## 3 Near-Infrared Silicon Absorption Physics
### Photoconductivity and Linear Absorption
#### 3.1.1 Mid-Bandgap Absorption (MBA)  
Defects always exist in semiconductors. They may be extrinsic, involving foreign atoms (_i.e._, impurities) or intrinsic due to crystalline defects. Impurities can be intentionally introduced as dopant or recombination centers in order to make a semiconductor useful for fabricating a device, or they can be unintentionally incorporated during crystal growth or device processing, with undesirable effects. By crystalline defect, one generally means any region where the microscopic arrangement of atoms differs drastically from that of a perfect crystal. Defects are classified into point defects, line defects and cluster defects. Among the most important kind of defects we mention: vacancies and interstitials (point defects), dislocations (line defects) [21, 22], divacancies and interstitial clusters (cluster defects) [23]. It is well known that the band diagram of a perfect crystal semiconductor consists of a valence band and a conduction band separated by the band gap, with no energy levels within the band gap. When theperiodicity of the single crystal is perturbed by foreign atoms or crystal defects, discrete energy levels are introduced into the band gap. Usually, we can separate defects in two broad categories: shallow impurities and deep centers. The former are characterized by levels with binding energies much smaller than the energy gap of the host semiconductor, while the latter are characterized by electronic levels located near the middle of the bandgap. The simplest type of shallow impurity is substitutional. In an energy level diagram, an electron from a substitutional impurity lies closer to the conduction band than a valence electron. A photon whose energy is at least equal to the energy difference can elevate an electron from an impurity level to the conduction band. The ionization energy is a few meV, permitting the absorption in the far-infrared regime. Another simple case is that of interstitial impurities, which produce a second type of optical effect: impurity changes in the local lattice environment, resulting in an alteration of vibrational behaviour in the atoms located in the neighbourhood of each impurity. This can lead to new infrared vibrational modes [21,22]. Defects called generation-recombination centers lie deep in the band gap. They act as recombination centers when there are excess carriers in the semiconductor, and as generation centers when carrier density is below its equilibrium value. For single crystal semiconductors, like silicon and germanium, deep level impurities are usually metallic impurities, but they can be crystal imperfections, such as dislocations, vacancies or interstitials. For the most part, they are undesirable. Occasionally, however, they are deliberately introduced to alter a device characteristic. Absorption of light is one of the most successful methods for investigating the impurity electronic state. The simplest information which can be obtained from absorption measurements is the energetic position of the impurity level within the forbidden energy gap of the semiconductor. This process can also be observed using photoconductivity measurements in bulk materials and photocurrent measurements on p-n as well as Schottky junctions. Extrinsic photoconductivity offers a convenient means of detecting and studying impurities and localized centers. Also, by changing the type of impurity in a given material, the spectral sensitivity can be adjusted over a wide wavelength range. Obvious applications for infrared detection exist and encourage a strong interest in this field [18-20]. As an example of optical absorption due to defect introduction, it is interesting to report the experimental results carried out by [PERSON] and [PERSON] in 1959 [24]. In Figure 1, curve A shows the intrinsic absorption edge in un-implanted high-resistivity silicon; curves B and C are two of the curves obtained on a sample after successive deuteron irradiations.