Peak Energy Shift Research Articles

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.

Effect of Thermal Annealing on the Photoluminescence Properties of a GaInNAs/GaAs Single Quantum Well

ABSTRACTWe report on the effect of thermal annealing on the photoluminescence properties of a Ga0.65In0.35N0.02As0.98/GaAs single quantum well. Thermal annealing is shown to decrease the strong nitrogen-induced localization effects observed at low temperatures and to reduce the full width at half maximum of the emission peak. It also induces a strong blue shift of the emission peak energy, which is thought not to arise from an In-Ga interdiffusion alone, as it is much larger than in a nitrogen-free reference single quantum well.

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.

Steady-state and time-resolved photoconductivity measurements of minority carrier lifetime in ZnTe

Minority carrier lifetime in ZnTe has been determined from steady-state and time-resolved photoconductivity (PC) measurements. Three types of single crystal p-ZnTe (EG=2.26 eV) grown by the Bridgman technique were studied: (i) as-grown, before and after hydrogen passivation, (ii) Zn annealed, and (iii) In-doped semi-insulating. Steady-state photoconductivity was studied between 80 and 300 K and showed that for as-grown samples, the lifetime went through a sharp maximum of 4.5×10−7 s at 220 K, decreasing to 2.5×10−8 s at 300 K. For hydrogen passivated samples, the low temperature behavior was similar but the lifetime remained high at 4.5×10−7 s at 300 K, due to passivation of deep acceptors OTe. Time-resolved photoconductivity measurements gave a value of 4.6×10−8 s for as-grown ZnTe in reasonably good agreement and 3.2×10−7 s for Zn annealed and 3.1×10−7 s for SI-ZnTe. The radiative recombination constant B was thus found to be 1.4×10−9 and 4×10−10 cm3 s−1, respectively, at 300 K for as-grown and Zn-annealed samples. PC spectral response studies showed a maximum at 2.41 eV at 300 K corresponding to the main valence–conduction band transition as well as a feature near 3.2 eV corresponding to the splitoff band. Slight shift in peak energy to 2.43 eV occurred on H surface passivation due to reduction of surface recombination velocity.

Unoccupied electronic structure ofLi2CuO2

We have investigated the unoccupied electronic structure of quasi-one-dimensional ${\mathrm{Li}}_{2}{\mathrm{CuO}}_{2}$ using polarization-dependent x-ray absorption spectroscopy. Although an isotropic ``in-plane'' situation is expected for the O $1s$ absorption edge, due to the single oxygen site in the edge-sharing chain geometry, we observe a clear energy shift of peaks derived from the upper Hubbard band, dependent on the direction of the light polarization. For the possible explanation of this observed shift, alternative scenarios are discussed, based upon the electronic structure as obtained from calculations within the local-density approximation, the effects of coupling to phonons during the core-level excitation process, as well as on the polarization-dependent charge distribution in the final state resulting in varying contributions from intersite electron-electron interactions.

Investigation of the Oxidation Effect of Porous Silicon During Electroluminescence by In Situ FTIR

In situ FTIR spectra of porous silicon are measured during electroluminescence (EL). It was found that oxidation process of porous silicon surface occurs during EL, which is the main reason of the EL intensity decrease and red shift of peak energy with time.

Optical transition and ionicity of C60/amine charge-transfer complexes studied by optical spectroscopy

Abstract We have investigated absorption spectra of charge-transfer (CT) complexes of C60 with various aromatic amines with different ionization potentials. An absorption band due to CT transition is observed in the visible region; with increasing ionization potential of amines, the peak energy shifts to the higher energy side and the oscillator strength of CT transition increases. We extract degrees of charge transfer by analyzing the transition energy of the CT band as a function of ionization potential and Raman spectra. The experimental results show that the CT complexes studied here have a neutral character in their ground state.

Mechanism of photoluminescence of Si nanocrystals in SiO2 fabricated by ion implantation: the role of interactions of nanocrystals and oxygen

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 and the temperature 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, a photoluminescence band, peaking just below 1.7 eV was observed. The peak energy of the photoluminescence was found to be affected by the dose of implanted Si ions and the temperature during ion implantation, 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.

Quantum confinement effects on the electronic structure of Si(001) ultrathin films: Energy shifts of optical band edges and luminescence in crystalline and amorphous Si films

The electronic structure for Si(001) ultrathin films terminated by ${\mathrm{SiO}}_{4}$ species is calculated by the extended H\"uckel-type nonorthogonal tight-binding method and compared to experimental results for amorphous $\mathrm{Si}(a\ensuremath{-}\mathrm{Si})/{\mathrm{SiO}}_{2}$ superlattices grown on (001)Si substrates by molecular beam epitaxy (MBE) [Lockwood et al., Phys. Rev. Lett. 76, 539 (1996)]. A remarkable coincidence in the energy shifts of the valence band maximum (VBM) and conduction band minimum (CBM) between the calculation and experiment strongly suggests that the MBE-grown $a\ensuremath{-}\mathrm{Si}$ well layers are almost crystalline and can have a CBM projected in the (001) growth direction and a local minimum of the conduction band as in the crystalline Si(001) films. The similarity of the thickness dependence of observed photoluminescence (PL) peak energy shifts to that of calculated energy-gap shifts indicates that the observed PL should be due to direct recombination of excitons between the VBM and the projected CBM of extended states quantized in the $a\ensuremath{-}\mathrm{Si}$ well layers as in the crystalline Si(001) ultrathin films. It is found that the binding energy of excitons (100 meV) related to the PL can be a combination of the increased Coulomb energy induced by a compression of wave functions for two-dimensional excitons and the one caused by a reduction in dielectric constant due to confinement in the growth direction.

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.

Pressure-induced energy level crossings and narrowing of photoluminescence linewidth in self-assembled InAlAs/AlGaAs quantum dots

We present a study of the hydrostatic-pressure dependence of the photoluminescence from In0.5Al0.5As/Al0.25Ga0.75As self-assembled quantum dots. Three distinct regions of quantum-dot peak-energy shift with pressure are observed and are attributed to energy level crossings and band mixing effects. In addition, a large reduction in photoluminescence linewidth with applied pressure is noted.

The Procedure to Realize Two-Dimensional Quantum Dot Superlattices: From Incoherently Coupled to Coherently Coupled Quantum Dot Arrays

We will describe, from both the theoretical and experimental points of view, a procedure leading to the realization of two-dimensional quantum dot superlattices (2D QDSLs). Two kinds of laterally coupled quantum dot arrays in which the exciton motions are coherent and incoherent are studied by photoluminescence (PL) measurements. The formation of minibands with coherent exciton motion is demonstrated in the large red shift of the PL peak energy as well as the dramatic narrowing of the PL linewidth. Moreover, we indicate, using excitation-dependent PL spectra as an example, that the exciton coherence can be reduced by various scattering mechanisms.

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.

Contributions to understanding the optical properties of partially ordered (Al 0.3Ga 0.7) 0.52In 0.48P

AlGaInP was deposited lattice matched to GaAs at growth temperatures between 720 and 780°C (a parameter range used for LED structures) by MOVPE using the carrier gases nitrogen and hydrogen and (0 0 1) substrates with different misorientations. We will report on a comparative study of the optical and structural properties of these layers using photoluminescence spectroscopy (PL) at 300 and 2 K and transmission electron microscopy (TEM). In principle, a higher amount of order and a lower PL-emission energy was observed for the nitrogen samples compared to the hydrogen samples. Ordering affects the room temperature spectra: broadening is observed. The optical quality of the layers for LED applications is therefore difficult to assess by PL. The influence of domain size on PL peak energy shifts will be presented.

Energy Distribution of Field Emitted Charged Particles during Beam Propagation

The energy distribution during beam propagation in a diode space has been calculated quantitatively for cold cathode electrons and liquid metal ions. Especially, the energy spread and the peak energy shift dependence on the emission surface temperture for liquid gallium ions are verified to be the order of 1 eV even for a temperature increment of ∼100 K.

Visible photoluminescence from a-Si : H/SiO 2 superlattices fabricated by UHV evaporation

Hydrogenated amorphous-Si/SiO 2 (a-Si:H/SiO 2) superlattices with different a-Si : H thickness in the range of a few nanometers have been fabricated by ultra high vacuum evaporator (UHV evaporator). The photoluminescence (PL) of our superlattices is observed in the visible spectral region and the peak energy shifts to higher energy as the a-Si : H layer thickness decreases. The temperature dependence of the PL spectra reveals four sub-bands by fitting. Bands at 2.2, 1.9, 1.65 and 1.45 eV are detected and are attributed to E′δ centers, nonbridging-oxygen–hole centers (NBOHC), Si/SiO 2 interface and a-Si : H layer, respectively. We explain the overall blueshift of the PL spectra by the modification of the contribution of these sub-bands.

Dependence of the photoluminescence of Tl 2InGaS 4 layered crystal on temperature and excitation intensity

The emission band spectra of Tl 2InGaS 4 layered crystals were investigated in the 10–120 K temperature range and in the 540–860 nm wavelength range using photoluminescence (PL). The peak energy position of the emission band is located at 1.754 eV (707 nm) at 10 K. The emission band has a half-width of 0.28 eV and an asymmetric Gaussian lineshape. The increase of the half-width of the emission band, the blue shift of the emission band peak energy and the quenching of the PL with increasing temperature is explained using the configuration coordinate model. The blue shift of the emission band peak energy and the sublinear increase of the emission band intensity with increasing excitation intensity is explained using the inhomogenously spaced donor–acceptor pair recombination model.