Selective Excitation Photoluminescence Research Articles

Probing of local alloy disorder in InGaN using Er3+ ions

We report on studies to probe the local lattice disorder in InGaN:Er epilayers using the 1.54μm emission of Er3+ ions. The InGaN layers were doped during MOCVD growth with Er to aconcentration of about 2.3%. Site selective optical spectroscopy and excitation photoluminescence spectroscopy were used to show that in InGaN:Er two classes of the emitting Er centers can be distinguished: (i) one leading to emission of Er3+ which is nearly the same as in GaN:Er, except nonuniform broadening and (ii) another that yields new emission features not having its counterpart in GaN:Er. The latter emission is interpreted as originating from Er-complexes involving one or more In atoms in the second coordination sphere. The observed fluorescence line broadening and wavelength shifts in the emission wavelength indicate the extent of the disorder in the metallic sublattice.

Investigation of Intrinsic Carbon-Related Defects in 4H-SiC by Selective-Excitation Photoluminescence Spectroscopy

Emission of carbon-related defects is investigated by means of selectively-excited photoluminescence in high purity 4H-SiC electron-irradiated with very low dose. Two new centers with clearly associated phonon replicas are observed, one of which is tentatively assigned to the carbon split interstitial at hexagonal site. The temperature dependence of the spectrum is also studied and indicates that at least some of the observed luminescence lines arise from recombination of excitons bound to isoelectronic centers.

Energy dependent carrier relaxation in self‐assembled InAs/GaAs quantum dots

AbstractWe performed the selective excitation photoluminescence (SEPL) spectroscopy studies on InAs/GaAs self‐assembled quantum dots (QDs). Under different excitation energies, different groups of QDs are selected and emit light. The excited carriers relax to the ground state through different mechanisms when excited at different energies. Three distinct regions with different mechanisms in carrier excitation and relaxation are identified in the emission spectra. These three regions can be categorized, from high energy to low energy, as continuum absorption, electronic state excitation, and multi‐phonon resonance. The QDs' special joint density‐of‐state tail extending from the wetting layer peak facilitates the carrier relaxation and was suggested to explain these spectral results. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Energy-dependent carrier relaxation in self-assembled InAs quantum dots

Selective excitation photoluminescence spectroscopy was employed to study InAs/GaAs self-assembled quantum dots (QDs). Under different excitation energies, different groups of QDs are selected and then emit light. The excited carriers relax to the ground state through different mechanisms when excited at different energies. Three distinct regions with different mechanisms in carrier excitation and relaxation are identified in the emission spectra. These three regions can be categorized from high energy to low energy, as continuum absorption, electronic state excitation, and multiphonon resonance. The special joint density of state tail of the QD that extends from the wetting layer band edge facilitates carrier relaxation and is posited to explain these spectral results.

Carrier dynamics in submonolayer InGaAs∕GaAs quantum dots

Carrier dynamics of submonolayer InGaAs∕GaAs quantum dots (QDs) were studied by microphotoluminecence (MPL), selectively excited photoluminescence (SEPL), and time-resolved photoluminescence (TRPL). MPL and SEPL show the coexistence of localized and delocalized states, and different local phonon modes. TRPL reveals shorter recombination lifetimes and longer capture times for the QDs with higher emission energy. This suggests that the smallest SML QDs are formed by perfectly vertically correlated two-dimensional InAs islands, having the highest In content and the lowest emission energy, while a slight deviation from the perfectly vertical correlation produces larger QDs with lower In content and higher emission energy.

Concentration dependence of photoluminescence properties in poly[(2-methoxy,5-octoxy)1,4-phenylenevinylene] thin films

Photoluminescence and selective excitation photoluminescence measurements at room temperature have been performed on poly[(2-methoxy,5-octoxy) 1,4-phenylenevinylene] (MO-PPV) thin films, which are prepared from MO-PPV chloroform solutions of different concentrations. The position of the S0→S1 absorption peak shows red-shift and broadened relative to that in an MO-PPV solution form due to the solidification effect, while no relative shifts between the absorption spectra of these film samples are observed. A long wavelength emission component near 630 nm has been identified as S2→S0 vibronic transition through the Gaussian decomposition method and confirmed by below-gap PL and selective-excitation PL experiments. This second vibronic component cannot be observed in the spectra of thick films. The PL efficiency of MO-PPV thin film is also investigated through comparison with that of an MEH-PPV thin film and explained by the side substituent effect.

Absorption and luminescence characteristics of optically thin conjugated polymer films

Poly[(2-methoxy,5-octoxy)1,4-phenylenevinylene] (MO-PPV) optically thin films have been prepared from their chloroform solutions of different concentrations. Then their UV–Vis absorption (Abs), photoluminescence (PL), and selective-excitation photoluminescence (SEPL) spectra have been measured at room temperature. The position of S0 → S1 absorption peak shows red shift and broadened relative to that in MO-PPV solution form due to solidation effect, while no relative shifts between the absorption spectra of these film samples are observed. A “long-wavelength tail” appears in Abs spectra which should attribute to the disorder morphology of these thin films. A long-wavelength emission component near 630 nm has been identified as S2 → S0 vibronic band through Gaussian decomposition method and confirmed by below-gap PL and selective-excitation PL experiments. This absorption tail and the second vibronic band cannot be observed in thick films. The PL efficiency of MEH-PPV thin film and that of MO-PPV thin film are also compared and explained by the side substituent effect, which can be proved both by theoretical simulation of interchain interactions and by comparing their morphology in film forms. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photophysical Properties of Lyotropic Liquid Crystalline Polybenzazoles and Evidence of Aggregate Formation for Polybenzazoles in Methanesulfonic Acid

The photophysical properties of the lyotropic liquid crystalline polybenzazoles [poly(p-phenylene benzobis(oxazole)) (PBO), poly(2,5-benzoxazole) (ABPBO), and their random copolymer PBO−ABPBO] and their model compounds in solutions with methanesulfonic acid (MSA) were investigated in detail using UV absorption and emission spectroscopy. It was found that the absorption wavelength and emission wavelength can be well modulated by the composition of PBO in the copolymer from the lowest value for pure ABPBO to the highest for PBO. The photoluminescence spectra at various concentrations were also studied in detail. Both excitation spectra and emission spectra show significant changes in shape and position of the peaks. With the increase of the concentration, the intensity of emission is depressed, and the highly structured emission spectra gradually change to featureless, red-shifted, and broad spectra. The peak position in excitation spectra also shows a slight red shift. Another dramatic change in excitation spectra with concentration is a second broad peak that shows up in the intermediate concentration range. The change in emission spectra can be attributed to the existence of the interchain aggregates, which can be excited from the ground state to the excited-state directly. This explanation is further supported from the evidence in a poor solvent, selective excitation, and time-resolved photoluminescence spectroscopy.

Study of optical properties in GaAs1-xSbx/GaAssingle quant um wells

GaAsSb/GaAs single quantum wells (SQWs) grown by molecular beam epitaxy are studied by selectively-excited photoluminescence (SEPL) measurement. For the first time, we have simultaneously observed the PL from both type Ⅰ and type Ⅱ transitions in GaAsSb/GaAs heterostructure in the SEPL. The two transitions exhibit different PL behaviours under different excitation energy. As expected, the peak energy of type I emission remains constant in the whole excitation energy range we used, while type Ⅱ transition shows a significant blue shift with increasing excitation energy.The observed blue shift is well explained in terms of electron-hole charge separation model at the interface. Time-resolved(TR) PL exhibits more type Ⅱ characteristic of GaAsSb/GaAs QW. Moreover, the results of the excitation-power-dependent PL and TRPL provide more direct information on the type-Ⅱ nature of the band alignment in GaAsSb/GaAs quantum-well structures. By combining the experimental results with some simple calculations, we have obtained the strained and unstrained valence band offsets of Qv＝1.145 and Q0v＝0.76 in our samples, respectively.

Optical Study of Localized and Delocalized States in GaAsN/GaAs

ABSTRACTTaking advantages of short pulse excitation and time-resolved photoluminescence (PL), we have studied the exciton localization effect in a number of GaAsN alloys and GaAsN/GaAs quantum wells (QWs). In the PL spectra, an extra transition located at the higher energy side of the commonly reported N-related emissions is observed. By measuring PL dependence on temperature and excitation power along with PL dynamics study, the new PL peak has been identified as a transition of the band edge-related recombination in dilute GaAsN alloy and delocalized transition in QWs. Using selective excitation PL we further attribute the localized emission in QWs to the excitons localized at the GaAsN/GaAs interfaces. This interface-related exciton localization could be greatly reduced by a rapid thermal annealing.

Residual donors in wurtzite GaN homoepitaxial layers and heterostructures

AbstractShallow donors in GaN epilayers grown by metalorganic chemical vapour deposition (MOCVD) and by molecular beam epitaxy (MBE) on sapphire, SiC and GaN substrates have been studied by selectively excited photoluminescence (SPL) and far‐infrared (FIR) absorption. The spectroscopic studies of neutral donor bound excitons allow us to determine the D0X rotational excited state spectra in a large domain of tensile and compressive strain fields depending on the substrates used for the growth. The energy transitions between the ground state and the n = 2 states of residual donors are determined from the spectra of the two‐electron replica of D0X lines in accordance with resonant electronic Raman scattering results. The comparison between FIR results and the two‐electron spectroscopy shows, that silicon is the main residual donor (Ed = 30.4 meV in strain‐free samples) in both MOVPE and MBE layers. A second unknown donor (Ed = 31.8 meV) is frequently found in GaN/SiC grown by MOCVD. A third deeper level (Ed = 33.1 meV) is rarely detected by two‐electron spectroscopy but could be associated with a weak neutral‐donor bound exciton shifted down by 0.9 meV from the main SiGa related D0X line in MBE homoepitaxial layers.

Optical Properties of Al0.70Ga0.30As:Er,Yb

ABSTRACTErbium (Er) ions were co-implanted with ytterbium (Yb) into Al0.70Ga0.30As substrates and we realized an increase in the intensity of Er intra-4f-shell luminescence. The photoluminescence (PL) intensity of Er-related dominant peak (1538.2nm) was enhanced by co-implanted Yb. The thermal quenching was improved. PL intensity of Yb-related emission was decreased. We studied the transfer energy and the optical sensitization of Yb ions co-implanted with Er ions in Al0.70Ga0.30As. Energy transfers from 2F5/2 (the first excited state) → 2F7/2 (the ground state) of Yb3+ to 4I13/2 (the first excited state) → 4I15/2 (the ground state) of Er3+ were observed by PL excitation (PLE) and selectively excited PL (SPL).

Far-infrared and selective photoluminescence studies of shallow donors in GaN hetero- and homoepitaxial layers

Shallow donors in GaN epilayers grown by metalorganic chemical vapor deposition on sapphire and GaN substrates have been studied by selectively excited photoluminescence (SPL) and by far-infrared (FIR) spectroscopy. A comparison of FIR and SPL results reveals a small splitting between the 2s and 2p donor states, interpreted as partly being due to the nonspherical crystal symmetry. The utilization of the selective excitation of neutral donor bound excitons I2 allows the identification of the I2 rotational excited states. An interpretation of two-electron spectra in GaN involving excited states of I2 is proposed.

Optical properties of nanostructures self-organized in CdSe/ZnSe fractional monolayer superlattices

We report on optical studies of excitonic transfer and localization in a CdSe/ZnSe fractional monolayer superlattice with an embedded deeper ZnCdSe quantum well (QW). Time-resolved selective excitation photoluminescence measurements reveal a two-step nature of the energy relaxation process. The first stage includes hot-exciton cascade relaxation assisted by emission of ZnSe LO phonons, which results in the formation of a narrow nonthermal distribution of free excitons. Different mechanisms compete to destroy the distribution, including optical- and acoustic-phonon-assisted exciton localization by fluctuations of the random potential, as well as tunneling escape of hot free excitons towards the embedded deeper QW in the sample.

Broadening of the excitonic mobility edge in a macroscopically disordered CdSe/ZnSe short-period superlattice

Selective excitation photoluminescence measurements performed in CdSe/ZnSe superlattices with an embedded ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}\mathrm{Se}$ quantum well reveal the limitation of the mobility edge concept to describe the dynamics of localized excitons in this system. Both localized and extended excitonic states are found to coexist in a certain energy range. We suggest that this behavior is governed by a macroscopic band-gap modulation imposed on the microscopic random localization potential.

Anomalous magnetophotoluminescence as a result of level repulsion in arrays of quantum dots

Selectively excited photoluminescence (SPL) of an array of self-organized In 0.5Ga 0.5As quantum dots has been measured in a magnetic field up to 11T. An anomalous magnetic field sensitivity of the SPL spectra has been observed under conditions for which the regular photoluminescence spectra are almost insensitive to the magnetic field due to large inhomogeneous broadening. The anomalous sensitivity is interpreted in terms of the repulsion of excited levels of the dots in a random potential. A theory presented to describe this phenomenon is in excellent agreement with the experimental data. The data estimated the correlation in the positions of excited levels of the dots to be 94%. The magnetic field dependence allows the determination of the reduced cyclotron effective mass in a dot. For our sample we have obtained m em h (m e + m h) = 0.034m 0 .

A compensating donor with a binding energy of 57 meV in nitrogen-doped ZnSe

A compensating deep donor with a binding energy of 57 meV in ZnSe:N epilayers has been studied by means of photoluminescence and selectively excited photoluminescence (SPL) spectroscopy. The emission of 2.766 eV due to transitions between the deep donors and free holes (DdF) was observed at 4 K under strong excitation conditions. The emission at 2.681 eV due to transitions between deep donors and nitrogen acceptors (DdAP) is attributed to the same deep donor as that of the DdF emission through a detailed SPL study. It is also demonstrated that the SPL technique is important for studying the deep levels in ZnSe:N.

Photoluminescence and raman studies of high quality CdTe:I Epilayers

Low-temperature photoluminescence (PL) studies of iodine-doped CdTe epilayers have been performed. A compensating acceptor center which gives rise to deep-level PL emission at 1.491 eV is identified. From selective excitation PL studies, we assign this 1.491 eV line to the recombination of an associate donor-acceptor close pair, consisting of nearest neighbor substitutional sodium and iodine atoms (NaCd-ITe). This neutral defect complex has a localized mode of 36.5 meV, which is much larger than the bulk CdTe lattice mode of 21.3 meV. The electronic energy level associated with this defect is 115 meV below the conduction band. Also, we use a combination of selective excitation PL and Raman spectroscopies to determine the ionization energy of the isolated shallow iodine donor (ITe) in CdTe. We find that the donor binding energy of this anion-site hydrogenic donor is 15.0 (±0.2) meV.

Optical and crystalline properties of Yb implanted InP

The effect of the annealing temperature on the optical and crystalline properties of 2 MeV Yb+ ion-implanted InP are systematically studied by photoluminescence (PL), photoluminescence excitation (PLE), selectively excited photoluminescence (SPL), and Raman scattering measurements. PL measurements present that the recrystallization of the implanted layers and the optical activation of Yb3+ begin at 450 °C and 500–550 °C, respectively. It is demonstrated from the Raman experiments that there is a continuous restoration in radiation damages when the as-implanted samples are annealed at temperatures between 450 °C and 550 °C. However, beyond 550 °C, the crystalline quality near the surface is shown to be degraded. For an explanation of this point, a comparison with optical micrographs is conducted. From the PLE spectra of Yb-related luminescence, it is concluded that the highest crystalline quality of Yb+ implanted layers could be achieved by annealing at 750 °C for 15 min. In the PLE spectra, a band observed earlier at 1.409 eV (8800 Å) is recognized to be split into two peaks at 1.409 eV (8798 Å) and 1.408 eV (8806 Å), denoted by B1 and B2, respectively. A convictive assignment for these two peaks is proposed. Furthermore, an unusual broad band at 1.24 eV, which is observed only in SPL spectrum when the excitation is set on B1 energy, is described.

Photoluminescence of porous silicon

Excitation spectroscopy of the luminescence, selective excitation photoluminescence, time-resolved photoluminescence and time-decay measurements of the photoluminescence have been carried out on various porous silicon samples. A very rich phenomenology of the luminescence has been measured. Strong evidence has been found for emission processes due to a strongly disordered system both topologically and energetically. No definitive responses about the origin of the luminescence in p-Si are found but some promising points for further investigations are underlined.