Novel chlorine-based chemistry and implementation hardware for the growth of lithium niobate and related complex metal oxides
Carver, Alexander Gilman
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Oxide related research has increased as standard oxides reach their operational limits and new classes of devices are imagined that can only be realized through the use of man-made compounds. Many of these devices require high quality films in order to reach their highest potential. Molecular beam epitaxy (MBE) is poised to become a key producer of high quality oxides. One of the most promising oxides is lithium niobate, LiNbO3, which can potentially deliver novel electronic, optic, and hybrid devices not currently possible. Growing lithium niobate using MBE is difficult. Several concepts are presented that will make this task easier. First, high temperature refractory metals can be delivered to the substrate through a novel use of low temperature chloride compounds such as niobium (V) chloride. This chloride chemistry allows low temperature sources to deliver high temperature materials to the substrate. Second, a precision, vapor-phase source and control system is prototyped for these chloride compounds achieving improved flux accuracy and expanding the capability of standard MBEs to support many sources. Chloride sources have high vapor pressures and are sensitive to temperature changes causing flux drift. The vapor-phase source removes the temperature sensitivity and eliminates thermal drifts. Third, a novel method of measuring flux with spontaneous ionzation current has been developed. This design utilizes a low noise design to measure femtoamp currents generated as an evaporant spontaneously ionizes. The measured current with additional predicted data has the potential for directly counting the atoms evaporated and controlling evaporation from a source. The design is sensitive enough to detect outgassing of the cell and cell "spitting" or other non-idealities. Monitoring these non-idealities can help improve other processes by ensuring the cell is fully outgassed and stable. Finally, a miniaturized RF induction cell prototype is shown that can eliminate the need for incandescent filaments in an oxide based MBE. The RF cell has the potential to increase reliability of MBEs for oxide work and achieve higher operating temperatures without the need for densely wound incandescent filaments or electron beam sources.