Properties Of Two-dimensional Electron Gas Research Articles

Dependence of structural and electrical properties of AlGaN/GaN HEMT on Si(111) on buffer growth conditions by MBE

Multiple AlGaN/GaN heterostructure has been grown on Si(111) substrate by molecular beam epitaxy with different buffer growth conditions. Its influence on physical and electrical properties of two-dimensional electron gas (2DEG) has been investigated. Correlation between growth temperature variation in AlN intermediate layer and thick GaN buffer layer on 2DEG transport property has been observed. Besides the variation in growth temperatures, dissimilar partial doping in the thick GaN buffer has also been studied. Impact of different silicon substrate doping has been examined to inspect the electrical properties of high electron mobility transistors (HEMT). DC characteristics of large area fabricated HEMT have been compared in terms of current, transconductance, and linearity for power amplifier applications.

Thermodynamic properties of Landau levels in InSb two-dimensional electron gas

This work investigates the influence of low temperature, external magnetic field and broadened Landau levels on the thermodynamic properties of two-dimensional electron gas (2DEG) of InSb. The interplay between these two physical parameters on the magnetic field dependence of the chemical potential and the magnetization is calculated. In the absence of a complete theory that explains the Landau level broadening, experimental and theoretical studies in literature perform different model calculations of this parameter. Here it is presented that different broadening parameter of Gaussian-shaped Landau levels cause width variation in their contribution to interlevel and intralevel excitations.

A 2 nm low temperature GaN spacer to improve the transport properties of two-dimensional electron gas in AlGaN/InAlN/AlN/GaN heterostructures

AlGaN layers were grown on InAlN/AlN/GaN heterostructure with various temperatures. The two-dimensional electron gas density and mobility in heterostructure with high temperature AlGaN were deteriorated which is attributed to the reverse polarization field of AlGaN and the degradation of InAlN structural quality and surface morphology. A 2 nm low temperature GaN spacer was inserted to reduce the risk of InAlN degradation. The improved structures demonstrated an increase of two-dimensional electron gas density and mobility compared to the heterostructures without the GaN spacer, which results in the reduction of sheet resistance. Particularly, at 855 °C, an enhancement of mobility in the improved structure indicates that the heterostructure is relatively stable at this temperature. At higher temperature of 920 °C, significant increase of sheet resistance indicates a sharp degradation of InAlN quality.

Two-dimensional electron transport in AlGaN/GaN heterostructures

We present a theoretical study of electron transport properties of two-dimensional electron gas in AlGaN/GaN heterostructures. By assuming a drifted Fermi–Dirac distribution and taking into account all major scattering mechanisms, including polar optical and acoustic phonons, background impurities, dislocation and interface roughness, the momentum- and energy-balance equations derived from Boltzmann equation are solved self-consistently. The dependence of the electron drift velocity and electron temperature as a function of the applied electric field are obtained and discussed.

Spin-polarized two-dimensional electron gas in undoped MgxZn1−xO/ZnO heterostructures

Quantum transport properties of two-dimensional electron gas (2DEG) in undoped MgxZn1−xO/ZnO heterostructures grown by metal organic vapor phase epitaxy have been investigated. A large zero-field spin-splitting energy more than 15 meV in the 2DEG is determined at 1.6 K. Meanwhile, ferromagnetism is observed in the heterostructures. The findings reveal that the 2DEG is spin polarized at zero magnetic fields. It is believed that the exchange interaction between the itinerant electrons in the two-dimensional channel and the magnetic polarons in the MgxZn1−xO barrier around the interface results in the spin polarization of the 2DEG.

Oscillations of Low-Field Magnetoresistivity of Two-Dimensional Electron Gases in Al0.22Ga0.78N/GaN Heterostructures in a Weak Localization Region

Low-field magnetotransport properties of two-dimensional electron gases (2DEGs) are investigated in Al0.22Ga0.78N/GaN heterostructures. By means of a tilting magnetic field, unexpected oscillations of magnetoresistivity are observed in a weak localization region. Qualitative understanding based on Altshuler—Aronov—Spivak oscillations is proposed for the case of interface disorder in Al0.22Ga0.78N/GaN heterostructures.

Two-dimensional electron gas properties by current-voltage analyses of Al0.86In0.14N/AlN/GaN heterostructures

We report on the current transport properties of AlInN/AlN/GaN high electron mobility transitors with different AlN interlayer thickness. We determined the two-dimensional electron gas (2DEG) properties directly from simple current-voltage measurements, carried out with two Schottky contacts in a planar back-to-back configuration. A model has been developed to straightforwardly extract the 2DEG electrical properties from room-temperature current-voltage curves, and we correlated them to the effects of varying AlN thickness. The 2DEG properties calculated from current-voltage analyses are in very good agreement with results obtained with standard Hall measurements.

Terahertz optical-Hall effect characterization of two-dimensional electron gas properties in AlGaN/GaN high electron mobility transistor structures

The free-charge carrier mobility, sheet density, and effective mass of a two-dimensional electron gas are exemplarily determined in the spectral range from 640 GHz to 1 THz in a AlGaN/GaN heterostructure using the optical-Hall effect at room temperature. Complementary midinfrared spectroscopic ellipsometry measurements are performed for analysis of heterostructure constituents layer thickness, phonon mode, and free-charge carrier parameters. The electron effective mass is determined to be (0.22±0.04)m0. The high-frequency sheet density and carrier mobility parameters are in good agreement with results from dc electrical Hall effect measurements, indicative for frequency-independent carrier scattering mechanisms of the two-dimensional carrier distribution.

Transport properties of two-dimensional electron gas in lattice-matched InAlN/GaN and InAlN/AlN/GaN materials

The lattice-matched InAlN/GaN structure is one kind of emerging material with high conductivity and used in GaN-based high electron mobility transistors (HEMTs). The transport properties of lattice-matched InAlN/GaN structure and InAlN/AlN/GaN structure are studied. The samples are grown using pulsed metal organic chemical vapor deposition on sapphire substates. Both structures show temperature-dependent Hall mobilities with a typical behavior of two-dimensional electron gas (2DEG). Theoretical analysis of the temperature dependence of mobility is carried out based on the comprehensive consideration of various scattering mechanisms such as acoustic deformation-potential, piezoelectric, polar optic phonon, dislocation, alloy disorder and interface roughness scattering. It is found that the dominant scattering mechanisms are the interface roughness scattering and the polar optic phonon scattering for both structures at room temperature. The insertion of AlN spacer layer into InAlN/GaN interface exempts 2DEG from alloy disorder scattering, more importantly results in a better interface, and restrains greatly interface roughness scattering. The influence of sheet density on 2DEG mobility is also considered, and the upper limit of density-dependent 2DEG mobility is given for lattice-matched InAlN/GaN and InAlN/AlN/GaN structures and compared with many reported experimental data.

Impact of Al content on transport properties of two-dimensional electron gas in GaN/AlxGa1−xN/GaN heterostructures

The influence of the Al content on the mobility of the two-dimensional electron gas (2DEG) in GaN/AlxGa1−xN/GaN heterostructures is studied by employing the ensemble Monte Carlo method. Using two interface polarization charge models, we calculate the room temperature low-field mobility at different Al compositions of the barrier layer ranging from 8% up to 35%. All relevant scattering mechanisms are considered to provide a quantitative description of the measured mobilities. We show that 2DEG transport in the heterostructures is mostly affected by dislocation scattering for all Al contents examined. The role of alloy scattering and interface roughness scattering is also discussed.

Short range scattering mechanism of type-II GaSb/GaAs quantum dots on the transport properties of two-dimensional electron gas

We have studied the scattering process of AlGaAs/GaAs two-dimensional electron gas with the nearby embedded GaSb/GaAs type-II quantum dots (QDs) at low temperature. Quantum Hall effect and Shubnikov–de Haas oscillation were performed to measure the electron density n2D, the transport lifetime τt, and the quantum lifetime τq under various biased gate voltage. By comparing measured results of QDs sample with that of reference sample without embedded QDs, mobilities (transport mobility μt and quantum mobility μq) dominated by GaSb QDs scattering were extracted as functions of n2D. It was found that the ratios of τt to τq were varying within the range of 1–4, implying the scattering mechanism belonging to the sort of short-range interaction. In the framework of Born approximation, a scattering model considering rectangular-shaped potential with constant barrier height was successfully applied to explain the transport experimental data. In addition, an oscillating ratio of τt/τq with the increasing n2D was predicted in the model.

Improved electrical properties of the two-dimensional electron gas in AlGaN/GaN heterostructures using high temperature AlN interlayers

The electrical properties of two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures using high temperature (HT) AlN interlayers (ITs) grown on c-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD) have been investigated. It is found that the electrical properties (electron mobility and sheet carrier density) are improved compared with those in the conventional AlGaN/GaN heterostructures without HT AlN ITs, and the improved 2DEG properties result in the reduction of the sheet resistance. The results from high resolution X-ray diffraction (HRXRD) and Raman spectroscopy measurements show that HT AlN ITs increase the in-plane compressive strain in the upper GaN layer, which enhances the piezoelectric polarization in it and consequently causes increasing of 2DEG density at the AlGaN/GaN interface. Meanwhile, the compressive strain induced by HT AlN ITs leads to a less tensile strain in AlGaN barrier layer and causes positive and negative effects on the sheet carrier density of 2DEG, which counteract each other. The HT AlN ITs reduce the lattice mismatch between the GaN and AlGaN layers and smooth the interface between them, thus increasing the electric mobility of 2DEG by weakening the alloy-related interface roughness and scattering. In addition, the surface morphology of AlGaN/GaN heterostructures is improved by the insertion of HT AlN ITs. The reason for the improved properties is discussed in this paper.

Epitaxial graphene on SiC(0001) and : from surface reconstructions to carbon electronics

Graphene with its unconventional two-dimensional electron gas properties promises apathway towards nanoscaled carbon electronics. Large scale graphene layers for a possibleapplication can be grown epitaxially on SiC by Si sublimation. Here we report on the initialgrowth of graphene on SiC basal plane surfaces and its relation to surface reconstructions.The surfaces were investigated by scanning tunneling microscopy (STM), low energyelectron diffraction (LEED), angle-resolved ultraviolet photoelectron spectroscopy(ARUPS) and x-ray photoelectron spectroscopy (XPS). On SiC(0001) the interface ischaracterized by the so-called reconstruction. The homogeneity of this phase is influenced by the preparation procedure.Yet, it appears to be crucial for the quality of further graphene growth. We discuss the roleof three structures with periodicities , (6 × 6) and (5 × 5) present in this phase. The graphitization process can be observed bydistinct features in the STM images with atomic resolution. The number ofgraphene layers grown can be controlled by the conical band structure of theπ-bands around the point of the graphene Brillouin zone as measured by laboratory-based ARUPS using UVlight from He II excitation. In addition we show that the spot intensity spectra in LEEDcan also be used as fingerprints for the exact determination of the number of layers for thefirst three graphene layers. LEED data correlated to the ARUPS results allow for an easyand practical method for the thickness analysis of epitaxial graphene on SiC(0001)that can be applied continuously during the preparation procedure, thus pavingthe way for a large variety of experiments to tune the electronic structure ofgraphene for future applications in carbon electronics. On graphene grows without the presence of an interface layer. The initial graphenelayer develops in coexistence with intrinsic surface reconstructions of the surface. In high resolution STM measurements we show atomically resolved graphene layers on topof the (3 × 3) reconstruction with a Moiré type modulation by a large superlattice periodicitythat indicates a weak coupling between the graphene layer and the substrate.

Characterizations of GaN films and AlGaN/GaN heterostructures on vicinal sapphire (0 0 0 1) substrates grown by MOCVD

GaN films and AlGaN/GaN heterostructures grown on vicinal sapphire (0 0 0 1) substrates by metalorganic chemical vapor deposition (MOCVD) are investigated. It is found that surface morphologies of GaN films depend on the vicinal angle, however, they are not sensitive to the inclination directions of the substrate. The optimized vicinal angle for obtaining excellent surface morphology is around 0.5°. This conclusion is also confirmed by characterizing the electrical property of two-dimensional electron gas (2DEG) in the AlGaN/GaN heterostructure.

Low-temperature field-effect and magnetotransport properties in a ZnO based heterostructure with atomic-layer-deposited gate dielectric

A top-gate field-effect device with atomic-layer-deposited Al2O3 dielectric was fabricated to investigate magnetotransport properties of two-dimensional electron gas (2DEG) at a semi-insulating ZnO–Mg0.12Zn0.88O double heterostructure grown by laser molecular-beam epitaxy. Hall mobility monotonically increased as the density of accumulated electrons increased. The highest mobility at 2K was recorded to be 5000cm2V−1s−1 at a 2DEG density of 1.2×1012cm−2, which is comparable to the previously reported value for a metallic ZnO∕Mg0.2Zn0.8O heterostructure. Insulator-to-metal transition was observed at a critical density of 6×1011cm−2. The metallic-state channel exhibited Shubnikov–de Haas oscillations, demonstrating an electric-field tunable quantum device based on transparent oxide semiconductor.

Spin polarization control via magnetic barriers and spin-orbit effects

We investigate the spin-dependent transport properties of two-dimensional electron gas (2DEG) systems formed in diluted magnetic semiconductors and in the presence of Rashba spin-orbit interaction in the framework of the scattering matrix approach. We focus on nanostructures consisting of realistic magnetic barriers produced by the deposition of ferromagnetic strips on heterostructures. We calculate spin-dependente conductance of such barrier systems and show that the magnetization pattern of the strips, the tunable spin-orbit coupling, and the enhanced Zeeman splitting have a strong effect on the conductance of the structure. We describe how these effects can be employed in the efficient control of spin polarization via the application of moderate fields. PACS numbers: 71.70.Ej, 73.23.Ad, 72.25.-b, 72.10.-d Spin-orbit coupling in semiconductors intrinsically connects the spin of an electron to its momentum, 1 providing a pathway for electrically initializing and manipulating electron spins for applications in spintronics 2,3 and spin-based quantum information processing. 4 This coupling can be regulated with quantum confinement in semiconductor heterostructures through band structure engineering, as well as by the application of external electric fields, as in the celebrated spin field-effect transistor proposed by Datta and Das. 5 Using diluted magnetic semiconductors (DMS) in such systems provides an additional degree of control of the transport properties. In particular, when an external magnetic field is applied, the magnetic dopant spins align, giving rise to a strong exchange field that acts on the electron spin. This sd exchange interaction between the electron spin in the conduction band and the localized magnetic ions induces a giant Zeeman splitting. In this communication we investigate the spindependent transport properties of two-dimensional electron gas (2DEG) systems formed in diluted magnetic semiconductors and take into account the electric-field– dependent Rashba spin-orbit (SOI) interaction. We focus our attention on nanostructures consisting of realistic magnetic barriers produced by the deposition of ferromagnetic strips near the heterostructures, 6 providing a relatively strong inhomogeneous magnetic field on the 2DEG. We show how the conductance of the 2DEG depends strongly on the magnetization pattern of the strips, as well as on geometry and externally applied electric fields. We demonstrate that significant spin polarization (exceeding 50%) can be obtained at low temperatures for ferromagnetic strips of typical dimensions and magnetization.

Chiral two-dimensional electron gas in a periodic magnetic field: Persistent current and quantized anomalous Hall effect

We study the energy spectrum and electronic properties of two-dimensional electron gas in a periodic magnetic field of zero average with symmetry of triangular lattice. We demonstrate how the structure of electron energy bands can be changed with the variation in the field strength so that we can start from nearly free-electron gas and then transform it continuously to a system of essentially localized chiral electron states. We find that the electrons near some minima of the effective potential are responsible for occurrence of dissipationless persistent currents creating a lattice of current contours. The topological properties of the electron energy bands are also varied with the intensity of periodic field. We calculated the topological Chern numbers of several lower-energy bands as a function of the field. The corresponding Hall conductivity is nonzero, and when the Fermi level lies in the gap, it is quantized.

Electron transport properties of In0.53Ga0.47As/In0.52Al0.48As quantum wells with two occupied subbands

Magnetotransport properties of two-dimensional electron gas have been investigated for three In0.53Ga0.47As/In0.52Al0.48As quantum well samples having two occupied subbands with different well widths. When the intersubband scattering is considered, we have obtained the subband density, transport scattering time, quantum scattering time and intersubband scattering time, respectively, by analyzing the result of fast Fourier transform of the first derivative of Shubnikov-de Haas oscillations. It is found that the main scattering mechanism is due to small-angle scattering, such as ionized impurity scattering, for the first subband electrons.

Quantum dots as tunable scatterers for 2D- and 1D-electron systems

We investigate the influence of charged, self-assembled InAs quantum dots on nearby, weakly coupled two- and one-dimensional electron gases. While the transport properties of two-dimensional electron gases (2DEG) are only weakly affected by the tunable charge in the quantum dots, we find that in laterally patterned samples the effect of coupling to the dot states is greatly enhanced. Even negative differential conductance can be observed close to the 2D–1D transition.

AlGaN/GaN HEMTs regrown by MBE on epi-ready semi-insulating GaN-on-sapphire with inhibited interface contamination

In this work, we show that, by carefully designing the subsurface Fe doping profile in SI-GaN templates grown by MOVPE and by optimizing the MBE regrowth conditions, a highly resistive GaN buffer can be achieved on these epi-ready GaN-on-sapphire templates without any addition of acceptors during the regrowth. As a result, high-quality high electron mobility transistors can be fabricated. Furthermore, we report on the excellent properties of two-dimensional electron gas and device performances with electron mobility greater than 2000 cm 2/V s at room temperature and off-state buffer leakage currents as low as 5 μA/mm at 100 V.