# Germanium Single Crystals for Photonics
## 1 Introduction  
Germanium is the first material based on which solid-state electronics products were manufactured and the development of microelectronics began [1, 2]. To do this, it was necessary to develop a technology for purifying raw materials from impurities and growing high-quality single crystals, which led to an active process of a wide variety of studies of its physical and chemical properties. Over several years, thousands of articles and dozens of monographs devoted to this material have been published. The properties of Ge turned out to be extremely sensitive to the content of even minor concentrations of impurities, especially electroactive ones. Therefore, at the first stage of research, one of the main directions was the technology of obtaining the purest material. Later, it became clear that their structural perfection has a significant impact on the operational properties of single crystals. At the next stage, in connection with practical application, as well as economic factors, it became necessary to obtain single crystals of maximum possible size. As a result, at present, there already exists an industrial technology for producing dislocation-free germanium single crystals [3]. The content of electroactive impurities has been reduced to a level of \(10^{-10}\,\mathrm{cm}^{-3}\) and even less [3]. The diameter of the grown single crystals of quite decent quality has been brought to the value of 500 mm [4]. Figure 1 shows a photograph of a block of antireflected output windows made of Ge single crystals with a diameter of 420 mm of the experimental 4-beam laser complex [5, 6]. Large-sized germanium single crystals for this task were obtained by the method of directional crystallization on a seed crystal. The growth direction was <111>; the grown crystals had the shape of adisk with a diameter of 420 mm and a thickness of 65 mm. The method was developed and is being implemented at Tver State University. The method is conservative; the main advantage is the absence of crystal stretching; the presence of a limited amount of melt; a uniform temperature field, providing low thermal stresses in the crystal. Initially, the main consumer of germanium was semiconductor electronics [1; 2]. A significant number of different microelectronics components were created on the basis of germanium. Industrial production of dozens of brands of single-crystal germanium has been established, differing from each other in the content of alloying additives, structural perfection, which fundamentally changing the operational properties of the material [3; 4; 7]. When such industrial production was established, it turned out that n-type germanium is one of the best optical materials for the infrared (IR) region of the spectrum [8]. It is transparent in the range of 1.8-23 \(\upmu\)m, and in the range of 2-11 \(\upmu\)m, absorption losses were reduced to a level of \(\sim\)0.015-0.02 cm\({}^{-1}\)[8; 9; 10; 11]. However, in the range of 11-23 \(\upmu\)m, a number of phonon absorption bands were discovered, sharply limiting its use in this range; a transparency window was also revealed in the THz range (\(\sim\)100-300 \(\upmu\)m) [12]. For two atmospheric transparency windows (ranges 3-5 \(\upmu\)m and 8-14 \(\upmu\)m), active development of various optical devices began in the post-war years, the most widespread of which were thermal imaging devices [1]. However, the development of IR technology was largely constrained by the shortage of transparent optical materials, in connection with which crystalline germanium (mono- and polycrystals) began to be widely used in these devices. Figure 1: Block of output windows made of Ge single crystals with a diameter of 420 mm of the experimental 4-beam laser complex. Reprinted with permission from Ref. [6]. 2013, Center for Scientific Periodicals NUST MISIS. In the visible region, there is a significant number of different optical materials, mainly based on different brands of glass, and there are proven technologies for their production, processing and operation [13].