Bound Exciton Recombination Research Articles

Composition dependence of electronic defects in CuGaS2

AbstractCuGaS2 films grown by physical vapor deposition were studied by photoluminescence (PL) spectroscopy, using excitation intensity and temperature‐dependent analyses. We observed free and bound exciton recombinations, three donor‐to‐acceptor (DA) transitions, and deep‐level transitions. The DA transitions at ~2.41, 2.398, and ~2.29 eV are attributed to a common donor level ~35 meV and two shallow acceptors at ~75 and ~90 meV and a deeper acceptor at 210 meV above the valence band. This electronic structure is similar to those of other chalcopyrite materials. The observed DA transitions are accompanied by several phonon replicas. The Cu‐rich and near‐stoichiometric CuGaS2 films are dominated by transitions involving the acceptor at 210 meV. All films show deep‐level transitions at ~2.15 and 1.85 eV due to broad deep defect bands. The slightly Cu‐deficient films were dominated by intense transitions at ~2.45 eV, which were attributed to excitonic transitions, and a broad defect transition at 2.15 eV.

Photoluminescence spectroscopy of excitonic emission in CsPbCl3 perovskite single crystals

We analyzed low-temperature photoluminescence (PL) and the band edge excitonic emission in bulk halide perovskite CsPbCl3 single crystals. Strong near band edge excitonic emission is observed at 420 nm and a series of excitonic states was observed. Temperature-dependent PL measurements on cesium lead chloride over a wide temperature range from 13 K to 295 K revealed significant electron phonon coupling with a Huang-Rhys parameter ranging from 1.34 to 3.38. A primary experimental finding is that the near band edge PL emission shows existence of multiple sharp features at low temperatures that are attributable to free excitons, bound excitons, and their phonon replicas. The PL also shows evidence of band tailing effects due to disorder. We investigated the recombination mechanism of photogenerated charge carriers by means of time-resolved PL (TR-PL) spectroscopy at low temperatures and various excitation powers and observed competing mono- and bimolecular recombination processes. A dynamic model is proposed to describe the TR-PL. We observed both long-lived and short-lived recombination where the short-lived time decay is attributed to free exciton recombination, whereas the long-lived decay to bound exciton recombination involving traps due to intrinsic disorder.

Characterization of sol gel Zn1-xCaxO thin layers deposited on p-Si substrate by spin-coating method

Abstract Thin films Zn1-xCaxO (0 ≤ x ≤ 6%) on a p-Si substrate are elaborated by sol-gel process and spin coating. X-ray diffraction displays a hexagonal wurtzite structure with an increase of lattice parameters confirming the substitution of Zn2+ by Ca2+. The estimation of crystallite sizes along the three main crystallographic planes is practically constant (22 nm) suggesting a spherical symmetry shape of the crystallites which is confirmed by SEM. UV–visible reflectance spectra attest a band gap tuning from 3.144 to 3.262 eV which is confirmed by the PL measurement at 2 K for Zn0.94Ca0.06O sample by the appearance of a broad band around 3.508 eV. Although the luminescence of the samples is weak, we well distinguish the free exciton emission (FXA) positioned at 3.376 eV and the bound exciton recombination (D0X) around 3.362 eV.

Transfer of Direct to Indirect Bound Excitons by Electron Intervalley Scattering in Cs2AgBiBr6 Double Perovskite Nanocrystals

Lead-free halide double perovskites have emerged as a non-toxic alternative to the heavily researched lead-based halide perovskites. However, their optical properties and the initial charge carrier relaxation processes are under debate. In this study, we apply time-resolved photoluminescence and differential transmission spectroscopy to investigate the photo-excited charge carrier dynamics within the indirect band structure of Cs2AgBiBr6 nanocrystals. Interestingly, we observe a high energetic emission stemming from the direct band gap, besides the previously reported emission from the indirect band gap transition. We attribute this emission to the radiative recombination of direct bound excitons. This emission maximum redshifts nearly 1 eV within 10 ps due to electron intervalley scattering, which leads to a transfer of direct to indirect bound excitons. We conclude that these direct bound excitons possess a giant oscillator strength causing not only a pronounced absorption peak at the optical band gap energy but also luminescence to occur at the direct band gap transition in spite of the prevailing intervalley scattering process. These results expand the understanding of the optical properties and the charge carrier relaxation in double perovskites, thus, facilitating the further development of optoelectronic devices harnessing lead-free perovskites.

Low-temperature photoluminescence spectroscopy of CH3NH3PbBrxCl3-x perovskite single crystals

Organic-inorganic halide perovskite (OIHP) has attracted tremendous attention due to its potential applications in optoelectronics such as light emitting device and photodetector. Here, we have grown high quality CH3NH3PbBrxCl3-x perovskite single crystals and investigated the luminescence mechanism of these materials. The results indicate that optical properties of CH3NH3PbBrxCl3-x perovskite materials are strongly depend on the temperature and the dope concentration. Free exciton recombination emission has dominant contribution to luminescence spectra of materials at orthorhombic structure, as bound exciton recombination emission to the spectra of materials at cubic structure. Based on temperature-dependent photoluminescence, the exciton binding energy of CH3NH3PbCl3 and CH3NH3PbBr0.3Cl2.7 are calculated as about 22.9 meV and 27.1 meV, respectively.

Temperature-Dependent Photoluminescence Emission from Unstrained and Strained GaSe Nanosheets.

Two-dimensional AIIIBVI layered semiconductors have recently attracted great attention due to their potential applications in piezo-phototronics and optoelectronics. Here, we report the temperature-dependent photoluminescence (PL) of strained and unstrained GaSe flakes. It is found that, as the temperature increases, the PL from both the strained (wrinkled) and unstrained (flat) positions show a prominent red-shift to low energies. However, for the flat case, the slope of PL energy versus temperature at the range of 163–283 K is about −0.36 meV/K, which is smaller than that of the wrinkled one (−0.5 meV/K). This is because more strain can be introduced at the freestanding wrinkled position during the temperature increase, thus accelerates the main PL peak (peak I, direct band gap transition) shift to lower energy. Additionally, for the wrinkled sheet, three new exciton states (peaks III, IV, and V) appear at the red side of peak I, and the emission intensity is highly dependent on the temperature variation. These peaks can be attributed to the bound exciton recombination. These findings demonstrate an interesting route for optical band gap tuning of the layered GaSe sheet, which are important for future optoelectronic device design.

Tuning the Competitive Recombination of Free Carriers and Bound Excitons in Perovskite CH3NH3PbBr3 Single Crystal

Excitation of perovskite CH3NH3PbBr3 crystal generates long-lived carriers, which radiatively recombine via free electrons and holes or localized excitons. Here, we investigate the competitive recombination of free carriers and bound excitons by tuning two-photon excitation wavelength. At excitation far from resonance, the CH3NH3PbBr3 perovskite single crystal shows the free carrier up-conversion emission because the exciton binding energy is comparable with the thermal energy at room temperature. At excitation near resonance (1.97 eV < hνexc < 2.16 eV), the anti-Stokes process with a consistent phonon energy of ∼186 meV is reported for the first time. Furthermore, when the anti-Stokes transition is resonant with localized exciton level below the band, highly efficient up-converted luminescence from the bound exciton recombination is observed. The finding that the excited state recombination kinetics vary versus excitation wavelength, in the present work, is helpful for developing the high-performance opt...

The influence of acoustic-dislocation interaction on intensity of the bound exciton recombination in initial and irradiated GaAsP LEDs structures

Acoustic-excitant interaction of GaAsP light emitting diodes (initial and irradiated by 2 MeV electrons) was studied. Structure based on GaAs1-хPх solid solutions, grown by epitaxy from the vapor phase, were the object of the research. It was observed that ultrasonic treatment (UST) results in the drop of the emitting intensity of structures, which relaxes to the previous values after ultrasound termination. The possible reason of observed changes concerning nonequilibrium dislocation clusters were discussed. Electron irradiation leads to the exponential drop of emitting intensity, which restores after UST much slower than initial one. Radiation degradation parameters τ0/Kτ of yellow and orange LEDs were found.

Uniaxial stress and Zeeman spectroscopy of the 3.324-eV Ge-related photoluminescence in ZnO

Recently observed photoluminescence (PL) in ZnO, positioned at 3.324 eV and known to be related to Ge impurities, is investigated here by uniaxial stress and Zeeman spectroscopy measurements. The 3.324-eV PL line shifts but does not split under uniaxial stress both parallel and perpendicular to the $c$ axis, indicating trigonal defect symmetry. This reinforces the findings of prior work that the defect center is related to a substitutional Ge impurity in ZnO. Applied magnetic fields result in linear splittings of the line into two components for fields parallel and perpendicular to the $c$ axis. This result, combined with the temperature dependence of the Zeeman spectra, enables the line to be assigned to neutral donor bound-exciton recombination, most likely at a partially compensated Ge${}_{\mathrm{Zn}}$ double-donor impurity.

Back Cover: Long delays of light in ZnO caused by exciton‐polariton propagation (Phys. Status Solidi B 7/2012)

ZnO has in recent years been a subject of extensive research efforts driven by the prospect for a variety of potential applications. Recently a dramatic decrease in the group velocity of light propagating through bulk ZnO was demonstrated by time-of-flight studies of laser pulse propagation [Phys. Rev. B 83, 245212 (2011)]. By directly comparing the propagation of both photoluminescence (PL) and laser pulses, Chen et al. show that this effect is caused by the formation of free-exciton polaritons and is determined by their dispersion, whereas contributions of the bound-exciton polaritons become important only in close proximity of the corresponding resonances (see the article on pp. 1307–1311). The cover figure presents transient PL images measured from bulk ZnO within the spectral range of bound exciton recombination (I6 and I7 lines) in conventional back-scattering geometry (a) and in a forward-transmission configuration (b), i.e. during time-of-flight. The solid line in (b) represents time delays after propagation of light through the ZnO media, calculated by taking into account the formation of the exciton polaritons.

Zeeman photoluminescence spectroscopy of isoelectronic beryllium pairs in silicon

Isoelectronic beryllium (Be) pair centers in silicon have been studied by photoluminescence spectroscopy under a magnetic field. The photoluminescence of the bound-exciton recombination at this center shows that the number of Zeeman split peaks is the smallest for the magnetic field applied along a 〈 100 〉 direction. This result provides direct evidence that the Be pairs orient themselves in 〈 111 〉 directions. The g values of the hole and electron in this bound exciton determined by fitting of the Zeeman diagrams support the shallow acceptor character of this isoelectronic center.

Luminescence and energy-transfer mechanisms inEu3+-doped GaN epitaxial films

We studied the photoluminescence (PL) and energy-transfer processes in ${\text{Eu}}^{3+}$-doped GaN epitaxial films by means of microscopic PL imaging spectroscopy. ${\text{Eu}}^{3+}$-doped GaN epitaxial films exhibited blue luminescence due to bound-exciton recombinations in GaN host crystals and red luminescence due to intra-$4f$ transitions of ${\text{Eu}}^{3+}$ ions. We found an anticorrelation between the exciton and ${\text{Eu}}^{3+}$ PL intensities in space-resolved PL images, indicating that energy transfer from GaN crystals to ${\text{Eu}}^{3+}$ ions determines the ${\text{Eu}}^{3+}$ luminescence intensity. PL and PL excitation spectra showed that efficient ${\text{Eu}}^{3+}$ luminescence is caused by two different excitation processes: energy transfer from the low-energy charge-transfer (CT) states to ${\text{Eu}}^{3+}$ ions or from the delocalized states above the band edge of GaN crystals to ${\text{Eu}}^{3+}$ ions. The energy-transfer process from the CT state to ${\text{Eu}}^{3+}$ ions dominates the ${\text{Eu}}^{3+}$ luminescence.

Bound and free excitons in ZnO. Optical selection rules in the absence and presence of time reversal symmetry

The correlation between ionized donor bound exciton recombinations and neutral donor bound exciton recombinations in ZnO has been investigated. The experimental data obtained by means of magneto-photoluminescence (MPL) concerning charge state and localization energies of ionized and neutral donor bound excitons are in good agreement with theoretical predictions. The optical selection rules in absence and presence of time reversal symmetry (TRS) are investigated. It is shown that the inclusion of extra degeneracy due to TRS reveals a number of new states of the same symmetries and essentially does not change the existing optical selection rules.

Dynamics of bound excitons versus thickness in freestanding GaN wafers grown by halide vapor phase epitaxy

The bound exciton recombination properties in freestanding GaN layers of various thicknesses grown by halide vapor phase epitaxy have been characterized by time-resolved spectroscopy. Improvement of the donor bound exciton (D0X) lifetime was observed with increasing GaN layer thickness up to ∼400μm, while for thicker layers the recombination time of D0X shows a tendency to saturate. The thickness-dependent behavior of the D0X decay can be understood in terms of competition between two nonradiative mechanisms: one of which is connected to structural defects, and consequently more important for thinner layers, while for layers with thickness above 400μm with low structural defect density the recombination time is limited by point defects such as impurities and vacancies.

Synthesis of CdS nanowire networks and their optical and electrical properties

High quality single-crystal CdS nanowire (NW) networks have been synthesized on Si(111)substrates via the chemical vapour deposition method. X-ray diffraction and selected areaelectron diffraction show that the NWs in the networks grow along the directions and their (0001) crystal planes are parallel to the Si(111) substrates.Room-temperature photoluminescence (PL) spectra of single CdS NWs in the networks aredominated by a near-band-edge emission and free from deep-level defect emissions. ThePLs resulting from free-exciton and bound-exciton recombinations are detected at 77 K.The results of the electrical transport measurement on the CdS NW networks show thatthe current can flow through different NWs via the cross-junctions. The resistivity, electronconcentration and electron mobility of single NWs in the networks are estimated by fitting theI–V curves measured on single NWs with the metal–semiconductor–metal model suggested byZhang et al (2006 Appl. Phys. Lett. 88 073102; 2007 Adv. Funct. Mater. at press).

Identification of donor-related impurities in ZnO using photoluminescence and radiotracer techniques

The results of photoluminescence measurements on ZnO implanted with stable and radioactive isotopes of Zn and Ga are presented. The donor-related exciton feature ${I}_{8}$ at $3.3600\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is suggested to be due to bound exciton recombination at Ga donors. The ${I}_{1}$ line at $3.3718\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is also likely to be due to Ga, and is attributed to ionized Ga donor bound exciton recombination. A feature at $3.3225\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is observed following transmutation of radioactive Ga into stable Ge, and is attributed to Ge. Finally, a damage-related band is observed in the region of $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ when the recoil energy of the decay is capable of dislodging the host atoms from their respective lattice sites.

Photoluminescence investigation of Be-doped Npn AlGaAs/GaAs heterojunction bipolar transistor structures

Highly complex Npn AlGaAs/GaAs single heterojunction bipolar transistor (HBT) layers with Be-doped base were investigated by photoluminescence (PL) spectroscopy. Room temperature PL shows only a broad peak of GaAs due to thermalization; 15 K PL shows five peaks. The peak at ∼1.481 eV is from a p-type GaAs base, that at ∼1.517 eV is from a low-doped GaAs layer and that at ∼1.55 eV is from a high-doped GaAs collector. The that at ∼1.849 eV is due to bound exciton recombination in an AlGaAs emitter, and that at ∼1.828 eV is due to the acceptor-related transition from the AlGaAs layer. The integrated intensity ratio of these two peaks can be used to investigate the Be outdiffusion behavior, thus optimizing the growth conditions of base. The DC current gain of the HBT structure with different growth conditions was found to be in good agreement with the PL results.

Shallow donors and acceptors in ZnO

In order to realize controlled p-type doping in ZnO the role of extrinsic and intrinsic donors has to be clarified. The extrinsic n-type dopants Al, Ga and In are commonly found in bulk ZnO crystals, but hydrogen also appears in relevant concentrations eventually controlling the residual n-type carrier concentrations in nominally undoped ZnO. The optical properties of excitonic recombinations in bulk, n-type ZnO are investigated by photoluminescence (PL). At liquid helium temperature the neutral donor–bound excitons dominate in the PL spectrum. Two electron satellite (TES) transitions of the donor–bound excitons allow us to determine the donor binding energies ranging from 46 to 73 meV. In the as-grown crystals a shallow donor with an activation energy of 30 meV controls the conductivity. Annealing annihilates this shallow donor which has a bound exciton recombination at 3.3628 eV. Correlated by magnetic resonance experiments we attribute this particular donor to hydrogen. These results are in line with the temperature-dependent Hall-effect measurements. The Al, Ga and In donor–bound exciton recombinations are identified based on doping and diffusion experiments, and using secondary ion mass spectroscopy. We report on the optical properties of the shallow nitrogen acceptor in ZnO incorporated by diffusion, by ion implantation and by in situ doping in epitaxial films.

Photoluminescence investigations on a native donor in ZnO

ABSTRACTThe shallow donor impurities in ZnO with binding energies between 46 and 56 meV have been studied in great detail in the recent years. They give rise to neutral donor bound exciton recombinations with the A- and B-valence bands, show rotator states and two-electron-satellite transitions. These properties allowed to establish the excited state splittings of the donors as well as confirming Hayne's rule in ZnO. So far they all seem to be of extrinsic origin, hydrogen, aluminum, gallium and indium in order of increasing binding energy. For many years it was common sense that intrinsic defects would dominate the n-type conductivity of ZnO. Interstitial zinc as well as oxygen vacancies should be double donors, and in order to contribute to the n-type-conduction they should have shallow levels, and low formation energies to be abundant. In PL-measurements at T∼100 K on various ZnO samples, single crystals as well as thin films, a luminescence around 3.31 eV was detected. Due to its line shape and temperature behaviour it is identified as bound-to-free recombination. If we assume that the 3.31 eV band with its level at EC ∼ 130 meV is the ++/+ level of the zinc interstitial we calculate for the binding energy of the +/0 level ∼ 130 meV, i.e. around 33 meV. Undoped Zn-rich epitaxial films grown by CVD show a dominant I3 recombination at 3.367 eV which according to Haynes rule is consistent with a shallow donor level at 33 meV. Moreover, they have free n-type carrier densities of 2×1019 cm−3 and as revealed by SIMS the common donor impurities (Al, Ga, In) cannot account for the high carrier densities.

Anomalous PL brightening caused by partial Auger recombination of a (D 0, X) complex during an impact ionization avalanche in n-GaAs

Partial Auger recombination of the (D 0, X) bound-exciton in n-GaAs at 4.2 K during an impact ionization avalanche under an applied pulse voltage has been investigated. The bright photoluminescence (PL)-pattern observed by applying the pulse voltage, characterizes well the formation of a current density filament. The observation of the bright PL spectra inside the current density filament gives a quite new result concerning the partial Auger recombination process of the (D 0, X) complex, leaving the neutral donor in the excited states.