Emission Peak Energy Research Articles

Optical constants of In(x)Ga(1-x)N (0 ≤ x ≤ 0.73) in the visible and near-infrared wavelength regimes.

Complex refractive indices of In(x)Ga(1-x)N epitaxial layers have been determined from analysis of data obtained by spectroscopic ellipsometry. The measurements were made in the wavelength range of 400-1687 nm. The samples were grown by plasma-assisted molecular beam epitaxy on (001) silicon substrate and are of the wurtzite crystalline form. A comparison of the fundamental absorption edge derived from analysis of measured data and the measured photoluminescence peak emission energy indicates a Stokes shift present in the alloys.

Temperature-Dependent Photoluminescence of Ag2Se Quantum Dots

The time-resolved photoluminescence spectroscopy was employed to analyze the optical properties of Ag2Se quantum dots with different diameters at temperatures of 80–360 K. The photoluminescence lifetime measurement disclosed that in the low-energy electronic structure there were two dominating emissive “in-gap” states associated with surface defect and intrinsic states, which were further confirmed by Gaussian fitting of the photoluminescence spectra. The temperature-dependent emission peak energy was fitted to phenomenological equations to extract the average phonon energy, the Huang–Rhys factor, and the excitonic acoustic phonon coupling coefficient. The relatively large phonon energy and small Huang–Rhys factor were demonstrated, which induced the small variation of emission peak energy in the low-temperature range. Meanwhile, the photoluminescence line width increased with temperature and was analyzed based on the standard equation describing the temperature dependence of the width of the ground state...

Spatially indirect radiative recombination in InAlAsSb grown lattice-matched to InP by molecular beam epitaxy

A photoluminescence (PL) spectroscopy study of the bulk quaternary alloy InAlAsSb is presented. Samples were grown lattice-matched to InP by molecular beam epitaxy and two different growth temperatures of 450 °C and 325 °C were compared. Interpolated bandgap energies suggest that the development of this alloy would extend the range of available direct bandgaps attainable in materials lattice-matched to InP to energies as high as 1.81 eV. However, the peak energy of the observed PL emission is anomalously low for samples grown at both temperatures, with the 450 °C sample showing larger deviation from the expected bandgap. A fit of the integrated PL intensity (I) to an I∝Pk dependence, where P is the incident power density, yields characteristic coefficients k = 1.05 and 1.18 for the 450 °C and 325 °C samples, respectively. This indicates that the PL from both samples is dominated by excitonic recombination. A blue-shift in the peak emission energy as a function of P, along with an S-shaped temperature dependence, is observed. These trends are characteristic of spatially-indirect recombination associated with compositional variations. The energy depth of the confining potential, as derived from the thermal quenching of the photoluminescence, is 0.14 eV for the 325 °C sample, which is consistent with the red-shift of the PL emission peak relative to the expected bandgap energy. This suggests that compositional variation is the primary cause of the anomalously low PL emission peak energy. The higher energy PL emission of the 325 °C sample, relative to the 450 °C sample, is consistent with a reduction of the compositional fluctuations. The lower growth temperature is therefore considered more favorable for further growth optimization.

Strong carrier localization effect in carrier dynamics of 585 nm InGaN amber light-emitting diodes

Temperature dependence and time-resolved photoluminescence (TRPL) have been carried out to study carrier dynamics for 585 nm InGaN amber light-emitting diodes (LEDs). It is found that in InGaN amber LEDs, peak emission energy only shows a slight blueshift from 588 to 575 nm, as temperature increased from 10 K to 300 K. Moreover, radiative recombination lifetime has demonstrated independent of temperature based TRPL results. These two features indicate that a strong carrier localization effect plays a dominant role in carrier dynamics for InGaN amber LEDs. Also, activation energy of 40.3 meV is obtained through Arrhenius plot of PL intensity versus temperature.

Spatially resolved study of polarized micro-photoluminescence spectroscopy on single GaAs nanowires with mixed zincblende and wurtzite phases

Localized and polarized photoluminescence spectra are observed in single GaAs nanowires with mixed zincblende and wurtzite phases, grown using molecular beam epitaxy. For low excitation intensities, the photoluminescence emission exhibits narrow spectral features predominantly polarized perpendicular to the nanowire axis. For high excitation intensities, the photoluminescence spectra transform into dominant broadened features, which exhibit different peak energies and polarization properties. The strongly polarized emission at high excitation intensities is identified as being due to a spatially direct transition in wurtzite sections of the nanowires. The analysis, including band structure calculations suggests that carriers in the wurtzite sections diffuse into regions where the average low-temperature peak emission energy and crystal field parameter are 1.535 eV and 20 meV, respectively.

Bandedge optical properties of MBE grown GaAsBi films measured by photoluminescence and photothermal deflection spectroscopy

The bandedge optical properties of GaAsBi films, as thick as 470nm, with Bi content varying from 0.7% Bi to 2.8% Bi grown by molecular beam epitaxy on GaAs substrates are measured by photoluminescence (PL) and photothermal deflection spectroscopy (PDS). The PDS spectra were fit with a modified Fernelius model which takes into account multiple reflections within the GaAsBi layer and GaAs substrate. Three undoped samples and two samples that are degenerately doped with silicon are studied. The undoped samples show a clear Urbach absorption edge with a composition dependent bandgap that decreases by 56meV/% Bi and a composition independent Urbach slope parameter of 25meV due to absorption by Bi cluster states near the valence band. The doped samples show a long absorption tail possibly due to absorption by gap states and free carriers in addition to a Burstein–Moss bandgap shift. PL of the undoped samples shows a lower energy emission peak due to defects not observed in the usually available thin samples (50nm or less) grown under similar conditions.

In situ etching for control over axial and radial III-V nanowire growth rates using HBr

We report on the influence of hydrogen bromide (HBr) in situ etching on the growth of InP, GaP and GaAs nanowires. We find that HBr can be used to impede undesired radial growth during axial growth for all three material systems. The use of HBr opens a window for optimizing the growth parameters with respect to the materials’ quality rather than only their morphology. Transmission electron microscopy (TEM) characterization reveals a partial transition from a wurtzite crystal structure to a zincblende upon the use of HBr during growth. For InP, defect-related luminescence due to parasitic radial growth is removed by use of HBr. For GaP, the etching with HBr reduced the defect-related luminescence, but no change in peak emission energy was observed. For GaAs, the HBr etching resulted in a shift to lower photon emission energies due to a shift in the crystal structure, which reduced the wurtzite segments.

Epitaxial growth and optical characterization of AlInGaN quaternary alloys with high Al/In mole ratio

AlInGaN quaternary epitaxial layers with high Al/In mole ratio were grown on GaN/sapphire templates by metalorganic chemical vapor deposition (MOCVD). The optical and structural properties were characterized by various kinds of technical tools. The full width at half-maximum of the (0002) X-ray diffraction rocking curves was found to decrease as the Al/In mole ratio was increased. The density and size of V-shaped defect pits observed on the surface of AlInGaN epitaxial layers with energy-dispersive X-ray spectroscopy could be reduced by lowering the MOCVD growth pressure. It was also discovered with the variable temperature photoluminescence spectra that the emission peak energy for the AlInGaN epitaxial layers demonstrated an S-shaped shift (red shift–blue shift–red shift) behavior with increasing the measurement temperature. This feature was more significant for those AlInGaN epitaxial layers with lower Al/In mole ratio.

Excitonic localization in AlN-rich AlxGa1−xN/AlyGa1−yN multi-quantum-well grain boundaries

AlGaN/AlGaN multi-quantum-wells (MQW) with AlN-rich grains have been grown by metal organic chemical vapor deposition. The grains are observed to have strong excitonic localization characteristics that are affected by their sizes. The tendency to confine excitons progressively intensifies with increasing grain boundary area. Photoluminescence results indicate that the MQW have a dominant effect on the peak energy of the near-bandedge emission at temperatures below 150 K, with the localization properties of the grains becoming evident beyond 150 K. Cathodoluminescence maps reveal that the grain boundary has no effect on the peak intensities of the AlGaN/AlGaN samples.

Excitation Spectra and Stokes Shift Measurements of Single Organic Dyes at Room Temperature.

We report measurements of excitation and emission spectra of single, polymer-embedded, perylene dye molecules at room temperature. From these measurements, we can derive the Stokes shift for each single molecule. We determined the distribution of excitation and emission peak energies and, thus, the distribution of single molecule Stokes shifts. Single molecule Stokes shifts have not been recorded to date, and the Stokes shift has often been assumed to be constant in single molecule studies. Our data show that the observed spectral heterogeneity in single molecule emission originates not only from synchronous energetic shifts of the excitation and the emission spectra but also from variations in the Stokes shift, speaking against the assumption of constant Stokes shift.

The effects of Si-doped prelayers on the optical properties of InGaN/GaN single quantum well structures

In this paper, we report on the effects of including Si-doped (In)GaN prelayers on the low temperature optical properties of a blue-light emitting InGaN/GaN single quantum well. We observed a large blue shift of the photoluminescence peak emission energy and significant increases in the radiative recombination rate for the quantum well structures that incorporated Si-doped prelayers. Simulations of the variation of the conduction and valence band energies show that a strong modification of the band profile occurs for the quantum wells on Si-doped prelayers due to an increase in strength of the surface polarization field. The enhanced surface polarization field opposes the built-in field across the quantum well and thus reduces this built-in electric field. This reduction of the electric field across the quantum well reduces the Quantum Confined Stark Effect and is responsible for the observed blue shift and the change in the recombination dynamics.

The nondestructive observation of correlation between optical characteristic and morphologic feature within capped ZnCdSe/ZnSe quantum dots system

The correlation between optical property and morphologic appearance within capped ZnCdSe/ZnSe quantum dots (QDs) system is investigated by photoluminescence (PL) spectrum measured at temperatures from 11 to 300K. The coexistent dense QDs and atypical two-dimensional non-uniform structures (neither wetting layer nor small platelet) are identified by their unique temperature evolutions of full width at half maximum (FWHM), distinct quenching rates of spectral integrated intensity and extra redshifts of emission peak energy. The divergent lateral exciton transfer modes between these two quantum structures are demonstrated by the differently morphologic-induced spectral features. The incompletely three-dimensional excitonic confinement within QDs assemblies is observed by comparing the carrier repopulation-induced redistribution of emission energy with that of isolated QDs system. In comparison with general surface detection methods of the uncapped sample, utilizing PL as spectral probe is a nondestructive technique to explore inner morphologic characteristics of capped multiple quantum structure within functional device.

An investigation into the spectral properties of bright Fermi blazars

We investigate the spectral properties of blazars detected with the Fermi-Large Area Telescope (LAT) in the high energy regime 100 MeV - 100 GeV. We find that over long timescales a log-parabola provides an adequate description of the spectrum in almost all objects and in most cases is significantly better than a simple power law or broken power law description. Broken power law descriptions appear to arise from two causes: confusion with nearby sources and as an artifact of older LAT instrument response functions. We create a light curve for 2FGLJ2253.9+1609 (3C 454.3), the brightest of the objects investigated. During the quiescent state we find the spectrum to be fairly stable and well-described by a log-parabola. There is some evidence that, on average, the peak energy of the inverse Compton emission is lower in the quiescent state than in the time-averaged state, suggesting that increases in flux are due to changing parameters within the jet as opposed to changes in an external photon field. However, no correlation between inverse Compton peak energy and flux is apparent. During high flux states, deviation of the spectral shape from a simple power law continues. In some cases a log-parabola provides a significantly better fit than a broken power law but in others the reverse is true.

Localization effects and band gap of GaAsBi alloys

The structural and optical properties of GaAs1−xBix alloys for x up to 0.108 have been investigated by high resolution X-ray diffraction and photoluminescence (PL). At room temperature (RT), the PL intensity of the GaAs0.97Bi0.03 sample was found to be ∼300 times higher than a GaAs control sample grown at the same temperature (400 °C). PL measurements carried out at 10 K show that when excitation power, Pex was increased from 0.11 to 1140 W cm−2, the PL peak energy blue-shifts by 80 meV while the full-width-at-half-maximum reduces from 115 to 63 meV. However, the PL peak emission energy becomes independent of the excitation power at RT. The results indicate the presence of localized energy states in the GaAs0.97Bi0.03 sample, which trap carriers at low temperatures and that the majority of the carriers become delocalized at RT. Furthermore, the temperature dependent PL also shows an S-shape behavior, which is a signature of localization effects. A theoretical model, which was derived by solving a rate equation was employed. The model successfully reproduces the observed S-shape behavior and the theory fits well with the experimental data. The RT band gap of GaAs1−xBix for x up to 0.108 has been plotted and compared with the literature.

Photoluminescence spectra of nitrogen-rich InN thin films grown on Si(110) and photoelectrochemical etched Si(110)

Indium nitride (InN) thin films were deposited on anisotropic silicon [Si(110)] and photoelectrochemical etched silicon [Psi(110)] substrates by reactive radio-frequency sputtering. All deposited films showed wurtzite nanocrystalline InN films with a (101) preferred growth orientation. The optical properties of the nitrogen-rich InN thin films were investigated under various deposition gas concentrations. Strong photoluminescence was observed in the 1.8 eV–1.98 eV energy range for nanocrystalline InN grown on Si(110) at room temperature. The lower energy emission peak was achieved for InN grown on PSi(110) samples.

Effect of substrate temperature on optical properties and strain distribution of ZnTe epilayer on (100) GaAs substrates

The photoluminescence properties of the ZnTe epilayers grown on (100) GaAs substrates with various substrate temperatures are investigated. The spectral characteristics of both bound and free excitonic emissions measured at different measurement temperatures and excitation power, show that compared with the too low (390°C) or too high (440°C) substrate temperature, the moderate substrate temperature (around 420°C) is suitable to obtain a high-quality epilayer due to its narrowest spectral width and smallest strain. With increasing excitation power the peak energy of the free excitonic emission decreases linearly, while the spectral width remains an approximate constant. This implies that the decreasing peak energy should be attributed to an increasing strain with increasing the effective excited depth, and the increasing strain only reduces the bandgap, but does not cause much influence on the crystalline quality for each epilayer. Furthermore, the increasing strain also causes the ground state free excitons splitting into the light hole and heavy hole free excitons.

Effect of surfactant Sb on In incorporation and thin film morphology of InGaN layers grown by organometallic vapor phase epitaxy

The effects of the surfactant Sb on InGaN grown by organometallic vapor phase epitaxy (OMVPE) were studied. Eight samples of InGaN were grown with Sb concentrations ranging from 0% to 2.5%. Characterization was done by photoluminescence (PL) and atomic force microscopy (AFM). An abrupt change in PL emission peak energy and surface morphology occurred at a certain critical Sb concentration. Above and below this threshold concentration two distinct regimes of surface morphology and PL emission characteristics were observed. This effect was interpreted as due to a surfactant-induced change of surface phase on the InGaN films.

Optical properties of InGaN‐based LEDs investigated using high hydrostatic pressure dependent techniques

AbstractHigh pressure electroluminescence (EL) measurements were carried out on blue and green emitting InGaN‐based light emitting diodes (LEDs). The weak pressure coefficient of the peak emission energy of the LEDs is found to increase with increasing injection current. Such behaviour is consistent with an enhancement of the piezoelectric fields under high pressure which become increasingly screened at high currents. A subsequent increase in the quantum confined Stark effect (QCSE) is expected to cause a reduction of the light output power as pressure is applied at a fixed low current density (∼10 A cm−2). A similar proportional reduction of light output power as pressure is applied at a fixed high current density (260 A cm−2) suggests that there is a non‐radiative loss process in these devices which is relatively insensitive to pressure. Such behaviour is shown to be consistent with a defect‐related recombination process which increases with increasing injection.

Controlled hydrothermal growth and optical characterization of wide band gap Be x Zn1−x O nanorod arrays

Wide band gap Be x Zn1−x O nanorod arrays on Zn foil substrates have been successfully prepared for the first time by a facile hydrothermal method at 80 °C for 24 h. The structure, morphology, and optical properties of the nanorod arrays were studied by X-ray diffraction (XRD), field emission scanning electron microscopy, and photoluminescence (PL). The composition x and band gap energy of the Be x Zn1−x O nanorod arrays were calculated from XRD and PL results, respectively. The results show that the PL emission peak energy increased with increasing x, as expected, and these materials should be suitable for developing UV-based optoelectronics.

Growth and Characteristics of a-Plane GaN/ZnO/GaN Heterostructure

AbstractThe crystal structure of a-plane GaN/ZnO heterostructures on r-plane sapphire was investigated by using the XRD and TEM measurment. It was found the formation of (220) ZnGa2O4 and crystal orientation of semipolar (10$\bar 1$3) GaN at GaN/ZnO interface. The epitaxial relation of normal surface direction are the sapphire (1$\bar 1$02) // a-GaN (11$\bar 2$0) and ZnGa2O4 (220) // semi-polar GaN (10$\bar 1$$\bar 3$). Beside, the emission peak energy of ZnO appears shift about 60 meV in the GaN/ZnO/GaN heterostructures due to the re-crystallization of ZnO layer with Ga or N atom and the formation of the localized state.