Gregory N. Parsons

Physical and electrical characterization of ultrathin yttrium silicate insulators on silicon

This article describes the oxidation of yttrium on silicon to form yttrium silicate films for application as high dielectric constant insulators. The high reactivity of yttrium metal with silicon and oxygen is utilized to form amorphous yttrium silicate films with a minimal interfacial silicon dioxide layer. Yttrium silicate films (∼40 A) with an equivalent silicon dioxide thickness of ∼11 A and k∼14 are formed by oxidizing yttrium on silicon. The physical properties of yttrium silicate films on silicon are investigated using x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The oxidation of yttrium silicide results in films nearly identical, although with a higher silicon fraction, to films formed by oxidation of yttrium on silicon. The oxidation of yttrium on silicon results in a competition for yttrium between silicide formation and oxidation. This competition yields yttrium silicate films for thin ( 80 A)...

Spatial atomic layer deposition: A route towards further industrialization of atomic layer deposition

Atomic layer deposition (ALD) is a technique capable of producing ultrathin conformal films with atomic level control over thickness. A major drawback of ALD is its low deposition rate, making ALD less attractive for applications that require high throughput processing. An approach to overcome this drawback is spatial ALD, i.e., an ALD mode where the half-reactions are separated spatially instead of through the use of purge steps. This allows for high deposition rate and high throughput ALD without compromising the typical ALD assets. This paper gives a perspective of past and current developments in spatial ALD. The technology is discussed and the main players are identified. Furthermore, this overview highlights current as well as new applications for spatial ALD, with a focus on photovoltaics and flexible electronics. 2012 American Vacuum Society.

Evidence of aluminum silicate formation during chemical vapor deposition of amorphous Al2O3 thin films on Si(100)

Using narrow nuclear reaction resonance profiling, aluminum profiles are obtained in ∼3.5 nm Al2O3 films deposited by low temperature (<400 °C) chemical vapor deposition on Si(100). Narrow nuclear resonance and Auger depth profiles show similar Al profiles for thicker (∼18 nm) films. The Al profile obtained on the thin film is consistent with a thin aluminum silicate layer, consisting of Al–O–Si bond units, between the silicon and Al2O3 layer. Transmission electron microscopy shows evidence for a two-layer structure in Si/Al2O3/Al stacks, and x-ray photoelectron spectroscopy shows a peak in the Si 2p region near 102 eV, consistent with Al–O–Si units. The silicate layer is speculated to result from reactions between silicon and hydroxyl groups formed on the surface during oxidation of the adsorbed precursor.

Stability of low-temperature amorphous silicon thin film transistors formed on glass and transparent plastic substrates

This article describes the formation of amorphous silicon thin film transistors (TFTs) on glass and flexible transparent plastic substrates using rf plasma enhanced chemical vapor deposition and a maximum processing temperature of 110 °C. Silane diluted with hydrogen was used for the preparation of the amorphous silicon, and SiH4/NH3/N2 or SiH4/NH3/N2/H2 mixtures were used for the deposition of the silicon nitride gate dielectric. The amorphous silicon nitride layers were characterized by transmission infrared spectroscopy and current-voltage measurements; the plastic substrates were 10 mil thick (0.25 mm) polyethylene terephthalate sheets. Transistors formed using the same process on glass and plastic showed linear mobilities ranging from 0.1 to 0.5 cm2/V s with ION/IOFF ratios⩾107. To characterize the stability of the transistors on glass, n- and p-channel transconductances were measured before and after bias stressing. Devices formed at 110 °C show evidence of charge trapping near the a-Si/SiNx interfa...

Low hydrogen content stoichiometric silicon nitride films deposited by plasma‐enhanced chemical vapor deposition

We have deposited silicon nitride films by plasma‐enhanced chemical vapor deposition (PECVD) at 250 °C with properties similar to films prepared at 700 °C by low‐pressure chemical vapor deposition (LPCVD). Films are prepared using silane and nitrogen source gases with helium dilution. The film properties, including N/Si ratio, hydrogen content and electrical quality are most sensitive to changes in the silane flow rate during deposition. For films deposited under optimized conditions at a substrate temperature of 250 °C, current versus voltage measurements in metal‐insulator‐semiconductor structures show the onset of carrier injection at 3–4 MV/cm, slightly lower than LPCVD films. When bias‐stressed to 2 MV/cm, capacitance versus voltage measurements show some hysteretic behavior and evidence for positive fixed charge, similar to LPCVD films. For the optimized films: N/Si=1.33±.02; refractive index (λ=6328 A)=1.980±0.01; dielectric constant (1 MHz) ∼7.5; density=2.7±0.1; and the etch rate in 10% buffered ...

Solar water splitting in a molecular photoelectrochemical cell.

Significance Solar water splitting into H2 and O2 with visible light has been achieved by a molecular assembly. The dye sensitized photoelectrosynthesis cell configuration combined with core–shell structures with a thin layer of TiO2 on transparent, nanostructured transparent conducting oxides (TCO), with the outer TiO2 shell formed by atomic layer deposition. In this configuration, excitation and injection occur rapidly and efficiently with the injected electrons collected by the nanostructured TCO on the nanosecond timescale where they are collected by the planar conductive electrode and transmitted to the cathode for H2 production. This allows multiple oxidative equivalents to accumulate at a remote catalyst where water oxidation catalysis occurs. Artificial photosynthesis and the production of solar fuels could be a key element in a future renewable energy economy providing a solution to the energy storage problem in solar energy conversion. We describe a hybrid strategy for solar water splitting based on a dye sensitized photoelectrosynthesis cell. It uses a derivatized, core–shell nanostructured photoanode with the core a high surface area conductive metal oxide film––indium tin oxide or antimony tin oxide––coated with a thin outer shell of TiO2 formed by atomic layer deposition. A “chromophore–catalyst assembly” 1, [(PO3H2)2bpy)2Ru(4-Mebpy-4-bimpy)Rub(tpy)(OH2)]4+, which combines both light absorber and water oxidation catalyst in a single molecule, was attached to the TiO2 shell. Visible photolysis of the resulting core–shell assembly structure with a Pt cathode resulted in water splitting into hydrogen and oxygen with an absorbed photon conversion efficiency of 4.4% at peak photocurrent.

Three-dimensional self-assembled photonic crystals with high temperature stability for thermal emission modification

Selective thermal emission in a useful range of energies from a material operating at high temperatures is required for effective solar thermophotovoltaic energy conversion. Three-dimensional metallic photonic crystals can exhibit spectral emissivity that is modified compared with the emissivity of unstructured metals, resulting in an emission spectrum useful for solar thermophotovoltaics. However, retention of the three-dimensional mesostructure at high temperatures remains a significant challenge. Here we utilize self-assembled templates to fabricate high-quality tungsten photonic crystals that demonstrate unprecedented thermal stability up to at least 1,400 °C and modified thermal emission at solar thermophotovoltaic operating temperatures. We also obtain comparable thermal and optical results using a photonic crystal comprising a previously unstudied material, hafnium diboride, suggesting that refractory metallic ceramic materials are viable candidates for photonic crystal-based solar thermophotovoltaic devices and should be more extensively studied.

Temperature-dependent subsurface growth during atomic layer deposition on polypropylene and cellulose fibers.

Nucleation and subsequent growth of aluminum oxide by atomic layer deposition (ALD) on polypropylene fiber substrates is strongly dependent on processing temperature and polymer backbone structure. Deposition on cellulose cotton, which contains ample hydroxyl sites for ALD nucleation and growth on the polymer backbone, readily produces a uniform and conformal coating. However, similar ALD processing on polypropylene, which contains no readily available active sites for growth initiation, results in a graded and intermixed polymer/inorganic interface layer. The structure of the polymer/inorganic layer depends strongly on the process temperature, where lower temperature (60 degrees C) produced a more abrupt transition. Cross-sectional transmission electron microscopy images of polypropylene fibers coated at higher temperature (90 degrees C) show that non-coalesced particles form in the near-surface region of the polymer, and the particles grow in size and coalesce into a film as the number of ALD cycles increases. Quartz crystal microbalance analysis on polypropylene films confirms enhanced mass uptake at higher processing temperatures, and X-ray photoelectron spectroscopy data also confirm heterogeneous mixing between the aluminum oxide and the polypropylene during deposition at higher temperatures. The strong temperature dependence of film nucleation and subsurface growth is ascribed to a relatively large increase in bulk species diffusivity that occurs upon the temperature-driven free volume expansion of the polypropylene. These results provide helpful insight into mechanisms for controlled organic/inorganic thin film and fiber materials integration.

Microcontact patterning of ruthenium gate electrodes by selective area atomic layer deposition

Patterned octadecyltrichlorosilane monolayers are used to inhibit film nucleation, enabling selective area atomic layer deposition (ALD) of ruthenium on SiO2 and HfO2 surfaces using bis-(cyclopentadienyl)ruthenium and oxygen. X-ray photoelectron spectroscopy indicated that OTS could deactivate film growth on thermal silicon oxide and hafnium oxide surfaces. The growth rate of ALD Ru is similar on various starting surfaces, but the growth initiation differed substantially. Metal-oxide-semiconductor capacitors were fabricated directly using the selective-area process. Capacitance measurements indicate the effective work function of ALD Ru is 4.84±0.1eV on SiO2, and the effective work function is reduced on HfO2∕SiO2 layers.

Atomic layer deposition and abrupt wetting transitions on nonwoven polypropylene and woven cotton fabrics.

Atomic layer deposition (ALD) of aluminum oxide on nonwoven polypropylene and woven cotton fabric materials can be used to transform and control fiber surface wetting properties. Infrared analysis shows that ALD can produce a uniform coating throughout the nonwoven polypropylene fiber matrix, and the amount of coating can be controlled by the number of ALD cycles. Upon coating by ALD aluminum oxide, nonwetting hydrophobic polypropylene fibers transition to either a metastable hydrophobic or a fully wetting hydrophilic state, consistent with well-known Cassie-Baxter and Wenzel models of surface wetting of roughened surfaces. The observed nonwetting/wetting transition depends on ALD process variables such as the number of ALD coating cycles and deposition temperature. Cotton fabrics coated with ALD aluminum oxide at moderate temperatures were also observed to transition from a natural wetting state to a metastable hydrophobic state and back to wetting depending on the number of ALD cycles. The transitions on cotton appear to be less sensitive to deposition temperature. The results provide insight into the effect of ALD film growth mechanisms on hydrophobic and hydrophilic polymers and fibrous structures. The ability to adjust and control surface energy, surface reactivity, and wettability of polymer and natural fiber systems using atomic layer deposition may enable a wide range of new applications for functional fiber-based systems.