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Electron Gas (electron + gas)

Distribution by Scientific Domains

Physics and Astronomy	84%
Polymers and Materials Science	7%

Distribution within Physics and Astronomy

Solid State Physics	84%
General & Introductory Physics	14%

Kinds of Electron Gas

two-dimensional electron gas

Selected Abstracts

Observation of a 2D Electron Gas and the Tuning of the Electrical Conductance of ZnO Nanowires by Controllable Surface Band-Bending

ADVANCED FUNCTIONAL MATERIALS, Issue 15 2009
Youfan Hu
Abstract Direct experimental evidence for the existence of a 2D electron gas in devices based on ZnO nanowires (NWs) is presented. A two-channel core/shell model is proposed for the interpretation of the temperature-dependent current,voltage (I,V) characteristics of the ZnO NW, where a mixed metallic,semiconducting behavior is observed. The experimental results are quantitatively analyzed using a weak-localization theory, and suggest that the NW is composed of a "bulk" semiconducting core with a metallic surface accumulation layer, which is basically a 2D electron gas in which the electron,phonon inelastic scattering is much weaker than the electron,electron inelastic scattering. A series of I,V measurements on a single NW device are carried out by alternating the atmosphere (vacuum, H2, vacuum, O2), and a reversible change in the conductance from metallic to semiconducting is achieved, indicating the surface accumulation layer is likely hydroxide-induced. Such results strongly support the two-channel model and demonstrate the controllable tuning of the ZnO NW electrical behavior via surface band-bending. [source]

What really happens with the electron gas in the famous Franck-Hertz experiment?

CONTRIBUTIONS TO PLASMA PHYSICS, Issue 3-4 2003
F. Sigeneger
Abstract The interpretation of the anode current characteristics obtained in the famous Franck-Hertz experiment of 1914 led to the verification of Bohr's predictions of quantised atomic states. This fundamental experiment has been often repeated, and nowadays is generally part of the curriculum in modern physics education. However, the interpretation of the experiment is typically based upon significant simplifying assumptions, some quite unrealistic. This is the case especially in relation to the kinetics of the electron gas, which is in reality quite complex, due mainly to non-uniformities in the electric field, caused by a combination of accelerating and retarding components. This non-uniformity leads to a potential energy valley in which the electrons are trapped. The present state of understanding of such effects, and their influence upon the anode characteristics, is quite unsatisfactory. In this article a rigorous study of a cylindrical Franck-Hertz experiment is presented, using mercury vapour, the aim being to reveal and explain what really happens with the electrons under realistic experimental conditions. In particular, the anode current characteristics are investigated over a range of mercury vapour pressures appropriate to the experiment to clearly elaborate the effects of elastic collisions (ignored in typical discussions) on the power budget, and the trapping of electrons in the potential energy valley. [source]

Observation of a 2D Electron Gas and the Tuning of the Electrical Conductance of ZnO Nanowires by Controllable Surface Band-Bending

Electronic and Magnetic Properties of SrTiO3/LaAlO3 Interfaces from First Principles

ADVANCED MATERIALS, Issue 26-27 2010
Hanghui Chen
Abstract A number of intriguing properties emerge upon the formation of the epitaxial interface between the insulating oxides LaAlO3 and SrTiO3. These properties, which include a quasi two-dimensional conducting electron gas, low temperature superconductivity, and magnetism, are not present in the bulk materials, generating a great deal of interest in the fundamental physics of their origins. While it is generally accepted that the novel behavior arises as a result of a combination of electronic and atomic reconstructions and growth-induced defects, the complex interplay between these effects remains unclear. In this report, we review the progress that has been made towards unraveling the complete picture of the SrTiO3/LaAlO3 interface, focusing primarily on present ab initio theoretical work and its relation to the experimental data. In the process, we highlight some key unresolved issues and discuss how they might be addressed by future experimental and theoretical studies. [source]

Grid-based density functional calculations of many-electron systems

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2008
Amlan K. RoyArticle first published online: 10 DEC 200
Abstract Exploratory variational pseudopotential density functional calculations are performed for the electronic properties of many-electron systems in the 3D cartesian coordinate grid (CCG). The atom-centered localized gaussian basis set, electronic density, and the two-body potentials are set up in the 3D cubic box. The classical Hartree potential is calculated accurately and efficiently through a Fourier convolution technique. As a first step, simple local density functionals of homogeneous electron gas are used for the exchange-correlation potential, while Hay-Wadt-type effective core potentials are employed to eliminate the core electrons. No auxiliary basis set is invoked. Preliminary illustrative calculations on total energies, individual energy components, eigenvalues, potential energy curves, ionization energies, and atomization energies of a set of 12 molecules show excellent agreement with the corresponding reference values of atom-centered grid as well as the grid-free calculation. Results for three atoms are also given. Combination of CCG and the convolution procedure used for classical Coulomb potential can provide reasonably accurate and reliable results for many-electron systems. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source]

Thomas,Fermi approximation for the quasi-two-dimensional electron gas

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2001
M. Moreno
Abstract To take into account static correlation effects in the quasi-two-dimensional electron gas a screened Coulombic interaction between particles is studied. The Thomas,Fermi approximation is used and the potential screening appears as a function of the Wigner,Seitz density parameter rs and the effective width t of the system. With the self-consistent field theory applied to the modified deformable jellium, the ground-state energy per particle and the conditions for electron localization are obtained in terms of the interparticle distance and the screening parameter ,. A critical minimum characteristic width tc is obtained; below tc no long-range order is obtained. For larger widths a stable localized state is predicted at finite densities. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 269,276, 2001 [source]

Optimization of homoepitaxially grown AlGaN/GaN heterostructures

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 10 2010
J. A. Grenko
Abstract We report on the growth of Al0.25Ga0.75N/GaN heterostructures on low dislocation density semi-insulating c -axis GaN substrates by metalorganic vapor phase epitaxy (MOVPE). A room temperature (RT) Hall mobility (µRT) up to 2065,cm2,V,1,s,1 at sheet density (ns) of 8.25,×,1012,cm,2 has been measured. This work compliments prior studies in which we observed a buffer-induced modulation of the RT two-dimensional electron gas (2DEG) ns and µRT by varying the GaN buffer layer thickness. Here, we focus on the optimization of the heterostructure 2DEG properties by elimination of silicon doping in the Al0.25Ga0.75N barrier and unintentional Al in the not-intentionally doped (n.i.d.) GaN buffer layer. The 15% improvement in µRT and ns relative to previous results is consistent with those predicted by Poisson solver calculations. Use of thick GaN buffers has minimized the theoretical mobility reduction based on intersubband scattering and has enabled us to determine the 2DEG sheet density associated with the polarization field () to be ,5,×,1012,cm,2. [source]

Spin-filtering effect in a two-dimensional electron gas under a local fringe field

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 12 2007
Seon-Gu Huh
Abstract We fabricated a spin-filtering device which consists of InAs based two-dimensional electron gas and two ferromagnetic pads. A fringe field at the edge of a ferromagnetic pad was used to induce spin polarization of the moving electrons in a two-dimensional electron gas. The direction of the fringe field was determined by the magnetization of the ferromagnetic pad, which was switched by using the external magnetic field. A current of which spins were aligned by one fringe field (polarizer) was filtered by the other fringe field (analyzer). The resistance of the device was in either a high state or a low state depending on the alignment of the fringe fields. The spin-filtering effect diminishes rapidly as the thermal energy exceeds the Zeeman splitting. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Hot phonons in GaN channels for HEMTs

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 10 2006
A. Matulionis
Abstract Accumulation of non-equilibrium longitudinal optical (LO) phonons (termed hot phonons) is considered as a possible cause for limitation of hot-electron drift velocity at high electric fields in a GaN biased channel with a two-dimensional electron gas (2DEG). Equivalent hot-phonon temperature and hot-phonon lifetime are estimated from experimental results on hot-electron noise at a microwave frequency. Dependence of the hot-phonon parameters on electron density and channel composition is investigated, and frequency performance of high-electron-mobility transistors (HEMTs) is discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Current imaging in quantum point contacts

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 6 2006
Alessandro Cresti
Abstract The experimental imaging of microscopic currents in two-dimensional electron gas based systems has been recently realized exploiting an ingenious use of scanning microscope tips. By means of the Keldysh Green's function formalism in a tight-binding framework, I study the electron transport in a model quantum point contact device, obtaining detailed maps of the local current distribution. The results are then compared with those obtained by a direct simulation of the experimental process, i.e. introducing a suitable external potential to reproduce the effect of the coupled microscopic tip on the overall conductance and the electron flow. The analysis of the differences between the two calculations helps to interpret the experimental maps and sheds light on the interference effects of the tip. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Strain-engineered novel III,N electronic devices with high quality dielectric/semiconductor interfaces

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003
M. Asif Khan
Abstract Since the early demonstration of 2D-electron gas [M. A. Khan et al., Appl. Phys. Lett. 60, 3027 (1992)] and a heterojunction field effect transistor (HFET) [M. Asif Khan et al., Appl. Phys. Lett. 63, 1214 (1993)] in III,N materials, rapid progress has been made to improve the DC and RF performance of GaN,AlGaN based HFETs. Stable and impressive microwave powers as high as 4,8 W/mm have been reported for device operation frequencies from 10 to 35 GHz. The key reason for these high performance numbers is an extremely large sheet carrier densities (>1 × 1013 cm,2) that can be induced at the interfaces in III,N hetereojunction [A. Bykhovsk et al., J. Appl. Phys. 74, 6734 (1993); M. Asif Khan et al., Appl. Phys. Lett. 75, 2806 (1999)]. These are instrumental in screening the channel dislocations thereby retaining large room temperature carrier mobilities (>1500 cm2/Vs) and sheet resistance as low as 300 ,/sq. These numbers and the high breakdown voltages of the large bandgap III,N material system thus enable rf-power approximately 5,10 times of that possible with GaAs and other competitor's technologies. We have recently introduced a unique pulsed atomic layer epitaxy approach to deposit AlN buffer layers and AlN/AlGaN superlattices [J. Zhang et al., Appl. Phys. Lett. 79, 925 (2001); J. P. Zhang et al., Appl. Phys. Lett. 80, 3542 (2002)] to manage strain and decrease the dislocation densities in high Al-content III,N layers. This has enabled us to significantly improve GaN/AlGaN hetereojunctions and the device isolation. The resulting low defect layers are not only key to improving the electronic but also deep ultraviolet light-emitting diode devices. For deep UV LED's they enabled us to obtain peak optical powers as high as 10 mW and 3 mW for wavelengths as short as 320 nm and 278 nm. Building on our past work [M. Asif Khan et al., Appl. Phys. Lett. 77, 1339 (2000); X. Hu et al., Appl. Phys. Lett. 79, 2832 (2001)] we have now deposited high quality SiO2/Si3N4 films over AlGaN with low interface state densities. They have then been used to demonstrate III,N insulating gate transistors (MOSHFET (SiO2) and MISHFET (Si3N4) with gate leakage currents 4,6 order less than those for conventional GaN,AlGaN HFETs. The introduction of the thin insulator layers (less then 100 Å) under the gate increases the threshold voltage by 2,3 V. In addition, it reduces the peak transconductance gm. However the unity cut-off frequency, the gain and the rf-powers remain unaffected as the gm/Cgs (gate-source capacitance) ratio remains unchanged. In addition to managing the defects and gate leakage currents we have also employed InGaN channel double heterojunction structures (AlInGaN,InGaN,GaN) to confine the carriers thereby reducing the spillover into trappings states. These InGaN based MOS-DHFETs exhibited no current-collapse, extremely low gate leakage currents (<10,10 A/mm) and 10,26 GHz rf-powers in excess of 6 W/mm. We have also demonstrated the scalability and stable operation of our new and innovative InGaN based insulating gate heterojunction field effect transistor approach. In this paper we will review the III,N heterojunction field-effect transistors progress and pioneering innovations including the excellent work from several research groups around the world. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Polarization effects in 2-DEG and 2-DHG AlGaN/AlN/GaN multi-heterostructures measured by electron holography

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2010
Q. Y. Wei
Abstract The electrostatic potential profiles and charge distributions in modulation-doped n-type and p-type AlGaN/AlN/GaN heterostructures have been measured by electron holography with high spatial resolution. For n-type two-dimensional electron gas structure a negative curvature and for p-type two-dimensional hole gas structure a positive curvature in the potential profile at the AlN/GaN interface were observed, which demonstrated the accumulation of two-dimensional carriers. The measured electrostatic potential profiles were also compared with the calculated band diagram in the heterostructures. [source]

Negative longitudinal magneto-resistance of layered crystals taking into account the spin splitting

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2006
S. R. Figarova
Abstract The influence of the spin splitting on the longitudinal magnetoresistance of a quasi-two-dimensional electron gas in layered crystals is theoretically investigated. At acoustic-phonon scattering, a general expression for the longitudinal magnetoresistance has been found. A negative longitudinal magnetoresistance due to the spin splitting has been revealed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Ultrafast decay of non-equilibrium (hot) phonons in GaN-based 2DEG channels

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2009
Arvydas MatulionisArticle first published online: 27 OCT 200
Abstract Fast and ultrafast decay of non-equilibrium longitudinal optical phonons (hot phonons) is discussed on examples of two-dimensional electron gas (2DEG) channels confined in nominally-undoped nitride and selectivelydoped arsenide heterostructures subjected to pulsed dc electric power. At a low,moderate power, a nonmonotonous dependence of the hot-phonon lifetime on the 2DEG density is found. The fastest decay takes place at ,2.5 × 1012 cm,2 in GaInAs 2DEG channel and at ,7 × 1012 cm,2 in GaN 2DEG channel. The result is discussed in terms of plasmon,LO-phonon resonance. The hot-phonon lifetime decreases when the supplied power is increased if the 2DEG density exceeds the resonance density. The shortest hot-phonon lifetime of 60 fs is obtained at/above 50 nW/electron for the lattice-matched AlInN/AlN/GaN 2DEG channel (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Enhancement in electrical properties of GaN heterostructure field-effect transistor by Si atom deposition on AlGaN barrier surface

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009
Norio Onojima
Abstract This study demonstrates that deposition of Si atoms on AlGaN barrier surfaces in GaN heterostructure field-effect transistors (HFETs) can modulate the electrical properties of the two-dimensional electron gas (2DEG). The results of Hall measurements performed using the eddy current and four-point van der Pauw methods showed that the sheet resistance of an AlGaN/GaN HFET sample without surface passivation increased from that of the unprocessed sample after post-metallization annealing at 820 °C for ohmic contacts. In contrast, the sheet resistance of the Si-deposited sample did not increase even after annealing. Furthermore, eddy current measurements for unprocessed wafers with and without Si deposition revealed that the sheet resistance can be reduced by depositing Si atoms, regardless of annealing. The effect of Si deposition on devices having a thin Al-rich barrier layer was found to be significant. The deposition of Si atoms (2 nm) on the AlN barrier surface in an AlN/GaN HFET (AlN 2 nm) resulted in a remarkable decrease in the sheet resistance from 60356 to 388 ,/sq. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Influence of barrier thickness on AlInN/AlN/GaN heterostructures and device properties

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009
H. Behmenburg
Abstract We report on structural and device properties of AlInN/AlN/GaN transistor heterostructures grown by metal organic vapour phase epitaxy (MOVPE) on 2, sapphire substrates with AlInN barriers of thicknesses between 4 nm and 10 nm. The In content and thickness of the thin AlInN barrier is shown to be well determinable by high-resolution X-ray diffraction (HRXRD). Room temperature Hall measurements yielded similar mobility between 1400 cm2V,1s,1 and 1520 cm2V,1s,1 on all samples and increasing sheet carrier concentration ns with rising barrier thickness resulting in a minimum sheet resistance value of 200 Ohm/,. The effect of surface passivation with Si3N4 on the electrical properties is investigated and found to strongly increase sheet carrier concentration ns of the two-dimensional electron gas (2DEG) to values above 2×1013cm,2. Characterization of transistors with gate length Lg of 1.5 ,m produced from the grown samples reveals high transconductance (gm) and a maximum drain current (ID) of 300 mS/mm and ,1 A/mm, respectively. For the sample with 4.6 nm barrier thickness, a reduced gate leakage current (IGL) and a absolute value of the threshold voltage (Vth) of -1.2 V is detected. Radio frequency (RF) measurements of passivated samples lead to maximum current gain cut-off frequencies ft of 11 GHz and maximum oscillating frequencies fmax of 25 GHz. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Analysis of minority carrier lifetime for InAlAs/InGaAs high electron mobility transistors by using 1.55-,m femto-second pulse laser

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2008
Hirohisa Taguchi
Abstract The minority carrier lifetime (,) of High electron mobility transistors (HEMTs) made using the InAlAs/InGaAs material system lattice-matched to the InP substrate had been obtained from optical response measurements with a 1.55-,m femto-second pulse laser where the laser was illuminated onto the backside of a wafer. The drain current of HEMTs associated with the optical pulse was detected using a digitizing oscilloscope, and , was estimated from the exponential dependence of drain current on time. In our current investigation, we found that , is dominated by the following modes: (1) the amount of time required for holes to transit across the channel toward the source, and (2) the amount of time required for the holes accumulated in the source region to recombine with two-dimensional electron gas (2DEG) through the Auger mechanism. Because the sheet concentration (ps) of holes accumulated in source region is low at a low source-to-drain voltage (VDS), Auger recombination is not predominant, and , was only dominated by the hole transit time. At a high VDS, ps became high enough for Auger recombination to occur and dominate ,. Furthermore, we investigated the optical power dependence of , where the optical power was supplied in a continuous wave (CW) to generate photo-excited holes in a steady state. The value of , decreased monotonically as VDS increased and saturated in as little as 6x10,10 s when the optical power was increased. The theoretical investigation was made to understand this saturation phenomenon. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Many body effects in the optical behavior of quantum well excitons

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2008
V. P. Kochereshko
Abstract Modulation-doped CdTe/Cd0.7Mg0.3Te quantum well structures with a two dimensional electron gas of low and moderate density (from ne = 3 × 1010 cm,2 up to ne , 1012 cm,2) have been studied. Photoluminescence spectra taken from the quantum well in magnetic fields from 0 T to 45 T at different electron densities were studied. The observed spectra were analyzed in terms of combined exciton-electron processes. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Piezoelectric actuation of all-nitride MEMS

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008
K. Tonisch
Abstract We present a MEMS technology based on (GaN/)AlGaN/GaN,heterostructures. Thereby the lower GaN layer represents the mechanical active layer, while the upper GaN and AlGaN layers supply the piezoelectrically active layers for actuation and the confinement of a 2D electron gas (at the lower interface). The 2DEG serves as back electrode for the piezoelectric actuation and as read,out, since it is modulated by the mechanical oscillation. The upper AlGaN and GaN layer both contribute to the total piezoelectric response, which was determined by piezoelectric force microscopy. The electrical field distribution throughout the heterostructure was determined by means of electroreflectance. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

AlGaN/GaN-based MEMS with two-dimensional electron gas for novel sensor applications

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008
F. Niebelschütz
Abstract Novel microelectromechanical resonators structures have been realized based on AlGaN/GaN heterostructures, which provide a basis for sophisticated sensor structures. There were grown on SiC substrates confining a two dimensional electron gas (2DEG). By means of the developed etching technology, freestanding resonators were patterned without degrading the sheet carrier concentration and electron mobility of the 2DEG inside the beams, which was confirmed by electrical measurements before and after the various process steps. As actuation and read out principle magnetomotive and piezoelectric effects were used, respectively. Due to the high sensitivity of the 2DEG and the chemical stability of the utilized materials these structures are suitable for chemical and biological sensor applications, where the sensitivity of the 2DEG on the surrounding environment acts as additional sensing signal, for example for simultaneous measurements of the viscosity and pH , value of a nanoliter droplet. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Nitride based nanotransistors as new sources and detectors of THz radiations

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008
A. El Fatimy
Abstract The plasma waves in gated two-dimensional electron gas have a linear dispersion law, similar to the sound waves. The transistor channel is acting as a resonator cavity for the plasma waves, which can reach frequencies in the Terahertz (THz) range for a sufficiently short gate length Field Effect Transistors (FETs). THz emission and detection by nanometer III-V transistors have been recently reported. In this work we report on THz emission and detection by nanometer GaN/AlGaN HEMTs. In particular, we show that specific GaN properties allow to observe THz emission up to room temperature. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Effect of electron-electron interaction on the diffusion current of spin-polarized electrons

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2008
Yutaka Takahashi
Abstract Electron-electron interaction modifies carrier transport in the spin-polarized system. The effects are investigated in the two-dimensional electron gas in semiconductor heterostructures. We find that the diffusion currents of spin-up and spin-down electrons are reduced, compared to the non-interacting values, by the momentum exchange between spin-up and spin-down electrons through their collisions (Spin Drag), and also by the electron energy renormalization arising from the manyparticle correlations. We numerically calculate the diffusion coefficients of spin-up and spin-down electrons separately in high-quality heterostructures of GaAs at low temperatures, including the effect of finite spin life time. Our calculations show that the diffusion coefficients are reduced down to less than half of their non-interacting values. We also find the negative diffusivity at low temperatures. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Fully unstrained GaN on sacrificial AlN layers by nano-heteroepitaxy

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2007
K. Tonisch
Abstract Usually, the fabrication of microelectromechanical systems (MEMS) requires unstrained or tensile strained active layers on a selectively removable sacrificial layer, since compressive strain causes instabilities due to buckling effects. For group III-nitride based MEMS, AlN is a promising material for sacrificial layers since it can be epitaxially overgrown and etched selectively to GaN. However, due to the larger lattice constants GaN is growing compressively strained on AlN. Nanoheteroepitaxy opens a way to yield fully unstrained, high quality epitaxial GaN layers on nanocrystalline AlN thin film by means of a 3D strain relaxation mechanism. For this purpose sputtered nanocrystalline AlN films were overgrown with single crystalline GaN and AlGaN/GaN layers by metalorganic chemical vapor deposition. The high quality of the layers is proven by an atomically flat surface and a 2D electron gas at the interface of the AlGaN/GaN heterostructure (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

XPS study of surface potential in AlGaN/GaN heterostructure with Cat-CVD SiN passivation

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2007
N. Onojima
Abstract AlGaN surface potentials in AlGaN/GaN heterostructures with and without SiN passivation were investigated using x-ray photoelectron spectroscopy (XPS). SiN films were formed on AlGaN surfaces by catalytic chemical vapor deposition (Cat-CVD), which has already been found to increase two-dimensional electron gas (2DEG) density. Based on a simple electrostatic analysis, the 2DEG density is expected to increase as the AlGaN surface potential decreases. This study experimentally demonstrates that a reduction in the AlGaN surface potential is actually induced by Cat-CVD SiN passivation. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Analysis of electrical properties of insulators (Si3N4, SiO2, AlN, and Al2O3)/0.5 nm Si3N4/AlGaN/GaN heterostructures

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2007
Narihiko Maeda
Abstract The electrical properties in AlGaN/GaN heterostructures with Si- and Al-based insulators (Si3N4, SiO2, AlN, and Al2O3) have been examined and analyzed. By insulators deposition, significant increase in the two-dimensional electron gas (2DEG) density (Ns) was observed with the order of Ns(Al2O3) > Ns(AlN) , Ns(SiO2) > Ns(Si3N4) > N0 (N0: Ns without insulators). As the result, the decrease in the sheet resistance (R) was observed; the smallest order of R was R(Al2O3) < R(AlN) < R(Si3N4) < R0 , R(SiO2) (R0: R without insulator). The insulators deposition effect has thus been shown to be significant and different among insulators. The increase in Ns was analyzed in terms of the change in the potential profile, and the observed differences in Ns among insulators have been interpreted. The band engineering including insulators is indispensable in understanding and designing AlGaN/GaN HFETs, since insulators are commonly used for the surface passivation as well as for the gate insulators, and the insulators deposition is to alter the essential device parameters such as the source resistance. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Co2MnSi as full Heusler alloy ferromagnetic electrode in magnetic tunneling junctions

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2006
G. Reiss
Abstract The discoveries of antiferromagnetic coupling in Fe/Cr multilayers by Grünberg, the Giant MagnetoResistance by Fert and Grünberg and a large tunneling magnetoresistance at room temperature by Moodera have triggered enormous research on magnetic thin films and magnetoelectronic devices. Large opportunities are especially opened by the spin dependent tunneling resistance, where a strong dependence of the tunneling current on an external magnetic field can be found. In order to obtain large magnetoresistance effects, materials with strongly spin polarized electron gas around the Fermi level have to be found. New materials with potentially 100% spin polarization will be discussed using the example of the full Heusler compound Co2MnSi. First, experimental aspects of the integration of this alloy in magnetic tunneling junctions will be addressed. With these junctions, we obtain up to 100% TMR at low temperature. The current status of this research will then be summarized with special regard to the complex diffusion mechanisms occurring in these devices and to the properties of the interfaces between the Heusler material and the insulator. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Characterisation of the electrical properties of solution-grown GaN crystals by reflectivity and Hall measurements

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2006
B. Birkmann
Abstract By using reflectivity and temperature resolved Hall measurements the electrical properties of low pressure solution grown (LPSG) GaN are determined. Hall measurements show that the material is degenerate. The reflectivity spectra are governed by the free electron gas in accordance with this finding. The charge carrier concentration is about 4 × 1019 cm,3 and the mobility 70 , 80 cm2/V s. These results are compared to gallium nitride synthesized by other solution or vapour phase growth techniques. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

High-field electron transport in AlGaN/GaN heterostructures

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2005
J. M. Barker
Abstract Experimental studies have been performed on the velocity-field characteristics of AlGaN/GaN heterostructures. A pulsed voltage input in combination with a four-point measurement was used in a 50 , environment to determine the drift velocity of electrons in the two-dimensional electron gas as a function of the applied electric field. These measurements show an apparent saturation velocity near 3.1 × 107 cm/s, at a field of 140 kV/cm. A comparison of these studies shows that the experimental velocities are close to previously published simulations based upon Monte Carlo techniques. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Temperature dependence of 2DEG and excitonic optical transitions in AlGaN/GaN heterostructures on SiC

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2005
C. W. Litton
Abstract Four (4) unique optical transitions are reported in both the emission and reflection spectra of high-quality AlGaN/GaN heterostructures. Study of the shifts of spectral peak energies and their intensity variations with temperature, reveal that these transitions arise from Free Exciton recombination and transitions between the A- and B-valence bands and the excited states of the 2-dimensional electron gas (2DEG) at the heterointerface. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Hot-electron transport in III,V nitride based two-dimensional gases

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2003
S. A. Vitusevich
Abstract We report on experimental and theoretical studies of low and high field transport in AlGaN/GaN two-dimensional electron gas (2DEG). Magnetotransport of 2DEG created as a result of polarization effects at the heterointerface has been studied. The velocity-electric field characteristics extracted from pulsed current,voltage measurements in AlGaN/GaN heterostructures are in good agreement with transport calculations up to fields as high as 100 kV/cm. [source]