High Resolution X-Ray Reflectometry and Diffraction of CaF 2 /Si(111) Structures Grown by Molecular Beam Epitaxy
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ABSTRACT CaF 2 layers were grown by molecular beam epitaxy on differently prepared Si( 11) substrates. X-ray reflectivity spectra were measured and fitted. From the fitting process, the thickness of the CaF 2 layer was precisely (within 1 A) determined and the CaF2/Si interface roughness was also obtained. This roughness was used as an evaluation parameter for the quality of the layers. The CaF 2/Si sample from which the intentional oxide was desorpted at 800'C inside the growth chamber exhibited the most clear x-ray reflectivity spectrum with very well resolved interference fringes. The epitaxial relations of the CaF 2/Si samples grown at temperatures between 250 and 700'C were determined from x-ray diffraction analysis. INTRODUCTION Epitaxy of stacked BaF2/CaF 2 buffer layers on silicon has received much attention in order to obtain the monolithic integration of lead salt detector arrays with silicon read-out circuits' 2 . Due to problems with the BaF2 layer during wet-processing techniques, the epitaxial growth of lead salt layers directly on CaF 2/Si has been tried. Recently, the fabrication of infrared sensor arrays of PbSnSe grown on Si with only a thin (-3 nm) CaF 2 buffer layer has been reported3 . The large difference in thermal expansion coefficients between the CaF 2 (also lead salts) and Si builds a tensile strain during cooling after growth. Normally, this strain is relieved through the gliding of dislocations which ultimately limits the infrared detector performance. The residual strain in CaF 2/Si structures has been mainly investigated by x-ray diffraction and Rutherford back scattering channeling 45 , and the dislocation dynamics by atomic force microscopy6. In this work, the structural properties of CaF2 grown on Si(1 11) are investigated by grazing incidence x-ray reflectivity analysis. For this purpose, CaF 2 layers were grown by molecular beam epitaxy (MBE) on differently prepared Si(l 11) substrates. By fitting the measured reflectivity spectra, the CaF 2 layer thickness was precisely determined and the roughness of the CaF 2/Si interface was obtained. Combining the reflectivity analysis with reflection high energy electron diffraction (RHEED) observations, the influence of the Si preparation processes on the quality of the CaF2 layers was evaluated. The epitaxial relations of the CaF2/Si samples grown at temperatures between 250 and 700"C were determined by scanning the asymmetrical (224) Bragg reflections at two opposite azimuthal directions. EXPERIMENTAL The CaF 2/Si samples were grown in a Riber 32P MBE system equipped with proper CaF 2 and BaF 2 effusion cells. The MBE system is dedicated for the growth of IV-VI compounds 661 Mat. Res. Soc. Symp. Proc. Vol. 484 01998 Materials Research Society
(PbTe-SnTe) on Si substrates using Ila fluoride buffer layers. The temperature of the CaF2 effusion cell was 1250'C which leads to a growth rate of approximately 1 nm/min at the substrate temperatures used (250 to 700 0C). The growth chamber had a background pressure below 10° Torr during the fluo
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