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Caractérisation du phosphore noir pour des applications optoélectroniques hyperfréquences
Abstract
Les dispositifs à base de silicium, industrialisés aujourd’hui pour les systèmes électroniques, atteignent leurs limites en termes de miniaturisation et de performances. La course à l’innovation et à la miniaturisation vise aujourd’hui à dépasser cette limite en intégrant de nouveaux matériaux dans les dispositifs, en couplant d’autres phénomènes physiques de l’optique à l’électronique haute fréquence. Le travail conduit pendant cette thèse porte sur la caractérisation du phosphore noir (bP) pour des applications dans le domaine de l'optoélectronique hyperfréquence avec une application spécifique aux interrupteurs microondes pilotés optiquement à 1,55 µm. La caractérisation du bP passe par le développement de techniques de fabrication de couches bidimensionnels de bP et également par la détermination de l'influence des matériaux annexes utilisés sur les propriétés de la couche. Cela a été couplé à une étude optique pour connaitre la réponse du bP à une excitation laser à 1,55 µm. La détermination de paramètres intrinsèques spécifiques du matériau tels que le temps de vie des photoporteurs, la résistivité et la permittivité a été conduite par l'intermédiaire d'expériences de caractérisation dans le domaine optique, radiofréquence et électronique (DC). Les résultats obtenus confirment l’intérêt du bP pour ce genre d'application et ont permis l'intégration du matériau dans le dispositif hyperfréquence visé. Les résultats obtenus lors de tests préliminaires présentés dans ce mémoire sont très encourageants et ouvre la voie à de nombreuses applications ultra-rapide à haute fréquence.
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Interest in two-dimensional (2D) materials is growing exponentially across various scientific and engineering disciplines owing to their fascinating electrical, optical, chemical, and thermal properties. As one of the newest members in the 2D nanomaterial family, black phosphorus (BP), has recently attracted much attention all over the world and shown great potential in novel nanoelectronics owing to its direct and narrow bandgap. Regarding to the scale production of BP and its related electronic devices, liquid-phase exfoliation reveals superior advances when compared with mechanical exfoliation. Remarkably, liquid-phase exfoliated BP flakes and quantum dots (QDs) exhibit exciting properties in electronic, energy storage, sensors, solar cells, photothermal effect, cancer theranostics and photonic devices. In this article, we discuss the exfoliation of BP in diverse solvents and extend the topic from BP flakes to QDs and summarize the significant advances recently made. We also outline future prospects of solution exfoliated BP nanomaterials, and comment on the challenges that need to be overcome for future applications.
We report on a scanning confocal Raman spectroscopy study investigating the strain-uniformity and the overall strain and doping of high-quality chemical vapour deposited (CVD) graphene-based heterostuctures on a large number of different substrate materials, including hexagonal boron nitride (hBN), transition metal dichalcogenides, silicon, different oxides and nitrides, as well as polymers. By applying a hBN-assisted, contamination free, dry transfer process for CVD graphene, high-quality heterostructures with low doping densities and low strain variations are assembled. The Raman spectra of these pristine heterostructures are sensitive to substrate-induced doping and strain variations and are thus used to probe the suitability of the substrate material for potential high-quality graphene devices. We find that the flatness of the substrate material is a key figure for gaining, or preserving high-quality graphene.
Neither of the two typical two-dimensional materials, graphene and single layer MoS 2 , are good enough for developing semiconductor logical devices. We calculated the electron mobility of 14 two-dimensional semiconductors with composition of MX 2 , where M (=Mo, W, Sn, Hf, Zr and Pt) are transition metals, and Xs are S, Se and Te. We approximated the electron phonon scattering matrix by deformation potentials, within which long wave longitudinal acoustical and optical phonon scatterings were included. Piezoelectric scattering in the compounds without inversion symmetry is also taken into account. We found that out of the 14 compounds, WS 2 , PtS 2 and PtSe 2 are promising for logical devices regarding the possible high electron mobility and finite band gap. Especially, the phonon limited electron mobility in PtSe 2 reaches about 4000 cm 2 ·V - 1 ·s - 1 at room temperature, which is the highest among the compounds with an indirect bandgap of about 1.25 eV under the local density approximation. Our results can be the first guide for experiments to synthesize better two-dimensional materials for future semiconductor devices.
An effective anisotropic tight-binding model is developed to analytically describe the low-energy electronic structure and optical response of phosphorene (a black phosphorus (BP) monolayer). Within the framework of the model, we derive explicit closed-form expressions, in terms of elementary functions, for the elements of the optical conductivity tensor of phosphorene. These relations provide a convenient parametrization of the highly anisotropic optical response of phosphorene, which allows the reflectance, transmittance, and absorbance of this material to be easily calculated as a function of the frequency of the incident radiation at arbitrary angles of incidence. The results of such a calculation are presented for both a free-standing phosphorene layer and the phosphorene layer deposited on a SiO 2 > substrate, and for the two principal cases of polarization of the incident radiation either parallel to or normal to the plane of incidence. Our findings (e.g., a ‘quasi-Brewster’ effect in the reflectance of the phosphorene/ SiO 2 > overlayer system) pave the way for developing a new, purely optical method of distinguishing BP monolayers.
In this paper, we show that a MoS2thin film formed from a mixture of pristine MoS2 monolayers and few-layer flakes deposited on a coplanar waveguide (CPW) is acting as an electrically tunable microwavematerial. In this respect, we have seen that up to 30 GHz, the transmission and reflection parameters of the CPW depend on the applied voltage. We have extracted from the measurements an equivalent circuit and have observed that the surface resistance is dependent on the DC applied voltage, as in the case of other two-dimensional materials such as graphene. So, the device is acting as a tunable matching network via an applied DC voltage.
The striking in-plane anisotropy remains one of the most intriguing properties for the newly rediscovered black phosphorus (BP) two-dimensional (2D) crystals. However, due to its rather low energy band gap, the optical anisotropy of few-layer BP has been primarily investigated in the near-infrared (NIR) regime. Moreover, the essential physics that determines the intrinsic anisotropic optical properties of few-layer BP, which is of great importance for practical applications in optical and optoelectronic devices, is still in the fancy of theory. Herein, we report the direct observation of the optical anisotropy of few-layer BP in the visible regime simply by using polarized optical microscopy. Based on the Fresnel equation, the intrinsic anisotropic complex refractive indices (n-iκ) in the visible regime (480 nm-650 nm) were experimentally obtained for the first time using the anisotropic optical contrast spectra. Our findings not only provide a convenient approach to measure the optical constants of 2D layered materials, but also suggest a possibility to design novel BP-based photonic devices, such as atomic-thick light modulator, including linear polarizer, phase plate and optical compensator in a broad spectral range extending to the visible window.
Black phosphorus has attracted significant attention due to its exceptional
electronic and optoelectronic properties, but its poor stability under ambient
conditions limits its use in applications. Despite several previous studies,
the degradation mechanism and especially the role of the different reactive
species present in air (oxygen, water vapor) still remains unclear. Here, we
systematically studied the degradation process of black phosphorus in different
environments (air; water with dissolved O2; and deaerated water) by optical
microscopy, analytical transmission electron microscopy, field-effect
transistor (FET) device characteristics, and electrochemical characterization.
Whereas the materials and devices exposed to air and oxygen-saturated water
showed signs of rapid degradation, immersion in deaerated water maintained
relatively stable morphologies and device characteristics. This enabled the use
of water as a solution top gate in FETs, which substantially enhanced the
charge carrier mobility from 230 cm2/Vs (back-gated in air) to 1260 cm2/Vs. Our
results, which show that oxygen plays the dominant role in the chemical
conversion of black phosphorus and that the material is stable in oxygen-free
water pave the way for its use in aqueous solutions (e.g., in photo-catalysis
or electrochemistry) and can guide future efforts to identify suitable
encapsulation strategies for black phosphorus for potential applications in
air.
Black phosphorus (BP) has recently been rediscovered as a new and interesting two-dimensional material due to its unique electronic and optical properties. Here, we study the linear and nonlinear optical properties of BP flakes. We observe that both the linear and nonlinear optical properties are anisotropic and can be tuned by the film thickness in BP, completely different from other typical two-dimensional layered materials (e.g., graphene and the most studied transition metal dichalcogenides). We then use the nonlinear optical properties of BP for ultrafast (pulse duration down to ∼786 fs in mode-locking) and large-energy (pulse energy up to >18 nJ in Q-switching) pulse generation in fiber lasers at the near-infrared telecommunication band ∼1.5 μ. We observe that the output of our BP based pulsed lasers is linearly polarized (with a degree-of-polarization ∼98% in mode-locking, >99% in Q-switching, respectively) due to the anisotropic optical property of BP. Our results underscore the relatively large optical nonlinearity of BP with unique polarization and thickness dependence, and its potential for polarized optical pulse generation, paving the way to BP based nonlinear and ultrafast photonic applications (e.g., ultrafast all-optical polarization switches/modulators, frequency converters etc.).
Isolating large-areas of atomically thin transition metal chalcogenide crystals is an important but challenging task. The mechanical exfoliation technique can provide single layers of the highest structural quality, enabling to study their pristine properties and ultimate device performance. However, a major drawback of the technique is the low yield and small (typically <10 um) lateral size of the produced single layers. Here, we report a novel mechanical exfoliation technique, based on chemically enhanced adhesion, yielding MoS2single layers with typical lateral sizes of several hundreds of microns. The idea is to exploit the chemical affinity of the sulfur atoms that can bind more strongly to a gold surface than the neighboring layers of the bulk MoS2 crystal. Moreover, we found that our exfoliation process is not specific to MoS2, but can be generally applied for various layered chalcogenides including selenites and tellurides, providing an easy access to large-area 2D crystals for the whole class of layered transition metal chalcogenides.
Recent work has demonstrated excellent p-type field-effect switching in exfoliated black phosphorus, but type control has remained elusive. Here, we report unipolar n-type black phosphorus transistors with switching polarity control via contact-metal engineering and flake thickness, combined with oxygen and moisture-free fabrication. With aluminium contacts to black phosphorus, a unipolar to ambipolar transition occurs as flake thickness increases from 3 to 13 nm. The 13-nm aluminium-contacted flake displays graphene-like symmetric hole and electron mobilities up to 950 cm(2) V(-1) s(-1) at 300 K, while a 3 nm flake displays unipolar n-type switching with on/off ratios greater than 10(5) (10(7)) and electron mobility of 275 (630) cm(2) V(-1) s(-1) at 300 K (80 K). For palladium contacts, p-type behaviour dominates in thick flakes, while 2.5-7 nm flakes have symmetric ambipolar transport. These results demonstrate a leap in n-type performance and exemplify the logical switching capabilities of black phosphorus.
Thin layers of black phosphorus have recently raised interest owing to their two-dimensional (2D) semiconducting properties, such as tunable direct bandgap and high carrier mobilities. This lamellar crystal of phosphorus atoms can be exfoliated down to monolayer 2D-phosphane (also called phosphorene) using procedures similar to those used for graphene. Probing the properties has, however, been challenged by a fast degradation of the thinnest layers on exposure to ambient conditions. Herein, we investigate this chemistry using in situ Raman and transmission electron spectroscopies. The results highlight a thickness-dependent photoassisted oxidation reaction with oxygen dissolved in adsorbed water. The oxidation kinetics is consistent with a phenomenological model involving electron transfer and quantum confinement as key parameters. A procedure carried out in a glove box is used to prepare mono-, bi- and multilayer 2D-phosphane in their pristine states for further studies on the effect of layer thickness on the Raman modes. Controlled experiments in ambient conditions are shown to lower the Ag(1)/Ag(2) intensity ratio for ultrathin layers, a signature of oxidation.
We present polarization-resolved transient transmission measurements on multi-layer black phosphorus. Background free two-color pump-probe spectroscopy
measurements are carried out on mechanically exfoliated black phosphorus flakes that have been transferred to a large-bandgap, silicon
carbide substrate. The blue-shifted pump pulse (780 nm) induces an increased transmission of the probe pulse (1560 nm) over a time scale commensurate with the measurement resolution (hundreds of fs). After the initial pump-induced transparency, the sign of the transient flips and a slower enhanced absorption is observed. This extended absorption is characterized by two relaxation time scales of 180 ps and 1.3 ns. The saturation peak is attributed to Pauli blocking while the extended absorption is ascribed to a Drude response of the pump-induced carriers. The anisotropic
carrier mobility in the black phosphorus leads to different weights of the Drude absorption, depending on the probe polarization, which is readily observed in the amplitude of the pump-probe signals.
This article reviews the potential of graphene and related two-dimensional (2D) materials for applications in micro- and nanoelectronics. In addition to graphene, special emphasis is placed on transition metal dichalcogenides (TMDs). First, we discuss potential solutions for application-scale material growth, in particular chemical vapor deposition. We describe challenges for electrical contacts and dielectric interfaces with 2D materials. The device-related sections in this review first weigh the pros and cons of semi-metal graphene as a field-effect transistor (FET) channel material for logic and radio frequency applications. This is followed by an introduction to alternate graphene switch concepts that utilize the particular properties of the material, namely tunnel FETs, vertical devices, and bilayer pseudospin FETs. The final section is dedicated to semiconducting TMDs and their integration in FETs using the examples of n-type molybdenum disulfide (MoS2) and p-type tungsten diselenide (WSe2).
Black phosphorus (bP) is the second known elemental allotrope with a layered
crystal structure that can be mechanically exfoliated down to atomic layer
thickness. We have fabricated bP naked quantum wells in a back-gated field
effect transistor geometry with bP thicknesses ranging from nm to
nm. Using an encapsulating polymer superstrate, we have suppressed bP
oxidation and have observed field effect mobilities up to 600 cm/Vs and
on/off current ratios exceeding . Importantly, Shubnikov-de Haas (SdH)
oscillations observed in magnetotransport measurements up to 35 T reveal the
presence of a 2-D hole gas with Schr\"odinger fermion character in an
accumulation layer at the bP/oxide interface. Our work demonstrates that 2-D
electronic structure and 2-D atomic structure are independent. 2-D carrier
confinement can be achieved in layered semiconducting materials without
necessarily approaching atomic layer thickness, advantageous for materials that
become increasingly reactive in the few-layer limit such as bP.
Few-layer black phosphorus (BP), also known as phosphorene, is poised to be
the most attractive graphene analogue owing to its high mobility approaching
that of graphene, and its thickness- tunable band gap that can be as large as
that of molybdenum disulfide. In essence, phosphorene represents the much
sought after high-mobility, large direct band gap two-dimensional layered
crystal that is ideal for optoelectronics and flexible devices. However, its
instability in air is of paramount concern for practical applications. Here, we
demonstrate air-stable BP devices with dielectric and hydrophobic
encapsulation. Microscopy, spectroscopy, and transport techniques were employed
to elucidate the aging mechanism, which can initiate from the BP surface for
bare samples, or edges for samples with thin dielectric coating highlighting
the ineffectiveness of conventional scaled dielectrics. Our pioneering
months-long studies indicate that a double layer of Al2O3 and hydrophobic
fluoropolymer affords BP devices and transistors with indefinite air-stability
for the first time, overcoming a critical material challenge for applied
research and development.
Single- and few-layer crystals of exfoliated MoTe2 have been characterized spectroscopically by photoluminescence, Raman scattering, and optical absorption measurements. We find that MoTe2 in the monolayer limit displays strong photoluminescence. Based on complementary optical absorption results, we conclude that monolayer MoTe2 is a direct-gap semiconductor, with an optical band-gap of 1.10 eV. This new monolayer material extends the spectral range of atomically thin direct-gap materials from the visible to the near infrared.
This letter evaluates temporal and thermal stability of a state-of-the-art
few-layer phosphorene MOSFET with Al2O3 surface passivation and Ti/Au top gate.
As fabricated, the phosphorene MOSFET was stable in atmosphere for at least 100
h. With annealing at 200{\deg}C in dry nitrogen for 1 h, its drain current
increased by an order of magnitude to approximately 100 mA/mm, which could be
attributed to the reduction of trapped charge in Al2O3 and/or Schottky barrier
at the source and drain contacts. Thereafter, the drain current was stable
between -50{\deg}C and 150{\deg}C up to at least 1000 h. These promising
results suggest that environmental protection of phosphorene should not be a
major concern, and passivation of phosphorene should focus on its effect on
electronic control and transport as in conventional silicon MOSFETs. With
cutoff frequencies approaching the gigahertz range, the present phosphorene
MOSFET, although far from being optimized, can meet the frequency and stability
requirements of most flexible electronics for which phosphorene is
intrinsically advantageous due to its corrugated lattice structure.
Using first-principles calculations, we study the electronic properties of
few-layer phosphorene focusing on layer-dependent behavior of band gap, work
function and band alignment and carrier effective mass. It is found that
few-layer phosphorene shows a robust direct band gap character, and its band
gap decreases with the number of layers following a power law. The work
function decreases rapidly from monolayer (5.16 eV) to trilayer (4.56 eV), and
then slowly upon further increasing the layer number. Compared to monolayer
phosphorene, there is a drastic decrease of hole effective mass along the ridge
(zigzag) direction for bilayer phosphorene, indicating a strong interlayer
coupling and screening effect. Our study suggests that 1). Few-layer
phosphorene with a layer-dependent band gap and a robust direct band gap
character is promising for efficient solar energy harvest. 2). Few-layer
phosphorene outperforms monolayer counterpart in terms of a lighter carrier
effective mass, a higher carrier density and a weaker scattering due to
enhanced screening. 3). The layer-dependent band edges and work functions of
few-layer phosphorene allow for modification of Schottky barrier with enhanced
carrier injection efficiency. It is expected that few-layer phosphorene will
present abundant opportunities for a plethora of new electronic applications.
Layered two-dimensional materials have shown novel optoelectronic properties
and are well suited to be integrated in planar photonic circuits. For example,
graphene has been utilized as a wideband photodetector. However, because it
lacks a band gap, graphene photodetectors suffer from extremely high dark
current8. In contrast, few-layer black phosphorous, the latest addition to the
family of 2D materials, is well-suited for photodetector applications due to
its narrow but finite energy gap. Here, we demonstrate a gated multilayer black
phosphorus photodetector integrated in a silicon photonic waveguide operating
in the near-infrared telecom band. In a significant advantage over graphene
devices, black phosphorus photodetectors can operate under a bias with very low
dark current and attain intrinsic responsivity up to 125 mA/W and 657 mA/W in
11.5nm and 100 nm thick devices, respectively. The photocurrent is dominated by
the photovoltaic effect with a high response bandwidth exceeding 3 GHz.
Graphene and transition metal dichalcogenides (TMDCs) are the two major types of layered materials under intensive investigation. However, the zero-bandgap nature of graphene and the relatively low mobility in TMDCs limit their applications. Here we reintroduce black phosphorus (BP), the most stable allotrope of phosphorus with strong intrinsic in-plane anisotropy, to the layered-material family. For 15-nm-thick BP, we measure a Hall mobility of 1,000 and 600 cm(2 )V(-1 )s(-1) for holes along the light (x) and heavy (y) effective mass directions at 120 K. BP thin films also exhibit large and anisotropic in-plane optical conductivity from 2 to 5 μm. Field-effect transistors using 5 nm BP along x direction exhibit an on-off current ratio exceeding 10(5), a field-effect mobility of 205 cm(2 )V(-1 )s(-1), and good current saturation characteristics all at room temperature. BP shows great potential for thin-film electronics, infrared optoelectronics and novel devices in which anisotropic properties are desirable.
Two-dimensional crystals are emerging materials for nanoelectronics. Development of the field requires candidate systems with both a high carrier mobility and, in contrast to graphene, a sufficiently large electronic bandgap. Here we present a detailed theoretical investigation of the atomic and electronic structure of few-layer black phosphorus (BP) to predict its electrical and optical properties. This system has a direct bandgap, tunable from 1.51 eV for a monolayer to 0.59 eV for a five-layer sample. We predict that the mobilities are hole-dominated, rather high and highly anisotropic. The monolayer is exceptional in having an extremely high hole mobility (of order 10,000 cm(2) V(-1) s(-1)) and anomalous elastic properties which reverse the anisotropy. Light absorption spectra indicate linear dichroism between perpendicular in-plane directions, which allows optical determination of the crystalline orientation and optical activation of the anisotropic transport properties. These results make few-layer BP a promising candidate for future electronics.
We investigate electrical transport and optoelectronic properties of field effect transistors
(FETs) made from few-layer black phosphorus (BP) crystals down to a few nanometers. In
particular, we explore the anisotropic nature and photocurrent generation mechanisms in BP
FETs through spatial-, polarization-, gate-, and bias-dependent photocurrent measurements.
Our results reveal that the photocurrent signals at BP-electrode junctions are mainly attributed
to the photovoltaic effect in the off-state and photothermoelectric effect in the on-state, and
their anisotropic feature primarily results from the directional-dependent absorption of BP
crystals.
Phosphorene is a unique single elemental semiconductor with two-dimensional
layered structures. In this letter, we study the transistor behavior on
mechanically exfoliated few-layer phosphorene with the top-gate. We achieve a
high on-current of 144 mA/mm and hole mobility of 95.6 cm2/Vs. We deposit Al2O3
by atomic layer deposition (ALD) and study the effects of dielectric capping.
We observe that the polarity of the transistors alternated from p-type to
ambipolar with Al2O3 grown on the top. We attribute this transition to the
changes for the effective Schottky barrier heights for both electrons and holes
at the metal contact edges, which is originated from fixed charges in the ALD
dielectric.
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Single-layer, large-scale two-dimensional material growth is still a challenge for their wide-range usage. Therefore, we carried out a comprehensive study of monolayer MoS 2 growth by CVD investigating the influence of growth zone configuration and precursors ratio. We first compared the two commonly used approaches regarding the relative substrate and precursor positions, namely, horizontal and face-down configurations where face-down approach is found to be more favorable to obtain larger flakes under identical growth conditions. Secondly, we used different types of substrate holders to investigate the influence of the Mo and S vapor confinement on the resulting diffusion environment. We suggest that local changes of the S to Mo vapor ratio in the growth zone is a key factor for the change of shape, size and uniformity of the resulting MoS 2 formations, which is also confirmed by performing depositions under different precursor ratios. Therefore, to obtain continuous monolayer films, the S to Mo vapor ratio is needed to be kept within a certain range throughout the substrate. As a conclusion, we obtained monolayer triangles with a side length of 90 µm and circles with a diameter of 500 µm and continuous films with an area of 850 µm × 1 cm when the S-to-Mo vapor ratio is optimized.
Complément pratique de l'ouvrage "Physique des semiconducteurs et des composants électroniques". - 2e et 3e cycles, écoles d'ingénieurs
A great interest has been lately initiated in the optoelectronics field for 2D materials with a tunable bandgap. Being able to choose the bandgap of a material is a huge progress in optoelectronics, since it would permit to overcome the limitation imposed by the graphene lack of energy bandgap, but also the restriction imposed by already used semiconductor whose bandgap are fixed and cannot apply for IR-NIR applications. From DFT simulations predictions, Black Phosphorus (bP) becomes a bidimensional semiconducting material with a direct tunable energy bandgap from 0.3 eV to 2 eV by controlling number of layers. This material also has a picosecond carrier response and exceptional mobilities under external excitation. Hence black phosphorus is a promising 2D material candidate for photoconductive switching under a NIR optical excitation as in telecommunication wavelength range of 1.55 μm. In this paper, material electromagnetic properties analysis is described in a large frequency band from optical to microwave measurements executed on different samples allowing energy bandgap and work function dependency to fabrication techniques, anisotropy and multiscale optoelectronic device realization by switch contact engineering and material passivation or encapsulation. Material implementation in microwave devices opens the route to new broadband electronic functionalities triggered by optics, thanks to light/matter extreme confinement degree. In this paper we present fabrication method of bP based microwave photoconductive switch, with a focus on black phosphorus Raman characterization, and obtained performances.
Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) and black phosphorus (BP) have beneficial electronic, optical, and physical properties at the few-layer limit. As atomically thin materials, 2D TMDCs and BP are highly sensitive to their environment and chemical modification, resulting in a strong dependence of their properties on substrate effects, intrinsic defects, and extrinsic adsorbates. Furthermore, the integration of 2D semiconductors into electronic and optoelectronic devices introduces unique challenges at metal-semiconductor and dielectric-semiconductor interfaces. Here, we review emerging efforts to understand and exploit chemical effects to influence the properties of 2D TMDCs and BP. In some cases, surface chemistry leads to significant degradation, thus necessitating the development of robust passivation schemes. On the other hand, appropriately designed chemical modification can be used to beneficially tailor electronic properties, such as controlling doping levels and charge carrier concentrations. Overall, chemical methods allow substantial tunability of the properties of 2D TMDCs and BP, thereby enabling significant future opportunities to optimize performance for device applications.
Gold-mediated exfoliation of ultralarge optoelectronically-perfect monolayers with lateral dimensions up to ≈500 μm is reported. Electrical, optical, and X-ray photo-electron spectroscopy characterization show that the quality of the gold-exfoliated flakes is similar to that of tape-exfoliated flakes. Large-area flakes allow manufacturing of large-area mono-layer transition metal dichalcogenide electronics.
The interaction between phosphorene and SiO2, Al2O3, and h-BN surfaces has been analyzed by first principles calculations. Our work demonstrates that phosphorene forms strong bonds with O-terminated SiO2 (0001) and Al- And O-terminated Al2O3 (0001) surfaces, and the structure of phosphorene changes drastically. We find that phosphorene adsorbs on the h-BN surface through van der Waals interactions. The 2D planar structure of h-BN is free of dangling bonds, which provides an ideal substrate for phosphorene to sit on. The bandgap of the phosphorene/h-BN system monotonically decreases with increasing vertical electric fields. Asemiconductor-to-metal transition occurs at about 6 V nm-1. The calculations suggest that phosphorene /h-BN heterostructures could provide a viable route to phosphorene-based electronic devices.
We present a microwave photoconductive switch based on exfoliated black phosphorus and strongly responding to a 1.55 lm optical excitation. According to its number of atomic layers, exfoliated black phosphorus presents unique properties for optoelectronic applications, like a tunable direct bandgap from 0.3 eV to 2 eV, strong mobilities, and strong conductivities. The switch shows a maximum ON/OFF ratio of 17 dB at 1 GHz, and 2.2 dB at 20GHz under 1.55-lm laser excitation at 50 mW, never achieved with bidimensional materials.
A laser diode integrated with a spot-size converter (SS-LD) and a waveguide photodiode (WGPD) have been developed for low cost optical modules. These devices involve a mechanism that matches their optical field with fiber or planar lightwave circuits (PLC). This improves the alignment tolerance of the optical coupling and allows the easy passive alignment approach. These devices were fabricated on a two-inch substrate using MOVPE and dry etching which were all fit for mass production. This reduces the cost of the devices. These devices offer high coupling efficiency to the single mode fiber and high reliability, so that they can be widely implemented in coming advanced optical networks.
Ambipolar black phosphorus MOSFETs with record n-channel extrinsic
transconductance are reported. The devices consist of multi-layer black
phosphorus aligned to a local back-gate electrode with 10-nm-thick HfO2 gate
dielectric. Before passivation, devices with 0.3-um gate length behaved as
p-MOSFETs with peak extrinsic transconductance, gm, of 282 uS/um at VDS = -2 V.
After passivation, the same devices displayed ambipolar behavior, and when
tested as n-MOSFETs, had peak gm = 66 uS/um at VDS = +2 V, and similar devices
on the same wafer had gm as high as 80 uS/um. These results are an important
step toward realization of high-performance black phosphorus complementary
logic circuits.
We report the preparation of thickness-controlled few-layer black phosphorus (BP) films through the modulated plasma treatment of BP flakes. Not only does the plasma treatment control the thickness of the BP film, it also removes the chemical degradation of the exposed oxidized BP surface, which results in enhanced field-effect transistor (FET) performance. Our fabricated BP FETs were passivated with polymethyl methacrylate (PMMA) immediately after the plasma etching process. With these techniques, a high field-effect mobility was achieved, 1150 cm2/Vs, with an Ion/Ioff ratio of ~100,000 at room temperature. Furthermore, a fabricated FET with plasma-treated few-layer BP that was passivated with PMMA was found to retain its I-V characteristics and thus to exhibit excellent environmental stability over several weeks.
We show that black phosphorus has room-temperature charge mobilities on the
order of 10 cmVs, which are about one order of magnitude
larger than silicon. We also demonstrate strong anisotropic transport in black
phosphorus, where the mobilities along the armchair direction are about one
order of magnitude larger than zigzag direction. A photocarrier lifetime as
long as 100 ps is also determined. These results illustrate that black
phosphorus is a promising candidate for future electronic and optoelectronic
applications.
Black phosphorus has recently emerged as a promising material for high
performance electronic and optoelectronic device for its high mobility, tunable
mid-infrared bandgap and anisotropic electronic properties. Dynamical evolution
of photo excited carriers and its induced change of transient electronic
properties are critical for materials' high field performance, but remains to
be explored for black phosphorus. In this work, we perform angle resolved
transient reflection spectroscopy to study the dynamical evolution of
anisotropic properties of black phosphorus under photo excitation. We find that
the anisotropy of reflectivity is enhanced in the pump induced
quasi-equilibrium state, suggesting an extraordinary enhancement of the
anisotropy in dynamical conductivity in hot carrier dominated regime. These
results raise enormous possibilities of creating high field, angle sensitive
electronic, optoelectronic and remote sensing devices exploiting the dynamical
electronic anisotropic with black phosphorus.
Anisotropic materials are characterized by unique optical response which is highly polarization dependent. Of particular interest are layered materials formed by the stacking of two-dimensional (2D) crystals which are naturally anisotropic in the direction perpendicular to the 2D planes. Black phosphorus (BP) is a stack of 2D phosphorene crystals, a highly anisotropic semiconductor with a direct band gap. We show that the angular dependence of polarized Raman spectra of BP is rather unusual, and can be explained only by considering complex values for the Raman tensor elements. This result can be traced back to the electron-photon and electron-phonon interactions in this material.
Unencapsulated, exfoliated black phosphorus (BP) flakes are found to chemically degrade upon exposure to ambient conditions. Atomic force microscopy, electrostatic force microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy are employed to characterize the structure and chemistry of the degradation process, suggesting that O2 saturated H2O irreversibly reacts with BP to form oxidized phosphorus species. This interpretation is further supported by the observation that BP degradation occurs more rapidly on hydrophobic octadecyltrichlorosilane self-assembled monolayers and on H-Si(111), versus hydrophilic SiO2. For unencapsulated BP field-effect transistors, the ambient degradation causes large increases in threshold voltage after 6 hours in ambient, followed by a ~103 decrease in FET current on/off ratio and mobility after 48 hours. Atomic layer deposited AlOx overlayers effectively suppress ambient degradation, allowing encapsulated BP FETs to maintain high on/off ratios of ~10^3 and mobilities of ~100 cm2/V-s for over two weeks in ambient. This work shows that the ambient degradation of BP can be managed effectively when the flakes are sufficiently passivated. In turn, our strategy for enhancing BP environmental stability will accelerate efforts to implement BP in electronic and optoelectronic applications.
In this article, we demonstrate enhanced electron and hole transport in few layer phosphorene field effect transistors (FETs) using Titanium as the source/drain contact electrode and 20nm SiO2 as the back gate dielectric. The field effect mobility values were extracted to be ~38cm2/Vs for electrons and ~172cm2/Vs for the holes. Based on our experimental data we also comprehensively discuss how the contact resistances arising due to the Schottky barriers at the source and the drain end effect the different regime of the device characteristics and ultimately limit the ON state performance. We also propose and implement a novel technique for extracting the transport gap as well as the Schottky barrier height at the metal-phosphorene contact interface from the ambipolar transfer characteristics of the phosphorene FETs. This robust technique is applicable to any ultra-thin body semiconductor which demonstrates symmetric ambipolar conduction. Finally, we demonstrate a high gain, high noise margin, chemical doping free and fully complementary logic inverter based on ambipolar phosphorene FETs.
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 film offers a
moderate bandgap of around 0.3 eV and high carrier mobility, leading to
transistors with decent on-off ratio and high on-state current density. Here,
we demonstrate the gigahertz frequency operation of black phosphorus
field-effect transistors for the first time. The BP transistors demonstrated
here show excellent current saturation with an on-off ratio exceeding 2000. 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 cut-off frequency fT of 12
GHz and a maximum oscillation frequency fmax 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 enabling
the future ubiquitous transistor technology that can operate in the multi-GHz
frequency range and beyond.
Thin-film transistors (TFTs) matured later than silicon integrated circuits, but in the past 15 years the technology has grown into a huge industry based on display applications, with amorphous and polycrystalline silicon as the incumbent technology. Recently, an intense search has developed for new materials and new fabrication techniques that can improve the performance, lower manufacturing cost, and enable new functionality. There are now many new options – organic semiconductor (OSCs), metal oxides, nanowires, printing technology as well as thin-film silicon materials with new properties. All of the new materials have something to offer but none is entirely without technical problems.
Phosphorene is a new family member of two-dimensional materials. We observed
strong and highly layer-dependent photoluminescence in few-layer phosphorene (2
to 5 layers). The results confirmed the theoretical prediction that few-layer
phosphorene has a direct and layer-sensitive band gap. We also demonstrated
that few-layer phosphorene is more sensitive to temperature modulation than
graphene and MoS2 in Raman scattering. The anisotropic Raman response in
few-layer phosphorene has enabled us to use an optical method to quickly
determine the crystalline orientation without tunneling electron microscope
(TEM) or scanning tunneling microscope (STM). Our results provide much needed
experimental information about the band structures and exciton nature in
few-layer phosphorene, paving the way for various optoelctronic and electronic
applications.