| Memory devices have gained growing popularity with the advance of integrated circuit industry in recent years.Phase-change random access memory(PCRAM)is one of the most promising next-generation memory technologies owing to the performance of nonvolatility,high data storage density,fast operation speed and long duration.PCRAM works in a simple way:phase-change materials transforms between its crystalline and amorphous phase reversibly according to electric pulses;as a consequence,the electrical resistivity would change back and forth,which represents 0 and 1.GeTe is a narrow-gap semiconductor and utilized as a phase-change material for its unique behavior of fast reversible phase transition between the amorphous and crystalline states,together with large physical properties contrast.High stability of the amorphous phase would benefit data retention at ambient temperature,while fast crystallization speed is required for the seek of rapid write and erase operations.Tremendous efforts have been devoted to tuning the stability of amorphous GeTe using transition metals.Cu would enhance the data retention of GeTe,while W could benefit both data retention and writing speed.However,the mechanism of transition metals tuning is ambiguous.In this thesis,the local structure,chemical bonding and phase transition process of Cu and W doped GeTe were demonstrated through first principles calculations and ab initio molecular dynamics(AIMD)simulations.The data were analyzed in terms of pair correlation functions,coordination numbers,bond angle distributions,ring statistics,charge density difference,electron localization functions and mean square displacements.The results would provide valuable clues for understanding the amorphous structures and revealing the doping mechanisms.Meanwhile,the intriguing structures,mechanical properties,magnetic and transportation properties of W doped cubic GeTe were investigated using first principles calculations.The results would provide valuable insights for developing new phase-change materials.Chapter 3 of this thesis had demonstrated enlarged differences between local structures of amorphous GeTe and Cu doped GeTe.Cu would increase the proportions of tetrahedally coordinated Ge as well as 3-fold rings.This would be the reason for higher stability of the amorphous phase and enhanced data encoding performances.The amorphous structures,re-crystallization process and bonding properties of W doped GeTe have been discussed in Chapter 4.W dopants are virtually static at room temperature,and help Ge and Te withstand thermal shock through strong chemical bonding,which enhance the stability of amorphous GeTe.While W displaces quite violently at elevated temperatures,releasing Ge and Te.GeTe crystallize rapidly due to the similarity of the local structures between the two phases.The results would help reveal the underlying machanisms of W dopants.W doped cubic GeTe were discussed in Chapter 5.Abnormal contraction of the cell when larger dopants substitute smaller hosts is discovered for the first time in similar systems to the best of our knowledge.According to first principles calculations,the outer shell of W would enter the not-fully-occupied 5p orbits of Te.W-Te bonds are shorter than Ge-Te bonds,which result in a shrinked cell.This observation is beyond the scope of classic Vegard’s law,and would provide new insights into the lattice parameters of substitutional solid solutions.Besides,W would introduce intriguing properties to GeTe.W increases the concentration of delocalized electrons,transforming GeTe into a conductor.W would induce magnetic moments when dispersed into GeTe.Besides,the toughness are also enhanced by W.The observations would be beneficial to widening the applications of GeTe. |
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