Thermodynamics and kinetics of solidification of Si-As solutions

In this paper we present results on the solidification behaviour of silicon-arsenic alloys in a wide range of velocity from 0.1 to 6 m/s obtained using short laser pulses and different substrate temperatures. Intermediate partitioning of arsenic between the equilibrium value and complete trapping has been measured. Experiments on substrates cut at different inclination angles from (111) towards (110) and (001) have shown a monotonic variation of the partition coefficient in both directions. The results are consistent with an interface broken in random length (111) steps. When increasing the liquid-solid interface velocity in the range 2–6 m/s no accumulation of impurity at the surface was detected; it is then possible to obtain arsenic concentrations much higher than the maximum solid solubility. Though, a limit to supersaturation has been found. Solidification experiments on supersaturated Si-As solid solutions in the range 1–3 at% on (100) and (111) silicon single crystals have shown that a critical velocity for the quenching of the amorphous phase exists which depends on arsenic concentration and on substrate orientation. Once the liquid undercooling is calculated from the interface velocity a single curve for the different substrate orientations was found. The results are consistent with a reduction of the difference in melting temperature between the amorphous and the crystalline phases. Extrapolating the results to zero undercooling we can estimate an arsenic concentration of about 15 at% for the difference between the free energy of the crystal to that of the amorphous to be zero; this concentration then represents the upper thermodynamical limit for supersaturation. The implications on the so-called To curve are discussed.