news digest ♦ Novel Devices
energy of charged excitons was compared with that of a quantum well structure of the same type of material, which was previously known to be ~1 meV, it was found to have a value more than 10 times larger.
This increase in many-body energy is due to a remarkable increase in the Coulomb force between the many-particle system which results from packing electrons in a 3-dimensional nano-space.
The researchers say that these results show, for the first time, the effect of confinement of a multi- electron state in a nano-space. From the viewpoint of applied technology, because electron correlation is also the source of diverse types of non-linear effect devices such as optical switching devices and lasers, if interaction intensity can be controlled using nanostructures, this can be expected to lead to the development of optical semiconductor devices which enable stable drive with low power consumption.
Further details of these results can be seen in the American scientific journal, Physical Review B, Vol. 82, Issue 20, Page 201301(R) (Nov. 15, 2010, DOI: 10.1103/PhysRevB.82.201301)
The paper is entitled “Energy renormalization of exciton complexes in GaAs quantum dots”, by M. Abbarchi et al.
Imec & Holst Develop NO2 Sensor Based on InAs Nanowires
The two research organizations have developed a vertically-grown InAs sensor for measuring ultra-low concentrations of NO2 which is an environmental pollutant.
Imec and Holst Centre have developed an innovative sensor for measuring ultra-low concentrations of NO2. Such sensors are important for applications that monitor environmental pollution resulting from traffic, and in general, from all combustion motors.
The sensor’s active components are arrays of vertically grown InAs nanowires. A typical sensor
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www.compoundsemiconductor.net January / February 2011
Figure gassensor: Artist impression of the NO2 gas sensor, showing the contacts and InAs nanowires.
The semiconductor nanowires are contacted ohmically using an air bridge construction (see picture). This construction has as advantage that it leaves the nanowire surface free for gas adsorption. Because of the small bandgap of InAs, it’s fairly easy to fabricate these ohmic contacts. The sensor can be reset, simply by applying a stronger current.
The sensor boasts several breakthroughs in nanowire technology. A key characteristic is that the vertical nanowires are electrically contacted in the locations on the substrate where they are grown. In other, comparable nanowire sensors, the nanowires have to be placed on the substrate after being grown elsewhere. Another major benefit of these sensing nanowires is that they function without heating, making them much more power-efficient
The new gas sensor has been developed in Holst Centre’s program for ultra-low-power sensors. As the next step, the researchers will increase the sensitivity of the sensor, as well as its detection selectivity. One goal is, for example, to make a sensor that can distinguish between NO2 and NO. Also, new manufacturing techniques will be investigated, with the aim to use cost-effective
would contain 500 such nanowires, and will be sensitive to NO2 concentrations of fewer than 100ppb at room temperature.
The sensor’s nanowires are about 3 microns in length and 50-100nm wide. They are made from InAs, which is well-suited for gas sensing, because it has an electron accumulation layer at the surface, making it sensitive to accumulated charges. Gas molecules adsorb onto the nanowires, changing the current that is flowing through the nanowires.
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