A method that chemically etches patterned arrays in the semiconductor gallium arsenide will make high-end optoelectronic devices easier to create. A team of researchers led by Xiuling Li, an electrical and computer engineering professor at the University of Illinois, developed the etching method in gallium arsenide, which is used in solar cells, lasers, LEDs, field effect transistors, capacitors and sensors. The researchers’ technique was described in Nano Letters. The physical properties of a semiconductor can vary depending on its structure; therefore, a semiconductor wafer is etched into structures that tune its electrical and optical properties and connectivity before it is assembled into chips. Two techniques are commonly used: “Wet” etching uses a chemical solution to erode the semiconductor in all directions, while “dry” etching uses a directed beam of ions to bombard the surface, carving out a directed pattern. Such patterns are required for high-aspect-ratio nanostructures, or tiny shapes that have a large ratio of height to width. High-aspect-ratio structures are essential to many high-end optoelectronic device applications. Scanning electron microscope image of nanopillars etched in gallium arsenide via metal-assisted chemical etching. (Images: Xiuling Li) While silicon is the most common material in semiconductor devices, materials in the III-V group are more efficient in optoelectronic applications, such as solar cells or lasers. Unfortunately, all of these materials can be difficult to dry-etch because high-energy ion blasts can damage the semiconductor’s surface. III-V semiconductors are especially susceptible to damage. To address this problem, Li’s group used metal-assisted chemical etching (MacEtch), a wet-etching approach they had previously developed for silicon. Unlike other wet methods, MacEtch works in one direction, from the top down. It is faster and less expensive than many dry-etch techniques, according to Li. Her group revisited the MacEtch technique, optimizing the chemical solution and reaction conditions for the III-V semiconductor gallium arsenide (GaAs). The process involved two steps. First, a thin film of metal is patterned on the GaAs surface. Then, the semiconductor with the metal pattern is immersed in the MacEtch chemical solution. The metal catalyzes the reaction so that only the areas touching metal are etched away, and high-aspect-ratio structures are formed as the metal sinks into the wafer. When the etching is done, the metal can be cleaned from the surface without damaging it. Metal-assisted chemical etching uses two steps. First, a thin layer of gold is patterned on top of a semiconductor wafer using soft lithography (left). The gold catalyzes a chemical reaction that etches the semiconductor from the top down, creating 3-D structures for optoelectronic applications (right). "The realization of high-aspect-ratio III-V nanostructure arrays by wet etching can potentially transform the fabrication of semiconductor lasers where surface grating is currently fabricated by dry etching, which is expensive and causes surface damage,” Li said. To create metal film patterns on the GaAs surface, the scientists used a patterning technique developed by John Rogers, the Lee J. Flory-Founder Chair and a professor of materials science and engineering at the university. Their research teams collaborated to optimize the method, called soft lithography, for chemical compatibility while protecting the GaAs surface. Soft lithography is applied to the whole semiconductor wafer, as opposed to small segments, creating patterns over large areas — without expensive optical equipment. By using soft lithography and MacEtch together, the teams were able to produce large-area, high-aspect-ratio III-V nanostructures at a minimal cost, Li said. Next, the researchers hope to further optimize conditions for GaAs etching and to establish parameters for MacEtch of other III-V semiconductors. They hope to demonstrate device fabrication, including distributed Bragg reflector lasers and photonic crystals. For more information, visit: www.illinois.edu