Multiphonon Emission Research Articles

Electronic Energy Transfer from Excitons Confined in Silicon Nanocrystals to Molecular Oxygen

AbstractWe report on efficient electronic energy transfer from excitons confined in silicon (Si) nanocrystals to molecular oxygen (MO). The remarkable photosensitizing properties of Si nanocrystal assemblies result from a broad energy spectrum of photoexcited excitons, a long triplet exciton lifetime and a highly developed surface area. Quenching of photoluminescence (PL) of Si nanocrystals by MO physisorbed on their surface is found to be most efficient when the energy of excitons coincides with the triplet-singlet splitting energy of oxygen molecules. Spectroscopic analysis of the quenched PL spectrum evidences that energy transfer is accompanied by multi-phonon emission. From time-resolved measurements the characteristic time of energy transfer is found to be in the range of microseconds. The dependence of PL quenching efficiency on the surface termination of nanocrystals is consistent with short-range resonant electron exchange mechanism of energy transfer. The energy transfer to oxygen molecules in the gaseous phase at elevated temperatures is demonstrated.

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.

Emission cross sections and spectroscopy of Ho/sup 3+/ laser channels in KGd(WO/sub 4/)/sub 2/ single crystal

The spectroscopic properties of Ho/sup 3+/ laser channels in KGd(WO/sub 4/)/sub 2/ crystals have been investigated using optical absorption, photoluminescence, and lifetime measurements. The radiative lifetimes of Ho/sup 3+/ have been calculated through a Judd-Ofelt (JO) formalism using 300-K optical absorption results. The JO parameters obtained were /spl Omega//sub 2/=15.35/spl times/10/sup -20/ cm/sup 2/, /spl Omega//sub 4/=3.79/spl times/10/sup -20/ cm/sup 2/, /spl Omega//sub 6/=1.69/spl times/10/sup -20/ cm/sup 2/. The 7-300-K lifetimes obtained in diluted (8/spl middot/10/sup 18/ cm/sup -3/) KGW:0.1% Ho samples are: /spl tau/(/sup 5/F/sub 3/)/spl ap/0.9 /spl mu/s, /spl tau/(/sup 5/S/sub 2/)=19-3.6 /spl mu/s, and /spl tau/(/sup 5/F/sub 5/)/spl ap/1.1 /spl mu/s. For Ho concentrations below 1.5/spl times/10/sup 20/ cm/sup -3/, multiphonon emission is the main source of non radiative losses, and the temperature independent multiphonon probability in KGW is found to follow the energy gap law /spl tau//sub ph//sup -1/(0)=/spl beta/exp(-/spl alpha//spl Delta/E), where /spl beta/=1.4/spl times/10/sup -7/ s/sup -1/, and /spl alpha/=1.4/spl times/10/sup 3/ cm. Above this holmium concentration, energy transfer between Ho impurities also contributes to the losses. The spectral distributions of the Ho/sup 3+/ emission cross section /spl sigma//sub EM/ for several laser channels are calculated in /spl sigma/- and /spl pi/-polarized configurations. The peak a /spl sigma//sub EM/ values achieved for transitions to the /sup 5/I/sub 8/ level are /spl ap/2/spl times/10/sup -20/ cm/sup 2/ in the /spl sigma/-polarized configuration, and three main lasing peaks at 2.02, 2.05, and 2.07 /spl mu/m are envisaged inside the /sup 5/I/sub 7//spl rarr//sup 5/I/sub 8/ channel.

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.

Multi phonon emission in InAs quantum wells studied with a free electron laser and high pulsed magnetic fields

The short pulsed, tunable output from the Dutch free electron laser FELIX, coupled with a 45 T pulsed magnet has been used to measure the non-radiative decay rates between Landau levels in n-type InAs/GaSb quantum wells. The decay rate is 2 ps −1 when the cyclotron resonance energy, E CR= E LO=30 meV, and only slightly less, 0.8 ps −1 when E CR=2 E LO when the principal relaxation pathway is the emission of multiple longitudinal optical (LO) phonons.

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.

Deep defect levels and exciton dissociation in conjugated polymers

We study the localized deep electronic level within the bandgap of conjugated polymers in the presence of structure or oxygen (carbonyl group) defects. The structure defect is modeled as a chain twist, including the chain ends as a special case. Analytic expressions for both the energy and the wavefunction of the deep levels and the itinerant levels are obtained and supplemented by clear intuitive pictures. Carbonyl group is treated numerically within the tight-binding models. The rates of free carrier capture and exciton dissociation through the defect level via multi-phonon emission are calculated. We conclude that the defect dissociation dominates the intrinsic dissociation through thermal activation, and is the primary carrier generation mechanism in photoconductivity. Our results explain the photoconductivity enhancement due to oxidation, as well as the recent observation on the temperature-independent photocurrent in the sweep-out regime.

Cyclotron resonance of a two-dimensional Wigner solid

Cyclotron resonance absorption from a two-dimensional electron solid formed on a free surface of liquid helium is analyzed by means of the memory function formalism. The presence of the low-frequency phonon mode of the Wigner solid in a strong magnetic field is shown to broaden significantly the electron dynamical structure factor (DSF) as a function of frequency, strengthening the contribution from multiphonon emission terms of the high-frequency mode. Thus in most cases the linewidth is formed by nonlinear terms of the DSF expansion in the cyclotron motion factor exp(−iωct) rather than by the linear terms analyzed previously. The inclusion of all these multiphonon terms changes the sign of the many-electron effect and agrees well with available experimental data, eliminating the discrepancy between theory and experiment.

Thulium-doped Ca4GdO(BO3)3 crystals. An investigation ofradiative and non-radiative processes

An extensive investigation of optical properties was carried out forTm3+-doped Ca4GdO(BO3)3 crystals in the 5-300 Ktemperature range. Based on absorption and emission spectra at 5 K, crystalfield levels of active ions have been located. Non-equivalent sites for Tm3+ in the Ca4GdO(BO3)3 matrix have been demonstrated.Lifetimes and the fluorescence dynamics were studied as a function oftemperature and concentration of thulium ions. The Judd-Ofelt analysis ofabsorption spectra and the Inokuti-Hirayama model for energy transfer betweenactivator ions provided information on radiative processes and non-radiativerelaxation of excited states. Additionally, multiphonon emission was takeninto account to explain a very strong quenching of the 3H4 and the3F4 fluorescence.

Study of thin gate dielectric films using deep level transient spectroscopy

This work studies the hole trap distributions at the thin oxide and nitrided-oxide interface using the Deep Level Transient Spectroscopy (DLTS) technique which was found to be not suitable for very thin oxide. By biasing the MIS capacitor at weak inversion, the fluctuation due to minority carrier capture is minimized and the weak DLTS signals in the thin-gate (16.8nm) capacitors are recorded. The proposed weak inversion technique also provides a hint for probing hole trap distribution near the valence band edge. However, a large offset in the trap level is found and needed to be calibrated. It is found in re-oxidized insulator that the highest interface trap level is about 3.5×1012eV−1cm−2 over the lower half band while for that of the nitrided oxide, it is just above half of the value. In addition, the capture cross-section for the nitrided samples is found to be of similar magnitude as well as temperature dependent. This phenomenon suggests that the states are of similar nature. The trends of the capture cross-section for the three samples support the model of lattice relaxation multiphonon emission.

Infrared to visible up-conversion study for erbium-dopedzinc tellurite glasses

Optical absorption and photoluminescence properties of Er3+-doped70TeO2-30ZnO glass are investigated. Judd-Ofelt intensityparameters of Er3+ have been determined to calculate the radiativetransition probabilities and the radiative lifetimes of excited states. Aninfrared to visible up-conversion was observed at room temperature in thistellurite glass system using a 797 nm excitation line. A study of the 4S3/2-4I15/2 transition (554 nm) versus power excitationprovided evidence for a two-step up-conversion process under this excitation.A red emission (663 nm) originating from the 4F9/2-4I15/2 transition has been observed as well. It was found that theefficiency of this up-conversion line is enhanced considerably with the Er3+ concentration relative to the green emission (554 nm). Thisbehaviour has been explained in terms of an energy transfer between excitedions. The temperature dependence of up-conversion intensity has been alsostudied in the range 40-310 K. It was found that the thermal quenching of thegreen emission (4S3/2-4I15/2) is large enoughcompared with those of the red transition (4F9/2-4I15/2 ). This thermal quenching has been discussed using the Riseberg andMoos model of multiphonon emission. It has been shown that the latter approachis not consistent with existing results. A complete analysis of thetemperature-dependent up-conversion has been made using an additional decayrate which may be attributed to a non-radiative energy transfer and/or acharge transfer through trapping impurities. A good agreement has beenachieved between measured and computed data.

Temporal evolution of resonant Raman-scattering in ZnCdSe quantum dots

We investigated ZnCdSe/ZnSe quantum-dot structures which include planar and coherently strained three-dimensional islands with different sizes. Optical excitation of these islands well below the ZnSe band gap leads to a resonant enhancement of the Zn0.7Cd0.3Se longitudinal-optical (LO) phonon-scattering efficiency and makes the 2LO and 3LO multiphonon emission observable. Resonant excitation with a power density of about 1.3 MW/cm2 using a micro-Raman setup results in an exponential decrease of the 1LO, 2LO, and 3LO intensity with irradiation time. This decay behavior is not observed for pure ZnSe crystals and can be avoided for the ZnCdSe/ZnSe structures using much lower excitation densities. The decrease in intensity is accompanied by a shift of the LO mode to higher frequencies resulting from a lower cadmium concentration in the alloy. From these experimental findings, we conclude that resonant excitation at a certain power density leads to cadmium out-diffusion from the planar quantum dots, which shifts the resonance away from the excitation energy.

Nonradiative electron-hole recombination by a low-barrier pathway in hydrogenated silicon semiconductors

A microscopic pathway for nonradiative electron-hole recombination by large structural reconfiguration in hydrogenated Si is found with first-principles calculations. Trapped-biexciton formation leads to a low-barrier reconfiguration of the H atom, accompanied by crossing of doubly occupied electron and hole levels in the band gap. This crossing represents the nonradiative recombination of the carriers, without multiphonon emission. The proposal provides a mechanism for carrier-induced H emission during metastable degradation of hydrogenated amorphous silicon.

Electron and hole relaxation pathways in semiconductor quantum dots

Femtosecond (fs) broad-band transient absorption (TA) is used to study the intraband relaxation and depopulation dynamics of electron and hole quantized states in CdSe nanocrystals (NC's) with a range of surface properties. Instead of the drastic reduction in the energy relaxation rate expected due to a ``phonon bottleneck,'' we observe a fast subpicosecond $1P$-to-$1S$ electron relaxation, with the rate exceeding that due to phonon emission in bulk semiconductors. The energy relaxation is enhanced with reducing the NC's radius, and does not show any dependence on the NC surface properties (quality of the surface passivation). These data indicate that electron energy relaxation occurs by neither multiphonon emission nor by coupling to surface defects, but is likely meditated by Auger-type electron-hole energy transfer. We use fs infrared TA to probe electron and hole intraband transitions, which allows us to distinguish between electron and hole relaxation pathways leading to the depopulation of NC quantized states. In contrast to the electron relaxation, which is controlled by NC surface passivation, the depopulation of hole quantized states is extremely fast (sub-ps-to-ps time scales) in all types of samples, independent of NC surface treatment (including NC's overcoated with a ZnS layer). Our results indicate that ultrafast hole dynamics are not due to trapping at localized surface defects such as a vacancy, but rather arise from relaxation into intrinsic NC states or intrinsically unpassivated interface states.

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.

Dislocation core effects on plasticity

The structure and/or the state of dislocation core often play a significant role in the glide process of the dislocation and hence control the plasticity of the crystal. We show various examples in this paper. (1) The yield stress of single crystals controlled by the Peierls mechanism for screw dislocation does not obey the Schmid law. (2) A narrow dissociation of the core can produce a plateau in the τy- T relation. (3) Duality of the core structure can produce «yield stress anomaly». (4) Multiphonon emission at the core of dislocation in semiconducting crystals by non-radiative recombination of electron-hole pairs enhances drastically the dislocation mobility.

Erbium electroluminescence excitation in amorphous hydrogenated silicon under thermally stimulated deep-center tunneling ionization

A study of the electroluminescence of erbium-doped, amorphous hydrogenated silicon, a-Si:H 〈Er〉, is reported. It has been found that the electroluminescence intensity at the wavelength λ=1.54 µm corresponding to the 4I13/2→4I15/2 intra-4f shell transition in Er passes through a maximum near room temperature. The unusual temperature and field dependences of the electroluminescence indicate electric-field induced multi-phonon tunneling emission of electrons from deep centers. The electroluminescence of Er3+ ions is due to their becoming excited as conduction-band electrons are captured by neutral dangling bonds (D0 centers), which form when erbium is incorporated into the amorphous matrix. This Auger process transforms the center from its neutral state, D0, to a negatively charged state, D−, and the energy released in the capture is transferred by Coulomb interaction into the erbium-ion 4f shell. The steady-state current through the electroluminescent structure is supported by the reverse process of multi-phonon tunneling-electron emission from the D− center to the conduction band. The proposed theoretical model is in a good agreement with experimental data.

Nonradiative capture of hot photoelectrons by multiphonon emission in highly excited polar semiconductors

Room-temperature recombination dynamics of hot electron-hole plasma in highly excited polar semiconductors is analysed. A rate equation with Arrhenius dependence of carrier lifetime on the effective temperature is employed to consider the capture by deep centres via multiphonon emission (MPE) with the nonequilibrium LO phonons taken into account. The luminescence-intensity dependence on the pump intensity under quasistationary excitation conditions, as well as the shape of the luminescence-intensity transient kinetics, are shown to be strongly dependent on the height of the localization barrier. Typical manifestation of carrier-temperature-activated capture by MPE is experimentally demonstrated both for quasi-steady-state and transient conditions in CdSe and CdS single crystals, respectively. Additionally, evidence, testifying that surface recombination in CdS nanocrystals embedded in a glass matrix exhibits features characteristic of nonradiative capture by MPE for carrier and phonon systems far from thermal equilibrium with the lattice, is presented. The relevant localization barriers are shown to lie within the 100–200 meV range.

Slow decay of photoconductivity caused by tin-related DX centers in AlGaAs

Capacitance decay after termination of light was measured on Alx Ga1 −xAs/GaAs heterostructures with p-n junctions. It has been shown that the decay can be well expressed by two curves of stretched exponential forms; exp[−(t/τ)β]. The temperature dependence of the characteristic timeτ of each of the two curves was determined in the range from 15K to 60K. The dependence follows the theoretical relation for electron capture dominated by tunneling via multiphonon emission. The estimated capture parameters show a strong electron-phonon coupling of two DX centers. Possible relation of these centers to tin is discussed.

Dense electron-hole plasma cooling due to second nonequilibrium-phonon bottleneck in CdS crystallites

Quantitative evidence on the existence of a second nonequilibrium-phonon bottleneck for dense electron-hole plasma cooling in a highly excited polar semiconductor is presented. The bottleneck is caused by recurrent fusion of nonequilibrium LO phonons from their decay products (daughter phonons). Carrier cooling was experimentally investigated in CdS, which offers a favorable phonon dispersion. Crystallites of 50-nm radius were utilized to prevent stimulated recombination and diffusion of photoexcited electron-hole plasma with a density around $1.5\ifmmode\times\else\texttimes\fi{}{10}^{19} {\mathrm{cm}}^{\ensuremath{-}3}.$ A transient of carrier effective temperature, deduced from time-resolved luminescence spectra, exhibit a slow-relaxation component with the time constant of 70 ps at room temperature. The transient was shown to be in quantitative consistence with the theoretical model based on Boltzmann equations for two generations of nonequilibrium phonons and degenerate-carrier energy rate equation with the energy income due to recombination effects (nonradiative capture via multiphonon emission, fermion self-heating, and band-gap renormalization) taken into account. The observed cooling rate agrees with two nonequilibrium-phonon bottlenecks with the depopulation time constants deduced from the available Raman data (0.5 ps for LO phonons and 12.4 ps for daughter phonons).