Direct Evidence for the Negative-U Property of the DX Center as Studied by Hydrostatic Pressure Experiments on GaAs Simultaneously Doped with Ge and Si

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DX centers in GaAs codoped with Ge and Si have been investigated under a hydrostatic pressure, where Ge acts as a DX center, and Si as a shallow donor. It is demonstrated that the number of electrons trapped by the Ge DX center at 22 kbar increases with Si concentration and tends to saturate at a certain value. The behavior substantiates the negative-U model of the DX center, where two electrons are bound to a DX center.