High-reflectivity AlxGa1−xN∕AlyGa1−yN distributed Bragg reflectors with peak wavelength around 350nm

${\mathrm{Al}}_{0.49}{\mathrm{Ga}}_{0.51}\mathrm{N}∕{\mathrm{Al}}_{0.16}{\mathrm{Ga}}_{0.84}\mathrm{N}$ distributed Bragg reflectors $\left(\mathrm{DBRs}\right)$ , designed for the ultraviolet spectral region, are reported. The crack-free structures are grown on sapphire substrates by metalorganic chemical vapor deposition. A peak reflectivity of $91\%$ at $353\mathrm{nm}$ with a stop-band width of $17\mathrm{nm}$ was obtained at $300\mathrm{K}$ using a $25\text{pair}$ $\mathrm{DBR}$ . Structures at $6\mathrm{nm}$ shorter wavelength show asymmetric spectra, a narrower stop band, and slightly decreased peak reflectivity. By studying the spectra as a function of temperature, the degradation is shown to arise from internal absorption at short wavelengths. Simulations based on transfer matrix theory agree well with the experimental data.