J. Murray Gibson

Direct measurement of strain in a Ge island on Si(001)

We report on a direct measurement of the strain in a single Ge “quantum dot” island grown on Si by chemical vapor deposition. This transmission electron microscopy method is reliable: without the need for detailed modeling of the strain field, it measures the maximum in-plane displacement. Good agreement is found between the experimental value of 0.86±0.17% average strain and finite element simulations assuming pure Ge. Thus no evidence of significant alloying with Si is observed.

Dome-to-pyramid shape transition in Ge/Si islands due to strain relaxation by interdiffusion

Ge islands were grown on Si(001) and then annealed at 650 °C for 0, 20, 40, and 60 min in a chemical-vapor deposition reactor following Ge deposition. This letter confirms the previous observations directly. By combining the ability to quantify strain with the ability to measure island dimensions in a transmission electron microscope, we were able to plot strain versus aspect ratio for the various annealing times. The islands first relax strain because of Si intermixing with the Ge epilayer causes the lattice mismatch to be lowered. Once the mismatch is sufficiently reduced, and thus the strain energy sufficiently reduced, it becomes favorable for the islands to reverse their shape back from domes to pyramids, thus reducing surface energy. This confirms the reversibility of island shape and thus the thermodynamics of the transition.

Fluctuation microscopy studies of medium-range ordering in amorphous diamond-like carbon films.

In this letter, we report fluctuation microscopy studies of medium-range ordering in amorphous diamond-like carbon films and the effect of annealing on this ordering. Annealed and unannealed diamond-like carbon films have almost identical short-range order. Our fluctuation microscopy results, however, indicate the presence of medium range order or clustering in the films on a lateral length scale that exceeds 1 nm. Within the clustered regions, the dominant local ordering appears to be diamond-like, and graphite-like ordering is not observed. Thermal annealing up to 600 °C leads to an increase in diamond-like clustering with no onset of graphite-like clustering. However, after high temperature annealing up to 1000 °C, graphite-like clustering becomes apparent as a result of the conversion of diamond-like carbon to graphite-like carbon. The results on the as-deposited films and films annealed up to 600 °C suggest that a spontaneous medium range ordering process occurs in diamond-like carbon films during an...

In situ transmission electron microscopy of AlN growth by nitridation of (0001) α-Al2O3

Using a novel ultrahigh vacuum transmission electron microscope (TEM) with in situ reactive molecular beam epitaxy system, we report the successful synthesis of epitaxial AlN on the (0001) sapphire surface upon exposure to ammonia. The substrate, an electron transparent sapphire foil, was annealed in oxygen at high temperature to eliminate damage induced during preparation and to create large atomically flat regions. The sample was then held at 950 °C in flowing ammonia inside the microscope for 2 h during which periodic observations of microstructural development were made. We report direct observation of the formation of epitaxial AlN with the orientation relationship (0001)AlN//(0001)sub, [1010]AlN//[1120]sub and present both TEM and atomic force microscope images of the sample before and after nitridation. The results are consistent with a diffusion-limited reaction model involving transport of oxygen and nitrogen ions through the growing AlN epilayer between the free surface and the unreacted α-Al2O3.

High-angle annular dark-field imaging of self-assembled Ge islands on Si(0 0 1)

Low-resolution high-angle annular dark-field (HAADF) imaging is applied to the study of coherent Ge islands on a Si(0 0 1) substrate. Experimental HAADF images reveal a complicated pattern for a coherent Ge island under (0 0 1) zone axis conditions due to strain-induced interband scattering between different Bloch-wave branches. This complicated pattern varies with objective aperture size and defocus because of the effect from the depth of field. This suggests that the strain field of a coherent Ge island can be mapped out in 3 dimensions using HAADF imaging. When samples are tilted away from dynamical conditions, image contrast agrees with the predictions from atomic number variation (Z contrast). Therefore, quantitatively compositional analysis is feasible under kinematical imaging conditions when strain contrast is suppressed. Simulations using multi-beam Bloch-wave theory agree well with the experimental images on the complicated strain-induced and through-focus images.

Evolution of coherent islands during strained-layer Volmer-Weber growth of Si on Ge(111)

Deposition of Si on Ge(111) at growth temperatures of 450--500{sup o}C by molecular beam epitaxy produces high densities (>10{sup 11} cm{sup -2}) of small (width {approx}10 nm) coherent three-dimensional Si islands. At intermediate temperatures, 550--600{sup o}C, islands become incoherent with the Ge(111) substrate when their widths exceed {approx}18 nm. The activation energy for the maximum island density prior to coalescence is {approx}1.7 eV over a wide temperature range 450--650{sup o}C.

In situ ultra-high vacuum transmission electron microscopy studies of nanocrystalline copper

Abstract We have built a particle production and transport system that allows the characterization of nanocrystals without exposure to contaminating atmospheres such as air. Nanocrystals (formed by inert gas condensation of a sputtered atom population) are transported in situ via the gas phase to an ultra-high vacuum transmission electron microscope (UHVTEM) equipped with a heating stage and gas exposure system. With this system, we can study various nanoparticle phenomena in real time and under clean conditions. In this paper we discuss the experimental design and preliminary studies using imaging and diffraction techniques. These include, the time-evolution of copper nanoparticle morphology and sintering behavior as a function of particle size, temperature, oxygen/atmosphere exposure and supporting substrate. In particular, we have observed immediate room temperature sintering of clean copper nanocrystals which does not occur with nanoparticles that have been exposed to oxygen. Furthermore, we have seen an interaction between copper nanocrystals and amorphous carbon which produces graphite shells. This shell formation process suggests a solid state analog to that seen when nanoparticles catalyze the growth of carbon fibers through a hydrocarbon atmosphere decomposition.

Breakdown of the weak-phase object approximation in amorphous objects and measurement of high-resolution electron optical parameters

Abstract Using multi-slice simulations of electron scattering we show that the validity of the weak-phase object approximation in amorphous thin films is resolution dependent. In particular, this has impact on the appearance of diffractograms from amorphous objects often taken far out-of-focus to measure the “information-limit” resolution, and other optical parameters. Such diffractograms may not show the expected oscillatory contrast transfer function beyond a certain spatial frequency, which is dependent on the specimen thickness. Despite this anomaly, measurements of the information limit from the highest frequency transfer should still be qualitatively valid. However, measurements of Cs for example, from such diffractograms could be in significant error.

TEM measurement of strain in coherent quantum heterostructures

We report on a transmission electron microscopy (TEM) technique that can be used to measure strain due to individual nanometer-scale coherent heterostructures such as quantum dots or inclusions. The measurement relies on two-beam imaging and on an approximation that employs a universal model for lattice plane bending. We demonstrate that analysis is simple and accurate. Using this method, we measured the average strain in dome-shaped Ge islands grown on Si (0 0 1). We found that the method of specimen preparation can significantly affect the observed strain in these islands.

Novel interactions of supported clusters: contact epitaxy

Abstract The study of clusters of ‘model’ metal systems such as Cu and Ag provide a valuable route to explore critical issues in materials epitaxy. Our investigations have led to observations of novel interactions between supported metal clusters in both homo- and heteroepitaxial configurations. In the experiments, clusters of both Cu and Ag were produced by inert gas condensation and deposited on the clean (001)Cu surface under ultrahigh vacuum. Following deposition, the Cu clusters were observed to be of initially random orientation on the substrate surface, undergoing reorientation upon annealing by a mechanism involving sintering and grain growth. In the case of Ag clusters, the formation of a heteroepitaxial layer between the particle and substrate was observed upon initial contact. The phenomenon, which we call ‘contact epitaxy’, may be understood from molecular dynamics simulations of a ‘soft impact’ between the nanoparticle and substrate which indicate that the ordered layers form within picoseconds of impact. The experiments were performed in an ultrahigh vacuum transmission electron microscope equipped with an in-situ nanoparticle sputtering system.