Modelling of infrared optical constants for polycrystalline low pressure chemical vapour deposition ZnO:B films

Abstract

Structural and optical properties of sol-gel derived Zn1-xMnxO nanoparticles AIP Conf.Doped zinc oxide films are of high interest in thin film solar cell technology for application as transparent conducting oxide. Rapid and detailed characterisation of ZnO thin film properties is required for quality control and optimisation of the deposited films. In the present work, a new model of dielectric functions based on the effective medium approximation (EMA) is developed and is applied for characterisation of polycrystalline boron doped zinc oxide (ZnO:B) films, deposited by low pressure chemical vapour deposition (LPCVD) technique onto glass substrates. The model takes into account that polycrystalline ZnO is considered to consist of crystal grains surrounded by depletion layers. Using this model and Fourier Transform Infrared Spectroscopy (FTIR) performed in reflection configuration over a wide mid-infrared spectral region (from 2 lm up to 25 lm), the properties of depletion layer and the bulk of the grains in ZnO can be rapidly characterised in detail, and the volume fraction of the depletion layer can be extracted. The results are in good agreement with previously presented theories of electron transport in polycrystalline materials. Using electrical measurements like conductivity and Hall techniques in addition to the optically determined parameters, predominant electron scattering mechanisms in polycrystalline films for different doping levels are identified. The measurements show the impact of the doping level on depletion layer of the crystallites. It is shown, furthermore, that under a water vapour rich environment the volume fraction of the depletion layer may increase up to 5 times and more, while the mobility of the charge carriers in the depletion layer drops drastically from about 31 cm 2 V À1 s À1 to about 8 cm 2 V À1 s À1 . This indicates that water vapour exposure causes an increase of the potential barrier in the grain boundary depletion layer, limiting the electron transport across the grain boundaries to a classical thermionic emission mechanism. V C 2013 American Institute of Physics. [http://dx.

Thin film zinc oxide deposited by CVD and PVD

“…As published elsewhere [15], the BZO shows a connected network of voids, where diffusion of small ions can take place. It has also been shown [16], that exposure of the films to damp/heat (80% rel. humidity, 80°C for several hours) leads to an increased depletion layer that surrounds the crystals.…”

Flash Lamp Annealing (FLA) of Magnetron Sputtered Low‐Temperature TCO Coatings

“…Optical transmittance in UV-VIS was performed with a Perkin Elmer Lambda 950 spectrometer, and FTIR reflection measurements were done in a Spectrum 100 Perkin Elmer spectrometer equipped with a 30° reflection accessory from Pike Technologies. The reflection spectra were fit with simulation software using a simple Drude model to determine the carrier concentration and carrier mobility [4].…”

Optical modeling of plasma-deposited ZnO films: Electron scattering at different length scales

“…27 Note that ZnO layers that have large volume fractions (>2%) of depletion region (i.e., the space charge region around grain boundaries) should not be modeled as a homogeneous layer in the frequency ranges used in this work. 28 Those layers would typically have low doping levels and/or small grain sizes; 28 therefore, they would also be very different from the layers under investigation in this work.…”

“…Note that influence of grain boundary scattering has been reported even at higher frequency ranges in the case of large depletion regions around the grain boundaries. 28 In this case, the assumption of a homogeneous material cannot be used and an effective medium approach is required. Combining such an approach with the interaction distance concept is outside the scope of this work since it is not applicable to the model systems under investigation.…”

“…By this choice the energy range below 0.2 eV where various phonon modes can be present is avoided, 50 and the material also does not have to be modeled as an EMA as long as volume fractions of the depletion region are below 2%. 28 …”