National technological priorities. Diamond extreme electronics
The purpose of this article is to present the results of the formation of innovative domestic technology of the electronic component base on the basis of a unique material for its electrical-physical, thermal-physical, optical and mechanical properties – a diamond that provides the creation of micro- and nanotechnology of a new generation with previously unattainable functional capabilities, modes and operating conditions (Fig.1). A developed fully domestic technological route from the growth of diamonds to the fabrication of components for electronics and photonics can be considered as a Russian challenge to achieve excellence in a highly intelligent science field with a long horizon of competitive implementation.
AS MAN-MADE "CARBON STAR"
Carbon (from the Latin "carboneum" – coal) is a fairly widespread chemical element in nature (the carbon content in the human body is 21%, in the earth's crust – 0.16%) that has an atomic-molecular energy conformism, which determines the structural-functional and physical-chemical diversity of carbon-containing materials (Fig.2), as well as their organic-inorganic convergence within the bio-technosphere.
The widely used term "carbon electronics" integrates an innovative scientific and technological basis in the field of synthesis, structure and formation of micro- and nanotechnology objects based on carbon-containing inorganic and organic materials and their hybrid convergent compositions (Fig.3).
The brightest representative of the "carbon community" is a diamond (from the Greek "adamas" – indestructible), whose carbon nature was unraveled at the end of the 18th century, remains to this day a unique costly material, in demand both in jewelry and in engineering. A diamond is known as a jewel – a brilliant (from the French "brilliant") and the hardest abrasive.
Synthetic diamond – a man-made copy of the natural mineral – has taken a stable position in the diamond market. At present, along with jewelry and abrasive diamond, considerable interest is attracted by the use of synthetic large (tens of carats) single crystals of sufficiently high structural perfection and purity for solving extreme engineering and technical problems. Let's present a brief analysis of the features of this material.
The diamond has a polymorphism, and its most widespread allotropic modification (98%) is a cubic diamond that has a face-centered cubic unit cell. Hexagonal diamond or "lonsdaleite" (named after the British crystallographer Kathleen Lonsdale that discovered it in 1966) crystallizes into a wurtzite lattice. If the source of natural classical cubic diamond is kimberlite pipes, hexagonal one is found in meteorite craters. It should be noted that the technology of obtaining a synthetic diamond's hexagonal modification is extremely complicated.
Let's consider properties of a cubic diamond that is the most widespread both in the nature and in industrial production. The molecular weight is 12 g/mole, the density of diamond is 3.5 g/cm3, the record relative hardness on the Mohs scale is 10 (more than 100 GPa, but varies depending on the base face of the diamond), which is ten times higher than that of corundum. The diamond is quite fragile, but has a high value of the modulus of elasticity (Young's modulus) – 1.2 · 1012 N/m2. Optical parameters are essential in characterizing the quality of a diamond, including "glitter" and a multi-colored "game" of faceted single crystals, and make a decisive contribution to the value of jewelry. The refractive index of diamond varies from 2.417 to 2.421; the angular dispersion is about 0.06; the reflectance is 0.172. The color of a diamond is one of the most important jewelry characteristics determined by its alloying and structural perfection. The following colorings of the diamond are known: colorless, yellow-brown, brown, black, gray, blue, green, red, pink, blue and very rarely lilac.
There is a classification that separates diamonds into types depending on the content of base impurities (nitrogen and boron) and their distribution in the crystal, which determines optical transparency in a wide range from hard ultraviolet to deep infrared wavelengths. The main impurity in the diamond is nitrogen (type I – nitrogen content up to 0.2%, type II – not more than 10–3%). Despite the possibility of nitrogen saturation to the level of 1018 cm-3, a natural diamond remains a dielectric whose resistance can vary in the range from 1013 to 1016 Ω · cm. This is caused by the fact that the energy depth of the donor (nitrogen) is very high – 1.7 eV, which limits the concentration of charge carriers at room temperature.
Prospects for the formation of semiconductor devices on the basis of diamond and their competitiveness for a number of energy and frequency parameters are determined by the possibility of obtaining a doped material of n- and p-types of conductivity. Doping also alters the color gamut of jewelry. Traditional diffusion processes for semiconductor technology are unsuitable because of the possibility of a diamond phase transition to graphite, since the diffusion of impurities requires high temperatures and significant process durations. Therefore, doping in the process of crystal growth, epitaxy, ion implantation or surface termination are used.
The basic set of alloying impurities for diamond is limited by boron (0.37 eV) as an acceptor and phosphorus (0.58 eV) and nitrogen (1.7 eV) as donors. At room temperature, these deep impurities have a sufficiently low degree of ionization, which determines the limitation of the number of free charge carriers. The only possible technological way to increase their quantity is to create a highly doped material, which ensures a reduction in the energy gap between the impurity zone and the valence band ceiling (p-type material) or the bottom of the conduction band (n-type material). If the activation energy is 370 meV (ionization degree is 0.2%) for a typical level of doping with boron (less than 1017 cm–3), then at an impurity concentration of 1020 cm–3 the activation energy has practically zero value, but at the same time the carrier mobility decreases catastrophically. A sharp decrease in the mobility of holes from 3800 cm2/V · s (NA ≈ 1015 cm–3) to 100 cm2/V · s occurs even at an impurity concentration of more than 1019 cm–3.
The technology for thermal modification of diamond surface by hydrogenation, that is, thermal or plasma treatment in hydrogen, allows the formation of a hole conducting channel up to 10 nm in thickness with a surface charge density up to 1013 cm–2 and an ultra-low carrier activation energy (23 meV). The mobility of charge carriers is 50–150 cm2/V · s. With regard to the use of phosphorus as a donor, it should be noted that it can be incorporated into a diamond, but, like the boron acceptor, it is deep, and at ordinary temperatures this source of electrons is extremely limited.
So-called color centers – nitrogen-vacancy (NV) and silicon-vacancy (SiV) – are important for quantum information photon systems based on diamond. Obviously, the alloying of diamond and other impurities from the IV group of the periodic system, for example, germanium, is of interest.
FOR EXTREME ELECTRONICS
Evaluating the properties of diamond as a material for extreme conditions and operating modes, one should turn to the generally accepted criteria (Table 1) characterizing the energy-frequency, switching and heat-dissipative capabilities of the material when creating a high-frequency and power electronic component base.
Prospects for the absolute leadership of diamond in extreme electronics determine its following record parameters:
• the critical electric field strength Ec = 10 MV/cm;
• thermal conductivity λ = 20 W/cm · K;
• the width of the band gap ΔЕ = 5,45 eV;
• Debye temperature TD = 1 860 K;
• velocity of sound propagation υs = 10 km/s.
A number of the most important electrical-physical parameters that determine the speed of the device, including Vs – the saturation rate for carrier drift in an electric field for electrons (1.6 ∙ 107 cm/s) and holes (1.1 ∙ 107 cm/s), and mobility charge carriers in undoped material (μn = 4 500 cm2/V · s, μp = 3 800 cm2/V · s) are at the level of similar parameters of semiconductor materials used in the creation of high-frequency devices. However, traditional materials – arsenide and gallium nitride – significantly lose to the diamond in terms of thermal conductivity, critical field strength and the width of the band gap. The latter parameter is one of the defining ones in terms of achieving the highest possible operating temperatures.
With regard to extreme micro-instrumentation, attention should also be paid to the record low expansion coefficient of diamond (10–6 K–1) and a relatively low value of the relative permittivity ε = 5.5.
DOMESTIC DIAMOND ROUTE "FROM THE CRYSTAL TO THE DEVICE"
Despite the discover the carbon nature of the diamond at the end of the 18th century, more than one and a half centuries passed before in Sweden, the USSR and the USA, in the early 1950s, synthetic diamonds were grown using two types of technologies: chemical vapor deposition CVD; the thermobaric method of HPHT (High Pressure, High Temperature) – the crystallization of diamond from the melt of carbon at high temperature and pressure in the presence of metal catalysts. Synthetic crystals grown by HPHT technology often have a habit in the form of a cube, and they are characterized by zonal (sectorial) color (Fig.4), determined by the anisotropy of the solubility of impurities.
The basis for the domestic development of a field-effect transistor based on diamond was the construction that provides the solution of two problems:
• сreation of an ultrathin nano-sized deep-layered heavily doped diamond layer, which is a source of charge carriers for a low-alloy layer with acceptable carrier mobility, which was realized within the epitaxy of a δ-doped boron layer about 2 nm thick;
• formation of a "control" gate insulator on the surface of a diamond δ-nanocomposite, which was realized by the precision low-temperature technology of the atomic-molecular chemical assembly of a perfect nanotubes layer of aluminum oxide.
As shutter material, platinum was used which, locally, with an acute-focused ion beam, was non-lithographically deposited onto the Al2O3 surface by an ionically-stimulated chemical reaction at the Helios Nanolab FIB station using an organometallic platinum compound.
The basic key operations for the formation of the domestic diamond MIS transistor are presented in Table 2. The hardware-technological realization of the generated and implemented technological route, the basic structure and output characteristics of the manufactured MIS-transistor diamond are explained in Table 3. Using as a gate dielectric the structurally perfect layer of aluminum oxide, obtained by the ALD method, provides effective control of the current in the channel and reduction of leakage currents. However, a sufficiently large control voltage is necessary, and there is no saturation of the output characteristic due to the manifestation of the known "deep tail" effect in the "δ-doped" layer.
The set of record parameters of a diamond predetermines the creation of commuting components with previously unattainable energy-frequency, energy-impulse parameters and resistance to severe operating conditions, including high temperature and radiation effects, as one of the promising niches for its effective application.
Table 4 presents a comparative analysis of the three types of switches that can be implemented on diamond using its record capabilities for various design solutions and functional purposes: generation of ultra-high power density, high-current and high-voltage switching, protection from external electromagnetic influences.
It should be noted that the Saint-Petersburg Electrotechnical University "LETI" carried out a complex of development of all types of switches presented on the basis of the nearest analogue of diamond – silicon carbide (Fig.5). Within the framework of scientific and technological cooperation with the IAP RAS, work was carried out to create micromechanical keys based on diamond in the interests of the Istok and field emission structures. In particular, the realization of the composite field-emission structure "silicon carbide/nanocrystalline diamond" (Figs. 6a and 6b) showed a sharp increase in stability and minimization of degradation processes (Fig.6c).
Priorities for the development of diamond electronics and photonics to ensure Russia's technological independence and competitiveness in the critical areas of the development of the electronic component base are reflected in Table 5, and the progressive material science trends in diamond technology are summarized in Fig.7.
The domestic scientific and technological school retains a certain international parity in this innovative sphere. Taking into account the declared strategic areas of the country's development, diamond is a bright innovative star for Russia. Integration of scientific and engineering and educational potentials of Istok, INREAL, New diamond technologies, Institute of Applied Physics RAS, Technological Institute for Superhard and Novel Carbon Materials, Institute of Nuclear Physics SB RAS, Saint-Petersburg Electrotechnical University "LETI" in the framework of innovative system of state-demanded diamond projects, it can allow to achieve real excellence in a highly intelligent science-intensive sphere with a long horizon of competitive implementation.
In conclusion, the author expresses gratitude to the domestic developers of diamond technology: A.L.Vikharev; M.P.Dukhnovsky; A.V.Kolyadin; V.A.Ilyin; V.I.Zubkov; A.V.Afanasyev; A.S.Ivanov; M.F.Panov; A.D.Kanareikin; as well as to Professor J.Butler (USA) for joint work, scientific and human communication, without which the appearance of this article would be impossible. ■
The work was carried out with the financial support of the Ministry of Education and Science of the Russian Federation (projects No. 14.B25.31.0021 and No. 03.G25.31.0243) and the grant of the RSF No. 15-19-30022.