3D Nanoarray Microporous Carbon Structure Achieves High Surface Area
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RESEARCH/RESEARCHERS
One-Dimensional Heterostructures Fabricated in InAs/InP Nanowhiskers Previously, researchers have grown one-dimensional semiconductor nanowhiskers and then made diodes and bipolar transistors between crossed, differently doped whiskers. However, researchers from the Solid-State Physics/Nanometer Consortium at Sweden’s Lund University have fabricated a heterostructure device within a single 40-nm-diameter InAs whisker. They used size-selected Au aerosol seed particles along with chemicalbeam epitaxy (CBE) to sprout their nanowhiskers from GaAs substrates. These tiny InAs pillars contain either one or a sequence of InP barriers, ranging from 1.5 nm to 100 nm thick, and boast atomically sharp interfaces between the layers. After depositing the Au seed particles into (111)B GaAs substrates, M.T. Björk and colleagues used CBE to grow the long whiskers at 420°C. The beam pressures were 0.3 mbar for the trimethyl indium (TMIn) source, 2.0 mbar for the tertiary butyl arsine source, and 3.0 mbar for the tertiary butyl phosphine source. The alternating InAs and InP layers were then grown by switching between the two Group V sources while the TMIn source was extinguished. As reported in the February 4 issue of Applied Physics Letters, the researchers found that the thicknesses were highly reproducible, and transmission electron microscopy (TEM) images showed that growth can occur in the 〈001〉 direction as well as in the more commonly reported 〈111〉B direction. The low growth rate (about 1 monolayer per second) and high vapor pressures of the Group V sources is responsible for the monolayer abruptness of the interfaces as imaged by high-resolution TEM, according to the researchers. The researchers report that measurements of thermionic emission across an InP heterobarrier are most telling. They demonstrated that the thermally excited currents follow the expected Arrhenius relation and deduced an InP barrier height (0.6 eV) that is very close to the bulk value. This result is gratifying, said the researchers, as it shows that the whiskers’ tiny cross sections permit efficient lateral relaxation of the two materials. This allows a combination of materials with dissimilar lattice constants that is not possible with ordinary epitaxy on a larger scale, due to, for example, the incorporation of misfit dislocations once the critical thickness of several monolayers is exceeded. The researchers also proposed new branches of physics phenomena and new families of device structures that can be 84
realized with these semiconductor nanowhiskers. “These results open the possibility of realizing novel 1-D systems, such as heterostructures integrated in scanning probe or field-emission tips, 1D–0D–1D resonant tunneling devices, and onedimensional superlattice arrays of intercoupled quantum dots,” said Björk, “and devices for quantum optics and photonics applications.” RICHARD N. LOUIE
Intermittency of CdSe QuantumDot Photon Emissions Follows a Power Law Distribution The development of sources that emit single photons se
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