Single-photon laser ionization time-of-flight mass spectroscopy detection in molecular-beam epitaxy: application to...

Single-photon laser ionization time-of-flight mass spectroscopy (TOF-MS) is used to monitor fluxes of As(4), As(2), and Ga, species that are important in molecular-beam epitaxy of GaAs. With this technique, fluxes of multiple chemical species above a substrate can be measured noninvasively and in real time during conventional molecular-beam epitaxy. Additionally, the geometry of the single-photon ionization TOF-MS permits simultaneous film-growth monitoring by using techniques such as reflection highenergyelectron diffraction (RHEED). Here gas-phase arsenic and gallium beams are ionized by a single 118-nm (10.5-eV) photon and detected with a TOF-MS. The 118-nm photons are produced by frequency tripling 355-nm light from a pulsed Nd:YAG laser in Xe. With single-photon ionization, less than 0.4% ofthe As(4)(+) signal fragments to As(2)(+). Neither As(4)(+) nor As(2)(+) fragments to As(+) at 118 nm. The relative ionization probability of As(4)/As(2) at 118 nm is approximately 4:1. This technique promises to be a powerful tool for analyzing most III-V and II-VI molecular-beam epitaxy growth species.