Biology Commons^™

A New Model Of Low Temperature Photoluminescence In Amorphous Semiconductors, Mathieu Kemp, Marvin Silver

Scanning Microscopy

Recent low temperature a-Si:H photoluminescence experiments show the presence of two peaks in the lifetime distribution, and a dependence of the efficiency on generation rate. These results contradict every existing model of amorphous semiconductor photoluminescence. The reason for the discrepancy is that every model predicts diffusive motion of the photo-generated pairs. We show how the inclusion of coulomb interaction between photocarriers, spin selection effects, and Auger recombination gives back agreement of theory with experiment. This new picture of the phenomenon also explains the transient behavior of the luminescence intensity.

Advances In Research Of Defects In Quartz Based On Luminescence, A. Halperin

Scanning Microscopy

The first part of this paper reviews the observation of the unconventional thermoluminescence {TL(M), where M = Li or Na} peaks observed in the glow curve of Li-and Na-containing quartz, and describes the full solution of the puzzling effect of the TL(M) peaks which needed a two irradiation procedure for their formation. Of interest is the intermediate warming (IW) method, which consisted of two irradiations and warming of the sample to 150-300 K between the two irradiations. This was explained as follows: the first low temperature irradiation produces {AlO₄/M/h}⁺ centers, when the now loosely bound M ions …

Thermoluminescence Of Zircon, Philibert Iacconi

Scanning Microscopy

The thermoluminescence (TL) of synthetic zircons into which some impurities have been individually insert-ed is investigated. The results obtained show that, after X-irradiation at 77K, the synthetic zircons present three kinds of thermoluminescent emissions. The first is related to the OH^- ions, the second is typical of the SiO₄^4-groups, and the third is characteristic of RE³⁺ ions with RE = Dy, Tb, Gd, Eu, or Sm (RE = rare earth).

The OH^- emission is a large band at 285 nm which appears at 115 and 160K.

The SiO₄^4- TL emission consists of …

Cathodoluminescence Of Carbonates: New Applications In Geology And Archaeology, V. Barbin

Scanning Microscopy

Several new applications of cathodoluminescence (CL) have been developed during the last few years, including: combined CL imaging and/or spectroscopy and CL observations of carbonates previously considered to be non-luminescent or of minor interest. Basically, two previously unstudied types of carbonates were investigated: marbles, and recent shells. From numerous classical white marble samples, cathodomicrofacies were defined and described. A single cathodomicrofacies is generally characteristic of a given area. This finding permits identification of the source of white marbles and has been successfully applied to ancient marbles from different collections. Despite the well established idea that CL in biogenic carbonates is …

Plagioclase Studies By Ionoluminescence (Il) And Particle-Induced X-Ray Emission (Pixe) Employing A Nuclear Microprobe, N. P. -O. Homman, C. Yang, K. G. Malmqvist, K. Hanghöj

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, termed ionoluminescence (IL), has been shown to be a very useful tool for investigating geological specimens when it is combined in a nuclear microprobe with a well-established, quantitative, trace element method such as Particle Induced X-ray Emission (PIXE). When plagioclases from the Skaergaard intrusion, East Greenland, were irradiated with protons, bluish luminescence was observed. Spectroscopic IL studies were undertaken with 1.5 and 2.5 MeV protons with power densities ranging from about 6 to 160 W/cm^{2 …}

Cathodoluminescence Of Some Synthetic Calcite Crystals. Investigation On The Role Played By Cerium, Rémy Chapoulie, Françoise Bechtel, Daniel Borschneck, Max Schvoerer, Guy Rémond

Scanning Microscopy

Cathodoluminescence (CL) from undoped and cerium-doped synthetic calcite is examined at room temperature and at liquid nitrogen temperature, using a scanning electron microscope. At room temperature, all of the samples exhibit a CL emission in the near ultra-violet (UV), centered at approximately 370 nm with some possible shoulders at 345 nm. At low temperature, the samples show an emission of CL in the near UV but with two well defined maxima for the Ce-doped crystals (460 ppm and 4600 ppm) peaking at 345 and around 380 nm. The third Ce-doped crystal (1.74%) presents a maximum at 375 nm and a …

Multi-Laboratory Results For The Cathodoluminescence Emission Spectrum From A Synthetic Zircon Standard, John M. Hanchar, Donald J. Marshall

Scanning Microscopy

The Standards Committee of the Society for Luminescence Microscopy and Spectroscopy (SLMS) circulated doped zircon crystals as a standard for comparison of cathodoluminescence (CL) emission spectra obtained at different laboratories. Eleven laboratories have submitted spectra acquired from this standard. The crystals are synthetic zircons doped with 1.4 wt.% of Dy₂O₃

The participating laboratories used a variety of CL instrumentation including cold cathode optical micro-scope attachments, hot cathode optical microscope attachments, and EMPA/SEM-based systems. Two laboratories provided both uncorrected and corrected spectra. All other spectra are uncorrected. A variety of different spectrometers/spectrographs and detector systems were used. Photomultiplier …

Can Cathodoluminescence And Scanning Electron Acoustic Microscopy Be Considered Complementary Techniques?, J. F. Bresse

Scanning Microscopy

Cathodoluminescence (CL) and Scanning Electron Acoustic Microscopy (SEAM) give complementary information on direct bandgap semiconductors when dopant impurities are introduced. CL deals with the electrical properties of the semiconductor and SEAM deals with the thermal and elastic properties and eventually with the piezoelectric properties in low doped III-V compounds. As function of the introduction of impurities for the doping of the semiconductor, the Near-Band-Edge (NBE) CL emission increases up to a maximum. and decreases when the impurities are no longer introduced in electrically active sites, but create complexes giving rise to the appearance of a Deep Level (DL) emission. The …

Luminescence Spectroscopy Of Semiconductor Surfaces And Interfaces, L. J. Brillson, S. Chang, A. D. Raisanen, I. M. Vitomirov

Scanning Microscopy

Low energy cathodoluminescence spectroscopy (CLS) employing incident electron energies in the range of a few kV or less enable measurement of electronic structure near semiconductor surfaces and interfaces. Coupled with photoluminescence spectroscopy (PL), the CLS technique has been extended to characterize electronic structure tens of nanometers below the free surface at metal-semiconductor and semiconductor-semiconductor junctions. CLS has revealed discrete, deep electronic states for clean and metallized semiconductor surfaces as a function of atomic ordering as well as vicinal surfaces as a function of misorientation. A combination of CLS and PL reveals deep level features associated with strain relaxation and dislocations …

Luminescence From Porous Silicon, Yasunori Mochizuki

Scanning Microscopy

Structural and optical properties of porous Si are reviewed with the main emphasis on the radiative recombination mechanisms. Behaviors of the visible photoluminescence and another intense luminescence process (the infrared luminescence) are discussed based on the available data provided by photoluminescence and related techniques. It is demonstrated that a further insight into the interrelation of these luminescence processes can be obtained by the optically-detected magnetic resonance method, in which non-radiative point defects (surface dangling bonds) are used as a local structural probe for the radiative states.

As for the model for the visible light emission, the implication of the quantum …

Scanning Tunneling Microscope-Induced Luminescence Spectroscopy On Semiconductor Heterostructures, M. Pfister, M. B. Johnson, U. Marti, S. F. Alvarado, H. W. M. Salemink, D. Martin, F. Morier-Genoud, F. K. Reinhart

Scanning Microscopy

Scanning tunneling microscope (STM)-induced luminescence is explored as a technique for the characterization of semiconductor quantum wells and quantum wire heterostructures. By injecting minority carriers into the cleaved cross section of these structures, luminescence excitation on a nanometer scale is demonstrated. Using spectrally resolved STM-induced luminescence for the tip placed at various positions across the cleaved heterostructure, it is possible to obtain local spectroscopic information on closely spaced quantum structures.

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 …

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 …

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 …

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 …

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 …

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.

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 …

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 …

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 …

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. …

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 …

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 …

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 …

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.

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 …

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 …

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 …