A.H. Verbruggen

Technology for high‐performance n‐channel SiGe modulation‐doped field‐effect transistors

A fabrication process for n‐channel SiGe modulation‐doped field‐effect transistors has been developed in which the strain in the heterostructure is completely preserved by taking into account an appropriate temperature range for the formation of ohmic contacts to source drain by implantation. It yields a low specific contact resistance of 7.1×10−5 Ω cm2 to be compared to 5.7×10−2 Ω cm2 for alloyed Au/Sb contacts. With the same thermal upper limit the application of different gate oxides has been studied. All oxides show negligible gate leakage enabling a superior gate voltage swing compared to devices with a Schottky gate. Application of thermal oxide grown at 650 °C yields a superior field‐effector transistor performance compared to devices with sputter and plasma‐enhanced chemical‐vapor deposition deposited oxides due to its lower and negative charge level. Gate recessing of 0.5 μm Schottky gates yields a transconductance increase up to 310 mS/mm.

Resistance of superconducting nanowires connected to normal-metal leads

Superconducting nanowires are resistive down to very low temperatures due to dynamical changes of the macroscopic phase (phase-slip). Thermally activated phase-slip (TAP) becomes more likely for reduced cross-sectional dimensions because the free-energy barrier scales with the area of the wire. Upon approaching T! 0, phase slip due to thermal activation disappears and resistivity persists only by macroscopic quantum tunneling through the free-energy barrier. These processes have recently been studied in suspended carbon nanotubes coated with a thin layer of a superconducting molybdenum-germanium sMoGed alloy, 1,2 and receive increased theoretical attention. 3,4 In a separate experiment 5 ropes of single-walled carbon nanotubes show signs of superconductivity, which should be strongly influenced by phase-slips as well. A second potential cause of low temperature resistance in superconducting wires is the penetration of a static electric field at normal-metal‐superconductor interfaces. It reflects the conversion of current carried by normal electrons into one carried by Cooper-pairs via Andreev-reflection. It has been studied extensively close to the critical temperature, where it is related to quasiparticle charge imbalance. 6,7 Although hardly experimentally studied, at very low temperatures a similar resistive contribution is expected to be present, reflecting a length of the order of the coherence length. Since the coherence length is a sizable portion of the resistive length of the nanowires it may contribute significantly to the measured two-point resistance. Here we report experimental results on the resistance of narrow superconducting wires connected to normal metal leads (NSN, for short). We find a strong contribution to the resistance due to the conversion processes, which is analyzed and understood in terms of the nonequilibrium theory for dirty superconductors.

Fabrication of metallic nanoconstrictions

Abstract Metallic nanoconstrictions with diameter down to 15 nm have been fabricated using silicon nitride membranes, electron-beam lithography and reactive ion etching. The silver constrictions investigated show high quality point-contact spectra, indicating that the devices operate in the ballistic transport regime.

Experimental study on magnetoresistance phenomena in n-type Si/SiGe quantum wires

The resistance of a quasi-ballistic wire fabricated in a high-mobility n-type Si/SiGe heterostructure is studied as a function of magnetic field in the temperature range 0.1 K-4.2 K. The wire has a length of 10 µm and a nominal width of 400 nm. When the magnetic field is increased from 0 T to 10 T we observe weak localization, reproducible resistance fluctuations and Shubnikov-de Haas oscillations. The various effects are superimposed on a background resistance showing the influence of diffuse sidewall scattering. All these effects are separated and analysed to give a quantitative and consistent picture of the electron transport. The magnitude of the various measured transport parameters indicates that the electron transport is quasi-ballistic. Size effects due to the influence of the confining boundary potential are observed and accounted for in the theoretical descriptions.

High‐resolution reactive ion etching and damage effects in the Si/GexSi1−x system

The reactive ion etching characteristics of epitaxial GexSi1−x(x≤0.25) alloys in SF6/O2/He and SiCl4/Cl2/He plasmas have been studied. A high‐resolution patterning process for Ge0.25Si0.75 using SiCl4/Cl2/He as a dry etch gas mixture has been developed. The etch rate and etch profile of Ge0.25Si0.75 are found to be strongly influenced by the substrate temperature and the oxygen content in the SF6/O2/He gas mixture. Conductance measurements of dry etched narrow wires in a Si/Ge0.2Si0.8 two‐dimensional hole gas at 4.2 K reveal less sidewall damage for gas mixture SiCl4/Cl2 compared with SF6/O2/He, SiCl4/Cl2/He, and SF6/O2. Scattering in the conducting wires is studied using weak localization theory.

Micron-sized Hall probes on a Si/SiGe heterostructure as a tool to study vortex dynamics in high-temperature superconducting crystals

We describe the fabrication of micron-sized Hall probes from a Si/SiGe heterostructure. The magnetic field response of the Hall probes shows a very high sensitivity of ∼60 Ω/kG. Below a temperature of 80 K, the Hall probes exhibit a highly linear field dependence of the Hall resistance. The onset of the quantum Hall effect at very low temperatures and high magnetic fields causes only small deviations from the linear field response. We demonstrate the performance of the device as a sensitive local magnetization probe in high-temperature superconducting crystals of Bi2Sr2CaCu2O8. With a linear array of Hall probes we track both the spatial and temporal evolution of the magnetization profile across the crystal. In this way surface and bulk contributions to the overall magnetization can be delineated.

Proximity effect in high voltage electron beam lithography on Ti/Pt/Au metallization

Abstract The point exposure distribution function (PEDF) for electron beam lithography on Ti/Pt/Au has been measured for 50 keV and 100 keV electrons. The PEDFs show strong non-Gaussian behaviour. The range of the backscattered electrons does not change whereas the backscatter coefficient is significantly reduced when the electron energy is increased from 50 keV to 100 keV. The data is used in calculations of the dose distribution for a typical Ti/Pt/Au metallization pattern of a microwave bipolar transistor. The results combined with the measured contrast function, provide information over the process parameters needed for well-defined resist patterns and confirm the narrow parameter range observed in preliminary experimental work.

High contrast marks for e-beam direct write made by reactive ion etching

Abstract Square wells, etched in silicon (111) substrates by reactive ion etching, are characterized as alignment marks for direct write electron beam lithography. In addition to compatibility with subsequent silicon bipolar processing steps, the contrast of the marks is comparable to etched V-shaped holes in silicon (100). The mark properties are investigated systematically by varying the size, the depth and the slope of the edges. Marks with a size smaller than 10 μm and a depth larger than about 1 μm have a sufficient contrast (>0.1) for automatic detection by a Philips EBPG-3 after deposition of 0.4 μm SiO 2 and 0.25 μm Ti/ 0.2 μm Pt/0.6 μm Au. Although the slope of the mark edges degrades after deposition of the layers, the marks allow an overlay accuracy between 50 to 100 nm. The application of the marks is not restricted to silicon (111) since the fabrication of the marks only depends on the anisotropic nature of reactive ion etching and this technique is applicable to many other substrates.

Fabrication of 35mm linewidth gold rings and observation of h/e and h/2e magnetoconductance oscillations

Abstract A detailed description is given of the fabrication of gold rings with a diameter of ∼700 nm and a linewidth between 25 and 50 nm. The fabrication technique is based on electron beam lithography in combination with lift-off. In contrast to previous work, the magnetoconductance of these rings shows at low magnetic field oscillations with flux period h/2e superimposed on oscillations with flux period h/e. At high magnetic fields, only the h/e oscillations are observed.

Metallization of submicron microwave bipolar transistors by electroplating

Abstract Electroplating is used to make dense Au metallization patterns with dimensions smaller than 0.5 μm in microwave bipolar transistors. The structures are grown in by electron beam lithography defined patterns in PMMA resist spun on top of a Ti/TiW-N/TiW/Au plating base. Test patterns consisting of 110 nm wide lines having a height of 500 nm are readily obtained. Transistors with an emitter width as small as 0.3 μm show good performance.