Comparative Optical Properties of Amorphous and Crystalline MoO3 Films by Spectroscopic Ellipsometry Study

Optical, structural and photoelectron spectroscopic studies on amorphous and crystalline molybdenum oxide thin films

The nearly unmatched conformality of Atomic Layer Deposition (ALD) renders it a powerful deposition technique for applications such as catalysis where high surface areas are critical, or for protective coatings on complex 3D surfaces such as powders or porous materials. In the last decade, ALD for energy storage has come into view. Advances in technology have pushed the requirements of energy storage devices, leading to novel lithium-ion battery (LIB) concepts such as 3D-structured electrodes, thin-film electrode and the combination of both. These concepts enable high-power and high-energy integrateable batteries, but require a conformal deposition technique for the current collectors, electrodes, solid electrolytes and interfacial coatings alike. Besides 3D structuring, materials design of the LIB electrodes is paramount to achieve higher power and higher energy densities, beyond traditional electrode materials such as LiCoO2, and LiMnO2which are limited to volumetric capacities of 550-700mAh/cm³. Traditionally, the main focus of materials development for batteries has been on crystalline compounds. However, several indications exist that higher energy and power density can be found in amorphous materials. ALD is not only excellently suited to deposit conformal films, but also able to grow high-quality amorphous films at low deposition temperatures, enabling the study of amorphous thin-film electrodes. In this work, ALD of vanadium oxides for applications as lithium-ion electrodes is envisioned. Vanadium oxides were amongst the first materials to be studied as LIB cathodes, but were never commercialised. V2O5 is a well-known LIB electrode, but the rather low volumetric energy density (when storing one lithium) or poor cyclability (when storing 2-3 lithium ions) have prevented its large-scale commercialisation in the past. Here, we demonstrate the ALD deposition of amorphous and crystalline V2O5, and two ‘flavours’ of amorphous VO2, from a single vanadium precursor (Tetrakis[ethylmethylamino] vanadium). We demonstrate the crystallisation of these films into crystalline films in the Wadsley series, and are able to unravel the influence of initial ALD process, atmosphere, temperature and substrate on the crystallization and oxidation of the VO2 films. With this in mind, we are able to obtain phase-pure crystalline vanadium oxides in the whole Wadsley series (VO2–V6O13–V4O9–V3O7–V2O5), which show very high energy densities up to more than double that of typical bulk cathodes (1380mAh/cm³ for V4O9). The amorphous initial states (amorphous VO2 and V2O5) are often not considered due to their lack of a clearly defined electrode potential. However, building on the nature of ALD to grow high-purity amorphous films, these were also investigated here. It was found that, besides also having very high capacities, their kinetics are excellent. Finally, the best candidates were examined on an ALD Pt-coated silicon micropillar array as 3D-structured thin-film electrodes. The 20x surface enhancement of the micropillar substrate enhanced the footprint capacity of the 40nm films from approximately 2-5µAh/cm² to 40-100µAh/cm², with the highest capacity for amorphous VO2(>100µAh/cm² for the 40nm film), while at the same time maintaining the excellent thin-film kinetics of the planar films, in effect simultaneously demonstrating the potential of ALD to deposit 3D thin-film electrodes as well as a tool to study amorphous and crystalline films. In conclusion, a wide range of crystalline and amorphous vanadium oxides were demonstrated from a single vanadium-precursor based ALD process. All films were examined as potential lithium-ion battery electrodes, and showed promise of very high volumetric capacities. In particular, the amorphous films and the crystalline VO2 (B) not only displayed very high capacities, but also excellent kinetics, enabling both high-energy and high-power electrodes. Figure 1

Study on the optical property and crystallinity of Ge90Te10 films deposited by electron beam evaporation

Raman microprobe spectroscopy was applied to the study of the photochromic properties of thin amorphous Moo3 (a-MoO3) films. The films became blue during UV irradiation, a process that involves a partial reduction of MoV1, and were bleached by electrochemical oxidation. Such coloration-bleaching cycles can be repeated many times. Raman peaks assigned to the three Mo-0 stretching modes (singly, doubly, and triply coordinated oxygen) were observed to shift reversibly during the coloration-bleaching cycles. Raman peak shifts occumng during the coloration process were consistent with predictions of Mo-0 bond strengths based on bond lengths in crystalline Moo3 and hydrogenated Moo3 presented in the literature. Disorder increased in the a-Mo03 structure as a result of the UV-induced coloration, as evidenced by increases in the Raman peak width, and decreased in the bleaching process. These changes were also reversible. We conclude that the local structure in the colored state in a-Moo3 is similar to that in the crystalline hydrogen bronze H,MoO3, where x I 0.36.

Structural, optical and mechanical properties of amorphous and crystalline alumina thin films

Amorphous and crystalline Bi3.25La0.75Ti3O12 thin films deposited on SiO2/Si(100) substrates have been prepared by a chemical solution deposition (CSD) process. X-ray diffraction shows that the crystal structure is a bismuth-layered perovskite structure with some preferred (117) orientation. The refractive indices and extinction coefficients of the amorphous and crystalline films were obtained by spectroscopic ellipsometry as a function of photo energy in the range from 1.8 to 4.0 eV. The dispersion of refractive indices was fitted by the Wemple–Didomenico single electronic oscillator dispersion mode. An interesting exponential absorption was found in the amorphous and crystalline BLT films, which may be ascribed to the substitution of lanthanum for bismuth. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical characterization of the coloration process in electrochromic amorphous and crystalline WO3 films by spectroscopic ellipsometry

The thermal conductivity of 2000-9000-\AA{}-thick amorphous and crystalline films of Ge and GeTe has been measured in the temperature range 100-550\ifmmode^\circ\else\textdegree\fi{}K. No thickness dependence has been observed in these films down to 2000 \AA{}. The results show that crystalline Ge and amorphous Ge and GeTe films have a negligible electronic contribution in the measured thermal conductivity. However, as expected, crystalline GeTe films exhibit significant electronic contribution which is \ensuremath{\sim}25% of the measured value of the thermal conductivity. The thermal conductivity of both amorphous Ge and GeTe increases slowly with increasing temperature in contrast to the rapid decrease in the crystalline films. In the latter case, the lattice component of the thermal conductivity decreases approximately inversely with temperature. The observed temperature dependence of the thermal conductivity can be understood on the basis of a combination of (i) umklapp-scattering mechanism, (ii) scattering due to defects, and (iii) scattering by grain boundaries. The last term, which is primarily responsible for the observed thermal resistance in the case of amorphous films, leads to a temperature-independent phonon mean free path. The observed increase of thermal conductivity with temperature for amorphous films is, therefore, attributed to the increase of specific heat with temperature. The values of the phonon mean free path in amorphous films as deduced from the thermal-conductivity data are \ensuremath{\sim}3 \AA{} in amorphous GeTe and \ensuremath{\sim}5 \AA{} in amorphous Ge films. These values suggest a relatively longer short-range order (coherently scattering regions) in amorphous Ge, as compared with amorphous GeTe.

Ln2O3 thin films with optically active f-electrons (Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb) have been grown on Si(100) and soda lime glass substrates by atomic layer deposition (ALD) using Ln(thd)3 (Hthd = 2,2,6,6-tetramethyl-3,5-heptanedione) and ozone as precursors. The temperature range for depositions was 200-400 °C. Growth rates were measured by spectroscopic ellipsometry and a region with a constant growth rate (ALD window) was found for Ln = Ho and Tm. All the compounds are grown as amorphous films at low temperatures, whereas crystalline films (cubic C-Ln2O3) are obtained above a certain temperature ranging from 300 to 250 °C for Nd2O3 to Yb2O3, respectively. AFM studies show that the films were smooth (rms < 1 nm) except for depositions at the highest temperatures. The refractive index was measured by spectroscopic ellipsometry and was found to depend on the deposition temperature. Optical absorption measurements show that the absorption from the f-f transitions depends strongly on the crystallinity of the material. The clear correlation between the degree of crystallinity, optical absorptions and refractive indices is discussed.

Comparison between thermal and plasma enhanced atomic layer deposition processes for the growth of HfO2 dielectric layers

Pulsed laser deposition of high Tc compounds onto unheated substrates, resulting in amorphous thin films, preserves to a great extent the composition of matter ejected from the target. This composition is of primary interest, both for understanding the dynamics of laser–target interaction and for practical (optimization) reasons. We have investigated the structure of amorphous and crystalline YBaCuO films obtained both in on-axis and off-axis deposition geometries, and correlated the results with optical and transport properties of these films. X-ray scattering reveals in amorphous films the existence of: (i) amorphous continuum of spatially disordered atoms, (ii) small (10–40 Å) amorphous clusters which can be considered as mesoscopic order fluctuations in the amorphous continuum, and (iii) slightly larger (50–250 Å) crystalline clusters exhibiting quasi-two dimensional (00l) or (11l) long range order. Crystalline films are predominantly (00l) oriented. Optical spectra of both crystalline and amorphous films show regions of enhanced attenuation caused by free charge carriers. Spectra of amorphous samples containing small crystalline clusters exhibit features which we relate to the electron localization caused by quantum size effects. Transport measurements are in good agreement with the structural and optical results. Conductivity of the on-axis films is 3–4 orders of magnitude higher than that of the off-axis films. The (nonlinear) conductivity of amorphous films increases with temperature and remains constant below 200 K. We suggest that besides the usual variable range hopping conduction mechanism, classical tunneling of charge carriers at constant energy between metallic (crystalline) clusters is present. Interestingly, the amorphous off-axis films exhibit a periodic repetition of the elements of atomic order or disorder along the direction of the plasma plume and undergo a structural transition of (11l)→(00l) type.

Boosting K-ion kinetics by interfacial polarization induced by amorphous MoO3- for MoSe2/MoO3-@rGO composites

HfO2 is a high-k material that is crucial in semiconductor devices. Atomic-level control of material processing is required for the fabrication of thin films of this material at nanoscale device sizes. Atomic layer deposition (ALD) and thermal atomic layer etching (ALE) allow the fabrication of ultra-thin films for semiconductor device processing. ALD is a well-known metal oxide thin film deposition technique that enables a high level of control over film thickness. Thermal ALE, which is growing in importance, uses self-limiting halogenation (e.g. HF exposure) producing a non-volatile modified layer. Subsequent ligand exchange reactions remove up to a monolayer of the metal oxide. This modern approach for controlled etching is the reverse of ALD. In gate oxides, the use of amorphous materials is better than crystalline films as the lack of grain boundaries allow for a high breakdown voltage. Amorphous materials have a lower density than crystalline materials.Experimental studies have shown that HfO2 amorphous films have higher etch rates per cycle than crystalline HfO2 films regardless of the fluorination reactant. Given that it is difficult to investigate ALE reactions directly using experimental techniques, first-principles-based atomic-level simulations using density functional theory (DFT) can give deep insights into the precursor chemistry and the reactions that drive the etch of different materials. This contribution presents first-principles density functional theory modelling to examine the etch chemistry of crystalline and amorphous films of HfO2. HF molecules adsorb on the surface of HfO2 by forming hydrogen bonds and may remain intact or dissociate to form Hf-F and O-H bonds. The adsorption of one HF molecule at the bare surface of both the crystalline and amorphous models results in dissociative adsorption at all binding sites. For multiple HF adsorption, we find mixed molecular and dissociative adsorption of HF molecules at the bare surfaces of the crystalline and amorphous models of HfO2. We also present a thermodynamic analysis approach to compare reaction models representing the self-limiting (SL) and continuous spontaneous etch (SE) processes taking place during an ALE pulse.

Zinc oxysulfide—Zn(O,S)—is a wide bandgap semiconductor with tunable electronic and optical properties, making it of potential interest as a buffer layer for thin film photovoltaics. Atomic layer deposition (ALD) of ZnS, ZnO, and Zn(O,S) films from dimethylzinc, H2O, and H2S was performed, and the deposited films were characterized by means of x-ray diffraction, x-ray photoelectron spectroscopy, and spectroscopic ellipsometry. With focus on the investigation of Zn(O,S) film growth characteristics and material properties, the ZnO/(ZnO + ZnS) ALD cycle ratios were systematically varied from 0 (ZnS ALD) to 1 (ZnO ALD). Notably, a strong effect ofthematerial properties on the optical characteristics is confirmed for the ternary films. The Zn(O,S) ALD growth and crystal structure resemble those of ZnS up to a 0.6 cycle ratio, at whichpoint XPS indicates 10% oxygen is incorporated into the film. For higher cycle ratios thefilm structure becomes amorphous, which is confirmed with XRD patterns and also reflected inthe optical constants as determined by spectroscopic ellipsometry; in particular, the optical bandgap transforms from direct type for the (cubic) ZnS like phase to a more narrow bandgap withamorphous characteristics, causing bandgap bowing. A direct bandgap is recovered atyethigherZnO/(ZnO + ZnS) cycle ratios, whereproperties converge toward ZnO ALD in termsof film growth rate, crystallinity, and composition.

The method of atomic layer deposition (ALD) is considered one of the primary candidates for the uniform and conformal deposition of ultrathin films vital for the continuous miniaturization in the semiconductor industry and related high-technology markets. By the virtue of two selflimiting surface reactions, the ALD technique yields an ultimate control of film growth in the sense that a submonolayer of material is deposited per so-called ALD cycle. With established materials being at the verge of industrial implementation, efforts are continuously undertaken to optimize and develop new ALD configurations and processes. So far, the main emphasis within the field of ALD has been on the materials characterization of the films by means of ex situ analysis. The research described in this thesis aims at the development of the relatively new configuration of plasma-assisted ALD and at in situ diagnostics studies of the (plasmaassisted) ALD processes. In plasma-assisted ALD, a plasma is used to activate the reactants in the gas phase in order to supply additional reactivity to the ALD chemistry. Plasma-assisted ALD is researched to provide benefits in the development of new ALD processes and materials. In particular, the opportunities to improve and tune the film properties, and to deposit films at reduced substrate temperatures have been addressed in this thesis. This work has been accompanied by studies using various in situ diagnostics, from which fundamental insight into the reaction mechanisms governing the ALD processes can be obtained. Moreover, in situ techniques provide the opportunity to monitor, optimize, and control the ALD process. In this work the use of in situ spectroscopic ellipsometry, transmission infrared spectroscopy, mass spectrometry, and optical emission spectroscopy has been demonstrated in studies of the plasma-assisted ALD processes of metal nitrides and metal oxides. The results of the film characterization obtained by these techniques have been corroborated and complemented by extensive ex situ analysis. In particular, the combination of in situ spectroscopic ellipsometry and the layer-by-layer ALD growth has been explored comprehensively. The merits of this in situ technique during ALD have been demonstrated by addressing various aspects relevant to ALD processes and materials. A large part of the work has concentrated on the plasma-assisted ALD process of the metal nitrides TiN and TaN. The merits of plasma-assisted ALD were observed in the deposition TiN films with excellent conductivity and low impurity content, even at low deposition temperatures. Furthermore, it was shown that by variation of the plasma condition in the ALD process of TaN, the film properties could be tailored from conductive, cubic TaNx;x??1 to semiconductive, amorphous Ta3N5. These aspects were clearly demonstrated by in situ spectroscopic ellipsometry, where the transition in TaNx phase could be distinguished by monitoring the energy dispersion in the optical constants. For the conductive films, the light absorption by free conduction electrons could be probed and that enabled extraction of the electrical film properties from the ellipsometry data. The latter was valuable to demonstrate electron-impurity scattering and finite size effects in TiN films. Furthermore, fundamental insight into the reaction mechanisms of plasma-assisted ALD process of TaN was obtained by detection of the volatile reaction by-products by mass spectrometry and optical emission spectroscopy. The possibilities for plasma-assisted ALD to improve the material properties and to deposit at reduced temperatures have been demonstrated for the process of Al2O3. The Al2O3 films were deposited at substrate temperatures down to room temperature and these films yielded good moisture permeation barrier properties as relevant for encapsulation purposes. The fundamental reaction mechanisms of this plasma-assisted ALD process were elucidated by transmission infrared spectroscopy in order to understand and further improve the film properties obtained at these reduced deposition temperatures. It was established that the surface chemistry is ruled by –CH3 and –OH surface groups created by the Al(CH3)3 precursor adsorption and the combustionlike reactions during the O2 plasma step, respectively. Moreover, infrared spectroscopy provided insight into the influence of deposition temperature on the material properties. It was shown that by prolonging the plasma exposure, i.e., by supplying more plasma reactivity to the ALD process, the surface chemistry at low temperatures was enhanced and the impurity content in the Al2O3 was reduced. In conclusion, the knowledge gained through the in situ diagnostic studies in this work is relevant to further develop the ALD technique. The insight obtained into the reaction mechanisms and the material properties of the ALD films in this work are particularly useful to further exploit the possibilities and opportunities of the plasma-assisted ALD technique in the synthesis of novel (complex) materials.