Optical Properties Of Defects Research Articles

Optical Properties of Defects in Solids via Quantum Embedding with Good Active Space Orbitals

Optical Properties of Defects in Solids via Quantum Embedding with Good Active Space Orbitals

Role of dopant local structure and defects in optical properties of SrHfO3:Ln3+ perovskite: experimental and theoretical studies

Role of dopant local structure and defects in optical properties of SrHfO3:Ln3+ perovskite: experimental and theoretical studies

Thermodynamic stability and optical properties of C-doping-induced defects in hexagonal boron nitride as potential single-photon emitters

Quantum information technologies have triggered enormous interest in single-photon emitters (SPEs), such as local defects in wide-band-gap semiconductors. Recently, visible single-photon emission was observed in carbon-doped hexagonal boron nitride (h-BN), yet the structure giving rise to the emission has not been unambiguously identified. Further progress in the rational design of SPEs requires a deep insight into all the ground-state properties, the electronic structure of the defects in question, and their relationship with the optical properties. To this end, we apply state-of-the-art computational methods to evaluate the stability and optical properties of defects that can be responsible for the above-mentioned experimental findings. For our analysis, we select seven possible C-based defects: (i) C substitutes B $({\mathrm{C}}_{\mathrm{B}})$, (ii) C substitutes N $({\mathrm{C}}_{\mathrm{N}})$, (iii) C substitutes N and nearest B and N are interchanged $({\mathrm{C}}_{\mathrm{N}}{\mathrm{N}}_{\mathrm{B}})$, (iv) C substitutes N and there is a nearby B vacancy $({V}_{\mathrm{B}}{\mathrm{C}}_{\mathrm{N}})$, and (v) three different configurations of ${V}_{\mathrm{N}}{\mathrm{C}}_{\mathrm{B}}$ (C substitutes B and a nearby N vacancy). We perform calculations of the electronic structure using the linear response approach ($GW$ method), followed by finding the solution to the Bethe-Salpeter equation to obtain the excitation spectra of dynamically stable defects. Our density-functional-theory-based calculations of the optimized geometries and their \ensuremath{\Gamma}-point phonon frequencies reveal that ${V}_{\mathrm{B}}{\mathrm{C}}_{\mathrm{N}}$ is dynamically unstable and undergoes a transformation to one corrugated ${V}_{\mathrm{N}}{\mathrm{C}}_{\mathrm{B}}$ structure. Likewise, the flat ${V}_{\mathrm{N}}{\mathrm{C}}_{\mathrm{B}}$ (with ${C}_{2\mathrm{v}}$ symmetry) lies on a very shallow and anharmonic minimum that would also relax toward the corrugated ${V}_{\mathrm{N}}{\mathrm{C}}_{\mathrm{B}}$ structure. Clearly, our results demonstrate how important is to attest to the dynamical stability of any proposed structure. Upon comparing our results of the considered defects with the reported experimental SPE spectra, we show that only the ${\mathrm{C}}_{\mathrm{N}}$ defect has an excitation spectrum fitting the conditions for the observed single-photon emission. Importantly, this defect is found to be the most thermodynamically stable among those under consideration.

ПОВЕРХНЕВА ЛЮМІНЕСЦЕНЦІЯ НАПІВПРОВІДНИКОВИХ КВАНТОВИХ ТОЧОК А2В6 (Огляд)

Semiconductor zero-dimensional nanocrystals – quantum dots (QDs) – have been increasingly used in various fields of opto- and nanoelectronics in recent decades. This is because of the exciton nature of their luminescence, which can be controlled via the well known quantum-dimensional effect. At the same time, at small nanocrystall sizes, the influence of the surface on the optical and structural properties of nanocrystals increases significantly. The presence of broken bonds of surface atoms and point defects – vacancies and interstial atoms – can both weaken the exciton luminescence and create new effective channels of radiant luminescence. In some cases, these surface luminescence becomes dominant, leading to optical spectra broadening up to the quasi-white light. The nature of such localized states often remains unestablished due to the large number of the possible sorts of defects in both of QD and its surrounding. In contrast to exciton luminescence, which can be properly described within effective-mass approximations, the optical properties of defects relay on chemical nature of both defect itsself and its surrounding, what cannot be provided by “hydrogen-type coulomb defect” approximation. Moreover, charge state and related to this lattice relaxation must be taken into account, what requires an application of atomistic approach, such as Density functioal theory (DFT). Therefore, this review is devoted to the study of surface (defect) states and related luminescence, as well as the analysis of possible defects in nanocrystals of semiconductor compounds A2B6 (CdS, CdZnS, ZnS), responsible for luminescence processes, within ab initio approach. The review presents the results of the authors' and literature sources devoted to the study of the luminescent characteristics of ultra-small (<2 nm) QDs.

Extrinsic Effects on the Optical Properties of Surface Color Defects Generated in Hexagonal Boron Nitride Nanosheets.

Hexagonal boron nitride (hBN) is a wide-band gap van der Waals material able to host light-emitting centers behaving as single photon sources. Here, we report the generation of color defects in hBN nanosheets dispersed on different kinds of substrates by thermal treatment processes. The optical properties of these defects have been studied using microspectroscopy techniques and far-field simulations of their light emission. Using these techniques, we have found that subsequent ozone treatments of the deposited hBN nanosheets improve the optical emission properties of created defects, as revealed by their zero-phonon linewidth narrowing and reduction of background emission. Microlocalized color defects deposited on dielectric substrates show bright (≈1 MHz) and stable room-temperature light emission with zero-phonon line peak energy varying from 1.56 to 2.27 eV, being the most probable value 2.16 eV. In addition to this, we have observed a substrate dependence of the optical performance of the generated color defects. The energy range of the emitters prepared on gold substrates is strongly reduced, as compared to that observed in dielectric substrates or even alumina. We attribute this effect to the quenching of low-energy color defects (these of energies lower than 1.9 eV) when gold substrates are used, which reveals the surface nature of the defects created in hBN nanosheets. Results described here are important for future quantum light experiments and their integration in photonic chips.

Defect Polaritons from First Principles.

Control over the optical properties of defects in solid-state materials is necessary for their application in quantum technologies. In this study, we demonstrate, from first principles, how to tune these properties via the formation of defect polaritons in an optical cavity. We show that the polaritonic splitting that shifts the absorption energy of the lower polariton is much higher than can be expected from a Jaynes-Cummings interaction. We also find that the absorption intensity of the lower polariton increases by several orders of magnitude, suggesting a possible route toward overcoming phonon-limited single-photon emission from defect centers. These findings are a result of an effective continuum of electronic transitions near the lowest-lying electronic transition that dramatically enhances the strength of the light-matter interaction. We expect our findings to spur experimental investigations of strong light-matter coupling between defect centers and cavity photons for applications in quantum technologies.

Temperature dependence of optical centers in Ib diamond characterized by photoluminescence spectra

The optical properties of defects in Ib diamond prepared by Chemical Vapor Deposition (CVD) and High Pressure High Temperature (HPHT) methods are studied by photoluminescence in a temperature range of 77– 297 K. The temperature dependence of color centers in diamond of 2.65 eV center, NV0, NV-, SiV-, 3H is studied. The zero-phonon line (ZPL) shift, intensity and full width at half maxima (FWHM) have a strong temperature dependence. The study shows that, due to the expansion of the lattice and the enhancement of the electron-phonon coupling strength, the ZPL shift can be described by the modified Varshni model which shows great potential in temperature sensing. The FWHM of ZPL matches a cubic function of temperature, this model can be employed to well describe the broadening of ZPLs in the defect-rich diamond. And the thermal quenching of absolute PL intensity can be described by the Arrhenius equation. Meanwhile, the difference of variable PL signal between CVD and HPHT samples is analyzed.

Defects and Properties in Polymorph of ZnO

In this work, polymorphic models of the structures of ZnO is considered. We have studied the defects and defects behaviours in Wurtzite, zinc blende, and rocksalt ZnO by using first principles and quantum simulation methods. We used density functional theory (DFT) and molecular dynamics (MD) simulation methods to investigate the structural and electronic properties of defect ZnO. For DFT calculations, a computer code within QUANTUM-ESPRESSO is used. Calculations of energies versus lattice constants display the optimum values of the lattice constants for crystal systems. Results are reported using a step-by-step approach, starting from the primitive cell of bulk ZnO, going to a slab model by including a surface charge defects. Generally, the behaviour of intrinsic defects in ZnO, electronic properties, formation energies and workfunction of ZnO were investigated. The effect of different surface absorbents and surface defects on the workfunction of ZnO were studied using DFT calculations. Anisotropy and optical properties of defects have also been studied.

Role of non-stoichiometric defects in optical properties of metal-selenide nanostructures

Metal-selenide nanostructures from different semiconductor families such as ZnSe, SnSe, and CuxSey were synthesized with different Se/cation ratios. The effects of non-stoichiometric defects such as vacancies and interstitials on their optical properties were investigated. X-ray diffraction patterns (XRD) indicated that the Se/Zn > 1 and Se/Cu > 1 conditions led to creating heterostructures of ZnSe/Se and CuxSey/Se, while, the sample with Se/Sn > 1 condition only showed a single phase of SnSe with border XRD peaks. Scanning electron microscopy (SEM) images of the samples showed different morphology for the ZnSe and CuxSey nanostructures with different conditions, while, SnSe samples had nanoparticles (NPs) morphology in all conditions. High-resolution transmission electron microscopy (HRTEM) images of SnSe samples showed that the SnSe NPs with Se/Sn = 1.2 condition were single-crystalline NPs, without any defects, while, the SnSe NPs with Se/Sn = 0.8 condition included vacancy and stacking fault defects. The optical band-gap values of the samples were not significantly changed by different Se/cation ratios. More details of the effects of non-stoichiometric defects on the optical properties of the samples were obtained by photoluminescence (PL) spectroscopy. The PL results of any type of samples showed that the non-stoichiometric defects could change the PL spectrum feature. Valence band spectroscopy results showed a shift in valence-band maximum (VBM) position in Se/cation>1 ratio in comparison to the Se/cation<1 ratio. However, this shift was bigger in the SnSe NPs and it was proposed that this bigger shift could be due to spin-orbit coupling (SOC) interaction, while, a valence-band offset was responsible for the VBM shift in the other nanostructures due to their heterostructure nature.

Emission properties of Ga2O3 nano-flakes: effect of excitation density

In the quest of developing high performance electronic and optical devices and more cost effective fabrication processes of monoclinic β-Ga2O3, new growth techniques and fundamental electronic and optical properties of defects have to be explored. By heating of dissolved metallic Ga in HCl in a NH3 and N2 atmosphere, nano-flake films of monoclinic β-phase Ga2O3 were grown as confirmed by XRD. From optical measurements, we observe two strong emissions. A red band peaking at ~2.0 eV and a UV band at ~3.8 eV. The band at ~2.0 eV is attributed to donor-acceptor pair recombination where the donor and acceptor level is suggested to be related to VO and nitrogen, respectively. By studying the dependence of the intensity of the UV band at 3.8 eV versus excitation density, a model is suggested. In the model, it is assumed that local potential fluctuations forming minima (maxima), where the carriers would be localized with a summarized band offset for conduction and valence band of 1 eV. The origin of the fluctuations is tentatively suggested to be related to micro-inclusions of different phases in the film.

电子辐照GaN基LED的缺陷光学性能研究

电子辐照GaN基LED的缺陷光学性能研究

Optical properties of defects in nitride semiconductors

Abstract

(Invited) Neutron Irradiation Effect on GaN-Based Materials

For the predominant property, GaN becomes a promising material for ultraviolet/ blue light-emitting diode, solar blind ultraviolet photodetector, radiation resistant particle detectors, and high power high temperature electronic devices. Because of the radiation hardness of GaN, those devices have the potential to be used in outer space and other radiation environment. Irradiation induced defects play the main role in the reliability of GaN based devices. Recently, a strong effort by several groups was devoted to the analyses of electrical and optical properties of defects and their interaction by irradiating GaN with electrons, ions, neutrons and protons [1–4], which is very important for the devices operating at extreme harsh environment. Positron annihilation spectroscopy (PAS) with a slow-positron beam is an efficient tool for the investigation of open-volume defects such as vacancies in semiconductors, at which they get trapped. In a vacancy the electron density is decreased which makes the positron lifetime longer and narrows the positron-electron momentum distribution. The latter can be observed by recording the Doppler broadening of the energy spectrum of the annihilation radiation. Especially the momentum distribution of the core electrons is sensitive to the chemical environment of an annihilation site. However, in the case of light elements such as O or N the annihilation probability with the core electrons is negligible. The samples used in the present work were grown on C-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD) with unintentionally doped and Si doped. A 20 nm low temperature GaN nucleation layer was initially grown, and 4 μm GaN film grown at 1050°C was followed. The sample was cut into small pieces with an area of about 10×10 mm2, and irradiated with 2MeV proton from heavy ion accelerator. The incident fluence is 1014, 1015, or 1016 cm-2, respectively. Positron annihilation spectroscopy, XRD, and Raman spectra were measured. Irradiation induced defect is vacancies which increased with incident proton. XRD result shows that the crystal-order was kept after proton irradiation. Raman spectra of the Si doped sample shows a “carrier removal” effect but there is not any change in the other spectra.

Development of interference absorptive filters on In4(Se3)1-х Te3х and CdSb basic crystals

Interference absorptive cutting light filters in the near and middle IR-region on the base of In 4 (Se 3 ) 1- х Te 3 х and CdSb crystals are obtained. Substrate crystals and films systems have been investigated by raster scanning microscopy, X-ray microanalysis, optical and electrophysical measurements to determine their suitability for developing effective cutting filters. The optical properties of defects in the crystals and substrates, their influence on the characteristics of the optical filter are studied. Low density of defect is observed in In 4 (Se 3 ) 1-x Te 3x crystals with [010] growth direction, which is prevailing for the manufacture of optical substrates. It is found that transparency of CdSb and ZnSb crystals and substrates is kept on cooling up to 77 K. The materials for film interference systems of filters different constructions and working range are chosen. Ge-SiO semiconductors are used as a film-forming material for the filters with cutoff in the middle infrared region and Te-SrF 2 in the more far regions. Ontained filters are characterized by the following parameters: cutoff slope k≥0,9, mean passband Т m ≥80 %, maximum transmission Т max ≥90 %. Spectral transmission characteristics of optical filters of various designs are measured, the condition of their mechanical stability and reproducibility of optical parameters are investigated.

Coupled Theoretical and Experimental Studies for the Radiation Hardening of Silica-Based Optical Fibers

We applied theoretical and experimental spectroscopy tools to ad hoc silica-based samples to characterize the influence of several dopants and of some drawing process parameters on their radiation sensitivities. We present in this paper, the recent advances and results occurring from our coupled approach. On the experimental side, we studied the doping influence on the response of optical fibers and showed that changing the drawing parameters has a negligible influence on the fiber response in the case of specialty fibers. We focus mainly on the ${rm SiE}^prime$ defect that is observed through Electron Paramagnetic Resonance (EPR) measurements in all canonical samples. On the theoretical side, we exhibit the improvements obtained in the calculations of electronic and optical properties of defects by using Many Body Perturbation Theory through the use of the GW approximation and the resolution of the Bethe-Salpeter equation instead of the Density Functional Theory (Local Density Approximation). To continue to strengthen the link between experiment and simulation, we have performed first-principles calculations of EPR parameters of some silica-based defects. The first results allowing for an attribution of EPR E' signals to structural models are presented. In particular, we confirm that the E'g center is originated by an unpaired electron in a sp3 state at a three-fold coordinated silicon atom.

Photoluminescence spectral dispersion as a probe of structural inhomogeneity insilica

We perform time-resolved photoluminescence measurements on point defects inamorphous silicon dioxide (silica). In particular, we report data on the decaykinetics of the emission signals of extrinsic oxygen deficient centres of thesecond type from singlet and directly excited triplet states, and we use themas a probe of structural inhomogeneity. Luminescence activity in sapphire(α-Al2O3) is studied as well and used as a model system to compare the optical properties ofdefects in silica with those of defects embedded in a crystalline matrix. Only fordefects in silica did we observe a variation of the decay lifetimes with emissionenergy and a time dependence of the first moment of the emission bands. Thesefeatures are analysed within a theoretical model, previously proposed in D’Amico et al (2008 Phys. Rev. B 78 014203), with an explicit hypothesis about the effect introduced bythe disorder of vitreous systems. Separate estimations of the homogeneous andinhomogeneous contributions to the measured emission linewidth are obtained.It is found that inhomogeneous effects strongly condition both the triplet andsinglet luminescence activities of oxygen deficient centres in silica, although thedegree of inhomogeneity of the triplet emission turns out to be lower than that ofthe singlet emission. Inhomogeneous effects appear to be negligible in sapphire.

Investigation of oval defects in (In)Ga(Al)As/GaAs heterostructures by spatially resolved photoluminescence and micro-cathodoluminescence

Oval defects, which are commonly found in epitaxial (In)Ga(Al)As heterostructures grown by molecular beam epitaxy (MBE) technique, are studied in this paper. The investigations of the morphology as well as the optical properties of defects were performed by scanning electron microscopy (SEM), cathodoluminescence (CL) and spatially resolved photoluminescence (SRPL). The conclusions are drawn as to the sources of defects, conditions of their appearance and their possible internal structure.

Conformational disorder and optical properties of point defects in vitreous silica

Disordered systems are characterized by the presence of local conformational heterogeneity, which reflects the complex landscape of the potential energy of the vitreous state. Optical properties of defects embedded in a vitreous matrix are also determined by the interaction with the surrounding environment; so the conformational disorder of the system induces spectral inhomogeneity. As a consequence, detailed experimental investigation of absorption and photoluminescence bands can give information on configurational substates around the chromophore. We focused our attention on B-type optical activity in silica glasses, characterized by a singlet emission and a triplet emission, connected via a phonon-assisted intersystem crossing (ISC) process. In particular, the ISC shows a strong dependence on excitation energy and a non-Arrhenius behaviour on varying the temperature. We take into account the observed thermal behaviour over a wide temperature range, assuming a distribution of the activation energies of the ISC process. This approach outlines the correlation between optical properties and conformational heterogeneity and indicates that the ISC process is driven by the entropic contribution, revealing the fundamental role of the dynamics of the surrounding matrix.

Electrical and optical properties of defects and impurities in ZnO

Advancements in ZnO device applications have fostered much interest in the electrical and optical activities of various defects and impurities in the material. Although it has long been known that Group III dopants, such as Al, make efficient donors, the roles of other impurities, such as H and N, are only recently being elucidated. The same is true of the simplest point defects, such as Zn and O vacancies and interstitials. Theoretical work has been essential in identifying and understanding various defects and impurities. For example, theory has shown that H is always a donor (not amphoteric), that the O vacancy is a deep donor, not shallow, and that the Zn interstitial is a shallow donor, in agreement with electron-irradiation (EI) experiments. Recent irradiation studies show that significant defect annihilations take place, even at low temperatures, thus showing why ZnO is so resistant to radiation effects. To develop applications involving electroluminescence, it will be necessary to identify a reliable acceptor dopant, and N, P, and As have been most thoroughly investigated so far. In fact, p-type samples with resistivities <1Ω-cm have been demonstrated, but certain questions remain unanswered.

Structural, electrical, and optical properties of defects in Si-doped GaN grown by molecular-beam epitaxy on hydride vapor phase epitaxy GaN on sapphire

Molecular-beam epitaxy (MBE) has been utilized to grow Si-doped GaN layers on GaN/sapphire templates prepared by hydride vapor phase epitaxy. An extensive set of characterization techniques is applied to investigate the layers. Positron annihilation experiments indicate that the samples contain open volume defects, most likely clusters of vacancies and possibly Ga vacancy-donor complexes. The number of vacancy clusters decreases, as Si concentration is increased. Photoluminescence spectra show that while the absolute intensity of both the yellow and ultraviolet (UV) band-edge transitions increase with Si doping, the intensity ratio of yellow-to-UV emission is decreased. Secondary ion mass spectrometry indicates that the impurity concentrations are in qualitative agreement with the carrier concentrations determined in electrical experiments. The data suggest further that silicon does not affect the diffusion of oxygen. Moreover, transmission electron microscopy reveals that MBE-grown GaN retains the threading dislocation structure of the GaN/sapphire substrate. The MBE layer is also shown to grow in the Ga face orientation, but some inversion boundaries are present in the epilayer.