Periodic Reporting for period 1 - TopRooT (Towards TOPological insulator-based electronic devices for ROOm Temperature operation)
Reporting period: 2023-06-01 to 2025-05-31
The increasing ubiquity of information and communications technologies (ICT) has an underlying and ever-increasing energy cost that is pointing to a scenario of unsustainability. This is particularly acute for Artificial Intelligence (AI) systems, like Large Language Models, whose fast rise in popularity is putting pressure on the energy supply chain due to the large amounts of energy required for their training and operation. These processes relay on data centres, which are expected to account for up to 3.1% of the electricity demand in Europe by 2030. This led to the recognition by the European Commission of the need to achieve climate-neutral data centres in its strategy on “Shaping Europe’s digital future”. The strategy recommends increasing energy efficiency, reusing waste energy, and using more renewable energy sources. One fundamental way of tackling the problem of energy efficiency is to radically change the paradigm of computing supporting AI, which in turn needs a new set of electronic devices and materials to overcome the limitations of current solutions. Topological insulators (TIs), a new class of emerging quantum materials with peculiar properties, could play a role in this electronics revolution. However, due to the challenges associated with the observation of their topological properties, TI research has been mostly restricted to cryogenic temperatures.
With this in mind, project TopRooT targeted the application of TIs for room-temperature electronic devices, with the objective of contributing to establish a clear path for creating and designing such devices. To this end, a crucial technology is the integration of TIs with topologically-trivial materials, such as ferromagnets (FM), which enables applications in spintronics and beyond. Therefore, the project focused on optimising TI/FM heterostructures for spin-orbit torque magnetic random access memory (SOT-MRAM), one of the possible candidates to support energy-efficient computing inspired by the brain, such as neuromorphic computing. TIs promise even higher energy efficiency than that obtained with conventional SOT materials. The objective is to find strategies to achieve an efficient interfacing of MBE-grown Bi2Se3 with relevant FM layers using conventional micro- and nanofabrication technologies and explore the integration of Bi2Se3 with nanoscale magnetic tunnel junctions.
At the end of the project, several fabrication routes to obtain Bi2Se3/CoFeB heterostructures and nanoscale magnetic tunnel junctions for integration with emerging SOT materials were explored and established. The project showed progress in the integration of TIs with ferromagnetic materials and conventional microfabrication techniques, but the transfer methods of the Bi2Se3 from the growth chamber to the subsequent deposition of the ferromagnetic layer need to be further optimized to harness the real potential of the Bi2Se3 for SOT-MRAM.
With this in mind, project TopRooT targeted the application of TIs for room-temperature electronic devices, with the objective of contributing to establish a clear path for creating and designing such devices. To this end, a crucial technology is the integration of TIs with topologically-trivial materials, such as ferromagnets (FM), which enables applications in spintronics and beyond. Therefore, the project focused on optimising TI/FM heterostructures for spin-orbit torque magnetic random access memory (SOT-MRAM), one of the possible candidates to support energy-efficient computing inspired by the brain, such as neuromorphic computing. TIs promise even higher energy efficiency than that obtained with conventional SOT materials. The objective is to find strategies to achieve an efficient interfacing of MBE-grown Bi2Se3 with relevant FM layers using conventional micro- and nanofabrication technologies and explore the integration of Bi2Se3 with nanoscale magnetic tunnel junctions.
At the end of the project, several fabrication routes to obtain Bi2Se3/CoFeB heterostructures and nanoscale magnetic tunnel junctions for integration with emerging SOT materials were explored and established. The project showed progress in the integration of TIs with ferromagnetic materials and conventional microfabrication techniques, but the transfer methods of the Bi2Se3 from the growth chamber to the subsequent deposition of the ferromagnetic layer need to be further optimized to harness the real potential of the Bi2Se3 for SOT-MRAM.
TopRooT focused on the potential of the topological insulator Bi2Se3 as spin-orbit torque (SOT) material for emerging magnetic memories known as SOT-MRAM. To study this, the project was initially divided in several work packages (WPs), related to the different stages of the project, from the growth of Bi2Se3 to the fabrication of nanoscale magnetic tunnel junctions.
WP1 was devoted to the growth of the topological insulator Bi2Se3 by molecular beam epitaxy on sapphire substrates. Different growth recipes were implemented and the resulting thin films characterized to decide which recipe would provide the best quality material to the subsequent interfacing with the ferromagnetic layers. A surface treatment of the sapphire substrate was chosen as the best approach due to a good balance of crystalline quality, electronic properties and good enough surface topography to accommodate the overlying very thin ferromagnetic layer. To explore other methods for tuning the electronic properties of Bi2Se3 thin films, ion implantation with Ca ions (p-type dopants in Bi2Se3) was employed and is currently being investigated.
In WP2, Bi2Se3/CoFeB heterostructures were successfully fabricated and test devices were patterned using conventional photolithography in a cleanroom environment. Different strategies for achieving a good interface between the Bi2Se3 and the CoFeB were explored, such as in-situ etching and capping with protection layers.
The characterization of the devices fabricated in WP2 was the focus of WP3, where two different electrical characterization techniques were employed to quantitatively measure the SOT efficiency: spin-torque ferromagnetic resonance and harmonic Hall voltage measurements. Both these methods were applied to the fabricated Bi2Se3/CoFeB heterostructures, where finite SOT efficiencies of the order of 3 % were measured, indicating the need to invest more in establishing better transfer methods.
WP4 was devoted to the integration of full magnetic tunnel junctions (MTJs) with the Bi2Se3 thin films. Fabrication of nanoscale MTJs was achieved and integration with SOT materials was explored in non-conventional architectures to allow future integration of emerging materials such as Bi2Se3.
Aligned with the project’s broader objective, a discussion and dissemination of the possible use of topological matter in novel electronics concepts aiming for room temperature operation was promoted by organizing a successful mini-Colloquium in the context of a European Conference that gathered several relevant researchers working on the topic in Braga.
WP1 was devoted to the growth of the topological insulator Bi2Se3 by molecular beam epitaxy on sapphire substrates. Different growth recipes were implemented and the resulting thin films characterized to decide which recipe would provide the best quality material to the subsequent interfacing with the ferromagnetic layers. A surface treatment of the sapphire substrate was chosen as the best approach due to a good balance of crystalline quality, electronic properties and good enough surface topography to accommodate the overlying very thin ferromagnetic layer. To explore other methods for tuning the electronic properties of Bi2Se3 thin films, ion implantation with Ca ions (p-type dopants in Bi2Se3) was employed and is currently being investigated.
In WP2, Bi2Se3/CoFeB heterostructures were successfully fabricated and test devices were patterned using conventional photolithography in a cleanroom environment. Different strategies for achieving a good interface between the Bi2Se3 and the CoFeB were explored, such as in-situ etching and capping with protection layers.
The characterization of the devices fabricated in WP2 was the focus of WP3, where two different electrical characterization techniques were employed to quantitatively measure the SOT efficiency: spin-torque ferromagnetic resonance and harmonic Hall voltage measurements. Both these methods were applied to the fabricated Bi2Se3/CoFeB heterostructures, where finite SOT efficiencies of the order of 3 % were measured, indicating the need to invest more in establishing better transfer methods.
WP4 was devoted to the integration of full magnetic tunnel junctions (MTJs) with the Bi2Se3 thin films. Fabrication of nanoscale MTJs was achieved and integration with SOT materials was explored in non-conventional architectures to allow future integration of emerging materials such as Bi2Se3.
Aligned with the project’s broader objective, a discussion and dissemination of the possible use of topological matter in novel electronics concepts aiming for room temperature operation was promoted by organizing a successful mini-Colloquium in the context of a European Conference that gathered several relevant researchers working on the topic in Braga.
During the rollout of TopRooT, topological insulators (Bi2Se3) grown by molecular beam epitaxy were interfaced with technologically relevant ferromagnetic layers (CoFeB) and test devices were fabricated using conventional microfabrication technology at INL’s cleanroom. Several strategies both on the material growth side and the fabrication side were tested to achieve progress in the integration of the topological materials in emerging spintronic device concepts based on spin-orbit torques. These strategies should not only inform further developments in this field, but also open pathways in new directions that benefit from the topological properties of Bi2Se3 and the magnetism put forth by the ferromagnets to have impact in energy-efficient computing solutions.
Ion implantation was explored for the post-growth doping of the Bi2Se3 in order to tune the electronic properties. The first tests enabled a range of ion fluences that can be used to implant Ca in thin Bi2Se3 films (30 nm) without completely etching the films. Further research will be conducted to assert the full potential of this approach. The successful implementation of this strategy for the tuning of the electronic properties should enable further engineering of the SOT efficiency by ion implantation, a popular semiconductor industry tool.
A novel fabrication path of MTJs for SOT-MRAM was explored that could provide significant advantages for the integration of emerging SOT materials over conventional approaches. A proof-of-concept is in progress at the end of the project and its success could have a positive impact on the wider spread of SOT-MRAM technology with emerging materials and future industrial implementation.
Ion implantation was explored for the post-growth doping of the Bi2Se3 in order to tune the electronic properties. The first tests enabled a range of ion fluences that can be used to implant Ca in thin Bi2Se3 films (30 nm) without completely etching the films. Further research will be conducted to assert the full potential of this approach. The successful implementation of this strategy for the tuning of the electronic properties should enable further engineering of the SOT efficiency by ion implantation, a popular semiconductor industry tool.
A novel fabrication path of MTJs for SOT-MRAM was explored that could provide significant advantages for the integration of emerging SOT materials over conventional approaches. A proof-of-concept is in progress at the end of the project and its success could have a positive impact on the wider spread of SOT-MRAM technology with emerging materials and future industrial implementation.