Pavel Hubík

Superconductive B-doped nanocrystalline diamond thin films: Electrical transport and Raman spectra

Electrical transport properties of thin boron doped nanocrystalline diamond films with thicknesses of 60–500nm have been studied. The Raman spectra measured exhibit Fano resonances, characteristic for B concentrations close to the metal-to-insulator transition. Upon increasing the B concentration, the sp2 carbon related Raman resonances vanish. In such boron-doped nanocrystalline diamond films, a positive magnetoresistance could be observed at liquid helium temperatures. The boron doped diamond films show conductivity similar to that of B-doped epitaxial diamond without any significant contribution of the grain boundary transport, leading to the superconductive transition in nanocrystalline diamond at ∼1.66K.

Weak localization in ultrananocrystalline diamond

In this letter we present results of magnetotransport measurements, carried out on heavily nitrogen-doped ultrananocrystalline diamond films, prepared by plasma-enhanced chemical vapor deposition. This material having at room temperature appreciably high electric conductivity (∼4200Sm−1) revealed surprisingly at Kelvin temperatures a giant negative magnetoresistance reaching up to 22% at 1.28K and at 8T. The analysis of experimental data has borne evidence of the fact that the transport in this subsystem has a character of low-dimensional disordered metal and it is controlled by quantum interference effects of electrons resulting in their weak localization.

Glassy, Amorphous and Nano-Crystalline Materials

Glassy, Amorphous and Nano-crystalline Material: Thermal Physics, Analysis, Structure and Properties includes twenty-one chapter contributions from an international array of distinguished academics based in Asia, Australia, Eastern and Western Europe, Russia, and the USA. The book provides a coherent and authoritative overview of cuttingedge themes involving the thermal analysis, applied solid-state physics and the microcrystallinity of selected materials and their macroand microscopic thermal properties. Selected chapters featured in the book include: Essential attributes of glassiness regarding the nature of non-crystalline solids; Aspects of vitrification, amorphization, disordering and the extent of nano-crystallinity; The basic role of thermal analysis in polymer physics; Classical and quantum diffusion and their application to the self-organized oscillatory reactions; Specificity of low temperature measurements applied to nano-crystalline diamante; Thermophysical properties of natural glasses at the extremes of the thermal history profile; Phenomenological meaning of temperature as background for the history and development of thermal analysis and calorimetry. Advanced undergraduates, postgraduates and researchers working in the field of thermal analysis and calorimetry will find this contributed volume invaluable.

Influence of growth rate on charge transport in GaSb homojunctions prepared by metalorganic vapor phase epitaxy

Dark current–voltage (I–V) characteristic measurement in the temperature range from −190 °C to 65 °C was carried out on GaSb p-n homojunctions prepared by low-pressure metalorganic vapor phase epitaxy. It was shown that the charge transport mechanism in these homojunctions is strongly affected by the growth rate of GaSb epitaxial layers. Samples prepared at higher growth rate (40 nm/min.) exhibit an anomalous low-temperature peak of tunneling current which can be explained by the presence of a narrow band of energies due to high concentration of native defects, probably GaSb antisites. The same defect levels are responsible for the generation–recombination current which dominates in these samples at higher temperatures. On the other hand, quite different behavior was found in the case of slowly grown (20 nm/min) samples. At sufficiently low temperatures, a current maximum near 50 mV of forward voltage points out a band-to-band tunneling as a prevailing transport mechanism. With increasing temperature, how...

Historical Roots and Development of Thermal Analysis and Calorimetry

Apparently, the first person which used a thought experiment of continuous heating and cooling of an illustrative body was curiously the Czech thinker and Bohemian educator [1], latter refugee Johann Amos Comenius (Jan Amos Komenský, 1592–1670) when trying to envisage the properties of substances. In his “Physicae Synopsis”, which he finished in 1629 and published first in Leipzig in 1633, he showed the importance of hotness and coldness in all natural processes. Heat (or better fire) is considered as the cause of all motions of things. The expansion of substances and the increasing the space they occupy is caused by their dilution with heat. By the influence of cold the substance gains in density and shrinks: the condensation of vapor to liquid water is given as an example. Comenius also determined, though very inaccurately, the volume increase in the gas phase caused by the evaporation of a unit volume of liquid water. In Amsterdam in 1659 he published a focal but rather unfamiliar treatise on the principles of heat and cold [2], which was probably inspired by the works of the Italian philosopher Bernardino Telesius. The third chapter of this Comenius’ book was devoted to the description of the influence of temperature changes on the properties of substances.

Synthesis, structure, and opto-electronic properties of organic-based nanoscale heterojunctions

Enormous research effort has been put into optimizing organic-based opto-electronic systems for efficient generation of free charge carriers. This optimization is mainly due to typically high dissociation energy (0.1-1 eV) and short diffusion length (10 nm) of excitons in organic materials. Inherently, interplay of microscopic structural, chemical, and opto-electronic properties plays crucial role. We show that employing and combining advanced scanning probe techniques can provide us significant insight into the correlation of these properties. By adjusting parameters of contact- and tapping-mode atomic force microscopy (AFM), we perform morphologic and mechanical characterizations (nanoshaving) of organic layers, measure their electrical conductivity by current-sensing AFM, and deduce work functions and surface photovoltage (SPV) effects by Kelvin force microscopy using high spatial resolution. These data are further correlated with local material composition detected using micro-Raman spectroscopy and with other electronic transport data. We demonstrate benefits of this multi-dimensional characterizations on (i) bulk heterojunction of fully organic composite films, indicating differences in blend quality and component segregation leading to local shunts of photovoltaic cell, and (ii) thin-film heterojunction of polypyrrole (PPy) electropolymerized on hydrogen-terminated diamond, indicating covalent bonding and transfer of charge carriers from PPy to diamond.

Deep defects in GaN/AlGaN/SiC heterostructures

Deep level transient spectroscopy (DLTS) measurements were carried out on GaN/AlGaN/SiC heterostructures prepared by low-pressure metalorganic vapor phase epitaxy. Si-doped n-GaN layers were grown using an n-AlGaN nucleation layer (8% and 30% of aluminum) on two kinds of p-type 4H-SiC substrates. The DLTS spectra of on-axis (0001) grown samples exhibit a dominant peak of a majority carrier trap with apparent activation energy close to 0.80 eV and capture cross section of about 5×10−14 cm2 regardless of the AlGaN composition. The energy of this deep level decreases with increasing electrical field due to Poole–Frenkel effect. Carrier capture kinetics indicates interacting point defects arranged along a line, probably a threading dislocation. Two additional traps (0.52 and 0.83 eV) were found in on-axis samples with 8% AlGaN composition. For 30% Al content, only a 0.83 eV level was detected. Majority carrier trap with activation energy of 0.66 eV was observed in the off-axis grown samples. This level is pro...

Selected topics related to the transport and superconductivity in boron-doped diamond

Abstract This contribution deals with a few topics closely related to the superconductivity in the heavily boron-doped diamond which are, in our opinion, not properly treated in the current literature. Attention is paid especially to the classification of metallic and insulating state, selection of pairing mechanism, limits of weak coupling approximation and to the influence of granularity on the superconducting transition.

High field transport in semi-insulating GaAs: A promising material for solid-state detectors

Semi-insulating GaAs detector structures have been studied by transport [current–voltage (I–V) curves] and scanning electron microscope voltage (potential) contrast methods. The saturated part of I–V curves, suitable for radiation detection, is due to the velocity controlled high field transport mechanism while the sharp increase of current with voltage is ascribed to the impact ionization. It is shown that a modified voltage contrast method is able to reveal the actual potential distribution and to contribute to the optimization of the radiation detector structures.

TRANSPORT CONSTITUTIVE RELATIONS, QUANTUM DIFFUSION AND PERIODIC REACTIONS

In this contribution we are discussing a class of linear phenomenological transport equations and in some cases also their relation to microphysical description of corresponding effects. Interestingly enough, in spite of practically identical forms of these constitutive relations there are large differences in their physical content; just such a large diversity of natural processes behind the same mathematical form should serve as a serious warning before making superficial analogies. On the other hand, besides quite obvious analogies there may be found also those much deeper and sometimes quite astonishing. Lesser known or even new aspects of this kind the reader can find especially in paragraphs dealing with Ohm’s law and with statistical interpretation of generalized Fick’s law. The congruence of the last one with the fundamental equation of quantum mechanics, the Schrodinger equation, opened the possibility to interpret the rather enigmatic “quantum” behaviour of periodic chemical reactions as a special kind of diffusion.