Tin-Based Semiconductors Show Experimental Promise

University Scientists Report Breakthroughs in Stannate Semiconductors

In the world of semiconductors, silicon has reigned supreme for over half a century. It is the foundation of microprocessors and memory devices which are now ubiquitous entities deeply involved in most aspects of our lives, from running our smartphones to reminding us when our cars need an oil change. However, silicon does have some shortcomings: it is opaque to visible light, and does not handle high levels of power well. So, in recent years, scientists have been working to discover new semiconductors, such as gallium nitride (GaN), that can provide these additional capabilities. Currently, scientists at the University of Minnesota are developing a new class of semiconductors that could greatly expand the application range of semiconductor technology, and the essential component of this new material might come as a surprise: it is tin. The charge is being led by Prof. Steven Koester from the Department of Electrical and Computer Engineering, and Prof. Bharat Jalan from the Department of Chemical Engineering and Materials Science.

TIN POISED TO TAKE ON A NEW ROLE

Tin is the key element in a class of complex oxide semiconductors known as “stannate perovskites.” Perovskites are materials with the chemical formula ABO3, where, in the case of stannates, the “B” element is tin. Although perovskites are interesting from the point of view of physics and have a wide range of uses in other fields, they do not have many room temperature electronics applications. However, stannates are set to change this; their high electron mobility and wide band gap give them several remarkable properties: they are transparent to visible light, they can sustain extremely high voltages over tiny distances in the material, and most importantly, they are great electrical conductors.

As Prof. Jalan notes, “In most complex oxides, electron ‘mobility’, or how easily electrons can move, is very low at practical temperature.  However, stannates have much higher electrical mobility among all perovskite materials.”

Papers recently published by University research teams led by professors Koester and Jalan have generated considerable excitement and interest in the use of tin-based semiconductors as transistors, a vital component in electronic devices. In “Depletion Mode MOSFET Using La-Doped BaSnO as a Channel,” the team demonstrated the use of barium stannate (BaSnO3) as a channel material in a field-effect transistor (FET). Critically, this work also details the role of dislocations (which, very crudely explained, occur when a crystal with a particular spacing between the atoms is grown on top of another crystal with a different atomic spacing) in BaSnO3-based FETs. The results point to ways by which the performance of BaSnO3 channel devices can be enhanced.

SHIFTING FOCUS

Among the stannate-based perovskites, BaSnO3 has garnered most of the recent attention due to its high mobility and its ability to be doped, or made electrically conductive by adding additional elements. However, lately Koester’s and Jalan’s teams have turned the spotlight on strontium stannate (SrSnO3), and for good reasons.

Pointing to the difference between barium stannate and strontium stannate, Prof. Koester says, “Strontium stannate is a particularly interesting material because it has an even wider band gap than barium stannate and thus can support even higher voltages.”

Pushing ahead their research into perovskites, the scientists have  demonstrated a SrSnO3 FET, the first demonstration of a transistor using this material. The results are presented in the paper, “Demonstration of a Depletion-Mode SrSnO n-Channel MESFET,” published in IEEE Electron Device Letters. The demonstration was made possible after the Jalan team recently learned how to dope SrSnO3, also a first for this material.

THE STRONTIUM STANNATE ADVANTAGE

The larger band gap present in SrSnO allows for visible light transparency, and greater breakdown voltage. Strontium stannate is also easier to grow on commercially available substrates because of its better lattice matching, compared to BaSnO. According to Jalan, this affords lower dislocation density, which entails the possibility of greater reliability of, and improved power handling capability by, the device.

Strontium stannate is also easier to grow on commercially available substrates, because of its better lattice matching, compared to barium stannate.

Other characteristics of this compound also suggest that it displays electron mobilities close to BaSnO3 and existing wide-gap semiconductors. For Prof. Koester’s team, the results of the recent demonstration of a SrSnO3 FETs are promising. The device displayed gain values that are a record for a stannate-based perovskite FET, and five times better than existing strontium titanate (SrTiO3)-based devices. SrTiO3, a more well-known perovskite, and regarded as the silicon among perovskites, has very low mobility, making it a weak candidate for transistors. While it is a worthwhile contender for many other applications and fundamental studies, its low mobility has hindered its widespread deployment in microelectronics.

LOOKING AHEAD

Even though the current performance of the SrSnO3 FETs still lags behind that of prevailing state-of-the-art GaN FETs, the research team believes the true potential lies in new capabilities that cannot be achieved in GaN. Perhaps the most important capability is the ability to integrate epitaxial ferroelectrics. SrSnO3 is closely lattice matched to one of the best known ferroelectric materials, barium titanate, and this could lead to applications with critical potential such as reconfigurable high power electronic devices.  

Based on the outcomes of their latest work, the University’s research teams are hopeful that SrSnO3 can open the door to incorporating a wide range of new properties, including ferroelectrics, piezoelectrics, and many others.  These ‘smart materials’ could lead to new kinds of reconfigurable electronics with a vast range of applications from military radars, to cell phone towers, to autonomous vehicles.

This research is primarily supported by the Young Investigator Program of the Air Force Office of Scientific Research (AFOSR), and the National Science Foundation through  its Ceramics program (CER) and the University’s MRSEC.

You can learn more about Prof. Steven J. Koester’s pioneering work here, and check his research lab here

More information about Professor Jalan’s research can be found here.