Mohan Krishnamurthy

Microstructural evolution during the heteroepitaxy of Ge on vicinal Si(100)

Microstructural evolution during the initial stages of islanding of Ge on vicinal Si(100) has been studied in situ with nanometer resolution in an ultrahigh‐vacuum scanning transmission electron microscope. Ge is deposited using molecular‐beam‐epitaxy (MBE) techniques on vicinal Si(100) misoriented 1° and 5° toward 〈110〉. For MBE‐type experiments, there is evidence for metastable growth of the Ge intermediate layer to much greater than the equilibrium critical thickness. The layer may grow up to seven monolayers thick before islanding in the Stranski–Krastanov growth mode. The presence of strong adatom sinks significantly alters the growth and size distribution of the islands when the spacing of these sinks is less than an adatom diffusion distance. Studies of the initial stages of islanding in solid‐phase MBE indicate that there is no long‐range adatom diffusion. There is an initial fast transformation from a disordered layer growth, followed by a sluggish growth of islands. We have studied the coarsenin...

Biassed secondary electron imaging in a UHV-STEM

Abstract A new dedicated ultra-high-vacuum scanning transmission electron microscope (UHV-STEM) has been developed for the NSF HREM facility at Arizona State University, in conjunction with VG Microscopes Ltd. This instrument is fitted with a specimen preparation chamber in which vacua of better than 5x10-10 mbar can be routinely achieved; the pressure inside the column is better than 10-10 mbar. The initial performance in various imaging modes is reported. Several techniques have been incorporated to obtain information from surfaces. Here we demonstrate the usefulness of biassed secondary electron imaging from both sides of thin transmission samples. In conjunction with conventional STEM imaging and analysis techniques, these methods can be used to correlate surface and subsurface information on samples with complex surface topography. Examples typical of catalytic and semiconductor applications are given.

Auger electron spectroscopy and microscopy with probe‐size limited resolution

High spatial resolution Auger electron spectra and images have been obtained by optimizing secondary electron collection efficiency in a scanning transmission electron microscope (STEM). We describe an ultrahigh vacuum, 100 keV STEM which is capable of collecting Auger electron spectra and images with edge resolutions of ≤5 nm. Typical spectra and images from the Ge/Si(100) and Ag/Si(100) film growth systems are presented and discussed. These images demonstrate high‐resolution elemental mapping which can be used in the study of surface processes on the nanometer scale.

Biassed secondary electron imaging of monatomic surface steps on vicinal Si(100) in a UHV STEM

Abstract Biassed secondary electron (b-SE) SEM images of steps on Si(100) surfaces misoriented 1° towards 〈110〉 have been obtained using a number of imaging geometries including normal incidence. The periodicity of the regularly spaced steps unambiguously identifies these as single-atom-high steps. High-efficiency secondary electron collection from the input surface of the sample makes it possible to obtain images of low contrast features on low-atomic-number samples in UHV. Simple topography is shown to be a likely mechanism for the contrast observed in the digitally acquired b-SE images. Images of step pinning at refractory particles during surface sublimation identify the up-step direction and are similar to those obtained by other surface microscopic techniques.

Heteroepitaxy of Ge on Vicinal Si(100)

The initial stages of germanium heteroepitaxy on vicinal Si(100) have been studied using in-situ deposition in a UHV STEM. Germanium was deposited using molecular beam techniques onto substrates misoriented 1° and 5* toward held at room temperature, 375°C and 525°C. Film thicknesses were in the range 4-6 ML, just greater than the stable intermediate layer of 3-4ML (1ML = 0.14nm). The Ge clusters were observed using biassed secondary electron (b-SE) imaging with nanometer resolution. Comparisons were made between deposition at the elevated temperatures, and room temperature deposition followed by anneals at the same temperatures. Annealing the low temperature deposits produces coarsening of the islands which is similar on the 1° and 5° samples. Island size distributions and other film growth parameters obtained from the 375°C and 525°C anneals indicate that the coarsening is different at these temperatures and is possibly affected by instabilities in the intermediate layer. Results of the high temperature depositions indicate that neither surface steps nor the edges of islands act as perfect sinks, and that diffusion distances are of the order of several microns. The nucleation density and size distributions are markedly different for deposition at 375°C and 525°C possibly due to competitive capture at strong sinks. In a parallel set of experiments in a standard UHV chamber, macroscopic wafer samples were analyzed with RHEED, Auger and secondary electron spectroscopy. These correlate well with the intermediate layer thicknesses previously reported in the literature, and the large contrast observed in the b-SE images. Ex situ TEM studies of samples grown in this chamber show islands with various contrast features including those of coherent strain.

High Temperature Corrosion Analysis of Inconel 617 | Thermal Energy Storage Capsules

The corrosion degradation of three Inconel 617 Alloy Thermal Energy Storage capsules filled with eutectic fluoride salts LiF-MgF2-KF, LiF-MgF2-NaF and LiF-MgF2 were analyzed. The ultrahigh purity fluoride salts were previously sealed inside the Inconel capsules under controlled processing conditions and thermally cycled in vacuum over a temperature differential of 200 K near 1000 K for 10,000 hr. It was determined that the microstructural degradation is minimal but some aluminum from the alloy may have been displaced into the salt. Comparisons are made with previously reported equilibrium calculations and a semiquantitative diffusion model is presented to support the possibility of diffusion control over the corrosion. The results indicate that Inconel 617 may be a suitable material for fluoride salts in thermal energy storage applications.

Studies of Surface Diffusion and Crystal Growth by SEM and STEM

Surface diffusion, crystal nucleation and growth are being studied using UHV-scanning electron (SEM) and scanning transmission electron (STEM) microscopy-based techniques. A simple technique, biassed secondary electron imaging (b-SEI) has been demonstrated to be sensitive to intermediate layers at the 0.1ML level, and also to surface step topography. Auger electron spectroscopy (AES), on a lateral scale sufficient to probe the chemical composition of intermediate layers between islands, has been developed at the 0.05ML level. Crystallographic information in the form of reflection (RHEED) and transmission (THEED) electron diffraction is also available. These technique developments are briefly reviewed and examples given.

Temperature Dependent Quasi-Periodic Facetting of AlAs Grown by MBE on (100) GaAs Substrates

High temperature (≥ 650°C) MBE growth of AlAs and AlAs/GaAs superlattices on (100) GaAs is shown to lead to quasi-periodic facetting. We demonstrate that the facetting is only due to the AlAs layers, and growth of GaAs on top of the facets replanarizes the surface. We show that the roughness between the AlAs and GaAs layers increases with increasing number of periods in the superlattice. The roughness increases to form distinct facets, which rapidly grow at the expense of the (100) surface. Within a few periods of the initial facet formation, the (100) surface has disappeared and only the facet planes are visible in cross-sectional transmission electron micrographs. At this point, the reflection high-energy electron diffraction pattern is spotty, and the specular spot is a distinct chevron. We also show that the facetting becomes more pronounced as the substrate temperature is increased from 620°C to 710°C. Atomic force micrographs show that the valleys enclosed by the facets can be several microns long, but they may also be only several nanometers long, depending on the growth conditions.

Microstructural Evolution of AG / Gaas (110)

We have monitored the microstructural evolution of Ag films grown on in situ cleaved GaAs(110) surfaces using nm spatial resolution secondary electron microscopy. Ag coverages between 1/2 ML and 256 ML were investigated for films deposited from an Ag Knudsen cell onto both room temperature (RT) and 250° C substrates. The initial stages of growth are markedly different for these two deposition temperatures. The room temperature grown films consist of approximately hemispherical islands while the 250° C films result in needle-like islands elongated along the direction. This elongation may be due to a number of factors including anisotropie surface diffusion or bonding along the and directions, corrugations in the GaAs(110) surface or better lattice matching of Ag(100) to the GaAs(110) direction. At higher coverages, the films appear similar when lengths are scaled to account for increased diffusion at higher temperatures. The islands in the RT film impinge at lower coverages than the 250° C film and the RT film becomes quasi-continuous near 256 ML. The 250° C film forms larger, more widely spaced islands which impinge but do not form a quasi-continuous film for even the highest coverage. Island size distributions will be used to discuss the growth mechanisms for these films.