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Effect of substrate temperature on the structural, optical, and photoconductive properties of co-evaporated ZnS1-xTex thin films

This study demonstrates that varying the substrate temperature during thermal co-evaporation effectively tunes the composition, crystal structure, and optoelectronic properties of ZnS1-xTex thin films, enabling their potential application in broadband visible photodetection.

Original authors: Camilo Andrés Velásquez Andrade, Nestor Jaime Torres

Published 2026-09-29

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Camilo Andrés Velásquez Andrade, Nestor Jaime Torres

Original paper licensed under CC BY 4.0 (https://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

Light is more than just what allows us to see; it is a tool that scientists use to probe the hidden nature of matter. In the world of semiconductors, materials that form the backbone of modern electronics and solar cells, the color of light a material absorbs or emits is directly tied to its internal structure. Think of these materials as having a specific energy threshold, a gate that light must be strong enough to open to trigger an electrical response. By mixing different elements together, researchers can tune this gate, shifting the material's sensitivity from one color of light to another. This ability to customize how a material interacts with light is crucial for building better sensors, cameras, and communication devices. The challenge lies in finding the right recipe to mix these elements so that the final product behaves exactly as intended, a task that often depends as much on how the material is grown as on what it is made of.

In a recent study, researchers set out to understand how the temperature of a glass surface affects the properties of a specific mixture of zinc, sulfur, and tellurium. They created thin films of this material, known as ZnS1-xTex, using a process called thermal co-evaporation. Imagine a high-tech vacuum chamber where solid powders of zinc sulfide and zinc telluride are heated until they turn into vapor. This vapor then rises and settles onto a glass slide, forming a microscopic layer. The researchers controlled the temperature of this glass slide, heating it to four different levels: 100, 150, 200, and 250 degrees Celsius. Their goal was to see if simply changing the heat of the glass would alter the chemical makeup of the film and, consequently, how it interacts with light.

The results were surprising and revealed that the temperature of the glass played a decisive role in determining the film's composition. When the glass was kept at 100 degrees Celsius, the resulting film was rich in tellurium. As the temperature rose to 200 degrees, the amount of tellurium dropped significantly, and the film became dominated by sulfur. However, when the temperature was pushed to 250 degrees, the trend reversed, and the tellurium content increased again. This non-linear behavior showed that the heat of the substrate does not just warm the material; it actively influences which atoms stick to the surface and which bounce off, creating a complex relationship between the growth conditions and the final chemical recipe.

These changes in chemical makeup led to distinct shifts in the physical and optical properties of the films. The researchers used X-rays to examine the crystal structure and found that all the films formed a cubic pattern, but the size of the tiny crystals within the film varied with temperature. More importantly, the temperature dictated how the films absorbed light. The films grown at lower temperatures absorbed light across a broad range of the visible spectrum, making them sensitive to many different colors. In contrast, the film grown at the highest temperature, 250 degrees Celsius, shifted its sensitivity toward longer wavelengths, responding more strongly to the red end of the spectrum. This means that by simply adjusting the heat during the manufacturing process, the researchers could tune the material to act as a detector for specific colors of light.

The study also looked at how these films glow when exposed to light and how they conduct electricity when illuminated. The film grown at the lowest temperature emitted a broad glow that could be broken down into two distinct colors, suggesting that the light was coming from multiple sources within the material. When the researchers measured the electrical current generated by light, they found that the films grown at 100 and 150 degrees responded to almost the entire visible spectrum, while the 250-degree film had a response that stretched further into the red. These findings confirm that the substrate temperature is a powerful lever for controlling the behavior of these materials. While the films are not yet ready to be used in commercial devices without further testing of their speed and stability, the study demonstrates a clear path toward creating custom light sensors by mastering the simple act of heating the glass during growth.

Technical Summary: Effect of Substrate Temperature on the Structural, Optical, and Photoconductive Properties of Co-evaporated ZnS1−xTex Thin Films

Problem Statement
II-VI compound semiconductors, particularly the ZnS1−xTex ternary alloy system, offer significant potential for optoelectronic applications due to the wide disparity in lattice constants and band-gap energies between their binary end members (ZnS and ZnTe). While the band-gap energy of ZnS1−xTex is known to exhibit a marked nonlinear dependence on composition (band-gap bowing) rather than simple linear interpolation, the precise control of this composition during thin-film deposition remains a challenge. Specifically, the influence of growth conditions, such as substrate temperature, on the final stoichiometry, crystal structure, and optoelectronic response of co-evaporated ZnS1−xTex films is not fully established. This study addresses the need to understand how substrate temperature affects the S/Te ratio and the resulting material properties, aiming to determine if deposition conditions can be used as a primary tool to tailor the spectral response of these films for broadband visible photodetection.

Methodology
The researchers deposited ZnS1−xTex thin films on Corning glass substrates using high-vacuum thermal co-evaporation. The process utilized separate Knudsen cells for ZnS and ZnTe precursors (99.995% purity), which were heated to approximately 1000 °C. Depositions were conducted for 18 minutes at a base pressure of ~1.0 × 10⁻⁵ mbar.

The study systematically varied the substrate temperature ( $T_s$ ) to four distinct levels: 100, 150, 200, and 250 °C, while keeping other deposition parameters constant. The films were characterized using the following techniques:

Compositional Analysis: Energy-dispersive X-ray spectroscopy (EDX) was used to determine atomic concentrations and calculate the Te fraction ( $x$ ).
Structural Analysis: X-ray diffraction (XRD) in Bragg–Brentano geometry was employed to identify crystal structure, preferred orientation, lattice parameters, and crystallite size (via the Scherrer equation).
Optical Characterization: Transmittance spectra (300–2500 nm) were measured to determine film thickness, refractive index (using Swanepoel's envelope method), and absorption coefficients. The optical band gap ( $E_g$ ) was calculated using the Tauc relation for direct allowed transitions.
Photoluminescence (PL): Room-temperature PL measurements were performed on the film deposited at 100 °C using a Hg lamp excitation source.
Photoconductivity: Spectral photoconductive responses were measured at room temperature using a halogen lamp and a monochromator, with Cu electrodes deposited in a parallel configuration (1 mm separation) and a 90 V bias applied.

Key Results

Composition: Contrary to a monotonic trend, the Te fraction ( $x$ ) varied significantly and non-linearly with substrate temperature. EDX analysis yielded $x$ values of 0.90, 0.67, 0.13, and 0.39 for films deposited at 100, 150, 200, and 250 °C, respectively. This indicates that substrate temperature strongly influences the incorporation or re-emission of S and Te species during growth.
Structural Properties: All films exhibited a polycrystalline cubic zinc-blende structure with a preferred (111) orientation. As $T_s$ increased, the lattice constant decreased from 6.081 Å (at 100 °C) to 5.914 Å (at 250 °C), consistent with the substitution of larger Te atoms by smaller S atoms, though the trend was not strictly parallel to the composition changes. Crystallite sizes ranged from 17.4 nm (at 200 °C) to 40.4 nm (at 100 °C).
Optical Properties: The optical band gap ranged from 2.19 eV to 3.13 eV. Notably, the band gap did not follow a simple monotonic relationship with the Te fraction; films with $x=0.90$ and $x=0.67$ both exhibited $E_g = 2.19$ eV, while the $x=0.13$ film showed 2.56 eV, and the $x=0.39$ film showed 3.13 eV.
Photoluminescence: The film with the highest Te content ( $x=0.90$ , 100 °C) displayed a broad emission band fitted with two Gaussian components centered at approximately 515 nm and 579 nm, suggesting multiple radiative recombination channels.
Photoconductivity: The spectral photoconductive response was highly dependent on $T_s$ . Films deposited at 100 and 150 °C showed broad responses across almost the entire visible range with maxima near 550 nm. The 200 °C film was restricted to ~400–650 nm. The 250 °C film exhibited a red-shifted maximum near 600 nm with a response extending to ~800 nm.

Significance and Claims
The authors conclude that substrate temperature serves as an effective parameter for modifying both the composition and the optoelectronic response of co-evaporated ZnS1−xTex thin films. The study demonstrates that the final alloy composition is not solely determined by the nominal precursor amounts but is significantly influenced by growth conditions, leading to non-monotonic variations in Te fraction.

The paper claims that these films possess tunable spectral photoconductive responses suitable for broadband visible photodetection. Specifically, the ability to shift the spectral response from the green region (~~550 nm) to the red/near-infrared region (~~600–800 nm) by adjusting the substrate temperature highlights the potential of this system for tailored photodetector applications. However, the authors maintain a modest stance regarding the immediate applicability of these films, noting that while the spectral tunability is established, actual device performance metrics—such as responsivity, specific detectivity, dark current, noise, temporal response, and stability—must still be quantified before these materials can be rigorously compared to established photodetector technologies. The work supports further investigation into ZnS1−xTex as a candidate material for broadband visible photodetection, provided these additional performance parameters are characterized.

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