Characterization study of GaN-based epitaxial layer and light-emitting diode on nature-patterned sapphire substrate

Abstract

Chemical wet etching on c-plane sapphire wafers by three etching solutions (H 3 PO 4 , H 2 SO 4 , and H 3 PO 4 /H 2 SO 4 mixing solution) was studied. Among these etching agents, the mixing H 3 PO 4 /H 2 SO 4 solution has the fastest etching rate (1.5 lm/min). Interestingly, we found that H 2 SO 4 does not etch the c-plane sapphire wafer in thickness; instead, a facet pyramidal pattern is formed on the c-plane sapphire wafer. GaN light-emitting diode (LED) epitaxial structure was grown on the sapphire wafer with the pyramidal pattern and the standard flat sapphire wafer. X-ray diffraction and photoluminescence measurement show that the pyramidal pattern on the sapphire wafer improved crystalline quality but augmented the compressive stress level in the GaN LED epilayer. The horizontal LED chips fabricated on the pyramidal-patterned sapphire wafer have a larger light output than the horizontal LED chips fabricated on the standard flat sapphire wafer by 20%.

AFM and SEM Study on Crystallographic and Topographical Evolutions of Wet-Etched Patterned Sapphire Substrate (PSS): Part II. Cone-Shaped PSS Etched in H2SO4and H3PO4Mixture with Varying Volume Ratio at 230°C

“…Our powder XRD results showed that these crystals are alunogen-Al 2 (SO 4 ) 3 • 17H 2 O (results not shown here). This agrees with the literature results, where it has been reported that alunogen crystals (Al 2 (SO 4 ) 3 • 17H 2 O) form on the flat c-plane sapphire substrate if the substrate is etched in H 2 SO 4 (98 wt%) at 230-400 • C. [19][20][21] When cubic-shaped alunogen crystals gradually grew (Figs. 4b and 4c), uncovered patterns evolved from triangular pyramids to hexagonal pyramids at 40 min, and then to hexagrams at 60 min.…”

AFM and SEM Study on Crystallographic and Topographical Evolutions of Wet-Etched Patterned Sapphire Substrate (PSS): Part III. Cone-Shaped PSS Etched in H2SO4and H3PO4Mixture at Various Temperatures

“…H 3 PO 4 dissolves sapphire by producing soluble AlPO 4 without any insoluble products. 28 It is well recognized that crystallographic planes with higher density of step-terrace structures and defects such as kinks are more active. 29,30 The E a of etching reactions in H 3 PO 4 for these planes should thus be lower.…”

“…H 2 SO 4 not only dissolves sapphire but also produces alunogens (Al 2 (SO 4 ) 3 • 17H 2 O). 28,31,32 Alunogens are insoluble in H 2 SO 4 and preferentially nucleate and grow on the reaction active sites (step edges and defects) of crystallographic planes. 23 At the same time, H 3 PO 4 in M5-1 can remove insoluble alunogens to expose the underlying fresh surface, thus enabling H 2 SO 4 and H 3 PO 4 to etch the exposed step-terrace structures of crystallographic planes.…”