Wei PLD Lab

Pulsed laser deposition is a powerful technique for growing epitaxial thin films of complex oxides, by virtue of its thermodynamic range and experimental flexibility. It has emerged as the method of choice for nanoscale materials research on complex oxides, particularly the transition-metal perovskites which have very rich phase diagrams and show a variety of correlated eletronic phenomena. These phenomena include high temperature superconductivity, colossal magnetoresistance and magnetoelectronic multiferroism, which may lead to a new generation of oxide-based microelectronic devices based on complex oxides.

Our pulsed laser deposition (PLD) system consists of an oxygen-compatible vacuum chamber made by Neocera Inc. and an excimer laser made by Lambdaphysik. The laser provides the high energies needed to ablate a wide range of complex oxides, such as YBa2Cu3O6+x, La0.7Ca0.3MnO3 and LaNiO3. The short pulse duration enables the plume to be deposited epitaxially, in a layer-by-layer manner, onto lattice-compatible substrate materials, such as LaAlO3 and SrTiO3. These oxide targets are 2-inch ceramic pucks, mounted on a multiple-target carousel, which enables pulse-by-pulse alternation of material during ablation, in a process known as heteroepitaxy. The size of the ablated plume is adjusted by varying the partial oxygen pressure in the chamber, thus controlling the deposition kinetics. The substrate temperature is also carefully controlled, to fine-tune the growth kinetics and chemical stoichiometry of the films.

The surface morphology of the the epitaxially grown films depends on the exact growth mode. Two typical modes are: step-flow growth, which lead to fishscale-like terraces; mount-clump growth, which produce rice-paddy surface structures. The surface quality can be characterized by imaging with either AFM or STM, with nanometer resolution. Film epitaxiality can be confirmed using x-ray diffraction with rocking-curve analysis. Overall film quality depends on the material and ablation conditions, and can be optimized through close feedback between synthesis and characterization. For the latter, our PPMS system enables measurement of various physical properties, such as resistivity and magnetization, versus both temperature and magnetic field.

We are currently installing a second laser deposition system, donated from IBM T.J. Watson Research Center. This state-of-the-art system has a high-power excimer laser with adjustable wavelength, plus an ultra-high vacuum chamber which is equipped with RHEED and atomic oxygen source. The differentially-pumped RHEED gun provides real-time monitoring of the film deposition, to enable true layer-by-layer growth in unit-cell blocks approaching atomic smoothness.