Total Photoluminescence Research Articles

Local Structure and Er3+ Emission from Pseudo-Amorphous GaN:Er Thin Films

AbstractWe report on strong Er3+ luminescence in the visible and infra-red regions at room temperature in amorphous GaN:Er thin films prepared by DC magnetron co-sputtering. The intensity of the Er3+ luminescence at 1.535 μm corresponding to 4I13/2 → 4I15/2 transitions is greatly enhanced after annealing at 750°C. In this material GaN crystallites have formed and embedded in the continuous amorphous matrix. The crystallites are 4 to 7 nm in diameter as analyzed by high resolution transmission electron microscopy. The absorption edge, extending three orders of magnitude in absorption coefficient in the spectral range from 0.5 to 3.5 eV, is superimposed on resonant absorption bands of Er3+ ions.The total photoluminescence spectrum consists of welldefined Er3+ luminescence peaks imposed on a broad band edge luminescence from the amorphous GaN host matrix.

Radiation-induced photoluminescence quenching of phenylene vinylene oligomer thin films

We have studied the radiation-induced photoluminescence (PL) quenching and the associated electronic structure modification of a light-emitting conjugated oligomer, namely, 1,4-bis[4-(3,5-di-tert-butylstyryl) styryl]benzene (4PV), in the absence of oxygen. We found that a short period of exposure to the radiation sources used in UV or x-ray photoelectron spectroscopies (UPS or XPS) resulted in total PL quenching in 4PV. Although there was no detectable change in the XPS C 1s core-level spectra UPS valence spectra indicated the disappearance of the vinylene peak upon exposure to UV radiation. This suggests the destruction of the vinylene groups, and the possible creation of radical species, which may serve as PL quenching sites. We also found that the modification in the electronic structure by Ca deposition masks that by radiation, although both quenches the PL effectively.

Optical Characterization of InAs Quantum Dots Fabricated by Molecular Beam Epitaxy

Optical characterization were performed on patterned InAs dots based on InAs/InAlAs hetero-structure and the self-assembled InAs dots grown on GaAs. Unexpectedly high peak energy shift in photoluminescence (PL) and cathodoluminescence (CL) was observed from InAs dots based on InAs/InAlAs heterostructure. PL results of self assembled InAs dots grown on GaAs also showed unexpectedly high peak energy. CL spectra from each single InAs dot suggested luminescence from highly strained dots contribute to shift the average luminescence peak towards higher energy. The high PL peak energy from the self-assembled InAs dots turned out to be composed of strain-induced band-gap widening (28%), carrier confinement effect (17%) in addition to unstrained band-gap energy (29%). 26% of the total PL peak energy remained unassigned.

Magnetic resonance in films and photodiodes based on poly-(phenyl-phenylene-vinylene)

Films of poly-(2-phenyl-1,4-phenylene-vinylene) (PPPV) and photodiodes with PPPV as an active layer were studied by optically (ODMR) and electrically (EDMR) detected electron-spin resonance (ESR). Two different signals were observed in ODMR: enhancement of the photoluminescence (PL) at g=2.01 due to recombination of the photogenerated polarons (s=1/2), and a half-field enhancement signal, attributed to the fusion of triplet excitons. Both processes lead to the formation of singlet excitons. The spectral dependence of the s=1/2 signal follows the low energy part of the PL spectrum, indicating that delayed recombination of distant polarons is influenced by ESR, whereas the cw PL contains both prompt and delayed contributions. The linewidth and the intensity of the ODMR signal depend on the PL excitation intensity. Both effects are due to a decrease of the recombination lifetime of the polaron pairs at higher intensities. The relative decrease of the short-circuit photocurrent ISC through a PPPV photodiode by ESR saturation is due to recombination of nonthermalized, nongeminate excess charge polarons in the active layer of the device. This effect is at least two orders of magnitude stronger than the enhancement of total PL at the same temperature. This feature is found to be common for conjugated polymers investigated so far, and reflects the fact that the total photogenerated ISC is spin dependent, whereas ODMR selects only the nongeminate portion of recombining species in the sample.

High temperature optical properties of GaAs/AlGaAs double heterostructures

We have studied the photoluminescence spectrum, intensity and decay time at temperatures up to 700 K in a series of GaAs/AlGaAs double heterostructures grown by liquid phase epitaxy (LPE). Several growth parameters were adjusted to optimize the photoluminescence intensity at high temperatures. We conclude that we can produce material of high quality, where the radiative recombination is dominant even at 700 K. A reduction of the total photoluminescence intensity at 700 K to 45% of the intensity of 300 K is explained as due to the presence of a weak competing non-radiative recombination process.

Temperature and doping dependence of the photon recycling effect in GaAs/AlGaAs heterostructures

We have calculated the effect of photon recycling, as a function of energy, temperature, and doping level in GaAs/AlGaAs double heterostructures. The calculations have been performed numerically using energy-dependent expressions for the absorption, emission, and refractive indices. Our calculations are compared to experiments, and we demonstrate the influence of the photon recycling on the photoluminescence line shape, both peak position and the high energy slope, total intensity, and decay time. The calculations show that the total emitted photoluminescence intensity should weakly increase with increasing temperature, if the internal quantum efficiency is 1. The experimentally observed intensity loss at high temperatures (700 K) is explained as due to a reduction of the internal quantum efficiency.

Free-carrier absorption and luminescence decay of porous silicon

We present time-resolved photoluminescence (PL) and photo-induced free-carrier absorption measurements on free-standing porous silicon flakes at room temperature. The number of free carriers was monitored by absorption of infrared beams, at 3.39 μm as well as at 1.3 μm wavelength. The excellent scaling between these two wavelengths during the complete decay yields clear evidence for an absorption related to free carriers as in bulk Si. Furthermore, the total PL yield in the decay, integrated over the PL spectrum, scales with the photo-induced carrier concentration. Using the bulk-Si free-carrier absorption cross-section we find that of the order of 15% of the initially excited carriers remain after a fast initial transient, whereas only a few per cent recombine radiatively. The competition between various recombination channels is discussed.

Exciton formation and hole-spin relaxation in intrinsic quantum wells.

We have investigated the exciton spin dynamics in quantum wells. In nonresonant excitation conditions, the initial degree of circular polarization of the exciton luminescence is higher than the excitation light polarization. This demonstrates that excitons are formed from nongeminate electron-hole pairs. The understanding of the exciton formation process allows us to propose a method to measure directly the hole spin-relaxation time (${\mathrm{\ensuremath{\tau}}}_{\mathit{h}}$) in intrinsic quantum wells. We show how the analysis of the total photoluminescence intensity under circularly and linearly polarized excitation allows us to measure ${\mathrm{\ensuremath{\tau}}}_{\mathit{h}}$, independently of the other spin-flip processes.

Simultaneous Detection of Radiative and Non-Radiative Recombination in Porous Silicon

ABSTRACTRadiative and non-radiative recombination in porous silicon (PSi) is measured under a wide injection range by detecting of photoluminescence (PL) and free-carrier absorption (FCA) decay. We used 2.34 eV and 140 ns light pulses for carrier excitation in PSi flakes. The excited carriers were probed by focused cw IR light and the carrier concentration was calculated by using the free-carrier absorption cross section of bulk (c-Si). The results demonstrate a scaling between the total PL yield in the S-band, integrated over wavelengths, and the free carrier concentration. At lower injections the observed free carrier and PL decay follow a similar, density-independent stretched-exponential law. At injections approaching 1018 cm−3, a fast recombination component appears reducing the lifetime by a factor of ten. This component is attributed to Auger recombination of separated e-h pairs on a set of Si subclusters.

Radiative lifetimes, quasi-Fermi-levels, and carrier densities in GaAs-(Ga,Al)As quantum-well photoluminescence under steady-state excitation conditions

A quantum-mechanical calculation of the carrier densities, electron and hole quasi-Fermi-levels, and various radiative decay times in GaAs-(Ga,Al)As quantum wells is performed, under steady-state excitation conditions, as functions of the cw laser intensity, temperature, well widths, and acceptor distribution in the well. We consider the radiative recombination of electrons with free holes and with holes bound at neutral acceptors. Our calculations---which have no free parameters---are in quantitative agreement in the intermediate laser-intensity regime at T=300 K with the results by Ding et al. [Appl. Phys. Lett. 60, 2051 (1992)], who obtained the carrier density for multiple asymmetric coupled quantum wells through a fitting procedure that reproduced the total experimental photoluminescence intensity. Results for the carrier-dependent e-h recombination decay time are in good agreement with experimental data by Bongiovanni and Staehli [Phys. Rev. B 46, 9861 (1992)].

Depth-dependent porous silicon photoluminescence

Porous silicon showing a two-layer structure is studied by stepwise peeling the surface layer to clarify the nonuniformity in photoluminescence (PL) emission as a function of depth. The upper layer is amorphous and luminesces at higher energy and efficiency. The deeper the depth or the lower the excitation energy Eex, the lower the PL peak Ep; but, at a low Eex, Ep is not sensitive to the depth. Both intrinsic PL emission and the variation of penetration depth with Eex contribute to the linear dependence of Ep on Eex, which is in contrast to the cases of a-Si:H and siloxene exhibiting thermalization gaps. The total PL excitation spectrum, the integrated PL intensities versus Eex, saturates rather than exhibiting a peak. Its leading edge profile is similar to that for the absorption spectrum, unchanged by the depth, and described by an Urbach tail with energy of 0.26 eV, which is 4–5 times larger than that of a-Si:H. The results can be understood based on silicon clusters embedded in amorphous silicon incorporating oxygen and/or hydrogen.

Optically Detected Magnetic Resonance Studies of Cd1-xMnxTe (x=0.095, 0.007)

The dominant mechanism responsible for the optical detection of the Mn 2 + magnetic resonance in Cd 1 x Mn x Te (x = 0.095, 0.007) is explained. By either change of the external magnetic freld or b y setting the conditions for the Mn 2 + magnetic resonance, we could change the relative efficiencies of the two competing excitonic recombination processes. By lowering magnetization at the magnetic resonance, recombination via the acceptor bound exciton channel, which is mainly nonradiative, is enhanced. Then, a large up to 50% decrease in the total photoluminescence efficiency was observed in the optically detected magnetic resonance experiment. Such observation allows for verification of the large efficiency of the Auger-type transition responsible for the nonradiative decay of the acceptor bound exciton.

Vertical transport in a GaAs/(Ga,Al)As superlattice containing an enlarged quantum well studied by photoluminescence in high in-plane magnetic fields

Vertical transport in a GaAs/(Ga,Al)As superlattice has been studied using photoluminescence and magnetophotoluminescence measurements with in-plane fields of up to 25 T at temperatures in the range 4-80 K. Vertical transport at 4 K (which is determined by exciton transport) has been found to be less efficient than free-carrier-like transport at high temperatures (47 and 80 K). Application of a high, in-plane magnetic field tends to reduce the hole transport considerably. In addition, a drastic enhancement of the total photoluminescence yield with in-plane field has been observed.

Characterization of recombination processes in multiple narrow asymmetric coupled quantum wells based on the dependence of photoluminescence on laser intensity

Continuous-wave intensity-dependent photoluminescence spectra of multiple narrow asymmetric coupled quantum wells at room temperature have been measured. At low laser intensity, the total photoluminescence intensity is primarily proportional to the square of the laser intensity due to dominant nonradiative recombination of free carriers at nearly saturated interface traps. At high laser intensity, however, the total photoluminescence intensity approaches a level proportional to the laser intensity due to radiative recombination of free carriers. Based on this transition behavior, which has been observed for the first time to the best of our knowledge, and our simple theory, the transition intensity, the nonradiative decay time of the carriers, and the intensity-dependent carrier density and photoluminescence quantum efficiency have been determined.

Photoluminescence Excitation Spectroscopy of Cd SxSe(1-x) Nanoparticles

ABSTRACTWe present photoluminescence excitation spectra for glasses doped with CdSxSe(1-x). The peak of the emission spectrum is blue shifted toward the bandgap as excitation energy flux is increased. Total photoluminescence emission intensity is observed to be significantly enhanced and blue shifted by exciting with photon energies greater than ∼4 eV. A simple model of the nanoparticle band structure is presented to account for the observed spectra.

Comparative photoluminescence study of hydrogenation of GaAs, AlxGa1−xAs, and AlAs

Low-temperature photoluminescence (PL) measurements of GaAs, AlxGa1−xAs, and AlAs samples grown by molecular beam epitaxy have been carried out to study the effects of hydrogen diffusion. Following exposure to a hydrogen plasma, the PL spectra of AlxGa1−xAs change. In particular, direct gap AlxGa1−xAs shows a strong increase in the total PL intensity whereas the PL spectra of indirect gap AlxGa1−xAs show an increase in the excitonic-related recombinations after hydrogenation; the binary compounds present less dramatic changes. We interpret our results in terms of hydrogen passivation of deep and shallow centers (DX), whose densities are higher for aluminum concentration near the direct to the indirect gap crossover.

Li-doped ZnSe epitaxial layers by ion implantation

Li+ ions were implanted into undoped high crystalline quality ZnSe epilayers grown by metalorganic vapor phase epitaxy. Thermal annealing after ion implantation was performed between 451 and 757 °C. The total photoluminescence intensity (measured at 77 K) of implanted layers before annealing was two orders of magnitude less than that of an as-grown one because of the Li+ ion damage. As the annealing temperature was increased, however, the damage was reduced and the photoluminescence intensities of two main peaks (band-edge emission and strong free to acceptor transition emission) became intense without generating deep centers. At 4.2 K, the only emission observed was associated with acceptor bound exciton and donor acceptor pair recombination in the near-band-edge region.

Doping superlattices in organometallic vapor phase epitaxial InP

Doping superlattices (nipi structures) have been grown in InP using organometallic vapor phase epitaxy in an atmospheric pressure reactor using trimethylindium and phosphine in a hydrogen ambient. The n-type and p-type dopants were diethyltellurium and dimethylzinc, respectively. The 4-K photoluminescence spectra at various excitation intensities are presented for a structure consisting of six 200-Å layers with doping levels of 1×1018 and 2×1018 cm−3 for the n and p layers. The luminescence peak is found to occur at energies considerably less than the band gap of InP and to move to higher energies with increased excitation intensity, as expected for doping superlattices where the band gap, which is indirect in real space, increases with increasing excited carrier concentration. The total photoluminescence signal decays in several steps, each exponential, with time constants ranging from 6×10−8 to 7×10−4 s at 4 K, typical of these spatially indirect band-gap materials.

Efficient luminescence band created in (Al, Ga)As multilayers by athermal laser processing

A new luminescence band is observed in (Al, Ga)As three-layer structures which were subjected to well-controlled CW Krion laser irradiation at power densities of 0.55 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The new band is shifted by 90 meV to lower energies with respect to the band-to-band recombination. The total photoluminescence efficiency at room temperature can be improved by as much as 80 percent. Measurements of the luminescence at 0.8 μm and of the infrared (IR) power emitted at 3.1 μm have been performed during processing. Maximum temperature rises of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">320-350\deg</tex> C for the laser heated zone have been evaluated from these measurements. The well-defined laser threshold power, necessary to create the new luminescence centers, does not depend on the substrate temperature in the range of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20-250\deg</tex> C. It does increase linearly with decreasing laser photon energy.