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ABSTRACT: Modulation doped GaAs-AlGaAs quantum well based structures are usually used
to achieve very high mobility 2-dimensional electron (or hole) gases. Usually
high mobilities ($>10^{7}{\rm{cm}^{2}\rm{V}^{-1}\rm{s}^{-1}}$) are achieved at
high densities. A loss of linear gateability is often associated with the
highest mobilites, on account of a 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
$\approx{6\times10^{11}\rm{cm}^{-2}}$ can be achieved while maintaining fully
linear and non-hysteretic gateability. We use these devices to understand the
possible mobility limiting mechanisms at very high densities.
11/2011;
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Advances in Condensed Matter Physics. 01/2011;
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ABSTRACT: 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 Onsager's reciprocity theorem.
Physical Review Letters 01/2009; 101(24):246801. · 7.37 Impact Factor
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ABSTRACT: We report our work on fabricating lithographically aligned patterned backgates on thin (50–60 μ m ) 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.
Journal of Applied Physics 01/2009; · 2.17 Impact Factor
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ABSTRACT: We report single layer resistivities of 2-dimensional electron and hole gases in an electron-hole bilayer with a 10nm barrier. In a regime where the interlayer interaction is stronger than the intralayer interaction, we find that an insulating state ($d\rho/dT < 0$) emerges at $T\sim1.5{\rm K}$ or lower, when both the layers are simultaneously present. This happens deep in the $"$metallic" regime, even in layers with $k_{F}l>500$, thus making conventional mechanisms of localisation due to disorder improbable. We suggest that this insulating state may be due to a charge density wave phase, as has been expected in electron-hole bilayers from the Singwi-Tosi-Land-Sj\"olander approximation based calculations of L. Liu {\it et al} [{\em Phys. Rev. B}, {\bf 53}, 7923 (1996)]. Our results are also in qualitative agreement with recent Path-Integral-Monte-Carlo simulations of a two component plasma in the low temperature regime [ P. Ludwig {\it et al}. {\em Contrib. Plasma Physics} {\bf 47}, No. 4-5, 335 (2007)] Comment: 5 pages + 3 EPS figures (replaced with published version)
12/2008;
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ABSTRACT: Transport measurements down to 50 mK have been performed on independently contacted electron–hole bilayers (EHBL) with a 10 nm barrier, close to the excitonic Bohr radius of the system (). Coulomb drag measured reveals a departure from that expected for two Fermi liquids, exhibiting an upturn that may be followed by a downturn or even a sign-reversal at the lowest temperatures. Concurrently an insulating state in the single-layer resistivities has been found for low sheet resistances (ρ≪h/e2), well away from where the metal–insulator transition has been previously observed in two-dimensional systems. We can unambiguously show that this insulating state is caused by the interlayer interaction, as opposed to disorder. We consider the possibility of excitonic and collective mode driven phases that have been anticipated in EHBLs.
Physica E: Low-dimensional Systems and Nanostructures.
