Steady state model for facet heating leading to thermal runaway in semiconductor lasers

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Abstract
A steady state model is presented which provides new insight into the thermal runaway process that leads to catastrophic damage of semiconductor lasers. We show that thermal runaway is preceded by a situation where two self consistent thermal steady state solutions exist at low output power, one stable and one unstable. When the output power is increased, the two solutions degenerate and disappear which means that the laser will enter thermal runaway. The steady state model consists of two parts: a three dimensional thermal model and a one dimensional model for the carrier diffusion towards the facet. The temperature dependence of both the heat sources and the thermal conductivity play the crucial role. Also ordinary bulk heating is shown to be an important factor. Both 0.88 μm GaAs lasers and 1.5 μm InGaAsP lasers are discussed and minimum values of surface recombination and output power needed for thermal runaway are given. Thermal runaway in GaAs lasers can be explained by the model for realistic values of surface recombination. However, the calculated values of needed output power are significantly higher than what is experienced in reality. Possible explanations for this discrepancy are given.