Life Sciences Commons^™

Strain Relaxation In Compositionally Graded Ingaas/Gaas Heterostructures, Karen L. Kavanagh, Rachel S. Goldman, Jessica C. P. Chang

Scanning Microscopy

Epilayer strain relaxation in the InGaAs/GaAs system occurs via two mechanisms, plastic deformation and/or surface roughening. Under conditions of two-dimensional growth, we find that compositionally graded InGaAs/GaAs (001) multi-layer buffer structures will plastically deform with < 110 > misfit dislocations approaching 100% strain relaxation. At higher growth temperatures, large-amplitude roughening is observed preferentially along the [110] direction, and the strain relaxation becomes asymmetric in the < 110 > directions. In single epilayers, the symmetry of the strain relaxation is dependent on the magnitude of the substrate offcut angle. In all cases, the epilayers develop a tilt about an in-plane axis in proportion to and opposite in …

Reconstructions Of C60 On The Ag(111)1x1 Surface, X. -D. Wang, S. Yamazaki, J. -L. Li, T. Hashizume, H. Shinohara, T. Sakurai

Scanning Microscopy

We report the scanning tunneling microscope (STM) study of the C₆₀ adsorption on the Ag(111)1x1 surface. The well-ordered C₆₀ monolayer with good quality was obtained by briefly annealing the multilayer C₆₀ at approximately 300°C. It is concluded that the (2√3)x(2√3)R30° reconstruction is energetically the most stable phase, while two other phases, "hex-a" and "hex-b" phases, are also observed, rotated by approximately 12.5 ± 1.5° and 47.5 ± 1.5° with respect to the stable (2√3)x(2√3)R30° phase. It is suggested that some specific stable adsorption sites are responsible for the pinning …

Atomic Step Organization In Homoepitaxial Growth On Gaas(111)B Substrates, Leo J. Schowalter, Kai Yang, Thomas Thundat

Scanning Microscopy

When homoepitaxial growth is performed on exactly oriented (singular) (111) GaAs substrates, while maintaining the √19 x √19 surface reconstruction, the originally flat surface spontaneously evolves vicinal (111) facets that are tilted approximately 2.5° toward the < 211 > azimuthal directions. These facets form pyramid-like structures where the distance between adjacent peaks can be varied from as little as 1 μm to tens of μm. When these surfaces are observed with atomic force microscopy (AFM), we find that they are extremely smooth with the observed tilt resulting from atomic steps which are spaced at approximately 7.5 nm. We have also studied growth on …

Strain Relaxation In Graded Ingaas And Inp Buffer Layers On Gaas (001), K. Eberl, K. Häusler, T. Shitara, Y. Kershaw, W. Sigle

Scanning Microscopy

We investigate compositionally graded In_xo≤x≤0.5Ga_1-xAs and InP buffer layers which are prepared by molecular beam epitaxy on (001) GaAs substrate. The initial In content x_o is equal to 0, 0.12, 0.18, 0.24, and 0.5 for the different samples. The In composition of the graded buffer increases linearly between x_o and 0.5 with a fixed slope of 50% In-content per μm. The idea was to combine the advantage of surface flatness in homogeneous buffer layers and the reduced density of threading dislocations on the surface for graded buffer layers. The best compromise in terms of …

Transmission Electron Microscopy, High Resolution X-Ray Diffraction And Rutherford Backscattering Study Of Strain Release In Ingaas/Gaas Buffer Layers, G. Salviati, L. Lazzarini, C. Ferrari, P. Franzosi, S. Milita, F. Romanato, M. Berti, M. Mazzer, A. V. Drigo, M. R. Bruni, M. G. Simeone, N. Gambacorti

Scanning Microscopy

Strain release and dislocation distribution in InGaAs/GaAs double heterostructures, step-graded and linear-graded buffer layers have been studied. A higher misfit dislocation density at the inner interface between the InGaAs layer and the substrate was found in all the samples. This corresponded to a strain release of the inner ternary layers much larger than predicted by equilibrium theories. The residual parallel strain of the external layers as a function of their thickness was found to follow a curve approximately of slope -0.5, in agreement with previous investigations on single InGaAs layers. This result has been interpreted as evidence that the elastic …

Bias Dependence Of The Depletion Layer Width In Semi-Insulating Gaas By Charge Collection Scanning Microscopy, A. Castaldini, A. Cavallini, C. Del Papa, M. Alietti, C. Canali, F. Nava, C. Lanzieri

Scanning Microscopy

A procedure for the evaluation of the depletion region width of a Schottky barrier diode made on semi-insulating materials has been assessed and applied to gallium arsenide nuclear detectors. This procedure, which makes use of the optical beam induced current method of charge collection scanning microscopy, allows the direct measurement of the depletion layer width. By taking into account the high resistivity of the material under examination and measuring the diode reverse current, it is possible to evaluate the actual voltage applied at the depletion layer boundaries. It was found that, at low actual bias values, the voltage dependence of …

Principles Of Semiconductor Surface Reconstruction, C. B. Duke

Scanning Microscopy

Semiconductor surfaces are known to reconstruct, i.e., their surface atomic geometries differ from those of the corresponding surface planes in the bulk material. For clean tetrahedrally coordinated semiconductors, these reconstructed geometries are shown to be predicted by five simple principles. These principles are illustrated by the specific examples of Si(100)-(2x1), Si(111)-(2x1), GaAs(100)-c(2x8), GaAs(111)-(2x2), and relaxed zincblende (110) surfaces. The concept of universal (i.e., material independent) semiconductor surface structures is introduced and shown to be characteristic of the cleavage surfaces of tetrahedrally coordinated compound semiconductors. The role of scanning tunneling microscopy in identifying and validating these principles is highlighted.

Mapping Metal Distributions In Thin Cryosections Without Scanning Transmission Electron Microscope With The Philips Electron Beam And Image Deflection (Ebid) Unit, A. J. Morgan, J. M. Brock, C. Winters, G. H. J. Lewis

Scanning Microscopy

The purpose of the present paper was to describe the use of a commercially-available, and relatively inexpensive, beam and image deflection unit that can facilitate digital X-ray (element) mapping in a standard transmission electron microscope not furnished with a STEM attachment. The test specimen was a thin freeze-dried section of the metal-sequestering chloragogenous tissue from the earthworm, Lumbricus rubellus, inhabiting a soil naturally contaminated with Pb, Zn and Cd. Qualitative maps obtained from this material confirmed the efficacy of the deflection unit, and revealed the presence of three compositionally distinct metal-accumulating compartments within the chloragocytes: (i) ovoid, electron-dense, phosphate-bearing and …

Role Of Surface Interactions In Determining Surface Structure And State Formation In Iii-V Semiconductors, H. E. Ruda, G. P. Jiang

Scanning Microscopy

GaAs(100), (110), and (111) surfaces are chosen as a vehicle to explain the plethora of surface relaxation and reconstruction phenomena seen for III-V compound semiconductors. These relaxation and reconstruction processes directly affect the formation of surface states. The occupation of these states, in turn, can have a profound influence on device performance. The purpose of this work is to attempt to provide a unified description of the phenomena responsible for surface relaxation and reconstruction on these surfaces. Our work makes use of an ab initio effective core potential scheme based on the Hartree Fock approximation. We discuss the critical steps …

Dislocation Nucleation And Propagation In Semiconductor Heterostructures, D. Cherns, S. Mylonas, C. T. Chou, J. Wu, D. E. Ashenford, B. Lunn

Scanning Microscopy

This paper considers misfit dislocation nucleation and propagation in dilute magnetic semiconductor heterostructures in the CdTe-ZnTe-MnTe system. It is shown that, where the deposit is in tension, 1/2 < 110 > dislocations with inclined Burgers vectors propagate by glide along interfacial < 110 > directions and may dissociate giving intrinsic stacking faults. In cases where the deposit is in compression, 1/2 < 110 > dislocations show no evidence of dissociation and propagate by extensive cross-slip to give networks of dislocations close to interfacial < 100 > directions.

Evidence for dislocation sources in ZnTe/GaSb films is presented. ZnTe films contained stacking fault pyramids, single Frank faults and a new type of "diamond defect" …

Surface Stress, Morphological Development, And Dislocation Nucleation During Sixge1-X Epitaxy, D. E. Jesson, S. J. Pennycook, J. -M. Baribeau, D. C. Houghton

Scanning Microscopy

Utilizing Ge marker layer experiments combined with atomic number contrast (Z-contrast) imaging, we have studied the evolving surface morphology of Si_xGe_1-x alloys during growth by molecular beam epitaxy. The marker layers map out the instability transition between planar two-dimensional (2D) growth and three-dimensional (3D) growth. The transition occurs via the gradual formation of a surface ripple as anticipated from instability theory. However, these undulations rapidly develop into crack-like surface instabilities which we simulate and explain by the mechanism of stress-driven surface diffusion. Finally, we model the large stresses associated with these features within a fracture mechanics formalism. …

Light Scattering And Electron Microscopy Study Of The Surface Morphology Of Gaas Films Grown By Molecular Beam Epitaxy, M. K. Nissen, C. Lavoie, S. Eisebitt, T. Pinnington, T. Tiedje

Scanning Microscopy

The surface morphology of thermally quenched GaAs films grown by molecular beam epitaxy on GaAs substrates has been studied by elastic light scattering, by scanning electron microscopy and by scanning tunneling microscopy (STM) in air. STM shows that the oxide-desorbed surface of GaAs is pitted, but smooths after deposition of a few hundred nanometers of material. Light scattering shows that, after the surface has smoothed, the power spectral density of the surface approaches a q^-2 dependence on spatial frequency over the spatial frequency range 0.2 μm^-1 < q < 20 μm^-1 that is accessible to the light scattering measurements at 488 nm. …

The Morphology And Misfit Dislocation Formation Characteristics Of Strained Heteroepitaxial Layers: Ex Situ And In Situ Growth Studies, A. G. Cullis

Scanning Microscopy

Under certain regimes of heteroepitaxial layer growth, misfit stresses can lead to very significant distortions in interface morphology, which can influence strain relief and subsequent misfit dislocation introduction. These phenomena have been clearly demonstrated in the case of SiGe/Si heteroepitaxy and the way in which surface SiGe growth ripples are accompanied by strain waves has been established. The ripples provide partial elastic relief of the layer misfit stress in a manner which has been correlated with theoretical expectations. The local stress variations ultimately may influence the formation and disposition of misfit dislocations in the strained layer structures. The present paper …

Ionoluminescence: A New Tool For Nuclear Microprobes In Geology, C. Yang, N. P. -O. Homman, K. G. Malmqvist, L. Johansson, N. M. Halden, V. Barbin

Scanning Microscopy

When an ion beam in the energy range of a few MeV/amu impacts on a mineral, visible light can often be observed. This light, induced by energetic ions, is termed ionoluminescence (IL). The intensity and wavelength of the ionoluminescent light provide information concerning the nature of luminescence centers, such as trace substituents and structural defects, found in the mineral. This makes IL a useful complement to other methods of ion beam analysis (IBA), such as particle induced X-ray emission (PIXE) and Rutherford backscattering (RBS), in characterizing geological samples. In the present study, a proton or alpha particle beam was used …

Ultrastructure Of Dentin Matrix In Heritable Dentin Defects, J. Waltimo, H. Ranta, P. -L. Lukinmaa

Scanning Microscopy

Heritable dentin defects form a group of diseases which exclusively affect dentin among the various dental tissues. While one type is associated with the generalized connective tissue disorder, osteogenesis imperfecta, other types occur as single traits. The clinical manifestations of the dentin defects vary from insignificant to severe enough to cause aesthetical and functional failure of the teeth. Scanning and transmission electron microscopic studies, reviewed in this paper, have markedly clarified the ultrastructure of the aberrant dentin matrix. Both similar and different changes seem to occur in the various forms of heritable dentin defects. Abnormalities in the appearance and organization …

Observations On Interactions Between Metal Clusters And Iii-V Semiconductor Substrates, Theodore D. Lowes

Scanning Microscopy

Interfacial reactions between deposited indium and gallium metals with GaAs(001) substrates are discussed. After Knudsen cell molecular beam epitaxy (MBE) deposition, samples were annealed in ultrahigh vacuum (UHV) and examined by ex situ electron microscopy. The resulting microstructure was compared to the microstructure of GaAs(001) substrates without metal deposition. It is shown that significant interactions occur between the deposited metal and substrate and that the final microstructure is consistent with the model for thermal decomposition of III-V compound semiconductor substrates.

Metastable Reconstructions On Si(111), Y. -N. Yang, E. D. Williams

Scanning Microscopy

We report unambiguous atomic scale evidence demonstrating that the atom density in the high temperature "1x1" phase of Si(111) is ~6% higher than the 7x7. Such evidence is provided by scanning tunneling microscopy (STM) observation of excess adatom density, and related island formation, on surfaces with very large terraces. The unusually large terraces were produced by heating the sample with DC current in the step-down direction at 1200°C. By trapping adatoms on the terraces through a quench, we have also created areas of metastable reconstructions, i.e., 9x9, 2x2, c2x4 and √3x√3, much larger than previously reported. For …

Boron Reconstructed Si(111) Surfaces Produced By B2o3 Decomposition, J. Nogami, S. Yoshikawa, J. C. Glueckstein, P. Pianetta

Scanning Microscopy

Scanning tunneling microscopy has been used to study the growth of boron on the Si(111) surface. Boron was deposited in the form of B₂O₃ which was decomposed by heating the substrate. With this technique, it is possible to control the B coverage, and also to produce the well known √3 x √3 reconstruction at annealing temperatures as low as 600°C. The optimal conditions for the formation of the √3 x √3 surface by B₂O₃ decomposition are given. In addition, the nature of the √3 x √3 surface over …

Epitaxial Growth Studies By Low Energy Electron Microscopy, E. Bauer

Scanning Microscopy

In spite of being a surface sensitive tool, low energy electron microscopy (LEEM) can also give information on interfaces. An example is the CoSi₂/Si(111) interface. Most of the work discussed in this paper, however, makes use of the high surface sensitivity of LEEM which makes this method an ideal tool for the study of the early growth stages in epitaxy, in particular of the growth dynamics and of the influence of misfit on the growth mode. Two prototype substrates, Mo(110) and Si(111), and three representative deposit metals, Cu, Au and Co, are used to illustrate the large variety …

Reflection High Energy Electron Diffraction (Rheed) Intensity Oscillations: Growth Modes And Growth Rates: A Critique, B. A. Joyce, X. M. Zhang, J. H. Neave, P. N. Fawcett, M. R. Fahy, K. Sato, I. Kamiya

Scanning Microscopy

The origin of and diffraction effects associated with reflection high energy electron diffraction (RHEED) intensity oscillations which occur during layer-by-layer growth of epitaxial thin films of III-V compounds by molecular beam epitaxy (MBE) are explained. It is shown that on (001) oriented substrates the period of the oscillations is in general a direct measure of the film growth rate which corresponds to the group III element flux. There are, however, exceptions to this simple concept including growth under group III rich-conditions, vicinal plane growth and growth from pulsed beams; each is considered.

On non-(001) low index orientations, the RHEED oscillation …

Microdissection And Measurement Of Polytene Chromosomes Using The Atomic Force Microscope, Curtis Mosher, Daniel Jondle, Linda Ambrosio, James Vesenka, Eric Henderson

Scanning Microscopy

A method to isolate specific regions of the Drosophila polytene chromosome using an atomic force microscope (AFM) was explored. The AFM was used for the microdissection of the locus of interest with much greater precision than standard microdissection techniques. The amplification of DNA isolated in this fashion by the polymerase chain reaction (PCR) is discussed. A study of the effect of hydration level on gross chromosome structure was carried out. It was shown that chromosome swelling is dependent upon humidity or the buffered medium. The significance of this swelling with respect to studies of chromosome structure under physiological conditions is …

The Principles Of Proton Probe Microanalysis In Biology, G. J. F. Legge, M. Cholewa

Scanning Microscopy

The proton microprobe, more correctly described as an ion microprobe which operates at MeV energies, complements its parent instrument the electron microprobe. This paper compares the basic principles and performance of the two instruments and relates the evolution of biological analysis on such ion microprobes to that on electron microprobes, covering the development of sample handling techniques and of data handling techniques and comparing beam damage studies. The paper describes the variety of techniques available to the ion microprobe - the initial techniques of Energy Dispersive X-ray analysis, Rutherford Back Scattering and Nuclear Reaction Analysis and the rapid evolution of …

X-Ray Absorption Studies Of Strain In Epitaxial (Si-Ge) Atomic Layer Superlattice And Alloy Films, T. Tyliszczak, A. P. Hitchcock, Z. H. Lu, J. -M. Baribeau, T. E. Jackman

Scanning Microscopy

The Si 1s (K-shell) X-ray absorption spectra of a series of strained Si_xGe_100-x alloy thin films and several {(Si)_m(Ge)_n}_p atomic layer superlattices (ALS) grown epitaxially on Si(100) and Ge(100) substrates have been investigated using plane polarized synchrotron radiation. Polarization dependent components of the signal are attributed to anisotropic states associated with strain-induced tetragonal distortions. The sense of the polarization is shown to be identical for all compositions (x = 25 to 92) of SiGe alloys grown on Si(100) substrates. The opposite polarization dependence is found to occur for all Si_xGe …

Interfacial Studies In Semiconductor Heterostructures By X-Ray Diffraction Techniques, J. -M. Baribeau, R. L. Headrick, P. Maigné

Scanning Microscopy

X-ray radiation is a non-destructive probe well suited to assess structural perfection of semiconductor material. Three techniques are used to study the interfacial roughness, period fluctuations and annealing-induced interdiffusion in various superlattice structures. Reflectivity of long period Si/Si_1-xGe_x multiple quantum wells reveals an asymmetry oriented along the direction of miscut in the interface roughness with the Si_1-xGe_x to Si interfaces being about twice as rough (0.5 versus 0.3 nm) as the Si to Si_1-xGe_x interfaces. For Si-Si_0.65Ge_0.35 multiple quantum wells, diffuse scattering is minimal for a growth temperature …

Reply By A. Carretero Et Al., Ana Carretero, H. Ditrich, M. Navarro, H. Splechtna, J. Ruberte

Scanning Microscopy

Dear Editor,

The study of vascular casts in the embryo by means of the scanning electron microscope represents the main subject of our research. We fully agree with DeRuiter and Gittenberger-de Groot in that corrosion casting is a valuable tool to study the early angiogenesis in embryos.

Znse Heteroepitaxial Growth On Si (100) And Gaas (100), D. K. Biegelsen, R. D. Bringans, J. E. Northrup, L. -E. Swartz

Scanning Microscopy

The early stages of ZnSe heteroepitaxy on Si(100), Si(100):As and GaAs(100) are compared and contrasted, based on results of scanning tunneling microscopy and photoemission spectroscopy. High Se reactivity with the substrate constituents leads to bulk phase formation which is detrimental to heteroepitaxy. As-termination of Si(100) not only passivates the surface, but also provides an ideal buffer for ZnSe overgrowth. Lacking a similar buffer layer, stoichiometric control of the GaAs(100) surface is investigated to find a means for controlled heteroepitaxy.

Balancing Surface Energy Terms For Stable Growth Of Planar Surfaces, M. Albrecht, P. O. Hansson, S. Christiansen, W. Dorsch, H. P. Strunk, E. Bauser

Scanning Microscopy

We investigate the driving forces that determine the growth mode of heteroepitaxial Ge layers grown from solution on Si substrates with orientations (001), (011) and (111) by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Using liquid phase epitaxy, we can study the influences of strain and surface energy terms independently on effects due to limited surface diffusion. In (001) and (011) orientated layers, {111} faceted islands form (Stranski-Krastanov growth). In contrast, (111) orientated layers grow in a two-dimensional step flow growth mode (Frank-van der Merwe growth).

We model these investigations in terms of energy minimisation considering surface energy …

Recent Surface Studies Using Biassed Secondary Electron Imaging, Rajendra Persaud, Hisato Noro, Muhammad Azim, Robert H. Milne, John A. Venables

Scanning Microscopy

The growth and surface diffusion of Cs on Si(100) and Ag on Fe(110) have been studied using biassed secondary electron imaging (b-SEI) and Auger electron spectroscopy (AES).

The b-SEI technique was found capable of detecting Cs on the Si surface with a 0.5% ML sensitivity. Unusual diffusion profiles containing linear sections were obtained for coverages (θ) < 1/2 ML. The general form of these profiles were reproduced using a 2-phase model, where Cs chains act as sources of mobile adatoms, in conjunction with a diffusion coefficient of the form D ~ θ (1- Cθ). This form of D, obtained from Boltzmann-Matano analysis, is consistent with diffusion theory including strongly repulsive Cs-Cs interactions. An adatom diffusion energy, E_d, = 0.47 ± 0.05 eV was found to be consistent with measurements …

Predicting Relaxation In Strained Epitaxial Layers, R. Beanland, D. J. Dunstan, P. J. Goodhew

Scanning Microscopy

Strained epitaxial semiconductor layers, much thicker than the critical thickness, have been used as "strain-relief" buffer layers for many years. The most successful structure developed so far dates back to the 1960's, and consists of a very thick ( ~30 μm) layer in which the misfit is gradually and continuously increased. These structures relax completely and have a sufficiently low threading dislocation density to allow a device structure to be grown on top. This process requires a very high growth rate to produce the buffer layer in a reasonable time, which is only provided by hydride vapourphase epitaxy. Recently, there …

Indicators Of Nutrient Limited Plankton Growth In Lakes Near Mount Saint Helens, Washington, Kurt Davis Carpenter

Dissertations and Theses

Several lakes located in the blast zone of the 1980 eruption of Mount St. Helens were studied to determine if the plankton in the lakes were limited in their growth by nitrogen or phosphorus availability. Long term nutrient enrichment experiments were performed on lake water from five lakes and measures of chlorophyll-a, carbon fixation, and nutrient uptake were used to evaluate the extent of limitation. Nutrient concentrations, ratios, and uptake from 14 lakes provided additional evidence for limitation by nitrogen and/or phosphorus. The physical, chemical, and biological characteristics of the lakes were also examined to monitor the return of these …