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HH9.01 - Theoretical and Experimental Understanding of Charge-Injection GeTe/Sb2Te3 Superlattice Phase Change Memory 
April 24, 2014   1:30pm - 2:00pm

The needs for storing and utilizing ‘big data’ are growing, and resistive switching non-volatile memory (NVM) has been widely investigated for replacing flash memory. A charge-injection GeTe/Sb2Te3 superlattice (SL) phase change memory (PCM) is proposed as a candidate NVM. Charge-injection PCM is “non-melting”, and its SET and RESET conditions differ from those of conventional Joule-heating (“melting”) PCM. Non-melting PCM operates on the basis of Ge atomic movement. Ge’s 4-fold state is the RESET state, a high resistance state (HRS), and the 6-fold state is the SET state, a low resistance state (LRS). Non-melting PCM’s theoretical programming energy was calculated to be less than that of melting PCM. First principle calculations verified that the difference in resistance was caused by the differences in local DOS and the transmission coefficients of HRS and LRS. In the RESET state, electrical conduction channels are disrupted by the isolation of GeTe layers from Sb2Te3 layers, resulting in HRS. First principle calculations also showed that the movement of Ge was driven by charge injection. The transition from LRS to HRS must be driven by electron injection, and the transition from HRS to LRS requires electron removal or hole generation. Previous works showed experimental evidence of non-melting resistance changes. Analysis by TEM showed the structural stability of GeTe/Sb2Te3 SL after an endurance of 1M cycles. A thermal analysis showed that the transition of GeTe/Sb2Te3 SL was different from the fcc to hcp transition of GeSbTe. Charge injection enhancement was supported by the observation of negative resistance states (NRS) (electron removal or hole injection) in SET operations. A DC sweep RESET also proved the charge injection enhancement since the RESET of melting PCM was only possible with an AC pulse. The stacking films of GeTe/Sb2Te3 must be carefully deposited for acquiring true performance of an accurate SL. Previous work with the optimized deposition technique showed that RESET current density was less than 10 MA/cm2, and the RESET voltage applied to the SL film is less than 0.5 V. Furthermore, a high quality GeTe/Sb2Te3 SL PCM film exhibited a RESET current of 70 uA and a SET speed of 10 ns when the contact electrode was 50 nm in diameter. These devices exhibited an endurance of 100 M cycles and multi-level cell (MLC) capabilities. The MLC operations were investigated by observing the NRS.In conclusion, this paper describes the current status of theoretical and experimental understanding of charge-injection GeTe/Sb2Te3 PCM. First principle calculations demonstrate the charge-injection enhanced Ge atom movement. Experimental results prove that non-melting resistance change and high-quality GeTe/Sb2Te3 films enable low-voltage (RESET < 0.5 V) and fast (SET = 10 ns) operations. An endurance of 100M cycles and MLC capabilities are also achieved.

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Fred Kavli Distinguished Lectureship in Nanoscience
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