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A reconsideration of the thermodynamics of phase‐change switching
Author(s) -
Tominaga Junji,
Wang Xiaomin,
Kolobov Alexander V.,
Fons Paul
Publication year - 2012
Publication title -
physica status solidi (b)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.51
H-Index - 109
eISSN - 1521-3951
pISSN - 0370-1972
DOI - 10.1002/pssb.201200350
Subject(s) - thermodynamics , materials science , entropy (arrow of time) , alloy , phase change memory , electrical resistivity and conductivity , statistical physics , thermodynamic free energy , phase transition , thermal , phase change , condensed matter physics , metallurgy , physics , quantum mechanics
Phase‐change random access memory (PCRAM) holds great promise as a future non‐volatile memory technology. All PCRAM devices have an inextricable relationship with thermodynamics. To make a memory cell crystallize or amorphize by injecting and removing thermal energy as a repetitive memory cycle results in the generation of entropy. In the study and development of PCRAM, the entropic part in the total energy has been neglected in the thermo‐dynamical system. In this paper, we discuss entropic energy losses through simple calculations based on the Sackur–Tetrode equation and compare the results with those obtained by first‐principles computer simulations. In addition, using a structure specific entropy generation model, we verify how much input energy is wasted in phase‐change memory cells fabricated with a Ge 2 Sb 2 Te 5 alloy‐layer or a simulation optimized [(GeTe) 2 (Sb 2 Te 3 ) 4 ] 7 multilayer structure. According to a detailed analysis, the entropy derived from application of the Sackur–Tetrode equation is in good agreement with results derived from first‐principles simulations. Our experimental device fabricated using an entropy‐controlled phase‐change design resulted in significant efficiency improvements. Relative to a Ge 2 Sb 2 Te 5 alloy‐layer device, the new multilayer device exhibited a 82% improvement in energy efficiency for a transition from the high‐ to the low‐resistivity state and with a 64% improvement in efficiency for a transition from the low‐ to the high‐resistivity state. This new entropic‐energy‐loss optimized device was able to return to the high‐resistivity state at approximately the same current as that required for the low‐resistivity state of the alloy device.