Surface Phase Metastability during Langmuir Evaporation.

We have directly imaged the spontaneous formation of metastable surface phase domains on GaAs(001) during Langmuir evaporation. Eventually, these metastable phases transform to the thermodynamically stable parent phase, producing a dynamic phase coexistence with a temperature dependent, time-averaged coverage. Monte Carlo simulations are used to identify the key kinetic processes and investigate the interplay between phase metastability and evolving surface morphology. This is used to explain the measured temperature dependence of the time-averaged coverage.

Selected energy dark-field imaging using low energy electrons for optimal surface phase discrimination.

We propose a general strategy for surface phase discrimination by dark-field imaging using low energy electrons, which maximizes contrast using diffraction spots, at selected optimal energies. The method can be automated to produce composite phase maps in real space and study the dynamics of complex phase transformations in real-time. To illustrate the capabilities of the technique, surface phases are mapped in the vicinity of liquid Ga droplets on the technologically important GaAs (001) surface.

Droplet Epitaxy Image Contrast in Mirror Electron Microscopy.

Image simulation methods are applied to interpret mirror electron microscopy (MEM) images obtained from a movie of GaAs droplet epitaxy. Cylindrical symmetry of structures grown by droplet epitaxy is assumed in the simulations which reproduce the main features of the experimental MEM image contrast, demonstrating that droplet epitaxy can be studied in real-time. It is therefore confirmed that an inner ring forms at the droplet contact line and an outer ring (or skirt) occurs outside the droplet periphery. We believe that MEM combined with image simulations will be increasingly used to study the formation and growth of quantum structures.

Origin of quantum ring formation during droplet epitaxy.

Droplet epitaxy of GaAs is studied in real time using in situ surface electron microscopy. The resulting movies motivate a theoretical model for quantum ring formation which can explain the origin of nanoscale features such as double rings observed under a variety of experimental conditions. Inner rings correspond to GaAs deposition at the droplet edge, while outer rings result from the reaction of Ga and As atoms diffusing along the surface. The observed variety of morphologies primarily reflects relative changes in the outer rings with temperature and As flux.

Characterizing the geometry of InAs nanowires using mirror electron microscopy.

Mirror electron microscopy (MEM) imaging of InAs nanowires is a non-destructive electron microscopy technique where the electrons are reflected via an applied electric field before they reach the specimen surface. However strong caustic features are observed that can be non-intuitive and difficult to relate to nanowire geometry and composition. Utilizing caustic imaging theory we can understand and interpret MEM image contrast, relating caustic image features to the properties and parameters of the nanowire. This is applied to obtain quantitative information, including the nanowire width via a through-focus series of MEM images.

Caustic imaging of gallium droplets using mirror electron microscopy.

We discuss a new interpretation of mirror electron microscopy (MEM) images, whereby electric field distortions caused by surface topography and/or potential variations are sufficiently large to create caustics in the image contrast. Using a ray-based trajectory method, we consider how a family of rays overlaps to create caustics in the vicinity of the imaging plane of the magnetic objective lens. Such image caustics contain useful information on the surface topography and/or potential, and can be directly related to surface features. Specifically we show how a through-focus series of MEM images can be used to extract the contact angle of a Ga droplet on a GaAs (001) surface.

Decomposition controlled by surface morphology during langmuir evaporation of GaAs.

When GaAs is heated in vacuum, it decomposes into Ga and As as it evaporates. Real-time in situ surface electron microscopy reveals striking bursts of "daughter" droplet nucleation and growth when coalescence of large "parent" droplets exposes nonplanar surface regions. We analyze the behavior, predicting a morphology-dependent congruent evaporation temperature. Based on this we propose a new approach for the self-assembly and positioning of quantum structures via droplet epitaxy, which we demonstrate at the proof-of-concept level.

Phase retrieval low energy electron microscopy.

We consider the utility of phase-retrieval methods in low energy electron microscopy (LEEM). Computer simulations are presented, demonstrating recovery of the terraced height profile of atomic steps. This recovery uses phase retrieval to decode a single LEEM image, incorporating the effects of defocus, spherical aberration and chromatic aberration. The ability of the method, to obtain temporal sequences of evolving step profiles from a single LEEM movie, is discussed.

Running droplets of gallium from evaporation of gallium arsenide.

High-temperature annealing of gallium arsenide in vacuum causes excess evaporation of arsenic, with accumulation of gallium as liquid droplets on the surface. Using real-time in situ surface electron microscopy, we found that these droplets spontaneously run across the crystal surface. Running droplets have been seen in many systems, but they typically require special surface preparation or gradient forces. In contrast, we show that noncongruent evaporation automatically provides a driving force for running droplets. The motion is predicted and observed to slow and stop near a characteristic temperature, with the speed increasing both below and above this temperature. The same behavior is expected to occur during the evaporation of similar III-V semiconductors such as indium arsenide.

Ga droplet morphology on GaAs(001) studied by Lloyd's mirror photoemission electron microscopy.

We apply Lloyd's mirror photoemission electron microscopy (PEEM) to study the surface shape of Ga droplets on GaAs(001). An unusual rectangular-based droplet shape is identified and the contact angle is determined in situ. It is shown that quenching does not appreciably affect droplet shape and ex situ measurements of the contact angle by atomic force microscopy are in good agreement with Lloyd's mirror PEEM. Extension of Lloyd's mirror technique to reconstruct general three-dimensional (3D) surface shapes and the potential use of synchrotron radiation to improve vertical resolution is discussed.

Imaging surface topography using Lloyd's mirror in photoemission electron microscopy.

We use Lloyd's mirror to modulate electron photoemission in photoemission electron microscopy. This results in the projection of Lloyd's fringes on to three-dimensional (3D) surface objects. An iterative reconstruction method is used to correct for distortions in the fringe pattern due to the cathode immersion lens, thereby providing a quantitative interpretation of surface shape. It is therefore possible to extract 3D height information directly from a two-dimensional, plan-view image. The technique is of sufficient intensity and contrast to study real-time changes in surface topography and we apply the method to study unusual contact-line dynamics during the reactive wetting of metal droplets.

Lateral motion of SiGe islands driven by surface-mediated alloying.

SiGe islands move laterally on a Si(001) substrate during in situ postgrowth annealing. This surprising behavior is revealed by an analysis of the substrate surface morphology after island removal using wet chemical etching. We explain the island motion by asymmetric surface-mediated alloying. Material leaves one side of the island by surface diffusion, and mixes with additional Si from the surrounding surface as it redeposits on the other side. Thus the island moves laterally while becoming larger and more dilute.

Tunable metastability of surface nanostructure arrays.

A Fokker-Planck equation is used to model the coarsening of surface nanostructure arrays. Metastable states are identified which are associated with a narrow size distribution and a coverage dependent mean island size. This is a general feature linked to nanostructures which, as a function of island size, are associated with a minimum in formation energy per atom and a positive chemical potential gradient. This has important implications for the self-organization of quantum dots.

Metastability of ultradense arrays of quantum dots.

We present a linear stability analysis of ultradense arrays of coherently strained islands against Ostwald ripening. Surprisingly, short-range elastic interactions are found to overcome the destabilizing contribution of surface energy, leading to a metastable array of quantum dots. Simulations of Ostwald ripening kinetics directly verify the existence of this metastable regime and confirm the nature of the most unstable mode for subcritical island coverage.

Crack-like sources of dislocation nucleation and multiplication in thin films.

With the combination of the height sensitivity of atomic force microscopy and the strain sensitivity of transmission electron microscopy, it is shown that near singular stress concentrations can develop naturally in strained epitaxial films. These crack-like instabilities are identified as the sources of dislocation nucleation and multiplication in films of high misfit. This link between morphological instability and dislocation nucleation provides a method for studying the basic micromechanisms that determine the strength and mechanical properties of materials.