Improved mode stability of long-wavelength GaInAsN vertical-cavity surface-emitting lasers by using the selective Ge-Coating layer

In this article, we demonstrate the fabrication and characteristics of the GaInAsN vertical-cavity surface-emitting lasers (VCSELs) with a stable and fundamental transverse mode by using the Ge antireflection (AR) coating, selective wet oxidation, and thick silicon oxide planarization processing techniques. The Ge-semiconductor interface provides a lower reflectivity, as compared with the semiconductor-air interface, and thus can be used as an AR coating to purify the lasing spectra for the oxide-confined VCSELs. The VCSELs with a 13 μm oxide aperture along with a 7 μm diameter Ge coating exhibit a maximum light output power of 0.63 mW and a single transverse mode with a side-mode suppression ratio over 30 dB at an injection current up to 15 mA. In addition, the SiO_x-planarized GaInAsN VCSELs show a clear and symmetric eye diagram operated at 1.25 Gb/s at 11.6 mA. These results show that such single-mode VCSELs are excellent candidates for use in high speed long-distance data communications.