Fine structure in the field effect mobility of MOS transistors

We have studied the fine structure in the field effect mobility of MOS transistors at 4.2 K. With applied substrate bias, the position in gate voltage of the structure is unchanged and the amplitude decreases. The position of the structure is also unchanged by normal magnetic fields of up to 150 kgauss. We have studied samples with four equivalent source-drain contacts. The structure depends strongly on the contacts. The structure in the field effect mobility appears to be due to the contacts and not a property of the inversion layer or surface states.     

Extrinsic limiting factors of carrier transport in organic field-effect transistors

Extrinsic factors to disturb the carrier transport in pentacene field-effect transistors (FETs), as a representative of the high-mobility organic FETs (OFETs), have been comprehensively analyzed by using atomic-force-microscope potentiometry (AFMP), microscopic four-point-probe field-effect transistor (MFPP-FET) measurement, and other techniques. In the first part, by mainly using AFMP as a powerful tool to reveal the potential distribution in working OFETs, we show how and how much the formation of source/drain electrodes influences the apparent field-effect mobility both for top- and bottom-contact configurations. In the second part, we show the influence of irregular grain structures and regular grain boundaries. The films grown both at very low and high temperature ranges contain distinctive insulating parts, which make the apparent mobility very low. Within the moderate growth temperature range, the intrinsic field-effect mobility obtained by MFPP-FET measurement is proportional to the average grain size. This behavior is well explained by the polycrystalline model with the diffusion theory. According to the observations in this work, it is obvious that these extrinsic limiting factors must be carefully excluded to discuss the intrinsic mechanism of the carrier transport in OFETs.     

有机场效应晶体管的非线性注入模型

有机场效应晶体管(Organic field effect transistor,OFET)的非线性特性是指其输出特性曲线在较低的漏极电压下出现类似于二极管的电压电流特性曲线,这种现象在有机场效应晶体管的实验研究中极为常见。Simonetti等通过引入随栅极电压变化的迁移率提出了模型并成功解释了这一现象,但实验中从器件转移特性得出的迁移率通常与栅极电压无关。本文通过引入常数迁移率对该模型进行改进,运用改进的模型研究了影响OFET非线性特性的主要因素,并对如何更加准确地获得器件参数进行了探究。     

Crystal growth of small-molecule organic semiconductors with nucleation additive

In this study, we employ a nucleation additive 4-octylbenzoic acid (OBA) with an optimized solvent evaporation method to regulate crystal orientation and grain width of small-molecule organic semiconductors. When 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was utilized as a benchmark material to mix with the additive, a self-assembled OBA interfacial layer was formed and promoted uniform deposition of nucleation seeds. As a result, the TIPS pentacene/OBA blend crystalline film exhibited crystal alignment in long range order, attributing to a 11-fold reduction of the crystal misorientation angle and a 4-fold increase of the grain width. We further discussed the important correlation between the effective hole mobility, grain boundaries, grain width and length, and nucleation sites. Organic thin film transistors were fabricated to test charge transport, yielding a hole mobility of up to 0.17 cm²/V. This work provides a new pathway to modulate the nucleation and crystallization events of organic semiconductors, and can potentially be applied to optimize the thin film morphology and electrical performance of organic semiconducting materials in general.     

Anomalous surface photoconductivity in hydrogenated amorphous silicon

The intensity and time dependence of the source-drain photocurrent is measured on hydrogenated amorphous silicon (a-Si:H) field effect transistors as a function of gate bias. The photocurrent increases rapidly, becomes weakly dependent on excitation intensity, and exhibits long decay times. A model in which the relaxation of the space charge dominates the photocurrent quantitatively predicts the experimental data. The results show that anomalous surface photoconductivity is often the dominant photoresponse of a-Si:H.     

Study of the motion of individual triple junctions in aluminum

The mobility of individual triple junctions in aluminum is studied. Triple junctions with 〈111〉, 〈100〉, and 〈110〉 tilt boundaries are studied. The data obtained show that, at low temperatures, the mobility of the system of grain boundaries with a triple junction is controlled by the mobility of the triple junction (the junction kinetics). At high temperatures, the system mobility is determined by the mobility of the grain boundaries (the boundary kinetics). There is a temperature at which the transition from the junction kinetics to the boundary kinetics occurs; this temperature is determined by the crystallographic parameters of the sample.     

Grain Boundary Dynamics: A Novel Tool for Microstructure Control

The reaction of grain boundaries to a wide spectrum of forces is reviewed. Curvature, volume energy and mechanical forces are considered. The boundary mobility is strongly dependent on misorientation, which is attributed to both grain boundary structure and segregation. In magnetically anisotropic materials grain boundaries can be moved by magnetic forces. For the first time a directionality of boundary mobility is reported. Flat boundaries can also be moved by mechanical forces, which sheds new light on microstructure evolution during elevated temperature deformation. Curvature driven and mechanically moved boundaries can behave differently. A sharp transition between the small and large angle boundary regime is observed. It is shown that grain boundary triple junctions have a finite mobility and thus, may have a serious impact on grain growth in fine grained materials. The various dependencies can be utilized to influence grain boundary motion and thus, microstructure evolution during recrystallization and grain growth.     

Laser crystallization for large-area electronics

Laser crystallization is reviewed for the purpose of fabrication of polycrystalline silicon thin film transistors (poly-Si TFTs). Laser-induced rapid heating is important for formation of crystalline films with a low thermal budget. Reduction of electrically active defects located at grain boundaries is essential for improving electrical properties of poly-Si films and achieving poly-Si TFTs with high performances. The internal film stress is attractive to increase the carrier mobility. Recent developments in laser crystallization methods with pulsed and continuous-wave lasers are also reviewed. Control of heat flow results in crystalline grain growth in the lateral direction, which is important for fabrication of large crystalline grains. We also report an annealing method using a high-power infrared semiconductor laser. High-power lasers will be attractive for rapid formation of crystalline films over a large area and activation of silicon with impurity atoms.     

Quantum interference effect in few-layered transition metal dichalcogenide

Van der Waals layered transition metal dichalcogenides (TMDCs), as atomically flat two-dimensional materials, have been studied extensively in both fundamental science and application fields in recent years. The reduced-dimensional properties of TMDCs not only provide a route for the fabricating of efficient field effect transistors and optoelectronic devices but also suggest the possibility of the devices that utilize quantum coherency. In this work, we characterize the electron transport properties of ReS₂, one of the TMDCs, at both room temperature and low temperature. Of particular note, we measured strong quantum conductance oscillations as a function of the gate voltages and source-drain voltages at reduced temperature, which is evidence of quantum coherent transport. This work unambiguously establishes ReS₂ as a promising candidate for future quantum materials.     

Grain boundaries of nanocrystalline materials – their widths, compositions, and internal structures

Nanocrystalline materials contain many atoms at and near grain boundaries. Sufficient numbers of Mössbauer probe atoms can be situated in grain boundary environments to make a clear contribution to the measured Mössbauer spectrum. Three types of measurements on nanocrystalline materials are reported here, all using Mössbauer spectrometry in conjunction with X-ray diffractometry, transmission electron microscopy, or small angle neutron scattering. By measuring the fraction of atoms contributing to the grain boundary component in a Mössbauer spectrum, and by knowing the grain size of the material, it is possible to deduce the average width of grain boundaries in metallic alloys. It is found that these widths are approximately 0.5 nm for fcc alloys and slightly larger than 1.0 nm for bcc alloys. Chemical segregation to grain boundaries can be measured by Mössbauer spectrometry, especially in conjunction with small angle neutron scattering. Such measurements on Fe-Cu and Fe₃Si-Nb were used to study how nanocrystalline materials could be stabilized against grain growth by the segregation of Cu and Nb to grain boundaries. The segregation of Cu to grain boundaries did not stabilize the Fe-Cu alloys against grain growth, since the grain boundaries were found to widen and accept more Cu atoms during annealing. The Nb additions to Fe₃Si did suppress grain growth, perhaps because of the low mobility of Nb atoms, but also perhaps because Nb atoms altered the chemical ordering in the alloy. The internal structure of grain boundaries in nanocrystalline materials prepared by high-energy ball milling is found to be unstable against internal relaxations at low temperatures. The Mössbauer spectra of the nanocrystalline samples showed changes in the hyperfine fields attributable to movements of grain boundary atoms. In conjunction with SANS measurements, the changes in grain boundary structure induced by cryogenic exposure and annealing at low temperature were found to be somewhat different. Both were consistent with a sharper density gradient between the crystalline region and the grain boundary region.     

Quantum galvanomagnetic properties of n-type inversion layers on Si(100) MOSFET

Transverse magnetoconductivity σ_xx and Hall effect in n-type inversion layers of Si(100) MOSFET are measured for various source-drain fields between 0.08 and 40 V/cm under magnetic fields up to 150 kOe at 1.4 K. Conductivity peaks in low Landau levels are in good agreement with theory. Effect of the source-drain field in the magnetoconductivity is found to be very important in higher Landau levels as well as in the appearance of the lowest Landau level peak. Immobile electrons are clearly observed in conductivity bottoms. Electrode geometry effect for Hall effect measurement under strong magnetic fields is discussed.     

Critical current of YBa(2)Cu(3)O(7-delta) low-angle grain boundaries

Transport critical current measurements have been performed on 5 degrees [001]-tilt thin film YBa(2)Cu(3)O(7-delta) single grain boundaries with the magnetic field rotated in the plane of the film, phi. The variation of the critical current has been determined as a function of the angle between the magnetic field and the grain boundary plane. In applied fields above 1 T the critical current j(c) is found to be strongly suppressed only when the magnetic field is within an angle phi(k) of the grain boundary. Outside this angular range the behavior of the artificial grain boundary is dominated by the critical current of the grains. We show that the phi dependence of j(c) in the suppressed region is well described by a flux cutting model.     

最小二乘拟合计算有机薄膜晶体管迁移率的研究

本文通过制备了一个基于并五笨为有源层的顶栅底接触OTFT器件获取电流电压实验数据,并运用电流电压特性曲线理论拟合计算方法计算其场效应迁移率.研究发现,采用不同的拟合方法得到的场效应迁移率值有较大的差异.若选取转移特性曲线线性区距中心1/2范围内测试点进行最小二乘拟合计算出的场效应迁移率能减少采用其他拟合方法的固有不准确性,而且与其他方法得到的场效应迁移率最接近. 关键词： 最小二乘拟合 场效应迁移率 有机薄膜晶体管     

Boundary Mobility and Energy Anisotropy Effects on Microstructural Evolution During Grain Growth

We have performed mesoscopic simulations of microstructural evolution during curvature driven grain growth in two-dimensions using anisotropic grain boundary properties obtained from atomistic simulations. Molecular dynamics simulations were employed to determine the energies and mobilities of grain boundaries as a function of boundary misorientation. The mesoscopic simulations were performed both with the Monte Carlo Potts model and the phase field model. The Monte Carlo Potts model and phase field model simulation predictions are in excellent agreement. While the atomistic simulations demonstrate strong anisotropies in both the boundary energy and mobility, both types of microstructural evolution simulations demonstrate that anisotropy in boundary mobility plays little role in the stochastic evolution of the microstructure (other than perhaps setting the overall rate of the evolution. On the other hand, anisotropy in the grain boundary energy strongly modifies both the topology of the polycrystalline microstructure the kinetic law that describes the temporal evolution of the mean grain size. The underlying reasons behind the strongly differing effects of the two types of anisotropy considered here can be understood based largely on geometric and topological arguments.     

Voltage-induced metal-insulator transition in polythiophene field-effect transistors

We have extensively studied the carrier transport in regio-regular polythiophene field-effect transistors (FETs) from room temperature to 4.2 K. At low temperatures, Zabrodskii plots (dlnsigma/dlnT) demonstrate that the gate voltage and source-drain voltage combine to induce the insulator-to-metal transition at a carrier density of 5x10(12) cm-2. The carrier transport in the insulating regime is well described by phonon assisted hopping in a disordered Fermi glass with Coulomb interaction between the hopping charge carrier and the opposite charge left behind, as described by Efros and Shklovskii.     

Linear grain growth kinetics and rotation in nanocrystalline Ni

We report three-dimensional atomistic molecular dynamics studies of grain growth kinetics in nanocrystalline Ni. The results show the grain size increasing linearly with time, contrary to the square root of the time kinetics observed in coarse-grained structures. The average grain boundary energy per unit area decreases simultaneously with the decrease in total grain boundary area associated with grain growth. The average mobility of the boundaries increases as the grain size increases. The results can be explained by a model that considers a size effect in the boundary mobility.     

Quantum transport in molecules and nanotube devices

Quantum transport is usually cast as an "open" scattering problem, for which available computational methods have not achieved the accuracy of methods for conventional "closed" problems. Here we cast quantum transport as a closed problem and demonstrate fully converged currents for the prototype benzene-dithiolate system. We further report results for carbon-nanotube field-effect transistors, highlighting differences with Si-based devices, e.g., band mixing caused by the gate electric field. We also find that the source-drain current exhibits an intrinsic saturation as a function of the gate voltage.     

Integrating functional oxides with graphene

Graphene–oxide hybrid structures offer the opportunity to combine the versatile functionalities of oxides with the excellent electronic transport in graphene. Understanding and controlling how the dielectric environment affects the intrinsic properties of graphene is also critical to fundamental studies and technological development of graphene. Here we review our recent effort on understanding the transport properties of graphene interfaced with ferroelectric Pb(Zr,Ti)O₃ (PZT) and high-κ HfO₂. Graphene field effect devices prepared on high-quality single crystal PZT substrates exhibit up to tenfold increases in mobility compared to SiO₂-gated devices. An unusual and robust resistance hysteresis is observed in these samples, which is attributed to the complex surface chemistry of the ferroelectric. Surface polar optical phonons of oxides in graphene transistors play an important role in the device performance. We review their effects on mobility and the high source-drain bias saturation current of graphene, which are crucial for developing graphene-based room temperature high-speed amplifiers. Oxides also introduce scattering sources that limit the low temperature electron mobility in graphene. We present a comprehensive study of the transport and quantum scattering times to differentiate various scattering scenarios and quantitatively evaluate the density and distribution of charged impurities and the effect of dielectric screening. Our results can facilitate the design of multifunctional nano-devices utilizing graphene–oxide hybrid structures.     

Theory of Triple Junctions Mobility in Crystals with Impurities

We consider the steady state migration of the triple junction in the tricrystal with impurities which segregate strongly at the grain boundaries. If the mobility of impurities inside grain boundaries is much higher than the rate of impurity atoms jumps from the grain boundary into the bulk, the triple junction migration causes the divergence of the impurity content at the triple point. We show that this divergence can be relaxed either by the non-equilibrium segregation at the growing grain boundary or by the formation of the inclusion of the impurity-rich phase at the triple point. In the former case the dihedral angle at the triple point differs considerably from its equilibrium value and is strongly temperature-dependent. However, the triple junction cannot be described as an individual object with its own mobility. In the latter case of the cavity formation at the triple point the triple junction can be characterized by its own mobility. It is shown that the dependence of the triple junction migration rate on the driving force is approximately linear at the low migration rates and highly nonlinear at high migration rates. Moreover, there is the maximal allowable steady-state migration rate of the system triple junction-inclusion. For the higher migration rates the jerky motion of the triple junction occurs. Both models are in a good agreement with the experimental data.     

Identification of grain boundaries as degradation site in n‐channel organic field‐effect transistors determined via conductive atomic force microscopy

One important figure of merit for the commercial usability of organic transistors (OFETs) is their electrical stability. With the aim of identifying the microscopic location of degradation sites within a transistor channel, we have investigated the bias stress stability of OFETs by electrical measurements as well as by conductive atomic force microscopy. Air‐stable n‐channel FETs based on a N,N′‐bis(2‐ethylhexyl)‐1,7(1,6)‐dicyano‐perylene[3,4:9,10]bis (dicarboximide) were fabricated to understand the relation between the thin‐film morphology, the substrate temperature during the vacuum de position with the aim to fabricate transistors with a mobility not dominated by interface traps. The devices showed a maximum carrier mobility of (0.12 ± 0.01) cm²/V s and an on/off ratio up to 10⁷. The electrical performance as well as the bias stress behavior of the semiconductor thin‐films is significantly influenced by grain boundaries. For example, the grain boundary resistance was found to increase upon electrical stress by more than 150% (from 2 ± 0.2 GΩ to 5 ± 1.5 GΩ), while the resistance within the grains remains unchanged. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)