Self-trapped Holes Research Articles

Electron spin resonance study of self-trapped holes in CdWO4 scintillator crystals

The self-trapping of holes at oxygen anions was studied by electron spin resonance in UV irradiated CdWO4 crystals. Analysis of superhyperfine interaction of the holes with W183 and C111,113d isotopes shows that the self-trapped hole is either delocalized in the space between two energetically equivalent nearest neighbor oxygen ions or tunnels between them. When the temperature increases above 40–50 K the self-trapped holes are thermally liberated and can be retrapped by oxygen ions perturbed by impurity ions. In case of the Nb5+ or Li+ stabilizing impurities the O− centers are thermally stable up to 160–170 K. The study of kinetic characteristics of the self-trapped holes suggests that holes leave oxygen ions by thermally assisted tunneling mechanism via two slightly different channels. Corresponding ionization probabilities are defined by the Arrhenius law with an average thermal ionization energy E=90(5) meV. Calculated pre-exponential factors, about 105 s−1, are small, which is consistent with the tunneling mechanism. Thermal stability and kinetic characteristics of the trapped holes are discussed in light of the scintillation and thermoluminescence characteristics of CdWO4.

Kinetic Monte Carlo Model of Scintillation Mechanisms in CsI and CsI(Tl)

We have developed a computational model of energy transfer processes in scintillators using the kinetic Monte Carlo (KMC) approach. In this publication, we focus on the alkali halide compound CsI both pure and doped with a range of thallium concentrations. The KMC model makes use of an explicit atomistic representation of the crystal lattice, activator sites, defect sites, and individual electron-hole pairs. The probability of individual diffusion, recombination, and scintillation events is calculated from rate equations parameterized with data published in the literature. Scintillation decay curves, relative intensities of emission peaks, and light yields are computed and found to be in good agreement with experimental data for a range of temperatures and thallium concentrations. This demonstrates that the KMC scintillation model is capable of reproducing both the kinetics and the efficiency of the scintillation process in CsI. In addition, novel predictions emerge from our simulations such as the diffusion distance distributions of self-trapped holes and excitons. Finally, the KMC scintillation model provides a framework for probing possible physical processes responsible for the nonlinear relationship between scintillation light yield and incident gamma-ray energy.

Electron and hole trapping in irradiated PbBr2:Tl single crystals

Several hole trapped centres, namely Tl2+, self-trapped hole and A-type centres, were observed by ESR in PbBr2:Tl single crystals after X-ray irradiation at 77K. The corresponding spectra are visible up to 200K for all three defect centres. The STH centres are observed at higher temperatures than in the undoped PbBr2 single crystals, probably due to the stabilising effect of the Tl+ impurities. The ESR parameters of the Tl2+ centre point to stronger covalency effects in PbBr2 than in the isostructural PbCl2 crystals. All hole trapped centres started decaying around 160K, probably due to recombination with nonparamagnetic electron trapped centres. Besides a small concentration of self-trapped electron centres, no other electron trapped centres were observed.

Optically stimulated desorption of “hot” excimers from pre-irradiated Ar solids

Electronically induced desorption from solid Ar pre-irradiated by a low-energy electron beam is investigated by activation spectroscopy methods —photon-stimulated exoelectron emission and photon-stimulated luminescence in combination with spectrally resolved measurements in the VUV range of the spectrum. Desorption of vibrationally excited argon molecules Ar2*(ν) from the surface of pre-irradiated solid Ar is observed for the first time. It is shown that desorption of “hot” Ar2*(ν) molecules is caused by recombination of self-trapped holes with electrons released from traps by visible-range photons. The possibility of optical stimulation of the phenomenon is evidenced.

Luminescence spectroscopy of excitons and antisite defects in Lu3Al5O12 single crystals and single-crystal films

The nature of the intrinsic luminescence of the lutetium aluminum garnet Lu3Al5O12 (LuAG) has been analyzed on the basis of time-resolved spectral kinetic investigations upon excitation of two model objects, LuAG single crystals and single-crystal films, by pulsed X-ray and synchrotron radiations. Due to the differences in the mechanisms and methods of crystallization, these objects are characterized by significantly different concentrations of LuAl antisite defects. The energy structure of luminescence centers in LuAG single crystals (self-trapped excitons (STEs), excitons localized near antisite defects, and LuAl antisite defects) has been established. For single-crystal LuAG films, grown by liquid-phase epitaxy from a Pb-containing flux, the energy parameters of the following luminescence centers have been determined: STEs in regular (unperturbed by the presence of antisite defects) sites of the garnet lattice and excitons localized near Pb2+ ions. The structure of the luminescence centers, related to the background emission of impurity Pb2+ ions, has also been established in the UV and visible ranges. It is suggested that, in contrast to the two-halide hole self-trapping, a self-trapped state similar to STEs in simple oxides (Al2O3, Y2O3) is formed in LuAG; this state is formed by self-trapped holes in the form of singly charged O− ions and electrons localized at excited levels of Lu3+ cations.

Recombination luminescence of oxygen-deficient centers in silica

The luminescence of silica glass, prepared by plasma chemical vapor deposition (PCVD) and quartz glass of type IV (trade mark KS-4V) methods, were studied while irradiated with pulses of ArF laser (193nm) light in the range of sample temperatures between 10 and 300K. The samples contain less than 0.1ppm metallic and hydroxyl impurities. The samples synthesized by PCVD were of two kinds. The first one (amorphous) was as-deposited from plasma at a substrate tube temperature of ∼1200°C. The second one (fused) was prepared from the first by the tube collapsing with an external burner. In this process, a section of the substrate tube with the deposited glass was installed in a lathe and processed at a temperature of ∼2100°C during ∼20min until the tube was transformed to a rod. After such processing, the rod was cooled down to room temperature in air at an average rate of about 400°C per min. The only observed luminescence possesses two broad bands, with not well defined position, one at 2.6–2.9eV (a blue band) and another in the range of 4.4eV (an UV band). There is a correspondence in luminescence properties between KS-4V silica and fused PCVD silica. Those bands have been attributed to oxygen deficient centers (ODC). No luminescence is observed in amorphous PCVD silica under irradiation with 193nm laser light. So, formation of the sample by melting at least stimulates formation of ODCs at 193nm. The blue band decays obeys to power law ∼t−1 and is detected in the range of time 10ns to 300μs. The UV band possesses a fast, practically repeating excitation pulse, and a slow component (∼30μs). The obtained new kinetics data are compared with known in literature for lone twofold-coordinated silicon having exponential decay for the blue band equal to 10ms and 4.5ns for the UV band. That shows the blue band of new studied samples under ArF laser possesses decay component faster and the UV band slower than that of the twofold-coordinated silicon center. This corresponds to the recombination process of luminescence excitation by laser. We propose a model of the processes as charge separation under excitation with creation of a nearest self-trapped hole and electron trapped on the twofold-coordinated silicon, modified by its surrounding atoms or ions. This pair is recombining then with luminescence.

Structure and ESR properties of self-trapped holes in pure silica from first-principles density functional calculations

The electronic structure and spectral properties of self-trapped holes (STH's) in $\mathrm{Si}{\mathrm{O}}_{2}$ are revisited using pure and hybrid density functional theory (DFT) approaches and cluster models. The structure and electron spin resonance (ESR) parameters [hyperfine coupling constants (hfcc's) and $g$ factors] have been determined for the two classical variants of STH's: ${\mathrm{STH}}_{1}$, consisting of a hole trapped at the $2p$ nonbonding orbital of an O atom bridging two Si atoms, and ${\mathrm{STH}}_{2}$, a metastable defect where the hole is delocalized over the $2p$ orbitals of two bridging O atoms. The computed ESR parameters fully support the experimental assignments based on the analysis of the ESR spectra. However, a nearly quantitative agreement between measured and computed hfcc's and $g$ factors is found only with a hybrid functional where 50% of the Hartree-Fock exchange is mixed in with the DFT exchange. Standard hybrid functionals, such as B3LYP, or pure DFT functionals, such as BLYP, fail completely in describing the two defects. According to these results, ${\mathrm{STH}}_{2}$ can form only in correspondence of specific O-Si-O angles of about 80\ifmmode^\circ\else\textdegree\fi{}--90\ifmmode^\circ\else\textdegree\fi{}, which classifies the defect center as a molecular polaron.

Modeling of light-induced defect creation in hydrogenated amorphous silicon

Light-induced defect creation in hydrogenated amorphous silicon $(a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H})$ is discussed in terms of a model based on the breaking of weak Si--Si bonds by self-trapping of holes under illumination. In this model, two separate dangling bonds, i.e., a normal dangling bond and a hydrogen-related dangling bond that is a dangling bond having hydrogen at a nearby site, are created under illumination. Further, we take into account dissociation of hydrogen atoms from hydrogen-related dangling bonds, termination of two types of dangling bonds by dissociated hydrogen atoms (metastable hydrogen atoms), insertion of dissociated hydrogen atoms into nearby weak Si--Si bonds, and formation of hydrogen molecules by collision of dissociated hydrogen atoms. Taking into account these processes, rate equations governing the kinetics of two types of dangling bonds and metastable hydrogen atoms under illumination are numerically solved for the case of continuous illumination. The calculated results are compared with experimental results taken from literature. Furthermore, we discuss the case of low-temperature illumination, the kinetics of light-induced dangling bonds given by stretched exponential function, and saturated density of light-induced dangling bonds in $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$.

Spectroscopic Study of $\gamma$-Ray and Pulsed X-Ray Radiation-Induced Point Defects in Pure-Silica-Core Optical Fibers

We investigate the radiation-induced effects on pure-silica-core (PSC) optical fibers. For this, we measured the radiation-induced attenuation (RIA) growth and decay kinetics in four fibers with different hydroxyl and chorine contents. Our results show that PSC fibers exhibit different transient and continuous radiation responses depending of the silica-glass composition. Self-trapped charges [self-trapped excitons (STEs) and self-trapped holes (STHs)] seem mainly responsible for the fiber transient responses (times shorter than 1 s after the X-ray pulse) in the ultraviolet to near infrared part of the spectrum (300-900 nm). As these defects are unstable at room temperature, the contribution of other defects, like non-bridging oxygen hole centers (NBOHC) increases with time. Finally, these stable defects mainly explain the fiber permanent responses. To complete our online RIA measurements, we also studied by confocal microscopy luminescence (CML) the spatial distribution of the stable radiation-induced emitting centers in both pristine and irradiated fibers. Our CML results, obtained with 514 and 633 nm laser excitation, showed that several optically-active centers are non-uniformly generated in the fiber cross-sections. The generation of NBOHC by breakage of Si-O-Si strained bonds is clearly enhanced in high-OH fibers, especially at their core and cladding interfaces. The generation mechanisms of other unidentified defects emitting around 600 nm seem to be affected by both the hydroxyl content in the fiber core and by another factor that is not related to the silica glass composition.

Laser light-stimulated exoelectron emission from solid Ar pre-irradiated by an electron beam

Spatially separated stable charge centers, self-trapped holes and trapped electrons, were generated in Ar cryocrystals by a low-energy electron beam. A combination of the cathodoluminescence (CL) and photon-stimulated exoelectron emission (PSEE) methods was used to monitor center formation and a selected relaxation channel—exoelectron emission. It was found that photon-promoted electron current decreased exponentially under irradiation with the laser operating in the visible range. The influence of the laser parameters (power and wavelength) on the characteristic lifetime of exoelectron emission is discussed. Effective bleaching of the low-temperature peaks of thermally stimulated exoelectron emission by the laser light in a visible range is observed.

Magnetic moment and coupling mechanism of iron-doped rutileTiO2from first principles

The magnetic ground state of Fe-doped rutile ${\mathrm{TiO}}_{2}$, an oxide-based dilute magnetic semiconductor, has been investigated within the hybrid-exchange approximation to density-functional theory. ${\mathrm{Fe}}_{x}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{O}}_{2}$ with $x=0.125$ has been simulated by means of a 24-atom supercell with Fe doped substitutionally for Ti and this is established as the dilute limit through explicit comparison with calculations on a 192-atom supercell $(x=0.0156)$. A detailed study of the nature and stability of the predicted ground state with respect to variations in the oxidation state of the Fe ion, the delocalization or self-trapping of holes donated to the lattice, and the treatment of electronic exchange and correlation is presented. The ground state is found to be well described by a model based on an ${\mathrm{Fe}}^{3+}\text{\ensuremath{-}}{d}^{5}$ in a high spin state coupled to a partially delocalized hole accommodated in the $2p$ states of neighboring oxygen ions. No evidence is found for ferromagnetic coupling suggesting that the observed ferromagnetism in this system is dependent upon additional structural and/or electronic defects.

Thermostimulated luminescence and electron spin resonance in X-ray- and photon-irradiated oxygen-deficient silica

Influences of oxygen-deficiency on radiation properties of high-purity, low-OH fused silica were studied. It is found that thermostimulated luminescence (TSL) peaks are different for photo (7.7 eV) and X-ray excitation at 77 K. X-ray excitation produces TSL peaks at 125 and 170 K corresponding to the anneal temperatures of two types of self-trapped holes centers STH 2 and STH 1, respectively, detected by electron spin resonance (ESR). Oxygen-deficiency apparently increases the number of electron traps, stabilizing a larger number of STHs in the continuous defect-free silica network than is observed in similarly X-irradiated stoichiometric silica glasses. Photoexcitation of oxygen-deficient glass at 7.7 eV produces TSL peaks at 105 and 200 K, presently attributed to STHs at perturbed sites in the immediate vicinity of oxygen-deficiency centers (ODCs). High temperature TSL peaks at 240 and 400 K are produced by both types of irradiations and are followed by ESR detection of E′ centers only. All observed TSL and ESR signals were proportional to the level of oxygen-deficiency. The main spectral band in TSL near 2.7 eV is a triplet–singlet transition, ascribable to the twofold-coordinated silicon center (ODC(II)) modified by its nearest structure. It is proposed that this recombination process results when a thermally detrapped STH encounters an electron trapped at the site of ODC(I), and is transmuted into a modified ODC(II):e −. In principle, any such ODC:e − defect should be paramagnetic. However, no trapped-electron centers were detected by ESR in the present experiments. Based on the recent work of others, the E α ′ centers that were observed are believed to be trapped hole centers. Thus, for reasons unknown, the postulated ODC(II):e − centers, which may be the primary electron traps in oxygen-deficient silicas, appear to be ESR-silent.

Reduction of the radiation-induced absorption in hydrogenated pure silica core fibres irradiated in situ with γ-rays

We studied the efficiency of a hydrogen-treatment in different pure silica core fibres irradiated by γ-ray in an attempt to decrease the radiation-induced absorption (RIA). It is observed that the effect of hydrogen already reduces the drawing-induced band commonly observed in low-OH fibres. During irradiation we observe a large increase of the radiation-induced OH growth with a corresponding significant reduction of the radiation-induced absorption at 0.63 μm. The presence of hydrogen also reduces the transient radiation-induced absorption at 0.7 μm sometimes observed in low-OH/low-chlorine fibres and believed to pertain to self-trapped holes (STHs).

Structure and spectroscopic properties of trapped holes in silica

We present the first consistent comparison of the theoretically predicted structures and spectroscopic characteristics of holes trapped in crystalline and amorphous silica. The structure and spectroscopic properties of Ge and [AlO 4] 0 hole centers in α-quartz and self-trapped holes (STH 1 and STH 2) in silica glass were calculated using an ab initio embedded cluster method. The calculated principal values of g-tensor and hyperfine constants as well as the positions of maxima of optical absorption bands are in good agreement with the experimental data. In the case of Ge and [AlO 4] 0 hole centers, as well as STH 1, the hole is almost fully localized on one O atom. In STH 2 the hole is delocalized over two bridging O atoms. The calculated principal values of the g-tensor and hyperfine constants of STH 1 and STH 2 support the assignment based on ESR studies. The calculated optical absorption energies of one-center holes in a amorphous silica agree with the experimentally observed absorption bands assigned to STH 1.

Electron, hole and exciton self-trapping in germanium doped silica glass from DFT calculations with self-interaction correction

Density functional theory (DFT) calculations were employed to understand the refractive index change in germanium doped silica glasses for the trapped states of electronic excitations induced by UV irradiation. Local structure relaxation and excess electron density distribution were calculated upon self-trapping of an excess electron, hole, and exciton in germanium doped silica glass. The results show that both the trapped exciton and excess electron are highly localized on germanium ion and, to some extent, on its oxygen neighbors. Exciton self-trapping is found to lead to the formation of a Ge E′ center and a non-bridging hole center. Electron trapping changes the GeO 4 tetrahedron structure into trigonal bi-pyramid with the majority of the excess electron density located along the equatorial line. The self-trapped hole is localized on bridging oxygen ions that are not coordinated to germanium atoms that lead to elongation of the Si–O bonds and change of the Si–O–Si bond angles. We carried out a comparative study of standard DFT versus DFT with a hybrid PBE0 exchange and correlation functional. The results show that the two methods give qualitatively similar relaxed structure and charge distribution for electron and exciton trapping in germanium doped silica glass; however, only the PBE0 functional produces the self-trapped hole.

The central role of oxygen on H +-irradiated Lu 2SiO 5 luminescence

The behavior of self-trapped defects (STDs) in ion-beam irradiated Lu 2SiO 5 (LSO) crystal has been investigated via temperature-dependent radioluminescence (RL) measurements. Production of oxygen vacancies is the major effect of H + irradiation on luminescencent properties of this phosphor. Luminescence centers for self-trapped exciton (STE) and self-trapped hole emission are assigned to oxygen vacancies and oxygen ions, respectively. Ion-induced structural damage modifies the thermal stability of the STDs and creates perturbed STEs. A striking effect of ion irradiation is the approximate factor-of-two enhancement of STE RL intensity that results from implantation of only a thin (∼250 nm) surface layer of LSO. This enhancement is attributed to ion-beam modification of a surface dead layer.

Comparative Study of Pulsed X-Ray and$gamma$-Ray Radiation-Induced Effects in Pure-Silica-Core Optical Fibers

We have investigated the variation of the optical absorption induced by pulsed (dose rate >108 rad/s) and continuous (<50 rad/s) gamma-ray exposures in pure-silica-core optical fibers. Tested multimode waveguides, designed with well-defined concentrations of hydroxyl groups and chlorine impurity, are possible candidates for integration in the plasma diagnostics of LMJ and ITER facilities. We evaluated their radiation-tolerance to low dose levels (<5times10 4 rad) in the visible and near-infrared parts of the spectrum. We measure the time dependent changes (10-9-10-1 s) of the radiation-induced attenuation (RIA) at fixed wavelengths and we complete these measurements with a spectral (450-1100 nm) analysis of these losses in the time range from 10-2 to 103 s. We compared the responses obtained under transient exposures with measurements done at same dose levels (5times103 rad) with lower dose rate (<102 rad/s). We showed that the radiation-induced changes are strongly dose rate dependent for this kind of optical fibers. Depending on the hydroxyl group concentration in the silica-glass, different point defects are shown to alter the fiber transmission after both irradiation types. The self-trapped holes play a particular role in the transient responses of optical fibers whereas non-bridging oxygen hole centers are mainly responsible for gamma-ray radiation-induced losses. The consequences of these different behaviors for the integration of optical fibers in the LMJ are discussed

Distributions of self-trapped hole continuums in silica glass

Photobleaching of self-trapped holes (STH) in low temperature UV-irradiated silica glass has been investigated by the electron spin resonance method. The bleaching time dependence of the decay of two kinds of STH, STH1, and STH2, could be well fitted by the stretched exponential function, and STH2 has a quicker decay than STH1. On the other hand, the decay becomes significant large when the photon energy increases from 1.5to2.0eV, and then keeps constant with a further increase of photon energy. The distributions of the STH continuums are estimated at the positions on top of the valence band, being 1.66±0.27eV for STH1 and 1.63±0.33eV for STH2. A possible recombination mechanism is proposed to explain the decay of STH signals.

Transient optical absorption in pulsed-X-ray-irradiated pure-silica-core optical fibers: Influence of self-trapped holes

We investigated pulsed-X-ray-induced attenuation in two pure-silica-core (PSC) optical fibers with low-chloride (Cl) and different hydroxyl (OH) contents. We measured at room temperature the temporal (100ms–1000s) and spectral (0.73–3.1eV) variation of the induced optical absorption after a 1MeV X-ray pulse in both low- and high-OH PSC multi-mode fiber samples. A component of the transient loss in the low-OH sample was found to comprise absorption bands at 1.63 and 1.88eV (760 and 660nm) arising from self-trapped holes (STHs). Already known to be unstable at room temperature, STHs appear to play a key role in the transient responses of low-OH/low-Cl pure-silica-core fibers in the visible/near-infrared part of the spectrum.

Self-trapped holes in pure-silica glass: A history of their discovery and characterization and an example of their critical significance to industry

It has long been assumed that hole self-trapping should take place in amorphous silicon dioxide (a-SiO2). However, no spectroscopic evidence for this was claimed before 1989, when the author used electron spin resonance (ESR) to identify self-trapped holes (STHs) in bulk samples of low-OH pure fused silica X irradiated ⩽100K, and Chernov et al. reported a low-temperature infrared absorption near 1600nm in irradiated pure-silica-core fibers, which they ascribed to STHs. Based on g values and 29Si and 17O hyperfine coupling constants measured by ESR, Griscom [D.L. Griscom, Phys. Rev. B 40 (1989) 4224; D.L. Griscom, J. Non-Cryst. Solids 149 (1992) 137] deduced the existence of two types of STHs: STH1 (a hole trapped on a single bridging oxygen) and STH2 (a hole delocalized over two equivalent bridging oxygens of the same SiO4 tetrahedron). The validity of Griscom’s models for STH1 and STH2 are supported by the ab initio calculations of Pacchioni and Basile [G. Pacchioni, A. Basile, Phys. Rev. B 60 (1999) 9990] and Gabriel [M.A. Gabriel, PhD dissertation, Department of Chemistry, University of Washington, Seattle, WA, in preparation]. In 1984, Nagasawa et al. reported that low-OH-pure-silica-core optical fibers γ irradiated at ∼300K exhibit metastable optical absorption bands at 660 and 760nm. Griscom [D.L. Griscom, J. Non-Cryst. Solids 349 (2004) 139] recorded these same bands in low-OH-pure-silica-core fibers γ irradiated at 77K, showing their isochronal annealing behaviors to correlate with his earlier ESR data for STHs in bulk silica and also with Harari et al.’s data [E. Harari, S. Wang, B.S.H. Royce, J. Appl. Phys. 46 (1975) 1310] for trapped positive charges in silica thin films following X irradiation at 77K. Sasajima and Tanimura [Y. Sasajima, K. Tanimura, Phys. Rev. B 68 (2003) 014204] established direct correlations of an induced band at 574nm with STH2 by performing both ESR and a variety of optical measurements on three types of high-purity bulk silicas following pulsed electron irradiations at 77K; however, these authors did not detect the 660 or 760nm bands. Yamaguchi et al. [M. Yamaguchi, K. Saito, A.J. Ikushima, Phys. Rev. B 68 (2003) 153204] demonstrated that the yield of ESR-detected STHs photoinduced in bulk pure-silica samples at 77K depends exponentially on fictive temperature (Tf). The present paper recounts the forgoing history in greater detail, while attempting to reconcile some seemingly disparate findings into a unified picture of STHs in silica. In the fall of 1998, a number of satellites in orbit were tumbling out of control due to failure of their HeNe ring-laser-gyro (RLG) attitude control systems. The author, acting in the capacity of pro-bono US government consultant, proposed the correct solution to this problem (replace Al-contaminated silica mirror coatings with high-purity ones). However, accelerated tests (short operation times at much-higher-than-normal laser powers) of the corrected devices failed to corroborate this fix. Thus, all further launches of satellites employing these RLGs remained grounded until the author was able to convince industry troubleshooters that (1) the accelerated test failures were due to the 660-nm STH band (which is induced even in the highest-purity silica coatings by 20-eV photons emitted by the laser plasma), (2) the strength of this band is initially proportional to ionizing dose rate (inevitably giving false positives in accelerated tests), and (3) this band eventually disappears after several months of irradiation, even at dose rates as low as 0.15Gy/s. All of these insights derived from curiosity-driven components of the author’s research.