Influence of Al Composition on Transport Properties of Two-Dimensional Electron Gas in Al_xGa_(1-x)N/GaN Heterostructures

Magnetotransport properties of two-dimensional electron gases （2DEG） in AlxGa1-x N/GaN heterostructures with different Al compositions are investigated by magnetotransport measurements at low temperatures and in high magnetic fields. It is found that heterostructures with a lower Al composition in the barrier have lower 2DEG concentration and higher 2DEG mobility.

Effective Qubit Emerging from the Nanoheterointerface Two-Dimensional Electron Gas Photodynamics

An “Eigenstate Adjustment Autonomy” Model, permeated by the Nanosystem’s Fermi Level Pinning along with its rigid Conduction Band Discontinuity, compatible with pertinent Experimental Measurements, is being employed for studying how the Functional Eigenstate of the Two-Dimensional Electron Gas (2DEG) dwelling within the Quantum Well of a typical Semiconductor Nanoheterointerface evolves versus (cryptographically) selectable consecutive Cumulative Photon Dose values. Thus, it is ultimately discussed that the experimentally observed (after a Critical Cumulative Photon Dose) Phenomenon of 2DEG Negative Differential Mobility allows for the Nanosystem to exhibit an Effective Qubit Specific Functionality potentially conducive to (Telecommunication) Quantum Information Registering.

Effects of layer stacking and strain on electronic transport in two-dimensional tin monoxide

Tin monoxide(SnO) is an interesting two-dimensional material because it is a rare oxide semiconductor with bipolar conductivity.However, the lower room temperature mobility limits the applications of SnO in the future.Thus, we systematically investigate the effects of different layer structures and strains on the electron–phonon coupling and phonon-limited mobility of SnO.The A2uphonon mode in the high-frequency region is the main contributor to the coupling with electrons for different layer structures.Moreover, the orbital hybridization of Sn atoms existing only in the bilayer structure changes the conduction band edge and conspicuously decreases the electron–phonon coupling, and thus the electronic transport performance of the bilayer is superior to that of other layers.In addition, the compressive strain of ε=-1.0% in the monolayer structure results in a conduction band minimum(CBM) consisting of two valleys at the Γ point and along the M–Γ line, and also leads to the intervalley electronic scattering assisted by the Eg(-1)mode.However, the electron–phonon coupling regionally transferring from high frequency A2uto low frequency Eg(-1)results in little change of mobility.

A comparison of the transport properties of bilayer graphene,monolayer graphene,and two-dimensional electron gas

We studied and compared the transport properties of charge carriers in bilayer graphene, monolayer graphene, and the conventional semiconductors （the two-dimensional electron gas （2DEG））. It is elucidated that the normal incidence transmission in the bilayer graphene is identical to that in the 2DEG but totally different from that in the monolayer graphene. However, resonant peaks appear in the non-normal incidence transmission profile for a high barrier in the bilayer graphene, which do not occur in the 2DEG. Furthermore, there are tunneling and forbidden regions in the transmission spectrum for each material, and the division of the two regions has been given in the work. The tunneling region covers a wide range of the incident energy for the two graphene systems, but only exists under specific conditions for the 2DEG. The counterparts of the transmission in the conductance profile are also given for the three materials, which may be used as high-performance devices based on the bilayer graphene.

Electron Transport Properties of Two-Dimensional Si_1P_1 Molecular Junctions

We focus on two new 21） materials, i.e., monolayer and bilayer silicon phosphides （Sil P1）. Based on the elastic- scattering Green＇s function, the electronic-transport properties of two-dimensional monolayer and bilayer Au- Si1P1-Au molecular junctions are studied. It is found that their bandgaps are narrow （0.16eV for a monolayer molecular junction and 0.26 e V for a bilayer molecular junction）. Moreover, the calculated current-voltage char- acteristics indicate that the monolayer molecular junction provides constant output current （20 hA） over a wide voltage range, and the bilayer molecular junction provides higher current （42 hA）.

Two-Dimensional Hybrid Model for High-Current Electron Beam Transport in a Dense Plasma

A two-dimensional hybrid code is developed to model the transport of a high-current electron beam in a dense plasma target. The beam electrons are treated as particles and described by particle-in-cell simulation including collisions with the target plasma particles. The background target plasma is assumed to be a stationary fluid with temperature variations. The return current and the self-generated electric and magnetic fields are obtained by combining Amp^re＇s law without the displacement current, the resistive Ohm＇s law and Faraday＇s law. The equations are solved in two-dimensional cylindrical geometry with rotational symmetry on a regular grid, with centered spatial differencing and first-order implicit time differencing. The algorithms implemented in the code are described, and a numerical experiment is performed for an electron beam with Maxwellian distribution ejected into a uniform deuterium-tritium plasma target.

Electron Transport Properties of Two-Dimensional Monolayer Films from Au-P-Au to Au-Si-Au Molecular Junctions

We investigate the electronic-transport properties of two-dimensional monolayer films from Au-P-Au molecular junction to Au-Si-Au molecular junction using elastic scattering Green＇s function theory. In the process of replacing the P atoms with Si atoms every other line from the middle of monolayer blue phosphorus molecular structure, the substitution of Si atoms changes the properties of Au-P-Au molecular junction significantly. Interestingly, the current value has a symmetric change as a parabolic curve with the peak appearing in Au-Si_1P_1-Au molecular junction, which provides the most stable current of 15.00 nA in a wide voltage range of 0.70-2.70 V.Moreover, the current-voltage characteristics of the structures indicate that the steps tend to disappear revealing the property similar to metal when the Si atoms dominate the molecular junction.

Magnetotransport properties of two-dimensional electron gas in AlGaN/AlN/GaN heterostructures

Magnetotransport measurements are carried out on the A1GaN/A1N/GaN in an SiC heterostructure, which demon- strates the existence of the high-quality two-dimensional electron gas （2DGE） at the A1N/GaN interface. While the carrier concentration reaches 1.32×10^13 cm^-2 and stays relatively unchanged with the decreasing temperature, the mobility of the 2DEG increases to 1.21 × 10^4 cm2/（V.s） at 2 K. The Shubnikov-de Haas （SdH） oscillations are observed in a magnetic field as low as 2.5 T at 2 K. By the measurements and the analyses of the temperature-dependent SdH oscillations, the effective mass of the 2DEC is determined. The ratio of the transport lifetime to the quantum scattering time is 9 in our sample, indicating that small-angle scattering is predominant.

TiO_(2)Electron Transport Layer with p-n Homojunctions for Efficient and Stable Perovskite Solar Cells

Low-temperature processed electron transport layer(ETL)of TiO_(2)that is widely used in planar perovskite solar cells(PSCs)has inherent low carrier mobility,resulting in insufficient photogenerated elec-tron transport and thus recombination loss at buried interface.Herein,we demonstrate an effective strategy of laser embedding of p-n homojunctions in the TiO_(2)ETL to accelerate electron transport in PSCs,through localized build-in electric fields that enables boosted electron mobility by two orders of magnitude.Such embedding is found significantly helpful for not only the enhanced crystallization quality of TiO_(2)ETL,but the fabrication of perovskite films with larger-grain and the less-trap-states.The embedded p-n homojunction enables also the modulation of interfacial energy level between perovskite layers and ETLs,favoring for the reduced voltage deficit of PSCs.Benefiting from these merits,the formamidinium lead iodide(FAPbI_(3))PSCs employing such ETLs deliver a champion efficiency of 25.50%,along with much-improved device stability under harsh conditions,i.e.,maintain over 95%of their initial efficiency after operation at maximum power point under continuous heat and illumination for 500 h,as well as mixed-cation PSCs with a champion efficiency of 22.02%and over 3000 h of ambient storage under humidity stability of 40%.Present study offers new possibilities of regulating charge transport layers via p-n homojunction embedding for high performance optoelectronics.

Electronic transport evolution across the successive structural transitions in Ni_(50-x)Fe_xTi_(50) shape memory alloys

TiNi-based shape memory alloys have been extensively investigated due to their significant applications,but a comprehensive understanding of the evolution of electronic structure and electrical transport in a system with martensitic transformations(MT) is still lacking.In this work,we focused on the electronic transport behavior of three phases in Ni_(50-x)Fe_xTi_(50)across the MT.A phase diagram of Ni_(50-x)Fe_xTi_(50) was established based on x-ray diffraction,calorimetric,magnetic,and electrical measurements.To reveal the driving force of MT,phonon softening was revealed using first-principles calculations.Notably,the transverse and longitudinal transport behavior changed significantly across the phase transition,which can be attributed to the reconstruction of electronic structures.This work promotes the understanding of phase transitions and demonstrates the sensitivity of electron transport to phase transition.

Ion heat transport in electron cyclotron resonance heated L-mode plasma on the T-10 tokamak

Anomalous ion heat transport is analyzed in the T-10 tokamak plasma heated with electron cyclotron resonance heating(ECRH) in second-harmonic extra-ordinary mode. Predictive modeling with empirical scaling for Ohmical heat conductivity shows that in ECRH plasmas the calculated ion temperature could be overestimated, so an increase of anomalous ion heat transport is required. To study this effect two scans are presented: over the EC resonance position and over the ECRH power. The EC resonance position varies from the high-field side to the low-field side by variation of the toroidal magnetic field. The scan over the heating power is presented with on-axis and mixed ECRH regimes. Discharges with high anomalous ion heat transport are obtained in all considered regimes. In these discharges the power balance ion heat conductivity exceeds the neoclassical level by up to 10 times. The high ion heat transport regimes are distinguished by three parameters: the ratio Te/Ti, the normalized electron density gradient R/■, and the ion–ion collisionality νii~*. The combination of high Te/Ti, high νii~*, and R/■=6-10 results in values of normalized anomalous ion heat fluxes up to 10 times higher than in the low transport scenario.

Two-dimensional electron gas characteristics of InP-based high electron mobility transistor terahertz detector

The samples of InxGa（1-x）As/In（0.52）Al（0.48）As two-dimensional electron gas（2DEG）are grown by molecular beam epitaxy（MBE）.In the sample preparation process,the In content and spacer layer thickness are changed and two kinds of methods,i.e.,contrast body doping andδ-doping are used.The samples are analyzed by the Hall measurements at 300 Kand 77 K.The InxGa1-xAs/In0.52Al0.48As 2DEG channel structures with mobilities as high as 10289 cm^2/V·s（300 K）and42040 cm^2/V·s（77 K）are obtained,and the values of carrier concentration（Nc）are 3.465×10^12/cm^2 and 2.502×10^12/cm^2,respectively.The THz response rates of In P-based high electron mobility transistor（HEMT）structures with different gate lengths at 300 K and 77 K temperatures are calculated based on the shallow water wave instability theory.The results provide a reference for the research and preparation of In P-based HEMT THz detectors.

Two-dimensional Boron Nitride for Electronics and Energy Applications

Two-dimensional(2D)boron nitride(BN),the so-called“white graphene,”has demonstrated a great potential in various fields,particularly in electronics and energy,by utilizing its wide bandgap(~5.5 eV),superior thermal stability,high thermal conductance,chemical inertness,and outstanding dielectric properties.However,to further optimize the performances from the view of structure-property relationship,the determinative factors such as crystallite sizes,layer thickness,dispersibility,and surface functionalities should be precisely controlled and adjusted.Therefore,in this review,the synthesis and functionalization methods including“top-down”and“bottom-up”strategies,and non-covalent and covalent modifications for 2D BN are systematically classified and discussed at first,thus catering for the requirements of versatile applications.Then,the progresses of 2D BN applied in the fields of microelectronics such as fieldeffect transistors and dielectric capacitors,energy domains such as thermal energy management and conversion,and batteries and supercapacitors are summarized to highlight the importance of 2D BN.Notably,these contents not only contain the state-of-the-art 2D BN composites,but also bring the current novel design of 2D BN-based microelectronic units.Finally,the challenges and perspectives are proposed to better broaden the scope of this material.Therefore,this review will pave an all-around way for understanding,utilizing,and applying 2D BN in future electronics and energy applications.

Electron mobility anisotropy in (Al,Ga)Sb/InAs two-dimensional electron gases epitaxied on GaAs (001) substrates

The electron mobility anisotropy in (Al,Ga)Sb/InAs two-dimensional electron gases with different surface morphology has been investigated.Large electron mobility anisotropy is found for the sample with anisotropic morphology,which is mainly induced by the threading dislocations in the InAs layer.For the samples with isotropic morphology,the electron mobility is also anisotropic and could be attributed to the piezoelectric scattering.At low temperature (below transition temperature),the piezoelectric scattering is enhanced with the increase of temperature,leading to the increase of electron mobility anisotropy.At high temperature (above transition temperature),the phonon scattering becomes dominant.Because the phonon scattering is isotropic,the electron mobility anisotropy in all the samples would be reduced.Our results provide useful information for the comprehensive understanding of electron mobility anisotropy in the (Al,Ga)Sb/InAs system.

Amorphous BaTiO_(3) Electron Transport Layer for Thermal Equilibrium-Governed γ-CsPbl_(3) Perovskite Solar Cell with High Power Conversion Efficiency of 19.96%

Compared to organic-inorganic hybrid perovskites,the cesium-based allinorganic lead halide perovskite(CsPbI_(3))is a promising light absorber for perovskite solar cells owing to its higher resistance to thermal stress.Nonetheless,additional research is required to reduce the nonradiative recombination to realize the full potential of CsPbI_(3).Here,the diffusion of Cs ions participating in ion exchange is proposed to be an important factor responsible for the bulk defects inγ-CsPbI_(3)perovskite.Calculations based on first-principles density functional theory reveal that the[PbI_(6)]^(4-)octahedral tilt modifies the perovskite crystallographic properties inγ-CsPbI_(3),leading to alterations in its bandgap and crystal strain.In addition,by substituting amorphous barium titanium oxide(a-BaTiO_(3))for TiO_(2)as the electron transport layer,interfacial defects caused by imperfect energy levels between the electron transport layer and perovskite are reduced.High-resolution transmission electron microscopy and electron energy loss spectroscopy demonstrate that a-BaTiO_(3)forms entirely as a single phase,as opposed to Ba-doped TiO_(2)hybrid nanoclusters or separate domains of TiO_(2)and BaTiO_(3)phases.Accordingly,inorganic perovskite solar cells based on the a-BaTiO_(3)electron transport layer achieved a power conversion efficiency of 19.96%.

Formation of two-dimensional electron gas at AlGaN/GaN heterostructure and the derivation of its sheet density expression

Models for calculating the sheet densities of two-dimensional electron gas （2DEG） induced by spontaneous and piezoelectric polarization in A1GaN/GaN, A1GaN/A1N/GaN, and GaN/A1GaN/GaN heterostructures are provided. The detailed derivation process of the expression of 2DEG sheet density is given. A longstanding confusion in a very widely cited formula is pointed out and its correct expression is analyzed in detail.

Influence of a two-dimensional electron gas on current-voltage characteristics of Al_(0.3)Ga_(0.7) N/GaN high electron mobility transistors

The J-V characteristics of AltGa1 tN/GaN high electron mobility transistors（HEMTs） are investigated and simulated using the self-consistent solution of the Schro dinger and Poisson equations for a two-dimensional electron gas（2DEG） in a triangular potential well with the Al mole fraction t = 0.3 as an example.Using a simple analytical model,the electronic drift velocity in a 2DEG channel is obtained.It is found that the current density through the 2DEG channel is on the order of 10^13 A/m^2 within a very narrow region（about 5 nm）.For a current density of 7 × 10^13 A/m62 passing through the 2DEG channel with a 2DEG density of above 1.2 × 10^17 m^-2 under a drain voltage Vds = 1.5 V at room temperature,the barrier thickness Lb should be more than 10 nm and the gate bias must be higher than 2 V.

Pseudohalide induced tunable electronic and excitonic properties in two-dimensional single-layer perovskite for photovoltaics and photoelectronic applications

Two-dimensional(2D) layered organic-inorganic hybrid perovskites have attracted much more attention for some applications than their three-dimensional(3D) perovskite counterparts due to their promising thermal and moisture stabilities.In particular, the 2D perovskite devices have shown better promise for optoelectronic applications.However, tunability of optoelectronic properties is often demanded to improve the device performance.Herein, we adopt a newly method to tune the electronic properties of 2D perovskite by introducing pseudohalide into the structure.In this work, we designed a pseudohalidesubstituted 2D perovskite by substituting the out-of-plane halide with pseudohalide and studied the electronic and excitonic properties of 2D-BA2MX4 and 2D-BA2MX2Ps2(M=Ge^(2+), Sn^(2+), and Pb^(2+);X=I;Ps=NCO, NCS, OCN, SCN, Se CN).We revealed the dependence of electronic properties including band gaps, composition of band edges, bonding characteristics, work functions, effective masses, and exciton binding energies on different pseudohalides substituted in 2D perovskite.Our results indicate that the substitution of pseudohalide in 2D perovskites is energetically favorable and can significantly affect the bonding characteristics as well as the CBM and VBM that often play major role in determining their performance in optoelectronic devices.It is expected that the pseudohalide substitution will be helpful in developing more advanced optoelectronic device based on 2D perovskite by optimizing band alignment and promoting charge extraction.

Molecular design towards two-dimensional electron acceptors for efficient non-fullerene solar cells

Non-fullerene polymer solar cells(NF-PSCs) have gained wide attention recently. Molecular design of non-fullerene electron acceptors effectively promotes the photovoltaic performance of NF-PSCs. However,molecular electron acceptors with 2-dimensional(2 D) configuration and conjugation are seldom reported.Herein, we designed and synthesized a series of novel 2 D electron acceptors for efficient NF-PSCs. With rational optimization on the conjugated moieties in both vertical and horizontal direction, these 2 D electron acceptors showed appealing properties, such as good planarity, full-spectrum absorption, high absorption extinction coefficient, and proper blend morphology with donor polymer. A high PCE of 9.76%was achieved for photovoltaic devices with PBDB-T as the donor and these 2 D electron acceptors. It was also found the charge transfer between the conjugated moieties in two directions of these 2 D molecules contributes to the utilization of absorbed photos, resulting in an exceptional EQE of 87% at 730 nm. This work presents rational design guidelines of 2 D electron acceptors, which showed great promise to achieve high-performance non-fullerene polymer solar cells.

The low-temperature mobility of two-dimensional electron gas in AlGaN/GaN heterostructures

To reveal the internal physics of the low-temperature mobility of two-dimensional electron gas （2DEG） in Al- GaN/GaN heterostructures, we present a theoretical study of the strong dependence of 2DEG mobility on Al content and thickness of AlGaN barrier layer. The theoretical results are compared with one of the highest measured of 2DEG mobility reported for AlGaN/GaN heterostructures. The 2DEG mobility is modelled as a combined effect of the scat- tering mechanisms including acoustic deformation-potential, piezoelectric, ionized background donor, surface donor, dislocation, alloy disorder and interface roughness scattering. The analyses of the individual scattering processes show that the dominant scattering mechanisms are the alloy disorder scattering and the interface roughness scattering at low temperatures. The variation of 2DEG mobility with the barrier layer parameters results mainly from the change of 2DEG density and distribution. It is suggested that in AlGaN/GaN samples with a high Al content or a thick AlGaN layer, the interface roughness scattering may restrict the 2DEG mobility significantly, for the AlGaN/GaN interface roughness increases due to the stress accumulation in AlGaN layer.