Morphological design of complex oxides during pulsed-laser deposition: The role of plasma-plume expansion

Complex oxides such as tin-doped indium oxide (ITO) are widely utilized as transparent conductors in a variety of functional devices. Typically, they are fabricated by sputtering, which often requires additional annealing to achieve high transparency and conductivity. Using pulsed laser deposition (PLD), both high transparency and high conductivity have been achieved without annealing, using instead selected gas species and pressures. However, the relative roles of Stranski-Krastanov-like and vapor-liquid-solid (VLS) growth modes during morphological transitions remain controversial. Here, we report on PLD of ITO in an inert-gas environment, identifying the role of plasma-plume expansion in the selection of VLS vs vapor-solid (VS) growth. For the lowest N₂ pressure, indium-tin droplet formation, followed by self-catalyzed VLS growth, is observed. With increasing N₂ pressure, a transition from VLS to VS growth is apparent. It is hypothesized that oxygen scattering at the lowest N₂ pressure induces a metal-rich plume, which leads to metal droplet formation, followed by VLS growth. As the N₂ pressure is increased, the plasma-plume and its metal-rich core are compressed, resulting in a transition to VS growth. This tunable compression of the plasma-plume offers a route to the morphological design of a wide range of functional complex oxide devices with tunable optical and electronic performance.