C. Steimer

Influence of Bi doping upon the phase change characteristics of Ge2Sb2Te5

The influence of Bi doping upon the phase change characteristics of Ge2Sb2Te5 alloys has been investigated using four-point-probe electrical resistance measurements, grazing incidence x-ray diffraction (XRD), x-ray reflectometry (XRR) and variable incident angle spectroscopic ellipsometry, a static tester and atomic force microscopy. For a Ge2Sb2Te5 alloy doped with 3% Bi, two transition temperatures are observed in the temperature dependent sheet resistance measurements at 136°C and 236°C, respectively. The evolution of structures upon annealing, investigated by XRD, reveals that the first transition is caused by the crystallization of the amorphous film to a NaCl-type structure, while the second transition is related to the transition to a hexagonal structure. The density values of 5.87±0.05gcm−3, 6.33±0.05gcm−3, and 6.41±0.05gcm−3 are measured by XRR for the film in the amorphous, NaCl-type, and hexagonal structure, respectively. Ultrafast crystallization, which is correlated with a single NaCl-structu...

Structural transformation of SbxSe100−x thin films for phase change nonvolatile memory applications

The structural transformation and transformation kinetics of SbxSe100−x films (60⩽x⩽70) were studied to investigate the feasibility of applying SbxSe100−x alloys in phase change nonvolatile memories. Temperature-dependent van der Pauw measurements, x-ray diffraction, x-ray reflectometry, and a static tester were used to determine the structure and transformation kinetics of the SbxSe100−x films. The sheet resistance difference between the amorphous and crystalline states was higher than 104Ω∕◻. The crystalline structure of the metastable phase of SbxSe100−x alloys, which plays a major role in fast crystallization, is similar to that of Sb2Te (rhombohedral structure). The transition temperature, sheet resistance, and activation energy for transformation decrease as the amount of Sb increases in the SbxSe100−x film. The density and thickness variation of the Sb65Se35 thin film were 5.9% and 5.4%, respectively. Applying the Kissinger method, the activation energies for crystallization were in the range from ...

CHALCOGENIDE ALLOYS AS A BASIS FOR NEW NON-VOLATILE RANDOM ACCESS MEMORIES

In the past decades chalcogenide alloys have been successfully employed for rewritable optical data storage. Beside the optical contrast upon phase change from the amorphous to the crystalline state utilized in optical data storage devices, chalcogenide alloys show an even more pronounced change in their electronic properties, namely their resistivity. This resistivity change is the basis of a new purely electronic memory device the so-called phase change random access memory (PCRAM). We present the electrical properties of one candidate material, namely Ge2Sb2Te5, and also demonstrate in a first experiment the switching operation of a single cell.

From a Fundamental Understanding of Phase Change Materials to Optimization Rules for Nonvolatile Optical and Electronic Storage

Phase change materials are commercially used in rewritable optical storage and investigated as non-volatile electronic storage. A short laser or current pulse of high intensity melts a sub-micron sized spot of crystalline material before quenching it to the amorphous state. A second pulse of lower intensity but longer duration may recrystallise and erase that bit. Since reflectivity and conductivity of the amorphous state are lower, a third even weaker laser or current pulse can be used to read out the state of the bit without changing it. As recrystallisation is the slowest process involved, materials with a small structural difference between the crystalline and amorphous phase promise higher data transfer rates. Such structural similarity however limits the optical and electronic contrast between the phases and the stability against spontaneous recrystallisation. This contradiction makes the development of phase change media a challenge that despite commercial applications still heavily relies on empirical approaches. This summary of recent experiments and ab-initio calculations reflects first steps toward an atomistic understanding of phase change materials.

OPTICAL AND ELECTRONIC DATA STORAGE WITH PHASE CHANGE MATERIALS: FROM CRYSTAL STRUCTURES TO KINETICS

We summarize the current understanding of how stoichiometry affects structure, optical and electronic properties and the transition kinetics of phase change materials for rewritable optical and nonvolatile electronic storage.

Phase Change Materials - From Structures to Kinetics

Phase change materials possess a unique combination of properties which include a pronounced property contrast between the amorphous and crystalline state, i.e. a high electrical and optical contrast. In particular the latter observation is indicative for a considerable structural difference between the amorphous and crystalline state. At the same time the crystallization of the amorphous state proceeds on a fast time scale. This raises the question how structure, properties and kinetics are related in phase change alloys. It will be demonstrated that only a small group of covalent semiconductors with octahedral-like coordination has the required property combination. This is related to their thermodynamic properties which govern the kinetics of crystallization.