A research team led by Jay Switzer at Missouri University of Science and Technology has discovered how to grow epitaxial films of cesium lead bromide (CsPbBr₃), lead iodide (PbI₂), sodium chloride (NaCl), and zinc oxide (ZnO) using spin coating. The results, published recently in Science, could represent a path to create high-quality thin films for devices using low-energy, straightforward fabrication methods.

Epitaxial films are often required for electronic and optical devices because of the lack of grain boundaries or defects that trap carriers or cause them to recombine. Films are typically grown using either chemical vapor deposition or molecular-beam epitaxy, both of which require high temperatures and ultrahigh vacuum. The need for specialized equipment in these techniques, however, makes growing epitaxial films very expensive.

To overcome this, researchers have investigated more economical methods for creating single-crystal thin films. Epitaxial films of perovskite materials have been formed by annealing thin films obtained by spin coating. Highly crystalline perovskite devices with large millimeter-sized grains have also been created by spin coating warm mixtures of precursor solutions at elevated temperatures.

Switzer’s group demonstrated that some materials can be grown epitaxially using spin coating alone. “We dissolved materials in supersaturated precursor solutions and then recrystallized them on the substrate surface during the spin-coating process,” Switzer says.  During spin coating, a thin layer in the solution is created on the surface of the substrate that encourages nucleation. The thickness of the film is related to the concentration of the precursor solution, although multiple applications can be used to increase the film thickness if the solubility is limited. In these multiple applications, the epitaxial nature of the film was preserved.

To demonstrate the versatility of the technique, four different films were grown. CsPbBr₃, a perovskite, was grown on a single crystal strontium titanate (SrTiO₃) substrate. PbI₂, a material used in solar cells, was grown on a thin epitaxial film of gold on single-crystal silicon (Au/Si). NaCl, which is frequently used as a sacrificial layer in device fabrication, and ZnO, a transparent conductor used in light-emitting diodes, were also grown on a Au/Si substrate. Most of the films required gentle heating of the precursor material, to temperatures between 80–120°C. Spin coating was performed at room temperature, except for the films of CsPbBr₃ and PbI₂. In- and out-of- plane x-ray diffraction confirmed the single-crystal nature of these films and their orientation relative to the substrate.

“This report is very impressive; they use a simple method to epitaxially deposit a single-crystalline material,” says Wanyi Nie of Los Alamos National Laboratory, who was not part of this work.

In the future, Switzer wants to expand the number of films that can be grown using spin coating. A metal sulfide thin film could potentially be formed by pairing a metallic precursor and sulfur-containing solute. When the solute reacts with the heated surface of the substrate in the spin coater, the solute could break down, releasing the sulfur. “This process would be a combination of spin coating and chemical bath deposition at the surface,” Switzer says.

“It’s a multidisciplinary approach,” says Nie, “that combines the methods from the organic community to produce highly crystalline materials from the inorganic community.”

There are many possible applications where epitaxial thin films fabricated using this method could be used, such as flexible epitaxial thin-film solar devices. Beyond any single application, however, Switzer hopes that his results are broadly accessible to the device community as a whole. “I hope this will bring epitaxy to the masses,” he says.

Read the abstract in Science.