Photo-Electroless-Etching of Wide Band Gap Material for Flexible Solid-State Devices

We report on photo electroless chemical (PEC) etching of III-Nitride material for thin layer lift-off for flexible solid-state light emitting devices. We investigate three different structures: 30 µm unintentionally doped GaN on sapphire, 30 µm n-type GaN on sapphire, and 200 nm InGaN on 4 µm unintentionally doped GaN on sapphire with different indium composition. Initially the three structures were etched as-grown for the same duration and the same conditions in a HF/CH₃OH/H₂O₂ solution under UV illumination. Scanning electron microscopy (SEM) measurements performed on the etched samples reveals porous structure and high density nanowire on the surface, as well as the formation of V-shaped pits. The etch rate in each structure were compared. Optical study were performed using photoluminescence (PL) and Raman spectroscopies to analyze PL peak shift and phonon frequency shift compared to the as-grown, which we attributed to the compressive strain relaxation and localized impurity formed in the crystalline structure after PEC etching. We also fabricated microcolumns of diameters ranging from 5 to 100-µm via inductively coupled plasma (ICP) etching process in the InGaN on GaN samples. Dry etching of micropillars exposes unintentionally doped GaN to the wet etching. We study the selectivity of GaN over InGaN in the PEC etching process. SEM images reveal lateral etching and porous GaN layer formation under the InGaN after wet etching. We compare etching process in the InGaN structure with and without microcolumns. PEC etching is sensitive to band bending in the surface of the semiconductor and electrolyte, when InGaN surface is in contact with HF solution, oxidation and reduction reaction happens at slow rate and leads to the porous structure in the surface. However, when the GaN under the InGaN is exposed, the etching in this layer is preferred due to the low resistivity and the confinement of the photogenerated minority in the upward band bending depleted region near the contact surface between GaN and electrolyte.

Undercut InGaN microdisks were lifted off for optical study via flexible polydimethylsiloxane (PDMS) material. Microdisks were then transferred on to a Si wafer for PL measurements. We also performed Raman study on the microdisks and observed a shift in the phonon frequency attributed to the relaxation of the compressive strain with respect to the as-grown. InGaN microdisks with higher indium composition were transferred on a blue LED for optical pumping. Optical measurements on transferred microdisks revealed typical prominent visible peaks. These observations confirm the promising application of this technique to selectively etch III-V semiconductor. The lift-off process is a compelling technique for next generation transferred substrate devices and III-Nitride micro phosphor fabrication.