Electron Mobility Values Research Articles

Peculiarities of Electrical Characteristics of Semi-Insulating CdTe-Сl crystals

Electrical properties of semi-insulating CdTe-Сl crystals, grown by the vertical Bridgman and the travelling heater method, have been studied. It is found that the travelling heater method provides electron conductivity of the crystals, and the vertical Bridgman method – hole conductivity. Specific resistance of the samples is of (108-109) Ohm×сm at 300 K, and Hall mobility of the holes and electrons is of (45 - 55) cm2/V·s and (10 - 20) cm2/V·s respectively. Very low values of electron mobility and an exponential temperature dependence of µn are due to drift barriers with a height of εb ≈ 0.20 eV. Formation of the barriers is caused by the fluctuations of the potential relief resulting from the microheterogeneity of the defect-impurity system. Quasi-photochemical reactions that reduce electron mobility after photo-excitation have been observed in n-CdTe-Cl samples. In p-CdTe-Cl samples, neither drift barriers, nor quasi-photochemical reactions were detected.

Prediction of Large Enhancement of Electron Mobility in Direct Gap Ge1−xSnx Alloy

Group IV semiconductor‐based alloy Ge1−xSnx can be grown on Si platform and a crossover from indirect (L valleys) to direct gap (Γ valley) occurs for x > 0.08. The direct gap alloy shows promise for use in high‐speed electronic and photonic devices. An earlier study predicted that the electron mobility in the alloy would increase by a few orders of magnitude above the value for pure Ge. In that work, however, electrons were assumed to occupy only the low effective mass Γ valley and only the deformation potential acoustic phonon and alloy scatterings were considered. In the present work, we give a more realistic estimate of the values of electron mobility in direct gap GeSn alloy, considering the Γ and L valleys and the phonon (deformation potential acoustic, optical and intervalley), alloy, and impurity scatterings. The effect of strain, both tensile and compressive, on the mobility is also studied. It is found that for higher alloy composition and suitable strain, mobility values ∼2 × 105 cm−2/V · s, more than 50 times higher than the value in pure Ge (3900 cm2/V · s) may be achieved. This is attributed to increased separation between Γ and L valleys and consequent reduction in intervalley scattering.

Growth of strained, but stable, graphene on Co

Mostly single-layer graphene with enhanced mechanical stability is demonstrated over polycrystalline Co film deposited onto a SiO2/Si substrate. Raman and electrical characterizations evidence that the continuous graphene layer can be easily transferred without the aid of any polymeric support, but preserving its quality. Better stability to the damaging action of the laser beam, as compared to the Cu-grown material is observed, together with electron mobility values in the 104cm2V−1s−1 range. It is suggested that most of the C atoms precipitate on surface during the first 10°C of cooling, because of the higher activation energy of C solubility into Co respect to Ni. Tensile stress is found to originate in graphene regions grown onto Co film discontinuities. Strain in the sheet is also observed after transfer. This opens possible routes to engineer the electronic spectrum through a control of the strain during growth.

Epsilon-near-Zero Modes and Surface Plasmon Resonance in Fluorine-Doped Cadmium Oxide Thin Films

In this report we demonstrate fluorine-doped CdO as a model infrared plasmonic material by virtue of its tunable carrier density, high mobility, and intense extreme-subwavelength plasmon–polariton coupling. Carrier concentrations ranging from 1019 to 1020 cm–3, with electron mobility values as high as 473 cm2/V·s, are readily achieved in epitaxial CdO films over a thickness range spanning 50 to 500 nm. Carrier concentration is achieved by reactive sputtering in an Ar/O2 atmosphere with trace quantities of CF4. Infrared reflectometry measurements demonstrate the possibility of near-perfect plasmonic absorption through the entire mid-IR spectral range. A companion set of reflectivity simulations are used to predict, understand, and optimize the epsilon-near-zero plasmonic modes. In the context of other transparent conductors, CdO exhibits substantially higher electron mobility values and thus sharp and tunable absorption features. This highlights the utility of high-mobility transparent conducting oxides as...

Suppressing Nucleation in Metal–Organic Chemical Vapor Deposition of MoS2 Monolayers by Alkali Metal Halides

Toward the large-area deposition of MoS2 layers, we employ metal-organic precursors of Mo and S for a facile and reproducible van der Waals epitaxy on c-plane sapphire. Exposing c-sapphire substrates to alkali metal halide salts such as KI or NaCl together with the Mo precursor prior to the start of the growth process results in increasing the lateral dimensions of single crystalline domains by more than 2 orders of magnitude. The MoS2 grown this way exhibits high crystallinity and optoelectronic quality comparable to single-crystal MoS2 produced by conventional chemical vapor deposition methods. The presence of alkali metal halides suppresses the nucleation and enhances enlargement of domains while resulting in chemically pure MoS2 after transfer. Field-effect measurements in polymer electrolyte-gated devices result in promising electron mobility values close to 100 cm2 V-1 s-1 at cryogenic temperatures.

Fabrication and investigation of P3HT:PCBM bulk heterojunction based organic field effect transistors using dielectric layers of PMMA:Ta2O5 nanocomposites

In this work, we have fabricated top-gate/bottom-contact type Organic Field Effect Transistors (OFETs). A poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend with 1:1 wt/wt ratio was used as the semiconducting layer, and poly(methyl methacrylate)(PMMA):Ta2O5 blends with varying concentration of Ta2O5 were used as dielectric layers of the devices. Interdigitated type source-drain electrodes on a glass substrate were coated with a semiconducting layer, which was followed by the deposition of dielectric layer and a top aluminum gate contact in sequence. Dielectric layers were prepared as the blends of PMMA:Ta2O5 with Ta2O5 wt/wt concentrations of 0%, 3%, 7%, 10%, 20% and 50%. Structural and electrical characterization of PMMA:Ta2O5 nanocomposite films were carried out using scanning electron microscope (SEM) and dielectric analyzer respectively. Dielectric constant of PMMA:Ta2O5 nanocomposites was found to increase with the increasing concentration of Ta2O5. Devices that were processed using 0wt% and 3wt% concentration of Ta2O5 exhibited a typical p-type OFET behavior with hole mobility of 2.68×10−3 and 3.42×10−3cm2/Vs respectively. Whereas, the devices with 7wt% and higher concentration of Ta2O5 showed electron mobility in addition to hole mobility, indicating ambipolar behavior of the devices. Furthermore, electron mobility was found to increase with increasing concentration of Ta2O5. The hole and electron mobility values were found to be 1.08×10−3 and 4.42×10−3cm2/Vs at 50% wt concentration of Ta2O5 respectively. Based on excellent saturation behavior and promising mobility values of the investigated solution processed devices, it is expected that the investigated devices can possibly be utilized in some practical applications, such as low-cost large area sensing arrays in future.

Prediction of High-Density and High-Mobility Two-Dimensional Electron Gas at Al<sub>x</sub>Ga<sub>1-x</sub>N/4H-SiC Interface

This work presents theoretical demonstration of two-dimensional electron gas (2DEG) at the interface between Al0.2Ga0.8N and 4H-SiC, based on the self-consistent solution of Schrödinger–Poisson equations. High sheet carrier density of 1.1×1013 cm-2 was obtained in the Al0.2Ga0.8N/4H-SiC heterostructure, which is comparable to the electron concentration in Al0.2Ga0.8N/GaN heterostructure. The current–voltage characteristics of a high-electron-mobility transistor (HEMT), based on the Al0.2Ga0.8N/4H-SiC heterostructure, show a saturated drain current of 1.5 A/mm at the gate voltage of 2 V and the transconductance of 194 mS/mm at -3.95 V. In spite of interface-roughness scattering and phonon scattering, the 2DEG at the AlxGa1-xN/4H-SiC interface exhibits high electron mobility values of 3365 cm2/ (V·s) at 77K and 1120 cm2/ (V·s) at 300K. These results indicate that AlxGa1-xN/4H-SiC heterostructure can significantly improve the mobility of SiC based power switching devices.

A Solution-Processable Liquid-Crystalline Semiconductor for Low-Temperature-Annealed Air-Stable N-Channel Field-Effect Transistors.

A new solution-processable and air-stable liquid-crystalline n-channel organic semiconductor (2,2'-(2,8-bis(5-(2-octyldodecyl)thiophen-2-yl)indeno[1,2-b]fluorene-6,12-diylidene)dimalononitrile, α,ω-2OD-TIFDMT) with donor-acceptor-donor (D-A-D) π conjugation has been designed, synthesized, and fully characterized. The new semiconductor exhibits a low LUMO energy (-4.19 eV) and a narrow optical bandgap (1.35 eV). The typical pseudo-focal-conic fan-shaped texture of a hexagonal columnar liquid-crystalline (LC) phase was observed over a wide temperature range. The spin-coated semiconductor thin films show the formation of large (≈0.5-1 μm) and highly crystalline platelike grains with edge-on molecular orientations. Low-temperature-annealed (50 °C) top-contact/bottom-gate OFETs have provided good electron mobility values as high as 0.11 cm2 (V s)-1 and high Ion /Ioff ratios of 107 to 108 with excellent ambient stability. This indicates an enhancement of two orders of magnitude (100×) when compared with the β-substituted parent semiconductor, β-DD-TIFDMT (2,2'-(2,8-bis(3-dodecylthiophen-2-yl)indeno[1,2-b]fluorene-6,12-diylidene)dimalononitrile). The current rational alkyl-chain engineering route offers great advantages for D-A-D π-core coplanarity in addition to maintaining good solubility in organic solvents, and leads to favorable optoelectronic/physicochemical characteristics. These remarkable findings demonstrate that α,ω-2OD-TIFDMT is a promising semiconductor material for the development of n-channel OFETs on flexible plastic substrates and LC-state annealing of the columnar liquid crystals can lower the electron mobility for transistor-type charge transport.

Communication—CMOS Thin-Film Transistors via Xe Flash-Lamp Crystallization of Patterned Amorphous Si

Flash-lamp annealing (FLA) has been investigated for crystallization of patterned amorphous silicon (a-Si) in the fabrication of NMOS and PMOS Thin-Film Transistors (TFTs) on display glass. Samples were exposed with a xenon flash irradiance of ∼30 kW/cm2 and pulse duration of 200 μs, with bolometer measurements showing an integrated energy of ∼6 J/cm2. Non-self-aligned TFTs fabricated from the resulting polycrystalline silicon demonstrated electron and hole channel mobility values in excess of 300 cm2/(Vs) and 100 cm2/(Vs), respectively. According to the authors’ knowledge, this is the first report of CMOS TFTs demonstrated using the FLA technique.

High accuracy magnetic field sensors with wide operation temperature range

n+InAs(Si) epitaxial thin films heavily doped by silicon and Hall effect magnetic field sensors based on this structures have been fabricated and studied. We have demonstrated the successful formation of highly doped InAs thin films (∼100 nm) with the different intermediate layer arrangement and appropriate electron mobility values. Hall sensors performance parameters have been measured in wide temperature range. Obtained sensitivity varied from 1 to 40 Ω/T, while the best linearity and lower temperature coefficient have been found in the higher doped samples with lower electron mobility. We attribute this to the electron system degeneracy and decreased phonon contribution to electron mobility and resistance.

A theoretical study of charge transport properties of trifluoromethyl (-CF3) substituted naphthalene (TFMNA) molecule

We present a density functional (DFT) study of the charge transport properties of CF3-naphthalene. Nature of charge transport is investigated using parameters such as reorganization energy (X), transfer integral (t), ionization potential (IP), electron affinity (EA), and carrier mobility (μ) computed through electronic structure calculations. We observe a decrease in X and IP from 2,6-DTFMNA to 1,5-DTFMNA, whereas, the EA is found to be enhanced, as a result p-type characteristics, with mild n-type signature, in the organic semiconductor gets increased. In addition, the HOMO-LUMO gap also gets reduced inferring more charge injection through the potential barrier. The maximum hole and electron mobility values for the substituted compound are obtained to be 2.17 cm2/ Vsec & 0.20 cm2/ Vsec, respectively.

(Invited) Manufacturing Microwave AlGaN/GaN High Electron Mobility Transistors (HEMTs) on Truly Bulk Semi-Insulating GaN Substrates

The substrate of choice for high power microwave GaN-based devices is silicon carbide. However recently novel semi-insulating truly bulk GaN substrates with excellent crystalline and electrical parameters have been developed by Ammono S..A. These allow to elaborate AlGaN/GaN heterostructures with high electron mobility values and density of two-dimensional electron gas. Developed processing steps, especially planar isolation by ion implantation and formation of low resistivity regrown ohmic contacts enabled fabrication of high quality devices. An 1000 mA/mm on-state current density along with low 4.4 Ω/mm on-state resistance were achieved. For the devices with rectangular, 0.8 μm gate length the fMAG and fs were 31 GHz and 22 GHz. The maximum output power density was more than 4.15 W/mm in S-band.

Modeling and estimation of process-induced stress in the nanowire field-effect-transistors (NW-FETs) on Insulator-on-Silicon substrates with high-k gate-dielectrics

An analytical model including the simultaneous impact of lattice and thermo-elastic constant mismatch-induced stress in nanowires on Insulator-on-Silicon substrate is developed. It is used to calibrate the finite-element based software, ANSYS, which is subsequently employed to estimate process-induced stress in the sequential steps of NW-FET fabrication. The model considers crystal structures and orientations for both the nanowires and substrates. In-plane stress components along nanowire-axis are estimated for different radii and fractions of insertion. Nature of longitudinal stress is observed to change when inserted fraction of nanowires is changed. Effect of various high-k gate-dielectrics is also investigated. A longitudinal tensile stress of 2.4 GPa and compressive stress of 1.89 GPa have been obtained for NW-FETs with 1/4th and 3/4th insertions with La2O3 and TiO2 as the gate-dielectrics, respectively. Therefore, it is possible to achieve comparable values of electron and hole mobility in NW-FETs by judiciously choosing gate-dielectrics and fractional insertion of the nanowires.

Impact of nanomaterial arrangement on the reliability and the electron mobility in AlGaN/GaN HEMTs

The obtained results demonstrate that the improvement of nanomaterial arrangement in AlGaN/GaN HEMT structures quantitatively characterized with the use of a multifractal parameter (the degree of disorder) results in the increase at several times in the electron mobility values at 2DEG channel in HEMT structures and the reliability of HEMT parameters.

Impact of InGaN back barrier layer on performance of AIInN/AlN/GaN MOS-HEMTs

In the present work, we have discussed the effect of InGaN back barrier on device performances of 100 nm gate length AlInN/AlN/GaN metal oxide semiconductor high electron mobility transistor (MOS-HEMT) device and a wide comparison is made with respect to without considering the back barrier layer. The InGaN layer is introduced in the intension to raise the conduction band of GaN buffer with respect to GaN channel so that there is an improvement in the carrier confinement and at the same time witnessed excellent high frequency performance. The simulations are carried out using 2D Sentaurus TCAD simulator using Hydrodynamic mobility model by taking interface traps into consideration. Due to high value of two-dimensional electron gas (2DEG) density and mobility in AlInN/AlN/GaN MOS-HEMT device, higher drain current density is achieved. Simulation are carried out for different device parameters such as transfer characteristic (Id-Vg), transconductance factor (gm), drain induced barrier lowering (DIBL), Subthreshold slope (SS), conduction band energy, transconductance generation factor (gm/Id) and electric field. We have also examined the RF performance such as, total gate capacitance (Cgg), current gain cutoff frequency (fT) and power gain cutoff frequency (fmax) of the proposed devices. Use of InGaN back barrier tends to increase threshold voltage towards more positive value, reduced DIBL, and improves SS and significant growth in (gm/Id) by 5.5%. It also helps to achieve better frequency response like substantial increase in fT up to 91 GHz with current gain 60 dB as compare to 67 GHz with 56 dB for the device without considering back barrier and increase in fmax up to 112 GHz with respect 94 GHz. These results evident that use of InGaN back barrier in such devices can be better solution for future analog and RF applications.

The digital filtration preprocessing of variable magnetic field Hall data before mobility spectrum analysis for p‐MCT films

AbstractThe variable‐field Hall data of p‐MCT films were filtered by weighting low‐pass digital filter with Blackman win‐dow before mobility spectrum analysis. The used cutoff frequencies lay in range 0.1‐0.7 Hz. It was obtained that the digital filtration allows determining of minor elec‐tron's mobility at low temperatures (77‐90 K). The electron mobility values, obtained by mobility spectrum analysis in conjunction with multicarrier fitting well agreed with result of photoconductivity in magnetic field for Faradays geometry.(© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Power Law of Phonon-Limited Electron Mobility in the 2-D Electron Gas of AlGaN/GaN Heterostructure

The reduction of electron mobility in AlGaN/GaN heterostructures follows the common power law, but with an unexpectedly high power coefficient. Following the experimental verification of the unusual power-coefficient value by a different measurement method, this brief presents an analysis that identifies the temperature dependence of the effective electron mass as the responsible physical mechanism for this effect. Based on this result, the measured values of electron mobility are used to calculate the effective mass of the electrons in AlGaN/GaN heterostructures over a wide temperature range, from 25 °C to 300 °C.

Dibenzothiophene-Substituted Fullerene Derivative as Electron Acceptor for Polymer Solar Cells.

A new fullerene derivative, [6,6]-dibenzo[b,d]thiophene-C61-butyric acid methyl ester (DBTC61BM) was synthesized from C60 using tosylhydrazone, and used as an electron-acceptor material for poly(3-hexylthiophene) (P3HT)-based organic photovoltaic cells. The synthesized DBTC61BM was used to modify the basic structure of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) by replacing the aromatic part with dibenzo[b,d]thiophene. The solubilities of DBTC61BM and PC61BM are similar; they have good solubilities in common organic solvents such as dichloromethane, chloroform, toluene, and 1,2-dichlorobenzene. The Stern-Volmer quenching constant (K(sv)) of DBTC61BM was 7.14 x 10(3) M(-1), and was correlated with the binding affinity between the fluorophore and a quencher. The lowest unoccupied molecular orbital energy level of DBTC61BM was -3.71 eV. The charge-carrier mobility of a P3HT:DBTC61BM blend film was determined using the space-charge-limited current method; the electron mobility value obtained for the P3HT:DBTC61BM blend film was 2.13 x 10(-4) cm2 V(-1) s(-1). Photovoltaic devices were fabricated using P3HT as the electron donor and DBTC61BM as the electron acceptor. Among the fabricated devices, photovoltaic cells with the structure ITO/PEDOT:PSS/P3HT:DBTC61BM/LiF/Al showed the highest power conversion efficiency, namely 3.23%, with an open-circuit voltage of 0.64 V, short-circuit-current density of 8.14 mA cm(-2), and fill factor of 0.59, under AM 1.5 G (100 mW cm(-2)) illumination.

Use of a cross-linkable or monolayer-forming polymeric buffer layer on PCBM-based n-channel organic field-effect transistors

Fabrication of organic field-effect transistors based on phenyl-C61-butyric acid methyl ester (PCBM) with dielectric bilayers of silicon dioxide and a hydroxyl-free polymer buffer layer was carried out. A polymer buffer layer, which was made of either a cross-linked divinylsilane-bis-benzocyclobutene (BCB) polymer or a polystyrene-based dimethylchlorosilane monolayer (b-PS), turned out to be very smooth and hydrophobic. Cured BCB and two types of b-PS monolayers (Mn = 8 and 108 kDa) covering SiO2 layers showed water contact angles of 91°, 83°, and 78° and roughness less than a nanometer. Ascribed to favorable properties of insulating layers, high electron mobility values, ranging from 0.073 to 0.10 cm2/(V s), and a low threshold voltage (5.3–8.5 V) were obtained.

(Invited) Decorating Nanocarbons with Perylenediimide Systems

Carbon-based nanostructured materials (fullerenes, carbon nanotubes or graphene, to name a few) are known to show electron-acceptor properties at the molecular scale, together with high values of electron mobilities in bulk. Thus, its implementation in active materials is an important issue in order to develop organic efficient full optoelectronic devices. One of the strategies employed in the improvement of the interesting optoelectronic properties of carbon nanostructures, is the doping, either in a covalent or a non-covalent way, with specially designed electron-donor/light harvesting moieties. These moieties act like light collectors, exciting themselves and promoting an electron to a higher energy state, which is rapidly deactivated through electron and/or energy transfer processes to the carbon-based nanostructure.Here we present different strategies for the covalent and non-covalent functionalization of nanocarbon materials. In these examples, the decorating agents, specially designed perylenediimides[1-4] (Figure 1) and others, whose electronic character has been modulated through the introduction of different substituents, are successfully employed to improve the light absorption behaviour in the UV and NIR regions of the, in principle, unabsorbing carbon nanoforms, thus yielding hybrid materials with potential applicability like active layers in organic photovoltaic devices. Acknowledgements. This work has been supported by the Spanish Ministerio de Economía y Competitividad, Generalitat Valenciana and the European FEDER funds (grants CTQ2011-26455, CTQ2014-55798-R, PROMETEO 2012/010 and ISIC/2012/008) References Martín-Gomis, L.; Rotas, G.; Ohkubo, K.; Fernández-Lázaro, F.; Fukuzumi, S.; Tagmatarchis, N.; Sastre-Santos, Á. Nanoscale 2015, 7, 7437-7444..Barrejón, M.; Pla, S.; Berlanga, I.; Gómez-Escalonilla, M. J.; Martín-Gomis, L.; Fierro, J. L.; Zhang, M.; Yudasaka, M.; Iijima, S.; Gobeze, H. B.; D’Souza, F.; Sastre-Santos, Á.; Langa, F. J. Mater.Chem. C. 2015, 3, 10215-10224.Pla, S.; Niemi, M.; Martín-Gomis, L.; Fernández-Lázaro, F.; Lemmetyinen, H.; Tkachenko, N. V.; Sastre-Santos, Á. Phys.Chem. Chem. Phys. Accepted. Martín-Gomis, L.; Karousis, N.; Petsalakis, I. D.; Ohkubo, K.; Fernández-Lázaro, F.; Tagmatarchis, N.; Fukuzumi, S.; Sastre-Santos Á. Submitted. Figure 1. Chemical structures of a) [60]Fullerene-Perylenediimide (C60-PDI, DB-1,7-Pyr-PDI-C60) and b) Double Wall Carbon Nanotube-Perylenedimide (DWCNT-PDI). Figure 1