Complex oxide growth using simultaneous in situ reflection high-energy electron diffraction and x-ray reflectivity: When is one layer complete?

Abstract

During layer-by-layer homoepitaxial growth, both the Reflection High-Energy Electron Diffraction (RHEED) intensity and the x-ray reflection intensity will oscillate, and each complete oscillation indicates the addition of one monolayer of material. However, it is well documented, but not well understood, that the phase of the RHEED oscillations is not constant and thus the maxima in the RHEED intensity oscillations do not necessarily occur at the completion of a layer. We demonstrate this using simultaneous in situ x-ray reflectivity and RHEED during layer-by-layer growth of SrTiO3. We show that we can control the RHEED oscillation phase by changing the pre-growth substrate annealing conditions, changing the RHEED oscillation phase by nearly 180• . In addition, during growth via pulsed laser deposition, the exponential relaxation times between each laser pulse can be used to determine when a layer is complete, independent of the phase of the RHEED oscillation.PACS numbers: 68.47. Gh,61.05.cf,81.15.Fg Thin-film growth changed dramatically more than three decades ago with the discovery of reflection high-energy electron diffraction (RHEED) intensity oscillations.1-3 During RHEED, a high-energy (≈ 10-30 keV) electron beam is fired at grazing incidence onto a growth surface and the intensity of the reflected beam is recorded. With RHEED, the incident electrons only interact with the topmost layer. During 2D monolayerby-monolayer growth, researchers discovered oscillations in the intensity of the reflected RHEED beam, and that the period of the oscillation corresponded to the addition of exactly one monolayer to the film. This discovery led to rapid implementation of RHEED systems for thinfilm growth, although largely restricted to semiconductor growth via molecular beam epitaxy due to the low pressures (< 10 −5 mbar) required to use RHEED. 4,5The landscape changed again nearly two decades ago with the development of high-pressure RHEED systems, allowing RHEED systems to operate at pressures as high as 1 mbar.6-8 Since this discovery, in situ RHEED characterization has become nearly ubiquitous in thinfilm growth systems, and it has been successfully implemented in a variety of growth techniques in addition to molecular beam epitaxy, such as sputtering 9 and pulsed laser deposition (PLD). 10Researchers have worked to understand RHEED intensity oscillations via a variety of different methods. In principle, the complete picture can only be understood using dynamical diffraction theories, 11,12 which often have to be modified for complicated growth conditions (e.g., including variations in the scattering potential 13 or small terrace sizes 14 ). Before resorting to a full model, it often suffices to describe the RHEED intensity oscillations as the interference between two layers via a kinematic scattering approximation, 15,16 though this model can become more complex when multiple layers are included 17 or other diffraction features such as Kikuchi lines are included.18 The other often-used simplification is the step ...

An efficient and reliable growth method for epitaxial complex oxide films by molecular beam epitaxy

“…In the growth of SrTiO 3 films using a shuttered method by molecular beam epitaxy (MBE), SrO and TiO 2 monolayers are deposited alternatively and the film is grown in a layer-by-layer manner. Typically, the RHEED intensity increases as the growth of SrO layer and decreases as the growth of TiO 2 layer [5, 6] although opposite phenomena were also reported [15]. During the calibration process, the precise shutter times of Sr and Ti sources for the deposition of full SrO and TiO 2 monolayers are obtained by stabilizing the RHEED intensity oscillation till it shows no clear amplitude change or overall intensity drift.…”

“…Using these calibrated shutter times, high quality SrTiO 3 films and SrTiO 3 based superlattices and interfaces can thus be grown [5][6][7][8]. However, the RHEED oscillation patterns of complex oxides are very complicated and minor variations of the electron beam incident angle or shutter times can result in significant deviations [5,6,15,16]. Therefore, it is very challenging and time consuming to establish a stable RHEED oscillation pattern in calibrating the growth parameters of complex oxides, hindering the growth of more sophisticated oxide heterostructures and interfaces.…”

Calculations of RHEED and RHEPD Rocking Curves for Growing Surfaces of Germanium

“…It seems that this observation may be helpful in the future for researchers who prepare thin films. This is because the precise determination of the time when the layer is completed is very important for technology of thin film growth [9].…”

Interfacial B-site atomic configuration in polar (111) and non-polar (001) SrIrO3/SrTiO3 heterostructures

“…2(a), the following microscopy and x-ray work showed the thickness of this layer to be 4 bilayers. It has been reported that discrepancies can exist between determining thicknesses from RHEED oscillations and other thickness methods, 26 so we attribute our sub-bilayer thickness discrepancy to this fact, and we therefore restrict our quantitative analysis hereafter to the substrate/film interfaces.…”