Cengiz S. Ozkan

Strain relaxation and defect formation in heteroepitaxial Si1−xGex films via surface roughening induced by controlled annealing experiments

Mechanisms of strain relaxation and defect formation during surface roughening in Si1−xGex films grown epitaxially on (100)Si substrates have been investigated by controlled annealing experiments. Epitaxial films 10 nm in thickness and containing 18% Ge, which are subcritical with respect to the formation of misfit dislocations, show strain relaxation through surface roughening on annealing at 850 °C, where surface grooves are aligned along 〈100〉 directions. Other films with 22% Ge and supercritical thicknesses have also been studied, where surface grooves are aligned along 〈110〉 directions.

Atomistic models of dislocation formation at crystal surface ledges in Si1-xGex/Si(100) heteroepitaxial thin films

Abstract Mechanisms of defect formation near surface ledges of a diamond cubic crystal subjected to compressive strain parallel to the surface are investigated as precursory processes to dislocation nucleation in Si1-xGex/Si(100) heteroepitaxial thin films under surface diffusion conditions. This study is motivated by our preliminary calculations of dislocation formation at surface ledges in a model crystal characterized by the 6–12 Lennard-Jones interatomic potential, and by our controlled annealing experiments on evolution of a Si1-xGex/Si(100) film from an atomically flat, defect-free, surface morphology to an undulating surface morphology with cusp-like surface features and dislocation formation at the cusp valley. When subjecting such films to high temperature anneals, we observed nucleation and growth of three types of dislocations: the 60° glide dislocations, the 90° Lomer-Cottrell dislocations with stair rod Shockley partials and twinned wedge disclinations with twofold ∑9 coincidence boundaries b...

Stress-driven surface evolution in heteroepitaxial thin films: Anisotropy of the two-dimensional roughening mode

/Si(001)heterostructures by use of methods of elastic analysis of undulated surfaces andperturbation analysis on the basis of global energy variations associated with surfaceevolution. Both methods have shown that the two-dimensional stage of surfaceroughening preferentially takes place in the form of ridges aligned along the twoorthogonal 〈100〉type directions. This prediction has been confirmed by ex situexperimental observations of surface evolution by use of atomic force microscopy andtransmission electron microscopy in both subcritically and supercritically thickSi

Integration of optoelectronic array devices for cell transport and sorting

Current biochip technologies typically rely on electrostatic or mechanical forces for the transport and sorting of biological samples such as single cells. In this paper we have investigated how optical pressure forces can be effectively used for the manipulation of cells and switching in a microfluidic system. By projecting the optical beams externally non-contact between the control devices and the sample chip is possible thus allowing the sample chips to be disposable which reduces the chance of cross-contamination. In one implementation we have shown that vertical cavity surface emitting laser (VCSEL) array devices used as parallel optical tweezer arrays can increase the parallelism of sample manipulation on a chip. We have demonstrated the use of a high-order Laguerre-Gaussian mode VCSEL for optical tweezing of polystyrene microspheres and live cells. We have also shown that optical pressure forces from higher- power sources can be used for the switching of particles within microfluidic channels. Both the attractive gradient force and the scattering force of a focused optical beam have been used to direct small particles flowing through junctions molded in PDMS. We believe that by integrating optical array devices for simultaneous detection and manipulation, highly parallel and low-cost analysis and sorting devices may be achieved.

A twinned wedge in a Si-Ge epitaxial film: Twofold Σ = 9 twinning

Abstract A wedge-like defect in a heteroepitaxial Si-22% Ge film on a Si substrate has been investigated. The defect is found to be a twofold Σ = 9 twin with elastic properties of a wedge disclination triangle. From the defect arrangement it follows that two distinct defect morphologies exist: in the first stage, a nascent wedge forms which has the properties of a single disclination; in the second stage, the triangle morphology is established (called a mature wedge) with the long-range elastic properties of an edge dislocation. The formation involves thermally activated nucleation of a nascent wedge, growth of the stable nucleus, transition from the nascent wedge to the mature twin, growth of the mature twin and eventual trapping of the defect. The processes (i) and (ii) can be understood by considering a single defect in an infinite half-space with a homogeneous stress field only. The growth of the nascent wedge is a self-similar process, and thus the transition (iii) requires the presence of inhomogene...

Massively parallel low-cost pick-and-place of optoelectronic devices by electrochemical fluidic processing

We describe a novel electrochemical technique for the nonlithographic, fluidic pick-and-place assembly of optoelectronic devices by electrical and optical addressing. An electrochemical cell was developed that consists of indium tin oxide (ITO) and n -type silicon substrates as the two electrode materials and deionized water (R = 18 MOmega) as the electrolytic medium between the two electrodes. 0.8-20-microm-diameter negatively charged polystyrene beads, 50-100-microm-diameter SiO(2) pucks, and 50-microm LEDs were successfully integrated upon a patterned silicon substrate by electrical addressing. In addition, 0.8-microm-diameter beads were integrated upon a homogeneous silicon substrate by optical addressing. This method can be applied to massively parallel assembly (>1000 x 1000 arrays) of multiple types of devices (of a wide size range) with very fast (a few seconds) and accurate positioning.

Studies of Morphological Instability and Dislocation Formation in Heteroepitaxial Si 1−x Ge x Thin Films Via Controlled Annealing Experiments

Heteroepitaxial Si 1−x Ge x thin films deposited on silicon substrates exhibit surface roughening via surface diffusion under the effect of a compressive stress which is caused by a lattice mismatch. In these films, surface roughening takes place in the form of ridges aligned along either or directions depending on the film thickness and composition. In this paper, we compare the relaxation behaviour of capped and uncapped heteroepitaxial Si 1−x Ge x thin films containing 22% Ge, with surface roughening being inhibited in films with a capping layer. Films with 50 nm thickness were deposited on bare silicon substrates in a LPCVD reactor. Annealing experiments were conducted in a Hydrogen atmosphere in the reactor chamber. Transmission electron microscopy and atomic force microscopy have been used to study the surface morphology and microstructure of these films. XRD measurements were conducted to determine the amount of relaxation in these films. In-situ transmission electron microscopy/annealing experiments have been performed on uncapped Si 1−x Ge x /Si structures to study the dynamics of surface roughening and defect formation in a vacuum ambient. Finally, we compare surface morphologies and defects in samples subjected to annealing in vacuum and H 2 ambients.

In-situ TEM observations of surface roughening and defect formation in lattice mismatched heteroepitaxial thin films

This paper focuses on in-situ transmission electron microscopy observations of surface roughening and defect formation in heteroepitaxial Si{sub 1{minus}x}Ge{sub x} thin films. Annealing experiments have been carried out in-situ in the microscope under a high vacuum environment. The authors comment on the sample preparation procedure for in-situ TEM experiments and explain the importance of having a sufficiently thick sample to have the stress state in the film unaltered. Experimental results of in-situ surface roughening are presented for subcritically and supercritically thick Si{sub 1{minus}x}Ge{sub x} films. They found that, in a vacuum environment, the kinetics of surface roughening and the resulting surface morphology are much different than in a hydrogen environment.