In practice, a thin layer of silicon is deposited on silicon doped with germanium, which leads to a distortion. Because of its small dimensions, an experimental determination of physical properties is very difficult. At this point, the major advantages of ab initio calculations come to their use. Without much effort, physical properties may be determined (structural, thermal, electronic, etc.), for which in other cases various experimental measurement arrangements were necessary. In addition to the calculations, the exact structures of the electronic properties were also determined. Conclusions about the relationship between crystal lattice and other physical quantities could be made. In chapter 1 the structure of SiGe is determined. Using the structure obtained from chapter 1, in chapter 2 the electronic properties of strained silicon are calculated. To demonstrate an allocation of 25% germanium, Si₃Ge is used.

Bracing to improve the conductivity

MOSFET's are the main important components of the semiconductor industry. Due to the decreasing feature size of these devices, the development encounters its limits. Unwanted side effects are inevitable. Another way to increase the performance of MOSFETs without down scaling them, is the tension of the silicon. Because of the tension it comes through a change in the band structure to an increase in conductivity. This allows the electrons to pass through the channel of the transistor and switch faster. However, applications of a mechanical stress on very small length scales are very difficult to achieve [1].

As a solution it could be used the doping with elements from the 5th Main group of the periodic table. Based on the same lattice structure and the different lattice constants of these elements, the introduction of germanium or carbon are leading to increase the crystal volume. Leaving now on such a SiGe structure, a thin layer of silicon to grow up, so this attempt to take on the larger lattice constant. This causes stretching in the appropriate level. On the 8th of June 2001 IBM announced the first use of strained silicon to increase and reached a speed by 35% of their chips.

To compare the results, the FCC lattice of Si and Ge were calculated for comparison. The volume of the 16-atom cells be V_Si = 2.1304 * 10³ Bohr³, V_GE = 2856 * 10³ Bohr³ and V_Si3Ge = 2163 * 10³ Bohr³. In fact of to the lower volume of the silicon cell, the cell gets larger at the doping with germanium (see Figure 2).

The expansion of the SiGe cell is now determining the stretch of the silicon lattice. Trying to keep its volume constant the silicon cell opposed to a stretching in the x-y- plane, a compression in the z-direction (see Figure 3).