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Al-induced crystallization of co sputtered hydrogenated amorphous germanium films, deposited at 220 °C, onto crystalline silicon substrates is investigated by Raman and infrared spectroscopies as a function of the Al concentration (2×10−6<[Al/Ge]<2.5×10−2). Aluminum induces partial crystallization of the films for metal concentrations smaller than ∼1.3 at. %. A sort of explosive crystallization of the films occurs within a narrow Al concentration range (∼1.3<[Al/Ge]<∼1.8 at. %). Raman spectra do not display any crystallization signal for metal concentrations above this narrow range. Data of the extended x-ray absorption fine structure of the coordination and of the local order around gallium, in Ga-doped a-Ge:H, are used to propose an overall picture of the microscopic mechanisms behind these results. A comparative analysis suggests that the crystallization seeds are fourfold-coordinated Al atoms sitting at the center of perfect tetrahedral Ge sites.

This work reports on the structural, optical, and transport properties of rf sputtered In-doped hydrogenated amorphous germanium thin films. It has been found that the incorporation of In induces important changes in the optoelectronic properties of the Alms. The experimental results may be explained in terms of acceptor levels produced by tetrahedrally coordinated In, in a way similar to In doping of c-Ge. The transition from n-type conduction of as-deposited samples to p-type conduction has been monitored through thermopower measurements. Indium concentrations of the order of 1'Po induce the pinning of the Fermi energy at 0.45 eV above the valence band edge and a thermally activated conductivity at room temperature having an activation energy of a few meV. The eKect is explained in terms of nearest-neighbor hopping between electron states produced by In atoms in a metal-like coordination at defective sites of the Ge network.

The aim of this work is to analyse the influence of spring length on the simple harmonic motion of a spring–mass system. In particular, we study the effect of changing the spring length on the elastic constant k, the angular frequency ω and the damping factor γ of the oscillations. To characterize the behaviour of these variables we worked with a series of springs of seven different lengths, in which the elastic constant was found by means of the spring-elongation measurement and ω was obtained from the measurement of the oscillation period T of a suspended mass. The oscillatory movement was recorded using a force sensor and the γ value was determined by the fit of the envelope oscillations. Graphical analysis of the results shows that k, ω and γ decrease when the natural spring length increases. This experiment can be performed with equipment normally found in undergraduate physics laboratories. In addition, through graphical analysis students can deduce some relationships between variables that determine the simple harmonic motion behaviour.

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We describe a simple procedure for the preparation of graphite-based conductive paint and determine its basic transport properties when applied, comparing them to those of pencil strokes. Ohm’s law was fulfilled on the applied paint, which makes it an ideal strategy to teach the relations between a resistor’s length, width and resistance. The conductive paint was used in the construction of RC circuits on paper in a simple and didactic format. Using only the paint and a piece of cardboard, a completely functional parallel plate capacitor can be constructed with different plate geometries; in particular, we painted circular and rectangular plates. The charge and discharge cycles of the two RC circuits painted were observed in the oscilloscope. We obtained characteristic times and estimated the value of the dielectric constant of paper, which serves as a dielectric between the plates of the capacitors. We found conductive paint to be a useful and easy method to teach basic electricity and circuit concepts in fundamental courses and lab practices because it allows one to visualise properties such as the dependence of resistance and capacitance with geometric factors using a specific material.

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