Effects of chemical treatment on barrier height and ideality factors of Au/GaN Schottky diodes

We have studied Au/n-GaN Schottky barrier diodes. GaN surfaces have been prepared by cleaning in HCl and (NH₄)₂S prior to metal deposition. The zero-biased barrier heights and ideality factors obtained from the current-voltage characteristics differ from diode to diode, although all the samples were prepared identically. The statistical analysis for the reverse bias C-V data yielded mean value of (1.35±0.04) eV for Schottky barrier height of HCl treated sample and (1.20±0.03) eV for (NH₄)₂S sample, where 9 dots were considered from each cleaning method. It was found that the barrier height values obtained from the C⁻²-V (1.43 eV) and I-V characteristics (0.89 eV) are different from each other by 0.54 eV. The inhomogeneous barrier heights were found to be related to the effect of the high series resistance on diode parameters (Akkiliç et al., 2004) [1].     

Study on anti-emission materials for non-emitting grid applications in microwave power tubes

Hafnium and platinum were deposited onto molybdenum grids by ion-beam assisted deposition method. Electron-emission characteristics from molybdenum grids with Hf and Pt films, which were contaminated by active electron-emission substances (Ba, BaO) of the cathode, were measured using analogous diode method. The surfaces of grids were analyzed by X-ray diffraction. The results revealed that the reaction between BaO and Hf formed BaHfO₃ compound, which greatly reduced the accumulation of BaO on the surface and accordingly decreased grid emission. In contrast, Ba were formed by the decomposition of BaO on the surface of Pt film under high temperature and re-evaporated from its surface, which reduced the active electron-emission substances on the surface of the grid and effectively restrained grid emission. Their mechanisms for grid-emission suppression are discussed and a good method to develop new grid-coating materials is suggested.     

Evaporation of atoms from femtosecond laser-heated gallium arsenide

The surface temperature of a GaAs crystal irradiated with 150 fs laser pulses is determined from the Maxwell velocity distribution of the evaporated atoms. The crystal is strongly superheated, and melting is observed to occur at a temperature 600–1000 K above the equilibrium melting point.     

Injection of holes at indium tin oxide/dendrimer interface: An explanation with new theory of thermionic emission at metal/organic interfaces

The traditional theory of thermionic emission at metal/inorganic crystalline semiconductor interfaces is no longer applicable for the interface between a metal and an organic semiconductor. Under the assumption of thermalization of hot carriers in the organic semiconductor near the interface, a theory for thermionic emission of charge carriers at metal/organic semiconductor interfaces is developed. This theory is used to explain the experimental result from Samuel group [J.P.J. Markham, D.W. Samuel, S.-C. Lo, P.L. Burn, M. Weiter, H. Baessler, J. Appl. Phys. 95 (2004) 438] for the injection of holes from indium tin oxide into the dendrimer based on fac-tris(2-phenylpyridyl) iridium(III).     

Enhanced two temperature modeling of ultrashort laser ablation for the investigation of thermionic emission characteristics

Characteristics of thermionic electron emission during femtosecond laser ablation of gold film are studied numerically. For the rigorous calculation of electron and lattice temperatures, an enhanced two-temperature model with transient thermal and optical properties is developed and it is demonstrated that the model predicts the damage threshold fluences closely matching experimental data. From the calculated electron emission characteristics, quantum efficiency and electron current density are estimated.     

Formation of Pt-C films by ion beam assisted deposition for the application of suppression thermo-electron emission

Platinum and carbon were deposited onto the surface of molybdenum grids simultaneously by ion beam assisted deposition. The structure of the Pt-C films was studied by XRD and Raman spectroscopy. The XRD results showed that Pt exhibited mixed strong (1 1 1) and weak (2 0 0) orientations. The Raman spectra showed that the carbon existed in the form of graphite-like phase. Electron emission characteristics from the Mo grid with and without Pt-C films were measured using analogous diode method. The results showed that electron emission from the Mo grid coated with Pt-C films was much less than that from the Mo grid without Pt-C films. The obtained results demonstrated that the Pt-C films are effective grid-coating materials for the application of suppression thermo-electron emission.     

The effect of substrate work function on work function reduction of Re/W alloy-coated impregnated cathodes

The work function reduction at a metal surface caused by adsorbed barium and oxygen is already known with the resulting expression for the overall work function being =_S–_R. Results from the Re/W alloy-coated cathodes exhibit that _R increases with substrate work function with a slope of 1.09 by linear regression. A qualitative explanation of the above results comes from considering the arrangement of barium and oxygen on the surfaces. A charge transfer model is proposed for two-dimensional barium and oxygen lying on a flat non-interacting substrate. Calculation shows that the dipole strength increases with the charge transfer below the intermediate states.     

Temperature dependence of the work function of free nanoparticle agglomerates

Starting point for this study was the experimentally found unusual high charge state of free submicron particles at elevated temperatures. By employing an aerosol setup with single electron counting efficiency it is shown for Pt and TiO₂ particles that the work function decreases with increasing temperature. Sign and amount of the work function decrease agree with the results of other experiments on extended surfaces, single crystals and clusters. On the basis of the reduced work function the observed high particle charge states are quantitatively understood.     

Positron wave function in ReO3 by the APW method

The wave function of a positron in ReO₃ is calculated using the augmented-plane-wave method. Due to the loosely-packed structure of ReO₃, the ground-stateГ ₁ wave function exhibits a marked anisotropy particularly around the oxygen ions, and a large fraction of a positron is distributed in the interstitial region. Experimental results of the positron annihilation 2γ correlations and the positron annihilation rates in ReO₃ are discussed based on the positron wavefunction.     

Numerical analysis of gate leakage current in AlGaN Schottky diodes

We studied theoretically the influence of the tunneling current on the leakage current in AlGaN Schottky diodes. It is shown that the most important conductance mechanism in these structures is the tunneling. The thermionic emission has lower influence on the total current practically throughout the whole reverse bias range and doping concentrations studied. For high doping concentrations we found very slow temperature dependence of the diode current.     

Current-temperature measurements of a SBD evaporated onto inductively coupled plasma cleaned germanium

Inductively coupled plasma (ICP) etching has been used primarily on compound semiconductors. There are however compelling reasons to study the effects of ICP etching on Ge. Pd Schottky barrier diodes (SBDs) were resistively evaporated onto Ge (1 1 1) that was ICP etched at a rate of 60 Å per minute for three or ten minute intervals. Although plasma cleaning is known to introduce defects that were observed with DLTS, the diodes exhibited excellent current-voltage characteristics when cooled down to 80 K. Current-temperature (IT) scans that were recorded from 20 K up to 300 K after cooling under reverse bias showed no effect of recombination/generation (RG). On the other hand, IT scans that were recorded after cooling under zero or forward bias clearly exhibited RG effects in the 100-240 K temperature range. This effect was found to be completely reversible. In addition, ICP etching leads to superior devices when compared to devices manufactured by RF sputter deposition.     

The dependence of barrier height on temperature for Pd Schottky contacts on ZnO

Temperature dependent current-voltage (I-V) and capacitance-voltage (C-V) measurements have been performed on Pd/ZnO Schottky barrier diodes in the range 60-300 K. The room temperature values for the zero bias barrier height from the I-V measurements (Φ_I-V) was found to be 0.52 eV and from the C-V measurements (Φ_C-V) as 3.83 eV. From the temperature dependence of forward bias I-V, the barrier height was observed to increase with temperature, a trend that disagrees with the negative temperature coefficient for semiconductor material. The C-V barrier height decreases with temperature, a trend that is in agreement with the negative temperature coefficient of semiconductor material. This has enabled us to fit two curves in two regions (60-120 K and 140-300 K). We have attributed this behaviour to a defect observed by DLTS with energy level 0.31 eV below the conduction band and defect concentration of between 4×10¹⁶ and 6×10¹⁶ cm⁻³ that traps carriers, influencing the determination of the barrier height.     

Bridging of lateral nanoelectrodes with a metal particle chain

Received: 4 September 1998/Accepted: 9 September 1998     

Cellular reactions of osteoblast-like cells to a novel nanocomposite membrane for guided bone regeneration

This study investigated the bioactivity and biocompatibility of hydroxyapatite nanoparticles (n-HA)/Polyamide-66 (PA66) nanocomposite membrane and expanded-polytetrafluoroethylene (e-PTFE) membrane (as control) to MG63 osteoblast-like cells. The attachment and proliferation of the cells on the porous surface of nHA/PA66 membrane and the surface of e-PTFE membrane were evaluated by scanning electron microscope (SEM) observation and the MTT assay. The bioactivity of the cells on the surface of the two membranes was evaluated by testing cell viability and alkaline phosphatase (ALP) activities. The results suggested that the bioresponse of MG63 osteoblast-like cells on the porous surface of nHA/PA66 membrane was better than the bioresponse on the opposite surface of e-PTFE membrane. Because of a better cell attachment manner, there is a potential utilization of the guided bone regeneration (GBR) membrane to substitute nHA/PA66 membrane for e-PTFE membrane.     

Work function variation versus alloy concentration for dispenser cathodes

The dependence of the work function (∅) of dispenser cathodes on the concentration of Ir and Re alloyed with the tungsten emitting surface has been determined for controlled porosity dispenser (CPD) cathodes and matrix type cathodes. ∅ versus concentration curves show a minimum with a distinct cusp near 50/50 concentration for CPD cathodes and near 40% tungsten for matrix cathodes. Our matrix cathode data (using Ir) is consistent (relative to the concentration at which a minimum occurs) with Os/W data derived from other work [6,15]. For all alloys the concentration at which the minimum occurs is independent of which noble metal is involved, but does depend on the nature of the tungsten substrate, and perhaps the deposition technique used to deposit the noble metal film. The data is interpreted in terms of a previously proposed model [7] involving enhanced bonding between Ba-Ir and O-W compared to Ba-W and O-Ir bonds. In addition we suggest a model which explains the cusp as well as other characteristics of the work function concentration curves in terms of a postulated surface phase transition.     

A self-aligning ultrasound sensor for detecting foreign bodies in glass containers

This paper introduces the principle and design of an ultrasonic transducer system with auto-alignment mechanism. The proposed system is used for detecting foreign bodies (FBs) in beverage containers. Variation in reflection amplitude is analyzed as a function of the ultrasound beam incident angle to beverage container surface. It is concluded that there exists a quadratic relationship between the strength of the reflected signal and the incident angle. Furthermore, a calculation for effective angular increment for searching the normal of a curved surface is introduced. Experiments conducted using the prototype demonstrated that FBs are detectable in various juices. Experiments also observed some false echo signals that occur due to curved container surface. Threshold in confined time region is therefore set to ensure no fail of detection in low signal to noise ratio. This design is also applicable to non-destructive inspection for metal canned food.     

Numerical predictions and experiments for optimizing hidden corrosion detection in aircraft structures using Lamb modes

To increase the sensitivity of Lamb waves to hidden corrosion in aircraft structures, a preliminary step is to understand the phenomena governing this interaction. A hybrid model combining a finite element approach and a modal decomposition method is used to investigate the interaction of Lamb modes with corrosion pits. The finite element mesh is used to describe the region surrounding the corrosion pits while the modal decomposition method permits to determine the waves reflected and transmitted by the damaged area. Simulations make easier the interpretation of some parts of the measured waveform corresponding to superposition of waves diffracted by the corroded area. Numerical results permit to extract significant information from the transmitted waveform and thus to optimize the signal processing for the detection of corrosion at an early stage. Now, we are able to detect corrosion pits down to 80-μm depth distributed randomly on a square centimeter of an aluminum plate. Moreover, thickness variations present on aircraft structures can be discriminated from a slightly corroded area. Finally, using this experimental setup, aircraft structures have been tested.     

Ultrasonic resonator with electrical discharge cell for ozone generation

The effect of ultrasound waves in an ultrasonic resonator cell on ozone generation by a hollow needle to plate electrical discharge enhanced by the airflow through the needle is experimentally investigated. It was found that the application of ultrasound waves increases ozone generation for discharge when the needle is negatively biased, and has no effect on ozone generation for the discharge when the needle is biased positively. We try to explain the influence of ultrasound on the discharge mechanisms.     

Thermal emission of electrons from highly excited sodium clusters

Positively charged sodium clusters can be easily ionized by a fs laser pulse of relatively low intensity (<10¹⁰ W/cm²), if the laser is in resonance with the plasmon excitation of the cluster. This ionization process was investigated in detail by measuring the kinetic energy distribution of electrons emitted from a size-selected Na₉₃ ⁺ as a function of the fs laser intensity. In all cases pure Boltzmann-like energy distributions were observed. A comparison with statistical theory shows that the emission is a purely thermal process. It is different to normal thermionic emission insofar as the electrons are emitted from a hot electron system which is only weakly coupled to a cold ionic background. The results demonstrate purely statistical behaviour of a small fermionic system even for very high excitation energy. Received: 25 May 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Comparison of continuous-wave (CW) and tone-burst (TB) excitation modes in vibro-acoustography: Application for the non-destructive imaging of flaws

This paper describes the application of continuous-wave (CW) and tone-burst (TB) vibro-acoustography (VA) experiments for imaging a flawed composite plate. For both modes, the ultrasound frequency is set at f₁ = 3 MHz and f₂ = 3 MHz + ∣Δf∣. The plate was placed at the focus of the transducer and scanned point-by-point over an area of 60 mm by 50 mm on its frontal face with an increment step equal to 0.25 mm/pixel. The resulting acoustic emission amplitude at ∣Δ f∣ is recorded. For the CW mode the difference frequency was set at ∣Δf∣ = 12.9 kHz. For the TB mode, the burst-emitted signal was 100 μs long at a pulse repetition frequency (PRF) of 100 Hz corresponding to bursts of 300 cycles at 3 MHz, and the difference frequency was set at ∣Δf∣ = 44 kHz. The resulting VA images readily show the shape of the flaws. The images also reveal considerable detail of internal substructures such as the fibers used to reinforce the plate. However, the CW VA image shows an artifact caused by the effect of ultrasound standing waves established between the plate and the concave surface of the transducer, resulting in masking some of the flaws. On the other hand, the TB-VA image is free from such artifact. Despite some advantages of using TB-VA, there are some limitations related to this mode. Advantages and limitations of using the two modes are discussed.