Ion implanted Back-gates developed for high-mobility Two-dimensional Electron systems

Abstract

Since discovering the quantum Hall effect, two-dimensional electron systems (2DEGs) in (perpendicular) magnetic fields are a field of intense research. With the continuous improvement of sample quality, multiple new exotic quantum phases have been discovered, some of which have potentially topological protected states with quantum computing applications. Almost all of the correlated phases are stabilized by electron-electron interaction, which can be controlled using sophisticated gating approaches. While the application of metallic top-gates is straightforward, back-gates are a technological challenge. The presented new development of planar structured back-gates based on ion implantation overcomes traditional designs' common problems, allowing the combination of multiple back-gates with very high-quality 2DEGs. Two techniques to fabricate patterned back-gates are investigated. One applies either oxygen or gallium ion implantation to passivate doped regions. Another approach utilizes the implantation of donors, tested with silicon, selenium, and tellurium, to form conducting areas in semi-insulating gallium arsenide. While both techniques allow the fabrication of conducting, patterned back-gates, passivation using oxygen implantation results in both the most reliable and least detrimental method, crucial for obtaining high mobility 2DEGs. The high-quality MBE-growth on patterned back-gated substrates has been demonstrated on a single 2DEG, equipped with ion-implanted back- and metallic top-gate. By tuning the gates to exactly center the wave-function of the 2DEG in the quantum well, a peak mobility of \SI{40E6}{\centi\meter\squared\per\volt\per\second} could be achieved. Additionally, the new gate design has overcome typical problems in magneto-transport measurements of gate-enriched 2DEGs. Bilayer systems, consisting of two closely-spaced 2DEGs, have shown an even richer phase diagram due to the additional degree of freedom. Preparing a bilayer system with both top- and back-gates allows to control their interaction energy and set individual 2DEG densities. Bilayer systems, with and without separated contacts to the individual layers, are tuned across a large parameter space. While in the investigated parameter range no novel quantum states were found, we believe the planar back-gate design holds great promise to produce controllable bilayers suitable to investigate the exotic (potentially non-Abelian) properties of correlated states.