Abstract
A central challenge in making two-dimensional (2D) material-based devices faster, smaller, and more efficient is to control their charge carrier density at the nanometer scale. Traditional gating techniques based on capacitive coupling through a gate dielectric cannot generate strong and uniform electric fields at this scale due to divergence of the fields in dielectrics. This field divergence limits the gating strength, boundary sharpness, and minimum feature size of local gates, precluding certain device concepts (such as plasmonics and metamaterials based on spatial charge density variation) and resulting in large device footprints. Here we present a nanopatterned electrolyte gating concept that allows locally creating excess charges by combining electrolyte gating with an ion-impenetrable e-beam-defined resist mask. Electrostatic simulations indicate high carrier density variations of Δn ∼ 1014 cm−2 across a length of only 15 nm at the mask boundaries on the surface of a 2D conductor. We implement this technique using cross-linked poly(methyl methacrylate), experimentally prove its ion-impenetrability and demonstrate e-beam patterning of the resist mask down to 30 nm half-pitch resolution. The spatial versatility enables us to demonstrate a compact mid-infrared graphene thermopile with a geometry optimized for Gaussian incident radiation. The thermopile has a small footprint despite the number of thermocouples in the device, paving the way for more compact high-speed thermal detectors and cameras.
Highlights
Modulation of charge carrier concentration of semiconductors lies at the heart of many electronic and optoelectronic device operation principles [1, 2]
Many novel device concepts rely on the ability to create metamaterials with spatial carrier density variations down to the nanometer scale, including for instance graphene with periodically doped nanodisk or nanoribbon arrays for complete optical absorption in the visible and near-infrared [15, 16], graphene with doped waveguide, bend and resonator patterns for a plasmon-based nanophotonic network [17], and superlattices based on graphene and other 2D materials for concepts such as electron beam supercollimation [18,19,20,21]
The discontinuity in the carrier density profile is caused by the difference in the electric permittivity between electrolyte medium and cross-linked poly(methyl methacrylate) (PMMA) mask
Summary
Modulation of charge carrier concentration of semiconductors lies at the heart of many electronic and optoelectronic device operation principles [1, 2]. Many novel device concepts rely on the ability to create metamaterials with spatial carrier density variations down to the nanometer scale, including for instance graphene with periodically doped nanodisk or nanoribbon arrays for complete optical absorption in the visible and near-infrared [15, 16], graphene with doped waveguide, bend and resonator patterns for a plasmon-based nanophotonic network [17], and superlattices based on graphene and other 2D materials for concepts such as electron beam supercollimation [18,19,20,21] Implementing these concepts calls for a gating method that allows for sharp p–n junctions with narrow depletion regions (∼15 nm), a small minimum gating feature size, large carrier density contrasts (1014 cm−2), strong in-plane electric fields (>1 × 108 V m−1), and the versatility to generate complex spatial doping profiles within a small area
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