K. T. Stevens

Identification of electron and hole traps in KH2PO4 crystals

Electron paramagnetic resonance (EPR) has been used to characterize a hole trap and several electron traps in single crystals of potassium dihydrogen phosphate (KH2PO4 or KDP). The paramagnetic charge states of these centers are produced by ionizing radiation (e.g., x rays or a 266 nm beam from a pulsed Nd:YAG laser) and are stable for days and even weeks at room temperature. One center consists of a hole trapped on an oxygen ion adjacent to a silicon impurity located on a phosphorus site. This defect has a small, but easily observed, hyperfine interaction with the adjacent substitutional proton. The other centers are formed when an electron is trapped at an oxygen vacancy. These latter defects are best described as (PO3)2− molecular ions, where the primary phosphorus nucleus is responsible for a large hyperfine splitting (500–800 G in magnitude). Five EPR spectra representing variations of these oxygen vacancy centers are observed, with the differences being attributed to the relative position of a nearb...

Characterization of defect-related optical absorption in ZnGeP2

A broad optical absorption band with a peak near 1 μm is present in most single crystals of ZnGeP2. These same crystals have an electron paramagnetic resonance (EPR) signal which has been assigned to singly ionized zinc vacancies. A direct correlation between the intensity of the optical absorption at 1 μm and the intensity of the EPR signal has been established using a set of ZnGeP2 crystals where this absorption varied widely. These results suggest that the singly ionized zinc vacancy acceptor plays a direct role in the electronic transition(s) responsible for the 1 μm optical absorption. In separate experiments, it was found that illuminating the ZnGeP2 crystals with a He–Ne laser (632.8 nm) while at temperatures near 25 K produces an increase in the absorption at 1 μm and an increase in the zinc vacancy EPR spectrum. These latter results provide further evidence that the absorption at 1 μm is associated with the singly ionized zinc vacancy acceptor.

Infrared absorption bands associated with native defects in ZnGeP2

An optical absorption investigation from 10 to 296 K has been performed on bulk crystals of ZnGeP2 grown by the horizontal-gradient-freeze method. We identify three broad absorption bands in the spectral range from 1 to 4 μm that are due to native defects. At low temperature, a band peaking near 1.2 μm and another band peaking near 2.2 μm have intensities that correlate. The strength of these two absorption bands can be increased or decreased by illuminating the crystal with selected laser wavelengths. The 2.2 μm band is strongly polarized, with its greatest intensity occurring when E is parallel to the c axis of the crystal. A third absorption band, peaking near 2.3 μm and extending from 1.5 μm to beyond 4 μm, was observed at low temperature, during and immediately after illumination. Comparison of photoinduced changes in absorption with photoinduced changes in electron paramagnetic resonance spectra allowed specific defects to be associated with each of the three absorption bands. Both the 1.2 and the 2...

Identification of the intrinsic self-trapped hole center in KD2PO4

The intrinsic “self-trapped” hole center in KD2PO4 crystals has been identified using electron paramagnetic resonance and electron-nuclear double resonance. These defects, labeled [D2PO4]o centers, can be formed at 77 K by irradiating with either 60 kV x rays or the fourth harmonic (266 nm) of a pulsed Nd:YAG laser. The hole is equally shared by two adjacent oxygen ions, and hyperfine interactions with one phosphorus and two equivalent deuterons are observed. The sample used in this investigation was approximately 80% deuterated, thus both [D2PO4]o and [HDPO4]o centers were detected, with the former being dominant. These intrinsic self-trapped hole centers are of interest because of their potential role in the transient optical absorption produced in KD2PO4 crystals at room temperature by intense 266 nm laser pulses.

Gallium vacancies in β-Ga2O3 crystals

The gallium vacancy, an intrinsic acceptor, is identified in β-Ga2O3 using electron paramagnetic resonance (EPR). Spectra from doubly ionized ( V G a 2 −) and singly ionized ( V G a −) gallium vacancies are observed at room temperature, without photoexcitation, after an irradiation with high-energy neutrons. The V G a 2 − centers (with S = 1/2) have a slight angular variation due to a small anisotropy in the g matrix (principal values are 2.0034, 2.0097, and 2.0322). The V G a 2 − centers also exhibit a resolved hyperfine structure due to equal and nearly isotropic interactions with the 69,71Ga nuclei at two Ga sites (the hyperfine parameters are 1.28 and 1.63 mT for the 69Ga and 71Ga nuclei, respectively, when the field is along the a direction). Based on these g-matrix and hyperfine results, the model for the ground state of the doubly ionized vacancy ( V G a 2 −) has a hole localized on one threefold-coordinated oxygen ion. The vacancy is located at one of the three neighboring gallium sites, and the r...

Electron paramagnetic resonance and electron–nuclear double-resonance study of Ti3+ centres in KTiOPO4

Electron paramagnetic resonance and electron–nuclear double resonance have been used to characterize four Ti3+ centres in undoped crystals of potassium titanyl phosphate (KTiOPO4 or KTP). These 3d1 defects (S = 1/2) are produced by ionizing radiation (either 60 kV x-rays or 355 nm photons from a tripled Nd:YAG laser), and form when the regular Ti4+ ions in the crystal trap an electron. Two of these trapped-electron centres are only observed in hydrothermally grown KTP and the other two are dominant in flux-grown KTP. Both of the Ti3+ centres in hydrothermally grown crystals have a neighbouring proton (i.e. an OH− molecule). In the flux-grown crystals, one of the Ti3+ centres is adjacent to an oxygen vacancy and the other centre is tentatively attributed to a self-trapped electron (i.e. a Ti3+ centre with no stabilizing entity nearby). The g matrix and phosphorus hyperfine matrices are determined for all four Ti3+ centres, and the proton hyperfine matrix is determined for the two centres associated with OH− ions. These Ti3+ centres contribute to the formation of the grey tracks often observed in KTP crystals used to generate the second harmonic of high-power, near-infrared lasers.

Identification of a Pb-related Ti3+ center in flux-grown KTiOPO4

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) have been used to identify a new Ti3+ center in KTiOPO4 crystals containing lead impurities. Many of the K+ vacancies in this set of KTP crystals are compensated nonlocally by Pb2+ ions substituting for K+ ions. During exposure to ionizing radiation (either 60 kV x rays or 355 nm photons from a tripled Nd:YAG laser), “free” electrons are trapped on Ti4+ ions near isolated Pb2+ ions, thus forming the perturbed Ti3+ ions observed with EPR and ENDOR. Four distinct Pb-related Ti3+ centers are formed by a 77 K irradiation, but only one remains after a 5 min anneal at 180 K. This latter defect, labeled the [Ti3+–Pb2+]A center, is thermally unstable above 250 K. Angular dependence data were used to determine the g matrix, one 207Pb hyperfine matrix, and two 31P hyperfine matrices for the [Ti3+–Pb2+]A center. More generally, we note that oxygen-vacancy-associated Ti3+ centers could not be formed in these Pb-containing KTP crystals...

Optical absorption and electron-nuclear double resonance study of Ni+ ions in AgGaSe2 crystals

Optical absorption, electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) have been used to characterize Ni+ ions substituting for Ag+ ions in AgGaSe2 crystals. These Ni+ ions are responsible for a strongly polarized optical absorption band peaking near 2.2 µm (the maximum absorption occurs with ). Phonon structure is resolved at low temperature. This infrared band, which is present even in undoped crystals because of trace amounts of Ni, can significantly degrade the performance of optical parametric oscillators using AgGaSe2. Sets of EPR and ENDOR angular dependence data were taken in the (010), () and (001) planes. This allowed the g matrix, the 61Ni hyperfine and nuclear quadrupole matrices, the 77Se hyperfine matrix for the four nearest neighbours and the 69,71Ga hyperfine and nuclear quadrupole matrices for four neighbours to be determined. Of the eight gallium near neighbours, only the four in the basal plane containing the Ni+ ion have a significant interaction. Large hyperfine interactions with 107,109Ag nuclei were not observed.

Optical and EPR characterization of point defects in bismuth-doped CdWO4 crystals

Abstract We suggest that the 550-nm “yellow” emission in CdWO4 is due to Bi3+ ions substituting for Cd2+ ions. The absorption band corresponding to this emission has a peak near 350 nm. This yellow emission was only observed in crystals that contained bismuth impurities. Electron paramagnetic resonace has identified two new centers in X-ray irradiated bismuth-doped CdWO4. One center is electron-like and is suggested to be a Bi2+ ion located on a Cd2+ site. The other center is assigned to a hole trapped on an oxygen ion adjacent to a Nb5+ ion substituting for a W6+ ion.

Electron paramagnetic resonance of platinum impurities in KTiOPO4 crystals

Single crystals of KTiOPO4 (KTP) often contain trace amounts of isolated platinum impurities. When present in sufficient concentration, these ions increase the KTP crystal’s susceptibility to form gray tracks during frequency doubling of high-power laser beams. Electron paramagnetic resonance (EPR) has been used to characterize three platinum centers in a crystal of KTiOPO4. Initially, the platinum ions are present in nonparamagnetic forms. Exposure to ionizing radiation (i.e., x rays or above-band-gap laser beams) at room temperature converts them into paramagnetic centers. Once formed, these centers are stable for weeks at room temperature; however, their EPR spectra can only be observed at temperatures near or below 30 K. An angular study provided principal values and principal directions for the g matrices and the platinum hyperfine matrices. The Pt(A) center has principal g values of 1.9397, 2.4463, and 2.5900 and is assigned to a Pt3+ ion substituting for a Ti4+ ion. In contrast, the Pt(B) center ha...