Composition-structure-property relationships in thermally evaporated ZnS thin films and Ag/n-ZnS/FTO Schottky barrier diodes

Sunlight is a powerful resource, but capturing it efficiently requires materials that can act as a clear, protective window while allowing electricity to flow freely. In the world of solar energy and electronic sensors, scientists often look for semiconductors that are transparent to visible light yet capable of conducting electrical current when triggered. One such material is zinc sulphide, a compound made of zinc and sulphur atoms. It is naturally abundant, non-toxic, and possesses a wide energy gap that makes it ideal for letting light pass through without absorbing it, a quality essential for the top layers of solar cells. However, the performance of zinc sulphide depends heavily on how it is built at the atomic level. If the atoms are arranged haphazardly or if the film is too thick or too thin, the material may become cloudy or resist the flow of electricity, rendering it useless for high-tech devices.

Researchers at the University of Jammu set out to find the perfect recipe for creating these zinc sulphide films. They used a technique called thermal evaporation, where solid zinc sulphide powder is heated in a vacuum until it turns into a vapor, which then settles onto a glass surface to form a thin, solid layer. By carefully adjusting two main variables—the thickness of the layer and the temperature of the glass during deposition—they aimed to discover which conditions produced the most orderly and electrically efficient films. Their goal was not just to make a film, but to understand how the physical structure of the material, the arrangement of its atoms, and its chemical makeup directly influenced its ability to transmit light and conduct electricity.

The team created a series of films with thicknesses ranging from 100 to 500 nanometers, a scale so small that a human hair is thousands of times wider. They also tested different substrate temperatures, heating the glass to various levels before and during the coating process. Using powerful microscopes and X-ray tools, they examined the internal structure of each film. They found that the arrangement of the atoms was not random; the films naturally formed a cubic crystal structure with a strong preference for a specific orientation, much like a stack of bricks all aligned in the same direction. However, this order was not uniform across all samples. The films that were 300 nanometers thick showed the most consistent and well-organized crystal structure, with larger, more uniform grains than the thinner or thicker versions.

Chemical analysis revealed that the balance between zinc and sulphur atoms shifted depending on how the film was made. In some samples, there was a slight excess of zinc, which created tiny defects in the crystal lattice. These defects, while seemingly imperfections, actually played a crucial role in the film's electrical behavior. The zinc-rich films acted as n-type semiconductors, meaning they carried a negative electrical charge. The researchers discovered that the 300-nanometer-thick film, deposited at a modest temperature of 323 Kelvin, offered the best overall performance. This specific sample exhibited a high degree of transparency, allowing up to 92 percent of visible light to pass through, and possessed a direct energy gap between 3.40 and 3.50 electron volts. Most importantly, it allowed electrical charges to move with a mobility of 49.3 square centimeters per volt-second, a measure of how easily electrons can travel through the material, while maintaining a low electrical resistance.

To prove that these films could function in real electronic devices, the researchers built simple diodes by placing a silver contact on top of the optimized zinc sulphide layer, which sat on a conductive glass base. These devices acted as Schottky barrier diodes, which are components that allow current to flow easily in one direction but block it in the other, a fundamental requirement for rectifying alternating current into direct current. The silver-zinc sulphide junctions performed exceptionally well, showing a strong ability to rectify current with a ratio of nearly 6 to 1 at a voltage of one volt. The electrical barrier at the interface between the metal and the semiconductor was measured to be around 0.67 to 0.72 electron volts, a value that indicates a high-quality, stable connection. The behavior of these diodes confirmed that the underlying zinc sulphide film was of high quality, with minimal defects interfering with the flow of electricity.

The study concluded that the relationship between the film's thickness, its temperature during creation, and its final properties is complex but manageable. While higher temperatures helped the atoms arrange themselves more neatly in some ways, the specific combination of a 300-nanometer thickness and a 323 Kelvin substrate temperature produced the most balanced material. This optimal film combined a wide energy gap, high transparency, and excellent electrical conductivity, making it a strong candidate for use as a window or buffer layer in solar cells and other optoelectronic devices. By fine-tuning the deposition process, the researchers demonstrated that it is possible to grow zinc sulphide films that are not only structurally sound but also electrically superior, offering a promising, non-toxic alternative to other materials currently used in the industry.