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

Abstract

Here we present a systematic study on crystallographic and topographical evolutions as well as etching rates of crystallographic planes of cone-shaped patterned sapphire substrate (PSS) etched in H 2 SO 4 and H 3 PO 4 mixture with a wide range of volume ratios (H 2 SO 4 :H 3 PO 4 = 1:0, 5:1, 3:1, 1:1, 1:3, and 0:1) at 230 • C. The Miller indexes of four major exposed crystallographic planes were determined as S 1 {11 0 5}, S 3 {45 1 38}, S 4 {11 0 12}, and S 6 {11 0 8}. The etching rates of crystallographic planes follow the order S 1 > S 3 > S 4 > c plane for H 2 SO 4 and H 3 PO 4 mixtures as well as S 1 > S 6 > c plane for H 3 PO 4 . A larger volume ratio (higher H 2 SO 4 concentration) favors a higher etching rate of c-plane and conversely lower etching rates of S 1 -, S 3 -, and S 4 -planes, thus causing a larger filling factor with the largest filling factor of 0.93 obtained at the volume ratio of 5:1. This work will contribute to revealing the etching mechanism and fabricating optimized PSS for enhanced performance of light-emitting diodes. GaN-based light-emitting diodes (LEDs), as high-brightness solidstate light sources, offer attractive advantages over conventional incandescent and fluorescent lamps.1 Nevertheless, a large mismatch in the refractive index as well as the lattice constant between GaN and sapphire crystals causes difficulty with low-light extraction efficiency (LEE) and internal quantum efficiency (IQE). 2,3 To solve these problems, patterned sapphire substrate (PSS), i.e., etched sapphire substrate with artificial periodical micro/nano structures, has been proposed and popularly used commercially. The geometrical shape and dimension of PSS strongly affect the LEE and IQE of LEDs.4-6 PSSs with various geometrical shapes, usually groove, 7 circular concave, 8 cone, 9 pyramid, 10 volcano, 11 or hemispheroid, 12 have been created using dry or wet etching methods.Wet etching of PSS, usually using H 2 SO 4 and H 3 PO 4 mixture at an elevated temperature, enables crystallographic specific etching. Compared with dry etching, more sophisticated patterns can be fabricated, such as pyramids or volcanos with multiple planes showing specific slant angles. [13][14][15][16] Efforts have been devoted to investigating the crystallographic and topographical evolutions of PSS to reveal the etching mechanism and fabricating optimized PSS. 13,[15][16][17] In Part I of this series, we reported the results about crystallographic and topographical evolutions of cone-shaped PSS (CPSS) etched in the H 2 SO 4 and H 3 PO 4 mixture (H 2 SO 4 :H 3 PO 4 = 3:1 at volume ratio) at 230• C. 15We also demonstrated that atomic force microscopy (AFM), thanks to its three-dimensional (3D) topography imaging capability, is an indispensable tool for fast and reliable topographic characterization of PSS samples. However, there is still much room left for systematic studies regarding crystallographic and topographical evolutions of PSS under a wide range of etching conditions such as different mixture ratios and etchant temperatu...

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

“…At 250 min, S 4 -zones were completely etched away, and then quasi-hexagrams changed to separated triangular pyramids exposed with only S 5 -zones. Compared with the etching at higher temperature of 503 K reported in Part II, 21 some noticeable differences can be found. First, the crystallographic and topographical evolutions of patterns are much slower at 473 K. For instance, triangular pyramids exposed with only S 5 -zones (Fig.…”

“…23 Careful comparisons about etching behaviors of PSS in H 2 SO 4 at various temperatures are essential for revealing the temperature dependence of the crystallographic and topographical evolutions as well as the etching mechanism. As reported in Part II of this series, when the etchants of pure H 2 SO 4 or with rather low H 3 PO 4 concentration (i.e., H 2 SO 4 :H 3 PO 4 = 1:0 and 10:1 at volume ratio) are used, many alunogen crystals (Al 2 (SO 4 ) 3 · 17H 2 O) and irregular protrusions are created on CPSS, 21 thus hindering observation of the crystallographic and topographical evolutions as well as determination of etching rates of crystallographic planes. Therefore, H 2 SO 4 -based etchant with a moderate H 3 PO 4 concentration, i.e., M5-1 (H 2 SO 4 :H 3 PO 4 = 5:1 at volume ratio), was chosen to avoid the interference of alunogens.…”

“…20,21 Then, the Miller index of S 5 -planes was determined. Triangular pyramids etched for 250 min (Figs.…”

“…[17][18][19][20][21] Efforts have been devoted to investigate the crystallographic and topographical evolutions of wet etched PSS. [19][20][21][22] In Part I of this series, we reported the strongly time-dependent crystallographic and topographical evolutions of cone-shaped PSS (CPSS) etched in one of ordinary etchants (H 2 SO 4 :H 3 PO 4 = 3:1 at volume ratio) at 230…”

“…21 Temperature is the other crucial factor in affecting the crystallographic and topographical evolutions as well as the etching rate of crystallographic planes of PSS. However, relevant reports in the literature are scarce.…”