Abstract
Herein, a systematic study of the morphology and local defect distribution in epitaxially laterally overgrown (ELO) AlN on c‐plane sapphire substrates with different off‐cut angles ranging from 0.08° to 0.23° is presented. Precise measurements of the off‐cut angle α, using a combination of optical alignment and X‐ray diffraction with an accuracy of ±5° for the off‐cut direction and ±0.015° for the off‐cut angle, are carried out. For ELO AlN growth, a transition from step flow growth at α < 0.14° with height undulations on the surface to step bunching with step heights up to 20 nm for α > 0.14° is observed. Furthermore, the terraces of the step‐bunched surface exhibit curved steps. An analysis of the local defect distribution by scanning transmission electron microscopy and a comparison with atomic force microscopy reveal a bunching of defects in line with the ELO pattern and a roughening of step edges in highly defective regions. In addition, a reduction in the threshold excitation power density for optically pumped ultraviolet‐C (UVC) lasers with smooth surface morphologies is observed.
Highlights
Introduction templatesIn a previous study, we compared epitaxially laterally overgrown (ELO) AlN/sapphireLight-emitting diodes (LEDs) and laser diodes (LDs) emitting in the ultraviolet-C spectral region between 200 and 280 nm have a wide range of applications such as water purification,[1] disinfection of surfaces, gas sensing,[2,3] and medical diagnostics.[4]templates on sapphire wafers with nominal off-cut angles of 0.1 Æ 0.1 and 0.2 Æ 0.1
Optically pumped laser structures consisting of AlGaN multiple quantum wells emitting between 270 and 275 nm embedded in Al0.7Ga0.3N waveguides and an AlGaN cladding layer[17] were grown on top of ELO AlN/sapphire templates
After AlN growth, processing, and overgrowth with ELO AlN, the surface morphology changes from a wavelike surface morphology to step bunching at a critical angle of α1⁄4 0.14∘ Æ 0.02∘ for the used AlN growth conditions
Summary
ELO AlN/sapphire occurs at very small off-cut angles between 0.1 and 0.2,[10] a highly accurate determination of the substrate off-cut is necessary to determine the critical angle for this transition. This technique is similar to the one described by Halliwell and Chua[11] extended by an additional alignment step of the wafer Using this alignment, the surface normal of the sapphire wafer can be adjusted parallel to the rotation axis of the diffractometer. By tilting the sample holder independently from the goniometer (ω/φ/χ), the precession radius was minimized, which corresponds to an adjustment of the surface normal parallel to the rotation axis of the diffractometer. After the alignment of the surface normal to the goniometer rotation axis, a map in ω and φ of the (0006) reflection of sapphire was acquired (Figure 2). To cover a broad range of off-cut angles, we selected eight wafers with off-cut angles varying from 0.08 to 0.1 Æ 0.23 for further investigations (see Figure 3, open symbols)
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