Giant resistance changes in (Al,Ga)As contact layers of GaAs/AlAs superlattices due to deep donors.

For transport studies on semiconductor superlattices under optical excitation, samples with highly Si-doped (Al,Ga)As contact layers are used due to their optical transparency. These samples exhibit a giant increase of the resistance at temperatures below 200 K. Likewise, the current plateau in the I–V characteristics, which contains the region of current self-oscillations, is shifted towards higher voltages. However, the oscillation frequencies remain unchanged. The current decreases during cooling by as much as seven orders of magnitude. The giant resistance can be compensated for at low temperatures by applying a high dc voltage or by weak illumination. The optically induced current transients are shown to be controlled by thermally activated processes. On the basis of the activation energies derived, it is suggested that the dramatic resistance changes are mainly due to deep donors, i.e., well-known DX centers, in the (Al,Ga)As contact layers. The effects are negligible, if GaAs contact layers are used. Possible DX centers at the AlAs/GaAs interfaces inside the superlattice structure can be therefore neglected. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]