Ultra-high mobility two-dimensional electron gas in a SiGe/Si/SiGe quantum well

Abstract

We report the observation of an electron gas in a SiGe/Si/SiGe quantum well with maximum mobility up to 240 m 2 /Vs, which is noticeably higher than previously reported results in silicon-based structures. Using SiO, rather than Al2O3, as an insulator, we obtain strongly reduced threshold voltages close to zero. In addition to the predominantly small-angle scattering well known in the high-mobility heterostructures, the observed linear temperature dependence of the conductivity reveals the presence of a short-range random potential.PACS numbers: 71.10. Hf, 71.27.+a, 71.10.Ay During last two decades, a big progress has been made in studying the behavior of strongly correlated electrons confined in two dimensional (2D) quantum wells. The metal-insulator transition, discovered initially in low-disorder silicon metal-oxide-semiconductor fieldeffect transistors (MOSFETs) and subsequently observed in many other strongly correlated two-dimensional systems [1], has attracted a great deal of attention and is still not completely understood [2]. It is generally agreed that for the zero-magnetic-field metallic state to exist in 2D, strong interactions between carriers are needed. Other effects of strong correlations include greatly enhanced spin susceptibility [3] and the effective mass [4]. The strength of interactions is usually characterized by the ratio of Coulomb (E c ) and Fermi (E F ) energies that is related to the dimensionless Wigner-Seitz radius r s :where n v is the valley degeneracy, m is the effective mass, ǫ is the dielectric constant, and n s is the electron density. The 2D electron system in silicon turned out to be a very convenient object for studies of the strongly correlated regime due to relatively high effective band mass (0.19 m e compared to 0.067 m e in n-type GaAs/AlGaAs heterostructures) and the existence of two almost degenerate valleys in the spectrum (n v = 2), which further enhances the correlation effects [5]. However, even in the highest quality Si MOSFETs, the maximum electron mobilities did not exceed 3 m 2 /Vs, and the effects of the * e-mail: melnikov@issp.ac.ru † e-mail: chee@cc.ee.ntu.edu.tw residual disorder mask the effects of interactions on the insulating side of the metal-insulator transition, where the formation of the Wigner crystal is expected. This calls for a new high-mobility silicon-based 2D system to be developed.In this Letter, we report the observation of the 2D electron gas in extremely low-disorder SiGe/Si/SiGe quantum wells with mobilities up to 240 m 2 /Vs, i.e., almost two orders of magnitude higher than those in the best silicon MOSFETs and noticeably higher than in previously fabricated SiGe heterostructures [6][7][8]. The main improvements to the mobility and threshold voltage, compared to previously fabricated SiGe/Si/SiGe quantum wells, come from using an ultrahigh-vacuum chemicalvapor-deposition (UHVCVD) instead of MBE and employing SiO, rather than Al 2 O 3 [9-11], as an insulator. Our samples are also characterized by an extremely high homogeneity of th...

Indication of band flattening at the Fermi level in a strongly correlated electron system

“…No mesa etching was used, and the 2D electron gas was created in a way similar to silicon MOSFETs (for details, see refs. 33 , 34 ). The maximum electron mobility in our samples reached 240 m 2 /Vs which is the highest mobility reported for this electron system 34 .…”

“…Bottom inset: the effective mass m F versus electron density determined by analysis of the temperature dependence of the amplitude of Shubnikov-de Haas oscillations, similar to ref. 34 . The dashed line is a guide to the eye.…”

Multiple metamagnetism, extreme magnetoresistance and nontrivial topological electronic structures in the magnetic semimetal candidate holmium monobismuthide

“…In the case of perfect compensation between electrons and holes, another prediction from the formula manifests that the magnetoresistance ratio strongly depends on the average mobility u ave , and can be rewritten as MR=(u ave B) 2 (u ave = u u e h ) [51]. Based on this formula, the obtained u ave is evaluated to be 2.1×10 4 cm 2 V −1 s −1 at 2 K. Although the average mobility of HoBi stands no superiority with respect to the carrier mobility of topological semimetals like Cd 3 As 2 and WTe 2 [52,53], it is still larger than those of the common magnetic metals, which could contribute to the extreme magnetoresistance of this compound.…”

Chapter 2. Metal-Insulator Transition in a Strongly Correlated Two-Dimensional Electron System

“…We stress that the ballistic regime introduced in Ref [9]. is not related to the wellknown ballistic transport, or Knudsen regime, where the mean free path is larger than the sample dimensions 2. The behaviors of the effective electron mass at the Fermi level and the energyaveraged effective electron mass are qualitatively different at low electron densities in the strongly correlated 2D system in SiGe/Si/SiGe quantum wells (see section 5).…”