Pressure Dependence Of Photoluminescence Research Articles

Applicability of Langmuir equation to oxygen pressure dependent photoluminescence from β-Ga2O3 nanostructures

β-Ga2O3 nanostructures were synthesized via vapor transport method on gold coated Silicon substrate in N2 ambient. The as synthesized products were investigated by grazing incident X-ray diffraction, scanning electron microscopy and photoluminescence (PL) spectroscopy. It is shown that the intensity of photoluminescence from the ensemble of β-Ga2O3 nanostructures in oxygen gas ambience is correlated with the oxygen pressure through the Langmuir equation. This correlation is found to be reversible and reproducible. This phenomenon, which was not observed in the bulk β-Ga2O3 single crystal, is attributed to the oxygen related shallow trap surface states of the nanostructures with energies at about 4.2 eV above the valance band. Based on the changes in the PL intensity with the oxygen pressures, a possible mechanism for the observed photoluminescence is suggested. The present results provide a route for room-temperature response of oxygen in the gallium oxide nanostructures.

Peculiarities in the pressure dependence of photoluminescence in InAlN

AbstractStudies of ambient‐pressure and high‐pressure behavior of photoluminescence (PL) for series of InxAl1−xN layers are presented. The measured evolution of PL energy (EPL) with x is characterized by a clear decrease of EPL and exhibits a strong bowing. This dependence corresponds to the predictions of ab initio calculations of the band‐gap energy changes EG with x. However, values of EPL are clearly lower than EG, for 0<x<0.3. For higher x, the measured EPL follows well the calculated EG. The experimentally determined pressure coefficient of PL energy (dEPL/dp) shows a complicated behavior for alloys with different In‐content. We found a strong reduction of dEPL/dp for 0<x<0.3 and a relatively constant magnitude of this coefficient for higher x. Moreover, for the lower x region, we observed dEPL/dp that can differ even by a factor two in samples with nominally very similar In‐content. The general tendency in dEPL/dp evolution with x corresponds to lower values than calculated dEG/dp for alloys with non‐uniform indium distribution. We propose two not necessary independent explanations of these experimental findings. First, due to non‐uniform In distribution (induced, e.g. by defects or non‐homogeneous strain) both EPL and dEPL/dp are reduced. Second, a similar behavior results from an involvement of the localized states, whose formation and contribution to PL can be induced by strain and/or native defects. In both hypotheses, the strain/defect density can significantly change around x ≈ 0.18 where InxAl1−xN layers are lattice matched to GaN template.

Packing Dependent Electronic Coupling in Single Poly(3-hexylthiophene) H- and J-Aggregate Nanofibers

Nanofibers (NFs) of the prototype conjugated polymer, poly(3-hexylthiophene) (P3HT), displaying H- and J-aggregate character are studied using temperature- and pressure-dependent photoluminescence (PL) spectroscopy. Single J-aggregate NF spectra show a decrease of the 0-0/0-1 vibronic intensity ratio from ~2.0 at 300 K to ~1.3 at 4 K. Temperature-dependent PL line shape parameters (i.e., 0-0 energies and 0-0/0-1 intensity ratios) undergo an abrupt change in the range of ~110-130 K suggesting a change in NF chain packing. Pressure-dependent PL lifetimes also show increased contributions from an instrument-limited decay component which is attributed to greater torsional disorder of the P3HT backbone upon decreasing NF volume. It is proposed that the P3HT alkyl side groups change their packing arrangement from a type I to type II configuration causing a decrease in J-aggregate character (lower intrachain order) in both temperature- and pressure-dependent PL spectra. Chain packing dependent exciton and polaron relaxation and recombination dynamics in NF aggregates are next studied using transient absorption spectroscopy (TAS). TAS data reveal faster polaron recombination dynamics in H-type P3HT NFs indicative of interchain delocalization whereas J-type NFs exhibit delayed recombination suggesting that polarons (in addition to excitons) are more delocalized along individual chains. Both time-resolved and steady-state spectra confirm that excitons and polarons in J-type NFs are predominantly intrachain in nature that can acquire interchain character with small structural (chain packing) perturbations.

High‐pressure Raman and photoluminescence of highly anisotropic CdS nanowires

AbstractRaman and photoluminescence of CdS nanowires of diameter 80 nm and lengths up to several tens of micrometers were studied at pressure up to 60 kbar using a Jobin‐Yvon T64000 micro‐Raman system in conjunction with the diamond‐anvil cell technique. The phase transition pressure of wurtzite to rock salt was observed at 38 kbar, which is higher than that of bulk CdS. In contrast with the transition pressure of different‐sized CdS nanocrystal, this elevated phase transition pressure cannot be explained well by the size effect. Thus the contribution of particle morphology of such a system, which represents the low‐energy surface structure, should be considered. The pressure dependence of photoluminescence is also discussed. Copyright © 2007 John Wiley & Sons, Ltd.

High pressure studies of radiative recombination mechanisms in InN

AbstractReported hydrostatic pressure measurements of Hall electron concentration and Hall mobility of InN suggest the presence of a localized donor state in the conduction band. Since localized‐state emission is rather inefficient, the involvement of such a localized state would be highly inadvantageous for device applications. Pressure‐dependent photoluminescence (PL) experiments of InN epilayers show PL pressure coefficients dE E/dp of 30.7 ± 2.0 meV/GPa and 21.4 ± 3.0 meV/GPa for InN epilayers with electron concentration of ∼5 × 1018 cm–3 and ∼5 × 1017 cm–3, respectively. These values are comparable with the InN band gap pressure coefficient dE G/dp of ∼25–30 meV/GPa (indicating regular band‐to‐band emission), and differ significantly from the value of 5–10 meV/GPa that should be expected in the case of localized‐state emission. Therefore, in the presented experiments no evidence of the involvement of localized donor states in radiative recombination processes in InN is found. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence of wurtzite ZnO under hydrostatic pressure

Photoluminescence (PL) spectra of single-crystal ZnO bulk under hydrostatic pressure are studied using the diamond-anvil-cell technique at room temperature. The PL spectrum of ZnO single crystal taken at atmospheric pressure was dominated by a strong near-band-edge exciton emission. The emission line was found to shift towards higher energy with increasing applied pressure. By examining the pressure-dependent PL spectral features, the pressure coefficient of the direct Γ band gap of the wurtzite ZnO was determined. The hydrostatic deformation potential of the band gap has also been deduced from the experimental results.

Localization Effects in InGaN/GaN Double Heterostructure Laser Diode Structures Grown on Bulk GaN Crystals

We demonstrate a double heterostructure (DH) nitride laser diode (LD) with an untypically wide 9.5 nm InGaN active region instead of the commonly used narrow InGaN quantum wells. Structures were grown on bulk GaN, which ensures low dislocation densities and therefore low concentrations of nonradiative recombination centers. The efficient screening of polarization induced electric fields in the structures under investigation, which contained heavily (1×1019 cm-3) Si doped barriers, was demonstrated by means of hydrostatic pressure dependent photoluminescence measurements on a simplified sample with identical active region. Since the detrimental separation of carriers by electric fields becomes more pronounced for wider InGaN active regions, efficient screening of polarization induced electric fields is essential for the investigated heterostructures. Optical and electrical parameters of this LD were comparable to those of comparable devices with typical QWs of ∼4 nm as active region. We observed a high thermal stability of the photoluminescence intensity and, via time-resolved photoluminescence, a relatively temperature-independent radiative decay time. These observations support the significance of carrier localization phenomena for the radiative recombination processes of the investigated structure. The implications of these results for DH LD structures are discussed.

Synthesis and Luminescence of ZnMgS:Mn<SUP>2+</SUP> Nanoparticles

Efficient green emission from ZnMgS:Mn2+ nanoparticles prepared by co-doping Mg2+ and Mn2+ ions into ZnS lattices has been observed. The synthesis is carried out in aqueous solution, followed by a post-annealing process, thus showing the features of less complexity, low cost, and easy incorporation of dopants. In comparison with the emission of ZnS:Mn2+ nanoparticles, which is located generally around 590 nm, the photoluminescence of ZnMgS:Mn2+ nanoparticles is blue-shifted by 14 nm in wavelength, leading to the enhanced green emission. The X-ray diffraction, electron spin resonance, and pressure dependent photoluminescence measurements suggest that the change of the crystal field caused by Mg2+ ionic doping and the lower symmetry in the nanoparticles may account for the blue-shift of the photoluminescence. The ZnMgS:Mn2+ nanoparticles with 1% Mn2+ doping exhibit the strongest luminescence, which could potentially meet the requirements for the construction of green light emitting diodes.

Pressure‐dependent photoluminescence study of CuGaSe2

AbstractWe present the results of a pressure‐dependent photoluminescence (PL) study on CuGaSe2 films grown on GaAs substrate by metalorganic vapor phase epitaxy. The low‐temperature PL spectra of the CuGaSe2 samples measured at atmospheric pressure are dominated by one near band edge exciton luminescence line and two strong and relatively broad emissions associated with donor acceptor pairs (DAP) transitions. All the observed luminescence emission lines shift toward higher energy with increasing pressure at the same rate. The nearly identical pressure coefficients of the two DAP emissions as compared to that of the exciton emission confirm the suggestion that the recombination processes associated with the DAPs involve one shallow effective‐mass donor and two different acceptor species with different binding energies and related to two different native defects. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

InP quantum dots embedded in GaP: Optical properties and carrier dynamics

The optical emission and dynamics of carriers in Stranski-Krastanow self-organized InP quantum dots embedded in a GaP matrix are studied. InP deposited on GaP (001) using gas-source molecular-beam epitaxy forms quantum dots for InP coverage greater than 1.8 monolayers. Strong photoluminescence from the quantum dots is observed up to room temperature at about 2 eV; photoluminescence from the two-dimensional InP wetting layer is measured at about 2.2 eV. Modeling based on the ``model-solid theory'' indicates that the band alignment for the InP quantum dots is direct and type I. Furthermore, low-temperature time-resolved photoluminescence measurements indicate that the carrier lifetime in the quantum dots is about 2 ns, typical for type-I quantum dots. Pressure-dependent photoluminescence measurements provide further evidence for a type-I band alignment for InP/GaP quantum dots at normal pressure with the GaP X states lying about 30 meV higher than the $\ensuremath{\Gamma}$ states in the InP quantum dots, but indicate that they become type II under hydrostatic pressures of about 1.2 GPa.

High-pressure study of deep emission band at GaAs/partially ordered GaInP interface

We have measured the photoluminescence (PL) spectra of GaAs/GaInP single quantum wells at pressures up to ∼5 GPa, and investigated the characteristics of the 1.46 eV deep emission band. It has a very long decay time of 200–400 ns. In addition, unlike the emission from the GaAs well, a strong blueshift of the spectral position with excitation intensity is observed. With increasing pressure, the deep emission shows a sublinear shift towards higher energy, while the GaAs well exhibits a linear shift. The pressure-dependent PL behavior at lower excitation intensity is rather similar to those observed for partially ordered GaInP alloys. These results suggest that the presence of ordered GaInP layers plays an important role in the radiative recombination at 1.46 eV, and the 1.46 eV deep emission is related to the interface transitions of electrons and holes localized at the heterointerface.

Pressure Dependence of Photoluminescence in GaAs/Partially Ordered GaInP Interface

We have measured the photoluminescence (PL) spectra of GaAs/GaInP single quantum wells at pressures up to ∼5 GPa, and investigated the characteristics of the 1.46 eV deep emission band. It has a very long decay time of 200–400 ns. In addition, at normal pressure, unlike the emission from the GaAs well, a strong blueshift of the spectral position with excitation intensity is observed. With increasing pressure, the deep emission shows a sublinear shift towards higher energy, while the GaAs well exhibits a linear shift. The pressure-dependent PL behavior at lower excitation intensity is rather similar to those observed for partially ordered GaInP alloys. These results suggest that the presence of ordered GaInP layers plays an important role in the radiative recombination at 1.46 eV, and the 1.46 eV deep emission is related to the interface transitions of electrons and holes localized at the heterointerface.

Time-Resolved Photoluminescence Study of GaAs/Ordered GaInP Interface under High Pressure

We have measured the photoluminescence (PL) spectra of GaAs/GaInP single quantum wells at pressure up to 5 GPa, and investigated the characteristics of the 1.46 eV deep emission band. It has a very long decay time of several hundred nanoseconds. In addition, unlike the emission from the GaAs well, a strong blue-shift of its peak energy with excitation intensity is observed. With increasing pressure, the emission peak shows a sublinear shift towards higher energy, while the GaAs quantum well exhibits a linear peak shift. The pressure dependence of the spectral peak position at higher excitation intensity tends to reflect that of the adjacent 1.49 eV emission band which has a faster decay profile. The pressure-dependent PL behavior at lower excitation intensity is rather similar to those observed for partially ordered GaInP alloys. These results suggest that the presence of ordered GaInP layers plays an important role in the radiative recombination at 1.46 eV and the deep emission is related to the transitions of electrons and holes localized at the GaAs/ordered GaInP heterointerface.

Near-band-edge photoluminescence emission in AlxGa1−xN under high pressure

We present results of pressure-dependent photoluminescence (PL) studies of single-crystal AlxGa1−xN epitaxial films grown on sapphire substrates by metalorganic chemical vapor deposition. PL measurements were performed under hydrostatic pressure using the diamond-anvil-cell technique. PL spectra taken from the AlxGa1−xN epitaxial films are dominated by strong near-band-edge luminescence emissions. The emission lines were found to shift linearly towards higher energy with increasing pressure. By examining the pressure dependence of the spectral features, the pressure coefficients for the PL emissions associated with the direct Γ band gap of AlxGa1−xN were determined. Our results yield a pressure coefficient of 4.0×10−3 eV/kbar for Al0.05Ga0.95N and 3.6×10−3 eV/kbar for Al0.35Ga0.65N.

Pressure Dependence of Photoluminescence Spectra in CuAlS2 Doped with Lanthanide.

Pressure and temperature dependence of photoluminescence (PL) spectra of ternary compound semiconductor CuAlS2 doped Er and Fe are measured. Under hydrostatic pressures, PL peaks originated in transitions of 4f electrons in Er shift to lower energy and new sharp peaks appears. These results are discussed in terms of crystalline fields and spin-orbit interaction. The broad peak assigned as self-activated (SA) PL shifts to high energy under high hydrostatic pressures which coefficient is the same sign and order as that of band gap of chalcopyrite type I-III-VI2 compounds. This result indicates that SA PL is related to Bloch bands.

High-pressure investigation of InGan quantum wells.

ABSTRACTWe have studied the pressure dependence of photoluminescence for various InGaN quantum wells of different Indium compositions. In contrast to classical quantum well systems such as GaAs/AlGaAs, the pressure coefficients of the photoluminescence peak position of wells of similar parameters can vary greatly, depending on conditions of growth. Generally, the pressure coefficient of the emission peak is much lower than the one predicted for a GaN-InN system. We show that this behavior can be related in most cases to the influence of the thick GaN layer compressibility on the strain in the InGaN quantum wells. However, in some cases (Nichia devices), the pressure coefficient of the emission peak seems to be unrelated to the shift of the energy gap of the material, suggesting the involvement of strongly localized electronic states

Pressure-dependent photoluminescence study of InxGa1−xN

We present the results of pressure-dependent photoluminescence (PL) studies of single-crystal InxGa1−xN (0⩽x<0.15) films grown on top of thick GaN epitaxial layers by metalorganic chemical vapor deposition with sapphire as substrates. PL measurements were performed at 10 K as a function of applied hydrostatic pressure using the diamond-anvil-cell technique. The luminescence emissions from the InxGa1−xN epifilms were found to shift linearly toward higher energy with increasing pressure. By examining the pressure dependence of the PL spectra, the pressure coefficients for the emission structures associated with the direct band gap of InxGa1−xN were determined. The values of the pressure coefficients were found to be 3.9×10−3 eV/kbar for In0.08Ga0.92N and 3.5×10−3 eV/kbar for In0.14Ga0.86N.

Band alignments in GaInP/GaP/GaAs/GaP/GaInP quantum wells

Pressure-dependent photoluminescence (PL) in several GaInP(ordered)-GaAs quantum well structures grown by metal organic vapor phase epitaxy is reported. Quantum well emission from GaAs is observed only in structures where thin (∼2 nm) GaP layers are inserted between the GaAs well and the GaInP barrier. By extrapolating the energies of the various inter and intralayer PL transitions observed under pressures (up to 5.5 GPa) to zero pressure, the different band offsets of the heterostructure have been determined.

Pressure dependence of photoluminescence in amorphous silicon nanopowder produced by plasma enhanced chemical vapour deposition

The photoluminescence emission in silicon powder grown by plasma enhanced chemical vapour deposition has a very unusual dependence on pressure. Its intensity diminishes exponentially, I PL I 0 exp(- P/P 0 ), where P 0 has a value of several pascals. Consequently it is detectable only under vacuum. This dependence is analysed within the framework ofa multistep multiphoton excitation process. It is shown that an exponential dependence on pressure of the dynamic constants (lifetimes and optical cross-sections involved in the model) can account qualitatively for all the experimental results. MST/3288

Pressure dependence of photoluminescence from ZnSe:Te-(CdSe) 1(ZnSe) 3 superlattice quantum wells

Pressure dependence of photoluminescence from ZnSe:Te(CdSe) 1,(ZnSe) 3 short period superlattice quantum wells is reported. In addition to the exciton band from the superlattice layers, strong bands for localized excitons self-trapped at single Te (Te 1) atom, double Te atoms (Te 2) and Te clusters (Te n, n ≥ 3) as well as for the free excitons in isoelectronic Te incorporated ZnSe layers are observed. Significant differences in the pressure and temperature dependencies of the observed exciton transitions are presented and discussed.