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Mechanisms of time-dependent-dielectric-breakdown (TDDB) in non-filamentary a-Si/TiO2 RRAM cell (a-VMCO) have been examined in this work, including defects generation in the grain boundary, defects clustering and different defects generation rates in a-Si and TiO2 layers. The unique feature of a bimodal Weibull distribution at low resistance state (LRS) and a single shallow slope distribution at high resistance state (HRS) cannot be explained by the above mechanisms. By using a combination of constant-voltage-stress (CVS), time-to-breakdown Weibull distribution and random-telegraph-noise (RTN) based defect profiling in devices of various sizes, layer thickness and processes, it is revealed that the defect profile is modulated when switching between HRS and LRS and the correlation of defect profile modulation with local defect generation rate can explain the difference in Weibull distributions at HRS and LRS. The transition from bimodal distribution at LRS to a single-steep- slope with thinner a-Si layer, and the good area scaling of Weibull distribution at HRS but not at LRS, can also be explained. The critical layers affecting the TDDB in a-VMCO are identified, providing useful guidance for device performance improvement.

An extensive investigation of the pre-existing and generated defects in amorphous-Si/TiO2 based non-filamentary (a-VMCO) RRAM device has been carried out in this work to identify the switching and degradation mechanisms, through a combination of random-telegraph-noise (RTN) and constant- voltage-stress (CVS) analysis. The amplitude of RTN, which leads to read instability, is also evaluated statistically at different stages of cell degradation and correlated with different defects, for the first time. It is found that the switching between low and high resistance states (LRS and HRS) are correlated with the profile modulation of pre-existing defects in the ‘defect-less’ region near the a-Si/TiO2 interface. The RTN amplitude observed at this stage is small and has a tight distribution. At longer stress times, a percolation path is formed due to defects generation, which introduces larger RTN amplitude and a significant tail in its distribution.

Selector device is critical in high-density cross-point resistive switching memory arrays for suppressing the sneak leakage current path. GexSe1-x based ovonic threshold switch (OTS) selectors have recently demonstrated strong performance with high on-state current, nonlinearity and endurance. Detailed study of its reliability is still lacking and the understanding on the responsible mechanisms is limited. In this work, for the first time, the endurance degradation mechanism of Ge-rich GexSe1-x OTS is identified. Accumulation of slow defects that remain delocalized at off-state and GeSe segregation/crystallization during cycling lead to the recoverable and non-recoverable leakage current, respectively. Most importantly, a refreshing program scheme is developed to recover and prevent the OTS degradation and the endurance can be therefore improved by more than five orders without adding additional material elements or process steps.

When strained Stranski-Krastanow islands are used as "self-assembled quantum dots," a key goal is to control the island position. Here we show that nanoscale grooves can control the nucleation of epitaxial Ge islands on Si(001), and can drive lateral motion of existing islands onto the grooves, even when the grooves are very narrow and shallow compared to the islands. A position centered on the groove minimizes energy. We use as prototype grooves the trenches which form naturally around islands. During coarsening, the shrinking islands move laterally to sit directly astride that trench. In subsequent growth, we demonstrate that islands nucleate on the "empty trenches" which remain on the surface after complete dissolution of the original islands.