AlGaN-based deep UV LEDs grown on sputtered and high temperature annealed AlN/sapphire

Abstract

The performance characteristics of AlGaN-based deep ultraviolet light emitting diodes (UV-LEDs) grown by metalorganic vapor phase epitaxy on sputtered and high temperature annealed AlN/sapphire templates are investigated and compared with LEDs grown on epitaxially laterally overgrown (ELO) AlN/sapphire. The structural and electro-optical properties of the devices on 350 nm sputtered and high temperature annealed AlN/sapphire show similar defect densities and output power levels as LEDs grown on low defect density ELO AlN/sapphire templates. After high temperature annealing of the 350 nm sputtered AlN, the full widths at half maximum of the (0002) and (101¯2) reflections of the high resolution x-ray diffraction rocking curves decrease by one order of magnitude to 65 arc sec and 240 arc sec, respectively. The curvature of the sputtered and HTA AlN/sapphire templates after regrowth with 400 nm MOVPE AlN is with −80 km−1 much lower than the curvature of the ELO AlN/sapphire template of −160 km−1. The on-wafer measured output powers of 268 nm LEDs grown on 350 nm sputtered and high temperature annealed AlN/sapphire templates and ELO AlN/sapphire templates were 0.70 mW and 0.72 mW at 20 mA, respectively (corresponding to an external quantum efficiency of 0.75% and 0.78%). These results show that sputtered and high temperature annealed AlN/sapphire provide a viable approach for the fabrication of efficient UVC-LEDs with reduced complexity and thus reduced costs.

High‐Temperature Annealing of Sputter‐Deposited AlN on (001) Diamond Substrate

“…Evidently, the crystallinity at a thickness of 600 nm was improved compared with that at 400 nm despite the cracking of the film, indicating that grain size was certainly increased by annealing, but that the film partially cracked through cooling down. Based on the relationship of (0001) [11][12][13][14][15][16][17][18][19][20] AlN || (001) [110] diamond confirmed by XRD, transmission electron microscope (TEM) and NBD at the AlN/diamond interface were performed as shown in Figure 5. [14] Therefore, the crystallinity of FFA sp-AlN/diamond structure can be further improved, e.g., by optimizing sputtering condition to prevent cracking at thick AlN.…”

“…XRD φ scans revealed that the conventional problem of the in-plane epitaxial relationship in hexagonal AlN/(001) diamond structure was stabilized in the single domain of (0001) [11][12][13][14][15][16][17][18][19][20] Crystallinity of the AlN film was significantly improved by the FFA, leading to the XRC-FWHM value of 1490 arcsec at FFA Sp-AlN thickness of 400 nm.…”

“…[16][17][18][19][20][21] We were able to achieve a value of 49 arcsec for the FWHM of the (0002) diffraction XRC, and of 287 arcsec in (10)(11)(12) diffraction. Previously, we have reported that a high crystalline quality of sputter-deposited AlN (Sp-AlN) films on sapphire substrates can be obtained by the annealing.…”

“…Hirama et al reported the fabrication of the AlGaN/GaN HEMT structure on (111) diamond substrates as having lower thermal resistance than using SiC substrates. Another problem resulting from the mismatch in the sixfold and fourfold rotation symmetry is the twin orientation of (0001) [11][12][13][14][15][16][17][18][19][20] [15] However, there is lack of clear evidence because they determined the epitaxial relationship on the basis of surface scanning electron microscopy (SEM) observation of GaN islands, rather than X-ray diffraction (XRD) or electronbeam diffraction. scale, though hexagonal AlN growth on a (001) diamond is difficult due to the difference in crystal structure and rotation symmetry.…”

“…TEM and NBD showed the existence of 20-25 nm thick epitaxial graphite at the interface of FFA Sp-AlN and diamond, exhibiting an epitaxial relationship of (0001)[11][12][13][14][15][16][17][18][19][20] AlN || (1-100) [0001] graphite || (001) [110] diamond.As a result of investigations on AlN/graphite/diamond structure, we suggest that the diffusion of O atoms from the AlN film into diamond is a driving force of transition from sp 3 to sp 2 bonds. This is due to the stabilized direction of C-C dimers by means of the diamond substrate with off-cut angle along[110].…”