THERMODYNAMICS AND KINETICS OF SOLIDIFICATION OF SI-AS SOLUTIONS

In this paper we present results on the solidification behaviour of si licon-arsenic alloys in a wide range of velocity from 0.1 to 6 m/s obt ained using short laser pulses and different substrate temperatures. I ntermediate 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 s hown a monotonic variation of the partition coefficient in both direct ions. The results are consistent with an interface broken in random le ngth (111) steps. When increasing the liquid-solid interface velocity in the range 2-6 m/s no accumulation of impurity at the surface was de tected; it is then possible to obtain arsenic concentrations much high er than the maximum solid solubility. Though, a limit to supersaturati on 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 ori entations was found. The results are consistent with a reduction of th e difference in melting temperature between the amorphous and the crys talline phases. Extrapolating the results to zero undercooling we can estimate an arsenic concentration of about 15 at% for the difference b etween the free energy of the crystal to that of the amorphous to be z ero; this concentration then represents the upper thermodynamical limi t for supersaturation. The implications on the so-called T-0 curve are discussed.