Piezoresistive materials

A piezo resistive material with a sufficiently large (10x) and hysteretic change in resistance occurring at low pressures is an essential component of the proposed memory device. The PETMEM device will first be demonstrated using presently-known rare earth monochalcogenides, deposited using highly-controlled state-of-the-art methods. Simultaneously, an adventurous materials innovation activity will explore properties of the versatile family of Heusler compounds, other chalcogenides and oxides and will identify the most promising materials for subsequent versions of the PETMEM.

 

Samarium Monosulfide

Samarium Monosulfide (SmS) is a rare earth monochalcogenide that, similar to SmSe and SmTe, exhibits switching behaviour between a semiconducting and a metallic state, during which the Sm ions change from divalent to trivalent state. In contrast to SmSe and SmTe where the transition is continuous, SmS exhibits a discontinuous phase transition at fairly low pressure (0.65 GBar). The effect was first described (for all 3 compounds) in 1970, and has been a subject of research since then, both on a fundamental level and industrially, resulting in more than 200 publications by 1978. In addition to pressure, the transition can also be induced by light or temperature.

Potential technological roles for SmS have been suggested, for strain detectors, optical switching, but also for data storage. The latter application, which is the subject of the present project, was first proposed by Pohl et al. in 1974, and patented by Eden in 1987. Local heating and phase change using laser pulses was used as the switching method in these publications.

Both SmS single crystals and thin films have been produced and studied. In 2000, a method for producing switchable films using sputter co-deposition from Sm2S3 and Sm targets was patented by Jin and Tanemura. Other deposition methods used include molecular beam epitaxy (MBE), spray pyrolysis, electrodeposition and reactive evaporation. The latter method is similar to the one used in preliminary experiments conducted in a collaboration between Ghent University (UGENT) and TU Delft.

While it is clear that many research groups have investigated this exciting material, there has been no targeted research towards the proposed application of this project. Specifically, very thin and uniform SmS layers will be deposited, using reactive e-beam deposition. This method has the advantage that is much easier to control than reactive co-sputtering, let alone RF sputtering, and that the deposition speed can be varied over a very wide range. In addition, we will aim for a specific switching behaviour as a function of pressure, adapted to the requirements set by the piezo-electric materials developed in the project. This can be achieved by alloying SmS with other rare earth elements.

 

 

Heusler compounds

One of the major challenges to achieving high-performance piezoresistive (PR) materials such as SmS is in the elaborate fabrication required. Techniques such as molecular beam epitaxial (MBE) or spray pyrolysis show disadvantages with regard to film homogeneity over a wide range. To overcome this obstacle for large scale production, either alternative preparation techniques (as above) or alternative materials are required.

 

The family of Heusler compounds has attracted a great deal of interest recently due to the high potential of designing new materials with outstanding properties such as topological insulators, thermoelectrics or half metallic ferromagnets, which are very interesting for novel devices not only in the field of spintronics but beyond it. Heusler compounds are a large class of compounds with more than 1000 members, and many unexplored properties. The ternary and quaternary compounds can be seen as relatives of the elemental and binary semiconductors with strongly bonded tetrahedral structures. Following the great tunability of this material class, this proposal aims to explore and design Heusler compounds with outstanding PR materials.

 

Based on our preliminary results, we already have identified promising materials within the family of Heusler compounds to be very attractive for use as PR materials. The half Heusler compound LiMnAs shows a giant thermo-resistance of more than 5 orders of magnitudes in a small temperature range between 80 and 250K (see Figure below). A similar giant change of the resistivity can also be expected by applying a high pressure, which is currently under investigation in MPG Dresden high-pressure subgroup.

The half Heusler compound LiMnAs with giant thermoresistance.
The half Heusler compound LiMnAs with giant thermoresistance.

By investigating the family of Heusler compounds for potential topological insulators we have shown that a band inversion takes place in the half Heusler YPdBi induced by a change of the lattice parameter, such as could happen under pressure. Therefore, we propose to probe materials similar to topological insulators such as YPdBi for use as piezoresistive materials in PETMEM devices. It is worth noting that Sm-chalcogenides are also of considerable current interest in the context of topological insulators.

 

Band structure calculations that were performed by our group show Mn2CoAl to be a spin gapless semiconductor. The half-metallic behaviour and the electro-resistance in this compound are very sensitive to pressure. Therefore, an applied pressure to such a compound should allow the gating of a current through this material. In contrast to the above-mentioned compounds, the different behaviour of the two spin channels could give the possibility of an additional degree of freedom that could for example be used as a reference signal.

 

Finally, MPG Dresden has shown that Heusler compounds offer a great tunability with regard to their electronic structure, which allows, therefore, to tailor their material properties. For example, by an elementary substitution of Mn by Fe in the predicted half-metallic Heusler compound Co2MnSi (CMS) to generate CFS or an Mn/Fe alloy (CMFS), MPG Dresden were able to control the density of states at the Fermi level EF to generate a high spin polarization. This systematic approach of combining materials advantages and avoiding disadvantages opens an enormous phase space for new PR materials with tailored properties such as a specific switching behaviour.

The development of new materials yields important advantages. The goal is to identify materials, which are easy to process, are reliable, have long-term usage and temperature stability. New materials with comparable or improved characteristic properties suitable for mass production using sustainable raw materials that have low cost will also be identified.