Showing papers on "Field effect published in 2013"
TL;DR: This work proposes a novel tunnelling heterostructure by replacing one of the metal electrodes in a normal FTJ with a heavily doped semiconductor, which suggests their great potential in non-destructive readout non-volatile memories.
Abstract: Ferroelectric tunnel junctions (FTJs), composed of two metal electrodes separated by an ultrathin ferroelectric barrier, have attracted much attention as promising candidates for non-volatile resistive memories. Theoretical and experimental works have revealed that the tunnelling resistance switching in FTJs originates mainly from a ferroelectric modulation on the barrier height. However, in these devices, modulation on the barrier width is very limited, although the tunnelling transmittance depends on it exponentially as well. Here we propose a novel tunnelling heterostructure by replacing one of the metal electrodes in a normal FTJ with a heavily doped semiconductor. In these metal/ferroelectric/semiconductor FTJs, not only the height but also the width of the barrier can be electrically modulated as a result of a ferroelectric field effect, leading to a greatly enhanced tunnelling electroresistance. This idea is implemented in Pt/BaTiO3/Nb:SrTiO3 heterostructures, in which an ON/OFF conductance ratio above 10(4), about one to two orders greater than those reported in normal FTJs, can be achieved at room temperature. The giant tunnelling electroresistance, reliable switching reproducibility and long data retention observed in these metal/ferroelectric/semiconductor FTJs suggest their great potential in non-destructive readout non-volatile memories.
509 citations
TL;DR: The high stability in electronic performance of the devices upon bending up to ±2.2 mm in compressive and tensile modes, and the ability to recover electrical properties after degradation upon annealing, reveal the efficacy of using 2D materials for creating highly flexible and transparent devices.
Abstract: A highly flexible and transparent transistor is developed based on an exfoliated MoS2 channel and CVD-grown graphene source/drain electrodes. Introducing the 2D nanomaterials provides a high mechanical flexibility, optical transmittance (∼74%), and current on/off ratio (>10(4)) with an average field effect mobility of ∼4.7 cm(2) V(-1) s(-1), all of which cannot be achieved by other transistors consisting of a MoS2 active channel/metal electrodes or graphene channel/graphene electrodes. In particular, a low Schottky barrier (∼22 meV) forms at the MoS2 /graphene interface, which is comparable to the MoS2 /metal interface. The high stability in electronic performance of the devices upon bending up to ±2.2 mm in compressive and tensile modes, and the ability to recover electrical properties after degradation upon annealing, reveal the efficacy of using 2D materials for creating highly flexible and transparent devices.
274 citations
TL;DR: It is shown that exchange bias is reversibly switched between two stable states with opposite exchange bias polarities upon ferroelectric poling of the BFO and a model is proposed to explain the control of exchange bias.
Abstract: We report the creation of a multiferroic field effect device with a BiFeO(3) (BFO) (antiferromagnetic-ferroelectric) gate dielectric and a La(0.7)Sr(0.3)MnO(3) (LSMO) (ferromagnetic) conducting channel that exhibits direct, bipolar electrical control of exchange bias. We show that exchange bias is reversibly switched between two stable states with opposite exchange bias polarities upon ferroelectric poling of the BFO. No field cooling, temperature cycling, or additional applied magnetic or electric field beyond the initial BFO polarization is needed for this bipolar modulation effect. Based on these results and the current understanding of exchange bias, we propose a model to explain the control of exchange bias. In this model the coupled antiferromagnetic-ferroelectric order in BFO along with the modulation of interfacial exchange interactions due to ionic displacement of Fe(3+) in BFO relative to Mn(3+/4+) in LSMO cause bipolar modulation.
253 citations
TL;DR: In this article, Molybdenum disulfide (MoS2) field effect transistors (FETs) were fabricated on atomically smooth large-area single layers grown by chemical vapor deposition.
Abstract: Molybdenum disulfide (MoS2) field effect transistors (FET) were fabricated on atomically smooth large-area single layers grown by chemical vapor deposition. The layer qualities and physical properties were characterized using high-resolution Raman and photoluminescence spectroscopy, scanning electron microscopy, and atomic force microscopy. Electronic performance of the FET devices was measured using field effect mobility measurements as a function of temperature. The back-gated devices had mobilities of 6.0 cm2/V s at 300 K without a high-κ dielectric overcoat and increased to 16.1 cm2/V s with a high-κ dielectric overcoat. In addition the devices show on/off ratios ranging from 105 to 109.
225 citations
TL;DR: The data suggest that the gap saturates at a large displacement field of D ~ 3 V/nm, in agreement with self-consistent Hartree calculations, and the band structure of the ABA trilayer continues to evolve with increasing D.
Abstract: We report on the transport properties of ABC and ABA stacked trilayer graphene using dual, locally gated field effect devices. The high efficiency and large breakdown voltage of the HfO2 top and bottom gates enable independent tuning of the perpendicular electric field and the Fermi level over an unprecedentedly large range. We observe a resistance change of 6 orders of magnitude in the ABC trilayer, which demonstrates the opening of a band gap. Our data suggest that the gap saturates at a large displacement field of D ∼ 3 V/nm, in agreement with self-consistent Hartree calculations. In contrast, the ABA trilayer remains metallic even under a large perpendicular electric field. Despite the absence of a band gap, the band structure of the ABA trilayer continues to evolve with increasing D. We observe signatures of two-band conduction at large D fields. Our self-consistent Hartree calculation reproduces many aspects of the experimental data but also points to the need for more sophisticated theory.
122 citations
TL;DR: In this article, a review of metal-insulator transition mechanisms in correlated electron materials and three-terminal field effect devices utilizing such correlated oxides as the channel layer is presented.
Abstract: Correlated electron systems are among the centerpieces of modern condensed matter sciences, where many interesting physical phenomena, such as metal-insulator transition and high-T c superconductivity appear. Recent efforts have been focused on electrostatic doping of such materials to probe the underlying physics without introducing disorder as well as to build field-effect transistors that may complement conventional semiconductor metal-oxide-semiconductor field effect transistor (MOSFET) technology. This review focuses on metal-insulator transition mechanisms in correlated electron materials and three-terminal field effect devices utilizing such correlated oxides as the channel layer. We first describe how electron-disorder interaction, electron-phonon interaction, and/or electron correlation in solids could modify the electronic properties of materials and lead to metal-insulator transitions. Then we analyze experimental efforts toward utilizing these transitions in field effect transistors and their ...
122 citations
TL;DR: In this paper, temperature dependent I-V measurements of short channel MoS2 field effect devices at high source-drain bias were presented, and the existence of an exponential distribution of trap states was observed.
Abstract: We present temperature dependent I-V measurements of short channel MoS2 field effect devices at high source-drain bias. We find that, although the I-V characteristics are ohmic at low bias, the conduction becomes space charge limited at high V-DS, and existence of an exponential distribution of trap states was observed. The temperature independent critical drain-source voltage (V-c) was also determined. The density of trap states was quantitatively calculated from V-c. The possible origin of exponential trap distribution in these devices is also discussed. (C) 2013 AIP Publishing LLC.
105 citations
TL;DR: The change in the optical bandgap supported by the Burstein-Moss theory could successfully show a mobility increase that is related to interstitial doping of alkali metal in ZTO semiconductors.
Abstract: Solution-processed and alkali metals, such as Li and Na, are introduced in doped amorphous zinc tin oxide (ZTO) semiconductor TFTs, which show better electrical performance, such as improved field effect mobility, than intrinsic amorphous ZTO semiconductor TFTs. Furthermore, by using spectroscopic UV-visible analysis we propose a comprehensive technique for monitoring the improved electrical performance induced by alkali metal doping in terms of the change in optical properties. The change in the optical bandgap supported by the Burstein-Moss theory could successfully show a mobility increase that is related to interstitial doping of alkali metal in ZTO semiconductors.
90 citations
TL;DR: In this article, the degradation of field effect transistors based on two-dimensional materials due to irradiation with heavy ions was investigated, and it was shown that the irradiation leads to significant changes of structural and electrical properties.
Abstract: We have investigated the deterioration of field effect transistors based on two-dimensional materials due to irradiation with swift heavy ions. Devices were prepared with exfoliated single layers of MoS2 and graphene, respectively. They were characterized before and after irradiation with 1.14 GeV U28+ ions using three different fluences. By electrical characterization, atomic force microscopy, and Raman spectroscopy, we show that the irradiation leads to significant changes of structural and electrical properties. At the highest fluence of 4 × 1011 ions/cm2, the MoS2 transistor is destroyed, while the graphene based device remains operational, albeit with an inferior performance.
85 citations
TL;DR: In this article, the transport properties of ABC and ABA stacked trilayer graphene using dual, locally gated field effect devices were investigated. And the authors showed that the gap saturates at a large displacement field of D ~ 3 V/nm, in agreement with selfconsistent Hartree calculations.
Abstract: We report on the transport properties of ABC and ABA stacked trilayer graphene using dual, locally gated field effect devices. The high efficiency and large breakdown voltage of the HfO2 top and bottom gates enables independent tuning of the perpendicular electric field and the Fermi level over an unprecedentedly large range. We observe a resistance change of six orders of magnitude in the ABC trilayer, which demonstrates the opening of a band gap. Our data suggest that the gap saturates at a large displacement field of D ~ 3 V/nm, in agreement with self-consistent Hartree calculations. In contrast, the ABA trilayer remains metallic even under a large perpendicular electric field. Despite the absence of a band gap, the band structure of the ABA trilayer continues to evolve with increasing D. We observe signatures of two-band conduction at large D fields. Our self-consistent Hartree calculation reproduces many aspects of the experimental data, but also points to the need for more sophisticated theory.
82 citations
TL;DR: The thermoelectric power factor of hole gas in individual Ge-Si core-shell nanowires with Ge core diameters ranging from 11 to 25 nm is experimentally studied to find the power factor is found to be closely correlated with the carrier mobility, which is higher than that of bulk Ge in one of the core- shell Nanowires studied here.
Abstract: We experimentally studied the thermoelectric power factor of hole gas in individual Ge–Si core–shell nanowires with Ge core diameters ranging from 11 to 25 nm. The Ge cores are dopant-free, but the Fermi level in the cores is pinned by surface and defect states in the epitaxial Si shell thereby doping the cores into the degenerate regime. This doping mechanism avoids the high concentration of dopants usually encountered in bulk thermoelectric materials and provides a unique opportunity to enhance the carrier mobility with suppressed ionized impurity scattering. Moreover, the carrier concentration in small diameter nanowires has also been effectively modulated by field effect, allowing one to probe the electrical conductivity and thermopower within a wide range of carrier concentrations, which is crucial to understand the thermoelectric transport behavior. We found that the thermopower of nanowires with Ge core diameters down to 11 nm still follows the behavior of bulk Ge. As a result, the power factor is ...
TL;DR: In this paper, the conduction channel of a single-crystal nanowires of the topological insulator Bi2Se3 can be used as the channel for a field effect transistor (FET), a basic circuit building block.
Abstract: Topological insulators are unique electronic materials with insulating interiors and robust metallic surfaces. Device applications exploiting their remarkable properties have so far been hampered by the difficulty to electrically tune the Fermi levels of both bulk and thin film samples. Here we show experimentally that single-crystal nanowires of the topological insulator Bi2Se3 can be used as the conduction channel in high-performance field effect transistor (FET), a basic circuit building block. Its current-voltage characteristics are superior to many of those reported for semiconductor nanowire transistors, including sharp turn-on, nearly zero cutoff current, very large On/Off current ratio, and well-saturated output current. The metallic electron transport at the surface with good FET effective mobility can be effectively separated from the conduction of bulk Bi2Se3 and adjusted by field effect at a small gate voltage. This opens up a suite of potential applications in nanoelectronics and spintronics.
TL;DR: A prototype graphene field effect sensor is examined for the study of the dielectric constant, pyroelectric coefficient, and ferroelectric polarization of 100-300 nm epitaxial (Ba,Sr)TiO3 thin films and how the polarization asymmetry and interface charge dynamics affect the electronic properties of graphene.
Abstract: We examine a prototype graphene field effect sensor for the study of the dielectric constant, pyroelectric coefficient, and ferroelectric polarization of 100–300 nm epitaxial (Ba,Sr)TiO3 thin films. Ferroelectric switching induces hysteresis in the resistivity and carrier density of n-layer graphene (n = 1–5) below 100 K, which competes with an antihysteresis behavior activated by the combined effects of electric field and temperature. We also discuss how the polarization asymmetry and interface charge dynamics affect the electronic properties of graphene.
TL;DR: The enhanced field effect mobility and stability obtained for the superlattice TFT devices were explained on the basis of layer-by-layer growth mode, improved crystalline nature of the channel layers, and passivation effect of Al2O3 layers.
Abstract: High-performance thin-film transistors (TFTs) are the fundamental building blocks in realizing the potential applications of the next-generation displays. Atomically controlled superlattice structures are expected to induce advanced electric and optical performance due to two-dimensional electron gas system, resulting in high-electron mobility transistors. Here, we have utilized a semiconductor/insulator superlattice channel structure comprising of ZnO/Al2O3 layers to realize high-performance TFTs. The TFT with ZnO (5 nm)/Al2O3 (3.6 nm) superlattice channel structure exhibited high field effect mobility of 27.8 cm2/Vs, and threshold voltage shift of only < 0.5 V under positive/negative gate bias stress test during 2 hours. These properties showed extremely improved TFT performance, compared to ZnO TFTs. The enhanced field effect mobility and stability obtained for the superlattice TFT devices were explained on the basis of layer-by-layer growth mode, improved crystalline nature of the channel layers, and passivation effect of Al2O3 layers.
TL;DR: A novel estimate of the electron mobility is given that is compared with the result of standard field-effect based mobility estimates and discussed in relation to the effect of charge traps in the devices.
Abstract: Millivolt range thermovoltage is demonstrated in single InAs nanowire based field effect transistors. Thanks to a buried heating scheme, we drive both a large thermal bias ΔT > 10 K and a strong field-effect modulation of electric conductance on the nanostructures. This allows the precise mapping of the evolution of the Seebeck coefficient S as a function of the gate-controlled conductivity σ between room temperature and 100 K. Based on these experimental data a novel estimate of the electron mobility is given. This value is compared with the result of standard field-effect based mobility estimates and discussed in relation to the effect of charge traps in the devices.
TL;DR: In this article, the Stern layer effect is taken into account for active control of surface charge property and electroosmotic flow in a silica-based nanochannel using a field effect transistor (FET).
Abstract: Active control of surface charge property and electroosmotic flow (EOF) in a silica-based nanochannel using a field effect transistor (FET) is analyzed for the first time taking the Stern layer effect into account. Approximations for surface charge property and EOF have been derived and validated by comparing their predictions with experimental data available in the literature. We show that, in addition to the background solution properties such as its pH and salt concentration, the field effect control of the zeta potential of the nanochannel wall and the EOF velocity depends highly on the surface capacitance of the Stern layer, stemming from the attraction of immobile counterions within that layer. The Stern layer effect becomes significant when the background salt concentration, solution pH, and/or the applied gate potential are relatively high. Results gathered provide valuable information for designing relevant gated nanofluidic devices for regulating ion, fluid flow, and biomolecule transport.
TL;DR: It is demonstrated that solvothermally synthesized Bi(2)Se(3) nanoplates are attractive for topological insulator studies, and pronounced ambipolar field effect is observed that demonstrates the flexible manipulation of carrier type and concentration for these nanostructures.
Abstract: A topological insulator is a new phase of quantum matter with a bulk band gap and spin-polarized surface states, which might find use in applications ranging from electronics to energy conversion. Despite much exciting progress in the field, high-yield solution synthesis has not been widely used for the study of topological insulator behavior. Here, we demonstrate that solvothermally synthesized Bi2Se3 nanoplates are attractive for topological insulator studies. The carrier concentration of these Bi2Se3 nanoplates is controlled by compensational Sb doping during the synthesis. In low-carrier-density, Sb-doped Bi2Se3 nanoplates, we observe pronounced ambipolar field effect that demonstrates the flexible manipulation of carrier type and concentration for these nanostructures. Solvothermal synthesis offers an affordable, facile approach to produce high-quality nanomaterials to explore the properties of topological insulators.
TL;DR: In this paper, single-layer graphene was transferred onto a PMN-PT substrate to investigate the transport properties of graphene-based field effect transistors (FETs) by ferroelectric gating.
Abstract: Single-layer graphene was transferred onto (1 – x)[Pb(Mg1/3Nb2/3)O3]–x[PbTiO3]0.3 (PMN-PT) substrate to investigate the transport properties of graphene-based field effect transistors (FETs) by ferroelectric gating. The graphene/PMN-PT FET exhibited p-type characteristics with a large memory window and an on/off current ratio of about 5.5 in air ambient conditions at room temperature. By prepoling the PMN-PT substrate, the FET showed a reduction in p-doping for the graphene/PMN-PT FET, implying the pre-polarization and the polarization reversal played an important part in the behaviors of graphene on PMN-PT. The observation of simultaneous rise in gate current with the dramatic transition in drain current suggested that the transport properties of graphene mainly stemmed from the coupling of the ferroelectric polarization to the charge carriers in graphene. The field effect mobility and the excess hole concentration were calculated to be about 4.52 × 103 cm2 V–1 s–1 and 6.74 × 1012 cm–2, respectively. Fur...
TL;DR: In this paper, the humidity dependent properties of a single Sb doped SnO2 nanowire field effect transistor (NWFET) were investigated under different relative humidities (RHs) at room temperature.
Abstract: This work reports the humidity dependent properties of a single Sb doped SnO2 nanowire field effect transistor (NWFET). The NWFET is fabricated by a lithography method on a highly doped silicon substrate as back gate covered by oxide as gate dielectric. The electric properties of the device under different relative humidities (RHs) at room temperature are investigated. The NWFET exhibits a field effect mobility of 108.7 cm2/(V s), a subthreshold swing of 70 mV/decade, and a drain current on/off ratio of 106. The threshold voltage shifts from −11.2 V to −14.6 V as RH increases from 22% to 40%. The NWFET exhibits sensitive behaviors to the humidity, which is promising for the application in humidity sensors.
TL;DR: In this article, a deterministic assembly of ultrathin metal oxide-semiconductor field effect transistors on bulk wafers with (111) orientation provides a route to high quality electronics on nearly any type of substrate.
Abstract: Deterministic assembly of ultrathin metal oxide-semiconductor field-effect transistors released from the surfaces of bulk wafers with (111) orientation provides a route to high quality electronics on nearly any type of substrate. Device parameters and bias stability characteristics from transistors on sheets of plastic confirm the effectiveness of the approach and the critical roles of thermally grown layers of silicon dioxide for the gate dielectrics and passivation layers. Systematic studies of the anisotropic etching processes used to release the devices illustrate capabilities into the sub-micron thickness regime, with beneficial effects on the bending stiffness and degree of bendability.
TL;DR: Al2O3/W heterogeneous nanopore arrays for field effect modulated nanofluidic diodes are fabricated by transferring self-organized nanopores of anodic aluminium oxide into a W thin film as discussed by the authors.
Abstract: We developed Al2O3/W heterogeneous nanopore arrays for field effect modulated nanofluidic diodes. They are fabricated by transferring self-organized nanopores of anodic aluminium oxide into a W thin film. The nanopores are ∼20 nm in diameter and 400 nm in length. After mild oxidation, approximately 10 nm WO3 grows on the surface of W, forming a conformal and dense dielectric layer. It allows the application of an electrical field through the surrounding W electrode to modulate the ionic transport across the entire membrane. Our experimental findings have potential applications in high throughput controlled delivery and electrostatic sorting of biomolecules.
TL;DR: In this paper, the influence of chemical and field effect passivation of a-Si:H films on the junction recombination at maximum power point conditions of silicon heterojunction solar cells is investigated.
TL;DR: In this article, the authors grow perpendicular L10-FePt films epitaxially on (001)[Pb(Mg1/3Nb2/3)]0.7-(PbTiO3)0.3 ferroelectric substrates.
Abstract: We grow perpendicular L10-FePt films epitaxially on (001)[Pb(Mg1/3Nb2/3)]0.7-(PbTiO3)0.3 ferroelectric substrates. Due to the magnetostriction effect, the out-of-plane coercivity (Hc⊥) of the L10-FePt varies with applied electric fields, showing an asymmetric butterfly-like loop. The Hc⊥ at the zero-electric-field state (Hc⊥,0) shows a nonvolatile change, depending on the direction of the poling electric field. The magnitude of nonvolatile magnetic anisotropy change, induced by the ferroelectric field effect, can be comparable to the anisotropy change induced by pure electric fields. The nonvolatile magnetic anisotropy change is inversely proportional to the FePt thickness and can be eliminated by inserting a metallic intermediate layer.
TL;DR: In this article, a new family of isomeric carbazolocarbazole derivatives, namely carbazolo[1,2-a] carbazole, carbazalo[3, 2-b]-carbazole and carbazlo[4, 3-c]-carbazole is presented.
Abstract: We report here the synthesis and characterization of a new family of isomeric carbazolocarbazole derivatives, namely carbazolo[1,2-a]carbazole, carbazolo[3,2-b]carbazole and carbazolo[4,3-c]carbazole. Thermal, optical, electrochemical, morphological and semiconducting properties have been studied to understand the influence of geometrical isomerism on the optoelectronic properties of these compounds. Different packing patterns have been observed by single crystal X-ray diffraction (XRD) which then correlate with the different morphologies of the evaporated thin films studied by XRD and Atomic Force Microscopy (AFM). The effect of N-substituents has also been evaluated for one of the isomers revealing a noticeable influence on the performance as organic semiconductors in Organic Field Effect Transistors (OFETs). A good p-channel field effect has been determined for N,N′-dioctylcarbazolo[4,3-c]carbazole with a mobility of 0.02 cm2 V−1 s−1 and Ion/Ioff ratio of 106 in air. These preliminary results demonstrate the promising properties of molecular carbazolocarbazole systems which should be further explored in the area of organic semiconducting materials.
TL;DR: Organic charge modulated FETs, whose features can be optimized for charge detection in liquid solutions, are focused on and the results of the measurement of different bio-related effects are shown.
Abstract: The ability of field effect transistors (FETs) to detect charge variations on the gate may be exploited for realizing chemo- and bio-sensors. In this paper, we focus our attention on a particular kind of field effect device, named organic charge modulated FETs, whose features can be optimized for charge detection in liquid solutions. The results of the measurement of different bio-related effects are shown. In particular, DNA hybridization and pH detection in liquid media are proposed. Finally, preliminary considerations about the applicability of these devices to the detection of the electrical activity of cells are also provided. The device has considerable potential for being employed as a reliable, high sensitivity, low cost technology for sensing signals derived from living systems.
TL;DR: In this article, the effect of surface roughness on the electrical characteristics of amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors was investigated.
TL;DR: In this article, the authors exploit the low density of electronic states of graphene to modulate the tunnel current flowing perpendicular to the atomic layers of a multi-layer graphene-boron nitride device.
Abstract: We exploit the low density of electronic states of graphene to modulate the tunnel current flowing perpendicular to the atomic layers of a multi-layer graphene-boron nitride device. This is achieved by using the electric field effect to raise the Fermi energy of the graphene emitter layer and thereby reduce the effective barrier height for tunneling electrons. We discuss how the electron charge density in the graphene layers and the properties of the boron nitride tunnel barrier determine the device characteristics under operating conditions and derive expressions for carrier tunneling in these highly anisotropic layered heterostructures.
TL;DR: This work fabricates multigated silicon nanowires (Si NWs) and demonstrates significant modulation of electrical conductivity and the Seebeck coefficient with gate bias, demonstrating that power factor for the gated Si NWs is similar to the highest values reported for n-type Si nanostructures despite charge transport only occurring at the NW surface.
Abstract: Electric-field-induced charge carriers typically exhibit greater mobility over carriers contributed by chemical dopants and offer a powerful mechanism for thermoelectric power factor enhancement. We fabricate multigated silicon nanowires (Si NWs) and demonstrate significant modulation of electrical conductivity and the Seebeck coefficient with gate bias. Because of the higher mobility of field-effect charge carriers, we demonstrate that power factor for the gated Si NWs is similar to the highest values reported for n-type Si nanostructures despite charge transport only occurring at the NW surface. Field-effect doping is a promising strategy for optimizing power factor and may result in significant power factor enhancement in smaller diameter Si NWs where high average carrier densities can be obtained with induced surface charge.
TL;DR: In this article, the structural, electrical and optical properties of high-performance, low-temperature and solution-processed alkali metal-doped ZnO TFTs were studied using various analytic instruments, including HR-TEM, AFM, XPS, EDS, electrical bias stability test and UV-vis spectroscopy.
Abstract: The structural, electrical and optical properties of high-performance, low-temperature and solution-processed alkali metal-doped ZnO TFTs were studied using various analytic instruments, including HR-TEM, AFM, XPS, EDS, electrical bias stability test and UV-vis spectroscopy. Furthermore, we successfully demonstrated that a change in the optical bandgap energy of Li-doped ZnO semiconductor films supported by Burstein–Moss theory can show a trade-off relationship between the field effect mobility of Li-ZnO TFTs and the Li doping concentrations. The relative broadening of the Eopt values, which are strongly related to the amount of excited electrons from the Fermi level in the valance band to the conduction band, was observed from the undoped ZnO film to the Li-doped ZnO film (10 mol%). The increase in the electron donor concentration was the dominant reason for the enhancement in the electron mobility of the alkali metal-doped ZnO TFTs.
TL;DR: In this article, an analytical model was developed that describes the Fermi level shift as a function of the gate voltage, of the substrate work function, and of the type and thickness of the dielectric spacer.
Abstract: We study how the Fermi energy of a graphene monolayer separated from a conducting substrate by a dielectric spacer depends on the properties of the substrate and on an applied voltage. An analytical model is developed that describes the Fermi level shift as a function of the gate voltage, of the substrate work function, and of the type and thickness of the dielectric spacer. The parameters of this model, that should describe the effect of gate electrodes in field effect devices, can be obtained from density functional theory (DFT) calculations on single layers or interfaces. The doping of graphene in metal| dielectric| graphene structures is found to be determined not only by the difference in work function between the metal and graphene and the dielectric properties of the spacer but potential steps that result from details of the microscopic bonding at the interfaces also play an important role. The doping levels predicted by the model agree very well with the results obtained from first-principles DFT calculations on metal| dielectric| graphene structures with the metals Al, Co, Ni, Cu, Pd, Ag, Pt, or Au, and a h -BN or vacuum dielectric spacer