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In-situ Silicon Doped hBN by High-Temperature Molecular Beam Epitaxy Enables Single Photon Emission

This paper demonstrates that in-situ silicon doping of hexagonal boron nitride (hBN) via high-temperature molecular beam epitaxy enables the creation of room-temperature single photon emitters across a broad spectral range, offering a viable pathway for quantum photonic device integration.

Original authors: Jiyun Kim, Nika Teran, Juliette Plo, Jonathan Bradford, Guillaume Cassabois, Amy F. M. Collins, Tin S. Cheng, Christopher J. Mellor, Shery L. Y. Chang, Sergei V. Novikov, Igor Aharonovich

Published 2026-06-02

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Jiyun Kim, Nika Teran, Juliette Plo, Jonathan Bradford, Guillaume Cassabois, Amy F. M. Collins, Tin S. Cheng, Christopher J. Mellor, Shery L. Y. Chang, Sergei V. Novikov, Igor Aharonovich

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a very special, ultra-thin sheet of material called hexagonal boron nitride (hBN). Think of this sheet as a pristine, white canvas. In the world of quantum physics, scientists want to paint tiny, glowing dots on this canvas. These dots are called quantum emitters, and they are special because they can release light one single photon (particle of light) at a time. This is the "holy grail" for building future quantum computers and ultra-secure communication networks.

For a long time, scientists knew how to make these dots appear naturally, but they didn't know exactly what was causing the glow. It was like seeing a lightbulb turn on in a dark room but not knowing if it was a candle, an LED, or a firefly. Most researchers had tried adding carbon (like soot) to the mix to make the lights appear, but they wanted to try something new.

The Experiment: Cooking with Silicon

The team in this paper decided to try a different ingredient: Silicon. Think of silicon as a different type of "seasoning" for their material soup.

They used a high-tech oven called Molecular Beam Epitaxy (MBE). Imagine this as a very precise kitchen where they shoot beams of atoms (Boron, Nitrogen, and their new Silicon seasoning) at a hot surface to grow the hBN sheet atom by atom.

To figure out the perfect recipe, they cooked the material at different temperatures, ranging from a "warm" 900°C to a "scorching" 1390°C, while keeping the amount of Silicon constant.

The Results: Finding the Sweet Spot

Here is what they discovered, using a few simple analogies:

The Temperature Matters:
- At low temperatures (900°C), the "soup" didn't cook right. The silicon didn't mix in well, and no glowing dots appeared.
- At medium temperatures (1000°C), a few dots appeared, but they were very specific and limited in color (mostly yellow-green).
- At high temperatures (1200°C and above), the magic happened. The silicon mixed perfectly with the material, creating a whole rainbow of glowing dots. These dots glowed in a wide range of colors (from blue to red) and were very bright and clear.
The "Single Photon" Proof:
The researchers tested these glowing dots to see if they were truly "single photon" emitters. They used a special test (like a traffic light counter) to ensure the dots weren't flashing two lights at once. The results showed that the dots were indeed flashing one photon at a time, which is exactly what you need for quantum technology.
It's Not Just the Surface:
Sometimes, glowing spots can be caused by dirt on the surface or the material underneath (the sapphire plate they grew the film on). To prove the dots were actually inside the hBN sheet, they carefully peeled the thin film off the plate and stuck it onto a new glass slide. The dots were still there, glowing just as brightly. This proved the "glow" was baked into the material itself, not just sitting on top.

What is Making the Light?

The scientists looked at the material under a super-powerful microscope (called ADF-STEM) that can see individual atoms. They saw that the silicon atoms had successfully swapped places with some of the boron atoms in the crystal structure.

They proposed a few theories on what exactly is glowing:

It could be a Silicon atom sitting alone in the crystal.
It could be a Silicon atom paired up with a missing atom (a vacancy) or even a tiny bit of carbon that was accidentally left in the mix.
They ruled out the idea that it was just the carbon they were used to seeing in other experiments, because their "pure" reference samples without silicon didn't glow the same way.

The Bottom Line

This paper is like discovering a new way to bake a cake. Before, everyone knew how to make a cake using chocolate chips (carbon) to get a specific flavor. This team showed that if you bake your cake at the right high temperature and add vanilla beans (silicon) instead, you get a whole new variety of delicious flavors (colors of light) that are perfect for making quantum devices.

They haven't built a quantum computer yet, but they have proven that they can reliably "grow" these special light sources inside the material by controlling the heat and adding silicon. This gives scientists a new, powerful tool to build the future of quantum technology.

Technical Summary: In-situ Silicon Doped hBN by High-Temperature Molecular Beam Epitaxy Enables Single Photon Emission

Problem Statement
Hexagonal boron nitride (hBN) is a leading host material for optically active quantum defects due to its wide bandgap in both 3D and 2D geometries and its ability to host bright emitters across the visible to near-infrared spectrum. However, the microscopic origins of many observed quantum emitters remain elusive. While carbon doping has been extensively studied and linked to specific emission lines (such as the 4.1 eV emission and B-centres), the introduction of other specific impurity species, particularly silicon, remains largely unexplored. Furthermore, identifying specific defect configurations that yield tailored emission lines in the visible spectral range remains a significant challenge.

Methodology
The authors employed High-Temperature Molecular Beam Epitaxy (HT-MBE) to achieve in-situ silicon doping of hBN.

Growth Process: A Si sublimation source was integrated into the HT-MBE system to simultaneously supply fluxes of Si, boron, and active nitrogen to the growth surface. This allowed for the direct incorporation of Si into the BN lattice sites during growth.
Systematic Variation: To investigate the mechanisms of Si incorporation, hBN films were grown on (0001) sapphire substrates across a wide temperature range (900 °C to 1390 °C) while maintaining a constant silicon flux. Samples were labeled S1 through S4 corresponding to increasing growth temperatures. A pristine hBN film grown at 1390 °C without Si (S0) and a sample with half the growth time at 1390 °C (S5) served as controls.
Characterization: The structural and chemical properties were analyzed using Atomic Force Microscopy (AFM), Spectroscopic Ellipsometry, and X-ray Photoelectron Spectroscopy (XPS). Optical properties were assessed via Photoluminescence (PL) mapping, confocal PL spectroscopy, and second-order photon autocorrelation measurements ( $g^{(2)}(\tau)$ ) using Hanbury Brown and Twiss (HBT) interferometry.
Verification: To confirm the emitters were intrinsic to the hBN film and not artifacts from the substrate or surface, a polyvinyl acetate (PVA)-assisted dry transfer method was used to move the Si-doped hBN films onto Si/SiO₂ substrates.
Microscopy: Annular Dark-Field Scanning Transmission Electron Microscopy (ADF-STEM) was utilized to visualize atomic columns and verify the presence of substitutional Si atoms.

Key Results

Structural Quality: AFM and ellipsometry confirmed that Si-doped hBN films maintained high crystalline quality, though surface roughness slightly increased compared to pristine films, likely due to lattice distortion from Si substitution. XPS analysis confirmed hBN stoichiometry close to 1:1, with Si content below the typical XPS detection limit (<1 atomic %), indicating successful low-concentration doping.
Temperature Dependence of Emission:
- S1 (900 °C): No single photon emitter (SPE) activity was observed, attributed to a regime dominated by non-radiative recombination channels.
- S2 (1000 °C): A low density of emitters was observed, confined to a narrow spectral range (570–580 nm).
- S3 (1200 °C) and S4 (1390 °C): A dramatic increase in both emitter density and spectral diversity was observed. Emitters were distributed continuously across 550–750 nm.
- S5 (1390 °C, reduced time): Exhibited similar emitter density and spectral distribution to S3 and S4, confirming that temperature, rather than just total growth time, is the critical factor.
Single Photon Nature: The observed emitters displayed narrow Zero Phonon Lines (ZPLs) with an average Full Width at Half Maximum (FWHM) of ~11 nm at room temperature. Second-order autocorrelation measurements confirmed antibunching with $g^{(2)}(0)$ values well below 0.5 (e.g., 0.05 for S3), verifying their nature as isolated single photon emitters.
Exclusion of Carbon: Control samples (S0) and UV-PL measurements showed no evidence of the 4.1 eV emission typical of carbon dimers, suggesting the observed SPEs are not primarily carbon-related, despite carbon being a common residual impurity.
Transferability: The SPE activity was preserved after transferring the hBN films to Si/SiO₂ substrates, confirming the emitters originate from the hBN film itself and are compatible with device integration.
Atomic Confirmation: ADF-STEM analysis revealed atomic columns with approximately twice the intensity of surrounding atoms, providing direct evidence of Si species occupying sublattice sites.

Significance and Claims
The paper demonstrates a robust method for in-situ silicon doping of hBN via HT-MBE, establishing an optimal growth temperature window (>1200 °C) to host a diverse range of quantum emitters without requiring post-growth annealing or plasma cleaning. The authors propose that the observed optical transitions likely arise from silicon-related defect complexes. Plausible candidates include:

Substitutional Si at boron sites ( $Si_B$ ).
Out-of-plane dimer configurations such as $Si_NSi_i^-$ and $C_NSi_i^-$ .
Donor-Acceptor Pair (DAP) recombination involving $Si_B^+$ as a donor and native B/N acceptor defects.

The study concludes that while a direct experimental correlation between specific silicon-related crystallographic configurations and individual optical lines remains unresolved, this work provides a critical step toward a comprehensive understanding of in-situ doping in hBN. It expands the range of possible defect configurations beyond carbon and offers new avenues for engineering quantum photonic applications through controlled impurity incorporation.

The Experiment: Cooking with Silicon

The Results: Finding the Sweet Spot

What is Making the Light?

The Bottom Line

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