AlN on Sapphire Based Quantum Emitters: Materials, Fabrication, and Optical Properties +

PAM-XIAMEN offers AlN on sapphire template for quantum emitter research. More wafer specifications, please contact: [email protected]

Aluminum Nitride (AlN), as a wide-bandgap semiconductor, has demonstrated significant potential in the field of quantum photonics in recent years. Its ultra-wide bandgap (~6.015 eV), excellent thermal stability, compatibility with established semiconductor processing, and the ability to achieve single-photon emission at room temperature make it an ideal platform for scalable quantum photonic integrated circuits. Particularly when AlN is epitaxially grown on sapphire substrates, its single-crystal quality and optical properties are further enhanced, laying the foundation for the research and application of high-performance Quantum Emitters (QEs).

1. Material Advantages of Sapphire-Based AlN and the Potential for Quantum Emitters

AlN is a Group III-nitride semiconductor with a wurtzite crystal structure. Its structure lacks inversion symmetry along the [0001] direction, leading to internal electrical polarization and a piezoelectric effect. This characteristic makes it not only attractive for microelectromechanical systems (MEMS) and high-power electronic devices but also provides a pathway for spin-state manipulation via strain. Growing AlN as a single-crystal film on sapphire substrates enables the production of high-quality, low-defect-density thin films, which is crucial for achieving stable and efficient quantum emission.

The sapphire substrate itself offers excellent chemical stability, high thermal conductivity, and transparency in the visible spectrum, providing an ideal template for AlN film growth. Studies have shown that commercially available c-plane AlN films grown on sapphire exhibit bright (>10⁵ counts/s), pure (g⁽²⁾(0)<0.2), and polarization-defined single-photon emission at room temperature. These emitters typically appear as diffraction-limited spots, clearly distinguishable in confocal scan maps. Furthermore, the relatively good lattice match between AlN and sapphire helps minimize interfacial defects, improving the overall optical quality of the material.

Fig. 1 Optical image of an AlN sample and confocal scanning intensity map

2. Fabrication of AlN Quantum Emitters and Defect Engineering

The origin of quantum emitters in sapphire-based AlN can be both intrinsic defects from the material growth and defects introduced through post-processing. Current primary fabrication methods include ion implantation, laser writing, and the direct utilization of intrinsic defects formed during epitaxial growth.

2.1 Ion Implantation

Ion implantation is a controllable method for introducing defects to create quantum emitters. Research indicates that irradiating AlN films with various ions such as He⁺, Al⁺, Zr⁺, and Kr⁺, followed by post-annealing, can effectively form luminescent defects. For instance, implantation with 60 keV Al⁺ ions followed by annealing at 600°C can significantly increase the areal density of emitters, with an optimal dose around 1×10¹³ cm⁻². Similarly, using 200 keV Zr⁺ ions at a fluence of 2×10¹⁴ cm⁻² can yield a high emitter density while maintaining low background fluorescence. The annealing step (typically at 1000°C in an inert atmosphere) is crucial for repairing lattice damage and activating the luminescent centers.

Fig. 2 Formation of luminescent defects via Al-ion implantation and thermal annealing

2.2 Intrinsic Defects and Laser Writing

Not all quantum emitters require foreign ion implantation. Intrinsic single-photon emitters with sparse distribution have been observed in as-grown AlN-on-sapphire samples. Their zero-phonon line (ZPL) energies are primarily concentrated in the 2.00-2.15eV range, with a narrow distribution, suggesting they may originate from the same type of crystal defect. Furthermore, femtosecond laser writing has been proven capable of fabricating quantum emitters with high Debye-Waller factors (~0.65) in freestanding AlN.

3. Optical Properties and Energy-Level Dynamics of AlN Quantum Emitters

Sapphire-based AlN quantum emitters exhibit rich and complex optical dynamics, which is key to understanding their physical origin and optimizing their performance.

3.1 Spectral Characteristics

At room temperature, most emitters show broad emission covering approximately 580-650nm, typically consisting of a resolvable zero-phonon line and a pronounced redshifted phonon sideband. At cryogenic temperatures (e.g., 5K), the ZPL sharpens, with linewidths as narrow as 0.16nm, and the Debye-Waller factor can exceed 15%.

Fig. 3 Room-temperature photoluminescence spectrum and g⁽²⁾ correlation histogram of a representative emitter in a Zr-implanted sample

3.2 Single-Photon Purity and Correlation Measurements

Second-order autocorrelation function g⁽²⁾(τ) measurements confirm their single-photon emission nature. The g⁽²⁾(0) values for most emitters are below 0.5, with some even below 0.1. For example, under pulsed excitation, g⁽²⁾(0) values as low as 0.05 have been achieved. The correlation histograms commonly show antibunching (nanosecond timescale) and bunching (hundreds of nanoseconds to microsecond timescale), indicating the presence of a complex multi-level system with metastable “dark states.”

3.3 Complex Energy-Level Structure and Dark-State Dynamics

Research suggests that a simple three-level model (ground state, excited state, one dark state) is insufficient to fully describe the dynamics of AlN quantum emitters. Photon Emission Correlation Spectroscopy (PECS) and Time-Resolved Photoluminescence (TRPL) analyses infer the existence of at least four dark states (or “shelving states”) that collectively govern the saturation, bunching behavior, and lifetime of the optical transition. These dark states may arise from different spin manifolds, internal charge states, or fluorescence intermittency caused by charging of nearby trap sites.

Fig. 4 PECS fitting revealing the presence of multiple dark states

Notably, different emitters exhibit strikingly opposite power-dependent behaviors: For QE A sample, increasing laser power enhances bunching, indicating population pumping into dark states; whereas for QE B, increasing power suppresses bunching, indicating population being “deshelved” from dark states, thereby achieving more efficient radiative emission at high power. This difference may be related to charge ionization and recombination processes.

Fig. 5 Opposite power-dependent bunching behaviors of different AlN emitters, QE A and QE B

4. Performance Optimization and Integration Prospects for AlN Quantum Emitters

4.1 Improving Saturated Emission Rate

The practical application of quantum emitters as single-photon sources is limited by their saturated emission rate, where dark-state dynamics are a key factor. An ideal emitter should minimize the time population spends trapped in dark states at high power. Simulations and experiments show that for emitters like QE B, which can be optically deshelved from dark states, efficient radiation and higher saturation count rates are maintained even at high power. Furthermore, employing a second laser at a different wavelength for repumping can effectively bring population back from the dark state to the bright transition, further enhancing brightness.

4.2 Photonic Integration

A major advantage of AlN lies in its mature photonic integration processes. Quantum emitters can be directly fabricated and integrated into AlN-on-sapphire photonic integrated circuits. Studies have shown off-chip count rates exceeding 6×10⁴counts/s for waveguide-integrated emitters. Elements such as distributed Bragg reflectors, spectral filters, and grating couplers can be designed to efficiently guide, filter, and output emitted photons.

Fig. 6 Confocal scan, correlation measurement, saturation curve of a waveguide-integrated emitter, and a conceptual diagram of the integrated device

4.3 Polarization Properties

Most AlN quantum emitters exhibit strong linear polarization. Interestingly, the dipole orientations for absorption and emission are often different, even orthogonal. This is likely due to different in-plane polarization selection rules for the excitation and emission transitions within their multi-level structure. This property is advantageous for implementing efficient polarization filtering and resonant control.

AlN on sapphire based quantum emitters represent a class of room-temperature solid-state single-photon sources compatible with mature semiconductor processing. Their complex multi-level dynamics and controllable fabrication paths via defect engineering have been preliminarily revealed. However, their precise atomic origins and spin properties remain unclear, and achieving deterministic control over their performance and location remains a key challenge. By deepening the understanding of defect physics and developing techniques for site-controlled fabrication and spin-state probing, this platform holds significant promise for playing an important role in scalable quantum information processing and sensing applications.

Whether you need AlN templates for research or for industrial applications, please contact us email at [email protected] and [email protected].

 

References:

1. Guo, Y., Hadden, J. P., Clark, R. N., Bishop, S. G., & Bennett, A. J. (2024). Emission dynamics of optically driven aluminum nitride quantum emitters. Physical Review B, 110(1), 014109.
2. Bishop, S. G., Hadden, J. P., Alzahrani, F. D., Hekmati, R., Huffaker, D. L., Langbein, W. W., & Bennett, A. J. (2020). Room-temperature quantum emitter in aluminum nitride. ACS photonics, 7(7), 1636-1641.
3. Lienhard, B., Jeong, K. Y., Moon, H., Iranmanesh, A., & Englund, D. R. (2020). Bright High-Purity Quantum Emitters in Aluminum Nitride Integrated Photonics.
4. Nieto Hernández, E., Yağcı, H. B., Pugliese, V., Aprà, P., Cannon, J. K., Bishop, S. G., … & Forneris, J. (2024). Fabrication of quantum emitters in aluminum nitride by Al-ion implantation and thermal annealing. Applied Physics Letters, 124(12).
5. Senichev, A., Martin, Z. O., Wang, Y., Matthiessen, O. M., Lagutchev, A., Htoon, H., … & Shalaev, V. M. (2024). Quantum emitters in aluminum nitride induced by heavy ion irradiation. APL Quantum, 1(3).