ATOMIC CONSTRUCTION OF OXIDE THIN FILMS BY LASER MOLECULAR BEAM EPITAXY

Abstract

Advancements in nanoscale engineering of oxide interfaces and heterostructures have led to discoveries of emergent phenomena and new artificial materials. Reactive molecular-beam epitaxy (MBE) and pulsed-laser deposition (PLD) are the two most successful growth techniques for epitaxial heterostructures of complex oxides. PLD possesses experimental simplicity, low cost, and versatility in the materials to be deposited. Reactive MBE employing alternately-shuttered elemental sources (atomic layer-by-layer MBE, or ALL-MBE) can control the cation stoichiometry precisely, thus producing oxide thin films of exceptional quality. There are, however, major drawbacks to the two techniques. Reactive MBE is limited to source elements whose vapor pressure is sufficiently high; this eliminates a large fraction of 4- and 5-d metals. In addition, the need for ozone to maintain low-pressure MBE conditions increases system complexity in comparison to conventional PLD. On the other hand, conventional PLD using a compound target often results in cation off-stoichiometry in the films. This thesis presents an approach that combines the strengths of reactive MBE and PLD: atomic layer-by-layer laser MBE (ALL-Laser MBE) using separate oxide targets. Ablating alternately the targets of constituent oxides, for example SrO and TiO2, a SrTiO3 film can be grown one atomic layer at a time. Stoichiometry for both the cations and oxygen in the oxide films can be controlled. Using Sr1+xTi1-xO3, CaMnO3, BaTiO3 and Ruddlesden–Popper phase Lan+1NinO3n+1 (n = 4) as examples, the technique is demonstrated to be effective in producing oxide films with stoichiometric and crystalline perfection. By growing LaAl1+yO3 films of different stoichiometry on TiO2-terminated SrTiO3 substrate at high oxygen pressure, it is shown that the behavior of the two-dimensional electron gas at the LaAlO3/SrTiO3 interface can be quantitatively explained by the polar catastrophe mechanism.