Friday, December 30, 2011

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.

Story Source:

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

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

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.

Story Source:

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

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

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.

Story Source:

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

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

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