Walter K. Njoroge

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...

Density changes upon crystallization of Ge2Sb2.04Te4.74 films

The density of sputtered Ge2Sb2.04Te4.74 thin films upon annealing has been precisely determined by x-ray reflection and compared to the values determined from x-ray diffraction (XRD) data. The film density increases in two steps around 130 and 280 °C upon annealing up to 400 °C. These increases are consequences of phase transitions from amorphous to NaCl type and from NaCl type to hexagonal structure, respectively, as revealed by XRD. Average density values of 5.87±0.02, 6.27±0.02, and 6.39±0.02 g/cm3 were measured for the amorphous, NaCl-type, and hexagonal phases, respectively. This corresponds to density changes upon crystallization of 6.8±0.2% and 8.8±0.2% for NaCl-type and hexagonal phases, respectively. The accompanying film thickness reductions were determined to be 6.5±0.2% and 8.2±0.2%, which compares very well with the density changes. The corresponding XRD values are determined to be 6.43–6.48 and 6.48 g/cm3 for NaCl-type and the hexagonal phases, respectively. This shows that nearly void-free...

Mechanical stresses upon crystallization in phase change materials

Crystallization processes in different Te alloys, employed in phase changematerials for optical data storage, have been investigated by in situ mechanical stress measurements. Upon crystallization a considerable stress buildup is observed, which scales with the volume change upon crystallization. Nevertheless the observed stress change only corresponds to approximately 9% of the stress estimated for a purely elastic transformation. Further evidence of stress relief phenomena comes from the temperature dependence of the stress in the crystalline and amorphous states. Ultrathin dielectric layers have a profound influence on the crystallization process as evidenced by simultaneous optical reflectance and mechanical stress measurements. This observation can be explained by heterogeneous nucleation of crystallites at the interface between the dielectric layer and the phase changefilm.

Crystallization kinetics of sputter-deposited amorphous AgInSbTe films

AgInSbTe films have recently attracted considerable interest as advanced materials for phase change recording. For this application the determination of crystallization kinetics is of crucial importance. In this work the temperature dependence of structural and electrical properties of sputtered AgInSbTe films has been determined. Temperature dependent measurements of the electrical resistance have been employed to study the kinetics of structural changes of these films. Upon annealing a major resistivity drop is observed at around 160 °C which can be attributed to a structural change as corroborated by x-ray diffraction. X-ray diffraction shows an amorphous phase for as-deposited films, while crystalline films with hexagonal structure (a=4283 A, c=16 995 A) are obtained upon annealing above 160 °C. By applying Kissinger’s method, an activation energy of 3.03±0.17 eV is obtained for the crystallization. X-ray reflection measurements reveal a density increase of 5.2%±0.2% and a thickness decrease of 5.5%±0...

Crystallization kinetics of Ge4Sb1Te5 films

Abstract The crystallization kinetics of sputtered Ge 4 Sb 1 Te 5 films has been investigated using temperature-dependent sheet resistance measurements in conjunction with structural investigations employing X-ray diffraction (XRD) and X-ray reflectometry (XRR). This work correlates the kinetics of crystallization of thin Ge 4 Sb 1 Te 5 films to the structural and density changes. The resistivity of the Ge 4 Sb 1 Te 5 samples decreases from 2 to 3.1×10 −5 Ω m upon annealing above the phase transition temperature. The transition temperature varies with the heating rate and has been determined to be approximately 453 K. Corroborative XRD patterns attribute the rapid resistivity change to the structural transformation from an amorphous as-deposited state to a crystalline rock salt structure ( a =5.974±0.002 A). Furthermore, it has been established that Ge 4 Sb 1 Te 5 forms no second crystal structure upon annealing to higher temperatures. The crystallization process has been quantified by determining the activation energy, E a , to be 3.48±0.12 eV using Kissingers analysis. XRR measurements yield density values of 5.59±0.02 and 6.14±0.02 g cm −3 for the amorphous and crystalline phases, respectively. This corresponds to a density increase of 9.3%, which causes a contraction of 9.1% in film thickness upon annealing, as revealed by XRR. The formation of an oxide layer has been observed to commence in the amorphous phase at 378 K and saturates for an oxide thickness of 1.7 nm at 413 K.

Physical properties of thin GeO2 films produced by reactive DC magnetron sputtering

Abstract We have studied optical and structural properties of GeOx films produced by reactive DC magnetron sputtering of Ge targets in an Ar/O2-mixture. Optical spectroscopy measurements of reflectance and transmittance of the films were employed to determine optical properties. The film structure was determined by X-ray diffraction measurements, while X-ray reflectometry was used to determine the thickness, density, and roughness of the films. The film topography was additionally characterized by atomic force microscopy. For appropriate oxygen flows highly transmitting GeOx films can be grown at rates up to 3 nm/s for power density of 2.6 W/cm2. Optical and structural properties of the films are closely correlated and can be controlled by the oxygen flow. With increasing oxygen flow an increasing growth rate of the oxide films is initially observed. The resulting films above 20 sccm and below 25 sccm O2 are transparent in the visible range, have a negligible roughness and are amorphous. A further rise in oxygen flow leads to a pronounced decrease in growth rate above 25 sccm and a further increase in the bandgap of the germanium oxide up to more than 48000 cm−1 (5.95 eV). This transition in film growth is accompanied by an increase in roughness and a structural transition leading to polycrystalline films of α-GeO2 (quartz-structure). Nevertheless, the film density is hardly affected by the structural change and remains rather constant around 3.65 g/cm3, which is 85% of the density of single crystalline α-GeO2. A model is presented which can account for the change of film properties.

Structural and optical properties of thin lead oxide films produced by reactive direct current magnetron sputtering

The structural and optical properties of thin lead oxide films were studied. Thin films were prepared by reactive dc magnetron sputtering of lead (Pb) targets in an Ar/O2 mixture. The structure has been determined by x-ray diffraction measurements, which show that crystalline lead oxide films of different composition (PbO, PbO1.44, Pb2O3, and PbO2) have been formed upon increasing oxygen flow (partial pressure). This result is confirmed by Raman spectroscopy. The effect of postdeposition annealing on the structural properties of PbO films reveals that the film structure is governed by both energetics and kinetics. X-ray reflectivity measurements were used to determine the thickness, density, and roughness of the films. The calculated film density values are almost equal to the bulk density of the material, showing that compact, nearly void free films are formed. The optical properties of the films have been studied from the reflectance and transmittance spectra recorded by optical spectroscopy measurement...

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.