Jean-René Martin

Autocatalytic Deposition of Gold and Palladium onto n ‐ GaAs in Acidic Media

Deposition of gold, palladium, and gold‐palladium alloys is obtained with the help of acidic electroless baths of low metal content. The baths are prepared by suitably mixing separate solutions containing the reductant (hydroxylamine hydrochloride) or one of the desired cations respectively, with suitable additives. The stability of the baths with time is good and their replenishment easy to realize. Provided that a suitable choice of mixture of the three solutions is made, they allow the fabrication of multilayered heterostructures which could be fruitfully used for ohmic contact preparation onto without any initial activation step. The deposition mechanism of gold and palladium and the effect of adding palladium to gold is discussed.

Study of a chaotic mixing system for DNA chip hybridization chambers

Numerical simulations of a micromixing system based on chaotic advection for improved deoxyribonucleic acid (DNA) chip hybridization are presented. To attain best chip performance, homogeneous dispersion of DNA molecules throughout the chamber in which the chip is placed is of primary importance. Poincare sections of a simple time-periodic flow, based on numerical simulations of the flow, are compared with visualizations in a scaled-up experiment, with good agreement. The influence on mixing efficiency of varying the period of the flow at fixed volume flow rate is studied and a trade off is found between the absence of regular islands and a small enough total sample volume. The results illustrate the potential for optimization of such devices based on numerical flow simulations.

Structural and Analytical Characteristics of Adsorbed Pd‐Sn Colloids

Mixed Pd-Sn colloidal systems are used to activate nonconductor surfaces for electroless metal deposition. The ways to prepare the colloidal solution (temperature, pH, reaction time) and the aging process have an effect on the size and the structure of the adsorbed Pd-Sn nuclei. The studies are supported by high resolution transmission electron microscopy observations, and selective area electron diffraction, energy dispersive x-ray spectroscopy, and Rutherford backscattering spectroscopy analyses

Hydrogen production by water photoelectrolysis with a powder semiconductor anode

Abstract It has been shown that a photoelectrolysis H cell with a powder semiconductor anode, a platinum collector electrode and a counter electrode, can operate. Hydrogen can be produced from water dissociation by this cell which combines the advantages of a large photosensitive surface as in photocatalysis and a separation of the products as in classical photoelectrolysis.

Gas sensor characterization through both contact potential difference and photopotential measurements

Abstract Two different electrical measurement methods have been used to study p-Si/SiO 2 /SnO 2 /Pd structures which could be used in gas sensors. One of the methods is the contact potential difference measurement which characterizes the surface of the structure. A new method using a modulated light beam permits the measurement of the photopotential. These two methods are described. The relation between the surface potential and the photopotential is examined.

Electroless Ni as a Refractory Ohmic Contact for n ‐ InP

We report on the realization of ohmic contacts for using Ni electroless deposition. A nickel film can be deposited by two wet methods: first, noncatalytically, to obtain a 100 nm Ni film, and now, catalytically, which avoids activating the contact. Catalytic deposition starts when a small sheet of Ni is laid on the substrate and the autocatalytic deposition goes on. The contact reactions to annealing treatment in junctions also are reported. Chemical depth profiling by Auger electron spectroscopy, low incidence x‐ray diffraction, and scanning electron microscopy (SEM) are used to study the morphology and the structure of the nickel films. Just after deposition, only compound is detected: this phase and interphase evolve with the annealing temperature to ternary and binary phases. The value of the specific contact resistance is in the annealing temperature range 350°C. This value does not evolve with high annealing temperatures and is held even after an annealing treatment at 400°C for 20 h or 500°C for 3 h conferring to this ohmic contact a refractory quality.

Behaviour of cryptophane molecules in gas media

In solution, synthetic molecules of the cryptophane family present the property of capturing molecular species selectively, the size of which is well adapted to the host cavity. Specially, cryptophane A, the smallest of the series exhibited an amazing affinity towards methane. This work aims to determine the feasibility of using cryptophanes as new chemical receptors in gas sensor applications. For this, the behaviour of cryptophanes deposited on substrates and put in contact with gas (methane, air, hydrogen) has been investigated. Preliminary results show the influence of the deposition technique on the device response. Elaboration of mixed Langmuir-Blodgett layers including cryptophanes seems to be a promising method and gives a specific signature of the interactions between cryptophanes A and gas. The amplitude and shape of the signal depend on the type of gas (methane, hydrogen, air).

Influence of surface modifications on semiconductor gas sensor behaviour

Abstract GasFET devices are based on the superficial conductivity change of semiconducting substrate in the presence of the gas to be detected. One way to improve the selectivity and the sensitivity of the gas sensor is to modify the surface with a catalytic metal film. Usually, this film is made by sputtering or a thermal evaporation process. In this study, using a chemical catalytic metal deposition method (from colloidal solution), we are able to vary the size and morphology of the metal islands in order to understand the key role of the metal. The effect of various surface modifications on the gas sensor behaviour in contact with the gas is investigated by measuring the surface potential variations using a vibrating capacitor technique.

Electroless deposition as a means of obtaining ohmic contacts: Au/Pd onto GaAs

Abstract Autocatalytic gold deposits onto palladium-activated GaAs substrates, prepared by chemical displacement, have been found to produce good quality ohmic contacts (in the 10 −5 Ω cm 2 range) provided that the Pd layer thickness and the annealing temperature are optimized. Pd plays the role of catalyst for the subsequent autocatalytic gold deposition; but by using thicker Pd films than necessary to fulfil this “chemical” part, this element also plays another prominent role in the metallurgy of the contact. A transmission electron microscopy (TEM) study has been carried out in order to characterize the growth of Pd nuclei on GaAs. The electrical measurements obtained on such alloyed Au/Pd/ n or p -type GaAs contacts are analyzed in term of a graded amorphous/crystalline junction.

Use of Au‐In‐Pd and Pd‐In Electroless Deposits for Ohmic Contacts on n ‐ GaAs

Codeposition of gold-palladium-indium and palladium-indium alloys onto n-GaAs substrates is obtained in an acidic medium by the electroless method. The preparation and characteristics of the different baths are presented. For the Pd-In alloys, a crystalline refractory Pd(In){sub 3} compound is initially formed, whereas after a 2 min annealing treatment at 470{degrees}C traces of InAs and PdGa are found. A high-quality ohmic contact having {rho} = 6.10{sup {minus} 6} {Omega} cm{sup 2} is obtained with annealing.