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Nanocrystals have twins
RESEARCH NEWS
Nanocrystals have twins DEFORMATION The high strength and hardness of nanocrystalline materials implies a different mode of plastic deformation from coarse-grained materials. Deformation twins, for example, are not observed in coarse-grained Al, but molecular dynamics (MD) simulations have recently indicated that they would occur with a small enough grain size. Mingwei Chen and coworkers at The Johns Hopkins University have reported the first experimental observations of deformation twinning in plastically deformed nanocrystalline Al [Science (2003) 300, 1275]. The researchers introduced large plastic strains into nanocrystalline Al films by using two deformation methods: microindentation and manual grinding. Partial dislocations in consecutive planes in a face-centered cubic crystal result in the formation of a twin band, which can be identified by a lattice that is a mirror of the host lattice. The team’s high-resolution transmission electron microscopy (HRTEM) studies reveal the presence
Current developments MICROSCOPY
Ben D. Schrag and Gang Xiao of Brown University have developed a new scanning microscopy technique for imaging the magnetic fields produced by current-carrying electronic circuits [Appl. Phys. Lett. (2003) 82 (19), 3272]. Using a mathematical algorithm, the magnetic field data is converted into a profile of the in-plane current densities within the sample. The submicron resolution images of current distribution produced by the new technique can be used to reveal flaws in microscopic conductors. “This microscope will allow manufacturers to find defects in each embedded wire in evertinier circuits,” believes Xiao. A nanometer-scale magnetoresistance sensor is key to the operation of the scanning microscope. The thermally stable sensor has an active area measuring only tens of nanometers in the primary direction and a linear response over a wide frequency range. Schrag and Xiao estimate it gives a spatial resolution on the order of 40 nm. The system is capable of imaging embedded circuits and is operated at room temperature. This gives real advantages over other magnetic imaging tools that require low temperatures or exposed current-carrying layers. The two researchers demonstrate the capabilities of the microscope by imaging conductors undergoing electromigration. The development of physical voids in passivated Al conductors can be deduced from the appearance of gaps in the current density profile. Schrag and Xiao are also using the microscope to detect the current-induced formation of pinholes in magnetic tunnel junction devices. They hope there might eventually be more applications for the scanning magnetic microscope, for example in finding internal cracks within aircraft, sensing biological agents, or recognizing counterfeit bills.
of such deformation twins in samples produced by both methods. The group believes that the HRTEM results corroborate the previous MD
Measuring dislocations
predictions and give credence to a
DEFECTS
picture of a transition in the mechanism of deformation as the grain size of a material gets smaller. In coarse-grained samples, perfect dislocations dominate. However, partial twinning dislocations become the preferential mode of deformation when the grain size is on the order of tens of nanometers. The generation of twin interfaces and stacking faults may have an important role in mediating deformation at small grain sizes. This could provide a physical explanation, at least in part, for the hardness of nanocrystalline materials.
14
July/August 2003
A detailed understanding of how strain fields surrounding defects influence electronic and mechanical properties would be of great benefit in many areas, especially for semiconductor devices. Wanting to develop the electron microscope as a tool for strain analysis at the atomic scale, researchers at the Centre National de Recherche Scientifique (CNRS) and CEA-Grenoble, France have accurately measured displacements around an edge dislocation in Si [Nature (2003) 423, 270]. The French group combined high-resolution electron microscopy with an image analysis technique first developed for optical interferometry. Improvements in sample preparation and the correction of optical distortions introduced by the microscope are crucial for the method’s success. They were able to measure the displacement field surrounding an edge dislocation and compare it to anisotropic elastic theory. The values agree to within 0.03 Å. “Indeed,” explain the researchers, “the results can be considered as an experimental verification of anisotropic theory at the near-atomic scale.”
Lens enhances resolution MICROSCOPY Researchers at the Universität Karlsruhe, Germany have enhanced far-field nanophotoluminescence (nano-PL) experiments by combining a hemispherical solid immersion lens (h-SIL) and a confocal microscope. This setup could have a number of unique applications in semiconductor spectroscopy [J. Appl. Phys. (2003) 93 (10) 6265]. Photoluminescence (PL) is an important technique for the investigation of electronic states. To be able to characterize semiconductor nanostructures, the diffraction-limited resolution of far-field optics can be improved by increasing the refractive index of the media around the sample. Heinz Kalt and colleagues did this by placing a h-SIL with a refractive index of 2.16 on the surface of the sample. The spatial resolution of the h-SILenhanced confocal microscope system is 0.4 times the wavelength of the incident laser light. “This resolution is nearly comparable to that of an apertureless scanning near-field optical microscope (SNOM) using uncoated fiber tips,” says Kalt. The introduction of the h-SIL also improves the collection efficiency of the microscope five times, making the system suitable for low signal levels. In addition, the researchers show the feasibility of time-resolved excitation and detection studies. One of the advantages of this far-field system over SNOM is the opportunity to detect PL at positions outside the local excitation spot. This enables the investigation of exciton transport in quantum wells, for example. Here, the spatial extent of exciton luminescence can be observed over time. The group also demonstrate single quantum dot spectroscopy and Kalt adds that SIL-enhanced nano-PL should be well suited to the study of propagating modes in photonic structures.
Nanocrystals have twins DEFORMATION The high strength and hardness of nanocrystalline materials implies a different mode of plastic deformation from coarse-grained materials. Deformation twins, for example, are not observed in coarse-grained Al, but molecular dynamics (MD) simulations have recently indicated that they would occur with a small enough grain size. Mingwei Chen and coworkers at The Johns Hopkins University have reported the first experimental observations of deformation twinning in plastically deformed nanocrystalline Al [Science (2003) 300, 1275]. The researchers introduced large plastic strains into nanocrystalline Al films by using two deformation methods: microindentation and manual grinding. Partial dislocations in consecutive planes in a face-centered cubic crystal result in the formation of a twin band, which can be identified by a lattice that is a mirror of the host lattice. The team’s high-resolution transmission electron microscopy (HRTEM) studies reveal the presence
Current developments MICROSCOPY
Ben D. Schrag and Gang Xiao of Brown University have developed a new scanning microscopy technique for imaging the magnetic fields produced by current-carrying electronic circuits [Appl. Phys. Lett. (2003) 82 (19), 3272]. Using a mathematical algorithm, the magnetic field data is converted into a profile of the in-plane current densities within the sample. The submicron resolution images of current distribution produced by the new technique can be used to reveal flaws in microscopic conductors. “This microscope will allow manufacturers to find defects in each embedded wire in evertinier circuits,” believes Xiao. A nanometer-scale magnetoresistance sensor is key to the operation of the scanning microscope. The thermally stable sensor has an active area measuring only tens of nanometers in the primary direction and a linear response over a wide frequency range. Schrag and Xiao estimate it gives a spatial resolution on the order of 40 nm. The system is capable of imaging embedded circuits and is operated at room temperature. This gives real advantages over other magnetic imaging tools that require low temperatures or exposed current-carrying layers. The two researchers demonstrate the capabilities of the microscope by imaging conductors undergoing electromigration. The development of physical voids in passivated Al conductors can be deduced from the appearance of gaps in the current density profile. Schrag and Xiao are also using the microscope to detect the current-induced formation of pinholes in magnetic tunnel junction devices. They hope there might eventually be more applications for the scanning magnetic microscope, for example in finding internal cracks within aircraft, sensing biological agents, or recognizing counterfeit bills.
of such deformation twins in samples produced by both methods. The group believes that the HRTEM results corroborate the previous MD
Measuring dislocations
predictions and give credence to a
DEFECTS
picture of a transition in the mechanism of deformation as the grain size of a material gets smaller. In coarse-grained samples, perfect dislocations dominate. However, partial twinning dislocations become the preferential mode of deformation when the grain size is on the order of tens of nanometers. The generation of twin interfaces and stacking faults may have an important role in mediating deformation at small grain sizes. This could provide a physical explanation, at least in part, for the hardness of nanocrystalline materials.
14
July/August 2003
A detailed understanding of how strain fields surrounding defects influence electronic and mechanical properties would be of great benefit in many areas, especially for semiconductor devices. Wanting to develop the electron microscope as a tool for strain analysis at the atomic scale, researchers at the Centre National de Recherche Scientifique (CNRS) and CEA-Grenoble, France have accurately measured displacements around an edge dislocation in Si [Nature (2003) 423, 270]. The French group combined high-resolution electron microscopy with an image analysis technique first developed for optical interferometry. Improvements in sample preparation and the correction of optical distortions introduced by the microscope are crucial for the method’s success. They were able to measure the displacement field surrounding an edge dislocation and compare it to anisotropic elastic theory. The values agree to within 0.03 Å. “Indeed,” explain the researchers, “the results can be considered as an experimental verification of anisotropic theory at the near-atomic scale.”
Lens enhances resolution MICROSCOPY Researchers at the Universität Karlsruhe, Germany have enhanced far-field nanophotoluminescence (nano-PL) experiments by combining a hemispherical solid immersion lens (h-SIL) and a confocal microscope. This setup could have a number of unique applications in semiconductor spectroscopy [J. Appl. Phys. (2003) 93 (10) 6265]. Photoluminescence (PL) is an important technique for the investigation of electronic states. To be able to characterize semiconductor nanostructures, the diffraction-limited resolution of far-field optics can be improved by increasing the refractive index of the media around the sample. Heinz Kalt and colleagues did this by placing a h-SIL with a refractive index of 2.16 on the surface of the sample. The spatial resolution of the h-SILenhanced confocal microscope system is 0.4 times the wavelength of the incident laser light. “This resolution is nearly comparable to that of an apertureless scanning near-field optical microscope (SNOM) using uncoated fiber tips,” says Kalt. The introduction of the h-SIL also improves the collection efficiency of the microscope five times, making the system suitable for low signal levels. In addition, the researchers show the feasibility of time-resolved excitation and detection studies. One of the advantages of this far-field system over SNOM is the opportunity to detect PL at positions outside the local excitation spot. This enables the investigation of exciton transport in quantum wells, for example. Here, the spatial extent of exciton luminescence can be observed over time. The group also demonstrate single quantum dot spectroscopy and Kalt adds that SIL-enhanced nano-PL should be well suited to the study of propagating modes in photonic structures.