Blueshift Of Peak Energy Research Articles

Comparison of the thermal interdiffusion phenomena in InGaAs/GaAs and InGaAs/AlGaAs strained heterostructures

The effect of 5 min high-temperature thermal annealing on InGaAs/GaAs strained superlattices and InGaAs/AlGaAs PHEMT structures grown by molecular beam epitaxy was studied using the Hall measurements, photoluminescence spectroscopy, and high-resolution X-ray diffraction. InGaAs/GaAs superlattices are shown to undergo photoluminescence peak energy blueshift after 700 °C annealing and distinguishable heterointerface roughening after 800 °C annealing. The magnitude of quantum well smoothing, caused by annealing induced III-group atom interdiffusion, was estimated experimentally using secondary ion mass spectrometry and X-ray reflectivity, and calculated from the modelled electron energy spectra in the diffused wells. InGaAs/AlGaAs PHEMTs appear to be more sensitive to annealing, demonstrating optical and structural changes of a similar nature to InGaAs/GaAs heterostructures, as well as transport properties degradation, at a wider range of annealing temperatures starting 500 °C.

Tailoring of stoichiometry and band-tail emission in PLD a-SiC thin films by varying He deposition pressure

The present work focuses on the effect of He pressure (vacuum, 10−2–5 mbar) on structural and optical properties of amorphous silicon carbide (a-SiC) thin films, deposited by pulsed laser deposition (PLD) technique onto fused silica. A stoichiometric change occurs in PLD a-SiC film, with increasing He deposition pressure, transforming a near stoichiometric SiC material to C-rich SiC films till 1 mbar of He pressure followed by drastic decrease in carbon (C) content at 5 mbar. The optical band gap of the films estimated using Tauc plot from UV–Vis–NIR transmittance spectra showed initial increase from 2.09 to 2.61 eV with increasing He pressure from 10−6 to 1 mbar, then decrease to 2.11 eV at 5 mbar. A broad band photoluminescence (PL) peak featured in each of the a-SiC films at room temperature. The PL spectra exhibited blue shift in peak energy from 1.8 to 2.2 eV with increase in atomic percentage of C from 49.4 to 62.4%. The origin of the PL exhibited in these films is clearly due to band tail emissions from amorphous SiC structures which has been explained by the observed variation of PL intensity and corresponding FWHM with band gap as well as Urbach energy.

Interaction between Water and Alkali Metal Ions and Its Temperature Dependence Revealed by Oxygen K-Edge X-ray Absorption Spectroscopy.

Interaction between water molecules and alkali metal ions in aqueous salt solutions has been studied by the oxygen K-edge soft X-ray absorption spectroscopy (XAS) in transmission mode. In the measurement of several alkali halide aqueous solutions with different alkali chlorides (Li, Na, and K) and different sodium halides (Cl, Br, and I), the pre-edge component arising from the hydration water molecules shows a blue shift in peak energy as strongly depending on cations but not on anions. In the temperature dependent measurement, the pre-edge component arising from water molecules beyond the first hydration shell shows the same behavior as that of pure liquid water. On the other hand, the pre-edge component arising from water molecules in the first hydration shell of Li+ ions is not evidently dependent on the temperature, indicating that the hydration water molecules are more strongly bound with Li+ ions than the other water molecules. These experimental results are supported by the results of radial distribution functions of the first hydration shell and their temperature dependence, evaluated by molecular dynamics simulations.

Advantages of InGaN/GaN multiple quantum wells with two-step grown low temperature GaN cap layers

Two-step grown low temperature GaN cap layers (LT-cap) are employed to improve the optical and structural properties of InGaN/GaN multiple quantum wells (MQWs). The first LT-cap layer is grown in nitrogen atmosphere, while a small hydrogen flow is added to the carrier gas during the growth of the second LT-cap layer. High-resolution X-ray diffraction results indicate that the two-step growth method can improve the interface quality of MQWs. Room temperature photoluminescence (PL) tests show about two-fold enhancement in integrated PL intensity, only 25 meV blue-shift in peak energy and almost unchanged line width. On the basis of temperature-dependent PL characteristics analysis, it is concluded that the first and the second LT-cap layer play a different role during the growth of MQWs. The first LT-cap layer acts as a protective layer, which protects quantum well from serious indium loss and interface roughening resulting from the hydrogen over-etching. The hydrogen gas employed in the second LT-cap layer is in favor of reducing defect density and indium segregation. Consequently, interface/surface and optical properties are improved by adopting the two-step growth method.

Temperature dependent electroluminescence from all-Si-nanocrystal p-i-n diodes grown on dielectric substrates

In this work, we demonstrate for the first time the electroluminescence (EL) from a mesa isolated p-i-n diode based on silicon nanocrystals (Si NCs) embedded in a SiO2 matrix fabricated on a dielectric substrate. The structure fabricated on a dielectric substrate ensures that the EL signal originates entirely from the Si NC material. A small offset between the EL (1.28 eV) and photoluminescence (PL) (1.33 eV) peak energies has been observed at room temperature. We attribute this discrepancy to the different subset of light-emitting Si NCs in EL and PL. A model classifying Si NCs into connected NCs and isolated NCs is proposed. Atom probe tomography is employed to visualize the existence of isolated NCs and connected NCs. This model has been further studied using temperature dependent EL and PL, where a blue-shift of peak energy is observed as the temperature is increased. The blue-shift is attributed to the temperature dependent transport between the two subsets of NCs and the quenching of the PL emission from the connected NCs at higher temperatures.

Coexistence of type-I and type-II band alignments in In0.46Al0.54As/Ga0.46Al0.54As self-assembled quantum dots

Optical properties of In0.46Al0.54As/Ga0.46Al0.54As quantum dots (QDs) have been investigated by photoluminescence (PL). At a low temperature of 8 K, the PL peak energy blue-shifts 44 meV and the linewidth broadens by 21 meV as the excitation intensity increases by four orders of magnitude. As the temperature increases, the QD spectra demonstrate a fast redshift and narrowing from ∼35 K. These observations have been explained by the type-II nature of QDs and the lateral carrier transfer due to electronic coupling between neighboring QDs. A special double exponential decay behavior indicates the coexistence of type-I and type-II band alignment in this QD sample.

Structural and optical analyses of AlxGa1−xN thin films grown by metal organic chemical vapor deposition

A series of AlxGa1−xN thin films with x = 0.20–0.60 were grown by metal organic chemical vapor deposition (MOCVD) on sapphire (0001) substrate using AlN buffer layer. High resolution X-ray diffraction (HRXRD) was performed for (0002), (0004), and (0006) reflections to investigate the threading dislocation density in variation with Al composition by X-ray analysis technique; Williamson–Hall (WH) plot. A symmetric high resolution 2θ–ω scans exhibit high crystal quality for all the AlGaN samples. A room temperature deep ultraviolet (DUV) photoluminescence (PL) spectroscopy (excitation at 248 nm) has also been employed to investigate the effect of various Al compositions on crystal structure of the thin film layers. It was observed that the band edge transition peak energy blueshifts from 3.87 eV for x = 0.23 to 4.55 eV for x = 0.47. In addition to the band edge transition, each spectrum also shows deep impurity transitions.

Growth mechanism and optical properties of InGaAs/GaAsSb Su-perlattice structures

In this paper, we report the growth of GaAsSb and its crystalline property under various Sb2/As2 flux ratios and growth temperatures. We simulated the incorporation difference between Sb2 and As2 by using a non-equilibrium thermodynamic model. Our study of GaAsSb growth has successfully yielded, high quality InGaAs/GaAsSb Type II superlattice for which the optical properties were characterized by photoluminescence at different excitation power and temperature. A blue-shift in luminescence peak energy with excitation power was observed and was described by a non-equilibrium carrier density model. We measured and analyzed the dependences of peak energy and integrated intensity on temperature. Two thermal processes were observed from intensity dependent photoluminescence measurements.

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.

Bismuth-induced effects on optical, lattice vibrational, and structural properties of bulk GaAsBi alloys

Bulk GaAs1 - xBix/GaAs alloys with various bismuth compositions are studied using power- and temperature-dependent photoluminescence (PL), Raman scattering, and atomic force microscopy (AFM). PL measurements exhibit that the bandgap of the alloy decreases with increasing bismuth composition. Moreover, PL peak energy and PL characteristic are found to be excitation intensity dependent. The PL signal is detectable below 150 K at low excitation intensities, but quenches at higher temperatures. As excitation intensity is increased, PL can be observable at room temperature and PL peak energy blueshifts. The quenching temperature of the PL signal tends to shift to higher temperatures with increasing bismuth composition, giving rise to an increase in Bi-related localization energy of disorders. The composition dependence of the PL is also found to be power dependent, changing from about 63 to 87 meV/Bi% as excitation intensity is increased. In addition, S-shaped temperature dependence at low excitation intensities is observed, a well-known signature of localized levels above valence band. Applying Varshni’s law to the temperature dependence of the PL peak energy, the concentration dependence of Debye temperature (β) and thermal expansion coefficient (α) are determined. AFM observations show that bismuth islands are randomly distributed on the surface and the diameter of the islands tends to increase with increasing bismuth composition. Raman scattering spectra show that incorporation of Bi into GaAs causes a new feature at around 185 cm-1 with slightly increasing Raman intensity as the Bi concentration increases. A broad feature located between 210 and 250 cm-1 is also observed and its intensity increases with increasing Bi content. Furthermore, the forbidden transverse optical (TO) mode becomes more pronounced for the samples with higher bismuth composition, which can be attributed to the effect of Bi-induced disorders on crystal symmetry.PACS78.55Cr 78.55-m 78.20-e 78.30-j

Effects of rapid thermal annealing on the optical properties of InAs/(In)GaAs quantum dots with different areal density

The effects of rapid thermal annealing on the optical properties of InAs/(In)GaAs quantum dots (QDs) with different areal density were investigated by photoluminescence (PL) measurement. The annealing results in PL peak energy blue-shift which strongly depends on QD areal density and capping layer. It is noticeable that low-density QDs and/or InGaAs-capped QDs are more sensitive to the annealing. We attribute the larger energy blue-shift from these samples to enhanced strain-driven diffusion and/or defect-assisted diffusion.

Rapid thermal annealing and photoluminescence of type-II GaSb single monolayer quantum dot stacks

The effects of rapid thermal annealing on the photoluminescence emission obtained from ten-layer stacks of GaSb/GaAs type-II single monolayer quantum dots and Stranski–Krastanow grown quantum rings have been studied and interpreted. Post-growth rapid thermal annealing was performed with proximity capping at temperatures from 550 °C to 800 °C, resulting in an increase in photoluminescence emission intensity and a blue shift in peak energy in both types of stacks, together with changes in the activation energy for thermal quenching. This behaviour originates from Sb–As intermixing and changes in morphology of the nanostructures formed using the two different growth mechanisms.

Impact of substrate nitridation on the photoluminescence and photovoltaic characteristics of GaN grown on p-Si (100) by molecular beam epitaxy

This report focuses on the structural and optical properties of the GaN films grown on p-Si (100) substrates along with photovoltaic characteristics of GaN/p-Si heterojunctions fabricated with substrate nitridation and in absence of substrate nitridation. The high resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), Raman and photoluminescence (PL) spectroscopic studies reveal that the significant enhancement in the structural as well as in the optical properties of GaN epifilms grown with silicon nitride buffer layer when compared with the sample grown without silicon nitride buffer layer. The low temperature PL shows a free excitonic (FX) emission peak at 3.51 eV at the temperature of 5 K with a very narrow line width of 35 meV. Temperature dependent PL spectra follow the Varshni equation well and peak energy blue shifts by ~63 meV from 300 to 5 K. Raman data confirms the strain free nature and reasonably good crystallinity of the films. The GaN/p-Si heterojunctions fabricated without substrate nitridation show a superior photovoltaic performance compared to the devices fabricated in presence of substrate nitridation. The discussions have been carried out on the junction properties. Such single junction devices exhibit a promising fill factor and conversion efficiency of 23.36 and 0.12 %, respectively, under concentrated AM1.5 illumination.

Temperature-dependent efficiency droop behaviors of GaN-based green light-emitting diodes

The efficiency droop behaviors of GaN-based green light-emitting diodes (LEDs) are studied as a function of temperature from 300 K to 480 K. The overall quantum efficiency of the green LEDs is found to degrade as temperature increases, which is mainly caused by activation of new non-radiative recombination centers within the LED active layer. Meanwhile, the external quantum efficiency of the green LEDs starts to decrease at low injection current level (< 1 A/cm2) with a temperature-insensitive peak-efficiency-current. In contrast, the peak-efficiency-current of a control GaN-based blue LED shows continuous up-shift at higher temperatures. Around the onset point of efficiency droop, the electroluminescence spectra of the green LEDs also exhibit a monotonic blue-shift of peak energy and a reduction of full width at half maximum as injection current increases. Carrier delocalization is believed to play an important role in causing the efficiency droop in GaN-based green LEDs.

Optical properties of coupled three-dimensional Ge quantum dot crystals

We report on optical properties of coupled three-dimensional (3D) Ge quantum dot crystals (QDCs). With increasing the vertical periodic number of the QDCs, the photoluminescence (PL) spectral linewidth decreased exponentially, and so did the peak energy blueshift caused by increasing excitation power, which are attributed to the electronic coupling and thus the formation of miniband. In the PL spectra, the relative intensity of the transverse-optical (TO) phonon replica also decreases with increasing the vertical periodic number, which is attributed to the increased Brillouin-zone folding effect in vertical direction and therewith the relaxation of indirect transition nature of exciton recombination. Besides, the optical reflectivity at the interband transition energy was much more reduced for the QDCs than for the in-plane disordered QDs grown with the same parameters, indicating a higher interband absorption of the QDCs due to the miniband formation.

Optical properties of wurtzite/zinc-blende heterostructures in GaN nanowires

The optical and structural properties of wurtzite GaN nanowires containing zinc-blende GaN inclusions of different thicknesses are investigated. Micro-photoluminescence spectra of single nanowires exhibit a series of narrow emission peaks with linewidth as low as 0.8 meV in the interval 3.1–3.42 eV. The peak energy blue-shifts with increasing excitation power following a ∼I1/3 law due to the progressive band filling and to the screening of the internal field. The quantum confinement in these type-II crystal phase heterostructures was simulated in the framework of a one-dimensional effective mass model, accounting for the internal electrical polarization of the wurtzite GaN. The predicted transition energies are in good agreement with the energy statistics realized on more than 30 single nanowire emission spectra.

Enhanced internal quantum efficiency of an InGaN/GaN quantum well as a function of silver thickness due to surface plasmon coupling

AbstractExcitation‐power‐dependent photoluminescence (PL) measurements have been carried out on a single InGaN quantum well structure with a different thickness of silver on its surface. This allows for investigation of surface plasmon (SP) coupling enhanced internal quantum efficiency as a function of the silver thickness. A maximal enhancement with a factor of up to 25 times in PL intensity has been observed on the sample with a silver film of 100 nm, compared with the as‐grown sample. A blue shift in peak energy has been observed as a function of silver thickness. Atomic force microscopy has been employed to examine the change in morphology of the silver films with different thickness. Finally an estimation of the internal quantum efficiency is made, showing ∼10.8 times enhancement on the sample with the 100 nm silver, compared to the as‐grown sample. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Correlation between luminescence and compositional striations in InGaN layers grown on miscut GaN substrates

The influence of the miscut angle of GaN substrate on compositional and optical properties of InxGa1−xN epilayers (0.05&amp;lt;x&amp;lt;0.1) was examined using x-ray diffraction, photoluminescence (PL), cathodoluminescence, and Z-contrast scanning electron microscopy. We show that single atomic steps bunch during growth of InGaN and form macrosteps. Indium is incorporated differently at treads and risers of these macrosteps, which causes the layer to decompose and induces the formation of compositional growth striations. Since the growth step density increases with growing miscut angle of the substrate, the average indium concentration decreases and the average PL peak energy blueshifts and broadens with increasing miscut angle. The presented work enables understanding on microscopic scale effects related to the inhomogeneous distribution of indium in InGaN layers on miscut substrates, which is significant from the point of view of optoelectronic applications.

Photoluminescence spectroscopy of nearly defect-free InN microcrystals exhibiting nondegenerate semiconductor behaviors

Nearly defect-free InN microcrystals grown on Si(111) substrates have been realized by plasma-assisted molecular beam epitaxy. High-resolution transmission electron microscope images reveal that these microcrystals exhibit single-crystalline wurtzite structure. Low temperature photoluminescence (PL) shows a strong emission peak at 0.679eV with a very narrow linewidth of 17meV at excitation power density of 3.4W∕cm2. Temperature-dependent PL spectra follow the Varshni equation well, and peak energy blueshifts by ∼45meV from 300to15K. Power-density-dependent PL spectroscopy manifests direct near-band-edge transition. A low carrier density of 3×1017cm−3 has been estimated from PL empirical relation, which is close to the critical carrier density of the Mott transition of 2×1017cm−3.

Strong optical nonlinearity in strain-induced laterally ordered In0.4Ga0.6As quantum wires on GaAs (311)A substrate

Strain-induced laterally ordered In0.4Ga0.6As on (311)A GaAs template quantum wires have been fabricated and identified with cross-section transmission electron microscopy technique to be of average length ∼1μm, and on average width and height of 23 and 2nm, respectively, under InGaAs coverage of six monolayers. The photoluminescence spectrum of a sample demonstrates unusually strong optical nonlinearity even at moderate excitation densities. The excitonic peak energy blueshifts by ∼25meV without essential contribution of the quantum wire excited states at elevating excitation density. Strong decrease of the polarization anisotropy and increase of the energy of excitonic photoluminescence are attributed to a combined action of the phase-space filling effects and the screening of the internal piezoelectric field by free carriers.