Evidence of a second acceptor state of the E center in Si(1-x)Ge(x)

We have found evidence of a second acceptor state of the E center in Si1−xGex by using positron annihilation spectroscopy. To achieve this, we studied proton irradiated n-type Si1−xGex with a Ge content of 10%–30% and a P dopant concentration of 1018cm−3, in which the number of Ge atoms around irradiation induced E centers was increased by annealing. When measuring the Doppler broadening of the annihilation line, the shape parameter S starts to decreases at 150K with decreasing measurement temperature. This indicates that a charge transition in the upper half of the Si1−xGex band gap, above the well known (0/−) level, takes place. Hence, we suggest that the increased concentration of germanium around the E center pulls down the localized second acceptor state into the Si1−xGex band gap, making the Ge decorated E center a more effective trap for conduction electrons.