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Research Alert: August 12, 2014

SRC, UC Davis explore new materials and device structures to develop next-generation “Race Track Memory” technologies

University of California, Davis researchers sponsored by Semiconductor Research Corporation (SRC), a university-research consortium for semiconductors and related technologies, are exploring new materials and device structures to develop next-generation memory technologies.

The research promises to help data storage companies advance their technologies with predicted benefits including increased speed, lower costs, higher capacity, more reliability and improved energy efficiency compared to today’s magnetic hard disk drive and solid state random access memory (RAM) solutions.

Conducted by UC Davis’ Takamura Research Group that has extensive experience in the growth and characterization of complex oxide thin films, heterostructures and nanostructures, the research involves leveraging complex oxides to manipulate magnetic domain walls within the wires of semiconductor memory devices at nanoscale dimensions. This work utilized sophisticated facilities available through the network of Department of Energy-funded national laboratories at the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory and the Advanced Light Source, Lawrence Berkeley National Laboratory.

“We were inspired by the ‘Race Track Memory’ developed at IBM and believe complex oxides have the potential to provide additional degrees of freedom that may enable more efficient and reliable manipulation of magnetic domain walls,” said Yayoi Takamura, Associate Professor, Department of Chemical Engineering and Materials Science, UC Davis.

Existing magnetic hard disk drive and solid state RAM solutions store data either based on the magnetic or electronic state of the storage medium. Hard disk drives provide a lower cost solution for ultra-dense storage, but are relatively slow and suffer reliability issues due to the movement of mechanical parts. Solid state solutions, such as Flash memory for long-term storage and DRAM for short-term storage, offer higher access speeds, but can store fewer bits per unit area and are significantly more costly per bit of data stored.

An alternative technology that may address both of these shortcomings is based on the manipulation of magnetic domain walls, regions that separate two magnetic regions. This technology, originally proposed by IBM researchers and named ‘Race Track Memory,’ is where the UC Davis work picked up.

Notre Dame paper offers insights into a new class of semiconducting materials

A new paper by University of Notre Dame researchers describes their investigations of the fundamental optical properties of a new class of semiconducting materials known as organic-inorganic “hybrid” perovskites.

The research was conducted at the Notre Dame Radiation Laboratory by Joseph Manser, a doctoral student in chemical and biomolecular engineering, under the direction of Prashant Kamat, Rev. John A. Zahm Professor of Science. The findings appear in a paper in the August 10 edition of the journal Nature Photonics.

The term “perovskites” refers to the structural order these materials adopt upon drying and assembling in the solid state.

“Hybrid perovskites have recently demonstrated exceptional performance in solid-state thin film solar cells, with light-to-electricity conversion efficiencies approaching nearly 20 percent,” Manser said. “Though currently only at the laboratory scale, this efficiency rivals that of commercial solar cells based on polycrystalline silicon. More importantly, these materials are extremely easy and cheap to process, with much of the device fabrication carried out using coating and or printing techniques that are amenable to mass production. This is in stark contrast to most commercial photovoltaic technologies that require extremely high purity materials, especially for silicon solar cells, and energy-intensive, high-temperature processing.”

Manser points out that although the performance of perovskite solar cells has risen dramatically in only a few short years, the scientific community does not yet fully know how these unique materials interact with light on a fundamental level.

Manser and Kamat used a powerful technique known as “transient absorption pump-probe spectroscopy” to examine the events that occur trillions of a second after light absorption in the hybrid methylammonium lead iodide, a relevant material for solar applications. They analyzed both the relaxation pathway and spectral broadening in photoexcited hybrid methylammonium lead iodide and found that the excited state is primarily composed of separate and distinct electrons and holes known as “free carriers.”

“The fact that these separated species are present intrinsically in photoexcited hybrid methylammonium lead iodide provides a vital insight into the basic operation of perovskite solar cells,” Manser said. “Since the electron and hole are equal and opposite in charge, they often exist in a bound or unseparated form known as an ‘exciton.’ Most next-generation’ photovoltaics based on low-temperature, solution-processable materials are unable to perform the function of separating these bound species without intimate contact with another material that can extract one of the charges. ”

This separation process siphons energy within the light absorbing layer and restricts the device architecture to one of highly interfacial surface area. As a result, the overall effectiveness of the solar cell is reduced.

Pairing old technologies with new for next-generation electronic devices

UCL scientists have discovered a new method to efficiently generate and control currents based on the magnetic nature of electrons in semiconducting materials, offering a radical way to develop a new generation of electronic devices.

One promising approach to developing new technologies is to exploit the electron’s tiny magnetic moment, or ‘spin’. Electrons have two properties – charge and spin – and although current technologies use charge, it is thought that spin-based technologies have the potential to outperform the ‘charge’-based technology of semiconductors for the storage and process of information.

In order to utilise electron spins for electronics, or ‘spintronics’, the method of electrically generating and detecting spins needs to be efficient so the devices can process the spin information with low-power consumption. One way to achieve this is by the spin-Hall effect, which is being researched by scientists who are keen to understand the mechanisms of the effect, but also which materials optimise its efficiency. If research into this effect is successful, it will open the door to new technologies.

The spin-Hall effect helps generate ‘spin currents’ which enable spin information transfer without the flow of electric charge currents. Unlike other concepts that harness electrons, spin current can transfer information without causing heat from the electric charge, which is a serious problem for current semiconductor devices. Effective use of spins generated by the spin-Hall effect can also revolutionise spin-based memory applications.

The study published in Nature Materials shows how applying an electric field in a common semiconductor material can dramatically increase the efficiency of the spin-Hall effect which is key for generating and detecting spin from an electrical input.

The scientists reported a 40-times-larger effect than previously achieved in semiconductor materials, with the largest value measured comparable to a record high value of the spin-Hall effect observed in heavy metals such as Platinum. This demonstrates that future spintronics might not need to rely on expensive, rare, heavy metals for efficiency, but relatively cheap materials can be used to process spin information with low-power consumption.

As there are limited amounts of natural resources in the earth and prices of materials are progressively going up, scientists are looking for more accessible materials with which to develop future sustainable technologies, potentially based on electron spin rather than charge. Added to this, the miniaturization approach of current semiconductor technology will see a point when the trend, predicted by Moore’s law, will come to an end because transistors are as small as atoms and cannot be shrunk any further. To address this, fundamentally new concepts for electronics will be needed to produce commercially viable alternatives which meet demands for ever-growing computing power.



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