Abstract
To realize AlGaN-based solar-blind ultraviolet distributed Bragg reflectors (DBRs), a novel tri-layer AlGaN/AlInN/AlInGaN periodical structure that differs from the traditional periodically alternating layers of high- and low-refractive-index materials was proposed and grown on an Al0.5Ga0.5N template via metal-organic chemical vapour deposition. Because of the intentional design of the AlInGaN strain transition layer, a state-of-the-art DBR structure with atomic-level-flatness interfaces was achieved using an AlGaN template. The fabricated DBR exhibits a peak reflectivity of 86% at the centre wavelength of 274 nm and a stopband with a full-width at half-maximum of 16 nm.
Highlights
To reduce the strain resulting from the lattice mismatch, a fully strain-relaxed Al0.5Ga0.5N template on a sapphire substrate was employed for fabrication of the distributed Bragg reflectors (DBRs) structure by MOCVD22
The thickness of each layer was accurately measured owing to the clear and abrupt interfaces and was found to be 16.0, 10.9, and 27.6 nm for the AlGaN, AlInN, and AlInGaN layers, respectively, as shown in the magnified image presented in the inset of Fig. 1; these thicknesses are consistent with the designed structure
The high-resolution transmission electron microscopy (TEM) image of the DBR in Fig. 2 shows that the high-quality AlGaN/AlInN interface and AlGaN surface with atomic-level flatness were achieved in our DBR structure, indicating atomic-layer control during the metal-organic chemical vapour deposition (MOCVD) growth of the high-Al-composition
Summary
A number of challenges still remain for achieving high-quality solar-blind UV DBRs. Here, we used a periodic AlGaN/AlInN/AlInGaN tri-layer structure instead of the traditional periodically alternating layers of high- and low-refractive-index materials to fabricate a solar-blind UV DBR. The AlGaN alloy was used as the high-refractive-index layer in the DBR. Higher refractive-index contrast will be obtained with a lower Al content in the AlGaN layer, resulting in a high-reflectivity DBR. The low-Al-content AlGaN alloys will increase residual absorption in the stopband, which degrades DBR performance. To retain the advantages of the high-refractive-index contrast and the low residual absorption, a relatively low Al content was chosen under the precondition that the bandgap of AlGaN was larger than the short-wavelength edge of the stopband
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