Study on Strain Characteristics of Freestanding GaN Membranes Released by Electrochemical Etching

Abstract

Gallium nitride （GaN） is a key semiconductor material widely used in micro-LEDs and optoelectronic devices. In InGaN/GaN-based micro-LEDs in particular, strain within the active region induces piezoelectric polarization and the quantum-confined Stark effect （QCSE）, reducing the spatial overlap of the electron and hole wavefunctions and thereby degrading the emission efficiency and shifting the emission wavelength. Moreover, as the device dimension decreases, the proportion of the sidewall region relative to the total area increases, making edge-induced strain relaxation more pronounced; this is a primary cause of the size-dependent variation in optical properties.

GaN thin films are generally grown heteroepitaxially on sapphire substrates, so a residual compressive strain develops due to the lattice mismatch and the difference in thermal expansion coefficients. The physical constraint imposed by the substrate masks the intrinsic strain relaxation behavior of GaN itself, limiting its clear analysis. Therefore, in order to exclude the influence of substrate-induced strain and to enable applications in micro-LEDs as well as various heterojunction and transfer-based devices, the fabrication of a freestanding GaN membrane separated from the sapphire substrate and a systematic analysis of its strain characteristics are required.

In this study, a freestanding u-GaN membrane was fabricated by electrochemical （EC） etching using a heavily doped n⁺-GaN layer as a sacrificial release layer, and the strain relaxation behavior before and after substrate removal was analyzed using photoluminescence （PL） and Raman E₂（high） mapping. By optimizing the EC etching conditions in a 0.3 M oxalic acid electrolyte, effective electropolishing of the n⁺-GaN sacrificial layer was achieved at 25 V for 30 min, and u-GaN membranes with sizes of 100, 50, and 25 μm were successfully released from the sapphire substrate.

The PL analysis showed that the PL peak red-shifted from 363.02 nm to 363.78 nm after membrane release, which is attributed to the relaxation of the residual compressive strain previously sustained by the sapphire substrate. The Raman E₂（high） mapping revealed that, for GaN mesas on the sapphire substrate, edge-induced strain relaxation became more evident as the mesa size decreased. In contrast, the released u-GaN membranes exhibited almost no center-to-edge variation, indicating that global strain relaxation occurred throughout the membrane after substrate removal. In addition, an H₃PO₄ treatment effectively removed the porous n⁺-GaN residue remaining after EC etching, reducing the RMS roughness from 8.8 nm to 1.86 nm.

These results demonstrate that electrochemical etching is an effective method for fabricating freestanding u-GaN membranes. Furthermore, the removal of the sapphire substrate was shown to transform the strain relaxation behavior of GaN from localized edge relaxation into global relaxation across the entire membrane. Consequently, the freestanding GaN membrane fabricated in this study serves as a useful analytical platform for understanding the size-dependent strain effects in micro-LEDs and is expected to function as a foundational structure that can be extended to heterojunction devices, transfer-based optoelectronic devices, and strain-relaxed GaN platforms.