Showing papers on "Photoluminescence published in 1979"

Photoluminescence of singlet oxygen in pigment solutions

Abstract: — Luminescence of 1O2 (1270 nm) accompanying energy transfer to oxygen from photoexcited (triplet) molecules of sensitizers in air saturated solutions has been investigated. The luminescence was observed in CC14, CS2 and freon with the use of porphyrins, chlorophylls, pheophytins and aromatic hydrocarbons as sensitizers. The lifetime and quantum yield of the luminescence depended on the nature of the solvents. pigments and their concentrations. The maximum values of these parameters were equal to 28 ± 5 ms and 5 ± 4 times 10--5, respectively. The quantum yield of 1O2 generation by pigments has been measured and the results used for determining the quantum yields of intersystem crossing in the pigment molecules. The rate constants of 1O2 reaction with different substances have been determined with the aid of luminescence quenching. It has been shown that along with β-carotene. Chls, pheophytins, and some porphyrins are also very active quenchers of 1O2, The quenching effect depends on their molecular structure and on the presence and nature of the central metal atom. Quenching 1O2 by the pigments is due mainly to a “physical” mechanism (without destruction of the pigments). The destructive “chemical” quenching is by 1–4 orders of magnitude less effective and is accompanied with photochemiluminescence of the pigments. The experiments on 1O2 generation and quenching indicate that energy of triplet states of bacteriochlorophyll and bacteriopheophytin is somewhat higher than that of 1Δg oxygen. The data demonstrate wide possibilities of the luminescence studied as a method for investigating 1O2 reactivity and photophysical properties of sensitizers.

The Effects of Crystal Field and Temperature on the Photoluminescence Excitation Efficiency of Ce3+ in YAG

Abstract: Absorption and luminescence spectra of YAG:Ce3+ crystals have been measured at various temperatures between 4.2° and 300°K. The high resolution, low temperature spectra show resolved fine structure. Zero‐phonon transitions have been assigned and the phonon replicas correlated with lattice phonon energies. The characteristic absorption band of YAG:Ce3+ near 460 nm decreases in intensity, whereas that near 340 nm increases with increasing temperature. This effect is explained in terms of a tetragonal crystal field splitting of both the 4f1 ground state and 5d1 excited states of Ce3+ in YAG. When allowance is made for changes in the absorption of pump light with temperature the photoluminescence quantum efficiency of Ce3+ in YAG is constant up to 300°K. The significance of these crystal field effects for the use of YAG:Ce as a high temperature lamp phosphor is discussed.

Fabrication of 6H‐SiC light‐emitting diodes by a rotation dipping technique: Electroluminescence mechanisms

Abstract: Various light‐emitting diodes of 6H–SiC, doped in the n layers with Al, Ga, or B acceptors and N donors, have been fabricated using a rotation dipping technique. A detailed study of electroluminescence mechanisms from ∼100 to 400 K has been carried out using photoluminescence, electroluminescence, and time‐resolved spectra. At low temperatures the radiation originates mainly in the p layer, and around room temperature in the n layer. The luminescence mechanisms are free‐exciton recombinations for the peak at ∼425 nm, donor‐acceptor pair recombinations for the main broad band, transitions within localized centers for the peak at ∼455 nm, and recombinations due to divacancies for the series of peaks beginning at ∼470 nm. The dependence of the electroluminescence efficiency on the N concentration in the n layers is studied and methods to improve efficiency are discussed. The maximum external quantum efficiencies obtained were 1.2×10−5 for Al‐doped, 9×10−6 for Ga‐doped, and 2.5×10−5 for B‐doped diodes, and we...

Photoluminescence in ZnSe grown by liquid‐phase epitaxy from Zn‐Ga solution

Abstract: Photoluminescence has been studied of epitaxial ZnSe layers grown from a pure Zn or a Zn‐Ga alloy solution on ZnSxSe1−x (0⩽x⩽1) single‐crystalline substrates. Intense blue emission bands at room temperature have been observed in the layers grown from a Zn‐Ga alloy solution. The spectrum of the layer at 4.2 K generally consists of a strong emission line due to radiative recombination of excitons bound to the neutral donor, I2, an associated donor‐acceptor pair emission band with LO‐phonon replicas, and a deep‐level emission, i.e., the so‐called SA emission band. The effect of Ga on the emission spectrum is to greatly increase the intensity of the blue part of the spectrum. On the basis of the comparison between the experimental results and theoretical calculations, it is tentatively proposed that the blue emisison bands at room temperature originate from the radiative recombination due to band‐to‐band transition.

Photoluminescence at dislocations in GaAs and InP

Abstract: The influence of dislocations on the radiative recombination of electron‐hole pairs in GaAs and InP is investigated by spatially resolved photoluminescence measurements. The shape of the spectra is unchanged at individual dislocations, while the overall intensity is lowered by about 30% in a region of 5–10 μm around the dislocations. This relative change of intensity is temperature independent in the investigated range 10–300 K. Deformation‐induced dislocations, prepared by plastic deformation at 600 °C, behave similarly to as‐grown dislocations. These statements hold for 60° dislocations and screw dislocations in both n‐type and p‐type material. Search for dislocation‐induced electronic levels in the band gap by deep‐level transient spectroscopy, optical absorption, and photoluminescence of GaAs samples with very high dislocation density gives no evidence for such levels. Our findings suggest that the quenching of the radiative recombination near dislocations is caused by an enhancement of the nonradiati...

Defect states in Ge chalcogenides observed by photoluminescence and ESR

Abstract: Photoluminescence studies of melt-quenched GexSe1−x (0.2 < x < 0.43) glasses reveal two unresolved components with broad linewidths and large Stokes shifts. The lines have differing excitation spectra, composition dependences and decay lifetimes. Two distinct singly occupied localized states are also observed by optically induced ESR, and are distinguished by different composition dependences, microwave saturation, excitation wavelength dependences and annealing temperatures. These states are interpreted as defect states with negative correlation energy, originating from Ge and Se atoms. The annealing properties of luminescence fatigue and induced ESR indicate that at least one of the defects is common to both experiments. The annealing kinetics of ESR is reported and supports a model of recombination by tunnelling between singly occupied sites. A large increase in the luminescence linewidth is observed at the composition Ge37Se63. It is suggested that the reason is an anomalously large disorder potential that originates from the introduction of GeGe bonding states at the valence band edge.

Photoluminescence and lifetime studies on plasma discharge a-Si

Abstract: A detailed study has been made of the photoluminescence (PL) behaviour of high TD, undoped, glow-discharge a-Si, prepared by the Dundee group. Our measurements include temperature and field quenching of the luminescence and PL decay in the region 2 ns to ∼20 μs. Some recombination models for a-Si are briefly reviewed in the light of these results. Preliminary life-time data on doped a-Si are also presented and used to discuss our earlier measurements on the PL efficiency of doped samples. We conclude that a distribution of radiative lifetimes is present in a-Si, due to disorder. The main competitive non-radiative process is tunnelling to defects.

Molecular beam epitaxial growth of low‐resistivity ZnSe films

Abstract: Single‐crystalline films of low‐resistivity ZnSe have been successfully grown by molecular beam epitaxy (MBE). The film shows good crystallinity, having a smooth and flat surface as revealed by RHEED and SEM investigations. The resistivity of as‐grown MBE ZnSe is low (typically ∼1 Ω cm) and the mobility of the film is relatively high (typically ∼300 cm2/V sec) at room temperature. The concentrations of the residual electrically active donor and acceptor centers are ND =8×1016 cm−3 and NA=6×1016 cm−3, respectively. The strong blue band‐edge photoluminescence peak is observed even at room temperature, and the luminescence intensity at longer wavelength is very weak.

Photoluminescence in Mn‐implanted GaAs—An explanation on the ∼1.40‐eV emission

Abstract: The emission properties of Mn‐implanted layers in GaAs are investigated with respect to changes in excitation intensity, temperature, substrate, and encapsulation. The substrates used were n‐type conducting and Cr‐doped semi‐insulating crystals. The depth‐dependent measurements of the Mn emission show an increase of the diffusion coefficient with dose for Si3N4 encapsulated samples, whereas a suppression of Mn diffusion is observed for SiO2 encapsulated samples. A large energy shift from ∼1.36 to 1.41 eV is observed for Mn emission at 4.2 °K at various excitation intensities. The energy shift increases with the concentration of compensating donors in the n‐type substrate material. The excitation‐dependent emissions are due to the donor‐acceptor pair type transitions in the normal and the random‐impurity‐potential‐disturbed energy states of the bands. Another emission becomes dominant at T≳∼30 °K following the quenching of the donor‐acceptor pair type transitions with an increase of the temperature. An activation energy 95±15 meV is obtained from the temperature quenching of the emission intensity and the energy shift is observed to follow the band‐gap variation. The possible radiative mechanisms are discussed.

The isoelectronic trap iodine in AgBr

Abstract: Iodine in AgBr behaves as an isoelectronic trap with the hole being bound first. The bound-exciton zero-phonon photoluminescence line is split by exchange into an A-line and a B-line separated by 0.1 meV. The Zeeman splitting at H=80 kG is isotropic in agreement with iodine being a substitutional point-like impurity in AgBr. In the low-temperature limit optic and acoustic phonons contribute to the phonon side wing with nearly the same coupling strength. At temperatures below 10K, up to n=11 phonons are observed in the phonon side band. The authors present experimental evidence that these are due to the emission of n optic phonons which, for n>3, are statistically independent. A multiphonon model which neglects the k dependence of transition rates is introduced; however, the model is useful only qualitatively.

Luminescent p‐GaAs grown by zinc ion doped MBE

Abstract: Low‐energy zinc ions have been incorporated in growing MBE GaAs layers producing heavily doped p‐type material. As‐grown layers retain a substantial amount of radiation damage as indicated by the absence of photoluminescence and low hole mobilities. A postgrowth anneal yields layers with low‐temperature luminescence and mobilities comparable to zinc doped LPE epitaxial layers. Annealed layer quality does not depend on substrate growth temperature or ion energy over the ranges 580–650 °C and 100–3000 eV, respectively. The measured zinc ion sticking coefficient has a value of ∼50% and depends only weakly on ion energy. The data suggest that the sticking coefficients are enhanced by a simple ion burial mechanism and not by an electrostatic attraction between the ion and surface atoms.

Iron impurities as non-radiative recombination centres in chalcogenide glasses

Abstract: Photoluminescence and E.S.R. studies have been carried out on pure and iron-doped As2S3 glass. The luminescence spectra, photoluminescence excitation spectra, temperature dependence of the luminescence efficiency, and luminescence fatiguing curves demonstrate that the iron impurities quench the luminescence efficiency by introducing competing non-radiative recombination centres. The E.S.R. measurements reveal that iron is a pervasive inadvertent impurity even in nominally pure chalcogenide glasses. It is suggested that non-radiative recombination occurs by transfer of an exciton's recombination energy to localized d-band excited states of an iron ion; the excited iron ion then relaxes non-radiatively by phonon emission.

The effects of impurities upon photoluminescence and optically induced paramagnetic states in chalcogenide glasses

Abstract: Photoluminescence (PL) and optically induced electron spin resonance (ESR) have been studied in As2Se3 glasses doped with Cu, Tl, I, Ag, In, and K and in B-doped As2S3 glass. In all cases there is no significant change in PL efficiency or intensity of induced ESR until dopant concentrations exceed ∼1 at.%. These results are in marked contrast the strong dependence of transient hole transport upon dopant concentration in the same glasses. These parallel but contrasting observations are discussed in terms of the predicted effects of dopants on defects in chalcogenide glasses postulated by Mott on the basis of the charged defect model of Mott, Davis and Street (MDS). PL and ESR studies of the AsSe glass system have revealed that oxygen contamination can severely quench PL efficiency and ESR intensity for As concentrations

Strain‐enhanced luminescence degradation in GaAs/GaAlAs double‐heterostructure lasers revealed by photoluminescence

Abstract: Photoluminescence degradation observed in room‐temperature‐lifetested GaAs/GaAlAs DH stripe lasers has been correlated with the strain field induced in the devices by the stripe‐defining oxide. A degradation mechanism based on strain‐ and nonradiative‐recombination‐enhanced point‐defect migration is proposed.

Radiative and non-radiative tunnelling in glow-discharge and sputtered amorphous silicon

Abstract: Photoluminescence decay measurements are reported for doped and undoped glow-discharge a-Si, and for a-Si prepared by sputtering in a hydrogenated atmo-sphere. It is concluded that a distribution of radiative lifetimes is present, due either to non-crystalline disorder or to a donor-acceptor pair mechanism. The main competitive mechanism is tunnelling to defects.

Pulsed infrared laser induced visible luminescence

Abstract: : Crystals of saccharin doped with p-toluenesulfonamide, courmarin, NaCl, and KCl are found to emit in the visible, following a 20 nsec pulse of high intensity 1060 nm radiation. The pulses were 0.5 to 4 J/sq.cm. in energy, corresponding to up to 200 MW peak power. The emissions in the first two cases are from molecular excited states, but with differences in detail from those induced by photoexcitation or found in triboluminescence. NaCl and KCl show neither photoemission or relevant triboluminescence, and the laser induced emission is essentially that found thermally or by mechanically stressing x- or gamma-ray irradiated crystals. It is attributed to electron-hole recombination. The mechanism of the laser induced emission may thus involve some combination of mechanical shock wave and of plasma ionization. The trivial explanations of photoexcitation by higher harmonics of the 1060 nm fundamental or through successive multiple photon processes can be ruled out. (Author)

Optical gain spectra of InGaAsP/InP double heterostructures

Abstract: We report the first optical gain measurements on InGaAsP/ InP double heterostructures with composition corresponding to an emission wavelength of about 1.3 μm at 300 K. At optical pumping levels of about 1 MW/cm2the maximum gain values of the best samples available are 800 cm-1at 2 K, 500 cm-1at 77 K, and 200 cm-1at 300 K. We conclude from low-intensity luminescence and absorption spectra, that the laser transition corresponds to free-carrier recombination between band-tail states, which are present even in not intentionally doped material. Although these tail states result in rather broad low-intensity luminescence, narrow gain spectra comparable to those of GaAs/GaAlAs double heterostructures are obtained.

Photoluminescence characterization of solution and lec grown InP

Abstract: Low temperature photoluminescence and electrical measurements on bulk grown polycrystalline and single crystal InP indicate the purity and crystal perfection to be equivalent to that of LPE and VPE samples

Determination of the valence‐band discontinuity of InP1−xGaxP1−zAsz (x∼0.13, z∼0.29) by quantum‐well luminescence

Abstract: By means of liquid‐phase epitaxy, undoped single‐quantum‐well layers of In1−xGaxP1−zAsz (x∼0.13, z∼0.29) of thickness Lz∼150 A are grown embedded in InP and are examined in photoluminescence. Hot‐electron recombination from Ec (InP) to bound holes in the quaternary quantum well leads to stimuated emission in a band ∼80 meV below Eg (InP) and thus to an estimate of ΔEv∼80 meV (ΔEc∼ΔEv) for the InP‐InGaPAs valence‐band discontinuity.