Donor levels and the microscopic structure of the DX center in n-type Si-doped AlxGa0.51-In0.49P grown by molecular-beam epitaxy

We have investigated donor levels and the local structure of DX centers in Si-doped AlxGa0.51-xIn0.49P grown by gas-source molecular-beam epitaxy. In a ternary alloy Ga0.51In0.49P, Si donors form only shallow donor states. In contrast, in quaternary alloys with x greater than or equal to 0.25 a deep electron trapping center is observed. Hall measurements reveal an activated behavior of the mobile electron concentration, and the thermal binding energy of the dominant donor state is similar to 0.1 eV when the Al fraction is x = 0.25. Illumination with infrared or red light results in persistent photoconductivity at T less than or equal to 120 K. The appearance of the DX level in the band gap around x approximate to 0.1 gives a consistent picture of the experimental findings. Positron annihilation spectroscopy shows that the Si DX center is a vacancylike defect with a local structure equivalent to that found earlier in AlxGa1-xAs. The very different core shell structures of the group-III (Ga, In) and group-V (P) atoms give direct evidence that the vacancy has P atoms as its nearest neighbors and we identify it as a vacancy in the group-III sublattice. The structural data give support to the vacancy-interstitial model, which predicts that the donor impurities can take two different configurations in sp-bonded semiconductors.