Gallium nitride (GaN), as a compound wide direct-bandgap semiconductor, plays an important role in many applications, including in high brightness light-emitting diodes (LEDs) for lighting and high electron mobility transistors (HEMTs) for high-power, high-frequency devices like cellular towers and radar. The demand for faster networks and higher bandwidths in wireless communication especially is driving the development of GaN-powered semiconductor devices with higher breakdown voltages than currently available. Specifically, this requires deep trench structures in GaN to form p–n junction columns.

The currently used inductively-coupled plasma reactive-ion etching (ICP-RIE) technique poses limitations on the fabrication process by introducing plasma damage to the functional surfaces, thus degrading device performance. On the other hand, photo-electrochemical etching (PEC) is a wet etching method that uses UV irradiation at the anode/electrolyte to promote oxidation of the targeted materials, with the oxide dissolving in an acid or base. It offers a potential route for a damage-free device fabrication process alternative due to its electrochemical oxidation process and excellent etching selectivity to mask materials. It especially slows the lateral etch rate for high-aspect-ratio microstructures.

In a recent issue of Applied Physics Express, a research team, led by Fumimasa Horikiri of SCIOCS Co., Ltd. and Tomoyoshi Mishima at Hosei University, introduced the use of PEC etching in deep trench structures of GaN with a titanium mask. “This paper describes the great potential of PEC etching for GaN required for superjunction [power semiconductor] devices in the near future,” says Horikiri, the lead author of this work; “[This work also demonstrates] the potential for fabricating GaN-microelectromechanical systems (MEMS); PEC etching of GaN is not [just] for optical and electrical devices.”

Photo-assisted anodic oxidation combined with the short carrier lifetime of GaN due to high dislocation densities gives PEC the ability to etch deeply but not laterally, since UV irradiation dissipates quickly into GaN and promotes oxidation mainly in the illumination direction, therefore benefiting semiconductor devices with high-aspect-ratio (>10) trench structures. The non-plasma fabrication process also reduces high energy ion damage to the etched surface. An earlier study (Japanese Journal of Applied Physics 57, 086502, 2018) by the same research team showed that the photoluminescence intensity of the near band edge remains the same before and after PEC etching.

The researchers demonstrated >600 etch selectivity of GaN with respect to a Ti mask, and less than 1 μm lateral etching for depth >30 μm. It is estimated that PEC etching on GaN can provide comparable etching depths with the same aspect ratio as ICP-RIE on silicon carbide (SiC), which is a common wide bandgap material for microelectromechanical systems (MEMS) applications. Horikiri suggests that GaN now has the potential to be used in MEMS devices besides optical and electronic devices.

“This work does provide an alternative processing route to conventional dry etching for GaN,” says Leonard Hsu, a senior technical specialist at Lam Research Corporation with expertise in plasma dry-etching, “but I am not convinced that PEC provides higher throughput and similar production productivity than plasma dry etching due to the complexity of its experimental setup.”

Read the article in Applied Physics Express.