Electron Drift Mobility Research Articles

Estimation of Electron Drift Mobility along the <i>c</i>-Axis in 4H-SiC by Using Vertical Schottky Barrier Diodes

Electron mobility along the c-axis is the most important in SiC because the current flows along this direction in vertical SiC devices. However, previous reports on the drift mobility along the c-axis are still limited because of the difficulty of sample preparation or analysis. In this study, the authors presented the method to estimate the electron drift mobility of a lightly-doped epitaxial layer by using SiC(0001) vertical Schottky barrier diodes (SBDs). For the analyses, the effects of current spreading and series resistance were carefully considered based on experimental results obtained from SBDs with various device parameters, leading to a more accurate estimation. The mobility along the c-axis was obtained as 1070 ± 290 cm2/Vs for a donor density of 1 × 1015 cm-3, and it was compared with the results by Hall effect measurement.

Electric field dependence of the electron drift velocity in n-type InxGa1-xAs1-yBiy epilayer

The effect of Bi incorporation into InxGa1-xAs lattice on the nanosecond pulsed electric field dependence of the drift velocity of electrons in n-type InxGa1-xAs1-yBiy alloys with various doping densities is investigated at room temperature. The electronic band structure of the alloys is calculated by Finite Element Methods (FEM). The electron temperature is determined from analytical modeling of the power dissipation mechanism. The drift velocity of the electrons (vdrift) does not saturate up to 5.4 kV/cm electric field for the lowest doping density sample. The sample is burned out at a higher electric field due to the impact ionization mechanism. However, the vdrift saturates at a high electric field region in the samples with higher doping densities, and the saturation of electric field values of vdrift depends on the doping density for these samples. The highest electron drift mobility (μdrift) is obtained as 6236 cm2/Vs for the sample with the lowest electron density. The heating of the only electrons through the applied electric field is not enough to initiate inter-valley transfer because of scatterings during transport. Hence, the electric field dependence on electron transport occurs only in the central valley. The effect of the scatterings on the electron transport is identified by considering ionized impurity and phonon scatterings.

La/Ge stacked nanosheets designed as optical resonators, microwave oscillators and 5 G/6 G gigahertz receivers

Glass/Ge and La/Ge stacked layers, 100 nm thick, were prepared via thermal evaporation under a vacuum pressure of 10−5 mbar. Structural analysis confirmed amorphous growth of Ge nanosheets. Optically coating Ge onto La improved light absorption, energy band gap, and dielectric responses. In addition, dielectric spectra fitting using Lorentz approach showed increased free electron density, plasmon frequency, and drift mobility (up to 1153 cm2/Vs) in Ge due to lanthanum presence. Moreover, LGA Schottky barrier devices (La/Ge/Au) exhibited tunneling-type current conduction with 0.815 eV barrier height and 40 nm width. Resistance spectra of these Schottky barriers displayed negative resistance near 0.40 GHz, while capacitance spectra showed resonance-antiresonance behavior. La/Ge and LGA interface can be utilized for optical receivers and microwave resonators in 5 G/6 G technologies.

Derivatives of Imidazole and Carbazole as Bifunctional Materials for Organic Light-Emitting Diodes.

New derivatives of carbazole and diphenyl imidazole for potential multiple applications were synthesized and investigated. Their properties were studied by thermal, optical, photophysical, electrochemical, and photoelectrical measurements. The compounds exhibited relatively narrow blue light-emission bands, which is favorable for deep-blue electroluminescent devices. The synthesized derivatives of imidazole and carbazole were tested as fluorescent emitters for OLEDs. The device showed deep-blue emissions with CIE color coordinates of (0.16, 0.08) and maximum quantum efficiency of 1.1%. The compounds demonstrated high triplet energy values above 3.0 eV and hole drift mobility exceeding 10-4 cm2/V·s at high electric fields. One of the compounds having two diphenyl imidazole moieties and tert-butyl-substituted carbazolyl groups showed bipolar charge transport with electron drift mobility reaching 10-4 cm2/V·s at electric field of 8 × 105 V/cm. The synthesized compounds were investigated as hosts for green, red and sky-blue phosphorescent OLEDs. The green-, red- and sky-blue-emitting devices demonstrated maximum quantum efficiencies of 8.3%, 6.4% and 7.6%, respectively.

Summary of the Basic Free Electron Transport Characteristics in Donor Doped Silicon

A present summary is assigned to present the transport characteristics of the free randomly moving (RM) electrons in silicon at any doping level by phosphorous donors. The application of the Fermi-Dirac statistics and stochastic description of the free RM electrons lead to obtaining the general expressions of conductivity, the effective density of the free RM electrons, their diffusion coefficient and the drift mobility, which are valid for silicon with any doping level. It is shown that drift mobility of the free RM electrons considerably exceeds the Hall mobility at heavy doping, and that the Einstein relation is fundamental and is conserved at any level of degeneracy. It is estimated what part of electrons in the conduction band of heavily doped silicon is not free and is coupled with phosphorous ions. The main conclusions and formulations can be applicable for holes in acceptor-doped silicon, and other homogeneous materials with one type of the free RM charge carriers as well.

Threading dislocation lines and their impact on bulk cubic boron nitride’s low-field electron transport response

Building upon Chilleri et al.’s [Solid State Commun. 352 (2022) 114776] relaxation-time approximation based low-field electron drift mobility formalism, we incorporate treatment of threading dislocation related scattering into this analytical framework. Then, for a reasonable range of threading dislocation line density selections, representative of boron nitride’s zinc-blende phase, the role that threading dislocation lines play in shaping the low-field electron transport response of this material is examined. The six scattering processes considered in this analysis include the ionized impurity, polar optical phonon, acoustic deformation potential, piezoelectric, inter-valley, and threading dislocation line related scattering processes. The impact of threading dislocations is assessed through evaluations of the scattering rates, the fractional scattering contributions, and the low-field electron drift mobility contributions associated with the various scattering processes. The dependence of the total low-field electron drift mobility on the threading dislocation line density is also examined. We conclude the analysis with a comparison with the results of experiment. Threading dislocation lines are found to be a significant contributor to bulk zinc-blende boron nitride’s low-field electron transport response.

High‐Field Electron‐Drift Velocity in n‐Type Modulation‐Doped GaAs0.96Bi0.04 Quantum Well Structure

The drift velocity (vdrift) of electrons in an n‐type modulation‐doped GaAs0.96Bi0.04/Al0.15Ga0.85As quantum well (QW) structure is determined for electric fields (F) ranging from ≈0.4 to 3.58 kV cm−1. The resulting vdrift characteristic exhibited a linear increase and reached ≈6 × 106 cm s−1 at low electric fields then almost saturated with increasing electric field. The electron drift mobility is determined as 2265 cm2 Vs−1 in the regime where the drift velocity is linear with respect to the electric field. The drift velocity saturates at ≈6.1 × 106 cm s−1 at the electric fields between ≈2.7 and 3.4 kV cm−1. Saturation of the drift velocity is attributed to the transfer of the electrons from the QW layer (GaAs0.96Bi0.04) with higher electron mobility to the barrier layer (Al0.15Ga0.85As) and satellite valley L‐valley with lower electron mobility, which initiates cooling of electrons via phonon scattering in the sample.

Charge transport mechanisms in free-standing devices with electrospun electrodes

Embedding electronic and optoelectronic devices in common, daily use objects is a fast developing field of research. New architectures are needed for migrating from the classic wafer- based substrates. Novel types of flexible PMMA/Au/Alq3/LiF/Al structures were obtained starting from electrospun polymer fibers. Thus, using an electrospinning process poly (methyl metacrylate) (PMMA) nanofibers were fabricated. A thin Au layer deposition rendered the fiber array conductive, this being further employed as the anode. The next steps consisted of the thermal evaporation of tris(8-hydroxyquinolinato) aluminum (Alq3) and aluminum deposition as the cathode. The Au covered PMMA nanofiber layer had a similar behavior with an indium tin oxide film i.e. low sheet resistance 10.6 Ω/sq and high transparency. The low electrode resistivities allow an electron drift mobility of about 10–6 cm2 V−1 s−1 at a low applied field, similar to the counterpart structures based on thin films. Concerning the relaxation processes in these structures, the Cole–Cole plots exhibit a slightly deformed semicircle, indicating a more complex equivalent circuit for the processes between metal electrodes and the active layer. This equivalent circuit includes reactance equivalent processes at the anode, cathode, in the active layer and most probably originates from the roughness of the metallic electrodes.

A low-field electron mobility analysis of cubic boron nitride

We examine zinc-blende boron nitride’s low-field electron transport response. In particular, within the framework of a relaxation-time approximation analysis, we probe how the individual scattering processes contribute to the total electron drift mobility. The scattering processes considered include the ionized impurity, polar optical phonon, acoustic deformation potential, piezoelectric, and inter-valley scattering processes. A comparison with the results of experiment is presented.

Vacancy-induced enhancement of electron–phonon coupling in cubic silicon carbide and its relationship to the two-temperature model

The electron–phonon coupling factor was calculated for both pristine and vacancy-rich 3C-SiC. Ab initio calculations were performed within the framework of the density functional perturbation theory. Wannier functions were used to interpolate eigenvalues into denser grids through the electron–phonon using Wannier code. The coupling factor was determined through calculations of the electron self-energy, electron–phonon relaxation time, and electronic specific heat. These parameters were extrapolated to high temperatures using a hybrid model which mixes band calculations for electrons below an energy cutoff with the free electron gas model for electrons above the energy cutoff. The electron relaxation times, specific heats, electron drift mobilities, and electron–phonon coupling factors were calculated as a function of electron temperature. Si and C vacancies were found to have a profound effect on electron–phonon coupling for all temperatures, while electronic specific heat capacity was found to be most affected at cryogenic temperatures. The electron drift mobility was calculated at different temperatures using the scattering time. Calculated mobilities were validated with Hall mobility measurements reported in the literature. The importance of structural defects on the electron–phonon coupling is discussed in the context of the two-temperature model, a model that has been widely used to understand aspects of the interaction of solids with pulsed laser irradiation and swift heavy ion irradiation.

Accurate Electron Drift Mobility Measurements in Moderately Dense Helium Gas at Several Temperatures

We report new accurate measurements of the drift mobility μ of quasifree electrons in moderately dense helium gas in the temperature range 26K≤T≤300K for densities lower than those at which states of electrons localized in bubbles appear. By heuristically including multiple-scattering effects into classical kinetic formulas, as previously done for neon and argon, an excellent description of the field E, density N, and temperature T dependence of μ is obtained. Moreover, the experimental evidence suggests that the strong decrease of the zero-field density-normalized mobility μ0N with increasing N from the low up to intermediate density regime is mainly due to weak localization of electrons caused by the intrinsic disorder of the system, whereas the further decrease of μ0N for even larger N is due to electron self-trapping in cavities. We suggest that a distinction between weakly localized and electron bubble states can be done by inspecting the behavior of μ0N as a function of N at intermediate densities.

Investigation of Photoexcitation Energy Impact on Electron Mobility in Single Crystalline CdTe.

The exceptional electronic properties of cadmium telluride (CdTe) allow the material to be used in a wide range of high energy radiation detection applications. Understanding the mechanisms of local carrier scattering is of fundamental importance to understand the charge transport in the material. Here, we investigate the effect of photoexcitation on electron transport properties in chlorine doped single crystalline cadmium telluride (SC-CdTe:Cl). For this purpose time of flight measurements were performed on SC-CdTe:Cl in order to study the electron drift mobility in the low injection regime. Measurements were made at the temperature intervals of 80 to 300 K, for an applied electric field between 270 and 1600 V/cm and for wavelengths of 532, 355 and 213 nm. We have found that the electron drift mobility was affected by the excitation energy for temperatures below 200 K. In addition, the measurements revealed that it is possible to determine impurity and shallow trap concentration by this method. The method proves to be extremely sensitive in measuring very low impurity levels and in identifying dominant scattering mechanisms.

Charge Transport and Space-Charge Formation in Cd1−xZnxTe1−ySey Radiation Detectors

The electron- and hole-transport properties in cadmium zinc telluride selenide (CZTS) crystals are studied using a laser-induced transient-current technique with pulsed and dc bias. The internal electric field profile and velocity of surface recombination are determined by Monte Carlo simulations of electron and hole transient currents combined with a numerical solution of the drift-diffusion equation coupled with Poisson's equation. Electron and hole drift mobilities of ${\ensuremath{\mu}}_{e}$ = 830 ${\mathrm{cm}}^{2}$/Vs and ${\ensuremath{\mu}}_{h}$ = 40 ${\mathrm{cm}}^{2}$/Vs, respectively, are determined. We also develop a simple technique for evaluating surface recombination directly from measured current waveforms without the need for numerical simulation. The good quality of the prepared detector at pulsed bias, with electron- and hole-mobility-lifetime products of $(\ensuremath{\mu}\ensuremath{\tau}{)}_{e}$ = 1.9 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{2}$/V and $(\ensuremath{\mu}\ensuremath{\tau}{)}_{h}$ = 1.4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{2}$/V, respectively, are observed. The formation of a positive space charge, originating from hole injection combined with a recombination level, is found. We observe a significant position dependence of the lifetime of electrons and holes in dc bias due to hole injection. The experiment is successfully fitted by a simple model dominated by a single deep recombination level with an energy of ${E}_{t}={E}_{C}\ensuremath{-}0.73\phantom{\rule{0.2em}{0ex}}\mathrm{eV}$; concentration of 7.3 \ifmmode\times\else\texttimes\fi{} ${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}3}$; and electron- and hole-capture cross sections of 3.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{2}$ and 6.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{2}$, respectively.

Calculation of Electron Drift Mobility at Low Electric Field in Hg1-xCdxTe Using Iterative Method

Calculation of Electron Drift Mobility at Low Electric Field in Hg_1-xCd_xTe Using Iterative Method

Triphenylethylene-based emitters exhibiting aggregation induced emission enhancement and balanced bipolar charge transport for blue non-doped organic light-emitting diodes

New triphenylethylene-based derivatives, containing 2,6-di-tert-butylcarbazole or 9,9-dimethyl-9,10-dihydroacridine as donor moieties and phenylene or tetrafluorophenylene linking bridges were synthesised by Heck coupling and characterized. The synthesized compounds emitted blue light in the solid state with emission maxima in the range of 478–487 nm and photoluminescence quantum yields of 16–29%. In thermogravimetric analysis they demonstrated sublimation with 5% weight loss temperatures in the range of 307–355 °C. They exhibited glass-forming properties with glass-transition temperatures of 89–123 °C. The solid-state ionization potentials and electrochemical band gaps ranged from 5.88 to 6.37 eV and from 3.30 to 3.57 eV, respectively. Three compounds demonstrated bipolar semiconducting properties. Derivative of triphenylethylene and 2,6-di-tert-butylcarbazole with phenylene bridge showed highest hole and electron drift mobilities reaching 3.07 × 10−3 cm2/Vs and 4.2 × 10−3 cm2/Vs respectively at high electric field. Utilizing compounds in organic light‐emitting diodes as emitters external quantum efficiency of 2% was observed.

Modeling novel effects in transient current measurements of single-crystal CVD diamond with carrier excitation by MeV α-particles

Temperature- and electric field-dependent transient current measurements with 4.6 MeV high-energy α-particle excitation were reported recently in commercially available CVD diamonds and were used to extract drift velocities and drift mobilities of electrons and holes [1–3]. These data showed unexpected characteristic temperature-dependent pulse current profiles which remained unexplained as well as other novel observations. Here we interpret all experimental findings by a detailed physical model. It demonstrates that the measured electron or hole currents consist of two components one of which is constant in intensity whereas the other is temperature-dependent due to thermal activation of carriers out of a low-temperature storage state shown to consist of electron-hole droplets (EHDs); this is a condensed phase of electrons and holes. EHDs form in regions with electrons and holes embedded between adjacent regions with electrons or holes alone. This local separation is caused by the electric field during carrier relaxation from the highly excited states. The model describes all experimental findings very satisfactorily, some of them quantitatively. Maxima in the transit time as a function of temperature were experimentally reported [2, 3] which successively become more pronounced at decreasing electric fields while shifting to lower temperatures. This observation is modeled by temperature-controlled re-population of the “slow” and “fast” conduction band valleys split apart by the applied electric field upon lifting the sixfold valley orientational degeneracy. The model suggests that the novel effects are characteristic of the very high excitation energy of the free excess carriers and would not be observable for near-band gap excitation.

Space charge formation in chromium compensated GaAs radiation detectors

Space charge formation in chromium-compensated GaAs sensors is investigated by the laser-induced transient current technique applying pulsed and DC bias. Formation of non-standard space charge manifested by an appearance of both negatively and positively charged regions in DC biased sensors was revealed during 5 ms after switching bias. Using Monte Carlo simulations of current transients we determined enhanced electron lifetime τ = 150 ns and electron drift mobility μd = 3650 cm2 Vs−1. We developed and successfully applied a theoretical model based on fast hole trapping in the system with spatially variable hole conductivity.

Prospects for high carrier mobility in the cubic germanates

High-mobility wide-band-gap oxides are essential materials for device applications in high-frequency and power electronics. We use first-principles calculations to evaluate the potential of two cubic perovskite germanates, SrGeO3 and BaGeO3, for these applications. We find their effective masses to be small, both for electrons and holes. The prospect of p-type conductivity renders the germanates attractive for complementary metal-oxide-semiconductor technology. We calculate the electron drift mobility in SrGeO3 to lie between 400 and 500 cm2V−1s−1 for a range of accessible carrier concentrations; this mobility is as much as 50% larger than in BaSnO3, which has the highest room-temperature electron mobility among perovskite oxides. Our results suggest that the germanates hold promise for high-mobility electronic applications.

Charge transport properties in CdZnTeSe semiconductor room-temperature γ-ray detectors

CdxZn1−xTeySe1−y (CZTS) has emerged as a next-generation compound semiconductor for high energy gamma-ray detection. In the present study, we report for the first time the hole transport property measurements in CZTS based gamma-ray detectors in the planar configuration. Current–voltage measurements revealed a bulk resistivity of ∼5 × 108 Ω cm and the fabricated detectors produced well-resolved 5486 keV alpha particle peaks, for both electrons and holes drifting alike, when pulse-height spectra (PHS) were recorded using a 241Am radiation source. The PHS measurements were enabled to measure the charge transport properties for both the charge carriers. The mobility–lifetime product (μτ) for electrons and holes was calculated to be 6.4 × 10−4 cm2 V−1 and 8.5 × 10−5 cm2 V−1, respectively, using a single polarity Hecht plot regression method. The pre-amplifier pulses were also recorded and processed digitally to obtain electron and hole drift mobilities of 692 cm2 V−1 s−1 and 55 cm2 V−1 s−1, respectively, using a time-of-flight method. The measured transport properties indicated the hole lifetime to be greater than the electron lifetime by a factor of ∼1.5. Gamma-ray PHS were recorded using fabricated detectors that showed tailing of the 662 keV photopeak due to hole trapping effects. Depth dependent PHS were digitally generated from 2D biparametric plots to reveal the effects of hole trapping on the gamma PHS at different detector depths. Digital correction procedures were applied to generate well-resolved PHS with an energy resolution of ∼2% for 662 keV γ-rays.

Simultaneous determination of electron and hole drift mobilities in working inverted organic solar cells: modulated photocurrent spectroscopy versus impedance spectroscopy

Applicability of modulated photocurrent (MPC) spectroscopy and impedance spectroscopy (IS) were examined both experimentally and numerically to simultaneous determination of electron and hole drift mobilities in working prototypical inverted organic photovoltaics (OPVs) of poly(3-hexylthiophene) (P3HT):1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) bulk heterojunction (BHJ). Charge carriers were photoinjected in MPC measurements and were injected from electrodes in IS. The P3HT:PCBM composition of the BHJ was chosen so as to observe both electron and hole transits separately. Electron and hole transits were simultaneously observed in the inverted OPVs by means of MPC measurements, while only hole transit was observed by means of IS. The device simulation shows that the reason that electron transit was not observed in the inverted OPVs by means of IS is due to the electron injection barrier of >0.2 eV inevitably existed in the inverted OPVs.