Wavelength dependent negative and positive persistentphotoconductivity in Sn δ-doped GaAsstructures

Abstract

The photoconductivity of GaAs structures δ-doped by Sn has been investigated for wavelengths λ = 650-1200 nm in the temperature interval T = 4.2-300 K. Theelectron densities and mobilities, before and afterillumination, have been determined by magnetoresistance,Shubnikov-de Haas effect and Hall effect measurements, in highmagnetic fields. For the heavily doped structures (Hall densitynH>2×1013 cm-2) we observe underillumination by light with wavelengths larger than the bandgapwavelength of the host material (λ = 815 nm atT = 4.2 K) first positive (PPPC) and then negative (NPPC)persistent photoconductivity. The NPPC is attributed to theionization of DX centres and PPPC is explained by the excitationof electrons from Cr impurity states in the substrate. Forλ<815 nm, in addition, the excitation of electronsover the bandgap of GaAs contributes to the PPPC. For thelightly doped structures (nH⩽2×1013 cm-2) the photoconductivity effect is always positive.