VCSEL arrays with low threshold currents and stable linear polarization

8 × 8 Arrays of vertical-cavity surface-emitting lasers (VCSELs) with lateral current injection via the doped GaAs cladding layers have been fabricated. To obtain low series resistances ohmic p-contacts were made to a p-doped GaAs cladding layer above the active MQW-region of the double mesa VCSEL-diodes. Lateral current confinement is made by introducing a 50 nm thick n-type GaAs current blocking layer into the p-doped cladding. The laser diodes are structured by selective removal of this layer in the cavity and subsequent growth of the top Bragg mirror in a second MBE sequence. The epitaxial overgrowth resulted in layers with unchanged structural and optical quality, when compared with layers grown in a single epitaxial run on planar growth surfaces. The overgrowth of the 50 nm steps leads to about 1 μm wide facets in the [011̄] direction. These VCSELs have threshold currents of 470 μA and a maximum output power of 5 mW for diodes with blocking layer diameters of 6 μm and 14 μm, respectively. The external differential quantum and power conversion efficiencies amount to 46 % and 16 %, respectively. Devices with 6 μm blocking layer diameter operate in single mode emission at all current levels, and show a stable linear polarization along the [011] crystal axis, due to the facetting mentioned above. The on-off ratio between the orthogonal directions is better than 20 dB. Internal losses and quantum efficiencies have been determined varying the device diameter and number of top DBR pairs. Device characteristics are very homogenous not only within one array but also over almost the whole two-inch wafer, e.g. threshold currents differ by less than 10 %.