A Non-Contacting Optical Probe of Electrical Transport Properties: Applications for Photovoltaics

Spectroscopic ellipsometry (SE) is a powerful technique which can be implement in ex-situ as well as in-situ real time measurement conditions for investigation and characterization of thin film photovoltaic (PV) relevant materials. Transport property measurement of the individual components in a complete PV device consisting of multiple semiconducting and metallic contact layers using direct electrical measurement is impossible as it requires a direct electrical contact. With direct electrical contact, there is substantial convolution between the contributions from these materials and the particular layer structure. Additionally, in multiple layer stacks material transport properties may be influenced by underlying layers. The grain boundary defects in polycrystalline thin films impact transport properties measurement and resulting device performance, and the underlying material may impact the particular grain structure and evolution. As long wavelength range ellipsometry is non-contacting, it is a potential alternative method to accurately determine transport properties in the polycrystalline thin films and individual layers in a device.In this dissertation, optical properties of sputtered aluminum doped ZnO (ZnO:Al) thin film in the form of complex dielectric function (ɛ = ɛ1 + iɛ2) spectra are studied from 0.4 meV to 5.89 eV using SE. Optical band gap and DC dielectric constant of the ZnO:Al film are found to be 3.62 ± 0.01 eV and 9.072, respectively. Resistivity (ρ), scattering time (τ), carrier concentration (N), and mobility (μ) are found to be (2.250 ± 0.007) × 10^-3 Ω-cm, 5.1 ± 0.7 fs, 20.8 cm2V-1s-1, and 1.3 × 10^20 cm-3, respectively. The values are in good agreement with literature for similar films. ρ and τ resulting from fitting to measurements are found to depend on the spectral range modeled, such that the inclusion of increasingly longer wavelengths results in a convergence to direct electrical measurement.Next, H-doped ZnO (ZnO:H) thin films prepared on thermal oxide coated silicon wafers by a novel supercycle approach of thermal atomic layer deposition technique are studied from 0.4 meV to 5.89 eV. The influence of H2 plasma treatment and substrate composition on optical and electrical properties of ZnO films are studied using SE and direct electrical measurements. A blue shift in the optical band gap energy, increase in the amplitude of phonon absorption, and increase in film conductivity are observed with decreasing cycle ratio (n) or more frequent H2 plasma treatments during deposition. Samples have been prepared using the same n but on bare thermal oxide, a 5 nm ZnO:H film, or a 5 nm intrinsic ZnO film to identify differences in ɛ and free carrier transport characteristics resulting from substrate composition. Effective carrier mass (m*) for the ZnO films are determined by combining results from Hall effect measurements with UV to THz spectra in ɛ.Sputtered ITO thin films prepared under similar conditions on soda lime glass and single crystal silicon wafers (c-Si) are used to study near infrared optical absorption variations during film growth. The presence of strong free carrier absorption and phonon absorption in the low photon energy spectral range provides an opportunity to disentangle tailing effects extending into the near infrared to visible spectral range. Toward that end, a model describing ITO film optical response in the form of ɛ from ex-situ ellipsometry collected over 0.4 to 4.1 meV, 0.035 to 0.4 eV, and 0.75 to 5.89 eV spectral range is developed to describe these features along with other higher energy electronic transitions. RTSE from 0.75 to 5.89 eV is used to track changes in film structure and optical response during thin film growth. Optical emission spectroscopy (OES) indicates the plasma is very stable during deposition implying that film property changes with thickness are not correlated with changes in plasma chemistry. Film ρ, μ, τ, and N are determined from the free carrier absorption component of ɛ. Increased near infrared absorption manifested in ɛ obtained from RTSE data analysis is hypothesized to originate from enhancement of OH-group phonon absorption due to the presence of water molecules in the chamber at the beginning of growth.Finally, transport properties (N, μ, m*) of materials of interest for PV devices are deduced from free carrier optical absorption using non-contacting optical Hall effect measurements. This new technique has the potential to determine transport properties of individual components even in complex multilayer PV device structures, which are usually inaccessible by other physical direct contact methods such as 4-point probe and electrical Hall effect. We build upon determining some or all of these transport properties for thin film polycrystalline PV absorber layers (CH3NH3PbI3, CdTe, and CuInSe2), and single crystal silicon (Si) wafers using optical Hall effect.