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Semiconductors are traditionaly either p-type or n-type, meaning that the mobile charge carriers in them are either "holes" in the valence band or electrons in the conduction band. Ambipolar conduction implies that the experimenter should be able to populate the same channel with either electrons or holes in a controlled manner. This has been shown to be possible in newer materials like Graphene and some organic semiconductors. "Ambipolarity" can open up new device possibilities as well as new ways to study fundamental scattering mechanisms in semiconductors. However, achieving this in a conventional high mobility structure like a GaAs-AlGaAs heterostructure/quantum well requires new thinking. It was realized, that to do this modulation doping must be given up and techniques to make an undoped heterostructure conduct, must be developed first. Such structures have been developed by only a few groups worldwide. They are of great interest to low temperature physicists working with Quantum Hall states and mesoscopic/nano structures in the ballistic regime. We discuss the reason behind this interest and the analysis of scattering mechanisms in such structures. Finally very recent experimental success in developing fully gate controlled ambipolar structures where both electron and hole mobilites exceed 1 million cm²/Vs at low temperatures (T~1Kelvin) are discussed. Such gated ambipolar structures can be used to analyse scattering mechanisms in ultra-high mobility 2dimensional electron and hole gases in a way that is not possible using other techniques. [ABSTRACT FROM AUTHOR]

We report on polarization-resolved photoluminescence studies of diverse excitonic complexes formed in GaAs quantum wells with a high-mobility two-dimensional hole gas in magnetic fields up to 23 T. Using two-beam illumination we decrease the hole concentration beyond the point of conversion from p- to n-type structures. We have demonstrated charge conversion between positive and negative exciton complexes (both free and bound to acceptors in the well). The switch between the electron and hole gases allowed us to distinguish between the emission lines from positive trions moving almost freely in the quantum well and bound to ionized acceptors in the barrier, which indicate their coexistence in the same well. [ABSTRACT FROM AUTHOR]

We have developed a method of fabricating very shallow, gateable, undoped 2-dimensional electron gases (2DEG) and making very low resistivity contacts to these. We studied the evolution of mobility as a function of the depth of the 2DEG (from 300 nm to 30 nm). We extract quantitative information about the background impurity concentration in GaAs and AlGaAs, the interface roughness and the charge in the surface states. Surface charge sets an intrinsic limit to the mobility of very shallow 2DEGs. It is probably impossible, to fabricate such shallow high-mobility 2DEGs using modulation doping due to the need to accommodate a spacer layer. [ABSTRACT FROM AUTHOR]

We present experimental evidence of a breakdown of localization at low temperatures in strongly interacting two-dimensional electrons systems with moderate background disorder. The observation is manifested by a transition from an activated transport behavior at high temperature to a metal-like (dρ/dT>0) behavior at low temperature T <=1 K. The results have been achieved in mesoscopic two-dimensional electron systems (2DES), where the disorder strength was systematically varied. [ABSTRACT FROM AUTHOR]

We have created single-photon detectors based on micron-size GaAs/AlGaAs resonant tunnelling diodes containing a layer of self-assembled InAs quantum dots. A quantum efficiency of 12.5% has been achieved with a dark count rate of about 10-3 per second in devices with 10nm AlGaAs barriers at 4K. With optimisation, we expect that the quantum efficiency of the devices can be significantly improved. This type of device could eventually find application in efficient quantum communication or computation. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

We present results of photon counting with a single photon detector based on field effect transistor with a layer of quantum dots in close proximity to the channel. Detection of a photon is achieved when photo-hole is captured by negatively charged InAs dot leading to a step-like increase of the current in the channel of the transistor. We use a transimpedance amplifier with a cryogenic stage and ac coupling on the input to convert the current steps of 1–2 nA height into voltage peaks with microsecond time resolution. We show that single photon counting with 0.3% efficiency, dark count rate below 10-8 ns-1 is then achieved, while the jitter limited by the circuit can be as low as 8.5 ns. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]