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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

High performance top-gated multilayer WSe2 field effect transistors

In this paper, high performance top-gated WSe 2 field effect transistor (FET) devices are demonstrated via a two-step remote plasma assisted ALD process. High-quality, low-leakage aluminum oxide (Al 2 O 3 ) gate dielectric layers are deposited onto the WSe 2 channel using a remote plasma assisted ALD process with an ultrathin (~1 nm) titanium buffer layer. The first few nanometers (~2 nm) of the Al 2 O 3 dielectric film is deposited at relatively low temperature (i.e. 50 °C) and remainder of the film is deposited at 150 °C to ensure the conformal coating of Al 2 O 3 on the WSe 2 surface. Additionally, an ultra-thin titanium buffer layer is introduced at the WSe2 channel surface prior to ALD process to mitigate oxygen plasma induced doping effects. Excellent device characteristics with current on–off ratio in excess of 106 and a field effect mobility as high as 70.1 cm 2 V –1 s –1 are achieved in a few-layer WSe 2 FET device with a 30 nm Al 2 O 3 top-gate dielectric. With further investigation and careful optimization, this method can play an important role for the realization of high performance top gated FETs for future optoelectronic device applications.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

The surface chemistry of the atomic layer deposition of metal thin films

Abstract In this perspective we discuss the progress made in the mechanistic studies of the surface chemistry associated with the atomic layer deposition (ALD) of metal films and the usefulness of that knowledge for the optimization of existing film growth processes and for the design of new ones. Our focus is on the deposition of late transition metals. We start by introducing some of the main surface-sensitive techniques and approaches used in this research. We comment on the general nature of the metallorganic complexes used as precursors for these depositions, and the uniqueness that solid surfaces and the absence of liquid solvents bring to the ALD chemistry and differentiate it from what is known from metalorganic chemistry in solution. We then delve into the adsorption and thermal chemistry of those precursors, highlighting the complex and stepwise nature of the decomposition of the organic ligands that usually ensued upon their thermal activation. We discuss the criteria relevant for the selection of co-reactants to be used on the second half of the ALD cycle, with emphasis on the redox chemistry often associated with the growth of metallic films starting from complexes with metal cations. Additional considerations include the nature of the substrate and the final structural and chemical properties of the growing films, which we indicate rarely retain the homogeneous 2D structure often aimed for. We end with some general conclusions and personal thoughts about the future of this field.

Materials Science↗
DOE OSTI2021

Changes in precipitation and air temperature contribute comparably to permafrost degradation in a warmer climate

Abstract Surface energy budgets of high-latitude permafrost systems are poorly represented in Earth system models (ESMs), yet permafrost is rapidly degrading and these dynamics are critical to future carbon-climate feedback predictions. A potentially important factor in permafrost degradation neglected so far by ESMs is heat transfer from precipitation, although increases in soil temperature and thaw depth have been observed following increases in precipitation. Using observations and a mechanistic ecosystem model, we show here that increases in precipitation hasten active layer development beyond that caused by surface air warming across the North Slope of Alaska (NSA) under recent and 21st century climate (RCP8.5). Modeled active layer depth (ALD) in simulations that allow precipitation heat transfer agreed very well with observations from 28 Circumpolar Active Layer Monitoring sites (R 2 = 0.63; RMSE = 10 cm). Simulations that ignored precipitation heat transfer resulted in lower spatially-averaged soil temperatures and a 39 cm shallower ALD by 2100 across the NSA. The results from our sensitivity analysis show that projected increases in 21st century precipitation deepen the active layer by enhancing precipitation heat transfer and ground thermal conductivity, suggesting that precipitation is as important an environmental control on permafrost degradation as surface air temperature. We conclude that ESMs that do not account for precipitation heat transfer likely underestimate ALD rates of change, and thus likely predict biased ecosystem responses.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

Stability of Pt/Al 2 O 3 -Based Electrode Langasite SAW Sensors with Al 2 O 3 Capping Layer and Yttria-Stabilized Zirconia Sensing Layer

Surface acoustic wave (SAW) devices have been shown recently to be an appropriate technology for harsh environment sensor applications, such as those in the automotive, aerospace, power plant, and metallurgic industries. Usages include temperature, pressure, torque, strain, gas, liquid, biochemical, and mass sensors in structural health monitoring, industrial process monitoring, environment checking and control, and condition based maintenance. For harsh environment high-temperature applications, battery-free wireless operation associated with sensor robustness, small profile, and mass fabrication capability make SAW sensors a very attractive solution. For high-temperature (< 800°C) gas sensor applications, the stability of SAW sensor platform including protective and sensing film layers must be guaranteed over time and under temperature cycling conditions. This paper reports on the performance and stability of langasite SAW resonator (SAWR) sensor platform using PtAl2O3-based electrode, with atomic layer deposited (ALD) Al2O3 protective layer and yttria-stabilized zirconia (YSZ) gas sensing layer. The effect of 50nm thick Al2O3 or YSZ layers on SAWR sensor resonant frequency and temperature sensitivity responses were investigated. Temperature sensitivities of SAWR sensors with and without ALD Al2O3 protective layer have been compared with numerical predictions for a bare substrate. Measured temperature sensitivities of SAWR sensors with and without the YSZ sensing layers have been compared against each other. In addition, the stability of the SAWR sensor frequency responses have been monitored during multiple 110-hour temperature cycling (over 620 hours in total) from 300°C to 750°C. The measured frequency deviation at 750ºC over six of these 110-hour cycling tests was 0.05%. The results obtained confirm that the presence of the ALD Al2O3 protective layer or the YSZ sensing layer do not compromise the stability of high-temperature harsh environment SAWR sensors, validating their use for the aforementioned applications.

42 ENGINEERING↗
DOE OSTI2018

Particle atomic layer deposition of alumina for sintering yttria-stabilized cubic zirconia

The addition of aluminum oxide (Al 2 O 3 ) as a sintering aid to yttria-stabilized zirconia (YSZ) reduces the required densification temperature. Sintering aids are incorporated using a number of processes which can lead to ambiguity when determining the effect of the sintering aid on the densification mechanism. In this study, a novel method for sintering aid addition, Particle Atomic Layer Deposition (ALD), was used to deposit an amorphous Al 2 O 3 thin film on YSZ particles. Transmission electron microscopy confirmed the deposition of conformal Al 2 O 3 thin films on the surface of the YSZ particles. The addition of Al 2 O 3 to YSZ reduced the temperature at which densification began by ~75 degrees C, and 2.2 wt% Al 2 O 3 addition resulted in a minimum activation energy for the intermediate stage of densification. This concentration is well in excess of the solubility limit of Al 2 O 3 in YSZ, showing that Al 2 O 3 does not enhance the densification of YSZ solely by dissolving into the YSZ lattice and activating volume diffusion. The addition of 0.7 wt% Al 2 O 3 with one Particle ALD cycle enhanced the ionic conductivity of YSZ by 23% after sintering at 1350 degrees C for 2 hours, demonstrating that dense parts with high oxygen ion conductivities can be produced after sintering at reduced temperatures. One Particle ALD cycle is a fast, easily scaled-up process that eliminates the use of solvents and has substantial cost/performance advantages over conventional processing.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Kinetic study on hot-wire-assisted atomic layer deposition of nickel thin films

High-purity Ni films were deposited using hot-wire-assisted atomic layer deposition (HW-ALD) at deposition temperatures of 175, 250, and 350 °C. Negligible amount of nitrogen or carbon contamination was detected, even though the authors used NH{sub 2} radical as the reducing agent and nickelocene as the precursor. NH{sub 2} radicals were generated by the thermal decomposition of NH{sub 3} with the assist of HW and used to reduce the adsorbed metal growth precursors. To understand and improve the deposition process, the kinetics of HW-ALD were analyzed using a Langmuir-type model. Unlike remote-plasma-enhanced atomic layer deposition, HW-ALD does not lead to plasma-induced damage. This is a significant advantage, because the authors can supply sufficient NH{sub 2} radicals to deposit high-purity metallic films by adjusting the distance between the hot wire and the substrate. NH{sub 2} radicals have a short lifetime, and it was important to use a short distance between the radical generation site and substrate. Furthermore, the impurity content of the nickel films was independent of the deposition temperature, which is evidence of the temperature-independent nature of the NH{sub 2} radical flux and the reactivity of the NH{sub 2} radicals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Growth mode evolution of hafnium oxide by atomic layer deposition

HfO{sub 2} thin films were deposited using tetrakis-ethylmethylamido hafnium and H{sub 2}O as precursors on silicon by atomic layer deposition (ALD). The morphology and microstructures at different ALD cycles were characterized by atomic force microscopy and high-resolution transmission electron microscopy. Based on the height–height correlation function and power spectral density function, quantitative analysis of surface morphologies was performed. Three characteristic dimensions (ξ{sub 1}, ξ{sub 2}, and ξ{sub 3}) corresponding to three surface structures, islands, local and global fluctuations, were identified. The evolution of ALD growth mode at range of the three critical scales was investigated, respectively. It suggests the transformation of growth mode from quasi two-dimensional layer-by-layer to three-dimensional island for global fluctuations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Atomic layer deposition of Hf{sub x}Al{sub y}C{sub z} as a work function material in metal gate MOS devices

As advanced silicon semiconductor devices are transitioning from planar to 3D structures, new materials and processes are needed to control the device characteristics. Atomic layer deposition (ALD) of Hf{sub x}Al{sub y}C{sub z} films using hafnium chloride and trimethylaluminum precursors was combined with postdeposition anneals and ALD liners to control the device characteristics in high-k metal-gate devices. Combinatorial process methods and technologies were employed for rapid electrical and materials characterization of various materials stacks. The effective work function in metal–oxide–semiconductor capacitor devices with the Hf{sub x}Al{sub y}C{sub z} layer coupled with an ALD HfO{sub 2} dielectric was quantified to be mid-gap at ∼4.6 eV. Thus, Hf{sub x}Al{sub y}C{sub z} is a promising metal gate work function material that allows for the tuning of device threshold voltages (V{sub th}) for anticipated multi-V{sub th} integrated circuit devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Solution based prompt inorganic condensation and atomic layer deposition of Al{sub 2}O{sub 3} films: A side-by-side comparison

A comparison was made of Al{sub 2}O{sub 3} films deposited on Si via prompt inorganic condensation (PIC) and atomic layer deposition (ALD). Current–voltage measurements as a function of annealing temperature indicate that the solution-processed PIC films, annealed at 500 °C, exhibit lower leakage and roughly equivalent breakdown strength in comparison to ALD films. PIC films are less dense than as-deposited ALD films and capacitance–voltage measurements indicate a lower relative dielectric constant. On the basis of x-ray photoelectron spectroscopy, transmission electron microscopy, and energy dispersive x-ray spectroscopy, it is found that the 500 °C anneal results in the formation of a ∼6 nm thick interfacial SiO{sub 2} layer at the Si interface. This SiO{sub 2} interfacial layer significantly affects the electrical performance of PIC Al{sub 2}O{sub 3} films deposited on Si.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Roll-to-roll atomic layer deposition process for flexible electronics encapsulation applications

At present flexible electronic devices are under extensive development and, among them, flexible organic light-emitting diode displays are the closest to a large market deployment. One of the remaining unsolved challenges is high throughput production of impermeable flexible transparent barrier layers that protect sensitive light-emitting materials against ambient moisture. The present studies deal with the adaptation of the atomic layer deposition (ALD) process to high-throughput roll-to-roll production using the spatial ALD concept. We report the development of such a process for the deposition of 20 nm thickness Al{sub 2}O{sub 3} diffusion barrier layers on 500 mm wide polymer webs. The process uses trimethylaluminum and water as precursors at a substrate temperature of 105 °C. The observation of self-limiting film growth behavior and uniformity of thickness confirms the ALD growth mechanism. Water vapor transmission rates for 20 nm Al{sub 2}O{sub 3} films deposited on polyethylene naphthalate (PEN) substrates were measured as a function of substrate residence time, that is, time of exposure of the substrate to one precursor zone. Moisture permeation levels measured at 38 °C/90% relative humidity by coulometric isostatic–isobaric method were below the detection limit of the instrument (<5 × 10{sup −4} g/m{sup 2} day) for films coated at web moving speed of 0.25 m/min. Measurements using the Ca test indicated water vapor transmission rates ∼5 × 10{sup −6} g/m{sup 2} day. Optical measurements on the coated web showed minimum transmission of 80% in the visible range that is the same as the original PEN substrate.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Atomic layer deposition TiO 2 -Al 2 O 3 stack: An improved gate dielectric on Ga-polar GaN metal oxide semiconductor capacitors

This research focuses on the benefits and properties of TiO 2 -Al 2 O 3 nano-stack thin films deposited on Ga 2 O 3 /GaN by plasma-assisted atomic layer deposition (PA-ALD) for gate dielectric development. This combination of materials achieved a high dielectric constant, a low leakage current, and a low interface trap density. Correlations were sought between the films’ structure, composition, and electrical properties. The gate dielectrics were approximately 15 nm thick and contained 5.1 nm TiO 2 , 7.1 nm Al 2 O 3 and 2 nm Ga 2 O 3 as determined by spectroscopic ellipsometry. The interface carbon concentration, as measured by x-ray photoelectron spectroscopy (XPS) depth profile, was negligible for GaN pretreated by thermal oxidation in O 2 for 30 minutes at 850°C. The RMS roughness slightly increased after thermal oxidation and remained the same after ALD of the nano-stack, as determined by atomic force microscopy. The dielectric constant of TiO 2 -Al 2 O 3 on Ga2O3/GaN was increased to 12.5 compared to that of pure Al 2 O 3 (8~9) on GaN. In addition, the nano-stack's capacitance-voltage (C-V) hysteresis was small, with a total trap density of 8.74 × 10 11 cm -2 . The gate leakage current density (J=2.81× 10 -8 A/cm 2 ) was low at +1 V gate bias. These results demonstrate the promising potential of plasma ALD deposited TiO 2 /Al 2 O 3 for serving as the gate oxide on Ga 2 O 3 /GaN based MOS devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Enhanced photoresponse of conformal TiO{sub 2}/Ag nanorod array-based Schottky photodiodes fabricated via successive glancing angle and atomic layer deposition

In this study, the authors demonstrate a proof of concept nanostructured photodiode fabrication method via successive glancing angle deposition (GLAD) and atomic layer deposition (ALD). The fabricated metal-semiconductor nanorod (NR) arrays offer enhanced photoresponse compared to conventional planar thin-film counterparts. Silver (Ag) metallic NR arrays were deposited on Ag-film/Si templates by utilizing GLAD. Subsequently, titanium dioxide (TiO{sub 2}) was deposited conformally on Ag NRs via ALD. Scanning electron microscopy studies confirmed the successful formation of vertically aligned Ag NRs deposited via GLAD and conformal deposition of TiO{sub 2} on Ag NRs via ALD. Following the growth of TiO{sub 2} on Ag NRs, aluminum metallic top contacts were formed to complete the fabrication of NR-based Schottky photodiodes. Nanostructured devices exhibited a photo response enhancement factor of 1.49 × 10{sup 2} under a reverse bias of 3 V.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Atomic layer deposition of magnesium fluoride via bis(ethylcyclopentadienyl)magnesium and anhydrous hydrogen fluoride

A new process has been developed to deposit magnesium fluoride (MgF{sub 2}) thin films via atomic layer deposition (ALD) for use as optical coatings in the ultraviolet. MgF{sub 2} was deposited in a showerhead style ALD reactor using bis(ethylcyclopentadienyl)magnesium and anhydrous hydrogen fluoride (HF) as precursors at substrate temperatures from 100 to 250 °C. The use of HF was observed to result in improved morphology and reduced impurity content compared to other reported MgF{sub 2} ALD approaches that use metal fluoride precursors as the fluorine-containing chemistry. Characterization of these films has been performed using spectroscopic ellipsometry, atomic force microscopy, and x-ray photoelectron spectroscopy for material deposited on silicon substrates. Films at all substrate temperatures were transparent at wavelengths down to 190 nm and the low deposition temperature combined with low surface roughness makes these coatings good candidates for a variety of optical applications in the far ultraviolet.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Designing high performance precursors for atomic layer deposition of silicon oxide

Conformal and continuous silicon oxide films produced by atomic layer deposition (ALD) are enabling novel processing schemes and integrated device structures. The increasing drive toward lower temperature processing requires new precursors with even higher reactivity. The aminosilane family of precursors has advantages due to their reactive nature and relative ease of use. In this paper, the authors present the experimental results that reveal the uniqueness of the monoaminosilane structure [(R{sub 2}N)SiH{sub 3}] in providing ultralow temperature silicon oxide depositions. Disubstituted aminosilanes with primary amines such as in bis(t-butylamino)silane and with secondary amines such as in bis(diethylamino)silane were compared with a representative monoaminosilane: di-sec-butylaminosilane (DSBAS). DSBAS showed the highest growth per cycle in both thermal and plasma enhanced ALD. These findings show the importance of the arrangement of the precursor's organic groups in an ALD silicon oxide process.

42 ENGINEERING↗
DOE OSTI2015

Electronic and optical device applications of hollow cathode plasma assisted atomic layer deposition based GaN thin films

Electronic and optoelectronic devices, namely, thin film transistors (TFTs) and metal–semiconductor–metal (MSM) photodetectors, based on GaN films grown by hollow cathode plasma-assisted atomic layer deposition (PA-ALD) are demonstrated. Resistivity of GaN thin films and metal-GaN contact resistance are investigated as a function of annealing temperature. Effect of the plasma gas and postmetallization annealing on the performances of the TFTs as well as the effect of the annealing on the performance of MSM photodetectors are studied. Dark current to voltage and responsivity behavior of MSM devices are investigated as well. TFTs with the N{sub 2}/H{sub 2} PA-ALD based GaN channels are observed to have improved stability and transfer characteristics with respect to NH{sub 3} PA-ALD based transistors. Dark current of the MSM photodetectors is suppressed strongly after high-temperature annealing in N{sub 2}:H{sub 2} ambient.

42 ENGINEERING↗
DOE OSTI2015

Enhancing the platinum atomic layer deposition infiltration depth inside anodic alumina nanoporous membrane

Nanoporous platinum membranes can be straightforwardly fabricated by forming a Pt coating inside the nanopores of anodic alumina membranes (AAO) using atomic layer deposition (ALD). However, the high-aspect-ratio of AAO makes Pt ALD very challenging. By tuning the process deposition temperature and precursor exposure time, enhanced infiltration depth along with conformal coating was achieved for Pt ALD inside the AAO templates. Cross-sectional scanning electron microscopy/energy dispersive x-ray spectroscopy and small angle neutron scattering were employed to analyze the Pt coverage and thickness inside the AAO nanopores. Additionally, one application of platinum-coated membrane was demonstrated by creating a high-density protein-functionalized interface.

42 ENGINEERING↗
DOE OSTI2015

Atomic layer deposition of titanium sulfide and its application in extremely thin absorber solar cells

Atomic layer deposition (ALD) of TiS{sub 2} is investigated with titanium tetrachloride and hydrogen sulfide precursors. In-situ quartz crystal microbalance and ex-situ x-ray reflectivity measurements are carried out to study self-limiting deposition chemistry and material growth characteristics. The saturated growth rate is found to be ca. 0.5 Å/cycle within the ALD temperature window of 125–200 °C. As grown material is found poorly crystalline. ALD grown TiS{sub 2} is applied as a photon harvesting material for solid state sensitized solar cells with TiO{sub 2} as electron transport medium. Initial results with Spiro-OMeTAD as hole conducting layer show ca. 0.6% energy conversion efficiency under 1 sun illumination.

36 MATERIALS SCIENCE↗