Thermal Strain Measurements in Epitaxial CoSi 2 /Si by Double Crystal X-Ray Diffraction
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THERMAL STRAIN MEASUREMENTS IN EPITAXIAL CoSi 2 /S1 BY DOUBLE CRYSTAL X-RAY DIFFRACTION GANG BAP, MARC-A. NICOLETr, THAD VREELAND JR.*, Q. YE-, Y. C. KAO*, K. L. WANG** *California Institute of Technology, Pasadena, CA 91125 'UCLA, Los Angeles, CA 90024 ABSTRACT The perpendicular x-ray strain of epitaxial CoSi 2 films grown on Si(ll1) substrates at - 600'C by MBE was measured at various temperatures. Within experimental error margins, the strain decreases linearly with rising temperature at a rate of (1.3±0.1) x 10-1/ 0 C from room temperature up to 600'C. Over that temperature range and the duration of a complete measurement (_- .5h to - 2h), these strain values remain reversible. At 593*C, the x-ray strain is -0.85%, which is about the strain that a stress-free CoSi 2 film on Si(111) would have at that temperature. This results show that the stress in the epitaxial CoSi 2 film is fully relaxed at the growth temperature. Strains below the growth temperature are induced in the film by the difference in the linear coefficient of thermal expansion of CoSi 2 and Si. They were calculated by assuming that the density of misfit dislocations formed at the growth temperature remains constant. The slope of the strain-temperature dependence obtained that way agrees with the measured slope if the unknown Possion ratio of CoSi2 is assumed to be VCoSi 2 = 0.35. A film stress of - 0.8 GPa at room temperature was calculated using the above value for the Possion ratio, 130 GPa for the Young modulus, and the measured x-ray strain.
INTRODUCTION The successful growth of an epitaxial CoSi 2 film on a Si(111) substrate holds promise for the fabrication of novel devices such as tunneling structures and metal/semiconductor superlattices. Since the initial experimentation on epitaxial growth of CoSi2 on Sil1l, progress in understanding and controlling the growth process and the imperfections has been achieved. Many important structural and electrical properties of the CoSi 2 films have been characterizedl2-3].. The atomic coordination of the epitaxial CoSi 2 film is rotated 1800 about the 1111] axis with respect to the Si(111) substrate (type-B)[4]. The lattice mismatch, f, between the CoSi 2 and Si at room temperature is -1.23%. The critical thickness, ta,, for pseudomorphic growth of CoSi 2 on Si(111) was reported to be - 30AN[s. The growth of CoSi 2 on porous Si(111) substrates was explored with the aim of increasing the critical thickness of the film'll. Double crystal diffractometry shows that epitaxial CoSi 2 films grown on Si substrates, with surfaces offset from {111}, are misoriented, i.e., there is a tilt angle between the 11111 directions of the CoSi2 film and the Si substrateIT]. Strain measurements on thick films (i.e., the film thickness, t 1 > t,,) by double crystal diffractometry at room temperature show that the perpendicular x-ray strain E± of epitaxial CoSi 2 films Mat. Res. Soc. Symp. Proc. Vol. 130. c1989 Materials Research Society
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has a typical value of about -1.65% over the range of film thickness tj from 100
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