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We identify a new mechanism of stress driven surface morphological evolution in strained semiconductor films. Surface roughness forms by a cooperative mechanism involving the sequential nucleation of islands and pits, which is distinct from the conventional view of ripple formation as an Asaro-Tiller-Grinfeld (ATG) instability. This mechanism is operative both during annealing and growth and competes with the ATG instability as a kinetic pathway to ripple formation. [S0031-9007(96)00853-8] PACS numbers: 68.35.MdThe morphological stability of stressed solids is a subject of considerable scientific and technological importance. It is directly relevant to several key issues in materials science ranging from stress corrosion cracking through to phase transformations and strained layer epitaxy. Since the pioneering work of Asaro and Tiller [1] and Grinfeld (ATG) [2], it is generally argued that above a critical wavelength l c the planar surface of a stressed solid is unstable to the formation of undulations [3][4][5]. This is because the energy reduction associated with elastic relaxation of the undulations exceeds the increase in surface energy. In the case of thin-film deposition, it is therefore envisioned that an initially planar film surface will gradually roughen in the growth direction over extended regions with a characteristic lateral wavelength. The observation of continuous surface ripple patterns on strained semiconductor layers over large areas would appear to give direct confirmation of this view [6,7]. Indeed, the patterns observed in cross section often closely resemble the sinusoidal roughness profiles used as a basis for the instability theory [8,9].In this Letter we reveal an entirely new mechanism of surface ripple formation, which is linked to the activated nature of island and pit formation. The ripple forms by a cooperative mechanism involving the sequential nucleation of islands and pits. This is very different from the gradual strain induced roughening mechanism normally envisioned [3-9] and has important implications for our fundamental understanding of the 2D to 3D transition of strained systems.To determine the mechanism of surface ripple formation we have studied the stress induced 2D to 3D transition in the technologically important Si x Ge 12x system. Our method involves a novel two stage process in which a thin (5 nm) planar Si 0.5 Ge 0.5 alloy layer is first grown at relatively low temperatures to ensure a nominally planar surface. The ripple morphology is then formed by a gentle postdeposition anneal at around 590 ± C for 5 min. This approach emulates equilibrium surface conditions, at least locally, as closely as possible. A map of the surface evolution during growth was obtained from atomic force microscopy (AFM) measurements of the ripple geometry at different temperature regions of one sample wafer.To capture the mechanism of surface ripple formation, use was made of the natural temperature gradient across the sample [10]. At the center of the wafer, corresponding to a temperature...

Thylakoids and photosystem I (PSI) reaction centers were imaged by scanning tunneling microscopy. The thylakoids were isolated from spinach chloroplasts, and PSI reaction centers were extracted from thylakoid membranes. Because thylakoids are relatively thick nonconductors, they were sputter-coated with Pd/Au before imaging. PSI photosynthetic centers and chemically platinized PSI were investigated without sputter-coating. They were mounted on flat gold substrates that had been treated with mercaptoacetic acid to help bind the proteins. With tunneling spectroscopy, the PSI centers displayed a semiconductor-like response with a band gap of 1.8 eV. Lightly platinized (platinized for 1 hr) centers displayed diode-like conduction that resulted in dramatic contrast changes between images taken with opposite bias voltages. The electronic properties of this system were stable under long-term storage.The photosynthetic apparatus in green plants and algae is contained in a unique cellular organelle, the chloroplast. The chloroplast is enclosed by a double membrane and contains thylakoids, which consist of stacked membrane disks (grana) and unstacked membrane disks (stromal region). Although scanning tunneling microscopy (STM) images of disrupted chloroplasts have been reported (1, 2), functional characterization of single isolated reaction centers has not yet been achieved. Thylakoids play an important role in electron transport during photosynthesis. With STM and scanning tunneling spectroscopy (STS), it is possible to investigate their operational electrooptical properties.Photosystem I (PSI) reaction centers are embedded in the thylakoid membrane. They drive the light-dependent transfer of electrons from plastocyanin (a copper-containing soluble protein located in the luminal space of chloroplast thylakoids) to ferredoxin (a [2Fe-2S]-containing soluble protein located in the chloroplast stroma). The PSI complex contains two high molecular mass subunits, the products of the psaA and psaB genes (the gene products for the chlorophyll a protein of PSI), and many low molecular mass subunits (3, 4). The PSI reaction center measured by electron microscopy is about 10 nm x 15 nm (5-7), or 7 nm x 12 nm after correction for attached detergent. Alekperov et al. (8) used STM to image photosynthetic reaction centers of purple bacteria and used the tip to transfer molecules or clusters of molecules from one area to another within the scanning range.The attachment of platinum on the reducing side of PSI has a significant effect on the electrical properties of PSI. This can be observed by transformed photobiocatalytic properties and a sustained steady-state vectorial flow of current (9, 10). In this paper, we describe the use of tunneling spectroscopy to characterize the electrical nature of bare and platinized PSI. For semiconductors, tunneling spectroscopy has been used to characterize surface states and to measure surface-state band gaps (11,12). We show that a band gap can be clearly seen in bare PSI, and platinized PSIs...