New technique makes it easier to etch semiconductors

 Creating semiconductor structures for high-end optoelectronic devices just got easier, thanks to University of Illinois researchers.

The team developed a method to chemically etch patterned arrays in the semiconductor gallium arsenide, used in solar cells, lasers, light emitting diodes (LEDs), field effect transistors (FETs), capacitors and sensors. Led by electrical and computer engineering professor Xiuling Li, the researchers describe their technique in the journal Nano Letters.

A semiconductor's physical properties can vary depending on its structure, so semiconductor wafers are etched into structures that tune their electrical and optical properties and connectivity before they are assembled into chips.

Semiconductors are commonly etched with two techniques: "Wet" etching uses a chemical solution to erode the semiconductor in all directions, while "dry" etching uses a directed beam of ions to bombard the surface, carving out a directed pattern. Such patterns are required for high-aspect-ratio nanostructures, or tiny shapes that have a large ratio of height to width. High-aspect-ratio structures are essential to many high-end optoelectronic device applications.

While silicon is the most ubiquitous material in semiconductor devices, materials in the III-V (pronounced three-five) group are more efficient in optoelectronic applications, such as solar cells or lasers.

Unfortunately, these materials can be difficult to dry etch, as the high-energy ion blasts damage the semiconductor's surface. III-V semiconductors are especially susceptible to damage.

To address this problem, Li and her group turned to metal-assisted chemical etching (MacEtch), a wet-etching approach they had previously developed for silicon. Unlike other wet methods, MacEtch works in one direction, from the top down. It is faster and less expensive than many dry etch techniques, according to Li. Her group revisited the MacEtch technique, optimizing the chemical solution and reaction conditions for the III-V semiconductor gallium arsenide (GaAs).

The process has two steps. First, a thin film of metal is patterned on the GaAs surface. Then, the semiconductor with the metal pattern is immersed in the MacEtch chemical solution. The metal catalyzes the reaction so that only the areas touching metal are etched away, and high-aspect-ratio structures are formed as the metal sinks into the wafer. When the etching is done, the metal can be cleaned from the surface without damaging it.

"It is a big deal to be able to etch GaAs this way," Li said. "The realization of high-aspect-ratio III-V nanostructure arrays by wet etching can potentially transform the fabrication of semiconductor lasers where surface grating is currently fabricated by dry etching, which is expensive and causes surface damage."

To create metal film patterns on the GaAs surface, Li's team used a patterning technique pioneered by John Rogers, the Lee J. Flory-Founder Chair and a professor of materials science and engineering at the U. of I. Their research teams joined forces to optimize the method, called soft lithography, for chemical compatibility while protecting the GaAs surface. Soft lithography is applied to the whole semiconductor wafer, as opposed to small segments, creating patterns over large areas -- without expensive optical equipment.

"The combination of soft lithography and MacEtch make the perfect combination to produce large-area, high-aspect-ratio III-V nanostructures in a low-cost fashion," said Li, who is affiliated with the Micro and Nanotechnology Laboratory, the Frederick Seitz Materials Research Laboratory and the Beckman Institute for Advanced Science and Technology at the U. of I.

Next, the researchers hope to further optimize conditions for GaAs etching and establish parameters for MacEtch of other III-V semiconductors. Then, they hope to demonstrate device fabrication, including distributed Bragg reflector lasers and photonic crystals.

"MacEtch is a universal method as long as the right condition for deferential etching with and without metal can be found," Li said.

The Department of Energy and the National Science Foundation supported this work.

Story Source:

The above story is reprinted from materials provided by University of Illinois at Urbana-Champaign.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Matt DeJarld, Jae Cheol Shin, Winston Chern, Debashis Chanda, Karthik Balasundaram, John A. Rogers, Xiuling Li. Formation of High Aspect Ratio GaAs Nanostructures with Metal-Assisted Chemical Etching. Nano Letters, 2011; 11 (12): 5259 DOI: 10.1021/nl202708d

Landmark discovery has magnetic appeal for scientists

The effect causes a dramatic change to how this material conducts electricity at very low temperatures. The discovery gives new insight into the mineral in which magnetism was discovered, and it may enable magnetite and similar materials to be exploited in new ways.

Ancient knowledge

"We have solved a fundamental problem in understanding the original magnetic material, upon which everything we know about magnetism is built," said Professor Paul Attfield of the Centre for Science at Extreme Conditions.

Magnetite's properties have been known for more than 2000 years and gave rise to the original concepts of magnets and magnetism.

The mineral has formed the basis for decades of research into magnetic recording and information storage materials.

The research was led by the University in collaboration with the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, where the experiments were conducted.

Their results were published in Nature.

Unexplained behaviour

In 1939, Dutch scientist Evert Verwey discovered that the electrical conductivity of magnetite decreases abruptly and dramatically at low temperatures.

At about 125 Kelvin, or minus 150 degrees Celsius, the metallic mineral turns into an insulator.

Despite many efforts, until now the reason for this transition has been debated and remained controversial.

X-ray experiment

The team of scientists fired an intense X-ray beam at a tiny crystal of magnetite at very low temperatures.

Their results enabled them to understand a subtle rearrangement of the mineral's chemical structure.

Electrons are trapped within groups of three iron atoms, where they can no longer transport an electrical current.

"This vital insight into how magnetite is constructed and how it behaves will help in the development of future electronic and magnetic technologies," Attfield said.

The research was funded by the Science and Technology Facilities Council, the Engineering and Physical Sciences Research Council, and the Leverhulme Trust.

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The above story is reprinted from materials provided by University of Edinburgh.

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Journal References:

Mark S. Senn, Jon P. Wright, J. Paul Attfield. Charge order and three-site distortions in the Verwey structure of magnetite. Nature, 2011; DOI: 10.1038/nature10704J. Paul Attfield. Condensed-matter physics: A fresh twist on shrinking materials. Nature, 2011; 480 (7378): 465 DOI: 10.1038/480465a

Largest ever gas mix caught in ultra-freeze trap

A team of scientists have made it easier to study atomic or subatomic-scale properties of the building blocks of matter (which also include protons, neutrons and electrons) known as fermions by slowing down the movement of a large quantity of gaseous atoms at ultra-low temperature.

This is according to a study recently published in The European Physical Journal D as part of a cold quantum matter special issue, by researchers from the Paris-based École Normale Supérieure and the Non-Linear Institute at Nice Sophia-Antipolis University in France.

Thanks to the laser cooling method for which Claude Cohen-Tannoudji, Steven Chu and William D. Phillips received the Nobel Prize in 1997, Armin Ridinger and his colleagues succeeded in creating the largest Lithium 6 (6Li) and Potassium 40 (40K) gas mixture to date. The method used involved confining gaseous atoms under an ultra-high vacuum using electromagnetic forces, in an ultra-freeze trap of sorts.

This trap enabled them to load twice as many atoms than previous attempts at studying such gas mixtures, reaching a total on the order of a few billion atoms under study at a temperature of only a few hundred microKelvins (corresponding to a temperature near the absolute zero of roughly -273 °C).

Given that the results of this study significantly increased the number of gaseous atoms under study, it will facilitate future simulation of subatomic-scale phenomena in gases. In particular, it will enable future experiments in which the gas mixture is brought to a so-called degenerate state characterised by particles of different species with very strong interactions. Following international efforts to produce the conditions to study subatomic-scale properties of matter under the quantum simulation program, this could ultimately help scientists to understand quantum mechanical phenomena occurring in neutron stars and so-called many-body problems such as high-temperature superconductivity.

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The above story is reprinted from materials provided by Springer Science+Business Media.

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Journal Reference:

A. Ridinger, S. Chaudhuri, T. Salez, U. Eismann, D. R. Fernandes, K. Magalhaes, D. Wilkowski, C. Salomon, F. Chevy. Large atom number dual-species magneto-optical trap for fermionic 6Li and 40K atoms. The European Physical Journal D, 2011; 65 (1-2): 223 DOI: 10.1140/epjd/e2011-20069-4

Amplifier helps diamond spy on atoms

 An 'amplifier' molecule placed on the tip of a diamond could help scientists locate and identify individual atoms, Oxford University and Singapore scientists believe.

The idea builds on ongoing work towards creating a diamond nanocrystal that can be used to detect an atom's incredibly weak magnetic field. Defects within the diamond hold electrons that act rather like a compass, lining up with even the very weak magnetic field emanating from the core of an atom.

Crucially this diamond compass can be 'read' by shining a pulse of laser light into the crystal giving information about the location and type of atom -- for instance telling the difference between a carbon and hydrogen atom and giving their exact location within a structure such as a virus or new material.

'The problem with this approach is that the 'compass' only behaves well if it is buried within the diamond: this makes it very difficult to get it close enough to a structure to detect an individual atom's magnetic field,' said Dr Simon Benjamin of Oxford University's Department of Materials and National University of Singapore. 'It's a bit like trying to grasp one particular marble out of a bucket of marbles whilst wearing an oven glove.

'The new research, which the team recently report in Physical Review Letters, calculates that by attaching another 'compass' -- the amplifier molecule -- to the tip of the diamond this will pass the information about an atom along to the compass inside the diamond that can then be read.

'Our calculations show for the first time how such an amplifier could be used to make a diamond probe sensitive enough to pinpoint and identify individual atomic cores,' said Dr Benjamin. 'If this can be made to work, the additional information we would gain would be rather like moving from black and white photographs of atoms to full colour.

'Dr Erik Gauger of Oxford University's Department of Materials and National University of Singapore, an author of the paper with Dr Benjamin, said: 'The device that we propose may well represent the limit of what is possible in terms of magnetic field sensitivity and resolution; if, as we hope, it allows direct identification of atoms by their core signatures, then it will be a revolutionary tool in chemistry, biology and medicine.'

The team believe that it may only be a couple of years before diamond probes are created that will reveal the world of the atom in unprecedented detail but that the small step of adding an amplifier could make such systems many times more powerful.

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The above story is reprinted from materials provided by University of Oxford.

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Journal Reference:

Marcus Schaffry, Erik Gauger, John Morton, Simon Benjamin. Proposed Spin Amplification for Magnetic Sensors Employing Crystal Defects. Physical Review Letters, 2011; 107 (20) DOI: 10.1103/PhysRevLett.107.207210

 

Functionalized graphene oxide plays part in next-generation oil-well drilling fluids

Graphene's star is rising as a material that could become essential to efficient, environmentally sound oil production. Rice University researchers are taking advantage of graphene's outstanding strength, light weight and solubility to enhance fluids used to drill oil wells.

The Rice University lab of chemist James Tour and scientists at M-I SWACO, a Texas-based supplier of drilling fluids and subsidiary of oil-services provider Schlumberger, have produced functionalized graphene oxide to alleviate the clogging of oil-producing pores in newly drilled wells.

The patented technique took a step closer to commercialization with the publication of new research this month in the American Chemical Society journal Applied Materials and Interfaces. Graphene is a one-atom-thick sheet of carbon that won its discoverers a Nobel Prize last year.

Rice's relationship with M-I SWACO began more than two years ago when the company funded the lab's follow-up to research that produced the first graphene additives for drilling fluids known as muds. These fluids are pumped downhole as part of the process to keep drill bits clean and remove cuttings. With traditional clay-enhanced muds, differential pressure forms a layer on the wellbore called a filter cake, which both keeps the oil from flowing out and drilling fluids from invading the tiny, oil-producing pores.

When the drill bit is removed and drilling fluid displaced, the formation oil forces remnants of the filter cake out of the pores as the well begins to produce. But sometimes the clay won't budge, and the well's productivity is reduced.

The Tour Group discovered that microscopic, pliable flakes of graphene can form a thinner, lighter filter cake. When they encounter a pore, the flakes fold in upon themselves and look something like starfish sucked into a hole. But when well pressure is relieved, the flakes are pushed back out by the oil.

All that was known two years ago. Since then, Tour and a research team led by Dmitry Kosynkin, a former Rice postdoctoral associate and now a petroleum engineer at Saudi Aramco, have been fine-tuning the materials.

They found a few issues that needed to be dealt with. First, pristine graphene is hard to disperse in water, so it is unsuitable for water-based muds. Graphene oxide (GO) turned out to be much more soluble in fresh water, but tended to coagulate in saltwater, the basis for many muds.

The solution was to "esterify" GO flakes with alcohol. "It's a simple, one-step reaction," said Tour, Rice's T.T. and W.F. Chao Chair in Chemistry as well as a professor of mechanical engineering and materials science and of computer science. "Graphene oxide functionalized with alcohol works much better because it doesn't precipitate in the presence of salts. There's nothing exotic about it."

In a series of standard American Petroleum Institute tests, the team found the best mix of functionalized GO to be a combination of large flakes and powdered GO for reinforcement. A mud with 2 percent functionalized GO formed a filter cake an average of 22 micrometers wide -- substantially smaller than the 278-micrometer cake formed by traditional muds. GO blocked pores many times smaller than the flakes' original diameter by folding.

Aside from making the filter cake much thinner, which would give a drill bit more room to turn, the Rice mud contained less than half as many suspended solids; this would also make drilling more efficient as well as more environmentally friendly. Tour and Andreas Lüttge, a Rice professor of Earth science and chemistry, reported last year that GO is reduced to graphite, the material found in pencil lead and a natural mineral, by common bacteria.

"The most exciting aspect is the ability to modify the GO nanoparticle with a variety of functionalities," said James Friedheim, corporate director of fluids research and development at M-I SWACO and a co-author of the research. "Therefore we can 'dial in' our application by picking the right organic chemistry that will suit the purpose. The trick is just choosing the right chemistry for the right purpose."

"There's still a lot to be worked out," Tour said. "We're looking at the rheological properties, the changes in viscosity under shear. In other words, we want to know how viscous this becomes as it goes through a drill head, because that also has implications for efficiency."

Muds may help graphene live up to its commercial promise, Tour said. "Everybody thinks of graphene in electronics or in composites, but this would be a use for large amounts of graphene, and it could happen soon," he said.

Friedheim agreed. "With the team we currently have assembled, Jim Tour's group and some development scientists at M-I SWACO, I am confident that we are close to both technical and commercial success."

Other authors of the paper are Rice graduate student Gabriel Ceriotti, former Rice research associates Kurt Wilson and Jay Lomeda, and M-I SWACO researchers Jason Scorsone and Arvind Patel.

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The above story is reprinted from materials provided by Rice University.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Dmitry V. Kosynkin, Gabriel Ceriotti, Kurt C. Wilson, Jay R. Lomeda, Jason T. Scorsone, Arvind D. Patel, James E. Friedheim, James M. Tour. Graphene Oxide as a High-Performance Fluid-Loss-Control Additive in Water-Based Drilling Fluids. ACS Applied Materials & Interfaces, 2011; : 111213103240001 DOI: 10.1021/am2012799