Frank Fu Fang

Observation of the fractional quantum Hall effect in Si/SiGe heterostructures

We have observed the fractional quantum Hall effect in the magnetotransport properties of a two‐dimensional electron gas in an n‐type Si/SiGe heterostructure. The effect was observed for filling factors ν=2/3 and ν=4/3 in samples whose mobilities at 1.4 K ranged from 37 000 to 85 000 cm2/V s.

Landau-level broadening and scattering time in modulation doped GaAs/AlGaAs heterostructures

Abstract Single particle relaxation time and the scattering time of the 2DEG in modulation doped GaAs/AlGaAs heterostructure are measured by Ladau respectively, for samples with spacer layer thickness 1.5–500 nm. The results are compared with the recent calculations of Das Sarma and Stern and excellent agreement is obtained. When the excited subband is occupied under persistent photoconductive (PPC) conditions, the relaxation time for the excited subband was found to be nearly three times longer than that for the ground subbands. The results are interpreted in terms of screened remote impurity scattering.

Two-dimensional hole gas in Si/SiGe heterostructures

Abstract Transport measurements of p-type modulation doped pseudomorphic heterostructures show that the interfaces of Si/SiGe in either growth direction have the same high quality. Magneto-transport results indicate that the degeneracy of the heavy and light hole bands at the zone center is lifted. SdH spectra show anomalous resolution of filling factors resulting from the interplay of spin splittings of the adjacent Landau levels anisotropy of the g -factor found for these structures. With a novel stress relieved superlattice substrate, the g -factor anisotropy was found to be greatly reduced.

A low-temperature insulating phase at for 2D holes in high-mobility heterostructures with Landau level degeneracy

Magneto-transport measurements of the 2D hole system (2DHS) in p-type Si-Si1-xGex heterostructures identify the integer quantum Hall effect (IQHE) at dominantly odd-integer filling factors v and two low-temperature insulating phases (IPs) at v = 1.5 and v less than or similar to 0.5, with re-entrance to the quantum Hall effect at v = 1. The temperature dependence, current-voltage characteristics, and tilted field and illumination responses of the IP at v = 1.5 indicate that the important physics is associated with an energy degeneracy of adjacent Landau levels of opposite spin, which provides a basis for consideration of an intrinsic, many-body origin.

Magnetotransport and cyclotron resonance in AlAs quantum wells

We have made the first measurements of cyclotron resonance and the Shubnikov-de Haas effect in two-dimensional electronic system confined to AlAs. Cyclotron resonance data show that the effective mass for (100)-oriented samples (0.5m0) is larger than for (111)-oriented samples (0.41m0). This indicates that the X point ellipsoids with their long axes parallel to the interface are occupied. This may be due to compressive stress in the AlAs. From the temperature and magnetic field dependence of the Shubnikov-de Haas oscillations we have also determined the effective mass (m(111)∗ = 0.49m0; m(100)∗ = 0.60m0) and the effective g-factor (g∗ = 3.1−3.4). We find fairly good agreement between measured and calculated values.

Magnetic-field-induced transitions in InAs/Ga1−xAlxSb heterostructures

Abstract We have studied the two-dimensional properties of high-mobility electron-hole systems in InAs quantum wells bounded by Ga 1− x Al x Sb. By changing the alloy composition x , we are able to vary the system from quasi-semimetallic (with unequal electron and hole carrier concentrations) to semiconducting (with only electrons). Furthermore, we have observed a magnetic-field-induced semimetal-to-semi-conductor transition which manifests itself in the magnetotransport properties of samples with both carriers. This transition can be understood by considering the continuity of the Fermi energy across the InAs/Ga 1− x Al x Sb interface and the interdependence of the electron and hole densities.

Effect of biaxial stress on Si(100) inversion layers

Biaxial compressive stress was produced on (100)Si MOS structures by differential thermal expansion coefficients between Si and epoxy. It was shown that the heavy mass band was shifted to the lower energy than the light mass band. However, the expected valley degeneracy of 4 was not observed. The SdH spectrum in this system shows phase reversal at about 4−5 × 1012 cm−2 due to exchange interaction enhanced spin splitting. It is shown that under certain conditions, transition between heavy to light mass bands and vice versa, can be observed in a single sample and the former appears to be nearly first order.

2DEG in strained Si/SiGe heterostructures

Abstract Low-temperature magnetotransport properties for the recent n-type modulation-doped Si/SiGe heterostructures are reported. The biaxially tensile-stressed (100)Si surface layers on stress-relieved SiGe substrates are shown to have all the well known MOSFET characteristics but with more than an order of magnitude higher mobility. The four-fold degeneracy of the ground subband can be completely resolved at a magnetic field as low as 2 T. FQHE at 2 3 and 4 3 are clearly discerned and the activation energy at the plateaus is about 1 K at 10 T. The ratios between scattering time and single-particle relaxation time are 4–10 for a great number of samples examined. Low-temperature mobility appears to be capped below 2 × 10 5 cm 2 V −1 s −1 at present, as reported from several laboratories. Comparison of data and model calculations of Stern and Laux for the temperature-dependent mobility are made. Fixed charge and surface roughness appear to be the limiting parameters for the samples currently examined.

Effect of subband splitting on Si inversion layers

The subband structure of (100)Si inversion layers can be altered by either uniaxial stress or substrate bias due to deformation potential and quantizing surface field, respectively. The reduced splitting between E0 and E0 caused by compressive stress is shown to be exactly compensated by the additional surface field via substrate bias at high compressive stress. At low compressive stress (≲ 109 dyn cm−2) and low carrier concentration (< 1012 cm 2) we observe a marked decrease of electron mobility at 4.2 K. The decrease in mobility can be recovered by suitable negative substrate bias. We attribute these results to the influence of the density of state tails of E0 subband on the free electrons in the E0 subband. At high stress, E0 and E0 cross. The measured Ef crossing point from E0 to mixed subband at various stresses and depletion charges agrees well with Takada and Andos calculation. The results for [001] and [011] stresses indicate further splitting of the E0 subband for the former. For (111) surfaces, the mobility increases with either eompressive or tensile stress in [011] direction. From the point of view of the influence of tail states of the higher subbands, this even function dependence of μ(P) seems to support 6-valley degeneracy in the absence of sttess.

On the effective mass and collision time of (100) Si surface electrons

Abstract We have measured the apparent mass of (100) Si surface electrons by thermal broadening of SdH oscillations of inversion and accumulation layers for a wide range of substrate doping level, geometry, interface charge density, magnetic field strength and substrate bias. It is shown that the apparent mass values determined in this way are very sensitive to these parameters and others which are not yet understood. Collision broadening of the SdH oscillations has been studied in terms of scattering time τ and compared with zero field mobility as well as average magnetoconductance relaxation times. Fair agreements were obtained between these quantities.