940-nm VCSELs grown by molecular beam epitaxy on Ge(001)

Abstract

Vertical-cavity surface-emitting laser (VCSEL) structures emitting near 940 nm were grown by solid source molecular beam epitaxy (MBE) on Ge(001) substrates. The VCSEL MBE-growth was realized upon a virtual substrate composed of GaAs on Ge grown by melatorganic vapour phase epitaxy (MOVPE). In situ monitoring during MBE growth employed multispectral reflectometry and magnification-inferred curvature imaging for real-time growth analysis. Curvature measurements revealed progressive compressive stress, while optical reflectivity data confirmed uniform layer growth and accurate stopband formation. Fabricated devices with mesa diameters of 35-40 $μ$m, corresponding to oxide apertures of approximately 11-16 $μ$m, exhibited room-temperature lasing under continuous-wave bias with threshold currents below 3 mA. To the best of our knowledge, this is the first demonstration of monolithically integrated 940 nm VCSELs grown on Ge substrates by MBE. These results confirm the viability of MBE-grown VCSELs on Ge with in situ process control for scalable optoelectronic integration.

Figures (5)

• Figure 1: In-situ wafer curvature using the MIC method during the complete MBE growth of the VCSEL structure on GaAs substrate (a) and on germanium (b). Corresponding lattice mismatch calculated with the Stoney law for each individual VCSEL layers grown on GaAs (c) and Ge (d). The datapoints color corresponds to its Al concentration in AlGaAs alloy.
• Figure 2: (a) In situ wafer reflectometry during the complete epitaxial growth of a VCSEL structure by MBE. (b) real-time detected characteristic VCSEL wavelengths during growth: DBR sidelobes edges (black crosses), DBR center wavelength (blue), Fabry-perot cavity dip (red). The alternating layers with high index contrast (GaAs/AlGaAs) are illustrated in grey and white shading.
• Figure 3: Atomic force microscopy (AFM) images of VCSEL epitaxial samples surface grown on Ge, with scanning areas of (a) $2.0 \times 2.0\,\mu m^2$ ($R_q=8.56\,nm$) and (b) $10 \times 10\,\mu m^2$ ($R_q=9.67\,nm$).
• Figure 4: Normal incidence reflectivity spectrum showing a stopband centered at 942.7 nm with a width of 93.6 nm. This confirms accurate optical thickness control in the DBR stack.
• Figure 5: LIV characteristic of a 35 µm-diameter VCSEL measured under continuous-wave electrical injection at 25 °C.