Lithium Perchlorate as a Solid/Liquid State Oxygen Source for Molecular Beam Epitaxy

Abstract

In order to grow metastable transition metal suboxides, LiClO4 is theorized, designed, and tested as an alternative oxygen source for molecular beam epitaxy growth of oxide films. The material is found to decompose into a gas of both molecular and atomic oxygen beginning at approximately 250 °C, with an Arrhenius vapor pressure relationship across the measured range of less than 400 °C to avoid exothermic decomposition. A method for analyzing reaction rates is demonstrated using in situ Auger Electron Spectroscopy, and LiClO4 is compared to O2 showing a comparable reaction rate at four times lower growth pressure indicating the possibility for higher growth rates than are possible with O2. The decomposition kinetics of LiClO4 are discussed and solutions to instabilities are hypothesized for future studies on the material. A new style of MBE effusion cell for the evaporation of high vapor pressure sources is also presented, utilizing a two-zone heating system and aperture plate to resolve radiative heating issues common to materials evaporated close to room temperature. Discussion follows the design and testing of the cell, which exhibits acceptable long term stability over a 3 week testing period. These two sources are used, along with lithium, to grow thin films of multiple niobium oxidation states, from pure BCC niobium to LiNbO3. The most important films are high quality LiNbO2 which are grown using a method outlined in this work. The growth space is studied showing the preferential nucleation and growth of LiNbO2 at 950 – 1000 °C and a lithium flux of approximately 3×10-7 torr beam equivalent pressure.