Temperature-dependent 29Si incorporation during deposition of highly enriched 28Si films

In this study, we examine the mechanisms leading to Si-29 incorporation into highly enriched Si-28 films deposited by hyperthermal ion beams at elevated temperatures in the dilute presence of natural abundance silane (SiH4) gas. Enriched Si-28 is a critical material in the development of quantum information devices because Si-28 is free of nuclear spins that cause decoherence in a quantum system. We deposit epitaxial thin films of Si-28 enriched in situ beyond 99.999 98% Si-28 onto Si(100) using an ion-beam deposition system and seek to develop the ability to systematically vary the enrichment and measure the impact on quantum coherence. We use secondary ion mass spectrometry to measure the residual Si-29 isotope fraction in enriched samples deposited from approximate to 250 degrees C up to 800 degrees C. The Si-29 isotope fraction is found to increase from <1 x 10(-6) at the lower temperatures, up to >4 x 10(-6) at around 800 degrees C. From these data, we estimate the temperature dependence of the incorporation fraction s of SiH4, which increases sharply from about 2.9 x 10(-4) at 500 degrees C to 2.3 x 10(-2) at 800 degrees C. We determine an activation energy of 1.00(8) eV associated with the abrupt increase in incorporation and conclude that below 500 degrees C, a temperature-independent mechanism such as activation from ion collisions with adsorbed SiH4 molecules is the primary incorporation mechanism. Direct incorporation from the adsorbed state is found to be minimal.