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New technique simplifies GaAs etching

Posted: 26 Dec 2011 ?? ?Print Version ?Bookmark and Share

Keywords:GaAs? etching? nanostructure? optoelectronic device?

A team of researcher from the University of Illinois (UI) has developed a method to chemically etch patterned arrays in the semiconductor gallium arsenide (GaAs) used in solar cells, lasers, LEDs, FETs, capacitors and sensors. Xiuling Li, electrical and computer engineering professor at UI, led the group who believed the semiconductor structures will push the development of high-end optoelectronic devices.

A semiconductor's physical properties can vary depending on its structure, so semiconductor wafers are etched into structures that tune their electrical and optical properties and connectivity before they are assembled into chips. Semiconductors are commonly etched with two techniques: '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.

semiconductor wafer

Metal-assisted chemical etching uses two steps. First, a thin layer of gold is patterned on top of a semiconductor wafer with soft lithography (left). The gold catalyzes a chemical reaction that etches the semiconductor form the top down, creating 3D structures for optoelectronic applications (right). (Graphic by Xiuling Li)

While silicon is the most ubiquitous material in semiconductor devices, materials in the III-V group are more efficient in optoelectronic applications, such as solar cells or lasers. Unfortunately, these materials can be difficult to dry etch, as the high-energy ion blasts damage the semiconductor's surface. III-V semiconductors are especially susceptible to damage.

To address this problem, Li and her group turned to 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 GaAs.

The process has 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.

GaAs

A scanning electron microscope image of 'nanopillars' etched in GaAs. (Image by Xiuling Li).

"It is a big deal to be able to etch GaAs this way," Li stated. "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."

To create metal film patterns on the GaAs surface, Li's team used a patterning technique pioneered by John Rogers, the Lee J. Flory founder chair and a professor of materials science and engineering at UI. Their research teams joined forces 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 areaswithout expensive optical equipment.

"The combination of soft lithography and MacEtch make the perfect combination to produce large-area, high-aspect-ratio III-V nanostructures in a low-cost fashion," said Li.

The researchers hope to further optimize conditions for GaAs etching and establish parameters for MacEtch of other III-V semiconductors. Then hope to demonstrate device fabrication, including distributed Bragg reflector lasers and photonic crystals.

"MacEtch is a universal method as long as the right condition for deferential etching with and without metal can be found," added Li.





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