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Institute
Aluminum nitride films prepared by plasma atomic layer deposition using different plasma sources
(2018)
Aluminum nitride (AlN) thin films are promising for versatile applications in optoelectronics, electronics, piezoelectrics, and acoustics due to their remarkable properties such as wide band gap, high dielectric constant, low electrical conductivity, good piezoelectric coefficient and high ultrasonic velocity. We present a comparative study of AlN films grown by plasma-enhanced atomic layer deposition at 350°C silicon wafers in the SENTECH SI ALD LL system using TMA and NH3 where either a capacitively coupled plasma (CCP) or a direct PTSA (planar triple spiral antenna) source was applied. The films were characterized by ellipsometry, XPS and electrical measurements. The layer properties are discussed concerning the varied ALD process parameters. In general, the process using the direct PTSA source delivered films with higher refractive index and better homogeneity over the wafer achieving also higher growth rates per cycle (GPC) in reduced total cycle durations. Films with refractive index in the range of 2.05 and permittivity around 8 could be realized with a GPC of 1.54 Å/cycle.
The growth of samaria thin films on copper sheets has been chemically studied by in situ X-ray photoelectron spectroscopy (XPS). The early stages of growth (0.1-14 ML) were studied by consecutive evaporations of Sm by chemical vapor deposition followed by XPS measurements. Subsequently, samaria films of different thicknesses, namely 0.1, 1, and 14 ML, were oxidized at room temperature (RT). The evolution of the sample morphology was determined through inelastic peak shape analysis (IPSA) using the QUASES software as an indirect method to study the relationship between Sm oxidation state and its surface arrangement. Our results show that samarium grows as 2D islands up to 2ML, which is followed by 3D growth. Chemical analysis indicates that at low coverages (<0.5ML) Sm is already oxidized, leading to the appearance of Sm3+ as the only oxidation state. The increase in the intensity of the O1s peak with time and the absence of spectral changes in the Cu2p and LMM Auger (substrate) indicate that the transformation is mainly due to adventitious oxidation of the layer. With further deposition at RT the metallic state Sm0 appears at higher coverages, which is readily postoxidized by subsequent exposure to molecular oxygen, leading to complete oxidation. No intermediate oxidation states (Sm2+) were observed, in contrast to the reported prevalence of Sm2+ on single crystal surfaces during the early stages of growth.
CeOx-Cu inverse catalysts have been shown to convert CO2 into valuable chemicals through catalytic hydrogenation. The catalytic activity may further be enhanced by alloying ceria with trivalent, catalytically active metals, such as Sm, promoting the formation of Ce3+ active sites. In this work, the structural and chemical properties of (111)- and (100)- oriented CeOx islands alloyed with samarium were explored by low-energy electron microscopy and X-ray photoemission electron microscopy. After Sm deposition on the as-grown CeOx islands, the near-surface region of (100)-oriented CeOx is reduced after exposure to H2 at 470 ∘C, whereas the deeper layers as well as the whole (111)-oriented islands retain the Ce4+ state. Subsequent reoxidation with O2 leads to the complete Ce4+ state recovery, suggesting the healing of oxygen vacancies. Additional annealing at 470 ∘C induces samarium diffusion into the ceria matrix. Yet, subsequent exposure to H2 reduces neither the (111)- nor the (100)-oriented CeSmOx islands, suggesting a quite unexpected stability of this system.
We have used low-energy electron microscopy (LEEM), micro-illumination low-energy electron diffraction (µLEED) supported by ab initio calculations, and X-ray absorption spectroscopy (XAS) to investigate in-situ and in real-time the structural properties of Sm2O3 deposits grown on Ru(0001), a rare-earth metal oxide model catalyst. Our results show that samarium oxide grows in a hexagonal A-Sm2O3 phase on Ru(0001), exhibiting a (0001) oriented-top facet and (113) side facets. Upon annealing, a structural transition from the hexagonal to cubic phase occurs, in which the Sm cations exhibit the +3 oxidation state. The unexpected initial growth in the A-Sm2O3 hexagonal phase and its gradual transition to a mixture with cubic C-Sm2O3 showcases the complexity of the system and the critical role of the substrate in the stabilization of the hexagonal phase, which was previously reported only at high pressures and temperatures for bulk samaria. Besides, these results highlight the potential interactions that Sm could have with other catalytic compounds with respect to the here gathered insights on the preparation conditions and the specific compounds with which it interacts.
A comparative study of thin aluminum nitride (AlN) films deposited by plasma-enhanced atomic layer deposition in the SENTECH SI ALD LL system applying either a direct inductively coupled plasma (ICP) or an indirect capacitively coupled plasma (CCP) source is presented. The films prepared with the ICP source (based on a planar triple spiral antenna) exhibit improved properties concerning the growth rate per cycle, total cycle duration, homogeneity, refractive index, fixed and mobile electrical charges, and residual oxygen content compared to the CCP source, where the comparison is based on the applied plasma power of 200 W. The increase of the plasma power to 600 W in the ICP process significantly reduces the residual oxygen content and enhances the electrical breakdown field. The AlN layers grown under these conditions, with a growth rate per cycle of 1.54 Å/cycle, contain residual oxygen and carbon concentrations of about 10% and 4%, respectively, and possess a refractive index of 2.07 (at 632.8 nm).
AlN has remarkable properties (wide band gap, low electrical and thermal conductivity, high dielectric constant, piezoelectricity) and is attractive for (opto)electronic and sensor applications. However, high oxygen content within nitride films is always a critical issue due to the thermodynamically favorable oxidation against nitridation resulting in deteriorated materials properties. In order to clarify whether the oxidation is a surface-limited or a bulk process elemental depth profiling is essential. In this work XPS in combination with Ar+ sputtering is applied to carry out depth profiling of AlN films prepared by plasma-enhanced atomic layer deposition using different parameters (plasma source, power and pulse duration). Particularly, the Al2p core levels are analyzed where the signals are decomposed into four components, representing weaker contributions of pure AlN and aluminum oxide phases as well as stronger signals of mixed oxygen-rich and nitrogen-rich phases. After sputtering (providing access to the deeper part of the film) the pure AlN phase content increases while the pure aluminum oxide content stays relatively constant. These issues are discussed with regard to the preparation parameters employed and accompanying XRD and electrical measurements.
The use of cerium oxide for hydrogen sensing is limited by the low electrical conductivity of layers deposited from a ceria target. To increase the electrical conductivity, partially reduced cerium oxide layers were obtained from a metallic cerium target by reactive magnetron sputtering. The proportions of the oxidation states Ce3+, present in reduced species, and Ce4+, present in fully oxidized species, were determined by ex-situ XPS. For electrical characterization, films were deposited on planarized tungsten finger electrodes. IV curves were measured over several days to investigate possible influences of oxygen and humidity on electrical conductivity. The morphological stability of the layers under ambient conditions was investigated by microscopical methods. The XPS results show a significant amount of Ce3+ in the layers. The electrical conductivity of as-grown samples is several orders of magnitude higher than that of samples grown from a ceria target. However, the conductivity decreases over time, indicating an oxidation of the layers. The surface morphology of the samples was found to be changing drastically within days, leading to partial delamination.