LONDON – Researchers at the University of Pennsylvania (Philadephia, Pennsylvania) led by Professor I-Wei Chen have developed a resistive-switching memory device based on silicon dioxide.
The key to the memory is the inclusion of atomically dispersed platinum on a random basis, the variation of the distances between the platinum atoms and their relationship to a quantum mechanical property known as the electron localization length.
Silicon dioxide is generally considered an insulator but in films of nanometer-scale thickness and random dispersion of platinum atoms it can be made to display a memory effect. The development of a resistive-switching non-volatile memory based on the CMOS-compatible material offers the prospect of an ReRAM memory with useful properties but a simpler material structure to some alternative proposals based on metal-oxide films.
The University of Pennsylvania research has been reported in a number of learned papers on different aspects of the technology and on models of the behavior during 2011. The behavior is ascribed to the tunneling of electrons between atomically dispersed platinum in amorphous silicon dioxide thin films between platinum and molybdenum electrodes. It is said that the trapped electrons and the local Coulomb barriers which can also be created can "choke off" the electron passage in the nearby nanometallic paths, making non-volatile memory possible. Nanoparticles of platinum, which may not even be metallic according to their optical responses, are sometimes present at higher platinum concentrations but are not required for the operation of the device.
Professor Chen is due to present an update on the technology and
comparison with other ReRAM systems at a one-day symposium on emerging
non-volatile memory technologies due to be held Friday April 6 in Santa
The electron transport system through the material is generally applicable to random metal-insulator mixes and has been demonstrated in silicon oxide and silicon nitride glasses with platinum inclusions as well as in perovskite transition metal oxides.
Professor Chen told EE Times that the materials are quite easy to make by the use of co-sputtering on to a substrate, although the exact composition together with final film thicknesses are significant in tuning the memory effect and voltage scheme. The research group has made individual devices with sizes down to 20-micron by 20-micron with film thicknesses of between 5-nanometer and 30-nm to 40-nm. "I see no evidence why it should not scale," Professor Chen said.
Having read his student's thesis, I think what they got is a standard filamentary RRAM, with pre-defined filaments, marked by Pt and pores characteristic of co-sputtered films. Filaments can consist of charge-trapping defects of course. But the defects they show are nm-scale rather than atomic.
I understand that there are characteristics of resistance switching behavior and V-I-R curves that can be used to distinguish between filamentary and non-filamentary behavior. These are things which I did not go into here.
I suggest that those who are interested in more detail get along to the TI Conference Center at
2900 Semiconductor Drive, Santa Clara, California on Friday, April 6. OR try and get hold of Professor Chen's slide set from the event, perhaps from the IEEE.
Resistron-If you fabricate a device with a volume fraction of Pt in the range 30% close to the value of the percolation limit, (i.e 33%), where the probability of a continuous path becomes 1, then there will be in 3D many possible similar discontinuous shorter paths. I think it is wrong to describe this as a standard filamentary RRAM, whatever that is. Sure you could lump these under your definition of "predefined filaments". The results in work already published suggest a good correlation of resistance with area, suggesting the insulating state is NOT single filament like many of the reported RRAM and ReRAMs. I think you need to understand the role of the spreading resistance in the values reported by Prof Wei and colleagues for the conducting state, without that you might conclude the results for the conducting state are substantially independent of area and are therefore for a single conducting filament.
When operating near the 33% volume fraction limit there is also a probability of many paths where there are short continuous chains of Pt atoms that do not reach the distance between the electrodes, so they might be considered as nano metallic strands, there can still be traps between those chains that allow the device to work as reported, even if the strands become negatively charged the will offer some form of Coulombic repulsion.
I would suggest if you have the opportunity you follow Peter Clarke's advice here above and attend Prof Wei's presentation. Also read the report of the work from on Rice NV RAM, reported in EETimes today-that is clearly a single filament device. Your standard filamentary RRAM if you like.
I see the distinction you are going after, a distributed vs. more focused filament path. But the way to show this is to compare R vs. size. If R is weakly dependent on size, as was shown in the thesis, it is more likely to be at least not so evenly distributed as would be imagined. I'll check this against the foils from the event, if they are available soon.
This paper and 11 other exciting talks are scheduled for the IEEE San Francisco Bay Area Nanotechnology Council's 8th Annual Full Day Symposium -"Emerging Non-volatile Memory Technology" on April 6th. Register at www.ieee.org/nano where you can find the abstracts for the other papers
Some more details from this group are at Adv. Mat. 23, 3847-3852 (2011). It largely reflects what has been reported here. However, capacitance measurements are missing which should be normally used to evaluate the charge trapping.