Roman Yatskiv

Transport properties of metal-semiconductor junctions on n-type InP prepared by electrophoretic deposition of Pt nanoparticles

Electrical properties of highly rectifying Pt/InP junctions fabricated by electrophoretic deposition of Pt nanoparticles are investigated at different temperatures by the measurement of current–voltage and capacitance–voltage characteristics. The forward I–V characteristics of the junction are described by thermionic emissions theory at low forward bias (3kT/q 0.2 V. The reverse I–V characteristics are analysed in the scope of the thermionic emission model in the presence of shunt resistance. Electrical characteristics of these diodes are sensitive to gas mixtures with a low hydrogen concentration and show an extremely fast response and recovery time.

Thermal stability study of semimetal graphite n-InP and n-GaN Schottky diodes

The electrical properties of highly rectifying semimetal-graphite Schottky contacts fabricated by printing colloid graphite on n-type InP and GaN are investigated as a function of annealing temperature by current–voltage and capacitance–voltage techniques. As-deposited Schottky diodes exhibit excellent current–voltage rectifying characteristics of 7.5 × 107 and 1.9 × 1011 with Schottky barrier height of 1.13 and 1.29 eV at room temperature for InP and GaN, respectively. The key aspect of this technique, compared with conventional vacuum evaporation, is low deposition energy process, leaving the surface undisturbed.

Graphite/CdMnTe Schottky diodes and their electrical characteristics

The first Schottky diodes based on n-CdMnTe crystals with pronounced rectifying properties are investigated. It is shown that the I–V characteristics of the diodes fabricated by printing colloidal graphite can be described by the Sah–Noyce–Shockley theory of generation–recombination in the space charge region. Exponential increase of forward current with voltage is limited by a relatively low barrier height at the graphite/CdMnTe contact (~ 0.4 eV) and a significant series resistance of the crystal bulk (~ 106 Ω at room temperature). Tunneling due to high concentration of uncompensated impurities in investigated Cd0.9Mn0.1Te crystals (7 × 1017 cm−3) does not allow increasing the reverse bias voltage to the values needed for the operation of x- and γ-ray detectors. High concentration of uncompensated donors is interpreted by the fact that a certain part of the Mn atoms does not substitute for Cd but plays a role of over-stoichiometric impurities. In the case of the presence of a thin intermediate insulator layer in the graphite/CdMnTe contact, a rapid increase in the current for both polarities of high voltage due to the Frenkel–Poole emission is observed. The obtained results shed light on the problems of technology of growing and post-processing CdMnTe crystals regarded as a prospective material for x- and γ-ray detectors.

Temperature-dependent properties of semimetal graphite-ZnO Schottky diodes

Highly rectifying semimetal graphite/ZnO Schottky diodes with a low-ideality-factor (1.08 at 300 K) were investigated by temperature-dependent current-voltage measurements. The current transport was dominated by thermionic emission between 300 and 420 K and the extracted barrier height followed the Schottky-Mott relation. A Richardson constant (A** = 0.272 A cm−2K−2) extracted from the Richardson plot shows nearly linear characteristics in the temperature range 300–420 K.

Hydrogen sensors based on electrophoretically deposited Pd nanoparticles onto InP

Electrophoretic deposition of palladium nanoparticles prepared by the reverse micelle technique onto InP substrates is addressed. We demonstrate that the substrate pre-deposition treatment and the deposition conditions can extensively influence the morphology of the deposited palladium nanoparticle films. Schottky diodes based on these films show notably high values of the barrier height and of the rectification ratio giving evidence of a small degree of the Fermi level pinning. Moreover, electrical characteristics of these diodes are exceptionally sensitive to the exposure to gas mixtures with small hydrogen content.

Schottky barriers based on metal nanoparticles deposited on InP epitaxial layers

Fabrication of high-quality Schottky barriers on InP epitaxial layers prepared by liquid-phase epitaxy from rare-earth treated melts is reported. The Schottky structures are based on metal nanoparticles and a graphite layer deposited from colloidal solutions onto epitaxial layers with varying carrier concentration. The structures have notably high values of the barrier height and of the rectification ratio giving evidence of a small degree of the Fermi-level pinning. Electrical characteristics of these diodes are shown to be extremely sensitive to the exposure of gas mixtures with small hydrogen content.

Analysis of the resonant tunneling diode with the stepped pre-barrier

Resonant-tunnelling diodes (RTD) incorporating an emitter stepped pre-barrier are studied both theoretically and experimentally. The simulation of I-V characteristics of modified AlAs-GaAs double barrier RTD grown by molecular beam epitaxy with the stepped pre-barrier is presented. An 1D quantum transport simulator Wingreen based on the nonequilibrium Green functions (NEGF) is used in our case. Our result show that the coupling between energy levels in the emitter quantum-well and the main quantum well leads to the plateau behaviour of the I-V curves. The two plateau regions on the I-V characteristics have been observed in experimental and simulation results.

ZnO-Based Gas Sensors Prepared by EPD and Hydrothermal Growth

Arrays of vertically well aligned ZnO nanorods (NRs) were prepared on nanostructured ZnO films using a low temperature hydrothermal method. We propose the use of the low cost, environmentally friendly electrophoretic deposition technique (EPD) as seeding procedure, which allows the obtaining of homogeneous, well oriented nanostructured ZnO thin films. ZnO nanorod arrays were covered with graphite in order to prepare graphite/ZnO NRs junctions. These nanostructured junctions showed promising current-voltage rectifying characteristics and gas sensing properties at room temperature.

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.

Study of Schottky barriers prepared by deposition of colloidal graphite and Pt nanoparticles on InP and GaN

Schottky barriers were prepared by mechanical deposition of colloidal graphite on polished surfaces of InP or GaN single crystals. Before applying the graphite, the surface was sparsely covered with Pt nanoparticles deposited electrophoretically. Diodes, consisting of the Schottky barrier and an ohmic contact made on the plain surface, were studied by current voltage characteristics. The diodes showed high rectification ratio and electron transport governed by thermionic emission with a high Schottky barrier height indicating negligible Fermi level pinning. By testing with the flow of calibrated gas consisting of 10−6 parts of hydrogen and rest nitrogen, the current voltage characteristics showed high sensitivity to hydrogen. The sensitivity represents more than three orders-of-magnitude improvements over the best results published previously by other authors. Besides extremely high sensitivity, the both types of diodes have short response and recovery times after hydrogen exposure, are temporally stable and low production price which properties make them prospective for practical applications as hydrogen sensors. The advantage of pricier GaN diodes is their potential use at high temperatures.