<p></p><p>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 cm<sup>2</sup>/Vs was recently
observed when an 8N Ga (total nominal impurity
concentration of ~10 ppb) source
was used compared to 25
million cm<sup>2</sup>/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(O<sub>2</sub>),
the equilibrium P(GeO)-P(O<sub>2</sub>) vapor species diagram was calculated
from thermodynamics. The analysis shows
that even very dilute Ge in Ga of ~ 1 ppm concentration can be <a>actively oxidized in the extremely low P(O<sub>2</sub>) of
MBE</a>. In order to prove active oxidation of Ge, molecular distillation of 7N
Ga was performed in <a>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(O<sub>2</sub>)
of 3 x 10<sup>-6</sup> torr and the total pressure of 10<sup>-5</sup> 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. </a></p><br><p></p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8044856 |
| Date | 14 May 2019 |
| Creators | Kyungjean Min (6635897) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/IMPURITY_CONTROL_AND_ANALYSIS_OF_ULTRA-PURE_GALLIUM_FOR_INCREASING_MOBILITY_IN_GALLIUM_ARSENIDE_GROWN_BY_MOLECULAR_BEAM_EPITAXY/8044856 |
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