Low-Loss Tunnel Junction Integration in Mid-Infrared VCSELs

Mid-infrared (Mid-IR) vertical cavity surface-emitting lasers (VCSELs) are sought after for spectroscopy and gas sensing in the 3–5 μm range due to their compact design, single-mode operation, and potential for energy-efficient performance. State-of-the-art mid-infrared (mid-IR) VCSELs demonstrated so far fall into two categories: those using multi-stage interband cascade laser (ICL) active regions with current confinement via mesa etching [1], and those employing buried tunnel junctions (BTJ) with multi-quantum well active regions [2]. While ICL-based designs benefit from lower threshold current density through active region scaling, they suffer from significant current spreading across the mesa. On the other hand, BTJ-based VCSELs rely on a structured interface between highly doped GaSb and In.Aso.e.Sbo.,» materials to confine current and guide optical modes within the mesa centre, cf. Fig. 1(a) [2]. However, due to high optical absorption produced in the tunnel junction, this method requires precise placement of the tunnel junction at the node of the optical field in the laser cavity. Since there is uncertainty in the laser active region and mirror layers thickness during molecular beam epitaxy (MBE) growth high BTJ loss limits processing yield. In this work, we demonstrate mid-IR BTJ VCSELs with a tunnel junction based on a semi-metallic interface between GaSb and an InAs/AISb superlattice, shown in Fig. 1(b). This design, similar to the tunnel junction used in ICLs [1], it significantly reduces optical losses at mid-IR wavelengths and does not require precise placement at the mode node for VCSEL operation. The new buried tunnel junctions are patterned by dry etching mesas of varying radii in the GaSb-InAs/AISb tunnel junction layer. A methane-oxygen etching recipe at 60°C was developed to precisely etch the tunnel junction layer. After etching, the sample undergoes chemical cleaning followed by high temperature deoxidation (550°C for 10 minutes, using a thermocouple) in MBE. A GaSb spreading layer and InAsSb contact layer are then overgrown to complete the VCSEL cavity. The schematic of the fabricated BTJ VCSELs is shown in Fig. 1(a). It is similar to the traditionally used BTJ VCSEL structure [2], except for the BTJ configuration.