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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Ethane dehydrogenation on pristine and AlO x decorated Pt stepped surfaces

In this work, ethane dehydrogenation on pristine and AlO x decorated Pt surfaces was studied using density functional theory (DFT) calculations and reaction kinetics experiments. Following the study of adsorption of fourteen distinct C 2 H x and CH x species, the whole reaction network for ethane dehydrogenation on the terrace and the step of the pristine Pt(433) surface was systematically analyzed using DFT. We show that the under-coordinated step edge sites bind C 2 H x and CH x species more strongly than terrace sites. These under-coordinated step sites facilitate deep dehydrogenation, resulting in accumulation of coke precursors on the surface and catalyst deactivation. We demonstrate that by decorating the step with AlO x species the step edge sites can be blocked, leading to substantial suppression of deep dehydrogenation and coke formation. These findings were corroborated by reaction kinetics experiments where a four-fold enhancement of the turnover frequency for ethylene formation was measured on 10-cycle atomic layer deposited (ALD) alumina overcoated Pt/γ-Al 2 O 3 catalyst compared to Pt/γ-Al 2 O 3 without decoration with ALD-alumina. Our results suggest that AlO x decorated Pt is a promising catalyst for alkane dehydrogenation and could have applications, including in endothermic cooling in hypersonic flights.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Atomic layer deposition for membrane interface engineering

In many applications, interfaces govern the performance of membranes. Structure, chemistry, electrostatics, and other properties of interfaces can dominate the selectivity, flux, fouling resistance, and other critical aspects of membrane functionality. Control over membrane interfacial properties, therefore, is a powerful means of tailoring performance. In this minireview, we discuss the application of atomic layer deposition (ALD) and related techniques in the design of novel membrane interfaces. We discuss recent literature in which ALD is used to 1) modify the surface chemistry and interfacial properties of membranes, 2) tailor the pore sizes and separation characteristics of membranes, and 3) enable novel advanced functional membranes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Temporal and thermal evolutions of surface Sr-segregation in pristine and atomic layer deposition modified La 0.6 Sr 0.4 CoO 3-δ epitaxial films

The bulk-to-surface Sr segregation can seriously compromise the stability of oxygen electrocatalysis in La 1-x Sr x CoO 3-δ and limit its practical applications such as in solid oxide fuel cells. We show via in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) that the surface Sr-segregation is a kinetically fast process and the equilibrium surface Sr-concentration follows Arrhenius law from 250 to 520 °C at a fixed p O 2 = 1 × 10 -3 atm. We also show that application of a nanoscaled, atomic layer deposition (ALD) derived ZrO 2 overcoat can effectively suppress the Sr-segregation by reducing the surface concentration of oxygen vacancies. Electrochemical impedance spectroscopy (EIS) study further confirms that the ALD-ZrO 2 -coated LSCo epitaxial film exhibits a much lower and more stable polarization resistance than the uncoated one at 550 °C for >300 hours, suggesting that Sr-segregation is the source of the higher resistance.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Accurately constituting robust interfaces for high-performance high-energy lithium metal batteries

High-energy lithium metal batteries (LMBs) have received ever-increasing interest. Among them, coupling lithium metal (Li) with nickel-rich material, LiNi x Mn y Co z O 2 (NMCs, x ≥ 0.6, x + y + z = 1), is promising because Li anodes enable an extremely high capacity (∼3860 mA h g −1 ) and the lowest redox potential (−3.04 V vs. standard hydrogen electrode), while NMCs can achieve a much higher capacity of ∼200 mA h g −1 and lower cost than those of LiCoO 2 . However, the resultant Li‖NMC cells have been hindered from commercialization due to a series of challenges related to the interface stability of both Li anodes and NMC cathodes. Specifically, Li anodes suffer from Li dendritic growth and the formation of solid electrolyte interphase (SEI), while NMC cathodes suffer from the formation of cathode electrolyte interphase (CEI) and other interface-related issues, including transition metal dissolution, oxygen release, cracking, and so on. To tackle these issues, recently, two sister techniques, atomic and molecular layer deposition (ALD and MLD), have emerged and exhibit tremendous capabilities to accurately constitute robust interfaces to achieve high-performance Li‖NMC LMBs. They can uniquely develop uniform and conformal films as surface coatings of LMBs in a precisely controllable mode at the atomic/molecular level, while proceeding with film deposition at low temperatures (e.g., ≤250 °C). In this Feature Article, we review the latest research progress in developing novel surface coatings via ALD and MLD for Li‖NMC LMBs and discuss outcomes for pursuing high performance.

25 ENERGY STORAGE↗
DOE OSTI2013

Improving electrical performance and bias stability of HfInZnO-TFT with optimizing the channel thickness

RF magnetron sputtered HfInZnO film and atomic layer deposition (ALD) Al{sub 2}O{sub 3} film were employed for thin film transistors (TFTs) as channel layer and gate insulator, respectively. To achieve HfInZnO-TFT with high performance and good bias stability, the thickness of HfInZnO active layer was optimized. The performance of HfInZnO-TFTs was found to be thickness dependent. As the HfInZnO active layer got thicker, the leakage current greatly increased from 1.73 × 10{sup −12} to 2.54 × 10{sup −8} A, the threshold voltage decreased from 7.4 to −4.7 V, while the subthreshold swing varied from 0.41 to 1.07 V/decade. Overall, the HfInZnO film showed superior performance, such as saturation mobility of 6.4 cm{sup 2}/V s, threshold voltage of 4.2 V, subthreshold swing of 0.43 V/decade, on/off current ratio of 3 × 10{sup 7} and V{sub th} shift of 3.6 V under V{sub GS}= 10 V for 7200 s. The results demonstrate the possibility of fabricating TFTs using HfInZnO film as active layer and using ALD Al{sub 2}O{sub 3} as gate insulator.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Zinc-oxide charge trapping memory cell with ultra-thin chromium-oxide trapping layer

A functional zinc-oxide based SONOS memory cell with ultra-thin chromium oxide trapping layer was fabricated. A 5 nm CrO{sub 2} layer is deposited between Atomic Layer Deposition (ALD) steps. A threshold voltage (V{sub t}) shift of 2.6V was achieved with a 10V programming voltage. Also for a 2V V{sub t} shift, the memory with CrO{sub 2} layer has a low programming voltage of 7.2V. Moreover, the deep trapping levels in CrO{sub 2} layer allows for additional scaling of the tunnel oxide due to an increase in the retention time. In addition, the structure was simulated using Physics Based TCAD. The results of the simulation fit very well with the experimental results providing an understanding of the charge trapping and tunneling physics.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Impact of N{sub 2} and forming gas plasma exposure on the growth and interfacial characteristics of Al{sub 2}O{sub 3} on AlGaN

The interface and atomic layer deposition (ALD) of Al{sub 2}O{sub 3} on the annealed, N{sub 2} plasma and forming gas (N{sub 2}:H{sub 2}) exposed Al{sub 0.25}Ga{sub 0.75}N surface was studied using in situ X-ray photoelectron spectroscopy and low energy ion scattering spectroscopy. Exposure of the Al{sub 0.25}Ga{sub 0.75}N surface to the plasma treatments is able to remove spurious carbon, and readily facilitate uniform ALD Al{sub 2}O{sub 3} nucleation.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Atomic layer deposition of Al{sub 2}O{sub 3} on germanium-tin (GeSn) and impact of wet chemical surface pre-treatment

GeSn is quickly emerging as a potential candidate for high performance Si-compatible transistor technology. Fabrication of high-ĸ gate stacks on GeSn with good interface properties is essential for realizing high performance field effect transistors based on this material system. We demonstrate an effective surface passivation scheme for n-Ge{sub 0.97}Sn{sub 0.03} alloy using atomic layer deposition (ALD) of Al{sub 2}O{sub 3}. The effect of pre-ALD wet chemical surface treatment is analyzed and shown to be critical in obtaining a good quality interface between GeSn and Al{sub 2}O{sub 3}. Using proper surface pre-treatment, mid-gap trap density for the Al{sub 2}O{sub 3}/GeSn interface of the order of 10{sup 12} cm{sup −2} has been achieved.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Subcutaneous oxidation of In{sub 0.53}Ga{sub 0.47}As(100) through ultra-thin atomic layer deposited Al{sub 2}O{sub 3}

Stability of oxide/semiconductor interfaces during device fabrication is critically important, particularly for adoption of new semiconductor channel materials, such as III-V compounds. Unintentional oxidation of an underlying In{sub 0.53}Ga{sub 0.47}As(100) surface through atomic layer deposited (ALD) Al{sub 2}O{sub 3} layers of varying thickness is investigated. Oxygen annealing of 1 ∼ 2 nm thickness Al{sub 2}O{sub 3} layers at 300 °C or higher and large-dose water vapor exposure during the ALD-Al{sub 2}O{sub 3} process at 300 °C produces InGaAs surface oxidation. This subcutaneous oxidation of InGaAs increases the Al{sub 2}O{sub 3}/InGaAs interface defect density as observed in suppressed band-edge photoluminescence and in capacitance-voltage analysis, possibly by creating As dangling bonds at the InGaAs surface.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Compositional characterization of atomic layer deposited alumina

As the microelectronic industry demands feature size in the order of few and sub nanometer regime, the film composition and other film properties become critical issues and ALD has emerged as the choice of industry. Aluminum oxide is a material with wide applications in electronic and optoelectronic devices and protective and ion barrier layers. Al{sub 2}O{sub 3} is an excellent dielectric because of its large band gap (8.7eV), large band offsets with silicon. We have deposited thin layers of alumina on silicon wafer (p-type) for gate dielectric applications by ALD technique and compositional characterizations of the deposited thin films were done using EDS, XPS and FTIR spectra.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Fluidized-bed atomic layer deposition reactor for the synthesis of core-shell nanoparticles

The design of a fluidized bed atomic layer deposition (ALD) reactor is described in detail. The reactor consists of three parts that have all been placed in one protective cabinet: precursor dosing, reactor, and residual gas treatment section. In the precursor dosing section, the chemicals needed for the ALD reaction are injected into the carrier gas using different methods for different precursors. The reactor section is designed in such a way that a homogeneous fluidized bed can be obtained with a constant, actively controlled, reactor pressure. Furthermore, no filters are required inside the reactor chamber, minimizing the risk of pressure increase due to fouling. The residual gas treatment section consists of a decomposition furnace to remove residual precursor and a particle filter and is installed to protect the pump. In order to demonstrate the performance of the reactor, SiO{sub 2} particles have been coated with TiO{sub 2} using tetrakis-dimethylamino titanium (TDMAT) and H{sub 2}O as precursors. Experiments with varying pulse times show that saturated growth can be obtained with TDMAT pulse times larger than 600 s. Analysis of the powder with High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy confirmed that after 50 cycles, all SiO{sub 2} particles were coated with a 1.6 nm homogenous shell of TiO{sub 2}.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Band alignment of HfO{sub 2}/Al{sub 0.25}Ga{sub 0.75}N determined by x-ray photoelectron spectroscopy: Effect of SiH{sub 4} surface treatment

The band-alignment of atomic layer deposited (ALD)-HfO{sub 2}/Al{sub 0.25}Ga{sub 0.75}N was studied by high resolution x-ray photoelectron spectroscopy measurements for both the non-passivated and SiH{sub 4} passivated AlGaN surfaces. The valence band offset and the conduction band offset for the ALD-HfO{sub 2}/Al{sub 0.25}Ga{sub 0.75}N interface were found to be 0.43 eV and 1.47 eV, respectively, for the non-passivated sample, and 0.59 eV and 1.31 eV, respectively, for the SiH{sub 4}-passivated sample. The difference in the band alignment is dominated by the band bending or band shift in the AlGaN substrate as a result of the different interlayers formed by the two surface preparations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Growth of crystalline Al{sub 2}O{sub 3} via thermal atomic layer deposition: Nanomaterial phase stabilization

We report the growth of crystalline Al{sub 2}O{sub 3} thin films deposited by thermal Atomic Layer Deposition (ALD) at 200 °C, which up to now has always resulted in the amorphous phase. The 5 nm thick films were deposited on Ga{sub 2}O{sub 3}, ZnO, and Si nanowire substrates 100 nm or less in diameter. The crystalline nature of the Al{sub 2}O{sub 3} thin film coating was confirmed using Transmission Electron Microscopy (TEM), including high-resolution TEM lattice imaging, selected area diffraction, and energy filtered TEM. Al{sub 2}O{sub 3} coatings on nanowires with diameters of 10 nm or less formed a fully crystalline phase, while those with diameters in the 20–25 nm range resulted in a partially crystalline coating, and those with diameters in excess of 50 nm were fully amorphous. We suggest that the amorphous Al{sub 2}O{sub 3} phase becomes metastable with respect to a crystalline alumina polymorph, due to the nanometer size scale of the film/substrate combination. Since ALD Al{sub 2}O{sub 3} films are widely used as protective barriers, dielectric layers, as well as potential coatings in energy materials, these findings may have important implications.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Electrical response in atomic layer deposited Al:ZnO with varying stack thickness

We report on the effects of stacking of the macrocycles in atomic layer deposited (ALD) Al:ZnO thin films on the structural and electrical properties. There is a large change in the resistivity ranging from as high as 1.19 × 10{sup −3} Ω cm for 760 growth cycles film down to as low as 7.9 × 10{sup −4} Ω cm for the 4000 cycles. The electrical transport demonstrates a transition from a semiconductor behavior at 760 cycles to a metallic behavior in the 4000 cycle, due to an increase in electron scattering as well as increase in the carrier concentration. However, interestingly the carrier concentration sharply increases with increasing macrocycles containing Al and Zn, exhibiting a nearly metal-like behavior in thicker films. We anticipate that the change in Zn-vacancy, V{sub zn}, formation energy is related to the increase in surface area of the ALD precursor deposition plane. The increase in V{sub zn} density allows for more adsorption of Al-precursor into the doped monolayer, showing interesting electrical properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

C-H surface diamond field effect transistors for high temperature (400 °C) and high voltage (500 V) operation

By forming a highly stable Al{sub 2}O{sub 3} gate oxide on a C-H bonded channel of diamond, high-temperature, and high-voltage metal-oxide-semiconductor field-effect transistor (MOSFET) has been realized. From room temperature to 400 °C (673 K), the variation of maximum drain-current is within 30% at a given gate bias. The maximum breakdown voltage (V{sub B}) of the MOSFET without a field plate is 600 V at a gate-drain distance (L{sub GD}) of 7 μm. We fabricated some MOSFETs for which V{sub B}/L{sub GD} > 100 V/μm. These values are comparable to those of lateral SiC or GaN FETs. The Al{sub 2}O{sub 3} was deposited on the C-H surface by atomic layer deposition (ALD) at 450 °C using H{sub 2}O as an oxidant. The ALD at relatively high temperature results in stable p-type conduction and FET operation at 400 °C in vacuum. The drain current density and transconductance normalized by the gate width are almost constant from room temperature to 400 °C in vacuum and are about 10 times higher than those of boron-doped diamond FETs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Excellent c-Si surface passivation by low-temperature atomic layer deposited titanium oxide

In this work, we demonstrate that thermal atomic layer deposited (ALD) titanium oxide (TiO{sub x}) films are able to provide a—up to now unprecedented—level of surface passivation on undiffused low-resistivity crystalline silicon (c-Si). The surface passivation provided by the ALD TiO{sub x} films is activated by a post-deposition anneal and subsequent light soaking treatment. Ultralow effective surface recombination velocities down to 2.8 cm/s and 8.3 cm/s, respectively, are achieved on n-type and p-type float-zone c-Si wafers. Detailed analysis confirms that the TiO{sub x} films are nearly stoichiometric, have no significant level of contaminants, and are of amorphous nature. The passivation is found to be stable after storage in the dark for eight months. These results demonstrate that TiO{sub x} films are also capable of providing excellent passivation of undiffused c-Si surfaces on a comparable level to thermal silicon oxide, silicon nitride, and aluminum oxide. In addition, it is well known that TiO{sub x} has an optimal refractive index of 2.4 in the visible range for glass encapsulated solar cells, as well as a low extinction coefficient. Thus, the results presented in this work could facilitate the re-emergence of TiO{sub x} in the field of high-efficiency silicon wafer solar cells.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Investigation of the impact of insulator material on the performance of dissimilar electrode metal-insulator-metal diodes

The performance of thin film metal-insulator-metal (MIM) diodes is investigated for a variety of large and small electron affinity insulators using ultrasmooth amorphous metal as the bottom electrode. Nb{sub 2}O{sub 5}, Ta{sub 2}O{sub 5}, ZrO{sub 2}, HfO{sub 2}, Al{sub 2}O{sub 3}, and SiO{sub 2} amorphous insulators are deposited via atomic layer deposition (ALD). Reflection electron energy loss spectroscopy (REELS) is utilized to measure the band-gap energy (E{sub G}) and energy position of intrinsic sub-gap defect states for each insulator. E{sub G} of as-deposited ALD insulators are found to be Nb{sub 2}O{sub 5} = 3.8 eV, Ta{sub 2}O{sub 5} = 4.4 eV, ZrO{sub 2} = 5.4 eV, HfO{sub 2} = 5.6 eV, Al{sub 2}O{sub 3} = 6.4 eV, and SiO{sub 2} = 8.8 eV with uncertainty of ±0.2 eV. Current vs. voltage asymmetry, non-linearity, turn-on voltage, and dominant conduction mechanisms are compared. Al{sub 2}O{sub 3} and SiO{sub 2} are found to operate based on Fowler-Nordheim tunneling. Al{sub 2}O{sub 3} shows the highest asymmetry. ZrO{sub 2}, Nb{sub 2}O{sub 5}, and Ta{sub 2}O{sub 5} based diodes are found to be dominated by Frenkel-Poole emission at large biases and exhibit lower asymmetry. The electrically estimated trap energy levels for defects that dominate Frenkel-Poole conduction are found to be consistent with the energy levels of surface oxygen vacancy defects observed in REELS measurements. For HfO{sub 2}, conduction is found to be a mix of trap assisted tunneling and Frenkel-Poole emission. Insulator selection criteria in regards to MIM diodes applications are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Band alignment of HfO{sub 2}/In{sub 0.18}Al{sub 0.82}N determined by angle-resolved x-ray photoelectron spectroscopy

The band-alignment of atomic layer deposited (ALD)-HfO{sub 2}/In{sub 0.18}Al{sub 0.82}N was studied by high resolution angle-resolved X-ray photoelectron spectroscopy measurements. The band bending near the HfO{sub 2}/In{sub 0.18}Al{sub 0.82}N interface was investigated, and the potential variation across the interface was taken into account in the band alignment calculation. It is observed that the binding energies for N 1s and Al 2p in In{sub 0.18}Al{sub 0.82}N decreases and the corresponding extracted valence band offsets increases with increasing θ (i.e., closer to the HfO{sub 2}/In{sub 0.18}Al{sub 0.82}N interface), as a result of an upward energy band bending towards the HfO{sub 2}/In{sub 0.18}Al{sub 0.82}N interface. The resultant valence band offset and the conduction band offset for the ALD-HfO{sub 2}/In{sub 0.18}Al{sub 0.82}N interface calculated was found to be 0.69 eV and 1.01 eV, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗