The relationship of the D-X centre in Al_xGa_1-xAs and other III-V alloys with the conduction band structure

The electrical properties of the Al_xGa_1-xAs alloys for x>or=0.24 are dominated by deep donors which appear in undoped and doped crystals. The corresponding defects, which are now more commonly known as D-X centres, also behave as electron traps. The properties of these defects in undoped and doped alloys are reviewed. Results of Hall, transient capacitance, deep-level transient spectroscopy and thermally stimulated capacitance measurements on crystals grown by liquid phase epitaxy and molecular beam epitaxy are presented and discussed. The deep levels have a constant thermal activation energy of 0.21 eV in the direct band-gap region, while in the same composition range the defect binding energy, as determined from Hall measurements, increases monotonically. Beyond the crossover region, the thermal activation energy and binding energy assume similar values and both decrease monotonically with increase of x. These energy values and the electron capture properties of the defects can be consistently explained by involving the interaction of the defects with indirect L and X minima. Defects with similar properties are also identified in other alloy systems in which the L and X conduction minima are lowest in energy.