Atomic Structure of Ultrathin Erbium Silicides on Si(111)
- PDF / 458,865 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 2 Downloads / 186 Views
ABSTRACT The atomic structure of thin epitaxial erbium silicides on Si( 111) substrates has been studied in situ by means of surface X-ray diffraction and medium-energy ion scattering. In the sub-monolayer range, a two-dimensional (2D) silicide is formed within a 1 x 1 unit cell, with the Er atoms occupying T4 sites and a Si bilayer on top which is 1800 rotated with respect to the bulk (B-type). Silicide layers with a thickness of 3 monolayers exhibit a regular network of Si vacancies which release the compressive strain of the graphite-like Si layers between adjacent Er layers. This results in a V13 x V3 R 300 unit cell, in which 3 out of 5 Si atoms are displaced towards the vacancy, and in which 3 Er atoms relax away from the vacancy. INTRODUCTION The study of ErSil.7 silicides has attracted much interest since the demonstration of their epitaxial growth on Si(111).1, 2 ErSil.7 films grown on n-type Si(ll1) are characterized by unusually low Schottky barriers for which values as low as 0.28 eV have been reported.3 As a consequence, the possiblity of ohmic contacts with extremely low resistivity has been applied in Schottky diodes with nearly ideal rectifying properties, 4 and in tunable infrared sensors. 5 From a fundamental point of view, the low lattice mismatch between bulk ErSil. 7 and Si(l 11) of -1.2%6 enables the study of model Schottky interfaces with low interface defect densities. 7', For an understanding of such electrical properties, knowledge about the atomic structure of the formed interface is a prerequisite. For coverages below 1 monolayer of Er (1 ML =_0.78 x 1015 atoms/cm 2), an epitaxial twodimensional (2D) silicide is formed. 9 The structure of this unique silicide has been investigated by Auger electron diffraction,9 identifying the Si atoms above the Er atoms as forming bulk-like hexagonal rings which are rotated by 1800 with respect to the substrate (B-type, as opposed to unrotated A-type rings). A combination of theoretical work and photoemission studies has helped to solve the registry of the Er atom within the 1 x 1 unit cell.' 0 In principle, 4 different models are physically feasible: Er can sit on an H3 site (in the hollow between first layer substrate atoms), on a T4 site (above a second layer substrate atom), on a T site (on top of a first layer substrate atom) or on an S site (substitutional, replacing a first layer substrate atom). T4 appears to be the most probable site. This is confirmed by a recent surface extended X-ray absorption fine structure study (SEXAFS). 11 Thicker ErSil.7 films can be grown epitaxially on Si(l 11) in the AIB 2 structure, i.e., a stack of alternating Er and Si hexagons, where Si bilayers become compressed into graphite-like planes.6 They exhibit v3 x v3 R 300 diffraction patterns, commonly attributed to ordered networks of probed by extended vacancies in the Si plane. The Si-Er bond lengths of such layers have been 12 X-ray absorption fine structure (EXAFS), supporting the vacancy model. 281
Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research
Data Loading...
