A. F. Croxall

Anomalous Coulomb Drag in Electron-Hole Bilayers

We report Coulomb drag measurements on GaAs-AlGaAs electron-hole bilayers. The two layers are separated by a 10 or 25 nm barrier. Below T approximately 1 K 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 Onsagers reciprocity theorem.

Demonstration and characterization of an ambipolar high mobility transistor in an undoped GaAs/AlGaAs quantum well

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.

Experimental Progress towards Probing the Ground State of an Electron-Hole Bilayer by Low-Temperature Transport

Recently, it has been possible to design independently contacted electron-hole bilayers (EHBLs) with carrier densities <5×1010 cm2 in each layer and a separation of 10–20 nm in a GaAs/AlGaAs system. In these EHBLs, the interlayer interaction can be stronger than the intralayer interactions. Theoretical works have indicated the possibility of a very rich phase diagram in EHBLs consisting of excitonic superfluid phases, charge density waves, and Wigner crystals. Experiments have revealed that the Coulomb drag on the hole layer shows strong nonmonotonic deviations from a ∼𝑇2 behaviour expected for Fermi-liquids at low temperatures. Simultaneously, an unexpected insulating behaviour in the single-layer resistances (at a highly “metallic” regime with 𝑘𝐹𝑙>500) also appears in both layers despite electron mobilities of above ∼106cm2V−1s−1 and hole mobilities over ∼105cm2V−1s−1. Experimental data also indicates that the point of equal densities (𝑛=𝑝) is not special.

Switching between attractive and repulsive Coulomb-interaction-mediated drag in an ambipolar GaAs/AlGaAs bilayer device

We present measurements of Coulomb drag in an ambipolar GaAs/AlGaAs double quantum well structure that can be configured as both an electron-hole bilayer and a hole-hole bilayer, with an insulating barrier of only 10 nm between the two quantum wells. Coulomb drag resistivity is a direct measure of the strength of interlayer particle-particle interactions. We explore the strongly interacting regime of low carrier densities (2D interaction parameter rs up to 14). Our ambipolar device design allows a comparison between the effects of the attractive electron-hole and repulsive hole-hole interactions and also shows the effects of the different effective masses of electrons and holes in GaAs.

A complete laboratory for transport studies of electron-hole interactions in GaAs/AlGaAs ambipolar bilayers

We present GaAs/AlGaAs double quantum well devices that can operate as both electron-hole (e-h) and hole-hole (h-h) bilayers, with separating barriers as narrow as 5 nm or 7.5 nm. With such narrow barriers, in the h-h configuration, we observe signs of magnetic-field-induced exciton condensation in the quantum Hall bilayer regime. In the same devices, we can study the zero-magnetic-field e-h and h-h bilayer states using Coulomb drag. Very strong e-h Coulomb drag resistivity (up to 10% of the single layer resistivity) is observed at liquid helium temperatures, but no definite signs of exciton condensation are seen in this case. Self-consistent calculations of the electron and hole wavefunctions show this might be because the average interlayer separation is larger in the e-h case than the h-h case.

Patterned backgating using single-sided mask aligners: Application to density-matched electron-hole bilayers

We report our work on fabricating lithographically aligned patterned backgates on thin (50–60 μm) III-V semiconductor samples using single sided mask aligners only. Along with this we also present a way to photograph both sides of a thin patterned chip using inexpensive infrared light emitting diodes and an inexpensive (consumer) digital camera. A robust method of contacting both sides of a sample using an ultrasonic bonder is described. In addition we present a mathematical model to analyze the variation in the electrochemical potential through the doped layers and heterojunctions that are normally present in most GaAs based devices. We utilize the technique and the estimates from our model to fabricate an electron-hole bilayer device in which each layer is separately contacted and has tunable densities. The electron and hole layers are separated by barriers either 25 or 15 nm wide. In both cases, the densities can be matched by using appropriate bias voltages.

Linear non-hysteretic gating of a very high density 2DEG in an undoped metal–semiconductor–metal sandwich structure

Modulation-doped GaAs–AlGaAs quantum-well-based structures are usually used to achieve very high mobility two-dimensional electron (or hole) gases. Usually high mobilities (>107cm2 V−1 s−1) are achieved at high densities. A loss of linear gateability is often associated with the highest mobilities, on account of some residual hopping or parallel conduction in the doped regions. We have developed a method of using fully undoped GaAs–AlGaAs quantum wells, where densities ≈6 × 1011cm−2 can be achieved while maintaining linear and non-hysteretic gateability. The conducting channel of our device is induced entirely by a field-effect mechanism, when suitable voltages are applied to the top and bottom gates. We do not use any intentional dopants at all. Our method overcomes the problem of gating very high density two-dimensional electronic system. We show how these devices are useful for understanding mobility limiting mechanisms at very high densities and indicate the likely future applications.

N-type ohmic contacts to undoped GaAs/AlGaAs quantum wells using only front-sided processing: application to ambipolar FETs

We report the development of a simple and reliable, front-sided-only fabrication technique for n-type ohmic contacts to two-dimensional electron gases (2DEGs) in undoped GaAs/AlGaAs quantum wells. We have adapted the well-established recessed ohmic contacts/insulated metal gate technique for inducing a 2DEG in an undoped triangular well to also work reliably for undoped square quantum wells. Our adaptation involves a change in the procedure for etching the ohmic contact pits to optimise the etch side-wall profile and depth. As an application of our technique, we present a front-side-gated ambipolar field effect transistor (FET), where both 2D electron and hole gases can be induced in the same quantum well. We present results of low-temperature (0.3 K - 4 K) transport measurements of this device, including assessment of the n-type ohmic contact quality. On the basis of our findings, we discuss why the fabrication of these contacts is difficult and how our technique circumvents the challenges.

A complete laboratory for transport studies of electron-hole interactions in GaAs/AlGaAs systems

This work was funded by EPSRC EP/H017720/1 and EP/J003417/1 and European Union Grant INDEX 289968. A.F.C. acknowledges funding from Trinity College, Cambridge, and D.T. from St. Catherines College, Cambridge. I.F. acknowledges funding from Toshiba Research Europe Limited.

Data set accompanying the letter "A complete laboratory for transport studies of electron-hole interactions in GaAs/AlGaAs ambipolar bilayers"

Coulomb drag and magnetotransport data from the ambipolar GaAs/AlGaAs 2D bilayer devices described in the associated publication, measured by the authors at the Cavendish Laboratory, University of Cambridge UK, in the period October 2013 to April 2016. The data were measured at low temperature (90 mK to 4 K). The experimental methods are described in the associated publication.