Structural Evolution of Amorphous Ge 2 Sb 2 Te 5 Thin Films in the Transition to the Crystalline Phase
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1072-G02-05
Structural Evolution of Amorphous Ge2Sb2Te5 Thin Films in the Transition to the Crystalline Phase Riccardo De Bastiani1, Alberto Maria Piro1, Maria Grazia Grimaldi1, Emanuele Rimini2, Giuseppe Baratta3, and Giovanni Strazzulla3 1 Dipartimento di Fisica ed Astronomia, Università di Catania and MATIS CNR-INFM, 64 via S. Sofia, Catania, 95123, Italy 2 Dipartimento di Fisica ed Astronomia, Università di Catania and CNR-IMM, 64 via S. Sofia, Catania, 95123, Italy 3 Osservatorio Astrofisico di Catania, INAF, via S. Sofia 78, Catania, 95123, Italy ABSTRACT The crystallization kinetics of as-deposited amorphous Ge2Sb2Te5 thin films has been measured by in situ time resolved reflectivity. X-ray diffraction and Raman scattering analyses of partially transformed samples allowed to correlate the evolution of the transition to the structural modification in the long and short range configuration. The experimental results evidenced that during the early stages of crystallization there is a reduction of Ge-Te tetrahedral bonds, characteristics of the Ge coordination in amorphous Ge2Sb2Te5 films. INTRODUCTION The Ge–Sb–Te (GST) system, among chalcogenides semiconductors, has been widely used as storage media for optical memory disks and recently has been proposed for phase change memory (PCM) devices [1,2]. A recording film made of GST can be switched rapidly and reversibly between the crystalline phase and the amorphous phase by laser or electrical current pulses of suitable intensities and durations. Although several studies have focused on kinetics of transformations, very little is known about the dynamics and the local structural evolution of the amorphous alloy to the crystalline phase. In this work, the kinetics of the amorphous to crystal (a-c) transformation was measured during isothermal annealing by means of in situ time resolved reflectivity measurements (TRR) taking advantage of the different optical properties [3] between the two phases. These analyses have been related to the structural properties by means of X-ray diffraction and Raman scattering, in order to highlight the progressive changes occurring in the amorphous network. The experimental results reveal an alterations of the local order with a change in the coordination number of Ge atoms. The variation of the Raman signal visible in the early stage of the crystallization process is consistent with a reduction of Ge-Te tetrahedral bonds. EXPERIMENTAL Ge2Sb2Te5 films, 70 nm thick, were prepared by R.F. sputter deposition at room temperature (R.T.), from a stoichiometric target, over oxidized Si wafers. The crystallization of amorphous films was followed by TRR measurements using a low power He-Ne laser probe (P=5 mW λ= 633 nm) during annealing at 122 °C, at a pressure of ~10-6 mbar. The sample temperature was kept constant within 0.1 K. The structure of the films was examined by X-ray diffraction (XRD) using a SIEMENS D5005 diffractometer operating with Cu Kα (λ=0.15406 nm) radiation. To
maximize the signal and reduce background the measurements
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