Controlling the Structural and Optical Properties
of Ta<sub>3</sub>N<sub>5</sub> Films through Nitridation Temperature
and the Nature of the Ta Metal
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Abstract
The development of a reliable synthetic
route to produce high performance
Ta<sub>3</sub>N<sub>5</sub> photoanodes has been complicated by the
large number of synthetic parameters, notably nitridation conditions.
A systematic study of nitridation from 850 °C–1000 °C
reveals that, contrary to common knowledge, nitridation temperature
has little effect on the quality of the Ta<sub>3</sub>N<sub>5</sub> produced. Rather, it is the nature of the tantalum starting material
and substrate that play a key role. Ta<sub>3</sub>N<sub>5</sub> films
synthesized by thermal oxidation and subsequent nitridation of Ta
thin films on inert fused silica substrates exhibit identical structural
and optical properties, regardless of preparation temperature. The
optical spectra collected on these samples reveal clear, distinct
features that give insight into the electronic band structure. Films
grown in the same manner on Ta foils, however, reveal that textured
Ta<sub>2</sub>N is formed at the Ta<sub>3</sub>N<sub>5</sub>/Ta interface
even at low temperature, as shown by grazing incidence X-ray scattering.
Ta<sub>3</sub>N<sub>5</sub> on Ta foils is converted to bulk Ta<sub>5</sub>N<sub>6</sub> at 1000 °C, and the possible mechanisms
for these phase transitions are discussed