Defect-related Photoluminescence Bands Research Articles

Defect-related photoluminescence from ammono GaN

Photoluminescence (PL) from GaN substrates fabricated by the ammonothermal growth method was studied in a wide range of temperatures and excitation intensities, both with steady-state and time-resolved PL techniques. Three defect-related PL bands were detected: the ultraviolet luminescence band with the zero-phonon line at 3.27 eV, the Zn-related BL1 band with a maximum at 2.9 eV, and the yellow luminescence band (labeled YL2) with a maximum at 2.3 eV. The YL2 band belongs to an unknown defect and is different from the CN-related YL1 band. Its maximum blueshifts by 0.06 eV with increasing excitation intensity and redshifts by more than 0.1 eV with a time delay after a laser pulse. The YL2 band is preliminarily attributed to a defect complex containing the gallium vacancy.

Synthesis and properties of composite materials based on zinc oxide nanoparticles in an insulating matrix

A composite material based on zinc oxide nanoparticles synthesized by the chemical hydrothermal method and a polymer insulated matrix of sodium silicate was obtained. Free zinc oxide nanoparticles were formed by heating an equimolar solution of zinc nitrate and hexamethylenetetramine with different pH values (3–5). Nanoparticles were introduced into an aqueous sodium silicate solution and applied on a silicon substrate by centrifugation. Using scanning electron microscopy, we studied the structure and morphology of zinc oxide nanoparticles on a silicon substrate in unbound state and after their introduction into the sodium silicate matrix. It was found that the obtained particles have a hexagonal crystal lattice and a bimodal size distribution. After introduction in the matrix of sodium silicate, only smaller zinc oxide nanoparticles with an average diameter of 45 nm remain in it. Studying of the photoluminesclence spectra of a composite material, including zinc oxide nanoparticles obtained at different pH values of the initial solution, showed the presence of exciton and defectrelated photoluminescence bands with intensity’s maximums located at 383 and 590 nm, respectively. Zinc oxide nanoparticles in the stabilizing matrix of sodium silicate exhibit a higher relative intensity of exciton photoluminescence than unbound particles. The highest value of the relative intensity of exciton photoluminescence was achieved for a composite material including zinc oxide nanoparticles synthesized at pH=3,35 of the initial solution. The resulting composite materials can be used in optoelectronic and photovoltaic devices, and as an optical medium of non-cavity lasers.

Effect of Oxygen Mixture Ratio on the Properties of ZnO Thin-Films and n-ZnO/p-Si Heterojunction Diode Prepared by RF Sputtering

ZnO thin-films are grown on a p-Si(111) substrate by RF sputtering. The effects of growth temperature and O2 mixture ratio on the ZnO films are investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and roomtemperature photoluminescence (PL) measurements. All the grown ZnO thin films show a strong preferred orientation along the c-axis, with an intense ultraviolet emission centered at 377 nm. However, when O2 is mixed with the sputtering gas, the half width at half maximum (FWHM) of the XRD peak increases and the deep-level defect-related emission PL band becomes pronounced. In addition, an n-ZnO/p-Si heterojunction diode is fabricated by photolithographic processes and characterized using its current-voltage (I-V) characteristic curve and photoresponsivity. The fabricated n-ZnO/p-Si heterojunction diode exhibits typical rectifying I-V characteristics, with turn-on voltage of about 1.1 V and ideality factor of 1.7. The ratio of current density at ± 3 V of the reverse and forward bias voltage is about 5.8 × 103, which demonstrates the switching performance of the fabricated diode. The photoresponse of the diode under illumination of chopped with 40 Hz white light source shows fast response time and recovery time of 0.5 msec and 0.4 msec, respectively.

Determination of donor and DX center capture characteristics by pulsed photoluminescence

In AlN, several broad defect-related photoluminescence bands in the range from 1.6 eV to 2.4 eV have been observed and were associated with donor or DX– to deep electron trap transitions. By the variation of the length of sub-bandgap laser excitation pulses, we find remarkable changes in the wavelength and intensity of the photoluminescence band at 2.4 eV. Purely based on photoluminescence experiments with variable pulsed excitation, we can determine the capture cross sections for the capture of free electrons into the initial D+ and D0 states and the photon absorption cross section for the excitation of the deep electron trap.

Determination of the electron-capture coefficients and the concentration of free electrons in GaN from time-resolved photoluminescence.

Point defects in high-purity GaN layers grown by hydride vapor phase epitaxy are studied by steady-state and time-resolved photoluminescence (PL). The electron-capture coefficients for defects responsible for the dominant defect-related PL bands in this material are found. The capture coefficients for all the defects, except for the green luminescence (GL1) band, are independent of temperature. The electron-capture coefficient for the GL1 band significantly changes with temperature because the GL1 band is caused by an internal transition in the related defect, involving an excited state acting as a giant trap for electrons. By using the determined electron-capture coefficients, the concentration of free electrons can be found at different temperatures by a contactless method. A new classification system is suggested for defect-related PL bands in undoped GaN.

Emission of Cu-related complexes in ZnO:Cu nanocrystals

Photoluminescence (PL), its temperature dependence, scanning electronic microscopy (SEM) and X ray diffraction (XRD) have been applied for the comparative study of varying the emission, morphology and crystal structure of ZnO and ZnO:Cu nanocrystals (NCs) versus technological routines, as well as the dependence of ZnO:Cu NC parameters on the Cu concentration. A set of ZnO and ZnO Cu NCs was prepared by the electrochemical (anodization) method at a permanent voltage and different etching durations with follows thermal annealing at 400°C for 2h in ambient air. The size of ZnO NCs decreases from 300nm×540nm down to 200nm×320nm with etching duration increasing. XRD study has confirmed that thermal annealing stimulates the ZnO oxidation and crystallization with the formation of wurtzite ZnO crystal lattice. XRD method has been used for monitoring the lattice parameters and for confirming the Cu doping of ZnO Cu NCs. In ZnO Cu NCs four defect related PL bands are detected with the PL peaks at 1.95–2.00eV (A), 2.15-2.23 eV (B), 2.43–2.50eV (C) and 2.61–2.69eV (D). Highest PL intensities of orange, yellow and green emissions have been obtained in ZnO Cu NCs with the Cu concentration of 2.28at%. At Cu concentration increasing (≥2.28at%) the PL intensities of the bands A, B, C decrease and the new PL band peaked at 2.61–2.69eV at 10K appears in the PL spectrum. The variation of PL intensities for all PL bands versus temperature has been studied and the corresponding activation energies of PL thermal decay have been estimated. The type of Cu-related complexes is discussed using the correlation between the PL spectrum transformation and the variation of XRD parameters in ZnO Cu NCs.

Effects of surface modification on photoluminescence properties of self-assembled monolayer of CdSe quantum dots

We have investigated surface-modification effects on photoluminescence (PL) properties of the self-assemble monolayer of CdSe quantum dots from the viewpoint of the temperature dependence of PL properties. The band-edge PL band is strongly activated by the surface modification and is observed as the main PL band, contrary to the fact that the defect-related PL band is dominant in the as-grown sample. From the analysis of PL decay dynamics, it is demonstrated that the contribution of a bound exciton state to PL processes would suppress a trapping process to defects.

Effect of Rare-Earth Doping on Structural and Luminescent Properties of Screen-Printed ZnO Films

Over the past decades, zinc oxide has attracted considerable attention for its possible application in optoelectronics due to simultaneous observation of intense ultraviolet and visible emission offering the development of white light-emitting devices. However, in the most cases the native defects responsible for visible emission are not stable upon materials processing. Moreover, for white phosphors, efficient and controllable emission in specific spectral range is often required. This can be achieved in particular via materials doping with rare-earth (RE) ions. In the present study the results on effect of doping of screen-printed ZnO films with Sm3+ and/or Ho3+ions are presented. Photoluminescence (PL) and Raman scattering spectra as well as the X-ray diffraction patterns of undoped and doped films are examined in details versus sintering conditions and doping level. In the PL spectra of undoped ZnO films sintered at 400−700°C, the excitonic emission was observed only, whereas sintering at higher temperatures (up to 1200°C) resulted in the appearance of visible defect-related PL bands peaked at 540−600 nm. The most intensive defect-related emission was found in the films sintered at 1000°C and these latter were doped with rare-earth ions of different concentration in the range of 1·1019− 4·1020 cm-3. In the PL spectra of the RE-doped films, the corresponding RE emission was observed at low temperatures, but not at room temperature. Since the defect-related PL band caused by intrinsic defects in ZnO overlapped essentially with corresponding Ho and Sm PL bands, the PL bands peaked at about 700 nm due to 4G5/2→6H11/2 transitions in Sm3+ ions were observed only. At the same time, simultaneous codoping with both Sm3+ and Ho3+ ions produced significant decrease in the intensity of the UV and visible defect-related PL bands as well as the increase in the intensity of Sm3+ and Ho3+ PL components. In the PL excitation spectra of the PL band peaked at about 720 nm in addition to excitation caused by ZnO band-to-band absorption and absorption involving intrinsic defects of zinc oxide, a new characteristic absorption band at about 410 nm appeared. The effect of RE doping on the PL and PL excitation spectra is discussed in terms of the formation of RE ions complexes as well as energy transfer from ZnO host to RE ions.

Effect of Rare-Earth Doping on Structural and Luminescent Properties of Screen-Printed ZnO Films

The effect of Sm3+ and/or Ho3+ doping on structural and luminescent properties of screen-printed ZnO films sintered at 1000°C was investigated by photoluminescence (PL), PL excitation and Raman scattering methods. For all the films, ultraviolet excitonic and visible defect-related PL bands of ZnO were detected. The doping with Ho3+ ions produced an enhancement of PL in ZnO films, the excitonic PL intensity being increased prominently, while the co-doping with Sm3+ and Ho3+ ions resulted in PL decrease in ZnO films. Only for (Sm,Ho)- co-doped ZnO films, the rare-earth PL bands were detected. The reduction of Sm3+ to Sm2+ was observed demonstrating 5D0→7FJ radiative transitions. The mechanism of PL and PL excitation is discussed in terms of the formation of rare-earth complexes as well as energy transfer towards them from ZnO host.

HVPE GaN with Low Concentration of Point Defects for Power Electronics

ABSTRACTWe have studied photoluminescence (PL) from undoped GaN films grown by HVPE technique on sapphire. Several defect-related PL bands are observed in the low-temperature PL spectrum. The concentrations of the defects responsible for these PL bands are determined from the dependence of PL intensity on excitation intensity. The RL band with a maximum at 1.8 eV is often the dominant PL band in HVPE GaN. It is caused by an unknown defect with the concentration of up to ∼1017 cm-3. The concentrations of defects responsible for other defect-related PL bands rarely exceed 1015 cm-3.

Thermal annealing effects on ZnO films grown on graphene buffered Si substrates.

ZnO films deposited on SiO2/Si substrate with a graphene single layer (GSL) were studied by using an ultra-high vacuum sputter. The as-prepared films were annealed at temperature ranges from 500 degrees C to 800 degrees C for 1 min under ambient N2 gas. When the annealing temperature was increased up to 800 degrees C, the root mean square roughness of the ZnO/Si sample surface decreased down to 3.4 nm, and the grain sizes increased about 50.8 nm with a clear grain boundary. From the photoluminescence (PL) spectra, the high intensity of near-band-edge UV emission at 380 nm (3.26 eV) and the broad band emission between 450 and 650 nm, known as the visible defect related PL band, decreased with increasing annealing temperature up to 800 degrees C. The ZnO thin films on the growth on the GSL and post-annealing at 700 degrees C for 1 min under ambient N2 gas had the best structural and optical properties.

Emission of ZnO:Ag nanorods obtained by ultrasonic spray pyrolysis

Scanning electronic microscopy (SEM), X ray diffraction (XRD), photoluminescence (PL) and its temperature dependence have been studied in ZnO:Ag nanorods (NRs) prepared by the ultrasonic spray pyrolysis (USP) method. The time variation at the growth of ZnO:Ag films permits modifying the ZnO phase from the amorphous to crystalline, to change the size of ZnO:Ag NRs and to vary their emission spectra. PL spectra of ZnO:Ag NRs versus temperature has been investigated. This study reveals that the PL band related to the acceptor AgZn (LO phonon replicas of an acceptor bound exciton, ABE (2.877eV)), and its second-order diffraction peak (1.44eV) disappeared in the temperature range of 10–170K with the formation of free exciton (FX). The PL intensity of defect related PL bands decreases monotonously in the range 10–300K with the activation energy of 13meV. The PL band (3.22eV), related to the LO phonon replica of free exciton (FX-2LO) and its second-order diffraction peak (1.61eV) increase monotonously in the range 10–300K. FX related peak dominates in PL spectra at room temperature that testifies on the high quality of ZnO:Ag films prepared by the USP technology.

Size dependent emission stimulation in ZnO nanosheets

Photoluminescence (PL), X ray diffraction (XRD) and Raman scattering have been studied in crystalline ZnO nanosheets (NSs) of different sizes, estimated by scanning electronic microscopy (SEM). ZnO NSs with the size from the range of 60–600nm were created by the electrochemical (anodization) method and followed thermal annealing at 400°C for 2h in ambient air. XRD study confirms the wurtzite structure of ZnO NSs and has revealed that the lattice parameters increase monotonically with decreasing NS sizes. Simultaneously the intensity of a set of Raman peaks increases and Raman peaks shift into the low energy range. The surface phonon has been detected in smallest size ZnO NSs.Two types of PL bands deal with a set of phonon replicas of free excitons and the defect related emission have been detected in ZnO NSs. The intensity enhancement of exciton- and defect-related PL bands with decreasing ZnO NS sizes has been detected. The intensity stimulation of exciton-related PL bands is attributed to the realization of the week confinement and the exciton-light coupling with the formation of polariton in small size ZnO NSs of 67–170nm. The intensity rising of defect-related PL bands is attributed to the concentration enlargement of surface defects when the surface to volume ration increases at decreasing ZnO NS sizes. Numerical simulations of radiative lifetimes and exciton radiative recombination rates in ZnO NSs for different emission wavelengths have been done using the exciton-light coupling model. Then the experimental and numerically simulated PL results have been compared and discussed.

Enhancement of the photoluminescence in CdSe quantum dot–polyvinyl alcohol composite by light irradiation

The effect of photo-induced enhancement (more than a tenfold) of room temperature deep-trap photoluminescence (PL) in CdSe quantum dots (QDs) embedded in polyvinyl alcohol (PVA) film has been found and investigated by the PL and X-ray diffraction methods. The effect is observed under illumination of the QD/PVA composite with LED's light of 409 or 470nm at elevated temperatures and is shown to be caused by an increase of the activation energy of thermal quenching of defect-related PL. It is shown that thermal annealing of the composite by itself stimulates polymer crystallization and produces a small increase in the intensity of both the band-edge and defect-related PL bands of CdSe QDs. It is found that the effect of illumination decreases when the annealing temperature increases from 90°C to 120°C because thermal annealing at 120°C per se results in strong enhancement of room temperature deep-trap PL. The effect of photo-induced enhancement of defect-related PL is found to be irreversible and is assumed to be related to the change of QD surface defect passivation or surface defect rearrangement. This is ascribed to partial destruction of PVA matrix as a result of interaction of QD/PVA interface with photocarriers generated in the QDs due to LED's light absorption.

Structure and emission transformations in ZnO nanosheets at thermal annealing

Photoluminescence (PL), scanning electronic microscopy (SEM), Raman scattering and X-ray diffraction (XRD) have been studied in freshly prepared amorphous phase ZnO nanosheets (NSs) and in thermal annealed crystalline ZnO nanosheets. ZnO NSs were created by the electrochemical (anodization) method using different technological regimes. ZnO NSs are characterized by the amorphous phase in a freshly prepared state. Defect related PL bands with the peaks at 1.90–2.03, 2.49–2.51 and 2.80–2.85eV are detected in the amorphous ZnO phase. Then ZnO nanosheets have been annealed at 400°C for 2h in ambient air. Raman and XRD studies have shown that annealing stimulates the ZnO oxidation and crystallization with the creation of ZnO nanocrystals with the wurzite crystal lattice. The oxidation and crystallization processes are accompanied by a transformation of the PL spectra. In crystalline ZnO NSs four PL bands appear with the PL peaks at 1.55–1.60, 2.06–2.10, 2.52 and 3.10eV. The reasons for the emission transformation have been discussed. For some PL bands (1.90–2.03, 2.49–2.51 and 2.80–2.85eV) the nature of radiative defects has been clarified from the comparative study of the ZnO NS structure and emission properties. It is shown that the anodization method permits to obtain the ZnO nanocrystals by a simple controllable way with emission in a wide spectral range that is interesting for future application in room temperature “white” light-emitting diodes.

Photoluminescence Properties of Self-Assembled Monolayers of CdSe and CdSe/ZnS Quantum Dots

The photoluminescence (PL) properties of self-assembled monolayers (SAMs) of CdSe and CdSe/ZnS quantum dots (QDs) were investigated. From measurements of the temperature dependence of PL spectra, it is demonstrated that the thiol group, which is a constituent of the reagent for formation of the SAM, degrades the PL properties during the SAM formation process. With an additional dipping treatment in a Cd(ClO4)2 aqueous solution with pH = 10, the band-edge PL intensity is increased remarkably. The improvement of PL properties is attributed to QD surface modification with a Cd(OH)2 layer. On the other hand, the defect-related PL band is not observed at all, even at 10 K, in the SAMs of CdSe/ZnS core/shell QDs. This fact indicates that the ZnS shell layer prevents degradation of the PL properties during the SAM process.

ZnO grown by atomic layer deposition: A material for transparent electronics and organic heterojunctions

We report on zinc oxide thin films grown by atomic layer deposition at a low temperature, which is compatible with a low thermal budget required for some novel electronic devices. By selecting appropriate precursors and process parameters, we were able to obtain films with controllable electrical parameters, from heavily n-type to the resistive ones. Optimization of the growth process together with the low temperature deposition led to ZnO thin films, in which no defect-related photoluminescence bands are observed. Such films show anticorrelation between mobility and free-electron concentration, which indicates that low n electron concentration is a result of lower number of defects rather than the self-compensation effect.

Extremely low temperature growth of ZnO by atomic layer deposition

We report on the zinc oxide (ZnO) thin films obtained by the atomic layer deposition (ALD) method using diethyl zinc and water precursors, which allowed us to lower deposition temperature to below 200 °C. The so-obtained “as grown” ZnO layers are polycrystalline and show excitonic photoluminescence (PL) at room temperature, even if the deposition temperature was lowered down to 100 °C. Defect-related PL bands are of low intensity and are absent for layers grown at 140−200 °C. This is evidence that extremely low temperature growth by ALD can result in high quality ZnO thin films with inefficient nonradiative decay channels and with thermodynamically blocked self-compensation processes.

Photoluminescence properties related to localized states in colloidal PbS quantum dots

We have investigated photoluminescence (PL) properties of PbS quantum dots (QDs) prepared by a colloidal method. The QDs were dispersed into polyvinyl alcohol films in order to investigate the temperature dependence of optical properties. In the visible region, both absorption and PL peaks are clearly observed. The temperature dependence of the absorption energy in QDs is quite different from that in a bulk crystal; the dependence is quantitatively analyzed by considering a thermal-expansion effect on the quantum confinement in QDs. The PL spectra are composed of three PL bands which are classified into two types. Two PL bands among them with the Stokes shifts of 0.25 and 0.47 eV at 10 K are attributed to the defect-related PL bands, and another with the relatively small Stokes shift of 0.17 eV exhibits characteristics of shallow trapped states in the temperature dependence of PL.

Stimulation of excitonic and defect-related luminescence in porous SiC

This paper presents the results of porous SiC (PsiC) characterization using photoluminescence (PL) and scanning electronic microscopy. It is shown that the intensities of visible PL bands increase monotonically with the thickness rise of PSiC layers. Intensity enhancement for excitonic PL bands (2.86, 3.05 and 3.28 eV) is attributed to the recombination rate increasing (recombination time decreasing) due to electron–hole confinement effect in SiC nano-crystallites. The intensity increasing for defect-related PL bands (1.82, 2.10, 2.30 and 2.58 eV) is assigned to grow up of defect concentrations on the PSiC surface at the etching process.