Optical Phonon Energy Research Articles

Radiative recombination and electron-phonon coupling in lead-free CH3NH3SnI3 perovskite thin films

Solution-processed metal halide perovskites are attracting significant attention as high-performance photovoltaic materials. The electron-phonon interactions in lead halide perovskites are currently in the focus of interest because these interactions play an important role to the observed superior material properties and high device performance. Here, we investigated the electron-phonon coupling in lead-free $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Sn}{\mathrm{I}}_{3}$ thin films by analyzing the temperature-dependence of the photoluminescence (PL) spectra. The PL linewidth shows a superlinear dependence on the temperature, which suggests that the electron--longitudinal-optical (LO) phonon interaction is the dominating broadening mechanism of the optical transitions in $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Sn}{\mathrm{I}}_{3}$ near room temperature. Furthermore, from the analysis of the low-energy tail of the PL spectra by using the van Roosbroeck-Shockley relation, we determined the Urbach energy below the band gap and the optical-phonon energy that contributes to the optical absorption. This phonon energy coincides with the LO phonon energy determined from the PL width analysis. Our work quantitatively verifies that the electrons in tin halide perovskites efficiently couple with optical phonons, and demonstrates that their intrinsic optical properties at room temperature are comparable to those of lead halide perovskites implemented in high-efficiency solar cells.

Impact of temperature on exciton-cavity detuning and polariton relaxation kinetics in monolayer Tungsten Disulphide ([formula omitted]) based microcavity

Monolayer Tungsten Disulphide (WS2) with strongly confined 2D Wannier-Mott excitons exhibits a large binding energy and oscillator strength. This monolayer is currently emerging as a good candidate for studying strong light–matter coupling at high temperature. Here, we investigate the formation of exciton-polaritons in a monolayer WS2 based semiconductor microcavity under non-resonant excitation. Our results show that the cavity detuning changes from negative to positive values over a temperature range of 130–230 K, which allow the tuning of polariton dispersion. The Hopfield coefficient show that the Upper branch (UP) can be tuned from a more excitonlike (130 K), to a more photonlike (230 K) at small wave vector k. Thus, the UP states have a faster lifetime which enhances the relaxation mechanisms towards lower polariton (LP) states. A Rabi splitting of 40 meV is observed, which corresponds to energy of longitudinal optical (LO) phonon. To show how polaritons states are populated, we investigate a semiclassical model that treats the temporal dynamics of polaritons in which LO phonon-assisted polariton emission is taken into account. The results reveals that the UP occupation starts to decrease, for increasing the occupation factor in the LP branch.

Optical Properties of Bulk Gallium Oxide Grown from the Melt

Abstract Results of the studies of the properties of single-crystalline bulk β-Ga2O3 grown from the melt are presented. High chemical purity and phase uniformity of the grown material are demonstrated. Raman spectroscopy studies confirmed low energies of optical phonons in β-Ga2O3, which makes it a promising material for laser optics applications.

Optical absorption of Fröhlich polaron in monolayer transition metal dichalcogenides

We theoretically study the optical absorption of a Fröhlich polaron in monolayer transition metal dichalcogenides (TMDS) on different polar substrates using the Devreese-Huybrechts-Lemmens model, in which both the surface optical phonon modes induced by the polar substrate and the intrinsic longitudinal optical phonon modes have been taken into account. We find that the optical absorption occurs only as the energy of incident photon exceeds the energy of optical phonon. The behaviors of absorption are determined by the interplays between the strength of electron-optical phonon coupling and the energy of optical phonon. The amplitude of absorption can be tuned directly by the internal distance between TMDS and polar substrates. These theoretical results provide significant insight into the infrared absorption in two-dimensional materials.

Temperature dependence of long coherence times of oxide charge qubits

The ability to maintain coherence and control in a qubit is a major requirement for quantum computation. We show theoretically that long coherence times can be achieved at easily accessible temperatures (such as boiling point of liquid helium) in small (i.e., ~10 nanometers) charge qubits of oxide double quantum dots when only optical phonons are the source of decoherence. In the regime of strong electron-phonon coupling and in the non-adiabatic region, we employ a duality transformation to make the problem tractable and analyze the dynamics through a non-Markovian quantum master equation. We find that the system decoheres after a long time, despite the fact that no energy is exchanged with the bath. Detuning the dots to a fraction of the optical phonon energy, increasing the electron-phonon coupling, reducing the adiabaticity, or decreasing the temperature enhances the coherence time.

Characterization of the influences of morphology on the intrinsic properties of perovskite films by temperature-dependent and time-resolved spectroscopies.

Organic-inorganic halide perovskites have attracted enormous attention owing to their promising application in photovoltaic devices. The morphology of the perovskites is the key to driving the performance of perovskite devices, which necessitates a systematic study. In this work, two typical morphologies, i.e., flake and cube, of perovskite films are fabricated, and the temperature-dependent optical absorption and photoluminescence properties of the two types of perovskite film are systematically investigated. From the temperature-dependent spectra, both exciton and phase transition temperatures of the flake film are found to be about 10 K lower than those of the cube one. Meanwhile, the influences of the morphology on the exciton binding energy, optical phonon energy and polaron binding energy are quantitatively characterized. The exciton binding of the flake film is nearly three times smaller than that of the cube one, while the phonon coupling energy and the polaron binding energy of the former are about 5 meV and 2 meV larger than those of the latter. Furthermore, the results of photoluminescence lifetime and charge separation efficiency further reveal that the charge carrier kinetics in the two kinds of perovskite films is significantly different. The current study provides a theoretical framework to understand the fundamental physics of perovskites and to promote the design and enhancement of active materials for improved optoelectronic devices.

Nonlinear absorption and temperature-dependent fluorescence of perovskite FAPbBr3 nanocrystal.

Recent progress in solar cell and light-emitting devices makes halide perovskite a research hot spot in optics. In this Letter, the nonlinear absorption and fluorescence properties of FAPbBr3 nanocrystal, one typical organometallic halide perovskite, have been investigated via Z-scan measurements and a density-dependent photoluminescence (PL) spectrum. The FAPbBr3 nanocrystal exhibits nonlinear absorption under the excitation of 800nm, whose photon energy is below the bandgap of FAPbBr3. The significant absorption is experimentally confirmed to be induced by two-photon absorption (TPA), and the TPA coefficient is measured to be ∼0.0042 cm/GW. Moreover, the PL induced by TPA in FAPbBr3 nanocrystal shows different temperature-dependent behaviors in the range of 90 to 350K. The peaks of the PL spectrum remain nearly constant at 100-160 K, with a very shallow trough at around 150K, while a linear blue shift (0.496meV/K) of the spectrum is observed when temperature is above 160K. These temperature-dependent fluorescence behaviors can be ascribed to the structural phase transition at about 150K and the contribution of thermal expansion. Moreover, the exciton binding energy around 160meV and the optical phonon energy of 15.3meV are also extracted from the temperature-dependent PL data.

Vapor-Assisted Solution Approach for High-Quality Perovskite CH3NH3PbBr3 Thin Films for High-Performance Green Light-Emitting Diode Applications.

The vapor-assisted solution method was developed to prepare high-quality organic-inorganic halide perovskite CH3NH3PbBr3 (MAPbBr3) thin films. We detailedly investigated the effect of evaporation time and temperature of MABr powder on the microstructure, crystallinity, and optical characterizations of MAPbBr3 thin films, and a controllable morphology evolution with varying surface coverage was observed. Temperature-dependent and time-resolved photoluminescence measurements were carried out to investigate the optical transition mechanisms and carrier recombination dynamics of MAPbBr3 thin films. Our results revealed that no structural phase transition occurred within the heating process (10-300 K). In addition to the exciton-related emission, a trapped charge-carrier emission appeared at a critical temperature of 140 K. The corresponding temperature sensitivity coefficient of band gap, exciton binding energy, and optical phonon energy of the MAPbBr3 thin films were extracted from the experimental data. Furthermore, planar perovskite light-emitting diodes (PeLEDs) based on a Al/LiF/TPBi/MAPbBr3/NiO/ITO structure were fabricated, and a high-purity green emission at ∼532 nm with a low line width (25 nm) was achieved. The devices demonstrated remarkable performances with high luminance (6530 cd/m2), current efficiency (8.16 cd/A), external quantum efficiency (4.36%), and power efficiency (4.49 lm/W). This research will provide valuable information for the preparation of high-quality perovskite thin films, facilitating their future applications in novel high-performance PeLEDs.

Real-Time Observation of Exciton-Phonon Coupling Dynamics in Self-Assembled Hybrid Perovskite Quantum Wells.

Self-assembled hybrid perovskite quantum wells have attracted attention due to their tunable emission properties, ease of fabrication, and device integration. However, the dynamics of excitons in these materials, especially how they couple to phonons, remains an open question. Here, we investigate two widely used materials, namely, butylammonium lead iodide (CH3(CH2)3NH3)2PbI4 and hexylammonium lead iodide (CH3(CH2)5NH3)2PbI4, both of which exhibit broad photoluminescence tails at room temperature. We performed femtosecond vibrational spectroscopy to obtain a real-time picture of the exciton-phonon interaction and directly identified the vibrational modes that couple to excitons. We show that the choice of the organic cation controls which vibrational modes the exciton couples to. In butylammonium lead iodide, excitons dominantly couple to a 100 cm-1 phonon mode, whereas in hexylammonium lead iodide, excitons interact with phonons with frequencies of 88 and 137 cm-1. Using the determined optical phonon energies, we analyzed photoluminescence broadening mechanisms. At low temperatures (<100 K), the broadening is due to acoustic phonon scattering, whereas at high temperatures, LO phonon-exciton coupling is the dominant mechanism. Our results help explain the broad photoluminescence line shape observed in hybrid perovskite quantum wells and provide insights into the mechanism of exciton-phonon coupling in these materials.

Strong Carrier-Phonon Coupling in Lead Halide Perovskite Nanocrystals.

We highlight the importance of carrier–phonon coupling in inorganic lead halide perovskite nanocrystals. The low-temperature photoluminescence (PL) spectrum of CsPbBr3 has been investigated under a nonresonant and a nonstandard, quasi-resonant excitation scheme, and phonon replicas of the main PL band have been identified as due to the Fröhlich interaction. The energy of longitudinal optical (LO) phonons has been determined from the separation of the zero phonon band and phonon replicas. We reason that the observed LO phonon coupling can only be related to an orthorhombically distorted crystal structure of the perovskite nanocrystals. Additionally, the strength of carrier–phonon coupling has been characterized using the ratio between the intensities of the first phonon replica and the zero-phonon band. PL emission from localized versus delocalized carriers has been identified as the source of the observed discrepancies between the LO phonon energy and phonon coupling strength under quasi-resonant and nonresonant excitation conditions, respectively.

Competition between homogeneous and inhomogeneous broadening of orbital transitions in Si:Bi

We present results for the lifetime of the orbital transitions of Bi donors in Si, measured using both frequency domain and time-domain techniques, allowing us to distinguish between homogeneous and inhomogeneous processes. The proximity of the energy of the optically allowed transitions to the optical phonon energy means that there is an unusually wide variation in the lifetimes and broadening mechanisms for this impurity, from fully homogeneous lifetime-broadened transitions to fully inhomogeneously broadened lines. The relaxation lifetime (T1) of the states range from the low 10’s to 100’s of ps, and we find that there is little extra dephasing (so that T1 is of the order of T2/2) in each case.

Mid-infrared epsilon-near-zero modes in ultra-thin phononic films

We demonstrate strong, narrow-band selective absorption and subsequent selective thermal emission from ultra-thin planar films of polar materials at mid-infrared wavelengths. Our structures consist of AlN layers of varying thicknesses deposited upon molybdenum ground planes. We demonstrate coupling to the Berreman mode at frequencies at, or near, the longitudinal optical phonon energy of AlN. Samples are characterized experimentally by temperature-, angle-, and polarization-dependent Fourier transform infrared reflection and emission spectroscopy and modeled using a transfer matrix method approach. Strong, spectrally selective thermal emission, with near angle-independent spectral position, is demonstrated from an AlN layer with thickness t&amp;lt;λo/100.

Ultrafast Processes in Graphene: From Fundamental Manybody Interactions to Device Applications

AbstractA joint experiment‐theory investigation of the carrier dynamics in graphene, in particular in the energetic vicinity of the Dirac point, is reviewed. Radiation of low photon energy is employed in order to match the intrinsic energy scales of the material, i.e. the optical phonon energy (∼200 meV) and the Fermi energy (10‐20 meV), respectively. Significant slower carrier cooling is predicted and observed for photon energies below the optical phonon energy. Furthermore, a strongly anisotropic distribution of electrons in k‐space upon excitation with linearly polarized radiation is discussed. Depending on photon energy, the anisotropic distribution decays either rapidly via optical phonon emission, or slowly via non‐collinear Coulomb scattering. Finally, a room temperature operated ultra‐broadband hot‐electron bolometer is demonstrated. It covers the spectral range from the THz to visible region with a single detector element featuring a response time of 40 ps.

Charged excitons in monolayer WSe2 : Experiment and theory

Charged excitons, or X$^{\pm}$-trions, in monolayer transition metal dichalcogenides have binding energies of several tens of meV. Together with the neutral exciton X$^0$ they dominate the emission spectrum at low and elevated temperatures. We use charge tunable devices based on WSe$_2$ monolayers encapsulated in hexagonal boron nitride, to investigate the difference in binding energy between X$^+$ and X$^-$ and the X$^-$ fine structure. We find in the charge neutral regime, the X$^0$ emission accompanied at lower energy by a strong peak close to the longitudinal optical (LO) phonon energy. This peak is absent in reflectivity measurements, where only the X$^0$ and an excited state of the X$^0$ are visible. In the $n$-doped regime, we find a closer correspondence between emission and reflectivity as the trion transition with a well-resolved fine-structure splitting of 6~meV for X$^-$ is observed. We present a symmetry analysis of the different X$^+$ and X$^-$ trion states and results of the binding energy calculations. We compare the trion binding energy for the $n$-and $p$-doped regimes with our model calculations for low carrier concentrations. We demonstrate that the splitting between the X$^+$ and X$^-$ trions as well as the fine structure of the X$^-$ state can be related to the short-range Coulomb exchange interaction between the charge carriers.

Optical phonon dynamics and electronic fluctuations in the Dirac semimetal Cd3As2

Raman scattering in the three-dimensional Dirac semimetal $\mathrm{C}{\mathrm{d}}_{3}\mathrm{A}{\mathrm{s}}_{2}$ shows an intricate interplay of electronic and phonon degrees of freedom. We observe resonant phonon scattering due to interband transitions, an anomalous anharmonicity of phonon frequency and intensity, as well as quasielastic $(E\ensuremath{\sim}0)$ electronic scattering. The latter two effects are governed by a characteristic temperature scale ${T}^{*}\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ that is related to mutual fluctuations of lattice and electronic degrees of freedom. A refined analysis shows that this characteristic temperature corresponds to the energy of optical phonons which couple to interband transitions in the Dirac states of $\mathrm{C}{\mathrm{d}}_{3}\mathrm{A}{\mathrm{s}}_{2}$. As electron-phonon coupling in a topological semimetal is primarily related to phonons with finite momenta, the back action on the optical phonons is only observed as anharmonicities via multiphonon processes involving a broad range of momenta. The resulting energy density fluctuations of the coupled system have previously only been observed in low dimensional or frustrated spin systems with suppressed long range ordering.

A two-dimensional electron gas in donor–acceptor doped backward heterostructures

We propose a backward-diode heterostructure modified by a built-in acceptor-doped layer that forms an additional potential barrier decreasing the transverse transport of hot electrons to the substrate. According to calculations, this structure is characterized by (i) an energy difference between dimensional quantization levels that is several times the optical phonon energy in GaAs and (ii) increased linearity of transfer characteristics.

Impact of film thickness on optical and electrical transport properties of noncrystalline GeSe1.4Sn0.6 films

The optical and electrical properties for GeSe1.4Sn0.6 with a different film thickness of 145, 230 and 343nm were studied. Structural properties affirm the amorphous nature of these films at different thickness. Transmittance (T) and Reflectance (R) of these films are measured with different film thickness and then the other optical properties are calculated. The calculated linear optical properties illustrated that the optical band gap, phonon energy, refractive index and dielectric constants of these films are increasing with increasing film thickness. On the other hand, the calculated third-order nonlinear optical susceptibility is found to be a function of the film thickness of GeSe1.4Sn0.6 films. Regarding the electrical properties, both dc electrical conductivity, and current-voltage dependence are studied for GeSe1.4Sn0.6 with different film thickness. The dc electrical conductivity as a function of the temperature is verified the Mott and Davis model. The experimental and calculated results have shown that the present film has two regions on the current-voltage relation. In the ohmic district, the conduction has no traps of electron in the forbidden gap of present films. In the range of higher voltage, the current is expected a limited space charge which controlled by a single trap level.

Tunneling at room temperature in GaAs/AlGaAs asymmetric coupled double quantum wells observed via time resolved photoluminescence spectroscopy

We have demonstrated experimental evidence of non-resonant tunneling at room temperature in GaAs/AlGaAs asymmetric coupled double quantum wells (ACDQW’s) using time-resolved photoluminescence (TRPL) spectroscopy at 300 K. Two ACDQW samples (A and B) with a barrier thickness of 25 Å were grown via molecular beam epitaxy. The energy separation (ΔE) between the ground state of the conduction band of the wide well and that of the narrow well are 42.7 meV and 19.5 meV, for samples A and B respectively. The TRPL measurement revealed a double decay rate in sample A whose ΔE is greater than one GaAs longitudinal optical phonon energy (36 meV), suggesting a phonon assisted tunneling mechanism. The evidence of tunneling was supported by measuring the relative intensity of the PL contributions from the narrow and wide well at 10 K.

Impact of Recess Etching on the Temperature-Dependent Characteristics of GaN-Based MIS-HEMTs With Al2O3/AlN Gate-Stack

This paper studied the recess-etching effects on the temperature-dependent characteristics of AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) with Al2O3/AlN gate-stack. The 12.6 nm recess-etching resulted in voltage shift of capacitance-voltage curves by 2.4 V, improved peak field-effect mobility ( $\mu _{\text {FE}})$ from 1906 to 2036 cm2/ $\text{V}\cdot \text{s}$ , and increased peak transconductance ( ${g}_{\text {max}})$ from 272 to 353 mS/mm. The temperature-dependent measu-rement from 298 to 473 K showed decrease in maximum drain current, ${g}_{\text {max}}$ , and $\mu _{\text {FE}}$ for both devices with and without recess etching, attributed to thermal electron emission and carrier depletion effects. Recess etching did not degrade the thermal stability of carrier distribution and the transport properties. Temperature-dependent $\mu _{\text {FE}}$ analysis revealed that optical phonon scattering dominated the transport mechanism of Al2O3/AlN/ AlGaN/GaN MIS-HEMTs. Optical phonon energy of 74 and 77 meV were obtained for the devices with and without recess etching, respectively.

Effect of CH 3 NH 3 I concentration on the physical properties of solution-processed organometal halide perovskite CH 3 NH 3 PbI 3

Abstract Organic-inorganic halide perovskite CH 3 NH 3 PbI 3 films were synthesized by low-temperature solution method by using a two-step spin-coating technique. We detailedly investigated the effect of the concentration of CH 3 NH 3 I solution on the morphology and optical properties of CH 3 NH 3 PbI 3 films, and a regular morphology evolution was observed. Further, temperature-dependent photoluminescence measurement was performed, and a structural phase transition of the CH 3 NH 3 PbI 3 films at 150 K was revealed by examining the temperature dependence of the peak positions and widths. In addition, the corresponding exciton binding energy and optical phonon energy of the CH 3 NH 3 PbI 3 films were extracted in the high-temperature tetragonal phase based on the experimental data. It can be anticipated that our results obtained provide valuable information for the understanding on the optical properties of perovskite CH 3 NH 3 PbI 3 , and also their future applications in optoelectronic devices.