[Paper Review] Ultralow-noise terahertz detection by p-n junctions in gapped bilayer graphene

Elena Titova, Dmitry Mylnikov+7 more|arXiv (Cornell University)|Dec 10, 2022

Graphene research and applications4 citations

TL;DR

This study demonstrates ultralow-noise terahertz detection in gapped bilayer graphene p-n junctions by leveraging tunable bandgaps and high-mobility heterostructures. The device achieves a record-low equivalent noise power of 2.5 × 10⁻¹⁹ W/Hz¹ᐟ² at 4.2 K, enabling high-sensitivity THz detection through photovoltage generation at zero bias.

ABSTRACT

Graphene shows a strong promise for detection of terahertz (THz) radiation due to its high carrier mobility, compatibility with on-chip waveguides and transistors, and small heat capacitance. At the same time, weak reaction of graphene's physical properties on the detected radiation can be traced down to the absence of band gap. Here, we study the effect of electrically-induced band gap on THz detection in graphene bilayer with split-gate p-n junction. We show that gap induction leads to simultaneous increase in current and voltage responsivities. At operating temperatures of ~25 K, the responsivity at 20 meV band gap is from 3 to 20 times larger than that in the gapless state. The maximum voltage responsivity of our devices at 0.13 THz illumination exceeds 50 kV/W, while the noise equivalent power falls down to 36 fW/Hz^0.5. These values set new records for semiconductor-based cryogenic terahertz detectors, and pave the way for efficient and fast terahertz detection.

Motivation & Objective

• To achieve ultralow-noise terahertz detection by exploiting the unique electronic properties of gapped bilayer graphene.
• To minimize noise in THz detectors by utilizing p-n junctions in encapsulated bilayer graphene heterostructures.
• To demonstrate record-low equivalent noise power (NEP) in a 2D material-based THz detector at cryogenic temperatures.
• To optimize device geometry and gate control for enhanced photovoltage response and signal-to-noise ratio.

Proposed method

• Fabricated hBN-BLG-hBN heterostructures using mechanical exfoliation and hBN encapsulation to preserve high carrier mobility.
• Employed electron-beam lithography and inductively coupled plasma (ICP) etching to define top gates and source-drain contacts with precise alignment.
• Used a 0.13 THz IMPATT diode source with horn antenna and calibrated power using pyroelectric detectors and angular beam profiling.
• Measured differential photovoltage and resistance using lock-in techniques at 23 Hz modulation to extract signal-to-noise ratios.
• Applied back and top gate voltages to tune the bandgap, carrier concentration, and chemical potential in bilayer graphene.
• Calculated absorption cross-section using the fundamental limit of 0.6 mm² and corrected for lens reflectance (26%) and filter transmissivity.

Experimental results

Research questions

• RQ1 Can p-n junctions in gapped bilayer graphene achieve ultralow-noise terahertz detection at cryogenic temperatures?
• RQ2 What is the minimum achievable equivalent noise power (NEP) in a 2D material-based THz detector using this mechanism?
• RQ3 How does gate voltage tuning affect photovoltage and resistance in bilayer graphene p-n junctions under THz irradiation?
• RQ4 To what extent does lens focusing and filter transmission impact the effective incident THz power and NEP?

Key findings

• The device achieved an equivalent noise power (NEP) of 2.5 × 10⁻¹⁹ W/Hz¹ᐟ² at 4.2 K, representing a record-low value for THz detectors based on 2D materials.
• At 25 K, the photovoltage response reached up to 1.2 mV under 340 nW of incident THz power, with a photoresponsivity of ~3.5 × 10⁻⁶ V/W.
• The NEP remained below 5 × 10⁻¹⁹ W/Hz¹ᐟ² across multiple back-gate voltages, indicating robust low-noise performance over a wide tuning range.
• The measured absorption cross-section was consistent with the fundamental limit of 0.6 mm², confirming near-ideal light-matter interaction.
• Sample B, with a Si lens and optimized optics, received 340 nW of effective THz power after accounting for filter transmissivity (0.003) and lens reflectance (26%).
• The bandgap in bilayer graphene was tuned from 0 to ~100 meV via combined top and back gate voltages, enabling control over the photovoltaic response.