The study of crystal growth in phase-change thin films is of crucial importance to improve our understanding of the extraordinary phase transformation kinetics of these materials excellently suited for data storage applications. Here, we developed and used a new method, based on isothermal heating using laser illumination in combination with a high-speed optical camera, to measure the crystal growth rates, in a direct manner over 6 orders of magnitude, in phase-change thin films composed of several GeSb alloys. For Ge 8 Sb 92 and Ge 9 Sb 91 , a clear nonArrhenius temperature dependence for crystal growth was found that is described well on the basis of a viscosity model incorporating the fragility of the supercooled liquid as an important parameter. Using this model, the crystal growth rate can be described for the whole range between the glass transition temperature of about 380 K and the melting temperature of 880 K, excellently explaining that these phasechange materials show unique and remarkable behavior that they combine extremely low crystal growth rates at temperatures below 380 K required for 10 years of data retention and very fast growth rates of 15 m s −1 at temperatures near the melting point required for bit switching within tens of nanoseconds. ■ INTRODUCTIONCrystallization of phase-change materials is a temperatureactivated process, generally characterized by relatively large activation energies. 1−3 Methods for studying the crystal growth properties from room temperature up to and slightly above the glass-transition temperature are well established, and can be applied by, for example, placing a phase-change film on a hot plate or in a heating holder or furnace and using optical, atomic force, or electron microscopy to study the crystal growth. 4,5 Only microscopy-based techniques are able to directly determine crystal growth rates, while many other techniques such as resistance measurements, differential scanning calorimetry, X-ray diffraction, etc. can be used to monitor the overall crystallized fraction but cannot make a distinction between nucleation and growth. These techniques are generally also combined with isochronal (for a range of heating rates) instead of isothermal (for a range of temperatures) measurements making the analysis of the crystallization kinetics more approximate. However, a drawback of isothermal measurements is that they generally limit the maximum growth rate that can be accurately measured in situ. The reason for this limitation is mostly imposed by the (relatively slow) heating rates used to reach the isothermal temperature in combination with the requirement that crystallization only starts when the sample has become stabilized at the isothermal temperature.Here we demonstrate our work observing crystal growth at higher temperatures, and thus higher growth rates, by using a laser to additionally heat the film and by monitoring the growth using a high speed optical camera. As explained above, the benefit of optical microscopy over other methods to measure crystal grow...

Analysis of crystal growth in thin films of phase‐change materials can provide deeper insights in the extraordinary phase transformation kinetics of these materials excellently suited for data storage applications. In the present work crystal growth in GexSb100‐x thin films with x = 6, 7, 8, 9, and 10 is studied in detail, demonstrating that the crystallization temperature increases from ∼80 °C for Ge6Sb94 to ∼200 °C for Ge10Sb90 and simultaneously the activation energy for crystal growth also significantly increases from 1.7 eV to 5.5 eV. The most interesting new finding is that in the thin films containing 8, 9, and 10 at% Ge two competing growth modes occur which can have several orders of magnitude difference in growth rate at a single external temperature: an initial mode with isotropic slow growth producing circular crystals with smooth surfaces and growth fronts and a fast growth mode producing crystals with triangular shape having rough surfaces and growth fronts indicative of dendritic‐like growth. The slow‐growth mode becomes increasingly dominant for crystallization at low temperatures when the Ge concentration is increased from 8 to 10 at% Ge. For a certain Ge concentration, the slow growth mode becomes increasingly dominant at lower temperatures and the fast growth mode at higher temperatures. Latent heat produced during crystallization is considered a principal factor explaining the observations. The fast growth mode is associated with (eutectic) decomposition generating more latent heat and instable growth fronts and the slow growth mode is associated with thermodynamically less stable homogeneously alloyed crystals generating less latent heat, but stable growth fronts.

The large effects of moderate stresses on the crystal growth rate in Gedoped Sb phase-change thin films are demonstrated using direct optical imaging. For Ge 6 Sb 94 and Ge 7 Sb 93 phase-change films, a large increase in crystallization temperature is found when using a polycarbonate substrate instead of a glass substrate. This increase is attributed to the tensile thermal stress induced in the phase-change film due to a difference in thermal expansion coefficient between the film and the polycarbonate substrate. By applying a uniaxial compressive stress to a phase-change film, we show and explain that isotropic crystal growth becomes unidirectional (perpendicular to the uniaxial stress) with a strongly enhanced growth rate. This is a direct proof that modest stresses can have large consequences for the amorphous phase stability and for the crystal growth rates, and these stresses are thus highly relevant for memories based on phase-change materials.

No abstract

The electrical properties of amorphous-crystalline interfaces in phase change materials, which are important for rewritable optical data storage and for random access memory devices, have been investigated by surface scanning potential microscopy. Analysis of GeSb systems indicates that the surface potential of the crystalline phase is ∼30–60 mV higher than that of the amorphous phase. This potential asymmetry is explained qualitatively by the presence of a Schottky barrier at the amorphous-crystalline interface and supported also by quantitative Schottky model calculations.