T. M. Klapwijk

Field-effect transistors on tetracene single crystals

We report on the fabrication and electrical characterization of field-effect transistors at the surface of tetracene single crystals. We find that the mobility of these transistors reaches the room-temperature value of 0.4?cm2/V?s. The nonmonotonous temperature dependence of the mobility, its weak gate voltage dependence, as well as the sharpness of the subthreshold slope, confirm the high quality of single-crystal devices. This is due to the fabrication process that does not substantially affect the crystal quality.

Scaling of nano-Schottky-diodes

A generally applicable model is presented to describe the potential barrier shape in ultrasmall Schottky diodes. It is shown that for diodes smaller than a characteristic length lc (associated with the semiconductor doping level) the conventional description no longer holds. For such small diodes the Schottky barrier thickness decreases with decreasing diode size. As a consequence, the resistance of the diode is strongly reduced, due to enhanced tunneling. Without the necessity of assuming a reduced (non-bulk) Schottky barrier height, this effect provides an explanation for several experimental observations of enhanced conduction in small Schottky diodes.

Terahertz heterodyne receiver based on a quantum cascade laser and a superconducting bolometer

We report the first demonstration of an all solid-state heterodyne receiver that can be used for high-resolution spectroscopy above 2THz suitable for space-based observatories. The receiver uses a NbN superconducting hot-electron bolometer as mixer and a quantum cascade laser operating at 2.8THz as local oscillator. We measure a double sideband receiver noise temperature of 1400K at 2.8THz and 4.2K, and find that the free-running QCL has sufficient power stability for a practical receiver, demonstrating an unprecedented combination of sensitivity and stability.

Switch-on voltage in disordered organic field-effect transistors

The switch-on voltage for disordered organic field-effect transistors is defined as the flatband voltage, and is used as a characterization parameter. The transfer characteristics of the solution processed organic semiconductors pentacene, poly(2,5-thienylene vinylene) and poly(3-hexyl thiophene) are modeled as a function of temperature and gate voltage with a hopping model in an exponential density of states. The data can be described with reasonable values for the switch-on voltage, which is independent of temperature. This result also demonstrates that the large threshold voltage shifts as a function of temperature reported in the literature constitute a fit parameter without a clear physical basis.

Scaling behavior and parasitic series resistance in disordered organic field-effect transistors

The scaling behavior of the transfer characteristics of solution-processed disordered organic thin-film transistors with channel length is investigated. This is done for a variety of organic semiconductors in combination with gold injecting electrodes. From the channel-length dependence of the transistor resistance in the conducting ON-state, we determine the field-effect mobility and the parasitic series resistance. The extracted parasitic resistance, typically in the MΩ range, depends on the applied gate voltage, and we find experimentally that the parasitic resistance decreases with increasing field-effect mobility.

Space charge limited transport and time of flight measurements in tetracene single crystals: A comparative study

We report on a systematic study of electronic transport in tetracene single crystals by means of space charge limited current spectroscopy and time of flight measurements. Both I–V and time of flight measurements show that the room-temperature effective hole mobility reaches values close to μ≃1 cm2/V s and that, within a range of temperatures, the mobility increases with decreasing temperature. The experimental results further allow the characterization of different aspects of the tetracene crystals. In particular, the effects of both deep and shallow traps are clearly visible and can be used to estimate their densities and characteristic energies. The results presented in this article show that the combination of I–V measurements and time of flight spectroscopy is very effective in characterizing several different aspects of electronic transport through organic crystals.

Coulomb-blockade transport in single-crystal organic thin-film transistors

Coulomb-blockade transport—whereby the Coulomb interaction between electrons can prohibit their transport around a circuit—occurs in systems in which both the tunnel resistance, RT, between neighbouring sites is large (≫h/e2) and the charging energy, EC (EC = e2/2C, where C is the capacitance of the site), of an excess electron on a site is large compared to kT. (Here e is the charge of an electron, k is Boltzmanns constant, and h is Plancks constant.) The nature of the individual sites—metallic, superconducting, semiconducting or quantum dot—is to first order irrelevant for this phenomenon to be observed. Coulomb blockade has also been observed in two-dimensional arrays of normal-metal tunnel junctions, but the relatively large capacitances of these micrometre-sized metal islands results in a small charging energy, and so the effect can be seen only at extremely low temperatures. Here we demonstrate that organic thin-film transistors based on highly ordered molecular materials can, to first order, also be considered as an array of sites separated by tunnel resistances. And as a result of the sub-nanometre sizes of the sites (the individual molecules), and hence their small capacitances, the charging energy dominates at room temperature. Conductivity measurements as a function of both gate bias and temperature reveal the presence of thermally activated transport, consistent with the conventional model of Coulomb blockade.

Doubling of sensitivity and bandwidth in phonon cooled hot electron bolometer mixers

We demonstrate that the performance of NbN lattice cooled hot electron bolometer mixers depends strongly on the interface quality between the bolometer and the contact structure. We show experimentally that both the receiver noise temperature and the gain bandwidth can be improved by more than a factor of 2 by cleaning the interface and adding an additional superconducting interlayer to the contact pad. Using this we obtain a double sideband receiver noise temperature TN,DSB 5950 K at 2.5 THz and 4.3 K, uncorrected for losses in the optics. At the same bias point, we obtain an IF gain bandwidth of 6 GHz. ©2004 American Institute of Physics. @DOI: 10.1063/1.1667012 #

Enhanced tunneling across nanometer-scale metal–semiconductor interfaces

We have measured electrical transport across epitaxial, nanometer-sized metal–semiconductor interfaces by contacting CoSi2 islands grown on Si(111) with the tip of a scanning tunneling microscope. The conductance per unit area was found to increase with decreasing diode area. Indeed, the zero-bias conductance was found to be ? 104 times larger than expected from downscaling a conventional diode. These observations are explained by a model, which predicts a narrower barrier for small diodes and, therefore, a greatly increased contribution of tunneling to the electrical transport.

Enhanced telecom wavelength single-photon detection with NbTiN superconducting nanowires on oxidized silicon

Superconducting nanowire single-photon detectors (SNSPDs) have emerged as a highly promising infrared single-photon detector technology. Next-generation devices are being developed with enhanced detection efficiency (DE) at key technological wavelengths via the use of optical cavities. Furthermore, new materials and substrates are being explored for improved fabrication versatility, higher DE, and lower dark counts. We report on the practical performance of packaged NbTiN SNSPDs fabricated on oxidized silicon substrates in the wavelength range from 830 to 1700 nm. We exploit constructive interference from the SiO2/Si interface in order to achieve enhanced front-side fiber-coupled DE of 23.2 % at 1310 nm, at 1 kHz dark count rate, with 60 ps full width half maximum timing jitter.