Influence of annealing temperature on the structural, morphological and optical properties of sol–gel spin-coated NiO thin films on Si and SiO₂ substrates

Silicon is the bedrock of modern electronics, the material that powers everything from smartphones to solar panels. Yet, to make these devices work efficiently, engineers often need to pair silicon with other materials that can manage the flow of electricity in specific ways. One such material is nickel oxide, a ceramic-like substance that naturally conducts electricity in a particular direction, known as p-type conductivity. This property makes it a valuable partner for silicon in creating advanced sensors, solar cells, and memory devices. However, for nickel oxide to work well in these roles, it must be grown as a thin, uniform film with a highly ordered internal structure. If the atoms in the film are jumbled or stressed, the material becomes less effective. The key to organizing these atoms lies in heat. By heating the film after it is created, scientists can encourage the tiny crystals within it to grow larger and settle into a more perfect arrangement, but finding the exact temperature that achieves this without damaging the film is a delicate balancing act.

In a recent study, researchers at the National University of Uzbekistan set out to find this perfect balance. They created thin films of nickel oxide on two different types of silicon-based surfaces: one made of pure silicon and another coated with a layer of silicon dioxide, a common insulating material. Using a technique called sol-gel spin coating, which involves spinning a liquid chemical mixture onto the surface to spread it evenly, they built up the films layer by layer. Once the films were in place, they subjected them to heat treatment, or annealing, at three different temperatures: 400, 500, and 600 degrees Celsius. The goal was to watch how the internal structure and the way light interacts with the film changed as the heat increased, determining which temperature produced the highest quality material for future devices.

The researchers examined the films using powerful tools that could see the arrangement of atoms and the shape of the surface. At the lowest temperature of 400 degrees Celsius, the films were still quite disordered, composed of extremely tiny crystal clusters that were barely visible to the instruments. As the temperature rose to 500 degrees, these clusters began to merge and grow. By the time the films reached 600 degrees, the transformation was complete. The tiny crystals had grown significantly, reaching a size of roughly 10 to 11 nanometers, and the internal stress that had been squeezing the atoms out of alignment had relaxed. The films became dense and free of cracks, with a smooth surface made of fine, round grains. This level of order was not just a visual improvement; it meant the material was structurally much closer to a perfect, solid block of nickel oxide found in nature.

This structural improvement had a direct and measurable impact on how the films handled light. The researchers measured the energy gap, which is the specific amount of energy required for the material to absorb light and conduct electricity. On the pure silicon substrates, this energy gap widened steadily as the temperature increased, moving from 3.42 electron volts at 400 degrees to 3.55 electron volts at 600 degrees. On the silicon dioxide substrates, the energy gap remained consistently high, hovering between 3.63 and 3.66 electron volts. More importantly, the study looked at the "disorder energy," a measure of how messy the atomic structure is. At lower temperatures, this disorder was high, but at 600 degrees, it dropped significantly on both types of substrates. This drop confirmed that the heat had successfully removed the defects and irregularities that were hindering the material's performance.

The study explicitly ruled out the idea that the changes in light absorption were caused by the tiny size of the crystals themselves, a phenomenon known as quantum confinement. If that were the case, the energy gap would have behaved differently as the crystals grew. Instead, the data showed that the changes were driven by the healing of the crystal lattice and the removal of defects. The researchers found that 600 degrees Celsius was the optimal temperature for this process. At this heat, the films achieved their largest crystal size, their lowest internal stress, and the cleanest atomic arrangement. While the films on the two different substrates showed slight differences in their exact energy values, both reached their peak quality at this same temperature.

Ultimately, the work provides a clear roadmap for creating high-quality nickel oxide films for use with silicon technology. By simply adjusting the heat treatment to 600 degrees Celsius, manufacturers can produce films that are dense, crack-free, and highly ordered. These films are ready to be integrated into next-generation electronic components, where their improved structure will allow them to function more efficiently. The findings suggest that for anyone looking to build devices that combine nickel oxide with silicon, there is no need to guess at the right conditions; the path to the best material is straightforward and lies in reaching that specific, higher temperature.