Photo-Oxidation and the Absence of Photodarkening in Ge 2 Sb 2 Te 5 Phase Change Material
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0918-H02-04
Photo-Oxidation and the Absence of Photodarkening in Ge2Sb2Te5 Phase Change Material Bong-Sub Lee1,2, Ying Xiao1,2, Stephen G. Bishop2,3, John R. Abelson1,2, Simone Raoux4, Vaughn R. Deline4, Min-Ho Kwon5, Ki-Bum Kim5, Byung-ki Cheong6, Heng Li7, and P. Craig Taylor7 1 Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, 61801 2 The Coordinated Sciences Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL, 61801 3 Department of Electrical and Computer Engineering, University of Illinois at UrbanaChampaign, Urbana, IL, 61801 4 IBM Almaden Research Center, San Jose, CA, 95120 5 School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Korea, Republic of 6 Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul, 136791, Korea, Republic of 7 Physics Department, Colorado School of Mines, Golden, Co, 80401
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 GexSb20-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. INTRODUCTION Phase change materials like Ge2Sb2Te5 are widely investigated as the active media for rewriteable DVDs and phase-change random access memories [1]. The operation of those devices is based on the rapid transformation between amorphous and crystalline states. This phase change in a rewriteable optical disc is optically induced by an appropriate laser pulse, but this is still a thermally-driven process. Many traditional chalcogenides display purely photo-
induced effects, including photo-oxidation, photodarkening, photo-bleaching, and many others [2]. However, little has been reported on possible non-thermal photo-induced effects of the GeSb-Te system. Recently photodarkening was reported in compositions from the GexSb20-xTe80 alloy system [3,4], but we address several problems wit
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