Study On Acid Etching Behaviours Of Patterned Saphire Substrates For LED Application

Patterned sapphire substrates（PSS）, i.e., etched sapphire substrates with periodically patterned artificial structures, have been proved to be effective in improving the performanceof GaN-based LEDs. Regarding wet etching of sapphire substrates, the etching mechanism is still poorly understood. Taking c-plane sapphire as the model systems, we systematically study the effects of etchant and etching time on the wet etching using AFM and SEM. Continuous wet etching techniques were applied to investigate the morphological evolution and geometrical dimension of cone-shaped PSS. The accurate Miller indices and etching rates of etching facets are calculated. The reaults will have important implications for revealing the etching mechanism and the structure-performance relationship of LED devices.Chemical wet etching on c-plane sapphire wafers by three etching solutions（H₃PO₄, H₂SO₄, and H₃PO₄/H₂SO₄ mixing solution） was studied. Among these etching agents, three etching solutions were able to etch c-plane sapphire wafers and the fastest etching rate（15.46 nm/min） were obtained when the etchant was the mixing H₃PO₄/H₂SO₄ mixing solution. H₂SO₄ etchant were able to etch the c-plane sapphire wafer, and many cubic alunogen crystals（Al₂（SO₄）₃·17H₂O） formed on the c-plane sapphire wafer. Alunogen products prevented their bottom regions of sapphire from etching and grew in H₂SO₄ etchant. Therefore, three-dimensional pyramidal patterns with mutiple step struture were fabricated on the etched sapphire wafers.The wet etching behaviours of cone-shaped PSSs using three etching solutions（H₃PO₄, H₂SO₄, and H₃PO₄/H₂SO₄ mixing solution） were systematically investigated. The morphological evolution, Miller index and etching rate of the crystallographic planes of cone-shaped PSSs were studied by the combination of AFM and SEM characterizations. The cone-shaped pattens transformed to pyramid with mutiple planes in the first few minutes, and the patterns finally transformed to 3-fold symmetrical triangular pyramids in the H₃PO₄ etchant at 230 °C. In addition, the calculated Miller index of the crystallographic planes in two etching zones were（11 05） and（11 07）, and the size of patterns decreased with etching time. H₂SO₄ etchant can etch the cone-shaped PSS in the first few minutes. Then, many Al₂（SO₄）₃·17H₂O crystals formed on the PSS, and Al₂（SO₄）₃·17H₂O crystals protected their bottom regions of sapphire from being etching by H₂SO₄. Therefore, the morphological evolutions of PSS werecomplicated in H₂SO₄ etchant. In the mixing H₃PO₄/H₂SO₄ etchant, the cone-shaped pattens transformed to hexagonal pyramid on 20 minutes, and the patterns finally transformed to 3-fold symmetrical triangular pyramids on 28 minutes. In addition, the size of patterns decreases continuously with the etching time.