Current spreading and carrier diffusion in long-wavelength vertical-cavity surface-emitting lasers

Current spreading and carrier diffusion in vertical cavity surface-emitting lasers (VCSELs) critically affect the threshold current density, transverse gain profile, and optical modal characteristics of the devices. We have numerically analyzed these effects for four popular long-wavelength VCSEL structures with wafer-bonded GaAs-AlGaAs Bragg bottom mirrors. The results show that current confinement for p-mirror VCSELs is approximately twice as effective as the corresponding n-mirror VCSEL's although the confinement factor for all practical VCSEL structures studied here is considerably lower than the "ideal" structure. Calculations also show a strong dependence of current confinement on device aperture size and a tradeoff between the optical modal property and the current confinement efficiency for different apertured devices.