Mobility Two-dimensional Electron Research Articles

Parametric Analysis of Indium Gallium Arsenide Wafer-based Thin Body (5 nm) Double-gate MOSFETs for Hybrid RF Applications.

The electrical behavior of a high-performance Indium Gallium Arsenide (In- GaAs) wafer-based n-type Double-Gate (DG) MOSFET with a gate length (LG1= LG2) of 2 nm was analyzed. The relationship of channel length, gate length, top and bottom gate oxide layer thickness, a gate oxide material, and the rectangular wafer with upgraded structural characteristics and the parameters, such as switch current ratio (ION/IOFF) and transconductance (Gm) was analyzed for hybrid RF applications. This work was carried out at 300 K utilizing a Non-Equilibrium Green Function (NEGF) mechanism for the proposed DG MOSFET architecture with La2O3 (EOT=1 nm) as gate dielectric oxide and source-drain device length (LSD) of 45 nm. It resulted in a maximum drain current (IDmax) of 4.52 mA, where the drain-source voltage (VDS) varied between 0 V and 0.5 V at the fixed gate to source voltage (VGS) = 0.5 V. The ON current(ION), leakage current (IOFF), and (ION/IOFF) switching current ratios of 1.56 mA, 8.49Í10-6 μA, and 18.3Í107 μA were obtained when the gate to source voltage (VGS) varied between 0 and 0.5 V at fixed drain-source voltage (VDS)=0.5V. The simulated result showed the values of maximum current density (Jmax), one and twodimensional electron density (N1D and N2D), electron mobility (μn), transconductance (Gm), and Subthreshold Slope (SS) are 52.4 μA/m2, 3.6Í107 cm-1, 11.36Í1012 cm-2, 1417 cm2V-1S-1, 3140 μS/μm, and 178 mV/dec, respectively. The Fermi-Dirac statistics were employed to limit the charge distribution of holes and electrons at a semiconductor-insulator interface. The flat-band voltage (VFB) of - 0.45 V for the fixed threshold voltage greatly impacted the breakdown voltage. The results were obtained by applying carriers to the channels with the (001) axis perpendicular to the gate oxide. The sub-band energy profile and electron density were well implemented and derived using the Non-Equilibrium Green's Function (NEGF) formalism. Further, a few advantages of the proposed heterostructure-based DG MOSFET structure over the other structures were observed. This proposed patent design, with a reduction in the leakage current characteristics, is mainly suitable for advanced Silicon-based solid-state CMOS devices, Microelectronics, Nanotechnologies, and future-generation device applications.

Investigation of Electron Effective Mass in AlGaN/GaN Heterostructures by THz Spectroscopy of Drude Conductivity

Terahertz time domain spectroscopy (TDS) of the two-dimensional (2-D) electrons in various commercial AlGaN/GaN heterostructures was performed in the frequency range of 0.1–2.0 THz at selected temperatures of 80 and 300 K. Experimental transmission spectra were analyzed using analytical model derived for the thin conductive layer on dielectric substrate assuming a high-frequency Drude response of electrons. Two-dimensional electron mobility and density values found from the best fit parameters were compared with the results of independent Hall experiment demonstrating good agreement within uncertainty range of 10%. Thermal renormalization of the electron effective mass was revealed in all investigated samples manifesting as the increase of up to 1.45 ± 0.13 fold of nominal value obtained at cryogenic temperatures. Fast method based on measurement of the ratio of transmission spectra at two temperatures (80 and 300 K) was proposed to monitor the renormalization phenomenon in the samples with thin Drude conductivity layer where carrier density is relatively independent on temperature.

Density-dependent two-dimensional optimal mobility in ultra-high-quality semiconductor quantum wells

We calculate using the Boltzmann transport theory the density-dependent mobility of two-dimensional (2D) electrons in GaAs, SiGe, and AlAs quantum wells as well as of 2D holes in GaAs quantum wells. The goal is to precisely understand the recently reported breakthrough in achieving a record 2D mobility for electrons confined in a GaAs quantum well. Comparing our theory with the experimentally reported electron mobility in GaAs quantum wells, we conclude that the mobility is limited by unintentional background random charged impurities at an unprecedented low concentration of $\ensuremath{\sim}{10}^{13}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. We find that this same low level of background disorder should lead to 2D GaAs hole and 2D AlAs electron mobilities of $\ensuremath{\sim}{10}^{7}$ and $\ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}/\text{V}\phantom{\rule{0.16em}{0ex}}\text{s}$, respectively, which are much higher theoretical limits than the currently achieved experimental values in these systems. We therefore conclude that the current GaAs hole and AlAs electron systems are much dirtier than the state-of-the-art 2D GaAs electron systems. We present theoretical results for 2D mobility as a function of density, effective mass, quantum-well width, and valley degeneracy, comparing with experimental data.

Insights on the optoelectronic properties in two-dimensional Janus lateral In2SeTe/Ga2STe heterostructure

In this paper, the lateral heterostructures combined with monolayer Janus compounds Indium selenide telluride (In2SeTe) and Gallium sulfide telluride (Ga2STe) are investigated by first-principles calculations. Phonon spectrum analysis indicates that two Janus lateral In2SeTe/Ga2STe heterostructures combined in different ways can be formed steadily. The calculations show that two heterostructures exhibit type-II band alignment, which is key to the separation of photocarriers. The lateral In2SeTe/Ga2STe heterostructure has a direct band gap of 1.50 eV, in line with high-performance excitonic solar cells. The ternary systems show higher optical absorption coefficient in the visible light region and better two-dimensional electron mobility than the binary lateral heterostructure InS/InSe. Finally, the photoelectric conversion efficiency of the Janus In2SeTe/Ga2STe lateral heterostructure reaches a competitive 21%. Thus, the high conversion efficiency and suitable direct bang gap as well as the perfect band alignment may make the In2SeTe/Ga2STe lateral heterostructure a promising material for optoelectronics devices.

Effect of proton irradiation on the mobility of two-dimensional electron in AlGaN/AlN/GaN high electron mobility transistors at low temperature

The authors simulated the damage caused by proton irradiation to the device and analyzed the effect of proton irradiation on two-dimensional electron mobility taking various scattering mechanisms into account. Proton-irradiation simulation of the AlGaN/AlN/GaN HEMT device was carried out to obtain the irradiation simulation results by using SRIM software. Then, considering various scattering mechanisms, the authors established a model to simulate two-dimensional electron mobility under different proton energy and irradiation doses at low temperature. The theoretical data show that proton irradiation significantly decreased the mobility of a two-dimensional electron in a GaN-based HEMT at low temperature.

Effects of floating gate structures on the two-dimensional electron gas density and electron mobility in AlGaN/AlN/GaN heterostructure field-effect transistors

In this study, we have investigated the electrical properties of the AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) with floating gate structures using the measured capacitancevoltage (C-V) and current-voltage (I-V) characteristics. It is found that the two-dimensional electron gas (2DEG) density under the central gate cannot be changed by the floating gate structures. However, the floating gate structures can cause the strain variation in the barrier layer, which lead to the non-uniform distribution of the polarization charges, then induce a polarization Coulomb field and scatter the 2DEG. More floating gate structures and closer distance between the floating gates and the central gate will result in stronger scattering effect of the 2DEG.

Improved two-dimensional electron mobility in asymmetric barrier delta-doped GaAs/AlGaAs modulation-doped field-effect transistor structures

We study the enhancement of electron mobility μ in barrier delta-doped GaAs/AlGaAs quantum-well-based modulation-doped field-effect transistor (MODFET) structures. We asymmetrically vary the doping concentrations Nd1 and Nd2 in the barriers on the substrate and surface sides, respectively, to obtain a nonlinear enhancement of μ as a function of the well width w through multi-subband effects. We show that an increase in doping concentration increases the surface electron density Ns, which in turn enhances μ. Interchanging Nd1 and Nd2 leads to no change in Ns but rather, an enhancement of μ as a function of w for Nd2 > Nd1 owing to asymmetric variation of subband wave functions, thereby implying a higher channel conductivity in a surface-doped structure than in an inverted doped structure. By keeping (Nd1 + Nd2) unchanged, the conductivity of a single-channel MODFET, Nd1 (Nd2) ≠ 0 and Nd2 (Nd1) = 0, can be enhanced by considering a MODFET based on an asymmetrically doped (Nd1 ≠ Nd2 ≠ 0) quantum well structure. We show that the highest Ns and μ product for these structures occurs almost before the onset of the occupation of the second subband. Our analysis of the effect of asymmetric doping profiles on channel conductivity can be utilized for the performance improvement of MODFET-like devices.

Long-lived two-dimensional triplet magnetoexcitons in a Hall insulator

An experimental technique is developed to perform photoexcitation of an ensemble of translationinvariant triplet excitons, to manipulate this ensemble, and to detect the properties of its components. In particular, the influence of temperature on the radiationless decay during the relaxation of an exciton spin into the ground state of a Hall insulator at a filling factor ν = 2 is studied. The generation of photoexcited electrons and holes is controlled using photoinduced resonance reflection spectra, which makes it possible to estimate the density of light-generated electron–hole pairs and to independently control the self-consistent generation of electrons at the first Landau level and holes (vacancies) at the ground (zero) cyclotron electronic level. The existence of triplet excitons is established from inelastic light scattering spectra, which are used to determine the singlet–triplet exciton splitting. The lifetimes of triplet excitons, which are closely related to the relaxation time of an electron spin, are extremely long: they reach 100 μs in perfect GaAs/AlGaAs heterostructures with a high mobility of two-dimensional electrons at low temperatures. These long spin relaxation times are qualitatively explained, and the expected collective behavior of high-density triplet magnetoexcitons at sufficiently low temperatures, which is related to their Bose nature, is discussed.

Observation of Microwave-Induced Magnetoresistance Oscillations in a ZnO/Mg x Zn1–x O Heterojunction

Microwave-induced magnetoresistance and magnetoconductance oscillations in the two-dimensional electron system have been detected in a ZnO/MgxZn1–xO heterojunction. The structure has a high electron density of n = 6.5 × 1011 cm–2 and a very low mobility of 105 cm2 V–1 s–1. This clearly indicates that the mobility of two-dimensional electrons is not a critical parameter for the observation of microwave-induced oscillations. The samples were both Hall bars and Corbino disks. At least four oscillations have been resolved. Their amplitude increases with the magnetic field and all oscillations with numbers >1 have the same phase of 0.25. It has been shown that the microwave-induced photoelectromotive force is also a periodically oscillating function of the inverse magnetic field.

Numerical and experimental analyses of two-dimensional electron mobility in Al(In,Ga)N/AlGaN heterostructures

Two-dimensional electron mobilities in AlGaN-channel nitride heterostructures were numerically and experimentally analyzed. The calculation and experimental results indicated that mobility for InAlN/AlGaN heterostructures grown by metalorganic chemical vapor deposition was limited by interface roughness scattering as well as alloy disorder scattering. The mobility limits for AlGaN-channel heterostructures with a two-dimensional electron gas (2DEG) density of 3 × 1013/cm2 were estimated to be, for instance, 249 cm2 V−1 s−1 for an Al0.3Ga0.7N channel and 192 cm2 V−1 s−1 for an Al0.5Ga0.5N channel. The calculated values showed no major disagreement with the results reported on AlGaN-channel heterostructures.

Effects of GaN cap layer thickness on an AlN/GaN heterostructure

In this study, we investigate the effects of GaN cap layer thickness on the two-dimensional electron gas (2DEG) electron density and 2DEG electron mobility of AlN/GaN heterostructures by using the temperature-dependent Hall measurement and theoretical fitting method. The results of our analysis clearly indicate that the GaN cap layer thickness of an AlN/GaN heterostructure has influences on the 2DEG electron density and the electron mobility. For the AlN/GaN heterostructures with a 3-nm AlN barrier layer, the optimized thickness of the GaN cap layer is around 4 nm and the strained a-axis lattice constant of the AlN barrier layer is less than that of GaN.

Influence of the form of structure factor on the mobility of nondegenerate two-dimensional electrons

The temperature dependences of the mobility of nondegenerate two-dimensional electrons in scattering by a correlated distribution of impurity ions in AlxGa1 − xAs/GaAs heterostructures have been investigated. The cases where the influence of the first maximum of the structure factor on the scattering of electrons begins to dominate with increasing temperature have been considered. It has been found that the mobility of two-dimensional electrons decreases with increasing correlations in the spatial distribution of impurity ions. The influence exerted by the correlations and the width of the spacer layer on this effect has been analyzed.

Thermal broadening of electron mobility distribution in AlGaN/AlN/GaN heterostructures

In this paper we present a detailed study of the effects of temperature on the two-dimensional electron mobility distribution, using a fully numerical calculation in unintentionally doped AlxGa1−xN/AlN/GaN heterostructures. The analysis of our results clearly indicates that the effect of partial sub-band occupancy is considerable, especially at higher operating temperatures when more than one sub-band is occupied. By correlating with published experimental data, our results are consistent with the observed broadening of the mobility distribution with increasing temperature, which is shown to be a direct consequence of the temperature dependence of polar optical phonons. Although other scattering mechanisms also affect broadening of the mobility distribution, the contribution from these other individual scattering mechanisms is found to be independent of temperature. In particular, a comparison of our calculated results with published experimental data is shown to be in excellent agreement across the temperature range from 100 K to 300 K. By fitting our model to experimental results at low temperatures (95 K–125 K), we are able to extract detailed information related to surface roughness scattering, which is dominant in this temperature range. In particular, the average height of interface roughness is found to be 18 A.

Mobility of nondegenerate two-dimensional electrons in scattering by a correlated distribution of impurity ions

The temperature dependences of the inverse mobility of nondegenerate two-dimensional electrons in scattering by impurity ions in heterostructures with a narrow spacer have been investigated using the Al x Ga1 − x As/GaAs heterostructure as an example. Correlations in the arrangement of impurity ions have been taken into account in the model of hard spheres on a plane. The influence of the form of the structure factor in the electron mobility has been considered.

Al组分对MOCVD制备的AlxGa1-xN/AlN/GaN HEMT电学和结构性质的影响

采用金属有机化合物化学气相沉积(MOCVD)方法制备了不同Al组分(x=0.19,0.22,0.25,0.32)的Al x Ga1-x N/AlN/GaN结构的高电子迁移率晶体管(HEMT)材料。研究了Al x Ga1-x N势垒层中Al组分对HEMT材料电学性质和结构性质的影响。研究结果表明,在一定的Al组分范围内,二维电子气(2DEG)浓度和迁移率随着Al组分的升高而增大。然而,过高的Al组分导致HEMT材料表面粗糙度增大,2DEG迁移率降低,该实验现象在另一方面得到了原子力显微镜测试结果的验证。在最佳Al组分(25%)范围内,获得的HEMT材料的2DEG浓度和室温迁移率分别达到1.2×1013cm-2和1 680 cm2/(V·s),方块电阻低至310Ω/□。

Classical and quantum hall effect measurements in GaInNAs/GaAs quantum wells

We have performed magneto-transport experiments in modulation-doped Ga0.7In0.3NyAs1−y/GaAs quantum wells with nitrogen mole fractions 0.4%, 1.0% and 1.5%. Classical magnetotransport (resistivity and low-field Hall effect) measurements have been performed in the temperatures between 1.8 and 275K, while quantum Hall effect measurements in the temperatures between 1.8 and 47K and magnetic fields up to 11T.The variations of Hall mobility and Hall carrier density with nitrogen mole fractions and temperature have been obtained from the classical magnetotransport measurements. The results are used to investigate the scattering mechanisms of electrons in the modulation-doped Ga0.7In0.3NyAs1−y/GaAs quantum wells. It is shown that the alloy disorder scattering is the major scattering mechanism at investigated temperatures.The quantum oscillations in Hall resistance have been used to determine the carrier density, effective mass, transport mobility, quantum mobility and Fermi energy of two-dimensional (2D) electrons in the modulation-doped Ga0.7In0.3NyAs1−y/GaAs quantum wells. The carrier density, in-plane effective mass and Fermi energy of the 2D electrons increases when the nitrogen mole fraction is increased from y=0.004 to 0.015. The results found for these parameters are in good agreement with those determined from the Shubnikov-de Haas effect in magnetoresistance.

Influence of correlations in a flat distribution of impurity ions on the mobility of two-dimensional electrons at low temperatures

The temperature dependences of the electrical resistivity of degenerate two-dimensional electrons in scattering by impurity ions in heterostructures with a spacer of arbitrary width have been investigated using the AlxGa1 − xAs/GaAs heterostructure as an example. Correlations in the arrangement of impurity ions have been taken into account in the model of hard spheres on a plane.

Experimental and theoretical analysis of the temperature dependence of the two-dimensional electron mobility in a strained Si quantum well

The temperature dependence of the mobility of the two-dimensional electron gas (2DEG) in a silicon quantum well strained by Si0.7Ge0.3 relaxed buffer layer is determined precisely by a mobility spectrum analysis. The 2DEG mobility is 2780 cm2/V s at room temperature and, upon cooling, increases continuously to reach μ2DEG=7.4×104cm2/Vs at 7 K. A back gate installed on the sample changes the 2DEG concentration n successfully to establish μ2DEG∝n1.4 at the constant temperature T=10K, implying that the scattering at such low temperature is limited solely by the remote ionized impurity scattering. Based on this finding, theoretical analysis of the temperature dependence of μ2DEG is performed based on the relaxation time approximation using 2DEG wavefunctions and subband structures determined self-consistently and including three major scatterings; by intravalley acoustic phonons, intervalley g-processes of longitudinal optical (LO) phonons, and remote ionized impurities. The calculation included only three fitting parameters, the shear deformation potential (Ξu=9.5eV), LO phonon deformation potential for g-process scattering (D0=9.0×108eV/cm), and sheet density of remote ionized impurities that have been determined by quantitative comparison with our experimental results. The temperature dependence of μ2DEG calculated theoretically show excellent agreement with experimentally determined μ2DEG.

Electron mobility, Hall scattering factor, and sheet conductivity in AlGaN/AlN/GaN heterostructures

In this paper, we present a study of the effect of temperature on the two-dimensional electron mobility, Hall scattering factor, and sheet conductivity, using a fully numerical calculation in unintentionally doped AlxGa1−xN/AlN/GaN heterostructures. The analysis of our results clearly indicates that the effect of partial sub-band occupancy is considerable, especially at higher operating temperatures when more than one sub-band is occupied. The comparison of our calculated results with published experimental data is shown to be in good agreement.

Voltage-controlled sub-terahertz radiation transmission through GaN quantum well structure

We report on measurements of radiation transmission in the 0.220–0.325 THz frequency range through GaN quantum wells grown on sapphire substrates at nitrogen and room temperatures. Significant enhancement of the transmitted beam intensity with applied voltage is found at nitrogen temperature. This effect is explained by changes in the mobility of two-dimensional electrons under electric bias. We have clarified which physical mechanism modifies the electron mobility and we suggest that the effect of voltage-controlled sub-terahertz transmission can be used for the development of electro-optic modulators operating in the sub-THz frequency range.