Atomic Layer Deposition (ALD) has enormous potential for the synthesis of advanced heterogeneous catalysts with controlled composition and structure at the atomic scale. The ability of ALD to produce conformal oxide coatings on porous, high-surface area materials offers the possibility to provide completely new types of catalyst supports. At the same time ALD can achieve highly uniform catalytically active metal and oxide phases with (sub-) nanometer dimensions.

Vanadium oxide species supported on high surface area oxides are among the most important catalytic materials for the selective, oxidative conversion of hydrocarbons to useful chemicals. In our laboratory ALD has been used to synthesize both the catalytic vanadium oxide and the supporting oxide on both high surface powders and anodic aluminum oxide (AAO) nanoliths. These materials have been characterized by SEM, XRF, ICP, UV-Vis absorption spectroscopy, Raman spectroscopy and evaluated for the oxidative dehydrogenation (ODH) of cyclohexane.

More recently we have studied what we call “ABC-type” ALD in which metal nanoparticles and support materials are grown sequentially in each ALD cycle. This method makes possible metal deposition at lower temperatures than conventional AB-type ALD and exceptionally small particles, ca. 0.5 nm. Using additional ALD support layers at the conclusion of the growth, the metal particles are stabilized against sintering at high temperatures and reaction conditions.

Ruthenium is of particular interest to the semiconductor industry and others due to its low bulk resistivity (7μٟcm) and high work function (4.7ev). In addition, its complementary oxide, RuO₂, can exhibit high specific capacitance (up to 750 F/g) and conductivity (80-100 µΩŸcm), making it attractive for energy storage applications. We report results for Ru and RuO₂ ALD using a novel Ru cyclohexadienyl precursor and oxygen. This precursor is attractive because it is liquid at room temperature, stable in air and non-reactive with water, while its vapor pressure is similar to that of common ALD Ru precursors RuCp₂ and Ru(EtCp)₂, i.e., 0.1 Torr at 60°C and 1 Torr at 100°C. ALD Ru deposition was achieved in a wafer scale (100mm), cross-flow ALD reactor. Self-limiting ALD surface chemistry is observed between 250-300°C with a growth rate of ~0.5Å/cycle and across-wafer uniformity >98%. Four point probe measurements show a low sheet resistance of 16μٟcm. Ru nucleation is improved compared to RuCp₂ and Ru(EtCp)₂ based processes, with no nucleation delay on SiO₂ or TiO₂, a slight delay on Si, and a significant delay on Al₂O₃. Growth rates are constant with the number of deposition cycles. Conformality studies were conducted using high aspect ratio anodic aluminum oxide (AAO) thin films, using a thin TiO2 nucleation layer before Ru ALD. This Ru ALD process converts to a RuO₂ ALD process at higher oxygen partial pressure, with an oxide conductivity of ~80 µΩŸcm. Post-process thin film characterization using XRD, XPS and AFM will be also presented.

This work was supported by the US Department of Energy, Office of Basic Energy Sciences as part of an Energy Frontier Research Center.

Silicon nanowire (SiNW) ensembles with vertical geometry have been realized using wet chemical etching of bulk silicon wafers (n-Si(100)) with an etching hard mask of silver nanoparticles that are deposited by wet chemical electroless deposition on silicon surfaces.

The new concept of the solar cell is based on the semiconductor-insulator-semiconductor (SIS) layer sequence produced by Atomic Layer Deposition (ALD). A thin tunnelling oxide (SiO₂, Al₂O₃) with a thickness of 5-20 Å and a 300 nm transparent conductive oxide (Al doped ZnO or In doped SnO₂) around 1D silicon nanostructures have been realized using Plasma Assisted ALD approach (Oxford Plasma, OpAL).

The first prototype reached an open-circuit voltage of 80mV and a short-circuit current density of 23mA/cm².

The influence of the thickness and chemical nature of the tunnelling oxide will be discussed. Back side contacts of Ti/Al or Ti/Ag were realized using sputtering. From literature it is known that the planar SIS solar cell can reach an energy conversation efficiency of approx 15%. Absorption of visible and infra-red light is significantly enhanced in nanowires compared to planar layers of identical thickness. Thus, wet chemically etched silicon nanowires have the potential for even higher energy conversation efficiencies compared to the planar SIS solar cells. The morphology, crystallographic and surface structure, optical and solar cell properties will be presented and discussed in details.

Phosphors have many uses today, such as information display, medical imaging, and theft prevention. The phosphors are often used as powders, even though thin films offer higher resolution and better chemical stability. We have investigated the cathodoluminescent (CL) and photoluminescent (PL) properties of thin films of several phosphors (e.g. SrAl₂O₄:Eu²⁺,Dy³⁺; SiO₂:PbS; Gd₂O₂S:Tb³⁺; SiO₂:Ce³⁺,Tb³⁺ and Y₂SiO₅:Ce³⁺) that were ablation deposited onto Si (100) substrates using either conventional pulsed laser deposition (PLD) or pulsed reactive crossed beam laser ablation (PRCLA). Several deposition parameters were varied, including vacuum versus partial pressure of gas (O₂ or Ar), type of laser pulse, and substrate temperature using either a 307 nm XeCl or 248 nm KrF excimer laser. The CL spectrum and intensity were measured in vacuum from films irradiated with 2 keV electrons for a prolonged period of time, while PL data were collected in air under excitation by either a 325 nm HeCd laser or a monochromatized xenon lamp. Both the CL and PL intensities were strongly dependent on the deposition conditions and post-deposition annealing. Data from scanning electron microscopy (SEM) and atomic force microscopy (AFM) show that the major influence of the deposition conditions on the CL/PL intensity was through changes in the morphology and topography of the films, which affects light scattering and out-coupling. Finally, the CL intensity from the films decreased significantly during prolonged electron beam irradiation. The degraded CL intensity resulted from the formation of non-luminescent oxide layers on the film surfaces. The chemical composition and electronic states of the ‘dead’ layers were analyzed using x-ray photoelectron spectroscopy (XPS). The influence of the various deposition conditions on the luminescent intensities will be discussed. The mechanism leading to lower CL intensities will be concluded to be electron stimulated surface chemical reactions.