Oxygen-related Centers Research Articles

Optically stimulated luminescence and radiophotoluminescence in NaMgF3:Eu nanoparticles

Optically stimulated luminescence and radiophotoluminescence were observed in NaMgF3:0.1%Eu nanoparticles synthesised via a hydrothermal technique. X-ray irradiation created F-centres, and subsequent optical stimulation at 420 nm released the electrons and electron-hole recombination produced Eu2+ emissions. The integrated optically stimulated luminescence intensity was linear up to 180 Gy. Photoluminescence was observed from Eu2+ and Eu3+ valences, whereas optically stimulated luminescence only occurred via the Eu2+ sites. X-ray irradiation also resulted in Eu3+ → Eu2+ and O2− → O− radiophotoluminescence effects, where the luminescence intensity ratio, RPLEu2+/RPLO2−, indicated a large detection range (>1 kGy). The observation of optically stimulated luminescence in lanthanide-doped near tissue-equivalent NaMgF3 nanoparticles has potential applications including as high spatial resolution two- and three-dimensional dosimeters for radiotherapy dose verification and validation. Radiophotoluminescence from oxygen-related centres and Eu is advantageous because it can provide a record of the dose history as well as an additional measure of the dose.

Narrowband Quantum Light Emission from Oxygen-Related Color Centers in Hexagonal Boron Nitride

Narrowband Quantum Light Emission from Oxygen-Related Color Centers in Hexagonal Boron Nitride

DLTS analysis of interface and near-interface bulk defects induced by TCO-plasma deposition in carrier-selective contact solar cells

We investigate the electrical characteristics of defects at the SiO2/Si interface, within the adjacent Si crystal, and through the depth profile of the bulk defect using three-dimensional deep-level transient spectroscopy (3D-DLTS). These defects are introduced by the reactive plasma deposition technique employed for depositing transparent conductive oxides in the fabrication of carrier-selective contact-type solar cells. To control the surface potential near the Si surface, we apply a varying voltage to obtain DLTS signals as functions of both temperature and Fermi level at the SiO2/Si interface. Using machine learning for 3D-DLTS spectral analysis, we estimate the capture cross sections, energy levels, densities, and depth profiles of these process-induced defects. The experimental results indicate the existence of three types of electron traps within the bulk defects, ranging from the interface to a depth of ∼70 nm. The electrical properties of these bulk defects suggest the presence of oxygen-related defects within Si. On the other hand, regarding the properties of interface defects, the capture cross sections and the defect densities are estimated as a function of their energy levels. They suggest that the defects at the SiO2/Si interface are likely oxygen-related PL centers.

UV light induced processes in pure and doped AlN ceramics

AlN ceramics samples, pure and doped with Y 2 O 3 , Eu 2 O 3 and GaN, and produced from the same AlN powder, were studied for photoelectric effect, photoluminescence spectra and kinetics and thermoluminescence under irradiation with UV light from above- and below-bandgap spectral region. Common properties of all studied samples were found, such as presence of the complex UV-Blue and Red emission bands in PL and TL emission due to oxygen-related defects and manganese impurities, correspondingly; a new emission band at 320 nm, which is present in PL and absent in TL emission spectrum and is assigned to an oxygen-related centre; luminescence decay comprised of superposition of exponents of various duration from nanoseconds to hundreds of minutes. Among the studied samples the dopant-related luminescence was observed only in AlN:Eu 2 O 3 at 525 nm. No effect of the dopants on fading rate of the stored TL signal was observed. Such properties of the PL and TL as luminescence intensity and relative contribution of the emission bands manifested by individual samples were explained by influence of the dopant type and ceramics sintering procedure on generation and recharging of the intrinsic and impurity defects. • Pure and doped AlN ceramics show a photoelectric effect under UV light from the above- and below-bandgap spectral regions. • Luminescence spectra of all samples contain 400, 480 and 600 nm bands, in AlN:Eu additional 525 nm band is observed. • In all samples a novel luminescence band is found at 320 nm, preliminary ascribed to (V Al -2O N ) 0 centres. • Photoluminescence kinetics is characterized by superposition of exponents varied from nanoseconds to hundreds of minutes. • Sintering procedure and doping of AlN ceramics affect generation of the oxygen-related centres and charging state of defects.

Selective 6H-SiC White Light Emission by Picosecond Laser Direct Writing

Displaying a full or tuneable emission spectrum with highly efficient is significant for luminescent materials used in solid-state lighting. Silicon carbide (SiC) has potential for use in photoelectric devices that operate under extreme conditions. In this paper, we present a method to selectively modify the photoluminescence (PL) properties of SiC by ultrafast laser direct writing. Based on this method, visible white PL could be observed by the naked eye at room temperature under ultraviolet excitation. By increasing the laser power intensity from 40 to 80 MW/cm2, the PL of the irradiated samples increased and pure white sunlight-like emission with controlled colour temperature was realised. The optimised laser power intensity of 65 MW/cm2 achieved a desirable colour temperature similar to that of sunlight (x = 0.33, y = 0.33 and colour temperature of 5500 K) and suppressed blue emission. By direct laser irradiation along designed scanning path, a large-scale and arbitrary pattern white emission was fabricated. The origin of the white luminescence was a mixture of multiple luminescent transitions of oxygen-related centres that turned the Si-C system into silicon oxycarbide. This work sheds light on new luminescent materials and a preparation technique for next-generation lighting devices.

Temperature-dependent photoluminescence from CdS/Si nanoheterojunctions

CdS/Si nanoheterojunctions have been fabricated by growing nanocrystal CdS (nc-CdS) on the silicon nanoporous pillar array (Si-NPA) through using a chemical bath deposition method. The nanoheterojunctions have been constructed by three layers: the upper layer being a nc-CdS thin films, the intermediate layer being the interface region including nc-CdS and nanocrystal silicon (nc-Si), and the bottom layer being nc-Si layer grown on sc-Si substrate. The room temperature and temperature-dependent photoluminescence (PL) have been measured and analyzed to provide some useful information of defect states. Utilizing the Gauss–Newton fitting method, five emission peaks from the temperature-dependent PL spectra can be determined. From the high energy to low energy, these five peaks are ascribed to the some luminescence centers which are formed by the oxygen-related deficiency centers in the silicon oxide layer of Si-NPA, the band gap emission of nc-CdS, the transition from the interstitial cadmium (ICd) to the valence band, the recombination from ICd to cadmium vacancies (VCd), and from sulfur vacancies (Vs) to the valence band, respectively. Understanding of the defect states in the CdS/Si nanoheterojunctions is very meaningful for the performance of devices based on CdS/Si nanoheterojunctions.

Photocapacitance spectroscopy of InAlN nearly lattice-matched to GaN

We study the deep levels in InAlN nearly lattice-matched to GaN by photocapacitance spectroscopy. This technique allows the study of very deep levels having too slow thermal emission rates to be detected by other deep level spectroscopy techniques. We will identify a broad band of deep levels centered 1.7 eV below the InAlN conduction band edge. The deep level band is characterized by a negligible Franck-Condon shift and by a broadening parameter ΔE = 0.38 eV. Furthermore, we will show evidences for a second class of deep levels with optical ionization energy >2 eV, which will be attributed to previously reported oxygen-related DX centers.

Time-resolved photoluminescence characterization of oxygen-related defect centers in AlN

Time-resolved photoluminescence (PL) spectroscopy has been employed to investigate the emission characteristics of oxygen-related defects in AlN in the temperature region from 77 to 500 K. Two PL components with different decay constants are observed in the near-ultraviolet to visible regions. One is the PL component with decay time of <10 ns and its peak position shifts to longer wavelengths from ∼350 to ∼500 nm with increasing temperature up to 500 K. This PL component is attributed to the radiative relaxation of photoexcited electrons from the band-edge states to the ground state of the oxygen-related emission centers. In the time region from tens to hundreds of nanoseconds, the second PL component emerges in the wavelength region from 300 to 400 nm. The spectral shape and the decay profiles are hardly dependent on temperature. This temperature-independent PL component most likely results from the transfer of photoexcited electrons from the band-edge states to the localized excited state of the oxygen-related emission centers. These results provide a detailed insight into the radiative relaxation processes of the oxygen-related defect centers in AlN immediately after the photoexcitation process.

Investigations on the photoluminescence of the iron and cobalt doped fluoride-containing phosphate-based glasses and its defects-related nature

The influence of doping iron and cobalt ions on the formation of defects in the fluoride-containing phosphate-based glasses was studied comprehensively by means of analyzing the absorption and photoluminescence (PL) spectra as well as the electron spin resonance (ESR) spectra. The absorption and ESR spectra show that the defects, including phosphate-related oxygen hole center (POHC), oxygen-related hole centers (OHC), Fe3+, Co2+, PO3-EC, PO4-EC and fluorine-related non-paramagnetic color (FD) centers were formed during the glass preparation process. In addition, the analysis on the PL and absorption as well as excitation (PLE) spectra indicate that PO4-EC and OHC defects which have large absorption in the UV region result in the emission band peaking at 780 nm under the excitation at 351 nm for the cobalt doped (2ω absorptive) glass. Addition of small quantities of iron causes the effectively intense absorption towards the 1ω wavelength (1053 nm) due to the presence of Fe2+, but also a distinctively observed red-shift of the UV absorption edge was dominated by the charge-transfer transition of the existence of Fe3+. The experimental results demonstrate that Fe3+ suppress the formation of FD defects in the glasses, while promotes the formation of POHC defects. Besides, introduction of Co2+ inhibits the formation of POHC and PO4-EC defects, and Fe2+ promotes forming of POHC defects.

Effects of Gamma Radiation and Heat Treatment on the Photoluminescence of the Fluoride-Containing Phosphate-Based Glasses

We demonstrate, for the first time to our knowledge, two different balances around fluorine-related non-paramagnetic color (FD) centers in fluoride containing phosphate based optical filter glasses at low (190°C) and higher (390°C) heat-treatment temperature, respectively. The thermal bleaching experiment results indicate the heat-treatment bleached gamma radiation induced phosphate-related oxygen hole center (POHC) and oxygen-related hole center (OHC) to some extent due to the release and capture of the electrons in the conduction band. Through the gamma radiation and subsequent heat-treatment experiment, as well as the detailed analysis of photoluminescence and excitation spectra, a general model corresponding to the two different balances about the FD defects and the evolutionary mechanism of these existing defects in the fluoride-containing phosphate based glasses was proposed.

Localised transitions in luminescence of AlN ceramics

The photoluminescence (PL) and thermoluminescence (TL) properties of AlN ceramics revealed under UV irradiation are determined mainly by oxygen-related centres, giving rise to the UV (around 3.18 eV) and the Blue (2.58 eV) bands. It was found that the UV irradiation-generated donor–acceptor pairs (DAPs), responsible for the UV emission band, are randomly distributed with regard to separation distance. Luminescence properties of AlN are interpreted basing on the model of localised recombination involving electron tunnel transitions from the excited state of D to the ground state of A, proposed by Jain et al. (2012). The observed features of PL, afterglow and TL of AlN ceramics are explained by dependence of tunnelling recombination probability on separation distance between D and A implied by the used model.

Optical and Nonlinear Optical Properties of Nanostructured Aluminum Oxyhydroxide: The Effect of Thermal Treatment

We studied effect of thermal annealing at temperatures from 100 to 900 � C on the light scattering, photoluminescence and nonlinear optical properties of nanoporous aluminum oxyhydroxide, prepared by selective oxidation of the surface of liquid-metal fusion at room temperatures. A significant increase of the photoluminescence intensity accompanied by the spectrum expansion to longer wavelength was found in all annealed samples. The blue-shift behavior for the photoluminescence peaks and an appearance of new low-energy peak at 2.28 eV have been observed after annealing of the native material at 100 � C. This fact is explained by annealing-induced structural transformations of the material, which lead to redistribution of the luminescent oxygen-related centers. We consider that the annealing results in the formation of oxygen vacancies and divacancies, which gives the enhanced photoluminescence in the wide energy range. An impact of the annealing on photoinduced absorption variation under self-action of picosecond range laser pulses was observed. The water desorption and phase transformations after annealing at different temperatures result in modifications of the nonlinear optical response, which can be used for the optical diagnostics of structural reorganization of ultraporous media under thermal treatment. The study of both the liner optical and nonlinear optical properties reveals the morphological structure transformations in the investigated transition aluminas.

Thermoluminescence kinetics of oxygen-related centers in AlN single crystals

Excitation and emission spectra of thermoluminescence (TL) in bulk aluminum nitride single crystals irradiated by UV have been studied. TL has been found to be most effectively excited by the 5.04eV photons. The 3.44eV band caused by recombination processes with oxygen–vacancy (VAl−ON)-centers dominates in the TL spectrum. Besides, the 2.91 and 2.0eV emissions have been also observed. The TL mechanisms have been quantitatively analyzed in terms of formal kinetics of general order. On the basis of the obtained values and from their comparison with literature data it has been concluded that the main traps of charge carriers, responsible for the TL peak at 470К, are formed by the VN vacancy. To interpret the observed regularities, the model of TL has been proposed, which satisfactorily agrees with independent data for thermally and optically stimulated processes in aluminum nitride.

EPR study of gamma and neutron irradiation effects on KU1, KS-4V and Infrasil 301 silica glasses

Electron paramagnetic resonance (EPR) studies have been carried out on KU1 and KS-4V high purity quartz glasses and commercial silica Infrasil 301, irradiated with gamma rays up to a dose of 11.6MGy and neutron fluences of 1021 and 1022n/m2. Gamma irradiations produce a much higher concentration of defect centres (mainly E′, POR and NBOHC) for KU1 and I301 than for KS-4V silica. In contrast, neutron irradiation at the highest fluence produces similar concentrations in all silica types. These results agree to a good extent with those obtained in previous optical absorption measurements. Moreover, oxygen-related centres (POR and NBOHC) have been well characterized by means of electron paramagnetic resonance.

Specific features of photoelectric properties of layered films of amorphous hydrogenated silicon

Temperature dependences of photoconductivity of layered and conventional undoped films of amorphous hydrogenated silicon have been studied within a wide range of temperatures (130–420 K) and illumination intensities (0.1–60 mW cm−2). It is established that a higher photosensitivity of layered films compared with conventional films is governed by a low dark conductivity of layered films as a consequence of a deeper position of the equilibrium Fermi level in the band gap and the absence of temperature quenching of photoconductivity in these films. It is shown that these specific features of electrical and photoelectric properties of layered films can be attributed to a low concentration of silicon dangling bonds in comparison with the concentration of oxygen-related acceptor centers, which feature a larger capture coefficient for holes.

Generation and annihilation of boron–oxygen-related recombination centers in compensated p- and n-type silicon

The impact of boron–oxygen-related recombination centers as well as their defect kinetics have been intensely studied in boron-doped oxygen-rich p-type crystalline silicon. Experimental data for the defect in simultaneously boron- and phosphorus-doped compensated p- and n-type silicon, however, is sparse. In this study, we present time-resolved carrier lifetime measurements on Czochralski-grown silicon (Cz-Si) doped with both boron and phosphorus under illumination at 30 °C (defect generation) as well as at 200 °C in the dark (defect annihilation). The defect generation in compensated n-type Cz-Si is found to proceed on a similar time scale as the defect generation in (compensated) p-type Cz-Si. However, the shape of the carrier lifetime reduction during defect generation in compensated n-type silicon differs considerably from that in (compensated) p-type Cz-Si. The defect annihilation in compensated n-type Cz-Si is found to take up to 1000 times longer than in (compensated) p-type Cz-Si. In addition, we confirm a linear dependence of the normalized defect concentration Nt∗ on the net doping concentration p0 as well as a proportionality between the defect generation rate Rgen and the square of the net doping concentration p02 in compensated p-type Cz-Si. These results cannot be explained by the established BsO2i defect model, however, they agree with a recently proposed defect model in which the defect is composed of one interstitial boron atom and an interstitial oxygen dimer (BiO2i).

Luminescence properties of undopedLiBaAlF6 single crystals

This paper presents the results of the study of electronic excitations in undopedLiBaAlF6 single crystals by means of luminescence spectroscopy and complimentary opticalmethods. The intrinsic emission at 4.2 eV due to self-trapped excitons was identified.The fast nanosecond defect-related luminescence was revealed at 3.0 eV. Bothemissions degrade under electron beam irradiation, the most probable reasonof which is defect creation introducing an additional non-radiative relaxationchannel prohibiting energy transfer to luminescence centers. These defects canbe recovered and luminescence intensity restored at higher temperatures (>200 K). The permanent damage by electron beam irradiation results only in overall growth ofthe absorption coefficient in the whole 1.5–6.5 eV spectral region studied. Theanalysis of thermally stimulated luminescence glow curves in the temperaturerange of 5–410 K revealed two shallow charge carrier traps with the activationenergies of 0.22 and 0.33 eV, respectively. The luminescence of an impurity peaked at2.5 eV was found and tentatively assigned to an oxygen-related emission center.

Impact of oxygen on the permanent deactivation of boron–oxygen-related recombination centers in crystalline silicon

The carrier lifetime in boron-doped Czochralski-grown silicon is ultimately limited by light-induced boron–oxygen-related recombination centers. These centers can be permanently deactivated by illumination at elevated temperature (70–220 °C). However, the detailed defect reactions leading to permanent deactivation are still unresolved. In this work, we study the impact of oxygen on the deactivation process. We examine the dependence of the deactivation rate on the interstitial oxygen concentration as well as the impact of long-term annealing at 450 °C, leading to the generation of oxygen clusters acting as donors (thermal donors). We find a decrease in the deactivation rate with both increasing interstitial oxygen concentration and increasing thermal donor concentration, suggesting that oxygen is involved in the deactivation process.

Isotopic labeling study of oxygen diffusion in amorphous LaScO3 high-κ films on Si(100) and its effects on the electrical characteristics

The influence of post-deposition oxygen anneals on the properties of amorphous LaScO3 films on Si(100) is reported. The use of an isotopically (18O2) enriched atmosphere allowed to investigate the 16O–18O exchange and the oxygen diffusion across the dielectric layer. Such effects are connected to the formation of an interfacial layer. Oxygen annealing leads to nearly ideal capacitance–voltage curves, lower leakage currents and interface trap densities, as well as to κ-values up to 33 for the LaScO3 films. These results are attributed to the suppression of oxygen-related trap centers and the achievement of a stoichiometric oxygen content.

Luminescence processes induced by UV radiation in A1N nanotips and nanorods

The processes of the UV radiation induced photoluminescence, thermoluminescence and optically stimulated luminescence in the AlN nanotips and nanorods are studied in comparison with those in the AlN ceramics. The emission spectra of the UV radiation induced luminescence processes in the AlN nanostructures are similar to those of AlN ceramics, presumably originating from recombination processes with participation of the oxygen-related centres. In the nanostructures the luminescence processes occur mainly through the excitation of the host lattice, probably due to the smaller content of the randomly distributed defects in the lattice. The observed small mutual differences in the luminescence properties of nanorods and nanotips may be explained by the modifications of the lattice structure and different surface defects. Dosimetric properties of AlN nanostructures are inferior to those of AlN ceramics due to the lower thermostimulated luminescence (TL) and optically stimulated luminescence (OSL) response and the higher fading rate of the stored signal.