Morphology of Thin Films
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fundamental question is whether these seemingly diverse systems have an underlying universality - are they described by the same physics? In recent years, some progress has been made towards answering this question using experiments, analytic theories and computer simulations. Scaling regimes and universality classes have been identified for the simplest class of problems - those that do not involve nonlocal effects2 such as screening or shadowing. It is interesting to note, however, that disagreements remain between theory and experiment.' Another reason for interest in these problems is that they are prototypes of far from equilibrium physics thus having no Hamiltonian formulation. Our focus in this paper is to isolate just one problem - the growth of thin films - for a detailed two pronged attack. By controlling the range of angles of the incoming atoms, one can essentially tune the physics from local to non-local. Thin films prepared under low adatom mobility have common morphologies relatively independent of the material composition.3 -9 Changes in morphology of thin films are directly linked to corresponding wide variations in their resulting physical properties and have important ramifications in applications such as coated cutting tools, optical coatings and optical and magnetic storage media. In this report, we present results of experiments on vapour deposited films complemented by a continuum approach that allows a unified study of diverse growth problems. The theoretical approach incorporates surface tension, allows for overhanging configurations of the interface and permits a study of non-local growth phenomena involving shadowing. 71 Mat. Res. Soc. Symp. Proc. Vol. 355 0 1995 Materials Research Society
EXPERIMENT Figure l(a) shows a scanning electron micrograph of the fractured edge of a sputtered amorphous germanium (a-Ge) film grown on glass in a turbomolecular-pumped r.f. sputtering system with a base pressure of 2 x 10-7 Torr using argon purified with a titanium getter, and subsequently etched in 30% H202. The anisotropic columnar structure of the film interior grows obliquely with respect to the plane of the substrate when the incident vapor has a nonnormal average incident angle. On the other hand, figure l(b) shows a match stick like columnar growth with the columns growing normally when the range of arrival angles of the incoming atoms is isotropic. We have been able to link variations in the columnar morphology to concurrent ion bombardment during film growth suggesting that the surface energy, the nature of the chemical bonding and the interplay between the smoothing effects of the surface tension and the highly nonequilibrium nature of an energetically bombarded surface all play 3 crucial roles in determining the film morphology. -9 MODEL Previous theoretical studies of low adatom mobility film growth fall into two categories: 1) Models of ballistic aggregation of discrete particles10 do not naturally incorporate surface tension, while molecular dynamics simulations" are restricted to sma
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