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Bistability, local symmetries and charge states of Sn-related donors in AlxGa1-xAs and GaAs under pressure studied by Mossbauer spectroscopy

The bistable character of Sn donors AlxGa1-xAs for x>0.2 or in GaAs under pressure >2.4 GPa has been studied by 119Sn Mossbauer spectroscopy (MS). The shallow Sn donor state and the deep Sn DX state are observed to exist simultaneously and are readily distinguishable due to significantly different electronic configurations as manifested in their isomer shifts. An upper limit on the amount of non-cubic local lattice distortion at the Sn DX centre has been obtained through establishment of an upper limit on the local electric field gradient as determined from the quadrupole interaction. The MS data from GaAs under pressure, coupled with magnetotransport data, provide strong evidence that the Sn DX centre localizes at least two electrons in its ground state.