Electrical activation of carbon in GaAs : implantation temperature effects

Carbon was implanted into GaAs at the energy of 1 MeV with doses between 131013 and 2 31015 cm22 at temperatures of 80 K, nominal room temperature (RT), and 300 °C. A markedly higher electrical activation was obtained in the samples implanted at 80 K compared to those implanted at RT or 300 °C, attaining a maximum hole concentration of 231019 cm23. The redistribution of the C profile during rapid thermal annealing at temperatures from 700 to 950 °C for 10 s was found negligible, independently of the implantation temperature. Similar improvements in the electrical properties were also verified in samples implanted at 80 K with a lower energy of 60 keV. We consider that despite the light mass of C ions, the reduced dynamic annealing at 80 K allows the accumulation of an abundance of As vacancies, which assist the C activation as a p-type dopant

An experimental study on gold precipitation from leach solutions of technogenic gold-bearing raw materials

This paper presents the results of the study dedicated to the determination of the optimum parameters for the electrolytic gold precipitation from thiourea leach solutions. The leaching was carried out using technogenic gold-bearing raw materials (gold-bearing sands) of the Far East of the Russian Federation. The study focused on the influence of the below parameters of electrolytic precipitation: electrolyte composition, overall cathodic current density, electrode voltage, and structural design of an electrolytic cell. As the results of the experiments, the optimum electrolyte composition and parameters of the current (overall cathodic current density 1000 А/m2, electrode voltage 6 V) were determined. Under these parameters, it is possible to recover up to 93 % of gold from thiourea leach solutions.Keywords: technogenic gold-bearing raw materials, gold, thiourea leaching, electrolyte, cathodic current density, electrolytic cel

Homogeneous photometry and star counts in the field of 9 Galactic star clusters

We present homogeneous $V,I$ CCD photometry of nine stellar fields in the two inner quadrants of the Galactic plane. The lines-of-view to most of these fields aim in the direction of the very inner Galaxy, where the Galactic field is very dense, and extinction is high and patchy. Our nine fields are, according to several catalogs, centred on Galactic star clusters, namely Trumpler 13, Trumpler 20, Lynga 4, Hogg 19, Lynga 12, Trumpler 25, Trumpler 26, Ruprecht 128, and Trumpler 34. Apart from their coordinates, and in some cases additional basic data (mainly from the 2MASS archive), their properties are poorly known. By means of star count techniques and field star decontaminated Color-Magnitude diagrams, the nature and size of these visual over-densities has been established; and, when possible, new cluster fundamental parameters have been derived. To strengthen our findings, we complement our data-set with JHK$_{s}$ photometry from the 2MASS archive, that we analyze using a suitably defined Q-parameter. Most clusters are projected towards the Carina-Sagittarium spiral arm. Because of that, we detect in the Color Magnitude Diagrams of most of the other fields several distinctive sequences produced by young population within the arm. All the clusters are of intermediate or old age. The most interesting cases detected by our study are, perhaps, that of Trumpler 20, which seems to be much older than previously believed, as indicated by its prominent -and double- red clump; and that of Hogg 19, a previously overlooked old open cluster, whose existence in such regions of the Milky Way is puzzling.Comment: 37 pages, 16 eps figures, in press in New Astronom

Early stage morphology of quench condensed Ag, Pb and Pb/Ag hybrid films

Scanning Tunneling Microscopy (STM) has been used to study the morphology of Ag, Pb and Pb/Ag bilayer films fabricated by quench condensation of the elements onto cold (T=77K), inert and atomically flat Highly Oriented Pyrolytic Graphite (HOPG) substrates. All films are thinner than 10 nm and show a granular structure that is consistent with earlier studies of QC films. The average lateral diameter, $\bar {2r}$, of the Ag grains, however, depends on whether the Ag is deposited directly on HOPG ($\bar {2r}$ = 13 nm) or on a Pb film consisting of a single layer of Pb grains ($\bar {2r}$ = 26.8 nm). In addition, the critical thickness for electrical conduction ($d_{G}$) of Pb/Ag films on inert glass substrates is substantially larger than for pure Ag films. These results are evidence that the structure of the underlying substrate exerts an influence on the size of the grains in QC films. We propose a qualitative explanation for this previously unencountered phenomenon.Comment: 11 pages, 3 figures and one tabl

Electrical transport studies of quench condensed Bi films at the initial stage of film growth: Structural transition and the possible formation of electron droplets

The electrical transport properties of amorphous Bi films prepared by sequential quench deposition have been studied in situ. A superconductor-insulator (S-I) transition was observed as the film was made increasingly thicker, consistent with previous studies. Unexpected behavior was found at the initial stage of film growth, a regime not explored in detail prior to the present work. As the temperature was lowered, a positive temperature coefficient of resistance (dR/dT > 0) emerged, with the resistance reaching a minimum before the dR/dT became negative again. This behavior was accompanied by a non-linear and asymmetric I-V characteristic. As the film became thicker, conventional variable-range hopping (VRH) was recovered. We attribute the observed crossover in the electrical transport properties to an amorphous to granular structural transition. The positive dR/dT found in the amorphous phase of Bi formed at the initial stage of film growth was qualitatively explained by the formation of metallic droplets within the electron glass.Comment: 7 pages, 6 figure

Electrical Isolation Of A Silicon δ-doped Layer In Gaas By Ion Irradiation

The electrical isolation of a n-type δ-doped layer embedded into undoped GaAs was studied using proton or helium ion bombardment. The threshold dose for isolation Dth of the δ-doped layer was found to be ≈2 times higher than that predicted for thick doped layers of similar carrier concentration. The thermal stability of the isolation, i.e., the persistence of sheet resistance Rs at values > 109Ω/□ after subsequent thermal annealing, is limited to temperatures below 400°C. This temperature limit for the thermal stability Tsm is markedly lower than those observed in wider doped layers in which Tsm is ≅650°C. A previously isolated δ-doped layer presents p-type conductivity after annealing at temperatures >600°C . © 1999 American Institute of Physics.751319171919Shubert, E.F., (1990) J. Vac. Sci. Technol. A, 8, p. 2980Daniltsev, V.M., Irin, I.V., Murel, A.V., Khrykin, O.I., Shashkin, V.I., (1994) Inorg. Mater. (Transl. of Neorg. Mater.), 30, p. 948Zrenner, A., Koch, F., Ploog, K., (1987) Inst. Phys. Conf. Ser., 91, p. 171Van Der Pauw, L.J., (1958) Philips Res. Rep., 13, p. 1De Souza, J.P., Danilov, I., Boudinov, H., (1997) J. Appl. Phys., 81, p. 650Ziegler, J.F., Biersack, J.P., Littmark, U., (1985) The Stopping and Range of Ions in Solids, 1. , Pergamon, OxfordDe Souza, J.P., Danilov, I., Boudinov, H., (1998) J. Appl. Phys., 84, p. 4757De Souza, J.P., Danilov, I., Boudinov, H., (1996) Appl. Phys. Lett., 68, p. 535De Souza, J.P., Danilov, I., Boudinov, H., (1998) Radiat. Eff. Defects Solids, 147, p. 109Grandidier, B., Stiévenard, D., Nys, J.P., Wallart, X., (1998) Appl. Phys. Lett., 72, p. 245

Optical excitation of single- and multi-mode magnetization precession in Fe-Ga nanolayers

We demonstrate a variety of precessional responses of the magnetization to ultrafast optical excitation in nanolayers of Galfenol (Fe,Ga), which is a ferromagnetic material with large saturation magnetization and enhanced magnetostriction. The particular properties of Galfenol, including cubic magnetic anisotropy and weak damping, allow us to detect up to 6 magnon modes in a 120nm layer, and a single mode with effective damping _eff = 0.005 and frequency up to 100 GHz in a 4- nm layer. This is the highest frequency observed to date in time-resolved experiments with metallic ferromagnets. We predict that detection of magnetisation precession approaching THz frequencies should be possible with Galfenol nanolayers

Quasi freestanding graphene on SiC 0001 via cobalt intercalation of zero layer graphene

Modification of the electronic and crystal structure of zero layer graphene grown on 6H SiC 0001 after Co intercalation is reported. Using a wide range of techniques including angle resolved photoelectron spectroscopy, x ray photoelectron spectroscopy, Raman spectroscopy, low energy electron diffraction, we found that zero layer graphene on SiC transforms into graphene monolayer as a result of cobalt intercalation. The Dirac cone of amp; 960; band characteristic of quasi freestanding graphene is observed. In combination with high resolution transmission electron microscopy and atomic force microscopy data, we conclude that ultrathin silicide CoSi CoSi2 structure is formed between graphene and SiC substrate. Investigation of magnetic properties reveals ferromagnetic behavior with open hysteresis loop. The results of this work are the basis for further implementation of magneto spin orbit graphene on a semiconducting substrate and are important for the future application of such graphene in spintronic

Modeling of some nonstationary processes in tokamak plasmas

The results of modeling of shots from two series of experiments in two tokamaks with rather different geometric parameters are presented. The first one includes two shots from the spherical tokamak MAST with current ramp up, and the second one, a number of T-10 shots with periodic gas puffing. The modeling was performed with the ASTRA code in the framework of Canonical Profiles Transport Model (CPTM).Представлены результаты моделирования импульсов из двух серий экспериментов на двух разных токамаках, существенно отличающихся геометрическими параметрами. Первая серия включает два импульса с нарастанием тока сферического токамака MAST, а вторая – несколько импульсов Т-10 с периодическим газонапуском. Моделирование осуществлялось с помощью кода ASTRA в рамках транспортной модели канонических профилей (ТМКП).Представлені результати моделювання імпульсів з двох серій експериментів на двох різних токамаках, що істотно відрізняються геометричними параметрами. Перша серія включає два імпульси з наростанням струму сферичного токамака МАST, а друга – декілька імпульсів Т-10 з періодичним газонапуском. Моделювання здійснювалося за допомогою коду ASTRA в рамках транспортної моделі канонічних профілів (ТМКП)