Influence of Ge content on the optical properties of X and W centers in dilute Si-Ge alloys

Photoluminescence (PL) measurements, performed in Si and Si1-xGex alloys (x = 0.0069 and 0.0125) irradiated with protons and annealed between 100 and 650 degrees C, are combined with first-principles calculations to assess the assignment of the W and X lines to the trigonal and tetragonal forms of the tri- and tetra-interstitial defects, denoted I-3 and I-4, respectively. It is found that for the W line the annealing temperature that maximizes the PL intensity is independent of x, whereas for the X line a similar to 50 degrees C shift to higher temperatures was observed in the alloys when compared to pure Si. Analysis of the PL quenching mechanisms along with the calculations show that the radiative excited state for both defects comprises a pseudodonor state, where a trapped exciton combines a tightly bound hole and a diffuse electron. The broadening rate as a function of x for the X line is greater than that of the W line by about a factor of two. This is consistent with a stronger electronic localization in the X center than in the W center, as well as with the relative positions of the donor levels as obtained from the calculations for the I-3 and I-4 defects, respectively. The calculated change rates of donor levels with x are in qualitative agreement with the hole binding energy shifts obtained from the experiments. Our results support the assignment of I-3 and I-4 to the W and X centers, respectively.