Photo-Oxidation and the Absence of Photodarkening in Ge₂Sb₂Te₅ Phase Change Material

Abstract

Ge₂Sb₂Te₅ is under intense investigation for phase-change memory devices, including rewriteable DVDs where optical illumination is used to switch between the glassy and crystalline states. We investigate the influence of optical irradiation on the amorphous phase. Many chalcogenide glasses display photo-oxidation, photodarkening or photo-bleaching, but little has been reported on the Ge-Sb-Te system. Using spectroscopic ellipsometry (SE) and secondary ion mass spectrometry, we determine that the samples have a strong tendency to photo-oxidize; if this is not accounted for, then the analysis of SE data appears to show photodarkening. Other authors recently reported photodarkening in nonstoichiometric Ge_xSb_20−xTe₈₀ [Pamukchieva et al., Proc. SPIE 5581, 608 (2004); Pamukchieva et al., J. Optoelectron. Adv. Mater 7, 1277 (2005)], but our analysis suggests that the changes were actually the result of photo-oxidation. The oxide has lower values of (n, k) than Ge₂Sb₂Te₅, and can be etched by hydrofluoric acid or water. Our observation of negligible photodarkening is consistent with previous work that found less photodarkening in tellurides compared with selenides or sulfides, and that an increase in the mean coordination number, here by Ge addition, further reduces the photodarkening effect.