Femtosecond melting and ablation of semiconductors studied with time of flight mass spectroscopy

Using time-of-flight mass spectroscopy, we have investigated melting and ablation of gallium arsenide and silicon irradiated by femtosecond pulses. Below the ablation threshold the maximum surface temperature is obtained from the collisionless time-of-flight distributions of evaporated or sublimated particles. At the melting threshold, we estimate a temperature for the silicon surface which is approximately 500 K higher than the equilibrium melting temperature. In the fluence regime where melting is known to be a nonthermal process, we measure maximum surface temperatures in excess of 2500 K for both silicon and gallium arsenide, indicating rapid thermalization after nonthermal melting. At the ablation threshold, we estimated for both materials surface temperatures between 3000 and 4000 K. We observed a clear threshold-like effect in the number of detected particles, indicating the occurrence of a bulk effect. The flow parameters above the ablation threshold are discussed and compared to the different models of collisional expansion. For F (abl)<F<2F (abl), transition from the liquid state to the gas phase occurs through the two-phase regime. For F> 2F (abl), we find evidence that expansion takes place at temperatures that are higher than the critical temperature. Plasma formation appears only at fluences above 1 J/cm(2) (F> 5F (abl)). (C) 1999 American Institute of Physics. [S0021-8979(99)05506-1].