M.Sc Thesis


M.Sc StudentTammy Cohn
SubjectPulsed Laser Deposition of Nb-doped SrTiO3 Transperent
Electrodes
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Rothschild Avner


Abstract

Transparent electrodes are essential component in optoelectronic devices such as solar cells and light emitting diodes. At present nearly all transparent electrodes are made of transparent conducting oxides (TCO) of donor doped In2O3 (ITO), SnO2 (FTO), and ZnO (AZO or GZO). The limited selection of TCO leads to difficulties in matching the transparent electrode material to the photoactive layer. Consequently, there is often a misalignment in the energy bands that leads to losses in performance. Therefore, new materials are required to offer a wider selection of transparent electrodes.

We developed a new TCO based on the Nb-doped SrTiO3 (STN). SrTiO3 (STO) is a wide band gap semiconductor which is typically transparent in the visible range and insulating at ambient temperatures. Doping STO by donor impurities such as Nb leads to the formation of itinerant electrons following the defect chemical reaction  (electronic compensation) or Sr vacancies following the reaction  (ionic compensation). In general, electronic compensation occurs at low oxygen pressures (< 10-15 Pa) and ionic compensation at high pressures. The conductivity of STN can be increased by controlling the preparation conditions, but at the same time it absorbs light and becomes opaque.

In this work we investigated the influence of the deposition conditions on the optical and electrical properties of STN thin films. Epitaxial STN thin films were deposited by Pulsed Laser Deposition (PLD) on STO single crystal substrates. The films were deposited in different oxygen partial pressures (10-3 and 70mTorr). The surface morphology of the STN films was examined by AFM and HRSEM, displaying smooth terraces similar to the substrate topography. The crystal structure of the STO films and the epitaxial relationships between the STN films and the (001) STO substrates was analyzed using High Resolution X-Ray Diffraction (HRXRD).

The STN films that were deposited in 10-3mTorr showed high conductivity, 1250 S/cm, but rather low transparency of 10%. On the other hand, the films that were deposited in 70mTorr presented high transparency, ~90%, and sufficient conductivity of 50 S/cm, corresponding to a sheet resistance of 4000Ω/□.

By depositing STN thin films on FTO-coated glass substrates (TEC) we were able to reduce the overall sheet resistance by 30 times to ~ 100 Ω/□ while maintaining high transparency of 90%.

To conclude, in this research we developed new TCO material based on STN, with electrical and optical properties within range of the state-of-the-art TCO materials that are being used today.