We present a study on electrical properties of Al/Si3N4/Si/GaAs structure studied by deep level transient spectroscopy and capacitance-voltage techniques Here, the Si means an ultrathin silicon layer with thickness of approximately 2 nm. We have modelled the presented structure, while emissions from deep levels at Si/GaAs interface, traps in the bulk of GaAs and from a quantum well (QW) possibly formed by the Si interlayer, were taken into the account. Four deep traps were identified in the structure with the following thermal activation energies: 0.04, 0.19, 0.40 and 0.68 eV. The energy levels 0.4 and 0.68 are related with As defects, while the energy levels 0.04 and 0.19 are associated with the presence of the Si interlayer. Based on the emission behaviour, an existence of the QW is not probable (can be excluded). Rather, parameters of the Si-related energy levels suggest the levels are induced by delta-doping of GaAs. (C) 2003 Elsevier Science B.V. All rights reserved.