Thermal modelling of transfer-bonded thin-film gallium arsenide laser diode

Two-dimensional temperature distributions of thin-film edge-emitting gallium arsenide (GaAs) 3 μm wide ridge lasers transfer-bonded to substrates with different thermal conductivities, k, were simulated in COMSOL. The thermal resistance, Rth, is compared with a simplified steady-state analytic expression. The effects of laser cavity length, thickness of dielectric passivation layer, contact metal layer thickness and submount material are investigated in order to reduce the thermal resistance of the laser when referenced to lasers with the native GaAs substrate. The simulations show the importance in reducing the GaAs substrate thickness especially for short cavity lengths. The Rth of a 200 μm long laser with the substrate fully removed is 37.5 K/W, compared with 230 K/W for the laser with a 100 μm thick GaAs substrate. Increased p-contact metal thickness and reduced dielectric layer thickness further reduce Rth. If alumina (k = 1.35 W/mK) is used as a submount, its thickness above a perfect heat sink should be minimised to decrease the junction temperature. A 10 μm thick silicon submount (k = 150 W/mK) above a perfect heat sink provides an acceptably low Rth.