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

TL;DR

This study investigates wavelength-dependent persistent photoconductivity in Sn delta-doped GaAs, revealing both positive and negative effects linked to different doping levels and illumination wavelengths, with implications for optoelectronic device design.

Contribution

It provides new insights into the mechanisms of persistent photoconductivity in doped GaAs, especially the roles of DX centers and impurity states at various wavelengths.

Findings

Negative photoconductivity occurs at wavelengths above 815 nm in heavily doped structures.

Positive photoconductivity is observed at all wavelengths in lightly doped structures.

Different mechanisms, such as DX center ionization and impurity excitation, explain the observed effects.

Abstract

The photoconductivity of GaAs structures delta-doped by Sn has been investigated for wavelengths lambda= 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 (n_H> 2x10^13 cm^-2) we observe under illumination by light with wavelengths larger than the band-gap wavelength of the host material (lambda= 815 nm at T= 4.2 K) first positive (PPPC) and then negative (NPPC) persistent photoconductivity. The NPPC is attributed to the ionisation of DX centres and PPPC is explained by the excitation of electrons from Cr impurity states in the substrate. For lambda< 815 nm in addition the excitation of electron over the band gap of GaAs contributes to the PPPC. For the lightly doped structures (n_H<= 2x10^13 cm^-2) the photoconductivity effect is always positive.