Layers of Metal Nanoparticles on Semiconductors Deposited by Electrophoresis from Solutions with Reverse Micelles
© to the authors 2007
Received: 15 May 2007
Accepted: 27 July 2007
Published: 16 August 2007
Pd nanoparticles were prepared with reverse micelles of water/AOT/isooctane solution and deposited onto silicon or InP substrates by electrophoresis. A large change of capacitance-voltage characteristics of mercury contacts on a semiconductor was found after Pd deposition. This change could be modified when the Pd deposition is followed by a partial removal of the deposited AOT. The deposited Pd nanoparticles were investigated by optical mictroscopy, SIMS and SEM. Finally, Schottky diodes with barrier height as high as 1.07 eV were prepared by deposition of Pd nanoparticles on n-type InP and by a partial removal of superfluous AOT. These diodes are prospective structures for further testing as hydrogen sensors.
KeywordsNanoparticles Electrophoresis Schottky barrier InP
Formation of stable, reproducible high-barrier metal/InP interfaces is an essential prerequisite for the development of high-speed electronic devices, charge-control devices, optoelectronic detectors and wave-guides. High quality metal/InP interfaces are demanded also for good performance of InP based particle detectors. Recently, sensors of hydrogen  or NO2 gas  based on metal/InP interfaces have been reported. Gas sensors have been used for industrial process controls, for detection of toxic environmental pollutants, in the area of human health, and for the prevention of hazardous gas leaks, which comes from the manufacturing processes. Harmful effects on human health of such gases as NO2 in the environment and the need for gas measurements in manufacturing processes has led to an increase in research of gas sensors. There are many types of gas sensors that have been used for the detection of various gases. However, these are gas sensors based on metal-semiconductor interfaces that are playing an important role in the detection of toxic pollutants and the control of industrial processes.
Recently, high-sensitive sensors of hydrogen-gas based on Pd-InP Schottky barriers have been reported . The authors claimed that the barriers were prepared by a simple electrophoretic deposition of reverse micelles water/Pd-nanoparticles-AOT/isooctane. In this paper we report, inspired by Ref. 3, the preparation of metal-InP interface by deposition of Pd nanoparticles. First, we obtained deposited layers of Pd nanoparticles which were electrically isolated from each other and from the semiconductor substrate. Obviously, remnants of AOT were deposited with the Pd nanoparticles and this was the cause of the electrical isolation. Thus first we have studied the layers deposited onto Si substrates which are less expensive than InP and tried to remove superfluous AOT. Finally, Schottky barriers on InP have been prepared by removing superfluous AOT from the deposited layer and by optimizing electrophoresis parameters.
Colloids with Pd Nanoparticles
Layers of Pd Nanoparticles on Si
Polished Si wafers with the doping concentration 4 × 1014 cm−3were used as substrates for electrophoretic deposition of Pd nanoparticles from colloid solutions at room temperature. The shape of the electrophoretic cell was designed to create a proper electric-field-gradient for deposition of electrically neutral metal particles of Pd. Layers of various thicknesses were prepared by varying the time of the deposition. When longer times were applied larger particles with the diameter about 100 nm were formed in the deposited layer, as observed by optical microscopy, besides the unresolved background presumably of 10 nm diameter Pd particles.
Schottky Barrier on InP
It is well known that metal-InP interfaces are usually subjected to a strong “Fermi level pinning” with the consequence of small barrier height on the metal interface with n-type InP . The barrier height value 1.07 eV of the prepared diode is the highest value ever reported as far as we know. It is even far higher then 829 meV achieved by the same method previously . The high value of the barrier height indicates a very low Fermi level pinning which is much promising for a good performance of the diode as a sensitive hydrogen sensor. Besides, a high quality of the diode is demonstrated by its ideality factor equal to 1.
The authors thank Dr. Svetla Vackova and MSc. Zdenek Jarchovsky for providing SEM images, MSc. Vaclav Malina for measurements of layer thicknesses and MSc. Ladislav Pekarek for providing InP crystals. The work has been financially supported by Academy of Sciences of the Czech Republic, grant KAN400670651 in the program Nanotechnology for Society.
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