Two-dimensional Electron Gas Structures Research Articles

Effect of surface potential pinning on strain behavior of AlGaN/GaN device structures

Understanding the varied strain effects in AlGaN/GaN devices is crucial for realizing optimized flexible electronics systems and strain sensors. Here, we report on the effects of surface potential pinning, altered by the deposition of device-relevant SiNx passivation and Ni gate layers, on the strain-dependent carrier density, ns, of AlGaN/GaN two-dimensional electron gas structures. Flexible van der Pauw samples were made by separating AlGaN/GaN layers from the sapphire growth substrate using a two-dimensional boron nitride van der Waals release layer and transferring them to flexible substrates. For bare surface samples, we observed relatively large decreases in ns with tensile strain (Δns of −2 × 1011 cm−2 at 0.1% uniaxial strain), indicating an unpinned AlGaN surface potential. For the SiNx and Ni covered samples, the ns-strain trends were nearly flat, indicating a more pinned surface potential. Additionally, sub-bandgap 400 nm light is shown to effectively pin the surface potential as evidenced by flattening the ns-strain trend, the mechanism of which we explain in terms of the persistent photoconductivity effect. These observations could have important implications in tuning strain sensors and minimizing device variability in flexible electronics.

Indirect Measurement of Electron Energy Relaxation Time at Room Temperature in Two-Dimensional Heterostructured Semiconductors.

Hot carriers are a critical issue in modern photovoltaics and miniaturized electronics. We present a study of hot electron energy relaxation in different two-dimensional electron gas (2DEG) structures and compare the measured values with regard to the dimensionality of the semiconductor formations. Asymmetrically necked structures containing different types of AlGaAs/GaAs single quantum wells, GaAs/InGaAs layers, or bulk highly and lowly doped GaAs formations were investigated. The research was performed in the dark and under white light illumination at room temperature. Electron energy relaxation time was estimated using two models of I-V characteristics analysis applied to a structure with n-n+ junction and a model of voltage sensitivity dependence on microwave frequency. The best results were obtained using the latter model, showing that the electron energy relaxation time in a single quantum well structure (2DEG structure) is twice as long as that in the bulk semiconductor.

Nucleus-free two-dimensional electron gas structure enhanced Raman scattering

Nucleus-free two-dimensional electron gas structure enhanced Raman scattering

Strain-induced changes in AlGaN/GaN two-dimensional electron gas structures with low surface state densities

We report on the effects of applied external strain on AlGaN/GaN two dimensional electron gas structures exhibiting a negative strain dependence of the sheet carrier density (ns). Flexible AlGaN/GaN heterojunctions, grown on two-dimensional boron nitride (BN)-on-sapphire templates, were released from the substrate via strain-induced separation at the weak BN van der Waals interface and then transferred to flexible substrates. By releasing the AlGaN/GaN layer from the substrate, residual strain was removed which allowed for isolation and study of the effects of the externally applied strain. By bending samples, uniaxial strain up to 0.15% was applied as measured by the shifts in the GaN E2H Raman mode. Hall effect measurements revealed a 2.5% decrease in ns with 0.11% applied tensile strain, which is contrary to the increase expected from the piezoelectric effect. The observed decrease in ns is attributed to a relatively large increase in the AlGaN surface barrier height. This effect, which is rarely reported, is observable due to a low surface state density (2.2 × 1012 cm−2 eV–1) in the samples. Illumination was found to dramatically alter the ns-strain dependence, an effect potentially related to detrapping of electrons in the GaN buffer.

MgZnO/ZnO Two-Dimensional Electron Gas Photodetectors Fabricated by Radio Frequency Sputtering

MgZnO/ZnO two-dimensional electron gas (2DEG) structures with ZnO annealed at various temperatures (600-900 °C) and photodetectors (PDs) with and without a 2DEG structure were fabricated using a radio frequency magnetron sputtering system. It was found that the carrier concentration and mobility increase with the annealing temperature owing to the improved crystalline in ZnO; however, high-temperature (800 °C or higher) annealing can degrade the crystalline of the ZnO layer. Hall measurements showed that compared with that of bulk ZnO, the sheet carrier concentration of the 2DEG sample increased from 1.3 × 1013 to 1.2 × 1014 cm-2, and the mobility was enhanced from 5.1 to 17.5 cm2/V s. This is because the channel layer is the total thickness (300 nm) in bulk ZnO, whereas the carriers are confined to a 45 nm region beneath the MgZO layer in the 2DEG sample, confirming the 2DEG behavior at the MgZnO/ZnO interface. The PDs with 2DEG structures demonstrate a higher ultraviolet (UV) response and a UV/visible rejection ratio that is six times larger than that of the PDs without a 2DEG structure. The 2DEG structure also induces a photocurrent gain, which results in a 240% quantum efficiency for the 310 nm incident wavelength. The related mechanism is elucidated with a band diagram.

Optical investigation of electrical spin injection into an inverted two-dimensional electron gas structure

We report on electrical spin injection from (Ga,Mn)As into a high-mobility two-dimensional electron gas confined at an (Al,Ga)As/GaAs interface. Besides standard nonlocal electrical detection, we use a magneto-optical approach which provides cross-sectional images of the spin accumulation at the cleaved edge of the sample, yielding spin decay lengths on the order of $2\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. In some cases we find a nonmonotonic bias voltage dependence of the spin signal, which may be linked to ballistic tunneling effects during spin injection. We observe a clear Hanle depolarization using a technique which is free of dynamic nuclear polarization effects. Fitting the data with the standard drift-diffusion model of spin injection suggests averaged in-plane spin lifetimes on the order of 1 ns.

Spin force and the generation of sustained spin current in time-dependent Rashba and Dresselhaus systems

The generation of spin current and spin polarization in a two-dimensional electron gas structure is studied in the presence of Dresselhaus and Rashba spin-orbit couplings (SOC), the strength of the latter being modulated in time by an ac gate voltage. By means of the non-Abelian gauge field approach, we established the relation between the Lorentz spin force and the spin current in the SOC system, and showed that the longitudinal component of the spin force induces a transverse spin current. For a constant (time-invariant) Rashba system, we recover the universal spin Hall conductivity of e8π, derived previously via the Berry phase and semi-classical methods. In the case of a time-dependent SOC system, the spin current is sustained even under strong impurity scattering. We evaluated the ac spin current generated by a time-modulated Rashba SOC in the absence of any dc electric field. The magnitude of the spin current reaches a maximum when the modulation frequency matches the Larmor frequency of the electrons.

A THEORETICAL INVESTIGATION OF THE ELECTRICAL PROPERTIES OF ZnMgO/ZnO HETEROJUNCTION FIELD EFFECT TRANSISTOR

In this paper, I have tried to analyze the electrical properties and the experimental data related to drain current–drain voltage of the Zn 0.7 Mg 0.3 O / ZnO / Zn 0.7 Mg 0.3 O heterojunction field effect transistor by use of Hoffman nonideal model theoretically. Also by use of different scattering mechanisms in two-dimensional electron gas structures, I have studied the electrical transport properties of this structure and most important effective parameters for controlling electron mobility in the range of 75 to 300 K have been studied theoretically.

Effects of Proton Irradiation on the Magnetoelectric Properties of 2DEG AlGaN/GaN Micro-Hall Sensors

The effects of irradiating AlGaN/GaN micro-Hall sensors with 380 keV protons on were investigated by magnetoresistance measurements. The sheet resistance increased after irradiation with a proton dose of 1×1013 cm−2 due to the decrease of carrier mobility rather than the decrease of sheet carrier density. Our experiments showed that AlGaN/GaN two-dimensional electron gas (2DEG) structures are good strong candidates for Hall sensors operable in harsh radiation environments.

Forward bias voltage controlled infrared photodetection and electroluminescence from a p-i-n quantum dot structure

We report a specially designed p-i-n quantum dot (QD) device in which both the intersubband and the interband transitions of the QD are controlled by the applied forward bias voltage. The design is achieved by using asymmetric AlGaAs barriers on both sides of only one QD layer and by placing two-dimensional electron gas structure on one side of the QD layer. Experimentally, at 77 K, the device can detect normal incidence infrared light of about 5.6 µm when the forward bias voltage is in between about 3 and 7 V, while it emits electroluminescence of about 1 μm when the applied forward bias voltage is larger than 11 V.

Plasma Wave Electronics: A Revival Towards Solid-State Terahertz Electron Devices

Researches on the interactions between drifting carrier plasma waves and electromagnetic waves were intensively studied theoretically and experimentally in 1960s to 1990s but faded out with inconclusive results. This study presents a series of fundamental works carried out by our group in order to re-evaluate the possibility of realizing a solid-state amplifier operating in THz region based on the drifting plasma wave interactions. Firstly, we described a new method to analyze the properties of semiconductor drifting plasma in a semiconductor-insulator structure based on the Transverse Magnetic (TM) mode analysis. The components of waves, the electromagnetic fields and the ω- and k- dependent effective permittivity which are used to describe the dielectric response of the semiconductor plasma to the TM surface wave excitation are derived. Then, the properties of semiconductor drifting plasma in a two-dimensional electron gas (2DEG) structure using the developed TM mode analysis are presented. Here, the electromagnetic fields and the ω- and k- dependent effective permittivity of the 2DEG drifting plasma are also determined. The analyzed device structure and formulation procedures to explain the interactions between drifting plasma waves in semiconductor and electromagnetic waves propagating through the interdigital slow-wave structure is then presented. Next, some preliminary experimental studies of plasma wave interactions using interdigital-gated n-AlGaAs/GaAs high-electron-mobility-structure carried out are presented. Finally, we concluded the findings or contributions of our work and some remarks for future research.

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

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.

The effect of structural parameters and growth temperature on the electron density in asymmetric double-2DEGs

We report on growth and low-temperature transport of Ga(Al)As-based asymmetric double two-dimensional electron gas (2DEG) structures consisting of a quantum well (QW) stacked on top of a normal heterojunction. Our data show that the electron density of both parallel 2DEGs can be reliably matched just by varying a single growth parameter namely the width of spacer 2 which determines the electron density in the QW. This finding is independent of growth temperature and QW width although segregation of Si dopant atoms especially at elevated growth temperatures has a remarkable impact on the QW electron density. The best sample with exact match of the carrier densities exhibits metrological quality of the quantized Hall resistance with a relative standard deviation of five parts in 109 from the nominal value RK-90/4 at filling factor 2.

Comparing the image potentials for intercalated graphene with a two-dimensional electron gas with and without a gated grating

We calculate the surface response function and the image potential of a system of layered two-dimensional (2D) electron gas structures. A point charge is placed at a distance away from the surface which lies in the $xy$- plane. These 2D layers are coupled through the Coulomb interaction and there is no interlayer electron hopping. The separation between adjacent layers can be adjusted to investigate the roles which the layer separation and the number of layers play on both the surface response function and the image potential. Specifically, we consider the system composed of graphene layers or the layered 2D electron gas (EG) formed at the interface of a semiconductor heterostructure such as GaAs/AlGaAs. We show that the image potential for graphene is qualitatively the same as for the 2DEG. We examine the way in which the image potential is modified by applying a one-dimensional periodic electrostatic potential (through a gated grating for modulation). The results indicate that the charge screening for graphene is not much different from the 2DEG.

Investigation of high mobility two-dimensional electron gas in AlGaN/GaN heterostructures grown by hydride vapor phase epitaxy (HVPE)

Abstract:Two-inch-diameter sapphire (0001) substrates were used for the growth of AlGaN/GaNstructure by hydride vapor phase epitaxy (HVPE). The two-dimensional electron gas(2DEG) structure consisted of a 2 mm undoped GaN layer substrate, followed by 30 nmthick Al0.3Ga 0.7N layer. Structural and optical properties of GaN layer werecharacterized by high resolution X-ray diffraction, transmission electron microscopy(TEM) and photoluminescence (PL) measurements. TEM images showed thedislocation density in the GaN layer was ~ 9 ´ 109 /cm2 . The full width at halfmaximum (FWHM) values of the x-ray rocking curve for GaN (0002) was 0.156 deg.PL spectra of our sample exhibited a predominant band-edge emission of the wurziteGaN epilayer near 3.36 eV with FWHM = 45 meV at 50 K and 75 meV at roomtemperature, respectively. The Hall effect measurements gave a sheet electronconcentration of 7.8´1012 cm-2 and a mobility of 850 cm2/Vs at 300K. The experimentaldata indicated that the HVPE grown GaN layers exhibited superior quality compared toGaN layers grown by metal organic chemical vapor deposition in prior art literatures.

Asymmetric double two-dimensional electron gas structures for electrical quantum metrology

We study growth and low temperature magnetotransport of Ga(Al)As based asymmetric double two-dimensional electron gas (2DEG) structures consisting of a quantum well stacked on top of a heterojunction. These structures allow for matching the electron densities in the two parallel 2DEGs by variation of a single growth parameter without the appearance of any parasitic transport channel. At filling factor 2, we observe a 2T wide quantum Hall plateau of metrological quality with a quantized resistance of one quarter of the resistance quantum. Such structures can be used for electrical quantum metrology and to fabricate vertically integrated circuits.

Spin-polarized resonant transport in a hybrid ferromagnetic–two-dimensional electron gas structure

We investigated the spin-polarized resonant transport in a hybrid high-electron-mobility transistor (HEMT) structure, with source and drain electrodes made of ferromagnetic (FM) material, while the channel consists of a highly doped ${n}^{++}$ AlGaAs-GaAs two-dimensional electron gas (2DEG). The electron transport in the FM layer is modeled using the spin-drift diffusion model, while across the 2DEG layer, ballistic transport is assumed, given the long mean free path within the 2DEG. By solving the two transport models self-consistently, we found that the transport properties of the device, such as the transmission probability, the spin injection (SI) efficiency, and the magnetoresistance (MR) ratio, all exhibit oscillatory behavior when the 2DEG layer width or the 2DEG Fermi energy is varied. The basis of these oscillations is the resonant transport across the 2DEG, which is reminiscent of the spin-polarized resonant tunneling (SPRT), observed recently in magnetic tunnel junctions (MTJs). The hybrid device has distinct advantages over the metal-based MTJ structures in the practical utilization of the SPRT effect. This is because the ballistic charge conduction through the 2DEG enables easy tunability of the MR ratio and SI efficiency, by varying the doping density and gate bias, while avoiding the exponential suppression of MR with barrier thickness, which occurs in MTJ devices. Numerically, the hybrid HEMT device is predicted to be capable of achieving maximum MR and SI ratios approaching 20% and 40%, respectively, at the crest of their respective oscillations.

Strained Silicon Two-dimensional Electron Gases On Commercially Available Si1-xGex Relaxed Graded Buffers

In this work, we describe the growth of two-dimensional electron gas (2DEG) structures in strained Si on commercially available SiGe relaxed graded buffers with 20% Ge fraction. Most work to date on 2DEG's focuses on x=0.3 and above for larger conduction band offsets. In this work we demonstrate the suitability of commercially available relaxed buffers for these structures, for eventual quantum computing applications based on the spin of electrons in Si. The use of commercially available buffers is desirable because of their very high quality and because it is often difficult for a single system to grow both the thick buffer and the thin precise layers for 2DEG's. 20% Ge buffers are used because higher Ge fractions are not easily commercially available. The modulation of the electron gases we grow using atomic layer deposited (ALD) Al2O3 is also demonstrated.

Spectral simulation of GaAs and InAs quantum-dot terahertz detectors designed for higher-temperature operation

The spectral properties and other performance characteristics of single-photon quantum-dot terahertz detectors incorporating single-electron transistors are analyzed for the cases of GaAs and InAs electron channels. By defining the electrostatically confined double-quantum-dot structures in shallow two-dimensional electron gas structures, these devices have the potential to operate at temperatures above the dilution refrigerator range. We calculate the interlevel plasmon absorption by using the random-phase approximation in conjunction with realistic band structures and three-dimensional confinement geometries. While the level separations increase with decreasing dot diameter, carrier depletion eventually limits the minimum size that is viable. The maximum detection frequency is about 1THz for GaAs channels, but increases to 1.5–2THz for InAs channels having a lower effective mass.

Growth of crack-free, carbon-doped GaN and AlGaN∕GaN high electron mobility transistor structures on Si (111) substrates by ammonia molecular beam epitaxy

The growth of C-doped GaN epilayers on p-Si (111) substrates by ammonia molecular beam epitaxy is reported. Highly insulating and crack-free 1.5-μm-thick C-doped GaN layers have been prepared using ionized methane as the dopant source. Using such a template, AlGaN∕GaN two-dimensional electron gas structures with a mobility of 1260cm2∕Vs for a sheet carrier density of 1.24×1013cm−2 have been achieved at room temperature. Fabricated devices demonstrated an excellent pinch-off characteristic as revealed by an on-to-off ratio higher than four orders of magnitudes and by very low leakage current (10μA∕mm at VDS=20V).