Wet-etched Patterned Sapphire Substrate Research Articles

Growth of GaN on {125}-like Facets of Patterned Sapphire Substrate

GaN has been grown on wet-etched patterned sapphire substrate (PSS) by metal-organic chemical vapor deposition. It has been found that GaN was initiated not only from bottom c-facet but also from E {125} facets of hexagonal patterns /pyramids. In this study, a vacancy-PSS on c-plane sapphire was used to investigate the growth of GaN on the E-like facets (E1 and E2) of distorted pyramids. The results show that the orientation relationship between E1-GaN and sapphire is ()GaN // (10)sapphire and [11]GaN // [110]sapphire. At the same time, that between E2-GaN and sapphire is (01)GaN // (30)sapphire and [021]GaN // [110]sapphire.

AFM and SEM Study on Crystallographic and Topographical Evolutions of Wet-Etched Patterned Sapphire Substrate (PSS): Part III. Cone-Shaped PSS Etched in H2SO4 and H3PO4 Mixture at Various Temperatures

The effect of etching temperature (473–543 K) on the crystallographic and topographical evolutions as well as the etching rate of crystallographic planes of cone-shaped patterned sapphire substrate (PSS) etched in both H2SO4-based (H2SO4:H3PO4 = 5:1 at volume ratio) and H3PO4 etchants was systematically studied. For H2SO4-based etchant, higher temperature favors larger etching rate ratio of slant planes (S1 {1 0 5}, S3 {4 1 38}, S4 {1 0 12}, and S5 { 1 0 37} planes) to c-plane, thus leading to smaller filling factor (FF) of patterns; the Arrhenius activation energy (Ea) of etching reactions for various crystallographic planes follows the order: Ea (c) < Ea (S1) < Ea (S3) < Ea (S4). For H3PO4, higher temperature favors smaller etching rate ratio of slant planes (S1 {1 0 5} and S6 {1 0 8} planes) to c-plane and larger FF of patterns; the Ea follows the order: Ea (S1) < Ea (S6) < Ea (c). This work will contribute to elucidating the etching mechanism and fabricating optimized PSS for enhanced performance of light emitting diodes.

AFM and SEM Study on Crystallographic and Topographical Evolutions of Wet-Etched Patterned Sapphire Substrate (PSS): Part II. Cone-Shaped PSS Etched in H2SO4 and H3PO4 Mixture with Varying Volume Ratio at 230°C

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 H2SO4 and H3PO4 mixture with a wide range of volume ratios (H2SO4:H3PO4 = 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 S1 {1 0 5}, S3 {4 1 38}, S4 {1 0 12}, and S6 {1 0 8}. The etching rates of crystallographic planes follow the order S1 > S3 > S4 > c plane for H2SO4 and H3PO4 mixtures as well as S1 > S6 > c plane for H3PO4. A larger volume ratio (higher H2SO4 concentration) favors a higher etching rate of c-plane and conversely lower etching rates of S1-, S3-, and S4-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.

AFM and SEM Study on Crystallographic and Topographical Evolution of Wet-Etched Patterned Sapphire Substrates (PSS): I. Cone-Shaped PSS Etched in Sulfuric Acid and Phosphoric Acid Mixture (3:1) at 230°C

A thorough understanding about the crystallographic and topographical evolution of wet-etched patterned sapphire substrates (PSS) is essential for revealing the etching mechanism and critical for fabricating optimized PSS for enhanced performance of GaN-based LED devices. As the first one of a series of three papers, using complementary scanning electron microscope (SEM) and atomic force microscope (AFM), we carried out a systematic study on the crystallographic and topographical evolution of cone-shaped PSS etched with an ordinary etchant (H2SO4:H3PO4 = 3:1 at volume ratio, 230°C) which enables crystallographic specific etching. The topography of patterns was found to evolve from normal cones to truncated cones, to triangular pyramids with multiple families of exposed crystallographic planes, then to hexagonal and triangular pyramids with a single family of planes. The Miller indexes of three major exposed crystallographic planes, with decreasing slant angles, were determined to be {1 0 5}, {4 1 38} and {1 0 12} with the last two being reported for the first time to our best knowledge. It was clearly demonstrated that AFM, thanks to its 3D topography imaging capability compared to SEM and focused ion beam, can be an indispensable tool for fast and reliable characterization of routine PSS samples.

Morphologies and plane indices of pyramid patterns on wet-etched patterned sapphire substrate

A wet etching process was employed to investigate the evolution of pyramids on the patterned sapphire substrate (PSS). It has been found that the PSS comprised a hexagonal pyramid covered with six facets {34¯17} when disk-shaped SiO2 mask still remained. Three facets {11¯05} were exposed when mask was etched away. In this study, a continuous etching process was performed. It was found that another six facets {45¯130} and another three facets {11¯010} appeared on the bottom and the top of pyramid. Finally, when the etching time reached around 5min, most of pyramids on the PSS disappeared.

The Formation of Smooth Facets on Wet-Etched Patterned Sapphire Substrate

In this study, rectangle-shaped SiO2 hard masks with various orientations were employed to find various facets on wet-etched patterned sapphire substrate (PSS). Seven facets (A, B, B1, B2, D1, D2 and E) were observed after etching. The surfaces of A, B and E-facets were smooth. Their plane indexes were , and , respectively. On the other hand, the surfaces of B1, B2, D1 and D2-facets were not smooth, with some ambiguous stripes, which were investigated by using “zigzag triangle” hard mask. A large triangle-mask was employed to investigate smooth facets and the GaN epitaxial behavior. It was found that most of the growth of zincblende GaN was initiated not from A and B-facets but E-facets.

Evolution of Bottom c-Plane on Wet-Etched Patterned Sapphire Substrate

Wet-etched pattern sapphire substrate (PSS) has been employed to improve the epitaxy of GaN-based LEDs. It was found that the crystal quality and performance of LEDs improved with decrease in c-plane areas of PSS. However, further decrease in bottom c-plane areas makes epitaxy of GaN film very difficult. In this research, the evolution of bottom c-plane was investigated through a systematic study. It was found that epitaxy difficulty might be due to the appearance of 6C facets and the disappearance of bottom c-plane.

Surface Modification on Wet-Etched Patterned Sapphire Substrates Using Plasma Treatments for Improved GaN Crystal Quality and LED Performance

A surface-modified wet-etched pyramidal patterned sapphire substrate (PSS) was used to fabricate blue-light InGaN-based light-emitting diodes (LEDs). The pyramidal PSSs were modified using CF4 plasma with different treatment times. It was found that the plasma treatment created a slightly roughened surface pattern, which not only suppressed lateral growth from the incline of the pattern, but also enhanced vertical growth from mesa and groove of the pattern. A clear reduction of GaN dislocation density in PSS LEDs was confirmed by electron microscopy, X-ray diffraction and etch-pit-density measurements. Additionally, the output power of plasma-treated PSS LEDs was 25% higher than that of untreated PSS LED for a 20 mA injection current. The enhanced output power was attributed to the reduction in dislocation density of the GaN film and the enhancement of light scattering by surface-modified PSSs.

Self-Assembled Periodic Silica Nanosphere Arrays on Wet-Etched Patterned Sapphire Substrate for a High-Light-Extraction-Efficiency Light-Emitting Diode

We have developed a periodical self-assembled method of patterning the sapphire substrate with silica nanospheres for light-emitting diode (LED) applications. Using this method, silica nanospheres were directed to self-assemble in a periodic fashion on a wet-etched patterned sapphire substrate (WPSS). Moreover, we fabricated an InGaN/GaN-based LED on the silica-nanosphere-assembled WPSS. The light output power of the resultant LED was about 2.23 and 1.7 times higher than that of the LEDs grown on planar sapphire and WPSS, respectively. Moreover, the light extraction efficiency of the silica-nanosphere-assembled WPSS LED was 37% higher than that of conventional LED.

Enhanced Photoluminescence of InGaN/GaN Green Light-Emitting Diodes Grown on Patterned Sapphire Substrate

Green InGaN/GaN based light-emitting diodes (LEDs) are fabricated both on planar and wet-etched patterned sapphire substrates by metalorganic vapour phase epitaxy (MOVPE). Their photoluminescence (PL) properties of the two samples are studied. The results indicate that the PL integral intensity of the green LED on the patterned substrate is nearly two times of that on the planar one within the whole measured temperature range. The enhanced PL intensity in the green LED on the patterned substrate is shown completely contributed from the extraction efficiency, but not from the internal quantum efficiency. The conclusion is supported by temperature-dependent PL analysis on the two samples, and the mechanisms are discussed.

Fabrication and Characterization of GaN -Based LEDs Grown on Chemical Wet-Etched Patterned Sapphire Substrates

Characteristics of -based light-emitting diodes (LEDs) grown on the chemical wet-etched patterned sapphire substrates (CWE-PSS) with different crystallography-etched facets were investigated. An improvement of 40% on the overall quantum efficiency was achieved by adopting this CWE-PSS scheme. A Monte Carlo ray-tracing method was employed to derive the optimized condition of sapphire etching time, and the calculated result demonstrated the same trend with real device measurement. Adopting the CWE-PSS in LEDs could not only improve the epitaxial quality but also increase the extraction quantum efficiency due to crystallography-etched facets efficiently scattering the guided light to enter the escape cone on the top of device surfaces. Finally, we observed better aging behaviors of CWE-PSS LEDs, which could be due to the reduction of threading dislocations of the epitaxial layers.

Enhancing the output power of GaN-based LEDs grown on wet-etched patterned sapphire substrates

GaN-based light-emitting diodes (LEDs) with emitting wavelength of 450 nm were grown on patterned sapphire substrates (PSSs) fabricated by chemical wet etching. The crystallography-etched facet was {1-102} R-plane with a 57deg against {0001} C-axis and had superior capability for enhancing light extraction efficiency. The light output power of the PSS LED was 1.15 times higher than that of the conventional LED at an injection current of 20 mA. The output power and external quantum efficiency were estimated to be 9 mW and 16.4%, respectively. The improvement was attributed not only to geometrical shapes of {1-102} crystallography-etched facets that efficiently scatter the guided light to find escape cones, but also to dislocation density reduction by adopting the PSS growth scheme