Graphene/h-BN/ZnO van der Waals tunneling heterostructure based ultraviolet photodetector

Novel Van Der Waals Heterostructures Based on Borophene, Graphene-like GaN and ZnO for Nanoelectronics: A First Principles Study

At present, the combination of 2D materials of different types of conductivity in the form of van der Waals heterostructures is an effective approach to designing electronic devices with desired characteristics. In this paper, we design novel van der Waals heterostructures by combing buckled triangular borophene (tr-B) and graphene-like gallium nitride (GaN) monolayers, and tr-B and zinc oxide (ZnO) monolayers together. Using ab initio methods, we theoretically predict the structural, electronic, and electrically conductive properties of tr-B/GaN and tr-B/ZnO van der Waals heterostructures. It is shown that the proposed atomic configurations of tr-B/GaN and tr-B/ZnO heterostructures are energetically stable and are characterized by a gapless band structure in contrast to the semiconductor character of GaN and ZnO monolayers. We find the phenomenon of charge transfer from tr-B to GaN and ZnO monolayers, which predetermines the key role of borophene in the formation of the features of the electronic structure of tr-B/GaN and tr-B/ZnO van der Waals heterostructures. The results of the calculation of the current–voltage (I–V) curves reveal that tr-B/GaN and tr-B/ZnO van der Waals heterostructures are characterized by the phenomenon of current anisotropy: the current along the zigzag edge of the ZnO/GaN monolayers is five times greater than along the armchair edge of these monolayers. Moreover, the heterostructures show good stability of current to temperature change at small voltage. These findings demonstrate that r-B/GaN and tr-B/ZnO vdW heterostructures are promising candidates for creating the element base of nanoelectronic devices, in particular, a conducting channel in field-effect transistors.

Upconversion under Photon Trapping in ZnO/BN Nanoarray: An Ultrahigh Responsivity Solar-Blind Photodetecting Paper

Solar-blind photodetectors (PDs) are widely applicable in special, military, medical, environmental, and commercial fields. However, high performance and flexible PD for deep ultraviolet (UV) range is still a challenge. Here, it is demonstrated that an upconversion of photon absorption beyond the energy bandgap is achieved in the ZnO nanoarray/h-BN heterostructure, which enables the ultrahigh responsivity of a solar-blind photodetecting paper. The direct growth of ultralong ZnO nanoarray on polycrystalline copper paper induced by h-BN 2D interlayer is obtained. Meanwhile, strong photon trapping takes place within the ZnO nanoarray forest through the cyclic state transition of surface oxygen ions, resulting in an extremely high absorption efficiency (> 99.5%). A flexible photodetecting paper is fabricated for switchable detections between near UV and deep UV signals by critical external bias. The device shows robust reliability, ultrahigh responsivity up to 700 A W^-1 @ 265-276 nm, and high photoconductive gain of ≈2 × 10³ . A negative differential resistance effect is revealed for driving the rapid transfer of up-converted electrons between adjacent energy valleys (Γ to A) above the critical bias (3.9 V). The discovered rationale and device structure are expected to bring high-efficiency deep UV detecting and future wearable applications.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

Toward h-BN/GaN Schottky Diodes: Spectroscopic Study on the Electronic Phenomena at the Interface

Hexagonal boron nitride (h-BN), together with other members of the van der Waals crystal family, has been studied for over a decade, both in terms of fundamental and applied research. Up to now, the spectrum of h-BN-based devices has broadened significantly, and systems containing the h-BN/III-V junctions have gained substantial interest as building blocks in, inter alia, light emitters, photodetectors, or transistor structures. Therefore, the understanding of electronic phenomena at the h-BN/III-V interfaces becomes a question of high importance regarding device engineering. In this study, we present the investigation of electronic phenomena at the h-BN/GaN interface by means of contactless electroreflectance (CER) spectroscopy. This nondestructive method enables precise determination of the Fermi level position at the h-BN/GaN interface and the investigation of carrier transport across the interface. CER results showed that h-BN induces an enlargement of the surface barrier height at the GaN surface. Such an effect translates to Fermi level pinning deeper inside the GaN band gap. As an explanation, we propose a mechanism based on electron transfer from GaN surface states to the native acceptor states in h-BN. We reinforced our findings by thorough structural characterization and demonstration of the h-BN/GaN Schottky diode. The surface barriers obtained from CER (0.60 ± 0.09 eV for GaN and 0.91 ± 0.12 eV for h-BN/GaN) and electrical measurements are consistent within the experimental accuracy, proving that CER is an excellent tool for interfacial studies of 2D/III-V hybrids.

Novel electrical properties and applications in kaleidoscopic graphene nanoribbons

As one of the representatives of nano-graphene materials, graphene nanoribbons (GNRs) have more novel electrical properties, highly adjustable electronic properties, and optoelectronic properties than graphene due to their diverse geometric structures and atomic precision configurations. The electrical properties and band gaps of GNRs depend on their width, length, boundary configuration and other elemental doping, etc. With the improvement of the preparation technology and level of GNRs with atomic precision, increasing number of GNRs with different configurations are being prepared. They all show novel electrical properties and high tunability, which provides a broad prospect for the application of GNRs in the field of microelectronics. Here, we summarize the latest GNR-based achievements in recent years and summarize the latest electrical properties and potential applications of GNRs.

Engineered tunneling layer with enhanced impact ionization for detection improvement in graphene/silicon heterojunction photodetectors

Here, an engineered tunneling layer enhanced photocurrent multiplication through the impact ionization effect was proposed and experimentally demonstrated on the graphene/silicon heterojunction photodetectors. With considering the suitable band structure of the insulation material and their special defect states, an atomic layer deposition (ALD) prepared wide-bandgap insulating (WBI) layer of AlN was introduced into the interface of graphene/silicon heterojunction. The promoted tunneling process from this designed structure demonstrated that can effectively help the impact ionization with photogain not only for the regular minority carriers from silicon, but also for the novel hot carries from graphene. As a result, significantly enhanced photocurrent as well as simultaneously decreased dark current about one order were accomplished in this graphene/insulation/silicon (GIS) heterojunction devices with the optimized AlN thickness of ~15 nm compared to the conventional graphene/silicon (GS) devices. Specifically, at the reverse bias of -10 V, a 3.96-A W^-1 responsivity with the photogain of ~5.8 for the peak response under 850-nm light illumination, and a 1.03-A W^-1 responsivity with ∼3.5 photogain under the 365 nm ultraviolet (UV) illumination were realized, which are even remarkably higher than those in GIS devices with either Al₂O₃ or the commonly employed SiO₂ insulation layers. This work demonstrates a universal strategy to fabricate broadband, low-cost and high-performance photo-detecting devices towards the graphene-silicon optoelectronic integration.

Van der Waals contacted MoOx staked ZnO/GaN vertical heterostructured ultraviolet light emitting diodes

Since the discovery of two dimensional (2D) materials, there has been a gold rush for van der Waals integrated 2D material heterostructure based optoelectronic devices. Van der Waals integration involves the physical assembly of the components of the device. In the present work, we extended van der Waals integration from 2D materials to three-dimensional (3D) materials, and herein we uniquely designed a van der Waals contacted light emitting diode based on MoO_x staked ZnO/GaN heterostructure. The presence of the MoO_x layer between n-type ZnO and p-type GaN leads to the confinement of electrons and an increase in the electron charge density at n-type ZnO. The n-type MoO_x, a well-known hole injection layer, favors the availability of holes at the ZnO site, leading to the efficient recombination of electrons and holes at the ZnO site, which results in predominant high-intensity UV-EL emission around 380 nm in both forward and reverse bias.

Fast Response Characteristics of Flexible Ultraviolet Photosensors with GaN Nanowires and Graphene

We report the fast response characteristics of flexible ultraviolet photosensors with GaN nanowires (NWs) and a graphene channel. The GaN NWs used as light-absorbing media are horizontally and randomly embedded in a graphene sandwich structure in which the number of bottom graphene layers is varied from zero to three and the top is a fixed single layer of graphene. In the response curve of the photosensor with a double-layer bottom graphene, as obtained under pulsed illumination with a pulse width of 50 ms and a duty cycle of 50%, the rise and decay times were measured as 24.1 ± 0.1 and 28.2 ± 0.1 ms, respectively. The eye-crossing percentage was evaluated as 52.1%, indicating no substantial distortion of the duty cycle and no pulse symmetry problem. The rise and decay times estimated from an equivalent circuit analysis represented by resistances and capacitances agree well with the measured values. When the device was under the bending condition, the rise and decay times of the photosensor were comparable to those in the unbent state.

A synergetic enhancement of localized surface plasmon resonance and photo-induced effect for graphene/GaAs photodetector

Photodetectors based on graphene/GaAs heterostructure were fabricated and demonstrated for application in self-powered photodetection. Then, Si quantum dots (QDs) were spin-coated onto the surface of the devices to enhance the built-in field by photo-induced doping, because of the tunable Fermi level (E F) of graphene and shallow junction of the heterojunction. Additionally, Au nanoparticles working as a light trapping structure were used to the enhance quantum efficiency of the Si QDs and the optical absorption of the heterojunction, benefitting from localized surface plasmon resonance. Therefore, a large-area photodetector under self-powered conditions achieved a high performance i.e. responsivity (1.81 × 105 V W-1), detectivity (2.0 × 1012 Jones), fast response speed (<0.04 ms), and on-off ratio (6 × 103). The high voltage responsivity opens a promising pathway to ultra-weak light detection, and facilities the development of novel sensors.

Are graphene-Bi2Te3 van der Waals heterostructure-based saturable absorbers promising for solid-state Q-switched lasers?

This Letter compared the absorption characteristics of a homemade graphene-Bi₂Te₃ (G-B) van der Waals heterostructure to a Bi₂Te₃ topological insulator (TI) with a similar preparation method and number of layers. The results indicate that the G-B heterostructure can tremendously enhance the modulation depth and saturable intensity. In addition, a passively Q-switched laser at 1.06 μm with a G-B heterostructure as a saturable absorber (SA) was demonstrated for the first time, to the best of our knowledge. Compared to Bi₂Te₃ TI, the G-B heterostructure Q-switched laser had better laser performance, indicating that a G-B heterostructure is a promising SA candidate for a 1 μm laser.

A High Current Density Direct-Current Generator Based on a Moving van der Waals Schottky Diode

Traditionally, Schottky diodes are used statically in the electronic information industry while dynamic or moving Schottky diode-based applications are rarely explored. Herein, a novel Schottky diode named "moving Schottky diode generator" is designed, which can convert mechanical energy into electrical energy by means of lateral movement between the graphene/metal film and semiconductor. The mechanism is based on the built-in electric field separation of the diffusing carriers in moving Schottky diode. A current-density output up of 40.0 A m^-2 is achieved through minimizing the contact distance between metal and semiconductor, which is 100-1000 times higher than former piezoelectric and triboelectric nanogenerators. The power density and power conversion efficiency of the heterostructure-based generator can reach 5.25 W m^-2 and 20.8%, which can be further enhanced by Schottky junction interface design. Moreover, the graphene film/semiconductor moving Schottky diode-based generator behaves better flexibility and stability, which does not show obvious degradation after 10 000 times of running, indicating its great potential in the usage of portable energy source. This moving Schottky diode direct-current generator can light up a blue light-emitting diode and a flexible graphene wristband is demonstrated for wearable energy source.

Gate tunable surface plasmon resonance enhanced graphene/Ag nanoparticles-polymethyl methacrylate/graphene/p-GaN heterostructure light-emitting diodes

By combining the surface plasmon enhancement technique with gating effect, a tunable blue lighting emitting diode (LED) based on graphene/Ag nanoparticles (NPs)-polymethyl methacrylate (PMMA)/graphene/p-GaN heterostructure has been achieved. The surface plasmon enhancement is introduced through spin-coating Ag nanoparticles on graphene/p-GaN heterostructure while the gating effect is demonstrated through a graphene/PMMA/graphene sandwich structure, where the top graphene layer acts as the gate electrode. Compared with initial graphene/p-GaN heterostructure LEDs, the electroluminescence (EL) emission intensity of Ag NPs/graphene/p-GaN heterostructure LEDs has been largely enhanced, attributing to the surface plasmon resonance (SPR) of Ag nanoparticles. The EL emission intensity of graphene/Ag NPs-PMMA/graphene/p-GaN heterostructure LEDs can further be gate-tunable effectively through exerting a static voltage between the sandwich structure, which tunes the Fermi level of graphene contacting with p-GaN. These results indicate that through sophisticated design, graphene/Ag NPs-PMMA/graphene/p-GaN heterostructure LEDs can be a potential candidate for many essential electronic and optoelectronic applications.

Graphene-Based Semiconductor Heterostructures for Photodetectors

Graphene transparent conductive electrodes are highly attractive for photodetector (PD) applications due to their excellent electrical and optical properties. The emergence of graphene/semiconductor hybrid heterostructures provides a platform useful for fabricating high-performance optoelectronic devices, thereby overcoming the inherent limitations of graphene. Here, we review the studies of PDs based on graphene/semiconductor hybrid heterostructures, including device physics/design, performance, and process technologies for the optimization of PDs. In the last section, existing technologies and future challenges for PD applications of graphene/semiconductor hybrid heterostructures are discussed.

High-efficiency energy transfer in perovskite heterostructures

Here, we report the energy transfer in (PEA)₂PbI₄/MAPbBr₃ perovskite heterostructures. Under two-photon excitation, the photoluminescence (PL) emission of the (PEA)₂PbI₄ flake is nearly completely quenched, while that of the MAPbBr₃ microplate is greatly increased (6.5 folds higher) in the heterostructure. The opposite variation character of the PL emissions is attributed to the radiative energy transfer from the (PEA)₂PbI₄ flake to the MAPbBr₃ microplate. The radiative energy transfer occurs on an ultrafast timescale with a high efficiency (~100%). In addition, a strongly thickness- and wavelength-dependent interlayer interaction is observed under one-photon excitation. This work advocates great promise for revealing the interlayer interaction of perovskite heterostructures and developing high-performance optoelectronic devices.

ZnO quantum dot-doped graphene/h-BN/GaN-heterostructure ultraviolet photodetector with extremely high responsivity

A ZnO quantum dot  photo-doped graphene/h-BN/GaN-heterostructure ultraviolet photodetector with extremely high responsivity of more than 1915 A W^-1 and detectivity of more than 1.02 × 10¹³ Jones (Jones = cm Hz^1/2 W^-1) has been demonstrated. The interfaced h-BN layer increases the barrier height at the graphene/GaN heterojunction, which decreases the dark current and improves the on/off current ratio of the device. The photo-doping effect increases the barrier height and carrier concentration at the graphene/h-BN/GaN heterojunction, thus the responsivity is improved from 1473 A W^-1 to 1915 A W^-1 and the detectivity is improved from 5.8 × 10¹² to 1.0 × 10¹³ Jones. Moreover, all of the responsivity and detectivity values are the highest values among all the graphene-based ultraviolet photodetectors.

Improvement of n-type conductivity in hexagonal boron nitride monolayers by doping, strain and adsorption

The n-type conductivity of h-BN monolayers is improved significantly via doping, applying strain and alkali-metal atom adsorption.