Piezoelectric-potential-controlled polarity-reversible Schottky diodes and switches of ZnO wires

Using a two-end bonded ZnO piezoelectric-fine-wire (PFW) (nanowire, microwire) on a flexible polymer substrate, the strain-induced change in I-V transport characteristic from symmetric to diode-type has been observed. This phenomenon is attributed to the asymmetric change in Schottky-barrier heights at both source and drain electrodes as caused by the strain-induced piezoelectric potential-drop along the PFW, which have been quantified using the thermionic emission-diffusion theory. A new piezotronic switch device with an "on" and "off" ratio of approximately 120 has been demonstrated. This work demonstrates a novel approach for fabricating diodes and switches that rely on a strain governed piezoelectric-semiconductor coupling process.     

Temperature dependence of current—voltage characteristics of Au/n-GaAs epitaxial Schottky diode

The influence of temperature on current-voltage (I-V) characteristics of Au/n-GaAs Schottky diode formed on n-GaAs epitaxial layer grown by metal organic chemical vapour deposition technique has been investigated. The dopant concentration in the epitaxial layer is 1 X 10¹⁶ cm^-3. The change in various parameters of the diode like Schottky barrier height (SBH), ideality factor and reverse breakdown voltage as a function of temperature in the range 80–300 K is presented. The variation of apparent Schottky barrier height and ideality factor with temperature has been explained considering lateral inhomogeneities in the Schottky barrier height in nanometer scale lengths at the metal-semiconductor interface     

Electrical characterization of n/p-type nickel silicide/silicon junctions by Sb segregation

In this paper, n/p-type nickel-silicided Schottky diodes were fabricated by incorporating antimony atoms near the nickel silicide/Si junction interface and the electrical characteristics were studied through measurements and simulations. The effective Schottky barrier height (SBH) for electron, extracted from the thermionic emission model, drastically decreased from 0.68 to less than 0.1 eV while that for hole slightly increased from 0.43 to 0.53 eV. In order to identify the current conduction mechanisms, the experimental current-temperature-voltage characteristics for the n-type diode were fitted based on various models for transport of charge carrier in Schottky diodes. As the result, the large change in effective SBH for electron is ascribed to trap-assisted tunneling rather than barrier height inhomogeneity.     

Mechanical-electrical triggers and sensors using piezoelectric micowires/nanowires

We demonstrate a mechanical-electrical trigger using a ZnO piezoelectric fine-wire (PFW) (microwire, nanowire). Once subjected to mechanical impact, a bent PFW creates a voltage drop across its width, with the tensile and compressive surfaces showing positive and negative voltages, respectively. The voltage and current created by the piezoelectric effect could trigger an external electronic system, thus, the impact force/pressure can be detected. The response time of the trigger/sensor is approximately 10 ms. The piezoelectric potential across the PFW has a lifetime of approximately 100 s, which is long enough for effectively "gating" the transport current along the wire; thus a piezoelectric field effect transistor is possible based on the piezotronic effect.     

金属/半导体肖特基接触模型研究进展

在分析理想金属/半导体肖特基接触的基础上,概述了一般情形下肖特基接触的形成机理和影响因素。金属/半导体间的界面层使得肖特基势垒高度(SBH)对功函数的依赖减弱,也导致SBH与外加偏压有关。研究证实,多种因素,如界面晶向、原子结构、化学键和结构不完整性等,都会造成SBH的空间不均匀分布。该特性在肖特基接触中普遍存在,并对基于肖特基结的器件工作有显著影响。     

Tunable Reverse‐Biased Graphene/Silicon Heterojunction Schottky Diode Sensor

A new chemical sensor based on reverse‐biased graphene/Si heterojunction diode has been developed that exhibits extremely high bias‐dependent molecular detection sensitivity and low operating power. The device takes advantage of graphene's atomically thin nature, which enables molecular adsorption on its surface to directly alter graphene/Si interface barrier height, thus affecting the junction current exponentially when operated in reverse bias and resulting in ultrahigh sensitivity. By operating the device in reverse bias, the work function of graphene, and hence the barrier height at the graphene/Si heterointerface, can be controlled by the bias magnitude, leading to a wide tunability of the molecular detection sensitivity. Such sensitivity control is also possible by carefully selecting the graphene/Si heterojunction Schottky barrier height. Compared to a conventional graphene amperometric sensor fabricated on the same chip, the proposed sensor demonstrated 13 times higher sensitivity for NO₂ and 3 times higher for NH₃ in ambient conditions, while consuming ～500 times less power for same magnitude of applied voltage bias. The sensing mechanism based on heterojunction Schottky barrier height change has been confirmed using capacitance‐voltage measurements.     

Fiber-optic strain-gauge manometer up to 100 MPa

A practical realization of a novel pressure transducer utilizing a fiber-optic strain sensor and an active element configured to simulate an infinite cylinder with free ends is described. The deformation of such a cylinder depends uniquely on pressure acting from its inside and is independent of the stress resulting from the attachment of the device to the pressure system. The fiber-optic strain sensor is permanently bound to the external surface of the cylinder and as such is fully isolated from the high pressure region. The sensing element of the device consists of a highly birefringent (HB) polarization-maintaining optical-fiber strain gauge. The device was characterized at ambient temperatures for pressure up to 100 MPa. The sensor has inherent advantages such as immunity to electromagnetic interference, safety, direct compatibility with optical data transmission systems, simplicity and cost-effectiveness. It does not require any fiber-optic leadthrough and has significantly increased sensitivity (around 0.1 MPa^-1) compared to similar devices based on electrical strain gauges     

Pd-oxide- Al/sub 0.24/Ga/sub 0.76/As (MOS) high electron mobility transistor (HEMT)-based hydrogen sensor

An interesting hydrogen sensor based on an Al/sub 0.24/Ga/sub 0.76/As Schottky barrier high-electron mobility transistor with a catalytic Pd metal/oxide/semiconductor is fabricated and demonstrated. In comparison with traditional Schottky diodes or capacitance-voltage type hydrogen sensors, the studied device exhibits larger current variation, lower hydrogen detection limit, and shorter transient hydrogen response time. Besides, good hydrogen-sensing properties, such as significant drain current change, threshold voltage shift, and transconductance change of transistor behaviors, are obtained. Therefore, the studied device provides the promise for high-performance solid-state hydrogen sensors, optoelectronic integrated circuits, and microelectromechanical system applications.     

Enhanced performance of GaN nanobelt-based photodetectors by means of piezotronic effects

GaN ultraviolet （UV） photodetectors （PDs） have attracted tremendous attention due to their chemical stability in harsh environments. Although Schottky- contacted GaN-based UV PDs have been implemented with better performance than that of ohmic contacts, it remains unknown how the barrier height at local Schottky contacts controls the sensors＇ performance. In this work, the piezotronic effect was employed to tune the Schottky barrier height （SBH） at local contacts and hence enhance the performances of Schottky-contacted metal-semiconductor- metal （MSM） structured GaN nanobelt （NB）-based PDs. In general, the response level of the PDs was obviously enhanced by the piezotronic effect when applying a strain on devices. The responsivity of the PD was increased by 18%, and the sensitivity was enhanced by from 22% to 31%, when illuminated by a 325 nm laser with light intensity ranging from 12 to 2 W/cm2. Carefully studying the mechanism using band structure diagrams reveals that the observed enhancement of the PD performance resulted from the change in SBH caused by external strain as well as light intensity. Using piezotronic effects thus provides a practical way to enhance the performance of PDs made not only of GaN, but also other wurtzite and zinc blende family materials.     

Post-annealing behavior of Ni/Au Schottky contact on non-polar a-plane GaN

The effects of annealing on the performance of Schottky devices on a-plane GaN/r-plane sapphire were investigated. The results show that the post-anneal Schottky barrier height (SBH) increased with increasing annealing temperature, reaching a peak increase of 43% at 500 °C. A further increase in the anneal temperature above 500 °C degraded the SBH. The ideality factor displayed a weak dependence on post-annealing temperature until rising dramatically at a post-annealing temperature of 600 °C. The degradation at 600 °C post-annealing temperature can be attributed to the formation of Nickel-Gallides. In-situ current-voltage characteristics obtained between 15 °C and 165 °C revealed that both the ideality factor and SBH were stable up to 165 °C with increasing in-situ measurement temperature.     

Schottky contacts of refractory metal nitrides on gallium nitride using reactive sputtering

Schottky contacts of refractory metal nitrides formed by reactive sputtering on n-type gallium nitride (GaN) were electrically evaluated, including film resistivity, Schottky characteristics and thermal stability. For the metal nitrides of TiN_x, MoN_x and ZrN_x, resistivities of 108.3, 159.0 and 270.0 μΩcm were obtained, respectively. Current-voltage (I-V) characteristics showed that the ideality factor varied from 1.03 to 1.16, while the Schottky barrier height (SBH) varied from 0.66 to 0.79 eV for the three kinds of Schottky contacts. Especially for the ZrN_x contact, the ideality factor and SBH were improved after annealing at 800 °C for 30 s. Schottky contact utilizing a refractory metal nitride on GaN shows its potential to develop thermally stable GaN devices.     

Comparative study of I-V characteristics of the ICB deposited Ag\-Si(111) and Ag\p-Si(100) Schottky junctions

The room temperature I-U characteristics of the ionized cluster beam, ICB, deposited Ag\-Si(111) and Ag\p-Si(100) Schottky barrier junctions, for non zero Ag ions acceleration voltage U_a > 0 V, have been investigated. The effective Schottky barrier height (SBH) is observed to decrease if Ag is deposited on n-doped Si substrate but for Ag\p-Si junction shows an increase, relatively to the corresponding SBH values of U_a = 0 V ICB deposited junctions, respectively.     

应变传感器计量特性的校准方法

为了合理评价应变传感器（应变计）的计量特性,讨论了应变计的相关国家标准和技术规范中计量特性的不同之处,并以振弦式应变计为例,归纳出应变计的计量特性参数。对应变计校准现状进行分析,设计了一种基于激光干涉法的高精度校准装置,标距范围可以达到500mm,并提出了系列计量特性的高精度计算方法。通过试验对应变计的计量特性进行测试,并评估了综合误差的校准不确定度U=0.10%（包含因子k=2）。     

Barrier inhomogeneities in titanium Schottky contacts formed on argon plasma etched p-type Si0.95Ge0.05

Electrical properties of Ti Schottky barrier diode fabricated on argon plasma etched p-type Si_0.95Ge_0.05 were studied using current–voltage (I–V) over a wide temperature range (100–300 K). The transport properties of the junction were analyzed by investigating the temperature dependence of both the effective Schottky barrier height (Φ _0bp) and the ideality factor (n). It is shown that the ideality factor increases and the Schottky barrier height (SBH) decreases with decreasing temperature. This abnormal temperature dependence of the Φ _0bp and n is explained on the basis of a thermionic emission conduction mechanism with Gaussian distributed barrier heights due to the barrier height inhomogeneities at the metal-p-Si_0.95Ge_0.05 interface. From the linear plot of the experimental SBH versus 1/T, a homogeneous SBH ( $\overline{\varPhi }_{0bp}$ ) and a zero-bias standard deviation (σ_0s) values of approximately 0.55 eV and 67 mV, respectively were computed. Furthermore the modified Richardson plot according to the Gaussian distribution model resulted in a homogeneous SBH ( $\overline{\varPhi }_{0bp}$ ) and a Richardson constant (A*) of 0.55 eV and 35 A/cm² K², respectively. The A* value obtained from this plot is in very close agreement with the theoretical value of 32 A/cm² K² for p-type Si_0.95Ge_0.05. Furthermore, the SBH is found to decrease linearly as the interface states density (N _ss) increases. It is proposed that the lateral inhomogeneities of the SBH are actually attributed to the distribution of the interface states which are in turns resulting from the plasma etching induced defects beneath the Si_0.95Ge_0.05 surface.     

Illumination-dependent free carrier screening effect on the performance evolution of ZnO piezotronic strain sensor

Performance modulation of ZnO optoelectronic devices in the presence of proper piezoelectric polarization charges has been widely reported, whereas relatively less work has been performed about the influence of photoexcitation on piezotronics. In this study, we experimentally investigated the performance evolution of ZnO piezotronic strain sensor under various 365 nm UV irradiation densities. The device demonstrated a response ratio of ~200 under no illumination and under ?0.53% compressive strain, and the response time is approximately 0.3 s. However, tremendous performance degradation was observed with the increase in the illumination density, which is attributed to the UV-modulated change in the free electron concentration and Schottky barrier height. It was observed that increased carrier density intensifies the screening effect and thus, the modulation ability of piezo-polarization charges weakens. Meanwhile, the deterioration of rectifying behavior at the interface under UV illumination also jeopardizes the device performance.

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Piezotronic effect enhanced Schottky-contact ZnO micro/nanowire humidity sensors

A ZnO micro/nanowire has been utilized to fabricate Schottky-contacted humidity sensors based on a metal-semiconductor-metal （M-S-M） structure. By means of the piezotronic effect, the signal level, sensitivity and sensing resolution of the humidity sensor were significantly enhanced when applying an external strain. Since a higher Schottky barrier markedly reduces the signal level, while a lower Schottky barrier decreases the sensor sensitivity due to increased ohmic transport, a 0.22% compressive strain was found to optimize the performance of the humidity sensor, with the largest responsivity being 1,240%. The physical mechanism behind the observed mechanical-electrical behavior was carefully studied by using band structure diagrams. This work provides a promising way to significantly enhance the overall performance of a Schottky-contact structured micro/nanowire sensor.     

Electrical transport of bottom-up grown single-crystal Si(1-x)Ge(x) nanowire

In this work, we fabricated an Si(1-x)Ge(x) nanowire (NW) metal-oxide-semiconductor field-effect transistor (MOSFET) by using bottom-up grown single-crystal Si(1-x)Ge(x) NWs integrated with HfO(2) gate dielectric, TaN/Ta gate electrode and Pd Schottky source/drain electrodes, and investigated the electrical transport properties of Si(1-x)Ge(x) NWs. It is found that both undoped and phosphorus-doped Si(1-x)Ge(x) NW MOSFETs exhibit p-MOS operation while enhanced performance of higher I(on)～100?nA and I(on)/I(off)～10(5) are achieved from phosphorus-doped Si(1-x)Ge(x) NWs, which can be attributed to the reduction of the effective Schottky barrier height (SBH). Further improvement in gate control with a subthreshold slope of 142?mV?dec(-1) was obtained by reducing HfO(2) gate dielectric thickness. A comprehensive study on SBH between the Si(1-x)Ge(x) NW channel and Pd source/drain shows that a doped Si(1-x)Ge(x) NW has a lower effective SBH due to a thinner depletion width at the junction and the gate oxide thickness has negligible effect on effective SBH.     

Highly Sensitive and Stretchable Resistive Strain Sensors Based on Microstructured Metal Nanowire/Elastomer Composite Films

High sensitivity and high stretchability are two conflicting characteristics that are difficult to achieve simultaneously in elastic strain sensors. A highly sensitive and stretchable strain sensor comprising a microstructured metal nanowire (mNW)/elastomer composite film is presented. The surface structure is easily prepared by combining an mNW coating and soft‐lithographic replication processes in a simple and reproducible manner. The densely packed microprism‐array architecture of the composite film leads to a large morphological change in the mNW percolation network by efficiently concentrating the strain in the valley regions upon stretching. Meanwhile, the percolation network comprising mNWs with a high aspect ratio is stable enough to prevent electrical failure, even under high strains. This enables the sensor to simultaneously satisfy high sensitivity (gauge factor ≈81 at >130% strain) and high stretchability (150%) while ensuring long‐term reliability (10 000 cycles at 150% strain). The sensor can also detect strain induced by bending and pressure, thus demonstrating its potential as a versatile sensing tool. The sensor is successfully utilized to monitor a wide range of human motions in real time. Furthermore, the unique sensing mechanism is easily extended to detect more complex multiaxial strains by optimizing the surface morphology of the device.     

Photoelectron spectroscopic investigation of InN and InN/GaN heterostructures

The surface band diagram of InN and band structure of the InN/GaN interface were studied using ultraviolet photoemissive yield spectroscopy and X-ray photoemission spectroscopy (XPS). The surface work function and the difference between the Fermi level and the conduction band minimum of InN were determined by ultraviolet photoemissive yield measurement. The band offsets and surface band bending were determined using XPS. Both spectra proposed downward band bending of the InN surface. Moreover, the Schottky barrier height (SBH) of the InN/GaN interface is determined (1.5 eV). Comparison of the measured SBH with our previous results by electrical measurement is discussed. The physical quantities derived in this work provide important information for use in future studies of InN and InN/GaN heterostructures.     

Gate‐Tunable Graphene–WSe2 Heterojunctions at the Schottky–Mott Limit

Metal–semiconductor interfaces, known as Schottky junctions, have long been hindered by defects and impurities. Such imperfections dominate the electrical characteristics of the junction by pinning the metal Fermi energy. Here, a graphene–WSe₂ p‐type Schottky junction, which exhibits a lack of Fermi level pinning, is studied. The Schottky junction displays near‐ideal diode characteristics with large gate tunability and small leakage currents. Using a gate electrostatically coupled to the WSe₂ channel to tune the Schottky barrier height, the Schottky–Mott limit is probed in a single device. As a special manifestation of the tunable Schottky barrier, a diode with a dynamically controlled ideality factor is demonstrated.