Field-effect Transistor Measurements Research Articles

In situ tracking anisotropic photocarrier dynamics in two-dimensional ternary Ta2NiSe5 via digital micromirror device-based pump-probe microscopy

In two-dimensional (2D) material studies, tracking the anisotropic ultrafast carrier dynamics is essential for the development of optoelectronic nano-devices. Conventionally, the anisotropic optical and electronic properties are investigated via either polarization-dependent Raman spectroscopy or field-effect transistors measurements. However, study of the anisotropic transient carrier behaviors is still challenging, due largely to the lack of picosecond-resolved acquisition or programmable scanning capabilities in the current characterization systems. In this work, we select Ta2NiSe5 as a model system to investigate the ultrafast anisotropic transportation properties of photo-excited carriers and transient polarized responses via a digital micromirror device (DMD)-based pump-probe microscope, where the probe beam scans along the armchair and zigzag directions of a crystal structure via binary holography to obtain distinct carrier diffusion coefficients, respectively. The results reveal the nonlinear diffusion behaviors of Ta2NiSe5 in tens of picoseconds, which are attributed to the interplay between excited electrons and phonons. The trend of the measured local polarization-dependent transient reflectivity is consistent with the polarized Raman spectra results. These results show that the DMD-based pump-probe microscope is an effective and versatile tool to study the optoelectronic properties of 2D materials.

Continuous Template Growth of Large-Scale Tellurene Films on 1T'-MoTe2.

Use of a template triggers an epitaxial interaction with the depositing material during synthesis. Recent studies have demonstrated that two-dimensional tellurium (tellurene) can be directionally oriented when grown on transition metal dichalcogenide (TMD) templates. Specifically, employing a T-phase TMD, such as WTe2, restricts the growth direction even further due to its anisotropic nature, which allows for the synthesis of well-oriented tellurene films. Despite this, producing large-area epitaxial films still remains a significant challenge. Here, we report the continuous synthesis of a 1T'-MoTe2 template via chemical vapor deposition and tellurene via vapor transport. The interaction between helical Te and the 1T'-MoTe2 template facilitates the Te chains to collapse into ribbon shapes, enhancing lateral growth at a rate approximately 6 times higher than in the vertical direction, as confirmed by scanning electron microscopy and atomic force microscopy. Interestingly, despite the predominance of the lateral growth, cross-sectional transmission electron microscopy analysis of the tellurene ribbons revealed a consistent 60-degree incline at the edges. This suggests that the edges of the tellurene ribbons, where they contact the template surface, are favorable sites for additional Te absorption, which then stacks along the incline angle to expand. Furthermore, controlling the synthesis temperature, duration, and preheating time has facilitated the successful synthesis of tellurene films. The resultant tellurene exhibited hole mobility as high as ∼400 cm2/V s. After removing the underlying metallic template with plasma treatment, the film showed a current on/off ratio of ∼103. This ratio was confirmed by two-terminal field-effect transistor measurements and supported by near-field terahertz (THz) spectroscopy mapping.

Impact of Gate Voltage on Mobility of Charge Carriers in Conductive Channel of Organic Molecular Semiconductors: Transport Landscape Assessed by Alternating and Direct Current Mode Techniques

AbstractThe gate voltage dependence of the charge carrier mobility is investigated by the microwave resonance technique in transport channels at various organic semiconductors‐insulator interfaces. Although the microwave based measurement is free from the Schottky‐barrier, the vertical charge transport, and the minority carrier trapping, which have been regarded as the origin of the nonlinearity in field effect transistor (FET) measurements, the obtained charge carrier mobility is impacted significantly by the applied gate bias. Atomic force microscopy (AFM) and X‐ray diffractometry (XRD) measurements revealed that the observed nonlinearity indeed originates from the surface disorder in even polycrystalline materials including conjugated polymers. The concerted assessment of the microwave resonance and the FET measurements reveals that 1) injected carriers below the threshold voltage are not trapped despite of the absence of the drain current, 2) the only charge carrier mobility at the 1st layer is assessed by FET measurements, and 3) the threshold voltage of conjugated polymers is relatively low due to the intra‐domain connection through tie molecules.

Microwave graphitic nitrogen/boron ultradoping of graphene

Insufficient carrier concentration and lack of room temperature ferromagnetism in pristine graphene limit its dream applications in electronic and spintronic chips. While theoretical calculations have revealed that graphitic ultradoping can turn graphene into semiconducting and room temperature ferromagnetic, the exotic set of thermodynamic conditions needed for doping result in defects and functionalities in graphene which end up giving significant electronic scattering. We report our discovery of microwave ultradoping of graphene with N > 30%, B ~ 19%, and co-doping to form BCN phases (B5C73N22, B8C76N16, and B10C77N13). An unprecedented level of graphitic doping ~95% enhances carrier concentration up to ~9.2 × 1012 cm−2, keeping high electronic mobility ~9688 cm2 V−1s−1 intact, demonstrated by field effect transistor measurements. Room temperature ferromagnetic character with magnetization ~4.18 emug−1 is reported and is consistent with our DFT band structure calculations. This breakthrough research on tunable graphitic ultradoping of 2D materials opens new avenues for emerging multi-functional technological applications.

Three-Dimensional Surface Treatment of MoS2 Using BCl3 Plasma-Derived Radicals.

The realization of next-generation gate-all-around field-effect transistors (FETs) using two-dimensional transition metal dichalcogenide (TMDC) semiconductors necessitates the exploration of a three-dimensional (3D) and damage-free surface treatment method to achieve uniform atomic layer-deposition (ALD) of a high-k dielectric film on the inert surface of a TMDC channel. This study developed a BCl3 plasma-derived radical treatment for MoS2 to functionalize MoS2 surfaces for the subsequent ALD of an ultrathin Al2O3 film. Microstructural verification demonstrated a complete coverage of an approximately 2 nm-thick Al2O3 film on a planar MoS2 surface, and the applicability of the technique to 3D structures was confirmed using a suspended MoS2 channel floating from the substrate. Density functional theory calculations supported by optical emission spectroscopy and X-ray photoelectron spectroscopy measurements revealed that BCl radicals, predominantly generated by the BCl3 plasma, adsorbed on MoS2 and facilitated the uniform nucleation of ultrathin ALD-Al2O3 films. Raman and photoluminescence measurements of monolayer MoS2 and electrical measurements of a bottom-gated FET confirmed negligible damage caused by the BCl3 plasma-derived radical treatment. Finally, the successful operation of a top-gated FET with an ultrathin ALD-Al2O3 (∼5 nm) gate dielectric film was demonstrated, indicating the effectiveness of the pretreatment.

Realizing metallicity in Sr2IrO4 thin films by high-pressure oxygen annealing

Perovskite iridates are a promising material platform for hosting unconventional superconductivity. Transport measurements of Sr2IrO4 thin-film field-effect transistors are expected to provide irrefutable evidence for the existence of superconductivity. However, these experiments have revealed a remarkably robust insulating state over wide electron and hole doping ranges; this finding is in contrast to the case of the bulk material, in which metallicity appears upon moderate electron doping by substituting cations in place of Sr. The nature of this robust insulating state and whether any metallic state can be realized in the Sr2IrO4 thin film are two remaining challenges that preclude further progress in the search for superconductivity in this system. Here, we show that this insulating state is enhanced in Sr2IrO4 thin films by thermal annealing under vacuum conditions, while it can be destroyed upon annealing in an oxygen atmosphere within restricted ranges of oxygen pressure, annealing temperature and ion substitution levels. The resulting films exhibit metallic transport behavior near room temperature and a metal–insulator crossover at ~200 K. Our results point to the potentially important roles of the oxygen vacancies at different atomic sites in the formation of the robust insulating state and the new metallic state and to their interplay in the Sr2IrO4 thin film. This finding opens new possibilities in the search for unconventional superconductivity by further tailoring the as-found metallic state in properly oxygen-annealed Sr2IrO4 thin films.

Metal Halide Perovskite Photo-Field-Effect Transistors with Chiral Selectivity.

Organic-inorganic (hybrid) metal halide perovskites (MHPs) incorporating chiral organic ligand molecules are naturally sensitive to left- and right-handed circular polarized light, potentially enabling selective circular polarized photodetection. Here, the photoresponses in chiral MHP polycrystalline thin films made of ((S)-(-)-α-methyl benzylamine)2PbI4 and ((R)-(+)-α-methyl benzylamine)2PbI4, denoted as (S-MBA)2 PbI4 and (R-MBA)2PbI4, respectively, are investigated by employing a thin-film field-effect transistor (FET) configuration. The left-hand-sensitive films made of (S-MBA)2PbI4 perovskite show higher photocurrent under left-handed circularly polarized (LCP) light than under right-handed circularly polarized (RCP) illumination under otherwise identical conditions. Conversely, the right-hand-sensitive films made of (R-MBA)2PbI4 are more sensitive to RCP than LCP illumination over a wide temperature range of 77-300 K. Furthermore, based on FET measurements, we found evidence of two different carrier transport mechanisms with two distinct activation energies in the 77-260 and 280-300 K temperature ranges, respectively. In the former temperature range, shallow traps are dominant in the perovskite film, which are filled by thermally activated carriers with increasing temperature; in the latter temperature range, deep traps with one order of magnitude larger activation energy dominate. Both types of chiral MHPs show intrinsic p-type carrier transport behavior regardless of the handedness (S or R) of these materials. The optimal carrier mobility for both handedness of material is around (2.7 ± 0.2) × 10-7 cm2 V-1 s-1 at 270-280 K, which is two magnitudes larger than those reported in nonchiral perovskite MAPbI3 polycrystalline thin films. These findings suggest that chiral MHPs can be an excellent candidate for selective circular polarized photodetection applications, without additional polarizing optical components, enabling simplified construction of detection systems.

Terahertz Electromodulation Spectroscopy for Characterizing Electronic Transport in Organic Semiconductor Thin Films

Terahertz (THz) spectroscopy is a well-established tool for measuring the high-frequency conductance of inorganic semiconductors. Its application to organic semiconductors, however, is challenging, because of the low carrier mobilities in organic materials, which rarely exceed 10cm2/Vs. Furthermore, low charge carrier densities in organic field-effect devices lead to sheet conductivities that are often far-below the detection limits of conventional THz techniques. In this contribution, we present the application of THz electromodulation spectroscopy for characterizing charge transport in organic semiconductors. Pulses of THz radiation are transmitted through organic field-effect devices and are time-resolved by electro-optic sampling. A differential transmission signal is obtained by modulating the gate voltage of the devices. This controls charge injection into the semiconductors, where the charge carriers reduce the THz transmission by their Drude response. Advantageous is that a nearly noise-free differential transmission can be obtained. Furthermore, electromodulation allows to sense specifically either injected electrons or holes. Because the method exclusively probes transport of mobile carriers, it provides access to fundamental transport properties, which are difficult to access with conventional characterization methods, such as conductance measurements of organic field-effect transistors. The outstanding property that a relative differential signal is measured allows to obtain charge carrier mobilities with high reliability. Mobilities as small as 1cm2/Vs can be probed, which makes THz electromodulation spectroscopy an attractive tool for studying charge transport in most technologically relevant organic semiconductors.

Fully Encapsulated and Stable Black Phosphorus Field‐Effect Transistors

AbstractBlack phosphorus (BP) has quickly gained popularity in the scientific community owing to its interesting semiconducting properties, such as direct bandgap, high mobility, and intrinsic ambipolar behavior. However, its sensitivity to oxygen, moisture, and other air species has restricted its integration into active devices. Here, the lithography‐free via‐encapsulation scheme to fabricate fully‐encapsulated BP‐based field‐effect transistors (FETs) is employed. The full encapsulation is achieved by sandwiching the BP layers between the top and bottom hexagonal boron nitride (hBN) layers; top hBN passivating the BP layer from the environment and bottom hBN acting as a spacer and suppressing charge transfer to the BP layer from the SiO2 substrate. The embedded via‐metal electrodes allow the authors to perform reliable electrical measurements of the BP FETs. Based on these results, it is found that the electronic properties of the via‐encapsulated BP FETs are significantly improved compared to unencapsulated devices. This further establishes that the via‐contacting scheme leads to superior results compared to graphene‐hBN heterostructures and bare hBN layers combined with evaporated metal contacts (both use top and bottom hBN to encapsulate BP) by revealing higher mobility, lower hysteresis, and long‐term ambient‐stability in BP FETs.

Probing Carrier Dynamics in sp3-Functionalized Single-Walled Carbon Nanotubes with Time-Resolved Terahertz Spectroscopy.

The controlled introduction of covalent sp3 defects into semiconducting single-walled carbon nanotubes (SWCNTs) gives rise to exciton localization and red-shifted near-infrared luminescence. The single-photon emission characteristics of these functionalized SWCNTs make them interesting candidates for electrically driven quantum light sources. However, the impact of sp3 defects on the carrier dynamics and charge transport in carbon nanotubes remains an open question. Here, we use ultrafast, time-resolved optical-pump terahertz-probe spectroscopy as a direct and quantitative technique to investigate the microscopic and temperature-dependent charge transport properties of pristine and functionalized (6,5) SWCNTs in dispersions and thin films. We find that sp3 functionalization increases charge carrier scattering, thus reducing the intra-nanotube carrier mobility. In combination with electrical measurements of SWCNT network field-effect transistors, these data enable us to distinguish between contributions of intra-nanotube band transport, sp3 defect scattering and inter-nanotube carrier hopping to the overall charge transport properties of nanotube networks.

Compact Super Electron-Donor to Monolayer MoS2.

The surface functionalization of two-dimensional (2D) materials with organic electron donors (OEDs) is a powerful tool to modulate the electronic properties of the material. Here we report a novel molecular dopant, Me-OED, that demonstrates record-breaking molecular doping to MoS2, achieving a carrier density of 1.10 ± 0.37 × 1014 cm-2 at optimal functionalization conditions; the achieved carrier density is much higher than those by other OEDs such as benzyl viologen and an OED based on 4,4'-bipyridine. This impressive doping power is attributed to the compact size of Me-OED, which leads to high surface coverage on MoS2. To confirm, we study tBu-OED, which has an identical reduction potential to Me-OED but is significantly larger. Using field-effect transistor measurements and spectroscopic characterization, we estimate the doping powers of Me- and tBu-OED are 0.22-0.44 and 0.11 electrons per molecule, respectively, in good agreement with calculations. Our results demonstrate that the small size of Me-OED is critical to maximizing the surface coverage and molecular interactions with MoS2, enabling us to achieve unprecedented doping of MoS2.

Experimental investigation of the mobility in the ITIC single crystal based field-effect transistor

The A-π-A electron acceptors, e.g. ITIC and its derivatives have attracted tremendous attention for optoelectronic applications since they can catch up with or even surpass the high performances based on the fulleren-based devices. The charge transport in the A-π-A electron acceptor depends heavily on the molecular packing structure. Tremendous theoretical works have been performed to investigate the correlation between the intermolecular arrangement and the mobility of A-π-A electron acceptors. The studies have simulated the mobility comparison between ITIC crystal and thin film. Although some researchers have measured the mobility of the ITIC thin films using space charge limit current (SCLC) technique and field-effect transistor (FET) measurement, the direct mobility measurements of ITIC crystals remain scarce. In this work, the ITIC single crystal nanosheets were grown by the solvent-non-solvent interfacial precipitation method, providing an effective way to investigate the mobility difference between the ITIC amorphous films and the ITIC single crystals. The measurements unravel that the resulting ITIC cystal has an edge-to-face stacking structure and it is anisotropic. The corresponding OFETs have been fabricated to investigate their mobilities: the single crystal devices exhibit mobility up to 5.4 × 10−2 cm2V−1s−1 while the amorphous ITIC film has a mobility of 2.1 × 10−3 cm2V−1s−1. This experimental result is consistent with the previous theoretical prediction and demonstrates that the crystalline ITIC has a more effective charge transport path along the b crystallographic axis than the amorphous film. This work provides a method to investigate the intrinsic charge transport property of the ITIC crystal.

Mid-IR Intraband Photodetectors with Colloidal Quantum Dots

In this paper, we investigate an intraband mid-infrared photodetector based on HgSe colloidal quantum dots (CQDs). We study the size, absorption spectra, and carrier mobility of HgSe CQDs films. By regulating the time and temperature of the reaction during synthesis, we have achieved the regulation of CQDs size, and the number of electrons doped in conduction band. It is experimentally verified by the field effect transistor measurement that dark current is effectively reduced by a factor of 10 when the 1Se state is doped with two electrons compared with other doping densities. The HgSe CQDs film mobility is also measured as a function of temperature the HgSe CQDs thin film detector, which could be well fitted by Marcus Theory with a maximum of 0.046 ± 0.002 cm2/Vs at room temperature. Finally, we experimentally discuss the device performance such as photocurrent and responsivity. The responsivity reaches a maximum of 0.135 ± 0.012 A/W at liquid nitrogen temperature with a narrow band photocurrent spectrum.

New metallophthalocyanines including benzylphenoxy groups and investigation of their organic-field effect transistor (OFET) features

In this study, metal and metal-free novel phthalocyanines containing peripheral and non-peripheral tetra 2-benzylphenoxy groups were synthesized. The compounds were characterized by UV–Vis, FT-IR, 1H NMR, and MALDI-TOF mass spectrometry as well as elemental analysis. These new phthalocyanines exhibited excellent solubility in most organic solvents, and their redox behavior was investigated in different solvents such as dimethyl sulfoxide (DMSO) and dichloromethane (DCM). The redox behavior of the peripheral and non-peripheral phthalocyanine compounds 1a-c and 2a-c was determined by cyclic voltammetry and in situ spectroelectrochemistry. According to organic field-effect transistors (OFETs) measurements, the peripheral and non-peripheral phthalocyanine-cobalt complexes which have higher mobility than others were utilized top-gate bottom-contact OFETs fabrication. The output characteristics of the device show that its mobility is approximately 5 × 10−2 cm2/Vs with p-type accumulation.

Radiation-hardened property of single-walled carbon nanotube film-based field-effect transistors under low-energy proton irradiation

Strong C–C bonds, nanoscale cross-section and low atomic number make single-walled carbon nanotubes (SWCNTs) a potential candidate material for integrated circuits (ICs) applied in outer space. However, very little work combines the simulation calculations with the electrical measurements of SWCNT field-effect transistors (FETs), which limits further understanding on the mechanisms of radiation effects. Here, SWCNT film-based FETs were fabricated to explore the total ionizing dose (TID) and displacement damage effect on the electrical performance under low-energy proton irradiation with different fluences up to 1 × 1015 p/cm2. Large negative shift of the threshold voltage and obvious decrease of the on-state current verified the TID effect caused in the oxide layer. The stability of the subthreshold swing and the off-state current reveals that the displacement damage caused in the CNT layer is not serious, which proves that the CNT film is radiation-hardened. Specially, according to the simulation, we found the displacement damage caused by protons is different in the source/drain contact area and channel area, leading to varying degrees of change for the contact resistance and sheet resistance. Having analyzed the simulation results and electrical measurements, we explained the low-energy proton irradiation mechanism of the CNT FETs, which is essential for the construction of radiation-hardened CNT film-based ICs for aircrafts.

Correlating Charge Transport Properties of Conjugated Polymers in Solution Aggregates and Thin-Film Aggregates.

The role of solution aggregates on the charge transport process of conjugated polymers in electronic devices has gained increasing attention; however, the correlation of the charge carrier mobilities between the solution aggregates and the solid-state films remains elusive. Herein, three polymers, FBDOPV-2T, FBDOPV-2F2T, and FBDOPV-4F2T, are designed and synthesized with distinct aggregation behavior in solution. By combining contact-free ultrafast terahertz (THz) spectroscopy and field-effect transistor measurements, we track the charge carrier mobility of the aggregates of these polymers from the solution to the thin-film state. Remarkably, the mobility of these three polymers is found to follow nearly the same trend (FBDOPV-2T>FBDOPV-2F2T≫FBDOPV-4F2T) in both solutions and thin-film states. The quantitative mobility correlation indicates that the charge transport properties of solution aggregates play a critical role in determining the thin-film charge transport properties and final device performance. Our results highlight the importance of investigating and controlling solution aggregation structures towards efficient organic electronic devices.

Ultrathin platinum diselenide synthesis controlling initial growth kinetics: Interfacial reaction depending on thickness and substrate

Platinum diselenide (PtSe2) is an emerging transition-metal dichalcogenide showing both strong interlayer coupling and remarkable electrical properties. Because field-effect transistors (FETs) recently fabricated using PtSe2 films grown by various methods have not shown the high mobilities and on–off ratios compared with exfoliated PtSe2 and predicted by theoretical calculations, the PtSe2 growth mechanism must be better understood to prevent the degradation of electrical properties and improve the electrical performances of PtSe2-based devices. Therefore, molecular beam epitaxy is used in the current study to grow PtSe2 films, and a correlation is demonstrated between the initial PtSe2 growth kinetics and the electrical performances of PtSe2-based devices. X-ray photoelectron spectroscopy and annular dark field–scanning transmission electron microscopy show that metallic Platinum (Pt) is present in the thin PtSe2 films, as explained by the density functional theory calculations comparing Pt–Pt bond with Pt–Se bond. Electrical measurements of the PtSe2-based FETs show that the electrical performances are closely correlated with the initial growth kinetics of the PtSe2 films, indicating that removing Pt (which degrades device performance) is the most important aspect of growing thin PtSe2 films to optimize device performance.

Electrochemically functionalized single-walled carbon nanotubes for ultrasensitive detection of BTEX vapors

Single-walled carbon nanotubes (SWNTs) have been functionalized electrochemically with porphyrin and were employed as the channel to fabricate the chemical field-effect transistor. Tetraphenylporphyrin was chosen for functionalization purpose. The primary aim of this study was to evaluate the sensing performance of functionalized SWNTs for the detection of BTEX vapor and to understand the underlying mechanism for the detection. The functionalization showed a significant enhancement in the sensing response for all analytes starting from 1.25 ppm to 15 ppm of concentration. A sub-ppm limit of detection was also observed. The systematic study of field effect transistor (FET) measurements indicates that the sensing mechanism is dominated by the electrostatic gating effect.

Size Discrimination of Carbohydrates via Conductive Carbon Nanotube@Metal Organic Framework Composites.

Traditional chemical sensing methodologies have typically relied on the specific chemistry of the analyte for detection. Modifications to the local environment surrounding the sensor represent an alternative pathway to impart selective differentiation. Here, we present the hybridization of a 2-D metal organic framework (Cu3(HHTP)2) with single-walled carbon nanotubes (SWCNTs) as a methodology for size discrimination of carbohydrates. Synthesis and the resulting conductive performance are modulated by both mass loading of SWCNTs and their relative oxidation. Liquid gated field-effect transistor (FET) devices demonstrate improved on/off characteristics and differentiation of carbohydrates based on molecular size. Glucose molecule detection is limited to the single micromolar concentration range. Molecular Dynamics (MD) calculations on model systems revealed decreases in ion diffusivity in the presence of different sugars as well as packing differences based on the size of a given carbohydrate molecule. The proposed sensing mechanism is a reduction in gate capacitance initiated by the filling of the pores with carbohydrate molecules. Restricting diffusion around a sensor in combination with FET measurements represents a new type of sensing mechanism for chemically similar analytes.

Electrical Double Percolation of Polybutadiene/Polyethylene Glycol Blends Loaded with Conducting Polymer Nanofibers.

The critical phenomena of double percolation on polybutadiene (PB)/polyethylene glycol (PEG) blends loaded with poly-3-hexylthiophene (P3HT) nanofibers is investigated. P3HT nanofibers are selectively localized in the PB phase of the PB/PEG blend, as observed by scanning force microscopy (SFM). Moreover, double percolation is observed, i.e., the percolation of the PB phase in PB/PEG blends and that of the P3HT nanofibers in the PB phase. The percolation threshold (φcI) and critical exponent (tI) of the percolation of the PB phase in PB/PEG blends are estimated to be 0.57 and 1.3, respectively, indicating that the percolation exhibits two-dimensional properties. For the percolation of P3HT nanofibers in the PB phase, the percolation threshold (φcII) and critical exponent (tII) are estimated to be 0.02 and 1.7, respectively. In this case, the percolation exhibits properties in between two and three dimensions. In addition, we investigated the dimensionality with respect to the carrier transport in the P3HT nanofiber network. From the temperature dependence of the field-effect mobility estimated by field-effect transistor (FET) measurements, the carrier transport was explained by a three-dimensional variable range hopping (VRH) model.