Increase in the diffusion length of minority carriers in Al$_x$Ga$_{1-x}$N ($x$ = 0–0.1) fabricated by ammonia molecular beam epitaxy

The room-temperature diffusion length of minority carriers in $n$-Al$_{0.1}$Ga$_{0.9}$N layers grown by ammonia molecular beam epitaxy on sapphire (0001) substrates used in structures for ultraviolet photodetectors is studied. Measurements were performed using the spectral dependence of the photocurrent recorded in a built-in $p$–$n$ junction for thin samples and using the induced electron-current procedure for films up to 2 $\mu$m thick. The results show that the hole diffusion length in n-AlGaN films is 120–150 nm, which is larger than in GaN films grown under similar growth conditions by a factor of 3–4. This result can be associated with the larger lateral sizes characteristic of hexagonal columns in AlGaN layers grown by molecular beam epitaxy. No increase in the hole diffusion length is observed for thicker films.