Nonequilibrium Carrier Distribution Research Articles

(Invited) Analogy between Semiconductor Junction Devices and Single Ion, Solid Polymer Electrolytes

Semiconductor electrochemistry using electronic junctions within the solid to carry out electrochemical reactions at the solid-liquid interface. The photogeneration of electron-hole pairs within the semiconductor creates a non-equilibrium distribution of charge carriers within the semiconductor for conversion of optical energy to chemical energy. In an analogous way, ion conducting solid polymers can form anion-cation solid junctions creating a separation of anions and cations which can be used to carry out electrochemical reactions under different conditions, such as different pH values at different surfaces. The characteristics and design of solid-state ion junctions will be described along with examples of the synthesis and formation of anion/cation solid state polymer junctions.

Broadband nonlinear refraction transients in C-doped GaN based on absorption spectroscopy

Optical nonlinear response and its dynamics of wide-bandgap materials are key to realizing integrated nonlinear photonics and photonic circuit applications. However, those applications are severely limited by the unavailability of both dispersion and dynamics of nonlinear refraction (NLR) via conventional measurements. In this work, the broadband NLR dynamics with extremely high sensitivity (λ/1000) can be obtained from absorption spectroscopy in GaN:C using the refraction-related interference model. Both the absorption and refraction kinetics are found to be significantly modulated by the C-related defects. Especially, we demonstrate that the refractive index change Δn of GaN:C is negative and can be used to realize all-optical switching applications owing to the large NLR and ultrafast switching time. The NLR under different non-equilibrium carrier distributions originates from the capture of electrons by CN+ defect state, while the absorption modulation originates from the excitation of tri-carbon defects. We believe that this work provides a better understanding of the GaN:C nonlinear properties and an effective solution to broadband NLR dynamics of transparent thin films or heterostructure materials.

Emergent energy dissipation in quantum limit

Energy dissipation is of fundamental interest and crucial importance in quantum systems. However, whether energy dissipation can emerge without backscattering inside topological systems remains a question. As a hallmark, we propose a microscopic picture that illustrates energy dissipation in the quantum Hall (QH) plateau regime of graphene. Despite the quantization of Hall, longitudinal, and two-probe resistances (dubbed as the quantum limit), we find that the energy dissipation emerges in the form of Joule heat. It is demonstrated that the non-equilibrium energy distribution of carriers plays much more essential roles than the resistance on energy dissipation. Eventually, we suggest probing the phenomenon by measuring local temperature increases in experiments and reconsidering the dissipation typically ignored in realistic topological circuits.

Evidence of hot carrier extraction in metal halide perovskite solar cells

AbstractThe presence of hot carriers is presented in the operational properties of an (FA,Cs)Pb(I, Br, Cl)3 solar cell at ambient temperatures and under practical solar concentration. Albeit, in a device architecture that is not suitably designed as a functional hot carrier solar cell. At 100 K, clear evidence of hot carriers is observed in both the high energy tail of the photoluminescence spectra and from the appearance of a nonequilibrium photocurrent at higher fluence in light J–V measurements. At room temperature, however, the presence of hot carriers in the emission at elevated laser fluence is shown to compete with a gradual red shift in the PL peak energy as photoinduced halide segregation begins to occur at higher lattice temperature. The effects of thermionic emission of hot carriers and the presence of a nonequilibrium carrier distribution are also shown to be distinct from simple lattice heating. This results in large unsaturated photocurrents at high powers as the Fermi distribution exceeds that of the heterointerface controlling carrier transport and rectification.

Leveraging plasmonic hot electrons to quench defect emission in metal-semiconductor nanostructured hybrids.

Modeling light-matter interactions in hybrid plasmonic materials is vital to their widening relevance from optoelectronics to photocatalysis. Here, we explore photoluminescence (PL) from ZnO nanorods (ZNRs) embedded with gold nanoparticles (Au NPs). A progressive increase in Au NP concentration introduces significant structural disorder and defects in ZNRs, which paradoxically quenches defect related visible PL while intensifying the near band edge (NBE) emission. Under UV excitation, the simulated semi-classical model realizes PL from ZnO with sub-bandgap defect states, eliciting visible emissions that are absorbed by Au NPs to generate a non-equilibrium hot carrier distribution. The photo-stimulated hot carriers, transferred to ZnO, substantially modify its steady-state luminescence, reducing NBE emission lifetime and altering the abundance of ionized defect states, finally reducing visible emission. The simulations show that the change in the interfacial band bending at the Au-ZnO interface under optical illumination facilitates charge transfer between the components. This work provides a general foundation to observe and model the hot carrier dynamics and strong light-matter interactions in hybrid plasmonic systems.

Eco-Friendly Zn-Ag-In-Ga-S Quantum Dots: Amorphous Indium Sulfide Passivated Silver/Sulfur Vacancies Achieving Efficient Red Light-Emitting Diodes.

I-III-VI quantum dots (QDs) and derivatives (I, III, and VI are Ag+/Cu+, Ga3+/In3+, and S2-/Se2-, respectively) are the ideal candidates to replace II-VI (e.g., CdSe) and perovskite QDs due to their nontoxicity, pure color, high photoluminescence quantum yield (PLQY), and full visible coverage. However, the chaotic cation alignment in multielement systems can easily lead to the formation of multiple surface vacancies, highlighted as VI and VVI, leading to nonradiative recombination and nonequilibrium carrier distribution, which severely limit the performance improvement of materials and devices. Here, based on Zn-Ag-In-Ga-S QDs, we construct an ultrathin indium sulfide shell that can passivate electron vacancies and convert donor/acceptor level concentrations. The optimized In-rich 2-layer indium sulfide structure not only enhances the radiative recombination rate by preventing further VS formation but also achieves the typical DAP emission enhancement, achieving a significant increase in PLQY to 86.2% at 628 nm. Moreover, the optimized structure can mitigate the lattice distortion and make the carrier distribution in the interior of the QDs more balanced. On this basis, red QD light-emitting diodes (QLEDs) with the highest external quantum efficiency (EQE; 5.32%) to date were obtained, providing a novel scheme for improving I-III-VI QD-based QLED efficiency.

Ultrafast broadband carrier and exciton dynamics of Fe-related centers in GaN

The effect of Fe defects on carrier recombination and two-photon induced ultrafast exciton dynamics in GaN crystals were investigated using femtosecond transient absorption spectroscopy. The absorption kinetics exhibited completely different characteristics under different nonequilibrium carrier concentrations and distributions. The carrier recombination mechanisms under different excitation conditions can be interpreted by the model of Fe defect-related energy level. The absorption under one-photon excitation originates from the intraband transition of free holes, and the capture of holes by Fe2+ leads to the decay of the absorption. The ultrafast excitonic state [Fe2+,hVB] formation time under two-photon excitation results in a rapid absorption decay (<20 ps), while the long tail of the absorption response could be attributed to long-lived excitonic state (>100 ns). These findings are crucial to the applications of GaN for ultrafast optoelectronics and integrated nonlinear optics.

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.

Transverse spin effects in electron transport

In the samples of nonmagnetic Al, Pt, and W metals of an asymmetric shape, which causes a nonequilibrium distribution of charge carriers, the transverse spin contributions of the spin-orbit interaction to the Hall and Nernst-Ettingshausen effects were studied by direct electric measurement. It is found a difference in the behavior of the spin contributions of thermal diffusion and electric nature in a magnetic field. The dependence of this behavior on the band structure that controls the profiles of spin magnetization, which is established at the edges of the sample by the accumulation of spins, is shown. An oscillographic visualization of the spin Hall effect on alternating current was carried out.

Theory of non-equilibrium ‘hot’ carriers in direct band-gap semiconductors under continuous illumination

The interplay between the illuminated excitation of carriers and subsequent thermalization and recombination leads to the formation of non-equilibrium distributions for the ‘hot’ carriers and to heating of both electrons, holes and phonons. In spite of the fundamental and practical importance of these processes, there is no theoretical framework which encompasses all of them and provides a clear prediction for the non-equilibrium carrier distributions. Here, a self-consistent theory accounting for the interplay between excitation, thermalization, and recombination in continuously-illuminated semiconductors is presented, enabling the calculation of non-equilibrium carrier distributions. We show that counter-intuitively, distributions deviate more from equilibrium under weak illumination than at high intensities. We mimic two experimental procedures to extract the carrier temperatures and show that they yield different dependence on illumination. Finally, we provide an accurate way to evaluate photoluminescence efficiency, which, unlike conventional models, predicts correctly the experimental results. These results provide a starting point towards examining how non-equilibrium features will affect properties hot-carrier based application.

Optical Gain in Solids after Ultrafast Strong-Field Excitation.

Multiphoton excitation of a solid by a few-cycle, intense laser pulse forms a very nonequilibrium distribution of charge carriers, where occupation probabilities do not necessarily decrease with energy. Within a fraction of the pulse, significant population inversion can emerge between pairs of valence-band states with a dipole-allowed transition between them. This population inversion leads to stimulated emission in a laser-excited solid at frequencies where the unperturbed solid is transparent. We establish the optimal conditions for observing this kind of strong-field-induced optical gain.

Improvement of the timing properties of Ce-doped oxyorthosilicate LYSO scintillating crystals

The aim of this work has been to improve the time resolution of radiation detectors for future high-energy physics experiments and medical imaging applications. Ce-doped oxyorthosilicate Lu2SiO5:Ce (LSO) and mixed oxyorthosilicate Lu1.6Y0.4SiO5:Ce (LYSO) have been investigated as prospective scintillators for such high-time-resolution applications. A differential optical absorption technique with sub-picosecond time resolution upon selective excitation of Ce3+ ions to different excited states has been adopted to study carrier dynamics in these scintillators, and coincidence time resolution measured using 511 keV γ-quanta has been exploited to test their timing properties. A delay in population of the emitting level of Ce3+ has been observed, and is interpreted in terms of electron trapping, which is more pronounced in mixed yttrium-containing LYSO crystals due to composition fluctuations. It is shown that the delay, which affects the luminescence response time, can be eliminated by co-doping of LYSO:Ce with calcium at concentrations as low as 5 ppm. The faster kinetics of electron transfer correlates with a better coincidence time resolution. Thermalization and spatial distribution of non-equilibrium carriers has been studied theoretically to link the results obtained by the time-resolved differential optical absorption technique with the behavior of the non-equilibrium carriers generated by irradiation.

Fundamental limits of hot carrier injection from metal in nanoplasmonics

Abstract The evolution of non-equilibrium carriers excited in the process of decay of surface plasmon polaritons (SPPs) in metal is described for each step – from the generation of carriers to their extraction from the metal. The relative importance of various carrier-generating mechanisms is discussed. It is shown that both the generation of carriers and their decay are inherently quantum processes as, for realistic illumination conditions, no more than a single SPP per nanoparticle exists at a given time. As a result, the distribution of non-equilibrium carriers cannot be described by a single temperature. It is also shown that the originally excited carriers that have not undergone a single electron-electron scattering event are practically the only ones that contribute to the injection. The role of momentum conservation in carrier extraction is discussed, and it is shown that, if all the momentum conservation rules are relaxed, it is the density of states in the semiconductor/dielectric that determines the ultimate injection efficiency. A set of recommendations aimed at improving the efficiency of plasmonic-assisted photodetection and (to a lesser degree) photocatalysis is made in the end.

On the Theory of a Photocurrent Burst in an Intrinsic Photoresistor upon Longitudinal and Transverse Illumination

In this paper, we analyze the intrinsic photoconductivity burst of semiconductors at an increase in the recombination center concentration upon uniform and non-uniform weak illumination along the electric field. An equation for the concentration distribution of nonequilibrium carriers at an arbitrary illumination profile along the electric field is derived without using the quasi-neutrality approximation. Based on this equation, the bursts in photoelectric gain of the transverse and longitudinal photoresistors should significantly differ from each other under any recombination conditions at current contacts due to the photo-induced space charge. If the carrier photogeneration is not uniform, then, unlike uniform photogeneration, the photoelectric gain burst depends on the polarity of the applied voltage. The quasi-neutrality approximation does not produce these results. An analytical expression for the maximum value of the electron photoelectric gain depending on the recombination center concentration is derived in the case of an exponential photogeneration profile and pulling-out contacts. The relationship that was found between the concentrations of nonequilibrium electrons and holes allows derivation of an analytical expression for the maximum value of the hole photoelectric gain.

Charge Density Wave Melting in One-Dimensional Wires with Femtosecond Subgap Excitation

Charge density waves (CDWs) are symmetry-broken ground states that commonly occur in low-dimensional metals due to strong electron-electron and/or electron-phonon coupling. The nonequilibrium carrier distribution established via photodoping with femtosecond laser pulses readily quenches these ground states and induces an ultrafast insulator-to-metal phase transition. To date, CDW melting has been mainly investigated in the single-photon regime with pump photon energies bigger than the gap size. The recent development of strong-field midinfrared sources now enables the investigation of CDW dynamics following subgap excitation. Here we excite prototypical one-dimensional indium wires with a CDW gap of ∼300 meV with midinfrared pulses at ℏω=190 meV with MV/cm field strength and probe the transient electronic structure with time- and angle-resolved photoemission spectroscopy. We find that the CDW gap is filled on a timescale short compared to our temporal resolution of 300fs and that the band structure changes are completed within ∼1 ps. Supported by a minimal theoretical model we attribute our findings to multiphoton absorption across the CDW gap.

Femtosecond pump-probe spectroscopy investigation of carrier dynamics in GaAlAs photocathods

Based on the study of GaAs photocathode, the carrier dynamics of reflection-mode GaAlAs photocathode has been investigated. In this paper, the reflection-mode GaAlAs/GaAs photocathode is grown by metal organic chemical vapor deposition (MOCVD) epitaxial technology. The femtosecond pump-probe spectroscopy is performed to measure carrier dynamics in GaAlAs and GaAs photocathods. After laser excitation the surfaces of GaAlAs/GaAs photocathode, the reflectivity of photocathode surface is changed. At the same time, the dynamic processes of the non-equilibrium carrier distribution in semiconductor material is obtained by the transient reflectance spectrum. Compared with GaAs photocathode, the Al element in GaAlAs photocathode is unfavorable to photoemission under some conditions, while it solves the problem that the spectrum response range of binary compounds is not adjustable. The change trend of initial relaxation transient of GaAs photocathode in the 22 ps range is the same as GaAlAs, while the change of GaAs and GaAlAs relaxation state is significantly different with the change of delay time after 22 ps. The results show that the GaAlAs increases band gap due to a significantly slower than GaAs photocathode in initial process. Meanwhile, this paper provides a reference for the preparation of the GaAlAs photocathode that only respond to blue and green light range.

Cascade capture of charge carriers in highly doped semiconductors

We analyze the cascade capture of charge carriers due to the interaction with acoustic phonons in highly doped semiconductors using a model that describes the recombination of photo-ionized carriers as a continuous relaxation of carriers in the energy space at both positive and negative energies in the field of a set of impurity ions. Such description enables simultaneous calculation of non-equilibrium carrier distribution formed by interaction with acoustic phonons in the presence of impurity traps, and the time of recombination in a wide range of concentrations of capture centers and phonon temperatures. Additionally, we calculated the time of cascade recombination in the presence of fast scattering processes forming a Maxwellian distribution of free carriers. We show that experimentally observed concentration and temperature dependence of carrier life times in highly doped semiconductors can be described within the model of the cascade capture to uniformly spaced capture centers, and the main factor that determines the regime of cascade capture is the ratio of the thermal energy and the energy of the overlap of impurity potentials.

Femtosecond pulse generation beyond photon lifetime limit in gain-switched semiconductor lasers

Femtosecond semiconductor lasers are ideal devices to provide the ultrashort pulses for industrial and biomedical use because of their robustness, stability, compactness and potential low cost. In particular, gain-switched semiconductor lasers have significant advantages of flexible pulse shaping and repetition rate with the robustness. Here we first demonstrate our laser, which is initiated by very strong pumping of 100 times the lasing threshold density, can surpass the photon lifetime limit that has restricted the pulse width to picoseconds for the past four decades and produce an unprecedented ultrashort pulse of 670 fs with a peak power of 7.5 W on autocorrelation measurement. The measured phenomena are reproduced effectively by our numerical calculation based on rate equations including the non-equilibrium intraband carrier distribution, which reveal that the pulse width is limited by the carrier–carrier scattering time, instead of the photon lifetime.

Valley Hall transport of photon-dressed quasiparticles in two-dimensional Dirac semiconductors

We present a theory of the photovoltaic valley-dependent Hall effect in a two-dimensional (2D) Dirac semiconductor subject to an intense near-resonant electromagnetic field. Our theory captures and elucidates the influence of both the field-induced resonant interband transitions and the nonequilibrium carrier kinetics on the resulting valley Hall transport in terms of photon-dressed quasiparticles (PDQs). The non-perturbative renormalization effect of the pump field manifests itself in the dynamics of the PDQs, with a quasienergy spectrum characterized by dynamical gaps δη (η is the valley index) that strongly depend on field amplitude and polarization. Nonequilibrium carrier distribution functions are determined by the pump field frequency ω as well as the ratio of intraband relaxation time τ and interband recombination time τrec. We obtain analytic results in three regimes, when (I) all relaxation processes are negligible, (II) τ ≪ τrec, and (III) τ ≫ τrec, and display corresponding asymptotic dependences on δη and ω. We then apply our theory to 2D transition-metal dichalcogenides, and find a strong enhancement of valley-dependent Hall conductivity as the pump field frequency approaches the transition energies between the pair of spin-resolved conduction and valence bands at the two valleys.

Polarization and plasmons in hot photoexcited graphene

We present a robust and exact method for calculating the polarization function and plasmon dispersion of graphene, for an arbitrary (isotropic) non-equilibrium carrier distribution, within random phase approximation (RPA). This is demonstrated for a range of carrier distributions, including hot carrier distributions which occur within the femtoseconds following photoexcitation. We show that qualitatively different behaviour from the equilibrium case can occur. As the polarization function determines dynamic screening, its calculation shall be essential to quantifying carrier-carrier scattering channels for graphene far from equilibrium.