Acoustic Deformation Potential Research Articles

Photoconductivity calculations of bilayer graphene from first principles and deformation-potential approach

We report first-principles calculations of electron-phonon coupling in bilayer graphene and the corresponding contribution to carrier scattering. At the phonon $\Gamma$ point, electrons with energies less than 200 meV are scattered predominantly by LA$^\prime$ and TA$^\prime$ modes while higher-energy electron scattering is dominated by optical phonon modes. Based on a two-temperature model, heat transfer from electrons with an initial temperature of 2000 K to the lattice (phonons) with an initial temperature of 300 K is computed, and in the overall relaxation process, most of this energy scatters into K-point phonon optical modes due to their strong coupling with electrons and their high energies. A Drude model is used to calculate photoconductivity for bilayer graphene with different doping levels. Good agreement with prior experimental trends for both the real and imaginary components of photoconductivity confirms the model's applicability. The effects of doping levels and electron-phonon scattering on photoconductiviy are analyzed. We also extract acoustic and optical deformation potentials from average scattering rates obtained from density functional theory (DFT) calculations and compare associated photoconductivity calculations with DFT results. The comparison indicates that momentum-dependent electron-phonon scattering potentials are required to provide accurate predictions.

Electron Mobility in Molybdenum Disulfide: From Bulk to Monolayer

Herein, the low‐field mobility in bulk, multilayer, and monolayer of molybdenum disulfide (MoS2) is calculated using a quantum kinetic approach. Analytical expressions are obtained for mobility of 3D and 2D electrons for scattering by out‐of‐plane flexural and in‐plane acoustic phonons (deformation potential and piezoelectric scattering), nonpolar and polar optical (PO) phonons, charged impurities, and surface roughness. It is shown that the known experimental Hall data in bulk MoS2 should be analyzed in the presence of two types of free electrons contributing to the charge transport: 3D electrons in the conduction band and 2D electrons in the quantum layers. Calculations show that the electron mobility in bulk, multilayer, and monolayer MoS2 is mainly limited by acoustic (deformation potential) and PO phonons scatterings in a wide temperature range of 100–700 K. For temperatures below 100 K, electron mobility is determined by the ionized impurity scattering.

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.

First principles calculations of intrinsic mobilities in tin-based oxide semiconductors SnO, SnO2, and Ta2SnO6

The development of high-performance p-type oxides with high hole mobility and a wide bandgap is critical for the applications of metal oxide semiconductors in vertically integrated CMOS devices [Salahuddin et al., Nat. Electron. 1, 442 (2018)]. Sn2+-based oxides such as SnO and K2Sn2O3 have recently been proposed as high-mobility p-type oxides due to their relatively low effective hole masses, which result from delocalized Sn s-orbital character at the valence band edge. Here, we introduce a promising ternary Sn-O-X compound, Ta2SnO6, which exhibits strong valence band dispersion and a large bandgap. In order to evaluate the performance of this oxide as a p-type semiconductor, we perform first-principles calculations of the phonon-limited room-temperature carrier mobilities in SnO, SnO2, and Ta2SnO6. Electron relaxation time is evaluated, accounting for the scatterings from acoustic deformation potentials and polar optical phonons (POP), within the isotropic and dispersionless approximation. At room temperature, the electron/hole mobilities in a given material (SnO, SnO2, and Ta2SnO6) are found to be limited by POP scattering. SnO2 shows high room-temperature electron mobility of 192 cm2/(V s), while SnO and Ta2SnO6 exhibit impressive hole mobilities, with the upper limit at 60 and 33 cm2/(V s), respectively. We find that carrier effective mass largely accounts for the differences in mobility between these oxides with correspondingly different POP scattering rates. The theoretically predicted intrinsic mobilities of each material will provide the upper limit to the real mobilities for their device applications. Our findings also suggest a necessity of further investigation to identify even higher mobility p-type oxides with smaller hole effective masses.

Thermoelectric properties improvement in quasi-one-dimensional organic crystals

The charge and energy transport in highly conducting quasi-one-dimensional organic crystals of p-type tetrathiotetracene-iodide, TTT2I3, and of n-type tetrathiotetracene-tetracyanoquinodimethane, TTT(TCNQ)2, is studied. Two electron-phonon interactions are considered simultaneously. One interaction is of the acoustic deformation potential type and the other one is of polaronic character. Charge transport along the conducting molecular chains is bandlike, whereas in the transversal directions, it is of the hopping type. It is shown that due to a partial compensation of these interactions for a narrow interval of states in the one-dimensional conduction band, the relaxation time is of Lorentzian shape and shows a distinct dependence on carrier energy with a pronounced maximum. The scattering of charge carriers on adjacent molecular chains and by impurities and structural defects limits the height of this maximum. However, rather high relaxation times might be anticipated in the case of perfect single crystals. As the carriers in these states show an enhanced mobility, this will lead to a simultaneous increase of electrical conductivity and Seebeck coefficient. It is proposed that, if the above-mentioned crystals could be accomplished by means of sufficient purification and by optimization of the carrier concentration, so that the Fermi level is close to energetic states for which the relaxation time has a maximum, one might achieve values for the thermoelectric figure of merit ZT∼5 in crystals of tetrathiotetracene-iodide and ZT∼1.5 in those of tetrathiotetracene-tetracyanoquinodimethane.

Electronic Transport Characteristics of a Graphene Nanoribbon Based p–n Device

Electronic transport in terms of its parameters, such as electron effective mass, electron mobility, deformation potential (DP), electron Fermi velocity and energy band gap, is investigated for a boron and nitrogen codoped armchair graphene nanoribbon (aGNR) based p–n device via determination of electron interaction with acoustical phonons by deformation potential (DP) scattering mechanism and piezoelectric (PZ) scattering mechanism under the impact of finite electric field at low temperature regime. The variation of acoustical deformation potential (ADP) with electron Fermi velocity for a particular temperature has been studied. The dependence of electron mobility on the doping concentration as well as electron effective mass and lattice temperature have been observed for a boron and nitrogen doped graphene nanoribbon p–n device for its nanoelectronic and photovoltaic applications. The current-voltage characteristics have also been studied for the p–n device under the forward biased condition. The net electron mobility (ADP + PZ) for the region of boron doped aGNR is observed much higher in comparison with the region of nitrogen-doped graphene for a particular temperature. Moreover, it has also been observed that the electron mobility increases with boron doping in the P-region, but decreases with an increase in nitrogen concentration in the N-region of the armchair graphene nanoribbon based p–n device. The band gap shows considerable variation with impurity concentration for the width of nanoribbons less than 40 nm with respect to pristine graphene, whereas for that with widths of more than 50 nm to up to 100 nm there is no impact of concentration on the band gap.

Theoretical Investigation on Electron Mobility in AlInGaN/InGaN Heterostructures

The dependences of electron mobility in AlInGaN/InGaN heterostructure on the barrier and channel alloy compositions and on temperature are investigated including six scattering processes: acoustic deformation potential (DP) scattering, piezoelectric field (PE) scattering, polar optical phonons (PO) scattering, dislocation impurity (DIS) scattering, interface roughness (IRF) scattering, and alloy disorder (ADO) scattering. The results show that ADO scattering is the most important scattering mechanism, and specifically channel alloy disorder gets severer than barrier alloy disorder except for InGaN channels with very low indium content (near 0) or extremely high indium mole fraction (near 1). The variations of the barrier strain, two‐dimensional electron gas (2DEG) density, 2DEG mobility, and conductivity in AlInGaN/In0.04Ga0.96N heterostructure with full barrier alloy composition are summarized. The results indicate that relatively large aluminum content and small indium mole fraction are desired for higher conductivity. By comparing the temperature‐dependent transport properties of Al0.83In0.13Ga0.04N/InGaN heterostructures with different InGaN compositions, we find that it is the ADO scattering and PO scattering that determine 2DEG mobility change and the mobility exhibits a weaker dependence on temperature with increasing indium mole fraction in InGaN channel.

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

At temperatures above 100 K, a two-dimensional electron gas generated at the AlGaAs/GaAs heterointerface can be characterized by the three dominant scattering mechanisms: acoustic deformation potential, polar acoustic phonon and polar optical phonon. An analytical model describing the two-dimensional electron gas mobility controlled by these scattering processes as a function of the electron concentration and the temperature was developed and integrated into a device simulator package using a built-in C language interpreter. The electrical characteristics of a simple AlGaAs/GaAs high electron mobility transistor were simulated using either the derived or a conventional bulk mobility model and the results were compared.

Noise performance of avalanche transit-time devices in the presence of acoustic phonons

Through this paper, the effects of acoustic phonons on the noise performance of avalanche transit-time devices have been investigated and reported. For this study, a double-drift-region silicon-based impact avalanche transit-time diode has been considered at operating frequencies of 94 GHz, 140 GHz and 220 GHz. To analyze the acoustic phonon effects on noise performance, the interactions of charge carriers with acoustic deformation potential and piezoelectric acoustic phonons have been considered in addition to all possible types of scattering events. These effects have been analyzed through a numerical expression for the ionization rate of charge carriers and incorporated in the noise analysis. The noise performance is evaluated in terms of noise spectral density (NSD) and noise measure (NM). The results show that due to acoustic phonons, values of NSD and NM significantly increase.

Magnetotransport Study on Iron Doped Novel 2D Nanoribbons via Electron – Acoustical Phonon Interactions

The electron transport parameters such as electron energy relaxation rate and phonon limited electron resistivity for iron (transition metal) doped 2D nanoribbons of armchair graphene nanoribbon (aGNR) and h-boron nitride nanoribbon (h-BNNR) have been calculated via hot electron acoustical phonon interactions on the basis of acoustical deformation potential (ADP) coupling mechanism. We have performed the investigation for the lower concentration ([Formula: see text]%) of iron doping under the influence of externally applied magnetic field at low temperature to room temperature regime. The hot electron acoustical phonon relaxation rates are observed with electric field and under constant applied magnetic field. The doping of iron increases the electron energy relaxation rate with respect to their pristine counter parts. Moreover, the pristine h-BNNR exhibits less electron energy relaxation rate with respect to pristine aGNR. Upon applying magnetic field on Fe doped armchair GNR as well as Fe-doped h-BNNR the electron energy relaxation rate reduces down to a considerable extent with respect to their pristine counterparts. Moreover, under the impact of magnetic field, the acoustical phonon restricted electrical resistivity of Fe-doped GNR is considerably low compared to pristine GNR.

Electron mobility in β-Ga2O3: an ensemble Monte Carlo study

Numerical simulations are performed to evaluate electron mobility in β-Ga2O3. Following scattering mechanisms were found to be important: acoustic deformation potential, ionized impurity, and polar optical phonons. In β-Ga2O3, a large primitive unit cell (containing ten atoms) leads to multiple phonon modes, which complicate the mobility calculation. Here, by restructuring an in-house ensemble Monte Carlo simulator, we were able to include and examine the effects of all relevant longitudinal optical phonon modes. For low electrical fields at 300 K, we report an electron mobility of 110 cm2/V s. Also, in the range of 150‒500 K, our simulation results match very well with the reported Hall mobility measurement data.

Acoustic deformation potentials of n -type PbTe from first principles

We calculate the uniaxial and dilatation acoustic deformation potentials, $\Xi^{\text{L}}_{u}$ and $\Xi^{\text{L}}_{d}$, of the conduction band L valleys of PbTe from first principles, using the local density approximation (LDA) and hybrid functional (HSE03) exchange-correlation functionals. We find that the choice of a functional does not substantially affect the effective band masses and deformation potentials as long as a physically correct representation of the conduction band states near the band gap has been obtained. Fitting of the electron-phonon matrix elements obtained in density functional perturbation theory (DFPT) with the LDA excluding spin orbit interaction (SOI) gives $\Xi^{\text{L}}_u = 7.0$~eV and $\Xi^{\text{L}}_d = 0.4$~eV. Computing the relative shifts of the L valleys induced by strain with the HSE03 functional including SOI gives $\Xi^{\text{L}}_u = 5.5$~eV and $\Xi^{\text{L}}_d = 0.8$~eV, in good agreement with the DFPT values. Our calculated values of $\Xi^{\text{L}}_u$ agree fairly well with experiment ($\sim 3-4.5$~eV). The computed values of $\Xi^{\text{L}}_d$ are substantially smaller than those obtained by fitting electronic transport measurements ($\sim 17-22$~eV), indicating that intravalley acoustic phonon scattering in PbTe is much weaker than previously thought.

Electron mobility of two-dimensional electron gas in InGaN heterostructures: Effects of alloy disorder and random dipole scatterings

InGaN has a smaller electron effective mass and is expected to be used as a channel material for high-electron-mobility transistors. However, it is an alloy semiconductor with a random distribution of atoms, which introduces additional scattering mechanisms: alloy disorder and random dipole scatterings. In this work, we calculate the electron mobility in InGaN- and GaN-channel high-electron-mobility transistors (HEMTs) while taking into account acoustic deformation potential, polar optical phonon, alloy disorder, and random dipole scatterings. For InGaN-channel HEMTs, we find that not only alloy disorder but also random dipole scattering has a strong impact on the electron mobility and it significantly decreases as the In mole fraction of the channel increases. Our calculation also shows that the channel thickness w dependence of the mobility is rather weak when w > 1 nm for In0.1Ga0.9N-channel HEMTs.

Mobility of Two‐Dimensional Hole Gas in H‐Terminated Diamond

The two‐dimensional hole gas (2DHG) induced at H‐terminated diamond surface provides the most widely used room‐temperature surface electrical conductance of diamond semiconductors. Temperature and hole sheet density dependences of the mobility of 2DHG in H‐terminated diamond are investigated for the first time considering four scattering mechanisms: surface impurity (SI) scattering, acoustic deformation potential (AC) scattering, nonpolar optical phonon (NOP) scattering, and surface/interface roughness (SFR/IFR) scattering. The calculation results are then compared with the experimental data, where the best agreement is obtained with the effective coupling constant of the non‐polar optical phonon Dnop and the correlation length L with values of of 1.2 × 1010 eV cm−1 and 2 nm, respectively. The theoretical data show that the SI scattering dominates the 2DHG mobility at a relatively large range of the temperature and the hole density due to the proximity of the surface impurities to the 2DHG, while the NOP scattering becomes important as the temperature increases further.

Coupling of quantum well states and phonons in thin multilayer Pb films on Si(111)

Density functional theory calculations for the electronic and phononic band structures of Pb/Si(111) thin films with a thickness of 4 and 5 monolayers (ML) are performed. We employ a Si(111)$(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ unit cell to model the Pb films including the Si substrate, and identify quantum well (QW) states for film thicknesses between 3 and 6 ML. The calculations show that the quantum-confined state closest to the Si band gap acquires the character of a quantum well resonance with a major part of its wave function extending into the Si(111) substrate. This finding explains the unusually low dispersion of this state and its lacking sensitivity to phonons in the 5 ML Pb film. Moreover, several unoccupied QW states are identified in the calculations and are assigned to previously observed features in structurally simpler freestanding Pb films. The calculated phonon band structures of the Pb/Si(111)$(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ films display stiff surface phonon modes in the 2.3--2.5 THz range. The electron-phonon coupling strength in the quantum-confined states is addressed by means of deformation-potential theory using the calculated atomic displacements of $\mathrm{\ensuremath{\Gamma}}$-point phonons. It is found that both the acoustic shear deformation potential as well as the optical deformation potentials of unoccupied QW states are sizable. Comparing the results for 4 and 5 ML Pb films, we conclude that the optical deformation potentials are generally larger for the 4 ML film. The occupied QW resonance in the 5 ML Pb film shows weak electron-phonon coupling, in qualitative agreement with the small experimentally observed lifetime broadening of this state. Our results form the basis for addressing the role of electron-phonon scattering for the lifetime of unoccupied QWs acting as intermediate states in two-photon photoemission from Pb/Si(111) films.

Electron Transport Parameters Study for Transition Metal-Doped Armchair Graphene Nanoribbon via Acoustical Phonon Interactions

Electron transport parameters such as electron effective mass, Fermi velocity of an electron and electron mobility are calculated for transition metal [manganese (Mn), cobalt (Co)]-doped armchair graphene nanoribbon (aGNR) via polar acoustical phonon [piezoelectric (PZ)] scattering and acoustical deformation potential (ADP) scattering under a high electric field and different doping concentrations. Moreover, the effect of dopant site on these electron transport parameters is also investigated. It is observed that the electron effective mass is reduced significantly in doped aGNR in comparison to pure GNR. It is observed that the net electron mobility contributed by both ADP and PZ mechanisms for Mn-doped aGNR as well as Co-doped aGNR varies in similar fashion as semiconductors wherein the net electron mobility (ADP + PZ) for Mn-doped aGNR is greater than that for the Co-doped graphene nanoribbon. Moreover, it is found that there is no impact of variation in dopant site on the electron transport parameters considered in this study.

Comparison of Hot Electron Mobility in h-Boron Nitride Nanosheets and Graphene under Manganese Doping

The electron mobility is calculated for h-BN nanosheets (h-BNNSs) and graphene with and without doping of manganese at high electric fields via acoustical deformation potential (ADP) scattering mechanism and piezoelectric scattering (Polar Acoustical Phonon (PAP) mechanism at low temperatures. Calculation includes the variation of electron Fermi energy and effective mass with high electric fields and with variation of Mn concentrations. Comparison of mobility in both the cases of with and without doping is carried out. It is observed that the net electron mobility due to both ADP and PAP mechanisms in graphene is much larger than that for h-BNNS for both the cases of with and without doping of manganese at low temperatures.

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.

Quantitative Analysis of Scattering Mechanisms in Highly Crystalline CVD MoS2through a Self-Limited Growth Strategy by Interface Engineering

The electrical performance of highly crystalline monolayer MoS2 is remarkably enhanced by a self-limited growth strategy on octadecyltrimethoxysilane self-assembled monolayer modified SiO2 /Si substrates. The scattering mechanisms in low-κ dielectric, including the dominant charged impurities, acoustic deformation potentials, optical deformation potentials), Fröhlich interaction, and the remote interface phonon interaction in dielectrics, are quantitatively analyzed.

Polycrystalline bismuth films: Correlation between grain structure and electron transport

Grain structure and the temperature dependences of resistivity, magnetoresistance, Hall and Seebeck coefficients measured in the range from 4 to 300 K were investigated for polycrystalline bismuth films obtained by the melt spinning (MS) and electrochemical deposition (ECD) methods. Charge‐carrier concentration and mobilities were calculated assuming the carrier scattering on acoustic deformation potential as the dominant scattering mechanism, parabolicity of holes dispersion law, implying the Lax model for L‐band electrons and neglecting the influence of L‐band holes on conductivity. The experimental results and calculations have demonstrated that the electrical properties of the Bi films studied are strongly affected by the grain‐boundary density.