Ab Initio Study Of Carbon-related And Self-interstitial Defects In Si And SiGe Alloy Semiconductors

Si_1-xGe_x alloys are important semiconductor materials for the microelectronics industry, due to the possibility of band-gap engineering leading to high-speed electronic devices and the compatibility with conventional silicon technology. The interstitial-mediated transient enhanced diffusion (TED) of dopant becomes a big concern in the Si- and Si_1-xGe_x- based semiconductor manufacture with the reducing of device sizes. Ion implantation is still a vitally important tool for doping in the manufacture of integrated devices. During postimplant thermal anealings, small interstitial clusters can be formed around the projected range of implant. Many studies have also shown that the dissolution of small interstitial clusters is responsible for the TED. Fortunately, Carbon, a common impurity introduced by ion-implantation, had been found recently to be a very efficient interstitial trap to suppress the TED in Si and Si_1-xGe_x semiconductor materials. Therefore, the studies on the properties of carbon-related defects and interstitial cluster defects in Si and Si_1-xGe_x alloy become very urgent and important.With the SIESTA code, we perform ab initio total-energy calculations based on the local-density approximation (LDA) of density-functional theory (DFT) to study the properties of C_iC_s pair, C_iO_i pair and W defect in Si and Si_1-xGe_x alloy. Through the calculation, we have gotten the structures of these defects in the materials. The formation energies of C_iC_s (A-type), C_iC_s (B-type), C_iO_i and W defect were 4.602 eV, 4.367 eV, 3.84 eV, and 8.27 eV, respectively.In the Si_1-xGe_x alloys a direct Ge-C or Ge-O bonds were unlikely to be formed. The substitution of Ge for Si atom took place at distance far from the core area of the defects. But the direct connection of Si and Ge in W defect cannot be excluded for the low formation energy. In the Si_1-xGe_x alloy containing C_iO_i defect, Ge atom is apt to substitute the Si atom in the [110] direction.Si_1-xGe_x alloy is a complete solid solution and has almost the same valence electron density with pure Si. Meanwhile, the adding of Ge atom in alloy has small affect on the valence electron densities of C_iC_s and C_iO_i defects. In the Si_1-xGe_x alloy, the formation energy of W defect increased with the increasing of Ge content. When the Ge content is bellow 4.69%, W defect's formation energy changed a little. But the formation energy of W defect increased quickly with the adding of Ge atoms when Ge content is more than 4.69%. Different with W defect, the formation energies of C_iC_s and C_iO_i defects were insensitive to the Ge content. The variation range is smaller than 0.15 eV. These results may stem from the stress compensation between smaller radii atoms (i.e. C and O) and larger radii atom (i.e.Ge). Through the investigation of characteristics of the defects, we found that the properties of defects in Si_1-xGe_x alloy with smaller Ge content were similar with pure Si system. But in Si_1-xGe_x alloy with higher Ge content, the C-iC_s, C_iO_i and W defect had different behavior.The energy differences of A form and B form C_iC_s defects were between 0.235 eV and 0.220 eV with less Ge contents, and reduced sharply to around 0.1 eV for high Ge contents. The adding of Ge atoms increased the stability of A-type C_iC_s defect and leaded to the increasing of proportion of A-type defects in the total C_iC_s defect. While isothermal annealing at 1000K and above, the structure of A-type C_iC_s defect in pure Si can overcome the barrier between the defects and translate to B-type defect. During annealing, the behavior of A-type C_iC_s defect in Si_1-xGe_x alloy depended on the annealing temperatures, Ge content in the alloy and the sites of Ge atoms.