Deep donor levels (DX centers) in III–V semiconductors: Recent experimental results

Abstract

The DX center, the lowest energy state of the donor in AlxGa1−xAs with x≥0.22, is responsible for the reduced conductivity as well as the persistent photoconductivity observed in this material at low temperature. Extensive studies of the properties of this deep level in Si-doped AlGaAs are reviewed here. It has been shown that the characteristics of the DX center remain essentially unchanged when it is resonant with the conduction band (x≤0.22) with the result that, independent of other compensation mechanisms, the DX center limits the free carrier concentration in Si-doped GaAs to a maximum of about 2·1019 cm−3. Recent measurements suggesting that the lattice relaxation involves the motion of the Si atom from the substitutional site toward an interstitial site are also presented. Evidence for the negative U model, that the DX level is the two electron state of the substitutional donor, is discussed.