Solar-blind Al$_{x}$Ga$_{1-x}$N ($x>$ 0.45) $p$–$i$–$n$ photodiodes with a polarization-$p$-doped emitter

Polarization-induced p-type doping of AlGaN layers with high aluminum content during plasmaassisted MBE growth has been studied. It is shown that a gradient of the AlN molar fraction in AlGaN (composition gradient) on a level of 0.005 nm$^{-1}$ must be set in order to obtain a hole concentration of $\sim$10$^{18}$ cm$^{-3}$ (measured by the $C$–$V$ method) in Al$_{x}$Ga$_{1-x}$N:Mg ($x$ = 0.52–0.32) layers with dopant concentration [Mg] = 1.3 $\times$ 10$^{18}$ cm$^{-3}$. $p$–$i$–$n$ photodiodes based on AlGaN heterostructures with such layers as p-emitters showed maximum photoresponsitivity in the solar-blind wavelength range ($\lambda$ = 281 nm) about 35 and 48 mA/W at reverse bias voltage $U$ = 0 and -5 V, respectively, and exhibited a dark current density of 3.9 $\times$ 10$^{-8}$ A/cm$^{2}$ at $U$ = -5 V.