Metal oxides play important roles in many critical applications such as photovoltaics, gas sensors and coatings for architectural glass. These applications require that thin oxide films be doped with small amounts of a different chemical element that improves the electrical conductivity of the film without changing its structure. For example, fluorine-doped tin oxide is used as an electrically conductive coating on glass in photovoltaic cells and in gas sensors. When tin oxide combines with an equal amount of barium oxide, a ceramic compound known as barium stannate forms. It has a more complex structure known as a perovskite, and its properties are extremely dependent on the perfection of the film. Initial experiments on barium stannate demonstrate remarkable electrical conductivities that can enable the development of a new class of microelectronics based on metal oxides. Depositing films with exact elemental ratios that are free of defects is difficult. This research studies the fundamental factors controlling the deposition of high quality tin oxide and barium stannate films. An important broader impact of this combined experimental and computational research is the production of videos that can be used to explain to students and the general public the events involved in the deposition processes.
Through a combination of experimental and theoretical/computational techniques, the researchers are identifying fundamental factors at the atomic and molecular level of detail that influence the rates and quality of crystal growth in binary and ternary tin oxides. Mechanistic knowledge gained from this work is aiding in the design of new molecular precursors and deposition processes. In addition, a separate chemometric approach offers a potentially powerful tool for rapid in-silico screening of alternative precursors with the goal of prioritizing experiments to focus on molecular precursors having a high likelihood of utility. An initial group of six tin precursors, including metal alkyls, amides and nitrates, is being used to deposit tin oxides in an ultrahigh vacuum reactor. Growth rates, measured in situ by using time-dependent intensity oscillations in the reflection high energy electron diffraction peaks, are studied as a function of substrate temperature, precursor and oxygen pressure, aiming to elucidate the deposition kinetics to be used as the basis for designing depositions of ternary tin oxides having the perovskite structure. In these depositions, molecular precursors are used as the tin source and effusion cells are used as sources for the group-two elements with the ultimate goal to understand the underlying design principles for chemical precursors that lead to self-regulating stoichiometry control.
