Innovations in Polarimetry Make Thin-Film Semiconductors More Feasible

February 16, 2017 by Robin Mitchell

A new polarimetry method allows discovery of properties of thin-film semiconductors made of transition metal dichalcogenides.

Researchers from Ludwig-Maximilians-Universitat (LMU), Rice University, and Los Alamos National Laboratory have developed a new polarimetry method to discover properties of thin-film semiconductors made of transition metal dichalcogenides.

A New Material and Market

The world of electronics has undergone a dramatic shift in the past decade. For years, electronics consisted of through-hole parts which were mounted on pieces of wood (breadboards) where wires were connected to component legs to make connections. As electronics became more popular and circuits became more complex, it was clear that mounting components on pieces of wood was not going to be sufficient.

In fact, the world’s first printed circuit board (PCB) was used in a radio and was created by Paul Eisler in 1942. With the PCB in production, it was only a few years before the transistor would be invented, which led to the creation of modern electronics, revolutionizing modern life.


A 1920s-era radio. It's amazing how far electronics have really come


So, what has made the changes in the past decade more dramatic than those of the past 100 years? The past 100 years have been about finding out what can be made with electronics and how to make life easier. The past decade, however, has seen electronic integration like never before.

The Internet has become such a massive presence that people are becoming dependent on it (PDF), the IoT concept is seeing electronics integrated into the simplest of objects, and there is even a demand for wearable electronics that can keep us connected while being unnoticed. However, wearable electronics is a market that is still in its infancy mainly due to the current technology available. One crucial property that wearable electronics will need to feature is flexibility so that, when worn, the movement of the body will not affect the functionality of the device. Therefore, the pressure on researchers and companies to produce such flexible devices is intense.

Some flexible electronics do exist which show real promise for the wearable market, but one type of device that remains inflexible is the most important: the semiconductor processor. Semiconductors are arguably the most important 20th-century invention (in my opinion, the transistor is humanity's greatest achievement next to bacon), but they still rely heavily on materials such as silicon and germanium which have crystalline properties. The crystalline property of silicon makes it a very brittle material and therefore inflexible (any twisting in such a material can cause fractures and shatter like glass), so for truly flexible devices, a flexible semiconductor is needed.

Not all is lost, however, as there have been significant discoveries in flexible semiconductors. Graphene, a material that has been widely reported on in the news, has shown promise of being a future material for flexible electronics due to its strength, suppleness, and semiconductor-like properties. One other material, however, molybdenum disulfide is a more likely candidate as it is not only flexible but also has strong semiconducting properties (flexible transistors have even been made with it).



MoS2 devices. Image courtesy of Stanford University.

A New Polarimetry Method

Molybdenum disulfide is just one example of novel thin-film semiconductors which are atoms thick. Such materials fall under a special category called transition metal dichalcogenide monolayers (PDF) which are materials that consist of a transitional metal and two chalcogen atoms (i.e., sulphur, selenium, and tellurium).

To discover more of their properties and further increase our understanding of such novel thin-film semiconductors, researchers from Ludwig-Maximilians-Universitat Munich (LMU), Rice University, and the Los Alamos National Laboratory (LANL) have studied polarimetry (PDF) on the thin monolayer crystalline semiconductors. What they have discovered may open up a massive potential for such materials.


The transition metals and chalcogens. Image courtesy of the King Abdullah University of Science and Technology


Originally, their experiments with the materials demonstrated that such thin-films interact strongly with light. When the films are exposed to polarized light, the electrons become excited which in turn creates charge carriers. These charge carriers exhibit left- or right-handed circuit motion whose angular momentum is quantized. This quantized angular momentum can be used in the same fashion as quantum mechanical spin. Therefore, such thin-film semiconductors may even see applications in quantum computing as well.

However, the results obtained from the thin-film semiconductors could not be reliably repeated by other researchers which has led to controversy. To address this, the research group has developed a new polarimetry method for rapidly and efficiently measuring properties of thin-films. This allows for the characterization of such materials to be more economical which is vital for mass production of flexible products.

Alexander Högele from LMU was quoted as saying, "Response to polarized light turns out to be very sensitive to the quality of the crystals, and can thus vary significantly within the same crystal. The interplay between crystal quality and valley polarization will allow us to measure rapidly and efficiently those properties of the sample that are relevant for applications based on the valley quantum degree of freedom."

With these significant strides, the future of thin-film semi-conductors is looking significantly brighter and could result in a significant leap forward for wearable devices and other technologies.


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The idea of being able to quickly and economically characterize thin-film properties may seem insignificant but with the way that quantum computing is heading and with how individual atomic structures can make or break a device, such economic measurement may be vital for the innovation of devices in the future.