Karel Zdansky

Layers of Metal Nanoparticles on Semiconductors Deposited by Electrophoresis from Solutions with Reverse Micelles

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.

Highly sensitive hydrogen sensor based on graphite-InP or graphite-GaN Schottky barrier with electrophoretically deposited Pd nanoparticles

Depositions on surfaces of semiconductor wafers of InP and GaN were performed from isooctane colloid solutions of palladium (Pd) nanoparticles (NPs) in AOT reverse micelles. Pd NPs in evaporated colloid and in layers deposited electrophoretically were monitored by SEM. Diodes were prepared by making Schottky contacts with colloidal graphite on semiconductor surfaces previously deposited with Pd NPs and ohmic contacts on blank surfaces. Forward and reverse current-voltage characteristics of the diodes showed high rectification ratio and high Schottky barrier heights, giving evidence of very small Fermi level pinning. A large increase of current was observed after exposing diodes to flow of gas blend hydrogen in nitrogen. Current change ratio about 700,000 with 0.1% hydrogen blend was achieved, which is more than two orders-of-magnitude improvement over the best result reported previously. Hydrogen detection limit of the diodes was estimated at 1 ppm H2/N2. The diodes, besides this extremely high sensitivity, have been temporally stable and of inexpensive production. Relatively more expensive GaN diodes have potential for functionality at high temperatures.

Electrophoresis deposition of metal nanoparticles with reverse micelles onto InP

Abstract Nanolayers were deposited onto surfaces of n-type InP single crystal wafers by electrophoresis from reverse micelle colloid solutions containing palladium nanoparticles. Two types of nanolayers were deposited, by applying a positive potential or a negative potential on the InP wafer. Further, wafers with nanolayers were annealed in high vacuum at 400 °C. The nanolayers were studied using capacitance–voltage characteristics on a mercury probe, by atomic force microscopy and by secondary-ion mass spectroscopy. Two types of Schottky-like diodes were prepared on wafers with the two types of nanolayers and studied by current–voltage characteristics. The diodes with nanolayers deposited by applying the positive potential, which contained Pd nanolayers, showed characteristics with better rectifying properties than the other type of diodes. Correlations among measured characterizations were found.

Palladium nanoparticles on InP for hydrogen detection

Layers of palladium (Pd) nanoparticles on indium phosphide (InP) were prepared by electrophoretic deposition from the colloid solution of Pd nanoparticles. Layers prepared by an opposite polarity of deposition showed different physical and morphological properties. Particles in solution are separated and, after deposition onto the InP surface, they form small aggregates. The size of the aggregates is dependent on the time of deposition. If the aggregates are small, the layer has no lateral conductance. Forward and reverse I-V characteristics showed a high rectification ratio with a high Schottky barrier height. The response of the structure on the presence of hydrogen was monitored.

Performance of lead iodide nuclear radiation detectors with the introduction of rare earth elements

Lead iodide has been recognized as a promising material for room temperature radiation detectors. It has a wide band-gap (∼ 2.3 eV), high atomic numbers (82, 53) and it is environmentally very stable compared to mercuric iodide. Electrical and optical properties of lead iodide grown crystals purified under the influence of selected rare earth elements have been investigated. Photo-luminescence and capacitance-voltage measurements have been performed using different rare earth elements.

Role of Rare-Earth Elements in the Technology of III-V Semiconductors Prepared by Liquid Phase Epitaxy

First applications of rare-earth (RE) elements in semiconductor technology are rooted in radiation tolerance improvements of silicon solar cells and purification of GaP crystals. The idea was later adopted in the technology of germanium and compound semiconductors. Since the 1980’s, considerable attention has been directed towards REs applications in III-V compounds both for epitaxial films and bulk crystals (Zakharenkov et al., 1997). The uniqueness of REs arises from the fact that the lowest-energy electrons are not spatially the outermost electrons of the ion, and thus have a limited direct interaction with the ion’s environment. The shielding of the 4f electrons by the outer filled shells of 5p and 5s electrons prevents the 4f electrons from directly participating in bonding (Thiel et al., 2002). The RE ions maintain much of the character exhibited by a free ion. This non-bonding property of the 4f electrons is responsible for the well-known chemical similarity of different REs. Since transitions between the electronic states of the shielded 4f electrons give rise to spectrally narrow electronic transitions, materials containing REs exhibit unique optical properties. By careful selection of the appropriate ion, intense, narrow-band emission can be gained across much of the visible region and into the near-infrared (Kenyon, 2002). Inspired by the striking results accomplished in the field of optical amplifiers and lasers based on REdoped fibres (Simpson, 2001), substantial research activity has been recently carried out on RE-doped semiconductor materials for optoelectronics (Klik et al., 2001). In most cases, however, achieving effective doping of III-V compounds by REs during growth from the liquid phase has proven difficult; the high chemical reactivity and the low solid solubility are the main restrictions on introducing RE atoms into the crystal lattices (Kozanecki & Groetzschel, 1990). On the other hand, the enhanced chemical affinity of REs towards most species of the shallow impurities leads to the formation of insoluble aggregates in the melt. Under suitable growth conditions, these aggregates are rejected by the growth front and are not incorporated into the grown layer: gettering of impurities takes place. Especially Si and main group-six elements acting as shallow donors in III-V semiconductors are effectively gettered due to REs high affinity towards them (Wu et al., 1992). Removal of detrimental impurities is of vital importance in applications such as PIN 13

High Detection Performance of Particle Detectors Based on SI InP Doped With Ti and Zn

In this work, we present results of measurements of spectra of alpha-particles carried out on semi-insulating (SI) InP detectors. The detectors were fabricated from Ti and Zn co-doped SI InP crystals grown by Czochralski technique. Tests of detectors performance with alpha-particles emitted from 241Am (5.48 MeV) radioactive source are reported. Excellent detectors performance has been evaluated by values of charge collection efficiency 99% and energy resolution ~ 1% obtained from the spectra measured at 230 K. Such high performance can be explained by InP doping with suitable Ti atoms and co-doping with low concentration of Zn acceptors sufficient for the full compensation of shallow donors, to reach SI properties. The electron mobility of reported SI InP detectors co-doped with Ti and Zn has been found to be equal to 2510 cm2 V-1 s-1 at room temperature. This value is much higher than that obtained previously with our SI InP co-doped with Ti and Mn or with InP crystals converted to SI state by temperature annealing. Excellent detector performance and high electron mobility makes the reported InP material promising to be used for the detection of X-rays and gamma-radiation.

Evaluation of Semiinsulating Annealed InP:Ta for Radiation Detectors

InP crystals were grown by the Czochralski technique. They were purified by inclusion of Ta into the growth melt and then converted to the semiinsulating state by annealing. Various annealing regimes were examined to find the optimum material for radiation detectors. Temperature dependent Hall measurements were carried over the range from 300 to 430 K and the activation energy of the impurity responsible for the semiinsulating state was determined from the slope of a straight line plot to be 0.75 eV from the conduction band edge. This energy is different from the activation energy, 0.65 eV, of Fe2+ in InP which was observed for annealed InP grown with an admixture of Fe. An InP wafer selected for fabrication of prototype particle detectors was lapped and chemomechanically polished on the both sides to a final thickness of 0.25 mm. The detectors themselves were fabricated by deposition of circular metal electrodes of 1 mm diameter on both sides of the wafer, using vacuum evaporation of Ni/Ge/Au. The performance of these particle detectors was characterised by pulse-height spectra obtained with alpha particles emitted from 241Am. The maximum of the spectral line measured at 300 K corresponded to an 85% charge collection efficiency when 100 V voltage was applied

Temperature change of Hall and Seebeck coefficient sign in InP doped with transition metals

Semi-insulating InP samples doped with transition metals were grown by the Czochralski technique. Both Hall and Seebeck effect measurements show the change of Hall and Seebeck coefficient sign at low temperatures. Electron hopping between iron or titanium ions in the nearest sites is assumed to cause the change. The theoretical calculations based on the mixed conductivity model are in good agreement with experimental values. Different temperatures for the change of sign of Seebeck or Hall coefficients for different samples are related to the activation energy of deep Ti donor and Fe acceptor energy levels, and also to the acceptor-to-donor compensation ratio.

EPD of Reverse Micelle Pd and Pt Nanoparticles onto InP and GaN for High-Response Hydrogen Sensors

We investigated properties of nanolayers electrophoretically deposited (EPD) onto semiconductor indium phosphide (InP) or gallium nitride (GaN) single crystals from colloid solutions of metal palladium (Pd), platinum (Pt) or bimetallic Pd/Pt nanoparticles (NPs) in isooctane. Colloids with metal NPs were prepared by reaction of metal compounds with the reducing agent hydrazine in water confined to reverse micelles of surfactant AOT.. Chopped DC electric voltage was applied for the time period to deposit metal NPs, only partly covering surface of the wafer. The deposits were image-observed by scanning electron microscopy (SEM)..Diodes with porous Schottky contacts were made by printing colloidal graphite on the NPs deposited surface and making ohmic contact on the blank side of the wafer. The diodes showed current-voltage characteristics of excellent rectification ratio and barrier height values close to Schottky-Mott limit, which was an evidence of negligible Fermi level pinning. Large increase of current was observed after switching on a flow of gas blend hydrogen in nitrogen (H2/N2). The diodes were measured with various H2/N2 in the range from 1000 ppm to 1 ppm of H2. Current change ratios about 106 and about 10 were achieved with 1000 ppm and 1 ppm H2/N2.