Research Bits: Oct. 22, 2013
Size matters in the giant magnetoresistance effect in semiconductors
Professors at Georgia State University reported that a giant magnetoresistance effect depends on the physical size of the device in the GaAs/AlGaAs semiconductor system.
In research that is supported by grants from the U.S. Department of Energy and the U.S. Army Research Office, Dr. Ramesh Mani, professor of physics and astronomy, studied the magnetoresistance in flat, very thin sheets of electrons in the ultra high quality GaAs/AlGaAs semiconductor with his colleagues Annika Kriisa from Emory University and Werner Wegscheider from the ETH-Zurich in Switzerland.
The researchers found that the change in the resistance or resistivity with the magnetic field depends on the size of the device. They demonstrated that, under the application of a magnetic field, wide devices develop a smaller and quicker change, while small devices develop a bigger but slower change in the resistivity. The resistance or resistivity of a material to the flow of electricity is a technologically important property, especially in semiconductors.
This research team developed a model to understand the observations and deduced that when the semiconductor system becomes of even better quality, the change in the resistance under the application of a magnetic field will become even bigger. Indeed, the change might become so big that the resistance vanishes entirely in the small magnetic field.
Thin film semiconductors that will drive production of next-generation displays
When an oxide film contains metal with low bond dissociation energy, the thin film absorbs or desorbs oxygen easily and the conductivity of the film changes. For example, zinc has very low bond dissociation energy, so a thin film using zinc absorbs or desorbs oxygen easily when heated or cooled. This finding suggests that the manufacturing conditions for oxide semiconductors can be controlled by focusing on the bond dissociation energy. In fact, the research team confirmed that film deposition conditions can be broadened by adding silicon oxide with high bond dissociation energy to indium oxide. We also confirmed stabilization of thin-film conductivity in post-deposition heat treatment.
The research results are expected to be effective not only for reducing the power consumption of displays which consume about half of the power in rapidly diffusing smartphones, but also for achieving higher frequencies to realize higher-definition TVs. Additionally, the thin film developed in this research contributes to conserving precious resources by not using zinc, which is a trace element of concern, or high-cost gallium which is used in large quantities for galvanized steel sheets or as a rubber vulcanizing agent, while it also enables the manufacture of flat panel displays not affected by wild fluctuations in raw material prices.
Ultraviolet light to the extreme
When you heat a tiny droplet of liquid tin with a laser, plasma forms on the surface of the droplet and produces extreme ultraviolet (EUV) light, which has a higher frequency and greater energy than normal ultraviolet.Now, for the first time, researchers have mapped this EUV emission and developed a theoretical model that explains how the emission depends on the three-dimensional shape of the plasma. In doing so, they found a previously untapped source of EUV light, which could be useful for various applications including semiconductor lithography, the process used to make integrated circuits.
In the experiments, Andrea Giovannini and Reza Abhari from ETH-Zurich in Switzerland blasted a 30-micron-diameter droplet of tin with a high-powered laser 6,000 times a second. They measured the spatial distribution of the resulting EUV emission and found that 30 percent of it came from behind the region of the droplet that was struck by the laser. According to their model, this unexpected distribution was due to the fact that the plasma partially surrounding the droplet was elongated in the direction of the laser pulse.
Devices that produce narrow beams of EUV for purposes like in semiconductor lithography use mirrors to focus the emission. But, until now, no one knew to collect the EUV light radiating from behind the droplet.