Fabrication and Characterization of Electrostatic Quantum Dots in a Si/SiGe 2D Electron Gas, Including an Integrated Read-out Channel

A new fabrication technique is used to produce quantum dots with read-out channels in silicon/silicon-germanium two-dimensional electron gases. The technique utilizes Schottky gates, placed on the sides of a shallow etched quantum dot, to control the electronic transport process. An adjacent quantum point contact gate is integrated to the side gates to define a read-out channel and thus allow for noninvasive detection of the electronic occupation of the quantum dot. Reproducible and stable Coulomb oscillations and the corresponding jumps in the read-out channel resistance are observed at low temperatures. The fabricated dot combined with the read-out channel represent a step towards the spin-based quantum bit in Si/SiGe heterostructures.Comment: 3 pages, 4 fig

Sputtered Gold as an Effective Schottky Gate for Strained Si/SiGe Nanostructures

Metallization of Schottky surface gates by sputtering Au on strained Si/SiGe heterojunctions enables the depletion of the two dimensional electron gas (2DEG) at a relatively small voltage while maintaining an extremely low level of leakage current. A fabrication process has been developed to enable the formation of sub-micron Au electrodes sputtered onto Si/SiGe without the need of a wetting layer.Comment: 3 pages, 3 figure

Observation of a (2X8) surface reconstruction on Si_(1-x)Ge_x alloys grown on (100) Si by molecular beam epitaxy

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 finding

Band offsets in Si/Si1–x–yGexCy heterojunctions measured by admittance spectroscopy

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

Measurement of band offsets in Si/Si1–xGex and Si/Si1–x–yGexCy heterojunctions

Realization of group IV heterostructure devices requires the accurate measurement of the energy band offsets in Si/Si1–xGex and Si/Si1–x–yGexCy heterojunctions. Using admittance spectroscopy, we have measured valence-band offsets in Si/Si1–xGex heterostructures and conduction-band and valence-band offsets in Si/Si1–x–yGexCy heterostructures grown by solid-source molecular-beam epitaxy. Measured Si/Si1–xGex valence-band offsets were in excellent agreement with previously reported values. For Si/Si1–x–yGexCy our measurements yielded a conduction-band offset of 100 ± 11 meV for a n-type Si/Si0.82Ge0.169C0.011 heterojunction and valence-band offsets of 118 ± 12 meV for a p-type Si/Si0.79Ge0.206C0.004 heterojunction and 223 ± 20 meV for a p-type Si/Si0.595Ge0.394C0.011 heterojunction. Comparison of our measured band offsets with previously reported measurements of energy band gaps in Si1–x–yGexCy and Si1–yCy 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

Electronic properties of Si/Si1–x–yGexCy heterojunctions

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

Characterization of Si/Si_(1-y)C_y superlattices grown by surfactant assisted molecular beam epitaxy

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

Epitaxial silicon grown on CeO2/Si(111) structure by molecular beam epitaxy

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

Deep-level transient spectroscopy of Si/Si1–x–yGexCy heterostructures

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