William Hayes

Absorption of Oxygen in Silicon in the Near and the Far Infrared

Previous work on oxygen in silicon has shown that oxygen dissolves interstitially in silicon forming a complex which may be approximately described as Si2O. Absorption bands of Si : O occur at 517, 1136 and 1203 cm-1 and these have been assigned by earlier authors to the v2 (symmetric bending), v3 (antisymmetric stretch) and v1 (symmetric stretch) normal modes of vibration of Si2O. The present investigation confirms the v3 origin of the 1136 cm-1 band (the well-known 9μm band) but we disagree with the earlier assignments of the 517 and 1136 cm-1 bands. The results reported here are relevant to organic siloxanes. We have extended the investigation of Si : O into the far infrared and we find sharp absorption lines at 29.3, 37.8, 43.3 and 49.0 cm-1 which we have assigned to the v2 mode of Si2O. The isotope shift due to l8O has been observed in the far infrared spectrum. Effects of uniaxial stress on the 29.3 cm-1 line have been investigated and are found to be consistent with the assignment to the v2 mode. The main features of the far infrared spectrum are accounted for with a simple anharmonic potential which ignores coupling of the Si2O to the crystal lattice. We have investigated effects of uniaxial stress on the 517, 1136 and 1203 cm-1 bands of Si2O. Our stress results for the 1136 cm-1 band are consistent with the earlier v3 assignment. Using our normal mode description, we conclude that the 1203 cm-1 band is a combination band involving v3 and v2 excitations. We have not been able to give a clear cut assignment to the 517 cm-1 band, but we suggest that v1 type excitation may be involved. The appendix describes the stress splitting of the 836 cm-1 band of the silicon A centre in electron irradiated Si : O and our results confirm an earlier model for this centre. In all cases investigated here, the stress splittings arise from raising the orientational degeneracy of the oxygen complex.

Raman scattering and theoretical studies of Jahn—Teller induced phase transitions in some rare-earth compounds

The rare-earth crystals DyVO4, DyAsO4 and TbVO4 undergo structural phase transitions at 14, 12 and 34 K respectively induced by a cooperative Jahn-Teller effect. An experimental investigation of the phase changes has been made using Raman scattering methods. For purposes of comparison, the Raman spectra of DyPO4, YVO4 and GdVO4 (which do not undergo phase transitions) have also been measured. A detailed theory of the phase transitions has been developed which accounts for the electronic and vibrational Raman spectra. The lowest four electronic levels of Dy3+ and Tb3+ are treated in a pseudospin formalism, and the theory is discussed in terms of mixed phonon and pseudo-spin excitations. The singly degenerate κ ≈ 0 optic phonons are not measurably affected by the transitions, but splittings of up to 20 cm-1 developed in the doubly degenerate Eg phonon modes in the Raman spectra owing to anharmonic couplings with the electronic states. Splittings are also observed in the lowest electronic levels of the rate earth ions below the transition temperature; these electronic modes effectively constitute the soft mode which causes the transitions to occur. A molecular field model is first used in the theoretical analysis. The equations of motion of the coupled excitations are then found and the energies, line widths and dispersion of the mixed excitations are discussed. The connexion between the static strain and the acoustic phonon mode coupling is investigated, and a thermodynamic treatment of the elastic constants is given. From the Raman data, it appears that in TbVO4 the acoustic mode coupling is stronger than the optic mode coupling.

Paramagnetic resonance of S-state ions in calcium fluoride

Paramagnetic resonance has been observed in Mn2+, Eu2+ and Gd3+ ions in single crystals of calcium fluoride grown from the melt. The Mn2+ ion has a very small cubic-field Stark splitting (a = + 0.6 ± 0.4 x 10-4 cm-1), and a fluorine hyperfine structure of overall splitting of about 60G due to some covalent bonding with the fluorine ligands. The Eu2+ spectrum has cubic symmetry with splitting parameters b4 = 57.9 + 0.2, b6 = 0.5 + 0.2 ( x 10-4 cm-1), but the Gd3+ spectrum has tetragonal symmetry with much larger Stark splittings; neither ion shows a resolved fluorine structure. The manganese and europium hyperfine structures are closely the same as in other salts with small covalent bonding.

An investigation of the Ni+ ion in irradiated LiF and NaF

The effects of ionizing radiations on LiF: Ni and NaF: Ni crystals have been investigated by paramagnetic resonance methods. Nickel dissolves substitutionally in LiF and NaF in the divalent state and is associated with charge compensating positive ion vacancies or vacancy aggregates. When the crystals are exposed to ionizing radiations at room temperature the Ni2+(3d8, 3F) ions trapelectrons forming Ni+(3d9, 2D) ions which are, in general, associated with a variety of vacancy configurations. Eight different Ni+ complexes have been investigated in both LiF and NaF and detailed models are proposed for some of these. The annealing behaviour of the Ni+ complexes in the room temperature and low temperature irradiated crystals is described. The symmetry of the Ni+ complexes is less than cubic because of the Jahn -Teller effect and associated vacancies and both |x2 — y2y and |3z2 — r2y ground states are observed. The ground state of unassociated Ni+ ions is |3z2 — r2>. The fluorine hyperfine structure of the Ni+ spectra has been investigated. Effects are observed in both the g values and the fluorine structure which are attributed to zero point vibrational admixtures of the |x2 — y2> and |3z2 — r2> states. The effects of the nickel impurity on F- and M- band formation in the crystals are described in an appendix.

Brillouin scattering, ultrasonic and theoretical studies of acoustic anomalies in crystals showing Jahn-Teller phase transitions

DyVO4 and TbVO4 undergo a Jahn-Teller induced crystallographic phase change at 14 and 34 K respectively, and elastic constants of these materials behave anomalously at the transition temperature TD. Brillouin scattering measurements on DyVO4 show that the elastic constant (c11 - c12) falls to zero at TD. The electron lattice interaction which drives the transition in this material is predominantly of short range and a cluster theory is used to interpret the measurements. The elastic constant c66 of TbVO4 also has an anomaly. It falls to zero at TD when measured at ultrasonic frequencies but only falls to a finite value when measured at the higher frequencies used in Brillouin scattering. This frequency dependence of the elastic constants is explained using both thermodynamic and microscopic theories.

Experimental and Theoretical Studies of Effects of Anharmonicity and High-Temperature Disorder on Raman Scattering in Fluorite Crystals

Some crystals with the fluorite structure, e.g. CaF2, SrF2, BaF2, SrCl2 and PbF2, are known to exhibit a specific heat anomaly at temperatures Tc well below the melting point. It is generally assumed that this anomaly is associated with the development of extensive disorder in the anion sublattice. To study this disorder we have carried out polarized Raman scattering investigations of these crystals in the temperature range 4-1420 K. Fluorite crystals have one Raman-active phonon with T2g symmetry. We find th at the position and shape of this line below Tc can be explained in detail by using third- and fourth-order anharmonicity. At Tc and above additional scattering develops on the low-energy side of the T2g phonon and this is accounted for by a theory of defect-induced scattering which includes effects of both anion vacancies and interstitials. Both force-constant and polarizability changes are considered in a nearest-neighbour approximation.

An investigation of the effects of X-rays on undoped and on hydrogen doped alkaline earth fluorides

We have investigated, using e.p.r. methods, the effects of X-rays on undoped and hydrogendoped alkaline earth fluorides. We find that irradiation of undoped CaF2 at any temperature does not produce F centres in observable concentration. Irradiation of undoped SrF2 and BaF2 at 77 °K produces Vk centres and trapped electron centres which anneal out below room temperature. The undoped SrF2 and BaF2 crystals are extremely difficult to colour with room temperature X-irradiation due, apparently, to recombination of electrons and holes. Irradiation of SrF2:H and BaF2:H crystals at 77 °K produces hydrogen atoms in fluorine sites (Hg centres) and trapped electron centres. The rate of introduction of the electron centres into the crystals is greater by a factor of about 3 than in the undoped crystals. Irradiation of CaF2:H at 77 °K also produces Hg centres and trapped electron centres but the rate of generation of the electron centres in these crystals is greater than in undoped CaF2 by a factor of at least 25. The origin of this difference in the behaviour of CaF2:H and the behaviour of SrF2:H and BaF2:H and the nature of the response of alkaline earth fluorides to X-rays are discussed. The use of polarized bleaching light to extract information from the optical absorption spectrum of irradiated CaF2:H is described in an appendix.

Raman study of phase transitions in rare earth vanadates

Abstract The phase transitions in DyVO4 at about 14K and in TbVO4 at about 33K have been investigated by Raman scattering. The Eg phonons are split below the transition temperature but the singlet phonon modes are not appreciably affected. No soft phonon mode is observed but large changes are seen in the low lying electronic levels. These results appear to be a consequence of a cooperative Jahn-Teller effect.

MAGNETO-OPTICAL PROPERTIES OF F CENTRES IN ALKALINE EARTH FLUORIDES.

The position of the F band peak is determined in additively coloured CaF2, SrF2 and BaF2 by the Faraday rotation method. However, in additively coloured crystals, the F band is normally overlaid by absorption due to higher F aggregate centres and a determination of the spin orbit coupling constant in the excited 2P state of the F centre is not feasible since a precise knowledge of the F band optical density is required. It is found that trapped electron centres are produced by X-irradiation at 77 °K of undoped SrF2 and BaF2 crystals and hydrogen doped CaF2, SrF2 and BaF2 crystals and the spin orbit coupling is determined for these centres. Paramagnetic resonance measurements show (Bessent, Hayes, Hodby & Smith 1968) that the trapped electron centres are closely related to the normal F centres found in additively coloured crystals. Wavefunctions obtained from a point ion model (Bennett & Lidiard 1965) are used to calculate the spin orbit coupling for the 2P state of the F centre in CaF2, SrF2 and BaF2. For comparison with theory it is assumed that the spin orbit coupling for the irradiation induced trapped electron centres is not appreciably different from that for normal F centres and we obtain good agreement between theory and experiment.

AN INVESTIGATION OF X-IRRADIATED KCl:H AND NaCl:H.

The e.p.r. spectrum of a new hydrogen atom centre called the U3 centre has been observed after X-irradiation of KC1 :H crystals at low temperatures and has been assigned to hydrogen atoms in chlorine sites. The centre bleaches thermally at 145 °K with an activation energy of 043 ± 0.03 eV. Optical bleaching experiments indicate that the centre has a weak absorption band peaking at 2-9 eV. The centre cannot be produced by ultraviolet irradiation. A production mechanism involving Vk centres is proposed and is discussed in detail. The U3 centre has also been observed in NaCl:H.