Optical Detection Of Electron Paramagnetic Resonance Research Articles

Spectroscopy of Proton Implanted GaN

Vibrational spectroscopy, photoluminescence, and optically detected electron paramagnetic resonance (ODEPR) have been used to characterize the defects produced in undoped and Sidoped GaN by the implantation of hydrogen. Several new vibrational bands were found near 3100 cm−1 in GaN that had been implanted with protons. These frequencies are close to those predicted for VGa−Hn complexes, leading to the tentative assignment of the new lines to VGa defects decorated with different numbers of H atoms. The proton implantation also produces an infrared PL band centered at 0.95 eV and the ODEPR spectrum labeled LE1, both of which were seen previously for electron-irradiated GaN.

Optically detected electron paramagnetic resonance of Ni-related defects in synthetic diamond crystals

Synthetic diamond crystals grown using a solvent catalyst that contains Ni were studied by optical detection of electron paramagnetic resonance (ODEPR) using the magnetic circular dichroism of the optical absorption (MCDA). The MCDA spectra in the infrared spectral region consist of a derivative-like line at 1.06 eV, single sharp lines at 1.29 and 1.40 eV, a complex phonon-split band with the zero-phonon line (ZPL) at 1.69 eV and a derivative-like doublet structure superimposed at 1.72 eV. All of these optical absorptions were shown to originate from paramagnetic defects. Using ODEPR it was possible to unambiguously assign the 1.72 eV doublet to the NE4 centre and the 1.40 eV line to the NIRIM-2 centre. The defect responsible for the ZPL at 1.06 eV has the same symmetry as the NIRIM-2 centre with a slightly larger -value. The NE4 centre was assigned to substitutional associated with a carbon vacancy in a nearest-neighbour position whereas the NIRIM-2 centre was assigned to interstitial with a distortion along a [111] axis.

Optical properties of hole centres in alkali halides: I. Investigation with optical detection of paramagnetic resonance

The spectra of the magnetic circular dichroism of the absorption (MCDA) and of optically detected electron paramagnetic resonance (ODEPR) of hole centres have been investigated in a number of alkali halides. Several new absorption bands were found using excitation spectra of the ODEPR lines. The number of the MCDA transitions of in the bromides and especially in the iodides is larger than predicted by the molecular orbital picture discussed so far in the literature. An explanation will be given in part II by Cabria et al on the basis of calculations including the spin-orbit effects of the ligands.

Spectroscopy of Proton Implanted GaN

AbstractVibrational spectroscopy, photoluminescence, and optically detected electron paramagnetic resonance (ODEPR) have been used to characterize the defects produced in undoped and Si-doped GaN by the implantation of hydrogen. Several new vibrational bands were found near 3100 cm-1 in GaN that had been implanted with protons. These frequencies are close to those predicted for VGa-Hn complexes, leading to the tentative assignment of the new lines to VGa defects decorated with different numbers of H atoms. The proton implantation also produces an infrared PL band centered at 0.95 eV and the ODEPR spectrum labeled LEI, both of which were seen previously for electron-irradiated GaN.

Zero-field splitting and line shape of the ODMR of self-trapped excitons in NaBr

The optically detected electron paramagnetic resonance (ODMR) spectrum of self-trapped excitons (STEs) in NaBr was measured using a microwave frequency of 25.9 GHz and compared with that measured previously at 24.0 GHz. In lock-in detected ODMR only one structureless non-Gaussian line was observed for each frequency. From the field positions of the lines obtained in the previous and new measurements a zero-field splitting of D = 1.7 T was derived. On the basis of transient ODMR measurements in KBr it is shown that the shape of the ODMR line does not follow the -centre nuclear spin statistics of the STE because of the dependence of the lifetimes on the nuclear spin states. Similar effects seem to occur in all alkali bromides.

EPR and ENDOR Investigations of Shallow Impurities in SiC Polytypes

Investigations of nitrogen donors in 6H-, 4H- and 3C-SiC using conventional electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and optical detection of EPR and ENDOR as well as optical absorption and emission spectroscopy are reviewed and critically discussed. An attempt is presented to interpret the experimentally found large differences in hyperfine interactions of the 14N nuclei on the various inequivalent sites in the different polytypes of SiC in terms of valley–orbit splittings and “central-cell corrections” in the framework of the effective mass theory (EMT). P-doping by neutron transmutation in 6H-SiC resulted in various P-related EPR spectra previously associated with shallow P donors and P–vacancy complexes. In analogy to the new interpretation of the N donor spectra in various polytypes, it is proposed that all P-related spectra found hitherto in 6H-SiC are due to isolated P donors in ground and excited EMT states. A detailed discussion is presented of the electronic structure of B acceptors, as determined by EPR and in particular by ENDOR investigations: The B atom itself has only very little unpaired hole density, while the hole resides mainly on a neighbouring relaxed C atom B acceptors have a rather “deep” character and pronounced dynamical properties. A discussion of the present understanding of the so-called deep B centre (D centre) is also given. In contrast to B, the Al acceptor behaves as expected from the effective mass theory. It shows, however, two optical absorption bands identified by optical detection of EPR which are related to an ionization transition to the valence band and another transition, probably to a V impurity.

Optically detected electron paramagnetic resonance of the shallow aluminium acceptor in 4H- and 6H- silicon carbide

Aluminium doped 4H- and 6HSiC was investigated with optically detected electron paramagnetic resonance (ODEPR). In addition to the known photoluminescence-detected EPR spectra new optical absorption spectra were observed measuring the magnetic circular dichroism (MCDA). With the MCDA-detected EPR the Al hyperfine interactions of all Al defects on three inequivalent sites in 6HSiC and two sites in 4HSiC could be resolved. The effective electron spin of all Al acceptors observed was determined to be S = 1 2 . The small spin density at the Al acceptors as well as the optical absorption transitions are discussed.

ODEPR of indium colour centres in the X- irradiated storage phosphor KBr:In

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 In 2+ 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-In 2+ centre pairs as was assumed previously. Considerable similarities to the storage phosphor BaFBr :Eu 2+ were found for the photostimulated emission and read-out properties.

Optically detected ESR studies of recombination processes in a-Si:H

A study of recombination processes in high-quality (intrinsic) and light-soaked hydrogenated amorphous silicon (a-Si:H) using the technique of optically detected electron spin resonance (ODESR) is reported. A time-domain detection scheme is employed, instead of the conventional lock-in technique, to record the ODESR lineshapes, which effectively minimizes the well-known interference between the ‘enhancing’ and ‘quenching’ signals. Monitoring the photoluminescence (PL), below 1.1 eV, in the intrinsic sample, a nearly symmetric enhancing signal ( g = 2.008) related to radiative recombination of spin pairs in shallow band-tail states and an asymmetric quenching signal ( g = 2.005) related non-radiative recombination of spin pairs in deep band tail states are observed. A quenching signal at half-field ( g ≌ 4) is also observed. The light-soaked sample exhibits an additional signal ( g = 2.006) which is related to the dangling bonds. The PL excitation energy, E x, is varied from above to below the optical gap to study possible changes in the recombination processes. It is found that the ODESR lineshapes are essentially the same for E x > 1.5 eV and some subtle changes occur for E x < 1.5 eV. The signal intensity, however, depends strongly on the excitation energy for a constant electron-hole pair generation rate.

Optically detected ESR studies of a-Si:H

Spin-dependent recombination processes in high-quality a-Si:H have been reported by optically detected electron spin resonance (ODESR) with the excitation energy (E x) varied from above the optical band gap to well below the gap. The ODESR lineshapes are obtained by measuring the microwave induced photoluminescence (PL) transients as functions of the magnetic field.

Magneto‐Optical Properties of F Center in KCI:SH−

AbstractThe magnetic circular dichroism (MCD) of F centers in KCI:SH− has been measured in absorption in the 1sσg → 2pyπu transitions at 493 and 509 nm, with fields up to 5 T and in the temperature range 1.5 K &lt; T &lt; 77 K. Within the limit of detection, no MCD is observed in the near infrared transition 1ss̀g → 2ps̀u as well as in both emissions 2pσu → Iss̀g and 2ps̀g. The optical detection of EPR in the F ground state presents an isotropic single band with g = 1.965 ± 0.007. The spin‐lattice relaxation measured at H = 0.32 T is typical of a direct process T−1 = 4.3 × 10−2 coth (gμBH/2kBT). The spectral variation of the MCD is calculated using perturbation theory to first order. The Hamiltonian includes the spin‐orbit interaction in the 2pπu excited state and the orbital molecular wave functions are obtained by a linear combination of 1s and 2p atomic orbitals. The calculated MCD is in good agreement with the observed one, for the spin‐orbit interaction strength σz = 3.6 meV.

Anomalous spin-lattice relaxation in InP:Fe3+ and BaFBr:Eu2+ examined with optical detection of electron paramagnetic resonance

The Eu2+ defect in BaFBr and the Fe3+ defect in InP were investigated by optical detection of electron paramagnetic resonance (ODEPR). For both defects allowed ( Delta ms=+or-1) and forbidden ( Delta ms=+or-2) ODEPR transitions were observed. Some of the allowed ODEPR transitions showed an anomalous change of sign indicating a spin polarization enhancement. This anomalous behaviour of both spin systems (S=7/2 for Eu2+ in BaFBr and S=5/2 for Fe3+ in InP) arises from spin-lattice relaxations within the Eu2+ and Fe3+ Zeeman levels which operate faster for forbidden ( Delta ms=+or-2) transitions than for allowed transitions. Using the appropriate rate equations describing the two spin systems and taking into account strong forbidden spin-lattice relaxation, the measured ODEPR effects were explained.

Arsenic antisite-related defects in low-temperature MBE grown GaAs

GaAs layers grown by the molecular beam epitaxy (MBE) method at low temperatures (200 degrees C) and also MBE samples grown at 300 degrees C highly doped with Be or Si show high concentrations of As antisite-related defects in the optical absorption and in the magnetic circular dichroism of the absorption (MCDA). With optical detection of EPR it is shown that these antisite-related defects have properties similar to those of the EL2 defects except for the EL2 bleaching characteristics and the so called zero phonon line. Their spin-lattice relaxation time is strongly reduced compared with that of EL2+. These differences may be related to their high concentrations, which are of the order of approximately 1019-1020 cm-3. In MBE samples grown at higher temperatures (325 degrees C, 400 degrees C) a new As antisite-related defect was detected with a reduced 75As hyperfine splitting as compared with that of EL2+. This new As antisite-related defect has properties very similar to those of another As antisite-related defect previously detected in horizontal Bridgman n-type GaAs. A comparison of four different As antisite-related defects with similar reduced 75As hyperfine splittings is presented.

Optically detected magnetic resonance of the luminescent state in NaF:Cu +

Optically detected electron paramagnetic resonance (OD-EPR) Spectra at 22.9, 75.0, and 94.9 GHz of the luminescent state of Cu + ions in NaF are reported. The EPR spectra do not support the current T lg( 3E g, 3d 94s) “atomic” assignment of the luminescent state, but point to a paramagnetic species with g values in the range of 2.0–2.5, a very small or no zero-field splitting and a spin density in the Cu 4s oribtal lower than 0.04.

Optical detection of EPR and electron-nuclear double resonance for substitutional MnIn-(d5) in InP:Sn.

An isotropic optically detected magnetic-resonance (ODMR) peak at g=1.997 is found in as-grown Sn-doped InP while monitoring the magnetic circular dichroism (MCD) of an absorption band at \ensuremath{\sim}900 nm. No fine or hyperfine structure is resolved. Optically detected electron-nuclear double resonance (ODENDOR) reveals spectra arising from a central Mn core (I=5/2, with isotropic hyperfine interaction A/h=-166.0\ifmmode\pm\else\textpm\fi{}0.3 MHz) and neighboring P and In shells. The pattern and spacings of these lines reveal that the defect is isolated ${\mathrm{Mn}}^{\mathrm{\ensuremath{-}}}$(${\mathit{d}}^{5}$), substituting for In, with S=5/2. A surprise observation is that the MCD, ODMR, and ODENDOR disappear abruptly when the material is converted to high-resistivity n type by electron irradiation.

ODMR of stoichiometry defects in III–V semiconductors

Abstract Optically detected electron paramagnetic resonance (ODEPR) and electron nuclear double resonance (ODENDOR), using the magnetic circular dichroism of the optical absorption (MCDA), are applied to investigate the microscopic structure and optical properties of antisite defects on both sublattices in III–V semiconductors. This article reviews briefly the results obtained for GaAs and GaP and presents for the first time the MCDA/ODENDOR identification of the isolated arsenic antisite defect in GaAs in low temperature electron irradiated crystals kept at 77 K. Its properties are compared with those of the EL2 defect and it is clearly shown that EL2 cannot be identified with the isolated AsGa antisite defect. EL2 can be formed by annealing the irradiated sample to 520 K where As interstitials are mobile, confirming an earlier ODENDOR identification of EL2 being an AsGaAsi pair defect. AsGa related defects in n-doped MOVPE grown AlxGa1−xAs epitaxial layers were identified for the first time with MCDA and ODEPR spectra.

Optically detected electron paramagnetic resonance and fluorescence study on geminate ion recombination in low-density polyethylene doped with pyrene

The optically detected electron paramagnetic resonance (ODEPR) spectrum of pyrene-doped low-density polyethylene has been studied. This is the first ODEPR spectrum observed on an ordinary polymer, and its intensity was comparable to that of spectra recorded in viscous solutions. The dependence of the ODEPR and fluorescence spectra on the concentration of pyrene as well as temperature led us to deduce the geminate recombination process in this inhomogeneous material. We found that: (1) charge recombination occurs mainly through electron hopping between the solute pyrenes. (2) Hole hopping between the pyrenes following hole transfer from the polymer matrix to the solute on the ODEPR timescale is allowed only at high temperatures and high solute concentrations.

ODMR investigations of DX centres in Sn- and Si-doped AlxGa1-xAs

In a thick (100 mu m) Sn-doped Al0.35Ga0.65As layer and an 11 mu m Si-doped Al0.41Ga0.59As layer the magnetic circular dichroism (MCDA) of the optical absorption, the optically detected electron spin resonance (ODEPR) and the photoconductivity (only in the Sn layer) have been measured simultaneously. All signals appear together after the photoionization of the DX centre and are directly correlated to each other. The ODEPR spectra measured in the MCDA bands show the hyperfine doublets due to the magnetic 117Sn and 119Sn isotopes (I=1/2) in Sn-doped samples and an ODEPR line (I=0) in Si-doped layers. The 4.7% abundant 29Si (I=1/2) isotopes could not be observed. The authors show that the MCDA is due to a neutral (Si0, Sn0) donor, a deep level which is stable at 1.6 K in the dark. This DX0 state needs thermal energy or light to capture an electron to form the negative-U DX- ground state which contains one donor dopant in a large lattice configuration.

Multiquantum resonances in low-field optically detected ESR of radical-ion pairs

Multiquantum resonances in low-field optically-detected electron spin resonance (ESR) spectra of radiation-induced radical-ion pairs of aromatic acceptors in liquid and some polymeric solutions have been studied experimentally and theoretically. Corresponding resonances occur in spectra under the action of relatively intense radiofrequency (RF) pumping, when usual signals are inverse due to spin-locking effect. Special experiments have been carried out to prove that such resonances are not caused by the apparatus. The dependencies of resonance shifts and intensities on the radiofrequency field amplitude for various polarizations of the RF field have been studied. The case of pumping with two independent RF generators has also been analysed. A general theory for multiquarium optically detected ESR spectra of radicalion pairs in liquid solutions has been developed, with spin relaxation processes being neglected. The theory has been applied to calculating multiquantum resonance parameters in transverse RF fi...

EL2 and anion antisite defects in plastically deformed GaAs

We report on optical absorption its magnetic circular dichroism (MCD), optically detected electron spin-resonance (ODESR), and electron nuclear double-resonance (ODENDOR) investigations of plastically deformed semi-insulating GaAs. By plastic deformation arsenic antisite defects are created which show a similar ODESR pattern as EL2 defects present in the material prior to deformation. EL2 and the new antisite defects can be distinguished by their different spectral dependence of the MCD. The new antisite defect formation starts at 2% deformation and is investigated as a function of the degree of deformation; additional EL2 defects are not created. With ODENDOR it is shown that the atomistic structure of the EL2 defects changes in the deformed GaAs.