🔬 materials science

Modification of optical properties in ultrathin ALD-grown NbN layers using oxygen plasma treatment

This study demonstrates that oxygen plasma treatment of 10 nm ALD-grown NbN films induces surface oxidation, evidenced by a new absorption feature at 4.9 eV and a reduction in effective carrier concentration and mobility as treatment duration increases.

Original authors: Mustafa Teke, Nitesh Yerra, Alireza Sadeghi, Mario Ziegler, Detlef Born, Nan Du, Marco Diegel, Uwe Hübner, Sahitya V. Vegesna, Heidemarie Krüger

Published 2026-10-26

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CC BY 4.0

Original authors: Mustafa Teke, Nitesh Yerra, Alireza Sadeghi, Mario Ziegler, Detlef Born, Nan Du, Marco Diegel, Uwe Hübner, Sahitya V. Vegesna, Heidemarie Krüger

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

In the world of modern technology, some materials are prized not for what they do, but for what they refuse to do. Niobium nitride is one such material. It is a metal that, when cooled to extreme cold, loses all electrical resistance, becoming a superconductor. This property makes it invaluable for building the most sensitive light detectors in existence, devices capable of counting individual photons of light. These detectors are the eyes of next-generation quantum computers and deep-space telescopes. However, for these devices to work perfectly, the niobium nitride must be grown in incredibly thin layers, and its surface must remain pristine. Even a tiny amount of oxygen, which is everywhere in the air, can sneak into the material and change how it interacts with light. Understanding exactly how this happens is crucial for engineers trying to build better sensors.

Researchers at the Leibniz Institute of Photonic Technology and Friedrich Schiller University Jena set out to understand this delicate interaction. They grew ultra-thin films of niobium nitride, just ten nanometers thick, on silicon wafers. To simulate the effects of oxygen exposure in a controlled way, they subjected these films to a stream of oxygen plasma for varying lengths of time: zero seconds, twenty seconds, forty seconds, and sixty seconds. Using a sophisticated optical technique that measures how light bounces off and passes through the material, they mapped out the changes in the film's properties. Their goal was to see if and how the oxygen altered the material's ability to conduct electricity and absorb light, which are the two key traits needed for high-performance photon detectors.

The results revealed a clear transformation. As the treatment time increased, the material began to behave less like a perfect metal and more like a slightly different substance. The most striking change appeared in the way the film absorbed light at high energies. After the oxygen treatment, a new, distinct feature emerged in the absorption spectrum near 4.9 electron volts. This feature was absent in the untreated samples and indicates that the oxygen had chemically reacted with the surface of the niobium nitride, creating a layer of niobium oxide. This is not merely a surface stain; it represents a fundamental shift in the electronic structure of the topmost atoms. The researchers observed that the original absorption patterns of the pure niobium nitride shifted slightly, while this new oxygen-related absorption band grew stronger with longer exposure times.

Beyond the surface chemistry, the treatment also changed how the free electrons inside the material moved. In a metal, these electrons act like a fluid that carries energy and light. The study found that as the oxygen treatment time increased, the number of these free electrons dropped. Specifically, the concentration of charge carriers fell from 1.20 × 10²² per cubic centimeter in the untreated sample to 1.03 × 10²² per cubic centimeter in the sample treated for sixty seconds. At the same time, the ease with which these electrons could move through the material, known as mobility, also decreased. This suggests that the oxygen atoms, by bonding with the niobium, were effectively trapping some of the electrons or scattering them, making it harder for the electrical current to flow smoothly.

These microscopic changes had a measurable effect on the macroscopic behavior of the film. The researchers calculated that the treated films became slightly less reflective, particularly at higher energy levels. In the context of superconducting detectors, this is a significant finding. If a detector reflects too much light, it misses the photons it is supposed to catch. The study suggests that by carefully controlling the oxidation of the surface, it might be possible to tune the material to absorb more light rather than bouncing it away. While the oxygen treatment reduced the superconducting properties slightly by depleting the free carriers, it simultaneously enhanced the material's ability to interact with light in a specific way.

The work provides a clear picture of how oxygen plasma modifies the optical and electronic landscape of niobium nitride. It confirms that the treatment creates a distinct oxide layer that introduces new ways for the material to absorb energy, while simultaneously reducing the density of mobile electrons. For scientists designing these ultra-sensitive detectors, this offers a new tool. Instead of viewing surface oxidation as a defect to be avoided at all costs, they may be able to use controlled oxidation to fine-tune the optical absorption of their devices. The study does not claim to have solved the problem of building perfect detectors, but it establishes a solid foundation for understanding how the surface chemistry of these films dictates their performance, opening the door to more precise engineering of the next generation of quantum sensors.

Technical Summary: Modification of Optical Properties in Ultrathin ALD-grown NbN Layers Using Oxygen Plasma Treatment

Problem Statement
Niobium nitride (NbN) is a critical material for superconducting nanowire single-photon detectors (SNSPDs), hot-electron bolometer mixers, and microwave resonators due to its superior electrical, optical, and superconducting properties. However, these properties are highly sensitive to film thickness, microstructure, and composition. In ultrathin films, surface oxidation can reduce the effective thickness of the superconducting layer and alter the near-surface composition, potentially depleting delocalized carriers. While oxidation is known to occur, a precise characterization of how controlled oxygen exposure modifies the optical constants, dielectric function, and free-carrier response of ultrathin NbN films grown by Atomic Layer Deposition (ALD) remains necessary to optimize device performance.

Methodology
The study utilized 10 nm nominal thickness NbN films grown on Al2O3-buffered Si(100) substrates via Plasma-Enhanced Atomic Layer Deposition (PEALD). The deposition employed a (tert-Butylimido)-tris(diethylamino)niobium (TBTDEN) precursor and H2 plasma. Four identical samples were subjected to oxygen plasma treatment for varying durations: 0 s (untreated), 20 s, 40 s, and 60 s.

Structural characterization was performed using X-ray Reflectivity (XRR) to determine layer thicknesses (NbN, Al2O3, and native SiO2) prior to optical modeling. Optical properties were investigated using Spectroscopic Ellipsometry (SE) across two spectral ranges: ultraviolet–visible–near-infrared (1.24–6.20 eV) and infrared (0.041–0.827 eV).

The SE data were modeled using a Drude–Lorentz dispersion model within a multi-layer optical structure (Si substrate, native SiO2, Al2O3, and NbN). The layer thicknesses derived from XRR were fixed parameters to ensure reliable extraction of the dielectric function. The model incorporated three Lorentz oscillators to describe bound-electron (interband) transitions and one Drude term to account for free-carrier response.

Key Results

Structural Stability: XRR analysis confirmed that the nominal NbN thickness remained stable (ranging from 11.4 to 12.9 nm) across all treatment times, with no significant change in the underlying Al2O3 buffer or native SiO2 layers.
Emergence of New Absorption Feature: A distinct absorption feature appeared near 4.9 eV in the samples treated with oxygen plasma (20 s, 40 s, and 60 s), which was absent in the untreated sample. This feature was modeled using a second UV Lorentz oscillator. Its amplitude decreased with longer treatment times, while the primary UV interband transition (originally at ~7.33 eV) exhibited a red shift and reduced amplitude.
Free-Carrier Reduction: The Drude analysis revealed a systematic reduction in effective free-carrier concentration ( $N$ $N$ ) and mobility ( $\mu$ $μ$ ) as treatment time increased.
- $N$ decreased from $1.20 \times 10^{22}$ cm $^{-3}$ (0 s) to $1.03 \times 10^{22}$ cm $^{-3}$ (60 s).
- $\mu$ decreased from 2.44 to 2.27 cm $^2$ V $^{-1}$ s $^{-1}$ .
- Consequently, DC conductivity decreased, and resistivity increased.
Optical Constants and Reflectivity: The real part of the dielectric function ( $\epsilon_1$ ) shifted from slightly negative (metallic behavior) in the untreated sample to positive after treatment, attributed to the interplay between the reduced Drude contribution and positive dispersive Lorentz terms. The effective reflectivity ( $R_{eff}$ ) decreased with increasing treatment time, particularly in the high-energy region (>7 eV).

Key Contributions
The paper provides a quantitative optical characterization of oxygen-induced modifications in ALD-grown NbN films. The primary contributions include:

The identification of a specific 4.9 eV absorption band linked to the formation of Nb–O bonds and surface NbOx species (likely Nb2O5).
The demonstration that oxygen plasma treatment acts as a controlled mechanism to deplete free carriers and reduce mobility in NbN films without significantly altering the bulk film thickness.
The establishment of a Drude–Lorentz model that successfully separates the contributions of interband transitions (including the new oxygen-related transition) and free-carrier dynamics in these ultrathin films.

Significance and Claims
The authors state that these results provide a basis for understanding how oxygen plasma treatment affects the optical and electronic properties of NbN thin films. Specifically, the observed reduction in reflectivity and the modification of free-carrier properties suggest a potential pathway for enhancing photon absorption in NbN-based SNSPDs. In such devices, boundaries with reduced superconductivity can exhibit normal-conducting behavior that increases reflectivity; the study suggests that controlled oxidation might mitigate this by lowering reflectivity. The work does not claim to have demonstrated improved detector performance directly but establishes the optical mechanisms that could be leveraged for such optimization.

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