Black Phosphorus Devices Research Articles

Van der Waals Encapsulation by Ultrathin Oxide for Air-Sensitive 2D Materials.

The ambient stability is one of the focal points for applications of 2D materials, especially for those well-known air-sensitive ones, such as black phosphorus (BP) and transitional metal telluride. Traditional methods of encapsulation, such as atomic layer deposition of oxides and heterogeneous integration of hexagonal boron nitride, can hardly avoid removal of encapsulation layer when the 2D materials are encapsulated for further device fabrication, which causes complexity and damage during the procedure. Here, a van der Waals encapsulation method that allows direct device fabrication without removal of encapsulation layer is introduced using Ga2O3 from liquid gallium. Taking advantage of the robust isolation ability against ambient environment of the dense native oxide of gallium, hundreds of times longer retention time of (opto)electronic properties of encapsulated BP and MoTe2 devices is realized than unencapsulated devices. Due to the ultrathin high-κ properties of Ga2O3, top-gated devices are directly fabricated with the encapsulation layer, simultaneously as a dielectric layer. This direct device fabrication is realized by selective etching of Ga2O3, leaving the encapsulated materials intact. Encapsulated 1T' MoTe2 exhibits high conductivity even after 150 days in ambient environment. This method is, therefore, highlighted as a promising and distinctive one compared with traditional passivation approaches.

Influence of N2 plasma treatment on properties of black phosphorus devices in space electronic systems

In order to improve the country’s comprehensive national strength and seize space resources, the implementation of new space systems requires the use of advanced technology in key applications of microelectronics. To further improve device performance, black phosphorus (BP) is used to overcome feature size limitations for its atomic thickness. BP has excellent physical properties such as in-plane anisotropy, thickness-dependent direct band gap and high carrier mobility. However, the performance control of phosphene is a major challenge in practical applications. In order to tune the BP performance, various theoretical and experimental studies on the doping mechanism and strategies of BP have been proposed and reported. In this work, the performance of BP can be effectively tuned by N2 plasma treatment. By changing the power and processing time, the on-state current and mobility of the device can be effectively improved. This simple and efficient doping technique provides a valuable way to realize high performance BP thin film transistors.

Role of Phonon Scattering on the Transport and Performance of an N-Channel Monolayer Black Phosphorus Transistor

We investigate the influence of phonon scattering on the transport properties and performance metrics of a monolayer n-channel black phosphorus transistor within a four-band tight binding Hamiltonian, employing a recursive Green's function algorithm and Buttiker probe scattering model. Our analysis reveals that electron-phonon scattering significantly degrades the on-state current, while its effects in the subthreshold region are found to be negligible. Further examination identifies optical phonons as the primary contributors to the degradation of on-state current, with acoustic phonons playing a less prominent role. The ballisticity of the device declines from 42% to 24% when transitioning from solely acoustic phonon scattering to the combined influence of acoustic and optical phonons. Expanding the placement of Buttiker probes from beneath the gate region to cover the entire path from source to drain results in a further 48% reduction in on-state current. The on-state current exhibits a parabolic relationship with the inverse Kelvin temperature. To quantify the effects of phonon scattering on device performance, we assess the key parameters, transconductance and unity current gain frequency. Phonon scattering is observed to severely impact both the parameters. The on-state transconductance declines from its ballistic value of 24.9 mS/µm to 3.99 mS/µm when both acoustic and optical phonons are concurrently active. Similarly, the unity current gain frequency decreases from 1.18 to 0.2 THz due to phonon scattering. Additionally, our analysis reveals that approximately 7–9% of the total power dissipated within the device is attributed to phonon scattering effects, while the remainder is released through thermalization in the device's contacts. Phonon scattering is shown to induce both lattice cooling and heating, depending on the presence or absence of potential barriers. When a potential barrier exists in the channel, electrons injected from the source experience lattice cooling before the barrier region and lattice heating after crossing the barrier. Including the source and drain contact resistances in our model unveils that achieving a contact resistance value of approximately 100 Ω-µm is crucial for the effective functioning of black phosphorus devices.

Gate-Controlled Metal to Insulator Transition in Black Phosphorus Nanosheet-Based Field Effect Transistors

Carrier–carrier interactions or disorders strongly affect the quantum localization–delocalization of carriers which leads to the metal to insulator transition (MIT) in two-dimensional (2D) systems. However, the physical origin of MIT in 2D systems remains controversial. Here, we report the MIT in black phosphorus (BP) nanosheet-based devices with and without the encapsulation of hexagonal boron nitride (hBN). In hBN encapsulated BP devices, we perform critical scaling analysis to elucidate the microscopic origin of 2D MIT by considering the significant role of carrier–carrier interactions over disorder. We find the critical exponents zν = 2.49 ± 0.05 and zν = 2.65 ± 0.06 in metallic and insulting phases, respectively, supporting the quantum percolation (zν = 7/3). In hBN unencapsulated BP devices, the Mott variable range hopping dominates the charge transport in the insulating phase, suggesting that the disorder plays a significant role over the carrier–carrier interactions. The extracted conductivity exponent of 1.31 ± 0.01 at 10 K approaches the 2D percolation exponent value of 4/3, which supports the classical percolation-based 2D MIT. Our findings pave the way toward the utilization of BP with and without hBN encapsulation as a model system with which to study the 2D MIT as well as various classical and quantum transports in 2D nanoelectronic devices.

Ultratough Hydrogen-Bond-Bridged Phosphorene Films.

The rapid development of flexible electronic devices, especially based on 2D materials, has triggered the demand for high-strength materials. Mono- or few-layer phosphorene with excellent electronic properties has attracted extensive attention. However, phosphorene is affected by its low Young's modulus when applied to flexible electronic devices. Here, a strategy via ion intercalation to significantly improve the mechanical properties of black phosphorus to generate hydrogen-bond-bridged phosphorene films with Young's modulus as high as 316GPa is reported. This value is several times larger than the theoretical values of 166GPa in the zigzag direction, 44GPa in the armchair direction, and the averaged Young's modulus among all directions of 94GPa. The impact of intercalation on mechanical properties is also explored. Experimental nanoindentation results obtained by atomic force microscopy indicate that the relationship between the ratio of intercalated ions to phosphorus atoms and the corresponding Young's modulus satisfies the formula . Furthermore, a flexible NO2 gas sensor device based on this ultratough material presents excellent performance, even after 10000 bending cycles. The results provide new insight into the potential for practical applications of black phosphorus devices.

Understanding random telegraph noise in two-dimensional BP/ReS2 heterointerface

Black phosphorus (BP)-based broken gap heterojunctions have attracted significant attention mainly owing to its wide thickness-dependent Fermi level, offering opportunities to demonstrate various carrier transport characteristics and high performing optoelectronic applications. However, the interfacial effects on the carrier scattering mechanism of the two-dimensional (2D) broken gap heterojunctions are unclear. Herein, we discuss the origin of random telegraph noise of multilayer BP/ReS2 heterojunction diode, in particular, at the direct tunneling (DT) conduction regime. The gate-tunable diode characteristic of BP/ReS2 heterojunction allows one to unveil systematically the transition of the charge fluctuation mechanism from drift-diffusion to the DT regime. Unlike individual BP and ReS2 devices, the current noise histogram obtained from the BP/ReS2 heterojunction device exhibits exclusively two dominant peaks at the DT regime. We ascribed this distinct low-frequency noise feature representing the presence of random telegraph signal to the BP/ReS2 interfacial traps by taking into account of the inherent direct tunneling current conduction mechanism. In addition, the electrostatic bias-dependent power spectrum density manifests clearly that the dominant scattering mechanism is the carrier number fluctuation rather than tunneling barrier height fluctuation at the BP/ReS2 heterointerface. This study elucidates the carrier transport and the charge fluctuation mechanism at the 2D heterostructure interface.

Lithography-free and high-efficiency preparation of black phosphorous devices by direct evaporation through shadow mask

Two-dimensional (2D) materials including black phosphorus (BP) have been extensively investigated because of their exotic physical properties and potential applications in nanoelectronics and optoelectronics. Fabricating BP based devices is challenging because BP is extremely sensitive to the external environment, especially to the chemical contamination during the lithography process. The direct evaporation through shadow mask technique is a clean method for lithography-free electrode patterning of 2D materials. Herein, we employ the lithography-free evaporation method for the construction of BP based field-effect transistors and photodetectors and systematically compare their performances with those of BP counterparts fabricated by conventional lithography and transfer electrode methods. The results show that BP devices fabricated by direct evaporation method possess higher mobility, faster response time, and smaller hysteresis than those prepared by the latter two methods. This can be attributed to the clean interface between BP and evaporated-electrodes as well as the lower Schottky barrier height of 20.2 meV, which is given by the temperature-dependent electrical results. Furthermore, the BP photodetectors exhibit a broad-spectrum response and polarization sensitivity. Our work elucidates a universal, low-cost and high-efficiency method to fabricate BP devices for optoelectronic applications.

Recent Progress in Improving the Performance of Infrared Photodetectors via Optical Field Manipulations.

Benefiting from the inherent capacity for detecting longer wavelengths inaccessible to human eyes, infrared photodetectors have found numerous applications in both military and daily life, such as individual combat weapons, automatic driving sensors and night-vision devices. However, the imperfect material growth and incomplete device manufacturing impose an inevitable restriction on the further improvement of infrared photodetectors. The advent of artificial microstructures, especially metasurfaces, featuring with strong light field enhancement and multifunctional properties in manipulating the light–matter interactions on subwavelength scale, have promised great potential in overcoming the bottlenecks faced by conventional infrared detectors. Additionally, metasurfaces exhibit versatile and flexible integration with existing detection semiconductors. In this paper, we start with a review of conventionally bulky and recently emerging two-dimensional material-based infrared photodetectors, i.e., InGaAs, HgCdTe, graphene, transition metal dichalcogenides and black phosphorus devices. As to the challenges the detectors are facing, we further discuss the recent progress on the metasurfaces integrated on the photodetectors and demonstrate their role in improving device performance. All information provided in this paper aims to open a new way to boost high-performance infrared photodetectors.

Emergence of Quantum Tunneling in Ambipolar Black Phosphorus Multilayers without Heterojunctions

AbstractNegative differential resistance (NDR) is an exotic quantum tunneling phenomenon that is exhibited in narrow p‐n junctions with heavy doping concentrations. However, the presence of multiple heterojunctions in a conventional tunneling device often hampers the observance of NDR and a deep understanding of its origin, particularly in 2D van der Waals heterojunctions. Herein, the emergence of quantum tunneling at the charge neutrality point (VCNP) in ambipolar multilayered black phosphorus (BP) transistors without heterojunctions is reported. The nearly identical electron and hole carrier densities at VCNP in the presence of a drain bias (VD) result in a lateral p‐i‐n configuration inside the BP multilayers similar to that in a tunneling field‐effect transistor. The variation of the local carrier density profile and tunneling barrier with VD at VCNP drives a sharp enhancement of the activation energy and local resistance, which consequently allows to observe band‐to‐band tunneling at up to 340 K. The enhancement of the local doping profile along the BP channel and the NDR behavior in the fabricated reconfigurable top‐gate BP device with an h‐BN top‐dielectric provide further evidence for the origin of NDR in 2D ambipolar materials.

High Photoresponse Black Phosphorus TFTs Capping with Transparent Hexagonal Boron Nitride.

Black phosphorus (BP), a single elemental two-dimensional (2D) material with a sizable band gap, meets several critical material requirements in the development of future nanoelectronic applications. This work reports the ambipolar characteristics of few-layer BP, induced using 2D transparent hexagonal boron nitride (h-BN) capping. The 2D h-BN capping have several advantages over conventional Al2O3 capping in flexible and transparent 2D device applications. The h-BN capping technique was used to achieve an electron mobility in the BP devices of 73 cm2V−1s−1, thereby demonstrating n-type behavior. The ambipolar BP devices exhibited ultrafast photodetector behavior with a very high photoresponsivity of 1980 mA/W over the ultraviolet (UV), visible, and infrared (IR) spectral ranges. The h-BN capping process offers a feasible approach to fabricating n-type behavior BP semiconductors and high photoresponse BP photodetectors.

Photocurrent Generation Mechanisms in Molybdenum‐Contacted Semiconducting Black Phosphorus and Contributions from the Photobolometric Effect

An analysis of the optoelectronic properties and photocurrent generation mechanisms in 2D multilayer crystallites of black phosphorus (BP) is conducted from 350 K down to cryogenic temperatures T of ≈6 K using a broad‐band white light source, and the results are compared to another semiconducting van der Waals solid, MoS2. Both the BP and MoS2 devices are contacted with Mo at the bottom as the electrode metal. The Mo–BP interface yields a low Schottky barrier ≈ −28.3 meV and a high photoresponsivity R of ≈2.43 × 105 A W−1 at a source‐drain bias voltage of ≈0.5 V (300 K, and incident optical power ≈3.16 μW cm−2). This article is the first to highlight the empirical use of Mo as a contact metal with BP. The high R is attributed to the low at the interfaces, in conjunction with the large effective area of a bottom contact architecture that increases the collection of photoexcited carriers and reduces thermal dissipation into the substrate from the largely suspended 2D nanodiaphragm. The results show the promise of BP to potentially advance thermoelectric and optoelectronic devices stemming from this monoelemental, direct bandgap 2D van der Waals solid.

Equilibration and filtering of quantum Hall edge states in few-layer black phosphorus

We realize p-p'-p junctions in few-layer black phosphorus (BP) devices, and use magneto-transport measurements to study the equilibration and transmission of edge states at the interfaces of regions with different charge densities. We observe both full equilibration, where all edge channels equilibrate and are equally partitioned at the interfaces, and partial equilibration, where only equilibration only takes place among modes of the same spin polarization. Furthermore, the inner p'-region with low-doping level in the junction can function as a filter for highly doped p-regions which demonstrates gate-tunable transmission of edge channels.

Surface Functionalization of Black Phosphorus with a Highly Reducing Organoruthenium Complex: Interface Properties and Enhanced Photoresponsivity of Photodetectors.

In this work, a heterostructure obtained by vacuum evaporation of a strong molecular n-dopant, [RuCp*(mes)]2 , onto black phosphorus (BP) is reported, along with the systematic investigation of the interfacial structure and properties by various in situ characterization techniques. Ultraviolet photoelectron spectra (UPS) showed a large decrease in the work function of BP and a new peak within the bandgap, which is attributed to electron transfer from dopants to the underlying BP. The electrons trapped at the interface act as hole traps and induce photogating effect so that a photodetector based on BP-organoruthenium complex heterostructure demonstrates a photoresponsivity of 5.5 mA W-1 and an EQE of 1.3 % at 515 nm, a tenfold improvement compared to the pristine BP device.

High performance near infrared photodetector based on in-plane black phosphorus p-n homojunction

Black phosphorus (BP) has been considered as an appealing candidate for broadband photodetection from the visible to the infrared part of the spectrum owing to its tunable direct band gap and high carrier mobility. However, compared with other typical materials, the photoresponsivity of BP devices is rather weak. In this work, a near infrared photodetector with high performance based on in-plane BP p-n homojunction was investigated. The strong built-in electric field of the p-n junction can be modulated through bias and gate voltage, where the photo generated electron-hole pairs can be effectively separated, thus, leads to the enhanced photocurrent and faster photoresponse. Compared with the pristine BP, the photoresponsivity of the BP p-n homojunction under 1550 nm displays a 50-fold increment. The response time of the in-plane BP p-n homojunction photodetector exhibits two orders of magnitude improvement than that of the pristine BP and other lateral homogeneous BP p-n junction. Fascinatingly, the BP p-n photodetector also displays visible-infrared dual-band detection with well separated spectral response, opening up a feasibility of the intelligent identification of infrared targets.

Sensitive Terahertz Detection and Imaging Driven by the Photothermoelectric Effect in Ultrashort-Channel Black Phosphorus Devices.

Terahertz (THz) photon detection is of particular appealing for myriad applications, but it still lags behind efficient manipulation with electronics and photonics due to the lack of a suitable principle satisfying both high sensitivity and fast response at room temperature. Here, a new strategy is proposed to overcome these limitations by exploring the photothermoelectric (PTE) effect in an ultrashort (down to 30 nm) channel with black phosphorus as a photoactive material. The preferential flow of hot carriers is enabled by the asymmetric Cr/Au and Ti/Au metallization with the titled‐angle evaporation technique. Most intriguingly, orders of magnitude field‐enhancement beyond the skin‐depth limit and photon absorption across a broadband frequency can be achieved. The PTE detector has excellent sensitivity of 297 V W−1, noise equivalent power less than 58 pW/Hz0.5, and response time below 0.8 ms, which is superior to other thermal‐based detectors at room temperature. A rigorous comparison with existing THz detectors, together with verification by further optical‐pumping and imaging experiments, substantiates the importance of the localized field effect in the skin‐depth limit. The results allow solid understanding on the role of PTE effect played in the THz photoresponse, opening up new opportunities for developing highly sensitive THz detectors for addressing targeted applications.

Tunneling devices based on graphene/black phosphorus van der Waals heterostructures

Vertically stacked devices of two-dimensional layered materials (2DLMs) based on van der Waals heterostructures (vdWHs) have recently attracted considerable attention due to their good properties. A tunneling structure is presented in this paper that, unlike other tunneling structures, has no specific insulating two-dimensional materials, such as boron nitride. The tunneling structure is comprised of graphene and black phosphorus. Black phosphorus is chemically active, and can be easily oxidized in the air to form an insulating layer. A tunneling device was produced based on this characteristic of black phosphorus. The insulation layer was an oxide layer formed by the oxidation of black phosphorus. The structure takes advantage of the easy oxidation ability of black phosphorus. The presence of a black phosphorus oxide layer was determined by XPS analysis. The tunneling characteristics of the overlay structure were determined by measuring the current-voltage (Id-V) curve of the device. Simulation studies showed that the oxidation layer was responsible for the tunneling effect. Compared with other black phosphorus devices, the photoelectric properties of the proposed device were greatly improved.

Tellurene Photodetector with High Gain and Wide Bandwidth.

Two-dimensional (2D) semiconductors have been extensively explored as a new class of materials with great potential. In particular, black phosphorus (BP) has been considered to be a strong candidate for applications such as high-performance infrared photodetectors. However, the scalability of BP thin film is still a challenge, and its poor stability in the air has hampered the progress of the commercialization of BP devices. Herein, we report the use of hydrothermal-synthesized and air-stable 2D tellurene nanoflakes for broadband and ultrasensitive photodetection. The tellurene nanoflakes show high hole mobilities up to 458 cm2/V·s at ambient conditions, and the tellurene photodetector presents peak extrinsic responsivity of 383 A/W, 19.2 mA/W, and 18.9 mA/W at 520 nm, 1.55 μm, and 3.39 μm light wavelength, respectively. Because of the photogating effect, high gains up to 1.9 × 103 and 3.15 × 104 are obtained at 520 nm and 3.39 μm wavelength, respectively. At the communication wavelength of 1.55 μm, the tellurene photodetector exhibits an exceptionally high anisotropic behavior, and a large bandwidth of 37 MHz is obtained. The photodetection performance at different wavelength is further supported by the corresponding quantum molecular dynamics (QMD) simulations. Our approach has demonstrated the air-stable tellurene photodetectors that fully cover the short-wave infrared band with ultrafast photoresponse.

First principles modeling of pure black phosphorus devices under pressure.

Black phosphorus (BP) has a pressure-dependent bandgap width and shows the potential for applications as a low-dimensional pressure sensor. We built two kinds of pure BP devices with zigzag or armchair conformation, and explored their pressure-dependent conductance in detail by using first principles calculations. The zigzag BP devices and the armchair BP devices exhibit different conductance–pressure relationships. For the zigzag BP devices conductance is robust against stress when the out-of-plane pressure ratio is less than 15%, and then increases rapidly until the conductive channels are fully opened. For the armchair pure BP devices conductance decreases at first by six orders of magnitude under increasing pressure and then increases quickly with further increase of pressure until the devices enter the on-state. This shows that the pure zigzag BP devices are more suitable for the application as flexible electronic devices with almost constant conductance under small pressure, while armchair BP devices can serve as bidirectional pressure sensors. Real-space distributions of band alignments were explored to understand the different pressure-related properties. We fitted a set of parameters based on the results from the empirical Wentzel–Kramers–Brillouin method, which provides an effortless approximation to quantitatively predict the pressure-related behaviors of large pure BP devices.

High-speed black phosphorus field-effect transistors approaching ballistic limit.

As a strong candidate for future electronics, atomically thin black phosphorus (BP) has attracted great attention in recent years because of its tunable bandgap and high carrier mobility. Here, we show that the transport properties of BP device under high electric field can be improved greatly by the interface engineering of high-quality HfLaO dielectrics and transport orientation. By designing the device channels along the lower effective mass armchair direction, a record-high drive current up to 1.2 mA/μm at 300 K and 1.6 mA/μm at 20 K can be achieved in a 100-nm back-gated BP transistor, surpassing any two-dimensional semiconductor transistors reported to date. The highest hole saturation velocity of 1.5 × 107 cm/s is also achieved at room temperature. Ballistic transport shows a record-high 36 and 79% ballistic efficiency at room temperature and 20 K, respectively, which is also further verified by theoretical simulations.

Tunable Schottky barriers in ultrathin black phosphorus field effect transistors via polymer capping

It is still a great challenge to avoid the degradation of ultrathin black phosphorus (BP) since its discovery in 2014. Various methods have been explored to stabilize the properties of ultrathin BP through capping technology or chemical passivation. Besides, the large metal–semiconductor contact resistance is also one of the critical issues. The two problems hinder the further development of ultrathin BP devices. Herein, we demonstrate that polymethyl methacrylate (PMMA) capping can not only enhance the durability of the ultrathin BP effectively and nondestructively, but also tune the effective Schottky barriers (SBs) formed at the interfaces between the metal and semiconductor dramatically. Particularly, the Schottky barrier (SB) for electron injection from metal to semiconductor is decreased by ~13 meV and the performance of the BP field effect transistor (FET) is strongly enhanced with the current on/off ratio increased by 6.8 times for the hole conduction after the PMMA capping. In addition, after the electron beam irradiation to the PMMA layer, the charge neutral point of the BP FET exhibits remarkable negative shift resulting in the electron dominated semiconductor channel at zero gate voltage. Furthermore, through partially capping the BP channel, a prototype of BP p-n diode was demonstrated with a maximum rectification factor of 21.3. The diode performs quite well with just a quarter of the BP channel capped by the PMMA layer. Our findings suggest that the PMMA capped ultrathin BP would be a promising choice for future device applications.