Jiří J. Mareš

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 −190u200a°C to 65u200a°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...

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.

Electrical properties of Mn-doped GaSb

By analyzing Hall measurements, we have found that Mn is embedded in GaSb grown by the Czochralski method in an electrically active form, with a distribution coefficient of about 0.011. In addition to the Hall measurements, we also studied the d.c. electrical conductivity for several Mn concentrations at temperatures ranging from 4.2 to 300 K. Mn forms an acceptor level lying about 0.016 eV about the valence band. The Mn concentration dependence in the impurity conduction regime (below 30 K) has shown a transition to metallic conduction at a concentration of 8 × 10 23 m −3 , coinciding with the critical concentration in the Mott-Hubbard theory. An alternative to the Mott-Hubbard formula containing only universal constants and quantities directly experimentally observed is derived

Electrostatic screening and experimental evidence of a topological phase transition in a bulk quantum Hall liquid

The electrostatic screening of a medium disordered quantum Hall liquid was investigated using an electric field penetration (EFP) technique. At sufficiently low temperatures and at magnetic fields corresponding to the integral quantum Hall regime with even filling factors, two topologically different phases of the bulk electron liquid have been identified. By means of elementary analysis of experimental data it has further been shown that the transition between these phases reveals features typical of a Kosterlitz–Thouless type phase transition. Finally, the validity of the presented physical picture has been supported by means of numerical simulations of local filling factor topography.

Deep levels in GaAs due to Si δ doping

δ(Si)-doped GaAs samples grown by metalorganic vapor phase epitaxy are studied by capacitance–voltage and deep level transient spectroscopy (DLTS) techniques. A detailed analysis of the DLTS signal (including spatial profiles) is performed. DLTS spectra exhibit a clear development depending on the sheet dopant concentration ranging from 5×1014 to 2×1016u200am−2. Two observed peaks do not change its activation energy with the doping level while their amplitude increases rapidly when the doping rises. We assign them to defects generated by high silicon concentration, probably related to gallium vacancy. Another peak in the most densely doped sample seems to correspond to the DX level which is occupied near the δ layer. Peculiar features of the EL2 level are observed in δ-doped GaAs and explained by the band bending due to the dopant sheet. No indication of the emission from the quantum confinement states is found in DLTS spectra taken at temperatures 80–400 K.

Decrease in free carrier concentration in GaSb crystals using an ionized hydrogen atmosphere

Abstract GaSb single crystals were grown under a flow of ionized hydrogen using the Czochralski method without encapsulant. The optimal position of a deuterium lamp was studied, and it has been found that the lamp position is not the main factor, in principle. However, better results were achieved in the case of the lamp position being several millimeters above the solid/liquid interface. In addition, it seems to be very likely that donors are passivated more than acceptors, which is confirmed by the growth of Te-doped GaSb. The low Te-doped crystals were p-type in the whole volume, and the free carrier concentration was almost the same and very homogeneous from the top to the bottom of the GaSb crystal, 1.8 − 2.3 × 1016 cm−3.

Reflections on how to evaluate the professional value of scientific papers and their corresponding citations

It is inevitable that the ´publish or perish´ paradigm has implications for the quality of research published because this leads to scientific output being evaluated based on quantity and not preferably on quality. The pressure to continually publish results in the creation of predatory journals acting without quality peer review. Moreover the citation records of papers do not reflect their scientific quality but merely increase the impact of their quantity. The growth of sophisticated ´push -button´ technologies allows for easier preparation of publications while facilitating ready-to-publish data. Articles can thus be compiled merely through combining various measurements, usually without thought to their significance and to what purpose they may serve. Moreover any deep-rooted theory which contravenes mainstream assumptions is not welcomed because it challenges often long-established practice. The driving force for the production of an ever growing number of scientific papers is the need for authors to be recognised in order to be seriously considered when seeking financial support. Funding and fame are distributed to scientists according to their publication and citation scores. While the number of publications is clearly a quantitative criterion, much hope has been placed on citation analysis, which promised to serve as an adequate measure of genuine scientific value, i.e. of the quality of the scientific work.

Self-organized Periodic Processes: From Macro-layers to Micro-world of Diffusion and Down to the Quantum Aspects of Light

Self-similarity and the orderly crystal (often dendritic) growth are an important parts of nature as well as the source of solid-state thermal chemistry under nonequilibrium (undercooling) conditions providing theoretical roots of chemical swinging clock. Such oscillation processes known in chemistry and biology apply for systems far from equilibrium involving special cases of oscillations extending from the self-organized periodic chemical reactions (such as Liesegang’s or Belousov–Zhabotinsky’s reactions) to ordered solid-state processes, from liquids to atmosphere, from macro to micro, indispensable in biology. The chapter deals with a remarkable problem of thermal physics, unresolved for more than 70 years, concerning class of diffusion-controlled periodic chemical reactions, where macroscopically observed diffusion action attains, with appreciable accuracy, the value of Planck’s quantum. Because the classical and quantum diffusions are processes, which are indistinguishable in the configuration space, a quantum criterion in terms of diffusion constants is valid. This criterion enables one to find out conditions under which the quantum behaviour of self-organized periodic reactions can be observed. Examples are shown for the subcritical and critical oscillatory regimes; a special kind of self-organized Liesegang’s rings—annual growth rings of a trunk of larch tree is discussed. The text even involves a thinkable hypothesis of the light self-organization based on the previously analysed principle on least time (Fermat) and of the least action (Maupertuis). It was already noticed by Galileo who opened this problem aware that the cycloid curve yields the quickest descent leading to the so-called brachistochrone. The chapter contains 130 references.

Radial space-charge-limited electron flow in semi-insulating GaN:Fe

Semi-insulating iron-doped gallium nitride (GaN:Fe), a promising material for microwave electronics and optoelectronics, is characterized by means of current-voltage curves measured under non-equilibrium charge carrier injection conditions. To increase the electric field and enable local characterization of GaN:Fe, measurements are performed using a two-electrode configuration including a point contact with a diameter of 30 μm and large-area alloyed counter contact. Experimental evidence for various radial flow regimes corresponding to the universal 3/2 and quadratic power laws and to the so-called trap-filled-limit is presented. Furthermore, it is shown that the space-charge-limited current injected from the point contact may serve as a probe to measure both local and average scale density of electrically active traps, i.e., parameters that are necessary for GaN:Fe to be applied and optimized.