Eliminating structural material imperfections to improve electronics

Wide bandgap semiconductors like gallium nitride show promise for high-temperature, high-powered devices, but their performance and durability can be hindered by structural imperfections called threading dislocations, characterized by a missing line of atoms. Their incidence can be reduced using, for example, top-down fabrication methods, which are subtractive processes to carve gallium nitride into predetermined structures. However, to significantly reduce the density of dislocations by these methods, too much material needs to be removed, resulting in a fill factor that is too low for applications in which capacity is determined by the cross-sectional area of the active region.

A new study in Journal of Applied Physics has introduced a novel hybrid method that enables researchers to fabricate gallium nitride nanocolumns with low dislocation densities without compromising array fill factor.

To form the nanocolumn arrays, the researchers grew gallium nitride films on sapphire wafers, fabricated nanocolumns with top-down methods, then corroded the nanocolumns using a hydroxide solution, which shrinks them laterally. Absolute dislocation density decreased proportionally to the array fill factor. Afterwards, the nanocolumns were returned to the reactor chamber for lateral overgrowth. This step recovered the array fill factor and, importantly, eliminated dislocations further by providing them with enough energy to bend into the areas between the nanocolumns. Transmission electron microscopy imaging enabled the researchers to characterize these nanocolumns at the micro- and nanoscale.

Author Vitaly Zubialevich said this process could be taken to the “extreme.” By shrinking the diameters of the nanocolumns to the minimum extent possible, most dislocations within the material could be eliminated.

The authors plan to continue this work for core-shell devices such as optoelectronic emitters and photoelectrodes for water splitting.

Source: “Reduction of threading dislocation density in top-down fabricated GaN nanocolumns via their lateral overgrowth by MOCVD,” by Vitaly Z. Zubialevich, Mathew McLaren, Pietro Pampili, John Shen, Miryam Arredondo-Arechavala, and Peter J. Parbrook, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/1.5110602 .