Polysilicon Memory Switching: Electrothermal-Induced Order

Polysilicon memory switching is demonstrated in vertical n-i-p diodes with TiN contacts and with feature sizes of 150 nm. An increase of more than three orders of magnitude of the forward current at +2 V is achieved after the application of a +8 V programming pulse. A physical change in the polysilicon is confirmed by different wet-etch rates of unprogrammed and programmed diodes. By comparing the ideality factors of the TiN-contacted diodes before and after programming, it is shown that the programming pulse increases the degree of order in the polysilicon. The mechanism by which the programming pulse increases order is electrothermal heating of the Si. When the metal contact to the polysilicon is changed from TiN to TiSi₂, memory switching is eliminated. The TiSi₂ contact seeds the crystallization of more ordered polysilicon during device fabrication compared to crystallization with TiN contacts, as shown by transmission electron microscopy