Anomaly in the Ga-Si phase diagram : nonretrograde solubility of Ga in Si layers grown by liquid-phase epitaxy

Silicon layers have been grown on Si substrates from Ga-rich solutions by liquid-phase epitaxy at temperatures between 615 and 1220 K. The diffusion coefficient of Si in liquid Ga is estimated from layer thickness data to be DGaSi=0.0235 exp(−4016 K/T) cm2/s. The incorporation of Ga in the Si layers was investigated by means of neutron-irradiation, double-crystal x-ray rocking curve, Auger recombination, infrared reflection, Rutherford backscattering, and Hall effect experiments. Below 850 K the normal retrograde solid solubility of Ga in Si changes to an anomalous nonretrograde solubility. The doping level increases from p=5×1018 cm−3 to 5×1020 cm−3 at 615 K. The concentrations of net carriers, ionized Ga impurities, and total Ga atoms are identical over the entire temperature range, showing that Ga atoms are incorporated on lattice sites. Imperfections such as dislocations, stacking faults, or precipitates are not responsible for such high levels. The occurrence of Si vacancies or microsegregation due to lateral growth are possible driving forces causing this anomalous behavior. Excess free-energy contributions resulting from internal stress produced by lattice mismatch may also provide a mechanism for producing this effect.