| Title: | Optymalizacja wzrostu epitaksjalnego azotku niobu do zastosowań w elektronice nadprzewodzącej |
| Project leader: | Artur Lachowski |
| Laboratory: | Laboratory of Semiconductor Characterization (NL-12) |
| Project number: | UMO-2025/56/C/ST5/00383 |
| Implementation date: | 27.08.2025 26.08.2028 |
| Total funding granted: | 1 058 971 zł |
| Funding for the entity: | 1 058 971 zł |
Project description
Superconducting materials serve as an excellent platform for fabricating quantum-based devices, including qubits, logic circuits, superconducting quantum interference devices (SQUIDs), and single-photon detectors. These devices rely on Josephson junction (JJ) structures, which consist of a superconducting/normal/superconducting material sequence.
However, conventionally used Al/AlOx/Al JJs face significant limitations due to the amorphous nature of AlOx, which
introduces defect states leading to qubit decoherence, as well as aluminum’s susceptibility to oxidation degrading
parameters of devices. A promising alternative is niobium nitride (NbN). The cubic δ-NbN phase, with its high
superconducting transition temperature (Tc) of 17 K and excellent resistance to oxidation, has the potential to surpass Albased JJs. Furthermore, the structural similarities between the cation sublattices of δ-NbN and semiconducting III-nitrides (GaN, AlN), combined with favorable lattice matching, allow for the epitaxial integration of these materials. This integration creates new paths for precise control over NbN/III-nitride heterostructures, facilitating the development of advanced superconducting quantum devices. While NbN is traditionally grown using sputter epitaxy, MBE offers superior control over doping, purity, and heterostructure integration. However, achieving high-quality δ-NbN on wurtzite substrates is challenging due to the tendency for twin formation, which degrades surface morphology and limits the fabrication of thin barriers in JJs.
This project aims to grow high-quality δ-NbN layers and δ-NbN/III-nitride/δ-NbN Josephson junction structures using molecular beam epitaxy (MBE). The primary focus is on developing twin-free layers and minimizing surface roughness by optimizing growth conditions. To identify the most suitable platform for NbN growth via MBE, various substrates will be investigated. Initially, the study will concentrate on the best lattice-matched options, such as native AlN and hightemperature annealed (HTA) sapphire templates, which offer an exceptionally low lattice mismatch of just 0.2%.
Additionally, a novel approach will be explored for growing cubic NbN on m-plane GaN and AlN crystals. Due to the
symmetry of these non-polar III-nitride substrates, twinning should be naturally suppressed, enabling the fabrication of high-quality layers with superior structural properties. Such advancements are expected to open new possibilities for next-generation device designs. Furthermore, silicon substrates with different MBE-grown buffer layers (TiN, AlN) will be employed for the development of JJ heterostructures. This strategy aims to provide a cost-effective alternative while maintaining a significantly higher layer quality and control over the growth process than current techniques, making it a promising solution for scalable superconducting device fabrication.
Furthermore, the project aims to optimize MBE NbN growth under indium-rich conditions. Unlike conventional nitrogenrich growth, this approach has already shown promising results by mitigating the detrimental effects of twin formation in NbN layers grown on c-plane bulk GaN substrates. Indium-rich conditions have led to larger twin-grain sizes, suppression of columnar growth, significantly smoother surfaces, and improved Tc compared to nitrogen-rich conditions. These advancements enabled the first successful fabrication of an MBE-grown NbN/InAlN/NbN Josephson junction with a critical current density of 1 kA/cm² and non-hysteretic I-V characteristics.
The next critical task is the growth of a thin (~1 nm), continuous III-nitride barrier on top of the NbN layer, followed by
the overgrowth of an additional NbN layer. This process requires developing a transition procedure between NbN and IIInitride growth conditions that preserves the NbN layer’s morphology and phase δ-NbN stoichiometry, ensuring smooth interfaces.
Beyond growth optimization, the project also seeks to understand how growth conditions and substrate choice influence microstructure, nucleation mechanisms, and defect formation in NbN layers. To achieve this, state-of-the-art transmission electron microscopy techniques will be employed, providing an in-depth structural analysis of the layers. The findings from this research will not only advance NbN-based superconducting devices but also contribute to a deeper understanding of the physics governing superconducting heterostructures.
However, conventionally used Al/AlOx/Al JJs face significant limitations due to the amorphous nature of AlOx, which
introduces defect states leading to qubit decoherence, as well as aluminum’s susceptibility to oxidation degrading
parameters of devices. A promising alternative is niobium nitride (NbN). The cubic δ-NbN phase, with its high
superconducting transition temperature (Tc) of 17 K and excellent resistance to oxidation, has the potential to surpass Albased JJs. Furthermore, the structural similarities between the cation sublattices of δ-NbN and semiconducting III-nitrides (GaN, AlN), combined with favorable lattice matching, allow for the epitaxial integration of these materials. This integration creates new paths for precise control over NbN/III-nitride heterostructures, facilitating the development of advanced superconducting quantum devices. While NbN is traditionally grown using sputter epitaxy, MBE offers superior control over doping, purity, and heterostructure integration. However, achieving high-quality δ-NbN on wurtzite substrates is challenging due to the tendency for twin formation, which degrades surface morphology and limits the fabrication of thin barriers in JJs.
This project aims to grow high-quality δ-NbN layers and δ-NbN/III-nitride/δ-NbN Josephson junction structures using molecular beam epitaxy (MBE). The primary focus is on developing twin-free layers and minimizing surface roughness by optimizing growth conditions. To identify the most suitable platform for NbN growth via MBE, various substrates will be investigated. Initially, the study will concentrate on the best lattice-matched options, such as native AlN and hightemperature annealed (HTA) sapphire templates, which offer an exceptionally low lattice mismatch of just 0.2%.
Additionally, a novel approach will be explored for growing cubic NbN on m-plane GaN and AlN crystals. Due to the
symmetry of these non-polar III-nitride substrates, twinning should be naturally suppressed, enabling the fabrication of high-quality layers with superior structural properties. Such advancements are expected to open new possibilities for next-generation device designs. Furthermore, silicon substrates with different MBE-grown buffer layers (TiN, AlN) will be employed for the development of JJ heterostructures. This strategy aims to provide a cost-effective alternative while maintaining a significantly higher layer quality and control over the growth process than current techniques, making it a promising solution for scalable superconducting device fabrication.
Furthermore, the project aims to optimize MBE NbN growth under indium-rich conditions. Unlike conventional nitrogenrich growth, this approach has already shown promising results by mitigating the detrimental effects of twin formation in NbN layers grown on c-plane bulk GaN substrates. Indium-rich conditions have led to larger twin-grain sizes, suppression of columnar growth, significantly smoother surfaces, and improved Tc compared to nitrogen-rich conditions. These advancements enabled the first successful fabrication of an MBE-grown NbN/InAlN/NbN Josephson junction with a critical current density of 1 kA/cm² and non-hysteretic I-V characteristics.
The next critical task is the growth of a thin (~1 nm), continuous III-nitride barrier on top of the NbN layer, followed by
the overgrowth of an additional NbN layer. This process requires developing a transition procedure between NbN and IIInitride growth conditions that preserves the NbN layer’s morphology and phase δ-NbN stoichiometry, ensuring smooth interfaces.
Beyond growth optimization, the project also seeks to understand how growth conditions and substrate choice influence microstructure, nucleation mechanisms, and defect formation in NbN layers. To achieve this, state-of-the-art transmission electron microscopy techniques will be employed, providing an in-depth structural analysis of the layers. The findings from this research will not only advance NbN-based superconducting devices but also contribute to a deeper understanding of the physics governing superconducting heterostructures.