Effect of exciton-phonon coupling in the calculated optical absorption of carbon nanotubes

We find that the optical properties of carbon nanotubes reflect remarkably strong effects of exciton-phonon coupling. Tight-binding calculations show that a significant fraction of the spectral weight of the absorption peak is transferred to a distinct exciton+phonon sideband, which is peaked at around 200 meV above the main absorption peak. This sideband provides a distinctive signature of the excitonic character of the optical transition. The exciton-phonon coupling is reflected in a dynamical structural distortion, which contributes a binding energy of up to 100 meV. The distortion is surprisingly long ranged, and is strongly dependent on chirality.     

Enhanced instability of strained alloy films due to compositional stresses

A single-component strained film is known to be unstable to the stress-driven morphological instability. Here, we determine how the instability is modified in an alloy film by considering the effect of compositional stresses due to an atomic size difference. We find that the coupling of composition to stress always makes the film more unstable to the formation of stress-driven surface undulations. The destabilization is greatest over a range of intermediate deposition rates.     

Nonuniform composition profile in In0.5Ga0.5As alloy quantum dots

We use cross-sectional scanning tunneling microscopy to examine the shape and composition distribution of In0.5Ga0.5As quantum dots (QDs) formed by capping heteroepitaxial islands. The QDs have a truncated pyramid shape. The composition appears highly nonuniform, with an In-rich core having an inverted-triangle shape. Thus the electronic properties will be drastically altered, relative to the uniform composition generally assumed in device modeling. Theoretical analysis of the QD growth suggests a simple explanation for the unexpected shape of the In-rich core.     

Mobile ambipolar domain in carbon-nanotube infrared emitters

We spatially resolve the infrared light emission from ambipolar carbon-nanotube field-effect transistors with long-channel lengths. Electrons and holes are injected from opposite contacts into a single nanotube molecule. The ambipolar domain, where electron and hole currents overlap, forms a microscopic light emitter within the carbon nanotube. We can control its location by varying gate and drain voltages. At high electric fields, additional stationary spots appear due to defect-assisted Zener tunneling or impact ionization. The laterally resolved measurement provides valuable insight into the transistor behavior, complementary to electronic device characteristics.     

Periodically changing morphology of the growth interface in Si, Ge, and GaP nanowires

Nanowire growth in the standard <111> direction is assumed to occur at a planar catalyst-nanowire interface, but recent reports contradict this picture. Here we show that a nonplanar growth interface is, in fact, a general phenomenon. Both III-V and group IV nanowires show a distinct region at the trijunction with a different orientation whose size oscillates during growth, synchronized with step flow. We develop an explicit model for this structure that agrees well with experiment and shows that the oscillations provide a direct visualization of catalyst supersaturation. We discuss the implications for wire growth and structure.     

Lateral motion of SiGe islands driven by surface-mediated alloying

SiGe islands move laterally on a Si(001) substrate during in situ postgrowth annealing. This surprising behavior is revealed by an analysis of the substrate surface morphology after island removal using wet chemical etching. We explain the island motion by asymmetric surface-mediated alloying. Material leaves one side of the island by surface diffusion, and mixes with additional Si from the surrounding surface as it redeposits on the other side. Thus the island moves laterally while becoming larger and more dilute.