rus
Issue #2/2023
A. V. Naumov, V. V. Startsev
Germanium as a Photonics Substance: from Lenses to Dislocation-Free Wafers
Germanium as a Photonics Substance: from Lenses to Dislocation-Free Wafers
DOI: 10.22184/1993-7296.FRos.2023.17.2.114.132
The article considers the process of development of germanium single crystal growth technology by the Czochralsky method, which allowed the application of germanium properties in IR optics and in gamma radiation detection. It is expected that germanium may return to optoelectronics again: recent developments in the cultivation of dislocation-free crystals have shown that germanium is a promising material for next-generation nanoscale electronic devices and for the integration of optical functions on logic circuits.
The article considers the process of development of germanium single crystal growth technology by the Czochralsky method, which allowed the application of germanium properties in IR optics and in gamma radiation detection. It is expected that germanium may return to optoelectronics again: recent developments in the cultivation of dislocation-free crystals have shown that germanium is a promising material for next-generation nanoscale electronic devices and for the integration of optical functions on logic circuits.
Теги: germanium germanium single crystal growth technologies ir-optics photonic integrated circuits германий ик-оптика технологии роста монокристаллов германия фотонные интегральные схемы
Germanium as a Photonics Substance: from Lenses to Dislocation-Free Wafers
A. V. Naumov, V. V. Startsev
JSC “Design Bureau “Astrohn”, Lytkarino, Moscow Region, Russia
The article considers the process of development of germanium single crystal growth technology by the Czochralsky method, which allowed the application of germanium properties in IR optics and in gamma radiation detection. It is expected that germanium may return to optoelectronics again: recent developments in the cultivation of dislocation-free crystals have shown that germanium is a promising material for next-generation nanoscale electronic devices and for the integration of optical functions on logic circuits.
Key words: germanium, germanium single crystal growth technologies, photonic integrated circuits, IR-optics
The article received: November 18, 2022
The article accepted: December 03, 2022
Once a pioneer in the history of electronics, germanium is again of great interest as a material for many products, including for the field of optoelectronic and electronic applications. D. I. Mendeleev’s prediction of the properties of ecasilicon 15 years before its discovery in 1886 by the German chemist Clemens Alexander Winkler (who named this element germanium) was one of the legendary events in chemistry of the XIX century. Winkler obtained germanium from argyrodite (4Ag2S-GeS2), a rare mineral containing 5–7 wt. % germany. The content of germanium in the earth’s crust is 7.10–4 % of its mass, which is more than the content of, for example, silver, but germanium is a very dispersed element. Ge is present in zinc, lead, copper-zinc ores and coal. Such a large dispersion of Ge is explained by the fact that it can behave as a chalcophilic, lithophilic or siderophilic element.
From sulfide zinc or lead ores, as well as low-energy coals, where germanium is contained in the range from thousandths to tenths of a percent, the following are successively obtained: germanium concentrate (with a germanium content of 5 to 30%), germanium tetrachloride (GeCl4), germanium oxide (GeO2), poly- and single crystals of germanium. Germanium tetrachloride GeCl4 is used as a component for glass production in fiber-optic communication lines (FOCLs), GeO2 is used as an integral part of catalysts for polymerization of PET plastics (Poly Ethylene Terephalate). Night vision devices in the IR range use poly- and monocrystalline germanium windows and lenses. Germanium monocrystalline wafers are used for electronic devices and solar cells. Especially pure germanium is used for nuclear radiation detectors.
Thus, there are several different Ge markets sequentially located along the technological chain of production: germanium dioxide of different purity, germanium tetrachloride, zone-refined polycrystalline ingots, single crystals, optical blanks, sensors and wafers for microelectronics (Fig.1). These markets live largely independent lives, experiencing their own ups and downs.
Present-day global production
and prices
The global production of germanium (outside of Russia and China) is based on the associated extraction of germanium from zinc sulfide, lead-zinc and, more rarely, copper-zinc ores. When processing sulfide-zinc ores containing from 0.01 to 0.015% Ge, the ores are fired. Germanium, cadmium, lead evaporate, vapor condenses and collects in an electrostatic precipitator filter. Germanium concentrate is fired and dissolved in hydrochloric acid. Germanium tetrachloride is distilled from the solution, which is sent for further refining.
Obtaining Ge by burning coal is a method adopted for extracting germanium in Russia and China. Practically all germanium (from 70% to 95%, depending on the combustion modes) contained in combustion goal, condenses on fly ash in the form of GeO2, germanates and silicogermanates. The main amount of germanium settles on ash particles less than 10–20 microns in size. The capture is carried out in bag filters or electrofilters.
In 2021, the total production of germanium and its compounds is approximately estimated by the USGS at 140 tons in terms of germanium, plus about 30% recovered by recycling. Figure 2 shows the dynamics of primary germanium output for 1998–2021 (according to USGS data) and prices (according to metal-pages data).
The production costs for the extraction and refinement of germanium are very high with any extraction technology and its price is traditionally one of the highest for dispersed metals, and remains so even in times of crisis. Currently, the price of polycrystalline zone-refined germanium ranges from 1200–1300 $/kg.
Application areas of germanium
and prospects thereof (Table 1)
The first industrial use of germanium compounds apparently occurred in 1923, when American researchers L. M. Dennis and A. W. Laubengayer added GeO2 to the glass instead of SiO2, which changed the dispersion and refractive index of the glass. To this day, glasses with a high germanium content are used for the manufacture of pass-through optics of IR equipment.
Catalysts
Germanium dioxide is used as a catalyst component at the polycondensation stage in the manufacture of synthetic fibers and PET (polyethylene phtholate) resins, which, in turn, are used for food packaging. The share of germanium for the production of catalysts tends to decrease.
Fiber optics
The core of optical fibers consists of (SiO2+GeO2), which ensures complete internal reflection of the signal at the core-shell interface and low energy losses during transmission (Fig.3). The FOCL is the main consumer of high-purity germanium tetrachloride. Now the sector is growing by 8–10% per year and further growth is expected.
IR equipment
Germanium is traditionally a material for the manufacture of lenses and windows of infrared optical systems for military purposes, designed to detect objects by their own radiation in the range of 2–16 microns (Fig.4). Systems designed to detect objects by their own radiation, both military and civilian, operate in this range.
The main requirements for single crystal germanium as an optical material are maximum transparency, high optical uniformity and minimum number of defects. In the operating wavelength range from 2.5 to 11 microns at room temperature, the absorption coefficient should be < 0.02 cm‑1 [1,2]. In Ge, absorption on free charge carriers prevails, the peculiarity of which is that the absorption cross section of photons by holes is practically an order of magnitude greater than the value of this parameter for electrons [3,4]. In this regard, in order to exclude the influence of holes generated by acceptor-type background impurities, in addition to deep purification, germanium doping with donor additives with concentrations between 4 × 1013 and 1 × 1015 cm‑3 is used. For monocrystalline Ge, this results in an absorption coefficient of less than 0.02 cm‑1 at room temperature. So germanium doped with antimony, with a specific electrical resistance from 3 to 40 ohms · cm, with a proper degree of purification, in the absence of small–angle boundaries, and with a dislocation content of less than 104 cm‑2 has a transparency of about 46.0%, and the absorption coefficient near the edge of the absorption band at a wavelength of 10.6 microns is 0.015–0.035 cm‑1. Optical quality germanium should also be optically homogeneous and isotropic, i. e. the uniformity of the refractive index should be very high (usually Δn < 10–4), and double refraction (≤1 mm/cm) should be minimized. [14]. In addition, to ensure low double refraction, residual stresses in the crystal should be minimized [15]. Residual stresses are the result of plastic deformation, which removes thermal stresses exceeding the critical shear stress when the crystal is cooled above the crystal/melt interface [16]. It is generally assumed that in order to minimize these thermal stresses, this interface (crystallization front) should be as flat as possible during growth, which corresponds to a vanishingly small radial temperature gradient in the melt. These conditions are met by selecting appropriate heat shields and/or heaters above the melt surface.
Among the new applications of germanium for IR systems, it should be noted safety systems in conditions of poor visibility for cars and anti-epidemic thermal imagers for monitoring the temperature of people in the stream. By 2030, the global market of germanium lenses is expected to grow almost 3 times. (Fig. 5).
Semiconductor gamma radiation detectors
The principle of operation of semiconductor detectors is based on the formation of electron-hole pairs in the crystal volume during the passage of radiation through the material. Under the influence of the applied voltage, the charges move to the electrodes and are recorded as an electrical signal, the value of which is determined by the absorbed radiation energy in the thickness of the material, and accordingly, in the case of complete energy absorption, the device works as a radiation spectrometer.
Semiconductor ionizing radiation detectors are made of various materials: germanium (Ge), silicon (Si), etc. Germanium detectors have the highest resolution. Gamma-ray spectrometers based on high-purity germanium (HPG) are an indispensable tool in many nuclear physics research and applications. They provide a record energy resolution compared to all scintillators, ionization chambers and other semiconductor detectors (in particular, lanthanum bromide, the best of the existing scintillators in energy resolution, is 15–20 times inferior to HPG, depending on the size of the crystal and the spectral region). Therefore, HPG has practically no alternatives in the following tasks: identification of radionuclides in a mixture, detection of small concentrations of radioactive substances, quantitative analysis of the isotopic composition of complex samples. Currently, HPG spectrometers are used in such fields as: radioecology and radiation monitoring (water, air, soil, food, etc.), nuclear power engineering (monitoring of the radiation situation at nuclear power plants, radioactive waste and spent nuclear fuel management, etc.), scientific research (in the field of nuclear physics, gamma-astronomy, planetary science, elementary particle physics). There are promising new applications of germanium spectrometers using neutrons – a baggage scanning system based on the method of labeled neutrons (detection of explosives and contraband).
Germanium crystals for gamma radiation detection must have an electrically active concentration of impurities up to 109–2×1010 cm‑3 (depending on the final size of the detector). Therefore, a quartz crucible of high purity is used, the growing crystal is blown with high purity hydrogen. Completely dislocated high-purity germanium grown in a hydrogen medium is unsuitable for the manufacture of detectors due to the deep level of EV + 0.072 eV with a concentration of about 1011 cm‑3 [17,18]. This center has been identified as a divalent hydrogen complex (V2H) [19]. Experience has shown that if a crystal contains at least 100 uniformly distributed dislocations per cm‑2, then the concentration of V2H is too small to degrade the characteristics of the detector. However, if the local dislocation densities exceed 104 cm‑2, then the dislocations themselves act as charge capture centers [20]. Meeting the requirement that the dislocation density everywhere ranges from 100 to 104 cm‑2 in volumes that can exceed 500 cm3 is a big problem with crystal growth, because in a sense, growing a crystal with a controlled dislocation density is a more complex applied task than growing a dislocation-free crystal.
The history of the emergence and development of electronic germanium (Table 2)
The birth of semiconductor electronics is usually counted from the time of the discovery of transistors: 1947 – the first point-contact transistors, 1949 – junction transistors. In the process of developing crystal detectors during World War II, the US National Defense Research Council in 1942 required a material or materials with semiconductor properties that could be manufactured in significant quantities, while being of a high degree of purity and easily processed. In the fact that germanium was chosen an important role was played by the work of Carl Lark-Horovitz from Perdue University USA. He was choosing between the semiconductors known by that time – silicon, germanium and lead sulfide (PbS), and was able to predict that despite the rarity and difficulty of obtaining germanium, it was this element, according to the totality of the properties studied by that time, and the achieved purity level which was the first candidate for this role. From this moment, the industrial production and application of germanium begins.
After the first point transistor was assembled in 1947 by physicists Wallter Brattain and John Bardin on polycrystalline equipment, the enormous potential of solid-state microelectronics became obvious. Since the mid‑1940s, the development of solid-state microelectronics in the Soviet Union began. In terms of scale, costs and results, this project was comparable to the creation of a rocket and space complex. The beginning of work on semiconductor materials at the State Institute of Rare Metals (Giredmet, Moscow) falls on 1947, when the task was set to provide high-purity germanium for solid-state electronics that was in the beginning of its development. Original technologies have been developed for the extraction of germanium from coking and power coal processing products, as well as mudstones and iron ores. Eventually, the industry began to use the method of obtaining germanium from coal. For the first time in the USSR, the production of germanium was established on an industrial scale. These works made it possible to meet the needs of the country in domestic production and were the basis for obtaining the initial product for growing single crystals by the Czochralsky method.
Growth of single crystals by the Czochralsky method
Since 1947, the race to improve the technology of obtaining germanium crystals for the manufacture of transistors began. Bell Labs employees Gordon Teal and John B. Little used the Czochralsky method of extraction of crystals from the melt, already known by that time for other materials, and in 1948 they grew the first single crystals of germanium (Fig. 6).
Germanium single crystals are grown from the melt today mainly by the Czochralsky method, which refers to the methods of directed crystallization and consists in pulling the seed from the melt together with the single crystal growing on it. The germanium melt is located in a quartz or graphite crucible. The resistive heater and the quartz crucible stand are made of graphite, and the heat shields are made of graphite–based materials. [2] One of the advantages of the method is the possibility of obtaining dislocation-free single crystals with the correct orientation, ordered crystal structure, certain optical and electrical parameters, and high purity of the single crystal. Laboratory studies of the process of growing single crystals of germanium by the Czochralsky method began in the USSR in the early 50s and were carried out simultaneously in several places – Ioffe Physico-Technical Institute, Lebedev Institute of Physics under the Academy of Sciences (FIAN), Baykov Institute of Metallurgy (IMET), Siberian Institute of Physics and Engineering (SFTI), etc. In 1950, laboratory samples of germanium triodes were developed at FIAN (B. M. Vul, A. V. Rzhanov, V. S. Vavilov et al.), at Leningrad Physico-Technical Instiute (LFTI) (V. M. Tuchkevich, D. N. Nasledov) and at the Institute of Radio Technologies and Electornics (IRE) of the USSR Academy of Sciences (S. G. Kalashnikov, N. A. Penin, etc.). The first industrial Ge single crystals from domestic raw materials and on domestic equipment were obtained at Giredmet in 1956.
Modern growth plants (Fig.7) designed for growing germanium crystals from a melt are equipped with control systems, where the main parameters are the temperature of the heater and the melt, the level of the melt in the crucible, as well as the diameter of the crystal being grown. Temperature measurement is carried out, as a rule, by pyrometers; laser triangulation sensors or weighing systems are used to determine the melt level; and to control the diameter of the crystal being grown the television or digital video systems with digital data processing facilities are used.
The growing process is automated from the moment of single crystal seeding until the end of the process. Today germanium crystals are obtained in the orientation [111] and [100]:
for optical blanks – with a diameter of 20–300 mm, depending on the requirements of the optical system,
for electronic applications – with a diameter of up to 300 mm, but crystals with a diameter of 100–150 mm are practically widely used.
Today in Russia there are the following manufacturers of Germanium products and Germanium single crystals: Germanium JSC (part of the Shvabe Concern)- the largest producer of germanium products since Soviet times, the capacity along the entire technological chain for processing is up to 30 tons/year, starting from concentrate and Ge-containing waste. In recent years, it has been processing 15–18 tons of raw materials, including “customer” raw materials. This corresponds to the company’s 80–90% share in the Russian market. The main products produced by Germanium JSC are: polycrystalline germanium (granules, ingots of GPZ (germanium polychrystallic zone-refined) 6N), monocrystalline germanium in orientation [111] and [100], mono-Ge plates, blanks for infrared optics made of poly- and monocrystalline germanium (diameter up to 300 mm), germanium dioxide (including electronic and catalyst quality), GeCl4(4N, 6N, OB‑6N).
LLC “Germanium and Applications” (a private enterprise founded in 2006) collects fly ash with a high Ge content from the places where the coals of the Pavlovsky opencast are burned. This provides the company with raw materials for Ge production in the amount of up to 10 tons/year in the form of single crystals, optical blanks, etc. The main products are: monocrystalline germanium in orientation [111], mono-Ge plates [111], [211], [110], blanks for infrared optics made of poly- and monocrystalline germanium (diameter up to 300 mm), germanium dioxide (grades DG-T, DG-B, DG-C), GeCl4 (grades B and C).
JSC “Design Bureau “Astron” has been producing monocrystalline germanium in orientation [111] according to TU 48–4–293–82 for its own needs since 2017, manufacturing optical lenses for thermal imagers and windows for microbolometers of its own production.
Features of obtaining
low-dislocation crystals
for electronic applications
Gallium arsenide and germanium have only a slight discrepancy in the lattice parameters (Table. 3), therefore germanium meets one of the main criteria that should be considered as an alternative wafer for the growth of compounds III–V. The problems of the germanium wafer – the mismatch of the lattice parameters, the different structure of the lattices – are solved by creating a buffer layer at the working layer-wafer interface. In addition, Ge wafers have certain advantages over GaAs substrates: higher crystallographic perfection, high mechanical strength, greater simplicity of germanium recycling processes for reuse. These factors have led to the widespread use of Ge plates as a wafer for GaAs/Ge solar cells for space telecommunications satellites [21], and also make Ge a viable competitor for other GaAs devices. The possibility of using Ge wafers instead of GaAs for the manufacture of transistors with high electron mobility (HEMT) [22,23], LEDs [23] and VSCEL laser diodes is shown. The current standard diameter for Ge wafers for the growth of III–V epitaxial structures for such applications is 100 mm.
The absence of dislocations makes it possible to grow high-quality GaAs epitaxial layers on a Ge wafer. In addition, dislocations reduce the service life of the p-n junction.
The formation of dislocations increases the Gibbs free energy in the crystal and, therefore, is thermodynamically unfavorable. Therefore, it is physically possible to grow dislocation-free crystals. When the seed crystal is immersed in the melt, dislocations occur due to high thermal stresses caused by a temperature shock. These dislocations can spread into a growing crystal, especially in the case of a large crystal diameter. Thermoelastic stresses in a growing crystal are the main cause of dislocations, especially in the case of large crystals. Due to the high stresses and temperatures in the crystal, these dislocations are not limited to their own glide planes but receive enough energy to propagate to neighboring glide planes through the processes of crawling. Since in crystals of the diamond structure (111)-planes are the main glide planes that are inclined to the crystal axis in the <100> or <111> direction, dislocations will slide and end on the crystal surface, provided that the crystal diameter is reduced to a very small size, so that a small residual stress may not be able to move dislocations or create new ones, and provided that no new dislocations are formed at the interface. This is a well-known Dash necking method [26], who was the first to report the growth of dislocation-free Si and Ge ingots. The Dash necking method is widely used to obtain dislocation-free Si and Ge crystals. Dislocation-free growth is relatively stable even for large-diameter crystals, despite high thermal stresses. The reason for this is the relatively high energy required to create the first dislocation in the crystal. If the shear stresses along the main slide planes at no point exceed the critical value for the nucleation of dislocations or for the growth of very small dislocation loops, the crystal will remain free from macroscopic dislocations. However, compared to the growth of silicon crystals, the growth of dislocation-free Ge ingots of large diameter is a much more difficult task, since the thermal conductivity of Ge is lower, so large thermoelastic stresses are generated in a growing crystal, and the critical shear stress is lower, so dislocations are easier to form and multiply. Also, the problem is more difficult for Ge than for Si, since both the density of Ge is higher, and the tensile strength is lower than for Si (Table 4). Optimizing the parameters of crystal growth, especially the process of forming the constriction and the upper cone to the full diameter of the crystal, the design of the hot zone and maintaining the thermal stability of the crystal growth system, it was possible to grow dislocation-free germanium crystals with a diameter of up to 300 mm. Table 5 shows the characteristics of the obtained plates:
Solar panels based on multistage high-efficiency solar cells using InGaP/InGaAs/Ge are used for onboard power supplies of telecommunications space satellites (Fig.9). The development is carried out by Sharp (Japan), Emcore Photovoltaics (USA), Azur space (Germany), Cesi (Italy), Spectrolab (USA), NPP Kvant JSC, Saturn JSC (Russia). Such solar panels will ensure the achievement of the active life of spacecraft for 15 years or more with an increase in the energy capacity of spacecraft by more than 2 times.
Currently, Umicore (Belgium) and AXT Inc. (USA) are the largest manufacturers of wafers. The main type of wafers produced are wafers with a diameter of 100 mm and 150 mm. Umicore offered the first 150 mm plate for VCSEL in 2020 and became the first to release a 200 mm plate for VCSEL in 2021 (Fig.10)
Conclusion
Along with long-established applications as single crystals in infrared optics and gamma ray detection, dislocation-free germanium has proved to be a promising substrate material for GaAs-based optoelectronic devices. The development of dislocation-free wafers with a diameter of up to 300 mm demonstrates the potential compatibility of germanium with modern technology for manufacturing silicon devices. It can be expected that the introduction of germanium for the next generations of CMOS transistors will accelerate. In addition, germanium, subject to compatibility with Si and AIII-BV, becomes a promising material for electron-photonic integrated circuits. This may mean that the demand in germanium will grow (Fig.11).
ABOUT AUTHORS
Naumov Arkady V., Head of the Research Area, Joint Stock Company “Optical and Mechanical Design Bureau Astrohn”, Lytkarino, Moscow region, Russia.
ORCID: 0000–0001–6081–8304
Startsev Vadim V., Candidate of Technical Sciences, Chief Designer, ASTROHN Technology Ltd, https://astrohn.ru, Lytkarino, Moscow region, Russia.
ORCID ID: 0000–0002–2800–544X
A. V. Naumov, V. V. Startsev
JSC “Design Bureau “Astrohn”, Lytkarino, Moscow Region, Russia
The article considers the process of development of germanium single crystal growth technology by the Czochralsky method, which allowed the application of germanium properties in IR optics and in gamma radiation detection. It is expected that germanium may return to optoelectronics again: recent developments in the cultivation of dislocation-free crystals have shown that germanium is a promising material for next-generation nanoscale electronic devices and for the integration of optical functions on logic circuits.
Key words: germanium, germanium single crystal growth technologies, photonic integrated circuits, IR-optics
The article received: November 18, 2022
The article accepted: December 03, 2022
Once a pioneer in the history of electronics, germanium is again of great interest as a material for many products, including for the field of optoelectronic and electronic applications. D. I. Mendeleev’s prediction of the properties of ecasilicon 15 years before its discovery in 1886 by the German chemist Clemens Alexander Winkler (who named this element germanium) was one of the legendary events in chemistry of the XIX century. Winkler obtained germanium from argyrodite (4Ag2S-GeS2), a rare mineral containing 5–7 wt. % germany. The content of germanium in the earth’s crust is 7.10–4 % of its mass, which is more than the content of, for example, silver, but germanium is a very dispersed element. Ge is present in zinc, lead, copper-zinc ores and coal. Such a large dispersion of Ge is explained by the fact that it can behave as a chalcophilic, lithophilic or siderophilic element.
From sulfide zinc or lead ores, as well as low-energy coals, where germanium is contained in the range from thousandths to tenths of a percent, the following are successively obtained: germanium concentrate (with a germanium content of 5 to 30%), germanium tetrachloride (GeCl4), germanium oxide (GeO2), poly- and single crystals of germanium. Germanium tetrachloride GeCl4 is used as a component for glass production in fiber-optic communication lines (FOCLs), GeO2 is used as an integral part of catalysts for polymerization of PET plastics (Poly Ethylene Terephalate). Night vision devices in the IR range use poly- and monocrystalline germanium windows and lenses. Germanium monocrystalline wafers are used for electronic devices and solar cells. Especially pure germanium is used for nuclear radiation detectors.
Thus, there are several different Ge markets sequentially located along the technological chain of production: germanium dioxide of different purity, germanium tetrachloride, zone-refined polycrystalline ingots, single crystals, optical blanks, sensors and wafers for microelectronics (Fig.1). These markets live largely independent lives, experiencing their own ups and downs.
Present-day global production
and prices
The global production of germanium (outside of Russia and China) is based on the associated extraction of germanium from zinc sulfide, lead-zinc and, more rarely, copper-zinc ores. When processing sulfide-zinc ores containing from 0.01 to 0.015% Ge, the ores are fired. Germanium, cadmium, lead evaporate, vapor condenses and collects in an electrostatic precipitator filter. Germanium concentrate is fired and dissolved in hydrochloric acid. Germanium tetrachloride is distilled from the solution, which is sent for further refining.
Obtaining Ge by burning coal is a method adopted for extracting germanium in Russia and China. Practically all germanium (from 70% to 95%, depending on the combustion modes) contained in combustion goal, condenses on fly ash in the form of GeO2, germanates and silicogermanates. The main amount of germanium settles on ash particles less than 10–20 microns in size. The capture is carried out in bag filters or electrofilters.
In 2021, the total production of germanium and its compounds is approximately estimated by the USGS at 140 tons in terms of germanium, plus about 30% recovered by recycling. Figure 2 shows the dynamics of primary germanium output for 1998–2021 (according to USGS data) and prices (according to metal-pages data).
The production costs for the extraction and refinement of germanium are very high with any extraction technology and its price is traditionally one of the highest for dispersed metals, and remains so even in times of crisis. Currently, the price of polycrystalline zone-refined germanium ranges from 1200–1300 $/kg.
Application areas of germanium
and prospects thereof (Table 1)
The first industrial use of germanium compounds apparently occurred in 1923, when American researchers L. M. Dennis and A. W. Laubengayer added GeO2 to the glass instead of SiO2, which changed the dispersion and refractive index of the glass. To this day, glasses with a high germanium content are used for the manufacture of pass-through optics of IR equipment.
Catalysts
Germanium dioxide is used as a catalyst component at the polycondensation stage in the manufacture of synthetic fibers and PET (polyethylene phtholate) resins, which, in turn, are used for food packaging. The share of germanium for the production of catalysts tends to decrease.
Fiber optics
The core of optical fibers consists of (SiO2+GeO2), which ensures complete internal reflection of the signal at the core-shell interface and low energy losses during transmission (Fig.3). The FOCL is the main consumer of high-purity germanium tetrachloride. Now the sector is growing by 8–10% per year and further growth is expected.
IR equipment
Germanium is traditionally a material for the manufacture of lenses and windows of infrared optical systems for military purposes, designed to detect objects by their own radiation in the range of 2–16 microns (Fig.4). Systems designed to detect objects by their own radiation, both military and civilian, operate in this range.
The main requirements for single crystal germanium as an optical material are maximum transparency, high optical uniformity and minimum number of defects. In the operating wavelength range from 2.5 to 11 microns at room temperature, the absorption coefficient should be < 0.02 cm‑1 [1,2]. In Ge, absorption on free charge carriers prevails, the peculiarity of which is that the absorption cross section of photons by holes is practically an order of magnitude greater than the value of this parameter for electrons [3,4]. In this regard, in order to exclude the influence of holes generated by acceptor-type background impurities, in addition to deep purification, germanium doping with donor additives with concentrations between 4 × 1013 and 1 × 1015 cm‑3 is used. For monocrystalline Ge, this results in an absorption coefficient of less than 0.02 cm‑1 at room temperature. So germanium doped with antimony, with a specific electrical resistance from 3 to 40 ohms · cm, with a proper degree of purification, in the absence of small–angle boundaries, and with a dislocation content of less than 104 cm‑2 has a transparency of about 46.0%, and the absorption coefficient near the edge of the absorption band at a wavelength of 10.6 microns is 0.015–0.035 cm‑1. Optical quality germanium should also be optically homogeneous and isotropic, i. e. the uniformity of the refractive index should be very high (usually Δn < 10–4), and double refraction (≤1 mm/cm) should be minimized. [14]. In addition, to ensure low double refraction, residual stresses in the crystal should be minimized [15]. Residual stresses are the result of plastic deformation, which removes thermal stresses exceeding the critical shear stress when the crystal is cooled above the crystal/melt interface [16]. It is generally assumed that in order to minimize these thermal stresses, this interface (crystallization front) should be as flat as possible during growth, which corresponds to a vanishingly small radial temperature gradient in the melt. These conditions are met by selecting appropriate heat shields and/or heaters above the melt surface.
Among the new applications of germanium for IR systems, it should be noted safety systems in conditions of poor visibility for cars and anti-epidemic thermal imagers for monitoring the temperature of people in the stream. By 2030, the global market of germanium lenses is expected to grow almost 3 times. (Fig. 5).
Semiconductor gamma radiation detectors
The principle of operation of semiconductor detectors is based on the formation of electron-hole pairs in the crystal volume during the passage of radiation through the material. Under the influence of the applied voltage, the charges move to the electrodes and are recorded as an electrical signal, the value of which is determined by the absorbed radiation energy in the thickness of the material, and accordingly, in the case of complete energy absorption, the device works as a radiation spectrometer.
Semiconductor ionizing radiation detectors are made of various materials: germanium (Ge), silicon (Si), etc. Germanium detectors have the highest resolution. Gamma-ray spectrometers based on high-purity germanium (HPG) are an indispensable tool in many nuclear physics research and applications. They provide a record energy resolution compared to all scintillators, ionization chambers and other semiconductor detectors (in particular, lanthanum bromide, the best of the existing scintillators in energy resolution, is 15–20 times inferior to HPG, depending on the size of the crystal and the spectral region). Therefore, HPG has practically no alternatives in the following tasks: identification of radionuclides in a mixture, detection of small concentrations of radioactive substances, quantitative analysis of the isotopic composition of complex samples. Currently, HPG spectrometers are used in such fields as: radioecology and radiation monitoring (water, air, soil, food, etc.), nuclear power engineering (monitoring of the radiation situation at nuclear power plants, radioactive waste and spent nuclear fuel management, etc.), scientific research (in the field of nuclear physics, gamma-astronomy, planetary science, elementary particle physics). There are promising new applications of germanium spectrometers using neutrons – a baggage scanning system based on the method of labeled neutrons (detection of explosives and contraband).
Germanium crystals for gamma radiation detection must have an electrically active concentration of impurities up to 109–2×1010 cm‑3 (depending on the final size of the detector). Therefore, a quartz crucible of high purity is used, the growing crystal is blown with high purity hydrogen. Completely dislocated high-purity germanium grown in a hydrogen medium is unsuitable for the manufacture of detectors due to the deep level of EV + 0.072 eV with a concentration of about 1011 cm‑3 [17,18]. This center has been identified as a divalent hydrogen complex (V2H) [19]. Experience has shown that if a crystal contains at least 100 uniformly distributed dislocations per cm‑2, then the concentration of V2H is too small to degrade the characteristics of the detector. However, if the local dislocation densities exceed 104 cm‑2, then the dislocations themselves act as charge capture centers [20]. Meeting the requirement that the dislocation density everywhere ranges from 100 to 104 cm‑2 in volumes that can exceed 500 cm3 is a big problem with crystal growth, because in a sense, growing a crystal with a controlled dislocation density is a more complex applied task than growing a dislocation-free crystal.
The history of the emergence and development of electronic germanium (Table 2)
The birth of semiconductor electronics is usually counted from the time of the discovery of transistors: 1947 – the first point-contact transistors, 1949 – junction transistors. In the process of developing crystal detectors during World War II, the US National Defense Research Council in 1942 required a material or materials with semiconductor properties that could be manufactured in significant quantities, while being of a high degree of purity and easily processed. In the fact that germanium was chosen an important role was played by the work of Carl Lark-Horovitz from Perdue University USA. He was choosing between the semiconductors known by that time – silicon, germanium and lead sulfide (PbS), and was able to predict that despite the rarity and difficulty of obtaining germanium, it was this element, according to the totality of the properties studied by that time, and the achieved purity level which was the first candidate for this role. From this moment, the industrial production and application of germanium begins.
After the first point transistor was assembled in 1947 by physicists Wallter Brattain and John Bardin on polycrystalline equipment, the enormous potential of solid-state microelectronics became obvious. Since the mid‑1940s, the development of solid-state microelectronics in the Soviet Union began. In terms of scale, costs and results, this project was comparable to the creation of a rocket and space complex. The beginning of work on semiconductor materials at the State Institute of Rare Metals (Giredmet, Moscow) falls on 1947, when the task was set to provide high-purity germanium for solid-state electronics that was in the beginning of its development. Original technologies have been developed for the extraction of germanium from coking and power coal processing products, as well as mudstones and iron ores. Eventually, the industry began to use the method of obtaining germanium from coal. For the first time in the USSR, the production of germanium was established on an industrial scale. These works made it possible to meet the needs of the country in domestic production and were the basis for obtaining the initial product for growing single crystals by the Czochralsky method.
Growth of single crystals by the Czochralsky method
Since 1947, the race to improve the technology of obtaining germanium crystals for the manufacture of transistors began. Bell Labs employees Gordon Teal and John B. Little used the Czochralsky method of extraction of crystals from the melt, already known by that time for other materials, and in 1948 they grew the first single crystals of germanium (Fig. 6).
Germanium single crystals are grown from the melt today mainly by the Czochralsky method, which refers to the methods of directed crystallization and consists in pulling the seed from the melt together with the single crystal growing on it. The germanium melt is located in a quartz or graphite crucible. The resistive heater and the quartz crucible stand are made of graphite, and the heat shields are made of graphite–based materials. [2] One of the advantages of the method is the possibility of obtaining dislocation-free single crystals with the correct orientation, ordered crystal structure, certain optical and electrical parameters, and high purity of the single crystal. Laboratory studies of the process of growing single crystals of germanium by the Czochralsky method began in the USSR in the early 50s and were carried out simultaneously in several places – Ioffe Physico-Technical Institute, Lebedev Institute of Physics under the Academy of Sciences (FIAN), Baykov Institute of Metallurgy (IMET), Siberian Institute of Physics and Engineering (SFTI), etc. In 1950, laboratory samples of germanium triodes were developed at FIAN (B. M. Vul, A. V. Rzhanov, V. S. Vavilov et al.), at Leningrad Physico-Technical Instiute (LFTI) (V. M. Tuchkevich, D. N. Nasledov) and at the Institute of Radio Technologies and Electornics (IRE) of the USSR Academy of Sciences (S. G. Kalashnikov, N. A. Penin, etc.). The first industrial Ge single crystals from domestic raw materials and on domestic equipment were obtained at Giredmet in 1956.
Modern growth plants (Fig.7) designed for growing germanium crystals from a melt are equipped with control systems, where the main parameters are the temperature of the heater and the melt, the level of the melt in the crucible, as well as the diameter of the crystal being grown. Temperature measurement is carried out, as a rule, by pyrometers; laser triangulation sensors or weighing systems are used to determine the melt level; and to control the diameter of the crystal being grown the television or digital video systems with digital data processing facilities are used.
The growing process is automated from the moment of single crystal seeding until the end of the process. Today germanium crystals are obtained in the orientation [111] and [100]:
for optical blanks – with a diameter of 20–300 mm, depending on the requirements of the optical system,
for electronic applications – with a diameter of up to 300 mm, but crystals with a diameter of 100–150 mm are practically widely used.
Today in Russia there are the following manufacturers of Germanium products and Germanium single crystals: Germanium JSC (part of the Shvabe Concern)- the largest producer of germanium products since Soviet times, the capacity along the entire technological chain for processing is up to 30 tons/year, starting from concentrate and Ge-containing waste. In recent years, it has been processing 15–18 tons of raw materials, including “customer” raw materials. This corresponds to the company’s 80–90% share in the Russian market. The main products produced by Germanium JSC are: polycrystalline germanium (granules, ingots of GPZ (germanium polychrystallic zone-refined) 6N), monocrystalline germanium in orientation [111] and [100], mono-Ge plates, blanks for infrared optics made of poly- and monocrystalline germanium (diameter up to 300 mm), germanium dioxide (including electronic and catalyst quality), GeCl4(4N, 6N, OB‑6N).
LLC “Germanium and Applications” (a private enterprise founded in 2006) collects fly ash with a high Ge content from the places where the coals of the Pavlovsky opencast are burned. This provides the company with raw materials for Ge production in the amount of up to 10 tons/year in the form of single crystals, optical blanks, etc. The main products are: monocrystalline germanium in orientation [111], mono-Ge plates [111], [211], [110], blanks for infrared optics made of poly- and monocrystalline germanium (diameter up to 300 mm), germanium dioxide (grades DG-T, DG-B, DG-C), GeCl4 (grades B and C).
JSC “Design Bureau “Astron” has been producing monocrystalline germanium in orientation [111] according to TU 48–4–293–82 for its own needs since 2017, manufacturing optical lenses for thermal imagers and windows for microbolometers of its own production.
Features of obtaining
low-dislocation crystals
for electronic applications
Gallium arsenide and germanium have only a slight discrepancy in the lattice parameters (Table. 3), therefore germanium meets one of the main criteria that should be considered as an alternative wafer for the growth of compounds III–V. The problems of the germanium wafer – the mismatch of the lattice parameters, the different structure of the lattices – are solved by creating a buffer layer at the working layer-wafer interface. In addition, Ge wafers have certain advantages over GaAs substrates: higher crystallographic perfection, high mechanical strength, greater simplicity of germanium recycling processes for reuse. These factors have led to the widespread use of Ge plates as a wafer for GaAs/Ge solar cells for space telecommunications satellites [21], and also make Ge a viable competitor for other GaAs devices. The possibility of using Ge wafers instead of GaAs for the manufacture of transistors with high electron mobility (HEMT) [22,23], LEDs [23] and VSCEL laser diodes is shown. The current standard diameter for Ge wafers for the growth of III–V epitaxial structures for such applications is 100 mm.
The absence of dislocations makes it possible to grow high-quality GaAs epitaxial layers on a Ge wafer. In addition, dislocations reduce the service life of the p-n junction.
The formation of dislocations increases the Gibbs free energy in the crystal and, therefore, is thermodynamically unfavorable. Therefore, it is physically possible to grow dislocation-free crystals. When the seed crystal is immersed in the melt, dislocations occur due to high thermal stresses caused by a temperature shock. These dislocations can spread into a growing crystal, especially in the case of a large crystal diameter. Thermoelastic stresses in a growing crystal are the main cause of dislocations, especially in the case of large crystals. Due to the high stresses and temperatures in the crystal, these dislocations are not limited to their own glide planes but receive enough energy to propagate to neighboring glide planes through the processes of crawling. Since in crystals of the diamond structure (111)-planes are the main glide planes that are inclined to the crystal axis in the <100> or <111> direction, dislocations will slide and end on the crystal surface, provided that the crystal diameter is reduced to a very small size, so that a small residual stress may not be able to move dislocations or create new ones, and provided that no new dislocations are formed at the interface. This is a well-known Dash necking method [26], who was the first to report the growth of dislocation-free Si and Ge ingots. The Dash necking method is widely used to obtain dislocation-free Si and Ge crystals. Dislocation-free growth is relatively stable even for large-diameter crystals, despite high thermal stresses. The reason for this is the relatively high energy required to create the first dislocation in the crystal. If the shear stresses along the main slide planes at no point exceed the critical value for the nucleation of dislocations or for the growth of very small dislocation loops, the crystal will remain free from macroscopic dislocations. However, compared to the growth of silicon crystals, the growth of dislocation-free Ge ingots of large diameter is a much more difficult task, since the thermal conductivity of Ge is lower, so large thermoelastic stresses are generated in a growing crystal, and the critical shear stress is lower, so dislocations are easier to form and multiply. Also, the problem is more difficult for Ge than for Si, since both the density of Ge is higher, and the tensile strength is lower than for Si (Table 4). Optimizing the parameters of crystal growth, especially the process of forming the constriction and the upper cone to the full diameter of the crystal, the design of the hot zone and maintaining the thermal stability of the crystal growth system, it was possible to grow dislocation-free germanium crystals with a diameter of up to 300 mm. Table 5 shows the characteristics of the obtained plates:
Solar panels based on multistage high-efficiency solar cells using InGaP/InGaAs/Ge are used for onboard power supplies of telecommunications space satellites (Fig.9). The development is carried out by Sharp (Japan), Emcore Photovoltaics (USA), Azur space (Germany), Cesi (Italy), Spectrolab (USA), NPP Kvant JSC, Saturn JSC (Russia). Such solar panels will ensure the achievement of the active life of spacecraft for 15 years or more with an increase in the energy capacity of spacecraft by more than 2 times.
Currently, Umicore (Belgium) and AXT Inc. (USA) are the largest manufacturers of wafers. The main type of wafers produced are wafers with a diameter of 100 mm and 150 mm. Umicore offered the first 150 mm plate for VCSEL in 2020 and became the first to release a 200 mm plate for VCSEL in 2021 (Fig.10)
Conclusion
Along with long-established applications as single crystals in infrared optics and gamma ray detection, dislocation-free germanium has proved to be a promising substrate material for GaAs-based optoelectronic devices. The development of dislocation-free wafers with a diameter of up to 300 mm demonstrates the potential compatibility of germanium with modern technology for manufacturing silicon devices. It can be expected that the introduction of germanium for the next generations of CMOS transistors will accelerate. In addition, germanium, subject to compatibility with Si and AIII-BV, becomes a promising material for electron-photonic integrated circuits. This may mean that the demand in germanium will grow (Fig.11).
ABOUT AUTHORS
Naumov Arkady V., Head of the Research Area, Joint Stock Company “Optical and Mechanical Design Bureau Astrohn”, Lytkarino, Moscow region, Russia.
ORCID: 0000–0001–6081–8304
Startsev Vadim V., Candidate of Technical Sciences, Chief Designer, ASTROHN Technology Ltd, https://astrohn.ru, Lytkarino, Moscow region, Russia.
ORCID ID: 0000–0002–2800–544X
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