Two-dimensional Electron Gas Layer Research Articles

Femtosecond coherent spectroscopy of four-wave mixing and photon echoes in a GaAs/AlGaAs heterostructure at room temperature

The results from experiments employing coherent femtosecond spectroscopy in a layer of two-dimensional electron gas at the boundary of the GaAs/AlGaAs heterojunction at room temperature are presented. The decay curves of primary femtosecond photon echo are obtained. The decoherence time in two-dimensional electron gas depends strongly on the power of the exciting pulse and varies from 36 to 54 fs. The dephasing time is studied for the first time as a function of the power of exciting pulses at room temperature. It is established that this dependence obeys the law T2 ∼ N−0.22, which differs from the typical law T2 ∼ N−1 for unscreened electron-electron interaction in semiconductor crystals. Analysis shows that electron-phonon interaction plays an important part along with electron-electron interaction. The induced spin gratings in the GaAs/AlGaAs heterostructure are studied with an eye to their possible application in spintronics.

Improvement of yield ratio of ohmic contact to GaAs/AlGaAs heterostructure by application of SiO2 protective layer

AbstractAll the contact pads of a quantized Hall resistance (QHR) device should have a low contact resistance (∼1 Ω) for the application of DC resistance standard devices at cryogenic temperatures. Corrosion of a GaAs/AlGaAs substrate occurs along the edge of the photoresist before the deposition of Ni/AuGe, and we assume that this corrosion degrades the contact resistance. A SiO2 protective layer is effective in preventing the corrosion of and protecting the surface of GaAs/AlGaAs substrate. With the use of this SiO2 protective layer, the yield ratio of the contact resistance to the twodimensional electron gas (2DEG) layer at low temperature (∼0.5 K) and high magnetic field (∼9 T) improves to nearly 100%. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Reliability Study and Device Modeling for p-HEMT Microwave Power Transistors

In this paper, the commercial 0.5-μm AlGaAs/InGaAs/GaAs pseudo-morphic high electron mobility transistors were subjected to both high-drain voltage and high-temperature stresses for investigating reliability issues. The results reveal that the stress-induced trapping phenomena near two-dimensional electron gas layer should be responsible for the different drain current collapses. The decay level of the DC and the small-signal characteristics increases with the stress voltage and/or the operation temperature. The self-consistent model was established through the de-embedded and the non-linear fitting processes, which can be used to estimate the DC and RF small-signal characteristics degradation under high-drain voltage and high-temperature stress.

A theoretical model to explain the mechanism of lightwave propagation through non-metallic nanowires

We propose a theoretical model to explain the mechanism of light wave propagation through non-metallic nanowires of sub-diffraction dimension. This model is assuming that unbound electrons on the surface of a non-metallic nanowire form an infinitesimally thin layer of quasi-free two-dimensional electron gas that is responsible for collective oscillation. Dispersion relations are obtained in the framework of hydrodynamic description and Maxwell equations. Previously undiscovered peculiar features of zero group velocity, different from conventional metal nanowire and metal-coated nanowires, have been found. Optimization of maximum power propagation through the nanowire has also been obtained.

Magneto-thermoelectric effects in two-dimensional quantum well: role of short-range potential

We have studied transverse thermomagnetic effects in a quantum well (QW) with parabolic potential in the presence of a magnetic field parallel to the two-dimensional electron gas layer. The calculation was carried out for the case of elastic electron scattering on short-range potential for degenerate and non-degenerate electron gas. It is shown that the reviewed mechanism of charge carriers' relaxation is essential for the electroconductivity at low temperatures. In the quantum limit, the dependencies of the transverse Nernst–Ettingshausen coefficient and the thermopower on the magnetic field strength, the temperature and the carrier density are determined and analyzed. We have showed that the magnetothermopower is not determined by the entropy only, as is the case for bulk specimens.

A modified scheme of charge sensitive infrared phototransistor

Charge sensitive infrared phototransistors (CSIP) realized in a GaAs/AlGaAs double quantum well (QW) structure have so far exploited the tunneling of excited electrons from an isolated island of upper QW to the lower two-dimensional electron gas layer. Another type of CSIP is developed by using a GaAs/AlGaAs double QW crystal, in which inter-QW tunneling is suppressed. Instead of “vertical” tunneling, excited electrons in the upper QW flow in and out the isolated island “laterally” via translational motion through gate-induced potential barriers. The scheme is demonstrated for wavelengths ≈14.6 μm but is suitable for expanding toward longer wavelengths.

High sensitivity and multifunctional micro-Hall sensors fabricated using InAlSb/InAsSb/InAlSb heterostructures

Further diversification of Hall sensor technology requires development of materials with high electron mobility and an ultrathin conducting layer very close to the material’s surface. Here, we describe the magnetoresistive properties of micro-Hall devices fabricated using InAlSb/InAsSb/InAlSb heterostructures where electrical conduction was confined to a 30 nm-InAsSb two-dimensional electron gas layer. The 300 K electron mobility and sheet carrier concentration were 36 500 cm2 V−1 s−1 and 2.5×1011 cm−2, respectively. The maximum current-related sensitivity was 2 750 V A−1 T−1, which was about an order of magnitude greater than AlGaAs/InGaAs pseudomorphic heterostructures devices. Photolithography was used to fabricate 1 μm×1 μm Hall probes, which were installed into a scanning Hall probe microscope and used to image the surface of a hard disk.

Nonlinear Plasma Waves in Coupled Two-Dimensional Electron Systems

The two layers of two-dimensional electron gas (2DEG) systems are discussed in order to examine how the properties of developing nonlinear plasma waves are changed by the presence of electrostatic coupling between the two layers. We consider the case where the two layers are sandwiched by two metallic gates. If the separation between the 2DEG layers and gates is shorter than the wavelength of the investigating plasma waves, the Korteweg–de Vries (KdV) equation well characterizes their propagation. It is found that the velocity of the KdV solitons increases as the separation between two 2DEG layers becomes small; therefore, when the solitons couple with neighboring grating, they generate high-frequency Smith–Purcell radiations more effectively than the solitons supported by the single-layer system.

What is obvious may not be trivial

Researchers find that tunneling between the two layers of a bilayer two-dimensional electron gas is proportional to their area. Although the result may seem intuitive it poses a challenge to current theory.

Spin-orbital coupling effect on Josephson current through a superconductor heterojunction

We study the spin-orbital coupling effect on the Josephson current through a superconductor (SC) heterojunction, consisting of two s-wave superconductors and a two-dimensional electron gas (2DEG) layer between them. The Rashba-type (RSOC) and/or Dresselhaus-type (DSOC) of spin-orbital coupling are considered in the 2DEG region. By using a tight-binding model and Green’s function method, we calculate the dc supercurrent flowing through the junction and find that the critical current Ic exhibits a damped oscillation with both the strength of SOC and the layer length of 2DEG; especially, the strength ratio between RSOC and DSOC can also induce switching between the 0 state and the π state of the SC/2DEG/SC junction as well. This 0-π transition results from the fact that SOC in a two-dimension system can lead to a pseudomagnetic effect on the flowing electrons like the effect of a ferromagnet, since the time-reversal symmetry of the system has already been broken by two SC leads with different macroscopic phases.

Mesoscopic Structures for Microwave-THz Detection

Properties of microwave detectors of various design on the base of MBE grown GaAs and AlGaAs structures are discussed in this paper: simple asymmetrically shaped structures with heavily doped GaAs and AlGaAs layers of nanometric thickness as well as diodes with two-dimensional electron gas layers. Novel models of the detectors with partially gated two-dimensional electron gas layer as well as with small area GaAs/AlGaAs heterojuction are discussed to demonstrate difierent ways to increase the voltage sensitivity of the detectors of electromagnetic radiation in GHz{THz frequency range.

Bistable Voltage Transition Using Spin-Orbit Interaction in a Ferromagnet-Semiconductor Hybrid Structure

A traveling electron in the high mobility layer with asymmetric charge distribution induces an effective magnetic field that creates an imbalance of spin-up and spin-down energy levels. If a ferromagnet contacts a two-dimensional electron gas layer, the potential depends on the magnetization direction of the ferromagnetic detector. Two different potential levels can be assigned to ldquo0rdquo and ldquo1rdquo states of nonvolatile memory. This new concept of nonvolatile memory using spin-orbit interaction does not need complex magnetic tunneling layers and additional word writing lines.

Ohmic contacts to pseudomorphic HEMTs with low contact resistance due to enhanced Ge penetration through AlGaAs layers

AuGe/Ni ohmic contacts are used as source and drain electrodes of pseudomorphic HEMTs (pHEMTs). High alloying temperatures are generally believed to be necessary to enhance penetration of the alloy materials through the AlGaAs layers in order to establish a very low resistance path for the source–drain currents to access the two-dimensional electron gas (2DEG) layer. Here we have performed alloying experiments in the temperature range of 390–450 °C, and the contact resistance was determined using transfer length method measurements. Germanium diffusion was studied using backside secondary ion mass spectrometry. During our study, we have observed that doping of the channel by germanium is possible even at lower temperatures. But alloying at lower temperatures does not appreciably enhance the concentration throughout the different device layers below the contact pads. Hence, unlike MESFET alloying, higher alloying temperatures are essential for increasing the doping concentration so as to reduce the contact resistance and overcome the resistance of the AlGaAs layers.

Interface resistance dependence of spin transport in a ferromagnet–semiconductor hybrid structure

The spin transport signals from NiFe and Co into two-dimensional electron gas layers are measured for various thicknesses of transmission barriers. A stable and reproducible electrical detection of spin transport was obtained only when the barrier thickness is less than 10 nm. The typical interface resistance to observe spin signals in this experiment is about 0.5–250 Ω, which is a neither transparent nor a severe tunneling limit. The optimal interface resistance depends on the ferromagnetic materials, but severe tunneling barrier is not proper for fully electrical spin transport. Device size is also a critical factor to decide the proper range of interface resistance.

Optimizing the physical contribution to the sensitivity and signal to noise ratio of extraordinary magnetoresistance quantum well structures

For applications to extraordinary magnetoresistance (EMR) quantum well sensor design, the electron areal density n2D, the mobility μ, and the products n3D1∕2μ2 and n3D1∕2μ5∕2 are key physical parameters to be optimized for enhanced device sensitivity and signal to noise ratio. We model the electron areal density and carrier mobility in a two-dimensional electron gas layer developed in a δ-doped AlInSb∕InSb heterostructure. The nonparabolic band structure due to the nature of the small energy band gap of InSb is accounted for. The detailed description of the energy dispersion and the energy dependent effective mass are obtained by the k∙P method of band structure calculation. The transport properties are calculated by including contributions of scattering from ionized impurities, the background neutral impurities, the deformation potential acoustic phonons, and the polar optical phonons. We calculate the dependencies of n2D, μ, n3D1∕2μ2, and n3D1∕2μ5∕2 on temperature, spacer layer thickness, doping density, and the quantum well thickness. This has important implications for EMR sensor design.

Electronically modulated two-dimensional plasmons coupled to surface plasmon modes

We present a calculation for a two-dimensional (2D) electron gas layer interacting with a slab of conductive material. We treat the plasmons in the slab in the local limit and obtain the frequency of the coupled mode corresponding to the extended 2D plasmon interacting with the background plasmons in the presence of a conducting surface. The dispersion equation of a double quantum well is obtained and we show how the split symmetric and antisymmetric modes are formed and modified by the localized surface plasmon. For a single layer, we show that when a one-dimensional (1D) periodic electrostatic potential is applied to the surface, each of the symmetric and antisymmetric modes will be further split by the interaction with the 1D modulation, leading to folding of plasmon dispersion curves for different modes. For double layers, we show that the coupled 2D and surface plasmons may result in radiated energy. Our analysis is based on a calculation of the surface response function obtained using a transfer-matrix method.

RF-Enhanced Contacts to Wide-Bandgap Devices

This letter proposes a novel approach to fabricate high-performance heterostructure microwave devices with nonohmic contacts. The contact can be as-deposited or made by "shallow" low-temperature annealing to form a low-height Schottky barrier while preserving the two-dimensional electron-gas layer (2DEG) at the heterointerface. Coupling between the metal and the 2DEG occurs via two paths: dc current injects through the barrier leakage current and ac-current component injects through capacitive coupling. Contacts with resistive/capacitive coupling have low microwave impedance and enhance the heterostructure field-effect transistor's maximum oscillation frequency, output power, and power-added efficiency as compared to resistive ohmic contacts

Unbalanced spin accumulation induced by spin Hall effect

Moving electrons of different spins experience skew scattering in a two-dimensional electron gas layer and the electron deviates different direction due to the spin-dependent deflection. If spin is randomly oriented, the amount of skew scattering on both sides will be same and the Hall voltage will read zero. In this experiment, the carrier concentrations of spin up and down are unbalanced because the spin is aligned by stray field from the ferromagnet. Therefore, the voltage probe reads charge accumulation asymmetry. Spin Hall voltage is functions of the spin alignment direction and the amount of spin polarization.

Transport property of insulating barrier in a ferromagnet-semiconductor hybrid system

Two-dimensional electron gas layer is essential for a developing spin-FET because of its high mobility and large spin–orbit coupling. The junction properties between a ferromagnet (FM) and a 2-DEG system are the most important factor. Two types of a 2-DEG layer, an InAs and a GaAs channel heterostructures, are fabricated to compare the junction properties of the two systems. A GaAs-based channel 2-DEG layer with Al2O3 tunneling layer using a new oxidization method is prepared. During the heat treatment at the furnace, the arsenic gas was evaporated and the top AlAs layer was converted to the aluminum oxide layer. An InAs channel 2-DEG layer with a semiconductor-based barrier and FM junction shows the ohmic behavior. In the potentiometric measurement, a spin–orbit coupling of 2-DEG layer is observed in the interface between a FM and an InAs channel 2-DEG layer, which proves the efficient spin transport junction.

Electrical and structural investigations of Ag-based Ohmic contacts for InAlAs∕InGaAs∕InP high electron mobility transistors

Ge∕Ag∕Ni and AuGe∕Ni∕Au Ohmic contacts on InAlAs∕InGaAs∕InP high electron mobility transistors with excellent contact resistance of 0.07Ωmm were obtained after annealing at 425 and 265°C, respectively. The Ag-based contacts have a large processing window of >130°C. Structural analyses confirm that Ag and Au protrusions created during annealing effectively linked the two-dimensional electron gas layer with the metal contacts to produce excellent Ohmic characteristics. The formation of liquid AuGe eutectic phase in AuGe∕Ni∕Au at 300°C is believed to cause overannealing. The eutectic temperature of Ag–Ge is ∼300°C higher leading to a higher optimum annealing temperature and a wider processing window for the Ge∕Ag∕Ni contacts.