Effect of as and se substitution on the phase change properties of gete based alloys

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PhD; 2022; Physics

Phase-change memory (PCM) is a key enabling technology for non-volatile electrical data storage at the nanometer scale. A PCM device consists of a small active volume of phase-change material between two electrodes. In PCM, data are stored using the electrical resistance contrast between a high-conductive crystalline phase and a low-conductive amorphous phase. An appealing attribute of PCM is that the stored data is retained for a very long time (typically 10 years at room temperature). This property could enable PCM to be used for non-volatile storage such as Flash and hard disk drives, while operating almost as fast as high-performance volatile memory such as dynamic random-access memory (DRAM). Primarily, chalcogenide alloys are used as an active element in PCM devices. The device performance relies on the structural variations, thus understanding the structural modulations with variations in composition is of great importance. Current work is focused on GeTe-based alloys Ge2Sb2−xAsxTe5 ( 0 ≤ x ≤ 2.0) and GeTe1−xSex ( 0 ≤ x ≤ 1.0). The objective is to study the effect of the substitution of smaller atoms in place of larger atoms on the structural and phase change properties. The substitution of As and Se is expected to influence the ionicity and hybridization capability of GeTe based alloys to a larger extent as predicted by Littlewoods ionicity - hybridization map for group IV-VI alloys. The As substituted for Sb in Ge2Sb2Te5 (GST) samples i.e. Ge2Sb2−xAsxTe5 , crystallize at higher temperatures compared to the parent GST. During the phase change, a direct transition from amorphous to the stable hexagonal structure for x > 1.0 has been observed. A distinct two-step transition in Sb rich samples and a single step transition for As rich samples are observed in R-T measurements with a high contrast in electrical resistivity. A composition-dependent Metal-Insulator Transition (MIT) is also observed in these samples. In GeTe1−xSex alloys, it is observed that with increasing Se substitution, the structure transforms from rhombohedral to orthorhombic, supported by Rietveld refinement analysis. In GeTe1−xSex thin films the transition temperature for amorphous to crystalline phase shows an increasing trend with the Se substitution. The contrast in electrical resistivity between the amorphous and crystalline states is 104 for GeTe, and with the Se substitution, the contrast increases considerably to 106 for GeTe0.5Se0.5. Devices fabricated with thin films show that the threshold current decreases with the Se substitution indicating a reduction in the power required for WRITE operation. The present study shows that the crystalline structure, resistivity, optical bandgap, transition temperature, data retention and threshold voltage can be effectively controlled and tuned by the substitution of Se for Te in GeTe and As for Sb in Ge2Sb2Te5, which is conducive for phase change memory applications.