Thermal impedance measurements of junction-down mounted single-side contact laser diodes

We designed and fabricated a 10 Gb/s edge-emitter DFB InP laser diode emitting at 1310 nm, and having the anode (p) and cathode (n) contacts fabricated on the same surface as the laser's ridge. This was made possible by epitaxially incorporating a buried contact layer under the laser ridge. The buried contact layer was designed to provide a similar series resistance as is typically obtained with a backside substrate contact. High-speed small-signal and large-signal measurements of the laser diode showed good 10 Gb/s operation. The thermal impedance of this single-side contact edge-emitting laser diode (SSC-LD) was measured using different mounting schemes. Primarily, a junction-down mounting was compared with a junction-up mounting and was found to have lower thermal impedance when the laser ridge was attached to the silicon optical bench (SiOB). The layout of the pads to which the laser diode bonded was also found to impact the thermal impedance, with the ridge contact being the major contributor to lowering the thermal impedance. Three variants of the laser were built with the active region spaced at different distances from the top of the ridge. We show that in the case of junction-down mounting the proximity of the laser's active region to the heat sink strongly affects the measured thermal impedance. 3D thermal simulations agreed well with the measured data and were used to estimate the variations in the thermal impedance due to laser diode bonding misalignment and amount of solder compression.