High quality window/plate shaped CaF2 single crystals have developed by vertical Bridgman method of with diameters of 50, 100, and 200 mm and 210 mm width, 200mm length. The CaF2 windows of 30mm diameter obtained from big crystals have evaluated the transmission spectra from 120 nm to 220nm by single beam VUV spectrometer. We could obtain bulk transmission spectra by subtracting surface reflection loss between tow kinds of thickness samples. The results show flat spectra without distinct absorption from 130nm to 220nm. In laser-damage tests with the fourth harmonic (4ω) of a Nd:YAG laser below 1NW/cm2 at the CaF2 window, a stable output of 600mW was obtained. ArF laser irradiation of 50mJ/cm2 and 6X104 pulses was tried and the degradation in transmission from 130nm to 220nm not observed. These data show that the radiation hardness of our CaF2 crystals is promising for deep ultra-violet laser applications.
Under ultra high vacuum, (beta) -FeSi2 thin films were formed by laser ablation method using poly crystal (beta) - FeSi2 as target material that was prepared by horizontal gradient freeze method. In order to compare the physical properties of thin films prepared by laser ablation with those of bulk crystal, (beta) -FeSi2 single crystal was prepared by chemical vapor transport method. The (beta) -FeSi2 plate-like and needle-like crystals were formed at 7 mg and 1.0 g of iodine quantity, respectively. To check a crystal symmetry and orientation, Laue transmission patterns were taken. Anisotropic Raman signals were observed from polarized Raman scattering measurements. Further, electron spin resonance measurement was carried out to examine the residual impurities and to determine g values. From (beta) -FeSi2 films during laser ablation growth, streaky signals were obtained in the RHEED observation. Highly oriented (202)/(220) (beta) -FeSi2 films were predominantly identified in XRD measurements. Raman scattering and optical absorption measurements for these layers revealed that the grown samples are nearly epitaxially-like and have approximately 0.85 eV as its direct optical band-gap.
Semiconductive iron disilicide (beta) -FeSi2 is an attractive material for optoelectronic and thermoelectric devices that can be integrated on Si substrates. Advantages arise from the direct band-gap, high absorption coefficient and high thermoelectric power figure of merit. We present here the semiconductor properties of (beta) -FeSi2 films on Si(100) substrate prepared by laser ablation (LA) method. We compare these results with those obtained from (beta) - FeSi2 films prepared by ion beam synthesis using high- energy ion implantation and electron beam deposition methods. As for laser ablation, two independent growth processes were adopted using two different target materials, The first one was Fe deposition on Si (100) substrate by LA using Fe target and subsequent high-temperature annealing leading to solid phase epitaxy. The second was LA using (beta) -FeSi2 bulk polycrystal as a target material which was grown by horizontal gradient freeze method. (beta) - FeSi2 films prepared by the two processes were heat- treated as a function of annealing temperature and duration time. Structural characterizations were made by reflection high-energy electron diffraction, x-ray diffraction, Raman scattering and optical absorption spectroscopy measurements at room temperature, which revealed that high-quality semiconducting (beta) -FeSi2 films can be fabricated by two LA processes.
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