Effect of a Lateral Overgrowth Process on the Strain Evolution of AlN Films Grown on a Nanopatterned Sapphire Substrate for Ultraviolet‐C Light‐Emitting Diode Applications

Abstract

Ultraviolet-C (UVC) light-emitting diodes (LEDs) based on AlGaN have been confirmed as an excellent potential candidate for various sterilization applications such as water/air purification, medical diagnostics, security, and so on. [1][2][3][4][5] In particular, the global outbreak of the COVID-19 has sparked even more research enthusiasm to UVC LEDs in both research and industry field due to their superior ability to eliminate severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) within second level. [6] However, in terms of device performance and mass production costs, UVC LEDs are still not comparable with GaN-based blue/green LEDs, and the main challenge is to achieve high crystalline quality and large-size AlN templates. The relatively large mismatch of lattice and thermal expansion between AlN and sapphire substrates results in the generation of numerous dislocations whose density is up to 10 10 cm À2 at the AlN/sapphire interface, which intensively affects the radiative property and reliability of UVC LEDs. [7,8] However, to realize high-performance UVC LED devices, AlN templates with threading dislocation (TD) density below 10 9 cm À2 are preferred. [1] To address this problem, epitaxial lateral overgrowth (ELOG) was proposed through the use of nanopatterned sapphire substrates (NPSSs), which subsequently became widespread due to the successful annihilation of TDs to %10 8 cm À2 . To further investigate the contribution of ELOG, NPSSs with various types of patterns were designed to eliminate dislocations. [9] Although the crystal quality has satisfied the subsequent requirements for epitaxy, the residual strain of the AlN template on NPSS is also quite important, because it greatly affects the warpage of wafer: excessive warpage leads to surface cracks and damages the uniformity of the chips on wafer. Previous studies have found that AlN on NPSS with hole-type pattern presents tensile strain which causes horrendous cracks. [10,11] In addition, the strain state also affects the surface roughness and the relaxation ratio of the upper AlGaN layer, which is accompanied by freshly generated dislocations. Thus, it is meaningful to understand how the NPSS pattern affects the strain evolution and further contributes to UVC LED light emission, which is crucial to optimizing device performance by modulating NPSS patterns.In this work, strain evolution of AlN templates grown on two types of NPSSs (hole type [HT] and pillar type [PT], respectively) is investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) reciprocal space map (RSM). It was found that the crystal merging of the AlN on HT-NPSS during ELOG leads to tensile strain, whereas the AlN on PT-NPSS, in which there is

Drive High Power UVC‐LED Wafer into Low‐Cost 4‐Inch Era: Effect of Strain Modulation

“…For comparison, a 4‐inch AlN template on NPSS (AlN/NPSS) with a hole‐type pattern was grown by metal–organic chemical vapor deposition (MOCVD) through performing the ELOG technique presented in our previous work. [ 2,38 ] Herein, the X‐ray diffraction (XRD) rocking curves of (002) and (102) planes of HTA‐AlN are shown in Figure S4 (Supporting Information), with the full width at half maximums (FWHM) of 52 and 197 arcsec, respectively. According to the mosaic model, the total threading dislocation density (TDD) is estimated as 5 × 10 8 cm –2 , [ 39 ] which is comparable to that of AlN/NPSS (TDD: ≈7 × 10 8 cm –2 , estimated from FWHM 002/102 = 95/254 arcsec).…”

Paving the Way for High‐Performance UVB‐LEDs Through Substrate‐Dominated Strain‐Modulation

“…This will effectively reduce the induced tensile stress in the ELOG stage. [ 48 ] NPSS with a larger hole diameter will provide larger air voids, thereby facilitating the relief of accumulated tensile stress and showing a tendency to present a compressively strained AlN, and vice versa. Thus, by tuning the apertures of the well‐designed NPSS, the introduced stress during epitaxial growth can be regulated through the variation of air voids.…”

Application of patterned sapphire substrate for III-nitride light-emitting diodes

“…Furthermore, the residual strain of AlN on hole-shaped NPSS was found to be tensile after complete coalescence, because NPSS patterns could directly influence the strain states associated with crystal merging behaviors during ELOG of AlN film. 145 As a result, AlN on hole-shaped NPSS with tensile strain facilitated the realization of high-quality epilayer with a relatively flat surface for fabrication of high-performance UV LEDs. 12a).…”