Oct 18, 2007
Close-up view of nanowires on silicon
Researchers have used a new ultra-high resolution transmission electron microscope (TEM) to closely examine the structure of group III-V nanowires grown on silicon substrates. The microscope, an FEI Titan 80-300 HB high-resolution TEM, is part of the new Canadian Centre for Electron Microscopy at McMaster University in Canada. This instrument is equipped with a sextupole-based aberration corrector of the imaging lens system providing a resolution of 0.75 Å. The new microscope was used to probe the structure of GaP/GaAsP heterostructured nanowires grown directly on silicon substrates for applications in quantum information processing.
Silicon is an ideal material for electronic devices due to its natural abundance, the existence of a native oxide useful for device fabrication, surface passivation by atomic hydrogen and its ease of doping. Although silicon is the workhorse material for microelectronics, its indirect bandgap is currently unsuitable for active device applications such as light-emitting diodes and diode lasers. In place of silicon, group III-V materials (such as GaAs), with their direct bandgaps and higher quantum efficiencies, currently dominate the optoelectronics market. They have applications in light-emitting diodes, multijunction solar cells and diode lasers. One of the challenges in modern device fabrication has been the integration of group III-V optoelectronics with silicon microelectronics. However, this integration has been challenged by the differences in crystal structure between silicon and III-V compounds that lead to crystal structure defects. These problems can be alleviated by growing structures on the nanometer scale where crystal structure differences can be easily accommodated. Researchers from the LaPierre group at McMaster University, Canada, have realized just this through the growth of nanowires with a layered GaP/GaAsP/GaP structure on silicon substrates. The nanowires were grown directly on silicon substrates by the now famous vapor-liquid-solid growth technique in which metal nanoparticles are used to seed the growth of the 1D nanowires. The composition of the nanowires was controlled by switching the source materials to produce a complex core-shell arrangement of GaAsP sandwiched between GaP. The GaAsP layer thickness can be controlled at the nanometer scale where quantum effects are inherent. This makes these nanowires ideally suited for active nanometer-scale optoelectronic devices, such as electrically driven single photon sources for quantum information processing.
The nanowires were examined at the atomic scale with the new Titan microscope at the Canadian Centre for Electron Microscopy at McMaster. It was clearly established that the nanowires had a wurtzite crystal structure with intermittent stacking faults wherein the arrangement of the atomic layers were interspersed with a zinc-blende structure. These stacking faults, which have long been a significant obstacle in the field of nanowire research due to their unfavorable effects on device functionality, were shown to be removed as a result of interruptions between the growth of adjacent layers.
In time, the authors plan to use the new Canadian facilities to investigate the structural, optical and electrical properties of single and bundled nanowires. A deeper understanding of the degree of quantum effects within the GaP/GaAsP/GaP nanowires will surely allow for improved control over the numerous foreseeable nanowire-based device applications.
About the author
Parsian Mohseni is a graduate student at McMaster University’s department of engineering physics, under the supervision of Prof. Ray LaPierre. Prof. LaPierre heads a Nanostructures and Optoelectronics Research Group and is a faculty member of the Centre for Emerging Device Technologies. His research involves fundamental physical processes associated with nanomaterials and nanodevices.