Sammendrag
Cr doped ZnS (ZnS:Cr) has been suggested as an intermediated band material based on density functional theory studies. We have investigated ZnS:Cr as a candidate intermediate band material. The ZnS and ZnS:Cr thin films were deposited by pulsed laser deposition and molecular beam epitaxy on Si and quartz substrates. In this work, we report an elaborate study of the optical properties of the films using spectroscopic ellipsometry (SE). SE data was mainly recorded using a dual rotating compensator spectroscopic ellipsometer (RC2), complemented by data from vacuum ultraviolet extended instrument (VUV-SE). The characteristic features of the ZnS:Cr are a direct band gap (E0) at approximately 3.7 eV, with E1 and E2 critical points around 5.7 and 7 eV. Excitonic effects are in line with the literature and observed to be strong in this material. The sub-band gap absorption appears as a broad feature that increases monotonously with increased doping at a given growth temperature. In this report we discuss three different approaches to extract and analyze the optical properties in terms of the complex dielectric function (ε = ε1 + iε2). First, the optical properties are extracted using parametric dispersion models. In order to correctly fit the highly important sub-band gap absorption features, parametric dispersion models incorporating a band gap are required. As a result, a reasonable robust approach was found to be the use of Tauc Lorentz oscillators to fit the above band features, while the below band gap features were fitted using Lorentzian or Gaussian oscillators. The problem with this approach is that the exact interpretation of the oscillator energies is not trivial. The second approach following Ghimire et al. [3], allow the use of standard oscillators assuming critical point parabolic bands (CPPB) models for the imaginary part of ε modified with a band-gap and an Urbach tail. The real part of ε is then calculated using a numerical Kramers Kronig transformation. Although the latter approach is not rigorously exact, it appears to be a better approach for physical interpretation of the data. Finally, direct inversion was also used to extract ε from the experimental data. The total sub-bandgap absorption is quantified by integration of the imaginary part of ε up to the band-gap. Critical point analysis is applied to the second derivative of numerically inverted ε. Problems in the direct inversion are discussed in terms of uniaxiality of certain columnar films.
Vis fullstendig beskrivelse
