  In this work GEM and single-hole Thick GEM structures, composed of different
coating materials, are studied. The used foils incorporate conductive layers
made of copper, aluminium, molybdenum, stainless steel, tungsten and tantalum.
The main focus of the study is the determination of the material dependence of
the formation of electrical discharges in GEM-based detectors. For this task,
discharge probability measurements are conducted with several Thick GEM samples
using a basic electronics readout chain. In addition to that, optical
spectroscopy methods are employed to study the light emitted during discharges
from the different foils. It is observed that the light spectra of GEMs include
emission lines from the conductive layer material. This indicates the presence
of the foil material in the discharge plasma after the initial spark. However,
no lines associated with the coating material are observed while studying spark
discharges induced in Thick GEMs. It is concluded that the conductive layer
material does not play a substantial role in terms of stability against primary
discharges. However, a strong material dependence is observed in the case of
secondary discharge formation, pointing to molybdenum coating as the one
providing increased stability.
