1,721,085 research outputs found
Atomic-scale electronics in semiconductors
No full textA dopant atom in a semiconductor, the solid state analogue of a hydrogen atom, has a Bohr radius of several nanometers. Because this length scale is close to being accessible by modern nanolithography, detection and control of charge and spin in a semiconductor down to the level of individual dopant atoms is within reach and provides the unique opportunity to study, manipulate, and utilize a single atom's wave function. We have performed electrical transport measurements across epitaxial defect-free nanometer-sized Schottky diodes. These were formed by self-assembled CoSi2-islands on Si(111) and contacted with the tip of a scanning tunneling microscope (STM). Greatly enhanced conductance was observed in diodes which were small compared to the Debye length in the semiconductor. The observed behavior can be understood qualitatively from a decreased barrier width for smaller diodes. On highly doped substrates, we find that individual dopant atoms even dominate the transport characteristics of our nanometer sized devices, due to their random distribution in the space charge region. The ability to observe the energy levels of single dopant atoms is essential for experimental studies of individual wave functions in a semiconductor. Preliminary results in a fabrication method for nano-devices approaching the size regime necessary for the observation of single dopants demonstrate the feasibility of our STM-based measurement method for this purpose. The most straightforward means to address an individual impurity is manipulation of its wave function with a gate. As a first approach to this problem, we theoretically studied the effect of a homogeneous electric or magnetic field on the energy levels of shallow impurities in silicon, taking the bandstructure into account. Furthermore, we used a description as hydrogen-like impurities for accurate computation of energy levels and lifetimes up to large electric fields. A similar description was used in a realistic device geometry, in which a small nearby gate influences a single dopant atom. This knowledge is particularly important for the development of a dopant-atom based quantum computer.Applied Science
A quasi-optical THz mixer: Based on a Nb diffusion-cooled hot-electron bolometer
No full textApplied Science
Superconductor-Insulator-Superconductor THz Mixer Integrated with a Superconducting Flux-Flow Oscillator
No full textApplied Science
Hotspot mixing in THz niobium superconducting hot electon bolometer mixers
No full textApplied Science
Charge transport in disordered organic field-effect transistors
No full textIn this thesis we study charge transport in organic semiconductors. We do this by focusing on the physical characterization of disordered organic field-effect transistors. It will be made clear that the disorder in the polymer films is crucial for the interpretation of the data. The field-effect transistor geometry allows variation of the charge carrier density in the semiconductor, without the presence of counter ions. Therefore, the transistor allows a rather clean study of the charge transport in organic semiconductors as a function of the charge carrier density and temperature. In the experiments we find that the organic transistors are in several respects not comparable to silicon MOSFETs. Therefore, in this thesis we redefine and re-evaluate basic transistor parameters, such as the threshold voltage, the field-effect mobility, the contact resistance and the dopant density. Subsequently, we study the charge transport as a function of charge density, temperature and electric field, giving insight into the charge transport mechanism. Based on our observations we propose as the main charge transport mechanism: multiphonon hopping of polaronic charge carriers in a Gaussian density of states. We investigate the electrical stability of the polymer layer in metal-insulator-semiconductor diodes, where we determine and analyse the dopant density changes as a function of oxygen and light exposure. The presence of contact resistances in the transistors is addressed by analysing the scaling behavior of the electrical characteristics as a function of the transistor channel length, and an empirical relation between the contact resistance and the charge carrier mobility in the polymer layer is observed. Finally, we discuss why typically only unipolar transistor behavior is observed experimentally, and we demonstrate ambipolar transistor behavior in organic field-effect transistors based on blends of organic semiconductors and on low band gap organic semiconductors.Applied Science
Electrodynamics of strongly disordered superconductors
No full textThin films of superconducting materials with a high resistance in the normal state, such as TiN, NbTiN, NbN and InO, are intensively studied from both an application and a fundamental point of view. A spatially inhomogeneous superconducting state can arise in these materials as a result of the strong disorder. This Thesis provides an experimental study of the evolution of the electromagnetic response with increasing disorder of superconducting thin films. We primarily investigate a series of disordered TiN films. The films are grown by means of plasma-enhanced atomic-layer deposition. We probe the electromagnetic response using superconducting microwave resonators. We find a gradual evolution of the electromagnetic response with disorder, deviating from the Mattis-Bardeen equations. This result might be attributed to changes in the quasiparticle density of states (DoS) induced by the disorder. We can describe the measured microwave response with a heuristic model which contains a disorder-dependent effective pair breaking parameter that modifies the DoS. Further, we compare the assumed DoS—used to describe the electrodynamics—to local tunnel spectra obtained using scanning tunneling spectroscopy. We find affirmative results for the lowest disordered film. However, we find a strong discrepancy for the most disordered film. Moreover, this film displays large variations in the local tunnel spectra. This result signals the breakdown of a model that is based on average properties, due to the emergence of a spatial inhomogeneous superconducting state.Kavli Institute of Nanoscience DelftApplied Science
Electrodynamic response of mesoscopic objects: Carbon nanotubes and superconducting nanowires
No full textKavli Institute of Nanoscience DelftApplied Science
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