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M. G. Borselli,
R. S. Ross,
A. A. Kiselev, E. T. Croke,
K. S. Holabird,
P. W. Deelman,
L. D. Warren,
I. Alvarado-Rodriguez,
I. Milosavljevic,
F. C. Ku,
W. S. Wong,
A. E. Schmitz,
M. Sokolich,
M. F. Gyure,
A. T. Hunter
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ABSTRACT: We have demonstrated few-electron quantum dots in Si/SiGe and InGaAs, with occupation number controllable from N = 0. These display a high degree of spatial symmetry and identifiable shell structure. Magnetospectroscopy measurements show that two Si-based devices possess a singlet N = 2 ground state at low magnetic field, and therefore, the twofold valley degeneracy is lifted. The valley splittings in these two devices were 270 and 120 μeV, suggesting the presence of atomically sharp interfaces in our heterostructures.
Applied Physics Letters 03/2011; 98(12):123118-123118-3. · 3.84 Impact Factor
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ABSTRACT: We report on the fabrication and characterization of a few-electron quantum dot controlled by a single gate electrode. Our device has a double-quantum-well design, in which the doping controls the occupancy of the lower well while the upper well remains empty under the free surface. A small air-bridged gate contacts the surface, and is positively biased to draw laterally confined electrons into the upper well. Electrons tunneling between this accumulation-mode dot and the lower well are detected using a quantum point contact (QPC), located slightly offset from the dot gate. The charge state of the dot is measured by monitoring the differential transconductance of the QPC near pinch-off. Addition spectra starting with N=0 were observed as a function of gate voltage. DC sensitivity to single electrons was determined to be as high as 8.6%, resulting in a signal-to-noise ratio of ~9:1 with an equivalent noise bandwidth of 12.1 kHz. Analysis of random telegraph signals associated with the zero to one electron transition allowed a measurement of the lifetimes for the filled and empty states of the one-electron dot: 0.38 ms and 0.22 ms, respectively, for a device with a 10 nm AlInAs tunnel barrier between the two wells. Comment: 3 pages, 3 figures
10/2009;
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ABSTRACT: The dependence of substitutional C fraction on growth temperature and
substrate orientation is measured for Si1-yCy alloy films grown on
(001) and (118) Si by molecular-beam epitaxy. Secondary ion mass
spectrometry and high-resolution x-ray diffraction were used to measure
the total C and the substitutional C concentrations, respectively, in
several samples prepared at temperatures between 450 and 650 degrees C.
The substitutional C fraction decreased rapidly with increasing
temperature in this range, regardless of orientation, and was slightly
lower for growth on (118) Si. Cross-sectional transmission electron
microscopy on (118)-oriented samples revealed a tendency for C to
concentrate periodically on (001) facets which formed immediately after
initiation of Si1-yCy growth. A kinetic Monte Carlo simulation based
upon enhanced diffusion of Si dimers in the presence of subsurface C
predicted a step instability leading to step bunching and the formation
of periodic surface features, as well as the accumulation of high C
concentrations on nearly (001) planes. (C) 2000 American Institute of
Physics. {[}S0003-6951(00)00831-7].
Applied Physics Letters 01/2000; 77(9):1310-1312. · 3.84 Impact Factor
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ABSTRACT: Realization of group IV heterostructure devices requires the accurate measurement of the energy band offsets in Si/Si <sub>1-x</sub> Ge <sub>x</sub> and Si/Si <sub>1-x-y</sub> Ge <sub>x</sub> C <sub>y</sub> heterojunctions. Using admittance spectroscopy, we have measured valence-band offsets in Si/Si <sub>1-x</sub> Ge <sub>x</sub> heterostructures and conduction-band and valence-band offsets in Si/Si <sub>1-x-y</sub> Ge <sub>x</sub> C <sub>y</sub> heterostructures grown by solid-source molecular-beam epitaxy. Measured Si/Si <sub>1-x</sub> Ge <sub>x</sub> valence-band offsets were in excellent agreement with previously reported values. For Si/Si <sub>1-x-y</sub> Ge <sub>x</sub> C <sub>y</sub> our measurements yielded a conduction-band offset of 100±11 meV for a n -type Si/Si <sub> 0.82 </sub> Ge <sub> 0.169 </sub> C <sub> 0.011 </sub> heterojunction and valence-band offsets of 118±12 meV for a p -type Si/Si <sub> 0.79 </sub> Ge <sub> 0.206 </sub> C <sub> 0.004 </sub> heterojunction and 223±20 meV for a p -type <font face=' ||
'roman'>Si/Si <sub> 0.595 </sub> Ge <sub> 0.394 </sub> C <sub> 0.011 </sub> heterojunction. Comparison of our measured band offsets with previously reported measurements of energy band gaps in Si <sub>1-x-y</sub> Ge <sub>x</sub> C <sub>y</sub> -
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and Si <sub>1-y</sub> C <sub>y</sub> alloy layers indicates that the band alignment is type I for the compositions we have studied and that our measured band offsets are in quantitative agreement with these previously reported results. © 1997 American Vacuum Society.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 08/1997; · 1.34 Impact Factor
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ABSTRACT: We have used admittance spectroscopy to measure conduction-band and valence-band offsets in Si/Si <sub>1-x</sub> Ge <sub>x</sub> and Si/Si <sub>1-x-y</sub> Ge <sub>x</sub> C <sub>y</sub> heterostructures grown by solid-source molecular-beam epitaxy. Valence-band offsets measured for Si/Si <sub>1-x</sub> Ge <sub>x</sub> heterojunctions were in excellent agreement with previously reported values. Incorporation of C into Si <sub>1-x-y</sub> Ge <sub>x</sub> C <sub>y</sub> lowers the valence- and conduction-band-edge energies compared to those in Si <sub>1-x</sub> Ge <sub>x</sub> with the same Ge concentration. Comparison of our measured band offsets with previously reported measurements of energy band gaps in Si <sub>1-x-y</sub> Ge <sub>x</sub> C <sub>y</sub> and Si <sub>1-y</sub> C <sub>y</sub> alloy layers indicate that the band alignment is Type I for the compositions we have studied and that our measured band offsets are in quantitative agreement with these previously reported results. © 1997 American Institute of Physics.
Applied Physics Letters 07/1997; · 3.84 Impact Factor
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ABSTRACT: Si/Si 0.97 C 0.03 superlattices grown on Si(001) substrates by Sb surfactant assisted molecular beam epitaxy are characterized by in situ reflection high energy electron diffraction (RHEED), atomic force microscopy, transmission electron microscopy (TEM), and high resolution x‐ray diffraction. The RHEED shows that, in the absence of Sb, the growth front roughens during Si 0.97 C 0.03 growth and smooths during subsequent Si growth. In contrast, when Sb is present, the growth front remains smooth throughout the growth. This observation is confirmed by cross‐sectional TEM, which reveals that for samples grown without the use of Sb, the Si/Si 0.97 C 0.03 interfaces (Si 0.97 C 0.03 on Si) are much more abrupt than the Si 0.97 C 0.03 /Si interfaces. In the case of Sb assisted growth, there is no observable difference in abruptness between the two types of interfaces. Atomic force microscopy micrographs of the Si 0.97 C 0.03 surface reveal features that could be the source of the roughness observed by RHEED and TEM. © 1996 American Vacuum Society
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 08/1996; · 1.34 Impact Factor
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ABSTRACT: We present evidence supporting the formation of a new, (2×8) surface reconstruction on Si 1-x Ge x alloys grown on (100) Si substrates by molecular‐beam epitaxy. Surfaces of Si 1-x Ge x alloys were studied using reflection high‐energy electron diffraction (RHEED) and low‐energy electron diffraction (LEED) techniques. RHEED patterns from samples with Ge concentrations, x, falling within the range 0.10–0.30 and grown at temperatures between 350 and 550 °C, exhibit n/8 fractional‐order diffraction streaks in addition to the normal (2×1) pattern seen on (100) Si. The presence of fractional‐order diffracted beams is indicative of an eight‐fold‐periodic modulation in electron scattering factor across the alloy surface. LEED patterns from surfaces of samples grown under similar conditions are entirely consistent with these results. In addition, the LEED patterns support the conclusion that the modulation is occurring in the direction of the dimer chains of a (2×1) reconstruction. We have examined the thermal stability of the (2×8) reconstruction and have found that it reverts to (2×1) after annealing to 700 °C and reappears after the sample temperature is allowed to cool below 600 °C. Such behavior suggests that the reconstruction is a stable, ordered phase for which the pair‐correlation function of surface Ge atoms exhibits an eightfold periodicity in the ‘‘1’’ direction of a Si‐like (2×1) reconstruction. We also present a simulation in the kinematic approximation, confirming the validity of our interpretation of these findings.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 08/1991; · 1.34 Impact Factor
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ABSTRACT: We have used x‐ray photoelectron spectroscopy to measure the valence band offset in situ for strained Si/Ge (100) heterojunctions and for AlSb/ZnTe (100) heterojunctions grown by molecular‐beam epitaxy. For the Si/Ge system, Si 2p and Ge 3d core level to valence band edge binding energies and Si 2p to Ge 3d core level energy separations were measured as functions of strain, and strain configurations in all samples were determined using x‐ray diffraction. Our measurements yield valence band offset values of 0.83±0.11 eV and 0.22±0.13 eV for Ge on Si (100) and Si on Ge (100), respectively. If we assume that the offset between the weighted averages of the light‐hole, heavy‐hole, and spin‐orbit valence bands in Si and Ge is independent of strain, we obtain a discontinuity in the average valence band edge of 0.49±0.13 eV. For the AlSb/ZnTe (100) heterojunction system, we obtain a value of -0.42±0.07 eV for the valence band offset. Our data also suggest that an intermediate compound, containing Al and Te, is formed at the AlSb/ZnTe (100) interface.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 08/1990; · 1.34 Impact Factor
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ABSTRACT: We have used x‐ray photoelectron spectroscopy to measure the valence‐band offset in situ for strained Si/Ge (100) heterojunctions grown by molecular beam epitaxy. Si 2p and Ge 3d core level to valence‐band‐edge binding energies and Si 2p to Ge 3d core level energy separations were measured as functions of strain, and strain configurations in all samples were determined using x‐ray diffraction. Our measurements yield valence‐band offset values of 0.83±0.11 eV and 0.22±0.13 eV for Ge on Si (100) and Si on Ge (100), respectively. If we assume that the offset between the weighted averages of the light hole, heavy hole, and spin‐orbit valence bands in Si and Ge is independent of strain, we obtain a discontinuity in the average valence‐band edge of 0.49±0.13 eV.
Applied Physics Letters 03/1990; · 3.84 Impact Factor
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ABSTRACT: High quality, coherently strained Si 1-x Ge x alloy layers are studied using high‐resolution x‐ray diffraction (HRXRD) and ex situ transmission electron diffraction. Several samples were grown at extremely low temperatures (310–330 °C) by molecular beam epitaxy. Sample thicknesses and alloy concentrations were chosen to span a range beginning just below to significantly above critical thicknesses previously reported for this system. HRXRD observations demonstrate a high degree of coherency in the as‐grown structures since measurements of the lattice constant parallel to the sample surface (a ‖ ) consistently yield the value for the (100)Si substrate. HRXRD from (004) planes used to measure a ⊥ typically yield a spectrum with several peaks for growths in excess of the critical thickness and single peaks for those below the critical thickness. The high degree of coherency observed in these samples suggests that chemical segregation is responsible for the observed x‐ray peaks.
Applied Physics Letters 02/1990; · 3.84 Impact Factor
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ABSTRACT: Strain relaxation in Si 1-x Ge x /Si superlattices and alloy films is studied as a function of ex situ anneal treatment with the use of x‐ray diffraction and Raman spectroscopy. Samples are grown by molecular‐beam epitaxy at an unusually low temperature (≊365 °C). This results in metastably strained alloy and superlattice films significantly in excess of critical thicknesses previously reported for such structures. Significant strain relaxation is observed upon anneal at temperatures as low as 390 °C. After a 700 °C, 2 h anneal, superlattices are observed to relax less fully (∼43% of coherent strain) than corresponding alloys (∼84% of coherent strain). Also, the strain relaxation kinetics of a Si 1-x Ge x alloy layer is studied quantitatively. Alloy strain relaxation is approximately described by a single, thermally activated, first order kinetic process having activation energy E a =2.0 eV. The relevance of our results to the microscopic mechanisms responsible for strain relaxation in lattice‐mismatched semiconductor heterostructures is discussed.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 08/1989; · 1.34 Impact Factor
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ABSTRACT: A study of the growth parameters governing the nucleation of metastable superconducting A15 V3Si on Si and A12O3 is presented. Nominally, 500Å films of V1-xSix were produced through codeposition of V and Si onto heated (111) Si and (1102) A12O3 substrates. Samples were prepared in a custom-built ultrahigh vacuum (UHV) chamber containing dual e-beam evaporation sources
and a high temperature substrate heater. V and Si fluxes were adjusted to result in the desired average film composition.
V0.75Si0.25 films prepared at temperatures in excess of 550° C on Si show significant reaction with the substrate and are nonsuperconducting
while similar films grown on A12O3 exhibit superconducting transition temperatures(@#@ Tc
@#@) approaching bulk values for V3Si (16.6-17.1 K). Codeposition at temperatures between 350 and 550° C results in superconducting films on Si substrates while
growth at lower temperatures results in nonsuperconducting films. Lowering the growth temperature to 400° C has been shown
throughex situ transmission electron microscopy (TEM) and Auger compositional profiling to minimize the reaction with the Si substrate while
still activating the surface migration processes needed to nucleate A15 V3Si. Variation of film composition aboutx = 0.25 is shown to result in nonsuperconducting films for highx and superconducting films with Tc approaching the bulk V value (5.4 K) for lowx. Finally, lowering the V0.75Si0.25 deposition rate is shown to raise Tc.
Journal of Electronic Materials 04/1989; 18(6):757-761. · 1.47 Impact Factor
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ABSTRACT: We have used admittance spectroscopy and deep-level transient spectroscopy to characterize electronic properties of Si/Si1–x–yGexCy heterostructures. Band offsets measured by admittance spectroscopy for compressively strained Si/Si1–x–yGexCy heterojunctions indicate that incorporation of C into Si1–x–yGexCy lowers both the valence- and conduction-band edges compared to those in Si1–xGex by an average of 107 ± 6 meV/% C and 75 ± 6 meV/% C, respectively. Combining these measurements indicates that the band alignment is type I for the compositions we have studied, and that these results are consistent with previously reported results on the energy band gap of Si1–x–yGexCy and with measurements of conduction band offsets in Si/Si1–yCy heterojunctions. Several electron traps were observed using deep-level transient spectroscopy on two n-type heterostructures. Despite the presence of a significant amount of nonsubstitutional C (0.29–1.6 at. %), none of the peaks appear attributable to previously reported interstitial C levels. Possible sources for these levels are discussed.
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ABSTRACT: We have used admittance spectroscopy to measure conduction-band and valence-band offsets in Si/Si1–xGex and Si/Si1–x–yGexCy heterostructures grown by solid-source molecular-beam epitaxy. Valence-band offsets measured for Si/Si1–xGex heterojunctions were in excellent agreement with previously reported values. Incorporation of C into Si1–x–yGexCy lowers the valence- and conduction-band-edge energies compared to those in Si1–xGex with the same Ge concentration. Comparison of our measured band offsets with previously reported measurements of energy band gaps in Si1–x–yGexCy and Si1–yCy alloy layers indicate that the band alignment is Type I for the compositions we have studied and that our measured band offsets are in quantitative agreement with these previously reported results.
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ABSTRACT: Si1–x–yGexCy/Si superlattices were grown by solid-source molecular beam epitaxy using silicon carbide as a source of C. Samples consisting of alternating layers of nominally 25 nm Si1–x–yGexCy and 35 nm Si for 10 periods were characterized by high-resolution x-ray diffraction, transmission electron microscopy (TEM), and Rutherford backscattering spectrometry to determine strain, thickness, and composition. C resonance backscattering and secondary ion mass spectrometries were used to measure the total C concentration in the Si1–x–yGexCy layers, allowing for an accurate determination of the substitutional C fraction to be made as a function of growth rate for fixed Ge and substitutional C compositions. For C concentrations close to 1%, high-quality layers were obtained without the use of Sb-surfactant mediation. These samples were found to be structurally perfect to a level consistent with cross-sectional TEM (< 10^7 defects/cm^2) and showed considerably improved homogeneity as compared with similar structures grown using graphite as the source for C. For higher Ge and C concentrations, Sb-surfactant mediation was found to be required to stabilize the surface morphology. The maximum value of substitutional C concentration, above which excessive generation of stacking fault defects caused polycrystalline and/or amorphous growth, was found to be approximately 2.4% in samples containing between 25 and 30% Ge. The fraction of substitutional C was found to decrease from roughly 60% by a factor of 0.86 as the Si1–x–yGexCy growth rate increased from 0.1 to 1.0 nm/s.
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ABSTRACT: Using electron beam evaporation, a Si/CeO2/Si(111) structure has been grown in a molecular beam epitaxy machine. In situ low energy electron diffraction, cross sectional transmission electron microscopy, selected area diffraction, and atomic force microscopy have been used to structurally characterize the overlying silicon layer and show it to be single crystalline and epitaxially oriented. Rutherford backscattering and energy dispersive x-ray analysis have been used to confirm the presence of a continuous 23 Å CeO2 layer at the interface. Rutherford backscattering and x-ray photoemission spectroscopy show an additional presence of cerium both at the exposed silicon surface and incorporated in low levels (~ 1%) within the silicon film, suggesting a growth mechanism with cerium riding atop the silicon growth front leaving behind small amounts of cerium incorporated in the growing silicon crystal.
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ABSTRACT: Si/Si0.97C0.03 superlattices were grown on Si(001) substrates by molecular beam epitaxy (MBE) to study the use of Sb as a surfactant during Si1–yCy growth. In situ reflection high energy electron diffraction (RHEED) shows that while carbon easily disrupts the two-dimensional growth of homoepitaxial Si, such disruption is suppressed for layers grown on Sb-terminated Si(001) surfaces. Cross-sectional transmission electron microscopy (TEM) reveals that for samples grown without the use of Sb, the Si/Si0.97C0.03 interfaces (Si0.97C0.03 on Si) were much more abrupt than Si0.97C0.03/Si interfaces. In the case of Sb-mediated growth, differences in abruptness between the two types of interfaces were not readily observable.
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ABSTRACT: Deep-level transient spectroscopy was used to measure the activation energies of deep levels in n-type Si/Si1–x–yGexCy heterostructures grown by solid-source molecular-beam epitaxy. Four deep levels have been observed at various activation energies ranging from 231 to 405 meV below the conduction band. The largest deep-level concentration observed was in the deepest level and was found to be approximately 2 × 10^15 cm^–3. Although a large amount of nonsubstitutional C was present in the alloy layers (1–2 at. %), no deep levels were observed at any energy levels that, to the best of our knowledge, have been previously attributed to interstitial C.
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ABSTRACT: Relaxation of coherent strain in Si1−xGex/Si heterostructures is studied quantitatively as a function of ex situ anneal procedures with the use of X-ray diffraction (XRD) and Raman scattering techniques. Highly strained Si1−xGex alloy films and superlattices of selected film thickness and composition have been produced through molecular beam epitaxial (MBE) growth at unusually low growth temperature (310–365 °C). Low MBE growth temperature leads to metastable, coherently strained films which subsequently relax substantially upon annealing at temperatures as low as 370 °C. Strain relaxation is observed to differ between superlattices and alloy films of equivalent average composition and overall film thickness. Superlattices are observed to relax only approximately 43% of their coherent strain, whereas the corresponding alloys relax approximately 84%. Thermal activation of strain relaxation is studied as a function of ex situ annealing time and temperature. The observed strain relaxation is described by a first-order kinetic process which has an activation energy near 2 eV. This value is close to the activation energy associated with dislocation glide in silicon.
Thin Solid Films.
