<title>Raman characterization of Te inclusions on CdTe surfaces using visible lasers</title>

Abstract

In this work, we report the behaviour of the CdTe surface studied by micro-Raman. The micro-Raman measurements were performed with different laser lines in regions of both transparency and opacity, using infrared and visible lasers respectively. Visible radiation leads to the formation of Te on the irradiated surface compromising any conclusion about the presence of Te inclusions in CdTe 2,3,4, a subject of controversy. We show that the power density and local temperature is directly connected with the intensity of Te modes on the irradiated surface. In this context, the increase of excitation power and exposure time of the sample surface accelerates the process. We also show that the process is too fast to allow, in most cases, for real time characterisation. Nevertheless in this work, it was possible to observe the growth of the Te optical modes during the measurement process by rigorous optical control of the beam characteristics and optical density. This result is the same in all random spectra obtained from different sample locations. We noticed that previously other authors5 irradiated CdTe and HgCdTe films with a nanosecond pulsed Ruby laser and attributed the decrease in electrical resistivity to the growth of a Te layer on the surface of the CdTe. Our work is a direct evidence for this result.

Micro‐Raman study of laser damage in CdTe

“…This means there is a significant formation of Te aggregates at the surface (optical damage). As we have shown in [4], the damage at these optical densities increases with exposure time for a while and then stabilizes. The spectra shown were recorded after a 30 min.…”

“…The observation of Te optical modes on the surface of CdTe using micro-Raman methods has been reported by several authors [1][2][3][4]. However the origin of these modes is a subject of controversy.…”

“…The authors concluded that the Te modes were due to small Te aggregates related with poor stoichiometry and disappeared gradually with the increase of cadmium content. In a previous work [4] we showed by micro-Raman that visible radiation leads to the formation of Te aggregates on the irradiated surface, compromising any conclusion about Te inclusions on CdTe surface, and speculated that heating by laser light could break chemical bonds and produce Cd evaporation, thus leaving an excess of Te on the surface.…”

Laser processing for thin film chalcogenide photovoltaics: a review and prospectus

“…15,[41][42][43][44][45] This was demonstrated by Soares et al using micro-Raman spectroscopy, where a distinct spectral line from elemental Te was seen to grow as a result of increasing intensity of the Raman excitation laser source. 44,45 An example of this phenomenon is shown in Fig. 5 from our own micro-Raman measurements where an asdeposited polycrystalline CdTe sample is continuously exposed to the excitation light with spectra taken every 5 min.…”