Spontaneous Formation of Ridges on Patterned Mesas and Their Role in the Evolution of Step Arrays
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Spontaneous Formation of Ridges on Patterned Mesas and Their Role in the Evolution of Step Arrays Kee-Chul Chang and Jack M. Blakely Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14850 ABSTRACT Mesa structures fabricated on Si(111) surfaces have been found experimentally to develop step arrays with large spacing of the order of a micron or more after annealing at temperatures where sublimation becomes important. Ridges around the edges initially develop during annealing and form barriers to step motion before eventually breaking down. This produces an array of steps of the same sign with a few wide terraces. Computer simulations using one dimensional Burton, Cabrera and Frank (BCF) theory including attachment-detachment rates and step-step repulsion for this configuration show that the terraces evolve under different dynamics depending on the terrace widths. For large terrace widths, sublimation dominates the step dynamics and the Ehrlich-Schwoebel effect is negligible. Sinusoidal terrace width distributions result in this case. The experimentally measured step distribution has such a sinusoidal shape suggesting that the step dynamics is sublimation dominated on the mesas after ridge breakdown.
INTRODUCTION Since crystals are composed of discrete atomic units, the height profile of a vicinal crystalline surface corresponds to a distribution of atomic steps. Technical limitations in orienting a crystal for polishing results in a slight ‘miscut’ of the crystal surface from the intended orientation. Thus, to control the surface height profile at an atomic level, it is necessary to manipulate the surface steps. Steps can also act as preferential adsorption sites due to the local changes in coordination and are known to have extremely important effects in nucleation and continued growth of homoand hetero-epitaxial layers. This provides addition motivation for the creation of surfaces with controlled step morphologies. Controlled manipulation of steps can be effected by a combination of patterning and thermal annealing. Patterning mesa or crater structures on a vicinal crystal surface results in a step array which is illustrated in figure 1a. The steps on the patterned structure can be divided into two different types. Inner steps are the enclosed steps that originate from the initial miscut of the crystal surface. Outer steps are the new steps at the boundaries of the patterns that are formed by etching. Thermal annealing will result in the movement and creation/destruction of both types of steps. In a previous paper[1], we analyzed the development of the inner steps and ignored the redistribution of the outer steps. In recent experiments on the ultrahigh vacuum annealing of mesa structures on Si(111) at temperatures over 925°C, we find that during the initial stages of annealing, the outer steps can reshape the inner steps to form ridges around the edges of the mesas. Subsequent breakdown of a portion of this ridge redistributes the steps on the mesa to create a
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