Two-dimensional Electron Gas In Presence Research Articles

Spin-polarized two-dimensional electron/hole gas at the interface of nonmagnetic semiconducting half-Heusler compounds: Modified Slater-Pauling rule for half-metallicity at the interface

Half-Heusler compounds with 18 valence electrons per unit cell are well-known nonmagnetic semiconductors. Employing first-principles electronic band-structure calculations, we study the interface properties of the half-Heusler heterojunctions based on FeVSb, CoTiSb, CoVSn, and NiTiSn compounds, which belong to this category of materials. Our results show that several of these heterojunction interfaces become not only metallic but also magnetic. The emergence of spin-polarization is accompanied by the formation of two-dimensional electron gas or hole gas at the interface. A qualitative analysis of the origin of the spin polarization at the interfaces suggests that strong correlations are responsible. For the cases of magnetic interfaces where half-metallicity is also present, we propose a modified Slater-Pauling rule similar to the one for bulk half-metallic half-Heusler compounds. Additionally, we calculate exchange parameters, Curie temperatures, and magnetic anisotropy energies for magnetic interfaces. Our study, along with recent experimental evidence confirming the presence of two-dimensional electron gas at CoTiSb/NiTiSn heterojunctions, may motivate future efforts to explore and realize device applications using these heterojunctions.

Analytic Model of Threshold Voltage (VTH) Recovery in Fully Recessed Gate MOS-Channel HEMT (High Electron Mobility Transistor) after OFF-State Drain Stress

Today, wide bandgap (WBG) GaN semiconductors are considered the future, allowing the improvement of power transistors. The main advantage of GaN is the presence of two-dimensional electron gas (2Deg) typically used as a conduction layer in normally-on and normally-off transistors. Concerning the normally-off family, several solutions are proposed. Among these, one of the most promising is the MIS-Gate technology that features a gate recess architecture allowing the semiconductor to physically cut off the 2Deg and drastically decrease gate–source leakage currents. The Vth relaxation characteristic, after voltage stress, has been investigated. It has been shown that the main impact is due to charges close to the gate dielectric/GaN interface, precisely dwelling within the dielectric or the GaN epitaxy. This work provides an analytical model of the Vth evolution of these MIS-GATE (metal insulator semiconductor gate) transistors fabricated on GaN-silicon substrate. This model allows the extraction of different trap energy levels from a temporary threshold voltage (Vth) shift after 650 V stress. Based on this method, it is possible to identify up to four different trap energy levels. By comparing state of the art methods, we show that these obtained energy levels are well correlated with either magnesium and carbon impurity or Ga and/or N vacancy sites in the GaN epitaxy.

AlGaN/GaN magnetic sensors featuring heterojunction 2DEG channel

Magnetic sensors based on the principle of Hall effect are widely used in various applications for detection and control of displacement, speed, angle, and rotation speed. Nowadays, the conventional Hall sensors are made of the narrow bandgap semiconductors. The typical ones are silicon (Si), indium antimonide (InSb), and gallium arsenide (GaAs). With the rapid increase in the need for high-temperature magnetic detections especially in the space exploration and thermonuclear power stations, they cannot meet the requirement of operation at high temperature due to the limitation of their natural characteristics such as low carrier mobility or small bandgap. In this work, the Hall sensors based on the third-generation semiconductor gallium nitride (GaN) were demonstrated by employing both the simulation and device fabrication schemes. The developed AlGaN/GaN heterojunction Hall sensors benefited from the presence of two-dimensional electron gas (2DEG) are small in size, and have an improved magnetic sensitivity and stability at high temperature. They exhibit a large current-related magnetic sensitivity up to 94.6 V A−1 T−1 and a low temperature coefficient less than 745 ppm K−1, which is obviously improved compared with those in the currently commercialised semiconductor Hall sensors. Moreover, the Hall sensors developed in this work can be widely used in various harsh environments with high temperature (>573 K) and intense radiation in future.

Deformation of the Fermi surface of a spinless two-dimensional electron gas in presence of an anisotropic Coulomb interaction potential

We consider the stability of the circular Fermi surface of a two-dimensional electron gas system against an elliptical deformation induced by an anisotropic Coulomb interaction potential. We use the jellium approximation for the neutralizing background and treat the electrons as fully spin-polarized (spinless) particles with a constant isotropic (effective) mass. The anisotropic Coulomb interaction potential considered in this work is inspired from studies of two-dimensional electron gas systems in the quantum Hall regime. We use a Hartree–Fock procedure to obtain analytical results for two special Fermi liquid quantum electronic phases. The first one corresponds to a system with circular Fermi surface while the second one corresponds to a liquid anisotropic phase with a specific elliptical deformation of the Fermi surface that gives rise to the lowest possible potential energy of the system. The results obtained suggest that, for the most general situations, neither of these two Fermi liquid phases represent the lowest energy state of the system within the framework of the family of states considered in this work. The lowest energy phase is one with an optimal elliptical deformation whose specific value is determined by a complex interplay of many factors including the density of the system.

Quantum Monte Carlo study of the two-dimensional electron gas in presence of Rashba interaction

We introduce a variant to the Diffusion Monte Carlo algorithm that can be employed to study the effects of the Rashba interaction in a many electron systems. Because of the spin--orbit nature of Rashba interaction a standard algorithm cannot be applied and therefore a specific imaginary time spin dependent propagator has been developed and implemented following previous work developed in the framework of nuclear physics. We computed the ground state energy of the 2D electron gas at different densities for several values of the Rashba interaction strength as a function of "Rashba spin states" polarization. Comparison is given with analytically known Hartree-Fock results and for the system in absence of Coulomb interaction.

Mapping electrostatic potential across an AlGaN/InGaN/AlGaN diode by electron holography

Electron holography has been used to image electrostatic potential variations across an AlGaN/InGaN/AlGaN heterojunction diode. Features in the energy profile have been interpreted in terms of the expected built-in voltage across the p-n junction of the diode as well as contributions from the spontaneous and piezoelectric polarization fields within the material. Additional profile features indicate the presence of two-dimensional electron gas at the upper interface of the InGaN well, and evidence for hole accumulation close to the lower interface.

Modulation-doped Cd1−xMnxTe/Cd1−yMgyTe quantum well structures with spatial in-plane profiling of the well width and the doping intensity

We report on the design and growth by molecular beam epitaxy of two types of graded Cd1−xMnxTe/Cd1−yMgyTe quantum well structures having a precisely controlled spatial profile of either the quantum well width and/or of the number of donors in one of the directions perpendicular to the growth axis. The existence of a spatially varying quantum well width was demonstrated by different wavelengths of the photoluminesce emitted from different regions of the sample. The presence of two-dimensional electron gas with a spatially varying concentration produced by graded modulation doping was evidenced by magneto-optical studies which revealed signatures of either excitons, or negatively charged exciton–electron complexes (X−), or Fermi-edge singularity, all in a single sample grown in one molecular beam epitaxy process. Such structures may be very useful for tunable wavelength radiation sources as well as in detailed studies of various physical characteristics of the quantum wells.

Novel CdTe/CdMgTe Graded Quantum Well Structures

We report on growth and magnetooptical studies of two types of novel CdTe/CdMgTe quantum well structures having a precisely controlled grading of either the quantum well width or the donor concentration in a direction perpendicular to the growth axis. The presence of two-dimensional electron gas of varying concentration produced by the graded modulation doping was evidenced by observation of negatively charged exciton-electron complexes (Χ). PACS numbers: 73.20.Dx, 78.55.Et

Growth and characterization of two-dimensional electron gas InGaP/GaAs heterostructures grown by low-pressure metalorganic chemical vapour deposition using tertiarybutylarsine and tertiarybutylphosphine

We report the growth of lattice-matched InGaP/GaAs heterostructures by metalorganic chemical vapour deposition (MOCVD) using TBA and TBP as group V sources. Transmission electron microscopy and X-ray diffraction showed that InGaP is ordered. Photoluminescence measurements at 13 K showed an emission at 1.917 eV which is close to the band gap reported for ordered InGaP. Shubnikov-De Haas oscillations observed at 4.2 K confirmed the presence of two-dimensional electron gas at the interface of this heterostructure. No persistent photoconductivity was observed in these structures.

Photoreflectance of AlxGa1−xAs and AlxGa1−xAs/GaAs interfaces and high-electron-mobility transistors

Photoreflectance is used to measure AlxGa1−xAs composition, and to determine carrier concentrations in Si-doped AlGaAs epilayers capped with GaAs. Undoped caps are generally depleted, and do not show a usual GaAs photoreflectance. However, photoreflectance from the cap/(doped AlGaAs) interface produces a broad signal which distorts the entire spectrum, making it hard to locate the GaAs and AlGaAs band edges precisely. A similar broad signal from modulation-doped heterostructures is apparently associated with samples that show the presence of two-dimensional electron gas.

Growth and characterization of GaAs on Si substrates grown by molecular-beam epitaxy

Single-crystal GaAs layers have been grown by molecular beam epitaxy (MBE) on (100) Si substrates. Surface morphology, defect density, and optical and electrical properties have been studied as a function of the growth parameters. The characterization techniques included photoluminescence, Hall effect, cross-sectional transmission electron microscopy, and X-ray diffraction. GaAs metal semiconductor field-effect transitors on Si exhibited transconductances of 128 mS∙mm−1 and current-gain cutoff frequencies as high as 19 GHz. Special heterostructures showed Shubnikov–de Haas oscillations at low temperature and plateaux in the Hall resistance, which confirmed the presence of two-dimensional electron gas in the heterostructure.