Injection laser diodes (LDs) in the III-N alloy system require the growth of heterostructures. Influenced by both lattice mismatch and thermal expansion coefficient mismatch in such structures, the alloy system is restrained from freely allowing full alloy selection for the best device performance. In the case of LDs emitting in the UV-A range, tensile strain limits the growth of adequately thick and/or high Al-mole-fraction AlGaN epitaxial layers on GaN substrates, and the growth of layers having thickness beyond the critical layer thickness results in cracking of the film. The inadequate thickness or index contrast of cladding layer (CL) to the active layer limits the LED or LD device performance due to low optical confinement, while the yield of the devices fabricated on the wafer is significantly reduced by the cracks. In this work, we describe a novel approach employing non-planar growth (NPG) that enables the deposition of crack-free AlGaN layers with relatively thick and high Al-mole-fraction on patterned GaN substrates with rectangular mesas with adequately large area for the fabrication of UV LEDs and injection laser devices. Cracking of AlGaN layers grown by MOCVD on (0001) GaN substrates with NPG mesas was evaluated. Stripe-shaped NPG mesas were formed on the GaN substrate with conventional dry etching. The direction of the stripe mesas was aligned to be parallel to the m axis of GaN. The widths of the NPG mesa were set to be 25 and 100 µm. AlxGa1-xN/AlyGa1-yN short period super lattices (SPSLs) were grown on the patterned GaN. The average Al mole fraction and total thickness of the AlGaN SPSL were set to be 0.16 and 1500 nm, respectively. Surface cracking after the growth was not observed on the AlGaN SPSL grown on the top of mesa, while the critical thickness for planar sample without mesa formation was 200 nm. Investigations with AlGaN with different Al mole fractions and thicknesses show that the surface cracking depends on the mesa width and depth. Mesas with narrower widths and/or deeper etching depth are more effective to prevent the cracking than others with either wider widths or shallower depths.[1,2] Additionally, the suppression of the formation of the crack is attributed to an anisotropic relaxation of the strain in the AlGaN SPSL which was enabled by the elastic deformation of GaN layer along the direction perpendicular to the stripe mesa. Our results clearly show that the NPG approach enables us to achieve thick AlGaN layers with high composition contrast to the substrate material with a thickness beyond critical layer thickness by relaxing the tensile stress in the AlGaN layers on the top of mesa. This is especially advantageous for fabrication of UV LDs as these devices require thick CLs with high refractive index contrast to the active layer to lower the lasing threshold. Using this approach, we have designed and grown a UV-A multiple-quantum-well LD heterostructure. The CL in upper and lower side of the active layers are composed of doped AlGaN SPSLs with average Al mole fraction and combined thickness of 0.16 and 1100 nm, respectively. The surface of the planar reference sample has a high density of cracks along the <11-20> directions while the NPG sample has no cracks on the top surface of several-mm-long NPG mesas. Test diodes device fabricated on the NPG sample before formation of the laser cavity structure show rectification with no leakage in a DC current-voltage measurement at 300K. Spontaneous light emission in UV-A range was successfully demonstrated by pulsed current injection at 300K. From these results, the NPG approach is proven to effectively suppress the cracking of relatively thick AlGaN layers with high Al mole fraction and thicknesses beyond the critical layer thickness and thus this approach is promising to achieve a high-performance UV-A LD and LEDs. [1] F. Mehnke et al., J. Appl. Phys. 131, 073103 (2022). [2] Y. Ando et al., J. Cryst. Growth 607, 127100 (2023).

Non-Planar Growth of Crack-Free High Al-Mole-Fraction AlGaN Heterostructures on Patterned GaN Substrates for Ultraviolet Light Emitting Diodes and Laser Diodes

M: Group III–Nitrides—Optical Emitters

technical paper

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Non-Planar Growth of Crack-Free High Al-Mole-Fraction AlGaN Heterostructures on Patterned GaN Substrates for Ultraviolet Light Emitting Diodes and Laser Diodes

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