To Trimethylaluminum Research Articles

Few-cycle atomic layer deposition to nanoengineer polybenzimidazole for H2/CO2 separation

Atomic layer deposition (ALD) creates uniform sub-nanometer films on a variety of surfaces and nanopore walls and has been used to modify polymers to improve surface affinity towards specific molecules, solvent resistance, and barrier properties to gases and vapors. Here, for the first time, we demonstrate that few-cycle ALD can be used to engineer functional polymers at a sub-nanometer scale to improve both molecular size-sieving ability and counterintuitively, gas permeability. Particularly, 1-cycle ALD treatment of polybenzimidazole (PBI) by sequential exposure to trimethylaluminum (TMA) and water vapor remarkably increases H2 permeability by 120% − 270% and H2/CO2 selectivity by 30% at 35–200 °C. The ALD not only deposits an AlOx layer on the surface but also enables the TMA to infiltrate and react with the bulk PBI to form an AlOx network, disrupting polymer chain packing and increasing chain rigidity. The membrane exhibits excellent stability when challenged with simulated syngas, overcoming the permeability/selectivity tradeoff for H2/CO2 separation. This study showcases a facile and scalable way of engineering polymeric membranes at a sub-nanometer level to improve molecular separation performance.

Aluminum Oxide at the Monolayer Limit via Oxidant‐Free Plasma‐Assisted Atomic Layer Deposition on GaN

AbstractAtomic layer deposition (ALD) is an essential tool in semiconductor device fabrication that allows the growth of ultrathin and conformal films to precisely form heterostructures and tune interface properties. The self‐limiting nature of the chemical reactions during ALD provides excellent control over the layer thickness. However, in contrast to idealized growth models, it is challenging to create continuous monolayers by ALD because surface inhomogeneities and precursor steric interactions result in island growth. Thus, the ability to create closed monolayers by ALD would offer new opportunities for controlling interfacial charge and mass transport in semiconductor devices, as well as for tailoring surface chemistry. Here, encapsulation of c‐plane gallium nitride (GaN) with ultimately thin (≈3 Å) aluminum oxide (AlOx) is reported, which is enabled by the partial conversion of the GaN surface oxide into AlOx using sequential exposure to trimethylaluminum (TMA) and hydrogen plasma. Introduction of monolayer AlOx decreases the work function and enhances reactivity with phosphonic acids under standard conditions, which results in self‐assembled monolayers with densities approaching the theoretical limit. Given the high reactivity of TMA with surface oxides, the presented approach likely can be extended to other dielectrics and III–V‐based semiconductors, with relevance for applications in optoelectronics, chemical sensing, and (photo)electrocatalysis.

Sequential Infiltration Synthesis of Electronic Materials: Group 13 Oxides via Metal Alkyl Precursors

The sequential infiltration synthesis (SIS) of group 13 indium and gallium oxides (In2O3 and Ga2O3) into poly(methyl methacrylate) (PMMA) thin films is demonstrated using trimethylindium (TMIn) and trimethylgallium (TMGa), respectively, with water. In situ Fourier transform infrared (FTIR) spectroscopy reveals that these metal alkyl precursors reversibly associate with the carbonyl groups of PMMA in analogy to trimethylaluminum (TMAl), however, with significantly lower affinity. This is demonstrated to have important kinetic consequences that dramatically alter the synthetic parameters required to achieve material growth. Ab initio density functional theory simulations of the methyl methacrylate monomer with group 13 metal alkyls corroborate association energy that is 3× greater for TMAl than for either TMIn or TMGa. As a consequence, the kinetics of activated diffusion within the film is observed to be far more rapid for TMIn and TMGa than for TMAl. Spectroscopic ellipsometry and scanning electron micros...

Investigation of industrial PECVD AlOx films with very low surface recombination

Plasma-enhanced chemical-vapor deposition systems are commonly used to deposit aluminum oxide (AlOx) on silicon wafers in the production of silicon solar cells. This study investigates the impact of the deposition conditions on the obtained surface passivation quality of p-type wafers. It is the first to investigate the impact of all the five main process parameters. The ratio between the microwave power and the total gas flow rate is identified as a critical condition to form thermally stable layers. We find that the most significant parameter for high quality surface passivation is the flow rate ratio of nitrous oxide (N2O) to tri-methyl-aluminum (TMA). Higher flow rate ratio ([N2O]/[TMA]) is required to achieve better passivation for fired wafers, whereas lower ratio is preferred when the firing process is not permissible. Elastic-recoil detection analysis reveals that the gas flow rate ratio has a significant impact on the likely direction of hydrogen released from the layer during firing (either to the interface with the silicon wafer or to the environment). Surprisingly, the atomic concentration of aluminum and oxygen is found to be almost stoichiometric regardless of the wide range of the gas flow rate ratios studied in this paper.

Inherent Area-Selective Growth and Nucleation Enhancement during Ru ALD Using the RuO4-Precursor and H2-Gas

Ruthenium is a candidate to replace copper in future sub-10 nm interconnects. At these dimensions the resistivity of Ru lines is expected to be lower compared to Cu due to the lower sensitivity to size effects.1 In addition, it is likely that Ru interconnects won’t require a diffusion barrier, and will show a better electromigration performance.2 At feature sizes below 10 nm it will be difficult to align subsequent lithography steps, and the conformality of the deposition method is increasingly important, such that area selective atomic layer deposition (ALD) of ruthenium is of high interest.3 We first report inherent area selective ALD of Ru on H-terminated Si (Si-H) versus SiO2, using the thermal RuO4 (ToRuSTM)/ H2-gas ALD process.4 In situ spectroscopic ellipsometry (SE) on blanket substrates shows that Ru growth initiation occurs from the first cycle on Si-H, while on SiO2 the growth is delayed, resulting in a substrate selectivity window of ~ 70 cycles. Area selective Ru ALD was evaluated using a patterned substrate of 1-10 µm wide Si-H lines separated by 10 µm wide SiO2 regions, and exposing it to 20 cycles of the RuO4 / H2-gas ALD process. Ex situ scanning electron microscopy (SEM) and cross section high resolution transmission electron microscopy (HRTEM) measurements show that a 4.5 nm Ru film could be deposited on the Si-H, with no Ru detected on the SiO2. In vacuo X-ray photoelectron spectroscopy (XPS) experiments showed that exposure of Si-H to a single RuO4 pulse leads to the oxidation of the Si surface, together with the deposition of RuO2. On SiO2 however, the surface is already oxidized, and in vacuo XPS shows that for the same exposure to RuO4 no Ru is deposited on the surface. Therefore, we propose that the mechanism behind the inherent substrate selectivity is the oxidation of the Si-H surface by RuO4. Secondly, we report for a methodology to enhance the nucleation of the RuO4 / H2-gas process on oxide substrates. In vacuo XPS and in situ SE experiments show that a single exposure of SiO2 to trimethylaluminum (TMA) makes the surface reactive towards RuO4, which allows for Ru growth initiation from the first cycle. We propose that this is due to the combustion of surface CH3-groups by RuO4. As TMA is known to be reactive towards many oxide substrates, this methodology presents a way to achieve Ru metallization of virtually any surface. [1] S. Dutta et al. IEEE Elec. Dev. Lett. 2017, 38, 949. [2] O. V. Pedreira et al . 2017 IEEE IRPS , Monterey, CA, 6B-2.1. [3] P. C. Lemaire et al. J. Chem. Phys. 2017, 146, 052811. [4] M. M. Minjauw et al. J. Mater. Chem. C. 2015, 3, 132.

Reconstruction at the interface of one cycle of trimethylaluminum and water on GaAs(111)A-2×2 from atomic layer deposition

During the initial stage of atomic layer deposition, the exposure of a GaAs(111)A-2×2 surface to trimethylaluminum (TMA) leads to occupying a Ga-vacancy site on the surface by a chemisorbed As-bonded aluminum with the loss of all methyl ligands. The water purge disrupts physisorbed TMA and initiates the growth of Al2O3. The subsequent growth of Al2O3 does not result in the oxidation of As, Ga, or Al in the Ga vacancy. The reconstructed Ga atoms retain the bulk-terminated positions resulting in a dangling bond, and all As atoms become four-fold coordinated. The correlation of an interfacial electronic structure with an electric performance is discussed for Al2O3 on GaAs(001)-2×4 and GaAs(001)-4×6.

Modification of Hematite Electronic Properties with Trimethyl Aluminum to Enhance the Efficiency of Photoelectrodes.

The electronic properties of hematite were investigated by means of synchrotron radiation photoemission (SR-PES) and X-ray absorption spectroscopy (XAS). Hematite samples were exposed to trimethyl aluminum (TMA) pulses, a widely used Al-precursor for the atomic layer deposition (ALD) of Al2O3. SR-PES and XAS showed that the electronic properties of hematite were modified by the interaction with TMA. In particular, the hybridization of O 2p states with Fe 3d and Fe 4s4p changed upon TMA pulses due to electron inclusion as polarons. The change of hybridization correlates with an enhancement of the photocurrent density due to water oxidation for the hematite electrodes. Such an enhancement has been associated with an improvement in charge carrier transport. Our findings open new perspectives for the understanding and utilization of electrode modifications by very thin ALD films and show that the interactions between metal precursors and substrates seem to be important factors in defining their electronic and photoelectrocatalytic properties.

Atomic Layer Deposition of Nanostructured Tunable Resistance Coatings: Growth, Characterization, and Electrical Properties

We have developed atomic layer deposition (ALD) methods to synthesize robust nanostructured composite coatings with tunable resistance comprised of conducting, metallic nanoparticles embedded in an amorphous dielectric matrix. These films are nominally composed of M:Al2O3 where M= W or Mo, and are prepared using alternating exposures to trimethyl aluminum (TMA) and H2O for Al2O3 ALD and alternating MF6/Si2H6 exposures for the metal ALD. By varying the ratio of ALD cycles for the metal and Al2O3 components in the film, we can precisely adjust the resistivity of these composite coatings over a very broad range (e.g. 1012-103 Ohm-cm). Furthermore, the self-limiting nature of ALD allows us to grow these nanocomposite films on a variety of complex substrates such as high aspect ratio porous 3D surfaces, MEMS devices, ceramics, grid structures and microchannel plates. Here we present a detailed ALD study of the nanostructured composite resistance coatings including in-situ material growth, microstructural characterization, and electrical measurements.

Atomic Layer Deposition of Nanostructured Tunable Resistance Coatings: Growth, Characterization, and Electrical Properties

We have developed ALD method to synthesize nanocomposite coatings comprised of conducting, metallic nanoparticles embedded in an amorphous dielectric matrix. These films are nominally composed of M:Al2O3 where M= W or Mo, and are prepared using alternating exposures to trimethyl aluminum (TMA) and H2O for the Al2O3 ALD and alternating MF6/Si2H6 exposures for the metal ALD. By varying the ratio of ALD cycles for the metal and the Al2O3 components in the film, we can tune precisely the resistance of these coatings over a very broad range (e.g. 1011-104 Ohm-cm). These films exhibit Ohmic behavior and resist breakdown even at high electric fields of 107V/m. Moreover, the self-limiting nature of ALD allows us to grow these films inside of high aspect ratio substrates and on complex, 3D surfaces. To investigate the ALD growth mechanism for the nanostructure composite films we employed in-situ quartz crystal microbalance (QCM), quadrupole mass spectrometry, and Fourier transform infrared (FTIR) absorption spectroscopy studies. For M:Al2O3 films, QCM showed that the metal ALD inhibits the Al2O3 ALD and vice versa. Despite this inhibition, the relationship between metal content and metal ALD cycle percentage was close to expectations. Depth profiling X-ray photoelectron spectroscopy showed that the M:Al2O3 films are uniform in composition and contained Al, O, and metallic Mo or W as expected, but also presence of moderate F and C. Attempts were also made to investigate the TMA as reducing precursor for MF6 and resulted in similar kind of resistive coatings but very significant F and C which shows influence on microstructure of the layers. Cross-sectional transmission electron microscopy (XTEM) revealed the film microstructure to be metallic nanoparticles (~1-2 nm) embedded in an amorphous matrix. The transport properties of these M:Al2O3 were studied as function of ratio of metal to Al2O3ALD cycles.We have utilized these nanocomposite coatings to functionalize capillary glass array plates to fabricate large-area MCPs suitable for application in large-area photodetectors. In addition, we have applied these films to serve as charge drain coatings in MEMS devices for a prototype maskless electron beam lithography tool, permitting high resolution electron beam patterns without charging artifacts. Here we will discuss the ALD growth, characterizations, and applications of ALD tunable resistive coatings.

Surface chemistry and Fermi level movement during the self-cleaning of GaAs by trimethyl-aluminum

The removal of the native oxides from NH4OH-cleaned p-GaAs (100) by exposure to trimethyl-aluminum (TMA) was studied by in situ photoelectron spectroscopy using synchrotron radiation. The reduction of high-valence As- and Ga-oxides occurred through different routes: while As3+ was reduced to As(1±Δ)+ suboxides (with 0 ≤ Δ ≤ 1), Ga3+ was directly removed. The surface Fermi level was shifted by about 100 meV towards the valence band edge upon TMA exposure. This indicates that removing the native oxide of GaAs by TMA is insufficient to create interfaces between GaAs and Al2O3 with defects densities below the 1012 cm−2 range.

Encapsulation and Chemical Resistance of Electrospun Nylon Nanofibers Coated Using Integrated Atomic and Molecular Layer Deposition

Nanofibers formed by electrospinning provide very large surface areas which can enhance material performance in filtration and product separation. In this work, we explore atomic layer deposition (ALD) as a means to coat and protect electrospun nylon-6 nanofibers. Exposing nylon to trimethyl aluminum (TMA) during ALD of aluminum oxide results in significant fiber degradation. Protecting fibers with a bilayer of ALD ZnO and an organic-inorganic hybrid polymer by molecular layer deposition maintains the shape of the original nanofibers, but chemical modification is still detected. These coating processes may help enable nanofibers with stable physical properties under chemical exposure.

In situ X-ray photoelectron spectroscopy characterization of Al 2O 3/GaSb interface evolution

GaSb(0 0 1) was treated with (NH 4) 2S x and the evolution of the interfacial chemistry was investigated, in situ, with monochromatic X-ray photoelectron spectroscopy (XPS), following heat treatment and exposure to trimethylaluminum (TMA) and deionized water (DIW) in an atomic layer deposition reactor. Elemental Sb (Sb–Sb bonding) as well as Sb 3+ and Sb 5+ chemical states were initially observed at the native oxide/GaSb interface, yet these diminished below the XPS detection limit after heating to 300 °C. No evidence of Ga–Ga bonding was observed whereas the Ga 1+/Ga–S chemical state was robust and persisted after heat treatment and exposure to TMA/DIW at 300 °C.

Influence of trimethylaluminum on the growth and properties of HfO2/In0.53Ga0.47As interfaces

The growth and the electrical properties of HfO2/In0.53Ga0.47As interfaces are characterized as a function of exposure to trimethylaluminum (TMA) prior to chemical beam deposition of HfO2 from an alkoxide precursor. It is shown that TMA can act as a surfactant for HfO2 growth for (2×4) but not for the group-III-rich (4×2) reconstructed surfaces. The Fermi-level can be unpinned by postdeposition forming gas anneals only for interfaces that were exposed to low doses of TMA at low temperatures. The results are discussed in the context of the interaction between TMA and III-V surfaces.

(Invited) Atomic Layer Deposition on Polymers: Applications to Physical Encapsulation of Electrospun Nylon Nanofibers

Nanofibers formed by electrospinning provide large surface areas which enable enhanced material performance in filtration and product separation. In this work, we explore atomic layer deposition (ALD) as a means to encapsulate and protect electrospun nylon-6 nanofibers. Exposing nylon to trimethyl aluminum (TMA) during ALD of aluminum oxide results in significant fiber degradation. Protecting fibers with a bilayer of ALD ZnO and an organic-inorganic hybrid polymer by molecular layer deposition maintains the shape of the original nanofibers, but chemical modification is still detected. These coating processes may help enable nanofibers with stable physical properties under chemical exposure.

Role of Gas Doping Sequence in Surface Reactions and Dopant Incorporation during Atomic Layer Deposition of Al-Doped ZnO

Aluminum incorporation into ZnO films during atomic layer deposition (ALD) is investigated using in situ quartz crystal microbalance and electrical conductance analysis. Chemical interactions between Zn and Al species during ZnO:Al ALD depend on the order of metal precursor exposure. Exposing the growing ZnO surface to trimethyl aluminum (TMA) impedes the subsequent ∼4 monolayers of ZnO growth. However, the extent of interaction can be reduced by performing the TMA exposure immediately following a diethyl zinc step, without an intermediate water exposure step, consistent with increased surface mixing of Zn and Al species. Infrared spectroscopy analysis of heavily aluminum doped ZnO shows features consistent with the presence of amorphous ZnAl2O4 bonding units. For more lightly doped films, mass spectroscopic depth profiling confirms nonuniform aluminum distribution, even after annealing at 500 °C. Film conductance measured during growth shows complex trends that are highly repeatable over multiple doping ...

Kinetic Modeling of the Effect of MAO/Zr Ratio and Chain Transfer to Aluminum in Zirconocene Catalyzed Propylene Polymerization

A kinetic model based on the coordination−insertion mechanism was developed to characterize metallocene-catalyzed propylene polymerization using two different catalyst systems: rac-Et(Ind)2ZrCl2/MAO (I/MAO) and rac-Et(4,7-Me2-1-Ind)2ZrCl2/MAO (II/MAO). Slurry propylene polymerizations were performed in a semibatch reactor at 40 °C to investigate the effects of propylene partial pressure and MAO/Zr ratio. The kinetic model accounts for the formation of regioirregularities, the occurrence of chain transfer to trimethylaluminum (TMA), and β-hydride chain transfer to both monomer and metal to predict the effects of propylene partial pressure and the MAO concentration on polymer molecular weight and the formation of isobutyl end groups. A systematic optimization strategy was applied to estimate the kinetic parameters from on-line measurements of the reaction rate and end-of-batch measurements of the molecular weights and percentages of end groups. The formation of 2,1-insertions was more frequent for catalyst...

Activation of 2,6-Bis(imino)pyridine Iron(II) Chloride Complexes by Methylaluminoxane: An Electrospray Ionization Tandem Mass Spectrometry Investigation

The activation of the [2,6-bis(1-((2,6-diisopropylphenyl)imino)ethyl)pyridine]iron(II) chloride complex (1) with methylaluminoxane (MAO) was investigated in tetrahydrofuran (THF) by means of electrospray ionization tandem mass spectrometry (ESI-MS) complemented by UV−visible spectroscopy studies. The four-coordinated cationic methyl iron(II) complex 2 ([LFe-Me]+, L = 2,6-bis(1-((2,6-diisopropylphenyl)imino)ethyl)pyridine) was identified in the ESI-MS experiment. In addition, the cationic monochloride iron(II) complex 3 ([LFe-Cl]+), the product of α-H transfer from 2 to trimethylaluminum (TMA) [LFe-CH2AlMe2]+ (4), and the cationic iron hydride complex 5 ([LFe-H]+) were identified. Correlation of the ESI-MS experiments with UV−visible studies revealed that 2 and 3 are present in the THF solution as their solvent adducts.

Dual Site Ethene/1-Hexene Copolymerization with MAO Activated (1,2,4-Me3Cp)2ZrCl2 and (Me5Cp)2ZrCl2 Catalysts. Possible Transfer of Polymer Chains between the Sites

Ethene homopolymerization and copolymerization in toluene solution with varying amounts of 1-hexene were catalyzed by the methylaluminoxane (MAO) activated zirconocene catalysts (1,2,4-Me3Cp)2ZrCl2 and (Me5Cp)2ZrCl2 and a mixture of these two in a 1:1 molar ratio. The dual site catalyst (1:1 mixture) has a homo- and copolymerization activity intermediate between the individual catalysts. Deviations from the expected average behavior taking into account the ca. 6-fold higher 1 h yield of (1,2,4-Me3Cp)2ZrCl2 are observed. The dual site system results in a copolymer with ca. 30% too low comonomer incorporation. The melting endotherm attributed to (Me5Cp)2ZrCl2 is found to be ca. 30−70% too high, and the observed molecular weights for the copolymers are higher than expected and even higher than for the two individual catalysts. Chain transfer to trimethylaluminum (TMA) is found to become less important at high comonomer concentrations concurrent with an increase of the concentration of vinyl and trans-vinylene unsaturations. Further, it is noteworthy that with the dual site system it is possible to produce ethene/1-hexene copolymers with narrow molecular weight distribution as a mixture of chains containing low and high comonomer contents. The observed discrepancies in the mixed system from the average of the (1,2,4-Me3Cp)2ZrCl2 and (Me5Cp)2ZrCl2 catalysts are provisionally explained by chain transfer between the sites coupled with an activity increase of the latter of these sites.

Chemical vapor deposition of nanometer-size aluminum features on silicon surfaces using an STM tip

We have deposited aluminum (Al) features on silicon (Si) surfaces using the lectron beam and electric field produced at the tip of a scanning tunneling microscope (STM), in both tunneling and field emission modes. Lines as small as 3 nm in width have been produced on crystal Si(001). Al deposition is accomplished by exposing the tip-sample junction to trimethylaluminum (TMA) gas while tunneling. The electron beam of the STM tip dissociates the TMA molecules adsorbed on the surface, resulting in the production of surface-bound Al and hydrocarbon gas. Independent Auger electron spectroscopy studies of the electron induced deposition of Al from TMA shows nearly pure Al deposits.

Trimethylamine alane for MOVPE of AlGaAs and vertical-cavity surface-emitting laser structures

The use of trimethylamine alane (TMAAl) as an alternative to trimethylaluminum (TMAl) for low-pressure metalorganic vapor-phase epitaxy (MOVPE) of AlGaAs thin films has been studied in detail. AlGaAs layers were grown in a horizontal reaction chamber at 20–110 mbar with linear flow velocities as high as 3 m/s and growth temperatures in the range 650 ≤ T g≤750°C. Wafer thickness uniformity is acceptable only for the highest linear flow velocities. The 12 K photoluminescence spectra of the layers exhibit intense and narrow bound-exciton emission throughout the growth temperature range investigated. The luminescence linewidths observed are the narrowest to date for MOVPE-grown Al x Ga 1- x As with x≥0.25. To assess the viability of this new source for the growth of advanced optoelectronic devices, several optically-pumped vertical-cavity surface-emitting laser structures were grown using TMAAl extensively. Room temperature lasing at 850 nm was reproducibly obtained, with a threshold pumping power comparable to similar structures grown by molecular beam epitaxy in our laboratories.