# Germanium Single Crystals for Photonics
## 3 On the Use of Germanium in Space  
Interest in Ge has increased in connection with the development of global satellite networks and other telecommunication projects [18]. For on-board power supply of artificial Earth satellites (AES), which form the basis of such projects, radiation-resistantphotoelectric converters (PEC) with high efficiency are required. They were developed based on epitaxial structures of AIII-BV GaIn (GaInAs) Ge on substrates of Ge single crystals. Such converters are significantly more expensive than the commonly used silicon ones, but in this case, their use is justified due to a much higher efficiency (more than 39%). The necessary requirements for Ge are a low dislocation density (at a level of no more than 200-250 cm\({}^{-2}\)) and the absence of dislocation defects, such as low-angle boundaries with a crystal diameter of \(\geq\)100 mm [63; 64].  
Ge-based optics are widely used in space, since this material has high radiation resistance [68]. Ge samples were irradiated with \({}^{60}\)Co \(\gamma\)-rays at 3800 rad/s (dose 10\({}^{8}\) rad), fast electrons with an energy of 1 MeV, in a nuclear reactor with a flow of slow and fast (up to 30%) neutrons at T = 200 \({}^{\circ}\)C. The IR transmission changed only after processing in the reactor. Thermal neutron irradiation of Ge leads to the formation of Ga and As atoms, which are electroactive impurities and the excess charge carriers thus formed increase the absorption coefficient. When irradiated at 200 \({}^{\circ}\)C, a significant proportion of radiation defects (point defects caused by fast neutrons and electrons, as well as gamma rays) are annealed. These are now well-proven results (using electrical methods, optical methods, EPR, etc.) [69; 70; 71; 72].  
With the development of laser technology, the range of necessary photoelectric converters (PECs) has changed significantly [73]. Such PECs are used for remote power supply of electronic devices and instruments, recharging of unmanned aerial vehicles, and wireless transmission of energy to an autonomous or remote object using laser radiation of different wavelengths \(\lambda\). There are also medical applications--for recharging implantable prostheses, sensors, and stimulators. The main task in designing and manufacturing such PECs is the conversion of laser radiation into electrical energy with minimal losses.  
The possibility of using germanium-based PECs to convert laser radiation with wavelengths of 850, 980, 1310, and 1550 nm is being considered [74; 75]. The wavelength of 1550 nm is of significant interest, as it falls into the transparency window of the atmosphere and optical fiber and is relatively safe for vision. This makes it possible to create wireless systems for receiving and transmitting energy in the atmosphere and systems based on fiber-optic communication lines.  
The efficiency of conversion of a monochromatic laser radiation (\(\lambda\) = 1550 nm) of germanium-based PECs significantly exceeds the coefficient of efficiency of similar solar cells. The theoretical maximum value of the monochromatic coefficient of efficiency for a given wavelength is \(\sim\)45% [76]. Unfortunately, the experimental results achieved so far are significantly inferior to the theoretical ones.  
The paper [76] examines the possibility of realizing high values of coefficient of efficiency with a relatively simple structure of a germanium-based converter and a reproducible manufacturing technology. Formation of a high-quality p-n junction and reproducibility of the technology were ensured by diffusion doping with zinc from the gas phase. With uniform irradiation of small-sized germanium-based PEC, a monochromatic (\(\lambda\) = 1550 nm) with coefficient of efficiency of 20% was experimentally obtained.