# Germanium Single Crystals for Photonics
## 2 Absorption and Scattering of Germanium in the IR Region of the Spectrum
A connection has been revealed between the crystal structure and electrical conductivity (and, consequently, absorption) of not only single crystals, but also optical polycrystals of Ge, as well as high-purity germanium (HPGe). The electrical conductivity of Ge polycrystals decreases with decreasing grain size, which is associated with a decrease in the mobility of charge carriers caused by their scattering at the crystallite boundaries. The electrical conductivity increases with decreasing crystallites size, and the lifetime of nonequilibrium charge carriers in it decreases due to an increase in the concentration of surface electron states [62]. Dislocation-free and low-dislocation germanium is grown for the production of ionizing radiation detectors, where crystals with a linear defects content (no more than 100 cm\({}^{-2}\)) and a concentration of electroactive impurities at the level of 10\({}^{9}\) cm\({}^{-3}\) are required. It turned out that one of the harmful impurities that affects the defect structure and properties of Ge single crystals is oxygen [O] [63; 64; 65; 66]. Oxygen in germanium, as a rule, is an atomic optically active interstitial impurity. The complexes formed by [O] in the crystal lattice of germanium are due to the presence of Ge-O-Ge quasimolecules, as well as various GeO\({}_{\rm x}\) precipitates, the sizes, concentrations, and shapes of which depend on the total concentration of [O], the growing conditions, and subsequent heat treatment. In works [65; 66], the authors established a connection between the oxygen concentration and the dislocation density, determined the content of optically active [O] in Ge crystals of various grades, and found that the presence of oxygen impurity has a noticeable effect on the structural perfection of single crystals and on the parameters of devices manufactured on its basis. The content of optically active [O] in Ge of different grades was determined using FTIR spectroscopy from the position of absorption bands in the phonon region. Regulation of the [O] content in the Ge crystal lattice is often achieved by doping with rare earth elements [67]. It has been shown that when Ge is doped with a number of lanthanides, they actively bind oxygen atoms into electrically neutral complexes, thus formally reducing the concentration of optically active [O] in the matrix by almost an order of magnitude. Modern technologies for growing germanium single crystals with low oxygen content in crystals should ensure an oxygen concentration at the level of 10\({}^{15}\) cm\({}^{-3}\), which implies growing in an atmosphere with a partial pressure of oxygen in the gas phase not exceeding 1.53\(\cdot\)10\({}^{-23}\) atm. (1.5\(\cdot\)10\({}^{-18}\) Pa). When Ge grows from a melt in the presence of oxygen in the gas phase, a reaction of Ge oxidation occurs with the formation of heterogeneous oxide inclusions GeO\({}_{2}\) in the melt, which are a source that stimulates not only the formation of dislocations, but also the growth of IR radiation scattering on these inclusions [66]. For example, HPGe can be doped with hydrogen during single-crystal growth using the Czochralski method in an H2 atmosphere, which reduces the presence of O-centers. The H\({}_{2}\) concentration in this case is \(\sim\)10\({}^{-15}\) cm\({}^{-3}\). The presence of a moderate density of dislocations, which are sinks, can facilitate the annihilation of some point defects. For single-crystal HPGe, the minimum density of linear dislocations and the maximum possible value of the concentration of optically active [O] were determined [13].