V. Foglietti

Investigation of SiGe-heterostructure nanowires

Transport characterizations of wires obtained by electron beam lithography and etching of (100) Si/SiGe heterostructures with a high-mobility two-dimensional electron gas are reported. Depending on the wire width, two different regimes for the electrical transport are found. Wires with a width larger than ~200xa0nm exhibit metallic behaviour in the quasi-ballistic regime. The conductance dependence on the wire width reveals the presence of a depletion layer, ~100xa0nm thick, on each etched side of the wire. The wires of width smaller than 200xa0nm have very large resistance and two different behaviours. The first kind of wires exhibit a zero-current region, compatible with a Coulomb blockade effect involving multiple tunnel junctions or with a space-charge limited current. Other wires are insulating up to applied voltages larger than 5-6xa0V and their I-V characteristics can be fitted by the functional dependence of voltage-induced tunnelling of Fowler-Nordheim type.

Three-dimensional shaping of sub-micron GaAs Schottky junctions for zero-bias terahertz rectification

We demonstrate a rectification mechanism based on quantum tunneling through the narrow Schottky barrier of a sub-micrometric Au/Ti/n-GaAs junction, which is capable of efficient power detection of free-space terahertz radiation beams even without an applied dc bias. Three-dimensional shaping of the junction geometry provides an enhanced zero-bias tunneling probability due to increased electric fields at the junction, resulting in cutoff frequencies up to 0.55 THz, responsivity up to 200u2009V/W, and noise equivalent power better than 10−9 W/Hz0.5 without applied dc bias.

EBL- and AFM-based techniques for nanowires fabrication on Si/SiGe

Abstract Two approaches for sub-100 nm patterning are applied to Si/SiGe samples. The first one combines electron beam lithography (EBL) and anisotropic wet etching to fabricate wires with triangular section whose top width is narrower than the beam size. Widths as small as 20 nm on silicon and 60 nm on Si/SiGe heterostructures are obtained. The second lithographic approach is based on the local anodization of an aluminum film induced by an atomic force scanning probe. Using atomic force microscopy (AFM) anodization and selective wet etching, aluminum and aluminum oxide nanostructures are obtained and used as masks for reactive ion etching (RIE). Sub-100 nm wide wires are fabricated on Si/SiGe substrates.

Reliable low noise DC-SQUID

A novel type of multiloop DC SQUID (superconducting quantum interference device) has been fabricated. The device is made of six different layers deposited on a 2-in silicon wafer; the layers are patterned by means of optical lithography. Every chip is a square 6.35 mm on a side and contains two SQUIDs and their input coils; the typical coil inductance is about 1 mu H. The junctions are 2 mu m/sup 2/ planar windows on SiO made of Nb-NbO/sub x/-PbAuIn with a current density of 500 A/cm/sup 2/ and V/sub m/ approximately=20 mV. The device was repeatedly cooled and stored in air for six months; after these thermal cycles no variations in critical current were observed. The measured mutual inductance between the input coil and SQUID is 2.1 nH, and the SQUID inductance is about 28 pH. The energy sensitivity referred to the input and in the flux-locked loop is approximately 1400 h in the white region with a 1/f corner frequency at 10 Hz. These SQUIDS are already operating in the gravitational wave experiment of the Rome group in Geneva. >

Nanofabrication of quantum wires on (100) Si and SiGe by shifted-resist pattern and anisotropic wet etching

A technique based on shifted resist pattern and anisotropic wet etching has been developed for fabrication of quantum wires on (100) silicon and silicon/germanium substrates. The Electron Beam Lithography exposure of a same pattern properly shifted is used to obtain a mask for the subsequent wet etching. This etching takes advantage on the strong rate dependency on plane orientation and allows V groove shaped thin wires formation.

Stray capacitance effect in superconducting quantum interferometers

The experimental results obtained with low noise dc superconducting quantum interference devices (dc‐SQUIDs) are sometimes not well understood. In particular, when the critical current is high, very irregular voltage versus flux characteristics can be measured. On the other hand, when the critical current is low, the characteristics follow perfectly the resistively shunted junction model. We have studied the dynamics of a dc‐SQUID in the presence of stray capacitance and shunting resistors imbalance. The equations describing a circuit model for this physical system are integrated numerically and the results are compared with voltage versus flux characteristics obtained from fabricated devices. We find that the theoretical analysis accounts very well for previously misunderstood experimental features of the dc‐SQUID. This model can give more physical sense to the various regimes of behavior of the dc‐SQUID giving more insight into the experimental results.

DC-SQUIDs fabricated by electron beam direct writing

An Electron Beam MicroFabricator (EBMF) has been used to write directly on a silicon wafer to obtain many equal chips containing dc-SQUIDs and superconducting test circuitry. We developed a complete lift-off technique on an electronic resist for all the seven needed layers of the process. The first layer, consisting of Au-Pd alloy, is used as resistor and for patterning the markers required to align the various layers. The calibration on such markers avoid the stitching problem due to the large chip dimensions (6.30×6.30 mm) as compared with the maximum range of the electron beam deflection. The direct writing permits alignment between the various layers better than 1um in any chip. The first test on the developed devices shows high reliability and flexibility of the overall process.

Detection of terahertz radiation by AlGaN/GaN field-effect transistors

High electron mobility transistors can work as room-temperature direct detectors of radiation at frequency much higher than their cutoff frequency. One open issue is how the radiation couples to the sub-wavelength transistor channel. Here, we studied the coupling of radiation to an AlGaN/GaN transistor with cut-off frequency of 30 GHz. Local irradiation with a Free Electron Laser source at 0.15 THz allowed us to selectively couple the signal to the channel through one transistor terminal at a time. Far-field experiments at 0.15–0.94 THz were also performed in order to study the nonlinear properties of the transistor channel.

10-um thin GaAs membrane manufactured by nonselective etching

The aim of this paper is to determine an optimum nonselective etching solution in order to manufacture an as thin as possible, uniform and high quality GaAs membrane. Three different etching systems in various proportions of the components were analyzed. A high quality 10 micrometer thin GaAs membrane was obtained using the [1(H3PO4)]: [1(CH3OH)]: [3(H2O2)] etching solution. The micromachined GaAs membranes are manufactured to be used as support for microwave circuits as well as in high temperature sensor applications.

Long term operation of low noise DC-SQUID coupled to a very high Q gravitational radiation detector

We have coupled a very low noise dc-SQUID to the gravitational radiation detector of the Rome group at CERN laboratories. The SQUID used is a multiloop thin-film device with an input inductance of 1.6 μH, loop inductance of 5 pH and coupling coefficient of 0.5. The gravitational radiation detector is composed by a 2.3 tons Aluminum cylinder mechanically coupled to a resonant capacitive transducer; this is matched to the SQUID by means of a large superconducting transformer. The signal to be detected is essentially composed by the two mode frequencies at about 1 kHz and with quality factors of the order of 4×106. To operate in a closed feedback loop mode we have used a particular setup in order not to degrade the performance of the system. The system operated for seven months with some interruptions due to refilling of liquid helium and various tests on the apparatus. The flux noise obtained was 1.5 to 3times10^{-6} Phi_{o}/sqrt{Hz} at 1 kHz with a linearity over 6 orders of magnitude and a long term stability of 1.5 times 10^{-8} Phi_{o} /hour.