SIMS analysis, under caesium bombardment, of Si in GaAs/(Al, Ga) as superlattices: Detection limit and depth resolution

Abstract

AbstractSilicon is widely used for n‐doping of GaAs/(Al, Ga)As layered structures grown by molecular beam epitaxy or metallo‐organic chemical vapour phase deposition for high speed and opto‐electronic devices, so the determination of Si doping levels in these structures is of primary importance. The secondary ion mass spectrometry technique has been revealed as a very powerful method for such a determination. But, in this particular case, some difficulties are encountered at low doping levels because of the presence of ions interfering with the main silicon isotope at mass 28. This paper deals with the search for satisfactory experimental conditions for such a silicon analysis under caesium primary ion bombardment, the main requirements being the detection limit and the depth resolution. It will be shown that AIH molecular ions are the interfering species and two approaches have been explored to avoid this interference: use of high mass resolving power facilities of a double focusing mass spectrometer, or experimentation at low vacuum conditions. A compromise is ineviatable between maximum sensitivity in the 1–2 1015 cm−3 range by detecting negative secondary ions and good depth resolution, equal to about, 40 Å, by using positive secondary ion collection. The ultimate performances of the technique are limited by the silicon surface segregation process and factors like the initial surface morphology of the samples, the flatness, and parallelism with the surface, of the crater bottoms and the beam induced roughness.