Ambipolar High Mobility Hexagonal Transistors on Hydrogen-Terminated Silicon (111) Surfaces

TL;DR

This paper reports the fabrication and characterization of ambipolar high-mobility hexagonal transistors on hydrogen-terminated Si(111) surfaces, demonstrating tunable electron and hole systems, high mobilities, and quantum Hall effects with symmetry analysis.

Contribution

The work introduces ambipolar transistors on Si(111) with high mobility and explores their symmetry and quantum Hall phenomena, a novel approach for silicon-based 2D systems.

Findings

Achieved electron and hole densities up to ~7.8×10^{11} cm^{-2}

Observed high electron mobility of 1.76×10^5 cm^2/Vs at 300 mK

Detected fractional quantum Hall states at high magnetic fields

Abstract

We have fabricated ambipolar transistors on chemically prepared hydrogen-terminated Si(111) surfaces, in which a two-dimensional electron system (2DES) or a two-dimensional hole system (2DHS) can be populated in the same conduction channel by changing the gate voltage of a global gate applied through a vacuum gap. Depending on the gate bias, ion implanted n$^+$ and p$^+$ regions function either as Ohmic contacts or as in-plane gates, which laterally confine the carriers induced by the global gate. On one device, electron and hole densities of up to $7.8\times10^{11}$ cm$^{-2}$ and $7.6\times10^{11}$ cm$^{-2}$ respectively are obtained. The peak electron mobility is $1.76\times10^5$ cm$^{2}$/Vs, and the peak hole mobility is $9.1\times10^3$ cm$^{2}$/Vs at 300 mK; the ratio of about 20 is mainly due to the very different valley degeneracies (6:1) of electrons and holes on the Si(111) surface. On another device, the peak electron mobility of $2.2\times10^5$ cm$^{2}$/Vs is reached at 300 mK. These devices are hexagonal in order to investigate the underlying symmetry of the 2DESs, which have a sixfold valley degeneracy at zero magnetic field. Three magnetoresistance measurements with threefold rotational symmetry are used to determine the symmetry of the 2DESs at different magnetic field. At filling factor $1 < \nu < 2$, the observed anisotropy can be explained by a single valley pair occupancy of composite fermions (CFs). Qualitatively the CFs preserve the valley anisotropy, in addition to the twofold valley degeneracy. At magnetic field up to 35 T, the 2/3 fractional quantum Hall state is observed with a well developed hall plateau; at $\nu<2/3$, the three magnetoresistances show a large anisotropy (50:1). We also show that device degradation is not a serious issue for our measurements, if the device is kept in vacuum or a nitrogen gas environment and its time in air is minimized.