UK team moves UltraRAM to silicon substrate
The Lancaster research team, under Professor Manus Hayne, has been working on the memory – dubbed UltraRAM – for several years. The researchers published a paper in Nature Scientific Reports in June 2019 described the use of resonant tunnelling to create a barrier that can be opened and closed with the application of a small voltage. This was followed by a paper in the January 2020 edition of Transactions on Electron Devices (see UK reports progress with III-V non-volatile memory).
However, the latest paper – UltraRAM: A low-energy, high-endurance, compound-semiconductor memory on silicon published in Advanced Electronic Materials – reports on the move to manufacturing on silicon wafers, a necessary step for cost-effective commercial manufacturing.
UltraRAM combines the non-volatility of flash memory with speed, energy of efficiency of DRAM albeit at the cost of complexity in manufacturing.
The UltraRAM devices on silicon outperformed previous generations of the technology manufactured on GaAs wafers. These demonstrated extrapolated storage of at least 1,000 years and program-erase cycling endurance of at least 10 million cycles.
“UltraRAM on silicon is a huge advance for our research, overcoming very significant materials challenges of large crystalline lattice mismatch, the change from elemental to compound semiconductor and differences in thermal contraction,” said Professor Manus Hayne of the Department of Physics at Lancaster, in a statement.
To produce their devices the team used molecular beam epitaxy to deposit an AlSb nucleation layer to seed the growth of a GaSb buffer layer followed by the III-V layers that form the quantum-well memory. Fabricated single-cell memories show clear 0/1 logic-state contrast after up to 10ms of program/erase pulses of approximately 2.5 V. These devices had gate lengths of 10 micron and 20 micron.
The numbers are expected to be a lot better for UltraRAM devices made at the leading edge.
Due to the low voltages required and the low capacitance per unit area of the device compared to DRAM, logic state switching energies of 10^-17 joules are predicted for 20nm feature size UltraRAM memories, which is two and three orders of magnitude lower than DRAM and flash respectively.
The research is reported in the journal Advanced Electronic Materials.
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