Fabrication and Characterization of Nano-porous GaN Template for Strain Relaxed GaN Growth

Abstract

A simple and cost-effective Si-doped porous GaN is fabricated by UV-enhanced electrochemical etching. An optimum current density of 20 mA/cm² applied for an hour in dilute NaOH solution produces a high density of uniform pores. Cross-section TEM reveals that etching takes place along the (0001) direction. A red shift of 0.7 cm⁻¹ in the E₂(high) phonon peak of GaN from micro-Raman indicates a relaxation of compressive stress in the porous GaN surface with respect to the underlying single crystalline epitaxial GaN. Subsequent growth of GaN layer on the porous template results in air gap formation, which is believed to serve as sinks for dislocations for reducing residual strain in the film. Reduction of FWHM of the XRD rocking curve as much as 0.033° and double intensity of the PL spectrum confirm the crystalline and optical quality improvement of the overgrown GaN layer as compared to as-grown. A red shift of ~0.4 cm⁻¹ towards the stress-free GaN also indicates a relaxation of compressive stress in the overgrown GaN layer.