Multiphonon Emission Process Research Articles

Trap-Assisted Auger-Meitner Recombination from First Principles.

Trap-assisted nonradiative recombination is known to limit the efficiency of optoelectronic devices, but the conventional multiphonon emission (MPE) process fails to explain the observed loss in wide-band-gap materials. Here, we highlight the role of trap-assisted Auger-Meitner (TAAM) recombination and present a first-principles methodology to determine TAAM rates due to defects or impurities in semiconductors or insulators. We assess the impact on efficiency of light emitters in a recombination cycle that may include both TAAM and carrier capture via MPE. We apply the formalism to the technologically relevant case study of a calcium impurity in InGaN, where a Shockley-Read-Hall recombination cycle involving MPE alone cannot explain the experimentally observed nonradiative loss. We find that, for band gaps larger than 2.5eV, the inclusion of TAAM results in recombination rates that are orders of magnitude larger than recombination rates based on MPE alone, demonstrating that TAAM can be a dominant nonradiative process in wide-band-gap materials. Our computational formalism is general and can be applied to the calculation of TAAM rates in any semiconducting or insulating material.

Correlated insulator collapse due to quantum avalanche via in-gap ladder states

The significant discrepancy observed between the predicted and experimental switching fields in correlated insulators under a DC electric field far-from-equilibrium necessitates a reevaluation of current microscopic understanding. Here we show that an electron avalanche can occur in the bulk limit of such insulators at arbitrarily small electric field by introducing a generic model of electrons coupled to an inelastic medium of phonons. The quantum avalanche arises by the generation of a ladder of in-gap states, created by a multi-phonon emission process. Hot-phonons in the avalanche trigger a premature and partial collapse of the correlated gap. The phonon spectrum dictates the existence of two-stage versus single-stage switching events which we associate with charge-density-wave and Mott resistive phase transitions, respectively. The behavior of electron and phonon temperatures, as well as the temperature dependence of the threshold fields, demonstrates how a crossover between the thermal and quantum switching scenarios emerges within a unified framework of the quantum avalanche.

Multiphonon quantum dynamics in cavity optomechanical systems

The multiphonon quantum dynamics in laser-pumped cavity optomechanical samples containing a vibrating mirror is investigated. Especially, we focus on dispersive interaction regimes where the externally applied coherent field frequency detuning from the optical resonator frequency is not equal to the mirror's oscillating frequency or to its multiples. As a result, for moderately strong couplings among the involved subsystems, the quantum dynamics of this complex system is described by multiphonon absorption or emission processes, respectively. Particularly, we demonstrate efficient ways to monitor the phonon quantum dynamics via photon detection. The possibility to extract the relevant sample parameters, for instance, the coupling strength between the mechanical mirror and the electromagnetic field, is also discussed.

Impacts of carrier capture processes in the thermal quenching of photoluminescence in Al–N co-doped SiC

High concentrations of aluminum (Al) and nitrogen (N) dopants of 6H SiC have been achieved by a fast sublimation growth process. The Al–N co-doped 6H-SiC layer exhibits a strong light-blue photoluminescence emission at low temperatures due to emissions from DI centers and donor acceptor pairs (DAP). The photoluminescence quenching mechanisms of those emissions are different. The decrease of free carrier capture cross-section as temperature increases according to the cascade capture process causes quenching of the photoluminescence emission form DI centers. Emission from Al–N DAP centers exhibits an exponential quenching with activation energy of (95 ± 10) meV. This is attributed to a competing hole capture by non-radiative defect in a multiphonon emission process.

First Principles Approach to Analyze Defect-induced Multiphonon Transition at the Si-SiO₂ Interface

The hole capture coefficients of defects through the non-radiative multiphonon emission process are calculated by using the first principle approach. Defects in the silicon bulk and the Si-SiO₂ interface are considered in the calculation. From the defect energy level and the phonon modes, which are calculated by using the density functional theory code, the transition rate is rigorously evaluated. The calculated hole capture parameters of the defect states exhibit physically reasonable behaviors within smaller magnitudes.

Shift and broadening of emission lines in Nd3+:YAG laser crystal influenced by input energy

Spectroscopic properties of the flashlamp-pumped Nd 3+:YAG laser as a function of input energy were studied over the range of 18–75 J. The spectral widths and shifts of quasi-three-level and four-level inter-Stark emissions within the respective intermanifold transitions of $^{\mathrm {4}}\mathrm {F}_{\mathrm {3/2}}\to ^{\mathrm {4}}{\kern -2.7pt}\mathrm {I}_{\mathrm {9/2}}$ and $^{\mathrm {4}}\mathrm {F}_{\mathrm {3/2}}\to ^{\mathrm {4}}{\kern -2.7pt}\mathrm {I}_{\mathrm {11/2}}$ were investigated. The emission lines of $^{\mathrm {4}}\mathrm {F}_{\mathrm {3/2}}\to ^{\mathrm {4}}{\kern -2.7pt}\mathrm {I}_{\mathrm {9/2}}$ shifted towards longer wavelength (red shift) and broadened, while the positions and linewidths of the $^{\mathrm {4}}\mathrm {F}_{\mathrm {3/2}}\to ^{\mathrm {4}}{\kern -3.5pt}\mathrm {I}_{\mathrm {11/2}}$ transition lines remained constant by increasing the pumping energy. This is attributed to the thermal population as well as one-phonon and multiphonon emission processes in the ground state. This phenomenon degrades the output performance of the lasers.

Shift and broadening of quasi three level emission lines in Nd3+:YAG laser crystal influenced by input energy

The pumping energy effect on fluorescence emission properties of Nd3+:YAG laser host pumped by flashlamp was studied over the range of 18–75J. The spectral widths and shifts of the quasi three level inter-Stark transitions at 938.5 and 946.0nm lines within the respective intermanifold transitions of 4F3/2→4I9/2 were investigated. The quasi three level emission lines shifted toward the longer wavelength (red shift) and broadened by increasing pumping energy. This was attributed to the thermal population as well as one-phonon and multi-phonon emission process in the ground state. These phenomena obstruct the output performance of quasi three level lasers.

Breakdown of the linear acousto-optic interaction regime in phoxonic cavities.

The limits of validity of the linear photoelastic model are investigated in a one-dimensional dual photonic-phononic cavity, formed by alternating layers of a chalcogenide glass and a polymer homogeneous and isotropic material, which supports both optical and acoustic resonant modes localized in the same region. It is shown that the linear-response regime breaks down when either the acoustic excitation increases or the first-order acousto-optic interaction coupling element vanishes by symmetry, giving rise to the manifestation of multiphonon absorption and emission processes by a photon. Our results provide a consistent interpretation of different aspects of the underlying physics relating to nonlinear acousto-optic interactions that can occur in such cavities.

The role of C-H and C-C stretching modes in the intrinsic non-radiative decay of triplet states in a Pt-containing conjugated phenylene ethynylene

The intrinsic non-radiative decay (internal conversion) from the triplet excited state in phosphorescent dyes can be described by a multi-phonon emission process. Since non-radiative decay of triplet excitons can be a significant process in organic light-emitting diodes, a detailed understanding of this decay mechanism is important if the overall device efficiency is to be controlled. We compare a deuterated Pt(II)-containing phenylene ethynylene with its non-deuterated counterpart in order to investigate which phonon modes control to the non-radiative decay path. We observe that deuteration does not decrease the non-radiative decay rate. A Franck-Condon analysis of the phosphorescence spectra shows that the electronic excitation is coupled strongly to the breathing mode of the phenyl ring and the C≡C carbon stretching modes, while high-energy C-H or C-D stretching modes play an insignificant role. We, therefore, associate the internal conversion process with the carbon-carbon stretching vibrations.

Resonant Raman Spectra of CdTe/ZnTe Self Assembled Quantum Dots

In this paper we present Raman scattering and photoluminescence spectra measured on CdTe/ZnTe self assembled quantum dots. The photoluminescence spectrum has shown two main emission peaks, both connected with existing of quantum dots. One presents direct deexcitation to ground state and the other is optical phonon (ω = 207.3 cm−1)-assisted deexcitation. The registered multiphonon emission process depends on temperature. At low temperature phonon spectra shows line at 450 cm−1 (TA+2LO in ZnTe) and 595 cm−1 (2LO+TO(Γ ) in ZnTe).

Raman spectra of CdTe/ZnTe self-assembled quantum dots

In this paper, we present Raman scattered spectra and results of photoluminescence (PL) measurements of CdTe/ZnTe self-assembled quantum dots (SAQD). The PL spectrum displayed two main emission peaks, both connected with the existing of QDs. One presents direct deexcitation to ground state and the other is optical phonon (ω=204.2cm−1)-assisted deexcitation. The registered multiphonon (MP) emission process depends on temperature. At low temperature, one-phonon spectra shows line at 200.4 and 210.3cm−1 (ZnTe LO mode confined by degenerate superlattices), 217cm−1 (TA+LO in ZnTe) and 386cm−1 (LO+TO(Γ) in ZnTe).

Raman spectroscopy of Pb 0.75Sn 0.25Te(In) single crystal

In this work we present galvanomagnetic measurements and Raman scattering spectra of Pb 0.75Sn 0.25Te + 1.2 at.% In. Positions and concentrations of different In impurity states are obtained using the Gibbs distribution and results of galvanomagnetic measurements. In Raman spectra, modes at about 90, 170, 340, 510 and 570 cm −1 were observed at all temperatures. These modes were discussed in terms of multiphonon emission (MPE) process on DX-like centers.

Blockade of free carriers by hopping carriers leading to the low-frequency current oscillations in semi-insulating GaAs

Semi-insulating GaAs samples show low-frequency current oscillations (LFO) under high dc voltage. These spontaneous oscillations are caused by traveling high-electric-field domains and can be explained based on the well-accepted model of electron trapping in deep levels by multiphonon emission processes. This paper aims to explain today the most important open question about LFO, namely, the cause of the very slow propagation of the domain resulting in the low frequencies. Our claim is that hopping conduction acts as a low mobility barrier for the free carriers leading to the formation of the electric-field domains. The barrier blocks, through a Coulomb shield, the flow of the high mobility free carriers leading to a charge accumulation, which produces the current pulse in the external circuit when the domain reaches the anode where it is destroyed. LFO occurs, therefore, only in the regime of band and hopping mixed conduction mechanisms.

Single-step multiphonon emission model of spin-dependent exciton formation in organic semiconductors

The maximum efficiency in organic light-emitting diodes (OLEDs) depends on the ratio, $r=k_S/k_T$, where $k_S$ ($k_T$) is the singlet (triplet) exciton formation rate. Several recent experiments found that r increases with increasing oligomer length from a value $r \approx 1$ in monomers and short oligomers. Here, we model exciton formation as a multi-phonon emission process. Our model is based on two assertions: (i) More phonons are emitted in triplet formation than in singlet formation. (ii) The Huang-Rhys parameter for this phonon emission is smaller in long oligomers than in short ones. We justify these assertions based on recent experimental and theoretical data.

Multiphonon emission process on DX‐like centers in indium doped Pb 0.75 Sn 0.25 Te

In this work we present Raman scattering spectra of Pb0.75Sn0.25Te + 1.2 at% In. Modes at about 90, 170, 340, 510, and 570 cm–1 were observed at all temperatures. These modes were discussed in terms of a multiphonon emission (MPE) process on DX-like centers.

Experimental measurement of ultracold neutron production in superfluid 4He

The absolute production rate of ultracold neutrons (UCN) produced by the interaction of a cold neutron beam with superfluid helium has been measured over an incident energy range of 0.7 to 4 meV. The neutrons are reduced in energy to become UCN by creating phonon(s) in the superfluid. The separate roles played by single and multi-phonon emission processes have been identified. Detection and identification of UCN, those neutrons with energies less than ∼250 neV and which can be stored in material bottles, were carried out using solid-state silicon detectors set within the superfluid helium. With a cold neutron flux of 2.62×10 7 neutrons cm −2 s −1 A ̊ −1 at 8.9 Å in the superfluid, the single-phonon production rate of UCN was measured to be (0.91±0.13) cm −3 s −1, a value close to theoretical prediction. Multi-phonon emission processes for UCN production by higher energy neutrons were also observed and, in the beam used for this work at ILL, they contributed (24±2)% to the overall UCN production rate.

Ultrafast relaxation dynamics of photoexcited states in semiconducting single-walled carbon nanotubes

We have experimentally investigated the relaxation dynamics of photoexcited states in semiconducting single-walled carbon nanotubes by means of femtosecond spectroscopy. The photoexcited electrons and holes are nonradiatively recombined via multi-phonon emission process within ∼1 ps . The tube diameter dependence of decay time suggests that the electron–phonon interaction is enhanced by decreasing the tube diameter.

Vibrational relaxation rates for H on a Si(1 0 0):(2×1) surface: a two-dimensional model

The results of calculations based on perturbation theory of vibrational relaxation rates due to coupling to substrate phonons for hydrogen atoms adsorbed on a Si(1 0 0):(2×1) surface are presented. For this purpose a two-dimensional model is adopted in which both the H–Si stretching and bending motions are included. It is shown that within this model the multiphonon emission and absorption processes play a negligible role. The calculated lifetimes agree well with available experimental data.

The energy gap law for triplet states in Pt-containing conjugated polymers and monomers.

The energy gap law established for aromatic hydrocarbons and rare earth ions relates the nonradiative decay rate to the energy gap of a transition through a multiphonon emission process. We show that this energy gap law can be applied to the phosphoresce of a series of conjugated polymers and monomers for which the radiative decay rate has been enhanced through incorporation of a heavy metal. We find that the nonradiative decay rate from the triplet state T(1) increases exponentially with decreasing T(1)-S(0) gap for the polymers and monomers at 300 and 20 K. Comparison of the nonradiative decay of polymers with that of their corresponding monomers highlights the role of electron-lattice coupling.

Radiative and nonradiative recombination in polymerlikea−C:Hfilms

Localized electronic states corresponding to \ensuremath{\pi} electrons in hydrogenated amorphous carbon have been investigated using photoluminescence (PL) and optical-absorption spectroscopies. Carbon films contain \ensuremath{\pi}-bonded ``grains'' with different configurations which spatially confine photogenerated electron-hole pairs. A model of PL in $a\ensuremath{-}\mathrm{C}:\mathrm{H},$ including excitonlike radiative recombination and electron tunneling to nonradiative acceptor centers at constant energy, is proposed to explain the following results. The broad PL emission spectra are composed of three bands (at 2.20--2.40 eV, 2.65 eV, and 2.95 eV, independent of the excitation energy) arising from different structural units containing $\mathrm{C}\mathrm{=}\mathrm{C}$ double bonds. Resonance features have been evidenced in the PL excitation spectra and attributed to excitation of confined electron-hole pairs similar to confined exciton optical transitions. Excitonic behavior is consistent with the redshift of peak (b) excitation resonance observed as a function of increasing gap (decreasing \ensuremath{\varepsilon}). Relative values of the radiative rates deduced from PL efficiency indicate that the exciton radius has the same order of magnitude for all three emissive centers. Nonradiative recombination cannot be explained by multiphonon emission processes. The dependence of PL efficiency on the density of acceptor sites (deduced from $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ optical transitions) can rather be described by electron tunneling from a confined exciton toward nonradiative localized ${\ensuremath{\pi}}^{*}$ states arising from distorted \ensuremath{\pi}-bonded sites. This mechanism accounts for the anticorrelation of the PL efficiency with the optical gap, the decrease of the decay time as a function of emission energy, and the relative enhancement of the blue emission (2.95-eV peak) as a function of decreasing optical gap. In contrast, the band-tail model proposed earlier leads to unrealistically low values of the electron Bohr radius.