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Register a new userCombined atomic force microscope and electron-beam lithography used for the fabrication of variable-coupling quantum dots
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We have combined direct nanofabrication by local anodic oxidation with conventional electron-beam lithography to produce a parallel double quantum dot based on a GaAs/AlGaAs heterostructure. The combination of both nanolithography methods allows to fabricate robust in-plane gates and Cr/Au top gate electrodes on the same device for optimal controllability. This is illustrated by the tunability of the interdot coupling in our device. We describe our fabrication and alignment scheme in detail and demonstrate the tunability in low-temperature transport measurements.
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Strong confinement of charges in few electron systems such as in atoms, molecules and quantum dots leads to a spectrum of discrete energy levels that are often shared by several degenerate quantum states. Since the electronic structure is key to understanding their chemical properties, methods that probe these energy levels in situ are important. We show how electrostatic force detection using atomic force microscopy reveals the electronic structure of individual and coupled self-assembled quantum dots. An electron addition spectrum in the Coulomb blockade regime, resulting from a change in cantilever resonance frequency and dissipation during tunneling events, shows one by one electron charging of a dot. The spectra show clear level degeneracies in isolated quantum dots, supported by the first observation of predicted temperature-dependent shifts of Coulomb blockade peaks. Further, by scanning the surface we observe that several quantum dots may reside on what topologically appears to be just one. These images of grouped weakly and strongly coupled dots allow us to estimate their relative coupling strengths.
A new method of preparation of radio-frequency superconducting quantum interference devices on MgB2 thin films is presented. The variable-thickness bridge was prepared by a combination of optical lithography and of the scratching by an atomic force microscope. The critical current of the nanobridge was 0.35 uA at 4.2 K. Non-contact measurements of the current-phase characteristics and of the critical current vs. temperature have been investigated on our structures.
We present a novel shadow evaporation technique for the realization of junctions and capacitors. The design by E-beam lithography of strongly asymmetric undercuts on a bilayer resist enables in-situ fabrication of junctions and capacitors without the use of the well-known suspended bridge[1]. The absence of bridges increases the mechanical robustness of the resist mask as well as the accessible range of the junction size, from 0.01 to more than 10000 micron square. We have fabricated Al/AlOx/Al Josephson junctions, phase qubit and capacitors using a 100kV E- beam writer. Although this high voltage enables a precise control of the undercut, implementation using a conventional 20kV E-beam is also discussed. The phase qubit coherence times, extracted from spectroscopy resonance width, Rabi and Ramsey oscillations decay and energy relaxation measurements, are longer than the ones obtained in our previous samples realized by standard techniques. These results demonstrate the high quality of the junction obtained by this controlled undercut technique.
We use an atomic force microscope (AFM) to manipulate graphene films on a nanoscopic length scale. By means of local anodic oxidation with an AFM we are able to structure isolating trenches into single-layer and few-layer graphene flakes, opening the possibility of tabletop graphene based device fabrication. Trench sizes of less than 30 nm in width are attainable with this technique. Besides oxidation we also show the influence of mechanical peeling and scratching with an AFM of few layer graphene sheets placed on different substrates.
We present a technique to fabricate ultrathin (down to 20 nm) uniform electron transparent windows at dedicated locations in a SiN membrane for in situ transmission electron microscopy experiments. An electron-beam (e-beam) resist is spray-coated on the backside of the membrane in a KOH- etched cavity in silicon which is patterned using through-membrane electron-beam lithography. This is a controlled way to make transparent windows in membranes, whilst the topside of the membrane remains undamaged and retains its flatness. Our approach was optimized for MEMS-based heating chips but can be applied to any chip design. We show two different applications of this technique for (1) fabrication of a nanogap electrode by means of electromigration in thin free-standing metal films and (2) making low-noise graphene nanopore devices.
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