Publications
162 results found
Ksendzov A, Pike WT, Larsson A, 1993, Interband optical transitions between confined and unconfined states in quantum wells, Physical Review B, Vol: 47, Pages: 2228-2232, ISSN: 0163-1829
We have measured and calculated the interband optical absorption in single quantum wells (SQW's) due to transitions between confined states and unconfined continuum states (C-U transitions), and compared it to absorption due to transitions involving pairs of confined states (C-C transitions). Our sample was an In0.25Ga0.75As/GaAs SQW with a 61- well width as determined by electron microscopy. The computed spectra and the results of polarization-modulated absorption measurements reveal steplike absorption features with gradual turnon due to C-U transitions, as opposed to the sharp steps corresponding to the C-C transitions. The transitions of both types have comparable strength, demonstrating the importance of the C-U transitions, which have been largely neglected in previous calculations of the optical properties of quantum wells. Unlike our SQW results, published absorption spectra of uncoupled multiple quantum wells show that C-U transitions produce steplike absorption features with sharp onset. We use a simple model to demonstrate how the slow-turnon absorption line shapes characteristic of SQW's evolve into sharp steplike features as more wells are added. © 1993 The American Physical Society.
Pike WT, 1993, Energy-filtered microdiffraction in a dedicated scanning transmission electron microscope, Ultramicroscopy, Vol: 51, Pages: 117-127, ISSN: 0304-3991
Microdiffraction patterns have been recorded using existing parallel electron energy-loss spectroscopy instrumentation in a field emission scanning transmission electron microscope (STEM) operating at 100 kV. The patterns are recorded by a low-noise CCD camera coupled to a scintillator which is placed after the exit slit of a spectrometer. Three sets of quadrupoles between the scintillator and spectrometer are used to control the camera length. The patterns are recorded at extremely high resolution (0.06 mrad) and very low noise with an energy window of down to 1 eV. As recording of the whole pattern is undertaken in parallel, acquisition times are very short (a few seconds) compared to lower-resolution, higher-noise, serial methods. Using this technique zero-energy-loss microdiffraction patterns have been obtained from individual layers in Si/Si1-xGex multilayers using the sub-nanometer-size probe of STEM. The high resolution of the patterns permits strain quantification in these layers to a few parts in ten thousand using higher-order Laue zone (HOLZ) deficit lines. Using a defocussed probe, strain maps of the superlattice structure can be obtained. Patterns taken from near the edge of single crystals show coherence effects in both the zero-order disc and HOLZ lines. Energy-filtered microdiffraction patterns have been acquired from single crystals for losses of up to 200 eV. © 1993.
Fathauer RW, George T, Pike WT, 1992, Co diffusion and growth of buried single-crystal CoSi<inf>2</inf> in Si(111) by endotaxy, Journal of Applied Physics, Vol: 72, Pages: 1874-1878, ISSN: 0021-8979
At a growth temperature of 800°C, Co deposited on a Si capping layer exhibits oriented growth on buried CoSi2 grains on Si(111), a process referred to as endotaxy. This occurs preferentially to surface nucleation of CoSi2, provided the thickness of the Si cap is less than a critical value of ≊100 nm for deposition rates of 0.003-0.01 nm/s. The steady-state process is modeled using known values of the Co diffusion coefficient and solid solubility in Si, allowing some conclusions to be drawn regarding parameters relevant to CoSi2 epitaxy. Using this technique, single-crystal continuous layers of CoSi2 can be formed under a high-quality Si capping layer.
Rich DH, George T, Pike WT, et al., 1992, Cathodoluminescence and transmission electron microscopy study of dark line defects in thick In<inf>0.2</inf>Ga<inf>0.8</inf>As/GaAs multiple quantum wells, Journal of Applied Physics, Vol: 72, Pages: 5834-5839, ISSN: 0021-8979
The spatial distribution of the long-wavelength luminescence in thick In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) has been investigated using cathodoluminescence (CL) imaging and spectroscopy. The CL spectra show defect-induced broad bands between 1000≲λ≲1600 nm. These bands exhibit spatial variations which correlate with the dark line defects (DLDs) observed in the λ=950 nm exciton luminescence imaging. Transmission electron microscopy showed that [110]-oriented misfit dislocations occur primarily at the substrate-to-MQW and GaAs capping layer-to-MQW interfaces. The large spatial variation of the luminescence intensities indicates that the DLDs observed in CL images are caused by the presence of nonradiative recombination centers occurring in the MQW region located between the interface misfit dislocations. This study provides new information describing the origin and nature of DLDs and differs from previous models which have regarded the electronic nature of dislocation cores as the primary mechanism for inducing DLD radiative contrast in luminescence imaging of strained InGaAs/GaAs.
George T, Anderson MS, Pike WT, et al., 1992, Microstructural investigations of light-emitting porous Si layers, Applied Physics Letters, Vol: 60, Pages: 2359-2361, ISSN: 0003-6951
The structural and morphological characteristics of visible-light-emitting porous Si layers produced by anodic and stain etching of single-crystal Si substrates are compared using transmission electron microscopy and atomic force microscopy (AFM). AFM of conventionally anodized, laterally anodized and stain-etched Si layers show that the layers have a fractal-type surface morphology. The anodized layers are rougher than the stain-etched films. At higher magnification 10 nm sized hillocks are visible on the surface. Transmission electron diffraction patterns indicate an amorphous structure with no evidence for the presence of crystalline Si in the near-surface regions of the porous Si layers.
Fathauer RW, George T, Jones EW, et al., 1992, New class of Si-based superlattices: Alternating layers of crystalline Si and porous amorphous Si<inf>1-x</inf>Ge<inf>x</inf> alloys, Applied Physics Letters, Vol: 61, Pages: 2350-2352, ISSN: 0003-6951
Superlattices consisting of alternating layers of crystalline Si and porous amorphous Si1-xGex have been fabricated. This is accomplished by first growing a Si/Si0.7Ge0.3 superlattice by molecular beam epitaxy, followed by Ar-ion milling to form mesa structures, and finally by immersion in HF:HNO3:H2O. This solution creates a porous structure similar to that created by anodic etching, and a high selectivity is observed for the conversion of the alloy layers relative to the Si layers. The degree of selectivity is found to depend on alloy layer thickness and strain. Superlattices have been fabricated from 1-μm wide mesas with Si0.7Ge0.3 layers fully converted to 5-nm thick porous amorphous material.
Pike WT, Brown LM, Kubiak RAA, et al., 1991, The determination of strain in Si-Ge superlattices by electron diffraction in a scanning transmission electron microscope, Journal of Crystal Growth, Vol: 111, Pages: 925-930, ISSN: 0022-0248
The nanometre scale of the novel strained layer electronic devices now being grown requires characterisation techniques of a corresponding resolution. This work employs the subnanometre probe of a dedicated scanning transmission electron microscope to investigate individual layers in a cross-sectioned SiGe superlattice. Using recently developed instrumentation, microdiffraction patterns have been obtained at very high resolution and the strains in each layer quantified by analysing the position of the deficit higher order Laue zone lines in the zero order beam. The experimental patterns are fitted to computer simulations incorporating possible dynamical effects. The results from a 10 nm SiGe layer are shown to be in good agreement with bulk X-ray diffraction analysis, with an accuracy limited only by the fundamental constraints of diffraction from a laterally finite sample. Hence surface relaxation, a major complication with previous applications of electron microscopy to strain measurement, can be ignored for the specimen geometry that the small probe allows. It is anticipated that the technique can in future be applied to multilayer structures which are not amenable to bulk characterisation. © 1991.
Pike WT, McClean J, Potential for Characterization of the Dust Threat for In-Situ Resource Utilization, 47th Lunar and Planetary Science Conference
Pike WT, Calcutt S, Standley I, et al., A Silicon Seismic Package (SSP) for Planetary Geophysics, 47th Lunar and Planetary Science Conference
Hecht M, McClean J, Pike WT, et al., MOXIE, ISRU, and the History of In Situ Studies of the Hazards of Dust in Human Exploration of Mars, Dust in the Atmosphere of Mars and Its Impact on Human Exploration Workshop
McClean J, Pike WT, Estimation of the Saltated Particle Flux at the Mars 2020 In-Situ Resource Utilization Experiment (MOXIE) Inlet, Dust in the Atmosphere of Mars and Its Impact on Human Exploration Workshop
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