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The photostimulated luminescence (PSL) from Eu-doped fluorobromo- and fluorochlorozirconate glass-ceramics is reviewed. The PSL is attributed to the characteristic emission of Eu2+ present in BaX2 (X = Br, Cl) crystallites which form in the glass upon thermal processing. Both the photoluminescence intensity and the PSL efficiency increase with the size of the crystallites in a way that is consistent with a PSL-inert surface layer surrounding the crystallites. The increasing crystallite sizes also result in decreased optical transparency at the stimulation wavelength due to scattering. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Tourmaline with the general formula XY 3 Z 6(BO 3) 3Si 6O 18(OH,O) 3(OH,F) and the trigonal space group R3 m ( C 3 v 5) is known as a gemstone with great variety of colors. Some color centers are related to transition metal ions, and others to electron or hole traps. In this paper we report on a combined study using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and the optical detection of EPR (ODEPR) on a yellow color center produced by ?-irradiation in colorless Li-bearing elbaite tourmaline from Brazil. The color center is an O - hole trap center, which is stabilized within the plane spanned by three Y sites, and is located in the structural channels formed by Si 6O 18. We suggest that two of the Y sites are substituted by 27Al and the other by 6,7Li. During the irradiation process atomic hydrogen H 0 is also produced, which shows the same thermal stability as the hole center (250 °C). Therefore, we assign H 0 to be the local charge compensator for the hole trap. From the ODEPR measurements we conclude that the yellow color is caused by the O - hole center. The large negative isotropic Al superhyperfine interaction of the O - hole trap center is consistent with a calculation of the transferred hyperfine interactions by exchange polarization supporting the proposed defect model of an O - at the O 1 sites, whereby the O - is relaxed into the plane formed by three Y ions.

AbstractWe report the electronic properties, obtained by deep level transient spectroscopy (DLTS), of electron trap defects in n-GaN, grown by hydride vapour phase epitaxy (HVPE), metal-organic vapour phase epitaxy (MOVPE), reactive molecular beam epitaxy (RMBE) and, for the first time, epitaxial lateral overgrowth (ELO) MOVPE. Two prominent traps with energy levels at EC− 0.27 eV and EC− 0.61 eV, respectively, are introduced during all four methods. Their filling kinetics suggest that they are extended defects. On the other hand, the levels at EC− 0.65 eV and EC− 0.21 eV are only detected in HVPE and RMBE grown GaN, respectively, and appear to be characteristic of these methods.

AbstractThe structure of oxygen-related luminescence centres in nominally undoped and Y2O3-doped AIN ceramics were investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and optically-detected EPR. The photoluminescence-detected EPR lines having gvalues of 1.990 and 2.008 were attributed to neighbouring donor and acceptor pairs causing the recombination luminescence excited in the ultraviolet. The two EPR lines at g= 1.987 and g= 2.003, detected via the recombination luminescence in the afterglow, are thought to be due to a recombination between the same, but more distant donor and acceptor pairs. The donor is supposed to be an electron trapped at an oxygen impurity which substitutes for a nitrogen (ON)−. The defect structure of the acceptor was established by ENDOR to be a hole trapped at an ON-VAlcomplex (VAl= Al vacancy).

AbstractWe determine, using deep level transient spectroscopy, the electronic and annealing properties of defects introduced in MOVPE grown n-GaN by high energy electron, helium-ion and proton irradiation. As in other semiconductors these three particles introduce electrically active defects in n-GaN. There are 6 electron defects (ER1-ER6) introduced in the hexagonal n-GaN. A deep lying defect (1.04 eV), ER6, was detected and characterised for the first time. Defect ER3 which exhibits a field dependent emission rate has been reported to be the nitrogen vacancy. Defect ER2 appears to have a higher introduction rate when using protons. The deeper lying defects ER4-ER6 which are positioned reasonably close to one another have peak temperature positions which differ depending on the incident particle mass.

AbstractBy means of magnetic circular dichroism of the optical absorption and photostimulated luminescence (PSL) the X-ray induced formation of FAand F centres in RbBr:(Ga+, Li+) was investigated. It turnes out that RbBr:Ga+co-doped with 1% Li+in the melt reveals the largest red-shift of the PSL excitation bands to 790 nm, whereby the FAto F ratio increases up to about 10% with decreasing X-ray dose. This is more than statistically expected (6%). High X-ray doses destroy FAcentres. However, up to 15% of the simultaneously generated F centres could be converted into the FAspecies by appropriate bleaching with 633 nm light into the F centre absorption band.

AbstractColour centres in alkali halides such as F centres and H or VKcentres have been studied intensely for decades but have hardly found any applications. However, in activator-doped halides their generation mechanisms upon irradiation with X-rays, their room temperature stability and photostimulated recombination processes are the basis of important applications in digital radiography, which has recently become commercially available. An overview is given of the present understanding of the point defect mechanisms in the commercially used X-ray storage phosphor BaFBr:Eu2+and its shortcomings. Further developments in BaFBr-based X-ray storage phosphors are discussed. Promising cubic X-ray storage phosphor materials with high figures of merit and potentially higher spatial resolution, such as RbBr:Ga+, CsBr:Ga+, RbBr:Eu2+and CsBr:Eu2+have recently been studied in detail and an overview of the generation and recombination mechanisms of the electron and hole trap centres active in the information storage and read-out processes is presented. Finally, first results are shown of the use of glass ceramics as potential new storage material in particular for high-energy X-rays.

N-type silicon doped with gold and lithium was investigated by Electron Paramagnetic Resonance (EPR) and Deep Level Transient Spectroscopy (DLTS), using two sets of samples with different defect concentrations of both gold and lithium. Photoluminescence (PL) on both sets of samples allowed to correlate the results of EPR and DLTS measurements. It is shown that the gold–lithium donor state, Au2––(Li+)3, with trigonal symmetry is most likely the origin of the well known DLTS level L1 at EC – 0.41 eV.

The structure of oxygen-related luminescence centres in nominally undoped and Y2O3 doped AlN ceramics was investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and optically-detected EPR. The photoluminescence-detected EPR lines having g-values of 1.990 and 2.008 were assigned to a recombination between neighbouring donor and acceptor pairs. The two EPR lines at g = 1.987 and 2.003 detected via the recombination luminescence in the afterglow are thought to be due to a recombination between the same, but distant donor and acceptor pairs. The donor was previously speculated to be an electron trapped on an oxygen impurity which substitutes for a nitrogen on a regular lattice site. The defect structure of the acceptor was established by ENDOR to be a hole trapped on an ON–vAl complex (ON oxygen on a regular N site, vAl Al vacancy). The measured superhyperfine interaction is caused by two equivalent 27Al nuclei both with a = ±27.0 MHz.

It is well known that exposure of semiconductor surfaces to energetic particles introduce both optically and electrically active defects. Hydrogen and He-ion implantation has been used in GaN-based microelectronic processes. He-ion implantation produced high resistivity GaN at a fluence that is compatible with photoresist masking techniques. Heavier ion implantation has the added advantage that the depth resolution of the incident ions can be ‘tuned’ for specific applications (lifetime tailoring, etc.). Using deep level transient spectroscopy the defects in as-grown n-GaN as well as those introduced during high energy proton, He-ion and electron bombardment are characterised. Prior to irradiation, four electron defects (EO1–EO3 and EO5) were observed in the as-grown GaN. Two defects ER1 and ER2, not previously observed after electron irradiation, were observed after high energy electron irradiation. He-ion, proton and electron irradiation introduced a defect ER3. Defects ER1 and ER2 were also observed after proton bombardment, whereas two deeper lying defects (ER4 and ER5) were observed after the He-ion bombardment. The electronic properties, introduction rates and the annealing kinetics of the particle induced and as-grown major defects are presented. The influence of defect removal on the Schottky barrier diode properties are also discussed.

Sputter deposition is a versatile method of depositing metal layers onto semiconductors, but it introduces electrically active defects at and below the semiconductor surface. We have used deep level transient spectroscopy (DLTS) to study the electrical properties of defects, ES1–ES4, introduced in epitaxial n-GaN during sputter deposition of Au Schottky contacts. Two of these defects, ES1 and ES2, located at EC−0.22±0.02 eV and EC−0.30±0.01 eV, respectively, exhibit a metastable-like behaviour. ES1 and ES2 can both be removed under a zero bias annealing at temperatures of as low as 20 K and are re-introduced during reverse bias annealing at temperatures of 100–125 K and 115–140 K, respectively. The re-introduction of ES1 was found to follow first order kinetics with an activation barrier of (0.22±0.02) eV.

AbstractIn the nonstoichiometric X-ray storage phosphor BaF1.1Br0.9a new intrinsic hole centre was found by electron paramagnetic resonance (EPR). It is the anticentre of the F(Br−) centre with which it is generated simultaneously by X-irradiation. Bleaching into the F(Br−) band destroys it. From the simulation of the power EPR spectrum and the generation mechanism the hole centre is thought to be an H(F−2,F) centre. Magic angle spinning nuclear magnetic resonance (MAS-NMR) revealed that the F excess in non-stoichiometric BaF1.1Br0.9is due to F−on Br−sites, i.e. F−antisites. The generation of F(Br−) electron trap-hole trap pairs upon X-irradiation originates at the F−antisites.

AbstractThe optical absorption band of Tl2+hole centres has been investigated by measuring the magnetic circular dichroism of the absorption bands (MCDA) as well as MCDA-detected electron paramagnetic resonance (MCDA-EPR). Several new absorption bands were found using the excitation spectra of the MCDA-EPR lines of TI2+. In the iodides there is a total of 7 MCDA bands, more than predicted by the molecular orbital picture discussed so far in the literature.

AbstractAfter room temperature X-irradiation of normally-grown stoichiometric BaFBr single crystals two new oxygen hole centres were discovered and investigated by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR). One of these two new centres is an oxygen ion (O−) on a bromide site, however, with different super-hyperfine interactions compared to the one investigated previously. The second one is an oxygen molecule (O3-2) with one O atom on a fluoride site and one on the neighbouring bromide site, both sharing one hole. In total, there are now four different paramagnetic oxygen hole centres known is stoichiometric BaFBr single crystals.

AbstractWe investigated the recombination processes in the X-ray storage phosphor BaFBr: Eu2+via the detection of the microwave-induced changes in the recombination luminescence in a high magnetic field. Resonances of the F(Br−) centre, of the VK(Br2−) centre, of the Eu2+ground state and of a hitherto unknown hole centre were observed. The luminescence band of the F(Br−)-VK(Br2−) recombination peaks at about 400 nm, that of the F(Br−)-new hole centre recombination at about 350 nm.

With electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) the surrounding of the activator Eu2+ was investigated in SrFCl single crystals. The analysis of the Eu2+ superhyperfine interaction with the nuclei of the next fluorine shells yielded that Eu2+ substitutes for Sr2+ and is not shifted from a regular lattice site. This result settles previous conflicting conclusions about such a shift.

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Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) have been used to investigate the surrounding of the activator ion Eu2+ in single crystals of the X-ray storage phosphor BaFBr. An analysis of the Eu2+ superhyperfine interaction with the next fluorine neighbours showed that Eu2+ substitutes for Ba2+ and is not shifted from a regular position within experimental error, in spite of the large difference in ionic radii. EPR and ENDOR investigations on BaFBr:Eu2+ powders established that different methods of producing these materials have no influence on the local environment of Eu2+.

With electron nuclear double resonance the superhyperfine interactions of numerous 29Siand 13Clattice neighbours of 14Ndonors in 3C and 4H–SiC have been measured. It was attempted to explain them using the effective mass theory (EMT). The interactions of N donors on cubic sites in 3C–SiC can be well explained with EMT assuming N to be on C sites, but not on Si sites. N in 3C–SiC represents probably the only true EMT donor, since for shallow donors in Si EMT is not sufficient as shown previously. The interactions of N on the hexagonal sites in 4H–SiC are still reasonably well explained, while the EMT approach fails to explain N on the quasi-cubic sites. Comparison between 3C–SiC and 4H–SiC shows that the “central cell corrections” due to the chemical nature of N introduced into EMT previously for silicon is very small. The major contribution comes from inter-valley interactions.

In Sc-doped 6H–SiC epitaxial layers we observed several different electron paramagnetic resonance (EPR) spectra with a 45Schyperfine interaction pattern. The spectra are explained as being due to the isolated Sc0(S=12)acceptor on carbon sites but with different microscopic configurations. The spectra show a pronounced temperature dependence. At low temperatures the hole of the Sc0acceptor is located in a Sc0–Si bond either along the c-axis forming an axial centre or along one of the three other C–Si bonds giving rise to a monoclinic centre. At higher temperature the hole changes place rapidly between the three Sc–Si bonds resulting in an axial centre. All three Sc0hole configurations have different ionisation levels. The ionization level of the low-temperature monoclinic–high-temperature axial centre is significantly influenced by an entropy term. In 4H–SiC spectra were only observed from a single Sc0centre with monoclinic symmetry.

We show that a deep level, the ES1, introduced in n-GaN by sputter-deposition of gold Schottky contacts exhibits metastable-like behaviour during temperature cycling between 55 and 250K. The ES1 has an energy level 0.22±0.02eV below the conduction band. We provide some evidence that indicates that the defect responsible for the ES1 has a second energy level, the ES1∗, and that the metastable behaviour of the ES1 may be due to negative-U ordering of these two energy levels. Furthermore, field effect measurements indicate that the ES1 level has a donor character, while the ES1∗level is probably an acceptor.

We have used deep-level transient spectroscopy (DLTS) to study the electrical properties of defects introduced in epitaxial n-GaN during electron beam (EB) deposition of Ru Schottky contacts. DLTS revealed that EB deposition introduces at least two defects, Ee1 and Ee2, with energy levels at EC−(0.19±0.01)eV and EC−(0.92±0.04)eV, respectively, in the band gap. The defect Ee1 has a similar signature as a radiation induced defect in GaN, speculated to be the VN. The concentrations of Ee1 and Ee2 both decrease away from the interface into the GaN. The concentration of Ee2, the more prominent of the two defects, is estimated as about 1016cm−3at the GaN surface. The effect of the EB deposition induced defects on the current–voltage characteristics of the Schottky contacts thus formed is to introduce recombination and generation components to the reverse and forward currents, respectively.

Laser action of X‐rayed lead doped KMgF3crystals from an unknown defect was recently reported by Hörsch and Paus.1By applying optical detection of ESR and ENDOR, the structure of two very similar laser active Pb centres could be determined from the selectively measured hyperfine and superhyperfine interactions. They consist of a Pb+substituting for a K+next to an F−vacancy along <110> with a next nearest K+vacancy for charge compensation in two different positions (Pb+(1) centres). The absorption band for the excitation of the laser emission at 1.4eV could be identified from the optical excitation spectra and from a special magneto‐optical ESR technique. The structural properties, as well as the explanation of their laser properties within a crystal field model, are analogous to those of Tl0(1) centres in alkali halides and Pb+(1) centres in alkaline earth fluorides. A Pb++perturbed FAcentre was also observed.

Iron-aluminum pairs in silicon are investigated with conventional and optically detected electron paramagnetic resonance (EPR). For the trigonal and orthorhombic pairs known from previous EPR measurements we found for the first time optical absorption bands by measuring their magnetic circular dichroism of the absorption (MCDA). Direct experimental evidence is presented for the configurational bistability of both pairs by showing that the MCDA of the trigonal configuration can be transformed into that of the orthorhombic configuration by the combined effect of light and temperature. A new trigonal pair was discovered by conventional EPR having the same EPR intensity as the known one. Total energy calculations of various (Fei-Als) pair configurations show that two trigonal (Fei-Als)0 pairs with different Fei-Als separations have almost the same binding energy and should occur with the same probability. Fei+is always on a tetrahedral interstitial site, while Als-is nearest neighbor along <111> in one pair, second nearest neighbor in the other one with one silicon lattice site in between.

Single crystalline NaCl doped with Rh2+was investigated with electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR). An orthorhombic Rh2+centre was found similar to that observed previously in AgCl, where Rh2+substitutes for Na+forming an [RhCl6]4−complex elongated along a [001] axis with a charge compensating Na+vacancy in a second nearest Na+position in the equatorial plane. In addition, two further axial Rh2+centres were found, where the Na+vacancy is not in the nearby equatorial plane. In the powder ENDOR spectra of NaCl:Rh2+the interactions with axial second nearest 23Na and axial 103Rh nuclei could be resolved. Powder ENDOR spectra of AgCl:Rh2+precipitated from aqueous solution showed that the Rh2+defect is different from that generated in Rh2+-doped AgCl grown from the melt. The limitations and possibilities of powder ENDOR are discussed. © 1997 Elsevier Science Ltd

In BaFBr, a material which can be used as a storage phosphor for X-rays, two types of F centres can be created by additive coloration or by X-irradiation. The properties of both F centres which play an important role in the storage phosphor mechanism were investigated with optical and magneto-optical methods. The absorption bands, the emission bands, production, and stability of the F centres are discussed.

The chemical diffusion of oxygen and the structure of the electronic defects in undoped and transition metal doped YSZ single crystals have been investigated by a contactless relaxation technique and magneto-optical resonance (MCD(A))-studies. The local structure and the transport properties of YSZ (9.5 mol% Y2O3) are interrelated by the actual defect distribution of 7- and 6-fold coordinated Zr-defects with trigonal symmetry which mainly originate from Zr d- conduction band tail states and act as electron traps.The strong interaction between the ionic and electronic transport properties can be quantitatively explained by a modified ambipolar transport model. Experimentally, the diffusion behaviour of oxygen can be precisely followed by the optical relaxation technique.

In X-ray storage phosphors an image is formed and stored by generation of room temperature stable radiation-induced electron and hole trap centres. The image is read-out by recording a photo-stimulated luminescence (PSL) from a doped activator, generally stimulating the electron trap centres. The best-known and hitherto most efficient X-ray storage phosphor is BaFBr:EuZ+. However, the exact mechanism of its functioning is not yet understood. The present discussion of the storage and read-out mechanisms is critically reviewed. New results about the role of oxygen Contamination of BaFBr:Eu2+ are presented: if there is much less oxygen present than Edc, then the PSL efficiency decreases and the stimulation energy increases. A new efficient X-ray storage phosphor is presented: Cs2NaYF6 doped with trivalent rare earth activators. Its properties are described and preliminary results on X-radiation-induced radiation damage centres in undoped Cs2NaYF6 are presented.

The paramagnetic rare earth ions Yb3+, Ce3+ and Gd3+ and the transition metal ion Cr3+ were investigated in the cubic elpasolite crystal Cs2NaYF6 with ENDOR. The superhyperfine (shf) interaction parameters of the first 19F, 23Na and 133Cs shells were determined. In the case of Gd3+ they were analyzed in an overlap model of the transferred hyperfine interaction including exchange polarisation.

The results of measurements of the magnetic circular dichroism of the optical absorption (MCDA) and optically detected electron paramagnetic resonance (ODEPR) of X-irradiated KBr:In crystals are presented. The MCDA bands and ODEPR parameters of In°(1) centers and In2+ centres have been measured. The mechanism of the energy storage in KBr:In crystals is found not to be simply the formation of correlated F centre-In2+ centre pairs as was assumed previously. Considerable similarities to the storage phosphor BaFBr:Eu2+ were found for the photostimulated emission and read-out properties.

The temperature dependence of the electron nuclear double resonance (ENDOR) spectra of FH(CN-) centres in KCl was investigated in the temperature range between 10-220 K. At the lowest temperature of 10 K only one CN- orientation with respect to the F centre electron is present, in which the nitrogen is thought to be nearer to the F-electron than the carbon. With the very small thermal activation energy of 2.9 meV the opposite orientation is occupied. The superhyperfine interactions of those first shell K nuclei nearest to CN- and of the 13C interaction of the CN- molecule are strongly temperature dependent between 10 and 60 K, following an exponential law with a thermal activation energy of 4.2 meV. It is assumed that a soft local mode involving those two nearest K nuclei and the CN- is causing the strong temperature dependence. The shf interactions of 14N nuclei have not been seen, probably because of the dynamical effects.

Pair defects consisting of F centres and CN- molecules as substitutional impurity anions (FH(CN-) centres) exhibit a strong coupling between the F centres and the CN- molecules. This leads to an optical pumping of the CN- vibrations via the F centre absorption band. We performed an Electron Nuclear Double Resonance (ENDOR) investigation in order to analyse the microscopic structure of these aggregate centres. The CN- molecule occupies a [110] nearest anion position with respect to the F centre in two dynamic dipole orientations even at low temperature.

A semiempirical analysis of the sizes and signs of the small boron hyperfine interaction constants as determined previously by electron nuclear double resonance in boron-doped 6H silicon carbide shows that the B−SI-C+model of this centre can account for the highly unusual feature of small isotropic and anisotropic boron hyperfine constants that are positive and negative, respectively.

The photo-stimulated luminescence (PSL) of Eu2+-doped BaFBr is excited in the two F centre absorption bands produced by X-irradiation at room temperature. The PSL intensity decreases with decreasing temperature by about a factor of 10. After bleaching at low temperatures the PSL is nearly zero, it can be regenerated by warming the crystal to room temperature. The results are discussed with a new model of the PSL process.

Pairs of F centres and OH- molecules (FH(OH-) centres) exhibit a very interesting optical and thermal bistability in KBr. The analysis of our Electron Nuclear Double Resonance (ENDOR) investigation yields structure models. In both bistable configurations the OH- molecules reside on a next nearest anion site (4th shell) relative to the F centre. The difference lies in the orientation of the OH- dipole with respect to the defect pair axis. The thermal bistability will be explained tentatively as being entropy-driven.

In different types of GaAs (p-type and semi-insulating) electron-irradiated at 4.2 K and kept below 80 K, a dominant defect with a quartet hyperfine split spectrum was observed with optically detected electron paramagnetic resonance. The defect is attributed to a Gallium vacancy-related defect. It decays at about room temperature while an As antisite-related defect, identified previously as an anti-structure pair, AsGa with GaAs as a second nearest neighbour (nnn), is formed. The spectrum of the magnetic circular dichroism of the Ga vacancy-related defect is interpreted to originate from photo-ionisation transitions to the valence band.

Direct experimental evidence for the spatial correlation between paramagnetic defects can be obtained by measuring the dipole-dipole interaction-induced cross relaxation effects between the defects using optical detection of the electron paramagnetic resonance via the magnetic circular dichroism of the absorption. This novel experimental method is explained. It is applied to the formation of F and H centres during relaxation of the self-trapped exciton in KBr at 1.5 K. Direct experimental evidence is presented that F and H centres are separated by four lattice spacings along 〈110〉 directions. This correlation is lost after warming to 30 K when H centres can diffuse away.