DONOR LEVELS AND THE MICROSCOPIC STRUCTURE OF THE DX CENTER IN N-TYPESI-DOPED ALXGA0.51-IN0.49P GROWN BY MOLECULAR-BEAM EPITAXY

We have investigated donor levels and the local structure of DX center s in Si-doped AlxGa0.51-xIn0.49P grown by gas-source molecular-beam ep itaxy. In a ternary alloy Ga0.51In0.49P, Si donors form only shallow d onor states. In contrast, in quaternary alloys with x greater than or equal to 0.25 a deep electron trapping center is observed. Hall measur ements reveal an activated behavior of the mobile electron concentrati on, and the thermal binding energy of the dominant donor state is simi lar to 0.1 eV when the Al fraction is x = 0.25. Illumination with infr ared or red light results in persistent photoconductivity at T less th an or equal to 120 K. The appearance of the DX level in the band gap a round x approximate to 0.1 gives a consistent picture of the experimen tal findings. Positron annihilation spectroscopy shows that the Si DX center is a vacancylike defect with a local structure equivalent to th at found earlier in AlxGa1-xAs. The very different core shell structur es of the group-III (Ga, In) and group-V (P) atoms give direct evidenc e that the vacancy has P atoms as its nearest neighbors and we identif y it as a vacancy in the group-III sublattice. The structural data giv e support to the vacancy-interstitial model, which predicts that the d onor impurities can take two different configurations in sp-bonded sem iconductors.