Digital Alloying Fine-Tunes Spin-Orbit Coupling for Spintronics and Topology

The manipulation of electron spin, rather than charge, offers potential advantages in future electronic devices, a field known as spintronics. Achieving strong control over spin relies on phenomena like spin-orbit coupling, an interaction between an electron’s spin and its motion within a material. Researchers are now investigating how precisely engineered semiconductor structures can enhance this coupling. Jason T. Dong, Yilmaz Gul, and colleagues, reporting in a recent study, demonstrate a method for tailoring spin-orbit coupling in indium gallium arsenide (InGaAs) quantum wells through a technique called digital alloying. This approach allows for the creation of asymmetric interfaces and compositional grading within the well, effectively modifying the spin-orbit interaction. Their work, titled “Spin-orbit coupling in digital alloyed InGaAs quantum wells”, details how digital alloying can alter the spin-orbit coupling by up to 138 milli-electron volts per Angstrom, with changes attributable to modifications in the interfacial Rashba spin-orbit coupling, a quantum mechanical effect occurring at the material’s surface.

Strong spin-orbit coupling underpins advances in spintronics and quantum technologies, driving research into materials exhibiting this property. Spin-orbit coupling, a relativistic effect, links an electron’s spin to its motion, influencing its behaviour and proving invaluable in technologies reliant on manipulating electron spin, such as spintronics and topological quantum computing. Enhancing the strength of spin-orbit coupling in semiconductor heterostructures, particularly in indium gallium arsenide (InGaAs) quantum wells, holds promise for improved device performance.

Quantum wells, created by confining electrons within thin semiconductor layers, provide a platform for studying and exploiting spin-orbit coupling, where material composition, quantum confinement, and applied electric fields dictate its magnitude. Researchers explore methods to enhance this effect, but conventional approaches involving external electric fields can negatively impact charge carrier mobility due to increased scattering. This motivates investigation into alternative strategies, and digital alloying, a technique involving the creation of short-period superlattices, presents a promising avenue.

Digital alloys approximate random alloys, offering a pathway to engineer materials with tailored properties. The ability to create asymmetric interfaces and compositional gradients within the quantum well structure allows for precise control over the electronic band structure and, consequently, the strength of spin-orbit coupling. Researchers now employ digital alloying to engineer these quantum wells and manipulate spin-orbit interactions. Unlike traditional alloy formation, digital alloying constructs heterostructures with atomically abrupt interfaces and carefully defined compositional grading. This technique centres on the growth of InGaAs and aluminium gallium arsenide (AlGaAs) quantum wells using molecular beam epitaxy, a process where precisely controlled beams of atoms deposit onto a substrate.

Careful characterisation of the resulting quantum wells’ electronic behaviour is central to understanding the impact of digital alloying. Magnetotransport measurements, involving the study of electrical conductivity under applied magnetic fields, form the cornerstone of this investigation. Researchers observe magnetophonon resonance, where electrons scatter off lattice vibrations (phonons) in a magnetic field, and magneto-intersubband oscillations, revealing transitions between energy levels within the quantum well. These oscillations provide insights into the energy band structure and the strength of spin-orbit coupling, and complementary Hall effect measurements determine the carrier density and electron mobility. High electron mobility, exceeding 100,000 cm²/V·s in these digitally alloyed structures, is a key achievement.

Analysis of these experimental results relies heavily on theoretical modelling. Researchers employ k⋅p perturbation theory, a method used to calculate the band structure and spin-orbit coupling parameters, simplifying calculations through the effective mass approximation. This framework allows for a deeper understanding of how digital alloying modifies the band structure and, consequently, the spin-orbit coupling. Observed changes in spin-orbit coupling, reaching up to 138 milli-electron volts per Angstrom, are qualitatively explained by modifications to the interfacial Rashba spin-orbit coupling, a mechanism arising from structural asymmetry at the interface between different materials. Disentangling the contributions of Rashba and Dresselhaus spin-orbit coupling, two distinct mechanisms that contribute to spin splitting, remains a central focus, and investigations into scattering mechanisms limiting electron mobility are crucial.

The study establishes that digital alloying effectively tunes the band structure of the InGaAs/InAlAs quantum wells, and analysis of magneto-intersubband oscillations (MIRO) reveals changes in the spin-orbit coupling strength. MIRO are particularly sensitive to the energy levels and effective mass of charge carriers, providing a detailed probe of the electronic structure, and the research demonstrates a modification of spin-orbit coupling of up to 138 milli-electron volts per Angstrom, a significant alteration achieved through compositional control. These changes correlate strongly with modifications to the interfacial Rashba effect, a specific type of spin-orbit interaction occurring at interfaces, and the researchers attribute the observed effects to the asymmetric potential created by the digitally alloyed interfaces.

This work has implications for the development of spintronic devices, which leverage the spin of electrons alongside their charge for information processing and storage. The ability to precisely control spin-orbit coupling is essential for manipulating spin currents and creating novel device functionalities. Furthermore, the high-mobility quantum wells offer potential for building faster and more energy-efficient electronic components, and the principles demonstrated contribute to the ongoing exploration of materials for quantum computing applications where control of spin states is paramount.

The researchers utilise magnetotransport measurements, including detailed analysis of magneto-intersubband oscillations and Shubnikov-de Haas oscillations, to characterise the electronic properties and spin-orbit coupling strength. These techniques reveal a direct correlation between digital alloying and modifications to the spin splitting observed in the quantum wells. High electron mobility, exceeding 100,000 cm²/V·s, is maintained throughout this process, indicating that spin-orbit coupling engineering does not compromise carrier transport. Theoretical modelling, employing k⋅p perturbation theory and the effective mass approximation, supports the experimental findings, and calculations accurately predict the observed changes in spin-orbit coupling, attributing them to modifications in the interfacial Rashba component. Acknowledgement of non-parabolic band structure effects further refines the theoretical framework, enhancing its predictive power and accuracy.

Future work focuses on separating and quantifying the contributions of both Rashba and Dresselhaus spin-orbit coupling mechanisms to the overall spin splitting, and investigating the impact of varying alloy compositions and interface structures promises further optimisation of spin-orbit coupling strength and band structure engineering. Extending these techniques to explore topological insulator states within these heterostructures represents a logical next step, potentially leading to novel quantum devices. Furthermore, detailed investigation into the role of interface roughness and other scattering mechanisms remains crucial for maximising carrier mobility, and developing advanced growth techniques to minimise these imperfections will be essential for realising high-performance spintronic devices. Exploring the scalability of digital alloying for large-area device fabrication also presents a significant challenge and opportunity for future research.