Carrier-phonon Interaction Research Articles

Ensemble Monte Carlo transport studies of zinc-blende cuprous halides

We present a comprehensive theoretical analysis of intrinsic high-field transport in cuprous halides: CuCl, CuBr, and CuI. Ensemble Monte Carlo transport simulations were performed based on analytical approximations of the multi-valley and three-band electronic structure model. The deformation potentials, extracted from carrier–phonon interaction matrix elements calculated via the Wannier function approach, were employed to determine scattering rates. Our detailed analysis uncovers intriguing transport characteristics based on the complex valence band structures, resulting from spin–orbit coupling and band inversion. Remarkably, the hole mobility in CuI was exceptionally high, reaching up to 193 cm2/V s, while CuCl exhibited unusual temperature dependencies in hole transport. Additionally, the electron mobility in CuI was found to be 254 cm2/V s, indicating a minimal disparity between carrier mobilities, which is advantageous for optoelectronic applications.

Transient Optical Modulation in Vanadium and Selenium Doped MoS2 by Carrier–Carrier and Carrier–Phonon Interactions

AbstractDoping and alloying induce defect states in atomically thin transition metal dichalcogenides (TMDCs), leading to strong carrier–phonon interactions. The robust excitonic behavior of these layered materials can be modified by injecting a high density of charge carriers. However, comprehending the influence of carrier–phonon and carrier–carrier interactions on the optical properties of 2D materials is crucial for their optoelectronic and photonic applications. Here, transient absorption (TA) spectroscopy is employed to demonstrate the modulation of the transient optical behavior of TMDCs through doping and excitation near Mott density. The TA spectra reveal broadening attributed to carrier–carrier and carrier–phonon interactions, with the broadening being particularly pronounced in vanadium (V) doped TMDCs due to the hybridization of defect and exciton transitions. Analysis of TA kinetics suggests the involvement of various carrier species in the carrier dynamics of TMDCs, with the influence of mid‐gap carriers dominating at higher excitation densities. Nonetheless, the presence of strong carrier–phonon coupling in V‐doped TMDCs is demonstrated by temperature‐dependent Raman and photoluminescence spectroscopy. The results reveal that the enhanced coupling between acoustic phonons and carriers can lead to multiphonon emission. The findings of this study hold promise for controlling the optical response of TMDCs in ultrafast optoelectronic applications.

Signature of electrothermal transport in 18 nm vertical junctionless gate-all-around nanowire field effect transistors

Abstract Addressing temperature hot-spots resulting from self-heating effects (SHE) poses a significant challenge in the design of emerging nanoscale transistors, such as vertical junctionless nanowire field-effect transistors (VNWFETs), due to reduced thermal conductivity. Consequently, electrothermal modeling becomes crucial for a comprehensive understanding of the underlying physical mechanisms governing carrier degradation and thermal conduction in these nanoscale devices. In this study, we present an enhanced drift-diffusion model coupled with nonlocal Guyer-Krumhansl (GK) equations to accurately capture carrier-phonon interactions and explore the electrothermal characteristics of gate-all-around (GAA) VNWFETs. Pulsed current-voltage (I–V) measurements are employed to investigate the performance of a state-of-the-art 18nm VNWFET technology. Furthermore, we report on the influences of both trapping and SHE under high-bias conditions for varying pulse widths. Our findings reveal that optimization of mobility degradation mechanisms allows for improved control over the physical behavior of carrier transport in these emerging technologies. Through careful consideration of these factors, it becomes possible to enhance the overall performance of GAA VNWFETs, particularly in mitigating temperature hot-spots and addressing challenges associated with self-heating effects.

Electrical and thermal transport properties of Cr2Se3-Cr2Te3 solid-solution alloy system and estimation of optimal thermoelectric properties

Cr2Se3 is a layered narrow-bandgap semiconductor with a low lattice thermal conductivity of ∼1 W/mK, which can be considered as a potential thermoelectric material. A solid-solution alloy system with Cr2Te3 can be designed to manipulate the electrical and thermal transport properties. In this study, a series of Cr2Se3-Cr2Te3 solid-solution alloys (Cr2(Se1-xTex)3, x = 0, 0.33, 0.5, 0.67, 0.83, and 1) were synthesized, and their electrical and thermal transport properties were investigated. Regarding the electrical transport properties, the power factor (∼0.35 mW/mK2) of Cr2Se3 gradually and significantly decreased as the Te content (x) increased owing to the significant decrease in the Seebeck coefficient that resulted from the large increase in the carrier concentration. Regarding the thermal transport properties, the total thermal conductivity increased with x as a result of the large increase in the electrical conductivity, despite the continuous decrease in the lattice thermal conductivity. Consequently, the thermoelectric figure of merit (zT ∼ 0.20) of Cr2Se3 gradually and significantly decreased to 0.009 for Cr2Te3 at 700 K; thus, no enhancement of zT was observed from the experimental results of simple solid-solution alloying. On the other hand, the thermoelectric quality factors of the solid-solution compositions for x = 0.33–0.83 were found to be enhanced compared to that of the Cr2Se3 sample, implying that zT could be further enhanced by optimizing the carrier concentration. Indeed, enhanced zT values were estimated based on a single parabolic band model for the solid-solution compositions with x = 0.33–0.83 if the carrier concentration was optimized to ∼1019 cm−3, which was found to be owing to the increased nondegenerate and weighted mobility (inferring weaker carrier–phonon interaction). Therefore, it is suggested that the solid-solution alloying approach could provide a feasible strategy to enhance the thermoelectric performance by reducing carrier-phonon interaction when further combined with carrier concentration optimization.

Mixed quantum-classical modeling of exciton-phonon scattering in solids: Application to optical linewidths of monolayer MoS2.

We present a mixed quantum-classical framework for the microscopic and non-Markovian modeling of exciton-phonon scattering in solid-state materials and apply it to calculate the optical linewidths of monolayer MoS2. Within this framework, we combine reciprocal-space mixed quantum-classical dynamics with models for the quasiparticle band structure as well as the electron-hole and carrier-phonon interactions, parametrized against abinitio calculations, although noting that a direct interfacing with abinitio calculations is straightforward in principle. We introduce various parameters for truncating the Brillouin zone to select regions of interest. Variations of these parameters allow us to determine linewidths in the limit of asymptotic material sizes. The obtained asymptotic linewidths are found to agree favorably with experimental measurements across a range of temperatures. As such, our framework establishes itself as a promising route toward unraveling the non-Markovian and microscopic principles governing the nonadiabatic dynamics of solids.

Floquet nonadiabatic mixed quantum-classical dynamics in periodically driven solid systems.

In this paper, we introduce the Floquet mean-field dynamics and Floquet surface hopping approaches to study the nonadiabatic dynamics in periodically driven solid systems. We demonstrate that these two approaches can be formulated in both real and reciprocal spaces. Using the two approaches, we are able to simulate the interaction between electronic carriers and phonons under periodic drivings, such as strong light-matter interactions. Employing the Holstein and Peierls models, we show that strong light-matter interactions can effectively modulate the dynamics of electronic population and mobility. Notably, our study demonstrates the feasibility and effectiveness of modeling low-momentum carriers' interactions with phonons using a truncated reciprocal space basis, an approach impractical in real space frameworks. Moreover, we reveal that even with a significant truncation, carrier populations derived from surface hopping maintain greater accuracy compared to those obtained via mean-field dynamics. These results underscore the potential of our proposed methods in advancing the understanding of carrier-phonon interactions in various periodically driven materials.

Polaronic Mass Enhancement and Polaronic Excitons in Metal Halide Perovskites.

In metal halide perovskites, the complex dielectric screening together with low energy of phonon modes leads to non-negligible Fröhlich coupling. While this feature of perovskites has already been used to explain some of the puzzling aspects of carrier transport in these materials, the possible impact of polaronic effects on the optical response, especially excitonic properties, is much less explored. Here, with the use of magneto-optical spectroscopy, we revealed the non-hydrogenic character of the excitons in metal halide perovskites, resulting from the pronounced Fröhlich coupling. Our results can be well described by the polaronic-exciton picture where electron and hole interactions are no longer described by a Coulomb potential. Furthermore, we show experimental evidence that the carrier-phonon interaction leads to the enhancement of the carrier's effective mass. Notably, our measurements reveal a pronounced temperature dependence of the carrier's effective mass, which we attribute to a band structure renormalization induced by the population of low-energy phonon modes. This interpretation finds support in our first-principles calculations.

Energetic disorder dominates optical properties and recombination dynamics in tin-lead perovskite nanocrystals

Tin-lead alloyed perovskite nanocrystals (PNCs) offer a promising pathway toward low-toxicity and air-stable light-emitting devices. However, substantial energetic disorder has thus far hindered their lighting applications compared to pure lead-based PNCs. A fundamental understanding of this disorder and its impact on optical properties is crucial for overcoming this limitation. Here, using temperature-dependent static and transient absorption spectroscopy, we meticulously distinguish the contributions of static disorder (including defects, impurities, etc.) and dynamic disorder (carrier–phonon interactions). We reveal how these disorders shape band-tail structure and ultimately influence inter-band carrier recombination behaviors. Surprisingly, we find that static and dynamic disorder primarily control band-tail defect states and bandgap renormalization, respectively, which together modulate fast carrier trapping and slow band-band recombination rates. Furthermore, we link these disorders to the tin-induced symmetry-lowering distortions in tin-lead alloyed PNCs. These findings illuminate critical design principles for highly luminescent, low-toxicity tin-lead PNCs, accelerating their adoption in optoelectronic applications.

Noticeable influence of V-dopant on optoelectronic properties of ZnO films prepared by SILAR technique

Wide bandgap semiconductors such as Zinc Oxide (ZnO) have earned much attention on account of its interesting optical and electrical properties. Also, doping ZnO with suitable transition elements enables to tune its properties, making it suitable for applications like transparent conducting oxides and LEDs. To this aim, Vanadium-doped ZnO (V:ZnO) thin films are fabricated using the SILAR method and studied the structural, optical, vibrational, photophysical and electrical properties. Introducing vanadium creates lattice relaxation in the ZnO wurtzite structure, influencing its electronic band structure. The X-ray diffraction (XRD) analyses have verified the structural integrity of the vanadium-doped zinc oxide (V:ZnO) films, revealing their polycrystalline composition. The Raman spectroscopy shows the reduced phonon line widths due to the improved crystallite quality with the presence of vanadium. Furthermore, it has been noted that optical characteristics such as the bandgap and refractive index are sensitive to vanadium doping as a consequence of modified electronic structures. Hall-effect experiments have shown that undoped ZnO films possess a mobility of 1.31 cm2/Vs and a charge carrier density of 5.58 × 1013 cm−3. It has been observed that doping with V markedly influences these parameters of ZnO films. Then, the time-resolved photoluminescence measurements reveal the increased carrier lifetimes, suggesting the introduction of dopants mitigates carrier-phonon interactions. Such interactions need to be minimized to prevent the dissipation of carrier energy. In essence, the reported work demonstrates the potential of V:ZnO for optoelectronic applications.

Effect of temperature modulation on the performance of top contact bottom gate organic thin film transistor

This research article investigates the influence of temperature variations on the performance parameters of Top Contact Bottom Gate (TCBG) Organic Thin Film Transistors (OTFTs). Employing the Arrhenius equation and Schottky emission model, the study explores temperature-dependent charge carrier mobility and injection mechanisms. Analyzing a wide range of parameters, the research unveils a complex interplay between temperature and OTFT behavior. The linear and saturation mobilities exhibit a decline due to enhanced carrier-phonon interactions and lattice vibrations. Transconductance and ION/IOFF ratio display a concomitant decrease at higher temperatures, indicative of the heightened susceptibility of OTFT performance to temperature fluctuations. The depreciation in subthreshold slope and threshold voltage with rising temperatures underscores the influence of temperature on trap states. The research emphasizes the importance of considering temperature’s impact during OTFT design for diverse applications, enhancing the reliability and functionality of organic electronic devices.

Impacts of Exciton Binding Energy and Dielectric Confinement of Layered Lead Halide Perovskites on Carrier Relaxation and Exciton Phonon Interactions.

This work highlights the significance of dielectric confinements and exciton binding energy of hybrid layered perovskites (LPs) in controlling the carrier relaxation dynamics of LPs for designing efficient optoelectronic devices. The polarizability of organic spacer cations in LPs modulates the carrier-phonon and carrier-carrier interactions, which eventually control the carrier relaxation dynamics. Here, we have varied the alkyl-ammonium chain length in the LPs to change the dielectric confinement, and the first-principles calculations reveal that the long-chain organic spacer experiences stronger dielectric confinement in comparison to short-chain organic spacer cation-based LPs. Transient absorption spectroscopic analysis suggests that the larger dielectric confinement and higher exciton binding energy exhibit faster carrier relaxation dynamics. The enhanced exciton-phonon interaction leads to faster carrier relaxation dynamics. The much softer phonon modes are responsible for the higher up-conversion of acoustic modes to optical modes, which leads to slower carrier relaxation dynamics in n-butylamine (BA) based LPs.

Carrier–phonon interaction and anharmonic phonon decay in ZnS thin film studied by resonance Raman scattering

AbstractResonance Raman scattering was carried out on polycrystalline zinc sulfide (ZnS) thin film. The resonance Raman spectra revealed strong carrier–phonon interaction where longitudinal optical phonon (LO) with overtones up to the fourth order and its combination phonons with transverse acoustic (TA) phonons were observed, namely, nLO + mTA (where n and m are integers). The resonance Raman scattering processes were well explained within the framework of the cascade scattering model. The wavenumber of the LO phonon with increased temperature was dominated by anharmonic phonon decay. Detailed experimental data fittings showed that thermal expansion contribution to the nLO phonon overtones differed from the fundamental LO phonon due to the involvement of the scattering of the LO phonons at the Brillion zone edge. The negative value of the mode‐Grüneisen parameter for the TA phonon slowed down the wavenumber shift of the combination nLO + mTA phonon modes with increasing temperature compared to the nLO phonons. The anharmonic phonon decay of combination nLO + mTA phonons were dominated by anharmonic phonon decay of nLO phonons.

Phonons in the 1/f noise of topological insulators

In topological insulators, such as (Bi,Sb)2Te3 and BiSbTeSe1.6, the 1/f noise intensity features intriguing peaks, which develop at some specific temperatures. In search for their microscopic origin, we compared this noise structure with either phonon density of states or Raman spectrum of each topological insulator (TI), respectively. In (Bi,Sb)2Te3, the comparison revealed that the noise peaks track the van Hove singularities in the phonon density of states. The most intense noise peak observed in (Bi,Sb)2Te3 at 50 K is attributed to the thermal motion of the Bi atoms. Other less intense noise peaks are assigned to either a single phonon mode or multi-phonon combinations. We found that thermal vibrations of Bi and Te2 atoms in different symmetry directions are involved in most of the phonon combinations, which stand for the signature of the lattice anharmonicity in noise. The noise increase observed in (Bi,Sb)2Te3 and BiSbTeSe1.6 above a specific temperature threshold is attributed to the strengthening of the carrier–phonon coupling induced by anharmonicity. In the case of BiSbTeSe1.6, we show that all noise singularities are mirrored in the Raman spectrum of a structurally close TI (BiSbTeSe2) in the whole temperature range. This indicates that although transport can be at the surface or in the bulk or both of them, the carrier–phonon interaction is the only source of 1/f fluctuations in TIs. Inherently, these results imply that the microscopic origin of 1/f noise in solid is in the perpetual thermal motion of the atoms.

Enhanced thermoelectric properties in dual cation doped CuCrO2 nanocrystals mediated by magnon-carrier drag

The open-layered CuCrO2 delafossite has received great attention in medium-range temperature thermoelectric applications. The addition of Ni with Mg upsurged the carrier concentration and effective mass by 10-20 times owing to the multiple conduction bands in CuCrO2 hence increased mobility of 38.21 cm2/Vs was achieved. The power factor obtained is 350 μW/mK2 for the CuCr(MgNi)O2 (Mg=0.5, Ni=0.2 wt%) at 700 °C, which is above two times the Mg alone doped CuCrO2 nanocrystals. The lowest thermal conductivity of 3.43 W/Km and the highest ZT of 0.099 were also obtained for the same sample. The augmented Seebeck coefficient of 423 μV/K due to magnon-carriers interaction in the Cr3+ site, the enhanced conductivity of 2083 S/m, and reduced thermal conductivity caused by decreased carrier-phonon interactions are the reasons for the enhanced power factor and improved thermoelectric figure of merit.

Confinement and Exciton Binding Energy Effects on Hot Carrier Cooling in Lead Halide Perovskite Nanomaterials.

The relaxation of the above-gap ("hot") carriers in lead halide perovskites (LHPs) is important for applications in photovoltaics and offers insights into carrier-carrier and carrier-phonon interactions. However, the role of quantum confinement in the hot carrier dynamics of nanosystems is still disputed. Here, we devise a single approach, ultrafast pump-push-probe spectroscopy, to study carrier cooling in six different size-controlled LHP nanomaterials. In cuboidal nanocrystals, we observe only a weak size effect on the cooling dynamics. In contrast, two-dimensional systems show suppression of the hot phonon bottleneck effect common in bulk perovskites. The proposed kinetic model describes the intrinsic and density-dependent cooling times accurately in all studied perovskite systems using only carrier-carrier, carrier-phonon, and excitonic coupling constants. This highlights the impact of exciton formation on carrier cooling and promotes dimensional confinement as a tool for engineering carrier-phonon and carrier-carrier interactions in LHP optoelectronic materials.

Hot Carrier Cooling and Trapping in Atomically Thin WS2 Probed by Three-Pulse Femtosecond Spectroscopy

Transition metal dichalcogenides (TMDs) have shown outstanding semiconducting properties which make them promising materials for next-generation optoelectronic and electronic devices. These properties are imparted by fundamental carrier-carrier and carrier-phonon interactions that are foundational to hot carrier cooling. Recent transient absorption studies have reported ultrafast time scales for carrier cooling in TMDs that can be slowed at high excitation densities via a hot-phonon bottleneck (HPB) and discussed these findings in the light of optoelectronic applications. However, quantitative descriptions of the HPB in TMDs, including details of the electron-lattice coupling and how cooling is affected by the redistribution of energy between carriers, are still lacking. Here, we use femtosecond pump-push-probe spectroscopy as a single approach to systematically characterize the scattering of hot carriers with optical phonons, cold carriers, and defects in a benchmark TMD monolayer of polycrystalline WS2. By controlling the interband pump and intraband push excitations, we observe, in real-time (i) an extremely rapid "intrinsic" cooling rate of ∼18 ± 2.7 eV/ps, which can be slowed with increasing hot carrier density, (ii) the deprecation of this HPB at elevated cold carrier densities, exposing a previously undisclosed role of the carrier-carrier interactions in mediating cooling, and (iii) the interception of high energy hot carriers on the subpicosecond time scale by lattice defects, which may account for the lower photoluminescence yield of TMDs when excited above band gap.

Ultrafast decay of carrier momentum anisotropy in graphite

Time- and angle-resolved photoelectron spectroscopy is employed to study the thermalization of a nonequilibrium carrier distribution in the Dirac cone of graphite generated upon absorption of linearly polarized near-infrared laser pulses. The data reveal a characteristic time constant of $(20\ifmmode\pm\else\textpm\fi{}3)\phantom{\rule{0.16em}{0ex}}\mathrm{fs}$ for the decay of a photoinduced momentum anisotropy in the nascent carrier population. Additionally, the nascent carrier spectral peak shows a downshift in energy by $\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{m}\mathrm{eV}$ within the first $30\phantom{\rule{0.16em}{0ex}}\mathrm{f}\mathrm{s}$ after excitation, which is associated with the emission of strongly coupled ${A}_{1}^{\ensuremath{'}}$ optical phonons at $K$. In agreement with previous findings for graphene, a pronounced momentum anisotropy is also observed for an intermediate quasithermalized state of the carrier population. A fully thermalized distribution (with respect to energy and momentum) is finally established on a characteristic timescale of $(40\ifmmode\pm\else\textpm\fi{}10)\phantom{\rule{0.16em}{0ex}}\mathrm{fs}$. The results underscore the complexity of carrier thermalization on ultrafast timescales resulting from the interplay of carrier-carrier and carrier-phonon interaction.

Phonon-induced transition between entangled and nonentangled photon emission in constantly driven quantum-dot–cavity systems

Entangled photon pairs are essential for many applications in quantum technologies. Recent theoretical studies demonstrated that different types of entangled Bell states can be created in a constantly driven four-level quantum emitter-cavity system. Unlike other candidates for the realization of the four-level emitter, semiconductor quantum dots unavoidably interact with their environment, resulting in carrier-phonon interactions. Surprisingly, phonons change the entanglement of emitted photon pairs in a qualitative way, already at low temperatures on the order of 4 K. While one type of Bell state can still be generated using small driving strengths, the other type is suppressed due to phonon interactions in strongly-confined quantum dots. The degree of entanglement decreases with rising temperature and driving strength until it vanishes at a certain parameter value. Because it remains zero afterward, we encounter a phonon-induced transition between entangled and nonentangled photon emission that resembles a phase transition. The transition occurs at temperatures below 30 K and, independent of the driving strength, the concurrence as a function of the reduced temperature is found to obey a power law with exponent one near the transition point.

High-performance infrared photodetection beyond bandgap limitation based on surface plasmon resonance in sub-stoichiometry molybdenum oxide nanostructures

High-performance photodetectors in the near-infrared (NIR) regime are essential for many advanced applications, such as optical communication, intelligent driving, and imaging system. However, conventional photoconductive infrared detectors commonly suffer from slow response speed and narrow spectral response. Here, we demonstrate a high performance NIR photodetector based on plasmonic sub-stoichiometry molybdenum oxide (MoO3−x) nanostructures/graphene heterostructure. Empowered by surface plasmon resonance induced near-field enhancement in MoO3−x and the subsequent hot-electron injection (HEI), a fast response time (rise time ∼6.7 μs, decay time ∼12.5 μs), high responsivity (3.3 A/W), low noise equivalent power (∼4.9 pW/Hz1/2), as well as wide response range from visible light to NIR is obtained at room temperature. The weak carrier–phonon interaction in graphene prevents the relaxation of injected hot electrons and enables efficient electron extraction. The response speed is nearly four orders of magnitude improved compared with other graphene-based hybrid devices with similar device structures. Moreover, the interfacial HEI breaks the bandgap limits of molybdenum oxide and further extends the response spectrum of the device to conventional band (C-band) of optical communication. Our proposed device architecture offers new strategy for developing high-performance infrared photodetectors.

Carrier mobility and high-field velocity in 2D transition metal dichalcogenides: degeneracy and screening

The effect of degeneracy and the impact of free-carrier screening on a low-field mobility and a high-field drift velocity in MoS2 and WS2 are explored using an in-house ensemble Monte Carlo simulator. Electron low field mobility increases to for MoS2 and to for WS2 when temperature decreases to and carrier concentration is around . In the case of holes, best mobility values were and , reached at similar temperature and carrier concentration conditions while at room temperature these fall to and for MoS2 and WS2, respectively. The carrier screening effect plays a major role at low fields, and low and intermediate temperatures, where a combination of large occupancy of primary valleys and carrier–phonon interactions dominated by relatively low energy exchange processes results in an enhanced screening of intrinsic scattering. For electrons, degeneracy yields to transport in secondary valleys, which plays an important role in the decrease of the low field mobility at high concentrations and/or at room temperature. The high-field drift velocity is not much affected by carrier screening because of an increased carrier scattering with surface optical polar phonons, favouring larger phonon wavevector interactions with small dielectric function values.