Statistical analysis in the negative-U model of donors in AlxGa1-xAs : Si

Hall measurements were performed on molecular beam epitaxy grown AlxGa1-xAs :Si in the temperature range 77-300 K. The DX center has been detected thro ugh the observation of persistent photoconductivity at low temperature. Two statistics have been developed, using the negative-U model of Chadi and Ch ang, to analyze the temperature dependence of Hall electron densities. The first statistic is derived assuming that the conduction electrons originate exclusively from the DX center. The second statistic, however, supposes th e existence of shallow donors and acceptors in addition to Si-DX centers. T he concentrations of these centers are treated as fitting parameters. We ha ve investigated, on the other hand, the pinning of the Fermi level E-F and the trend of the probability f(DX)(-) of Si to be in the DX- state versus t emperature using the previous statistics. We will attempt to explain all th ese results. (C) 2000 American Institute of Physics. [S0021-8979(00)05210-5 ].