Russian electronic components for extreme conditions: silicon carbide industry founded by LETI
•for a long time, Russia did not apply any systemic approach to the development of its own modern industry of manufacturing semiconductor silicon carbide and SiC-based ECB whilst different agencies and organizations have incurred considerable cumulative economic expenses on research and development in this area;
•it is necessary to develop the Russian silicon-carbide industry as part of the solutions to the problems of ECB import substitution and to ensure the parity in the technologies that define the country’s scientific and technological excellence and security.
In early 2016, when the strategic development areas of LETI were determined under the TOP-100 program for competitiveness improvement of the Russian leading universities, "Carbon Electronics" was chosen as one of the priority areas. This area was chosen as a priority because carbon is a widespread natural chemical element characterized by atomic and molecular energy conformism, determining the structural-functional and physico-chemical diversity of carbon containing materials, as well as organic-inorganic convergence in the biotechnosphere.
When the priority area was determined, the following target market segments of science-intensive innovative products were identified:
•silicon carbide and its compositions, such as silicon carbide – graphene, silicon carbide – diamond;
•diamond and diamond-based epitaxial compositions;
•2D and 3D nanoscale carbon: graphene, nano-tubes;
•polymers and biopolymers: structuring and shaping, additive printing and bionic technologies.
The primary expected outcome determining the effectiveness of the Carbon Electronics Interdisciplinary Center of Excellence is the implementation of the innovative development model with the creation at the University of competitive technological niches, which will have their infrastructural and practical realization in technological lines and projections focused on diamond, silicon carbide, flexible printed electronics.
The ambitious projects and rapid knowledge transformation from research to production phases require the concentration of competencies, infrastructural resources and professional elite. The selection of the competitive national innovational environment in the technological niches of silicon electronics as one of the priority areas has determined the need to conduct a detailed systemic analysis of the contemporary problems and achievements in the silicon carbon industry with the aim of positioning LETI within the market of science-intensive and highly demanded products. The purpose of this article is to provide a systemic analysis of the key issues and progressive trends in the silicon carbide technology and SiC-based ECB.
Since the reader needs a systematized understanding of the progress in the silicon-carbide industry, this article is mostly made up of tables and charts with generalized data that should provide for the visibility and practical use of the analytical results. The most significant current trends are illustrated by way of examples of the national industry solutions developed at the St-Petersburg Electrotechnical University in the field of growth processes and prospective ECB only for the last few years. Detailed description of earlier solutions is provided in papers [6–9].
Silicon Carbide as Material for Electronic Equipment
The industrial history of silicon carbide dates back to 1893, when E. Acheson offered a sublimation technology for producing abrasive material by evaporating a mixture based on carbon and silicon (quartz sand, in essence). The high-temperature synthesis formed a silicon carbide sinter mostly composed of a binary chemical compound – silicon carbide, in the form of aggregated crystals. This abrasive material with a high Mohs hardness (9.2 to 9.3) was called carborundum. The further history of silicon carbide development as a material for semiconductor technology is described in Table 1.
The first results of growing 3D mono-crystals – SiC ingots were presented by the researchers of Leningrad Electrotechnical Institute, Yu.Tairov and V.Tsvetkov in 1976 at the First European Conference on Crystal Growth from Gas Phase in Zurich (Switzerland). The full description of the new method of SiC growth, titled in the world practice as "the LETI method" by analogy with "the Lely method", was published in 1978  in the international magazine Crystal Growth. The method was based on the classical scheme of condensing supersaturated vapor on silicon carbide used as a seed mono-crystal.
In the course of a few years, the researchers used the technology of basic crystal-seed proliferation, initially achieved by the Lely method, and managed to grow 3D mono-crystal silicon-carbide ingots with a diameter of several inches. The size evolution of the silicon-carbide mono-crystals, grown using the LETI method, which determined the worldwide transition to industrial technologies of integral-group creation of devices based on silicon carbide, is illustrated in Fig.1.
The physico-chemical and technological characteristics of SiC as a material used for electronic equipment are shown in Table 2. The comprehensive analysis of the basic semiconductor materials in respect of their properties that determine the functional purpose and achieved parameters of ECB is provided in Fig.2.
The ranking provided in Fig.2 shows the proximity of silicon carbide to such promising materials as nitrides of gallium and aluminum, and diamond. The determining features from the perspective of achieving extremal operation modes and conditions are the band gap, Debye temperature, thermal conductivity, critical electric field, and carrier drift saturation speed.
The most important criteria of the material’s generalized qualities related to the ECB operation modes are Johnson, Kejes and Baliga, which take into account the actually acceptable thermal or electrical load. They are characterized by the formulas – "switching (generated) power × frequency", "working voltage × electric current density", or in the form of "attainable rate of voltage rise" in impulse systems. All of the above basic characteristics of the material are included in these evaluation criteria of the attainable parameters of power, high-frequency, and impulse electronic devices.
The SiC resistance to various affecters is often described in the literature based on the Debye temperature, which may in fact be detailed into a number of major extremal influencing factors (Table 3).
Key Problems of SiC Technology
The SiC resistance to external affecters determines the technological complexity of synthesis, processing and modification of this material. The contemporary key problems in the SiC-based ECB industrial development are systematized in Table 4.
Particularly noteworthy growth problems are associated with producing both heavily doped mono-crystal substrates for SiC-based power electronics and high-ohmic substrates, which are the basic substrates for microwave devices based on heterostructural compositions of GaN/AlN/SiC. The low-ohmic features of the substrate are achieved by doping mono-crystals during the growth induced by nitrogen. However, certain concentrations form defects which deteriorate the structural characteristics of the material. This negatively affects the quality of SiC epitaxial layers, which is unacceptable in the manufacture of high-voltage high-current electronic instruments.
The frequently applied additional doping of the growing mono-crystal with compensating admixture for production of high-ohmic substrates (ρ>108 Ω·cm) used in microwave electronics does not provide the required substrate characteristics during the operation in extreme microwave modes. This necessitates the use of high-purity non-doped substrates, which are actually unique not only for their price parameters but also for availability, in particular, for the domestic consumer.
When describing the development of LETI’s SiC crystal growth solutions, the following projects deserve particular attention:
creation in cooperation with Sector Ltd. of a completely new national facility for growing SiC mono-crystals (Fig.3), which can produce ingots (Fig.4) with a diameter of 6 inches;
conducting studies on silicon carbide growth using the substrates with unconventional crystallographic orientations ;
study of reversible growth polytypism processes in the SiC-AlN system in the context of rare polytypes matrix replication (Fig.5).
The silicon carbide epitaxial growth processes were analyzed and developed by the staff of St-Petersburg Electrotechnical University with the help of a foreign experimental facility, and since the end of 2015, this work has been conducted only at the University as this process was determined as part of the technological projection for manufacturing high-voltage power electronic devices. The researchers defined the most critical aspects for the homoepitaxy control based on the following major criteria: purity and doping level of the material; structural perfection and growth rate of epitaxial layers; surface morphology. These generalized views are shown by the chart in Fig.6.
The evolution of the SiC gas-phase epitaxy processes is shown in Table 5.
The doping process is controlled in three main ways:
•selection of the doping precursor (n-type donors: nitrogen, N2, NH3, phosphorus, PH3; p-type acceptors: aluminum and gallium, metalloorganic Al(CH3)3, Ga(CH3)3) and their concentrations determined by the gas flow velocity;
•ratio of Si/C (n-type), C/Si (p-type);
•crystallographic orientation of (0001) Si or (0001) C (the solubility of admixtures, for example, aluminum, on the Si-face is higher by one order of magnitude than the solubility of admixtures on the C-face).
The most popular technology when creating SiC-based devices using the absolutely prevalent epitaxial technology is changing the type of doping admixture in the growing reactor without its "decompression", directly during the process of epitaxial growth. The modern epitaxial reactor owned by LETI enables this process, including the automatic loading of substrates.
There is no doubt that the development of silicon carbide industry is crucial for Russia as one of the priorities for solving the problems of ECB import substitution and ensuring the parity in technologies that define the country’s scientific and technological competitiveness and security. Competitive developments of electronic components on SiC, which were designed in LETI for extreme conditions, will be discussed in the second part of the article.