"Real-Time Studies of Epitaxial Oxide Using Surface X-ray Diffraction"

Gyula Eres, Oak Ridge National Laboratory

Recent reports of unusual electronic properties that include quasi-2D electron gas and superconductivity at the interface region between the two insulating oxides strontium titanate (STO) and lanthanum aluminate (LAO) have focused new attention on the important topic of interface sharpness.  From purely kinetic standpoint the formation of sharp interfaces during film growth is governed by two atomic surface transport processes: intralayer transport, also known as surface diffusion, and interlayer transport, which describes the ability of adatoms to descend a step edge.  Of the two, interlayer transport is clearly the more exotic process. Its existence has never been in dispute.  But, because direct experimental measurements of interlayer transport are difficult, it is a source of ongoing controversy in the theoretical literature.  The recent emergence of direct measurement capabilities is proving that the experimental side is not immune to similar controversy.   In this talk I describe the use of time-resolved surface x-ray diffraction to study interlayer transport in epitaxial growth of STO by pulsed laser deposition.  We use a CCD camera to measure the entire diffraction intensity map around the (0 0 ½) reflection.  From these maps we extract the diffuse scattering and its relationship to the sharp anti-Brag peak.  In a different configuration we use avalanche photodiode detectors to measure simultaneously the intensity at the specular (0 0 ½) and off-specular (0 1 ½) positions with microsecond range resolution.  The use of x-ray diffraction greatly simplifies growth kinetics studies because in the kinematic limit the x-ray intensity changes correspond directly to coverage changes.  Therefore, we can analyze the intensity transients using a model-independent approach that allows direct determination of the time-dependent surface coverages from the transient intensities. These measurements reveal two clearly distinct components of interlayer transport:  in addition to the familiar equilibrium interlayer transport occurring in thermal deposition, there is also an extremely fast interlayer transport component that originates from laser driven processes.  The fast interlayer transport is at least four orders of magnitude faster than thermal interlayer transport and is the dominant process in the formation of the epitaxial layers. Time-resolved diffuse scattering shows that suppressing the sluggish thermal interlayer transport that occurs after crystallization is essential for maintaining persistent layer-by-layer growth.  The ability to determine interlayer transport quantitatively gives a new perspective on the intriguing question of how pulsed laser deposition occurs from the energetic plume.  The new understanding of the growth kinetics directs us to a nonthermal growth regime in which layer-by-layer growth is possible four orders of magnitude faster than by any other oxide thin film growth technique.