Lattice mismatch dislocations in a preferentially sputtered alloy studied by scanning tunneling microscopy

Abstract

Scanning tunneling microscopy (STM) on a sputtered and annealed Pt25Ni7s(l 11) single crystal reveals a network of subsurface lattice mismatch dislocations caused by platinum enrichment due to preferential sputtering and recoil mixing. Atomically resolved STM topographs are compared with simulations of these dislocations using embedded atom potentials. This allows one to estimate the depth of the dislocations, and thus the thickness of Pt enrichment, which is three monolayers on the 500 eV Xe + sputtered and five monolayers on the Ar + sputtered surface, compatible with the depth of radiation damage. PACS numbers: 61.70.Jc, 61.80.Jh, 68.35.Dv, 68.35.Fx Lattice mismatch is a frequent phenomenon in the growth of thin films, where dislocations caused by the different lattice constants of substrate and film have been observed by transmission electron microscopy [1,2] and scanning tunneling microscopy (STM) [3,4]. In the present Letter we show for the first time a case of lattice mismatch dislocations which is due to enrichment of one alloy component by sputtering, and we are able to employ atomically resolved STM topographs for a full characterization of this dislocation system. Determination of dislocation density and comparison with simulations of individual dislocations yield both the composition and the thickness of the enriched layer. This offers a totally new method of surface analysis for such systems, based entirely on geometrical information.The STM analysis has been performed with a customized commercial instrument (Omicron micro-STM). Preparation and STM work were done in the same ultrahigh vacuum chamber with a base pressure below 1 x 10"" ,0 mbar. For sputtering a 500 eV Ar or Xe beam inclined 70° to the surface normal was used, yielding a current density of 0.7 juA/cm 2 . The sample was cleaned by repeated cycles of sputtering and annealing until no contaminations were detectable by low-energy ionscattering spectrometry (LEIS) below 770 K, and approximately 1% of a monolayer of sulfur at 1000 K. After annealing above 970 K the crystal showed large terraces separated by nearly straight and parallel monatomic steps and no dislocations.If the crystal is sputtered with a dose of some 10 16 ions cm" 2 and annealed thereafter at temperatures between 600 and 870 K, shallow ditches 0.2-0.5 A deep and 20-50 A wide appear on the surface. Some ditches run along or across monatomic steps without being influenced by the steps, which makes a pure surface phenomenon (restricted to the first layer) implausible. With higher ion dose, the number of these ditches grows, and they form a network of hexagons with triangles at their edges, as shown in Fig. 1. The contrast is mainly topographical, since /(z) measurements and variation of the tunneling voltage between ±0.5 mV and 10 V did not show any evidence of electronic (density of state) effects. STM images after oxygen adsorption did not show any differences between ditches and the flat regions, which would indicate different chemical composition [6].Atomic resolution top...

Absence of superconductivity in ultrathin layers of FeSe synthesized on a topological insulator

“…1(a)]; a similar observation has been made in a previous study [16]. We note that similar structures were previously also observed in binary alloys stemming from a buried dislocation network [24]. As Se migration during heating is responsible for forming the FeSe shows a superposition of several reciprocal lattices: in addition to the hexagonal pattern stemming from Bi 2 Se 3 , a square lattice appears in three domains each rotated by 120°.…”

Atomic structure of screw dislocations intersecting theAu(111)surface: A combined scanning tunneling microscopy and molecular dynamics study

“…The scanning tunneling microscope ͑STM͒ is the instrument of choice when it comes to atomic resolution imaging of the structure and dynamics of surfaces and features on surfaces. There were several STM investigations about dislocations about a decade ago [9][10][11][12][13][14][15][16] but at that time interpretation of the results was difficult, largely because theorists could not yet perform the necessary large-scale calculations to understand the measurements.…”

Surface alloys and alloy surfaces: the platinum-tin system

“…So it is possible, for instance, to observe the step distribution and height on the surface (one of the first reports in this field was on the NiAl(111) surface [86]). Later on [87,88] it was observed by STM that ion bombardment of the Pt1 N i ( 111) surface leads to the formation of a pattern of shallow ditches (some 0.2-0.5Å deep) that have been attributed to the dislocations generated by the lattice mismatch of the top layers and the bulk ones. The top layers are enriched in Pt by ion bombardment and hence have a different lattice constant.…”