N.E. Harff

Terahertz photoconductivity and plasmon modes in double-quantum-well field-effect transistors

Double-quantum-well field-effect transistors with a grating gate exhibit a sharply resonant, voltage tuned terahertz photoconductivity. The voltage tuned resonance is determined by the plasma oscillations of the composite structure. The resonant photoconductivity requires a double-quantum well but the mechanism whereby plasma oscillations produce changes in device conductance is not understood. The phenomenon is potentially important for fast, tunable terahertz detectors.

2×106 cm2/V s electron mobility by metalorganic chemical vapor deposition with tertiarybutylarsine

We demonstrate the metalorganic chemical vapor deposition (MOCVD) growth of two‐dimensional electron gases (2DEGs) with electron mobilities up to 2.0×106 cm2/V s at 0.3 K. These are the highest mobilities to date for MOCVD materials, and were achieved using a safer replacement precursor for arsine, tertiarybutylarsine (TBA). For structures grown using arsine, we obtained a maximum mobility of 1.0×106 cm2/V s, which although comparable to the best by MOCVD to date, is half that obtained using TBA. Our studies on thick GaAs and AlGaAs layers indicate that the use of TBA in place of arsine reduces both the carbon and donor impurity concentrations. Thus, TBA is not only a safe alternative to arsine, but also produces significantly purer films.

Parallel quantum point contacts fabricated with independently biased gates and a submicrometer airbridge post

Using an integrated airbridge and submicrometer gate post technology, coupled quantum point contacts (QPCs) arranged in a parallel configuration were fabricated. The airbridge and gate post are fabricated by e‐beam lithography and Ti/Au evaporation in a single step. Gate post diameters as small as 0.1 μm have been achieved. The two QPCs are fabricated with two conventional gates and a central airbridged gate, each of which can be biased independently. Conductance measurements clearly exhibit coupling of the two QPCs, as the quantized conductance steps are in units of 4 e2/h. Independent measurements of each QPC show conductance steps in units of 2 e2/h.

High quality single and double two‐dimensional electron gases grown by metalorganic vapor phase epitaxy

We demonstrate the metalorganic vapor phase epitaxy (MOVPE) growth of AlGaAs/GaAs two‐dimensional electron gases (2DEGs) with mobilities as high as 786 000 cm2/V s at a carrier density of 3.0×1011 cm−2 at 0.3 K. The mobility figures of merit (μ/n3/2) for these 2DEGs are the highest reported to date for MOVPE materials. These 2DEGs also exhibit the fractional quantum Hall effect (FQHE) with minima in longitudinal resistance corresponding to Landau level filling factors 2/3, 4/3, and 5/3. The temperature dependence and carrier density dependence of mobility were characterized, and the mobility was found to vary linearly with carrier density, implying that the mobility is probably limited by background ionized impurity scattering. A delta‐doped 2DEG was also compared with uniformly doped 2DEGs and was found to have a slightly higher mobility. Finally, we obtained high mobility in a coupled double 2DEG structure for 2D to 2D tunneling applications.

Magnetic breakdown and Landau level spectra of a tunable double-quantum-well Fermi surface

Abstract By measuring longitudinal resistance, we map the Landau level spectra of double quantum wells as a function of both parallel ( B ‖ ) and perpendicular ( B ⊥ ) magnetic fields. In this continuously tunable highly non-parabolic system, the cyclotron masses of the two Fermi surface orbits change in opposite directions with B ‖ . This causes the two corresponding ladders of Landau levels formed at finite B ⊥ to exhibit multiple crossings. We also observe a third set of Landau levels, independent of B ‖ , which arise from magnetic breakdown of the Fermi surface. Both semiclassical and full-quantum mechanical calculations show good agreement with the data.

Tuning a double quantum well Fermi surface with in-plane magnetic fields

Abstract A double quantum well (QW) subject to in-plane magnetic fields B∥ has the dispersion curves of its two QWs shifted in k-space. When the QWs are strongly coupled, an anticrossing and partial energy gap occur, yielding a tunable multi-component Fermi surface. We report measurements of the resultant features in the conductance, the capacitive density of states and giant deviations in the cyclotron effective masses.

Extreme field-induced cyclotron mass variations in coupled double quantum wells

Abstract We observe deviations in the cyclotron effective mass m c near the partial energy gap formed in strongly coupled GaAs double quantum wells (QWs) subject to in-plane magnetic fields B ⊥ . In k-space, B ⊥ shifts the two QW dispersion curves relative to one another, resulting in an anticrossing and opening the energy gap. This gives rise to large B ⊥ -tunable distortions in the Fermi surface and density of states. This system is thus unique in that the Fermi surface and energy position of the gap can be controlled by sweeping B ⊥ . Recently, Lyo has predicted that m c undergoes large variations as the partial energy gap is moved through the Fermi level by B ⊥ . By tilting our sample by a small angle θ, we introduce a small perpendicular magnetic field B ⊥ , in addition to B ⊥ , and analyze the temperature dependence of the resulting Shubnikov-de Haas oscillations to obtain m c ( B ⊥ ). Due to the strongly distorted dispersion near the gap, m c is suppressed by more than a factor of 3 near the upper gap edge, and enhanced by ∼50% near the lower gap edge, in excellent agreement with the theory of Lyo. We also observe the quantum Hall effect in a double QW at a high, constant B ⊥ .

Magnetic breakdown in double quantum wells

The authors find that a sufficiently large perpendicular magnetic field (B{sub {perpendicular}}) causes magnetic breakdown (MB) in coupled double quantum wells (QWs) that are subject to an in-plane magnetic field (B{sub {parallel}}). B{sub {parallel}} shifts one QW dispersion curve with respect to that of the other QW, resulting in an anticrossing and an energy gap. When the gap is below the Fermi level the resulting Fermi surface (FS) consists of two components, a lens-shaped inner orbit and an hour-glass shaped outer orbit. B{sub {perpendicular}} causes Landau level formation and Shubnikov-de Haas (SdH) oscillations for each component of the FS. MB occurs when the magnetic forces from B{sub {perpendicular}} become dominant and the electrons move on free-electron circular orbits rather than on the lens and hour-glass orbits. MB is observed by identifying the peaks present in the Fourier power spectrum of the longitudinal resistance vs. 1/B{sub {perpendicular}} at constant B{sub {parallel}}, an arrangement achieved with an in-situ tilting sample holder. Results are presented for two strongly coupled GaAs/AlGaAs DQW samples.

Giant effective mass deviations near the magnetic field-induced minigap in double quantum wells

The authors report major deviations in the electron effective mass m* near the partial energy gap, or minigap, formed in strongly coupled double quantum wells (QWs) by an anticrossing of the two QW dispersion curves. The anticrossing and minigap are induced by an in-plane magnetic field B{sub {parallel}} and give rise to large distortions in the Fermi surface and density of states, including a Van Hove singularity. Sweeping B{sub {parallel}} moves the minigap through the Fermi level, with the upper and lower gap edges producing a sharp maximum and minimum in the low-temperature in-plane conductance, in agreement with theoretical calculations. The temperature dependence of Shubnikov-de Haas (SdH) oscillations appearing in a tilted magnetic field yield a decreased m* {le} 1/3 m*{sub GaAs} near the upper gap edge, and indicate an increase in m* near the lower gap edge.