Peak Energy Shift Research Articles

Photoluminescence Study on Temperature Dependence of Band Gap Energy of GaAsN Alloys

We have studied the temperature dependence of photoluminescence (PL) spectra of GaAsN alloys. The PL peak energy shift due to the temperature change decreases with increasing N concentration of GaAsN alloys. The localized state emission partly contributes to the decrease in the PL peak energy shift. In addition, the small PL peak energy shift at high temperatures is due to the reduction in the temperature dependence of the band gap energy. From the analysis using the Bose-Einstein statistical expression, the average phonon energy is much larger than that expected from the linear interpolation between GaAs and GaN, indicating that the interaction between electrons and phonons localized at N atoms plays an important role in the reduction of the temperature dependence of the band gap energy of GaAsN alloys.

Characteristic photoluminescence properties of Si nanocrystals in SiO2 fabricated by ion implantation and annealing

We have measured the implantation dose dependence as well as the oxidation effect of the photoluminescence behaviour of Si nanocrystals in SiO2 layers fabricated by ion implantation and a subsequent annealing step. After annealing at high temperature, a characteristic photoluminescence band, peaking just below the 1.7 eV was observed. The peak energy and the intensity of the photoluminescence were found to be affected by the dose of implanted Si ions, but to be independent of annealing time and excitation photon energy. We also present experimental results of an oxidation induced continuous peak energy shift of the photoluminescence peak up to around 1.8 eV. This peak energy, however, was found to return to its previous position with re-annealing. These results indicate that whilst the excitation photons are absorbed by Si nanocrystals, the emission is not simply due to electron–hole recombination inside the Si nanocrystals, but is related to the presence of defects, most likely located at the interface between the Si nanocrystals and the SiO2, for which the characteristic energy levels are affected by cluster–cluster interactions or the roughness of the interface.

UV and green photoluminescence from spark-processed zinc

Photoluminescence (PL) and scanning electron microscopy studies (SEM) have been performed on spark-processed zinc (sp-Zn). Furthermore, temperature-dependent PL spectra, decay dynamics, and X-ray photoelectron spectroscopy (XPS) have been taken. The area just below the spark-processing electrode is highly fragmented and heavily oxidized. The PL spectrum of sp-Zn peaks at 3.3 eV, that is, in the UV spectral range, and has the characteristics of the band-gap emission of bulk ZnO. Further, the surrounding area of sp-Zn displays a green PL having a maximum intensity near 2.3 eV. The green luminescence band of sp-Zn shows an unusual red shift in the PL peak energy with decreasing temperature. The possible mechanisms causing the observed PL properties of sp-Zn are discussed.

Effects of rapid thermal annealing and SiO2 encapsulation on GaNAs/GaAs single quantum wells grown by plasma-assisted molecular-beam epitaxy

Effects of rapid thermal annealing and SiO2 encapsulation on GaNAs/GaAs single quantum wells grown by plasma-assisted molecular-beam epitaxy were studied. Photoluminescence measurements on a series of samples with different well widths and N compositions were used to evaluate the effects. The intermixing of GaNAs and GaAs layers was clearly enhanced by the presence of a SiO2-cap layer. However, it was strongly dependent on the N composition. After annealing at 900 °C for 30 s, a blueshift up to 62 meV was observed for the SiO2-capped region of the sample with N composition of 1.5%, whereas only a small blueshift of 26 meV was exhibited for the bare region. For the sample with the N composition of 3.1%, nearly identical photoluminescence peak energy shift for both the SiO2-capped region and the bare region was observed. It is suggested that the enhanced intermixing is mainly dominated by SiO2-capped layer induced defects-assisted diffusion for the sample with smaller N composition, while with increasing N composition, the diffusion assisted by interior defects become predominant.

InGaAs/GaAs quantum wells and quantum dots on (111)B orientation

We report a magneto optical characterization of InGaAs/GaAs quantum well (QW) and quantum dot (QD) structures grown on (111)B GaAs substrates. The photoluminescence (PL) peak shift under high excitation intensity is used to distinguish QW from QD structures together with atomic force microscopy (AFM) imaging. The binding energy in (111)InGaAs/GaAs QW is about 5 meV. The extent of the wave function obtained from the diamagnetic shift of the PL peak energy is consistent with the result calculated by the k.p method. The InGaAs/GaAs QD lateral confinement energy and the dot size are also estimated from the diamagnetic shift of the PL lines. The lateral confinement energy is estimated as 7 meV. The mean radius of the InGaAs QD is about 14 nm and the dot height is about 6 nm, which is in good agreement with the results as revealed in AFM imaging.

Large Red Shift of PL Peak Energy in High Growth Rate a-Si:H Prepared by Hot-Wire CVD

ABSTRACTWe characterized the electronic states and microstructure of high-growth-rate a-Si:H films by employing photoluminescence (PL) and Raman spectroscopies. The growth rate was from 50 to 115 Å/s compared to the standard rate of less than 10 Å/s. For the high-growth-rate a-Si:H films, we observed typical a-Si:H features in Raman but new features in PL. The new PL features are: a) the PL peak energy is as low as ∼1.15 eV compared to the standard ∼1.4 eV at 80 K; and b) the total intensity is more than one order of magnitude higher then the standard. We suggest that the nano-scale microstructure may be responsible for the anomalous PL features.

Near-field Scanning Optical Microscopy and Electron Microprobe Microscopy Investigations of Immiscibility Effects in Indium Gallium Phosphide Grown by Liquid Phase Epitaxy

ABSTRACTWe have used Near-field Scanning Optical Microscopy (NSOM) and Electron Probe Microanalysis (EPMA) to study the topographic and microscopic optical properties of indium gallium phosphide (In1−xGaxP) samples grown by Liquid Phase Epitaxy (LPE) on gallium arsenide substrates. Photoluminescence (PL) intensity images gathered using NSOM exhibit strong, highly localized variations in the optical properties of these samples that are seen to occur roughly in registry with the surface topography. Shifts in the PL peak position (by 27 meV) occur across highly mismatched samples with high In content, whereas no shifts were seen for In1−xGaxP films with a nearly lattice matched composition. Compositional fluctuations lead to these PL peak energy shifts, measured by NSOM with a resolution of 250 nm. 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.

Mechanism for the Correlated Swinging of Photoluminescence Intensity and Peak Energy Shift with Sizes of Nanoscale Si Particles**The project supported by National Natural Science Foundation of China

Based on the quantum confinement-luminescence center model, we focus on the relationship between the photoluminescence (PL) spectra and sizes of nanoscale silicon particles.We find that when there are two kinds of luminescence centers (LC) in the oxide layer surrounding the silicon particles, both the integrated PL intensity and the spectral peak position swing with reducing the sizes of silicon particles, which is different from the monotonous blueshift of peak position predicted by quantum confinement model. By changing the concentrations of LC we find the correlation between the spectral peak position and the integrated PL intensity when the size of silicon particles is reduced. The effects of other parameters such as the half width of size distribution and the position of photon emission levels of LC are also found to be important.

Growth of BGaN/AlGaN multi-quantum-well structure by metalorganic vapor phase epitaxy

We report on the epitaxial growth and the resulting optical characteristics of B 0.01Ga 0.99N/Al 0.12Ga 0.88N multi-quantum wells (MQW) grown by metalorganic vapor-phase epitaxy (MOVPE). The room-temperature photoluminescence (PL) of B 0.01Ga 0.99N/Al 0.12Ga 0.88N MQWs was observed at 360.4 nm. The calculated PL energy shift from GaN/6H-SiC was 50 meV. The PL peak energy shift resulted from a quantized energy shift (10 meV), the effect of boron composition (16 meV), and the effect of a smaller tensile residual strain along the a-axis (the effect of the smaller compressive residual strain along the c-axis of ∼0.17%) (24 meV). The emission intensity from the B 0.01Ga 0.99N/Al 0.12Ga 0.88N MQW structure at room temperature was three-fold higher than that of the B 0.01Ga 0.99N grown directly on a 6H-SiC substrate because of the tensile strain along the a-axis in the B 0.01Ga 0.99N/Al 0.12Ga 0.88N MQW structure.

High-quality thin film GaAs bonded to Si using SeS 2 — A new approach for high-efficiency tandem solar cells

Epitaxial GaAs thin-films grown by metal organic chemical vapour deposition technique have been successfully bonded to Si substrates and lifted-off from the original GaAs substrate using the epitaxial lift-off (ELO) technique. The GaAs thin films transplanted in this manner to Si substrate were examined for its quality by photoluminescence, double-crystal X-ray diffraction and Raman scattering studies. The low FWHM, no peak energy and peak frequency shift were associated with the high crystalline quality of the bonded films. We have obtained a minority-carrier lifetime of 9.09 ns in a double heterostructure bonded to Si, which is nearly three times higher than heteroepitaxial GaAs on Si. The electron transport across the bonded interface was studied by observing the current–voltage characteristic. The electrical behaviour was improved by applying a voltage greater than 10 V, necessary for current conduction.

Field-enhanced Stokes shifts in strained Si1−yCy/Si(001) quantum wells

Field-enhanced Stokes shifts (FES), i.e. a large downward shift of luminescence peak energy under transverse electric field in excess of quantum confined Stark shift, were observed in strained Si1−yCy/Si quantum wells. The microscopic mechanism of FES is attributed to a carrier heating due to carrier-carrier scattering under transverse electric field, which allows carriers to relax down to lower energy sites by jumping over kinetic barriers in lateral directions in an inhomogeneous potential of fluctuated heterointerfaces.

Magneto-Photoluminescence Study of InGaAs/GaAs Quantum Wells and Quantum Dots Grown on (111)B GaAs Substrate

InGaAs/GaAs quantum well (QW) and quantum dot (QD) structures grown on GaAs (111)B substrates under different growing temperatures are investigated by magneto-photoluminescence (PL) up to 15 T in both Faraday and Voigt configurations. The spatial extents of the carrier wave functions (ECWFs) are deduced from the diamagnetic shift of the PL peak energy. The binding energies of the InGaAs/GaAs QWs are evaluated to be about 5 meV. The QW ECWFs in the growth direction obtained by the diamagnetic shift are consistent with those calculated by the k ·p theory. The heights and radii of the InGaAs/GaAs QDs are also estimated from the ECWFs. In addition, we found that the in-plane ECWFs decreased slightly as the growth temperature was varied from 525 to 450°C. The ECWFs in the growth direction decreased when the growth temperature was varied from 525 to 480°C and then increased as the temperature was decreased to 450°C.

Magneto-photoluminescence study of InGaAs quantum dots fabricated by droplet epitaxy

We have successfully measured magneto-photoluminescence of InGaAs quantum dots (QDs) fabricated by droplet epitaxy with highly dense Ga droplets, termed separated-phase enhance epitaxy with droplets (SPEED). In the low magnetic field region, the PL peak energy shift increases linearly with square of the magnetic field. From the estimated Bohr radii for the QDs, it is found that the size of the QDs in the lateral direction is 2.7-times larger than that in the vertical direction. Moreover, using high magnetic region for estimating the detailed lateral size, the cyclotron radius around 25 T in the QDs becomes equal to the lateral size of the QDs. The cyclotron radius of 5.1 nm at 25 T suggests that the size of the InGaAs QDs in lateral direction might be as small as about 10 nm.

Fabrication of nanometer sized Si dot multilayers and their photoluminescence properties

Silicon nanometer sized dot multilayers have been prepared by repeating low-pressure chemical vapor deposition (LPCVD) for dot formation and thermal oxidation for dot isolation. The multilayers show efficient photoluminescence after annealing at 1000°C, which reduces Si dangling bond density measured by electron spin resonance (ESR). The measured luminescence peak energy shifts from 1.25 to 1.64 eV as the dot height decreases from 15 to <1 nm. The optical bandgap measured by photothermal deflection spectroscopy (PDS) has a blue shift from 2.0 to 2.7 eV when the dot height changes from 6 to <1 nm. The photoluminescence (PL) efficiency for both single-layered and multilayered dots increases by two orders of magnitude as the dot height decreases from 15 to 5 nm and becomes constant for dots smaller than 5 nm. These results indicate that the luminescence is not caused by the band-to-band transition, but caused by the recombination through radiative centers existing in the Si/SiO 2 interface region.

Photoluminescence spectrum of a-Si/SiO 2 and c-Si/SiO 2 quantum wells

We have studied photoluminescence (PL) properties of a-Si- and c-Si-based quantum wells. The PL peak energies of a-Si/SiO 2 and c-Si/SiO 2 quantum wells are blue-shifted from those of bulk a-Si and c-Si. With a decrease in the a-Si well thickness in a-Si/ SiO 2 quantum wells, the PL peak energy shifts to higher energy and the PL lifetime becomes shorter. The well-thickness dependence of the PL peak energy and the PL lifetime are clearly observed in a-Si/SiO 2 quantum wells. The confinement and localization of excitons in Si quantum structures will be discussed.

Excited-state absorption of excitons confined in CuCl quantum dots

We have investigated infrared transient absorption spectra under size-selective excitation of CuCl QDs embedded in NaCl matrices. The transient absorption consists of mainly two decay components. The spectra for the faster one, ascribed to the transition from the 1S-exciton to the 2P states, show a peak energy shift depending on the dot size. The results indicate that the Rydberg energy of the confined exciton has a size dependency. A comparison between the experimental data and a theoretical calculation has been carried out. The slower component does not depend much on the dot size and shows an anomalous long-lived time at low temperature. The properties of this slower component are also discussed.

Anomalous behavior of the high-density excitons and their nonlinear optical response in a quasi-two-dimensional space in a layered crystal BiI 3

Spatial and temporal behavior of excitons heavily excited in a quasi-two-dimensional space of a stacking fault interface in BiI 3 has been studied by space- and time-resolved luminescence measurements. The luminescence spectra show the peak-energy shift due to high-density effects. Under the heavier excitation, an anomalous propagation of excitons is observed on the temporal profiles of the luminescence intensity and the peak-shift values at each detecting position. From a comparison with a model calculation, it is seen that transport mechanisms of high-density excitons are basically explained by both diffusion and polariton transport suffering the scattering, while there exist high-density exciton–polaritons which propagate with the polariton group velocity. These results are discussed in terms of a collective motion and coherent transport of excitons.

Photoluminescence dynamics of amorphous Si/SiO2 quantum wells

We have studied photoluminescence (PL) spectrum and dynamics of amorphous silicon (a-Si) based quantum wells at low temperatures. In a-Si/SiO2 quantum-well samples with thin a-Si layers, the PL spectra appear in the visible spectral region. With a decrease of the a-Si well thickness, the PL peak energy shifts to higher energy and the PL lifetime becomes shorter. The well-thickness and temperature dependence of the PL lifetime show that the nonradiative recombination of carriers occurs at the a-Si/SiO2 interface and the lifetime is determined by the energy relaxation process in the a-Si well and the carrier diffusion to the interface. The quantum confinement and localization of carriers in a-Si/SiO2 quantum-well structures will be discussed.

The effect of electric field on the photoluminescence and absorption spectra of porous silicon

The effect of electric field on the photoluminescence and absorption spectra of porous silicon were investigated. An electric field applied perpendicular to the porous silicon growth direction shows a red shift in the photoluminescence peak energy and quenching of the photoluminescence intensity, whereas an electric field applied parallel to the growth direction shows photoluminescence quenching, but no clear indication of red shift. The absorption spectra also show a red shift of the absorption edge together with an increase in the absorption coefficient when an electric is field applied perpendicular to the porous silicon growth direction. The results are qualitatively similar to the quantum confined Stark effect, and provide additional support for quantum size effect as the mechanism of light emission in porous silicon.

Characteristic photoluminescence band in Si+-implanted SiO2 grown on Si wafer

A possible mechanism for the photoemission from Si nanocrystals in an amorphous SiO2 matrix fabricated by ion implantation is reported. We have measured the implantation dose dependence as well as the oxidation effect of the photoluminescence behavior of Si nanocrystals in SiO2 layers fabricated by ion implantation and a subsequent annealing step. After annealing, a photoluminescence band, peaking just below the 1.7 eV was observed. The peak energy of the photoluminescence was found to be affected by the dose of implanted Si ions, but to be independent of annealing time and excitation photon energy. We also present experimental results of an oxidation induced continuous peak energy shift of the photoluminescence peak, up to around 1.8 eV. This peak energy, however, was found to return to its previous position with re-annealing. These results indicate that, whilst the excitation photons are absorbed by Si nanocrystals, the emission is not simply due to electron–hole recombination inside the Si nanocrystals, but is related to the presence of defects, most likely located at the interface between the Si nanocrystals and the SiO2, for which the characteristic energy levels are affected by cluster–cluster interactions or the roughness of the interface.