Presence Of High Electric Field Research Articles

Luminescence properties of SrS:Ce3+

SrS:Ce is an intensively investigated phosphor due to its blue-green electroluminescence, which shows efficient blue emission after filtering. Recently reported devices based on this material have demonstrated a luminous efficiency of 1.6 lm/W. The luminescence properties of SrS:Ce,X (X=Na or Cl) have been studied on powders and thin films. It is shown that a high density of traps in SrS:Ce,X occurs. The interaction of Ce3+ with traps gives rise to a phosphorescence. An energy transfer from Ce3+ to traps is responsible for an observed luminescence quenching in the presence of high electric fields. Moreover, the traps are electrically active and are involved in the electroluminescence process. The observed energy transfer is proposed to be the dominant excitation mechanism of Ce3+ in electroluminescence. It is demonstrated for thin films that the defect density increases with doping; therefore, the luminescence yield is already limited at doping concentrations below the onset of the concentration quenching. Thus, the prepared SrS:Ce,Cl thin films show a lower photoluminescence yield than powders. It is concluded that an undisturbed Ce incorporation into SrS thin films has not been achieved so far, although high electroluminescence efficiencies (1.6 lm/W) have been obtained.

Heat Generation and Transport in Submicron Semiconductor Devices

The reduction of semiconductor device size to the submicrometer range leads to unique electrical and thermal phenomena. The presence of high electric fields (order of 107 V/m) energizes the electrons and throws them far from equilibrium with the lattice. This makes heat generation a nonequilibrium process. For gallium arsenide (GaAs), energy is first transferred from the energized electrons to optical phonons due to strong polar coupling. Since optical phonons do not conduct heat, they must transfer their energy to acoustic phonons for lattice heat conduction. Based on the two-step mechanism with corresponding time scales, a new model is developed to study the process of nonequilibrium heat generation and transport in a GaAs metal semiconductor field effect transistor (MESFET) with a gate length of 0.2 μm. When 3 V is applied to the device, the electron temperature rise is predicted to be more than 1000 K. The effect of lattice heating on electrical characteristics of the device shows that the current is reduced due to decrease in electron mobility. The package thermal conductance is observed to have strong effects on the transient response of the device.

Analysis of F-Region Incoherent Scatter Spectra during Periods of High Electric Fields.

In the presence of high electric fields in the auroral ionosphere, the ion distribution functions depart from their usual Maxwellian shape. The incoherent scatter radars have observed the signature of this phenomenon in the form of distorted spectra which have lost their double-humped characteristics. In consequence, the classical procedure of spectrum analysis, which assumes Maxwellian distributions to fit the plasma parameters, is no longer valid and has to be modified to account for a non-Maxwellian plasma. The observations of the European incoherent scatter radar EISCAT were at the origin of substantial developments, which are still in progress. We briefly review the main steps in the experimental and theoretical approaches of the non-Maxwellian behaviour of the ionospheric plasma, and we discuss particularly the most recently published results.

Photogeneration and Photoluminescence Studies in Bilayer Structures Containing Poly(P-Phenylene Vinylene) and Molecularly Doped Polymers

Abstract The photogeneration efficiency for photocarriers (holes and electrons) in poly(p-phenylene vinylene) (PPV) is measured in the presence of high electric fields, for bilayer sandwich devices containing molecularly doped polymers (MDP) and PPV. A hole transport diamine molecule (TPD) and an electron transport malononitrile molecule (BCFM) each dispersed in polycarbonate were used to study holes and electrons respectively. By performing photoluminescence (PL) experiments in the bilayer structures and hole collection experiments in single layers without the TPD:PC overcoat, we have concluded that generation of photocarriers in PPV is mainly a bulk mediated process, assisted by the applied electric field, and not sensitized by the presence of the MDP. The electric field dependence of photogeneration suggests that photoexcitation results in a strongly correlated electron-hole pair dissociated by the electric field.

I–V and π−A characteristics of ultrathin Langmuir-Blodgett films

The deposition conditions and electrical properties of C 22-quinolium(TCNQ) Langmuir-Blodgett (LB) films have been studied. The optimum conditions of film deposition were examined by a study of surface pressure-area (π− A) isotherms with variations of pH, temperature, barrier moving speed, dipping speed, spreading amount of solution etc. The LB films were deposited on a slide glass substrate and tested with several tools. The π− A isotherm shows that there might be two phase transitions near surface pressures of 30 and 45 mN m −1. It is thought that the phase just below 30 mN m −1 is a liquid phase and that above 30 mN m −1 is a solid phase. The LB films were made under the following conditions; surface pressure, 45 mN m −1; 25°C; pH 5.6; spreading amount 60–70 Å 2 per molecule; and dipping speed, 4 mm min −1. The electrical conductivities and conduction mechanism were investigated by measuring current-voltage ( I–V) characteristics along the vertical and horizontal direction of the film. We observed an anisotropy in conductivity between the vertical and horizontal direction of the film. The conductivity along the film surface direction is about 10 −7 S cm −1 and that of the perpendicular direction is about 10 −14 S cm −1 at room temperature. We saw non-ohmic conducting behaviour in the presence of high electric field along the vertical direction. When the electric field is high in this direction, a breakdown occurs in the I–V characteristics. We analysed the I–V characteristics in detail near the breakdown region by changing the type of applied voltage (step or pulse) as well as the temperature.

‘Switch-on’ phenomena in field-electron and field-ion microscopy

Recent work in high-field microscopy has demonstrated striking similarities in the behaviour of metal-insulator (M-I) composite microemitters when tested in field-electron emission and field-ion operation modes. The composite structures consisted of a metallic core, usually tungsten, covered by thin layers of various dielectric materials (polyethylene in the field-ion case). The main features and similarities in the properties of composite emitters using both field-electron emission and field-ion regimes include: (i) initial switch-on phenomena; (ii) subsequent electron emission current-voltage (I–V) characteristics that were both smooth and reproducible; (iii) emission I–V characteristics which exhibit hysteresis upon cycling the applied voltage; and (iv) saturated electron emission current behaviour and its analogy in the field-ion mode has been observed. This behaviour has been interpreted in terms of an emission mechanism which involves the formation of a conducting filament in the insulator. An analysis of the M-I interface formation under the presence of high electric field is also given.

Corona from bursting bubbles

The phenomenon of bubble bursting in the presence of high electric fields has been studied with the intent of determining the critical field for corona initiation. Various bubble sizes were produced in a solution of sodium chloride in order to model conditions under oceanic thunderstorms. The results show that the corona threshold is invesely dependent on the bubble diameter. The quantity of charge transferred during a corona event was measured and was found to depend on the bubble size and the electric field strength. This process could be one of the effective charge transfer mechanisms over the oceans under thunderclouds.

Transport in a-Si:H

In this paper we present and discuss transport data deduced from DC-dark conductivity and time-of-flight experiments on doped (phosphorus, boron) and intrinsic a-Si:H at low temperatures (10K<T<300K) in the presence of high electric fields (F≤5×10 5 V/cm). The field creates strong non-linearities, which change the conductivity by several orders of magnitude. The field strength necessary to increase the conductivity consideraby depends on the doping concentration and on the dopant

Incoherent scattering of radar waves in the auroral ionosphere in the presence of high electric fields, and measurement problems with the EISCAT Facility

We have studied the effect of a distribution function of ions disturbed by an electric field perpendicular to the geomagnetic field in the lower F region, on the ionic part of the spectrum of incoherently scattered radar waves. The distortion of the spectra from the shape they would normally have, if the ion distribution were at local Maxwellian equilibrium, increases with the intensity of the electric field and with the angle of the direction of observation of the radar with respect to the magnetic field. We determined as a function of the ion population and of the direction of observation the range of electric field values where the assumption of a bi‐Maxwellian ion distribution is good enough to provide reliable measurements of the electron temperature and density and of the ion anisotropy temperature. Out of that range, the derived plasma parameters have large errors whose magnitude is related to the intensity of the electric field, to the ion composition, to the ratio of the electron temperature to the ion temperature, and to the direction of observation with respect to the magnetic field.

Extreme ionospheric effects in the presence of high electric fields

In the presence of high-convection electric fields, theoretical con-siderations and computer models predict, among other effects, large increases in the ion temperature, the molecular ions and horizontal neutral winds in the high-latitude F region, but experi-mental evidence of these effects has been difficult to obtain. Here we report on measurements of very high perpendicular convection velocities of ∼3.8 km s−1, corresponding to electric fields of 190 mVm−1, that were made in the auroral F region of the ionos-phere with the tristatic incoherent scatter radar at EISCAT in northern Scandinavia under magnetically perturbed conditions. These high velocities were associated with very high ion tem-peratures of 12,000 K without being accompanied by commensurate changes in the electron temperature. The large increase of the ion temperature can be accounted for quantitatively by Joule heating. The constraint of energy conservation during this event implies a substantial increase of molecular ions in the F layer, attaining values of more than 70% at 250 km altitude. Similarly, momentum conservation allows us to estimate neutral winds of up to 600 m s−1 mainly in the zonal direction.

Ensemble Monte Carlo simulation of real space transfer (NERFET/CHINT) devices

The first self-consistent particle-field ensemble Monte Carlo model for real space transfer (NERFET/CHINT) devices will be presented. The simulations performed are in agreement with experiments and reproduce all prominent features of NERFET/CHINT structures such as, negative differential resistance, saturation of drain and substrate (injection) current at high source-to-drain voltages, and the negative transconductance ( ΔI D, sat / ΔV sub < 0) in the saturated drain current Negative differential resistance (NDR) can be achieved through real-space electron transfer which is based on the emission of electrons from one semiconductor layer into another semiconductor layer in the presence of high electric fields parallel to the layers (Hess, 1979). The major advantage of this mechanism over the Gunn effect is that the material parameters such as mobility ratios of adjacent layers, barrier heights, layer thicknesses can be engineered to control peak-to-valley ratios, device speeds and NDR threshold voltages. Recently a number of devices, namely negative differential resistance field effect transistor (NERFET) and charge injection transistor (CHINT), have been proposed and experimentally verified (Luryi, 1985). The purpose of this paper is to present our Monte Carlo model for the real space transfer transistors (NERFET/CHINT) and summarize our simulations which are in agreement with experiments.

Spectroscopic study of the electric field induced microstructural changes in poly(vinylidene fluoride)

Infra-red spectra (4000-400 cm −1) have been obtained for poly(vinylidene fluoride) films in the presence of high electric fields up to 2.6 MV cm −1 in strength. Both band intensity and polarization studies have revealed that dipolar re-orientation can be correlated with the macroscopic electric effect. The frequency shift as a function of field strength for the β phase bands can be attributed to the non-equivalent reorientation behaviour of the bands in the unit cell. However, the change in the frequency of the α bands shifted irreversibly with the applied electric field. These changes cannot be attributed to reorientation behaviour alone.

Tunneling in base-emitter junctions

Tunneling currents in reverse-biased base-emitter junctions are investigated and analyzed. Guided by a simple analytic theory, shallow n+-p junctions are designed with a variety of different concentration profiles. Measurements of the dc electrical characteristics indicate a significant Zener tunneling component in the reverse diode current. The appearance of tunneling is ascertained by the temperature dependence, which also allows a clear distinction of other current mechanisms. The sensitivity of the current to the details of the doping profile is theoretically explained in terms of the maximum electric field in the junction and verified by SIMS and C-V profiling techniques. The C-V data are analyzed in a novel way to obtain experimental data on the maximum electric field making the conclusions valid for any arbitrary junction. The implications of the presence of high electric fields in shallow junctions are discussed with respect to scaling bipolar transistors.

Effect of optical-phonon scattering on the energy distribution of hot electrons in semiconductors at low temperature

The variational method is used in finding the solution of the transport equation for a system of hot electrons inn-Ge atT=20 K in the presence of high electric field. The role of the emission of optical phonons by hot electrons together with the effect of electron capture by repulsive centres on the formulation of the distribution function are studied. It is shown that the emission of optical phonons plays a dominant role in the formulation of the distribution. The influence of electron capture is very small, it may become appreciable at higher trap concentrations. The obtained distribution function is then used in calculating the capture rate of electrons by negatively charged centres. It is shown that the capture rate increases with electric field.

The intra-collisional field effect in semiconductors. I. Analytic results

First-order time-dependent perturbation theory is used to derive expression for scattering rates in the presence of high electric fields. Analysis in terms of Houston functions suggests that scattering rates are approximately field independent, but a more accurate treatment in terms of Airy functions shows that at sufficiently high fields, the resonant width of the scattering vertex is limited by the domain of integration, and a field dependent suppression of scattering occurs. This effect occurs first for small-wavevector phonons, and appreciable effects are expected for fields exceeding 104-105 V cm-1. It is also suggested that the complicated retarded transport equations recently introduced into hot electron theory are not necessary to treat the intra-collisional field effect in the weak phonon coupling limit at fields of interest in conventional semiconductor transport. At fields where suppression of scattering is significant, it is found that additional scattering into field-induced band tails is also important.

Reduced effective recombination coefficient in the disturbed polar E-region

The effective recombination coefficient can be substantially reduced in the polar E-region in the presence of high electric fields. This is a consequence of the high electron temperatures which have been found to occur under these conditions. Calculations, of the total ion production rate and particle heating are directly affected by the reduced recombination coefficient.

High current density in diffusive mode in MHD generator plasmas

This paper presents an experimental investigation of current conduction phenomena in potassium seeded combustion products of liquefied-petroleum-gas and oxygen using matrix copper cathode with and without seed deposition. Voltage drops in the cathode sheath and current densities are studied as a function of cathode temperature for different applied voltages. Performance of the matrix cathode is compared with that of a planar cathode. The seed laden matrix cathode has been observed to sustain high current densities (∼1500 A/m 2) in the diffusive mode of conduction. This is attributed to the effective lowering of the work function of the matrix cathode by the presence of high electric fields on the cathode surface.

X-ray emission from a laser irradiated target in the presence of high electric field

The temporal characteristics of the X-ray emission from a laser irradiated Ai target in a 20 kV/cm electric field are measured with electronic detectors. Spatial and spectral distributions of the X-ray radiation are evaluated using a pinhole camera and filter technique.

Non-Maxwellian Electron Distribution Function in n-GaAs Determined from Electric Field-Dependent Photoluminescence Spectrum

The hot electron distribution function in n -type GaAs has been experimentally investigated by photoluminescence spectroscopy of the ( e , h ) radiative transition under the presence of high electric fields. The hot electron distribution function deduced from the emission spectrum at 77 K shows a clear kink around the electron energy corresponding to the L O phonon energy. The elaborate comparison between the experimental observation and Monte Carlo simulation confirms that the observed kink is due to the dominant L O phonon emission process in the hot electron system. The experimentally determined temperature and mean energy of electrons are compared with the theoretical estimation.

The transient response of quasi-two-dimensional semiconductors under hot electron conditions

In calculating the transport properties of semiconductors in the presence of high electric fields, one must consider the effects of both the momentum, energy, and population-transfer relaxation times of the individual valleys, or quantum levels in a quasi-two-dimensional system. In recent calculations of hot electron microwave conductivities of quasi-two-dimensional silicon 〈100〉 samples it was found that the microwave conductivity at a fixed bias electric field showed non-monotonic behavior when plotted as a function of frequency, and has a conductivity peak in the high frequency region. To further clarify this conductivity peak and its implications for velocity overshoot, we have carried out transient response solutions to the electron transport for both large-signal and small-signal conditions. In this paper, the results of these calculations for electrons in a 〈100〉 inversion layer are reported. The response of the electron gas to a large dc field step and to a small step applied upon a large dc bias field are calculated. Much of the velocity overshoot and ac conductivity peaking are related to the differential repopulation among the inequivalent valley sub-bands.