Homo- and Hetero- p–n Junctions Formed on Graphene Steps

p–n junction is a fundamental building block in modern electronic circuits. We report graphene p–n junctions formed by a one-step thickness-dependent surface treatment of mono-/bilayer graphene steps. The junction electronic properties are systemically studied by means of Kelvin probe force microscopy (KPFM) and transport measurements. Because of the dissimilar modifications to graphene electronic properties, the junctions behave distinctly, i.e., two-component resistance-like for organic charge transfer doping and Shottky-junction-like for covalent doping. By exploring the spatially potential distribution, we clarify the potential profiles as well as the transport attributes across the graphene p–n junction interface under lateral bias and electrical gating. Our results not only unveil the detailed properties of graphene p–n junction interface, but also gain an insight into its practical applications in nanoelectronics

Tunable Plasmon–Phonon Polaritons in Layered Graphene–Hexagonal Boron Nitride Heterostructures

We use infrared spectroscopy to explore the hybridization of graphene plasmons and hexagonal boron nitride (hBN) phonons in their heterostructures with different compositions. We show that the degree of plasmon–phonon hybridization and the slowing of the light group velocity within the infrared transparency window due to the plasmon–phonon destructive interference are dominated by hBN phonon oscillating strength, which can be tuned by varying the hBN thickness in a layer-by-layer manner. However, the plasmon oscillating strength in metallic graphene governs the magnitude of infrared extinction, which exceeds 6% at around 7 μm in a graphene/hBN/graphene heterostructure due to the strong plasmon dipole–dipole coupling. Our work demonstrates that the infrared optical responses of graphene–hBN heterostructures can be engineered by controlling the coupling strength of plasmon–phonon hybridization and the overall plasmon oscillating strength simultaneously, thus opening the avenue for the light manipulation and detection in the mid-infrared regime based on such layered heterostructures

Black Phosphorus Radio-Frequency Transistors

Few-layer and thin film forms of layered black phosphorus (BP) have recently emerged as a promising material for applications in high performance nanoelectronics and infrared optoelectronics. Layered BP thin films offer a moderate bandgap of around 0.3 eV and high carrier mobility, which lead to transistors with decent on–off ratios and high on-state current densities. Here, we demonstrate the gigahertz frequency operation of BP field-effect transistors for the first time. The BP transistors demonstrated here show respectable current saturation with an on–off ratio that exceeds 2 × 10³. We achieved a current density in excess of 270 mA/mm and DC transconductance above 180 mS/mm for hole conduction. Using standard high frequency characterization techniques, we measured a short-circuit current-gain cutoff frequency <i>f</i>_T of 12 GHz and a maximum oscillation frequency <i>f</i>_max of 20 GHz in 300 nm channel length devices. BP devices may offer advantages over graphene transistors for high frequency electronics in terms of voltage and power gain due to the good current saturation properties arising from their finite bandgap, thus can be considered as a promising candidate for the future high performance thin film electronics technology for operation in the multi-GHz frequency range and beyond

Improving the Performance of Graphene Phototransistors Using a Heterostructure as the Light-Absorbing Layer

Interfacing light-sensitive semiconductors with graphene can afford high-gain phototransistors by the multiplication effect of carriers in the semiconductor layer. So far, most devices consist of one semiconductor light-absorbing layer, where the lack of internal built-in field can strongly reduce the quantum efficiency and bandwidth. Here, we demonstrate a much improved graphene phototransistor performances using an epitaxial organic heterostructure composed of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and pentacene as the light-absorbing layer. Compared with single light-absorbing material, the responsivity and response time can be simultaneously improved by 1 and 2 orders of magnitude over a broad band of 400–700 nm, under otherwise the same experimental conditions. As a result, the external quantum efficiency increases by over 800 times. Furthermore, the response time of the heterostructured phototransistor is highly gate-tunable down to sub-30 μs, which is among the fastest in the sensitized graphene phototransistors interfacing with electrically passive light-absorbing semiconductors. We show that the improvement is dominated by the efficient electron–hole pair dissociation due to interfacial built-in field rather than bulk absorption. The structure demonstrated here can be extended to many other organic and inorganic semiconductors, which opens new possibilities for high-performance graphene-based optoelectronics

Waveguide-Integrated MoTe₂<i>p</i>–<i>i</i>–<i>n</i> Homojunction Photodetector

Two-dimensional (2D) materials, featuring distinctive electronic and optical properties and dangling-bond-free surfaces, are promising for developing high-performance on-chip photodetectors in photonic integrated circuits. However, most of the previously reported devices operating in the photoconductive mode suffer from a high dark current or a low responsivity. Here, we demonstrate a MoTe2p–i–n homojunction fabricated directly on a silicon photonic crystal (PC) waveguide, which enables on-chip photodetection with ultralow dark current, high responsivity, and fast response speed. The adopted silicon PC waveguide is electrically split into two individual back gates to selectively dope the top regions of the MoTe2 channel in p- or n-types. High-quality reconfigurable MoTe2 (p–i–n, n–i–p, n–i–n, p–i–p) homojunctions are realized successfully, presenting rectification behaviors with ideality factors approaching 1.0 and ultralow dark currents less than 90 pA. Waveguide-assisted MoTe2 absorption promises a sensitive photodetection in the telecommunication O-band from 1260 to 1340 nm, though it is close to MoTe2’s absorption band-edge. A competitive photoresponsivity of 0.4 A/W is realized with a light on/off current ratio exceeding 104 and a record-high normalized photocurrent-to-dark-current ratio of 106 mW–1. The ultrasmall capacitance of p–i–n homojunction and high carrier mobility of MoTe2 promise a high dynamic response bandwidth close to 34.0 GHz. The proposed device geometry has the advantages of employing a silicon PC waveguide as the back gates to build a 2D material p–i–n homojunction directly and simultaneously to enhance light–2D material interaction. It provides a potential pathway to develop 2D material-based photodetectors, laser diodes, and electro-optic modulators on silicon photonic chips