PL Transitions Research Articles

Critical points in photoluminescence spectra and their relation with phase transition in Nb-doped SrTiO3

Luminescence spectra are extremely sensitive to variations in structural environment; thus, result of structural change, such as phase transition, can be observed via luminescence intensity. The temperature dynamic of photoluminescence in the Nb-doped SrTiO3 demonstrates two critical points at 115 and 160 K, which correspond to temperatures of structural phase transitions for the bulk and the surface of SrTiO3, respectively. The absence of a hysteresis effect in the photoluminescence emission points out the correspondence of the critical points to a second-order phase transition. Similar critical behaviours were also observed in oxygen-deficient SrTiO3, confirming a relationship between the PL and phase transition. The existence of peaks in the temperature coefficient of resistivity at the same temperatures also confirms the correlation between photoluminescence and phase transition in the Nb-doped SrTiO3, providing a simple non-contact method to detect phase transitions in luminescence materials.

Analysis of Photoluminiscence in the NCSI-DMA System

Silicon nanocrystalline particles (ncSi) were fabricated from the Si swarf using the beads milling method. Observed photoluminiscence spectra (PL) of the ncSi in hexane with the dimethylanthracene molecules (DMA) show photoluminescence peaks at energies of 2.55, 2.75, 2.92, and 3.09 eV. The shape of PL spectra corresponds to the vibronic structure of adsorbed DMA molecules. The PL intensity of the ncSi-DMA system increases by ~3000 times by adsorption of DMA on Si nanoparticles. The PL enhancement results from an increase in absorption probability of incident light by DMA caused by adsorption on the surface of ncSi. Theoretical model of the PL experiment was constructed and resulting model parameters were used in analysis of possible PL transitions and charge transfer processes.

Exciton and Trion Dynamics in Bilayer MoS2.

The control of exciton and triondynamics in bilayer MoS2 is demonstrated, via the comodulations by both temperature and electric field. The calculations here show that the band structure of bilayer MoS2 changes from indirect at room temperature toward direct nature as temperature decreases, which enables the electrical tunability of the K-K direct PL transition in bilayer MoS2 at low temperature.

Relevant characteristics of undoped GaMnN grown by using molecular beam epitaxy

GaN:Mn epilayers were grown on Al2O3 substrate by using molecular beam epitaxy (MBE). Xray diffraction (XRD) data showed intrinsic GaMnN and precipitates including MnGa and MnN. PL transitions (e, Mn) and (D, Mn) related to Mn were remarkably activated. Clear ferromagnetic hysteresis loops were obtained in both samples, which means a good formation of ferromagnetic coupling. The M-T curves revealed a curie temperature (T c ) of 140 ~ 170 K which is intrinsic to GaMnN and a Tc of above 300 K which is due to the MnGa and the MnN precipitates. The samples clearly displayed a magnetic resonance at a field of around 200 - 400 mT. Electron spin resonance (ESR) data showed that the shift of (H center ) (i.e., H center = 337 - H center [mT]) were greater than 20 mT for samples, and the appearance of a Hcenter with a positive H center is indicative of the samples having obvious ferromagnetism. The incorporated Mn ions are in a 2+ ionic state (i.e., Mn2+) because Mn2+ with a spin state of S = 5/2 typically exhibits a magnetic resonance with g ≈ 2 when Mn is doped into semiconductors.

Temperature-dependent photoluminescence of structurally-precise quantum-confined Au25(SC8H9)18 and Au38(SC12H25)24 metal nanoparticles.

Temperature-dependent photoluminescence of structurally precise Au25(SC8H9)18 and Au38(SC12H25)24 monolayer-protected cluster (MPC) nanoparticles were studied using energy-resolved, intensity-integrated, and time-resolved spectroscopy. Measurements were carried out at sample temperatures spanning the range from 4.5 to 200 K following electronic excitation using 3.1 eV pulsed lasers. The integrated PL intensity for Au25(SC8H9)18 increased sharply by 70% as the sample temperature was increased from 4.5 to 45 K. The PL intensity was statistically invariant for temperatures between 45 and 65 K but was quenched when the sample temperature was raised above 65 K. For both MPC samples, the global PL emission included several components. Each PL component exhibited an increase in emission energy when the sample temperature was increased from 4.5 to 40 K. This unexpected behavior may imply that MPCs in the 1 nm domain have negative expansion coefficients. Quantitative analysis of PL emission energies and peak widths obtained at sample temperatures greater than 45 K indicated MPC nonradiative relaxation dynamics are mediated by coupling to low-frequency vibrations associated with the ligand shell that passivated the nanoclusters, which accounted for the low emission yields at high sample temperatures. Contributions from two different vibrational modes were identified: Au(I)-S stretching (200 cm(-1)) and Au(0)-Au(I) stretching (90 cm(-1)). Analysis of each PL component revealed that the magnitude of electronic-vibration coupling was state-specific, and consistently larger for the high-energy portions of the PL spectra. The total integrated PL intensity of the Au25(SC8H9)18 MPC was correlated to the relative branching ratios of the emission components, which confirmed decreased emission for recombination channels associated with strong electron-vibration coupling and high emission yields for low emission energies at low temperature. The efficient low-energy emission was attributed to a charge-transfer PL transition. This conclusion was reached based on the strong correlation between temperature-dependent intensity-integrated and time-resolved emission measurements that revealed an ∼3.5-5.5 meV activation barrier to nonradiative decay. These findings suggest that nanoscale structure and composition can be modified to tailor the optical and mechanical properties and electronic relaxation dynamics of MPC nanostructures.

Optical gain characteristics of non-polar Al-rich AlGaN/AlN quantum well structures

Optical properties of non-polar Al-rich AlGaN/AlN quantum well (QW) structures were investigated using the multiband effective-mass theory and non-Markovian optical model with the many-body effect. These results are compared with those of the c-plane, i.e., the (0001)-oriented QW structures. The theoretical PL transition wavelength is found to agree well with the experimental result. The optical gain for the x′-polarization is much larger than that for the y′-polarization because the optical matrix element for the x′-polarization is larger than for the y′-polarization. The x′-direction in the non-poalr plane corresponds to c-axis direction. Also, the optical gain for the x′-polarization is shown to decrease with increasing Al composition. This is mainly attribute to the fact that the optical matrix element is decreased due to the reduction in the electron-hole overlap for QW structures with higher Al contents.

Black silicon with high density and high aspect ratio nanowhiskers

The physical properties of black silicon (b-Si) formed on Si wafers by reactive ion etching inchlorine plasma are reported in an attempt to clarify the formation mechanism and theorigin of the observed optical and electrical phenomena, which are promising for a varietyof applications. The b-Si consisting of high density and high aspect ratio sub-micronlength whiskers or pillars with tip diameters of well under 3 nm exhibits strongphotoluminescence (PL) both in the visible and the infrared, which is interpreted inconjunction with defects, confinement effects and near band-edge emission. Structuralanalysis indicates that the whiskers are all crystalline and encapsulated by a thin Sioxide layer. The infrared vibrational spectrum of Si–O–Si bondings in terms oftransverse-optic (TO) and longitudinal-optic (LO) phonons indicates that disorder inducedLO–TO optical mode coupling can be an effective tool in assessing the structuralquality of the b-Si. The same phonons are likely coupled to electrons in visibleregion PL transitions. Field emission properties of these nanoscopic features aredemonstrated indicating the influence of the tip shape on the emission. Overallproperties are discussed in terms of the surface morphology of the nanowhiskers.

Theory of Neutral Divacancy in SiC: A Defect for Spintronics

We investigate the neutral divacancy in SiC by means of first principles calculations and group theory analysis. We identify the nature of the PL transitions associated with this defect. We show that how the spin state may be manipulated optically in this defect.

The Elusive Excited Quintet 5D of Tb(III): A Source of Luminescence and Resonance Energy Transfer in Terbium Compounds

The understanding of electronic structure of atomic and molecular term states involved in spectroscopic transitions is aided by projecting combinations of micro-configurations to multi-electron states with “good” quantum numbers of angular momenta. In rare-earth (RE) compounds, atomic term labels are justifiably carried over to compounds, because the 4f orbitals are localized at the RE centers, and the transitions between them are modified by crystal-field splittings that are small compared to the spin−orbit interactions within the RE atom. The source of photoluminescence in Tb(III) compounds is lowest-lying excited quintet 5D4 originating from the [Xe]4f8 configuration, in contrast to other D-states originating from the [Xe]4f75d configuration, sometimes confused with the 5D4(4f8) state. The 5D4 state not only is missing in available databases (NIST Atomic Spectra Database Levels, http://physics.nist.gov/PhysRefData/ASD/levels_form.html; Moore, C. E. Atomic Energy Levels as Derived from the Analyses of Optical Spectra; Vol. I−III; National Bureau of Standards Circular, 467, U.S. Government Printing Office: Washington, DC, 1949−1958; Martin, W. C. Atomic Energy Levels—the Rare Earth Elements; NSRDS-NBS 60; U.S. Department of Commerce, National Bureau of Standards: Washington, DC, 1978), but also its structure in terms of combinations of the 4f8 microstates has so far eluded interpretation. We present a shortcut method using the shift operators arriving at the 5D4(4f8) state, which turns out to be a linear combination of no fewer than 6 Slater determinants. We then compare the Tb(III) PL transitions 5D4 → 7FJ, J = 6, 5, 4, 3 in compounds as diverse as TbCl3(aq), Tb(III)-Y zeolite, and Tb(III) polystyrene sulfonate, all at similar wavelengths but showing features sensitive to chemical environment.

Theoretical study of optical properties in deep ultraviolet Al-rich AlGaN/AIN quantum wells

Optical properties of deep ultraviolet Al-rich AlGaN/AlN quantum wells (QWs) were theoretically investigated by using the multiband effective-mass theory. The theoretical PL transition wavelength is found to agree well with the experimental result. The internal field in AlGaN/AlN QW structures with lower Al contents is much larger than that in QW structures with higher Al contents. The intensity of the spontaneous emission spectra is shown to be improved with increasing Al content because the optical matrix elements are largely enhanced due to the reduced internal field.

Novel blue emitters based on π -conjugated block copolymers

A series of new phenyl-based conjugated copolymers has been synthesized and investigated by vibrational and photoluminescence spectroscopy (PL). The materials are: poly(1,4-phenylene-alt-3,6-pyridazine) (COP-PIR), poly(9,9-dioctylfluorene)-co-quaterphenylene (COP-PPP) and poly[(1,4-phenylene-alt-3,6-pyridazine)-co-(1,4-phenylene-alt-9,9-dioctylfluorene)] (COP-PIR-FLUOR), with 3.5% of fluorene. COP-PPP and COP-PIR-FLUOR have high fluorescence quantum yields in solution. Infrared and Raman spectra were used to check the chemical structure of the compounds. The copolymers exhibit blue emission ranging from 2.8 to 3.6 eV when excited at E exc = 4.13 eV. Stokes-shift values were estimated on pristine samples in their condensed state from steady-state PL-emission and PL-excitation spectra. They suggest a difference in the torsional angle between the molecular configuration of the polymer blocks at the absorption and PL transitions and also in the photoexcitation diffusion. Additionally, the time-resolved PL of these materials has been investigated by using 100 fs laser pulses at E exc = 4.64 eV and a streak camera. Results show very fast biexponential kinetics for the two fluorene-based polymers with decay times below 300 ps indicating both intramolecular, fast radiative recombination and migration of photogenerated electron–hole pairs. By contrast, the PL of COP-PIR is less intense and longer lived, indicating that excitons are confined to the chains in this polymer.

Optical property of modulated n-doped ZnSe/BeTe type-Ⅱ quantum wells

We report the optical properties of the modulated n-doped ZnSe/BeTe/ZnSe type-Ⅱ quantum wells. The reflection spectra have shown typical negatively charged exciton features only in a doped sample. The luminescence spectra and the polarization anisotropy depend strikingly on both the n-doping into the barrier layers and an applied external electric field perpendicular to the layer. These are explained by screening of the built-in electric field with n-doping and Stark effects due to the applied electric field. The electron density as well as the mass was determined in high magnetic field (up to 160 T) cyclotron-resonance measurements. It was found that the observed indirect PL transition occurs via the charged excitons in a type-II quantum configuration.

Optical and electrical properties of Te doped AlGaAsSb/AlAsSb Bragg mirrors on InP

We present a comparative study carried out on the optical and electrical characteristics of undoped and Te doped AlGaAsSb/AlAsSb Bragg mirrors with 6.5 pairs of layers and bulk undoped and Te doped AlGaAsSb epilayers alloys lattice matched on InP, grown by molecular beam epitaxy, using SIMS, photoluminescence, reflectivity and IxV techniques. The temperature dependence of PL transitions observed in the Bragg mirrors are similar to that observed in bulk samples and associated with the donor and acceptor recombinations in alloys with electrostatic potential fluctuations described by quasi-donor-acceptor-pair (QDAP) models. We verified by SIMS the presence of a macro fluctuation in the Te profile concentrations in the growth direction of the doped Bragg mirror. The influence of doping, alloy and macrofluctuation of incorporated Te on the optical and electrical properties of doped Bragg mirror is analyzed. The AlGaAs/AlAsSb seems a very attractive option for VCSEL technology.

Magnetophotoluminescence study of GaxIn1−xP quantum wells with CuPt-type long-range ordering

Magnetophotoluminescence (PL) measurements are performed on GaxIn1−xP∕AlGaInP quantum wells with CuPt-type long-range ordering. Curve-fitting and second-order derivative operations are conducted, respectively, on the PL spectra, and two different PL components are identified with an energetic separation of about 5∼25meV. Obvious diamagnetic shift can be observed only for the PL component with higher energy, indicating the excitonic feature of the related PL transition. The in-(001)-plane exciton reduced effective mass and binding energy are evaluated based solely on the excitonic PL component, by a quasi-two-dimensional exciton effective mass approximation. The results suggest that the ordering competes with compressive strain, and enlarges the exciton reduced effective mass and binding energy. The enlargement is discussed and a comparison with previous reports is given.

PL due to Discrete Gap Levels in Some Chalcogenide Glasses- A Configurational Coordinate Diagram Illustration

ABSTRACTPhotoluminescence (PL) studies were carried out on a-Se and a few Ge20Se80−xBix and Ge20Se70−xBixTe10 bulk glassy semiconductors at 4.2 K with Ar+ laser as excitation source. While a-Se and samples with lesser at% of Bi show fine structured PL with a large Stokes shift, samples with higher at% of Bi did not show any detectable PL. The investigations show at least three radiative recombination transitions. Features extracted by deconvoluting the experimental spectra show that the discrete gap levels associated with the inherent coordination defects are involved in the PL transitions. Absence of PL in samples with higher Bi at% are explained on the basis of nonradiative transition mechanisms. Overall PL mechanism involving gap levels in chalcogenide glasses is illustrated with the help of a configurational coordinate diagram.

Temperature dependence of the photoluminescence properties and band gap energy of InxGa1−xAs/GaAs quantum wells

The effective band gap energy of InxGa1−xAs/GaAs strained quantum wells (QWs) is investigated by photoluminescence spectroscopy (PL) in the range 12–295 K. The temperature dependence of the band gap energy of strained QWs correlates well with that of bulk InxGa1−xAs of similar composition. Deviations from the band gap variation of bulk material at low temperatures (12–90 K) are interpreted in terms of exciton localization. The differences ΔE(12 K) between the measured PL peak energies and the expected transition energies at 12 K (obtained by simulating the measured temperature dependence of the PL peak positions by the well-known Varshni relation) are suggested to be closely related to the Stokes shifts that often exist between PL and PL excitation spectra of QWs. A linear relation is found between the PL full-width at half-maximum measured at 12 K and ΔE for a range of QWs prepared under different growth conditions. Excitonic recombination is inferred to be dominant in the PL transitions at the highest temperatures investigated—even at room temperature.

Origin of two types of excitons in CdSe dots on ZnSe

Recent micro-PL and time-resolved PL data performed on CdSe dots embedded in ZnSe show compelling evidence that the PL emission results from two different kinds of states. We propose a model explaining the origin of these different states coexisting within a single dot. The main concept of the model is that, while the ground state of the heavy hole is confined to strain-induced potential pockets at the bottom of the island, the electrons and the first excited state in the valence band are distributed within the entire island. That difference in the degree of localization between the two heavy-hole band states is responsible for the different properties of PL transitions observed in our experiments.

A silicon molecular beam epitaxy system dedicated to device-oriented material research

Design, performance test, doping capability and grown material quality of a Balzers UMS 630 Si MBE system are reported, particularly concerning measures to obtain good quality of grown films. Good stability, reproducibility and uniformity of deposition rates (Si and Ge) and doping concentrations (Sb and B) have been obtained for growth on a 4 inch Si wafer with sample rotation using a mass-spectrometry controlled e-beam evaporation system, and home-made doping sources, respectively. The quality of grown undoped and modulation doped Si and SiGe layered structures were evaluated using high-resolution XRD, XTEM, SIMS, Hall, and PL measurements. Intense and sharp excitonic PL transitions and high carrier mobility obtained from the grown Si SiGe heterostructures and quantum wells grown at a wide substrate temperature range (320–650°C) indicate high crystalline quality of grown films. Finally, test HBT structures with a thin SiGe base have been made. Good dc characteristics and frequency performance were obtained.

Fourth-order quantum electrodynamic corrections of order α5mc2in neutral helium

The fourth-order quantum electrodynamic corrections of order asmc2 are made for the transition energies between excited states in neutral helium. It is found that our theory is consistent with the latest experiment by Sansonetti and Martin. In the past few years, extremely accurate measurements for the transition energies between excited states in neutral helium have been carried out from the point of view of tests of quantum electrodynamics by Giacobino and Biraben (1982), Hlousek et a1 (1983) and Sansonetti and Martin (1984). In a previous letter (Hata 1984), we have reported a radiative correction formula to the transition energy of 23P,-2'PI and have found that the correction term E ;,* derived by Araki (1957) is significant for explaining the observed transition energy of Sansonetti and Martin (1984). One of the important points is that the correction term E& derived from the proper interaction kernel in the external-field Bethe-Salpeter equation includes fourth-order quantum electro- dynamic corrections of order cy5mc2 including two-photon exchange processes. Other fourth-order quantum electrodynamic corrections of order cy5mc2 are due to the anomalous magnetic moment correction given by (see Araki 1957) which is included in the calculations of Accad et a1 (1971). Therefore we are able to test all the terms of the fourth-order quantum electrodynamic corrections of order cy5mc2 by investigating the transition energy of 23Pl-2'P,. Our result (Hata 1984) shows that the fourth-order quantum electrodynamic corrections of order cy'mc' are consistent with the result of Sansonetti and Martin (1984) at the present level of experimental accuracy. It may be worthwhile to note that Lewis and Serafino (1978) have extended the fourth-order quantum electrodynamic corrections to cy6mc2 for the 23P fine-structure intervals in neutral helium and their result was consistent with the experiment of Kponou et a1 (1981) and of Frieze et a1 (1981). The transition energy 2'P,-3'D2 has been also measured by Sansonetti and Martin (1984) who have estimated the radiative contribution to the 2'PI level from their experiment. However the result was not consistent with that estimated from the 23PI-2'Pl transition energy. They then concluded that this inconsistency was due to the inaccuracy of the theoretical atomic data.

Defect centers in high purity hydride VPE indium phosphide

InP vapor phase epitaxial layers with net donor concentrations in the 8×10 14 to 3×10 16 cm -3 range have been reproducibly grown with the In/HCl/PH 3/H 2 reactant system. Room temperature electron mobilities varied from 1500 to 3500 cm 2/V·s. Photoluminescence measurements at 77 K indicated transitions at 1.41, 1.37 and 1.33 eV in the high purity samples. The relative photoluminescence intensity ratio I(1.41)/ I(1.37) depended upon epitaxial growth conditions. In some of the VPE layers, a PL transition with a peak at 1.19 eV was observed. Deep level transient spectroscopy measurements were performed on as-grown, doped and electron irradiated VPE material. The relationship between deep level spectra and residual impurities was examined.