Direct Measurement of Ion Beam Induced, Nanoscale Roughening of GaN
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Direct Measurement of Ion Beam Induced, Nanoscale Roughening of GaN Bentao Cui and P. I. Cohen Department of Electrical and Computer Engineering University of Minnesota, Minneapolis, MN 55455 A. M. Dabiran SVT Associates, Inc, Eden Prairie, MN 55344 ABATRACT The formation of ion induced nanoscale patterns such as ripple, dots or pores can be described by a linear continuum equation consisting of a surface roughening term due to curvature-dependent sputtering or asymmetric attachment of vacancies, and a surface smoothing term due to thermal or ion-induced diffusion. By studying ion-induced dimple volume change using atomic force microscopy, we show a method to measure the ion-roughening coefficient. Using this method, we found the roughening coefficient ν was 45 nm2/sec at 730K for initial ion etchings with 300 eV Argon ions. Cathodoluminescence measurements indicated Ga-vacancy formation during ion bombardment. The activation energy for surface relaxation after ion etching was about 0.12 eV as measured by reflection high energy electron diffraction. INTRODUCTION Surface morphology evolution under ion bombardment has been studied intensively by Barabasi [1], Cahill [2], Chason [3], Erlebacher [4] and Harper [5] and for various material systems, including metals, semiconductors, and oxides. A general linear continuum equation can be used to describe the evolution of the surface height, ∂h = −ν 0 − ν∇ 2 h − B∇ 4 h ∂t
(1)
where first term ν0 is the erosion rate of an unperturbed planar surface, the second term describes the surface roughening with the factor ν, which we term the roughening coefficient, and the third term describes the surface smoothening by surface diffusion (thermal or ion induced) with a factor of B. In the case of thermal diffusion, B can be calculated from surface diffusivity and mobile species concentration [5]. The orientation of the surface ripple patterns [6] can be explained by the curvature dependent sputtering yield, the so called Bradley-Harper (BH) mechanism [5]. In our recent previous work [7], we observed the formation of ripple during the ion-assisted growth of GaN. Although the ripple orientation can be explained by the BH mechanism, the agreement with the theory was not complete, especially the ion energy and ion species effects on the magnitude of the feature size. Moreover, this linear BH theory cannot explain the observed saturation of ripple amplitude [4]. On the other hand, for crystalline materials, asymmetrical attachment kinetics [2] can dominate over the BH mechanism, due to the effect of a step edge barrier for the attachment of vacancies at ascending steps, similar to the Ehrlich-Schwoebel barrier for adatoms in epitaxial growth. In this paper, we introduce a direct method to measure the ion roughening coefficient ν. Compared to ν obtained from BH theory,
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our results were four orders of magnitude higher. This suggested that asymmetrical attachment kinetics could be the dominant surface roughening mechanism for crystalline GaN surface at the temperature
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