Self Assembled Monolayers SAMs
Innovation in Quantum Computing: Self-Assembled Monolayers Provide 80% Loss Reduction and Stable Coherence
By successfully reducing the troublesome buildup of native oxides on the surfaces of aluminum and niobium, researchers have made a substantial progress in improving the stability and functionality of superconducting quantum circuit. It has been shown that a unique strategy employing molecular self-assembled monolayers (SAMs) can stop oxide regrowth, a recurring problem in the development of quantum hardware, resulting in significantly better coherence and significant energy loss reductions. These discoveries open the door to quantum computing devices that are more durable, dependable, and scalable.
Stable quantum coherence has long been hampered by the intrinsic susceptibility of superconducting quantum circuits to environmental influences, especially the development of native oxide layers on metallic components such as aluminium (Al) and niobium (Nb). Two-level system (TLS) flaws, which interact with electric fields, absorb microwave energy, and eventually deteriorate the delicate quantum states necessary for computation, are largely sourced from these oxide layers. Unwanted relaxation and decoherence result from the buildup of these defects, with TLS flaws found in amorphous materials, oxides, and the interfaces surrounding superconductors accounting for a significant amount of coherence loss. It is widely acknowledged that air interfaces account for more than 80% of TLS losses in superconducting quantum devices.
Etching, encapsulation, plasma cleaning, and argon milling are examples of conventional mitigation techniques that have been investigated. Although TLS losses can be decreased by removing oxide, stopping their regrowth after being exposed to ambient conditions has proven to be a difficult task. For example, oxide regrowth makes etch cleaning an unsustainable approach. Although encapsulation with more metal layers has been contemplated, it has frequently been avoided due to worries about creating new flaws at the metal-superconductor interface.
Ultrathin magnesium (Mg) or ruthenium (Ru) capping layers have been tried to inhibit the production of tantalum (Ta) and niobium (Nb) oxides, respectively. Since irregular oxide development is the primary cause of TLS flaws, it has even been demonstrated that encouraging crystalline oxide formation might alleviate TLS defects. However, the performance of quantum devices is still constrained by the problem of uncontrolled native oxide production.
Recent studies have concentrated on molecular self-assembled monolayers (SAMs) as a unique and efficient passivation method in response to this pressing issue. Organic compounds called SAMs are well-known for their capacity to create coatings with a high surface coverage, uniformity, and stability. They have been thoroughly investigated for altering the physical and chemical characteristics of semiconductors and metals, such as corrosion resistance, work function, surface chemistry, and dielectric constant change. Despite being relatively recent, their use in quantum circuits has showed great potential.
The process is submerging the silicon substrates coated with aluminium (Al) or niobium (Nb) in SAM solutions in order to passivate newly made Al and Nb thin films. Because of their strong affinity for Al surfaces, two different kinds of SAMs per fluoro octyl triethoxy silane (PFOTS) and tetradecyl phosphonic acid (TDPA) were used in the aluminium investigation. Alkyl-phosphonate SAMs were effectively cultivated on thin films for niobium following the removal of preexisting oxides.
A wide range of analytical methods were used to verify the efficacy of SAM passivation for circuits made of aluminium:
- XPS: XPS testing indicated that the SAMs bonded and that aluminium oxide did not develop. Compared to untreated samples, SAM-treated samples had decreased surface oxide peak locations and intensity. The absence of adsorbed water peaks in treated samples showed that SAMs effectively blocked water penetration and reduced oxide peak envelopes. In the case of unaltered aluminium, the oxide thickness was reduced to 2.00 ± 0.30 nm for TDPA/Al and to PFOTS/Al, according to calculations based on XPS data. This indicates that SAMs do more than just obscure the XPS signal; they also successfully prevent further oxide production.
- Stability After Ageing: SAM-passivated aged samples showed just a slight increase in oxide thickness (0.30 nm for PFOTS and 0.23 nm for TDPA), indicating that the passivation remained impressively robust even after 15 days of ageing in ambient conditions. Unmodified aluminium, which was already saturated with oxides and showed no discernible alteration, stood in sharp contrast to this.
- Contact Angle Measurements: The SAM’s successful binding to the Al surface and its drastic change from a hydrophilic to a hydrophobic surface were further validated by water contact angle measurements. PFOTS/Al and TDPA/Al films demonstrated noticeably greater contact angles of 119.6° and 124.7°, respectively, than the unmodified Al films, which displayed a contact angle of 39.1°. By preventing water penetration, its hydrophobicity reduces contamination and oxide development. In contrast to PFOTS’s 8-carbon fluorocarbon chain, TDPA’s longer 14-carbon hydrocarbon chain and phosphonic acid group’s ability to create strong P–O–Al interactions allowed it to attain a greater contact angle, resulting in a denser and more ordered monolayer.
- Energy Dispersive X-ray Spectroscopy (EDS) and Scanning Electron Microscopy (SEM): These analyses demonstrated a notable decrease in oxide content on modified surfaces and offered additional visual and elemental evidence supporting the binding of the SAM to the Al surface and the mitigation of oxide growth.
Additionally, SAMs have demonstrated impressive effectiveness for niobium-based superconducting circuits:
- Significant Loss Reduction: By employing alkyl-phosphonate SAMs to effectively inhibit oxide regrowth, researchers were able to reduce loss at single-photon power levels in un-passivated resonators by almost 80%. Excellent temporal stability was exhibited by SAM-passivated resonators, which continued to function as a circuit even after being exposed to air for six days.
- Better Quality Factors: Compared to resonators that were just oxide-etched, the implementation of SAMs consistently improved the measured quality factors across a variety of resonators. At single-photon excitation power and millikelvin temperatures, this was clearly visible.
- Measuring SAM Loss: Researchers used a two-component TLS model to measure the distinctive TLS loss of the SAMs, obtaining an exact value of roughly 5×10⁻⁷. For upcoming circuit optimization, this value which was previously unknown is essential.
- Surface Characterization: A well-ordered molecular monolayer with a nanometre film thickness was verified by ellipsometry. A barrier against oxide regrowth and successful monolayer formation were confirmed by water contact angle measurements, which once more demonstrated a change from a hydrophilic to a hydrophobic surface.
Together, these results strongly imply that SAM-based passivation is a very promising technique for enhancing the performance of Al- and Nb-based superconducting quantum circuits and lowering microwave loss. Instead of native oxide growth, SAMs use thin, stable organic molecules with known characteristics to promote quantum coherence. SAM materials are compatible with well-established manufacturing procedures, making industrial-scale quantum circuit fabrication possible, especially in applications that need long-term device stability. A basic materials-based constraint in quantum hardware is solved by this idea, paving the way for large-scale, high-performance quantum computer systems.
