# Germanium Single Crystals for Photonics
## 1 Introduction
At present, for example, in medicine and metal science, various devices using X-ray radiation are widely used. High-quality single crystals with relatively high reflectivity are used as X-ray monochromators. They are oriented in such a way that the surface is parallel to the diffraction planes. The reflected monochromatic radiation obtained in this way is always polarized to a certain degree. The degree of polarization of the radiation depends on the perfection of the monochromator single crystal. Pure Ge is used for these purposes due to its high refractive index, as well as the achieved high technological level of perfection of the crystal structure [18]. Impurity Ge crystals are actively used to manufacture highly sensitive photodetectors for recording IR radiation [2]. Their operation is based on the excitation by a quantum of radiation of the carrier, located at the impurity level and passing into the conduction band. For different areas of the IR range, a corresponding doping additive is used. As a rule, for the operation of such photodetectors, it is necessary to use forced cooling, usually to the temperature of liquid nitrogen, and in some cases, liquid helium. The range of such photodetectors is quite extensive, and a discussion of their properties is reflected in specialized literature [30; 31; 32; 33; 34; 35]. It is important that a structured antireflective surface can be created for germanium-based photodetectors [36]. Nevertheless, we considered it possible to report on the properties of germanium photodetectors for recording radiation from powerful pulsed CO\({}_{2}\) lasers based on the photon hole drag effect [37; 38; 39; 40], since their properties turned out to be close to the subject of this publication. Unlike most traditional optical materials, Ge has specific features that must be taken into account during operation. First, it is highly sensitive to the impurity content in the crystal, which requires a careful approach to selecting the crystal parameters for a specific application. It is also necessary to take into account the lack of transparency in the visible region, the large value of the refractive index n = 4.0, and the presence of an exponential temperature dependence of the absorption coefficient. Ge single crystals for optical applications are usually grown from a melt using the Czochralski, Stepanov, directional crystallization, and other methods [1; 8; 18; 41]. Sometimes, when absorption and scattering losses in the optical system are insignificant, cheaper polycrystalline Ge is used, which has somewhat higher losses. For the production of germanium detectors of ionizing radiation, a technology has been developed for growing high-purity single crystals with a concentration of electroactive impurities of less than \(10^{10}\) cm\({}^{-3}\)[1; 3; 42; 43; 44; 45; 46]. High-purity germanium single crystals (HPGe) are used to create X-ray and gamma radiation detectors, as well as for fundamental research [42; 43]. Isotopically pure \({}^{76}\)Ge single crystals are planned to be used in the experiment <<Large Enriched Germanium by Neutrinoless Double Beta Decay>> (LEGEND) to search for neutrinoless double beta decay [44]. At the same time, background electrically neutral impurities, such as oxygen (the concentration of which can reach \(10^{17}\) cm\({}^{-3}\)), carbon, nitrogen, etc., are always present in germanium. Initially, it was believed that the presence of these impurities does not affect the semiconductor properties of germanium, so little attention was paid to methods of deep purification from these impurities. However, it turned out that the presence of oxygen in germanium single crystals leads to the formation of dislocations, microdefects, and thermal donors, which, in turn, affects the lifetime of nonequilibrium charge carriers and causes losses due to radiation scattering.