Gateless and reversible Carrier density tunability in epitaxial graphene devices functionalized with chromium tricarbonyl

Abstract

Monolayer epitaxial graphene (EG) has been shown to have clearly superior properties for the development of quantized Hall resistance (QHR) standards. One major difficulty with QHR devices based on EG is that their electrical properties drift slowly over time if the device is stored in air due to adsorption of atmospheric molecular dopants. The crucial parameter for device stability is the charge carrier density, which helps determine the magnetic flux density required for precise QHR measurements. This work presents one solution to this problem of instability in air by functionalizing the surface of EG devices with chromium tricarbonyl-Cr(CO) 3. Observations of carrier density stability in air over the course of one year are reported, as well as the ability to tune the carrier density by annealing the devices. For low temperature annealing, the presence of Cr(CO) 3 stabilizes the electrical properties and allows for the reversible tuning of the carrier density in millimeter-scale graphene devices close to the Dirac point. Precision measurements in the quantum Hall regime show no detrimental effect on the carrier mobility.

Atypical quantized resistances in millimeter-scale epitaxial graphene p-n junctions

“…6 However, the reduction of the electron density from the order of 10 13 cm −2 to the order of 10 10 cm −2 by the presence of Cr(CO) 3 considerably assists the p region to undergo its transition. 16 It should be briefly noted that the temporary dip in resistivity near t = 30000 s arises from another competing process to shift the carrier density, namely that of the applied heat, which as prescribed by other work, causes n-type doping in EG devices. 16 Transport measurements were performed at 4 K, allowing us to determine the lowtemperature longitudinal resistivity (ρ xx ), mobility (μ), and electron density in EG (n G ), where the latter two parameters are calculated by the following respective formulas:…”

“…[1][2][3][4] Epitaxial graphene (EG) on silicon carbide (SiC), which can be grown on the centimeter scale and is one of the many methods of synthesizing graphene, exhibits properties that render it suitable for large-scale or high-current applications such as the continued development of quantized Hall resistance (QHR) standards. [5][6][7][8][9][10][11][12][13][14][15] Though modernday standards using millimeter-scale EG have been shown to have long-term electrical stability in ambient conditions, 16 these devices are, in most cases, only able to output a single value of quantized resistance (ν = 2 plateau) to a degree of accuracy which warrants possible use in metrology. The corresponding value is: One milestone for graphene QHR standards would be the eventual accessibility of different resistance values that are well-quantized.…”

“…Research in developing materials for gating and preserving properties of large devices has seen limited success with amorphous boron nitride, [21][22] atomicallylayered high-k dielectrics, [23][24][25][26] Parylene, [27][28][29] and hexagonal boron nitride, [30][31] whereas other materials have been more successful, such as (poly)-methyl methacrylate ((P)MMA), ZEP520A photoresist, tetrafluoro-tetracyanoquinodimethane (F4TCNQ), and chromium tricarbonyl. 16,[32][33] For millimeter-scale constructions, one major issue was fabricating correspondingly large pnJs. One of the major challenges of mass producing such devices with more than one pnJ has been the required use of electron beam lithography, a costly and time-consuming technique, for the fabrication of junctions that are abrupt, with n-type and p-type regions separated by a width on the scale of several hundreds of nanometers or smaller.…”

Two-Terminal and Multi-Terminal Designs for Next-Generation Quantized Hall Resistance Standards: Contact Material and Geometry

“…The wet-etching procedure removes the Pd/Au in the defined areas without harming the EG. The finished devices are functionalized with chromium tricarbonyl [Cr(CO) 3 ], which provides tunable and uniform doping without the need for large-scale electrostatic gates [12].…”

Covalent Atomic Bridges Enable Unidirectional Enhancement of Electronic Transport in Aligned Carbon Nanotubes

“…Additionally, a variety of ligands can be used to stabilize the electronic properties of the -conjugated carbon surfaces as recently reported in a study of the ( 6 -graphene)Cr(CO)3 complex, which has been suggested as an exceptionally robust quantum Hall resistance standard. 50 with the grid localization used in the code, optimizes the efficiency of the calculation so that most of the time-consuming computations scale linearly with time. We utilized the dispersion-corrected PBE-TS exchange correlation functional 52,53 in all of our calculations since van der Waals interactions play a significant role in SWNT junctions, and further details on the PBE-TS approach for carbon nanostructures are described further in ref.…”