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

The α-particle energy loss method (AEL) has been implemented in situ to monitor film thickness and composition during growth of GaAs, InP and LaF 3 based materials by molecular beam epitaxy (MBE). In the AEL method, a 228 Th source is used to recoil implant a 5 mm diameter region of the surface of the wafers with the α-emitter daughter isotope 224 Ra prior to growth. The implanted nuclei decay with a half life of 3.7 days through a sequence of daughters which emit alpha particles at different energies. Deposition on the surface causes the emission lines to be shifted to lower energies due to energy loss in the film. For substrates marked with a low activity (∼ 30 kBq; similar to activity of smoke detectors) we are able to measure film thickness with ± 6 nm uncertainty and growth rate with ± 0.01 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 ternary laver film as well as the sticking coefficients rates directly at different growth temperatures.