Projects / Programmes
Ultrafast memory devices by molecular beam epitaxy
| Code |
Science |
Field |
Subfield |
| 1.02.01 |
Natural sciences and mathematics |
Physics |
Physics of condesed matter |
| Code |
Science |
Field |
| P002 |
Natural sciences and mathematics |
Physics |
| Code |
Science |
Field |
| 1.03 |
Natural Sciences |
Physical sciences |
TaS2, hidden quantum state, non-ergodic phase transitions, charge density wave, molecular beam epitaxy, tunneling microscopy, memory devices
Researchers (6)
Organisations (2)
Abstract
We propose to investigate the dynamical properties of the 'hidden' quantum state in 1T-TaS2 thin films. The investigation is inspired by the possible application to ultrafast memory devices, and if successful, could lead to a breakthrough for recording and storing information – as a faster and less energy consuming technology compared with other candidates for next-generation nonvolatile memory technologies such as PCM, STT-MRAM, RRAM (memristors).
A hidden state is a quantum state of matter that cannot be reached under ergodic conditions and, therefore does not match any of the known thermodynamic phases of the material. In 2014, as part of an ERC AdG project, our group at JSI reported the existence of such a state in the layered transition metal dichalcogenide (TMD) 1T-TaS2 crystal below 195 K (Stojchevska et al., Science, 2014). This has lead to the discovery of ultrafast switching between the ground state and the hidden state both optically (35 fs) and electrically (30 ps) (Vaskivskyi et al., Science Advances, 2015). Both are currently world records. Importantly, in both cases the switching is associated with a large change in electrical resistance, which makes it of great interest for ultrafast memory devices.
This simple reversible switching between the two states, that may as well be dubbed '0' and '1', can serve as a foundation for the construction of a memory element, provided some important developments are made, particularly increased operating temperature and the demonstration of thin-film technology based devices. The first measurements on an unoptimized memory device have shown that a switch of a single bit would require an energy of about 0.25 pJ, which is an order of magnitude better than phase change memory (PCM) and comparable to STT-MRAM. However, in order to really make a breakthrough, we propose to investigate the processes leading to device defects, size, energy consumption, and increase the operating temperature. To achieve this we must replace the current lab-scale exfoliated crystalline samples with epitaxially grown 1T-TaS2 thin films.
The infrastructure for this research has already been established, and earlier experiments, MBE growth of TaS2 and electrical switching along the c axis have been shown to work. For the synthesis of 1T-TaS2 thin films we will use a molecular beam epitaxy (MBE) system dedicated to growing TMDs (TaS2, MoS2, MoSe2, etc.), that was purchased for this purpose in 2012. Importantly, we also have access to many facilities with modern characterization equipment (AFM, XRD, SEM, TEM, ellipsometry) and nanolithographic processes for mounting the electrodes. Moreover, we have a state-of-the-art four-probe STM (Omicron LT Nanoprobe), which will enable us to monitor the changes in the charge density wave during the switching to the hidden state.
The focus of the project will be to manufacture thin film 1T-TaS2 c-axis devices in a crossbar configuration. This will allow the investigation of fundamental processes, such as in-plane and c-axis charge ordering, which is currently a hot research topic, and investigation of fundamental processes important for device performance. For application purposes down the road, this research will allow the development of large scale crossbar circuits and miniaturization to the 10-nanometer size, significantly decreasing the energy per bit, and vastly increasing the surface density of memory elements. The effect of substrate strain – which is known to influence Tc – will also be investigated.
Significance for science
The project results improve understanding of the hidden quantum state in dichalcogenides and thus indirectly help to elucidate the mechanism of similar phenomena also in other materials. A good knowledge of the mechanism of switching between different states of matter is the drive behind establishing the new generation of computer memory devices, which is important for the development of a new generation of cryogenic high-performance computing and on-chip memory element integration in quantum computers. Science more directly benefits from our findings during the search for best conditions for epitaxial growth of 1T-TaS2 polytype. These improve the understanding of the TaS2 phase diagram as well as the effect of different substrates on epitaxial or van-der-Waals epitaxial growth, as seemingly small differences produce substantially different results.
Significance for the country
The project contributes to the development of Slovenia as it helps keep up with the rest of the world in the field of hidden state research on one side, and promises a substantial technological breakthrough should the devices become small, fast and efficient enough. Members of the project group have organized or helped organize several international conferences on topics related to the project: - Flatlands beyond Graphene 2106 conference on Bled, Slovenia (5.-8. July 2016) covered also transition metal dichalcogenides, - two Nonequilibrium Phenomena in Quantum Systems conferences on Krvavec, Slovenia (17.-21. December 2016 and 17.-20. December 2017) have covered hidden quantum states. This helped us to reaffirm our role on the world map of related fields.
Most important scientific results
Final report
Most important socioeconomically and culturally relevant results
Interim report,
final report
