Auto-Interstício em ligas SiGe

Abstract

Recent progress in nanotechnology applied to the development of new (nano)devices is mainly attributed to the research works in a large number of semiconducting materiais. Within this class of semiconducting materiais, the SixGe\_x alloy has attracted special attention due to the possibility of tunning its electronic and structural properties by controlling the alloy concentration. During the growth process of SixGe 1_x, it has been verified the diffusion of impurities as well as the formation of defects as, vacancies, self- interstitials, and stacking faults. For instance, the formation of Si or Ge self-interstitials has been verified during the ion-implantation process in these materiais. In this work, we performed first-principles calculations of the formation energies, electronic and structural properties of self-interstitials in Si, Ge, and SixGei_x alloys as a function of the alloy concentration. The calculations were performed in the framework of the density function theory, within the local density approximation, and the electron-ion interaction was treated by using norm-conserving pseudopotentials. The SixGe\-x alloys were described by using the Special Quasirandom Structures (SQS). Initially we calculated the formation energies of self-interstitials in pure Si and Ge, considering the following atomic configurations: Split[110], Hexagonal, Displaced-Hexago- nal, and Tetrahedral. In Si, we find that the Split[110] configuration is the energetically most stable, followed by the Displaced-Hexagonal (0.3 Â along the [111] direction) with a total energy difference of 0.02 eV compared with the Split[110]. On the other hand, the Displaced-Hexagonal configuration is not energetically stable in Ge, being the Split[l 10] configuration the energetically most stable one, followed by the Tetrahedral configuration by a total energy difference of 0.28 eV. Very recently, a new and energetically more stable than the previous self-interstitial structural models has been investigated, called four-fold coordinated defect (FFCD).'The energetic stability of the FFCD defect can be attributed to the four-fold coordination of the interstitial atom. Our calculations support the formation of FFCD in Si and Ge. In Si the FFCD exhibits a formation energy of 0.76 eV lower compared with the Split[110], and in Ge the FFCD is 0.78 eV lower in energy also compared with the Split[110]. In addition, we determined the ionization leveis of the Split[110] self-interstitial in Ge, where we find that the (+/0) levei is 0.08 eV below the valence band maximum (VBM), i.e. resonant within the valence band, and the (0/—) levei lies at 0.37 eV above the VBM. These results are in contrast with the recent experimental investigation. We next have investigated the formation of self-interstitials in the SixGe\-x alloys, for alloy (Si) concentrations of 0.50 and 0.85. We have studied self-interstitials of Si and Ge for different atomic arrangements in the nearest neighbor (NN) sites. We have considered three different concentrations of Si/Ge at the NN sites: Si-rich, Ge-rich and Sío.bGcq,5. For both alloy concentration, x — 0.50 and 0.85, we verify that the Split[l 10] of Ge, with Sia.^Geo.s at the NN sites, exhibits the lowest formation energy, followed by the Displaced-Hexagonal of Si with the NN sites rich of Si. We do not find the FFCD in those SixGei_x alloys.