Size and dimensionality effects in superconducting Mo thin films

Molybdenum is a low Tc, type I superconductor whose fundamental properties are poorly known. Its importance as an essential constituent of new high performance radiation detectors, the so-called transition edge sensors (TESs) calls for better characterization of this superconductor, especially in thin film form. Here we report on a study of the basic superconducting features of Mo thin films as a function of their thickness. The resistivity is found to rise and the critical temperature decreases on decreasing film thickness, as expected. More relevant, the critical fields along and perpendicular to the film plane are markedly different, thickness dependent and much larger than the thermodynamic critical field of Mo bulk. These results are consistent with a picture of type II 2D superconducting films, and allow estimates of the fundamental superconducting lengths of Mo. The role of morphology in determining the 2D and type II character of the otherwise type I molybdenum is discussed. The possible consequences of this behaviour on the performance of radiation detectors are also addressed

Thickness dependence of critical temperature in Mo/Au bilayers

We report on the sensitivity of the superconducting critical temperature (TC) to layer thickness, as well as on TC reproducibility in Mo/Au bilayers. Resistivity measurements on samples with a fixed Au thickness (dAu) and Mo thickness (dMo) ranging from 50 to 250 nm, and with a fixed dMo and different dAu thickness are shown. Experimental data are discussed in the framework of Martinis model, whose application to samples with dAu above their coherence length is analysed in detail. Results show a good coupling between normal and superconducting layers and excellent TC reproducibility, allowing to accurately correlate Mo layer thickness and bilayer TC.

Fabrication process of Mo/Au TES at RT

We report on the fabrication details of TES based on Mo/Au bilayers. The Mo layer is deposited by radio frequency (RF) sputtering and capped with a sputter deposited thin Au protection layer. Afterwards, a second Au layer of suitable (lower) resistivity is deposited ex‐situ by e‐beam evaporation, until completion of the total desired Au thickness. The deposition was performed at room temperature (RT) on LPCVD Si3 N4 membranes. Such a deposition procedure is very reproducible and allow controlling the critical temperature (Tc) and normal electrical resistance (RN ) of the Mo/Au bilayer. The process is optimized to achieve low stress bilayers, thus avoiding the undesirable curvature of the membranes. Bilayers are patterned using photolithographic techniques and wet etching procedures. Mo superconducting paths are used to contact the Mo/Au bilayers, thus ensuring good electrical conductivity and thermal isolation. The entire fabrication process let to stable and reproducible sensors with required and tunable functional properties

Thermal effects in Ni/Au and Mo/Au gate metallization AlGaN/GaN HEMT's reliability

AlGaN/GaN high electron mobility transistors (HEMT) are key devices for the next generation of high-power, high-frequency and high-temperature electronics applications. Although significant progress has been recently achieved [1], stability and reliability are still some of the main issues under investigation, particularly at high temperatures [2-3]. Taking into account that the gate contact metallization is one of the weakest points in AlGaN/GaN HEMTs, the reliability of Ni, Mo, Pt and refractory metal gates is crucial [4-6]. This work has been focused on the thermal stress and reliability assessment of AlGaN/GaN HEMTs. After an unbiased storage at 350 o C for 2000 hours, devices with Ni/Au gates exhibited detrimental IDS-VDS degradation in pulsed mode. In contrast, devices with Mo/Au gates showed no degradation after similar storage conditions. Further capacitance-voltage characterization as a function of temperature and frequency revealed two distinct trap-related effects in both kinds of devices. At low frequency (< 1MHz), increased capacitance near the threshold voltage was present at high temperatures and more pronounced for the Ni/Au gate HEMT and as the frequency is lower. Such an anomalous “bump” has been previously related to H-related surface polar charges [7]. This anomalous behavior in the C-V characteristics was also observed in Mo/Au gate HEMTs after 1000 h at a calculated channel temperatures of around from 250 o C (T2) up to 320 ºC (T4), under a DC bias (VDS= 25 V, IDS= 420 mA/mm) (DC-life test). The devices showed a higher “bump” as the channel temperature is higher (Fig. 1). At 1 MHz, the higher C-V curve slope of the Ni/Au gated HEMTs indicated higher trap density than Mo/Au metallization (Fig. 2). These results highlight that temperature is an acceleration factor in the device degradation, in good agreement with [3]. Interface state density analysis is being performed in order to estimate the trap density and activation energy.

Tres iglesias sajonas fortificadas en Transilvania, Rumanía: Hosman, Pelişor y Moşna

Saxon colonization in Transylvania occurred from 12th Century with the establishment of the populations with certain degree of freedom and autonomy. External threats over this territory obliged to the fortification of their churches, mainly between 15th and 16th Centuries with the advance of Turkish Empire. The fortification of the churches consists on the building of walls with gates and flanking towers, and also with the incorporation of several defensive elements over the apses. Most outstanding characteristics of this fortified system are the homogeneity in typology, construction and polyorcetic, without detracting of the singularity of each one of the more of 150 conserved churches. The article presents an actual state of the art. Also, it develops the main historical elements and it explains the architectonic typology and the constructive characteristics. At last, defensive elements are analyzed from the comparative and deep study through three proper examples: the churches of Hosman, Pelisor and Mosna. La colonización sajona de Transilvania se produjo a partir del siglo XII con el asentamiento de poblaciones con cierto grado de libertad y autonomía. Las amenazas externas sobre este territorio obligaron a la fortificación de sus iglesias, principalmente entre los siglos XV y XVI ante el avance del imperio turco. La fortificación de las iglesias consta de la construcción de murallas con puertas y torres de flanqueo, además de la incorporación de diversos elementos poliorcéticos sobre las cabeceras de las iglesias. Las características más relevantes de este sistema fortificado es la homogeneidad tipológica, constructiva y poliorcética sin menoscabo de la singularidad de cada una de las más de 150 iglesias conservadas. El artículo presenta un estado de la cuestión actualizado, además de desarrollar los elementos históricos más relevantes, explicar la tipología arquitectónica y las característica constructivas y analizar los elementos poliorcéticos desde el estudio comparado y profundo de tres ejemplos característicos: las iglesias de Hosman, Pelisor y Mosna.

Characterization of a Mo/Au thermometer for ATHENA

The first dark characterization of a thermometer fabricated with our Mo/Au bilayers to be used as a transition edge sensor is presented. High-quality, stress-free Mo layers, whose thickness is used to tune the critical temperature (TC ) down to 100 mK, are deposited by sputtering at room temperature (RT ) on Si3N4 bulk and membranes, and protected from degradation with a 15-nm sputtered Au layer. An extra layer of high-quality Au is deposited by ex situ e-beam to ensure low residual resistance. The thermometer is patterned on a membrane using standard photolithographic techniques and wet etching processes, and is contacted through Mo paths, displaying a sharp superconducting transition (α ≈ 600). Results show a good coupling between Mo and Au layers and excellent TC reproducibility, allowing to accurately correlate dM o and TC . Since dAu is bigger than ξM for all analyzed samples, bilayer residual resistance can be modified without affecting TC . Finally, first current to voltage measurements at different temperatures are measured and analyzed, obtaining the corresponding characterization parameters.

Cu(In,Ga)Se2 absorber thinning and the homo-interface model: Influence of Mo back contact and 3-stage process on device characteristics

Thinning the absorber layer is one of the possibilities envisaged to further decrease the production costs of Cu(In,Ga)Se2 (CIGSe) thin films solar cell technology. In the present study, the electronic transport in submicron CIGSe-based devices has been investigated and compared to that of standard devices. It is observed that when the absorber is around 0.5 μm-thick, tunnelling enhanced interface recombination dominates, which harms cells energy conversion efficiency. It is also shown that by varying either the properties of the Mo back contact or the characteristics of 3-stage growth processing, one can shift the dominating recombination mechanism from interface to space charge region and thereby improve the cells efficiency. Discussions on these experimental facts led to the conclusions that 3-stage process implies the formation of a CIGSe/CIGSe homo-interface, whose location as well as properties rule the device operation; its influence is enhanced in submicron CIGSe based solar cells.

Optical absorption and applications of the ABO4 (A=Ca, Pb and B= Mo, W) semiconductors

Ternary molybdates and tungstates ABO4 (A=Ca, Pb and B= Mo, W) are a group of materials that could be used for a variety of optoelectronic applications. We present a study of the optoelectronic properties based on first-principles using several orbitaldependent one-electron potentials applied to several orbital subspaces. The optical properties are split into chemical-species contributions in order to quantify the microscopic contributions. Furthermore, the effect of using several one-electron potentials and orbital subspaces is analyzed. From the results, the larger contribution to the optical absorption comes from the B-O transitions. The possible use as multi-gap solar cell absorbents is analyzed.