High Electron Drift Velocity Research Articles

Intermittency, bursty turbulence, and ion and electron phase-space holes formation in collisionless current-carrying plasmas

In the previous studies of nonlinear saturation of the Buneman instability caused by high electron drift velocity relative to ions, the phase-space holes and the plateau on the electron velocity distribution function were identified as features of the saturation stage of instability [notably in the paper by Omura et al., J. Geophys. Res. 108, 1197 (2003)]. We have performed a much longer simulation of the Buneman instability and observed a secondary instability. This secondary instability generates fast electron-acoustic waves. By analyzing the phase-space plot of ions and electrons, we show that the fast electron heating and the formation of the plateau of electron velocity distribution function are not due to the quasi-linear diffusion but due to the nonlinear interaction of ion- and electron-acoustic solitary waves (phase-space holes) by exchange of trapped electrons in each wave. We also report the details on the intermittent and bursty nature of turbulence driven by this instability.

New optimized HIT solar cell using wide-bandgap nc-3C-SiC:H as a window layer

This study focuses on the investigation of HIT solar cell using nanocrystalline hydrogenated cubic silicon carbide (nc-3C-SiC:H) as a window layer. The selection of nc-3C-SiC:H is motivated by its wide bandgap, high electron drift velocity, and exceptional optical, electrical, and structural properties. Employing SCAPS-1D simulator, a comprehensive simulation analysis of thin nc-3C-SiC:H-based solar cell was conducted. By analyzing the current–voltage and quantum efficiency curves, the impact of doping concentration in the window layer was examined, covering a range from [Formula: see text] to [Formula: see text]. A rapid degradation of [Formula: see text] was observed for high doping concentrations. Additionally, the temperature dependence of external DC parameters for the temperatures 273, 300, and 318[Formula: see text]K was explored under AM1.5G and AM1.5D sunlight spectra. The application of two distinct light spectra resulted in the DC parameters exhibiting minimal variation or similarity. The study also took into account the influence of window thickness variation, temperature, and doping concentration. The findings demonstrate a remarkable influence of temperature and doping concentration on the DC parameters. [Formula: see text] value of 30.29[Formula: see text]mA/cm2 was achieved with a 3-nm thickness and an optimal efficiency of 19.48% was registered with a doping concentration of up to [Formula: see text], showcasing the promising potential of nc-3C-SiC:H.

Nondestructive microstructural investigation of defects in 4H-SiC epilayers using a multiscale luminescence analysis approach

The development of metal oxide semiconductor field effect transistors (MOSFETs) utilizing epitaxially grown 4H-SiC has accelerated in recent years due to their favorable properties, including a high breakdown field, high saturated electron drift velocity, and good thermal conductivity. However, extended defects in epitaxial 4H-SiC can affect both device yields and operational lifetime. In this work, we demonstrate the importance of a multiscale luminescence characterization approach to studying nondestructively extended defects in epitaxial 4H-SiC semiconducting materials. Multiscale luminescence analysis reveals different aspects of excess charge carrier recombination behavior based on the scale of a particular measurement. Combining measurements of the same extended defect area at different scales tells us more about the essential nature of that defect and its microstructure. Here, we use photoluminescence imaging and cathodoluminescence spectrum imaging to investigate the recombination behavior of several different types of extended defects, including stacking faults, inclusions, and basal plane dislocations. A detailed understanding of the optoelectronic properties of extended defects in epitaxial SiC helps elucidate the microstructure of extended defects and can provide pathways to mitigate detrimental changes during device operation related to their evolution, such as the recombination enhanced dislocation glide effect that affects SiC-based MOSFETs.

GaN Schottky diode on sapphire substrate for THz frequency multiplier applications

Gallium Nitride, because of its high breakdown field, and peak and saturated electron drift velocity, high chemical stability and high thermal conductivity, is a very promising material as candidate for Schottky varactor diodes for next-generation high-power frequency multipliers. Similarly to the huge interest in the field of HEMT for high frequency and power application, GaN Schottky diodes is of a high interest for the next technological step for frequency multipliers for THz generations, replacing GaAs Schottky diodes. Due to a lack of research focused on GaN Schottky diodes for frequency multiplication, the design and fabrication of GaN frequency multipliers remains a technological challenge. In this paper, we present GaN Schottky diodes fabricated on a heteroepitaxial GaN on sapphire. The barrier height obtained is around 0.53 ​eV, and it increases up to 0.73 ​eV after Schottky contact annealing; and the ideality factor is equal to 1.1 before annealing and 1.2 after annealing. The resulting series resistance is very low, 7Ω and 1Ω, before and after annealing, respectively.

Growth evolution of N-polar indium-rich InAlN layer on c-sapphire via strain relaxation by ultrathin AlON interlayer

InAlN as a functional inorganic material is a promising alternative to the commonly used InGaN in tunnel diodes and optoelectronic devices, due to its tunable wider range of energy bandgap (0.65–6.2 eV), thus empowering utilization of the whole solar spectrum. Moreover, high electron drift velocity and carrier concentration are considered as the most desirable prerequisite of indium-rich InAlN. N-polar indium-rich InAlN could be more beneficial due to the reverse direction of the polarization compared to Ga-polar. However, unanswered questions persist concerning growth evolution of N-polar indium-rich InAlN grown by organometallic chemical vapor deposition (OMCVD). In this study, energy dispersive X-ray spectroscopy (EDX) and high-angle annular dark-field (HAADF) imaging are used to characterize N-polar In0.60Al0.40N layer at nanometer scale in order to determine the evolution of the layer on (0001) sapphire substrate. Long nitridation of sapphire substrate leading to the formation of ~2 nm AlON ultrathin interlayer, which relaxes strain at the InAlN/sapphire interface with assistance of a low-temperature AlN interlayer is observed. EDX analysis confirms that after strain relaxation of InAlN layer, the indium-incorporation has only a weak dependence on the polarity of the layer. The incorporation of indium at preferential sites is also discussed at length.

MBE growth of entire-indium-composition-tunable In<italic><sub>x</sub></italic>Ga<sub>1-<italic>x</italic></sub>N alloy

As a direct band-gap semiconductor, ternary nitride alloy In x Ga1- x N has a tunable energy band-gap from 3.43 eV (for GaN) to 0.64 eV (for InN) with increasing In content, which covers the spectrum from 0.36 to 1.9 μm. In x Ga1- x N alloy has high electron drift velocity and absorption coefficient, which shows great potential application in devices such as full spectrum solar cells and light-emitting devices. Due to lack of proper substrate, In x Ga1- x N is usually grown on sapphire or GaN template. In this article, we review the growth and properties of InN and InGaN with tuable In composition grown on sapphire substrate and GaN/sapphire template by MBE. A boundary-temperature-controlled method is proposed, which successfully lead to a high-electron-mobility InN layers on sapphire substrate by MBE. The temperature-controlled epitaxy method used to grow In x Ga1- x N layers with tunable In composition on GaN/sapphire template.

Hot-electron real-space transfer and longitudinal transport in dual AlGaN/AlN/{AlGaN/GaN} channels

Real-space transfer of hot electrons is studied in dual-channel GaN-based heterostructure operated at or near plasmon–optical phonon resonance in order to attain a high electron drift velocity at high current densities. For this study, pulsed electric field is applied in the channel plane of a nominally undoped Al0.3Ga0.7N/AlN/{Al0.15Ga0.85N/GaN} structure with a composite channel of Al0.15Ga0.85N/GaN, where the electrons with a sheet density of 1.4 × 1013 cm−2, estimated from the Hall effect measurements, are confined. The equilibrium electrons are situated predominantly in the Al0.15Ga0.85N layer as confirmed by capacitance–voltage experiment and Schrödinger–Poisson modelling. The main peak of the electron density per unit volume decreases as more electrons occupy the GaN layer at high electric fields. The associated decrease in the plasma frequency induces the plasmon-assisted decay of non-equilibrium optical phonons (hot phonons) confirmed by the decrease in the measured hot-phonon lifetime from 0.95 ps at low electric fields down below 200 fs at fields of 4 kV cm−1 as the plasmon–optical phonon resonance is approached. The onset of real-space transfer is resolved from microwave noise measurements: this source of noise dominates for 8 kV cm−1. In this range of fields, the longitudinal current exceeds the values measured for a mono channel reference Al0.3Ga0.7N/AlN/GaN structure. The results are explained in terms of the ultrafast decay of hot phonons and reduced alloy scattering caused by the real-space transfer in the composite channel.

Field-effect transistors based on cubic indium nitride.

Although the demand for high-speed telecommunications has increased in recent years, the performance of transistors fabricated with traditional semiconductors such as silicon, gallium arsenide, and gallium nitride have reached their physical performance limits. Therefore, new materials with high carrier velocities should be sought for the fabrication of next-generation, ultra-high-speed transistors. Indium nitride (InN) has attracted much attention for this purpose because of its high electron drift velocity under a high electric field. Thick InN films have been applied to the fabrication of field-effect transistors (FETs), but the performance of the thick InN transistors was discouraging, with no clear linear-saturation output characteristics and poor on/off current ratios. Here, we report the epitaxial deposition of ultrathin cubic InN on insulating oxide yttria-stabilized zirconia substrates and the first demonstration of ultrathin-InN-based FETs. The devices exhibit high on/off ratios and low off-current densities because of the high quality top and bottom interfaces between the ultrathin cubic InN and oxide insulators. This first demonstration of FETs using a ultrathin cubic indium nitride semiconductor will thus pave the way for the development of next-generation high-speed electronics.

Quantum transport and spin properties of the carriers in Ⅲ-nitride heterostructures

Owing to their advantages of wide direct-band gap, excellent physical and chemical stability, high saturated electron drift velocity, and high dielectric breakdown field, Ⅲ-nitride semiconductor materials are the most favorite materials in developing high-frequency, high-temperature and high-power electronic devices and optical devices. Owing to their advantages of long spin relaxation time and high Curie temperature, Ⅲ-nitride semiconductor materials are the most favorite materials in developing spintronic devices. The magnetotransport methods at low temperatures and the circular photogalvanic effect at room temperature are introduced. The research status of the quantum transport and spin properties of the carriers in GaN-based heterostructures is reviewed.

Monte Carlo Simulations of High-Speed InSb–InAlSb FETs

Self consistent Monte Carlo simulations which include impact ionization are used to study the high-speed potential of InSb field-effect transistors. It is found that the impact ionization has a strong influence on the performance of InSb for high speed. The ionization leads to a high electron drift velocity and substrate bias can be used to extract the holes which are generated in the channel. Residual hole density within the channel, however, limits the maximum speed. The substrate bias and buffer doping are critical for extracting holes from the channel without inducing excess ionization. Simulations yield a peak cutoff frequency of 820 GHz with a 0.03125-/spl mu/m gate, a source to drain voltage of 0.58, and a sheet doping density of 1.7/spl times/10/sup 12/ cm/sup -2/.

Exploring the limits of superlattice miniband engineering using inverse scattering

We show how inverse scattering theory can be used to design superlattice structures with specified energy versus crystal momentum dispersion relations. We illustrate the method by calculating the superlattice unit cell required to maximize the curvature of the lowest miniband near the centre of the minizone. This type of structure should have a high electron drift velocity and might therefore be useful for high frequency applications.

Epitaxial Growth of 3C-SiC on T-shape columnar Si Substrates

AbstractCubic silicon carbide (3C-SiC) is a suitable semiconductor material for high temperature, high power and high frequency electronic devices, because of its wide bandgap, high electron mobility and high saturated electron drift velocity. The usage of Si substrates has the advantage of large area substrates for the growth of 3C-SiC layers. However, large lattice mismatch between 3C-SiC and Si (>20%) has caused the generation of defects such as misfit dislocations, twins, stacking faults and threading dislocations at the SiC/Si interface. Lateral epitaxial overgrowth (ELOG) of 3C-SiC on Si substrates using SiO2 has been reported to reduce the defect density. In this report, epitaxial growth of 3C-SiC on T-shape patterned (100) Si substrates has been investigated to reduce interfacial defects.

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.

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.

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.

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.

Subband Effects on High‐Field Electron Transport in Quasi‐One‐Dimensional Systems

AbstractA detailed calculation of electron transport in AlxGa1‐xAs/GaAs quantum wires by the use of a balance‐equation approach, considering all possible acoustic and optical phonon scattering and taking fifteen subbands into account, is reported. Crossover from one dimension (1D) to three dimensions (3D) in both low‐ and high‐field regimes is studied by investigating mobility and electron temperature as functions of the radius of quantum wire. At low temperatures, the linear mobility of a quasi‐1D electron system may be ten times higher or ten times lower than that of a 3D system depending on the relative position of the Fermi energy to subbands. At very high electric field, the difference in the transport behavior of quantum wires with differing radii and that of a 3D system disappears as a result of high electron temperature and drift velocity.

Monte Carlo simulation of 0.35- mu m gate-length GaAs and InGaAs HEMTs

A first-principles theoretical comparison of the performances of identical Al/sub 0.32/Ga/sub 0.68/As/GaAs and Al/sub 0.15/Ga/sub 0.15/As/In/sub 0.15/Ga/sub 0.85/As/GaAs pseudomorphic HEMTs based on an ensemble Monte Carlo simulation coupled with a two-dimensional Poisson solver is presented. The Monte Carlo calculations incorporate the full physics of the device operation. As a test of the accuracy with which the calculations successfully model a real device, the calculated current-voltage characteristic of the pseudomorphic device is first compared to recent experimental measurements made on a comparable structure, showing excellent agreement over a full range of gate and drain biases. The device performance as measured in terms of the transconductance, cutoff frequency, and current-voltage characteristics of both devices can then be compared as a function of gate and drain bias. It is found that the pseudomorphic HEMT outperforms the conventional GaAs/AlGaAs device by approximately a factor of two in the above-mentioned criteria. The greatly improved performance of the pseudomorphic HEMT over the GaAs device stems predominantly from greater confinement of the electrons within the high-mobility two-dimensional system, higher electron drift velocity, and greater gamma valley confinement. >

The analysis of defect structures and substrate/film interfaces of diamond thin films

Diamond is an excellent candidate material for use in selected electronic and wear resistant coating applications due to its superior hardness, strength and thermal conductivity as well as its high electron drift velocity, chemical and thermal stability, radiation hardness and optical transmission. Electronic devices of particular interest include those having high-power, -frequency and -temperature applications, as well as those for chemically harsh and/or high radiation flux environments. The recent development of techniques for growth of crystalline diamond films at low pressures using common hydrocarbon and H2 gases has created the potential for growing thin films for such devices or wear resistant coatings and a host of related applications. In this research, diamond thin films grown from a low pressure methane-hydrogen gas mixture by microwave plasma enhanced chemical vapor deposition (CVD) have been examined by various transmission electron microscopy (TEM) techniques including bright and dark field, high resolution (HREM), selected area diffraction (SAD) and electron energy loss spectroscopy (EELS). Columnar growth of polycrystalline grain structure, twins, stacking faults, dislocations and intermediate layers were characteristic of the diamond films. No sp2 bonding character in the grains, defects or grain boundaries was detected by EELS.