Laser desorption ionization time-of-flight mass spectrometry of erbium-doped Ga-Ge-Sb-S glasses.

Rationale: Rare earth-doped sulphide glasses in the Ga-Ge-Sb-S system present radiative emissions from the visible to the middle infrared range (mid-IR) range, which are of interest for a variety of applications including (bio)-chemical optical sensing, light detection, and military counter-measures. The aim of this work was to reveal structural motifs present during the fabrication of thin films by plasma deposition techniques as such knowledge is important for the optimization of thin film growth.
Methods: The formation of clusters in plasma plume from different concentrations of erbium-doped Ga5Ge20Sb10S65 glasses (0.05, 0.1, and 0.5 wt. % of erbium) using laser (337 nm) desorption ionization (LDI) was studied by time-of-flight mass spectrometry (TOF MS) in both positive and negative ion mode. The stoichiometry of the Ga(m)Ge(n)Sb(o)S(p)(+/-) clusters was determined via isotopic envelope analysis and computer modelling.
Results: Several Ga(m)Ge(n)Sb(o)S(p)(+/-) singly charged clusters were found but, surprisingly, only four species (Sb3S4(+/-), GaSb2S(p)(+/-) (p = 4, 5), Ga3Sb2S7(+/-) ) were common to both ion modes. For the first time, species containing rare earths (GaSb2SEr(+) and GaS6 Er2(+)) were identified in the plasma formed from rare earth-doped chalcogenide glasses, directly confirming the importance of gallium presence for rare earth bonding within the glassy matrix.
Conclusions: The local structure of Ga-Ge-Sb-S glasses is at least partly different from the structure of species identified in plasma by mass spectrometry, as deduced from Raman scattering spectroscopy analysis; these glasses are mainly formed by [GeS4/2]/[GaS4/2] tetrahedra and [SbS3/2] pyramids. Extended X-ray absorption fine structure measurements show that Er(3+) ions in Ga-Ge-Sb-S glasses are surrounded by 7 sulphur atoms.
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