Annual review of condensed matter physics

2D material based photodetectors have attracted many research projects due to their unique structures and excellent electronic and optoelectronic properties. These 2D materials, including semimetallic graphene, semiconducting black phosphorus, transition metal dichalcogenides, insulating hexagonal boron nitride, and their various heterostructures, show a wide distribution in bandgap values. To date, hundreds of photodetectors based on 2D materials have been reported. Here, a review of photodetectors based on 2D materials covering the detection spectrum from ultraviolet to infrared is presented. First, a brief insight into the detection mechanisms of 2D material photodetectors as well as introducing the figure-of-merits which are key factors for a reasonable comparison between different photodetectors is provided. Then, the recent progress on 2D material based photodetectors is reviewed. Particularly, the excellent performances such as broadband spectrum detection, ultrahigh photoresponsivity and sensitivity, fast response speed and high bandwidth, polarization-sensitive detection are pointed out on the basis of the state-of-the-art 2D photodetectors. Initial applications based on 2D material photodetectors are mentioned. Finally, an outlook is delivered, the challenges and future directions are discussed, and general advice for designing and realizing novel high-performance photodetectors is given to provide a guideline for the future development of this fast-developing field.

Progress, Challenges, and Opportunities for 2D Material Based Photodetectors

Z. Jiang, 2, ∗ Y. Zhang, H. L. Stormer, 3, 4 and P. Kim Department of Physics, Columbia University, New York, New York 10027, USA National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA Bell Laboratories, Alcatel-Lucent, Murray Hill, New Jersey 07974, USA (Dated: February 1, 2008)

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The Nature of Quantum Hall States near the Charge Neutral Dirac Point in Graphene

With the continuous exploration of 2D transition metal dichalcogenides (TMDs), novel high-performance devices based on the remarkable electronic and optoelectronic natures of 2D TMDs are increasingly emerging. As fresh blood of 2D TMD family, anisotropic MTe2 and ReX2 (M = Mo, W, and X = S, Se) have drawn increasing attention owing to their low-symmetry structures and charming properties of mechanics, electronics, and optoelectronics, which are suitable for the applications of field-effect transistors (FETs), photodetectors, thermoelectric and piezoelectric applications, especially catering to anisotropic devices. Herein, a comprehensive review is introduced, concentrating on their recent progresses and various applications in recent years. First, the crystalline structure and the origin of the strong anisotropy characterized by various techniques are discussed. Specifically, the preparation of these 2D materials is presented and various growth methods are summarized. Then, high-performance applications of these anisotropic TMDs, including FETs, photodetectors, and thermoelectric and piezoelectric applications are discussed. Finally, the conclusion and outlook of these applications are proposed.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201700231

Electronic and Optoelectronic Applications Based on 2D Novel Anisotropic Transition Metal Dichalcogenides

2D transition metal dichalcogenides (TMDCs) have attracted considerable attention due to their impressively high performance in optoelectronic devices. However, efficient infrared (IR) photodetection has been significantly hampered because the absorption wavelength range of most TMDCs lies in the visible spectrum. In this regard, semiconducting 2D MoTe2 can be an alternative choice owing to its smaller band gap ≈1 eV from bulk to monolayer and high carrier mobility. Here, a MoTe2/graphene heterostructure photodetector is demonstrated for efficient near-infrared (NIR) light detection. The devices achieve a high responsivity of ≈970.82 A W−1 (at 1064 nm) and broadband photodetection (visible-1064 nm). Because of the effective photogating effect induced by electrons trapped in the localized states of MoTe2, the devices demonstrate an extremely high photoconductive gain of 4.69 × 108 and detectivity of 1.55 × 1011 cm Hz1/2 W−1. Moreover, flexible devices based on the MoTe2/graphene heterostructure on flexible substrate also retains a good photodetection ability after thousands of times bending test (1.2% tensile strain), with a high responsivity of ≈60 A W−1 at 1064 nm at VDS = 1 V, which provides a promising platform for highly efficient, flexible, and low cost broadband NIR photodetectors.

Near-Infrared Photodetectors Based on MoTe2 /Graphene Heterostructure with High Responsivity and Flexibility.

MoTe2 with a narrow band-gap of ∼1.1 eV is a promising candidate for optoelectronic applications, especially for the near-infrared photo detection. However, the photo responsivity of few layers MoTe2 is very small (<1 mA W−1). In this work, we show that a few layer MoTe2-graphene vertical heterostructures have a much larger photo responsivity of ∼20 mA W−1. The trans-conductance measurements with back gate voltage show on-off ratio of the vertical transistor to be ∼(0.5–1) × 105. The rectification nature of the source-drain current with the back gate voltage reveals the presence of a stronger Schottky barrier at the MoTe2-metal contact as compared to the MoTe2-graphene interface. In order to quantify the barrier height, it is essential to measure the work function of a few layers MoTe2, not known so far. We demonstrate a method to determine the work function by measuring the photo-response of the vertical transistor as a function of the Schottky barrier height at the MoTe2-graphene interface tuned by elec...

Enhancing photoresponsivity using MoTe2-graphene vertical heterostructures

The rather surprising position of the Kirkendall pores in Nb3Sn superconductor grown by bronze technique is clarified. It is shown that Sn is the fastest diffusing species, which seems to be unusual considering the crystal structure of Nb3Sn. Further, we explain the dramatic change in the growth rate and the activation energy for growth of the product phase, when the Sn composition in the Cu(Sn) bronze alloy is changed from 7 to 8 at. %.

Diffusion and growth mechanism of Nb3Sn superconductor grown by bronze technique

Fabrication of the out-of-plane atomically sharp p–n junction by stacking two dissimilar two-dimensional materials could lead to new and exciting physical phenomena. The control and tunability of the interlayer carrier transport in these p–n junctions have a potential to exhibit new kind of electronic and optoelectronic devices. In this article, we present the fabrication, electrical, and opto-electrical characterization of vertically stacked few-layers MoTe2(p)–single-layer MoS2(n) heterojunction. Over and above the antiambipolar transfer characteristics observed similar to other hetero p–n junction, our experiments reveal a unique feature as a dip in transconductance near the maximum. We further observe that the modulation of the dip in the transconductance depends on the doping concentration of the two-dimensional flakes and also on the power density of the incident light. We also demonstrate high photo-responsivity of ~105 A/W at room temperature for a forward bias of 1.5 V. We explain these new findings based on interlayer recombination rate-dependent semi-classical transport model. We further develop first principles-based atomistic model to explore the charge carrier transport through MoTe2–MoS2 heterojunction. The similar dip is also observed in the transmission spectrum when calculated using density functional theory–non-equilibrium Green’s function formalism. Our findings may pave the way for better understanding of atomically thin interface physics and device applications. p–n heterojunctions based on certain two-dimensional transition metal dichalcogenides display an unusual dip in the current–voltage characteristic. A team led by Anindya Das at the Indian Institute of Science in Bangalore fabricated p–n junctions made of few layers of MoTe2 and a single layer of MoS2 and observed an unexpected dip in the junction current as a function of the back gate voltage. The researchers could modulate the intensity and position of this dip feature by changing the power density of the incident light. They also performed theoretical calculations that qualitatively captured the behavior of the dip, which could be attributed to interface recombination of charge carriers. These findings further the understanding of the physics of thin-film interfaces, which can stimulate the improvement of devices.

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Photo-tunable transfer characteristics in MoTe 2 –MoS 2 vertical heterostructure

Superconductivity and the quantum Hall effect are distinct states of matter occurring in apparently incompatible physical conditions. Recent theoretical developments suggest that the coupling of the quantum Hall effect with a superconductor can provide fertile ground for realizing exotic topological excitations such as non-Abelian Majorana fermions or Fibonacci particles. As a step toward that goal, we report observation of Andreev reflection at the junction of a quantum Hall edge state in a single layer graphene and a quasi-two-dimensional niobium diselenide (${\mathrm{NbSe}}_{2}$) superconductor. Our principal finding is the observation of an anomalous finite-temperature conductance peak located precisely at the Dirac point, providing a definitive evidence for inter-Landau-level Andreev reflection in a quantum Hall system. Our observations are well supported by detailed numerical simulations, which offer additional insight into the role of the edge states in Andreev physics. This study paves the way for investigating analogous Andreev reflection in a fractional quantum Hall system coupled to a superconductor to realize exotic quasiparticles.

Inter-Landau-level Andreev Reflection at the Dirac Point in a Graphene Quantum Hall State Coupled to a NbSe 2 Superconductor

Transport properties of graphene-superconductor junction have been studied extensively to understand the interplay of the relativistic Dirac quasiparticles and superconductivity. Though shot noise measurements in graphene have been performed to realize many theoretical predictions, both at zero magnetic field as well as quantum Hall (QH) regime, its junction with superconductor remain unexplored. Here, we have carried out the shot noise measurements in an edge contacted bilayer graphene--niobium superconductor junction at zero magnetic field as well as QH regime. At the Dirac point we have observed a Fano factor $\ensuremath{\sim}1/3$ above the superconducting gap $(\mathrm{\ensuremath{\Delta}})$ and a transition to an enhanced Fano factor $\ensuremath{\sim}0.5$ below the superconducting gap. By changing the carrier density we have found a continuous reduction of Fano factor for both types of carriers; however, the enhancement of Fano factor within the superconducting gap by a factor of $\ensuremath{\sim}1.5$ is always preserved. The enhancement of shot noise is also observed in the QH regime, where the current is carried by the edge state, below the critical magnetic field and within the superconducting gap. These observations clearly demonstrate the enhanced charge transport at the graphene-superconductor interface.

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A K Paul

Papers

Enhancing photoresponsivity using MoTe2-graphene vertical heterostructures

Diffusion and growth mechanism of Nb3Sn superconductor grown by bronze technique

Photo-tunable transfer characteristics in MoTe 2 –MoS 2 vertical heterostructure

Inter-Landau-level Andreev Reflection at the Dirac Point in a Graphene Quantum Hall State Coupled to a NbSe 2 Superconductor

Enhanced shot noise at bilayer graphene -- superconductor junction