W. Scharff

A hybrid method for determination of optical thin film constants

Abstract A new method for optical thin film constant determination from spectrophotometric data is presented. The method combines features of single-wavelength methods as well as multiwavelength methods and thus represents a hybrid method specially designed for the analysis of amorphous thin film materials with more or less involved absorption behaviour, such as amorphous hydrogenated carbon (a-C:H), amorphous hydrogenated silicon (a-Si:H) and related materials. The unambiguity and continuity of solution contours are guaranteed without the postulation of any analytical dispersion law, by introducing specially constructed mathematical terms into the merit function to be minimized. The accuracy in determining refractive indices and film thicknesses is comparable to that achieved by curve-fitting methods. Weak inhomogeneities in film thickness may be taken into account.

The position of the fundamental absorption edge and activation energies for thermally activated electrical conductivity in amorphous carbon layers

Abstract Amorphous carbon layers (a-C, a-C:H) with a hydrogen content between 3 at.% and 25 at.% were deposited by plasma decomposition processes, sputtering and evaporation. Their mass density values were obtained from a flotation method. The refractive index and absorption coefficient were calculated from spectrophotometric data. Special attention was paid to the Urbach tail and Taucs plot absorption regions. The electrical conductivity was investigated in the temperature range T = 80–350 K. The conductivity values of all types of layers are discussed in terms of thermally activated conduction processes. In this sense all layers behave like semiconductors. For interpreting the conductivity values of the high gap layers, a Davis-Mott model with broad band tails was applied. However, this model was insufficient for fitting the conductivity data of samples with vanishing gaps. Reproduction of the conductivity values of these layers was possible in terms of a band model considering a structureless band at a position of 10–50 meV above the Fermi level. The conductivity of low gap samples (optical gaps around 0.3 eV) could only be fitted by a superposition of the conductivity laws following from both models. Fortuitously, the superposition of these functions yields a temperature dependence very similar to Motts T − 1 4 law between 50 K and 300 K, which may be an explanation of this widely observed behaviour.

Diamond-like amorphous carbon films prepared by r.f. sputtering in argon

Abstract In depositing thin carbon films by the r.f. sputtering of a graphite target in argon, an optimal parameter window was found in which it is possible to prepare adherent, IR-transparent, insulating, hard layers. Approximations for the sputtering process (total sputtering yield, energy of the sputtered carbon particles on the substrate) are demonstrated. A characteristic dependence of the film properties on the target voltage, an essential sputtering parameter, exists. In all cases, the target voltage influences the bombardment of the growing film by charged energetic particles. An anisotropic layer behaviour was established for more strongly absorbing films by three different methods of investigation. Finally, preliminary considerations concerning the film structure were made.

Plasma-assisted CVD of diamond films by hollow cathode arc discharge

Abstract The preparation of polycrystalline diamond particles and thin diamond films by deposition in the plasma discharge of a hollow cathode arc has been investigated. Although this discharge is known as a very simple and effective system for plasma generation, there has been little information about the growth of diamond by means of this method up to now. This type of discharge combines thermal and plasma dissociation of the reactant gases for diamond deposition. The temperature of the deposition area can be easily controlled by the electron current to the substrate and thus no auxiliary heating system is required, in contrast to many other diamond deposition methods. Furthermore, we assume additional decomposition of the reactant gases at the surface of the substrate due to the strong electron bombardment. As-deposited diamond particles and films were investigated by various electron microscopy methods, Raman spectroscopy and optical methods. They clearly show a polycrystalline structure with good crystallinity and quality. The diamond growth rate reaches about 2 μm h−1 on a deposition area of about 4 cm2.

Spectroscopic properties of i-C:H layers in the middle IR range

Abstract The optical parameters n and k of i-C:H layers produced by microwave plasma deposition and from an ion-plating deposition system on silicon substrates are determined by T and R measurements in the IR range. A computer model is given; the multiple solutions, the influence of errors in experimental values and the absorption peaks of the i-C:H layers in the IR range are discussed.

Noble gas ion assisted evaporation of carbon

Abstract Hard, adherent amorphous carbon films were formed by condensation of evaporated carbon particles with additional noble gas ion bombardment of the growing film (ion assisted evaporation (IAE)). Ion energy, ion mass, angle of ion incidence and ion-to-atom arrival rate are the main parameters influencing the film growth. The effects of these parameters on the resulting structural (electron energy loss spectroscopy and density determination) and mechanical (Vickers hardness measurements) film properties were studied. In addition, a systematic investigation of the corrosion protection potential of the carbon IAE films was carried out. It could be shown that the corrosion protection and the hardness of the films are strongly dependent on the angle of ion incidence and the ion-to-atom arrival ratio. The latter quantity and the ion energy also affect the film density and structure. The experimental results are discussed in the context of simulations of ion-atom interactions. In general, the depth and volume of the activated region and the amount of transferred ion energy to the carbon matrix atoms mainly affect the film growth and the resulting film properties.

Optical and electrical properties of amorphous hydrogenated carbon layers deposited by dc plasma decomposition

Abstract The optical properties of hard dc plasma deposited amorphous carbon layers have been investigated from the middle IR up to the visible spectral region. Refractive indices and absorption coefficient behaviour have been determined by means of spectral transmittance and reflectance measurements. The layers show a transparency window in the middle IR in the wavenumber range 1700 cm−1 Planar and transverse conductivities of the layers have been determined at room temperature. In addition, the conductivity of representative samples has been investigated in dependence of the temperature in the range of 80 K up to 350 K. A correlation between the thus-obtained activation energies for thermally-activated hopping in the band tails and the Urbach parameters of the layers could be established.

Determination of the optical constants of fine grained diamond layers on silicon substrates using curve-fitting procedures

Abstract The optical constants of fine-grained diamond layers have been investigated by spectrophotometric means in the near-IR and visible spectral ranges. The layers were deposited by a microwave chemical vapour deposition technique on 〈100〉 and 〈111〉 oriented silicon substrates. For determining the optical parameters, measurements of the directed and diffuse reflectance and transmittance of the layer-on-substrate system have been carried out. The calculation of the optical constants has been accomplished by a curve-fitting procedure. For that purpose, a quadratic error function has been minimized by the method of conjugated gradients. To reduce the multiplicity of solutions, the refractive index has been assumed to increase with increasing wavenumber following a quadratic dispersion law, while the loss coefficient is assumed to increase exponentially with increasing wavenumber. The investigation of experimental spectra has shown that the method works successfully in application to layers with a surface r.m.s. roughness less than approximately 100 nm. For such layers, refractive index values have been obtained which are slightly lower than those of natural diamond, while the loss coefficients are much higher because of surface and bulk scattering.

Nonuniformity of the dielectric response of amorphous carbon layers: correlation with atomic composition and structure

The potentially wide range of optical constants obtained from differently deposited carbon layers with diverse atomic structure has been the subject of an intensive research in recent years. The optical properties are well known to depend strongly on the level of contamination and the degree of atomic order in the layer material. The purpose of the present paper is to sum up the results of our investigations of optical and electrical properties of numerous differently deposited carbon layers and to relate them to the results of measurements of mass density, the atomic composition of the layers and electron diffraction measurements. In particular, estimation formulas are provided to relate the IR refractive index to the mass density and the hydrogen concentration of the layers. In addition, special attention is paid to the exponential absorption region.

Two-dimensional optical imaging spectroscopy in a dc driven Mercury-neon-argon glow discharge

Imaging spectroscopy is a powerful tool for visualization of plasma phenomena. A novel type of imaging spectrometer based on an acoustic optical tunable filter (AOTF) was used to select several emission lines for imaging from a mercury: rare gas glow discharge driven by direct current (dc) current. Images taken from this discharge show different locations of light origin for different emission lines. The spatial and wavelength resolutions of the spectrometer are evaluated.