Rare-earth-doped TiO Thin Films Grown on III-V Semiconductors Enable Hybrid Quantum Photonic Interfaces with Sub-300pm Roughness

The development of efficient quantum photonic networks hinges on the ability to create both reliable single-photon sources and long-lasting quantum memories, a challenging combination of functionalities. Henry C. Hammer, Caleb Whittier, and Nathan A. Helvy, from The University of Iowa and McMaster University, alongside colleagues including Christopher Rouleau and Nabil D. Bassim, now demonstrate a significant advance in this field. The team successfully grows rare-earth-doped titanium dioxide thin films directly onto gallium arsenide and gallium antimonide semiconductors, overcoming long-standing difficulties related to material incompatibility. This achievement preserves the desirable properties of both material systems and creates smooth, high-quality interfaces essential for building low-loss nanophotonic devices. By carefully controlling growth conditions, the researchers establish a crucial platform for integrating quantum memories with semiconductor photon sources, representing a key step towards scalable, hybrid photonic chips and ultimately, more powerful quantum communication networks.

Erbium Doping and Oxide Epitaxial Growth

Scientists are actively exploring the incorporation of erbium into various materials, particularly oxides and semiconductors, to create materials with specific optical properties for applications in quantum information, lasers, and sensing. A central focus is understanding how erbium ions interact with the surrounding material and controlling their optical transitions. Simultaneously, researchers are developing techniques to grow thin films of materials on substrates with differing crystal structures and thermal properties, crucial for creating high-quality materials with controlled characteristics and minimizing defects, strain, and interface roughness. Materials like strontium titanate, gallium arsenide, and various oxides are frequently investigated.

Controlling defects, such as oxygen vacancies and impurities, is a major theme, as these significantly impact optical and electrical properties. Researchers are developing methods to create or eliminate specific defects to tailor material behavior. Understanding and controlling the optical transitions of erbium ions is also central, with efforts focused on minimizing energy loss and broadening of optical signals. Factors like the surrounding crystal structure, interactions with vibrations, and the presence of defects are all investigated. The quality of surfaces and interfaces is critical for successful growth and device performance, leading to research on surface preparation and passivation techniques.

Strain engineering, the deliberate application of stress to a material, is used to tune its properties, particularly the optical characteristics of erbium ions. This research points to a highly active field focused on creating advanced materials with tailored optical and quantum properties. The challenges are significant, requiring precise control over material composition, structure, and defect chemistry. The potential applications are broad, ranging from quantum information processing to advanced sensing and optoelectronics.

Erbium-Doped Films Grown on III-V Semiconductors

Scientists successfully grew erbium-doped titanium dioxide films directly on both gallium arsenide and gallium antimonide substrates using pulsed laser deposition. This work addresses a critical challenge in quantum photonics by combining materials typically incompatible due to differences in their crystal structures and optimal growth conditions. The team carefully controlled the growth process, employing surface preparation steps and a buffer layer to enable crystalline film formation at temperatures compatible with existing III-V semiconductor quantum dots. Measurements confirm that surface roughness was maintained below 300 picometers, creating smooth interfaces suitable for fabricating low-loss nanophotonic structures.

The research demonstrates a remarkable ability to tune the crystalline phase of the titanium dioxide films. By controlling growth temperature and adapting interface preparation, scientists successfully grew either anatase or rutile titanium dioxide, two distinct crystal structures. Analysis using modeling explains the preferential growth of anatase on gallium arsenide, revealing the underlying mechanisms governing film orientation. Reflection high-energy electron diffraction confirmed the crystalline quality of the films both during and after growth, providing real-time feedback on the process. Further characterization using Raman spectroscopy and cryogenic photoluminescence excitation spectroscopy verified both the crystal phase and the optical activation of the incorporated erbium ions. These measurements confirm that the erbium ions are effectively incorporated into the titanium dioxide lattice and exhibit the desired optical properties for quantum memory applications. Achieving this integration, which combines the strengths of semiconductor photon sources and rare-earth ion memories, required careful control of growth conditions, specifically substrate preparation, oxygen levels, and temperature. The team established that employing an arsenic cap and oxygen-deficient buffer layer enables the creation of smooth, high-quality titanium dioxide films with a preferred crystalline structure. The study links microscopic growth processes to the optical properties of the resulting material, revealing how strain relaxation and defects influence optical coherence.

Cross-sectional imaging confirms that maintaining interfacial integrity is crucial for preventing unwanted phase transitions and promoting the desired crystalline structure. This work establishes a foundation for creating monolithically integrated, telecom-compatible rare-earth-doped oxide films with III-V semiconductors, paving the way for advanced on-chip quantum photonic technologies. Researchers acknowledge that decoupling the effects of substrate-induced strain and material imperfections on the rare-earth ions remains a challenge. Future research will focus on spatially engineered doping profiles and the introduction of diffusion barriers to further refine film quality and isolate the factors influencing optical coherence. Precise control of plasma conditions and oxygen levels during growth also represents a promising avenue for improvement, ultimately enabling the creation of scalable and high-performance integrated photonic devices.