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The effective mass at the Fermi level is measured in the strongly interacting two-dimensional (2D) electron system in ultra-clean SiGe/Si/SiGe quantum wells in the low-temperature limit in tilted magnetic fields. At low electron densities, the effective mass is found to be strongly enhanced and independent of the degree of spin polarization, which indicates that the mass enhancement is not related to the electrons' spins. The observed effect turns out to be universal for silicon-based 2D electron systems, regardless of random potential, and cannot be explained by existing theories. [ABSTRACT FROM AUTHOR]

We find an unusual anisotropy of the inplane field magnetoresistance with higher resistance in the parallel orientation of the field, Bǁ, and current, I, in ultra-high mobility SiGe/Si/SiGe quantum wells. The anisotropy depends on the orientation between the inplane field and the current relative to the crystallographic axes of the sample, and is a consequence of the ridges on the quantum well surface. For the parallel or perpendicular orientations between current and ridges, a method of converting the magnetoresistance measured at I⊥Bǁ into the one measured at I ǁ Bǁ is suggested and is shown to yield results that agree with the experiment. [ABSTRACT FROM AUTHOR]

The increase in the resistivity with decreasing temperature followed by a drop by more than one order of magnitude is observed on the metallic side near the zero-magnetic-field metal-insulator transition in a strongly interacting two-dimensional electron system in ultra-clean SiGe/Si/SiGe quantum wells. We find that the temperature T max , at which the resistivity exhibits a maximum, is close to the renormalized Fermi temperature. However, rather than increasing along with the Fermi temperature, the value T max decreases appreciably for spinless electrons in spin-polarizing (parallel) magnetic fields. The observed behaviour of T max cannot be described by existing theories. The results indicate the spin-related origin of the effect. [ABSTRACT FROM AUTHOR]

We report low-temperature magneto-transport measurements of an undoped Si/SiGe asymmetric double quantum well heterostructure. The density in both layers is tuned independently utilizing top and bottom gates, allowing the investigation of quantum wells at both imbalanced and matched densities. Integer quantum Hall states at total filling factor ν T = 1 and ν T = 2 are observed in both density regimes, and the evolution of their excitation gaps is reported as a function of the density. The ν T = 1 gap evolution departs from the behavior generally observed for valley splitting in the single layer regime. Furthermore, by comparing the ν T = 2 gap to the single particle tunneling energy, Δ SAS , obtained from Schrödinger–Poisson (SP) simulations, evidence for the onset of spontaneous interlayer coherence is observed for a relative filling fraction imbalance smaller than ∼ 50 %. [ABSTRACT FROM AUTHOR]

The broadband absorption of metal-oxide-semiconductor SiGe/Si quantum dot infrared photodetectors is demonstrated using boron δ doping in the Si spacer. The peak at 3.7–6 μm results from the intersubband transition in the SiGe quantum dot layers. The other peak at 6–16 μm mainly comes from the intraband transition in the boron δ-doping wells in the Si spacers. Since the atmospheric transmission windows are located at 3–5.3 and 7.5–14 μm, broadband detection is feasible using this device. The δ doping in SiGe quantum dots and Si0.9Ge0.1 quantum wells is also investigated to identify the origin of the absorption. [ABSTRACT FROM AUTHOR]

The back-channel-etch amorphous InGaZnO (a-IGZO) double-layer thin-film transistor (DL-TFT) consists of an a-IGZO layer with no oxygen flow (NOF) as a top layer and an a-IGZO layer with oxygen flow (OF) as a bottom layer. The DL-TFT demonstrates the field-effect mobility of 19 cm2/V–s, which is $1.6\times $ of the NOF and the OF single-layer TFTs (SL-TFTs) at the overdrive voltage of 18 V and the drain voltage of 0.1 V. Several measurements are performed to obtain the band alignment between the NOF and the OF layer for further understandings of the mechanism of the improvement in the field-effect mobility. Tauc method, X-ray photoelectron spectroscopy, and Kelvin probe force microscopy measurements were used to extract the optical bandgaps, the Fermi level position, and the work function of the NOF and the OF, respectively. The conduction band difference of 0.28 eV between the NOF and the OF layer is obtained. The carriers in the DL-TFT are confined in the NOF layer by quantum confinement, where the OF layer serves as the barrier to reduce the Coulomb scattering and the surface roughness scattering. [ABSTRACT FROM AUTHOR]

The recovery of light-induced degradation of the tandem micromorph solar cell by applying reverse bias is compared with the single-junction amorphous silicon solar cell. The illuminated current density-voltage characteristics and external quantum efficiency show that the degradation of both the micromorph and the amorphous silicon cells can be recovered by applying sufficient reverse bias. The micromorph cell was recovered at smaller reverse bias than amorphous silicon cell. The abundant H in the microcrystalline silicon bottom cell of the micromorph cell can act as a reservoir to repair the defects in the amorphous silicon top cell at the reverse bias. This is responsible for small recovery bias of tandem cells. [ABSTRACT FROM AUTHOR]

We have experimentally studied the fractional quantum Hall effect in SiGe/Si/SiGe quantum wells in relatively weak magnetic fields, where the Coulomb interaction between electrons exceeds the cyclotron splitting by a factor of a few XX. Minima of the longitudinal resistance have been observed corresponding to the quantum Hall effect of composite fermions with quantum numbers p = 1, 2, 3, and 4. Minima with p = 3 disappear in magnetic fields below 7 T, which may be a consequence of the intersection or even merging of the quantum levels of the composite fermions with different orientations of the pseudo-spin, i.e., those belonging to different valleys. We have also observed minima of the longitudinal resistance at filling factors ν = 4/5 and 4/11, which may be due to the formation of the second generation of the composite fermions. [ABSTRACT FROM AUTHOR]

We report the measurements of the effective g factor of low-density two-dimensional holes in a Ge quantum well. Using the temperature dependence of the Shubnikov-de Haas oscillations, we extract the effective g factor in a magnetic field perpendicular to the sample surface. Very large values of the effective g factor, ranging from ~13 to ~28, are observed in the density range of 1:4 × 1010 cm-2- 1:4 × 1011 cm-2. When the magnetic field is oriented parallel to the sample surface, the effective g factor is obtained from a protrusion in the magneto-resistance data that signify full spin polarization. In the latter orientation, a small effective g factor, ~1:3 - 1:4, is measured in the density range of 1:5 × 1010 cm-2-2 × 1010 cm-2. This very strong anisotropy is consistent with theoretical predictions and previous measurements in other 2D hole systems, such as InGaAs and GaSb. [ABSTRACT FROM AUTHOR]

Ge/strained GeSn/Ge quantum wells are grown on a 300mm Si substrate by chemical vapor deposition. The direct bandgap emission from strained GeSn is observed in the photoluminescence spectra and is enhanced by Al2O3/SiO2 passivation due to the field effect. The electroluminescence of the direct bandgap emission of strained GeSn is also observed from the Ni/Al2O3/GeSn metalinsulator-semiconductor tunneling diodes. Electroluminescence is a good indicator of GeSn material quality, since defects in GeSn layers degrade the electroluminescence intensity significantly. At the accumulation bias, the holes in the Ni gate electrode tunnel to the strained n-type GeSn layer through the ultrathin Al2O3 and recombine radiatively with electrons. The emission wavelength of photoluminescence and electroluminescence can be tuned by the Sn content. [ABSTRACT FROM AUTHOR]

We report the magneto-transport study and scattering mechanism analysis of a series of increasingly shallow Si/SiGe quantum wells with depth ranging from ~ 100 nm to ~ 10 nm away from the heterostructure surface. The peak mobility increases with depth, suggesting that charge centers near the oxide/semiconductor interface are the dominant scattering source. The power-law exponent of the electron mobility versus density curve, μ nα, is extracted as a function of the depth of the Si quantum well. At intermediate densities, the power-law dependence is characterized by α ~ 2.3. At the highest achievable densities in the quantum wells buried at intermediate depth, an exponent α ~ 5 is observed. We propose and show by simulations that this increase in the mobility dependence on the density can be explained by a non-equilibrium model where trapped electrons smooth out the potential landscape seen by the twodimensional electron gas. [ABSTRACT FROM AUTHOR]

We report the observation of an electron gas in a SiGe/Si/SiGe quantum well with maximum mobility up to 240 m²/Vs, which is noticeably higher than previously reported results in siliconbased structures. Using SiO, rather than Al2O3, as an insulator, we obtain strongly reduced threshold voltages close to zero. In addition to the predominantly small-angle scattering well known in the high-mobility heterostructures, the observed linear temperature dependence of the conductivity reveals the presence of a short-range random potential. [ABSTRACT FROM AUTHOR]

We report the design, the fabrication, and the magneto-transport study of an electron bilayer system embedded in an undoped Si/SiGe double-quantum-well heterostructure. Combined Hall densities (nHall) ranging from 2.6 × 1010 cm-2 to 2.7 × 1011 cm-2 were achieved, yielding a maximal combined Hall mobility (μHall) of 7.7 × 105 cm²/(V · s) at the highest density. Simultaneous electron population of both quantum wells is clearly observed through a Hall mobility drop as the Hall density is increased to nHall > 3.3 × 1010 cm-2, consistent with Schrödinger-Poisson simulations. The integer and fractional quantum Hall effects are observed in the device, and single-layer behavior is observed when both layers have comparable densities, either due to spontaneous interlayer coherence or to the symmetric-antisymmetric gap. [ABSTRACT FROM AUTHOR]

The interaction between the quantum dot (QD) level and the 2D continuum of a quantum well (QW) is investigated theoretically. Due to interference from these states, the Fano-like resonance states appear in the 2D continuum of the QW. Also, the free-electron energy band of the QW is changed because of QD potential influence. We have calculated the position and the broadening of Fano resonances, and also modified the band structure of free states in the QW. The obtained results are used further for research on tunneling from the QW to the QD. [ABSTRACT FROM AUTHOR]

In this paper, we present our study of the maximum electron density, nmax, accessible via low-temperature transport experiments in enhancement-mode Si/Si1-xGex heterostructure field-effect transistors. Experimentally, we find that nmax is much higher than the value obtained from self-consistent Schrödinger-Poisson simulations and that nmax can be changed only by changing the Ge concentration in the Si1-xGex barrier layer, not by varying the barrier layer thickness. The discrepancy between experiments and simulations is explained by a non-thermal-equilibrium tunneling-limited model. [ABSTRACT FROM AUTHOR]

We report the observation of a two-dimensional electron gas (2DEG) in a Si quantum well with mobility 1.6×106 cm2/V s at carrier densities n≥1.5×1011/cm2. The 2DEG, which resides in an undoped Si/SiGe heterostructure, is capacitively induced using an insulated-gate field-effect transistor (IGFET) device structure; its mobility is determined from transport and quantum Hall effect measurements at 0.3 K. Our IGFET device makes it now possible to access by transport experiments the low electron density regime down to n∼1×1010/cm2. [ABSTRACT FROM AUTHOR]

The ultrathin strained Si0.2Ge0.8 quantum well channel (∼5 nm) directly grown on Si substrates is demonstrated with low defect density and high hole mobility. The quantum well Si0.2Ge0.8 channel reveals an ∼3.2 times hole current enhancement and an ∼3 times hole mobility enhancement as compared with the bulk Si channel. The output current-voltage characteristics under the external mechanical strain confirm the compressive strain in the channel. The external compressive strain further enhances the hole mobility in a Si0.2Ge0.8 channel. [ABSTRACT FROM AUTHOR]