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Depth Control of Room-Temperature Quantum Emitters in Gallium Nitride

This paper identifies that naturally occurring quantum emitters in gallium nitride are confined near the substrate interface, limiting their optical coupling, and demonstrates that inserting a thin low-temperature GaN interlayer enables the precise, sub-60 nm depth control of bright, room-temperature quantum emitters, thereby overcoming integration barriers for efficient cavity-enhanced quantum emission.

Original authors: Alexandros Bampis, Johann Stachurski, Anna Schwab, Jean-François Carlin, Raphaël Butté, Nicolas Grandjean

Published 2026-08-27

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CC BY 4.0

Original authors: Alexandros Bampis, Johann Stachurski, Anna Schwab, Jean-François Carlin, Raphaël Butté, Nicolas Grandjean

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

To build a future where information travels securely across the globe, scientists are racing to create devices that can send individual particles of light, known as photons, one by one. These single-photon sources are the fundamental building blocks of quantum communication networks, which promise to be unhackable. However, for these networks to work in the real world, the devices must operate at room temperature, rather than requiring the extreme cold of liquid helium that most current technologies need. Nature offers a potential solution in the form of tiny flaws, or defects, inside solid crystals. When light hits these specific flaws, they glow with a single, pure color. Among the various materials being studied, a hard, blue crystal called gallium nitride has emerged as a particularly strong candidate because it can host these glowing defects and function perfectly at room temperature. The challenge, however, has been that these glowing spots appear randomly inside the crystal, making it nearly impossible to connect them to the tiny mirrors and channels needed to capture and direct their light.

A team of researchers at the Swiss Federal Institute of Technology in Lausanne has now solved this problem of randomness by learning exactly where these glowing defects form and how to place them where they are needed. In their study, they discovered that in gallium nitride grown on a standard sapphire base, these natural glowing defects are not scattered throughout the material. Instead, they are clustered tightly near the bottom, right where the crystal meets the sapphire. This location is a dead end for engineers; it is too deep to be reached by the optical structures designed to collect the light, meaning the potential of these defects has been largely wasted. By tracing the growth process step by step, the team identified that these defects are born during a specific, low-temperature phase of the crystal's creation, a thin layer that is usually buried at the very bottom.

Armed with this knowledge, the researchers developed a method to move this birth process to any depth they choose. They inserted a thin, low-temperature layer of gallium nitride into the middle of a much thicker crystal, effectively creating a new "floor" where the glowing defects would form. They proved this works by placing this special layer five micrometers deep inside a crystal and successfully finding the glowing spots exactly there. They repeated the experiment on a different type of crystal base, and the result was the same: the defects formed precisely where the low-temperature layer was placed, regardless of what the crystal was sitting on. This means scientists can now engineer these quantum light sources to sit in the perfect spot, such as right in the middle of a tiny electrical diode or inside a microscopic optical cavity, without being stuck at the bottom of the material.

The glowing spots created by this new method are just as bright and pure as the natural ones found at the bottom of the crystal. They emit light with a sharp, well-defined color and can be turned on and off incredibly fast, flashing more than a million times per second. Crucially, they emit light one photon at a time, a strict requirement for quantum communication. The researchers also confirmed that the surface of the crystal remains perfectly smooth after this process, which is essential for building the delicate optical structures needed to trap and guide the light. Furthermore, the layer where these defects form is electrically neutral, allowing it to be integrated into standard electronic devices without causing interference. This breakthrough transforms gallium nitride from a material with hidden, inaccessible potential into a platform where quantum light sources can be built with precision, paving the way for practical, room-temperature quantum devices that could one day revolutionize how we secure our digital world.

Technical Summary: Depth Control of Room-Temperature Quantum Emitters in Gallium Nitride

Problem Statement
Radiative point defects in gallium nitride (GaN) have emerged as promising candidates for room-temperature single-photon emission, operating across the visible to telecom O-band. Despite their potential, the integration of these quantum emitters (QEs) into photonic structures remains limited. Experimental photon extraction efficiencies are significantly lower than simulated predictions. A primary bottleneck is the lack of spatial control over emitter formation; naturally occurring QEs are believed to form randomly within the epilayer. Efficient coupling to photonic cavities (e.g., micropillars, Bragg gratings) and waveguides requires precise positioning of emitters both in-plane and in-depth. Without this control, emitters cannot be optimally aligned with optical modes or placed within specific device regions, such as the intrinsic layer of p-i-n diodes.

Methodology
The authors employed metalorganic vapor-phase epitaxy (MOVPE) to grow GaN epilayers on c-plane sapphire and free-standing (FS) GaN substrates. The study utilized a systematic approach to identify the origin of QE formation and engineer their depth:

Characterization of Natural Emitters: Room-temperature micro-photoluminescence (µ-PL) was used to map the spatial and spectral properties of naturally occurring QEs in standard GaN-on-sapphire templates. Depth-resolved µ-PL, combined with a high numerical aperture oil-immersion objective and piezo-stage Z-scanning, was used to determine the vertical distribution of emitters.
Growth Step Analysis: To pinpoint the formation mechanism, the authors simplified the standard GaN growth process step-by-step. They analyzed samples containing only the nitridated sapphire substrate, the low-temperature (LT) nucleation layer (NL) as-deposited, the annealed NL, and the full high-temperature (HT) GaN layer.
Engineered Depth Control: Based on the identification of the LT layer as the formation site, the authors designed samples with a thin LT GaN interlayer inserted at specific depths within a thick HT GaN buffer. This was tested on both sapphire and FS GaN substrates.
Verification and Localization: The depth and localization of intentionally introduced emitters were verified using cross-sectional scanning electron microscopy (SEM), atomic force microscopy (AFM), and inductively coupled plasma reactive-ion etching (ICP-RIE) combined with repeated µ-PL mapping. Secondary ion mass spectrometry (SIMS) and capacitance-voltage (C-V) profiling were used to correlate emitter presence with impurity concentrations and net doping levels.

Key Results

Origin and Natural Location: Naturally occurring QEs in GaN-on-sapphire are not randomly distributed but are localized near the GaN/sapphire interface. Specifically, they form during the deposition of the LT nucleation layer (~30 nm thick) and become optically active following the subsequent annealing step. Emitters were not detected in the nitridated sapphire alone or in the HT GaN layer grown without an LT interlayer.
Optical Properties: Naturally occurring emitters exhibit narrow zero-phonon line (ZPL) linewidths (median ~4.2 nm), high photon count rates (>2 MHz), strong antibunching ( $g^{(2)}(0)$ as low as 0.06), and high Debye-Waller factors (0.69–0.98).
Controlled Depth Insertion: By inserting a 30 nm LT GaN interlayer at arbitrary depths within HT GaN buffers, the authors successfully generated QEs at selected depths with a vertical accuracy better than 60 nm. This was demonstrated on both sapphire and FS GaN substrates, proving the method is substrate-independent.
Preserved Quality: Intentionally introduced emitters retain optical properties comparable to natural ones, including ZPLs spanning 530–920 nm, saturation count rates exceeding 1.5 MHz, and high Debye-Waller factors.
Material Quality: The GaN epilayer fully coalesces within less than 250 nm above the LT interlayer, resulting in a smooth surface (RMS roughness ~0.94 nm). This is crucial for integrating the emitters into photonic structures.
Impurity Correlation: SIMS measurements indicate high concentrations of carbon, oxygen, and hydrogen in the LT layer. Oxygen concentration correlates with defect density, suggesting oxygen may be part of the defect complex, though this requires further validation.
Doping Levels: C-V profiling established an upper limit for the net doping concentration of the LT interlayer at $1 \times 10^{16} \text{ cm}^{-3}$ , indicating the layer is highly compensated and effectively intrinsic.

Significance and Claims
The paper claims that identifying the LT nucleation layer as the source of QE formation allows for the deliberate engineering of emitter depth. This capability addresses the critical limitation of stochastic emitter positioning, enabling the placement of QEs within the intrinsic regions of p-i-n diode architectures and within GaN-based optical cavities.

The authors state that these results mark a "decisive step toward efficient emitter-cavity coupling" and enable the realization of cavity-enhanced quantum emission in GaN at room temperature. By achieving sub-60 nm positioning accuracy and ensuring full coalescence of the epilayer, the work establishes a foundation for the scalable fabrication of quantum photonic devices. The substrate-independent nature of the method further supports the integration of GaN QEs into high-quality optical cavities, such as those built on FS GaN with distributed Bragg reflectors (DBRs). The low net doping of the LT layer is highlighted as advantageous for reducing spectral diffusion and enabling electrical tuning in future device architectures.

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