Carrier Scattering Research Articles

Suppressed Carrier Scattering in CdS-Encapsulated PbS Nanocrystal Films

One of the key challenges facing the realization of functional nanocrystal devices concerns the development of techniques for depositing colloidal nanocrystals into electrically coupled nanoparticle solids. This work compares several alternative strategies for the assembly of such films using an all-optical approach to the characterization of electron transport phenomena. By measuring excited carrier lifetimes in either ligand-linked or matrix-encapsulated PbS nanocrystal films containing a tunable fraction of insulating ZnS domains, we uniquely distinguish the dynamics of charge scattering on defects from other processes of exciton dissociation. The measured times are subsequently used to estimate the diffusion length and the carrier mobility for each film type within the hopping transport regime. It is demonstrated that nanocrystal films encapsulated into semiconductor matrices exhibit a lower probability of charge scattering than that of nanocrystal solids cross-linked with either 3-mercaptopropionic acid or 1,2-ethanedithiol molecular linkers. The suppression of carrier scattering in matrix-encapsulated nanocrystal films is attributed to a relatively low density of surface defects at nanocrystal/matrix interfaces.

Scattering of carriers by charged dislocations in semiconductors

The scattering of carriers by charged dislocations in semiconductors is studied within the framework of the linearized Boltzmann transport theory with an emphasis on examining consequences of the extreme anisotropy of the cylindrically symmetric scattering potential. A new closed-form approximate expression for the carrier mobility valid for all temperatures is proposed. The ratios of quantum and transport scattering times are evaluated after averaging over the anisotropy in the relaxation time. The value of the Hall scattering factor computed for charged dislocation scattering indicates that there may be a factor of two error in the experimental mobility estimates using the Hall data. An expression for the resistivity tensor when the dislocations are tilted with respect to the plane of transport is derived. Finally, an expression for the isotropic relaxation time is derived when the dislocations are located within the sample with a uniform angular distribution.

Parameters of the Constant-Energy Surface and Features of Charge Carrier Scattering of Bi2Te3-Based Epitaxial Films

Galvanomagnetic properties of epitaxial Bi0.5Sb1.5Te3 films grown by the hot-wall technique were investigated in magnetic fields of H = 5 T to 14 T through the temperature interval of 10 K to 300 K. The results were analyzed in terms of a many-valley model of the energy spectrum and anisotropic charge carrier scattering. The degeneracy parameter βd, and ratios of components of the reciprocal effective-mass tensor \( {\mathord{\scriptscriptstyle\leftrightarrow}\over \alpha} \) and relaxation time tensor \( {\mathord{\scriptscriptstyle\leftrightarrow}\over \tau } (\varepsilon ) \) were estimated. Substantial anisotropy of charge carrier scattering was observed in the investigated temperature interval. Anisotropy of charge carrier scattering along bisector axes is enhanced, as compared with that of corresponding bulk samples. High charge carrier mobility and higher angular factor of the temperature dependence of mobility promote enhancement of the figure of merit Z of the films as compared with that of corresponding bulk solid solutions.

Atomic-Scale Evidence for Potential Barriers and Strong Carrier Scattering at Graphene Grain Boundaries: A Scanning Tunneling Microscopy Study

We use scanning tunneling microscopy and spectroscopy to examine the electronic nature of grain boundaries (GBs) in polycrystalline graphene grown by chemical vapor deposition (CVD) on Cu foil and transferred to SiO(2) substrates. We find no preferential orientation angle between grains, and the GBs are continuous across graphene wrinkles and SiO(2) topography. Scanning tunneling spectroscopy shows enhanced empty states tunneling conductance for most of the GBs and a shift toward more n-type behavior compared to the bulk of the graphene. We also observe standing wave patterns adjacent to GBs propagating in a zigzag direction with a decay length of ~1 nm. Fourier analysis of these patterns indicates that backscattering and intervalley scattering are the dominant mechanisms responsible for the mobility reduction in the presence of GBs in CVD-grown graphene.

Total Ionizing Dose Induced Charge Carrier Scattering in Graphene Devices

We investigate total ionizing dose (TID) effects in graphene field effect transistors comprised of chemical vapor deposition grown graphene transferred onto trimethylsiloxy(TMS)-passivated SiO2 Si substrates. TID exposure with a positive gate bias increases the concentration of positive oxide trapped charges near the SiO2/TMS/graphene interface making Coulomb-potential scatterer limited mobility more apparent. In particular, we observe asymmetric degradation in electron and hole mobility, the former degrading more rapidly. Consistent with the electron-hole puddle description, we observe an increase in intrinsic electron carrier density that varies linearly with the oxide trapped charge density, while the hole carrier density remains largely unaltered. These effects give rise to an increasing minimum conductivity.

Grain Boundary Carrier Scattering in ZnO Thin Films: a Study by Temperature-Dependent Charge Carrier Transport Measurements

In the present investigation, the effects of annealing in oxygen atmosphere on the electrical and optical properties of pulsed direct-current (DC) magnetron-sputtered ZnO thin films have been studied. With annealing, the electrical conductivity was found to increase from 2.3 S cm to 123 S cm. The optical transparency was also found to improve from 83% to 90%. The improvement in the electrical properties with annealing is attributed to the reduction of the grain boundary potential from 34 meV to 8 meV. X-ray photoelectron spectroscopy (XPS) measurements revealed oxygen deficiency in as-deposited films, whereas adequate incorporation of oxygen was observed in the annealed films. Ultraviolet photoelectron spectroscopy (UPS) measurements showed a shift of +0.8 eV in the Fermi level with annealing. This shift indicates significant changes in the electrical conductivity with annealing, which is due to an increase in the carrier concentration.

Effect of Phase Transition on the Thermoelectric Properties of Ag<sub>2</sub>Te

Ag2Te is well-known as a silver ion conductor. In this compound, a phase transition occurs at around 420K and silver ions jump to interstitial sites repeatedly at the point of starting the phase transition. We consider that the active movement of silver ions would have influence on the scatterings of both charge carriers and heat carrying phonons in Ag2Te. In order to evaluate the effect of the silver ion conduction on the thermoelectric properties of Ag2Te, the Seebeck coefficient, electrical resistivity, thermal conductivity and Hall coefficient of polycrystalline bulk samples of Ag2Te were examined in the temperature range from room temperature to 650K. The electrical resistivity and the Seebeck coefficient dramatically changed at around 420K due to the change of the carrier concentration before and after the phase transition. However, the carrier mobility and the lattice thermal conductivity showed no remarkable change at around the phase transition temperature. These results imply that the thermoelectric transport properties of Ag2Te were affected by the change of the crystal structure rather than the presence or absence of the movement of silver ions. [doi:10.2320/matertrans.E-M2012815]

Carrier Scattering from Dynamical Magnetoconductivity in Quasineutral Epitaxial Graphene

The energy dependence of the electronic scattering time is probed by Landau level spectroscopy in quasineutral multilayer epitaxial graphene. From the broadening of overlapping Landau levels we find that the scattering rate 1/τ increases linearly with energy ϵ. This implies a surprising property of the Landau level spectrum in graphene-the number of resolved Landau levels remains constant with the applied magnetic field. Insights are given about possible scattering mechanisms and carrier mobilities in the graphene system investigated.

Quantum and classical thermoelectric transport in quantum dot nanocomposites

Quantum dot nanocomposites are potentially high-efficiency thermoelectric materials, which could outperform superlattices and random nanocomposites in terms of manufacturing cost-effectiveness and material properties because of the reduction of thermal conductivity due to the phonon-interface scattering, the enhancement of Seebeck coefficient due to the formation of minibands, and the enhancement of electrical conductivity due to the phonon-bottleneck effect in electron-phonon scattering for quantum-confined electrons. In this paper, we investigate the thermoelectric transport properties of quantum dot nanocomposites through a two-channel transport model that includes the transport of quantum-confined electrons through the hopping mechanism and the semiclassical transport of bulk-like electrons. For the quantum-confined electrons whose wave functions are confined in the quantum dots with overlapping tail extending to the matrix, we develop a tight-binding model together with the Kubo formula and the Green’s function method to describe the transport processes of these electrons. The formation of minibands due to the quantum confinement and the phonon-bottleneck effect on carrier-phonon scattering are considered. For transport of bulk-like electrons, a Boltzmann-transport–equation-based semiclassical model is used to describe the multiband transport processes of carriers. The intrinsic carrier scatterings as well as the carrier-interface scattering of these bulk-like electrons are considered. We then apply the two-channel transport model to predict thermoelectric transport properties of n-type PbSe/PbTe quantum dot nanocomposites with PbSe quantum dots uniformly embedded in the PbTe matrix. The dependence of thermoelectric transport coefficients on the size of quantum dots, interdot distance, doping concentration, and temperature are studied in detail. Due to the formation of minibands and the phonon-bottleneck effect on carrier-phonon scattering, we show that simultaneous enhancement of electrical conductivity and Seebeck coefficient can be realized in quantum dot nanocomposites. Our study could shed some light on the design of high-efficiency thermoelectric materials for energy conversion and thermal management.

Scattering of current carriers on a Mo(110) surface covered with hydrogen and deuterium submonolayers

The paper reports the experimental study of current carrier transitions between surface and bulk energy states and their influence on the magneto-resistances of an atomically pure Mo(110) plate and a similar plate covered with adsorbed hydrogen and deuterium monolayer films. Effect of adsorbate submonolayer ordering on the surface scattering of current carriers is investigated. The experiment employs the static skin effect technique under ultrahigh vacuum conditions.

BiTe 系熱電材料における散乱因子による熱電特性向上と n 型 ZnSb 熱電材料

BiTe 系熱電材料における散乱因子による熱電特性向上と n 型 ZnSb 熱電材料

Evaluation of Charge Mobility in Organic Materials: From Localized to Delocalized Descriptions at a First‐Principles Level

The carrier mobility for carbon electronic materials is an important parameter for optoelectronics. We report here some recently developed theoretical tools to predict the mobility without any free parameters. Carrier scatterings with phonons and traps are the key factors in evaluating the mobility. We consider three major scattering regimes: i) where the molecular internal vibration severely induces charge self-trapping and, thus, the hopping mechanism dominates; ii) where both intermolecular and intramolecular scatterings come to play roles, so the Holstein-Peierls polaron model is applied; and, iii) where charge is well delocalized with coherence length comparable with acoustic phonon wavelength, so that a deformation potential approach is more appropriate. We develop computational methods at the first-principles level for the three different cases that have extensive potential application in rationalizing material design.

Control of bombardment energy and energetic species toward a superdense titanium nitride film

TiN deposited by dc magnetron sputtering has been widely used as a hard mask material for dielectric patterning in multilevel Cu interconnects. Typically inside a “poison-mode” regime, the film density is 4.5–4.9 g/cm3. The microstructure, varying from columnar structure to nanocrystalline, is controlled by both thermodynamics and surface kinetics through ionization, substrate bias, target voltage, etc. A relatively low density film can be correlated with a porous columnar structure, low mechanical robustness, and weak resistance to plasma etching. However, with controlled growth, an applied substrate bias does not create resputtering and crystal defects. Instead, the authors create film with a maximum density of 5.3 g/cm3. In this high density film, carrier scatterings through grain boundary are greatly suppressed and the film resistivity is as low as 95 μΩ cm, which brings additional benefits as a conductive capping layer. As it is deposited at room temperature, the process minimizes the thermal budget to the underlying low-k dielectric materials to be patterned.

Semiclassical model for thermoelectric transport in nanocomposites

Nanocomposites (NCs) has recently been proposed and experimentally demonstrated to be potentially high-efficiency thermoelectric materials by reducing the thermal conductivity through phonon-interface scattering and possibly by increasing the Seebeck coefficient through energy-selective carrier scattering (low-energy filtering) or quantum-size effect of electrons. In this paper, we develop a Boltzmann transport equation based semiclassical electron transport model to describe the thermoelectric transport processes in semiconductor NCs. This model considers multiband transport of electrons and holes with both the intrinsic carrier scatterings and the carrier-interface scattering. A relaxation-time model is developed for carrier-interface scattering. After fitting the model with bulk thermoelectric alloys that gives reasonable material input parameters for bulk alloys, which are close to handbook values, the model is further validated by comparing the modeled thermoelectric properties with recently reported measurement values of thermoelectric properties in high efficiency NCs. The model is then applied to predict thermoelectric properties of both the particle-host-type and the particle-particle-type semiconductor NCs such as the $p$-type $({\text{Bi}}_{\text{y}}{\text{Sb}}_{2\text{-y}}{\text{Te}}_{3})\text{\ensuremath{-}}({\text{Bi}}_{0.5}{\text{Sb}}_{1.5}{\text{Te}}_{3})$ NCs and the $n$-type $({\text{Mg}}_{2}{\text{Si}}_{\text{y}}{\text{Ge}}_{1\text{-y}})\text{\ensuremath{-}}({\text{Mg}}_{2}{\text{Si}}_{0.6}{\text{Ge}}_{0.4})$ NCs. The dependence of thermoelectric transport coefficients on the size of nanoconstituent, doping concentration and temperature are studied. Our study could shed some light to optimally design high-efficiency thermoelectric NCs which could contribute to solar-thermal utilization or waste heat recovery.

Scattering of charge carriers in graphene induced by topological defects

We study the scattering of graphene quasiparticles by topological defects, represented by holes, pentagons and heptagons. For holes, we found that at low concentration they give a negligible contribution to the resistivity. Whenever pentagons or heptagons are introduced we realize that a fermionic current is scattered by defects.

Structural and electrical properties of single crystalline and bi-crystalline ZnO thin films grown by molecular beam epitaxy

C-oriented ZnO epitaxial thin films are grown separately on the a-plane and c-plane sapphire substrates by using a molecular-beam epitaxy technique. In contrast to single crystalline ZnO films grown on a-plane sapphire, the films grown on c-plane sapphire are found to be bi-crystalline; some domains have a 30° rotation to reduce the large mismatch between the film and the substrate. The presence of these rotation domains in the bi-crystalline ZnO thin film causes much more carrier scatterings at the boundaries, leading to much lower mobility and smaller mean free path of the mobile carriers than those of the single crystalline one. In addition, the complex impedance spectra are also studied to identify relaxation mechanisms due to the domains and/or domain boundaries in both the single crystalline and bi-crystalline ZnO thin films.

Effects of Basal Stacking Faults on Electrical Anisotropy of Nonpolar a-Plane ($11\bar{2}0$) GaN Light-Emitting Diodes on Sapphire Substrate

We report on the effects of basal stacking faults (BSFs) on the electrical anisotropy and the device characteristics of nonpolar a-plane GaN (1120) light-emitting diodes (LEDs) on r-plane (1102 ) sapphire substrates. The sheet resistance in the direction parallel to the c-axis [0001] is 18%-70% higher than the one in the direction parallel to the m-axis [1100 ]. The anisotropic conductivity of faulted a-plane GaN films can be explained by carrier scatterings from BSFs. It is also shown that the output power of nonpolar a-plane GaN LEDs are significantly influenced by the presence of BSFs, which laterally hampers the carrier transport in the n-GaN layer, especially in the direction parallel to the c-axis in faulted nonpolar nitride films.

Photoluminescence and excitation spectra of Cu(Al xGa 1− x)S 2 films grown by vapor phase epitaxy

Chalcopyrite Cu(Al x Ga 1 −x )S 2 alloy films were successfully grown on GaP substrates by vapor phase epitaxy using metallic chlorides (CuCl, GaCl 3 and AlCl 3) and H 2S as source materials. Photoluminescence (PL) spectra of these films taken under a low excitation density using a super high pressure Hg lamp exhibited broad emissions in the orange region. Photoluminescence excitation (PLE) measurements revealed that these broad emissions are effectively excited at the photon energies of A- and the BC-exciton energies. Under the high excitation density using the pulsed XeCl laser, these alloy films showed the exciton related emissions composed of biexciton recombination, exciton–exciton and exciton–carrier scatterings. The influence of the compositional fluctuation was observed on the increase of the full-width at half maximum (FWHM) for the exciton related emission with increase in composition of x.

Effect of Carrier Scattering Phenomena on Drain Current Variability in Si MOSFETs

not Available.

Comparison for Thermoelectric Properties of BiTe Based Semiconductor Processed by the Mechanical Alloying and the High Pressure Torsion after Melt Grown by the Vertical Bridgman Method

Bi 2 Te 3 -related thermoelectric semiconductors were prepared by the mechanical alloying (MA) and the vertical Bridgman (VBM) - High Pressure Torsion (HPT) methods. Structures and electric properties of these alloys were then investigated. These samples had a nominal composition of Bi 0.5 Sb 1.5 Te 3.0 with 0.07 mass% excess Te. The MA sample was sintered into a compact after mechanical alloying by hot-pressing while the VBM - HPT sample was subjected by HPT after being cut into a disk from a melt-grown ingot by the vertical Bridgman method (VBM). The orientation factors of the MA and VBM - HPT samples were 0.054 and 0.525, respectively, indicating that the preferred orientation was formed by VBM - HPT. Average grain sizes of both samples were approximately 2 μm. By estimating the slope of carrier mobility versus temperature the carrier scattering factor of the MA and VBM - HPT samples were estimated to be -2.8 and -1.4. These values are indicative of carrier scatterings by acoustic phonons and by the interaction between acoustic and optical phonons, respectively. The maximum power factor obtained for the isotropic MA sample was 4.1 × 10 -3 W m -1 K -2 at 310 K and this was about twice the 2.1 × 10 -3 W m -1 K -2 at 440K obtained for the anisotropic VBM - HPT sample.