Emission Peak Energy Research Articles

Photoluminescence Studies of Porous ZnO Nanorods

Porous ZnO nanorods were prepared by thermal annealing at 700 °C in argon atmosphere. Temperature-dependent photoluminescence (PL) was carried out to investigate the optical properties of the porous ZnO nanorods. The average diameter and density of the pores formed on the surface of the porous ZnO nanorods are 25 nm and 1.2×1010 cm-2, respectively. Six peaks at 3.380, 3.366, 3.338, 3.267, 3.227, and 3.198 eV, corresponding to free exciton (FX), neutral donor bound exciton (D0X), first-order longitudinal optical phonon replica of free exciton (FX-1LO), FX-2LO, FX-3LO, and FX-4LO emissions, are obtained at low temperature (10 K). The FX emission peak energy of the porous ZnO nanorods exhibits an anomalous behavior as the temperature is increased to 100 K. The activation energy of the porous ZnO nanorods is found to be about 8.6 meV, and the values of fitting parameters to Varshni's empirical equation are α= 6×10-4 eV/K and β= 680 K.

Photoluminescence of samples produced by electroless wet chemical etching: Between silicon nanowires and porous structures

AbstractSamples containing silicon nanowires (Si‐NWs) and highly porous structures (P‐Si) were prepared by electroless wet chemical etching (EWCE) of crystalline silicon wafers using various etching parameters. Photoluminescence (PL) measurements were performed with excitation at 488 nm and a photon energy flux of 337 mW cm−2. According to the diameters of the Si‐NWs (>10 nm), from quantum confinement (QC) theory no shift in PL peak energy compared to the bandgap of crystalline silicon is expected. However, PL measurements show peak emission energies ranging between 1.4 and 1.6 eV. After further treatment of the samples with HF, substantial PL emission was still detectable with the measured PL peak pinned at 1.4 eV irrespective of etching time. We explain the observations by the hypothesis that the persistent part of PL emission is generated by nanocrystals located at the rough sidewalls of the Si‐NWs or residing within the porous sample structure. The part of the PL, which was present before HF treatment, but vanished after the treatment, is attributed to the presence of silicon suboxide surrounding the Si‐NWs or covering other Si surfaces. This hypothesis is explored by means of three sample series, prepared with different preparation parameters. In the first series the time used during the initial metallization step in order to prepare an Ag nanoparticle layer on the top surface was varied, in the second series the etching time was the changed parameter and in the third series the HF to H2O2 concentration ratio was varied.magnified image Strong visible orange colored PL of a sample produced by EWCE starting from a heavily doped wafer (n‐type c‐Si (111), As as dopant) and excited at 337 nm (the sample was mounted on a glass substrate. Blue luminescence is due to the substrate).

Droplet Epitaxy of InN Quantum Dots on Si(111) by RF Plasma-Assisted Molecular Beam Epitaxy

InN quantum dots (QDs) were fabricated on Si(111) substrate by droplet epitaxy using an RF plasma-assisted MBE system. Variation of the growth parameters, such as growth temperature and deposition time, allowed us to control the characteristic size and density of the QDs. As the growth temperature was increased from 100 C to 300 degrees C, an enlargement of QD size and a drop in dot density were observed, which was led by the limitation of surface diffusion of adatoms with the limited thermal energy. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to assess the QDs size and density. The chemical bonding configurations of InN QDs were examined by X-ray photo-electron spectroscopy (XPS). Fourier transform infrared (FTIR) spectrum of the deposited InN QDs shows the presence of In-N bond. Temperature-dependent photoluminescence (PL) measurements showed that the emission peak energies of the InN QDs are sensitive to temperature and show a strong peak emission at 0.79 eV.

Surface localized exciton emission from undoped SnO2 nanocrystal films

We report the UV photoluminescence properties of SnO2 nanocrystalline films. A free exciton decay centered at 3.7 eV and a strong surface localized exciton emission peak at 3.3 eV have been observed at room temperature. The peak energy of the surface localized exciton emission exhibits a redshift with increasing temperature and a blueshift with increasing excitation intensity. The surface localized exciton emission is considered to originate from the radiative recombination of exciton within the surface region of SnO2 nanocrystals. The surface defects and local disorder are believed to be responsible for the formation of band tail states at the conduction band and potential well within the band tails.

High-redshift Fermi blazars

With the release of the first year Fermi catalogue, the number of blazars detected above 100 MeV lying at high redshift has been largely increased. There are 28 blazars at z>2 in the "clean" sample. All of them are Flat Spectrum Radio Quasars (FSRQs). We study and model their overall spectral energy distribution in order to find the physical parameters of the jet emitting region, and for all of them we estimate their black hole masses and accretion rates. We then compare the jet with the accretion disk properties, setting these sources in the broader context of all the other bright gamma-ray or hard X-ray blazars. We confirm that the jet power correlates with the accretion luminosity. We find that the high energy emission peak shifts to smaller frequencies as the observed luminosity increases, according to the blazar sequence, making the hard X-ray band the most suitable for searching the most luminous and distant blazars.

Sb antisite defects in InSb epilayers prepared by metalorganic chemical vapor deposition

High quality InSb epilayers were grown epitaxially on InSb (100) by metalorganic chemical vapor deposition. Different growth temperatures and V/III ratios were employed in the growth and their effects on the quality of the grown films were studied. Low temperature photoluminescence spectra of the samples revealed that in addition to the main band-to-band emission around 5.4μm, an emission peak around 5.87μm was also be observed. Our results indicate that the low energy emission peak was originated from the antisite SbIn defects which can be removed by reducing the V/III ratio.

Optical characteristics of a-plane ZnO/Zn0.8Mg0.2O multiple quantum wells grown by pulsed laser deposition

We reported optical properties of a-plane ZnO/ZnMgO multiple quantum wells (MQWs) structure grown by the pulse laser deposition system. The emission peak energy of a-plane ZnO/ZnMgO MQWs kept invariant in the power-dependent photoluminescence (PL) measurement, indicating the nonpolar characteristics due to the lack of built-in electric fields. Large exciton binding energy of 68 meV was deduced and no apparent S-curve appeared in temperature-dependent PL results, demonstrating less carrier localization effect in a-plane ZnO/ZnMgO MQWs. Large difference in electronic transition levels of 45 meV due to the valence band splitting was observed in the polarization dependent absorption spectrum. Furthermore, the high degree of polarization of 92% and 56% at 20 and 300 K in PL emission of a-plane ZnO/ZnMgO MQWs were obtained.

III-nitride nanopyramid light emitting diodes grown by organometallic vapor phase epitaxy

Nanopyramid light emitting diodes (LEDs) have been synthesized by selective area organometallic vapor phase epitaxy. Self-organized porous anodic alumina is used to pattern the dielectric growth templates via reactive ion etching, eliminating the need for lithographic processes. (In,Ga)N quantum well growth occurs primarily on the six {11¯01} semipolar facets of each of the nanopyramids, while coherent (In,Ga)N quantum dots with heights of up to ∼20 nm are incorporated at the apex by controlling growth conditions. Transmission electron microscopy (TEM) indicates that the (In,Ga)N active regions of the nanopyramid heterostructures are completely dislocation-free. Temperature-dependent continuous-wave photoluminescence of nanopyramid heterostructures yields a peak emission wavelength of 617 nm and 605 nm at 300 K and 4 K, respectively. The peak emission energy varies with increasing temperature with a double S-shaped profile, which is attributed to either the presence of two types of InN-rich features within the nanopyramids or a contribution from the commonly observed yellow defect luminescence close to 300 K. TEM cross-sections reveal continuous planar defects in the (In,Ga)N quantum wells and GaN cladding layers grown at 650–780 °C, present in 38% of the nanopyramid heterostructures. Plan-view TEM of the planar defects confirms that these defects do not terminate within the nanopyramids. During the growth of p-GaN, the structure of the nanopyramid LEDs changed from pyramidal to a partially coalesced film as the thickness requirements for an undepleted p-GaN layer result in nanopyramid impingement. Continuous-wave electroluminescence of nanopyramid LEDs reveals a 45 nm redshift in comparison to a thin-film LED, suggesting higher InN incorporation in the nanopyramid LEDs. These results strongly encourage future investigations of III-nitride nanoheteroepitaxy as an approach for creating efficient long wavelength LEDs.

Low temperature photoluminescence and cathodoluminescence studies of nonpolar GaN grown using epitaxial lateral overgrowth

Low temperature cathodo- and photoluminescence has been performed on nonpolar a-plane GaN films grown using epitaxial lateral overgrowth. In films overgrown at a low V–III ratio, the emission spectrum is dominated by “yellow” and “blue” luminescence bands, attributed to recombination at point defects or impurities. The intensity of this emission is observed to decrease steadily across the window region along the −c direction, possibly due to asymmetric diffusion of a point defect/impurity species. When overgrown at a higher V–III ratio, the near band edge and basal-plane stacking fault emission intensity increases by orders of magnitude and a donor–acceptor pair band is observed. Using monochromatic cathodoluminescence imaging, the various emission features are correlated with the microstructure of the film. In particular, the peak energy of the basal-plane stacking fault emission is seen to be blueshifted by ∼15 meV in the wing relative to the window region, which may be related to the different strain states in the respective regions.

Optical properties of bulk‐like GaN nanorods grown on Si(111) substrates by rf‐plasma assisted molecular beam epitaxy

AbstractWe have investigated the optical properties of bulk‐like GaN nanorods by cathodoluminescence measurements at room temperature. The bulk‐like nanorods were grown on a Si (111) substrate by rf‐plasma assisted molecular beam epitaxy and are characterized be being surrounded by a compact region. A weak and broad emission line near 3.36 eV was observed at an accelerating voltage of 5 kV. This emission occurs at a lower energy than the emission from unstrained GaN nanorods that protrude from the compact region. Upon increasing the electron beam energy, and hence investigating regions deeper in the sample, the peak emission energy redshifts due to the strain at the substrate interface. The behaviour of the emission line is further investigated with cathodoluminescence measurement on single nanorod's (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of rapid thermal annealing effect on CdZnO thin films grown on Si substrate

CdZnO thin films were grown on Si (100) substrates by plasma-assisted molecular beam epitaxy. As-grown samples show near band edge emissions at 1.87, 2.03, and 2.16 eV, respectively, while the emission peak energy dramatically increases to up to ultraviolet region with increasing rapid thermal annealing temperature. Room temperature photoluminescence (PL), and temperature dependent PL show phase separations in the samples after the annealing process. Secondary ion mass spectroscopy measurements show redistribution of Cd in the as-annealed sample, which is believed to be the reason of PL peaks shift.

Carrier localization and nonradiative recombination in yellow emitting InGaN quantum wells

InGaN quantum wells, with luminescence in the yellow region of the visible spectrum, have been studied using conventional and time-resolved cathodoluminescence. We observe the absence of strong localization effects and a relatively high internal quantum efficiency of ∼12%, which are unexpected for InGaN in this-long wavelength emission range. We have also observed a steady decrease of the peak emission energy, and a continuous increase in the radiative recombination lifetime with temperature up to 100 K. These two features are manifestations of recombination due to nonlocalized excitons. Nonradiative recombination centers, with activation energy of ∼6 meV, appear to constitute the main mechanism limiting the internal quantum efficiency of these films.

Emission processes in colloidal PbSe/PbS core-shell quantum dots

The present study describes a thorough investigation of the ground-state exciton emission of PbSe/PbS core-shell colloidal quantum dots (CQDs) with different shell-thickness/core-radius ratio, by recording the photoluminescence (PL) spectra and the corresponding emission decay processes over a temperature range from 1.4 K to 300 K. These spectra were compared with that of the corresponding PbSe core CQDs. All PL spectra showed variation in the emission intensity and peak energy, including two distinct abrupt transitions, upon the increase of the temperature. However, the transitions appeared at different temperatures in the core-shell structures with respect to that of the core. Further on, the PL-decay processes in the core materials showed a single exponent behavior, while the emission of the core-shell CQDs, decayed as a stretched exponent, with an effective lifetime of ∼1 μs, at room temperature, slightly longer than that of the corresponding core.

Synthesis of ZnS whiskers and their photoluminescence properties

ZnS whiskers were successfully synthesized by the chemical vapor deposition method with the assistance of CuS micro-spheres. The composition, morphology and structure of the samples were characterized by X-ray diffraction, X-ray energy dispersive spectroscopy, scanning electron microscopy, and the temperature-dependent photoluminescence (PL) spectrum over a temperature range from 10 to 250 K was studied. The results show that the as-synthesized ZnS whiskers have an average length of 0.3 mm and diameter of 2.5 μm with a cubic zinc-blende structure. There exist three emission bands in the blue, green and yellow regions, and the emission mechanism is discussed. As the temperature increases, the temperature-dependent PL spectrum shows anomalous behavior, where distinct inverted V-shaped characters of blue and green emission integrated intensity and an inconspicuous S-shape of blue emission peak energy are observed. The transition mechanism is discussed.

Study of the Excitation Power Dependent Internal Quantum Efficiency in InGaN/GaN LEDs Grown on Patterned Sapphire Substrate

The mechanisms of the excitation power dependent internal quantum efficiency in InGaN/GaN multiple quantum wells (MQWs) LEDs grown on the planar and the patterned sapphire substrates (PSS) at temperature of 15 and 300 K were investigated. From observation the tendency of emission peak energy and carrier lifetime variation in MQWs with different excitation power for both LED samples, we conclude the internal quantum efficiency would increase as coulomb screening effect dominates at lower carrier injection stage and decrease due to the band-filling effect at higher density stage. At room temperature, the majority of the initial injected carriers would be first consumed by the thermal activated nonradiative centers that hinder the further achievement of high-efficiency LED devices. Experimentally, the internal quantum efficiency of the LED grown on the PSS is ~70% and that of the LED grown on the planar sapphire substrate is ~62%. For the LED grown on the PSS, the observed higher internal quantum efficiency is due to the larger activation energy Therefore, the reduction of dislocation defects and the prevention of injected carriers escaping from extended states would be a promising prospective for InGaN/GaN MQWs LEDs to achieve high internal quantum efficiency.

Isoelectronic centers and type-II quantum dots: Mechanisms for the green band emission in ZnSeTe alloy

Te-doped and ZnTe-doped diluted ZnSeTe samples were grown by using metal-organic chemical vapor deposition. The green lights emitted from these two samples in the photoluminescence (PL) measurement have different peak emission energies and can be attributed to different emission mechanisms. For the Te-doped sample, the PL emission band is excitation laser power independent, the lifetime of the PL emission is short, and the PL emission could be attributed to the emission originated from Te isoelectronic centers. For the ZnTe-doped sample, the energy of the green emission band is laser power dependent, the PL lifetime is much longer than the Te-doped sample, and the PL emission could be attributed to the emission from type-II ZnTe∕ZnSe quantum dots. The results demonstrate that there are two different mechanisms responsible for the green PL emission in ZnSeTe system and our growth methods provide a way of selecting one of the two mechanisms for the green light emission in this system.

A-plane GaN layer grown on (3 0 2) (-LiAlO 2 by MOCVD

Single phase a-GaN layer has been grown on the small lattice mismatch (3 0 2) (-LiAlO 2 (LAO) substrate. The epitaxial relationships between a-plane GaN and (3 0 2) LAO are [ 1 1 ¯ 0 0 ] GaN | | [ 2 0 3 ] LAO = − 2.86 % and [ 0 0 0 1 ] GaN | | [ 0 1 0 ] LAO = − 0.31 % . The diffraction angle for GaN ( 1 1 2 ¯ 0 ) layer presents a 0.12% extension parallel to the growth direction [ 1 1 2 ¯ 0 ] due to lattice mismatch and thermal mismatch. The compressive stress is gradually released with the a-GaN layer growth indicated by the E 2 frequency of Raman spectra. The a-GaN layer has high transmittance (85%), sharp absorption edge and observed excitonic absorption. The photoluminescence (PL) shows that the donor acceptor pair emission peak energy is ∼3.41 eV with full width at half maximum value (FWHM) of 105 meV and weak yellow peaks originate from the incorporation of Li into GaN as a result of the decomposition of the substrate while ramping up temperatures. These results show that (3 0 2) LAO is a promising substrate for fabricating high-quality a-GaN layer and device.

Optical investigations of Berthelot-type properties in quaternary AlInGaN multiple quantum well heterosystems

The anomalous Berthelot-type optical properties of quaternary AlInGaN heterostructure with different quantum well pairs have been investigated systematically in this study. A microscopic model based on the luminescence observations of a comparison between an Arrhenius-type radiative process and a Berthelot-type nonradiative process has been proposed to elucidate the temperature dependence of the optical and transport behavior in nanocrystalline and amorphous semiconductors. The photoluminescence of the AlInGaN heterostructures is also found to exhibit unique luminescence features such as S-shaped emission peak energy and W-shaped full-width at half-maximum (FWHM) curves over a broad range of temperature. We investigated in detail the aspects of luminescence that would provide significant information regarding the degree of disorder in AlInGaN-based semiconductor heterosystems, leading to the appearance of the Berthelot-type behavior. The increase of quantum-well pairs apparently will cause the incorporation of indium atoms in the AlInGaN nanostructures, resulting in augmentation of the degree of crystalline randomization. In other words, the higher degree of disorder in AlInGaN heterostructures, the longer the static microbarrier width and the more obvious carrier localization effects may be observed.

In adlayer mediated molecular beam epitaxial growth and properties of a-plane InN on freestanding GaN

The role of the In adlayer on the morphological and structural properties of nonpolar a-plane InN films was elucidated during the plasma-assisted molecular beam epitaxy on freestanding GaN. Reflection high energy electron diffraction during In adsorption experiments on a-plane InN surfaces revealed a stable In adlayer coverage of ∼2 ML. This In adlayer-mediated growth was responsible for achieving atomically smooth surfaces (rms roughness of <1 nm), phase-pure material with lower x-ray rocking curve widths (Δω<0.5°), lower crystal mosaic tilt/twist, and decreased stacking fault densities, compared to N-rich conditions. The photoluminescence peak emission and band gap energy of the a-plane InN films were ∼0.63 and ∼0.7 eV, respectively.

Phosphorescence Probes of Molecular Mobility, Oxygen Permeability, and Dynamic Site Heterogeneity in Amorphous Soybean Glycinin

The physical properties of amorphous biomolecules are important to the stability of low-moisture foods and pharmaceuticals. In the amorphous solid state, slow molecular motions are suitable for study by phosphorescent techniques. We use phosphorescence of erythrosin B dispersed in soy glycinin films to characterize the molecular mobility, oxygen permeability, and dynamic heterogeneity of the protein matrix. Films are spread from concentrated solutions of probe/protein at mole ratios of 0.045/1. Measurements as a function of temperature are made of phosphorescence intensity decays, emission spectra, and time-resolved emission spectra. Decays are fit with a stretched exponential function and both lifetimes and stretching exponents decrease with temperature. Arrhenius analysis of non-radiative quenching constants suggests that the protein matrix undergoes a broad softening transition between 70 and 120C during which additional modes of molecular motion are activated. The stretching exponent, a measure of the breadth of distribution of lifetimes and hence probe site heterogeneity, decreases gradually with temperature up to 70C and more steeply at higher temperatures, providing evidence of the onset of softening at 70C. Oxygen quenching rates, calculated from a comparison between emission lifetimes in the presence and absence of oxygen, vary roughly linearly with collisional quenching rates, which suggests that the local molecular mobility responsible for collisional quenching also modulates oxygen permeability. Delayed emission spectra are fit to a double lognormal function that provides peak emission energy and bandwidth. Bandwidth increases dramatically above 70C, which reflects an increase in the width of the distribution of energetically distinct matrix environments and provides further evidence of softening. The emission spectra blue shift with increasing delay time providing strong evidence that probes reside in distinct sites that vary in molecular mobility. Research supported by CSREES.