Synthesis of magnetic tunnel junctions with full in situ atomic layer and chemical vapor deposition processes

Magnetic tunnel junctions, i.e. the combination of two ferromagnetic electrodes separated by an ultrathin tunnel oxide barrier, are core elements in a large variety of spin-based devices. We report on the use of combined chemical vapor and atomic layer deposition processes for the synthesis of magnetic tunnel junctions with no vacuum break. Structural, chemical and morphological characterizations of selected ferromagnetic and oxide layers are reported, together with the evidence of tunnel magnetoresistance effect in patterned Fe/MgO/Co junctions.     

Atomic layer deposition of MnO using Bis(ethylcyclopentadienyl)manganese and H2O

Manganese oxide (MnO) atomic layer deposition (ALD) was accomplished using sequential exposures of bis(ethylcyclopentadienyl)manganese (Mn(CpEt)₂) and H₂O. Rutherford backscattering analysis revealed a nearly 1:1 atomic ratio for Mn:O in the MnO ALD films. X-ray diffraction determined that the films were crystalline and consistent with the cubic phase of MnO. Quartz crystal microbalance (QCM) measurements monitored the mass deposition rate during MnO ALD and verified self-limiting reactions for each reactant. Extremely efficient reactions were observed that required reactant exposures of only 3 × 10⁴ L (1 L = 1.33 × 10^− 4 Pa s). X-ray reflectivity (XRR) studies were used to confirm the QCM measurements and determine the film density and film thicknesses. The MnO ALD film density was 5.23 g/cm³. The growth per cycle was investigated from 100-300 °C. The largest MnO ALD growth per cycle was 1.2 Å/cycle at 100 °C and the growth per cycle decreased at higher temperatures. Transmission electron microscopy images observed the conformality of MnO films on ZrO₂ nanoparticles and confirmed the growth per cycle observed by the XRR studies. Fourier transform infrared spectroscopy was used to study the -CpEt? and -OH? surface species during MnO ALD and also monitored the bulk vibrational modes of the growing MnO films. The results allowed a growth mechanism to be established for MnO ALD using Mn(CpEt)₂ and H₂O. Only 54% of the Mn sites are observed to retain the -CpEt? surface species after the Mn(CpEt)₂ exposure. Efficient MnO ALD using Mn(CpEt)₂ and H₂O should be useful for a variety of applications where metal oxides are required that can easily change their oxidation states.     

In situ studies of the atomic layer deposition of thin HfO2 dielectrics by ultra high vacuum atomic force microscope

We studied in situ the initial stages of atomic layer deposition (ALD) of HfO₂ by an ultra high vacuum atomic force microscope working in frequency-modulation mode. The ALD cycles, made by using tetrakis-di-methyl-amido-Hf and water as precursors, were performed on the Si(001)/SiO₂ substrate maintained at 230 °C. After each ALD cycle we studied the influence of the HfO₂ growth on the surface height histogram, the root mean square roughness, the surface fractal dimension and the autocorrelation function. This detailed analysis of the surface topography allowed us to confirm the completion of the first HfO₂ layer after four ALD cycles.     

Atomic layer deposition of Ru thin film using N2/H2 plasma as a reactant

Ruthenium (Ru) thin films were grown by atomic layer deposition using IMBCHRu [(η6-1-Isopropyl-4-MethylBenzene)(η4-CycloHexa-1,3-diene)Ruthenium(0)] as a precursor and a nitrogen-hydrogen mixture (N₂/H₂) plasma as a reactant, at the substrate temperature of 270 °C. In the wide range of the ratios of N₂ and total gas flow rates (fN₂/N₂ + H₂) from 0.12 to 0.70, pure Ru films with negligible nitrogen incorporation of 0.5 at.% were obtained, with resistivities ranging from ~ 20 to ~ 30 μΩ cm. A growth rate of 0.057 nm/cycle and negligible incubation cycle for the growth on SiO₂ was observed, indicating the fast nucleation of Ru. The Ru films formed polycrystalline and columnar grain structures with a hexagonal-close-packed phase. Its resistivity was dependent on the crystallinity, which could be controlled by varying the deposition parameters such as plasma power and pulsing time. Cu was electroplated on a 10-nm-thick Ru film. Interestingly, it was found that the nitrogen could be incorporated into Ru at a higher reactant gas ratio of 0.86. The N-incorporated Ru film (~ 20 at.% of N) formed a nanocrystalline and non-columnar grain structure with the resistivity of ~ 340 μΩ cm.     

Characteristics of atomic layer deposition grown HfO2 films after exposure to plasma treatments

Ultra thin HfO₂ films were grown by the atomic layer deposition (ALD) technique using tetrakismethylethylaminohafnium (Hf[N(CH)₃(C₂H₅)]₄) and ozone (O₃) as the precursors and subsequently exposed to various plasma conditions, i.e., CCP (capacitively coupled plasma) and MMT (modified magnetron typed plasma) in N₂ or N₂/O₂ ambient. The conventional CCP treatment was not effective in removing the carbon impurities, which were incorporated during the ALD process, from the HfO₂ films. However, according to the X-ray photoelectron spectroscopy measurements, the MMT treated films exhibited a significant reduction in their carbon contents and the efficient incorporation of nitrogen atoms. Although the incorporated nitrogen was easily released during the post-thermal annealing of the MMT treated samples, it was more effective than the CCP treatment in removing the film impurities. Consequently, the MMT treated samples exhibited excellent electrical properties as compared to the as-deposited HfO₂ films, including negligible hysteresis (flatband voltage shift), a low leakage current, and the reduced equivalent oxide thickness of the gate stack. In conclusion, MMT post treatment is more effective than conventional CCP treatment in improving the electrical properties of high-k films by reducing the carbon contamination and densifying the as-deposited defective films.     

Structural and optical studies of ZnS nanocrystal films prepared by sulfosalicylic acid (C7H6O6S)-assisted galvanostatic deposition with subsequent annealing

Zinc sulfide (ZnS) semiconductor nanocrystal films have been prepared on indium tin oxide coated glass substrates by sulfosalicylic acid (C₇H₆O₆S)-assisted galvanostatic deposition with subsequent annealing. The deposition was performed at 10 mA cm^− 2 in acidic electrolytes containing 15-30 mM Zn(CH₃COO)₂, 20 mM Na₂S₂O₃, 200 mM LiCl, 0.375 mM Na₂SO₃, and 0 or 0.2 mM C₇H₆O₆S. Results show that the presence of C₇H₆O₆S can suppress the precipitation of Zn and S impurity phases during the ZnS deposition process. As the [C₇H₆O₆S] = 0.2 mM and [Zn^2 +] = 20 mM, the deposited ZnS film exhibits only hexagonal structure with an ideal Zn/S atomic ratio of 1.03 and a close-packed granular morphology. But its band gap about 2.86 eV is narrower than the common value of ZnS, probably due to the existence of some spurious acetate species and defect states. By annealing the film at 400 °C for 60 min, its band gap increased up to 3.70 eV, despite that its crystalline phase transformed into cubic structure which usually shows the narrower band gap than hexagonal ZnS. The significant band gap widening could be ascribed to the degradation of spurious acetate species and the reduction of various possible defect states in the annealing process.     

Atomic layer deposition of palladium films on Al2O3 surfaces

We examined the atomic layer deposition (ALD) of Pd films using sequential exposures of Pd(II) hexafluoroacetylacetonate (Pd(hfac)₂) and formalin and discovered that formalin enables the efficient nucleation of Pd ALD on Al₂O₃. In situ quartz crystal microbalance measurements revealed that the Pd nucleation is hampered by the relatively slow reaction of the adsorbed Pd(hfac)₂ species, but is accelerated using larger initial Pd(hfac)₂ and formalin exposures. Pd nucleation proceeds via coalescence of islands and leaves hfac contamination at the Al₂O₃ interface. Pd films were deposited on the thermal oxide of silicon, glass and mesoporous anodic alumina following the ALD of a thin Al₂O₃ seed layer and analyzed using a variety of techniques. We measured a Pd ALD growth rate of 0.2 Å/cycle following a nucleation period of slower growth. The deposited films are cubic Pd with a roughness of 4.2 nm and a resistivity of 11 μΩ cm at 42 nm thickness. Using this method, Pd deposits conformally on the inside of mesoporous anodic alumina membranes with aspect ratio ∼1500 yielding promising hydrogen sensors.     

Atomic layer deposition of thin films of ZnSe—structural and optical characterization

Thin films of sphalerite-type ZnSe were grown by atomic layer deposition (ALD) from elemental Zn and Se precursors. These films, grown on various substrates, show bright blue ‘edge’ emission accompanied by donor-acceptor pair emissions in the blue, green and red spectral regions. Red, green and blue emissions mixed together give a white color, with a color temperature between 2400 and 4500 K depending on a layer thickness and temperature. ZnSe grown by ALD is in consequence a promising material for the fabrication of semiconductor-based white light emitting thin film electroluminescence displays.     

Native oxide consumption during the atomic layer deposition of TiO2 films on GaAs (100) surfaces

The consumption of the surface native oxides is studied during the atomic layer deposition of TiO₂ films on GaAs (100) surfaces. Films are deposited at 200 °C from tetrakis dimethyl amido titanium and H₂O. Transmission electron microscopy data show that the starting surface consists of ~2.6 nm of native oxide and X-ray photoelectron spectroscopy indicates a gradual reduction in the thickness of the oxide layer as the thickness of the TiO₂ film increases. Approximately 0.1-0.2 nm of arsenic and gallium suboxide is detected at the interface after 250 process cycles. For depositions on etched GaAs surfaces no interfacial oxidation is observed.     

Structure and stimulated emission of a high-quality zinc oxide epilayer grown by atomic layer deposition on the sapphire substrate

A high-quality ZnO epilayer was grown on the (0001) sapphire substrate by atomic layer deposition (ALD) and followed by high-temperature rapid thermal annealing (RTA). The layer-by-layer growth and low deposition temperature of ALD, as well as the RTA treatment, prevent the formation of columnar structures in the ZnO epilayer. A distorted ZnO layer at the ZnO/sapphire interface, which relaxes the misfit in ZnO leads to a low threading dislocation density in the ZnO epilayer. Optically-pumped stimulated emission was achieved with a low-threshold intensity of 153 kW/cm² at room temperature. The good crystalline quality and low-threshold stimulated emission indicate that the ALD approach is appropriate for preparing high-quality ZnO epilayers.     

Nanocone SiGe antireflective thin films fabricated by ultrahigh-vacuum chemical vapor deposition with in situ annealing

In this study, we fabricated nanocone-presenting SiGe antireflection layers using only ultrahigh-vacuum chemical vapor deposition. In situ thermal annealing was adopted to cause SiGe clustering, yielding a characteristic nanocone array on the SiGe surface. Atomic force microscopy indicated that the SiGe nanocones had uniform height and distribution. Spectrophotometric measurements revealed that annealing at 900 °C yielded SiGe thin films possessing superior antireflective properties relative to those of the as-grown SiGe sample. We attribute this decrease in reflectance to the presence of the nanostructured cones. Prior to heat treatment, the mean reflectance of ultraviolet rays (wavelength < 400 nm) of the SiGe thin film was ca. 61.7%; it reduced significantly to less than 28.5% when the SiGe thin film was annealed at 900 °C. Thus, the drop in reflectance of the SiGe thin film after thermal treatment exceeded 33%.     

Textured strontium titanate layers on platinum by atomic layer deposition

Formation of textured strontium titanate (STO) layers with large lateral grain size (0.2-1 μm) and low X-ray reflectivity roughness (~ 1.36 nm) on Pt electrodes by industry proven atomic layer deposition (ALD) method is demonstrated. Sr(t-Bu₃Cp)₂, Ti(OMe)₄ and O₃ precursors at 250 °C were used to deposit Sr rich STO on Pt/Ti/SiO₂/Si ∅200 mm substrates. After crystallization post deposition annealing at 600 °C in air, most of the STO grains showed a preferential orientation of the {001} plane parallel to the substrate surface, although other orientations were also present. Cross sectional and plan view transmission electron microscopy and electron diffraction analysis revealed more than an order of magnitude larger lateral grain sizes for the STO compared to the underlying multicrystalline {111} oriented platinum electrode. The combination of platinum bottom electrodes with ALD STO(O₃) shows a promising path towards the formation of single oriented STO film.     

Low temperature atomic layer deposition of high-k dielectric stacks for scaled metal-oxide-semiconductor devices

Dielectric stacks of aluminium oxide and zirconium oxide grown by atomic layer deposition at low temperatures between 110 °C and 200 °C from trimethylaluminum/water and tetrakis-(diethylamino)zirconium/water, respectively, are investigated regarding their applicability in Metal-Oxide-Semiconductor (MOS) devices. We characterize the dielectric stacks regarding their electrical qualification in MOS devices and their thermodynamical behaviour, and refer these results to layer structure and morphology. As a result, we can show that the deposition of the stacks at low temperatures (110 °C to 150 °C) in combination with reductive/oxidative post-deposition annealing lead to amorphous dielectrics with remarkably high permittivity, with very low leakage currents in the range of 10^− 7 A/cm², and excellent capacitance-voltage characteristics.     

Temperature dependence of SiO2 film growth with plasma-enhanced atomic layer deposition

The growth per cycle (GPC) temperature dependence was investigated for SiO₂ films prepared by plasma-enhanced atomic layer deposition (PEALD). During preparation of PEALD-SiO₂ using bis-diethyl-amino-silane, the GPC was saturated via increasing the precursor dose time and flow rate. The saturated GPC decreased with increasing deposition temperature. GPC saturation curves as a function of precursor dose time were analyzed by a two-step adsorption model, where the amino-silane reversibly adsorbed (physisorption) during the first step, and then irreversibly adsorbed (chemisorption) on the SiO₂ surface upon reaction with surface OH absorbents. This model is in good quantitative agreement with the saturation curve. The GPC value was determined by the surface reaction of amino-silane with OH sites, whose surface density was decreased by increasing the deposition temperature. The GPC saturation became slower with increasing deposition temperature, because the desorption rate of the physisorbed precursor increased with increasing temperature.     

Influence of Si(1 0 0) surface pretreatment on the morphology of TiO2 films grown by atomic layer deposition

The effect of water plasma treatment of both hydrophobic and hydrophilic Si(1 0 0) surfaces has been studied using infrared spectroscopy to monitor the various surface species present. Exposure to a water plasma results in a significant increase in the concentration of H-bonded hydroxyls and hydrides. Both atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM) of TiO₂ films deposited by atomic layer deposition at 300 °C, show that the morphology of the films is dependent on the nature of the initial surface. XTEM of the early stages of growth showed that coatings on hydrophilic substrates deposited as initially amorphous and continuous films, which crystallised with further growth. However, the hydrophobic substrate produced island growth of small, crystalline grains. AFM images of 23-nm thick films showed that films deposited on hydrophobic and hydrophilic Si consisted of 35–100 and 150–350 nm crystallites, respectively. A film on water plasma treated Si, closely resembled that on the hydrophilic surface, indicating that hydroxyl groups are responsible for directing the film growth.     

Deposition of HfO2 thin films on ZnS substrates

HfO₂ thin films with columnar microstructure were deposited directly on ZnS substrates by electron beam evaporation process. SiO₂ thin films, deposited by reactive magnetron sputtering, were used as buffer layers, HfO₂ thin films of granular microstructure were obtained on SiO₂ interlayer by this process. X-ray diffraction patterns demonstrate that the as-deposited HfO₂ films are in an amorphous-like state with small amount of crystalline phase while the HfO₂ films annealed at 450 °C in O₂ for 30 min and in Ar for 150 min underwent a phase transformation from amorphous-like to monoclinic phase. Antireflection effect in certain infrared wave band, such as 3–6 μm, 4–12 μm, 4–8 μm and 3–10 μm, can be observed, which was dependent on the thickness of thin films. The cross-sectional images of HfO₂ films, obtained by field emission scanning electron microscopy, revealed that there was no distinct morphological change upon annealing.     

Multi-orientation patterned deposition of BaTiO3 thin films using an Au buffer layer

The control of crystallographic orientation for ferroelectric oxide thin films grown on single crystal substrates has been investigated. We find that perovskite BaTiO₃ has unusual orientation distributions when deposited on (100)- and (110)-oriented SrTiO₃ single crystal substrates that have predeposited patterned Au regions. BTO areas deposited on gold islands were found to have a (111) orientation, whereas those deposited directly on STO had the same orientation as the substrate. Based on this method, we can select, locate, and pattern an oxide film to have different orientation distributions, and thereby engineer films with the complete property anisotropy in-plane.     

Effects of atomic layer deposited HfO2 compact layer on the performance of dye-sensitized solar cells

The performance of dye-sensitized solar cells (DSSCs) is limited by the back-reaction of photogenerated electrons from the photoelectrode back into liquid electrolyte. An atomic layer deposited (ALD) hafnium oxide (HfO₂) ultra thin compact layer was grown on the surface of the transparent conducting oxide (TCO) and its effects on the DSSC performance were studied with dark and illuminated current-voltage and electrochemical impedance spectroscopy measurements. It was found that this compact layer was effectively blocking the back-reaction of electrons from TCO to the liquid electrolyte, resulting in the overall photoconversion efficiency being enhanced by 66% compared to a DSSC with a conventional sol-gel processed TiO₂ compact layer. Reasons for the improved photovoltaic performance were attributed to passivation of the TCO surface, better electronic quality of the compact layer material and the higher compactness, shown by atomic force microscopic images, obtained from gas-based deposition methods. Also, an increased short-circuit current density suggests that the interfacial resistance for the injection of electrons from the porous nanoparticle network to TCO was reduced. Further, the theory of electron recombination at the TCO/compact layer/electrolyte interface was developed and used to explain the improved DSSC performance with an ALD HfO₂ compact layer.     

Thermophilic biofiltration of H2S and isolation of a thermophilic and heterotrophic H2S-degrading bacterium, Bacillus sp. TSO3

Thermophilic biofiltration of H₂S-containing gas was studied at 60 °C using polyurethane (PU) cubes and as a packing material and compost as a source of thermophilic microorganisms. The performance of biofilter was enhanced by pH control and addition of yeast extract (YE). With YE supplement and pH control, H₂S removal efficiency remained above 95% up to an inlet concentration of 950 ppmv at a space velocity (SV) of 50 h⁻¹ (residence time = 1.2 min). H₂S removal efficiency strongly correlated with the inverse of H₂S inlet concentrations and gas flow rates. Thermophilic, sulfur-oxidizing bacteria, TSO3, were isolated from the biofilter and identified as Bacillus sp., which had high similarity value (99%) with Bacillus thermoleovorans. The isolate TSO3 was able to degrade H₂S without a lag period at 60 °C in liquid cultures as well as in the biofilter. High H₂S removal efficiencies were sustained with a periodic addition of YE. This study demonstrated that an application of thermophilic microorganism for a treatment of hot gases may be an economically attractive option since expensive pre-cooling of gases to accommodate mesophilic processes is not required.     

Thermal properties of TiO2 films grown by atomic layer deposition

We study the thermal properties of amorphous TiO₂ thin films of various thicknesses t, grown by atomic layer deposition. The thermo-optic coefficient dn/dT and the temperature coefficient dρ/dT of film density ρ are determined from ellipsometric data in wavelength range 380 < λ < 1800 nm with the Cauchy model and the Lorentz-Lorenz relation. It is found that dn/dT exhibits negative values for films with t < 150 nm and positive values for thicker films, while no significant changes in the two coefficients take place if t < 200 nm. A qualitative physical explanation based on porosity of the thin films is suggested. Films with t = 60 nm are illustrated in detail at λ = 640 nm: the room-temperature values of the coefficients are found to be dn/dT = − 3.1 × 10^− 5 °C^− 1 and dρ/dT = − 4.8 × 10^− 5g cm^− 3° C^− 1.