Low Excitation Densities Research Articles

Observation of biexcitonic effect in single ZnCdSe quantum well under very low excitation density

Under an excitation density of 0.4 W/cm2, a strong biexcitonic peak was observed in the photoluminescence (PL) spectrum of a single quantum well which was grown in submonolayer deposition mode. Its intensity was found to increase superlinearly with excitation density. The shape of the biexcitonic peak was found to be similar to that of biexcitons in CuCl and CdS crystal and a binding energy of 9.8 meV was obtained. When the temperature increased from 10 to 31 K, the biexcitonic peak decreased considerably whereas the excitonic peak increased slightly.

Time-resolved photoluminescence investigations of cubic GaN layers and crystals up to room temperature

We compare the recombination dynamics of GaN single crystals to that of epitaxial cubic GaN layers on GaAs(001) using picosecond photoluminescence (PL) spectroscopy. At low temperatures, the band-edge PL decay time of the layer is much shorter than that of the crystals, evidencing the importance of nonradiative processes in the case of the layer. However, at room temperature both the emission spectra and their decay times of layer and crystals are almost identical. At 300 K the decay times are short (10 ps) for low excitation density and increase to values of 100 ps at moderate excitation density reflecting the saturation of nonradiative recombination channels.

Magnetically Enhanced Exciton-Exciton Correlations in Semiconductors

We present investigations of fs time resolved coherent wave mixing under high magnetic field. Our experiments reveal a new regime at high magnetic field and low excitation density dominated by the Coulomb interaction. This regime is inconsistent with the semiconductor Bloch equations. A model which includes exciton-exciton correlation successfully describes many features of this regime. {copyright} {ital 1997} {ital The American Physical Society}

Optical-Gain Measurements on GaN and Alx Ga1-xN Heterostructures

ABSTRACTOptical gain processes in thin GaN and AlGaN are compared by means of gain spectroscopy using the stripe length method and high-excitation photoluminescence, both performed at various densities and temperatures. We find that inelastic excitonic scattering processes and biexciton decay are important at low temperatures and low excitation densities Both materials are similar in that increasing the excitation density results in gain spectra dominated by the electron-hole plasma and phonon-assisted band-to-band recombination. These also prevail at high temperatures.

Luminescence Properties of Si-Doped GaN and Evidence of Compensating Defects As the Origin of the Yellow Luminescence

AbstractWe investigate the optical properties of n-type Gallium Nitride (GaN) with concentrations ranging from 5×1016 to 7×1018 cm−3. The near-band edge ultraviolet (UV) transition increases monotonically with the doping concentration. The photoluminescence linewidth of the near-bandgap optical transition increases from 47 to 78 meV as the doping concentration is increased. The broadening is modeled by taking into account potential fluctuations caused by the random distribution of donor impurities. Excellent agreement is found between experimental and theoretical results. We also investigate the origin of the yellow luminescence in GaN. At low excitation densities the experimental ratio of the UV-to-yellow photoluminescence does not change significantly as the doping concentration is increased by two orders of magnitude. Analysis of the luminescence in terms of a theoretical model indicates that the yellow luminescence is due to compensating impurities or defects.

High Quality ZnSe and Zn(Se,S) Produced by Low Temperature Physical Vapour Transport

High quality, large (25 mm in diameter) crystals of ZnSe and Zn(Se,S) were grown by low-temperature physical vapour transport method and characterized by various techniques, i.e. energy dispersive X-ray fluorescence spectrometry, double-crystal X-ray diffraction rocking curve FWHM, reflectivity measurements in the region of free exciton, electrical measurements and photoluminescence measurements for both low and high excitation densities. The measurement of the photoluminescence related to the neutral-donor-bound exciton revealed superlinear log(photoluminescence intensity) versus log(excitation intensity) dependence, which indicates stimulated emission phenomena.

Rapid carrier relaxation in self-assembled InxGa1-xAs/GaAs quantum dots.

Carrier capture and relaxation processes in self-assembled 15-nm ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As/GaAs quantum dots are investigated by means of time-resolved photoluminescence spectroscopy. In a systematic study of photoluminescence rise times and barrier decay times (variation of temperature, excitation energy, and excitation density) we aim to identify the physical mechanisms responsible for fast carrier capture and relaxation in quantum dots. Both processes are separated by using appropriate excitation energies. Carrier capture and relaxation are shown to proceed with rates as high as \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at low temperature even if less than one electron-hole pair per dot and excitation pulse is created. We interpret our results in terms of multiphonon processes at low excitation densities and in terms of Auger processes at high excitation densities. \textcopyright{} 1996 The American Physical Society.

Exciton interaction effects in the emission spectra of single free-standing InGaAs/GaAs quantum dots

We report on highly spatially resolved photoluminescence studies on individual free-standing InGaAs/GaAs quantum dots with diameters between 100 and 40 nm. Single dots with spacings of 20 μm between adjacent structures have been developed by low-voltage electron-beam lithography and wet chemical etching. Photoluminescence spectra taken at low excitation densities systematically shift to higher energies with decreasing dot diameter. With increasing excitation power, the emission includes additional peaks at higher energies due to transitions between the second and third quantized levels. Simultaneously, a renormalization of the transition energies by up to 2 meV and a significant broadening of the spectra are observed. The energy renormalization and spectral broadening are discussed on the basis of Coulomb interaction between excitons inside the dot and carrier pairs in the surrounding GaAs barriers.

Transport analysis of the thermalization and energy relaxation of photoexcited hot electrons in Ge-doped GaAs.

A very fast broadening, thermalization, and cooling of an initially narrow photoexcited electron distribution was recently observed in the time- and frequency-resolved electron-acceptor luminescence of p-type bulk GaAs:Ge for near-band-gap excitation at low excitation densities and temperatures. As the short thermalization times could not be explained by the standard rates for the conventional carrier-carrier, carrier-impurity, and carrier-phonon scatterings, unconventional types of thermalizing carrier interactions were originally suggested to interpret the data. In contrast, the present analysis restricts itself to the conventional scattering mechanisms, but attempts to explain the fast thermalization and cooling rates by accounting for the finite experimental pulse-repetition rates and the resulting presence of neutral donors of the amphoteric germanium, causing a very effective additional electronic energy-relaxation channel via impact ionizations of Ge donors. \textcopyright{} 1996 The American Physical Society.

Nonequilibrium optical-phonon population by sequential resonant tunneling in GaAs-AlAs superlattices.

We report on the observation of nonequilibrium GaAs LO phonons generated by the relaxation of hot electrons due to sequential resonant tunneling in strongly coupled superlattices. Raman-scattering experiments were performed at 5 K with low optical excitation density. The anti-Stokes LO-phonon intensity, which gives a measure of the nonequilibrium LO-phonon population, exhibits maxima at the resonant tunneling voltages. The transport data show that the electrical heating power increases linearly with voltage. Our results suggest that hot-electron cooling via acoustical phonon emission is suppressed under sequential resonant tunneling conditions.

Electronic structure and energy relaxation in strained InAs/GaAs quantum pyramids

We perform experimental and theoretical studies of the electronic structure and relaxation processes in pyramid shaped InAs/GaAs quantum dots (QDs), grown by molecular beam epitaxy in the Stranski-Krastanow growth mode. Structural properties are characterized with plan view and cross section transmission electron microscopy. Finite difference calculations of the strain and the 3D Schrödinger equation, taking into account piezoelectric and excitonic effects, agree with experimental results on transition energies of ground and excited states, revealed in luminescence and absorption spectra. We find as relative standard deviation of the size fluctuation ξ=0.04; the pyramid shape fluctuates between {101} and {203} side facets. Carrier capture into the QD ground state after carrier excitation above barrier is a very efficient process. No luminescence from excited states is observed at low excitation density. Energy relaxation processes in the zero-dimensional energy states are found to be dominated by phonon energy selection rules. However, multi-phonon emission (involving GaAs barrier, InAs wetting layer, InAs QD and interface modes) allows for a large variety of relaxation channels and thus a phonon bottleneck effect does not exist here.

Femtosecond degenerate four‐wave mixing study of exciton coherence in wide‐gap II–VI quantum wells

AbstractThis paper studies coherent excitons in the presence of alloy disorder by femtosecond degenerate four‐wave mixing on ternary (ZnCd)Se/ZnSe quantum wells. The use of very low excitation densities enables to uncover an impurity‐mediated exciton dephasing time of (16 ± 2) ps and a related homogeneous exciton width of only 80 μeV. At higher densities, the coherence loss speeds up dramatically due to exciton‐exciton interaction. Separating the contributions from inhomogeneous (photon echo) and homogeneous broadening to the spectrally resolved decay transients allows to deduce the energy dependence of the dephasing time across the disorder broadened exciton ensemble. Further, pronounced biexciton resonances are observed in the signal spectrum which exhibit characteristic polarization properties. For cross‐polarized pulses, the biexciton signal shows a striking stimulation when the excitation density is increased.

Time-resolved exciton dynamics and stimulated emission from [formula omitted] multiple quantum well structures

Time-resolved luminescence has been used to investigate the dynamics of excitonic recombination and stimulated emission in ZnCdSe ZnSe multiple quantum wells where the exciton binding energy is close to the optical phonon energy. At low excitation densities strong excitonic recombination is observed with simple exponential decay kinetics. At densities near threshold, however, bilinear recombination kinetics are found with a large rate constant ( R = 1.8 × 10 −3 s −1 cm 2), and a dramatic reduction in the lifetime is observed. Stimulated emission occurs at an energy ∼ 18 meV below the exciton absorption peak, and shifts to lower energy with increasing excitation density up to three times the threshold density. These observations are consistent with recombination in a dense exciton system, which gives way at high density to a correlated electron-hole plasma.

Blue avalanche upconversion in YAG:Tm

Abstract We report on the investigation of the photon avalanche effect in YAG: 5 at% Tm at room temperature leading to intense blue emission after pumping at 616.4 nm. A theoretical model based on rate equations reproduces quite well the experimental results under low and intermediate excitation density. Above this, an extra blue intensity is observed and additional terms should be included in the model.

Photon-hopping conduction and collectively induced transparency in a photonic band gap.

We study the dynamics of photon-hopping conduction between impurity atoms in a photonic band gap via resonance dipole-dipole interaction (RDDI) in the low-excitation density limit. This is done both in the absence and in the presence of a localized dielectric defect mode. The random impurity atom positions are modeled by means of a Gaussian random distribution of RDDI's with variance J and atomic line fluctuations with variance \ensuremath{\delta}. By numerical calculation, we demonstrate the occurrence of a number of collective quantum features of photon-hopping conduction. These include collective enhancement of the energy-transfer rate and collectively induced transparency within the dielectric cavity mode. These effects are shown to depend sensitively on the variances J and \ensuremath{\delta}. Our results suggest that RDDI mediated hopping conduction may protect atomic excitation energy from dissipation through nonradiative relaxation channels.

Near band-edge photoluminescence in relaxed Si1−xGex layers

Near band-gap photoluminescence was observed at low temperatures from relaxed Si1−xGex layers with 0.17<x<0.32 grown on Si(001) by ultrahigh vacuum chemical vapor deposition. The luminescence from undoped samples was dominated at low temperature and low excitation densities by recombination of excitons bound to alloy fluctuations exhibiting the smallest full width at half-maximum, 2.44 meV, reported for relaxed epitaxial Si1−xGex layers. Excitons bound to phosphorous and boron were also observed as was free exciton recombination.

Dephasing processes in II–VI quantum dots

AbstractThe loss of phase coherence in II–VI quantum dots is investigated using experimental methods working both in the frequency and in the time domain. Femtosecond differential absorption experiments reveal narrow resonances at very early times after excitation and low excitation densities for CdSe quantum dots. A substructure is detected and explained by transitions from the series of confined hole states. By nondegenerate four‐wave mixing experiments (NDFWM) the dephasing time is determined for CdS quantum dots with radii larger than the Bohr radius. Temperature and excitation intensity dependent dephasing times between ≈︁ 20 fs and some 100 fs are measured and attributed to electron‐hole pair–phonon coupling and carrier–carrier scattering.

Carrier-carrier scattering in photoexcited quantum wells: inadequacy of the two-particle collision model at low densities

Carrier-carrier scattering of a near-bandgap excited electron-hole plasma in an intrinsic GaAs quantum well is studied using two different Monte Carlo (MC) techniques. In the first one, the two-particle collisions are modelled using a screened Coulomb interaction. In the second one, the many-particle interaction is incorporated through classical molecular dynamics (MD). At low excitation densities (2*1010cm-2) molecular dynamics gives a much slower thermalization of the plasma than the 'two-particle collision' technique. This difference is ascribed to the breakdown of the two-particle collision model, because the screening of Coulomb interaction at low densities is too weak to limit the range of the two-particle interaction to the mean interparticle distance. The agreement of 'two-particle collision' simulations with experiment seems to be fortuitous. The disagreement of MD with experiment implies that the theory is still incomplete.

Electron-hole scattering in highly doped p-GaAs after femtosecond optical excitation

The ultrafast relaxation of minority electrons in highly doped p-GaAs ( rho =1.0*1019cm-3) has been investigated through femtosecond lime-resolved luminescence. For low excitation densities the hole plasma temperature stays at 300 K and the transient luminescence spectra reveal the rapid cooling of the minority electrons within the first picoseconds. The electron-hole energy transfer is much larger (up to 10-7W) than the known electron-LO phonon scattering rate, which allows the quantitative determination of the electron-hole energy transfer.

Hot-carrier photoluminescence from GaAs V-groove quantum wires

We report measurements of hot-carrier photoluminescence from GaAs V-groove quantum wires. CW spectra reveal distinct one-dimensional subbands: at low excitation density recombination from laterally confined nv=1 states is dominant, which shows that carriers relax efficiently to the lowest subband; nv=2 and 3 recombination becomes pronounced at higher temperature and density, which indicates inhibited relaxation, possibly due to band filling. Time-resolved photoluminescence measurements at 10 K show a rise time of approximately 100 ps, which is due to energy relaxation. At later times the intensity of the nv=1 line shows a monotonic decay at low carrier density, but there is a nv pronounced plateau at high density that extends for approximately 200 ps. These results provide clear evidence of carrier relaxation and band filling in quantum wires.