Real-time control of MBE film growth using Reflection High Energy Electron Diffraction (RHEED) oscillations allows precise layer-by-layer growth using real-time surface structure information. Many complex functional oxides of device importance, however, require tight stoichiometry control. RHEED- Total Reflection Angle X-ray Spectroscopy (RHEED-TRAXS) can provide real-time chemical information and thus has the potential to achieve real time stoichiometry control. When incident RHEED electrons with energy in the range of 12-20 keV graze the sample surface at approximately a 2° angle, characteristic x-rays which are representative of the film surface stoichiometry are emitted. By measuring the x-rays at or close to their total reflection angle, RHEED-TRAXS is reported to probe only the top 20Å of group V elements [1].

The goal of the RHEED-TRAXS chamber modification design is to incorporate a non-UHV compatible x-ray detector into the chamber, and ensure highly accurate detector positioning within 0.01° through a 4° angle range. For real time operation, a shielding strategy that is transparent to x-rays must be developed to protect the detector from fouling during MBE processing. As a high volume of x-rays are excited by the RHEED electrons, collimation is necessary to control the incoming x-ray flux and avoid detector overload.

Our system uses a Nor-Cal PMXY-600-400-2 ±1 inch X-Y stage to achieve 0° to 4.65° angular positioning of the detector. Differential pumping with a Leybold TURBOVAC 50 L/s turbo molecular pump is used to achieve UHV compatibility. Due to limited chamber space, off-the-shelf shielding options such as shutters would cut down the detector movement and thus reduce the maximum detection angle by 50%. To avoid this, we combined detector fouling protection and collimation by mounting the detector with a custom made half-nipple covered with a removable aperture cap which provides both collimation and beryllium foil shielding support. The ability to remove the aperture cap allows the Be foil to be replaced when needed and also allows the flexibility control net x-ray flux by changing the aperture size. The system has been used for real-time study of thin film deposition by MBE, and the results will be presented.

1. Braun, W. and K.H. Ploog, Real-time surface composition and roughness analysis in MBE using RHEED-induced X-ray fluorescence. Journal of Crystal Growth, 2003. 251(1-4): p. 68-72.