Increasing Excitation Density Research Articles

Observation of many-body effects in the excitonic spectra of semiconductor quantum wires

We report the observation of a blue shift of very sharp luminescence lines in GaAs/AlGaAs quantum wires at increasing excitation density. These sharp lines correspond to recombination of one-dimensional excitons localized in local potential minima due to interface disorder. The evolution of the fine-structured photoluminescence spectra with increasing exciton density is interpreted in terms of many-body effects which lead to an increase of the exciton self-energy and to the formation of excitonic complexes

Pump-probe spectroscopy of band tail states in metalorganic chemical vapor deposition-grown InGaN

Nanosecond nondegenerate optical pump-probe experiments have been performed on InGaN thin films and InGaN/GaN multiple quantum wells. Bleaching of absorption of the localized band tail states was observed with increasing excitation density (Iexc) of the pump pulse. The dynamics of the bleaching was found to depend on the localization depth of the band tail states and on Iexc. With high Iexc, large blueshifts in the spontaneous emission luminescence peaks were also observed, the magnitude of which was again found to depend on the localization depth of the band tail states. Stimulated emission is observed from the samples with increasing Iexc and correlates with significant changes in the behavior of the absorption bleaching. The observed bleaching dynamics of the band tail states are well explained by considering the effective lifetime of the band tail states as measured by time-resolved photoluminescence experiments.

Compensation effects in Mg-doped GaN epilayers

The compensation of Mg-doped GaN is systematically studied by low-temperature photoluminescence (PL) using a series of samples with different Mg concentrations. Strongly doped samples are found to be highly compensated in electrical measurements. The compensation mechanism is directly related to the incorporation of Mg. Three different deep donor levels are found at 240±30, 350±30 and 850±30meV from the conduction band. With increasing excitation density in photoluminescence measurements these levels are subsequently neutralized giving rise to a deep unstructured donor–acceptor pair (DAP) emission. These emissions exhibit a blue shift that saturates at higher densities, thus excluding band fluctuations as the prime mechanism for the low energy of the observed DAP bands.

Mg concentration dependence of optical properties in GaN : Mg

Photoluminescence spectra of MBE-grown GaN:Mg samples have been studied as a function of temperature, excitation density, and Mg concentration. For highly doped samples a red shift of the donor–acceptor pair transition line was observed with increasing Mg concentration. With increasing excitation density, the emission was found to shift to higher energy. This is explained by the formation of mini-bands originating from the discrete acceptor levels.

Carrier dynamics in type-II GaSb/GaAs quantum dots

The optical properties and dynamics of charge carriers in self-organized arrays of type-II (staggered band lineup) GaSb/GaAs quantum dots are studied. Interband absorption from type-II quantum dots is evidenced; the energetic positions of quantum dot absorption peaks coincide with those apparent in photoluminescence spectra. (Sb,As) intermixing with an antimony diffusion length of about 1 nm is found to make an important contribution to the observed transition energies. Dipole layer formation and quantum dot state filling contribute to the luminescence blueshift with increasing excitation density. The recombination rate of electrons with localized holes drastically depends on the average carrier density. When several carriers are localized at each dot, decay time constants around 5 ns, quite similar to type-I systems, are observed. Individual, spatially indirect excitons decay with much larger time constants close to 1 \ensuremath{\mu}s. The decay time of quantum dot luminescence is independent of the temperature in the measured range $T<~65\mathrm{K}$ as expected for zero-dimensional excitons.

Gain to absorption conversion by increasing excitation density in excitonic waveguides

Exciton waveguiding was proposed to improve optical confinement of II–VI lasers. Strong resonant exciton absorption induces a refractive index enhancement (Δ n) on the low-energy side of the absorption peak. By stacking sheets of submonolayer (SML) CdSe insertions into a ZnSe matrix, we fabricated a region in ZnSe with strong resonant exciton absorption. In this region, the k-selection rule is lifted due to exciton localization by the SML islands. Therefore, a zero-phonon lasing mechanism resonant to the exciton waveguiding region could be realized. Using the variable-stripe-length method, we study the intensity-dependent optical gain of the ZnSe/CdSe submonolayer superlattices in a ZnS 0.06Se 0.94 matrix.

Gain to absorption conversion by increasing excitation density in excitonic waveguides

Gain to absorption conversion by increasing excitation density in excitonic waveguides

Space-charge effects in type-II strained layer superlattices

The low-temperature photoluminescence (PL) spectra of a range of CdS-CdSe and CdS-ZnSe type-II superlattices was measured under both cw and quasi-cw excitation over the excitation range from mW cm −2 to MW cm −2. In all cases, a single PL band due to the recombination of separated electrons and holes in the type-II bandstructure was observed. As the excitation density increases from a low value, the PL peak shifts logarithmically towards the blue. Total shifts of up to 200 meV for a CdS-CdSe sample of 10 nm period are observed, with a corresponding shift-factor of 30 meV per decade of excitation density. The shift factor increases in a regular fashion with the superlattice period. A comparison with the literature shows that a logarithmic dependance of the blue shift with increasing excitation density appears to be a universal feature of type-II SL, even in the absence of strain.

Exciton-exciton collisions and conversion of interwell excitons in GaAs/AlGaAs superlattices

The ratio of the densities of intra-and interwell excitons in a symmetric system of coupled quantum wells — a superlattice based on a GaAs/AlGaAs heterostructure — is investigated over a wide range of optical excitation power densities. Conversion of interwell excitons into intrawell excitons as a result of exciton-exciton collisions is observed at high exciton densities. Direct evidence for such a conversion mechanism is the square-root dependence of the interwell exciton density on the optical excitation level. The decrease in the lifetime of interwell excitons with increasing excitation density, as measured directly by time-resolved spectroscopy methods, confirms the explanation proposed for the effect.

Optical gain in optically pumped cubic GaN at room temperature

We investigate the transient surface emission and the optical gain of cubic GaN on GaAs(001) upon pulsed optical pumping at room temperature. The initial decay time of the transient surface emission drastically decreases with increasing excitation density, reaching a value as short as 20 ps at a fluence of 50 μJ cm−2. This rapid decay suggests the presence of laterally amplified spontaneous emission. In fact, gain-stripe measurements of the edge emission reveal an optical gain exceeding 100 cm−1 at a fluence of 20 μJ cm−2.

Cathodoluminescence study of band filling and carrier thermalization in GaAs/AlGaAs quantum boxes

We have examined carrier thermalization, recombination, and band filling in GaAs/AlGaAs quantum boxes with low-temperature cathodoluminescence (CL). The temperature dependence of the quantum box CL intensity for T⩽ 90 K exhibits an Arrhenius behavior, as a result of carrier thermalization between the quantum box and surrounding barrier regions. The width of the quantum box luminescence is found to increase rapidly with an increasing excitation density and reveals an enhanced phase-space and real-space filling, in comparison to the behavior observed for quantum wells.

Influence of compensation on the luminescence of nitrogen-doped ZnSe epilayers grown by MOVPE

The luminescence of ZnSe grown by metalorganic vapour phase epitaxy and doped by a DC nitrogen plasma is investigated. With increasing N2 flux the donor-acceptor pair (DAP) band continuously develops into a structureless band peaking at 2.62 eV for highest doping levels. This broad band evolves back into a structured DAP band peaking at 2.698 eV with increasing excitation density. At high N concentrations and at large degree of compensation potential fluctuations become important for the spatially indirect DAP recombination. These fluctuations can easily be screened by optically excited carriers making the experimental conditions decisive for luminescence spectra of strongly doped ZnSe: N samples.

Intensity-dependent energy and lineshape variation of donor-acceptor-pair bands in highly compensated ZnSe:N

The photoluminescence (PL) of donor-acceptor-pairs (DAP) in ZnSe:NGaAs epilayers at different levels of nitrogen doping and various compensation ratios is studied. The PL spectra of moderately doped and weakly compensated samples show a DAP zero-phonon line and clearly distinguished phonon replica shifting to the red by 10 meV for strongly decreasing excitation density. At medium nitrogen doping and compensation level, well-structured, but red-shifted and broadened DAP emission shows a more pronounced dependence on excitation density. For highly doped and strongly compensated samples, the DAP band continuously develops from a structureless red-shifted broad emission to a well-structured one for increasing excitation density. More heavily doped and strongly compensated samples show strongly red-shifted and structureless broad band emissions for all excitation densities, and a very pronounced dependence of the maximum position on excitation density. We present a general explanation by charged impurities randomly distributed in highly compensated samples and causing a fluctuating potential affecting the bands and impurity levels.

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.

Radiative recombination in near-surface strained Si1−xGex/Si quantum wells

Influence of the surface on the radiative recombination is studied in near-surface (NS) strained Si1−xGex/Si quantum wells (QWs). The luminescence intensity of NSQWs is clearly diminished as the Si cap thickness decreases due to carrier capture to the surface. However, the luminescence diminution discloses a saturable behavior with increasing excitation density. Time-resolved measurements reveal selective electron capture to surface traps as being the controlling process in the luminescence attenuation, and there is no signature of quantum mechanical hole tunneling to the surface. Results of potential-biased luminescence lend further support to the electron-controlled picture of the luminescence attenuation in Si-based near-surface geometry.

Non-linear optics of hybrid excitons in organic-inorganic nanostructures

In a system comprising an organic layer coupled to a semiconductor layer, if the energy of the Frenkel exciton in the first is close to that of the Wannier exciton in the second, the lowest electronic excitations can be described as hybrid resonances. These new excitons give rise to very pronounced optical non-linearities because they have at the same time large oscillator strengths, typical of Frenkel excitons, and large radii, typical of Wannier excitons. The results of a microscopic calculation of the optical properties of such a system are presented and discussed: compared with the usual semiconductor quantum wells, enhancements of two orders of magnitude both in the size of the susceptibility (linear part) and in its relative change with increasing excitation density (Kerr non-linearity) are predicted.

Intensity-dependent energy and line shape variation of donor–acceptor-pair bands in ZnSe:N at different compensation levels

We show that energy position and line shape of donor–acceptor-pair luminescence bands in ZnSe:N/GaAs epilayers depend very sensitively on excitation density and compensation. A continuous development from structureless red-shifted broad to well structured donor–acceptor-pair (DAP) bands is observed for increasing excitation density. The red shift is explained by the fluctuating potential affecting the bands and impurity levels and is caused by random distribution of charged impurities in highly compensated samples. The shift is reduced when these charge fluctuations are diminished due to an increasing number of impurities being neutralized via light-induced carrier excitation. These effects have not been taken into account in previous work concerning doped II-VI materials; however, they have to be considered when evaluating the frequently used hypothesis of a deep donor in ZnSe:N as an explanation of low-energy broadband DAP emission. The influence of band fluctuations on the behavior of the DAP luminescence and excitation spectra is qualitatively discussed.

Impulsive Softening of Coherent Phonons in Tellurium.

We investigate the dynamics of coherent optical phonons in tellurium after high-density excitation with femtosecond laser pulses. The data show a continuous redshift of the phonon frequency with increasing excitation density. Experiments with double-pulse excitation prove that the observed frequency shift is of purely electronic origin. We demonstrate that coherent phonons allow monitoring the pathway to nonthermal laser-induced melting of crystalline materials.

Type-II band alignment in Si/Si1-xGex quantum wells from photoluminescence line shifts due to optically induced band-bending effects: Experiment and theory.

We compare measured energy shifts of photoluminescence lines in Si/${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ quantum wells as a function of excitation power with theoretical calculations to conclude that the band alignment of the heterointerface is type II. Experimentally, we study molecular-beam epitaxy-grown fully strained Si/${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ single quantum wells with Ge fractions from 10% to 36%. These show significant blueshifts of the luminescence at increasing excitation density. Theoretically, we calculate self-consistently transition energy changes taking into account band bending due to the photoinduced charge carriers. Only for type-II band alignment are the experimental and theoretical results compatible.

Passivation of InGaAs/InP surface quantum wells by ion-gun hydrogenation

We have investigated the optical properties of an InGaAs/InP surface quantum well before and after room-temperature low-energy ion-gun hydrogenation. The luminescence efficiency of the surface quantum well was enhanced by up to two orders of magnitude after hydrogenation. Our experiments also reveal that the nonradiative recombination centers at the etched surface can be saturated by increasing excitation density for the photoluminescence measurement. To ‘‘unmask’’ the effects of the saturation of recombination sites, for a true comparison of passivation effects brought about by different surface treatments, an excitation density below 1 W/cm2 is required.