STM studies of island nucleation during hyperthermal atom deposition
- PDF / 187,585 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 50 Downloads / 170 Views
W16.6.1
STM studies of island nucleation during hyperthermal atom deposition Joshua M. Pomeroy* and Joel D. Brock Cornell Center for Materials Research *currently with Los Alamos National Laboratory ABSTRACT We report fundamental changes in island nucleation dynamics as the kinetic energy of the constituent particles used for film grown is increased. A hyperthermal energy ion beam-line with precise control over ion kinetic energy was used to grow copper islands on a Cu(100) substrate. Dramatic increases in island densities were observed with increasing kinetic energy from thermal energies to 150 eV. We find that sputter erosion and the formation of adatomvacancy pairs contribute to this increase. In addition, variations in flux and temperature suggest that the mean-field scaling exponent is sensitive to atomistic mechanisms activated by the ion beam. INTRODUCTION Molecular beam epitaxy (MBE) of metal thin films has enjoyed a rich and diverse history of academic and industrial accomplishments. Both experimentalists and theorists have devoted vast amounts of time and resources in an effort to understand the roles of basic environmental parameters during thin film growth. This effort has led to three generalized modes of film growth: three dimensional roughening, layer-by-layer (two-dimensional) growth, and step-flow growth. Three-dimensional growth occurs when second-layer nucleation occurs prior to coalescence in the first layer, and conversely, layer-by-layer growth occurs when first layer coalescence occurs prior to second layer nucleation. Step-flow growth is the result of newly deposited adatoms attaching to pre-existing substrate features without nucleation, i.e. vicinal steps that sweep across the sample. While step-flow growth is very smooth, it requires high temperatures that accelerate inter-diffusion and chemical activity, which is often undesirable. Two-dimensional growth is generally the most desirable, since many hetero-structures are grown in a planar geometry. Growth modes can be predicted as a function of only substrate temperature and deposition rate (flux) in a simplified phase diagram,1 as is shown in Figure 1 for a system with a non-zero Ehrlich-Schwoebel barrier (inter-layer diffusion). The solid line separating the “3D” and “2D” phases of growth is an intrinsic property of a particular material and face, representing the set of temperature and growth rates resulting in second layer nucleation occurring just as the first layer begins to coalesce. This condition is also the point at which both the probability for adatom descent approaches zero and the islands’ separation goes to zero. We will show that mechanisms activated by hyperthermal energy beams can shift this boundary to the dashed line, thereby increasing the range of phase space resulting in two-dimensional growth, or, perhaps, creating a two-dimensional growth mode in a material which otherwise would not support one. The boundary between “Step-flow” and “2-D” or “3-D” is determined by the particular sample being used. This boundary is d
Data Loading...
