eV Line Research Articles

A broad absorption feature in the X-ray spectrum of the isolated neutron star RBS1223 (1RXS J130848.6+212708)

X-ray spectra of the isolated neutron star RBS1223 obtained with the instruments on board XMM-Newton in December 2001 and January 2003 show deviations from a Planckian energy distribution at energies below 500 eV. The spectra are well fit when a broad, Gaussian-shaped absorption line with σ = 100 eV and centered at an energy of 300 eV is added to an absorbed blackbody model. The resulting equivalent width of the line is −150 eV. However, the spectral resolution at these low energies of the EPIC detectors and the lower statistical quality and restricted energy band of the RGS instruments are not sufficient to exclude even broader lines at energies down to 100 eV or several unresolved lines. The most likely interpretation of the absorption feature is a cyclotron absorption line produced by protons in the magnetic field of the neutron star. In this picture line energies of 100-300 eV yield a magnetic field strength of 2-6 × 10 13 G for a neutron star with canonical mass and radius. Folding light curves from different energy bands at a period of 10.31 s, which implies a double peaked pulse profile, shows different hardness ratios for the two peaks. This confirms that the true spin period of RBS1223 is twice as long as origi- nally thought and suggests variations in cyclotron absorption with pulse phase. We also propose that changes in photo-electric absorption seen in phase resolved spectra of RX J0720.4−3125 by Cropper et al. (2001), when formally fit with an absorbed blackbody model, are caused instead by cyclotron absorption varying with pulse phase.

A model of HPHT color enhancement mechanism in natural gray diamonds

In the present paper HPHT-processing of natural gray graphite containing diamonds is studied and a new model of the color enhancement mechanism is proposed. It is proposed that there occurs a polymorphous transformation of graphite inclusions into diamond during HPHT-processing. During this transformation hydrogen, the content of which depends to a great extent on the number of graphite inclusions, is transferred from graphite to diamond getting localized on the nearest structural defects with uncompensated negative charges. In the optical spectra this is accompanied by an anomalous increase in the intensity of the hydrogen-related lines in the IR spectrum. It has been observed that the intensity increases in the 2.988 eV line attributed to N3 centers after HPHT-processing at an unusually low temperature of 1800 °C. In natural brown diamonds the intensity increase of the 2.988 eV line takes place at higher HPHT temperatures (over 2000 °C).

Luminescence study of (112̄0) GaN film grown by metalorganic chemical-vapor deposition

Investigations on photoluminescence properties of (112̄0) GaN grown on (11̄02) Al2O3 substrate by metalorganic chemical-vapor deposition are reported. Several emission lines not reported before are observed at low temperature. The sharp peak at 3.359 eV is attributed to the exciton bound to the neutral acceptor. Another peak at 3.310 eV represents a free-to-bound, probably a free electron-to-acceptor, transition. The 3.241 and 3.170 eV lines are interpreted as phonon replica lines of the 3.310 eV line. The phonon energy is 70 meV, consistent with the energy of transverse optical E1 phonon. The optical properties of the lines are analyzed.

Spectroscopic study of a supersonic plasma jet generated by an ICP torch with a convergent-divergent nozzle

A supersonic argon and argon-hydrogen (3-5{%}) plasma jet generated by an induction plasma torch is studied by means of the methods of optical emission spectroscopy. The torch was operated at the input power of 20 kW and near atmospheric pressure. The supersonic jet with a periodic structure of expansion and compression zones is created by expanding the plasma through the Laval nozzle into a chamber maintained at the pressure around 1.8 kPa. Atomic argon lines with the upper level energies ranging from 13.3 to 15.5 eV, continuum emission and Hβ line profile are used to evaluate plasma parameters such as temperature and electron number density. Analysis based on the Boltzmann diagram, line-to-continuum ratio, population of continuum extrapolated level and Stark broadening reveals various stages of departure from thermodynamic equilibrium in the plasma flow. It is shown, among others, that the temperature derived from Boltzmann diagram does not follow the jet structure and reliable determination of electron temperature is questionable. An addition of several percent of hydrogen results in a significant quenching of populations of atomic states and nonequilibrium behaviour of continuum radiation.

Effect of Remote Hydrogen Plasma Treatment on ZnO Single Crystal Surfaces

ABSTRACTWe have studied the effects of hydrogen plasma treatment on the defect characteristics in single crystal ZnO grown at Eagle-Picher by chemical vapor transport. Depth-dependent cathodoluminescence (CL) spectra, temperature-dependent (9–300 K) and excitation intensity-dependent photoluminescence (PL) spectra reveal significant changes resulting from unannealed exposure of n-type ZnO to a remote hydrogen plasma. Low temperature PL spectra show that this hydrogen exposure effectively suppresses the free-exciton transition and redistributes intensities in the bound-exciton line set and two-electron satellites with their phonon replicas. The resultant spectra after hydrogenation exhibit a new peak feature at 3.366 eV possibly related to a neutral donor bound exciton. A simple thermal analysis of the activation energy for the 3.366 eV line yields 5–10 meV. Hydrogenation also produces a violet 100 meV-wide peak centered at 3.16 eV. Remote plasma hydrogenation produces similar changes in room-temperature CL spectra: near-band edge emission intensity increases with hydrogenation. Furthermore, this new emission increases with proximity to the free ZnO surfaces, i.e., with decreasing the energy of the incident electron beam from 3.0 down to 0.5 keV. Subsequent annealing at 450 °C completely restores both the PL and CL spectra in the sub-band gap range. The appearance of a new bound-exciton feature at 3.366 eV with H plasma exposure, the near-surface nature of the spectral changes, and the reversibility of spectral features with annealing indicate a direct link between H indiffusion and appearance of a shallow donor.

Photoluminescence due to boron-related defect in solar cell silicon irradiated with 1 MeV electrons

We investigated a new photoluminescence line at 0.87 eV in B-doped Si irradiated with 1 MeV electrons. We identified its origin as a complex of an interstitial B and an interstitial O (B/sub i/-O/sub i/), one of the major defects responsible for the degradation of space solar cells. The relative intensity of the line increases with the electron fluence and with the B-concentration. The activation energy of the electronic level responsible for the line is estimated to be 0.30 and 0.26 eV from the spectral shape analysis and the temperature dependence, respectively. These values agree with the energy level of the Bi-Oi defect. The agreement of the thermal stability between the line and the Bi-Oi defect also proves the validity of the idea that the 0.87 eV line is due to the B/sub i/-O/sub i/ defect.

P-Type Doping of Cubic GaN by Carbon

Successful p-type doping of cubic GaN epilayers by carbon using an e-beam evaporation source is reported. At room temperature Hall-effect measurements of the C-doped cubic GaN epilayer gave hole concentrations and hole mobilities as high as 6 x 10 17 cm -3 and 200 cm 2 /Vs, respectively. The thermal activation energy of the C-acceptor is (215 ± 10) meV. Low temperature photoluminescence spectra show a new line appearing at 3.08 eV. The emission energy increases with increasing e-beam evaporation power. This 3.08 eV line is attributed to a C related donor acceptor (D 0 A 0 ) transition.

P-Type Doping of Cubic GaN by Carbon

Successful p-type doping of cubic GaN epilayers by carbon using an e-beam evaporation source is reported. At room temperature Hall-effect measurements of the C-doped cubic GaN epilayer gave hole concentrations and hole mobilities as high as 6 x 10 17 cm -3 and 200 cm 2 /Vs, respectively. The thermal activation energy of the C-acceptor is (215 ± 10) meV. Low temperature photoluminescence spectra show a new line appearing at 3.08 eV. The emission energy increases with increasing e-beam evaporation power. This 3.08 eV line is attributed to a C related donor acceptor (D 0 A 0 ) transition.

Defect-Related Donors, Acceptors, and Traps in GaN

Point defects have been created in GaN by various types of irradiation: electrons (1 and 2.5 MeV, and the spectrum from 90Sr); protons (0.15 MeV, 2 MeV, and 24 GeV); He ions (5.4 MeV); γ-rays (60Co); and sputtering and e-beam deposition of metals. They have been studied by temperature-dependent Hall-effect measurements (T-Hall), deep-level transient spectroscopy (DLTS), optically detected magnetic resonance (ODMR), positron annihilation spectroscopy (PAS), and photoluminescence (PL). Confirmed defect energies, and firm or tentative defect assignments, are as follows: T-Hall (donor at 0.06 eV, VN); DLTS (electron trap at 0.18 eV (thermal 0.06 eV), VN; electron trap at 0.9 eV, NI or GaI–X); ODMR (GaI and GaI–X); PAS (VGa); PL (0.85 eV band with 0.88 eV zero-phonon line, ON or ON–GaI; 0.93 eV band; 3.37 eV line; 3.39 eV line). Many of these defect signatures have also been observed in as-grown GaN. Dislocations, of the threading-edge type, are found to be acceptor-like in n-type GaN.

Thermoluminescence properties of nitrogen containing chemical vapour deposited diamond films

The behaviour of centres due to nitrogen impurities introduced during the growth process into the chemical vapour deposited (CVD) diamond lattice is reported for the first time by thermoluminescence (TL) studies between 300 and 670 K after 200–400 nm ultraviolet (deuterium lamp) illumination at 300 K (RT) in air. TL curves exhibit some glow peaks at 490, 520 and 620 K, characterized by activation energies of approximately 1.2, 1.4 and 1.9 eV, respectively. Spectral analysis of these peaks which reveals some differences due to nitrogen content in the films shows well defined emission bands and interesting features leading to a better knowledge of the broad red photoluminescence (PL) band observed in our films. Nitrogen addition during the growth of the CVD material leads to the quenching of both of the 1.68 eV line related to Si centre and the broad green band centred at 2.25 eV which were observed for the high quality film.

UV and Anti-Stokes Photoluminescence of 3H Centre in Electron-Irradiated CVD Diamond

Photoluminescence (PL) of the 3H centre was studied at different excitation energies for 8 MeV electron-irradiated CVD diamond at temperatures 90-290 K. A frequency up-conversion effect was observed at 2.41 eV excitation of the 3H centre luminescence (2.462 eV). A deviation of the temperature dependence of the anti-Stokes PL intensity from the Boltzmann distribution was noted at 90-130 K. A transformation of the 2.462 eV (503.5 nm) line into the 2.443 eV (507.6 nm) line in the PL spectra was observed for the first few minutes of laser pumping at an excitation photon energy of 3.82 eV.

First Principles Study of the Self-Interstitial Defect in Diamond

First principles techniques have been employed to examine the isolated self-interstitial in diamond. The assignment of the R2 EPR center to the self-interstitial has been questioned because of the small fine structure term. We have calculated the spin–spin interaction tensor, which resolves the problem. We have also modeled the optical proberties of I1, and propose an inversion-doubling process to give rise to the 6 meV splitting of the ground and first excited states. The 1.685 and 1.859 eV lines can then be understood as being transitions between states made up from the p-orbitals on the three-fold coordinated C atoms. Our calculated piezospectroscopic tensor also agress with experiment.

Photoluminescence study of GaN grown by pulsed laser deposition in nitrogen atmosphere

Low temperature photoluminescence measurements were performed on thin films of gallium nitride grown by pulsed laser deposition in a nitrogen atmosphere. Activation energies of 26.5 and 230 meV were derived from temperature dependent PL studies of the donor–acceptor (D–A) transitions. Furthermore, for the 3.41 eV line, two values of activation energy (13.5 and 25 meV) were observed at low and high temperatures. X-ray diffraction showed predominantly the wurtzite structure, with some evidence for cubic inclusions. We discuss these data in terms of the possibility of a mixed phase structure and examine evidence for the localization of the electrons and holes in the two phases.

High quality homoepitaxial CVD diamond for electronic devices

Abstract Recent achievements in homoepitaxial CVD diamond films for electronic devices have been discussed. We have successfully synthesized high-quality homoepitaxial diamond films with atomically flat surface by the microwave plasma chemical vapor deposition (CVD) using a low CH4 concentration of CH4/H2 gas system less than 0.15% CH4/H2 ratio and Ib (001) substrates with low-misorientation angle less than 1.5°. These films are atomically flat over an area as large as 4×4 mm2 and have shown a strong excitonic emission of 5.27 eV line, even at room temperature, with no essential emission lines in the visible light region in the cathodoluminescence (CL) spectra. Furthermore, high-quality Schottky junctions between Al and P type high-conductivity layers near the surface of these films have been obtained. Based on this growth method, we have also successfully synthesized B-doped diamond films using trimethylboron [B(CH3)3,TMB] gas as a B-doping source, whose Hall mobility is 1840 cm2/Vs at 290 K. Schottky junction fabricated by the B-doped diamond also shows excellent performances, indicating that the homoepitaxial diamond films presented here have a high potentiality for electronic devices.

Effect of CdCl2 Treatment on the Interior of CdTe Crystals

ABSTRACTAn essential processing step in CdTe/CdS polycrystalline solar cells is heat treatment in CdCl2. We present photoluminescence results from single crystals of CdTe that have been exposed to CdCl2 treatments at 387 C similar to those used in actual cell fabrication. Using sub band gap excitation from a tunable diode laser, we probe states in the interior of the crystal. We show that high-purity (99.998 percent) CdCl2 treatment results in the appearance of a 1.45 eV donor-acceptor transition that is likely due to a Cl-Cu center. Low purity (99.7 percent) CdCl2treatment results in the appearance of the 1.45 eV line and a 1.555 eV Cu-related emission. These results indicate that the CdCl2 treatment has an effect on the interior of CdTe grains, in addition to its already well established effect on grain boundaries in polycrystalline CdS/CdTe devices. They also imply that CdCl2 treatment may result in the incorporation of Cu into the CdTe grains. The results will be related to the effects of CdCl2 on polycrystalline CdS/CdTe devices that have been observed by other groups.This work is supported by NREL

Photoluminescence from polymer-like hydrogenated and nitrogenated amorphous carbon films

The mechanism of photoluminescence (PL) in hydrogenated amorphous carbon (a-C:H), and nitrogenated and hydrogenated amorphous carbon (a-C:H:N) thin films grown by radio frequency driven plasma enhanced chemical vapor deposition is still a subject of much debate. In this work, we investigate the PL signal obtained from a-C:H and a-C:H:N films, paying particular attention to the effect of nitrogen flow rate during growth, and postgrowth, ex situ annealing on the PL properties of the films. We also correlate the PL spectra to the electronic structure of the films. The films had a low paramagnetic defect density (1017 cm−3). The PL spectra were obtained using the 488 nm (2.54 eV) line of an argon ion laser, as the excitation source. It was observed that the nitrogenation of the films leads to the creation of new bands in the PL signal, which were correlated to the bond fraction of CN triple bonds, as measured by infrared spectroscopy.

Deep-level luminescence at 1.0 eV in 6H SiC

ABSTRACTResults from photoluminescence (PL) and Zeeman effect measurements of a PL center, labeled UD-1, in 6H SiC are presented. The spectrum consists of three no phonon-lines (NPLs) at 0.9952, 1.0015 and 1.0020 eV. The luminescence starts decreasing in intensity above 40 K and is completely quenched at 80 K. The observed Zeeman splitting reveals a spin one half of the ground state of the two highest energy lines. No splitting of the 0.9952 eV line is detected. The g║-value for the 1.0015 eV and 1.0020 e║ lines are g║ = 1:4 and g║ = 1:7, respectively. For both lines, g⊥ = 0. The C3v symmetry indicates that the UD-1 center is either a substitutional defect or a complex with its constituents lying along the c-axis of the lattice.

Current problems in diamond: towards a quantitative understanding

This paper discusses three major areas of current work on the properties of point defects in diamond: attempting to introduce a shallow donor centre, studying the rôle of transition metals, and achieving a quantitative understanding of radiation effects. Recent work on the first two topics is surveyed briefly. Published and new data on the problem of radiation damage are discussed in terms of: measuring the concentrations of the vacancy-related and self-interstitial centres; isotope effects on the vacancy and interstitial centres, including the identification of the self-interstitial 1.859 eV line as a vibrational feature; the rate of production of damage, including the influence of the nitrogen impurity; and the radiation-induced reactions. Some of the problems arising from the lack of current information are highlighted.

X-ray induced luminescence of high-purity, amorphous silicon dioxide

A comprehensive study of x-ray stimulated luminescence has been carried out on four types of high-purity, amorphous silica (a-SiO2). Both high OH and low OH as well as oxygen-excess and oxygen-deficient materials were studied. The room-temperature, visible x-radio luminescence (XRL) was measured continuously as a function of x-ray dose from zero to 400 Mrad volume average dose. In addition to the XRL measurements, electron paramagnetic resonance (EPR) was used to determine the concentrations of the two key radiation-induced defects, the E′ center and the nonbridging oxygen hole center (NBOHC). The XRL spectra were deconvolved into four Gaussian components with centers at 1.9, 2.2, 2.6, and 2.75 eV. The same centers and widths could be used to describe the spectra in all four types of a-SiO2, only the intensities varied. The 2.6 and 2.75 eV lines are strongly dose dependent, rising from near zero intensity at zero dose in all four materials. These two lines are strongly correlated with each other; they have essentially the same dependence on dose and sample type. This correlation suggests that these two lines are due to the same radiation-induced defect, or to closely related defects. The dose dependence and sample-to-sample variation of these two lines bear some similarities to the E′ concentrations. In contrast to the 2.6 and 2.75 eV lines, the 1.9 eV line has a high intensity at the lowest doses measurable. A simple phenomenological model is proposed to describe the 1.9 eV XRL line. This model involves two populations of defects; one population is present at zero dose and is assumed to be dose independent, while the second population is dose dependent. Evidence is presented that the dose-dependent defect is the NBOHC. The XRL due to the dose-independent population may be associated with a transient response to the x rays, or to a metastable defect; this population may not be observable in post-irradiation experiments such as EPR and conventional photoluminescence. Similar to the 1.9 eV line, the 2.2 eV line also has relatively high intensity at the lowest measurable x-ray dose. The behavior of this line is in general agreement with the self-trapped exciton model.

Defect luminescence of nitrogen-doped hydrogenated amorphous germanium thin films

In this letter we report results on photoluminescence (PL) of nitrogen-doped a-Ge:H thin films excited with the 2.4 eV line of an Ar + laser. The samples were prepared by rf sputtering a c-Ge target in a gaseous mixture of Ar+H 2+N 2. The PL of samples having an impurity concentration lower than 5×10 19 cm −3 was measured in the 110–180 K temperature range. The emission spectra show a broad band peaking at 0.65 eV, the origin of which is tentatively associated to a negatively charged dangling bond–valence band tail transition. The interpretation is consistent with available data on doping effects in a-Ge:H and with PL data reported in the literature.