Wavelength dependent negative and positive persistent photoconductivity in Sn δ-doped GaAs structures

摘要

The photoconductivity of GaAs structures b-doped by Sn has been investigated for wavelengths λ = 650-1200 nm in the temperature interval T = 4.2-300 K. The electron densities and mobilities, before and after illumination, have been determined by magnetoresistance, Shubnikov--de Haas effect and Hall effect measurements, in high magnetic fields. For the heavily doped structures (Hall density n_H ＞ 2 × 10~13 cm~-2) we observe under illumination by light with wavelengths larger than the bandgap wavelength of the host material (λ = 815 nm at T = 4.2 K) first positive (PPPC) and then negative (NPPC) persistent photoconductivity. The NPPC is attributed to the ionization of DX centres and PPPC is explained by the excitation of electrons from Cr impurity states in the substrate. For λ ＜ 815 nm, in addition, the excitation of electrons over the bandgap of GaAs contributes to the PPPC. For the lightly doped structures (n_H ≤ 2 × 10~13 cm~-2) the photoconductivity effect is always positive.