Energy Position Of Peak Research Articles

Deep levels studies of AlGaN/GaN superlattices

The results of microcathodoluminescence (MCL) and deep levels transient spectroscopy (DLTS) studies of AlGaN/GaN modulation doped field effect transistor (MODFET) structures and modulation doped and undoped AlGaN/GaN superlattices (SLs) grown by metalorganic chemical vapor deposition are presented. A strong blue shift in the energy position of the band-edge MCL peak in modulation doped SLs compared to undoped SLs is observed and attributed to screening of the built-in piezoelectric field in AlGaN/GaN SLs by the high density of two-dimensional electron gas in the GaN quantum wells. The dominant traps in AlGaN/GaN MODFETs and doped SLs with Al composition in the barrier close to 28% were found to be the electron traps with activation energy 0.9 eV located near the AlGaN/GaN interface. In undoped SLs similar traps were observed and their energy position was found to strongly depend on the Al composition in the barrier and on the strength of the electric field in space charge region of the Schottky diode used for DLTS measurements.

Photoluminescence of Ge(Si)/Si(0 0 1) self-assembled islands in the near infra-red wavelength range

The dependence of photoluminescence spectra of structures with GeSi/ Si(0 0 1) self-assembled nanoislands on growth temperature has been investigated. It was shown that the redshift of the island-related photoluminescence peak with a decrease of the growth temperature is associated with suppression of Si diffusion in the islands and an increase of Ge content in them. For the first time a photoluminescence signal from SiGe islands was observed at energies much lower than the Ge band gap. The energy position of the island-related photoluminescence peak is well described by the model of optical transition, which is indirect in real space. The photoluminescence signal at 1.55 μm from GeSi/ Si(0 0 1) self-assembled islands was obtained up to room temperature.

Temperature dependence of the photoluminescence of InGaAs/GaAs quantum dot structures without wetting layer

We analyze the photoluminescence temperature behavior of InGaAs/GaAs quantum dots grown by heterogeneous droplet epitaxy. Morphologically, these dots are nanocrystal InGaAs inclusions in the GaAs matrix, with a concave disk shape and, more important, no wetting layer is connecting the dots. The photoluminescence of the dots does not show any of the typical of the Stranski–Krastanov dots temperature properties, such as sigmoidal peak energy position and linewidth narrowing. We demonstrate that such behavior stems from the lacking of the thermally activated dot–dot coupling channel provided by the wetting layer thus preventing the establishment of a common quasiequilibrium in the whole dot ensemble.

Resonant Raman scattering in ZnS epilayers

Resonant Raman scattering spectra for ZnS epilayers grown by hot-wall epitaxy have been studied extensively. With a 325nm He–Cd laser as the excitation source, three resonant Raman lines were observed in photoluminescence spectra. The temperature dependence of energy peak position, half width, Raman shift and intensity of three resonant Raman lines which were related to the temperature dependence of the energy gap of ZnS were analyzed. The values of the longitudinal-optical (LO) phonon energy between neighboring resonant Raman lines were confirmed to be very close to LO phonon energy of bulk ZnS.

Molecular beam epitaxy growth mechanism and wire width control for formation of dense networks of narrow InGaAs quantum wires

Precise wire width control is important in forming dense networks of narrow InGaAs ridge quantum wires (QWRs) by selective MBE growth on patterned InP substrates. In this paper, SEM, TEM, AFM, CL and PL measurements were carried out to clarify the QWR formation mechanism and to establish the method of wire width control. Formation of the arrow-head shaped QWR is due to evolution of (114)A facets caused by instability of (113)A facets during growth of the InAlAs layer. From PL peak energy positions, it has been found that the effective wire width can be controlled by precisely adjusting the growth time, or the grown thickness of the lower InAlAs layer.

Spontaneous quantum dot formation at In xGa 1− xAs ∣ In yGa 1− yAs interfaces

We have performed reflectivity and photoluminescence measurements on a set of In x Ga 1− x As ∣ In y Ga 1− y As ∣ GaAs(001) stepped/asymmetric quantum wells with an intentionally abrupt change of indium composition ( x=0.149, y=0.064). These measurements have been compared against extensive spectroscopic data gathered on a set of symmetric quantum wells with similar structural parameters. For both symmetric and asymmetric quantum well samples, reflectivity spectra agree well with theoretical calculations both in terms of peak energy positions and line shapes. The photoluminescence spectra from the asymmetric quantum well samples are consistent with the hypothesis of three-dimensional exciton confinement induced by local and random potential fluctuations at the In-alloy ∣ In-alloy interface. A brief examination of the possible formation mechanisms leading to the formation of such ‘natural quantum dots’ at this poorly studied interface will be presented.

Near-field scanning optical microscopy investigation of immiscibility effects in In1−xGaxP films grown by liquid phase epitaxy

Near-field scanning optical microscopy (NSOM) and electron probe microanalysis (EPMA) were used to study the topographic and microscopic optical properties of indium–gallium–phosphide (In1−xGaxP) samples grown by liquid phase epitaxy on gallium–arsenide substrates. NSOM imaging found strong and highly localized variations in the photoluminescence (PL) intensity for samples that were highly lattice mismatched with the substrate. The topography and optical features were roughly spatially correlated for these samples. Shifts in the PL peak energy position (by as much as 27 meV) were found during scans across highly mismatched samples, whereas no shifts were seen for In1−xGaxP films with a nearly lattice matched composition. Compositional fluctuations were determined to be the cause of these PL peak energy shifts. EPMA provided corroborating evidence that compositional fluctuations are spatially correlated with the topography. These composition fluctuations arise from the known solid–solid miscibility gap in the In1−xGaxP system at temperatures used for the growth of these samples.

Two-photon-excited green emission and its dichroic shift of oriented thin-film CdS on glass formed by laser deposition

The photoluminescence of oriented thin-film CdS on glass formed by laser deposition was investigated employing 200 fs, 1.54 eV laser pulses at room temperature. The ultrafast excitation caused a two-photon absorption process, which results in purely green emission at the band gap. The spectra are fitted very well by the application of the van Roosbroeck–Shockley relation, density of states, and Urbach’s rule demonstrating the intrinsic character of the radiative recombination. It is further shown that the energy position of the emission peak depends on the polarization of the impinging laser beam due to the dichroism of the highly oriented films.

Interband Transitions in GaInNAs/GaAs Single Quantum Wells

ABSTRACTPhotoluminescence (PL) spectra of interband transitions in GaInNAs/GaAs single quantum wells grown by metalorganic chemical vapor deposition technique on semi-insulating GaAs substrates were measured at 77 K for several samples grown with different In composition and dimethylhydrazine (DMH)/III ratios. The results show that the PL intensity increases as the In mole fraction is increased from 0 to 25%, but the PL intensity is degraded for samples with an In mole fraction of 30% or higher. The peak position energies of the PL spectra were investigated as a function of the DMH/III ratio. Thermal annealing effect induced a blue-shift in the PL spectra peak position energy in samples grown with high DMH/III ratio.

Cathodoluminescence study of h– and c–GaN single crystals grown by a Na or K flux

Luminescence properties of hexagonal (h-) and cubic (c-) GaN freestanding single crystals were studied by means of cathodoluminescence spectroscopy. The h-GaN crystals of about 0.2–2 mm in dimension were synthesized at 750 °C by the reaction of Ga and N2 in a Na flux, while c-GaN crystals of about 0.3 mm or less in a K flux. The h-GaN showed rather strong band edge emission at room temperature compared with the crystal grown by using NaN3 as a nitrogen source. At 20 K, the band edge emission of h-GaN was split into four peaks. The main energy peak position was 3.478 eV, which was assigned as the A-free exciton emission. The energy position of the main peak of c-GaN was 3.268 eV. Assuming the binding energies of excitons in h- and c-GaN as 25 and 26 meV, respectively, the energy difference of bandgap between h-and c-GaN is estimated to be 209 meV. Since these crystals are free from strain from the substrates, the peak energies are reliable for the intrinsic GaN crystals. The full widths at half maximum of the emission of c-GaN were much broader than those of h-GaN, suggesting that the cubic phase is much defective compared with the hexagonal one.

Compositional variation of AlGaN epitaxial films on 6H-SiC substrates determined by cathodoluminescence.

High quality epitaxial films of AlxGa1−xN, grown on SiC substrates, were investigated using spatially resolved cathodoluminescence (CL), scanning electron microscopy, and atomic force microscopy. A variation in the observed peak energy position of the CL was related to alloy fluctuations. CL was used to reveal relative alloy fluctuations of approximately 1% on a sub-micrometer scale, with a precision difficult to surpass with other available techniques. By correlating data from the different techniques, a model was derived. The main feature of it is an alloy fluctuation on the micrometer scale, seeded during the initial growth and extending through the epitaxial film. These alloy fluctuations seems to be related to terrace steps (≈5 nm in height), formed preferentially at scratches on the SiC surface. This investigation indicates that the initial growth of epitaxial films is critical and structures formed at the beginning of the growth tend to persist throughout the growth. Further, a strain gradient from the SiC interface extending towards the surface, was observed.

Correlation of Light-induced Enhancement of Open-Circuit Voltage and Structural Change of Heterogeneous Silicon Solar Cells

AbstractWe observe a significant light-induced increase in the open-circuit voltage, Voc, of thin-film silicon solar cells whose intrinsic (i) layer consists of an amorphous and microcrystalline mixed phase. The increase depends on the i-layer thickness, the i-layer deposition temperature, the initial Voc values, and the light-soaking intensity. An increase of as large as 150 mV is observed. The original Voc is restored after subsequent thermal annealing. In-situ photoluminescence (PL) spectroscopy is used to investigate this metastable phenomenon. We find that the PL intensity and peak-energy position associated with the amorphous component of the heterogeneous material increase upon light soaking, suggesting a structural change. We propose that a reduction of microcrystalline volume fraction or size is responsible for the Voc enhancement.

Effect of Well Thickness on GaN/AlGaN Separate Confinement Heterostructure Emission

We investigated the effects of well thickness on spontaneous and stimulated emission (SE) in GaN/AlGaN separate confinement heterostructures (SCHs), grown by low pressure metal organic chemical vapor deposition. The SCH wells are unstrained and lattice matched to a GaN buffer layer. Our series of SCHs had GaN well thicknesses of 3, 5, 9, and 15 nm. We explain the spontaneous emission peak energy positions of the SCHs in terms of spontaneous and strain-induced piezoelectric polarizations. At 10 K, the carrier lifetime was found to be lowest for a 3 nm well, and the SE threshold was lowest for a 5 nm well. We show that the screening of the piezoelectric field and the electron-hole separation are strongly dependent on the well thickness and have a profound effect on the optical properties of the GaN/AlGaN SCHs. The implications of this study on the development of near- and deep-ultraviolet light emitters are discussed.

Effect of Well Thickness on GaN/AlGaN Separate Confinement Heterostructure Emission

We investigated the effects of well thickness on spontaneous and stimulated emission (SE) in GaN/AlGaN separate confinement heterostructures (SCHs), grown by low pressure metal organic chemical vapor deposition. The SCH wells are unstrained and lattice matched to a GaN buffer layer. Our series of SCHs had GaN well thicknesses of 3, 5, 9, and 15 nm. We explain the spontaneous emission peak energy positions of the SCHs in terms of spontaneous and strain-induced piezoelectric polarizations. At 10 K, the carrier lifetime was found to be lowest for a 3 nm well, and the SE threshold was lowest for a 5 nm well. We show that the screening of the piezoelectric field and the electron–hole separation are strongly dependent on the well thickness and have a profound effect on the optical properties of the GaN/AlGaN SCHs. The implications of this study on the development of near- and deep-ultraviolet light emitters are discussed.

Photoluminescence study of Nd:YAG laser-etched silicon

Photoluminescence (PL) spectra of porous silicon (PS) samples fabricated with laser-induced etching (LIE) in HF acid are analyzed using a quantum confinement model with an assumption that porous silicon consists of a distribution of nanocrystallites corresponding to the Gaussian function. The mean nanocrystallite size and size distribution parameters are studied here as a function of the laser power density and irradiation time during laser-induced etching. The photoluminescence peak position energy is found to vary between 1.8 and 2 eV as the laser power density or irradiation time of the Nd:YAG laser is varied. Furthermore, the study helps in gaining fresh insight into maximizing the photoluminescence yields from porous silicon by optimizing laser parameters in the etching process.

Well-thickness dependence of emission from GaN/AlGaN separate confinement heterostructures

We investigated the effects of well thickness on spontaneous and stimulated emission (SE) in GaN/AlGaN separate confinement heterostructures (SCHs), grown by low-pressure metalorganic chemical vapor deposition. The SCH wells are unstrained and lattice-matched to a GaN buffer layer. Our series of SCHs had GaN well thicknesses of 3, 5, 9, and 15 nm. We explain the spontaneous emission peak energy positions of the SCHs in terms of spontaneous and strain-induced piezoelectric polarizations. At 10 K, the carrier lifetime was found to be lowest for a 3 nm well, and the SE threshold was lowest for a 5 nm well. We show that the screening of the piezoelectric field and the electron-hole separation are strongly dependent on the well thickness and have a profound effect on the optical properties of the GaN/AlGaN SCHs. The implications of this study on the development of near- and deep-ultraviolet light emitters are discussed.

Photoluminescence properties of the Eu3+ activator ion in the TiO2 host matrix

Titania (TiO2) is employed as a host for the Eu3+ activator ion. Thin films were produced by the sol-gel method on silicon and corning glass substrates and, depending on the nature of the substrate, they present different crystalline structure. The films show an intense red photoluminescence associated with the D05→7F2 transition of the electronic structure of Eu3+. The photoluminescence presents better characteristics for the films deposited on silicon wafers. For above band gap excitation the emission from the TiO2 matrix is obscured by the luminescence of the Eu3+ ions. The peak energy position, the spectral shape, and the width are insensitive to changes of temperature in the 12–300 K range, making the TiO2:Eu3+ a very attractive system for technological applications.

The Role of Silicon Oxide Layers in Luminescence of Ensembles of Silicon Quantum Dots**The project supported by National Natural Science Foundation of China, the NMS-6 of China, National Science Foundation(DMR9803005) and INT 0001313 of USA

Based on the quantum confinement-luminescence center model, to ensembles of spherical silicon nanocrystals (nc-Si) containing two kinds of luminescence centers (LCs) in the layers surrounding the nc-Si, the relationship between the photoluminescence (PL) and the thickness of the layer is studied with the excitation energy flux density as a parameter. When there is no layer surrounding the nc-Si, the electron-heavy hole pair can only recombine inside the nc-Si, then the PL blueshift with reducing particle sizes roughly accords with the rule predicted by the quantum confinement model of Canham. When there presences a layer, some of the carriers may tunnel into it and recombine outside the nc-Si at the LCs to emit visible light. The thicker the layer is, the higher the radiative recombination rate occurred outside the nc-Si will be. When the central scale of the nc-Si is much smaller than the critical scale, the radiative recombination rate outside the nc-Si dominates, and visible PL will be possible for some nc-Si samples with big average radius, greater than 4 nm, for example. When there is only one kind of LC in the layer, the PL peak position does not shift with reducing particle sizes. All these conclusions are in accord with the experimental results. When there are two or more kinds of LCs in the layer, the PL peak position energy and intensity swing with reducing particle sizes.

How precisely can the energy of a marker peak in an RBS spectrum be determined?

We studied, how precisely the position of a marker peak in an Rutherford backscattering (RBS) spectrum can be determined. For that purpose, we chose the following approach. Repeatedly, the primary energy of a proton beam was set to one of three selected energies (100, 350 and 700 keV), RBS spectra of a thin Pt marker (on carbon) were measured by a conventional PIPS detector, and the energy position of the marker peak was determined by a least squares fit. During the measurement, the electronic system was kept at a constant temperature within ±0.1°C in order to avoid any signal drift. From a statistical evaluation of the peak position and of the actual primary energy information on the precision of both, energy setting and RBS evaluation was obtained. A relative uncertainty of 4×10 −4 for the energy setting, and an uncertainty of ≈1% of the detector FWHM for the evaluation of the marker position are found.

Effects of Hydrogen Dilution on a-Si:H and its Solar Cells Studied by Raman and Photoluminescence Spectroscopy

ABSTRACTa-Si:H films and their n-i-p solar cells were prepared using plasma-enhanced CVD. The samples were prepared with no-, low-, standard, and high-H dilution. Raman and photoluminescence (PL) were used to characterize the i-layer. The main results are (a) Raman shows typical a-Si:H mode except for a c-Si peak in the 450 nm-thick film with high-H dilution, and (b) PL shows two regimes. (I) Below the onset of microcrystallinity characterized by x-ray diffraction, a blue-shift of the 1.4 eV PL peak energy and a decrease of the band width occur. (II) Above the onset of microcrystallinity, the PL efficiency decreases by a factor of 4-5, and the PL peak energy is red-shifted toward 1.2 eV as the μc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to-microcrystalline structural transition in thin film solar cells.