The distillation and growth experiments analyzed in this paper were detailed in reference . Ingots of 8N Ga totaling 100 g were loaded to the crucible, which was subjected to insitu distillation for further purification at 1360 K for 16 h. Then 138 heterostructure samples were grown at 1160 K for a total of 400 h. After distillation and growth were finished, it was observed that 25% (25 g) of the initial Ga amount was lost from the effusion cell.
Fig. 1 MBE Ga conical (near-cylindrical) crucible (L0 = 1.04 cm, d0 = 1.90 cm)
The Ga crucible is slightly conical, nearly cylindrical. The dimensions of the crucible and the estimated values of diameter and initial depth of liquid, L, when the crucible is filled with 100 g liquid Ga are illustrated in Fig. 1.
The evaporated Ga amount was calculated by Langmuir-Knudsen equation and corrected with the transmission probability of Clausing Theory.
Results and Discussion
The heterostructure growth was performed in ultra-high vacuum of about 1 x 10-10 Torr (=mmHg = 7.6 x 10-3 Pa). For such high vacuum, the rate of distillation is controlled by the rate of evaporation of atoms from the surface of the melt and equilibrium is not maintained between the evaporation from liquid phase and the condensation from vapor phase. This distillation process under the high vacuum condition is called molecular distillation. When there is no returning condensation from vapor phase to liquid phase, the evaporation rate is maximum. The maximum evaporation rate is given by Langmuir equation written in SI unit as,
Here P is the saturation vapor pressure in Pascal, T is absolute temperature in K, and M is molecular or atomic weight in g/mol.