P1 Centers Research Articles

Inversion of the EPR signal from P1 centers in a synthetic diamond single crystal under normal conditions

The electron paramagnetic resonance (EPR) spectrum of a synthetic 0.15-carat diamond single crystal grown in a (Fe-Co-C) system using the temperature gradient method in a split-sphere type apparatus has been studied. It is established that the EPR signal from P1 centers (representing nitrogen atoms occurring at the C sites) present in the diamond crystal upon a high-temperature high-pressure (HTHP) treatment exhibits inversion when the microwave power in an H102 resonator is increased from 70 μW to 70 mW at a polarizing magnetic field scan rate from 0.5 to 5 mT/min. No such inversion is observed in the initial diamond (before the HTHP treatment).

Inversion of the electron spin resonance signal from a P1 center in a synthetic diamond crystal

We have studied the electron spin resonance (ESR) of a 0.59 carat synthetic diamond single crystal at room temperature. The crystal was grown on a “split-sphere” apparatus in the Fe-Ni-C system by the temperature gradient method. After high-temperature/high-pressure treatment of the diamond, it was observed that as the microwave power supplied to the sample increased from 70 μW to 70 mW in an H102 cavity, the ESR signal from the P1 center (a nitrogen atom substituting for carbon at a lattice point of the diamond crystal: C-form nitrogen) is inverted. In the original diamond (before high-temperature/high-pressure treatment), no inversion of the ESR signal was observed.

Gamma and UV Radiation-Induced Color Centers in Optical Fibers

Radiation-induced losses and paramagnetic centers were investigated in phosphorusdoped and P-free multimode germanosilicate optical fibers after g-rays (~1 MeV) and ultraviolet (5 eV) exposures. After both types of irradiation, the same defects seem to be responsible of the fiber absorption in the spectral range 400 to 1650 nm. In particular, the P1 centers and the Phosphorus Oxygen Hole centers are created in both cases in the phosphorus-doped fibers and explain the high permanent radiation-induced attenuation levels observed in this fiber type. Luminescence and electron spin resonance measurements (77 K, ~9.38 GHz) on irradiated samples confirm that the GeE’, SiE’ and NBOHC defects are also generated in the different irradiated samples. From this study, it seems that the pertinence of a multimode fiber for nuclear space or civil applications could be estimated through low-cost ultraviolet measurements.

Inverted EPR signal from nitrogen defects in a synthetic diamond single crystal at room temperature

Electron paramagnetic resonance (EPR) in diamond single crystals was studied. The crystals were grown using apparatuses of the “split-sphere” type in a Ni-Fe-C system using the temperature gradient method with a subsequent high-temperature high-pressure treatment. It was found that, after the high-temperature high-pressure treatment of a diamond sample, the EPR signal from the lattice defects containing nitrogen atoms became inverted with the growth of the microwave power in an H102 resonator. In a constant polarizing magnetic field, when the microwave power applied to the diamond was low, a resonance absorption by the nitrogen defects took place, whereas, when the microwave power was high, an emission was observed. The inversion of the EPR lines of a single nitrogen atom substituting for a carbon atom at a diamond lattice site could be caused by the presence of a nickel atom with an uncompensated magnetic moment at the adjacent tetrahedral interstitial site. In synthetic diamond crystals that were not subjected to high-temperature high-pressure treatment, the inversion of the EPR signal from nitrogen atoms (P1 centers, nitrogen in the C form) was absent.

The effect of high-pressure–high-temperature annealing on paramagnetic defects indiamond

The formation and annealing out of paramagnetic defects in high-pressureand high-temperature diamond are investigated through a comprehensiveheat treatment study of as-grown crystals. Diamond crystals are subjected toa sequence of isochronal anneals in the temperature range between 1550 and2000 °C and the concentrations of paramagnetic defects are determined by electron paramagneticresonance measurements after each annealing step. The quantitative data areanalysed in the light of recently published results on the behaviour of nickel and/ornitrogen defects upon annealing. Like the P1 and W8 centres, the NIRIM1nickel-related defects show a strong decrease in their concentration upon annealing at1900 °C,whereas the NIRIM2 centres seem to be rather stable. The evolution of the nickel-relatedcentre concentration through the annealing sequence shows that not all W8 defectstransform into paramagnetic nickel–nitrogen complexes. Indeed, most of the W8 defects aretransformed in other non-paramagnetic centres, with the AB1, AB3, and AB6 defects beingprobably by-products of this process. The relatively low concentration of nickel–nitrogenparamagnetic complexes formed upon annealing shows that their formation does notplay any fundamental role in the kinetics of the nitrogen aggregation during heattreatments.

13C Spin–Lattice Relaxation in Natural Diamond: Zeeman Relaxation at 4.7 T and 300 K Due to Fixed Paramagnetic Nitrogen Defects

13C spin–lattice relaxation times in the laboratory frame, ranging from 1.4 to 36 h, have been measured on a suite of five natural type Ia and Ib diamonds at 4.7 T and 300 K. Each of the diamonds contains two types of fixed paramagnetic centers with overlapping inhomogeneous electron paramagnetic resonance (EPR) lines. EPR techniques have been employed to identify these defects and to determine their concentrations and relaxation times at X-band. Spin–lattice relaxation behavior of 13C in diamonds containing paramagnetic P1, P2, N2, and N3 centers are discussed. Depending on the paramagnetic impurity types and concentrations present in each diamond, three different nuclear spin–lattice relaxation (SLR) paths exist, namely that due to electron SLR mechanisms and two types of three-spin processes (TSPs). The one three-spin process (TSP1) involves a simultaneous transition of two electron spins belonging to the same hyperfine EPR line and a flip of a 13C spin, while the other process (TSP2) involves two electron spins belonging to different hyperfine EPR lines and a 13C spin. It is shown that the thermal contact between the 13C nuclear Zeeman and electron dipole–dipole interaction reservoirs is field dependent, thus forming a bottleneck in the 13C relaxation path due to TSP1 at high magnetic fields.

Electronic Structure of Defect Centers P1, P2, and P4 in P-Doped SiO2

We have studied the ground-state properties of phosphorus-related dia- and paramagnetic point defects in P-doped silica and α-quartz. Density functional theory calculations have been performed to determine the structure, charge and spin distribution of the P1, P2, and P4 centers. P1 consists of a three-coordinated P atom substituting for a Si atom in the SiO2 lattice, [(O−)3P•]+, in radical form; P2 denotes a four-coordinated P, [(O−)2P•(−O)2]0; P4 has only two P−O bonds, [(O−)2P•]0. These centers originate from electron or hole trapping derived from diamagnetic precursors. The computed hyperfine coupling matrices of the paramagnetic centers fully confirm the proposed assignment of the P1, P2 and P4 structures to the observed EPR signals in P-doped silica and P2 in α-quartz.

EPR study of preferential orientation of crystallites in N-doped high quality CVD diamond

The directions of preferential growth of free-standing optical-quality CVD diamond wafers have been investigated with the help of electron paramagnetic resonance (EPR). EPR signals of the well-known P1 centre (substitutional nitrogen) have been used as a probe. A computer simulation of EPR spectra of preferentially oriented polycrystalline material allows the estimation of the real orientation and spatial distribution of crystallographic axes of crystallites in the samples oriented preferentially in the [110] direction.

Photo EPR Study of Trapping and Recombination Processes in Semi-Insulating 4H-SiC Crystals as Function of Temperature

ABSTRACTPhoto-Electron Paramagnetic Resonance (photo-EPR) measurements of semi-insulating (s.-i.) 4H SiC have been made at 37 GHz including photo excitation and photo quenching techniques in the temperature interval from 77 K to 50 K. At T = 77 K in the dark the EPR spectrum consists of a low intensity line due to boron on the cubic lattice site and a single line with isotropic g∥ = g⊥ = 2.0025 due to a carbon-related surface defect. During illumination with ultraviolet light the EPR lines of hexagonal boron and cubic nitrogen appear in the EPR spectrum and persist after the illumination is removed. Subsequent illumination of the sample with sub-band gap, visible, light resulted in the quenching of the EPR lines from nitrogen and appearance of the IP1EPR line with g∥ = 2.0048, g⊥ = 2.0030 caused by direct transfer of electrons from nitrogen donor to the P1 center. The lifetime of the photo-generated carriers trapped by the P1 centers is found to be more than 15- 20 hours after the photo-excitation was turned off. The deep donor P1 local center is suggested to be the as yet unidentified deep level located at EC – 1.1 eV which pins the Fermi level in this sample at this energy in the dark. As the temperature is lowered from 77K and the quasi Fermi level positions reach shallow donor and acceptor states, an additional EPR line, ID, with g∥ = 2.0063, g⊥ = 2.0006, appears at 50 K in the excitation EPR spectrum and is attributed to the antisite defect Si−c with an energy level shallower than nitrogen. At the same time the ratio of the photo-excited EPR line intensities responsible for boron on the cubic and hexagonal sites, IkB:IhB, returns to the value observed at 77 K and becomes equal to 0.4 at 50 K, showing that the concentration of boron in the hexagonal site is higher than on the cubic site.

Room-temperature field dependence of the electron spin–lattice relaxation times of paramagnetic P1 and P2 centers in diamond

Electron spin–lattice relaxation (SLR) times of P1 and P2 centers have been measured at 300 K at X-band and W-band for one synthetic (type-Ib) and three natural (type-Ia) diamonds using electron paramagnetic resonance spectroscopy. Bi-exponential SLR, with a fast and a slow component, was observed for the overlapping lines of samples with two types of impurity centers. Comparison of X- and W-band results for P2 centers suggests that these centers relax via a field-independent second-order Raman process with T 1e≈2.2 ms at room temperature.

Effect of Stress on Optical and ESR Lines in CVD Diamond

Correlation between the shape of Raman, photoluminescence (PL) and Electron Spin Resonance (ESR) signals in CVD diamond films was examined for both undoped and nitrogen-doped films. No correlation was observed between the shift of the diamond Raman line and its linewidth, even for the films produced under essentially the same growth conditions. It was found that the increase in the width of the diamond Raman peak interrelates with the broadening of zero-phonon PL lines and the changes in the structure of the ESR signal from the substitutional nitrogen (P1) center. Observed changes in the ESR spectra are interpreted in terms of reduction of the symmetry of the P1 center induced by internal stress.

Temperature dependence of spin-spin and spin-lattice relaxation times of paramagnetic nitrogen defects in diamond

Spin-lattice relaxation times of P1 centers in a suite of two natural type Ib, two synthetic type Ib, and one natural type Ia diamonds were measured at 9.6 GHz as a function of temperature in the range 300 K>T>4.2 K. An analysis of the results revealed that for three of the diamonds (two synthetic type Ib and the natural type Ia) spin-orbit phonon-induced tunneling is the main relaxation mechanism. In the case of the Ia diamond cross-relaxation takes place between P1 and P2 centers. In the natural type Ib samples a much more effective relaxation mechanism dominates at lower temperatures. Electron spin resonance spectra of the latter samples revealed the presence of N3 centers. It seems that the more effective relaxation mechanism is associated with the N3 centers and that the P1 centers relax via the N3 centers to the lattice at these temperatures.

Dynamic nuclear polarization of diamond. I. Solid state and thermal mixing effects

The dynamic nuclear polarization of 13C nuclei in a suite of seven natural type Ia and Ib diamonds, using continuous wave S- and X-band microwave radiation, is described. The 13C signal enhancement and polarization time have been measured for one of the type Ib diamonds as a function of magnetic field in the vicinity of the resonance field. The total paramagnetic impurity concentration (P1 and other centers) in this diamond is 2×1018 cm−3 (23 atomic parts per million), while the concentration of P1 centers is 9.3 ppm. Since the central electron spin resonance (ESR) linewidth HL is comparable with H0γC/γe, flip–flip and flip–flop forbidden transitions take place simultaneously. Consequently thermal mixing plays an important role in the 13C signal enhancement. However, the 13C spin-lattice relaxation rate is determined to a large extent by the solid state effect (forbidden transitions). The 13C polarization rates have been measured for the suite of diamonds by executing dynamic nuclear polarization (DNP) experiments on both hyperfine and central ESR lines. It is shown that the polarization rate is proportional to the paramagnetic impurity concentration of the sample, in agreement with the existing theory. It has been found that in type Ib diamonds with relatively low nitrogen impurity concentrations the dynamic nuclear polarization of a single hyperfine line yields an equilibrium 13C polarization that is one-quarter of that obtained in the case of dynamic nuclear polarization of the central line. In samples containing P1 and P2 centers (type Ia), or in type Ib samples with relatively high concentrations of P1 centers, the same equilibrium 13C polarization is obtained for the DNP of hyperfine and central transitions. This phenomenon is explained in terms of a model in which thermal contact is established between the electron Zeeman reservoir and the nuclear spin reservoirs via the spin–spin interaction reservoir if HL⩾H0γC/γe.

Damage production in silicon irradiated with 112 MeV Ar ions

Silicon samples were irradiated below 50 K with 112 MeV Ar ions. Defect production was investigated at room temperature by EPR and infrared optical absorption techniques. The EPR measurements reveal the presence of amorphous zones and neutral tetravacancies (Si–P3 center) in the as-irradiated samples. The infrared optical absorption measurements indicate the formation of divacancy. The concentration of divacancy increases with ion fluence and saturates at high fluence. The isochronal annealing behaviors of the radiation induced defect centers are found to be fluence dependent. For 1.0 × 10 14/cm 2 irradiated sample, the disappearance of the Si–P3 center is accompanied by the appearance of Si–P1 and Si–B2 centers and the growth of Si–A11 center. The Si–P1 center and the isolated amorphous regions are annealed out between 500°C and 550°C. The Si–A11 center is stable up to 600°C. For 8.0 × 10 14/cm 2 irradiated sample, only Si–P1 center and the line due to the continued amorphous layer are detected above 200°C. The recrystallization of the continued amorphous layer occurs at temperature higher than 600°C.

The dependences of ESR line widths and spin - spin relaxation times of single nitrogen defects on the concentration of nitrogen defects in diamond

Line widths and spin - spin relaxation times of P1 centres in synthetic Ib and natural Ia diamonds with concentrations of P1 and P2 centres covering the range 0.03 - 400 atomic parts per million have been measured. At concentrations higher than about ten atomic parts per million the line width is linearly dependent on the concentration. At lower concentrations the electron - dipolar contribution to the line width dominates and the width of the line remains constant. Since the pulse sequence employed for measurements eliminates the effects of inhomogeneous line broadening, of the line remains linearly dependent on the total paramagnetic impurity concentration, even at very low paramagnetic impurity concentrations.

Optical excitation of paramagnetic nitrogen in chemical vapor deposited diamond

Investigations of polycrystalline chemical vapor deposited diamond films by electron-spin-resonance (ESR), light-induced (L)ESR, and the constant photoconductivity method have identified dispersed substitutional nitrogen (P1 center) as the main paramagnetic form of N incorporated in the CVD diamond. The density of N-related paramagnetic states is strongly affected by illumination and heat treatment. It is found that the P1 center in CVD diamond gives rise to a deep donor state about 1.5 eV below the conduction band.

ESR and LESR Studies in CVD Diamond

AbstractCVD diamond films with nitrogen content varying from 10 ppm to 132 ppm have been studied by electron spin resonance (ESR), light-induced ESR (LESR) as well as spin-dependent conductivity (SDC). Two characteristic signals have been observed. A carbon-related defect line with g = 2.0029 ± 0.0002 and width 4 ± 1 G, is observed in ESR, LESR and SDC. The intensity of this line measured by ESR increases linearly with nitrogen content. For low-defect-density samples, or after illuminating the high-defect-density samples with UV light, a second signal is observed both in ESR and LESR, but not in SDC, with a central line at g = 2.0024 ± 0.001 and width 0.2 ± 0.1 G and related hyperfine satellites ≈30 G away from the central line. This line is assigned to isolated substitutional nitrogen, the so-called P1 center. The density of N-related paramagnetic states is strongly affected by illumination and heat treatments. Spin-dependent conductivity measurements show that the dark conductivity at room temperature in CVD-diamond is dominated by hopping at the g = 2.0029 defects.

ENDOR studies on the N1 di-nitrogen centre in diamond

New ENDOR measurements on the N1 centre confirm the N-C-N+ model for the defect. The N+ is in a substitutional site with approximately tetrahedral symmetry. The N-C fragment of the centre resembles the P1 centre, with slightly larger unpaired electron density on the nitrogen, and a smaller quadrupole interaction.

ENDOR at S-band (2–4 GHz) microwave frequencies

Theory, techniques, and applications of electron nuclear double resonance (ENDOR) at microwave frequencies lower than X band are considered. Low microwave frequency exploits the hyperfine enhancement effect, resulting in significantly higher effective RF field strengths than in a comparable X-band experiment. The microwave frequency dependences of the magnitude of the CW-ENDOR effect and of ENDOR resolution are discussed. A practical structure for ENDOR at S-band microwave frequencies, incorporating a loop-gap resonator, is described. Experimental results on 14N ENDOR of the P1 center in diamond and 1H matrix ENDOR of γ-irradiated sucrose are presented.

Electron spin resonance of a tri-nitrogen centre in Cape yellow type Ia diamonds

A di-nitrogen centre has been observed in gem quality Cape yellow diamonds while illuminated with ultraviolet light. Analysis of the spectrum shows that the paramagnetic electron is localised on two neighbouring nitrogens along (111) directions, which suggests that it is most likely a derivative of the non-paramagnetic A centre. Observation of this paramagnetic centre in many diamonds suggests an interplay between it, the A centre, the P2 (or optical N3) and possibly the P1 centre.