Temperature and Flux dependence of ion induced ripple: a way to study defect and relaxation kinetics during ion bombardm
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Temperature and Flux dependence of ion induced ripple: a way to study defect and relaxation kinetics during ion bombardment Wai Lun Chan, Eric Chason Division of Engineering, Brown University Providence, RI02912.
ABSTRACT We have measured the temperature and ion flux dependence of the ripple wavelength on a Cu(001) surface during low energy ion sputtering. We analyze these results in terms of a linear instability model and identify different experimentally observed behavior with different mechanisms of relaxation and surface defect kinetics. In a high temperature regime, diffusing species on the surface are mainly thermally induced while in a lower temperature range, the diffusing species are ion beam induced. At even lower temperature, thermal diffusion is deactivated and the surface relaxes through an athermal mechanism. We define a transition between different defects formation kinetics in temperature and flux phase space and discuss how the defect kinetics model can be extended to different materials system.
INTRODUCTION Ripple formation on material surfaces under low energy ion bombardment is a well known phenomenon [1, 2]. A linear instability model that consists of competition between roughening and smoothening mechanisms is often used to explain the pattern formation behavior in the early time stage. In this model, the surface height is viewed as a linear superposition of sinusoidal perturbations with wave vector kx and ky, where x is the direction parallel to the ion beam. Each perturbation either grows or decays exponentially with a rate r given by r (k x , k y ) = − ν x ( f )k x − ν y ( f )k y + S100 ( f , T )(k x + k y ) − B( f , T )(k x2 + k y2 ) 2 2
2
− B I , x ( f )k x4 − BI , xy ( f )k x2 k y2 − BI , y ( f )k y4
2
2
(1)
where T and f are temperature and ion flux respectively. νx and νy are roughening terms due to the curvature dependence of the sputter yield [3], S100 is the roughening term due to the presence of a diffusion barrier (Schwoebel barrier) on the step edges [4], B describes smoothing due to surface diffusion [5] and the BI’s are called ion-induced effective surface diffusion [6]. The parameters in eq. (1) depend on temperature and flux as shown in the equation. By maximizing eq. (1) with respect to the wave vectors kx and ky, the wave vector of the fastest growing perturbation can be determined. The reciprocal of this is the characteristic wavelength of the ripple. By measuring the temperature and flux dependences of the ripple wavelength, we can use this model to explore the kinetics of surface defects during ion bombardment.
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ROUGHENING MECHANISMS The linear instability model contains two mechanisms for the roughening of the surface. In the first, proposed originally by Bradley and Harper (BH) [3], the ion energy deposited by the impinging ion at the surface is proportional to the surface curvature. This results in a roughening term proportional to flux and independent of temperature (the ν term in eq. (1)). The second roughening mechanism is due to the Schwo
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