IMPURITY CONTROL AND ANALYSIS OF ULTRA-PURE GALLIUM FOR INCREASING MOBILITY IN GALLIUM ARSENIDE GROWN BY MOLECULAR BEAM EPITAXY
High mobility 2DEG (two-dimensional electron gas) confined in GaAs is a good platform to understand correlated electron systems and a promising candidate for qubit devices. For example, the non-Abelian feature of Fractional Quantum Hall state enabling topological quantum computation is only found in GaAs with high mobility. Theoretical calculations have shown that the mobility is inversely proportional to impurities in GaAs/AlGaAs heterstructures grown by Molecular Beam Epitaxy (MBE). In recent MBE experiments, the source Ga was found to be more important in the limitation of mobility than Al and As. A high mobility of 35 million cm2/Vs was recently observed when an 8N Ga (total nominal impurity concentration of ~10 ppb) source was used compared to 25 million cm2/Vs for a 7N Ga source. In addition, significant mobility increase was observed after in-situ distillation of the source Ga before growth. In order to clarify the mechanism of how the distillation contributed to the Ga purification, thus resulting in the mobility increase, the MBE in-situ distillation was analyzed by molecular distillation theory. Evaporation behavior of solvent Ga was analyzed including effects of evaporation from a crucible with receding liquid depth. Then impurity removal through molecular distillation was analyzed with molecular evaporation kinetics. The remaining 7N and 8N Ga after in-situ MBE distillation and growth were elementally analyzed by ICP-MS (Inductively Coupled Plasma Mass Spectrometry) and compared with analyses of the starting 7N and 8N Ga from same lots. Due to the increased detection limit of ICP-MS in metal analysis, the concentrations of most impurity elements reached the detection limit of ~1-10 ppb. However, unusual high concentration of 690 ppb Ge was found in the 7N Ga, exceeding the nominal concentration of 7N (100 ppb). Significant decrease in Ge concentration was found in the comparison of initial ultra-pure Ga and remaining Ga for both grades of 7N and 8N. The significant Ge losses cannot be explained by atomic Ge evaporation due to the low vapor pressure of Ge. However, a hypothesis of Ge evaporation as GeO(g) by Ge active oxidation was proposed. In order to test the active oxidation of very dilute Ge in Ga in the MBE conditions with very low P(O2), the equilibrium P(GeO)-P(O2) vapor species diagram was calculated from thermodynamics. The analysis shows that even very dilute Ge in Ga of ~ 1 ppm concentration can be actively oxidized in the extremely low P(O2) of MBE. In order to prove active oxidation of Ge, molecular distillation of 7N Ga was performed in a specially constructed high vacuum chamber. The 7N Ga with unusual high Ge concentration of 440 ppb (by GDMS analysis) was distilled for 16 h at 1360 K under the starting P(O2) of 3 x 10-6 torr and the total pressure of 10-5 torr. The chamber vacuum was monitored by Residual Gas Analyzer (RGA) and the residual Ga after 16 h distillation was analyzed by GDMS. In the GDMS analysis, significant Ge loss was found from 440 ppb to below the detection limit of 10 ppb, confirming Ge active oxidation hypothesis. The oxygen-assisted impurity removal in distillation also may be applicable to other impurities with high vapor pressure gaseous oxide, but low vapor pressure itself, such as Al, Si and Sn.