Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

Single layers of isolated, size-controlled silicon nanocrystals were prepared by thermal crystallization of a thin amorphous silicon layer sandwiched between silicon dioxide layers. A subsequent oxidation treatment ensured controlled increase in their lateral separation. The size of the nanocrystals, separation of the nanocrystals (from < 1 nm to ∼ 4 nm), stoichiometry of the resulting oxide and surface morphology were monitored with transmission electron microscopy, scanning transmission electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. Mesoscopic charge transport studies performed with an electrostatic force microscope (EFM) revealed rapid lateral transport of charges when the nanocrystals were tightly packed (< 1 nm average separation) and interconnected. As the inter-nanocrystal separation was increased, lateral charge transport was rapidly suppressed. Nanocrystals separated by up to 3.6 nm retained the injected charges in a well-defined localized region (∼ 62 nm diameter region) for a time of the order of several days. The ability to switch from a very short to a very long retention time using the same structure by simply changing the post-growth processing conditions is attractive for various applications involving charge transport and localization.

Hetero-epitaxy of crystalline perovskite SrTiO3 and BaTiO3 on GaAs(001) was achieved using molecular beam epitaxy. A number of surface and interface techniques were used to characterize the growth, structural, and chemical properties of epitaxial oxide films on GaAs. Reflection-high-energy-electron diffraction of SrTiO3 and BaTiO3 showed streaky diffraction features and low diffraction background through out growth, indicating that the epitaxial SrTiO3 and BaTiO3 films had good crystallinity. X-ray diffraction showed an epitaxial relationship between these perovskite oxides and GaAs(001) substrates. Transmission electron microscopy and scanning probe microscopy revealed an abrupt oxide/GaAs interface and smooth SrTiO3 and BaTiO3 surfaces. The success of growth of epitaxial perovskite oxides on GaAs enabled us to integrate a number of functional oxides such as piezoelectric and magnetic oxides epitaxially on GaAs substrates.

Low-k material integration issues that plague the microelectronics industry include the compromise in mechanical properties that one incurs in abandoning fully dense silica dieletrics. Typical elastic moduli of OSG low-k dieletric films are 2-10 GPa with corresponding hardnesses of 0.5 to 1.5 GPa. In the present study, the hardness and elastic modulus properties measured by nanoindentation of porous silica based low-k films are correlated with in initial estimates of density using a novel technique of spectroscopic ellispsometry. Transmission electron microscopy and X-ray photoelectron spectroscopy show the structural and chemical similarity of the films. Nanoindentation and spectroscopic ellipsometry results reflect significant deviations in material behavior from that expected from a simple model of silica (SiO2) with included voids or porosity, suggesting that the methyl groups are actively participating in the mechanical and optical properties of the material.

This paper presents an x-ray diffraction and transmission electron microscopy analysis of Al1−xInxAs grown by molecular beam epitaxy. Two samples grown on (001) InP at temperatures of 370 and 400 °C are characterized. The first, which contains a high density of twin lamellae, exhibits triple-period short-range ordering with a rather short correlation range normal to the (111) planes. Within these (individual) planes, the concentration, however, is uniform over a considerably greater distance, leading to a highly anisotropic scattering. This is the first observation of triple-period short-range ordering in a sample that exhibits 2 × 1 surface reconstruction. The second sample exhibits CuPt-type short-range ordering with scattering that is significantly streaked, suggestive of lamellar-shaped ordered domains. Both samples contain high densities of stacking faults leading to additional sharp streaking along symmetry-allowed 〈111〉 directions.

Mass-selected ion-beam deposition using 120 eV C+ ions has been used to grow a carbon film on a Si substrate held at 200° C. The structure of the film has been characterized by transmission electron microscopy and electron energy loss spectroscopy. The film is graphitic and highly oriented with the c-axis lying parallel to the substrate. Moreover, the film is under significant biaxial stress such that the graphitic layer spacing is reduced by 4% from that of ambient pressure graphite. This oriented structure evolves due to the mobility of the carbon atoms at 200 °C. The material is sufficiently crystalline on the nanometer scale so as to produce Bragg diffraction discs in a convergent beam electron diffraction pattern using a 2.5 nm probe.

Two simple and effective far-field-optics-based methods capable of isolating photoluminescence peaks of different individual self assembled quantum dots (SAQD's) with nanometer scale precision are presented. By using these methods, we performed the temperature and electric field dependent studies on the optical properties of SAQD's. We found temperature induced inter-dot carrier transfer among neighboring quantum dots (QD's) and observed the quantum confined stark effect (QCSE).

Boron nitride thin films on sapphire substrates were investigated for their tribological and optoelectronic applications. A gridless end Hall gun source and an electron cyclotron resonance (ECR) source were used for nitrogen species delivery while pure boron was evaporated at a rate of 0.2 Å/s. The surface stability of these thin films was investigated by high temperature annealing. Atomic force microscopy (AFM), friction force microscopy (FFM), and Knoop microhardness measurements were performed on the materials in order to assess their merits as tribological coatings. Finally, BN thin films were subjected to laser transient photoconductivity (TPC) experiments to determine both their optical laser damage threshold as well as their photoconductivity characteristics. For both single-pulse shot and multiple-pulse irradiation regimes, preliminary tests showed the higher the ion beam current used during growth (70–150 mA), the higher the optical damage threshold. The lower damage threshold was typical of BN films grown using an ECR plasma source and was measured to be in the range of ∼50 MW/cm2. Optical damage of films grown at ion beam currents above 100 mA was not observed at laser intensities up to few hundreds MW/cm2. A multiphoton excitation technique was utilized to obtain PC signals from this wide band gap material and preliminary results show that unusual PC voltage amplitudes as high as 0.5 V were observed.

The YBa2Cu3O7−x formation kinetics from a spray-roasted precursor powder containing Y2O3, BaCO3, and CuO was followed via in situ, time-resolved x-ray diffraction as a function of gas atmosphere and temperature. In inert atmospheres, BaCO3 and CuO form BaCu2O2 which subsequently reacts with Y2O3 to form YBa2Cu3O6. However, YBa2Cu3O6 decomposes at temperatures exceeding 725 °C with Y2BaCuO5 being one of the decomposition products. In oxidizing atmospheres, YBa2Cu3O7−x formation involves the BaCuO2. At high temperatures (800–840 °C), oxygen increases the yield of YBa2Cu3O6. A nuclei growth model assuming two-dimensional, diffusion-controlled growth with second-order nucleation rate fits the experimental data.

Iridium oxide films produced by reactive sputtering (SIROFs) haveconsiderably higher densities than those made by other techniques such asthe anodic reaction at metal surfaces which contain pores and Microvoids. Ithas been previously reported that SIROFs deposited with substratetemperatures of 300K are amorphous. Here we report x-ray and electrondiffraction measurements on SIROFs deposited on Al2O3,Si, NaCI and MgO substrates at 40°C. The x-ray diffraction patterns, I(Q),show “diffuse” structure that extends to beyond Q(=4εsinθ/) =10Â−1. We show that it is possible to reproduce qualitativelythe main features of the diffraction pattern by convolving the crystallinerutile powder pattern with a Lorentzian profile. The width of this profileis compatible with that determined from the Scherrer equation althoughsignificant peak shifts and texture are observed. This analysis reveals thatas-deposited SIROFs are crystalline with particle sizes in the range 25À to35Å rather than amorphous (for which no crystalline model would beappropriate). The electron microdiffraction data are also consistent with acrystallite size of a few nm and high resolution TEM reveals lattice fringesfrom crystallites few nm in size. The peak shifts, however, remain to beexplained.

The compound Bi2Sr2(GdxCa1−x)0.8Cu2O8+δ is examined by electron diffraction for several values of x. The incommensurate modulation which has been reported for x - 0 persists for all values of x with the wave vector increasing as x increases. A secondary modulation with a wave vector equal to one-half that of the primary is observed for x  0.85. This modulation manifests itself through satellite reflections that are highly streaked normal to the (001) planes and that flank certain select positions in reciprocal space. The appearance of this secondary modulation is accompanied by a change in the average crystal structure from B-centered to F-centered orthorhombic.

Using both x-ray and electron diffraction, we have studied the incommensurate modulation structure in single crystal Bi2Sr2CaCu2Ox. Electron diffraction clearly indicates a small a∗-axis component of the modulation wave vector k, along with the more familiar b∗-axis component (0.004a∗ + 0.212b∗). This tilt of the k-vector breaks the mirror symmetry perpendicular to a- and b-axes as well as the twofold symmetry about a- and b-axes. The modulated structure thereby has only a mirror plane normal to the c-axis. Intense x-ray streaking along the c-axis at several satcllite reflections was considerably more pronounced than at the fundamental reflections, indicating a stacking disorder along the c-axis predominantly in the modulated structure.

The cause of the stability of long period superstructures is still something of a mystery. Typically, two very different models have been proposed: according to model I, the period of the superstructure (or modulation) is determined by lowering of the electronic energy resulting from the formation of a new Brillouin zone. According to model II, competing short-range interactions tend to produce long-period structures, the wavelength of which is determined by configurational entropy considerations. Model I is exemplified by the Sato and Toth theory, apparently applicable to long-period superstructures in Cu-Au, for example. Model II is exemplified by the Axial Next Nearest Neighbor Ising Model, for which a low-temperature free energy expansion has recently been given by Fisher and Selke. The latter model appears to apply to long-period superstructures in Ag3Mg.