We report Coulomb drag measurements on GaAs-AlGaAs electron-hole bilayers. The two layers are separated by a 10 or 25nm barrier. Below T≈1K we find two features that a Fermi-liquid picture cannot explain. First, the drag on the hole layer shows an upturn, which may be followed by a downturn. Second, the effect is either absent or much weaker in the electron layer, even though the measurements are within the linear response regime. Correlated phases have been anticipated in these, but surprisingly, the experimental results appear to contradict Onsager's reciprocity theorem.
Undoped GaAs/AlGaAs heterostructures have been used to fabricate quantum wires in which the average impurity separation is greater than the device size. We compare the behavior of the Zero-Bias Anomaly against predictions from Kondo and spin polarization models. Both theories display shortcomings, the most dramatic of which are the linear electron-density dependence of the Zero-Bias Anomaly spin-splitting at fixed magnetic field B and the suppression of the Zeeman effect at pinch-off.PACS numbers: 72.10. Fk, 72.25.Dc, 73.21.Hb, 73.23.Ad Split gates [1] can be used to restrict transport from a two-dimensional electron gas (2DEG) to a ballistic onedimensional (1D) channel. This results in the quantization of the differential conductance G = dI/dV sd in units of G 0 = 2e 2 /h at zero magnetic field [2,3]. A shoulder on the riser of the first quantized plateau, the "0.7 anomaly" or "0.7 structure" [4], is not completely understood but generally acknowledged to result from electronelectron interactions. Although spin polarization models [5,6,7,8,9,10] and 1D Kondo physics models [11,12,13] can describe many experiments, neither can explain all phenomena associated with the 0.7 structure. One example is the so-called zero-bias anomaly (ZBA): a peak in G centered at V sd = 0 for G < 2e 2 /h when sweeping sourcedrain bias V sd at a fixed gate voltage V gate at low temperature T . Spin polarization models cannot alone predict its occurrence in quantum wires, although an embedded impurity near or in the 1D channel could produce a ZBA via the 0D Kondo effect [14,15,16,17]. On the other hand, in 1D Kondo physics models, a bound state forms when G < G 0 . In this context, a resonance observed by a non-invasive detector capacitively coupled to a quantum wire at threshold [18] as well as a triple-peaked structure in G at fixed V gate below the 0.7 structure [19] are consistent with the presence of a localized state in 1D channels.Systematically studying the ZBA in modulation-doped 2DEGs has proven difficult because of the large variability of its characteristics from device to device [20,21], probably due to the randomly fluctuating background potential caused by the ionized dopants, significant even with the use of large (≥75 nm) spacer layers. This disorder is so pervasive that one can be led to wonder whether the ZBA always results from interactions between conduction electrons and a random localized state near the 1D channel. However, disorder can be dramatically reduced in undoped GaAs/AlGaAs heterostructures where an external electric field (via a voltage V top on a metal top gate) electrostatically induces the 2DEG [22,23]. Figure 1(a) shows the advantages of this technique, particularly at low carrier densities (see also Fig. 3 in Ref. [22]), a regime most relevant for the ZBA. In this Letter, we report on the study of the ZBA in ten quantum wires fabricated in undoped GaAs/AlGaAs heterostructures. We demonstrate that an unsplit ZBA does not result from interactions between conduction electrons and a random localiz...
We demonstrate a method of making a very shallow, gateable, undoped 2-dimensional electron gas. We have developed a method of making very low resistivity contacts to these structures and systematically studied the evolution of the mobility as a function of the depth of the 2DEG (from 300nm to 30nm). We demonstrate a way of extracting quantitative information about the background impurity concentration in GaAs and AlGaAs, the interface roughness and the charge in the surface states from the data. This information is very useful from the perspective of molecular beam epitaxy (MBE) growth. It is difficult to fabricate such shallow high-mobility 2DEGs using modulation doping due to the need to have a large enough spacer layer to reduce scattering and switching noise from remote ionsied dopants.
We have fabricated AlGaAs/GaAs heterostructure devices in which the conduction channel can be populated with either electrons or holes simply by changing the polarity of a gate bias. The heterostructures are entirely undoped, and carriers are instead induced electrostatically. We use these devices to perform a direct comparison of the scattering mechanisms of two-dimensional (2D) electrons (µ peak = 4×10 6 cm 2 /Vs) and holes (µ peak = 0.8 × 106 cm 2 /Vs) in the same conduction channel with nominally identical disorder potentials. We find significant discrepancies between electron and hole scattering, with the hole mobility being considerably lower than expected from simple theory.Modulation-doped Al x Ga 1−x As/GaAs heterostructures have formed the starting point for innumerable studies of low dimensional electron and hole systems. 1In modulation doped heterostructures the spatial separation of the dopants and the channels significantly reduces scattering, so that very high electron or hole mobilities can be achieved at low temperatures.2 However, the type of charge carrier is determined by the dopants used during the heterostructure growth, which makes it very difficult to fabricate ambipolar devices that can operate with both electron and hole conduction. The ability to switch seamlessly between electrons and holes in the same device would be of interest for studies of scattering, interaction effects and spin related phenomena, since the two types of charge carriers have very different effective masses, bandstructures and spin properties.To create ambipolar devices we eschew conventional modulation doping techniques, using instead a gate electrode to populate the channel electrostatically. The challenge with this approach is to make good electrical contact to the 2D electrons or holes in the channel without forming an unwanted contact to the gate electrode which must overlap the ohmic contact. Hirayama et al. 3,4used ion-implantation to overcome this problem and fabricate ambipolar devices on AlGaAs/GaAs heterostructures. Thick high Al content AlGaAs diffusion barriers were used to suppress leakage between the top gate and the ohmic contacts. However, a high temperature anneal (∼ 800• C) is required after the ion implantation to activate the dopants, which is higher than the wafer growth temperature and may have adverse effects on the heterostructure and the carrier mobility. Here we describe ambipolar devices fabricated without the need for ion implantation and subsequent dopant activation anneals. With this device design we are able to make high quality ambipolar 2D systems, and compare the transport lifetime of electrons and holes formed in the same channel, with the same scattering potential.We have fabricated devices from a number of different undoped heterostructures, extending the approach described in Refs. 5 and 6 for making unipolar devices. Ohmic contacts are fabricated by standard optical lithography techniques, so there is no need for ion implantation. Here we present data from wafer B13520 grown
We report an ambipolar device fabricated in undoped GaAs/AlGaAs quantum wells (widths 10 and 25 nm) with front and backgates that allow almost two orders of magnitude in density to be accessed in the same device (7×109cm−2 to 5×1011cm−2). By changing the well width, the relative electron and hole mobilities can be tuned, approaching similar velocities. We describe an approach to fully characterize the quantum well, including the impurity backgrounds and both the upper and lower interfaces, making use of the ability to control the carrier density and the position of the wavefunction independently over a wide range.
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