Observation of ion-induced ripples in Cu(001)
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Observation of ion-induced ripples in Cu(001) Wai Lun Chan, Niravun Pavenayotin, Eric Chason Division of Engineering, Brown University Providence, RI 02912 ABSTRACT A linear instability theory proposed by Bradley and Harper accounts for many of the features of ripple formation on amorphous and semiconductor surfaces, but it is not consistent with measurements of sputtered metal surfaces. In the present work, we report observations of sputter ripples on Cu(001) surfaces which are similar to amorphous and semiconductor systems. By using higher ion flux and temperature than previously used, we have produced ripples in which the wavelength is independent of time and the amplitude increases exponentially during the early stages of growth. The ripple evolution was measured using in situ light scattering during sputtering.
INTRODUCTION Under low energy ion bombardment with a collimated beam, periodic structures such as ripples or mounds can be produced on a wide range of surfaces. In many amorphous and semiconductor systems (SiO2 [1], Si[2]) , the early stage of ripple formation is well-explained by a linear instability theory first proposed by Bradley and Harper [3] (BH model). In the BH model, the wavelength of the ripple does not change with ion fluence and the amplitude increases exponentially with time. The wavevector of the ripple may be parallel or perpendicular to the ion beam direction depending on the interaction of the ion with the solid and the angle of incidence of the ion beam. Although successful for many systems, this model is not consistent with many experimental studies on metal surfaces. In contrast with amorphous and semiconductors, the ion-induced pattern in metals may follow the symmetry of the crystal [4-6], not the ion beam direction. Moreover, the wavelength of the feature is not fixed but increases with the sputtering time [7]. Many of these features of the pattern formation have been modeled using the effect of diffusional barriers to the crossing of steps on metal surfaces [8]. However, the presence of these diffusional effects does not explain why sputter ripples of the type predicted by the BH model should not be observed on metal surfaces. In the present work, we show that ripples with constant wavelength with respect to time can in fact be produced on Cu (001) surface at a relative high flux (~1014 ions cm-2 s-1) and high temperature (~400-500K). The amplitude of ripples is found to grow exponentially with time in the early stage and the orientation of ripples is in agreement with the BH model.
BRADLEY-HARPER LINEAR INSTABILITY MODEL The BH model is based on early work by Sigmund [9] in which the sputter yield is determined to be proportional to the surface curvature. Together with the relaxation due to
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surface diffusion, it provides a
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