Multistage bipolar cascade vertical-cavity surface-emitting lasers: theory and experiment

We present an overview over our research on bipolar cascade vertical-cavity surface-emitting lasers (VCSELs) emitting at 980 nm wavelength, including the scaling properties and the influence of design variations on laser performance as well as strategies for GaAs Esaki junction optimization. We experimentally demonstrate high-performance two- and three-stage devices, the latter of which with 130% differential quantum efficiency. The derived analytical expressions for the scaling behavior are confirmed by measurement data and show a significant improvement in the steady-state as well as the dynamic performance with respect to active region stacking. From the investigated design variations, the influence of oxide apertures and active region spacing on laser performance is clearly extracted and reveals that current spreading is present in the cavity and can lead to severe limitations in optical performance. The GaAs tunnel diodes have been optimized with respect to the donor concentration and the additional incorporation of an In_0.1Ga_0.9As layer on the n⁺⁺-side, leading to a zero-bias specific resistance of about 1·10^-4 Ωcm².