High-field Transport Properties Research Articles

High-Field Electron Transport and High Saturation Velocity in Multilayer Indium Selenide Transistors.

Creating a high-frequency electron system demands a high saturation velocity (υsat). Herein, we report the high-field transport properties of multilayer van der Waals (vdW) indium selenide (InSe). The InSe is on a hexagonal boron nitride substrate and encapsulated by a thin, noncontinuous In layer, resulting in an impressive electron mobility reaching 2600 cm2/(V s) at room temperature. The high-mobility InSe achieves υsat exceeding 2 × 107 cm/s, which is superior to those of other gapped vdW semiconductors, and exhibits a 50-60% improvement in υsat when cooled to 80 K. The temperature dependence of υsat suggests an optical phonon energy (ℏωop) for InSe in the range of 23-27 meV, previously reported values for InSe. It is also notable that the measured υsat values exceed what is expected according to the optical phonon emission model due to weak electron-phonon scattering. The superior υsat of our InSe, despite its relatively small ℏωop, reveals its potential for high-frequency electronics, including applications to control cryogenic quantum computers in close proximity.

High-field charge transport and noise in p -Si from first principles

The parameter-free computation of charge transport properties of semiconductors is now routine owing to advances in the ab-initio description of the electron-phonon interaction. Many studies focus on the low-field regime in which the carrier temperature equals the lattice temperature and the current power spectral density (PSD) is proportional to the mobility. The calculation of high-field transport and noise properties offers a stricter test of the theory as these relations no longer hold, yet few such calculations have been reported. Here, we compute the high-field mobility and PSD of hot holes in silicon from first principles at temperatures of 77 and 300 K and electric fields up to 20 kV cm$^{-1}$ along various crystallographic axes. We find that the calculations quantitatively reproduce experimental trends including the anisotropy and electric-field dependence of hole mobility and PSD. The experimentally observed rapid variation of energy relaxation time with electric field at cryogenic temperatures is also correctly predicted. However, as in low-field studies, absolute quantitative agreement is in general lacking, a discrepancy that has been attributed to inaccuracies in the calculated valence band structure. Our work highlights the use of high-field transport and noise properties as a rigorous test of the theory of electron-phonon interactions in semiconductors.

TCAD Modeling of High-Field Electron Transport in Bulk Wurtzite GaN: The Full-Band SHE-BTE

Gallium Nitride (GaN) High-Electron Mobility Transistors (HEMTs) actually represent one of the best candidates for medium-high power and radio frequency applications. As they operate at large bias and electric fields, a comprehensive analysis of the high-field transport properties is fundamentals, as hot electrons are expected to play a relevant role for the device reliability. In this perspective, Technology Computer-Aided Design (TCAD) simulations can be a very useful tool for the understanding of the phenomena dominating hot-electron degradation mechanisms. The most-accurate modeling approaches are based on the direct solution of the Boltzmann equation, which is not actually available for the GaN material. In this work, the deterministic solution of the Boltzmann transport equation via the spherical-harmonics expansion (SHE-BTE), as incorporated in a commercial TCAD tool, has been extended to the analysis of GaN electrons. To this purpose, the details of the full-band structure has been derived from DFT calculations as in state-of-art literature works, and the electron density of states, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$g(E)$ </tex-math></inline-formula> , and group velocity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$u_{g}(E)$ </tex-math></inline-formula> , have been calculated for the SHE-BTE for the first time. In addition to this, an accurate calibration of the total scattering rate accounting for nonpolar acoustic and optical carrier-phonon interaction, Coulomb scattering and impact ionization has been carried out against available Monte Carlo data and experiments. The proposed model is also shown to correctly predict the temperature dependence of the electron impact-ionization coefficient and current density up to breakdown.

Simulation analysis of high-field carrier transport in wide-bandgap semiconductors considering tunable band structures and scattering processes

Though the high breakdown electric field of wide-bandgap semiconductors is usually attributed to their large bandgap, the impacts of other band structure parameters and scattering processes on impact ionization phenomena have not been clarified yet. This study computationally analyzes the effects of band structures and scattering rates on the high-field carrier transport properties such as impact ionization coefficients and drift velocity in wide-bandgap semiconductors. For that purpose, this study adopts Monte Carlo simulations in which the E–k dispersion and scattering rates are directly tuned. Simulations with varied band structures indicate that an average of the group velocity in the whole Brillouin zone is a dominant factor determining the impact ionization coefficients rather than the effective mass at the band edge. In addition, the Brillouin zone width has critical impacts when Bloch oscillations occur, which significantly suppress impact ionization. As for scattering mechanisms, the roles of inelastic scattering processes including impact ionization in energy relaxation are discussed. It is also revealed that elastic scattering contributes to energy relaxation processes through transitions of electrons to higher bands. This mechanism leads to the unintuitive positive temperature dependence of impact ionization coefficients when Bloch oscillations occur. These results obtained by the theoretical analyses in this study can serve as basic physical insight to understand the behaviors of impact ionization coefficients in wide-bandgap semiconductors.

Investigating the high field transport properties of Janus WSSe and MoSSe by DFT analysis and Monte Carlo simulations

Janus transition metal dichalcogenides with out-of-plane structural asymmetry have attracted increasing attention due to their exceptional potential in electronic and optical applications. In this study, we systematically investigated the electron–phonon interactions and related transport properties in monolayer Janus MoSSe and WSSe using the density-functional formalism. The electron–phonon scattering rates were obtained using Fermi’s golden rule and extended to the extraction of the effective deformation potential constants for further Monte Carlo treatment. From the results of the Monte Carlo analysis, we found that WSSe provides better performance with higher low-field mobility, while MoSSe shows a higher peak velocity at higher fields. In our results, both MoSSe and WSSe seem to be competitive with other previously studied 2D materials. These predictions provide a systematic perspective on the potential of Janus WSSe and MoSSe for electronic applications.

High electric field transport in GaAs0.51Sb0.49

The semiconductor alloy GaAsSb is commonly found in many types of semiconductor devices, ranging from high electron mobility transistors to solar cells. Yet, surprisingly little is known about its transport properties. Here, we theoretically determine the high field transport properties of electrons and holes in the alloy GaAs0.51Sb0.49 that is lattice matched to InP and, in particular, is used in a great many of these types of semiconductor devices.

Understanding high-field electron transport properties and strain effects of monolayer transition metal dichalcogenides

Monolayer transition metal dichalcogenides (MX2) are promising candidates for future electronics. Although the transport properties (e.g. mobility) at low electric field have been widely studied, there are limited studies on high-field properties, which are important for many applications. Particularly, there is lack of understanding of the physical origins underlying the property differences across different MX2. Here by combining first-principles calculations with Monte Carlo simulations, we study the high-field electron transport in defects-free unstrained and tensilely strained MX2 (M=Mo, W and X=S, Se). We find that WS2 has the highest peak velocity (due to its smallest effective mass) that can be reached at the lowest electric field (owing to its highest mobility). Strain can increase the peak velocity by increasing the scattering energy. After reaching the peak velocity, most MX2 demonstrates negative differential mobility (NDM). WS2 shows the largest NDM among unstrained MX2 due to the strongest effect of electron transfer from the low-energy small-mass valley to the high-energy large-mass valley. The tensile strain increases the valley separation, which on one hand suppresses the electron transfer in WS2, on the other hand allows the electrons to access the non-parabolic band region of the low-energy valley. The latter effect leads to an NDM for electrons in the low-energy valley, which can significantly increase the overall NDM at moderate strain. The valley-separation induced NDM in the low-energy valley is found to be a general phenomenon. Our work unveils the physical factors underlying the differences in high-field transport properties of different MX2, and also identifies the most promising candidate as well as effective approach for further improvement.

Limitations of ab initio methods to predict the electronic-transport properties of two-dimensional semiconductors: the computational example of 2H-phase transition metal dichalcogenides

Over the last few years, ab initio methods have become an increasingly popular tool to evaluate intrinsic carrier transport properties in 2D semiconductors. The lack of experimental information, and the progress made in the development of DFT tools to evaluate electronic band structures, phonon dispersions, and electron–phonon scattering matrix-elements, have made them a favored choice. However, a large discrepancy is observed in the literature among the ab initio calculated carrier mobility in 2D semiconductors. Some of the discrepancies are a result of the physical approximations made in calculating the electron–phonon coupling constants and the carrier mobility. These approximations can be avoided by using a sophisticated transport model. However, despite using appropriate transport models, the uncertainty in the reported carrier mobility is still quite large in some materials. The major differences observed between these refined model calculations are the ‘flavors’ of DFT (exchange-correlation functional, pseudopotential, and the effect of spin-orbit coupling) used. Here, considering several monolayer 2H-TMDs as examples, we calculate the low- and high-field transport properties using different ‘flavors’ of DFT, and calculate a range for the electron mobility values. We observe that in some materials the values differ by orders of magnitude (For example, in monolayer WS2 the electron low-field mobility varies between 37 $$\hbox {cm}^{2}/(\hbox {V}\,\hbox {s}$$ ) and 767 $$\hbox {cm}^{2}/(\hbox {V}\,\hbox {s}$$ )). We analyze critically these discrepancies, and try to understand the limitations of the current ab initio methods in calculating carrier transport properties.

Velocity saturation in La-doped BaSnO3 thin films

BaSnO3, a high mobility perovskite oxide, is an attractive material for oxide-based electronic devices. However, in addition to low-field mobility, high-field transport properties such as the saturation velocity of carriers play a major role in determining the device performance. We report on the experimental measurement of the electron saturation velocity in La-doped BaSnO3 thin films for a range of doping densities. The predicted saturation velocities based on a simple LO-phonon emission mode, using an effective LO phonon energy of 120 meV show good agreement with the measurements of velocity saturation in La-doped BaSnO3 films. Density-dependent saturation velocity in the range of 1.8 × 107 cm/s reducing to 2 × 106 cm/s is predicted for δ-doped BaSnO3 channels with carrier densities ranging from 1013 cm−2 to 2 × 1014 cm−2, respectively. These results are expected to aid the informed design of BaSnO3 as an active material for high-charge density electronic transistors.

Theory of High Field Transport in β-Ga2O3

We present a comprehensive review of high-field transport properties in an emerging and trending ultra-widebandgap semiconductor β-Ga2O3. The focus is on the theoretical understanding of the microscopic mechanisms that control the dynamics of farfrom-equilibrium electrons. A manifold of density functional calculations and Boltzmann theory based transport formalism unravels the behavior of the electron distribution under a varied range of external electric fields. The key high-field transport properties that govern electronic device performance, like velocity and ionization co-efficients, are enlightened in detail with physical insights. Anisotropies in the above transport co-efficients are probed from the microscopic investigation of bandstructure, electron-phonon interactions, and electron-electron interactions.

K-space imaging of anisotropic 2D electron gas in GaN/GaAlN high-electron-mobility transistor heterostructures

Nanostructures based on buried interfaces and heterostructures are at the heart of modern semiconductor electronics as well as future devices utilizing spintronics, multiferroics, topological effects, and other novel operational principles. Knowledge of electronic structure of these systems resolved in electron momentum k delivers unprecedented insights into their physics. Here we explore 2D electron gas formed in GaN/AlGaN high-electron-mobility transistor heterostructures with an ultrathin barrier layer, key elements in current high-frequency and high-power electronics. Its electronic structure is accessed with angle-resolved photoelectron spectroscopy whose probing depth is pushed to a few nanometers using soft-X-ray synchrotron radiation. The experiment yields direct k-space images of the electronic structure fundamentals of this system—the Fermi surface, band dispersions and occupancy, and the Fourier composition of wavefunctions encoded in the k-dependent photoemission intensity. We discover significant planar anisotropy of the electron Fermi surface and effective mass connected with relaxation of the interfacial atomic positions, which translates into nonlinear (high-field) transport properties of the GaN/AlGaN heterostructures as an anisotropy of the saturation drift velocity of the 2D electrons.

MgB2 wire diameter reduction by hot isostatic pressing—a route for enhanced critical current density

The effect of wire diameter reduction on the critical current density of pristine MgB2 wire was studied. Wires were treated by a hot isostatic pressing method at 570 °C and at pressures of up to 1.1 GPa. It was found that the wire diameter reduction induces an increase of up to 70% in the mass density of the superconducting cores. This feature leads to increases in critical current, critical current density, and pinning force density. The magnitude and field dependence of the critical current density are related to both grain connectivity and structural defects, which act as effective pinning centers. High field transport properties were obtained without doping of the MgB2 phase. A critical current density jc of 3500 A mm−2 was reached at 4 K, 6 T for the best sample, which was a five-fold increase compared to MgB2 samples synthesized at ambient pressure.

(Invited) Simulation Study of High Voltage Vertical GaN Nanowire Field Effect Transistors

A Nanostructured GaN Device Architecture for Power Applications Z. John Shen, Gourab Sabui, Vitaly Z. Zubialevich, Mary White, Pietro Pampili, Peter J. Parbrook, Mathew McLaren, Miryam Arredondo-Arechavala Gallium nitride (GaN) has emerged as a promising material for development of power semiconductor devices owing to its high critical electric field, large bandgap and superior transport properties. The unique properties of GaN have prompted realization of power devices both in a lateral (high electron mobility transistors (HEMTs)) and a vertical layout (bulk GaN on GaN devices). GaN power devices show extremely low conduction and switching losses and holds the key to extremely low-loss and high-efficiency power converters of the future. However, GaN power devices have been plagued with several inherent drawbacks preventing a ubiquitous adoption of GaN as the material of choice for power switches. The most critical trade-off has been the choice of substrate for the growth of GaN epitaxy: a high performance, high-cost native substrate against a low-cost, non-native substrate with associated reliability issues. For GaN to thrive as a superior successor to Si, a low cost, high performance epitaxy with improved reliability is expected moving forward. Current lateral HEMTs and vertical GaN on GaN devices are inadequate to address this trade-off. A novel nanostructured approach to GaN power devices is proposed in this paper. The unique property of GaN nanowires is its ability to elastically relax in the lateral direction, and accommodate lattice mismatch with non-native substrate through pseudomorphic growth without dislocation formation as compared to thin-film heterostructures. Experimental studies have shown self-induced GaN NWs grown with a high surface to volume ratio on a Si substrate have a non-polar side face and is virtually free of threading dislocations and other structural defects. Power devices grown on a nano-GaN epitaxy theoretically have the potential to bypass the reliability concerns associated with a non-native substrate but still deliver comparable or even superior performance, albeit at a low-cost. Schottky barrier diodes are designed and fabricated based on the nano-GaN concept. Fabricated diodes grown on a Sapphire substrate show distinct rectifying properties blocking voltages of 100 V. Despite repetitive biasing, the devices did not show any sign of current collapse. Design optimization of the nanowire Schottky barrier diode (NWSBD) is performed using 3D TCAD drift-diffusion simulations and semiconductor-oxide interaction was utilized to push the performance of the NWSBDs beyond the unipolar limit of GaN. The optimized NWSBD show the potential to block voltages upwards of 650 V working under present fabrication constraints of doping concentrations and pillar height. Simulated characteristics predict superior VRB 2/RON FoM, outperforming both the lateral HEMTs and the vertical GaN on GaN devices. A fully controlled, three terminal nanowire field effect transistor (NWFET) is also investigated using 3D TCAD drift-diffusion modeling. The NWFET can be operated in both normally-off and normally-on modes based on design and requirements. The 3D gate around the nanowires coupled with a strong dielectric REduced SURface Field (RESURF) effect allows this architecture to block voltages beyond the 1D unipolar material limit of GaN in the reverse bias. With proper design, highly desirable linear voltage to pillar height scaling can be achieved for the NWFET. Breakdown voltages of more than 900 V is predicted, with superior VRB 2/RON and QGD x RDS(ON) figure of merits. Despite lacking a body-diode, the NWFETs being quasi-symmetrical enables current conduction in the 3rd quadrant itself by self-commutated reverse conduction with a very low voltage drop. The nanowire architecture holds great potential to produce power rectifiers and controlled FETs with high performance, improved reliability and ruggedness and in a cost-effective way.

High-field transport properties of a P-doped BaFe2As2 film on technical substrate

High temperature (high-Tc) superconductors like cuprates have superior critical current properties in magnetic fields over other superconductors. However, superconducting wires for high-field-magnet applications are still dominated by low-Tc Nb3Sn due probably to cost and processing issues. The recent discovery of a second class of high-Tc materials, Fe-based superconductors, may provide another option for high-field-magnet wires. In particular, AEFe2As2 (AE: Alkali earth elements, AE-122) is one of the best candidates for high-field-magnet applications because of its high upper critical field, Hc2, moderate Hc2 anisotropy, and intermediate Tc. Here we report on in-field transport properties of P-doped BaFe2As2 (Ba-122) thin films grown on technical substrates by pulsed laser deposition. The P-doped Ba-122 coated conductor exceeds a transport Jc of 105 A/cm2 at 15 T for main crystallographic directions of the applied field, which is favourable for practical applications. Our P-doped Ba-122 coated conductors show a superior in-field Jc over MgB2 and NbTi, and a comparable level to Nb3Sn above 20 T. By analysing the E − J curves for determining Jc, a non-Ohmic linear differential signature is observed at low field due to flux flow along the grain boundaries. However, grain boundaries work as flux pinning centres as demonstrated by the pinning force analysis.

On the nature of high field charge transport in reinforced silicone dielectrics: Experiment and simulation

The high field charge injection and transport properties in reinforced silicone dielectrics were investigated by measuring the time-dependent space charge distribution and the current under dc conditions up to the breakdown field and were compared with the properties of other dielectric polymers. It is argued that the energy and spatial distribution of localized electronic states are crucial in determining these properties for polymer dielectrics. Tunneling to localized states likely dominates the charge injection process. A transient transport regime arises due to the relaxation of charge carriers into deep traps at the energy band tails and is successfully verified by a Monte Carlo simulation using the multiple-hopping model. The charge carrier mobility is found to be highly heterogeneous due to the non-uniform trapping. The slow moving electron packet exhibits a negative field dependent drift velocity possibly due to the spatial disorder of traps.

Monte Carlo Investigation of High-Field Electron Transport Characteristics in ZnMgO/ZnO Heterostructures

The high-field electron transport properties of ZnMgO/ZnO heterostructures considering multisubband conduction were systematically investigated with an ensemble Monte Carlo program developed by us. Electronic band structures were obtained by first-principles calculations using density functional theory, which provides more accurate parameters to calculate the electronic states and transport characteristics of 2-D electron gas. Wave functions and energy levels describing the subband structure were achieved to determine the 2-D scattering rates by solving the Schrodinger and Poisson equations self-consistently. The highest electron mobility gained at room temperature was approximately $4.23\times 10^{\mathrm {2}}$ cm2/Vs. The negative differential mobility was observed with a threshold electric field of approximately 80 kV/cm. The peak drift velocity of the Zn0.694Mg0.306O/ZnO heterostructure at 200 KV was about $2.011\times 10^{8}$ cm/s. This paper benefits the development of high-performance ZnMgO/ZnO heterostructure devices.

Analysis of high-field hole transport in germanium and silicon nanowires based on Boltzmann’s transport equation

In this paper, high-field hole transport in germanium nanowires was studied by using Boltzmann's transport equation in an atomistic framework. The scattering mechanisms taken into account are phonon and surface roughness. The hole drift velocities of [110], [111], and [112]-oriented germanium nanowires showed negative-differential characteristics, while that of the [001] nanowire did not. The behavior of hole drift velocity was analyzed based on the highly nonparabolic and orientation-dependent valence band structure. High-field hole transport properties in silicon nanowires were also calculated, and the differences between germanium and silicon nanowires were discussed, focusing mainly on momentum and energy relaxation times. The [110], [111], and [112] silicon nanowires showed faster hole drift velocity at high field than the germanium nanowires with the same orientation. This was attributed to faster energy relaxation in silicon nanowires, which mitigates the negative differential mobility in silicon nanowires compared to germanium nanowires.

High-field thermal transport properties of REBCO coated conductors

The use of REBCO coated conductors (CCs) is envisaged for many applications, extending from power cables to high-field magnets. Whatever the case, thermal properties of REBCO tapes play a key role for the stability of superconducting devices. In this work, we present the first study on the longitudinal thermal conductivity (κ) of REBCO CCs in magnetic fields up to 19 T applied both parallel and perpendicularly to the thermal-current direction. Copper-stabilized tapes from six industrial manufacturers have been investigated. We show that zero-field κ of CCs can be calculated with an accuracy of from the residual resistivity ratio of the stabilizer and the Cu/non-Cu ratio. Measurements performed at high fields have allowed us to evaluate the consistency of the procedures generally used for estimating in-field κ in the framework of the Wiedemann–Franz law from an electrical characterization of the materials. In-field data are intended to provide primary ingredients for the thermal stability analysis of high-temperature superconductor-based magnets.

Theory of Carriers Transport in III-Nitride Materials: State of the Art and Future Outlook

In this paper, we describe the state of the art in the numerical simulation of the carrier transport properties of GaN and its ternary alloys. We outline the characteristics of our state of the art full-band Monte Carlo model that includes a fitting parameter free approach to compute the carrier-phonon interaction and a full quantum mechanical model for multiband transport which is critical to understand the high-field transport properties of these materials. Finally, we provide several examples of applications of the model to the calculation of the low-field electron mobility of GaN and In0.18Al0.82N, drift velocity in GaN and the impact ionization coefficients in AlxGa1-xN alloys.

High field transport properties of a bilayer graphene

The high electric field transport properties namely, hot electron energy loss rate P, momentum loss rate Q, electron temperature Te and drift velocity Vd are studied theoretically in a bilayer graphene (BLG) by employing the momentum and energy balance technique. P and Q are investigated as a function of Te by considering the electron interaction with the acoustic phonons (APs) and the surface polar phonons (SPPs). In the Bloch–Grüneisen regime P (Q) due to APs is ~Te4 (Te2.5), with a new feature of a kink appearing due to the chiral nature of the electrons. The predicted Te4 is consistent with the recent experimental observation of heat resistance (Yan et al. Nature Nanotechnology 3 (2012) 472 [35]). Hot phonon effect is taken into account for SPPs. A dip has been observed in the hot phonon distribution of SPPs, a new feature, which is not found in conventional two-dimensional electron gas, and this can be attributed to the chiral nature of the electrons. P (Q) due to SPPs is found to be dominant at about Te>150 (180)K for a lattice temperature T=4.2K. It is observed that the hot phonon effect is found to reduce P and Q due to SPPs significantly. Te and Vd are calculated as a function of the electric field E by taking into account the additional channels for momentum relaxation due to Coulomb impurity (CI) and short-range disorder (SD). Te is found to increase with the increasing electric field and is significantly enhanced by the hot phonon effect. Low field Vd is found to be limited by CI, SD and APs and in the high field region it reaches a near saturation value. The hot phonon effect tends to reduce the value of Vd. The presence of disorders CI and SD reduces Vd significantly and in clean samples larger saturation velocity can be achieved at a relatively smaller E.