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Tellurium trioxide as an n-type wide-gap semiconductor with ultrahigh work functions from first principles

First-principles calculations reveal that $\eta$ -TeO $_3$ is a promising n-type wide-gap semiconductor with ultrahigh work functions (~8 eV) suitable for optoelectronic applications, provided that oxygen chemical potential is precisely controlled to suppress surface oxygen vacancies that would otherwise degrade its electronic performance.

Original authors: Tianwei Wang, Teruya Nagafuji, Kota Hanzawa, Hidenori Hiramatsu, Fumiyasu Oba

Published 2026-10-14

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CC BY 4.0

Original authors: Tianwei Wang, Teruya Nagafuji, Kota Hanzawa, Hidenori Hiramatsu, Fumiyasu Oba

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 electronics, the materials that make up our devices are often chosen for how easily they let electricity flow. But for the next generation of technology, scientists are looking for something different: materials that are wide-gap semiconductors. These are substances that normally block electricity but can be coaxed into conducting it under specific conditions, making them ideal for controlling the flow of energy in light-emitting devices and solar cells. A critical feature for these materials is something called the work function, which is essentially a measure of how tightly a material holds onto its electrons. A high work function means the material is very good at pulling electrons out of a circuit or pushing them in, a property that is essential for efficient energy transfer in advanced electronics. While some existing materials can achieve this, they often fall short of the extreme levels needed for the most demanding applications, prompting researchers to search for new candidates that combine a wide energy gap with an exceptionally high ability to manage electron flow.

A team of researchers has turned their attention to a specific chemical compound, beta-tellurium trioxide, to see if it holds the key to these extreme properties. Using powerful computer simulations that model the behavior of atoms from first principles, the team investigated the fundamental nature of this material. They found that beta-tellurium trioxide is indeed a wide-gap semiconductor, with a gap large enough to make it nearly transparent to visible light, a trait that would be perfect for clear, conductive layers in displays or solar panels. More importantly, the simulations revealed that this material possesses an ultrahigh work function, reaching values as high as 8 electron volts. This is significantly higher than many standard electrode materials currently in use, suggesting that beta-tellurium trioxide could serve as a superior layer for injecting or extracting electrical charges in next-generation devices.

The study also delved into how this material behaves when tiny impurities are introduced, a process known as doping, which is necessary to make a semiconductor conduct electricity. The researchers discovered that the material naturally resists becoming a conductor on its own, but it responds very well to specific additions. When atoms like hydrogen, lithium, sodium, copper, magnesium, zinc, or fluorine are added, they act as shallow donors, meaning they easily release electrons to carry current. This confirms that the material can be tuned to become an n-type semiconductor, which conducts electricity via negative charges. However, the team also found that the material is sensitive to missing oxygen atoms. While these missing atoms can trap electrons and reduce the material's efficiency, the simulations showed that creating them requires a significant amount of energy. This suggests that if the material is grown or treated in an environment rich in oxygen, these problematic defects can be kept to a minimum, preserving the material's excellent electronic properties.

Perhaps the most significant finding relates to the surface of the material, where the work function is actually measured and utilized. The researchers modeled various flat surfaces of the crystal and found that the ultrahigh work function is a real and robust feature, but it is fragile. The presence of missing oxygen atoms on the surface, even in small numbers, tends to lower the work function by disrupting the electrical landscape at the boundary. The simulations indicated that to maintain the material's record-breaking ability to manage electrons, it is crucial to prevent these oxygen vacancies from forming on the surface. This implies that practical applications would require careful control over the chemical environment during manufacturing, ensuring an abundance of oxygen to keep the surface perfect.

Ultimately, the work presents a compelling case for beta-tellurium trioxide as a promising candidate for the future of transparent electronics. The combination of its wide energy gap, its ability to be doped for conductivity, and its naturally ultrahigh work function makes it a strong contender for use in hole-injection layers and transparent electrodes. While the material's performance is highly dependent on keeping its surface free of oxygen defects, the study provides a clear roadmap for achieving this through precise control of the chemical conditions during synthesis. By understanding how the atoms arrange themselves and how they react to impurities and surface changes, scientists now have the guidance needed to potentially bring this high-performance material from the computer screen into real-world devices that power our lights and harvest our energy.

Technical Summary: Tellurium Trioxide as an n-type Wide-Gap Semiconductor with Ultrahigh Work Functions from First Principles

Problem Statement
The development of next-generation electronic and optoelectronic devices, such as light-emitting diodes and photovoltaic cells, requires wide-gap semiconductors with extreme work functions. These materials are critical for hole injection, extraction, and transport layers, as well as transparent electrodes. While materials like MoO $_x$ have been studied for their high work functions (6.8–6.9 eV for $\alpha$ -MoO $_3$ ), there is a need to identify and understand other oxide candidates that possess similar or superior electronic properties, specifically regarding band alignment and the stability of their electronic states against defects. Despite reports of electrical conduction in amorphous TeO $_3$ , the fundamental bulk, defect, and surface properties of crystalline $\beta$ -TeO $_3$ remain largely unexplored.

Methodology
This study employs first-principles calculations based on the generalised Kohn-Sham framework using the projector augmented-wave (PAW) method as implemented in the VASP code. The exchange-correlation term is approximated using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional with a Fock exchange mixing parameter of 0.29, calibrated to reproduce the experimental band gap of TeO $_3$ .

The computational approach includes:

Bulk Properties: Optimization of the primitive and conventional cells of $\beta$ -TeO $_3$ (trigonal $R\bar{3}c$ space group) to determine lattice parameters, band structure, and effective masses.
Defect Engineering: Investigation of native point defects (vacancies, interstitials, antisites) and extrinsic impurities (H, Li, Na, Cu, Mg, Zn, and F) using supercells (2 $\times$ 2 $\times$ 1 expansion). Defect formation energies are calculated under O-rich and O-poor thermodynamic limits, corrected for finite-size errors using the extended Freysoldt-Neugebauer-Van de Walle (eFNV) scheme.
Surface Properties: Construction of slab models for low-index nonpolar surfaces (e.g., (12 $\bar{3}$ 2), (11 $\bar{2}$ 0), (10 $\bar{1}$ 0)) with varying oxygen vacancy concentrations. Surface energies, electron affinities (EA), and ionisation potentials (IP) are evaluated to estimate work functions.
Carrier Statistics: Equilibrium carrier concentrations and Fermi levels are determined by self-consistently solving the charge neutrality equation, incorporating Boltzmann distributions for defect concentrations.

Key Contributions and Results

Bulk Electronic Structure:
- $\beta$ -TeO $_3$ is confirmed as a wide-gap semiconductor with an indirect band gap of 3.13 eV, consistent with experimental estimates (~3.1 eV).
- The conduction band minimum (CBM) is located at the $\Gamma$ point, primarily composed of Te-5s orbitals with O-2p contributions. The valence band maximum (VBM) lies between $\Gamma$ and $L$ , dominated by O-2p orbitals.
- The material exhibits relatively low electron effective masses (0.64 $m_0$ and 0.44 $m_0$ along different axes), comparable to established n-type oxides like ZnO and In $_2$ O $_3$ , suggesting suitability for n-type conductivity. Conversely, high hole effective masses indicate that shallow p-type doping is unlikely.
Defect Physics and Doping:
- Native Defects: Native oxygen vacancies ( $V_O$ ) act as deep traps, localizing electrons at neighboring Te sites rather than acting as shallow donors. Their formation energies are high (~2 eV under O-rich conditions), suggesting low concentrations in thermodynamic equilibrium and a minor role in carrier compensation.
- Extrinsic Dopants: Interstitial impurities (H, Li, Na, Cu, Mg, Zn) and substitutional F (on O sites) are identified as shallow donors. Specifically, H, Li, and F exhibit low formation energies and no in-gap transition levels, making them effective for n-type doping. The study suggests that unintentional H incorporation could account for the n-type conductivity observed in nominally undoped TeO $_3$ .
- Carrier Concentrations: With shallow donors (e.g., H, Li, F) quenched from high temperatures, carrier concentrations of at least $10^{16}$ cm $^{-3}$ are predicted, with Fermi levels positioned close to the CBM.
Surface Properties and Work Functions:
- Ultrahigh Work Functions: Ideal stoichiometric surfaces of $\beta$ -TeO $_3$ exhibit ultrahigh work functions, with electron affinities (EAs) ranging from 6.86 to 8.46 eV across the investigated orientations. The (12 $\bar{3}$ 2) surface is identified as the most stable stoichiometric orientation among those studied. These values exceed those of prototypical high-work-function materials like Pt (5.65 eV) and $\alpha$ -MoO $_3$ (~6.6–6.7 eV).
- Mechanism: The high work functions are attributed to the high cation oxidation state (+6 for Te) and significant surface dipole contributions from undercoordinated Te-O bonds pointing toward the vacuum.
- Impact of Oxygen Vacancies: The formation of ordered oxygen vacancies on surfaces is energetically favorable and significantly reduces the work function (by ~0.5 eV for low concentrations and up to ~1.5 eV for high concentrations) by disrupting the surface dipole layers. For instance, the EA of the (10 $\bar{1}$ 0) surface drops to 5.33 eV with high oxygen deficiency.

Significance and Claims
The paper claims that $\beta$ -TeO $_3$ is a promising candidate for n-type wide-gap semiconductor applications requiring ultrahigh work functions, such as hole injection/extraction/transport layers and transparent electrodes. The study highlights that the exceptional electronic performance of $\beta$ -TeO $_3$ is contingent upon the precise control of the oxygen chemical potential. To preserve the ultrahigh work functions and prevent carrier compensation, synthesis and processing must suppress surface oxygen vacancy formation, potentially requiring conditions with high oxygen chemical potentials (e.g., oxygen plasma treatment or ozone precursors). The findings provide a theoretical basis for carrier doping strategies and surface tuning to realize the practical potential of $\beta$ -TeO $_3$ in optoelectronic devices.

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