Crystalline silicon has a density of 2.3290 g/cm3 and a diamond cubic crystal structure with a lattice constant of 543.07 pm, .below Figure shows two different sections through a crystalline silicon lattice, which originally consisted out of three by three by three unit cells. The first surface shown in the below Figure (a) is the 100 surface, whose surface normal is the 100 direction. At a 100 surface, each Si atom has two back bonds and two valence electrons pointing to the front. The second surface, shown in below Figure (b), is the 111 surface. Here, every Si atom has three back bonds and one valence electron pointing towards the plane normal.
To understand the importance of these directions, we look at the electronic band dispersion diagram for silicon, shown in below Figure . On the vertical axis, the energy position of the valence and conduction bands is shown. The horizontal axis shows the crystal momentum, i.e. the momentum of the charge carriers. The white area represents the energy levels in the forbidden band gap. The band gap of silicon is determined by the lowest energy point of the conduction band at X, which corresponds to the 100 direction, and the highest energy value of the valence band, at G. The band gap energy is the difference between those two levels and is equal 1.12 eV, or 1107 nm, when expressed in wavelengths. 1107 nm is in the infrared part of the spectrum of light. This bandgap is an indirect bandgap, because the charge carriers must change in energy and momentum to be excited from the valence to the conduction band. As we can see, crystalline silicon has a direct transition as well. This transition has an energy of 3.4 eV, which is equivalent to a wavelength of 364 nm, which is in the blue spectral part.
Because of the required change in momentum, for for an indirect band gap material it is less likely that a photon with an energy exceeding the bandgap can excite the electron, with respect to a direct bandgap material like gallium arsenide (GaAs) or indium phosphite (InP). Consequently, the absorption coefficient of crystalline silicon is significantly lower than that of direct band gap materials, as we can see in below Figure. While in the visible part of the spectrum c-Si absorbs less than the GaAs and InP, below 364 nm, it absorbs
