Determining the bandtail shape of highly Si-doped Al$_{0.3}$Ga$_{0.7}$As using persistent photoconductivity

Highly Si-doped Al$_{0.3}$Ga$_{0.7}$As can be driven through the metal-insulator phase transition using persistent photoconductivity [1].~ Owing to the bi-stable nature of the Si donor, samples cooled in the dark are insulating. In the present work, an infrared LED is used to photodope the sample at 5K for a range of illumination times, which populates shallow states and provides a way to change the carrier concentration of the sample\textit{ in situ}.~ Measuring the carrier concentration as a function of temperature allows for the infinite temperature carrier concentrations and Hall activation energies to be extracted for various illumination times as the Fermi energy is tuned systematically.~ Application to Si- doped Al$_{0.3}$Ga$_{0.7}$As prepared by MBE allows one to infer the bandtail shape [2] in the energy range between the Fermi energy of the unilluminated sample and the mobility edge. [1] S. Katsumoto, et al. J. Phys. Soc. Jpn. 56, 2259 (1987) [2] I. Terry, et al. Solid State Commun. 84, 235 (1992)