Chalcogenide glasses

M. FONTANA; A. UREÑA; J.ROCCA; J.M. CONDE-GARRIDO; B.ARCONDO

Barcelona

Workshop; International Workshop on Disordered, Nanostructured and Nanocrystalline Materials; 2012

One of most important properties of some tellurium-based chalcogenide glasses is the optical and electrical switching between two situations: the glass and the crystalline state. Nowadays, this property is widely used in optical information technologies (CD, R-DVD). Moreover, a first step has been taken to use these materials in computer memories. The understanding of the glass to crystal transition and its transformation kinetics is essential for their application in non-volatile memories where the switching is electrical. This kind of memories is a key component of integrated circuits because they retain data when power is interrupted. GeSbTe is very important system that presents this property and it can be employed to make non-volatile memories, being Ge2Sb2Te5 the best alloy. The glass forming ability of this system, by rapid solidification from the liquid state, is restricted to a small composition range nearby the binary eutectic Ge15Te85. The crystallization kinetics of the samples was studied by means of differential scanning calorimetry under both isothermal and continuous heating regimes. The quenched samples and the crystallization products have been characterized by X-ray diffraction with Cu(Ka) radiation. The crystallization temperature, activation energy, crystallization enthalpy and the dependence of these properties on concentration are reported.   The glass forming range of this system is widely expanded when the samples are deposited as thin films. This geometry is essential for their technologies applications. Following these ideas, GeSbTe thin films (of about 100 nm thickness) were deposited using PLD (Pulsed Laser Deposition) using glass and metallic subtract. The crystallization temperature of Ge2Sb2Te5 thin films is about 160 °C and corresponds to amorphous-metastable crystal transition. The key of their application in non-volatile memories is that they can be switched rapidly between amorphous and crystalline phases by applying appropriate heat pulses. Specifically, thin film can be amorphized by pulsed heating (~10 ns) above the melting temperature (~600 °C) with subsequent rapid cooling. Re-crystallization is achieved by a slightly longer heat pulse (~10 ns) below the melting temperature but above the glass temperature. Our aim is to study, using electronic microscopy and nano-calorimetry the kinetic amorphous – crystal state. The heating rates of the nano-calorimetry are many orders of magnitude higher than standard calorimetry, typically 103-105 K/s; these thermal treatments are similar to those involucrate in non-volatile memories applications. We propose to study the crystallization kinetic of thin film (analyzing crystalline phases, chemical composition, interphase proportion and the crystal size) crystallizing under two ways: electrical heating and thermal heating. In first case, a cell metal/thin-film/metal will be built and nano-calorimetry will be used for the second case.