Pattern formation on surfaces undergoing low-energy ion bombardment is a common phenomenon. Here, a recently developed in situ spectroscopic light scattering technique was used to monitor periodic ripple evolution on Si(OO1) during Ar+ sputtering. Analysis of the rippling kinetics indicates that under high flux sputtering at low temperatures the concentration of mobile species on the surface is saturated, and, surprisingly, is both temperature and ion flux independent. This is due to an effect of ion collision cascades on the concentration of mobile species. This new understanding of surface dynamics during sputtering allowed us to measure straightforwardly the activation energy for atomic migration on the surface to be L2 f 0.1 eV. The technique is generalizable to any material, including high temperature and insulating materials for which surface migration energies are notoriously difficult to measure. PRL, 1999 Low energy (500 -LOO0 eV) ion bombardment is a common technique used in many thin film applications such as forming shallow junctions, sputter etching and deposition, ion beam assisted growth, reactive ion etching, and plasma assisted chemical vapor deposition. Under certain conditions, ion sputtering is known to produce patterns on surfaces. Features such as ripples, bumps or cones are common [ 1,2,3]. Typical length scales of these features are of order 10-1000 n m In some cases, these features are nuisances, such as in sample thinning for transmission microscopy or depth profiling by secondary ion mass spectroscopy. However these nano-scale patterns also hold promise in applications as varied as optical devices, templates for liquid crystal orientation, and strain-free patterned substrates for heteroepitaxial growth of quantum dots or wires.Rippling has been observed in amorphous materials (Si02 In this study, a recently developed in situ light scattering spectroscopic technique [7] was used to monitor ripple evolution on Si(OO1) during Arc sputtering. The technique allowed the first systematic study of the temporal and spatial evolution of both ripple wavelength and ripple amplitude as functions of both temperature and ion beam flux.Theoretical models describe rippling as arising from competition between ion beam roughening/etching and surface diffusion or viscous flow mediated relaxation 181. Our results here help illuminate the range of validity of these models. Additionally, we describe the method by which the activation energy for surface migration can be found.A salient feature of these experiments was our use of a very high ion flux compared to other rippling experiments. At high fluxes, the annihilation process for mobile species becomes dominated by collision cascades. Sputter-induced rippling can, in principle, occur on the surface of any material. As such, it holds great promise as a method for measuring the activation energy for surface migration on high temperature or insulating materials, which are generally notoriously difficult to measure.Rippling experiments were perf...

As-deposited thin films grown by vapor deposition often exhibit large intrinsic stresses that can lead to film failure. While this is an “old” materials problem, our understanding has only recently begun to evolve in a more sophisticated fashion. Sensitive real-time measurements of stress evolution during thin-film deposition reveal a generic compressive–tensile–compressive behavior that correlates with island nucleation and growth, island coalescence, and postcoalescence film growth. In this article, we review the fundamental mechanisms that can generate stresses during the growth of Volmer–Weber thin films. Compressive stresses in both discontinuous and continuous films are generated by surface-stress effects. Tensile stresses are created during island coalescence and grain growth. Compressive stresses can also result from the flux-driven incorporation of excess atoms within grain boundaries. While significant progress has been made in this field recently, further modeling and experimentation are needed to quantitatively sort out the importance of the different mechanisms to the overall behavior.

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We examine the fundamental phonon mechanisms affecting the interfacial thermal conductance across a single layer of quantum dots (QDs) on a planar substrate. We synthesize a series of Ge x Si 1−x QDs by heteroepitaxial self-assembly on Si surfaces and modify the growth conditions to provide QD layers with different root-meansquare (rms) roughness levels in order to quantify the effects of roughness on thermal transport. We measure the thermal boundary conductance (h K) with time-domain thermoreflectance. The trends in thermal boundary conductance show that the effect of the QDs on h K are more apparent at elevated temperatures, while at low temperatures, the QD patterning does not drastically affect h K. The functional dependence of h K with rms surface roughness reveals a trend that suggests that both vibrational mismatch and changes in the localized phonon transport near the interface contribute to the reduction in h K. We find that QD structures with rms roughnesses greater than 4 nm decrease h K at Si interfaces by a factor of 1.6. We develop an analytical model for phonon transport at rough interfaces based on a diffusive scattering assumption and phonon attenuation that describes the measured trends in h K. This indicates that the observed reduction in thermal conductivity in SiGe quantum dot superlattices is primarily due to the increased physical roughness at the interfaces, which creates additional phonon resistive processes beyond the interfacial vibrational mismatch.

A simple model is presented that predicts the kinetics of tensile stress evolution during the deposition of thin films that grow by the Volmer–Weber mechanism. The generation of a tensile stress was attributed to the impingement and coalescence of growing islands, while concurrent stress relaxation was assumed to occur via a microstructure-dependent diffusive mechanism. To model the process of island coalescence, finite element methods were employed and yielded average tensile stresses more consistent with experimental observations than those predicted using previously reported analytical models. A computer simulation was developed that models the process of film growth as the continuous nucleation of isolated islands, which grow at a constant rate to impinge and coalesce to form a continuous polycrystalline film. By incorporating the finite element results for stress generation and a microstructure-dependent stress relaxation model, the simulation qualitatively reproduced the complex temperature-dependent trends observed from in situ measurements of stress evolution during the deposition of Ag thin films. The agreement includes simulation of the decreasing stress relaxation rate observed during deposition at increasing temperatures.