I. Friedrich

Structural transformations of Ge2Sb2Te5 films studied by electrical resistance measurements

Temperature dependent measurements of the electrical resistance have been employed to study structural changes in sputtered Ge2Sb2Te5 films. The pronounced changes of film resistance due to structural changes enable a precise determination of transition temperatures and activation energies. Furthermore the technique is sensitive enough to measure the influence of ultrathin capping layers on the transformation kinetics. With increasing temperature the Ge2Sb2Te5 films undergo a structural change from an amorphous to rock salt structure (Fm3m) around 140 °C and finally a hexagonal structure (p3m) around 310 °C. Both structural changes are accompanied by a major drop of resistance. Applying the Kissinger method [Anal. Chem. 29, 1702 (1957)] the activation energy for crystallization to the rock salt structure is determined to be 2.24±0.11 eV, and for the phase transformation to the hexagonal phase to be 3.64±0.19 eV, respectively. A thin capping layer of ZnS–SiO2 leads to an increase of the first transition t...

Laser induced crystallization of amorphous Ge2Sb2Te5 films

The crystallization behavior of Ge2Sb2Te5 thin films has been analyzed by atomic force microscopy and optical reflection measurements on various time scales in order to determine the crystallization kinetics including the crystallization mechanism, the corresponding activation barrier, and the Avrami coefficient. On the minute time scale, thin amorphous films were isothermally crystallized in a furnace under a protective Ar atmosphere. From these measurements the activation energy for crystallization was determined to be (2.0±0.2) eV, in close agreement with previous studies using different techniques. The isothermal measurements also revealed a temperature dependent incubation time for the formation of critical nuclei, which is compared with recent theories. On the nanosecond time scale, Ge2Sb2Te5 was locally crystallized with a focused laser. Either crystalline spots of submicron size were generated in an as deposited amorphous film or amorphous bits in an otherwise crystalline film were recrystallized....

Atomic force microscopy study of laser induced phase transitions in Ge2Sb2Te5

Micron- and submicron-size changes induced by local laser heating in thin films of Ge2Sb2Te5 are studied by atomic force microscopy (AFM). This material is presently used for rewritable data storage that employs phase change recording. Reversible switching between the amorphous and crystalline states, which is accompanied by a considerable change in optical properties and film density, is exploited to store bits of information. The density change can be detected by AFM, which we use here to study the amorphization (writing) and recrystallization (erasure) of single bits. Both processes have been measured as a function of modification pulse power and duration. Morphology changes can be detected even if the phase change film is covered by a thin protective layer. AFM allows a precise determination of the bit size and bit depth, which characterizes the progress of the phase change in the direction of the surface normal. The present setup allows the correlation of the change in optical reflectance ΔR caused b...

Minimum time for laser induced amorphization of Ge2Sb2Te5 films

The minimum time t required to form an amorphous spot in a crystalline film of Ge2Sb2Te5 with NaCl structure was investigated for various applied laser powers P. An elementary power law of the form P∝t−0.5 is observed for pulse lengths larger than 10 ns which shows that amorphization occurs as soon as the melting temperature is reached. This implies that kinetic superheating does not occur on this time scale. The growth velocity of amorphous marks was inferred from atomic force microscopy (AFM) both parallel and perpendicular to the film plane. The growth in the vertical direction is shown to dominate the change in reflectivity and thus the size of the readout signal of data storage devices. The experimental data are compared with numerical calculation of the temperature field using finite element analysis. These calculations determine the position of the melt temperature isotherm and reproduce the depth and the area of the amorphous regions as inferred from AFM data.

Morphology and structure of laser-modified Ge2Sb2Te5 films studied by transmission electron microscopy

Abstract The morphology and crystal structure of laser-modified areas in an amorphous Ge2Sb2Te5 film have been investigated by transmission electron microscopy (TEM) and selected area electron diffraction (SAD). Two different types of crystallized areas are found by TEM. Low power laser irradiation leads to crystallization out of the solid phase. Whereas these crystalline areas are characterized by a fine grained morphology (dgrain=5–25 nm), the grain distribution of melt crystallized areas is rather different. It resembles the well known pattern of solidified alloy ingots but is formed on a different length- and timescale. The crystal structure of both the fine-grained crystalline area as well as of the melt-crystallized area is shown to be cubic and not the more complex hexagonal equilibrium structure of Ge2Sb2Te5. These findings enable a deeper understanding of fast nucleation and growth processes in phase change media.

The quest for fast phase change materials

Strategies are presented to identify phase change materials with fast transformation kinetics. A microscopic investigation of crystallization kinetics with a static tester and an atomic force microscope demonstrates that for different alloy compositions recrystallization proceeds either via growth from the crystalline rim or by nucleation and growth of critical nuclei. The influence of dielectric layers enables a further optimization of transformation kinetics.

Microscopic studies of fast phase transformations in GeSbTe films

Abstract : Vital requirements for the future success of phase change media are high data transfer rates, i.e., fast processes to read, write and erase bits of information. The understanding and optimization of fast transformations is a considerable challenge since the processes only occur on a submicrometer length scale in actual bits. Hence both high temporal and spatial resolution is needed to unravel the essential details of the phase transformation. We employ a combination of fast optical measurements with microscopic analyses using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The AFM measurements exploit the fact that the phase transformation from amorphous to crystalline is accompanied by a 6% volume reduction. This enables a measurement of the vertical and lateral speed of the phase transformation. Several examples will be presented showing the information gained by this combination of techniques.

Exploring the Limits of Fast Phase Change Materials

In the last decade a number of chalcogenide alloys, including ternary alloys of GeSbTe and quaternary alloys of InAgSbTe, have been identified which enable fast phase change recording. In the quest for materials with improved phase change kinetics we present two different approaches. By comparing alloys with well-defined stoichiometries the mechanisms which govern the transformation kinetics are determined. Optical and electrical measurements determine the activation energy for crystallization to 2.24 ± 0.11 eV for Ge 2 Sb 2 Te 5 and to 3.71 ± 0.07 eV for Ge 4 Sb 1 Te 5 , respectively. It is shown that for GeSbTe-alloys with different composition the activation energy increases linearly with increasing Ge content. Power-time- reflectivity change diagrams recorded with a static tester reveal that Ge 2 Sb 2 Te 5 , in agreement with previous data, recrystallizes by the growth of sub critical nuclei, while Ge 4 Sb 1 Te 5 grows from the crystalline rim surrounding the bit. To speed up the search for faster materials we employ concepts of combinatorial material synthesis by producing films with a stoichiometry gradient. Then laterally resolved secondary neutral mass spectroscopy (SNMS) combined with the static tester are used to identify the composition with superior properties for phase change applications.