T. Sajoto

Realization of a quasi‐three‐dimensional modulation‐doped semiconductor structure

We report the realization of a modulation‐doped quasi‐three‐dimensional electron system. The structure consists of a 2000‐A‐wide undoped AlxGa1−xAs well bounded by undoped (spacer) and doped layers of AlyGa1−yAs (y>x) on both sides. The alloy composition in the well (x) is varied quadratically so that the combined potentials due to the AlxGa1−xAs and the electric charge in the well produce a square potential well with a nearly uniform carrier density. Magnetotransport data reveal that the system contains ≂2.5×1011 cm−2 electrons, which occupy four electric subbands and have a low‐temperature mobility in excess of 1×105 cm2/V s indicating the high quality of the structure.

Effect of substrate temperature on migration of Si in planar‐doped GaAs

Quantum oscillations in the magnetoresistance of GaAs δ (or planar) doped with Si are analyzed to obtain the electron densities of the electric subbands. We compare these densities with the results of self‐consistently calculated subband structures of δ‐doped GaAs in which the spread of the dopant atoms (Si) in the growth direction is a fitting parameter. The results indicate that there is negligible spread in structures grown at a substrate temperature TS ≲530 °C, while in structures grown at higher TS there is measurable spread which increases with TS. For TS =640 °C, the Si spread is determined to be ≂220 A. An examination of the three‐dimensional Si densities in these layers indicates that the dominant mechanism for the spreading of Si for TS >600 °C is the migration of Si to satisfy the solid solubility limit.

Use of superlattices to realize inverted GaAs/AlGaAs heterojunctions with low‐temperature mobility of 2×106 cm2/V s

Reproducible realization of high quality inverted interfaces (GaAs on AlGaAs) grown by molecular beam epitaxy is reported. Effective use of thin‐layer GaAs/AlAs superlattices in place of an AlGaAs barrier was made to reduce the number of impurities and the roughness at these interfaces. The low‐temperature (≂4 K) mobility for electrons at these interfaces is as high as 2×106 cm2/V s for an electron density of ≂5×1011 cm−2—a factor of four improvement over the highest mobility reported for inverted interfaces.

Growth of low‐density two‐dimensional electron system with very high mobility by molecular beam epitaxy

We report on the growth of modulation‐doped GaAs/Alx Ga1−xAs heterostructures with mobilities (μ) on the order of 1×106 cm2 /V s (at 4.2 K) and areal densities (ns ) below 8×1010 cm−2 . In growing these structures we employ the atomic plane doping technique and ultrathick (>1000 A) spacer layers. The mobilities of these structures are the highest ever reported for low densities. Measurements of μ vs ns as a function of illumination or gate voltage indicate μ∼nαs behavior with α≂0.6 and, even for ns ≂1.4×1010 cm−2 , μ has a value in excess of 0.3×106 cm2 /V s.

High quality electron system with variable electron layer thickness in a parabolic quantum well

We report the realization of a high quality electron system with variable areal density (ns ) in a selectively doped, parabolic Ax Ga1−x As well. For each ns, quantum oscillations in the magnetoresistance are analyzed to obtain the electron densities of the electric subbands. These densities are in good agreement with the predictions of self‐consistent calculations of the subband structure. The data reveal that with increasing ns , the width of the electron system increases so that the effective three‐dimensional density and the Fermi energy remain essentially constant. The dependence of the low‐temperature electron mobility on ns is also reported.

Fractional quantum Hall effect in a high‐mobility GaAs/AlxGa1−xAs multiple quantum well heterostructure

We report the realization of a high mobility [μ≂4.8×105 cm2/(V s)] selectively doped GaAs/AlxGa1−xAs multiple quantum well structure with low density (ns ≂1.7×1011 cm−2 for each of the 85 wells) grown by molecular beam epitaxy. The activation energy for the fractional quantum Hall state at the Landau‐level filling factor ν=1/3 is Δ≂2 K, the highest value ever reported for any multiple quantum well structure. Such a structure is nearly ideal for studies of the thermal properties of the two‐dimensional electron system in the fractional quantum Hall regime.

Two‐dimensional electron system with extremely low disorder

We report on the growth of modulation‐doped GaAs/AlxGa1−x As heterostructures with extremely low disorder by molecular beam epitaxy. In growing these structures we employed the atomic plane doping technique and ultrathick (>1000 A) spacer layers with graded composition. The structures have mobilities (μ) on the order of 1×106 cm2/V s (at 4.2 K) for areal densities (ns) as low as 4×1010 cm−2. Quantum transport measurements in these structures exhibit new fractional quantum Hall states and demonstrate their exceptionally high quality.

Migration of Si IN δ-doped GaAs and AlxGa1 − x As: Effect of substrate temperature

Abstract We have investigated the effect of substrate temperature, T S , during growth by molecular beam epitaxy on the migration of Si atoms in δ- (or planar) doped GaAs and Al 0.25 Ga 0.75 As. Determinations of the extent of Si migration were made through analysis of quantum oscillations in the magnetoresistance and independently by using secondary ion mass spectroscopy (SIMS). Analysis of magnetotransport data in δ-doped GaAs samples indicates that there is negligible spread in structures grown at T S ⩽530 °C, while in structures grown at higher T S there is a measurable spread which increases with T S . For T S =640 °C, the Si spread is determ be = 180 A. A similar analysis performed on the magnetoresistance of δ-doped Al 0.25 Ga 0.75 As samples reveals that the Si spreads further in Al 0.25 Ga 0.75 As than in GaAs (for a comparable T S ). SIMS measurements performed on the same structures confirm these findings.

Observation of a giant dielectric constant in the re-entrant insulating phase of two-dimensional electrons

Abstract We have measured the radio frequency Reσxx and Imσxx of a high mobility two-dimensional electron system (2DES) and observed a giant dielectric constant (κxx > 1 × 104) in the reentrant insulating phase (RIP) around the 1 5 fractional quantum Hall liquid. Results from temperature and excitation-level studies provide new evidence for the Wigner crystal (WC) model of the RIP, where the WC is strongly pinned by residual impurities in close proximity to the 2DES.

Studies of the reentrant insulating phase around the 15 FQH liquid: I–V curves, noise, RF bias

We report on systematic studies of I–V and noise in the insulating phase around the 15 FQH liquid at B = 11.9 T. The noise and I–V data are correlated, and together they are consistent with a weakly pinned Wigner crystal for B below and immediately above the 15 FQH liquid. For B ⩾ 12.7 T, the system is in a more insulating regime, in which the noise vanishes, and which could be described as a Wigner glass. Preliminary results of a search for resonances or Shapiro steps in a 2DES are also presented.