Electrochemical etching of crystalline n-GaN in H 2 SO 4 :H 2 O 2 results in the formation of porous GaN. Scanning electron microscopy images revealed the presence of branches on the surface of porous GaN and showed the varying stages with etching time. The branches on the surface of the porous GaN that have been associated with Ga 2 O 3 have a significant enhancing effect on the photoluminescence intensity. Raman spectra of both as-grown and porous GaN exhibit phonon mode E 2 (high), A 1 (LO), A 1 (TO) and E 2 (low). There is a red shift in E 2 (high), indicating a relaxation of compressive stress in the porous GaN surface with respect to the underlying single-crystalline epitaxial GaN.Recently, porous semiconductors have stimulated much interest because they exhibit physical properties different from those of bulk crystals. The high surface area, band gap shift, and efficient luminescence suggest uses of porous semiconductor applications to extend over a wide range, from optoelectronics to chemical and biochemical sensors applications. After the discovery of visible light emission in porous Si at room temperature, 1 several researchers have investigated other compound semiconductors such as GaAs, 2 GaP, 3 and GaN (Ref. 4) using several techniques. Researchers have carried out relatively little work on III-V semiconductors, namely GaN. In contrast with Si, GaN exhibits a direct band gap, thus providing more efficient light emission and detection. Because of this direct band gap and its smaller size, porous GaN promise multiple advantages over Si counterpart.The most viable method to produce porous semiconductor or porous GaN is through electrochemical etching. The main factor in the electrochemical etching is the electrolyte. Because of their different chemical nature, wide ranges of aqueous electrolytes have been used for GaN. Photoelectrochemical etching of GaN was conducted in alkaline aqueous solutions containing inorganic KOH, 5 and in inorganic acids such as H 2 SO 4 (Ref. 6) and H 3 PO 4 (Ref. 7) or into halogen acids such as HF and HCl. 8 Different morphological pores have been reported in the literature, and it was concluded that the pores depend strongly on the nature of the commercialized substrate and on the etching conditions. Indeed, the results were different from one researcher to another because of considerable variations in the experimental conditions and because commercialized substrates always contain dislocation and other defects promoting nonhomogeneous nucleation. From a systematic study of the electrochemical dissolution of GaAs in HF, and HCl-based electrolytes, Schmuki et al. 9 discovered that, under certain chemical conditions, crystalline deposits might form on the p-GaAs skeleton produced by the electrochemical treatment. Researchers have conducted little work on the optical properties of these crystals. 2,10 In comparison, the study of porous GaN is still in its early stage; many fundamental properties are still not well understood.In the present study, we discovered that, under cer...