Properties of Si donors and persistent photoconductivity in AlGaN

The behavior of Si donors was studied in AlxGa1-xN films with composition 0 < x < 0.6. It is shown that the Si donors ionization energy increases from 18 meV for 0 < x < 0.1 to about 50 meV for x = 0.4 and does not exceed 90 meV for x = 0.6. Increase in Al composition is also accompanied by the growth of the density of defects with energy levels deeper than Si. Combined action of the two above effects leads to increased difficulty in n-type doping of AlGaN films with higher Al mole fractions. Persistent photoconductivity (PPC) is shown to be a characteristic feature of AlGaN samples of all compositions. It is shown that PPC in AlGaN is related to the presence of centers with a relatively high (0.1-0.2 eV) barrier for capture of electrons. In GaN and AlGaN with x < 0.1 such centers are not associated with Si donors per se. (C) 1998 Elsevier Science Ltd. All rights reserved.