Effective Mobility Values Research Articles

Capillary Electrophoresis as a Complementary Analytical Tool for the Separation and Detection of Nanoplastic Particles.

Capillary electrophoresis (CE) is presented as a technique for the separation of polystyrene nanoparticles (NPs, particle diameters ranging from 30 to 300 nm) through a bare fused silica capillary and ultraviolet detection. The proposed strategy was also assessed for other types of nanoplastics, finding that stronger alkaline conditions, with an ammonium hydroxide buffer (7.5%, pH = 11.9), enabled the separation of poly(methyl methacrylate), polypropylene, and polyethylene NP for the first time by means of CE for particle diameters below 200 nm. Particle behavior has been investigated in terms of its effective electrophoretic mobility, showing an increasing absolute value of effective electrophoretic mobility from the smaller to the larger sizes. On the other hand, the absolute value of surface charge density decreased with increasing size of NPs. It was demonstrated and quantified that the separation mechanism was a combination of linear and nonlinear electrophoretic effects. This work is the first report on the quantification of nonlinear electrophoretic effects on nanoplastic particles in a CE system.

Fully Deep‐UV Transparent Thin Film Transistors Based on SrSnO3

AbstractUltra‐wide bandgap semiconductors are gaining attention for their promising properties for UV optoelectronics and UV transparent electronics as well as high‐power applications. Among them, La‐doped SrSnO3 exhibits excellent properties both for deep‐UV transparent oxide semiconductors and deep‐UV transparent conducting oxide. Here, the demonstration of thin film transistors (TFTs) with full deep‐UV transparency is reported, including electrodes, gate oxide, and substrate. The lightly La‐doped SrSnO3 for the channel layer is grown on MgO (100) substrates with buffer layers by pulsed laser deposition. TFTs with a metal—insulator–semiconductor structure are fabricated using high‐k perovskite dielectric LaScO3 as the gate oxide. A degenerately La‐doped SrSnO3 is used as the gate, the source, and the drain electrodes to obtain good ohmic contact with the channel layer as well as UV transparency. The resultant device shows a field effect mobility value of ≈24 cm2 V−1 s−1 and an on/off ratio >106. The optical transmittance of the entire device (including the substrate) is found to be >75% at 300 nm in wavelength. Furthermore, the electrical characteristics of the device exhibit excellent stability under visible irradiation. This research highlights the potential of SrSnO3 in advancing the field of UV optoelectronics and UV transparent electronics.

Electrical Characteristics of Diamond MOSFET with 2DHG on a Heteroepitaxial Diamond Substrate

In this work, hydrogen-terminated diamond (H-diamond) metal-oxide-semiconductor field-effect-transistors (MOSFETs) on a heteroepitaxial diamond substrate with an Al2O3 dielectric and a passivation layer were characterized. The full-width at half maximum value of the diamond (004) X-ray rocking curve was 205.9 arcsec. The maximum output current density and transconductance of the MOSFET were 172 mA/mm and 10.4 mS/mm, respectively. The effect of a low-temperature annealing process on electrical properties was also investigated. After the annealing process in N2 atmosphere, the threshold voltage (Vth) and flat-band voltage (VFB) shifts to negative direction due to loss of negative charges. After annealing at 423 K for 3 min, the maximum value of hole field effective mobility (μeff) increases by 27% at Vth − VGS = 2 V. The results, which are not inferior to those based on homoepitaxial diamond, promote the application of heteroepitaxial diamond in the field of electronic devices.

Nondegenerate Polycrystalline Hydrogen-Doped Indium Oxide (InOx:H) Thin Films Formed by Low-Temperature Solid-Phase Crystallization for Thin Film Transistors.

We successfully demonstrated a transition from a metallic InOx film into a nondegenerate semiconductor InOx:H film. A hydrogen-doped amorphous InOx:H (a-InOx:H) film, which was deposited by sputtering in Ar, O2, and H2 gases, could be converted into a polycrystalline InOx:H (poly-InOx:H) film by low-temperature (250 °C) solid-phase crystallization (SPC). Hall mobility increased from 49.9 cm2V−1s−1 for an a-InOx:H film to 77.2 cm2V−1s−1 for a poly-InOx:H film. Furthermore, the carrier density of a poly-InOx:H film could be reduced by SPC in air to as low as 2.4 × 1017 cm−3, which was below the metal–insulator transition (MIT) threshold. The thin film transistor (TFT) with a metallic poly-InOx channel did not show any switching properties. In contrast, that with a 50 nm thick nondegenerate poly-InOx:H channel could be fully depleted by a gate electric field. For the InOx:H TFTs with a channel carrier density close to the MIT point, maximum and average field effect mobility (μFE) values of 125.7 and 84.7 cm2V−1s−1 were obtained, respectively. We believe that a nondegenerate poly-InOx:H film has great potential for boosting the μFE of oxide TFTs.

A novel two-dimensional boron–carbon–nitride (BCN) monolayer: A first-principles insight

The optical, electronic, and structural properties of a theoretically predicted new boron–carbon–nitride (BCN) two-dimensional monolayer have been explored using density functional theory calculations. The phonon dispersion, molecular dynamics simulation, the cohesive energy, and the Born criteria of elastic constant calculations of the BCN monolayer confirm its stability. The phonon spectrum illustrates an out-of-plane flexure mode with quadratic dispersion in the long-wavelength limit. The BCN monolayer is a semiconductor with a direct bandgap of 0.9 (1.63) eV determined via the Perdew–Burke–Ernzerhof (Heyd–Scuseria–Ernzerhof) functional. The same electron and hole effective masses and mobility values indicate the high recombination rate of electrons and holes. Meanwhile, the BCN monolayer can absorb ultraviolet radiation more effectively than visible light. Due to its interesting physical properties, the novel BCN monolayer could be a rather good candidate material for electro-optical applications.

Enhanced zirconia oxide dielectric quality of germanium p-channel metal oxide semiconductor field effect transistor by in-situ low temperature treatment in atomic layer deposition process

The gate dielectric quality of germanium (Ge) p-channel metal oxide semiconductor field effect transistor (MOSFET) is enhanced by using an in-situ low temperature treatment in atomic layer deposition (ALD) process in this work. With additional oxygen flow or remote oxygen-based plasma, the dangling bond, oxygen vacancy, and oxide traps in the ALD-formed zirconia oxide gate dielectric can be effectively passivated. Unlike the traditional post deposition annealing process that generally causes an increased equivalent oxide thickness (EOT) significantly, the in-situ low temperature treatment can lower the EOT value, reduce the gate leakage current, and improve the gate dielectric quality at the same time. As a result, the sample with an additional oxygen gas flow can exhibit a lower hysteresis value, better frequency dispersion characteristics, higher on/off current ratio, lower sub-threshold swing value, higher transconductance value, higher field effect mobility value, and better uniformity, simultaneously. This dielectric quality enhancement technique with in-situ low temperature process is promising to overcome the trade-off between EOT and gate leakage for the future Ge MOSFET devices.

Glass transition temperature from the chemical structure of conjugated polymers

The glass transition temperature (Tg) is a key property that dictates the applicability of conjugated polymers. The Tg demarks the transition into a brittle glassy state, making its accurate prediction for conjugated polymers crucial for the design of soft, stretchable, or flexible electronics. Here we show that a single adjustable parameter can be used to build a relationship between the Tg and the molecular structure of 32 semiflexible (mostly conjugated) polymers that differ drastically in aromatic backbone and alkyl side chain chemistry. An effective mobility value, ζ, is calculated using an assigned atomic mobility value within each repeat unit. The only adjustable parameter in the calculation of ζ is the ratio of mobility between conjugated and non-conjugated atoms. We show that ζ correlates strongly to the Tg, and that this simple method predicts the Tg with a root-mean-square error of 13 °C for conjugated polymers with alkyl side chains.

Band structure, effective mass, and carrier mobility of few-layer h-AlN under layer and strain engineering

Wide bandgap two-dimensional semiconductors are of paramount importance for developing van der Waals heterostructure electronics. This work reports the use of layer and strain engineering to introduce the feasibility of two-dimensional hexagonal (h)-AlN to fill the scientific and application gap. We show that such one- to five-layer h-AlN has an indirect bandgap, tunable from 2.9 eV for a monolayer to ∼3.5 eV for multilayer structures, along with isotropic effective masses and carrier mobilities between zigzag and armchair directions. With an increase in the layer number to bulk AlN, the bandgap will experience a transition from an indirect gap to direct gap. Surprisingly, high room-temperature mobilities of electrons and holes (of the order of 1000 cm2 V−1 s−1) in a relaxed monolayer h-AlN system and widely adjustable effective masses and carrier mobilities in a different layer h-AlN are observed. In the presence of strain engineering, the bandgap decreases obviously with an increase in tensile strain; meanwhile, the isotropy and value of effective mass or carrier mobility in monolayer h-AlN can also be modulated effectively; the hole mobilities in the armchair direction, especially, will be enhanced dramatically. With a tunable bandgap, high carrier mobilities, and modifiable isotropy, our results indicate that few-layer h-AlN has potential applications in future mechano-electronic devices.

Comparative study of hybrid perovskite phototransistors based on CVD-grown and spin-coated MAPbI3

In this study, the properties of methyl ammonium lead iodide (MAPbI3) hybrid organic-inorganic perovskite films grown by spin-coating and chemical vapor deposition (CVD) method were examined and compared. Atomic force microscopy analyses indicate that the CVD-grown MAPbI3 films exhibit a smoother surface compared to their spin-coated counterparts. Phototransistors incorporating CVD-grown MAPbI3 as the active layer exhibit superior electrical characteristics, with field effect mobility values higher by approximately one order of magnitude. The degradation of phototransistors under negative bias stress (NBS) and positive bias stress (PBS) in the presence of illumination was evaluated and discussed based on the charge trapping mechanism.

Low-temperature formation of high-mobility a-InGaZnOx films using plasma-enhanced reactive processes

This work demonstrated the low-temperature fabrication of high-mobility a-InGaZnOx (a-IGZO) films using plasma-enhanced reactive processing. The effect of the post-processing temperature on the performance of IGZO thin-film transistors (TFTs) using these a-IGZO films as a channel layer was also investigated. IGZO TFTs incorporating a-IGZO films plasma-treated at temperatures as low as 200 °C exhibited the expected TFT transfer characteristics, with field effect mobility values as high as 40 cm2 V−1 s−1. The fabrication of IGZO TFTs that exhibited homogeneous characteristics over large substrate areas was attempted, by assessing the effects of the plasma treatment on a-IGZO films deposited at various oxygen flow rate ratios. The electrical properties of IGZO TFTs using plasma-treated films were found to be dramatically improved, irrespective of the film deposition conditions prior to the plasma treatment. These results confirm the feasibility of producing IGZO TFTs with uniform characteristics over large substrates areas.

P‐194: Late‐News Poster: Highly Stable Thin‐Film Transistors Based on Amorphous Perovskite Semiconductors

In this work, Indium‐free amorphous Zn‐Ba‐Sn‐O (ZBTO) thin films and resulting thin‐film transistor (TFT) characteristics were investigated. ZBTO films were fabricated by co‐sputtering system, using ZnSnO3 (ZTO) and BaSnO3 (BTO) ceramic targets. The BaSnO3 was selected because of its relatively large heat of formation compared to ZTO, and therefore enhancing structural ordering and less defective structure. The applied BTO target power was varied to investigate Ba content effect on TFT performances. X‐ray photoelectron spectroscopy (XPS) and absorption spectra revealed that the subgap states including tail states were significantly suppressed by Ba incorporation. Moreover, it was found that the optimized ZBTO TFTs shown field effect mobility value over 20 cm2/Vs in saturation regime, and ultra‐stable stability (ΔVth < 1.0 V) under negative bias illumination stress (NBIS).

Inferring changes in π-stack mobility induced by aging from vibronic transitions in poly(3-hexylthiophene-2,5-diyl) films

The mobility of charge carriers in polymeric semiconductors is limited by their motion between individual planar conjugated chromophores and is affected by aging. Direct mobility measurements require extraction of charges generated by injection, electrical-field effect, photoexcitation or doping. Effective mobility values are therefore sometimes difficult to interpret and additional investigations help to shed light on the underlying mechanisms that cause temporal changes. Here we employ UV-vis spectroscopy as a simple and contact free monitoring technique to predict mobility changes across the π-stack. UV–vis absorption of polymers is dominated by a vibronic progression of absorption intensities caused by a vinyl or ring stretching molecular distortion. We conjecture that differences in the relative absorption of the origin of the progression indicate alterations in the average length of the stacked chromophores. Mobility changes can then be inferred via Marcus theory from the coupling of displaced chromophores, which limits charge transport across the stack. However, differences in the high energy part indicate changes in the reorganization energy affecting the mobility exponentially. We report on mobility changes inferred by the proposed approach from UV–vis spectra of poly(3-hexylthiophene-2,5-diyl) (P3HT) films during pre- or post-deposition aging. We show that the former leads to a mobility increase caused by the healing of stacking faults whereas the latter causes a decrease due to an increased reorganization energy. Moreover, we claim that for stacks with an average chromophore length larger then a typical displacement further aggregation in solution does not lead to films with improved mobility.

Strain-mediated stability and electronic properties of WS2, Janus WSSe and WSe2 monolayers

Monolayers of transition metal dichalcogenides (TMDs) have been proposed as the next generation electronic materials for nanoscale devices. We evaluated the thermodynamic stability of WS2, Janus WSSe and WSe2 monolayers under biaxial tensile and compressive strain using density functional approach. The phonon band structures of unstrained WS2, WSSe and WSe2 monolayers confirm their thermodynamic stability. The stability of these monolayers is investigated under strain and phonon softening is observed for acoustic and optical mode upto 8% tensile strain. The bond length reduction under compressive strain resulted in out of plane deformation, leading the instability of monolayers under compressive strain. The bond length W-S(Se) and bond angle W-S(Se)-W exhibit significant contribution in coupling strength between transition metal d-orbitals and chalcogen p-orbitals that mediated the electronic properties. The spin orbit coupling showed strong effect on splitting of the valence band for unstrained monolayers. The spin splitting in valence band increases with increasing the tensile strain and decreases with increasing the compressive strain. The effective masses and mobility values are 0.57me, 0.54me, 0.47me; and 0.059, 0.062, 0.072 m2V−1s−1 for unstrained WS2, WSSe and WSe2 monolayers, respectively. The tungsten dx2-y2 orbitals are mainly contributing to the conduction and valence band electronic states in compressive strained monolayers. In contrast under tensile strain tungsten dz2 orbitals contribute to conduction and valence band electronic states, causing the direct to indirect band gap transition in these monolayers. We observed that the impact of tensile strain is more sensitive as compared to that of compressive strain.

In-plane InSb nanowires grown by selective area molecular beam epitaxy on semi-insulating substrate

In-plane InSb nanostructures are grown on a semi-insulating GaAs substrate using an AlGaSb buffer layer covered with a patterned SiO2 mask and selective area molecular beam epitaxy. The shape of these nanostructures is defined by the aperture in the silicon dioxide layer used as a selective mask thanks to the use of an atomic hydrogen flux during the growth. Transmission electron microscopy reveals that the mismatch accommodation between InSb and GaAs is obtained in two steps via the formation of an array of misfit dislocations both at the AlGaSb buffer layer/GaAs and at the InSb nanostructures/AlGaSb interfaces. Several micron long in-plane nanowires (NWs) can be achieved as well as more complex nanostructures such as branched NWs. The electrical properties of the material are investigated by the characterization of an InSb NW MOSFET down to 77 K. The resulting room temperature field effect mobility values are comparable with those reported on back-gated MOSFETs based on InSb NWs obtained by vapor liquid solid growth or electrodeposition. This growth method paves the way to the fabrication of complex InSb-based nanostructures.

Field‐Dependent Electrical and Thermal Transport in Polycrystalline WSe2

AbstractOwing to their desirable electrical and thermoelectric properties, transition metal dichalcogenides (TMDs) have attracted significant attention. It is important to develop an easy synthetic method and a simple device fabrication process for TMDs. In this study, WSe2 films were synthesized on a large scale by thermally assisted conversion (TAC) of W films on SiO2/Si substrates at 600 °C. The TAC process yields homogeneous polycrystalline films of controlled thickness over large areas which have the advantage that they can be adapted for mass production for applications in electronics and thermoelectrics. In this regard, pre‐patterning of the deposited metal films allows for devices to be easily fabricated without any etch process. UV‐lithography‐defined W structures have been deposited and after conversion to WSe2 their electrical and thermoelectric properties have been studied. Using e‐beam lithography, a field effect transistor (FET) with a WSe2 channel was fabricated. This showed p‐type behavior and reasonable field effect mobility value. The thermoelectric properties of WSe2 thin films were analyzed by additionally integrating micro heating elements to the WSe2 FET. The maximum Seebeck coefficient and power factor (S2·σ) values were calculated to be ≈61 mV·K−1 (Vg = 45 V) and ≈1.3 nW·K−2·cm−1, respectively.

Study of GO-Cu2O and RGO-Cu nanocomposite monolayer sheets prepared by modified Langmuir Blodgett route

The modified Langmuir-Blodgett (MLB) technique has been improvised and extended to transfer GO-Cu2O nanocomposite monolayer sheets, by introducing Cu2+ ions into the subphase at room temperature. Morphological studies of as-transferred sheets revealed the presence of closely spaced GO monolayer sheets, with slightly enhanced roughness. XPS studies of as-transferred sheets confirmed the presence of copper, either as metallic Cu or Cu2O, along with significant Cu(OH)2 component, but TEM results confirmed the formation of Cu2O nanocrystallites of size (7 ± 2) nm, distributed uniformly over GO sheets. After heat treatment in vacuum at 400 °C, the nanocomposite sheets were covered with a uniform distribution of larger size nanoparticles. Based on Raman, XPS and TEM studies it has been confirmed that heat treatment at 400 °C in vacuum results in the formation of agglomerated Cu nanoparticles of size (23 ± 9) nm distributed uniformly over reduced graphene oxide (RGO) sheets. The electrical characterization of nanocomposite sheets on SiO2/Si in back-gated FET geometry revealed that the electrical conductivity of as-transferred GO-Cu2O sheets was similar to that usually observed for GO monolayer sheets. The RGO-Cu sheets also displayed electrical conductivity and field effect mobility values comparable to those reported for RGO sheets obtained by chemical/thermal reduction, and was unaffected by the presence of Cu nanoparticles.

Low temperature fabrication of CuxO thin-film transistors and investigation on the origin of low field effect mobility

Cuprous (Cu2O) and cupric (CuO) oxide thin films have been deposited by radio frequency magnetron sputtering with two different oxygen partial pressures. The as-deposited copper oxide films were subjected to post-annealing at 300 °C for 30 min to improve the microstructural, morphological, and optical properties of thin films. Optical absorption studies revealed the existence of a large number of subgap states inside CuO films than Cu2O films. Cu2O and CuO thin film transistors (TFTs) were fabricated in an inverted staggered structure by using a post-annealed channel layer. The field effect mobility values of Cu2O and CuO TFTs were 5.20 × 10−4 cm2 V−1 s−1 and 2.33 × 10−4 cm2 V−1 s−1, respectively. The poor values of subthreshold swing, threshold voltage, and field effect mobility of the TFTs were due to the charge trap density at the copper oxide/dielectric interface as well as defect induced trap states originated from the oxygen vacancies inside the bulk copper oxide. In order to study the distribution of the trap states in the Cu2O and CuO active layer, the temperature dependent transfer characteristics of transistors in the temperature range between 310 K and 340 K were studied. The observed subgap states were found to be decreasing exponentially inside the bandgap, with CuO TFT showing higher subgap states than Cu2O TFT. The high-density hole trap states in the CuO channel are one of the plausible reasons for the lower mobility in CuO TFT than in Cu2O TFT. The origin of these subgap states was attributed to the impurities or oxygen vacancies present in the CuO channel layer.

Charge Transport Limitations in Perovskite Solar Cells: The Effect of Charge Extraction Layers.

Understanding the charge transport characteristics and their limiting factors in organolead halide perovskites is of great importance for the development of competitive and economically advantageous photovoltaic systems derived from these materials. In the present work, we examine the charge carrier mobilities in CH3NH3PbI3 (MAPI) thin films obtained from a one-step synthesis procedure and in planar n-i-p devices based on these films. By performing time-of-flight measurements, we find mobilities around 6 cm2/V s for electrons and holes in MAPI thin films, whereas in working solar cells, the respective effective mobility values are reduced by 3 orders of magnitude. From complementary experiments on devices with varying thicknesses of electron and hole transport layers, we identify the charge extraction layers and the associated interfaces rather than the perovskite material itself as the major limiting factors of the charge carrier transport time in working devices.

Performance and stability of amorphous In-Ga-Zn-O thin film transistors involving gate insulators synthesized at low temperatures

This study consists of an investigation on the electrical characteristics of amorphous In-Ga-Zn-O (a-IGZO) thin-film-transistors (TFTs) using SiO2 and Al2O3 gate insulators deposited at temperatures lower than 200 °C. The SiO2 and Al2O3 layers were synthesized by radio-frequency magnetron sputtering and atomic layer deposition (ALD). The a-IGZO TFTs involving stacked Al2O3/SiO2 gate insulators exhibit relatively high performance compared to those based on single SiO2 dielectrics. The maximum field effect mobility and subthreshold swing (SS) values obtained were 6.96 cm2/Vs and 0.28 V/decade, respectively. The threshold voltage shift (ΔVth) under negative bias stress (NBS), negative bias illumination stress (NBIS) and positive bias stress (PBS) were −0.38V, −2.05 V and 1.98 V, respectively. Here it is suspected that the defect content in sputtered SiO2 is reduced when it is grown onto ALD Al2O3, thus enhancing the device performance and stability.

(Invited) Interface Engineering of High-k Dielectrics and Metal Contacts for High Performance Top-Gated MoS2 FETs

The 2D materials, in particular Transition Metals Di-chalcogenides (TMD), are potential candidates to scale CMOS technology beyond Silicon. The ability to deposit ultrathin TMD films, down to a mono-layer, offers excellent electrostatic control of the channel by the gate electrode. This enables aggressive scaling of the channel length of transistor, due to superior short channel performance. In particular, few layer Molybdinum Disulphide (MoS2), with a bandgap comparable to Silicon, has emerged as a promising candidate to to enable continued transistors scaling. However, high performance and reliable gate dielectric and contact formation continues to be an open problem. Since the impurity doping strategies for MoS2 are still not clear, the ohmic contact formation is limited by the nature of metal-semiconductor schottky contact. Despite the Fermi level pinning close to conduction band of MoS2, a low source/drain contact resistance value for NMOS transistor is limited by the surface state effects. The problem is exasperated due to large variations in contact resistance values within the wafer, possibly due to surface defects and stoichiometric variations. We demonstrate that appropriate surface preparation can drastically enhance the performance of ohmic contacts on MoS2. In particular, we present a surface-state engineering technique using sulfur treatment of MoS2 before the contact formation. The transistors (L=W= ~1μm) are formed on exfoliated few layer MoS2 flakes (5-7 nm thick). The flakes are treated with ammonium sulphide solution (NH4)2S (Sigma Aldrich, 40% solution in H2O) for 5 min at a temperature of 50 °C, followed by De-ionized water rinse and N2 blow-dry. Nickel (Ni) and Palladium (Pd) contacts are formed using lift-off process. Through extensive material and electrical characterization, we demonstrate that the sulfur treatment reduces to schottky barrier height by 81 meV (Ni) and 135 meV (Pd) due to stronger pinning of Fermi level. This in turn lowers the contact resistance by as much as 10 times with concomitant increase in mobility by a factor of 2. We demonstrate record drain current of 170μA/μm, with complete mitigation of contact resistance and hence drain current variability. Due to the inert basal plane of 2D materials, it is difficult nucleate the growth of dielectrics using CVD and/or ALD techniques, which are routinely used for high-k dielectric integration on Silicon. Although some techniques have been proposed to nucleate the dielectric growth on MoS2, the device performance is inferior due to interface defects. To circumvent this issue, we develop a very high performance HfO2 thin film using electron beam evaporation. The O2 flow rate is optimized during e-beam evaporation to achieve 30nm HfO2 films with k=19 and EOT=6.1 nm. The interface trap density is estimated to be 8.7x1011 /cm2, resulting in a very high field effect mobility value of 63 cm2/V-s. The HfO2 gate oxide results in a very low leakage current of 7x10-7 A/cm2. The top gated transistors fabricated with this dielectric yield one of the highest reported drain current value of 180 μA/μm, with near ideal subthreshold behaviour (60 to 75 mV/decade). We also utilize 30nm HfO2 dielectric as a substrate to deposit MOS2 flakes, instead of conventional 300nm SiO2 substrate. Finally we demonstrate an NMOS inverter circuit using HfO2 top-gated devices using a depletion mode NMOS transistor as a resistive load. The Inverter exhibits an output to input gain of about 9. Figure 1