High Drift Velocity Research Articles

Sheath expansion in a drifting, nonuniform plasma

A model is proposed for the sheath expansion into a nonuniform plasma with substantial drift velocity. We refer in particular to the streaming metal plasma produced by a vacuum arc discharge; this kind of plasma is characterized by its spatial nonuniformity and high ion drift velocity. It is shown that for a nonuniform plasma and high substrate bias voltage (>10 kV), the sheath thickness is significantly smaller than in a uniform plasma. High drift velocity also leads to a decrease in sheath thickness. For plasma immersion ion implantation carried out in a nonuniform plasma, the increase in ion implantation current with increasing voltage is different from that of a uniform plasma where the ion current is constant with voltage. This effect was found to be quantitative agreement with experiment. The decrease in sheath thickness with increasing plasma drift velocity can explain qualitatively the experimentally observed existence of a stable high-voltage sheath for a relatively long time.

High Etch Rate and Smooth Morphology Using a Novel Chemistry in Reactive Ion Etching of GaN

GaN-based materials are commonly considered as potential candidates for light sources in the green and blue spectral range. The bandgap energy of AlGaInN varies between 1.95 and 6.2 eV. Significant research has been performed on high-brightness blue light emitting diodes (LEDs) and laser diodes (LDs). For example, the advantages of using LEDs over conventional electric incandescent lighting are faster response time, longer life, lower energy consumption, and higher luminescence efficiency. Also GaN, a wide-gap material, presents a potential use for high-temperature and high-power microelectronic applications. The high electron drift velocity, low thermal-generation rate, and high breakdown field make GaN and its related materials the most technologically interesting electronic materials for field-effect transistor applications. However, the intrinsic characteristics of this material system and, more specifically, its inertness, due to an atomic gallium-nitrogen bond energy of 8.92 eV/atom, make processing possible only with dry etching techniques. Dry etching of GaN has been extensively investigated using a wide range of plasma-based machines and sources. Most authors use electron cyclotron resonance (ECR) plasma based on CH 4 -H 2 -Ar, Cl 2 -H 2 , or CH 4 -H 2 -Ar-Cl 2 1-3 or chemically assisted ion beam etching (CAIBE).4,5 Only a few reports describe dry etching of GaN using various chemistries in a more conventional reactive ion etching (RIE) and the number of papers dealing with a SiCl 4 -based chemistry is even lower.6 Experimental In this paper, we present the results of RIE etching of metallorganic vapor-phase epitaxy (MOVPE) grown GaN on a (0001) sapphire substrate. The experiments were performed in a conventional parallel plate and load-locked Oxford Plasmalab System 100. Various combinations have been investigated using SiCl 4 :Ar, and eventually adding SF 6 . The mask used in these experiments is a plasma-deposited SiN x layer ~300 nm thick, annealed at 450°C for 10 min. Patterning the mask was carried out either by wet etching in a buffered HF solution or by using a dry etching process with SF 6 :Ar [10:10 standard cubic centimeters per minute (sccm)] at 150 W and 40 mTorr. The etch rate of the SiN x mask was ~200 nm/min. The influence of gas flow, pressure, and radio-frequency (rf) power were investigated. The basic gas flow rate used was SiCl 4 :Ar (10:10) sccm. Most gas combinations were investigated at three rf power values (70, 105, and 140 W) and at three pressure values (20, 40, and 60 mTorr). These parameters, combined together, determine the dc voltage which is related to the acceleration energy to the active species.

Strong impact of neutrals on anomalous inward drift and width of steep gradient zone

The fundamental properties of the anomalous inward drift and the width of the steep gradient zone in H mode plasmas are explored. A special version of the 1.5-D BALDUR transport code is used to determine the profiles of the electron heat diffusivity and vin/D by transport analysis. The strong rise with radius of vin/D in the edge region is explained by a linear dependence on the neutral deuterium density n0, resulting in a new scaling expression vin(x)/D(x) = F0Zeff(x)n0(x)2x/(ρwxs2). Applying this in simulations reproduces the empirical fit of the vin/D profile not only in the edge plasma but also in the bulk plasma. Modellings with this scaling yield the observed flattening of density profiles with rising line averaged density. The decreasing penetration of deuterium atoms to the core causes a decline of the inward drift. The new scaling is shown to be compatible with gas oscillation experiments, while n0-independent scalings are not. This further explains the strong density profile peaking and rise of vin/D during and after pellet injection by the increase in neutral density. The width of the steep gradient zone is found to be connected with the penetration of neutrals at the edge and the presence of high inward drift velocities. The anomalous inward drift is attributed to ion dynamics, i.e. to the friction between fluctuating deuterons and deuterium atoms diffusing inward. A more general vin/D scaling including impurity effects is presented.

Double-Doppler wavelength conversion of infrared optical pulses by moving grating of refractive index in semiconductors

A time-domain model of an excited semiconductor medium as a moving Bragg grating was used to investigate a double-Doppler wavelength conversion of an infrared (λ0 = 1.5 μm) 0.8 ps width optical pulse. In order to calculate ultrafast electromagnetic transients, a time-domain Volterra integro-differential equation was used for simulations. It is shown here that the high drift velocity of carriers in an InAs semiconductor produces a 3.5 nm up and down (9.7 THz) conversion span. The converted pulse width and spectral density strongly depend on the contrast of permittivity modulation. The depth of modulation has to be at least 5–7. A −3 dB conversion efficiency can be gained with a depth of modulation more than 7 and a length of semiconductor layer greater than 20 μm.

Forward muon system for the DØ detector upgrade

The design and main parameters of the completly redesigned DØ Forward Angle MUon System (FAMUS: 1.0<|η|<2.0) for the next high luminosity Tevatron Collider run are reported. Results of the studies of trigger scintillation counters based on fast scintillator Bicron 404A and WLS bars SOFZ-105 are presented. We report about results of test beam studies of prototype counters including minimum ionizing particles detection efficiency, time resolution and amplitude response. Radiation ageing of scintillating materials for the doses up to 1Mrad, phototubes magnetic shielding in the fields of up to 700G and ageing of phototubes are presented. Mini-Drift Tubes (MDTs) are chosen as FAMUS tracking detectors. The detector is a drift wire chamber with a metallic cathode. The detector operates in proportional mode with a fast freon–methane gas mixture to provide high drift velocity, adequate counting rate and low ageing. A description of the performance of the MDT is given. Studies of two prototypes in test beams were performed at FNAL and JINR. Obtained coordinate accuracy is around 0.5mm r.m.s. All tests show robustness of MDT as tracking detector of the new muon system for a long period in high DØ background radiation conditions.

Emission and absorption of confined and interface optical phonons by electrons in the non-linear transport regime of a parabolic quantum wire

We present a detailed study of dissipation by electrons in the non-linear transport regime of harmonically confined quantum wires in GaAs/AlAs structures through emission and absorption of confined and interface optical phonons. An applied magnetic field perpendicular to the interfaces is used to modulate the natural harmonic frequencies in order to explore situations in and off resonance with the vibrational modes. Our results show that electrons absorb both confined phonons and interface phonons over wide angles and backwards, at low drift velocities. The emission occurs both forwards and backwards and dominates entirely for high drift velocities. The confined phonons play the dominant role in the dissipation, even for quantum wires inside thin quantum wells and with a magnetic field strength selected to make the harmonic levels in resonance with the AlAs-like interface mode.

Transmission Electron Microscopy Investigation of Defects in B-Implanted 6H-SiC

Silicon carbide is due to its wide bandgap, high saturated electron drift velocity, high electric breakdown field and high thermal conductivity a suitable material for electron devices operating at high temperatures, high powers and high frequencies.[1,2] In order for SIC to reach its full potential in device technology, doping is essential. Usually ion implantation is used for doping since diffusion is difficult in SiC. Boron is a useful material for implantation because of its low atomic weight and greater penetration depth than other accepters, yet very few studies have been conducted on B-implanted 6H-SiC. [3,4] In this investigation we have used transmission electron microscopy (TEM) to study structural defects that are found in B-implanted 6H-SiC layers.

Electron transport in wurtzite indium nitride

We present the velocity-field characteristics of wurtzite indium nitride, determined using an ensemble Monte Carlo approach. It is found that indium nitride exhibits an extremely high peak drift velocity at room temperature, 4.3×107 cm/s, at a doping concentration of 1.0×1017 cm−3. We also demonstrate that the saturation drift velocity of indium nitride, 2.5×107 cm/s, is comparable to that of gallium nitride, and much larger than that of gallium arsenide. Our results suggest that the transport characteristics of indium nitride are superior to those of gallium nitride and gallium arsenide, over a wide range of temperatures, from 150 to 500 K, and doping concentrations, up to 1.0×1019 cm−3. Hence, indium nitride has considerable potential for device applications.

High-Speed Response of Uni-Traveling-Carrier Photodiodes

The photoresponse of a uni-traveling-carrier photodiode (UTC-PD), which is configured with a neutral narrow-gap light absorption layer and a depleted wide-gap carrier collecting layer, is investigated by small-signal analysis. Drift-diffusion model was used for analyzing carrier dynamics in the absorption layer. For accurately predicting the frequency response, a boundary condition at the edge of the absorption layer was carefully treated by taking into account the electron thermionic emission velocity. High electron mobility in the absorption layer and high drift velocity in the carrier collecting layer associated with the velocity overshoot effect are both essential for short response times. Calculations performed on InP/InGaAsP UTC-PDs with the same absorption and carrier collecting layer thicknesses show that the response can be dominated by the electron transport in the absorption layer provided that the significant velocity overshoot occurs in the carrier collecting layer. Furthermore, a UTC-PD with a quasi-field in the absorption layer can generate a several times broader bandwidth than conventional pin PDs, while maintaining a similar internal quantum efficiency.

Hydrodynamic behaviour of a solute particle by molecular dynamics

The hydrodynamic behaviour of a single solute particle immersed in a solvent system of (N-1 particles, interacting through a repulsive Lennard-Jones potential, has been studied for several system sizes, ranging from N = 108 to 2048 particles using isothermal-isochoric molecular dynamics computer simulation. The solute particle was projected at a fixed relative velocity with respect to the host fluid. The computations show that the linear resistance force versus velocity behaviour, postulated by the Stokes law, is obeyed quite well even up to relatively large (i.e. near thermal) solute velocities. However, two finite size effects have been found for large solute particles (cα σB/σs > 1, where σBand σs the solute and solvent particles, respectively) when they are confined in such small periodic systems. At high drift velocity, there is a breakdown in this linear relationship for the smaller systems leading to a maximum in the opposing force on the solute particle. Als o there is a slow system size convergen...

Hydrodynamic behaviour of a solute particle by molecular dynamics

The hydrodynamic behaviour of a single solute particle immersed in a solvent system of (N-1 particles, interacting through a repulsive Lennard-Jones potential, has been studied for several system sizes, ranging from N = 108 to 2048 particles using isothermal-isochoric molecular dynamics computer simulation. The solute particle was projected at a fixed relative velocity with respect to the host fluid. The computations show that the linear resistance force versus velocity behaviour, postulated by the Stokes law, is obeyed quite well even up to relatively large (i.e. near thermal) solute velocities. However, two finite size effects have been found for large solute particles (cα σB/σs > 1, where σBand σs the solute and solvent particles, respectively) when they are confined in such small periodic systems. At high drift velocity, there is a breakdown in this linear relationship for the smaller systems leading to a maximum in the opposing force on the solute particle. Als o there is a slow system size convergence to the thermodynamic (N → ∞ effective Stokes c parameter, and when compared with the value given by the Stokes-Einstein relationship.

Single crystal growth of SiC and electronic devices

Single crystal growth of silicon carbide (Sic) and application to electronic devices are reviewed. In the crystal growth, bulk and homoepitaxial growth are picked up, and crystal quality and electrical properties are described. For electronic devices, various device processes are argued. Power devices based on Sic are stressed in this review. Bulk single crystals of SiC can be grown by a sublimation method, and large-area 6H-SiC and 4H-SiC single crystals are obtained. The occurrence of SiC polytypes is affected by the growth condition, and can be controlled successfully by optimizing these conditions. 6H-SiC is grown on 6H-SiC (0001) Si-faces, and 4H-SiC on 6H-SiC (0001) C-faces. The crystallinity of bulk crystals is investigated by reflection high-energy electron diffraction (RHEED) and X-ray analysis, and characterization is carried out in detail by optical and electrical measurement. Successful homoepitaxial vapor phase growth of SiC can be realized using off-axis (0001) substrates prepared by a sublimation method called “step-controlled epitaxy”. Since the crystallinity of epilayers is improved during the step-controlled epitaxy, this growth technique is a key for getting high-quality crystal surfaces. Impurity doping is controlled during homoepitaxial growth by employing impurity gases, such as N2, trimethylaluminum (TMA), and B2H6. A wide-range of carrier concentrations of 5 × 1013∼3 × 1018 cm−3 for n-type and 5 × 1016∼3 × 1020 cm−3 for p-type are realized. The impurity-incorporation mechanism in the step-controlled epitaxy is discussed based on the C/Si ratio dependence of impurity doping. Electrical properties of SiC grown by step-controlled epitaxy are determined precisely. A high electron mobility of 720 cm2/Vs is obtained in an undoped 4H-SiC epilayer with an electron concentration of 2.5 × 10l6 cm−3 at 300 K. This electron mobility is about two times higher than that of 6H-Sic (∼380 cm2/Vs). High breakdown fields of 1∼5 × 106 V/cm are obtained for both 6H- and 4H-SiC, one order of magnitude higher than those for Si. A high saturation electron drift velocity of 1.6 × 107 cm/s is obtained in 4H-Sic, which may make possible high performance of high-frequency 4H-SiC power devices. Impurity levels and deep levels are investigated by Hall effect, admittance spectroscopy, and DLTS measurement. Metal/4H-SiC Schottky barrier heights are characterized and a strong dependence on metal work function without strong “pinning” is elucidated. Device processes are described for ion implantation. Interface properties of SiO2/SiC are characterized in detail using metal-oxide-semicond.

Radio-Frequency Power Transistors Based on 6H- and 4H-SiC

In recent years, SiC has received a great deal of attention as a nearly ideal material for the fabrication of high-speed, high-power transistors. The high electric breakdown field of 3.8 × 106 V/cm, high saturated electron drift velocity of 2 × 107 cm/s, and high thermal conductivity of 4.9 W/cm K indicate SiC's potential for high-power, high-frequency operation. A wide bandgap should also allow SiC field-effect transistors (FETs) to have high radio-frequency (rf) output power at high temperatures.These material qualities have been verified through outstanding device performance. Recent results for SiC metal-semiconductor field-effect transistors (MESFETs) have included superior frequency and power performance, with power gain at frequencies as high as 40 GHz and power densities as high as 3.3 W/mm. This represents significantly higher operating frequencies and power densities than current Si rf power FET technology, and nearly three times the power density of GaAs MESFETs, which are currently used in many commercial rf power applications. Similarly, SiC static induction transistors (SITs) have much higher power densities than their Si counterparts and have recently been demonstrated in modules with as much as 470-W total pulsed output power. This article describes microwave device operation, discusses material properties needed for rf power generation, and summarizes state-of-the-art SiC high-frequency device performance. Emphasis is placed on MESFETs and SITs since they are currently the most mature SiC-based device technologies.

High resolution diagnostics on COMPASS-D and START

In this paper we describe three novel high resolution diagnostic systems on the Culham tokamaks. The first is a pair of high dispersion, short sampling time visible light spectrometers, employing a fast charge-coupled device (CCD) and a multianode photomultiplier as detectors, allowing unusually high bandwidth ion drift velocity measurements. Secondly, the application of an X-ray CCD to a Johann spectrometer for ion and electron core parameter characterisation is outlined. Lastly, a very high throughput, 30-channel Thomson scattering spectrometer system is described, which exploits the low f number lens design techniques, coated fibres and third-generation image intensifiers now available.

Velocity Overshoot And Ballistic Electron Transport In Wurtzite Indium Nitride

AbstractUsing an ensemble Monte Carlo approach, ballistic transport and velocity overshoot effects are examined in InN and compared with those in GaN and GaAs. It is found that the peak overshoot velocity is in general greater than both GaN and GaAs. Furthermore, the velocity overshoot in InN occurs over distances in excess of 0.4 μm, which is comparable to GaAs but is significantly longer than the overshoot in GaN. These strong overshoot effects, combined with a high peak drift velocity, large low-field mobility, and large saturation drift velocity, should allow InN based field effect transistors to outperform their GaN and GaAs based counterparts.

The Velocity-Field Characteristic Of Indium Nitride

AbstractWe determine the velocity-field characteristic of wurtzite indium nitride using an ensemble Monte Carlo approach. It is found that indium nitride exhibits an extremely high room temperature peak drift velocity, 4.2 × 107 cm/s, at a doping concentration of 1 × 1017 cm−3. This exceeds that of gallium nitride, 2.9 × 107 cm/s, by approximately 40 %. For our nominal parameter selections, the saturation drift velocity of indium nitride is found to be 1.8 × 107 cm/s. The device performance of this material, as characterized by the cut-off frequency, is found to superior to that of gallium nitride, gallium arsenide, and silicon.

Asymptotic Field Dependence of Approach to Saturation of Drift Velocity of Electrons in Liquid Xenon

Abstract The high electric field saturation drift velocity v sat of electrons in liquid xenon near the triple point is 3 × 103 m/s. This value is recovered if (Vsat d) κsat τsat =D. Here D is the self-diffusion coefficient of liquid xenon, τsat is the lower limit of phonon oscillation periods, and κsat is an electron energy-loss probability characteristic of scattering of field-driven electrons by the atoms of the liquid medium. Given the form of the saturation drift velocity, the asymptotic field (E) dependence of the approach to the drift velocity saturation is then explored by expanding κ(Eτ(E) in the form κ(Eτ(E) = κsat τsat + A 1/E γ + higher-order terms. From experimental results of Huang and Freeman, γ is found to be 4/3. This form cannot, of course, be extended into regimes of much lower electric field where a further physical mechanism contributes to the electron mobility.

Electric field heated electrons in a-Si:H — new features

The subnanosecond time-of-flight measurement at very high electric field (≤ 0.55 MV/cm) and at high temperatures (300 K < T < 450 K) allowed one to achieve the full deactivation of the electron drift mobility in a-Si:H and thus almost to eliminate the influence of shallow trapping. From the temperature dependence of high temperature drift mobility at low electric fields the value of band mobility ( μ 0 = 6.5 cm 2/V s) is determined, which appears to be temperature independent. The field dependence of the drift mobility indicates that the μ 0 has to decrease with increasing field when the drift velocity exceeds the sound velocity. At very high electric fields drift velocity saturation has been observed, which we explain by a model based on optical phonon emission, similar to the behaviour of hot carriers in crystalline semiconductors.

Electron swarm parameters in ramp electric fields

The behavior of electron swarms in gases in time-varying ramp electric fields is investigated using a Monte Carlo simulation. In particular, we consider the case of swarms in chlorine gas. Swarm parameters as a function of instantaneous E/N(E is the electric field and N is the gas number density) for different dE/dt are determined. At higher dE/dt, all the parameters as a function of E/N are shifted to larger E/N when compared to the steady-state case. The drift velocity curve develops a peak which shifts to higher E/N and increases in magnitude as dE/dt is increased. Next, we pay attention to the definition of the swarm parameters, where the parameters derived from kinetic theory and by analogy to experiment are compared. For the high dE/dt cases, differences in the swarm parameters at the same E/N values are caused by the inertia of the electrons and their transient beam-like high drift velocity, while discrepancies due to the difference between the definitions in the kinetic theory and in the analogy to experiment appear significantly.

Silicon carbide high-power devices

In recent years, silicon carbide has received increased attention because of its potential for high-power devices. The unique material properties of SiC, high electric breakdown field, high saturated electron drift velocity, and high thermal conductivity are what give this material its tremendous potential in the power device arena. 4H-SiC Schottky barrier diodes (1400 V) with forward current densities over 700 A/cm/sup 2/ at 2 V have been demonstrated. Packaged SITs have produced 57 W of output power at 500 MHz, SiC UMOSFETs (1200 V) are projected to have 15 times the current density of Si IGBTs (1200 V). Submicron gate length 4H-SiC MESFETs have achieved f/sub max/=32 GHz, f/sub T/=14.0 GHz, and power density=2.8 W/mm @ 1.8 GHz. The performances of a wide variety of SiC devices are compared to that of similar Si and GaAs devices and to theoretically expected results.