Surface Topography of Cocondensed Amorphous ZrCo-Alloy Films Investigated in Situ By UHV-STM
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ABSTRACT Amorphous ZrCo-alloy films, prepared by electron beam deposition in ultra high vacuum (UHV), are investigated in situ with scanning tunneling microscopy (STM) in constant current mode. The topographs, statistically analysed by height-height-correlation functions, show a surface roughness on two different length scales. The surface structures on a larger scale (7.0 to 12.Onm in diameter), which vary with substrate temperature, film composition and film thickness, are discussed in relation to surface tension and diffusion kinetics during film growth. The structures on a smaller scale (0.4 to 0.7nm in diameter) have to be discussed with respect to the limits of atomic resolution.
INTRODUCTION In the last several years there is great interest in resolving the atomic arrangement of amorphous solids using scanning tunneling microscopy (STM). For such investigations the system ZrCo is especially well suited because it can be condensed into the amorphous state over a wide range of composition and substrate temperature [1] (see Fig. 1). In order to avoid any impurities that might influence the interpretation of STM data, ultra high vacuum (UHV) is necessary for preparation and in situ investigation. Earlier investigations on amorphous film growth using mechanical stress measurements led to the assumption of a rough surface, changing with the thickness of the film [2]. Since modern imaging techniques like STM are available now it is conclusive to image the surface topography in real space. Ex situ STM investigations of amorphous film surfaces in air ambient support the existence of a specificially rough surface [3], but with in situ UHV investigations it might be possible even to resolve the atomic arrangement of amorphous structures. Applying calculations of the height-height correlation from the Ref. [4], the vertical and lateral surface structure with scattering experiments.
ZrCo T [K]
oamorph 0 kristcll x Krlstaillsatlonstemperatur
80Co
*
Soo
A*t
\6
400
0 300 0
Co
20
00
40
at% Zr
Zo roo Zr
Fig. 1: Determined crystallization temperatures for ZrCo-alloy films (Ref. [1]). image data, basic theories are reviewed in can be statistically analysed and compared
601 Mat. Res. Soc. Symp. Proc. Vol. 355 e1995 Materials Research Society
r
a) 5.0
b) 3.0
c) 4.0
-0.0 Fig. 2:(100 x 100)nm2 STM-images of amorphous ZrxColoo.x-alloy films a)X=20, b)X=35 and c)X=45. The height scale is in nm.
EXPERIMENTAL The preparation of the amorphous films and the in situ measurements with STM are performed within a complex UHV-system (Varian), consisting of an evaporation chamber and an analytical chamber separated by load locks. The residual gas pressure is below 2* 10-1 0 mbar, except for the preparation process, where it is rising up to 2 * 10-mbar. The amorphous structure of the films is verified by ea situ X-ray diffraction.
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The ZrCo-alloy fims ate cocondensed onto silicon substrates (covered with 7nm of native oxide) by two rate-controlled electron beam evaporators (total deposition rate 0.7nm/sec). The he
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