Kristin Bergum

Thin film fabrication and characterization of proton conducting lanthanum tungstate

Thin films of the proton conducting lanthanum tungstate phase, La28−xW4+xO54+δv2−δ, were fabricated by atomic layer deposition (ALD) and characterized by impedance spectroscopy. The films were prepared by combining the processes of deposition of La2O3 and WO3 using [La(thd)3 + O3] and [(tBuN)2(Me2N)2W(VI) + H2O]. Deposition of WO3 by the above mentioned precursor combinations is investigated in the range of 250–375 °C proving ALD growth at temperatures 275–325 °C. WOCl4 + H2O was evaluated for WO3 deposition. The observed surface sensitive growth resulted in limited film growth on selected substrates. The process of lanthanum tungstate deposition was optimized with respect to control of stoichiometry and 0.9 and 1.2 μm thick films were deposited on MgO and Pd for cross-plane and in-plane impedance spectroscopy measurements, respectively. Due to short-circuiting of films on Pd after heat treatment, an alternative deposition procedure was devised which reduced short-circuiting significantly. The films show primarily ionic conductivity under wet conditions. Oxide ions are the dominating conductors above 650 °C, while protons become more dominant at lower temperatures. There is a significant contribution from n-type electronic conductivity in highly reducing atmospheres.

Electronic properties of Au/Cu 2 O/n-type Si heterojunction for energy conversion

Au/Cu2O/n-type Si structures have been fabricated and characterized electrically. The structure yields a current rectification of almost 7 orders of magnitude. It is also shown that the structure performs as a metal/semiconductor junction when subjected to CV measurements. The Cu2O film, deposited by radio frequency magnetron sputtering is highly transparent for light of energy below the fundamental band gap, while concurrently highly absorbing of light with energy above the bandgap. It is concluded that the structure is well suited as the upper junction in a tandem junction solar cell configuration, utilizing an industrial standard Si solar cell as ‘substrate’.

Improving carrier transport in Cu 2 O thin films by rapid thermal annealing

Cuprous oxide (Cu2O) is a promising material for large scale photovoltaic applications. The efficiencies of thin film structures are, however, currently lower than those for structures based on Cu2O sheets, possibly due to their poorer transport properties. This study shows that post-deposition rapid thermal annealing (RTA) of Cu2O films is an effective approach for improving carrier transport in films prepared by reactive magnetron sputtering. The as-deposited Cu2O films were poly-crystalline, p-type, with weak near band edge (NBE) emission in photoluminescence spectra, a grain size of ~100 nm and a hole mobility of 2-18 cm2 V-1 s-1. Subsequent RTA (3 min) at a pressure of 50 Pa and temperatures of 600-1000 °C enhanced the NBE by 2-3 orders of magnitude, evidencing improved crystalline quality and reduction of non-radiative carrier recombination. Both grain size and hole mobility were increased considerably upon RTA, reaching values above 1 µm and up to 58 cm2 V-1 s-1, respectively, for films annealed at 900-1000 °C. These films also exhibited a resistivity of ~50-200 Ω cm, a hole concentration of ~1015 cm-3 at room temperature, and a transmittance above 80%.

Thin film Cu 2 O for solar cell applications

Cu₂O is a highly relevant material for application as a top cell layer in a tandem solar cell structure. Phase-pure Cu, Cu₂O and CuO have been deposited with reactive magnetron sputtering by varying the oxygen flow during the deposition from 0 to 2 sccm, using a total flow of 51 sccm. The Cu₂O films are highly transparent at wavelengths above 520 nm. The highest carrier concentration and mobility of the Cu₂O films were 2.1 × 10¹⁵ cm^-3 and 18 cm²/Vs, respectively, with a resistivity of 163 Ω cm. Two defect levels were discerned from temperature-dependent Hall measurements of 0.16 and 0.23 eV above the valence band maximum.