Abstract
Atomic defects and their dynamics play a vital role in controlling the behavior of non-volatile phase change memory materials used in advanced optical storage devices. Synthesis and structural analysis by XRD and Raman spectroscopy on α-GeTe single crystal with different sizes are reported. The spectroscopic measurements on micron and nano sized α-GeTe single crystal reveal the evolution of phonon confinement with crystal sizes of few hundred nanometers. The characteristic vibrational modes of bulk α-GeTe structure are found to downshift and asymmetrically broaden to lower frequency with decreasing the single crystal size. We attribute the observed downshift of Raman lines in α-GeTe is largely due to the presence of high concentration of atomic vacancies. The crystal size and temperature dependent Raman spectra provide explicitly the dynamics of vacancies on optical phonon confinement in α-GeTe structure. Thus, the observed large concentration of vacancies and their size dependency might influence the phase change phenomenon in GeTe based alloys.
Highlights
Phase-change random-access memory (PCRAM) is an emerging class of information storage technology based on the reversible switching between the amorphous and crystalline phase of the material.[1,2] Currently, the materials of compositions used commercially for the optical storage devices are located on the GeTe-Sb2Te3 pseudo-binary tie-line applied in non-volatile memories in the form of thin films with thickness range of nanometer scale
Geetanjali Kalra and Sevi Murugavela Department of Physics and Astrophysics, University of Delhi, Delhi – 110007, India (Received 5 February 2015; accepted 8 April 2015; published online 16 April 2015). Atomic defects and their dynamics play a vital role in controlling the behavior of nonvolatile phase change memory materials used in advanced optical storage devices
The Rietveld analysis shows that the obtained x-ray diffraction (XRD) pattern and the refinement converged with good statistics (JCPDS 47-1079; space group R3m)
Summary
Phase-change random-access memory (PCRAM) is an emerging class of information storage technology based on the reversible switching between the amorphous and crystalline phase of the material.[1,2] Currently, the materials of compositions used commercially for the optical storage devices are located on the GeTe-Sb2Te3 pseudo-binary tie-line applied in non-volatile memories in the form of thin films with thickness range of nanometer scale. Different theoretical groups have adopted the combined density functional and molecular dynamic simulation studies on GeTe and Ge-Sb-Te (GST) alloys to uncover the complete information about the dynamics of these defects and their interrelation with phase change properties.[10,11] In order to understand the nature of defects between the two states of PCM based materials, an experimental effort has been made by using positron lifetime spectroscopy on Ge-Te alloys.[12] These studies elucidated with varying compositions, the associated defect are multi vacancies or vacancy clusters (nanoscale voids) rather simple mono vacancies or dangling bond defects in Ge-Te glass and crystal Both the size of cavity and concentration of defects play an important role in the fast reversible phase transition process. The temperature and size dependent Raman spectra provide unambiguously the dynamics of defects/vacancies on optical phonon confinement in α-GeTe structure
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