R. Prioli

Structural analysis of TiO2 films grown using microwave-activated chemical bath deposition

Abstract TiO2 layer films were grown using the microwave (MW)-activated chemical bath deposition technique on two different indium tin oxide substrates. The TiO2 films are studied to determine their structural response when changing the MW heating power. Thickness (areal density), oxygen concentration profile, composition and surface homogeneity were determined using Rutherford backscattering spectrometry, nuclear reaction analysis and atomic force microscopy. The analysis showed that the composition, thickness and surface structure of the films are highly influenced by MW heating power. The substrate, acting as seed for nucleation, influences the layer thickness, indicating that a thinner layer of TiO2 is obtained for the more conducting substrates. The oxygen concentration profile is constant in the TiO2 layer at low MW heating, power (≈20%). The rugosity of the samples and the non-homogeneity increase with the MW heating power. If the MW heating power is high enough pinholes in the TiO2 layer of the order of the sample thickness are produced.

Strain-related optical properties of ZnO crystals due to nanoindentation on various surface orientations

Nanoindentations were performed on various crystallographic orientations of single crystal ZnO using a cono-spherical diamond tip with a radius of curvature of 260 nm. The crystal orientations were the (112¯0) a-plane, (101¯0) m-plane, and (0001) c-plane (Zn-face). The optical properties associated with nanoindentation have been investigated by cathodoluminescence. The load-displacement curves show that the c-plane is the most resistive to deformation, followed by the m-plane, and the a-plane. A large number of non-radiative defects are created directly below the indentation, regardless of the crystal orientation. Nanoindentation on the a- and m-plane crystals activates slip along the (0001) basal planes, creating a band of non-radiative defects as well as tensile strain along the basal planes. Compressive strain is observed perpendicularly to the basal planes due to an absence of easy-glide mechanisms in these directions. The nanoindentation on the c-plane crystal results in regions under tensile strain ...

Effect of native oxide mechanical deformation on InP nanoindentation

Native oxide has been found to have a noticeable effect on the mechanical deformation of InP during nanoindentation. The indentations were performed using spherical diamond tips and the residual impressions were studied by atomic force microscopy. It has been observed that in the early stages of mechanical deformation, plastic flow occurs in the oxide layer while the indium phosphide is still in the elastic regime. The deformed native oxide layer results in a pile-up formation that causes an increase in the contact area between the tip and the surface during the nanoindentation process. This increase in the projected contact area is shown to contribute to the apparent high pressure sustained by the crystal before the onset of plastic deformation. It is also shown that the stress necessary to generate the first dislocations from the crystal surface is ∼3 GPa higher than the stress needed for slip to occur when dislocations are already present in the crystalline structure.

Nanoscale dislocation patterning by scratching in an atomic force microscope

The nature of nanoscratching as a lithographic technique for site-selective generation of dislocations is investigated for use in the growth of nanostructures. Linear arrays of dislocations have been selectively introduced into (001) indium phosphide crystals by dragging a diamond tip in an atomic force microscope. The nature of plastic deformation is found to depend on the scratch direction. For ⟨110⟩ directions, anisotropic butterflylike structures with mostly screw dislocations indicate rotational motion in the vicinity of the advancing tip. For ⟨100⟩ directions, the dislocations do not propagate far from the surface, possibly due to interlocking between dislocations on different slip planes, with a surface morphology suggesting melting of the near surface region by frictional heat. These results indicate that growth of nanostructures should be highly dependent on the direction of the nanoscratch.

Plastic hardening in cubic semiconductors by nanoscratching

The effect of scratch proximity on the resistance to plastic deformation in InP (100) crystals under low normal loads has been studied using atomic force microscopy (AFM) and transmission electron microscopy. Plastic flow has been observed for scratches performed with an atomic force microscope along ⟨110⟩ and ⟨100⟩ crystallographic directions. Plastic hardening has been determined from AFM measurements of the scratch depth and width, as a function of the distance between parallel scratches. For relatively low loads, hardening is found to be independent of the crystallographic direction of the scratch. Significant hardening takes place for scratch separations of less than ∼80 nm. Analysis of the microstructure indicates that hardening occurs due to the interaction of dislocations generated at adjacent scratches and acting on different slip planes.

Growth of linearly ordered arrays of InAs nanocrystals on scratched InP

Linear arrays of InAs nanocrystals have been produced by metalorganic vapor phase epitaxy on scratches performed with an atomic force microscope tip along specific crystallographic directions of an (100) InP wafer. Scratches along ⟨110⟩ generate highly mobile defects that extend far from the scratch region along easy-glide directions. On the other hand, ⟨100⟩ scratches result in highly-localized plastic deformation, hardening, and possibly frictional heating. In both cases, growth of nanocrystals was observed only on the scratched areas. Random nucleation of nanocrystals is observed along ⟨110⟩ scratches, while linearly ordered growth occur along ⟨100⟩ scratches. We attribute these observations to the delocalized nature of the dislocations in the ⟨110⟩ case, giving the appearance of random nucleation, while highly localized crystal defects along the ⟨100⟩ scratch lines act as nucleation sites for the growth of linear arrays of nanocrystals.

Plasticity and optical properties of GaN under highly localized nanoindentation stress fields

Nanoscale plasticity has been studied on (0001) GaN thin films, using tips with very small radius of curvature. Cross-section transmission electron microscopy images of the nanoindentations indicate that the primary slip systems are the pyramidal {11¯01}⟨112¯3⟩ and {112¯2}⟨112¯3⟩, followed by the basal {0002}⟨112¯0⟩. Incipient plasticity was observed to be initiated by metastable atomic-scale slip events that occur as the crystal conforms to the shape of the tip. Large volumetric material displacements along the {11¯01}⟨112¯3⟩ and {112¯2}⟨112¯3⟩ slip systems were observed at an average shear stress of 11 GPa. Hexagonal shaped nanoindentation impressions following the symmetry of GaN were observed, with material pile-up in the ⟨112¯0⟩ directions. Spatially resolved cathodoluminescence images were used to correlate the microstructure with the optical properties. A large number of non-radiative defects were observed directly below the indentation. Regions under tensile stress extending from the nanoindentati...

Early stages of mechanical deformation in indium phosphide with the zinc blende structure

Nanoindentations were performed on a cubic semiconductor using a cono-spherical diamond tip with a 260 nm radius. The tip produces a single point of contact with the crystal surface allowing indentations with nano-scale dimensions. The early stages of deformation on (100) InP with the zinc-blende structure were observed to happen by the sequential introduction of metastable dislocation loops along the various slip planes directly beneath the point of contact. Locking of the dislocations loops forms a hardened region that acts as an extended tip during subsequent indentation, eventually leading to multiple bulk-like displacements (pop-in events) and to material pile up in the vicinity of the indentation pit. The first pop-in marks the transition of deformation from the nanometer to the micrometer scale.

Rutherford backscattering spectrometry analysis of TiO2 thin films

Abstract TiO 2 layers grown by microwave-activated chemical bath deposition (MW) and dip coating (DC), as well as by the combination of both techniques, were studied by Rutherford backscattering spectrometry (RBS), atomic force microscopy (AFM) and scanning electron microscopy (SEM). RBS analysis allows the determination of the stoichiometry and the thickness (in atoms/cm 2 ) of the TiO 2 layers. TiO 2 layers grown by DC have higher growth rates on a TiO 2 film obtained by MW compared to deposition directly onto an indium–tin oxide (ITO) substrate. TiO 2 layers grown by MW on a film obtained by DC have higher growth rates when compared to layers deposited onto ITO substrates. In this case, AFM analysis shows that the surface is rough and RBS reveals the presence of holes in TiO 2 films.

The effect of nanoscratching direction on the plastic deformation and surface morphology of InP crystals

The microstructure of (001) InP crystals scratched with a sharp diamond tip depends strongly on the scratching direction. The scratch surface is found to conform to the radius of curvature of the tip (∼60 nm) by the formation of atomic crystal steps produced by dislocation glide along {111} planes. ⟨110⟩ scratches lead to coherent local crystal lattice movement and rotation causing deep dislocation propagation into the crystal and irregular pileups at the sides of the scratch surface. ⟨100⟩ scratches lead to incoherent lattice movement causing dislocation locking that inhibits their propagation and results in regular pileups.