Acoustic Phonon Scatterings Research Articles

Scattering of coherent acoustic phonons by silica nanoparticles reveals the 3D-morphology of cells in solution down to nanometer thicknesses

In this work, we show that the use of silica nanoparticles improves the imaging and 3D-morphological measurement down to nanometer thicknesses of fixed cells in solution with picosecond ultrasonics (PU). Synchronized ultrafast fs-laser pulses are used to generate coherent acoustic phonons (CAPs) that evoke the Brillouin light scattering and enable the recording of the time-resolved Brillouin oscillations along with the propagation of the acoustic nanopulses through a thin transparent cell in solution. Silica nanoparticles, whose size matches the phonon wavelength at the frequency of the Brillouin scattering in the solution, are used to strongly scatter the CAPs in the solution. Suppressing the Brillouin signature of the surrounding liquid, this protocol improves significantly the PU imaging and makes it possible to measure the mechanical properties of a transparent cell, including the thin peripheral region where the thickness is less than the Brillouin wavelength, equal to half the probe light wavelength in the cell, and where crucial interaction of the cell with its surroundings occurs. We present experimental evidence of the considerable improvement in the cartography of the entire cell using nanoparticles. The intricate frequency dependence of Brillouin scattering and of resonances for a very thin cell is analyzed using a semi-analytical model leading to the challenging measurement of the 3D-morphology of the immersed cell at thicknesses down to 1 / 9 of the optical wavelength.

Strain tuned low thermal conductivity in Indium Antimonide (InSb) through increase in anharmonic phonon scattering - A first-principles study

InSb is a promising thermoelectric material and a decrease in its thermal conductivity (k) can further improve its thermoelectric performance. Using first-principles computations, we report a 29% decrease in k of InSb through strain. k value decreases from 18.8 W/mK for unstrained InSb to 13.4 W/mK for 5% biaxially compressed InSb at 300 K. This decrease in k is found to be due to a large increase in anharmonic scattering of longitudinal acoustic (LA) phonons in strained InSb by almost 330% in Brillouin-zone center, driven by shift of transverse acoustic (TA) phonons to lower frequencies in strained InSb. This shift leads to a simultaneous increase in number of decay scattering channels and a two-fold increase in the magnitude of three-phonon coupling elements, leading to the observed large increase in scattering rates of LA phonons. Scattering of transverse acoustic (TA) phonons is also found to increase by ~61% at Brillouin-zone edge, facilitated by an increase in number of absorption scattering channels. First-principles computations based on deriving interatomic force interactions from density-functional theory are used to provide detailed understanding of increase in phonon scattering rates in strained InSb.

Power loss mechanisms in n-type modulation-doped AlGaAs/GaAsBi quantum well heterostructures

We report on the power loss mechanisms of hot electrons in as-grown and annealed n-type modulation-doped Al0.15Ga0.85As/GaAs1-xBix (x = 0 and 0.04) quantum well structures considering acoustic phonon interactions via the deformation potential (non-polar) and piezoelectric (polar) scatterings. The two-dimensional (2D) electron gas is heated by applying various electric fields under a steady-state magnetic field, and the effect of the applied electric field on the Shubnikov de Haas (SdH) oscillations is analyzed to investigate the power loss mechanism. The temperature of hot electrons (Te) has been obtained by comparing the lattice temperature and applied electric field dependencies of the SdH oscillation amplitude. The hot electron temperature is almost the same for both Bi-free and Bi-containing samples except for the sample annealed at a higher temperature (700 °C) than the growth temperature of GaAsBi. The electron temperature dependence of power loss is analyzed using current theoretical analytic models derived for 2D semiconductors. We find that energy relaxation occurs in the intermediate temperature regime, including mixing of piezoelectric and deformation potential scattering. The power loss of hot electrons is found to be proportional to () with γ in the range from 2.4 to 4.2, which indicates that the hot electron relaxation is due to acoustic phonon scatterings via unscreened deformation potential and piezoelectric scattering. It is found that deformation potential scattering is dominant over piezoelectric scattering in the Bi-free sample, while the incorporation of Bi into the GaAs lattice makes these processes comparable. After thermal annealing at lower than growth temperature (350 °C), the scattering mechanism switches from deformation potential to piezoelectric scattering. After thermal annealing at higher than growth temperature (700 °C), the theoretical model does not fit to the experimental results due to degradation of the sample.

Theoretical evaluation of intrinsic mobility for two-dimensional semiconductor of BC2N: First-principles calculation

Abstract In this study, intrinsic mobility of monolayer BC 2 N is investigated by the first-principles calculation. First, it is calculated from deformation potential theory (DPT) based on longitudinal acoustic and optical phonon scatterings. The values from longitudinal acoustic phonon scattering are higher than those from optical phonon scattering, so the mobility from DPT is mainly determined the optical phonon scattering. At 300 K, electron mobility is 3 . 82 × 1 0 2 cm 2 V − 1 s − 1 along zigzag direction, and 3 . 41 × 1 0 1 cm 2 V − 1 s − 1 along armchair direction, while hole mobility is 3 . 48 × 1 0 2 cm 2 V − 1 s − 1 along zigzag direction, and 2 . 63 × 1 0 1 cm 2 V − 1 s − 1 along armchair direction. Then, we investigate the carrier mobility from the electron–phonon coupling (EPC) matrix elements. If the polarization effect is considered in the calculation, the mobility along zigzag direction is reduced to be 58 ∼ 110 cm 2 V − 1 s − 1 for electron, and 41 ∼ 70 cm 2 V − 1 s − 1 for hole. It is found from vibration modes that the longitudinal optical phonon scattering dominates the carrier mobility. Because some empirical models do not consider Frohlich interaction, the mobility is greatly over estimated. In this study, we reveal the factors that affect the mobility and give a more reasonable prediction for the mobility of monolayer BC 2 N .

Magneto-optical transport properties of monolayer transition metal dichalcogenides

We study the optical transport properties of the monolayer transition metal dichalcogenides (TMDCs) such as ${\mathrm{MoS}}_{2}$, ${\mathrm{WS}}_{2}$, ${\mathrm{MoSe}}_{2}$, and ${\mathrm{WSe}}_{2}$ in the presence of a magnetic field. The TMDCs band structures are obtained and discussed by using the effective massive Dirac model, in which the spin and valley Zeeman effects as well as an external electric field are included. The magneto-optical absorption coefficient (MOAC) is derived as a function of absorbed photon energy when the carriers are scattered by random impurities combined with the intrinsic acoustic and optical phonons in TMDCs and the surface optical (SO) phonons of substrates. Our result shows that the spin-splitting feature appeared in all four TMDC materials. The combination of strong spin-orbit coupling (SOC) and Zeeman fields has doubled the Landau levels but has not changed the energy gap of the TMDCs monolayer, which can be controlled by the electric field. Because of their strong SOC effect, the absorption spectrum in monolayer TMDCs is separated into two separate peaks caused by spin up and down. At the low temperature, the MOAC intensity via impurity scattering is the biggest followed by that of the SO phonons while the intrinsic acoustic and optical phonon scatterings display the smallest. For the monolayer TMDCs on substrates, ${\mathrm{SiO}}_{2}$ always shows its superiority in comparison with the others. Among the four TMDC materials, ${\mathrm{MoSe}}_{2}$ shows the biggest MOAC intensity, while ${\mathrm{WS}}_{2}$ has the biggest value of the absorbed photon energy. The full-width at half-maximum (FWHM) via impurity scattering achieves its highest value in ${\mathrm{WS}}_{2}$, while this occurs in ${\mathrm{MoSe}}_{2}$ and ${\mathrm{MoS}}_{2}$ via intrinsic acoustic and optical phonon scatterings, respectively. Our estimation of mobility from FWHM gives good agreement with the experimental results in ${\mathrm{WS}}_{2}$.

Full consideration of acoustic phonon scatterings in two-dimensional Dirac materials.

The in-plane acoustic phonon scattering in graphene is solved by considering fully inelastic acoustic phonon scatterings in two-dimensional (2D) Dirac materials for a large range of temperatures (T) and chemical potentials (μ). Rigorous analytical solutions and symmetry properties of Fermionic and Bosonic functions are obtained. We illustrate how doping alters the temperature dependence of acoustic phonon scattering rates. It is shown that the quasi-elastic and ansatz equations previously derived for acoustic phonon scatterings in graphene are limiting cases of the inelastic-scattering equations derived here. For heavily-doped graphene, we found that the high-T behavior of resistivity is better described by ρ(T,μ) ∝T(1 -ζaμ2/3(kBT)2) rather than a linear T behavior, and in the low T regime we found τ-1∝ (kBT)4 but with a different prefactor (i.e.∼3 times smaller) in comparison with the existing quasi-elastic expressions. Furthermore, we found a simple analytic "semi-inelastic" expression of the form τ-1∝ (kBT)4/(1 + cT3) which matches nearly perfectly with the full inelastic results for any temperature up to 500 K and μ up to 1 eV. Our simple analytical results agree well with previous first-principles studies and available experimental data. Moreover, we obtain an analytical form for the acoustic gauge field . Our analyses pave a way for investigating scatterings between electrons and other fundamental excitations with linear dispersion relation in 2D Dirac material-based heterostructures such as bogolon-mediated electron scattering in graphene-based hybrid Bose-Fermi systems.

Intrinsic Mobility by Acoustic Phonon Scattering of Two Dimensional Semiconductors

Intrinsic Mobility by Acoustic Phonon Scattering of Two Dimensional Semiconductors

Mobility of a two-dimensional electron gas in the AlGaN/GaN heterostructure: simulation and analysis

At temperatures higher than the room temperature, a two-dimensional electron gas (2DEG) formed at the AlGaN/GaN heterointerface can be characterized by the three dominant scattering mechanisms: acoustic deformation potential, polar acoustic phonon and polar optical phonon scatterings. An analytical model describing the 2DEG mobility limited by these scattering mechanisms as a function of the carrier concentration and the temperature was developed and integrated into a device simulator package using a C language interpreter. The model should be useful for heterostructure device simulators such as Blaze.

Scattering of GHz coherent acoustic phonons by silica nanoparticles in water probed with the time-domain Brillouin spectroscopy

Coherent acoustic phonons detected by time-domain Brillouin spectroscopy (TDBS) in a liquid medium are strongly attenuated when silica nanoparticles sediment in the liquid. The decrease in the optical signal is due to the strong acoustic scattering by silica nanoparticles whose size matches the wavelength of phonons at the frequency of the Brillouin shift. The control of the acoustic wave-front with nanoparticles thus strongly reduces the undesired Brillouin response of the liquid medium. This is demonstrated with a thin transparent film in water, mimicking a biological sample in a nutritive serum. While strong Brillouin scattering in the particle-free liquid hampers the measurement in the film, we show that TDBS can however be performed in a film of a thickness less than the optical wavelength when scatterers sediment in the liquid.

The Photon-Drag Effect in Cylindrical Quantum Wire with an Infinite Potential for the Case of Electrons – Acoustic Phonon Scattering

Abstract - The photon - drag effect with electrons – acoustic phonon scattering in cylindrical quantum wire with an infinite potential is studied. With the appearance of the linearly polarized electromagnetic wave, the laser radiation field and the dc electric field, analytic expressions for the density of the direct current are calculated by the quantum kinetic equation. The dependence of the direct current density on the frequency of the laser radiation field, the frequency of the linearly polarized electromagnetic wave and the temperature of the system is obtained. The analytic expressions are numerically evaluated and plotted for a specific quantum wire, GaAs/AlGaAs. The difference of the density of the direct current in quantum wires from quantum well and bulk semiconductor is due to potential barrier and characteristic parameters of system. These results are for every temperature and are new results.

Prediction of Large Enhancement of Electron Mobility in Direct Gap Ge1−xSnx Alloy

Group IV semiconductor‐based alloy Ge1−xSnx can be grown on Si platform and a crossover from indirect (L valleys) to direct gap (Γ valley) occurs for x > 0.08. The direct gap alloy shows promise for use in high‐speed electronic and photonic devices. An earlier study predicted that the electron mobility in the alloy would increase by a few orders of magnitude above the value for pure Ge. In that work, however, electrons were assumed to occupy only the low effective mass Γ valley and only the deformation potential acoustic phonon and alloy scatterings were considered. In the present work, we give a more realistic estimate of the values of electron mobility in direct gap GeSn alloy, considering the Γ and L valleys and the phonon (deformation potential acoustic, optical and intervalley), alloy, and impurity scatterings. The effect of strain, both tensile and compressive, on the mobility is also studied. It is found that for higher alloy composition and suitable strain, mobility values ∼2 × 105 cm−2/V · s, more than 50 times higher than the value in pure Ge (3900 cm2/V · s) may be achieved. This is attributed to increased separation between Γ and L valleys and consequent reduction in intervalley scattering.

Impact of inelastic phonon scattering in the OFF state of Tunnel-field-effect transistors

We study the impact of electron---phonon interaction on the subthreshold operation region of Tunnel-FETs by means of full-quantum simulations. Our approach is based on the nonequilibrium Green's function method, where acoustic and optical phonon scatterings are taken into account through the self-consistent Born approximation. Two device architectures are analyzed: InAs nanowire longitudinal Tunnel-FETs, and 2D vertical Tunnel-FETs based on either an GaSb/AlSb/InAs heterostructure or a MoS$$_2$$2/WTe$$_2$$2 van der Waals heterojunction. In InAs nanowire Tunnel-FETs with interface traps, electron---phonon interaction deteriorates the subthreshold swing by allowing trap-assisted tunneling at energies higher than the valence-band edge in the source. In vertical heterojunction Tunnel-FETs, optical phonon scattering increases the OFF current by inducing inelastic transition in the overlap region even in the absence of traps.

Rashba Effect and Carrier Mobility in Hybrid Organic-Inorganic Perovskites.

The outstanding photovoltaic performance in hybrid organic-inorganic perovskites (HOIPs) relies on their desirable carrier transport properties. In the HOIPs, strong spin-orbit coupling (SOC) and structural inversion asymmetry give rise to a giant spin splitting in the conduction and valence bands, that is, the Rashba effect (RE), a subject intensively studied in spintronics. Here we show that this giant RE can manifest itself in charge transport and is the key to understanding carrier mobility and its temperature dependence in the HOIPs. The RE greatly enhances acoustic-phonon scattering (APS) and alters the temperature dependence of carrier mobility from T(-3/2) to T(-1). Meanwhile, it reduces polar-optical phonon scattering (POPS). In CH3NH3PbI3, the carrier mobility is limited by the APS for temperatures up to 100 K, above which the POPS becomes dominant. The effective polar coupling is moderate, α = 1.1, indicating that band conduction is still a valid description of charge transport. Our results account for the observed carrier transport behaviors over the entire temperature range and highlight the importance of SOC in charge transport in the HOIPs.

Transport Properties of Graphene and Suspended Graphene with EMC: The Role of Various Scattering Mechanisms

The electronic transport properties of graphene and suspended (intrinsic) graphene sheets are studied using an ensemble Monte Carlo (EMC) technique. The combined scattering mechanisms that are taken into account for both cases are nonpolar optic and acoustic phonons, ionized impurity, interface roughness, and surface polar phonon scatterings. The effect of screening is also considered in the ionized impurity and surface polar phonon scatterings of electrons. A rejection technique is used in EMC simulations to account for the occupancy of the final states. Velocity-field characteristics of graphene and suspended graphene sheets are obtained using various values of acoustic deformation potential constants. The variation of electron mobility of graphene is studied as a function of electron concentration and its variation as a function of temperature are investigated for the case of suspended graphene. For the former case, the mobility increases with electron concentration first and after a certain value of electron concentration it begins to decrease, while for the latter case the mobility decreases almost linearly with temperature. The mobility results from EMC simulations are compatible with the existing experimental studies for the unsuspended graphene case.

A comparative study of transport properties of monolayer graphene and AlGaN-GaN heterostructure

The electronic transport properties of monolayer graphene are presented with an Ensemble Monte Carlo method where a rejection technique is used to account for the occupancy of the final states after scattering. Acoustic and optic phonon scatterings are considered for intrinsic graphene and in addition, ionized impurity and surface roughness scatterings are considered for the case of dirty graphene. The effect of screening is considered in the ionized impurity scattering of electrons. The time dependence of drift velocity of carriers is obtained where overshoot and undershoot effects are observed for certain values of applied field and material parameters for intrinsic graphene. The field dependence of drift velocity of carriers showed negative differential resistance and disappeared as acoustic scattering becomes dominant for intrinsic graphene. The variation of electron mobility with temperature is calculated for intrinsic (suspended) and dirty monolayer graphene sheets separately and they are compared. These are also compared with the mobility of two dimensional electrons at an AlGaN/GaN heterostructure. It is observed that interface roughness may become very effective in limiting the mobility of electrons in graphene.

Electronic properties and charge carrier mobilities of graphynes and graphdiynes from first principles

The sp1 + sp2 hybridized carbon allotropes, graphynes (GYs) and graphdiynes (GDYs), have attracted increased attention, and researches from both theoretical and experimental communities are emerging. Theoretical calculations show that the electronic properties of GYs and GDYs can be tuned by straining, cutting into nanoribbons with different widths and edge morphology, and applying external electric fields. Due to their unique electronic properties, GYs and GDYs exhibit charge carrier mobility as high as ∼104–105 cm2 V−1 second−1 at room temperature based on the first‐principle calculations and the Boltzmann transport equation. Interestingly, the charge carrier mobility in 6,6,12‐GY with double Dirac cone structure is found to be even larger than that in graphene at room temperature. Through an in‐depth description of electron–phonon couplings by density functional perturbation theory, it is suggested that the intrinsic charge carrier transport in these carbon allotropes is dominated by the longitudinal acoustic phonon scatterings over a wide range of temperatures, although scatterings with optical phonon modes cannot be neglected at high temperatures. The unique electronic properties of GYs and GDYs make them highly promising for applications in next generation nanoelectronics. WIREs Comput Mol Sci 2015, 5:215–227. doi: 10.1002/wcms.1213This article is categorized under: Structure and Mechanism > Computational Materials Science

Combined effect of edge roughness and phonon scattering on the electronic properties of ultra scaled graphene nano-ribbons

The effect of optical and acoustic phonon-scattering in the presence of line-edge-roughness (LER) on the electronic properties of ultra-scaled armchair graphene nano-ribbons (AGNRs) is investigated. Non-equilibrium Green’s function formalism (NEGF) is employed using a Hamiltonian formed from tight bonding model with consideration of first and third nearest neighbors. The combined effect of phonons and line edge roughness on the transmission, transport gap, and conductance are studied for different roughness strengths and AGNR lengths. Results show edge roughness slightly reduces the onset of optical phonon emission, acoustic phonons reduce off-state conductance and optical phonons reduce on-state conductance. In both cases, the degree and behavior of reduction is totally dependent on the intensity of edge roughness. Also, in the longer AGNRs with high edge roughness intensity, phonons increase the transport gap.

Scattering of flexural acoustic phonons at grain boundaries in graphene

We investigate the scattering of long-wavelength flexural phonons against grain boundaries in graphene using molecular dynamics simulations. Three symmetric tilt grain boundaires are considered: one with a misorientation angle of $17.9^\circ$ displaying an out-of-plane buckling 1.5 nm high and 5 nm wide, one with a misorientation angle of $9.4^\circ$ and an out-of-plane buckling 0.6 nm high and 1.7 nm wide, and one with a misorientation angle of $32.2^\circ$ and no out-of-plane buckling. At the flat grain boundary, the phonon transmission exceeds 95 % for wavelengths above 1 nm. The buckled boundaries have a substantially lower transmission in this wavelength range, with a minimum transmission of 20 % for the $17.9^\circ$ boundary and 40 % for the $9.4^\circ$ boundary. At the buckled boundaries, coupling between flexural and longitudinal phonon modes is also observed. The results indicate that scattering of long-wavelength flexural phonons at grain boundaries in graphene is mainly due to out-of-plane buckling. A continuum mechanical model of the scattering process has been developed, providing a deeper understanding of the scattering process as well as a way to calculate the effect of a grain boundary on long-wavelength flexural phonons based on the buckling size.

Carrier transport properties of nanocrystalline Er3N@C80

Electrical transport properties of the nanocrystalline Er3N@C80 with fcc crystal structure were characterized by measuring both temperature-dependent d.c. conductance and a.c. impedance. The results showed that the Er3N@C80 sample has characteristics of n-type semiconductor and an electron affinity larger than work function of gold metal. The Er3N@C80/Au interface has an ohmic contact behavior and the contact resistance was very small as compared with bulk resistance of the Er3N@C80 sample. The charge carriers in the sample were thermally excited from various trapped levels and both acoustic phonon and ionic scatterings become a dominant process in different temperature regions, respectively. At temperatures below 250 K, the activation energy of the trapped carrier was estimated to be 35.5 meV, and the ionic scattering was a dominant mechanism. On the other hand, at temperatures above 350 K, the activation energy was reduced to 15.9 meV, and the acoustic phonon scattering was a dominant mechanism. In addition, a polarization effect from the charge carrier was observed at low frequencies below 2.0 MHz, and the relative intrinsic permittivity of the Er3N@C80 nanocrystalline lattice was estimated to be 4.6 at frequency of 5.0 MHz.

Simulation of electron plasma instability in n+nn+ GaAs structure

High frequency microwave generation is feasible in n+nn+ structures because of the electron plasma instability in the device. The time and frequency behaviour of the current instability in the electron plasma of n+nn+ GaAs structure is simulated by Monte Carlo method. The simulation technique involves the particle-in-cell formalism. In the model for simulation, the energy dispersion band is taken to be parabolic and spherical. The other parameters like the effective mass, band gap etc. are taken from the standard literatures. The temperature is taken to be 25°K. Polar optical, acoustic phonon and impurity scatterings are taken into consideration. It is observed that the oscillation frequency in the THz. region is affected by the structure length. The optical phonon dominated current instabilities show high peaks alternating with extremely high frequencies in the time scale of pico-seconds. For longer structures, the instability is found to be dominated strongly by the optic phonon scatterings while for short ones it is mainly by the ballistic mode. Smaller is the structure length higher is the instability frequency. It may also be remarked here that the structure length does not affect the carrier behavior very close to the cathode of the structure.