F. Yubero

Low energy ion assisted film growth

This is an introductory manual for Ion Assisted Deposition (IAD) procedures of thin films. It is addressed to researchers, post-graduates and even engineers with little or no experience in the techniques of thin film deposition. It reviews the basic concepts related to the interaction of low energy ion beams with materials. The main procedures used for IAD synthesis of thin films and the main effects of ion beam bombardment on growing films, such as densification, stress, mixing, surface flattening and changes in texture are critically discussed. A description of some of the applications of IAD methods and a review of the synthesis by IAD of diamond-like carbon and cubic-boron nitride complete the book.

Preparation of transparent and conductive Al-doped ZnO thin films by ECR plasma enhanced CVD

The objective of this work is to optimise the deposition conditions of aluminium-doped ZnO by ECR plasma enhanced CVD to produce transparent and conductive thin films. Films were deposited by the reaction of Zn(C 2 H 5 ) 2 and Al(CH 3 ) 3 with pure oxygen plasma or mixtures of oxygen and hydrogen plasmas, obtained by an ECR downstream plasma source operated at low pressure (0.1-1 Pa). Control of the resistivity and UV-Vis transparency of the films was achieved by changing the aluminium dose in the film, the preparation temperature and the composition of the plasma (O 2 :H 2 ratio). Optical constants were determined by transmittance data in the UV-Vis region, all samples presenting very good optical characteristics (96-92% transmittance in visible range). On the other hand, resistivities, as obtained at room temperature by a four-point probe, ranged from > 10 +7 to 6×10 -3 Ω-cm, depending on the preparation conditions. Thus, the lowest resistivities were obtained for samples prepared with a plasma of H 2 +O 2 (2.0:1.5). Composition, microstructure, cristallinity and thickness of the films were characterised by XRF, XPS, RBS and XRD.

Electronic state characterization of SiOx thin films prepared by evaporation

SiOx thin films with different stoichiometries from SiO1.3 to SiO1.8 have been prepared by evaporation of silicon monoxide in vacuum or under well-controlled partial pressures of oxygen (P<10−6Torr). These thin films have been characterized by x-ray photoemission and x-ray-absorption spectroscopies, this latter at the Si K and L2,3 absorption edges. It has been found that the films prepared in vacuum consists of a mixture of Si3+ and Si+ species that progressively convert into Si4+ as the partial pressure of oxygen during preparation increases. From this spectroscopic analysis, information has been gained about the energy distribution of both the full and empty states of, respectively, the valence and conduction bands of SiOx as a function of the O∕Si ratio. The characterization of these films by reflection electron energy-loss spectroscopy (REELS) has provided further evidences about their electronic structure (band gap and electronic states) as a function of the oxygen content. The determination of the ...

Chemical stability of Sin+ species in SiOx (x<2) thin films

SiOx thin films have been prepared by evaporation of silicon monoxide powder in an ultrahigh vacuum chamber. The films are characterized by x-ray photoelectron spectroscopy (XPS), synchrotron photoemission, and x-ray absorption spectroscopy at the Si K edge. XPS shows that the films prepared by evaporation in ultrahigh vacuum have a SiO1.3 stoichiometry and are formed by Si3+(∼77%) and Si+(∼23%) species. Based on extended x-ray absorption fine structure analysis, the structure of these films has been described as formed by tetrahedra of the type Si–(Si, O3) and Si–(Si3, O), in agreement with the Si 2p photoelectron spectra. No significant amount of Si2+ species [i.e., Si–(Si2, O2)] tetrahedra) or elemental silicon were detected in these films. When SiOx thin films are prepared by evaporation of silicon monoxide in O2 atmosphere, the oxygen content in the film increases with the partial pressure of this gas. Under these conditions, Si4+ species are formed in detriment of the Si+ and Si3+ oxidation states. ...

Synthesis of SiO2 and SiOxCyHz thin films by microwave plasma CVD

Abstract Plasma enhanced chemical vapour deposition of SiO2 and polymeric SiOxCyHz thin films has been carried out at room temperature in a microwave electron cyclotron resonance (ECR) reactor. Si(CH3)3Cl has been used as volatile precursor of Si and pure oxygen as plasma gas. Plasma conditions were characterised by optical emission spectroscopy as a function of the relative flow rates of oxygen and precursor. The oxygen plasma was characterised by emission lines and bands due to O* and O2+ species whose relative intensity decreased as the flow rate of the precursor increased. Then, the plasma was dominated by the emission lines of H* species formed by dissociation of the precursor molecules. From the evolution of the intensity of the emission of oxygen and hydrogen lines as a function of the relative concentration of oxygen and precursor and by considering the composition and microstructure of the obtained thin films, a model is proposed for the decomposition mechanism of the precursor. According to this model, Si–Cl bond would dissociate in a first step. Then a series of reactions would follow with the activated oxygen species that, depending on the relative flow rate of oxygen, lead to the formation of SiO2 or a polymeric SiOxCyHz material. The chemical composition of the films was analysed by Rutherford backscattering spectroscopy (RBS), electron recoil detection analysis (ERDA) and X-ray photoelectron spectroscopy (XPS), while their structure and microstructure were investigated by means of transmission electron microscopy (TEM), atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FT-IR). It has been also shown that the SiOxCyHz thin films, with typical compositions such as Si:1, O:2, C:3.6, H:5.5, yield SiO2 thin films by exposure to a plasma of oxygen. These SiO2 thin films were smoother than the parent SiOxCyHz samples and the silica films prepared by PECVD under oxygen rich conditions.

Quantification of the H content in diamondlike carbon and polymeric thin films by reflection electron energy loss spectroscopy

A nondestructive method to determine the hydrogen content at the surface of diamondlike carbon and polymeric thin films is proposed. The method relies on the analysis of the elastic peak produced by backscattering of electrons from the hydrogen atoms present at the sample surface. Quantitative analysis of the H content at the surface is achieved through use of a phenomenological sensitivity factor for elastic electron backscattering by H atoms with respect to other atoms present at the surface of reference polymeric samples. The validity of the method is checked with elastic recoil detection measurements and infrared spectroscopy analysis of the same samples. The accuracy of the method in the determination of H content at the sample surface is estimated to be ±10%.

Ar stabilisation of the cubic/tetragonal phases of ZrO2 in thin films prepared by ion beam induced chemical vapour deposition

Abstract ZrO2 thin films have been prepared at room temperature by ion beam induced chemical vapour deposition (IBICVD). Two different sets of samples have been prepared depending on whether O2+ or mixtures O2++Ar+ ions were used for the decomposition of the precursor. The structure, microstructure, surface roughness and optical properties (i.e. refraction index) have been determined for the two sets of samples. The ‘as prepared’ samples were very compact and dense and had a very low surface roughness. The (O2+)–ZrO2 samples were transparent and had a high index of refraction (n=2.20 at λ=660 nm). On the contrary, the (O2+–Ar+)–ZrO2 films showed a greyish aspect after preparation, probably because of the existence of lattice defects in their structure. These defects were removed by annealing in air at T≥573 K. Then, the films became transparent and had a slightly lower refraction index than that of the O2+–ZrO2 films subjected to the same thermal treatments. For the (O2+–Ar+)–ZrO2 thin films, it was shown by X-ray photoelectron spectroscopy (XPS) and Rutherford back scattering (RBS) that Ar (4 at.%) remained incorporated within the oxide lattice even after annealing at T=773 K. The incorporation of Ar atoms within the ZrO2 network induced significant modifications in the crystallographic structure of the films. Thus, while X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) showed that the (O2+)–ZrO2 films consist of a mixture of monoclinic and tetragonal phases of zirconia, the (O2+–Ar+)–ZrO2 films depicted, at 298 K, a high degree of amorphisation. However, these films yielded a cubic/tetragonal phase after annealing at T>573 K. The role of the embedded Ar in inducing these structural changes is discussed.

Low temperature synthesis of dense SiO2 thin films by ion beam induced chemical vapor deposition

Abstract This paper presents a comparative study of SiO 2 thin films prepared at room temperature by ion beam induced chemical vapor deposition (IBICVD) and plasma enhanced chemical vapor deposition (PECVD) methods. The films are characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), Rutherford backscattering spectroscopy (RBS), electron recoil detection analysis (ERDA), nuclear reaction analysis (NRA), X-ray reflectometry and spectroscopic ellipsometry. While the films prepared by IBICVD are very compact and dense and have a high refractive index ( n =1.48 at λ=550 nm), those prepared by PECVD exhibit a lower refractive index value ( n =1.45 at λ=550 nm), lower density and have a higher surface roughness. The different microstructure and properties of the two sets of films are discussed in relation to the ballistic effects that occur by the action of the highly energetic ion beams (e.g. 400 eV) impinging on the surface of the films prepared by IBICVD.

Dielectric properties of Zr, ZrN, Zr3N4, and ZrO2 determined by quantitative analysis of electron energy loss spectra

The dielectric properties of Zr, ZrN, Zr3N4, and ZrO2 in the energy range from 1 to 80 eV were determined by quantitative analysis of electron energy loss spectroscopy in the reflection mode (REELS) using a recently proposed model. Collective excitations and electronic transitions are well characterized in the REELS spectra after analysis of the respective energy loss function, real and imaginary parts of the dielectric constant, and the optical joint density of states for the four compounds studied here.

Electrochromic behavior of WxSiyOz thin films prepared by reactive magnetron sputtering at normal and glancing angles

This work reports the synthesis at room temperature of transparent and colored W(x)Si(y)O(z) thin films by magnetron sputtering (MS) from a single cathode. The films were characterized by a large set of techniques including X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS), Fourier transform infrared (FT-IR), and Raman spectroscopies. Their optical properties were determined by the analysis of the transmission and reflection spectra. It was found that both the relative amount of tungsten in the W-Si MS target and the ratio O(2)/Ar in the plasma gas were critical parameters to control the blue coloration of the films. The long-term stability of the color, attributed to the formation of a high concentration of W(5+) and W(4+) species, has been related with the formation of W-O-Si bond linkages in an amorphous network. At normal geometry (i.e., substrate surface parallel to the target) the films were rather compact, whereas they were very porous and had less tungsten content when deposited in a glancing angle configuration. In this case, they presented outstanding electrochromic properties characterized by a fast response, a high coloration, a complete reversibility after more than one thousand cycles and a relatively very low refractive index in the bleached state.