The atomic and electronic structure and relaxation of the perfect (111) surface and several low-index surfaces of CaF2 are calculated using an ab initio Hartree-Fock method. We find marked differences between the bulk and surface valence band structures; however, the band gap is reduced by only 0.5 eV for the stepped surface. The (111) surface is found to exhibit no significant relaxation and we obtain a surface energy of 0.47 J m-2. The atomic structure of two types of stepped surface both having 0.32 nm high steps aligned along the [110] crystallographic direction is determined and the step energies are found to be 0.25 and 0.35 nJ m-1, respectively. Step energies for configurations involving other orientations of steps are considerably higher. The formation of double-height steps along [110], however, is found to be energetically favourable. A considerable amount of relaxation is found for stepped surfaces and we explain qualitatively why it results in a roughening of the surface in some cases. The energetic position of gap electronic states induced by steps on the (111) surface is found to be very similar to that for surface states of the perfect (111) surface; i.e. such states do not yield a significant density of states more than 0.5 eV above the bulk valence band maximum. We discuss how the step formation energies found here might explain surface topographical features found on cleaved and grown CaF2(111) surfaces.

http://iopscience.iop.org/article/10.1088/0953-8984/13/10/304/pdf

Theoretical modelling of steps on the CaF2(111) surface

An annealing method for a single crystal of fluoride is provided. The method includes the steps of removing at least one of attached objects and absorbed objects from the surface of the single crystal of fluoride to clean the surface, thereafter annealing the single crystal of fluoride, including heating the single crystal of fluoride and gradually cooling the heated single crystal of fluoride, and removing a deteriorated layer which is formed on the surface of the single crystal of fluoride during the annealing step.

Method for annealing single crystal fluoride and method for manufacturing the same

Time-resolved photoluminescence (PL) on excitation with an ArF excimer laser and IR–vacuum UV optical absorption induced by irradiation of an ArF excimer laser of commercially available CaF2 single crystals fabricated by four different manufacturers were measured. Intrinsic PL that was due to self-trapped excitons generated by band-to-band excitation by two-photon absorption processes was observed in all samples. In addition, most samples showed PL originating from traces of lanthanoid (Ln) impurities, Ce3+,Eu2+ (blue emission), or Tb3+ (yellow-green emission). The PL of Ce3+ and Eu2+ had no correlation with color-center formation; however, intense absorption bands arising from an F- center associated with a trivalent yttrium ion (YFC) were necessarily induced in the samples that showed yellow-green PL of Tb3+ under irradiation by ArF excimer laser light. When the yellow-green fluorescent samples were irradiated with ArF excimer laser light, the PL intensity of Tb3+ and the induced absorption that was due to YFC increased with the irradiated pulse number, and their growth was found to conform to a first-order kinetics. It was concluded that photoelectrons were released from Tb2+ ions by ArF excimer laser irradiation, and the resulting electrons were trapped at the site of a fluorine vacancy neighboring on Y3+. Therefore the intensity of yellow-green PL of Tb3+ is proposed as an index for diagnosis of resistance to ArF excimer laser damage in CaF2 single crystals.

Time-resolved photoluminescence for diagnosis of resistance to ArF excimer laser damage to CaF 2 single crystals

An object of the present invention is to provide a fluoride crystal having a high transmittance with respect to an excimer laser and an excellent resistance with respect to a high output laser, and a production method therefore. The fluoride crystal of the present invention contains at least one kind of atom selected from the group consisting of Zn, Cd, Pb, Li, Bi and Na with a content of 10 ppm or less, and has an internal transmittance of 70% or more with respect to 135 nm wavelength light. The method of the present invention of producing a fluoride crystal comprises conducting a refining step of adding a scavenger to a calcium fluoride raw material and refining the raw material at least once, and a crystal growth step of further adding the scavenger to the refined raw material and growing a crystal by using a crucible lowering method, wherein the amount of the scavenger to be added in the first refining step is 0.04 to 0.1 mol % based on the raw material, and the total amount of the scavenger to be added in the subsequent refining steps and the crystal growth step is 10% to 50% based on the amount of the scavenger to be added in the first refining step.

Fluoride crystal, optical article, and production method

In order to provide a fluoride refining method and a fluoride crystal manufacturing method that have great general-purpose properties and can reduce the manufacturing cost and to provide at a low cost a fluoride crystal, an optical part and an aligner the transmission characteristics of which are hardly degraded even when repeatedly irradiated with a high-output light of a short wavelength for a long term, there is provided a method of refining a fluoride comprising the heating step of heating a solid scavenger-added fluoride raw material to melt the raw material and the cooling step of cooling the molten fluoride material to solidify the melt, wherein the environment of a chamber housing the fluoride raw material is changed to such an environment that a gas in the chamber is discharged to the outside of the chamber more easily than the environment before the change, during the heating step.

V.L. Lyutskanov

Papers

Growth of ultra-violet grade CaF2 crystals and their application for excimer laser optics