In situ real time monitoring of thickness and composition in MBE using alpha particle energy loss

The alpha-particle energy loss method (AEL) has been implemented in situ to monitor film thickness and composition during growth of GaAs, InP and LaF3 , based materials by molecular beam epitaxy (MBE), In the AEL method, a Th- 228 source is used to recoil implant a 5 mm diameter region of the surface of the wafers with the alpha-emitter daughter isotope Ra-224 prior to growt h. The implanted nuclei decay with a half life of 3.7 days through a sequen ce of daughters which emit alpha particles at different energies. Depositio n on the surface causes the emission lines to be shifted to lower energies due to energy loss in the him. For substrates marked with a low activity (s imilar to 30 kBq: similar to activity of smoke detectors) we are able to me asure film thickness with +/- 6 nm uncertainty and growth rate with +/- 0.0 1 nm/s uncertainty in real time. By measuring the relative growth rates of the different materials, AEL also allows us to infer the composition of a t ernary laver Blm as weil as the sticking coefficients rates directly at dif ferent growth temperatures. (C) 1999 Published by Elsevier Science B.V. All rights reserved.