Ralf Detemple

Optical shortpass filters based on macroporous silicon

A promising class of optical filters is introduced, based on diffraction at small apertures. The filters consist of straight pores with diameters in the micrometer regime and a length of up to one millimeter through a silicon wafer. In contrast to Bragg, Woods, or glass filters, the light is not transmitted in matter but in the medium inside the pores. The filters therefore show a true shortpass characteristic. Due to constructive interference between the high number of pores in an array, macropore filters are of high optical quality and may replace conventional filters in imaging systems.

Identification of Te alloys with suitable phase change characteristics

At present, the discovery and development of phase change materials is mainly based upon empirical strategies and trial and error approaches. Here, we present a structural criterion that needs to be met to enable the mandatory fast recrystallization with sufficient optical contrast that characterizes suitable phase change materials. Comparing the behavior of AgInTe2 and AgSbTe2 films it is demonstrated that only the AgSbTe2 films, which show a cubic coordination, have sufficient density contrast, and hence, also optical contrast to allow phase change recording.

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.

Identifying Au-based Te alloys for optical data storage

Au18Sb23Te59 and Au19In26Te55 have been investigated to determine their suitability as phase change recording alloys. Recrystallization experiments identify Au18Sb23Te59 as a suitable phase change material with a recrystallization time of 110 ns and high optical contrast. Coupled to the high optical contrast is a considerable density increase of 4% upon crystallization which allows phase change recording for the Au18Sb23Te59 alloy. On the other hand no recrystallization has been observed optically for Au19In26Te55 due to its low optical contrast of less than 1%. This is related to a lower density contrast of 2%. The crystallization for the Au18Sb23Te59 and Au19In26Te55 alloys observed from temperature-dependent sheet resistance measurements have yielded transition temperatures of 113 and 175 °C, and activation barriers of 1.61±0.01 eV and 2.42±0.02 eV, respectively. We report a cubic structure (a=2.99±0.002 A) for the Au18Sb23Te59 alloy and a chalcopyrite structure (a=6.50±0.018 A and 12.27±0.025 A) for t...

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.