Elevated Carrier Densities Research Articles

Exploring the photoelectrocatalytic performance of spinel- and delafossite-like CuMM’O4 (M, M’ = Al, Ga, In) obtained from pseudobinary CuMO2-based solid solution

In this study, the focus revolves around the synthesis, characterization, and subsequent evaluation of CuMM’O4 (M, M’ = Al, Ga, In) photoelectrocatalysts, notable for their distinct spinel- and delafossite-like structures. These novel materials are derived from a pseudobinary CuMO2-based solid solution. Specifically, the delafossite-like CuAlInO4 and CuGaInO4 manifest a trigonal superstructure within the R3‾m space group, defined by structural motifs comprising [InO6] octahedra, [CuO5] hexahedra, and [AlO5] or [GaO5] hexahedra. This study spans a comprehensive analysis encompassing optical properties, electrical characteristics, electronic structure, and the ensuing photoelectrochemical performancer. This exploration integrates experimental analyses and density functional theory calculations to discern the intricacies of these materials. The assessed CuMM’O4 samples display favorable optical traits and band-edge positions conducive to facilitating visible-light-driven photoelectrochemical water-splitting. Among the evaluated samples, the delafossite-like CuGaInO4 photocathode stands out due to its notably elevated carrier density and reduced resistance for carrier transport, distinguishing it from CuAlGaO4 and CuAlInO4. Consequently, it exhibits superior photoelectrochemical activity. CuGaInO4, serving as an unadorned photocathode, demonstrates a noteworthy photocurrent density of 8.6 μA/cm2 under 0.61VRHE and 102.05 μA/cm2 at 0.15 VRHE. Moreover, it achieves a peak incident photon-to-current efficiency of approximately 1.86 % at 0.61 VRHE under a 480 nm wavelength.

Microscopic calculation of the optical properties and intrinsic losses in the methylammonium lead iodide perovskite system

For opto-electronic and photo-voltaic applications of perovskites, it is essential to know the optical properties and intrinsic losses of the used materials. A systematic microscopic analysis is presented for the example of methylammonium lead iodide where density functional theory is applied to calculate the electronic band structure as well as the dipole and Coulomb matrix elements. These results serve as input for a many-body quantum approach used to compute the absorption, photoluminescence, and the optical and Auger losses for a wide range of application conditions. To illustrate the theory, the excitonic properties of the material system are investigated and numerical results are presented for typical photo-voltaic operation conditions and for the elevated carrier densities needed for laser operation.

Giant excitation induced bandgap renormalization in TMDC monolayers

Optical absorption and gain spectra in MoS2 monolayers with thermal carrier distributions are calculated from the combined gap and Dirac-Bloch equations. It is shown that the excited carriers lead to a bandgap renormalization as large as 800 meV for a suspended monolayer MoS2. Above the critical density, optical gain is obtained over an approx. 400 meV broad spectral range above the gap. Whereas the absorption spectra in the low density regime are very sensitive to the dielectric environment, the spectra become purely intrinsic at elevated carrier densities.

Single-unit-cell layer established Bi2WO6 3D hierarchical architectures: Efficient adsorption, photocatalysis and dye-sensitized photoelectrochemical performance

Single-layer catalysis sparks huge interests and gains widespread attention owing to its high activity. Simultaneously, three-dimensional (3D) hierarchical structure can afford large surface area and abundant reactive sites, contributing to high efficiency. Herein, we report an absorbing single-unit-cell layer established Bi2WO6 3D hierarchical architecture fabricated by a sodium dodecyl benzene sulfonate (SDBS)-assisted assembled strategy. The DBS− long chains can adsorb on the (Bi2O2)2+ layers and hence impede stacking of the layers, resulting in the single-unit-cell layer. We also uncovered that SDS with a shorter chain is less effective than SDBS. Due to the sufficient exposure of surface O atoms, single-unit-cell layer 3D Bi2WO6 shows strong selectivity for adsorption on multiform organic dyes with different charges. Remarkably, the single-unit-cell layer 3D Bi2WO6 casts profoundly enhanced photodegradation activity and especially a superior photocatalytic H2 evolution rate, which is 14-fold increase in contrast to the bulk Bi2WO6. Systematic photoelectrochemical characterizations disclose that the substantially elevated carrier density and charge separation efficiency take responsibility for the strengthened photocatalytic performance. Additionally, the possibility of single-unit-cell layer 3D Bi2WO6 as dye-sensitized solar cells (DSSC) has also been attempted and it was manifested to be a promising dye-sensitized photoanode for oxygen evolution reaction (ORR). Our work not only furnish an insight into designing single-layer assembled 3D hierarchical architecture, but also offer a multi-functional material for environmental and energy applications.

The Role of Trap-assisted Recombination in Luminescent Properties of Organometal Halide CH3NH3PbBr3 Perovskite Films and Quantum Dots.

Hybrid metal halide perovskites have been paid enormous attentions in photophysics research, whose excellent performances were attributed to their intriguing charge carriers proprieties. However, it still remains far from satisfaction in the comprehensive understanding of perovskite charge-transport properities, especially about trap-assisted recombination process. In this Letter, through time-resolved transient absorption (TA) and photoluminescence (PL) measurements, we provided a relative comprehensive investigation on the charge carriers recombination dynamics of CH3NH3PbBr3 (MAPbBr3) perovskite films and quantum dots (QDs), especially about trap-assisted recombination. It was found that the integral recombination mode of MAPbBr3 films was highly sensitive to the density distribution of generated charge carriers and trap states. Additional, Trap effects would be gradually weakened with elevated carrier densities. Furthermore, the trap-assisted recombination can be removed from MAPbBr3 QDs through its own surface passivation mechanism and this specialty may render the QDs as a new material in illuminating research. This work provides deeper physical insights into the dynamics processes of MAPbBr3 materials and paves a way toward more light-harvesting applications in future.

Correlation between carrier localization and efficiency droop in AlGaN epilayers

Photoluminescence studies of carrier dynamics in AlGaN epilayers with different degrees of carrier localization and densities of nonradiative recombination centers show that the prevailing droop mechanism in AlGaN epilayers with strong carrier localization and comparatively high density of nonradiative recombination centers is enhanced nonradiative recombination due to the carrier delocalization at elevated carrier density. The photoluminescence was investigated under quasi-steady-state excitation in the temperature range from 8 to 300 K. The results proved that the onset of this droop effect is below the threshold for the droop due to high-density effects in the epilayers, such as carrier heating, phase space filling, nonradiative Auger recombination, and stimulated emission.

Many-body theory of carrier capture and relaxation in semiconductor quantum-dot lasers

In quantum-dot laser devices containing a quasi-two-dimensional wetting layer, a pump process initially populates the wetting-layer states. The scattering of carriers from these spatially-extended quasi-two-dimensional states into the quantum-dot states as well as the relaxation of carriers between the quantum-dot levels are studied theoretically. Based on the wave functions for the coupled quantum-dot/wetting-layer system interaction matrix elements are calculated for carrier-carrier Coulomb interaction and carrier-phonon interaction. Scattering rates for various capture and relaxation processes are evaluated under quasiequilibrium conditions. For elevated carrier densities in the wetting layer, Coulomb scattering provides processes with capture (relaxation) times typically faster than $10\phantom{\rule{0.3em}{0ex}}\mathrm{ps}\phantom{\rule{0.3em}{0ex}}(1\phantom{\rule{0.3em}{0ex}}\mathrm{ps})$. When energy conservation allows for interaction with LO phonons, comparable rates are obtained.

Physics of high-power InGaN/GaN lasers

The authors analyse the performance and device physics of nitride laser diodes that exhibit the highest room-temperature continuous-wave output power. The analysis is based on advanced laser simulation. The laser model self-consistently combines band structure and free-carrier gain calculations with two-dimensional simulations of wave guiding, carrier transport and heat flux. Material parameters used in the model are carefully evaluated. Excellent agreement between simulations and measurements is achieved. The maximum output power is limited by electron leakage into the p-doped ridge. Leakage escalation is caused by strong self-heating, gain reduction and elevated carrier density within the quantum wells. Built-in polarisation fields are found to be effectively screened at high-power operation. Improved heat-sinking is predicted to allow for a significant increase of the maximum output power.

Emission dynamics and optical gain of 1.3-μm (GaIn)(NAs)/GaAs lasers

The ultrafast emission dynamics of a 1.3-/spl mu/m (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser is studied by femtosecond luminescence upconversion. We obtain a minimum peak delay of 15.5 ps and a minimum pulse width of 10.5 ps. Laser operation with picosecond emission dynamics is demonstrated over a temperature range from 30 to 388 K. The bandgap shift with temperature of (GaIn)(NAs)/GaAs is determined to be about -2.9/spl middot/10/sup -4/ eV/K, which is smaller than for GaAs. Our measurements of the optical gain provide gain spectra similar to those of commercial (GaIn)(PAs)/InP-structures at moderate densities but broaden considerably for elevated carrier densities due to the stronger carrier confinement. We compare our experimental results with gain spectra calculated from a microscopic model and confirm the predictive capability of the model. The theoretical gain spectra are used as the input for a calculation of the temperature dependence of the (GaIn)(NAs)/GaAs surface-emitter emission which results in very good agreement with experiment.

Gain spectra of (GaIn)(NAs) laser diodes for the 1.3-μm-wavelength regime

Optical gain spectra of (GaIn)(NAs)/GaAs quantum-well lasers operating in the 1.3-μm-emission-wavelength regime are measured and compared to those of a commercial (GaIn)(AsP)/InP structure. Good agreement of the experimental results with computed spectra of a microscopic many-body theory is obtained. Due to the contributions of a second confined subband, a spectrally broad gain region is expected for (GaIn)(NAs)/GaAs at elevated carrier densities.

Quantum-kinetic dephasing in resonantly excited semiconductor quantum wells

We investigate resonant time-resolved four-wave mixing on GaAs quantum wells at $T=300\mathrm{K}$ and at elevated carrier densities using excitation with 11 fs optical pulses. The experiments are compared with non-Markovian quantum-kinetic calculations including carrier-carrier and carrier-LO-phonon scattering. A self-consistently determined two-time-dependent screened random-phase approximation Coulomb potential is used for the carrier-carrier scattering. In experiment and theory we find an unusual real-time response, which is far from an ideal photon echo. Surprisingly, we also find that the dependence of the decay time \ensuremath{\tau} of the four-wave-mixing signal versus the optically excited nonequilibrium electron-hole pair density ${n}_{\mathrm{eh}}$ is quantitatively similar to the corresponding behavior of bulk GaAs. Thus both data sets can be fitted by the same form ${\ensuremath{\tau}}^{\ensuremath{-}1}={\ensuremath{\tau}}_{0}^{\ensuremath{-}1}{+cn}_{\mathrm{eh}}^{1/3}.$ This is not consistent with previous experimental work in a more limited range of carrier densities, which determined different behavior for quasi-two-dimensional (2D) and 3D, i.e., exponents of 0.3 and $0.55\ifmmode\pm\else\textpm\fi{}0.04,$ respectively.

Carrier dynamics in the quantum kinetics regime

Theory has predicted that, on a short time scale, the dynamics of optically excited carriers in semiconductors can no longer be described by the semiclassical Boltzmann equation. The carriers rather behave like quantum mechanical waves and the Boltzmann kinetics have to be replaced by quantum kinetics. Here we briefly review recent experiments on the quantum kinetics of electron–LO–phonon scattering. The focus of this review, however, is on more recent experiments at elevated carrier densities which are within the regime of the quantum kinetics of carrier–carrier scattering.

Photon Echoes from Semiconductor Band-to-Band Continuum Transitions in the Regime of Coulomb Quantum Kinetics

We show that the coherent response of semiconductor band-to-band continuum transitions at roomtemperature, excited by 11 fs pulses at elevated carrier densities, deviates substantially from thatexpected for an ideal photon echo. A microscopic non-Markovian quantum kinetic theory of bothelectron-electron and electron-LO-phonon scattering reproduces these unusual experimental observationsqualitatively and, furthermore, the famous scaling of the decay time t with carrier density