Q2. What are the appealing characteristics of PCMs?
The most appealing characteristics of PCRAMs are their cyclability [13], their endurance and fast programming [14], while presenting an extremely easy scaling path [4].
Q3. What is the effect of air exposure on the crystallization of GST?
As in encapsulated films, crystallization starts with the formation of Ge grains, which further grow by dragging the Ge excess out of the matrix until its concentration is low enough to allow the GST phase to nucleate.
Q4. What is the temperature shift between heterogeneous and homogeneous GST?
The temperature shift of 50-60°C which is observed, from 330°C for the air-exposed films to 380°C for the encapsulated films, does correspond to the temperature shift reported between heterogeneous and homogeneous crystallizations of amorphous Ge.
Q5. What is the way to encapsulate the E-GST layer?
To encapsulate the E-GST layer, an ultra-thin (~14 Å) Ti-rich layer was deposited on its surface prior to TiN deposition to favor its adhesion.
Q6. What is the effect of air exposure on the crystallization of Ge-rich GST alloys?
In Ge2Sb2Te5 alloys, the selective consumption of Ge and Sb atoms due to oxidation and the stoichiometric imbalance which results not only promote the crystallization of the Te phase at low temperature but also to the partial crystallization of the GST alloy into the less rich Ge1Sb2Te4 phase [32, 40].
Q7. How long does the film stay amorphous?
The film left exposed to air fully crystallizes after only 5 minutes, while the film that was encapsulated stays amorphous when annealed at this temperature.
Q8. Where do the seeds that promote the nucleation of the Ge cubic phase originate?
Ge accumulation also in the vicinity of the SiO2/E-GST bottom interface, tend to demonstrate that the seeds that promote this heterogeneous nucleation of the Ge cubic phase involve oxygen, directly as an impurity atom or complex, or most probably through a crystalline Sb2O3 phase.
Q9. What is the origin of the lowering of T observed after air exposure?
While not as clearly demonstrated, it is reasonable to think that, also in this case, the origin of the lowering of Tχ observed after air exposure relies on the elemental redistribution, resulting from selective oxidation, and on the formation of Te-rich regionswhich are able to crystallize at low temperature [32] and provide seeds for the subsequent heterogeneous crystallization of GST phases.
Q10. What is the effect of surface oxidation on the crystallization of GST films?
This is why the complete crystallization of the films is accompanied by a massive transfer of Ge from the bulk towards the surface and a disruption of the initial stoichiometry of the film.
Q11. Where are the Te and Sb atoms accumulated?
most of the Te and Sb atoms initially present in the whole thickness of the film are now found accumulated in the last 50 nm, in the deepest part of the film, close to the bottom interface.
Q12. What is the diffraction pattern of the E-GST film?
the polycrystalline TiN layer deposited on the E-GST film is visible in the image of the encapsulated E-GST sample and detected in the associated diffraction pattern (Figure 1(b)).
Q13. What is the difference between the encapsulated and air-exposed films?
it is very probable that the 50-60°C shift of the crystallization temperature observed between the encapsulated and air-exposed films only reflects the change of nucleation mechanism of the Ge phase from homogeneous to heterogeneous nucleation.