Ralf Petrich

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.

The optical constants of the so-called “diamond-like” carbon layers and their description in terms of semiempirical dispersion models

Abstract Data on the optical response of amorphous carbon layers are presented. The layers investigated ranged from well insulating polymer-like layers via hard “diamond-like” layers to well conducting graphite-like layers. The spectral range investigated covers parts of the far infrared, the middle infrared, the near infrared and the visible spectral regions. The data are presented and discussed in terms of parameters of commonly accepted dispersion models and absorption assumptions (Penn model, lorentzian oscillator model, Urbach edge, Tauc edge). Special attention is paid to the refractive index dispersion behaviour. The dependence of the refractive index on the mass density and the hydrogen content is experimentally obtained and reproduced in terms of a lorentzian model. The influence of contaminations such as nitrogen and oxygen is investigated. As typical applications, the status of amorphous carbon as a material of protective optical coating, SLAR coating and as a spectrally selective absorber coating is very briefly discussed.

Modeling of transmittance, reflectance and scattering of rough polycrystalline CVD diamond layers in application to the determination of optical constants

Abstract The optical constants of CVD diamond layers on silicon substrates have been determined from spectrophotometric measurements. The rough surface of polycrystalline diamond layers causes scatter losses, which reduce the specular transmittance and reflectance of the layers. A model is derived to consider the effects of surface roughness in terms of surface scattering and coherence disturbance. Both effects can be described by means of a single parameter — the root mean square (rms) roughness. The model has proved useful to fit the experimental spectra. The optical and geometrical parameters obtained from the fit are in good agreement with the results of additional measurements, such as SEM, mass density determination. Raman spectroscopy and direct measurements of the position of the band gap of the samples.

The effect of nitrogenation on the IR optical constants of amorphous hydrogenated carbon layers

Abstract Nitrogenated amorphous carbon layers have been deposited by a dc plasma technique on silicon substrates. The optical constants of the film material have been determined from spectrophotometric measurements in the MIR/NIR spectral ranges. Special attention was paid to the parameters of the exponential absorption tail. The layers have been found to have IR refractive indices around 1.9. They are MIR transparent while showing considerable absorption in the NIR. Therefore, they may be of interest for the design of spectrally selective solar absorber coatings.

Optical performance of amorphous hydrogenated carbon layers : range of optical constants achievable

The optical properties of differently deposited amorphous carbon layers have been investigated by spectrophotometric means from the middle infrared up to the visible spectral region. The optical constants of the layers could be calculated by a numerical procedure that combines features of single-wavelength methods and multiwavelength methods. Depending on the deposition technique applied, the optical constants of the layers have been found to vary over a wide range. Thus, refractive index values could be established between 1.6 and 2.9, while IR-absorption coefficients vary between 20/cm and 40,000/cm. The optical performance of the layers is thus nonuniform and may be influenced by the selection of suitable deposition parameters. Due to the potentially wide range of optical constants of amorphous carbon, various possibilities exist for the application as an optical thin film material. Applications such as selective absorber coatings for solar physical purposes are discussed as well as mechanically and chemically resistant antireflection coatings on high-refractive index semiconducting materials (silicon, germanium).

Linear optical constants of ultrathin copperphthalocyanine films from transmittance and reflectance data: error function minimization when the film thickness is below 20 nm

Thin copperphthalocyanine layers have been deposited on quartz glass substrates and investigated by means of transmission and reflection spectroscopy. The film thickness ranged between 20 nm and the subnanometer region. The determination of the optical constants allowed the estimation of the oscillator strengths for the relevant molecular transitions. A thickness dependence of the Q-band absorption maximum position could be established for layers with a thickness below 5 nm. The contributions of several physical mechanisms to such lineshifts are discussed.

Optical performance of amorphous carbon layers: nonuniformity of transmittance, reflectance, and scattering

The optical properties of differently deposited amorphous carbon layers have been investigated by spectrophotometric means from the middle infrared up to the visible spectral region. The optical constants of the layers could be calculated by a numerical procedure which combines features of single-wavelength methods and multi-wavelength methods. Depending on the deposition technique applied, the optical constants of the layers have been found to vary over a wide range. Thus, refractive index values could be established between 1.6 and 2.9, while IR- absorption coefficients vary between 20/cm and 40,000/cm. Thus, the optical performance of the layers is nonuniform and may be influenced by selecting suitable deposition parameters. Due to the potentially wide range of optical constants of amorphous carbon, there are various possibilities for application as an optical thin film material. Applications as selective absorber coatings for solar physical purposes are discussed as well as applications as mechanically and chemically resistant and anti-reflection coatings on high-refractive index semiconducting materials (silicon, germanium).

Nitrogenation of amorphous carbon layers as a method for improving their performance as a spectrally selective absorber coating

Nitrogen-containing amorphous hydrogenated carbon layers have been deposited on silicon and aluminum by a dc plasma technique. The optical constants of the layers (refractive index and absorption coefficient), their solar absorptance and the thermal emittance at 373 K have been determined from spectrophotometric measurements. It is demonstrated that the solar absorptance of nitrogenated layers is higher than that of corresponding pure amorphous hydrogenated carbon layers, while the emittance values are comparable.

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.