Electron paramagnetic resonance (EPR) measurements have been made at X-band (f9.5 GHz) and W-band... more Electron paramagnetic resonance (EPR) measurements have been made at X-band (f9.5 GHz) and W-band (f95 GHz) of a-C:H films on (1 0 0) silicon substrates; the sample temperature was varied in the range 5-300 K. Two types of film were examined. The first type are amorphous hydrogenated carbon (a-C:H) films grown by plasma enhanced chemical vapour deposition (PECVD) with negative self bias voltages in the approximate range 100-500 V. The second type were initially highly polymeric-like a-C:H films grown on Si placed on the earthed electrode of a PECVD system but were subsequently implanted with either 6=10 cm B or 2=10 cm B ions. At both X-and W-band and throughout the temperature range 5-300 K 15 y2 q 16 y2 q the EPR signal of the carbon unpaired electrons consists of a single symmetric line with gs2.0026"0.0002. As the temperature is lowered, several samples develop a dependence on sample orientation of the external field required for resonance. This anisotropy is explained in terms of the demagnetising fields more usually encountered in ferromagnetic resonance.
Proceedings of 11th International Conference on Ion Implantation Technology
Relaxed Si,,Gex layers with germanium contents of 0.04, 0.13, 0.24 and 0.36 have been grown by MB... more Relaxed Si,,Gex layers with germanium contents of 0.04, 0.13, 0.24 and 0.36 have been grown by MBE and implanted under tightly controlled conditions with 2 MeV Si' ions over the dose range 1 x lo1' to 5 x 10'' Si+ cm". The introduction rate and characteristics of simple defects have been investigated by EPR and DLTS whilst the accumulation of this damage with increasing dose has been followed by RBS, XTEM and optical depth profiling up to the onset of amorphisation. It is found that the integrated damage increases whilst the critical dose for amorphisation decreases with increasing Ge content.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
EPR measurements have been made on (100) n type silicon wafers implanted with 200 keV t60+ ions i... more EPR measurements have been made on (100) n type silicon wafers implanted with 200 keV t60+ ions in the dose range (0.5-1.4)X lOi* 0+ cme2 and at implantation temperatures, T,, in the range 250-600 o C. Several types of defect are observed including E; centres. For a dose of 1.4~ lo'* 0 cm-* the E; concentration is found to increase from 1.5 x 1014 to 2.3 X lOI cmm2 as T, increases from 250 to 6OO"C. Measurements on lower dose samples indicate that E; defects are present in SiO, precipitates as well as in the buried SiO, layer. The E; EPR signal only starts to decrease for annealing temperatures S 450 o C.
ABSTRACT The effect of varying current density and pressure during arc generation on the yield an... more ABSTRACT The effect of varying current density and pressure during arc generation on the yield and purity of multiwalled nanotube containing carbon soot has been studied. Various soots were produced and characterized using electron paramagnetic resonance, transmission electron microscopy and thermo gravimetric analysis. It was found that both yield and purity increase as current density and pressure are increased. For a current density of 195 A/cm2 and pressure of 500 Torr of Helium, nanotube content of 54% was measured. .
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1992
Electron paramagnetic resonance and Rutherford backscattering have been used to investigate the d... more Electron paramagnetic resonance and Rutherford backscattering have been used to investigate the damage produced by implanting (100) silicon wafers with either 3 MeV Auf or 2 MeV Sif ions at liquid nitrogen temperature, 300 or 500 K (Au only). For samples implanted at liquid nitrogen temperature the EPR spectra include those of the neutral 4-vacancy (Si-P3), the silicon di-interstitial (Si-P6), the D centre (indicative of amorphous silicon) and a broad anisotropic resonance; their concentration in these samples is determined as a function of dose in the range 1013-10'5 Si+ cmm2 and 1012-1014 Auf cm-'. The EPR spectrum for any dose of Au + ions at the low implantation temperature is very similar to the equivalent spectrum for an approximately 20 times '+ larger dose of SI ions. At the lowest doses there is no measurable D centre concentration. For implants at 300 K of 5~10'~-5~ 10" Au+ crne2 and of 3~ 1Or5 Sif cmm2 an amorphous layer is produced and the EPR spectrum reveals only D centres. However, no such layer is present and point defects are revealed by EPR for an implant of 5 X 1014 Sic crnm2 at 300 K or 5 x 1014-5x 1015 Auf crnm2 at 500 K.
The allowed and forbidden hyperfine transitions of the S-state ion Mn2+ have been observed in sin... more The allowed and forbidden hyperfine transitions of the S-state ion Mn2+ have been observed in single crystals of sodium beta -alumina. Manganese was incorporated in the beta -alumina during its synthesis. The observed spectrum is shown to arise from three overlapping spectra which are due to manganese ions occupying different sites in the spinel-like blocks. The values of g, A and D are evaluated for Mn2+ in each of the occupied sites. The distribution of manganese within the beta -alumina is shown to be different from that obtained during diffusion doping.
The EPR and optical absorption spectra of X-ray irradiated single crystal of Na beta -alumina hav... more The EPR and optical absorption spectra of X-ray irradiated single crystal of Na beta -alumina have been examined. The most prominent absorptions are tentatively ascribed to an F+ centre at the O(5) oxygen ion site in the conduction plane. To this defect the authors assign an axially symmetric EPR spectrum which shows a resolved hyperfine structure due to interaction of the single unpaired spin with two equivalent 27Al(I=5/2) nuclei. The measured spin-Hamiltonian parameters are g/sub ///=2.0076(2), gperpendicular to =2.0032(2), A/sub ///=1.665(2) mT, Aperpendicular to =1.617(2) mT, the defect symmetry axis coinciding with the crystal c axis. From the EPR linewidth it is inferred that the hyperfine interaction with three equivalent 23Na nuclei is of order 0.2 mT. This defect is also optically active having a strongly polarised s to pz transition with mean energy EF=4.15(5) eV. The corresponding s to px,y optical transitions are not observed in the range of photon energies from 4.2-6.8 eV.
Electron Paramagnetic Resonance measurements have been made on silicon-on-silicon dioxide and sil... more Electron Paramagnetic Resonance measurements have been made on silicon-on-silicon dioxide and silicon-on-silicon nitride structures produced by ion implantation. In the former type of structure the EPR spectra have been ascribed to the oxygen vacancy centre E1” in the SiO2, the defect PMb0 at the Si/SiO2 interfaces and the amorphous silicon centre in the bulk silicon. A single line of width
Measurements of the Hall voltage have been made on several specimens of high purity tin at liquid... more Measurements of the Hall voltage have been made on several specimens of high purity tin at liquid helium temperatures and in magnetic fields up to 4·5 T. The field dependence has been studied for field directions 0 to 51° from the c axis. In the range 0 to 1½°, the behaviour has been explained in terms of the conversion by magnetic breakdown (MB) of electron orbits into a network of coupled hole orbits. The linking by MB of open orbits to form closed hole orbits accounts for the behaviour in the range 16° to 38°. The occurrence of oscillations in the Hall voltage provides further evidence for these interpretations.The breakdown field, B0 has been found to depend on height, kz, above the central (0.01) plane according to the relation B0=(0.9±0.1)+(4.0±0.3) × 10kz .
Electron paramagnetic resonance ͑EPR͒ measurements have been made of amorphous hydrogenated carbo... more Electron paramagnetic resonance ͑EPR͒ measurements have been made of amorphous hydrogenated carbon ͑a-C:H͒ films grown by plasma enhanced chemical vapor deposition ͑PECVD͒ with negative self-bias voltages V b in the approximate range 10-540 V. For V b Ͻ100 V, as the film changes from polymerlike to diamondlike, the changes in linewidth and shape are interpreted in terms of changes to two contributions-one due to dipolar interactions between the unpaired spins and one due to unresolved lines arising from hyperfine interactions with H 1. The former yields a Lorentzian line, the latter a Gaussian, and the resultant spectrum has the Voigt shape. The empirical relation ⌬B pp G ͑in Gauss͒ϭ͑0.18Ϯ0.05͒ϫ͑at. % H) between the peak-to-peak Gaussian contribution ͑in Gauss͒ ⌬B pp G and the hydrogen content in atomic percentage is obtained. For V b Ͼ100 V the linewidth is shown to be dominated by the dipolar interactions and exchange and it decreases as V b increases; the change is shown to arise primarily from a change in the exchange interaction. Evidence for this comes from measurements which show that the spin-lattice relaxation time appreciably shortens and the spin-spin relaxation time lengthens as the bias voltage is increased. The magnitude and variation with bias of the linewidth are consistent with the EPR signal originating from the-type radicals.
The electrical properties of amorphous carbon are governed by the high localization of the sp 2 s... more The electrical properties of amorphous carbon are governed by the high localization of the sp 2 states, and conventional methods of altering the sp 2 content result in macroscopic graphitization. By using ion beams we have achieved a delocalization of the states by introducing nanoclustering and hence improving the connectivity between existing clusters, as demonstrated by the increase in the conductivity by two orders of magnitude without modification of the band gap. At higher doses, paramagnetic relaxation-time measurements indicate that exchange effects are present. This unveils the possibility of amorphous carbon-based electronics by tailoring the ion-beam conditions, which we demonstrate in the form of a rectifying device.
Electron paramagnetic resonance (EPR) measurements have been made at X-band (f9.5 GHz) and W-band... more Electron paramagnetic resonance (EPR) measurements have been made at X-band (f9.5 GHz) and W-band (f95 GHz) of a-C:H films on (1 0 0) silicon substrates; the sample temperature was varied in the range 5-300 K. Two types of film were examined. The first type are amorphous hydrogenated carbon (a-C:H) films grown by plasma enhanced chemical vapour deposition (PECVD) with negative self bias voltages in the approximate range 100-500 V. The second type were initially highly polymeric-like a-C:H films grown on Si placed on the earthed electrode of a PECVD system but were subsequently implanted with either 6=10 cm B or 2=10 cm B ions. At both X-and W-band and throughout the temperature range 5-300 K 15 y2 q 16 y2 q the EPR signal of the carbon unpaired electrons consists of a single symmetric line with gs2.0026"0.0002. As the temperature is lowered, several samples develop a dependence on sample orientation of the external field required for resonance. This anisotropy is explained in terms of the demagnetising fields more usually encountered in ferromagnetic resonance.
Proceedings of 11th International Conference on Ion Implantation Technology
Relaxed Si,,Gex layers with germanium contents of 0.04, 0.13, 0.24 and 0.36 have been grown by MB... more Relaxed Si,,Gex layers with germanium contents of 0.04, 0.13, 0.24 and 0.36 have been grown by MBE and implanted under tightly controlled conditions with 2 MeV Si' ions over the dose range 1 x lo1' to 5 x 10'' Si+ cm". The introduction rate and characteristics of simple defects have been investigated by EPR and DLTS whilst the accumulation of this damage with increasing dose has been followed by RBS, XTEM and optical depth profiling up to the onset of amorphisation. It is found that the integrated damage increases whilst the critical dose for amorphisation decreases with increasing Ge content.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
EPR measurements have been made on (100) n type silicon wafers implanted with 200 keV t60+ ions i... more EPR measurements have been made on (100) n type silicon wafers implanted with 200 keV t60+ ions in the dose range (0.5-1.4)X lOi* 0+ cme2 and at implantation temperatures, T,, in the range 250-600 o C. Several types of defect are observed including E; centres. For a dose of 1.4~ lo'* 0 cm-* the E; concentration is found to increase from 1.5 x 1014 to 2.3 X lOI cmm2 as T, increases from 250 to 6OO"C. Measurements on lower dose samples indicate that E; defects are present in SiO, precipitates as well as in the buried SiO, layer. The E; EPR signal only starts to decrease for annealing temperatures S 450 o C.
ABSTRACT The effect of varying current density and pressure during arc generation on the yield an... more ABSTRACT The effect of varying current density and pressure during arc generation on the yield and purity of multiwalled nanotube containing carbon soot has been studied. Various soots were produced and characterized using electron paramagnetic resonance, transmission electron microscopy and thermo gravimetric analysis. It was found that both yield and purity increase as current density and pressure are increased. For a current density of 195 A/cm2 and pressure of 500 Torr of Helium, nanotube content of 54% was measured. .
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1992
Electron paramagnetic resonance and Rutherford backscattering have been used to investigate the d... more Electron paramagnetic resonance and Rutherford backscattering have been used to investigate the damage produced by implanting (100) silicon wafers with either 3 MeV Auf or 2 MeV Sif ions at liquid nitrogen temperature, 300 or 500 K (Au only). For samples implanted at liquid nitrogen temperature the EPR spectra include those of the neutral 4-vacancy (Si-P3), the silicon di-interstitial (Si-P6), the D centre (indicative of amorphous silicon) and a broad anisotropic resonance; their concentration in these samples is determined as a function of dose in the range 1013-10'5 Si+ cmm2 and 1012-1014 Auf cm-'. The EPR spectrum for any dose of Au + ions at the low implantation temperature is very similar to the equivalent spectrum for an approximately 20 times '+ larger dose of SI ions. At the lowest doses there is no measurable D centre concentration. For implants at 300 K of 5~10'~-5~ 10" Au+ crne2 and of 3~ 1Or5 Sif cmm2 an amorphous layer is produced and the EPR spectrum reveals only D centres. However, no such layer is present and point defects are revealed by EPR for an implant of 5 X 1014 Sic crnm2 at 300 K or 5 x 1014-5x 1015 Auf crnm2 at 500 K.
The allowed and forbidden hyperfine transitions of the S-state ion Mn2+ have been observed in sin... more The allowed and forbidden hyperfine transitions of the S-state ion Mn2+ have been observed in single crystals of sodium beta -alumina. Manganese was incorporated in the beta -alumina during its synthesis. The observed spectrum is shown to arise from three overlapping spectra which are due to manganese ions occupying different sites in the spinel-like blocks. The values of g, A and D are evaluated for Mn2+ in each of the occupied sites. The distribution of manganese within the beta -alumina is shown to be different from that obtained during diffusion doping.
The EPR and optical absorption spectra of X-ray irradiated single crystal of Na beta -alumina hav... more The EPR and optical absorption spectra of X-ray irradiated single crystal of Na beta -alumina have been examined. The most prominent absorptions are tentatively ascribed to an F+ centre at the O(5) oxygen ion site in the conduction plane. To this defect the authors assign an axially symmetric EPR spectrum which shows a resolved hyperfine structure due to interaction of the single unpaired spin with two equivalent 27Al(I=5/2) nuclei. The measured spin-Hamiltonian parameters are g/sub ///=2.0076(2), gperpendicular to =2.0032(2), A/sub ///=1.665(2) mT, Aperpendicular to =1.617(2) mT, the defect symmetry axis coinciding with the crystal c axis. From the EPR linewidth it is inferred that the hyperfine interaction with three equivalent 23Na nuclei is of order 0.2 mT. This defect is also optically active having a strongly polarised s to pz transition with mean energy EF=4.15(5) eV. The corresponding s to px,y optical transitions are not observed in the range of photon energies from 4.2-6.8 eV.
Electron Paramagnetic Resonance measurements have been made on silicon-on-silicon dioxide and sil... more Electron Paramagnetic Resonance measurements have been made on silicon-on-silicon dioxide and silicon-on-silicon nitride structures produced by ion implantation. In the former type of structure the EPR spectra have been ascribed to the oxygen vacancy centre E1” in the SiO2, the defect PMb0 at the Si/SiO2 interfaces and the amorphous silicon centre in the bulk silicon. A single line of width
Measurements of the Hall voltage have been made on several specimens of high purity tin at liquid... more Measurements of the Hall voltage have been made on several specimens of high purity tin at liquid helium temperatures and in magnetic fields up to 4·5 T. The field dependence has been studied for field directions 0 to 51° from the c axis. In the range 0 to 1½°, the behaviour has been explained in terms of the conversion by magnetic breakdown (MB) of electron orbits into a network of coupled hole orbits. The linking by MB of open orbits to form closed hole orbits accounts for the behaviour in the range 16° to 38°. The occurrence of oscillations in the Hall voltage provides further evidence for these interpretations.The breakdown field, B0 has been found to depend on height, kz, above the central (0.01) plane according to the relation B0=(0.9±0.1)+(4.0±0.3) × 10kz .
Electron paramagnetic resonance ͑EPR͒ measurements have been made of amorphous hydrogenated carbo... more Electron paramagnetic resonance ͑EPR͒ measurements have been made of amorphous hydrogenated carbon ͑a-C:H͒ films grown by plasma enhanced chemical vapor deposition ͑PECVD͒ with negative self-bias voltages V b in the approximate range 10-540 V. For V b Ͻ100 V, as the film changes from polymerlike to diamondlike, the changes in linewidth and shape are interpreted in terms of changes to two contributions-one due to dipolar interactions between the unpaired spins and one due to unresolved lines arising from hyperfine interactions with H 1. The former yields a Lorentzian line, the latter a Gaussian, and the resultant spectrum has the Voigt shape. The empirical relation ⌬B pp G ͑in Gauss͒ϭ͑0.18Ϯ0.05͒ϫ͑at. % H) between the peak-to-peak Gaussian contribution ͑in Gauss͒ ⌬B pp G and the hydrogen content in atomic percentage is obtained. For V b Ͼ100 V the linewidth is shown to be dominated by the dipolar interactions and exchange and it decreases as V b increases; the change is shown to arise primarily from a change in the exchange interaction. Evidence for this comes from measurements which show that the spin-lattice relaxation time appreciably shortens and the spin-spin relaxation time lengthens as the bias voltage is increased. The magnitude and variation with bias of the linewidth are consistent with the EPR signal originating from the-type radicals.
The electrical properties of amorphous carbon are governed by the high localization of the sp 2 s... more The electrical properties of amorphous carbon are governed by the high localization of the sp 2 states, and conventional methods of altering the sp 2 content result in macroscopic graphitization. By using ion beams we have achieved a delocalization of the states by introducing nanoclustering and hence improving the connectivity between existing clusters, as demonstrated by the increase in the conductivity by two orders of magnitude without modification of the band gap. At higher doses, paramagnetic relaxation-time measurements indicate that exchange effects are present. This unveils the possibility of amorphous carbon-based electronics by tailoring the ion-beam conditions, which we demonstrate in the form of a rectifying device.
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Papers by R. Barklie