Shift Of Photoluminescence Peak Energy Research Articles

Excitonic States of GaN/AlGaN Quantum Well Structures under High Density of Excitation

Photoluminescence of GaN/AlGaN quantum well structures was studied under high intensity of excitation. The blue shift of photoluminescence peak energy was observed when excitation intensity increased. The blue shift was most prominent for the wide wells, becoming smaller with decreasing the well width. We attribute the observed effect to the screening of the built-in electric field by photoexcited carriers, which leads to reducing of the initial red shift caused by the quantum confined Stark effect. The variation of exciton binding energy with carrier concentration also makes the contribution to the blue shift. The theoretical calculations of the blue shift were performed and compared with experimental data.

Time-Resolved Studies of InGaN/GaN Quantum Dots

We report on structural and optical properties of InGaN inclusions in a GaN matrix. High-resolution transmission electron microscopy (HRTEM) studies and HRTEM image processing indicate the spontaneous formation of ultra-dense arrays of InGaN disc-like nano-inclusions. Non-resonant pulsed excitation results in a broad photoluminescence (PL) peak, which shifts towards longer wavelengths with time, demonstrating also an increase in decay time at the peak maximum. In contrast to that, resonant excitation into the non-resonant PL maximum results in an evolution of a sharp resonant PL peak, having a spectral shape defined by the excitation laser pulse and a radiative decay time close to that revealed for PL under non-resonant excitation. No spectral diffusion or PL peak energy shift caused by reduced piezoelectric screening at lower overall concentrations of non-equilibrium carriers was found. These observations give a clear proof of the quantum dot nature of PL in our InGaN/GaN samples.

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.

Temperature Dependence of Transmission and Emission Spectra in MOCVD-Grown AlGaN Ternary Alloys

Optical absorption, emission, and carrier recombination characteristics of Al x Ga 1-x N epilayers (x = 0.17, 0.26, and 0.33) were systematically studied by means of transmission, photoluminescence (PL), and time-resolved PL spectroscopy, respectively. A typical energy-gap shrinkage behaviour with temperature was confirmed for all the Al x Ga 1-x N epilayers by transmission measurements. However, we observed anomalous PL temperature dependences such as a decrease-increase-decrease behavior of the PL peak energy shift and an increase-decrease-increase behavior of the spectral width broadening with increasing temperature. The anomalous temperature-induced emission shift is attributed to energy tail states due to alloy potential inhomogeneities in the Al x Ga 1-x N epilayers with large Al content.

Temperature Dependence of Transmission and Emission Spectra in MOCVD-Grown AlGaN Ternary Alloys

Optical absorption, emission, and carrier recombination characteristics of Al x Ga 1-x N epilayers (x = 0.17, 0.26, and 0.33) were systematically studied by means of transmission, photoluminescence (PL), and time-resolved PL spectroscopy, respectively. A typical energy-gap shrinkage behaviour with temperature was confirmed for all the Al x Ga 1-x N epilayers by transmission measurements. However, we observed anomalous PL temperature dependences such as a decrease-increase-decrease behavior of the PL peak energy shift and an increase-decrease-increase behavior of the spectral width broadening with increasing temperature. The anomalous temperature-induced emission shift is attributed to energy tail states due to alloy potential inhomogeneities in the Al x Ga 1-x N epilayers with large Al content.

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.

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.

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.

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.

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.

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.

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.

Dynamics of anomalous optical transitions inAlxGa1−xNalloys

We present a comprehensive study of the optical characteristics of ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}\mathrm{}$ epilayers $(0l~xl~0.6)$ by means of photoluminescence (PL), PL excitation, and time-resolved PL spectroscopy. For ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}\mathrm{}$ with large Al content, we observed an anomalous PL temperature dependence: (i) an ``S-shaped'' PL peak energy shift (decrease-increase-decrease) and (ii) an ``inverted S-shaped'' spectral width broadening (increase-decrease-increase) with increasing temperature. We observed that the thermal decrease in integrated PL intensity was suppressed and the effective lifetime was enhanced in the temperature region showing the anomalous temperature-induced emission behavior, reflecting superior luminescence efficiency by suppressing nonradiative processes. All these features were enhanced as the Al mole fraction was increased. From these results, the anomalous temperature-induced emission shift is attributed to energy tail states due to alloy potential inhomogeneities in the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}\mathrm{}$ epilayers with large Al content.

Correlation of Photoluminescence and Bandgap Energies with Nanocrystal Sizes in Porous Silicon

We have measured the crystallite sizes, the bandgap energies, and the photoluminescence (PL) energies in porous silicon (PSi) samples having a wide range of porosities and kept in different ambient conditions. The dependence of the bandgap energy on the crystallite size agrees with theory. For PSi samples exposed to air and containing crystallites smaller than 5 nm, the PL intensity increases by several orders of magnitude and the PL peak energy shifts from the near infrared to the red, in agreement with the quantum confinement model for the PL. For crystallites smaller than 3 nm, there is a Stokes shift between the excitonic bandgap and PL energies, which increases to several hundreds of meV for sizes ∼2 nm, indicating that, in PSi exposed to air, the PL is not due to free excitons. Before exposure to air, very high porosity PSi samples emit at shorter wavelengths than after exposure to air, suggesting that the Stokes shift depends on the surface chemistry.

Dynamics of Anomalous Temperature-Induced Emission Shift in MOCVD-grown (Al, In)GaN Thin Films

We present a comprehensive study of the optical characteristics of (Al, In)GaN epilayers measured by photoluminescence (PL), integrated PL intensity, and time-resolved PL spectroscopy. For not only InGaN, but also AlGaN epilayers with large Al content, we observed an anomalous PL temperature dependence: (i) an “S-shaped” PL peak energy shift (decrease-increase-decrease) and (ii) an “inverted S-shaped” full width at half maximum (FWHM) change (increase-decrease-increase) with increasing temperature. Based on time-resolved PL, the S shape (inverted S shape) of the PL peak position (FWHM) as a function of temperature, and the much smaller PL intensity decrease in the temperature range showing the anomalous emission behavior, we conclude that strong localization of carriers occurs in InGaN and even in AlGaN with rather high Al content. We observed that the following increase with increasing Al content in AlGaN epilayers: (i) a Stokes shift between the PL peak energy and the absorption edge, (ii) a redshift of the emission with decay time, (iii) the deviations of the PL peak energy, FWHM, and PL intensity from their typical temperature dependence, and (iv) the corresponding temperature range of the anomalous emission behavior. This indicates that the band-gap fluctuation responsible for these characteristics is due to energy tail states caused by non-random inhomogeneous alloy potential variations enhanced with increasing Al content.

Gas-source molecular beam epitaxial growth and thermal annealing of GaInNAs/GaAs quantum wells

In this paper, we report the growth of GaInNAs/GaAs quantum wells (QWs) by gas-source molecular beam epitaxy with a radio-frequency (RF) nitrogen radical beam source. The influence of growth temperature and N 2 flow rate on GaInNAs/GaAs QWs is examined. At the optimal growth condition, room-temperature photoluminescence (PL) at 1.3 μm is obtained for an as-grown Ga 0.7In 0.3N 0.02As 0.98/GaAs QW. The satellite peaks of the X-ray rocking curves are broader for Ga 0.7In 0.3N 0.03As 0.97/GaAs QWs than for N-free samples, presumably due to composition fluctuations and rougher interfaces, as confirmed by cross-sectional transmission electron microscopy (XTEM) images. Power-dependent PL measurements show that a higher incident laser power causes a larger PL peak blue shift for Ga 0.7In 0.3N 0.03As 0.97/GaAs QWs than Ga 0.7In 0.3As/GaAs QWs. Rapid thermal annealing (RTA) improves interface morphology and PL intensity, reduces the PL peak width, but causes a blue shift in PL peak energy. The optimal annealing temperature is higher for GaInNAs/GaAs QWs than GaInAs/GaAs QWs, indicating the former is thermally more stable. By using RTA, an improved 1.3 μm RT PL emission of a GaInNAs/GaAs QW with stronger intensity and narrower linewidth is obtained successfully.

Luminescent studies of alloy Zn xCd 1− xSe quantum dots grown on ZnSe by metalorganic chemical vapor-phase deposition

Zn x Cd 1− x Se alloy quantum dots (QDs) with x in the range 0–0.39 are grown by metalorganic chemical vapor-phase deposition on ZnSe. Cathodoluminescence (CL) and photoluminescence (PL) were used to study these self-assembled quantum dots. CL imaging and spectra show that clusters of QDs are efficient luminescent sites. A large red shift of the low-temperature PL peak energy of QDs, despite an increase in Zn, is attributed to a considerable increase in their size. This increase in size is consistent with the results of recent theoretical models. In forming the self-assembled QDs, mismatch strain is regarded as the fundamental driving force. When the strain changes, through a change in the composition of Zn x Cd 1− x Se, QDs of a different size are obtained. A decrease in size, in turn, results in stronger quantum confinement effects. The size of the QDs is very sensitive to small changes of strain. Even a minute reduction in the zinc content of the QDs, achieved through a lengthening of growth interruptions, produces an observable blue shift of luminescence, as a result of the strengthening of the quantum confinement energy.

Effects of ion implantation and thermal annealing on the photoluminescence in amorphous silicon nitride

When amorphous silicon nitride films are irradiated by a KrF excimer laser, they exhibit broad photoluminescence (PL) centered around 2.4 eV. The PL intensity gradually decreases and the PL peak energy shifts to a lower energy with an increase of the implanted dose of Ar+ ions. This means that the PL consists of two components with peak energies at 2.66 and 2.15 eV and that implantation-induced defects such as vacancies are not the PL centers. The PL intensity is found to decrease if the film was thermally annealed, while the decreased PL intensity of the ion-implanted film recovers by the thermal annealing. Based on these results, it is concluded that the defects generated by hydrogen release or bond breaking act as nonradiative recombination centers that quench the PL.

Dynamics of Anomalous Temperature-Induced Emission Shift in MOCVD-grown (Al, In)GaN Thin Films

AbstractWe present a comprehensive study of the optical characteristics of (Al, In)GaN epilayers measured by photoluminescence (PL), integrated PL intensity, and time-resolved PL spectroscopy. For not only InGaN, but also AlGaN epilayers with large Al content, we observed an anomalous PL temperature dependence: (i) an “S-shaped” PL peak energy shift (decrease-increase-decrease) and (ii) an “inverted S-shaped” full width at half maximum (FWHM) change (increase-decrease-increase) with increasing temperature. Based on time-resolved PL, the S shape (inverted S shape) of the PL peak position (FWHM) as a function of temperature, and the much smaller PL intensity decrease in the temperature range showing the anomalous emission behavior, we conclude that strong localization of carriers occurs in InGaN and even in AlGaN with rather high Al content. We observed that the following increase with increasing Al content in AlGaN epilayers: (i) a Stokes shift between the PL peak energy and the absorption edge, (ii) a redshift of the emission with decay time, (iii) the deviations of the PL peak energy, FWHM, and PL intensity from their typical temperature dependence, and (iv) the corresponding temperature range of the anomalous emission behavior. This indicates that the band-gap fluctuation responsible for these characteristics is due to energy tail states caused by non-random inhomogeneous alloy potential variations enhanced with increasing Al content.