AlGaN/GaN High Electron Mobility Transistor Structures ... - IEEE Xplore

IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 8, NO. 3, SEPTEMBER 2008

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AlGaN/GaN High Electron Mobility Transistor Structures: Self-Heating Effect and Performance Degradation Svetlana A. Vitusevich, Andrey M. Kurakin, Norbert Klein, Mykhailo V. Petrychuk, Andrey V. Naumov, and Alexander E. Belyaev

Abstract—This paper reports on the results of the experimental and numerical investigation into the self-heating effect in AlGaN/ GaN heterostructures grown on sapphire and SiC substrates. It shows that temperature increase has an opposite dependence on the buffer thickness for sapphire and SiC substrates. Noise spectroscopy is also used to monitor the self-heating effect. Moreover, it is shown that the room-temperature spectra can be used to determine the activation energy of the traps. An irreversible improvement in mobility and quantum scattering time is registered after the irradiation of AlGaN/GaN heterostructures at a total dose of 1 × 106 rad of 60 Co gamma rays. Index Terms—Heating, high-electron mobility transistors (HEMTs), noise measurement, reliability, spectroscopy.

I. I NTRODUCTION

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IGH-ELECTRON mobility transistor (HEMT) structures based on III-nitride materials have extensively been studied over the last few years [1]–[4] as promising materials for the development of high-speed communication systems. Much progress has been made on the design and fabrication of the structures and on understanding their unique transport properties. However, one of the main challenges still consists of finding an optimal layered structure to enhance the reliability of HEMTs [5], [6]. The performance degradation of a biased 2-D electron gas (2DEG) in AlGaN/GaN heterostructures depends on the properties of the material, which, in turn, are defined by changes in the parameter performance that are caused by the self-heating effect at a high level of dissipated power in the channel [7]–[10]. A significant reduction in the saturation drain current is achieved by increasing the temperature. This Manuscript received November 30, 2007; revised March 27, 2008. Current version published October 16, 2008. This work was supported in part by the Ukrainian Ministry of Education and Science, the Deutsche Forschungsgemeinshaft under Contract KL1342, and the Office of Naval Research under Grant N00014-01-0828. S. A. Vitusevich is with the Forschungszentrum Juelich, 52428 Juelich, Germany (e-mail: [email protected]). A. M. Kurakin is with the Institute of Bio- and Nanosystems, Research Center Juelich, 52425 Juelich, Germany (e-mail: [email protected]). N. Klein is with the Forschungszentrum Juelich, Institut fuer Bio- und Nanosysteme (IBN), and the Center of Nanoelectronic Systems for Information Technology (CNI), 52425 Juelich, Germany (e-mail: [email protected]). M. V. Petrychuk is with the Department of Radio Engineering, National Taras Shevchenko University of Kiev, 01033 Kiev, Ukraine (e-mail: [email protected]). A. V. Naumov and A. E. Belyaev are with the V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 03028 Kiev, Ukraine (e-mail: [email protected]; [email protected]). Digital Object Identifier 10.1109/TDMR.2008.2001684

is caused by the decrease in the saturation carrier velocity and 2DEG density in the HEMT [11]. The degradation rates of key parameters extracted from continuous- and pulsed-wave-mode measurements, with respect to the operation temperature, have already been reported for HEMT structures with sapphire and Si substrates [12]. Devices suffer from pronounced degradation when thin substrates reduce the thermal impedance due to the heat-spreading effect [13]. Different thermal impedance substrates have been employed to study heat dissipation in the structure [14]–[17]. It has been demonstrated that the temperature increase for similar devices on sapphire and SiC substrates at the same added power is about twice as high as that on sapphire-based substrates. To overcome the low thermal conductivity of sapphire, a flip-chip integration with a high-thermal-conductivity AlN substrate can be used [18]. However, this approach demands additional process steps, such as metal contact pad deposition and flip-chip bonding by simultaneously applying heat and pressure to the carrier substrate and the HEMT, followed by a thermal reflow of the solder material. The alternative to flip-chip technology for improving the performance of the devices is the use of a high-thermalconductivity SiC substrate. At the same time, SiC substrates are very expensive to produce. Therefore, the most important issue is finding a new optimized payload concept. In this paper, we report on our study of the self-heating effect, taking into account the thickness of the buffer layer. In addition, lowfrequency noise spectral characteristics and small doses of gamma radiation treatment are used to monitor the origin of the processes, which limit the reliability of the HEMT structures. This paper is organized as follows: Section II describes the HEMT structure simulation results. Section III presents the experimental details. In Section IV, we report on the transport and noise properties in AlGaN/GaN heterostructures and discuss our results. Section V outlines the main conclusions. II. H EAT D ISSIPATION S IMULATION Fig. 1 shows a schematic plot of the simulated device. The dimensions are typical of HEMT structures used for measurement transport and noise properties: a 2.5 × 2.5 mm2 substrate and buffer area for different calculated values of the height of the buffer layer. The active area is defined as a part of the channel between two transmission line model (TLM) contacts, with a length L of 25 μm and a width W of 100 μm.

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Fig. 1. Scematic of the simulated HEMT structures with a sapphire or SiC substrate. The active area is defined as a part of the channel between two TLM contacts.

Fig. 3. Calculated temperature rise in the channel of the TLM device structure of AlGaN/GaN HEMTs for a substrate thickness 300 μm as a function of the buffer thickness. (Circle) SiC substrate. (Square) Sapphire substrate.

Fig. 2. Calculated temperature rise in the channel of the TLM device structure of AlGaN/GaN HEMTs for a buffer layer thickness of 3 μm as a function of different substrate thicknesses. (Circle) SiC substrate. (Square) Sapphire substrate.

Fig. 4. Calculated temperature rise in the channel of the TLM device structure of AlGaN/GaN HEMTs for the sapphire substrate thickness of 300 μm as a function of the buffer thickness. (Open square) Results for the substrate with constant thermal conductivity. (Closed square) Results taking into account the temperature dependence of substrate thermal conductivity.

A common geometry is used for simulating heat dissipation and heat transfer in the TLM device. If we neglect the temperature dependence in the thermal conductivity of the buffer and substrate layers, the solution of the problem is reduced to the Laplace equation [19]. The boundary conditions can be set as follows: a constant heat flux q = Pdis /(W L) from the hot zone in the active area (conducting channel) on top of the buffer layer, where Pdis is the dissipated power; continuous heat flux at the buffer–substrate boundary; and adiabatic thermal conditions all over the remaining surface (q = 0), except at the bottom, where an isothermal condition is provided by a good thermal contact under a certain temperature T0 . These conditions allow us to determine the temperature distribution in the substrate and evaluate channel temperature T averaged over the channel area, i.e., Sch = W L. The temperature rise ΔT = T − T0 in the channel is related to Pdis through thermal impedance θ = ΔT /Pdis . The thermal impedances were obtained for L = 25 μm using the substrate thermal conductivity λ = 0.35 W/cmK for sapphire, λ = 1.2 W/cmK for the GaN buffer, and λ = 3.5 W/cmK for the SiC materials. The calculation results are shown in Figs. 2 and 3. The HEMTs designed on the sapphire and SiC substrates demonstrate similar substrate thickness dependence. In addition, a

strong dependence of the increase in temperature on buffer thickness was also revealed (Fig. 3). As shown in the figure, the self-heating of the HEMTs grown on sapphire can be reduced if a thicker buffer layer is used. It should be noted that the sapphire material has a strong temperature dependence on the thermal conductivity [20]. To study such an influence of temperature, we calculated the overheating temperatures for different buffer thicknesses, taking into account that the nonlinear heat transfer equation and its boundary conditions for temperature Tnl (x, y, z) can be reduced to linear form Tl (x, y, z), as in the case of constant thermal conductivity λ0 = λ(T0 ), by Kirchhoff T transformation [19]: Tl = T0 + (1/λ0 ) Tlnl λ(T )dT . The obtained results are shown in Fig. 4. As can be seen, at a buffer thickness of 1 μm, the difference in the overheating temperatures is about 90 K, but, at a thicker buffer of 10 μm, this difference becomes smaller, i.e., only about 30 K. The results demonstrate the advantages of using the structures with a thicker buffer layer. III. E XPERIMENTAL D ETAIL The investigated structures were grown using metal–organic chemical vapor deposition on sapphire (001) and SiC substrates.

VITUSEVICH et al.: AlGaN/GaN HEMT STRUCTURES: SELF-HEATING EFFECT AND PERFORMANCE DEGRADATION

Fig. 5. Measured I–V characteristics of different layer compositions for AlGaN/GaN HEMT structures with channel L = 25 μm and a width of 100 μm.

The structures have the following sequence of layers: Sample labeled thin sapphire substrate (TNS): TNS (300 μm), undoped GaN buffer (1.1 μm), undoped Al0.33 Ga0.67 N barrier (23 nm), and Si3 N4 passivation layer (320 nm). Sample labeled thick sapphire substrate (TKS): The TKS was grown with a layer structure that is similar to the sample TNS on 3-mm TKS. Sample labeled thick buffer layer (BFR): GaN nucleation layer (28 nm), GaN insulating buffer (7.5 μm), undoped AlN spacer (∼1 nm), undoped Al0.25 Ga0.75 N barrier (25 nm), and GaN cap layer (1.3 nm). Sample labeled SiC: The SiC substrate (SiC) was grown with a layer structure similar to that of the sample TNS on a 300-μm-thick SiC substrate. The structures are patterned to form TLM devices. Ungated TLM patterns were investigated in order to avoid the nonuniform electric field redistribution introduced by the gate metal layer. The conducting channel of the devices had a width of 100 μm and an intercontact length L of 25 μm. These dimensions were chosen in order to insure a negligibly small contribution of contact resistance. The TLM ohmic contacts were processed by Ti/Al/Ni/Au metallization annealed for 40 s at 800 ◦ C. The I–V characteristics were measured in the steadystate regime. Spectral noise measurements were simultaneously performed in the frequency range of 1–100 kHz using a low-noise preamplifier and spectrum analyzer HP 35670A. The structural characterization of the heterostructures was performed by X-ray diffraction (XRD) and secondary ion mass spectroscopy. Gamma irradiation was provided by 60 Co gamma rays at room temperature with doses in the range of 104 –106 rad and a flux of 102 rad/s. The average gamma-quanta energy was about 1.2 MeV. IV. M EASUREMENT R ESULT AND D ISCUSSION Experimental current–voltage (I–V ) characteristics are shown in Fig. 5. Two regions can clearly be observed on the I–V plots: 1) the linear ohmic region and 2) the nonlinear saturation region at higher voltages. As previously shown in [21], the appearance of the saturation region can be explained by a

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strong self-heating effect and a weaker contribution by the hotelectron effects. It was previously reported [22] that an increase in operational temperature results in a decrease in mobility, an increase in resistance, and a corresponding voltage drop in the HEMT channel. Reliable AlGaN/GaN-based devices for highpower or high-frequency applications should withstand hightemperature operating regimes without any loss in functionality or performance. The realization of such devices requires the optimization of the layer structure design. In order to find the best solution for wafer design, taking into account reliability and payload, we investigated the performance of several stateof-the-art AlGaN/GaN heterostructures. As shown in Fig. 5, standard TLM devices processed on different wafers show different performances, despite the fact that they were grown in the same conditions. In order to understand the influence that wafer design apparently has on device performance, we compared the simulation results of the overheating temperature in the active region in the TLM device with a common geometry (a two-layer structure with the substrate as the first layer and the buffer as the second layer), with heat sources uniformly distributed over a rectangular area (100 × 25 μm2 ) on top of the second layer. The calculated overheating temperature for the dissipated power per square micrometer of 0.24 mW, plotted in Fig. 2 as a function of the substrate thickness for the sapphire and SiC substrates, increases, with the substrate thickness increasing from 350 to 750 μm and more slowly increasing from 750 to 2700 μm. The same tendency is observed in experimentally obtained I–V characteristics. The saturation current of AlGaN/ GaN heterostructures grown on a SiC substrate is higher than that of AlGaN/GaN heterostructures grown on sapphire. It should be noted that an increase in substrate thickness may be responsible for a slight decrease in the performance of devices grown on thick substrates because of higher overheating temperatures at the same dissipated power. At the same time, the thicker sapphire substrate sample demonstrated about the same performance (taking into account 10% wafer from wafer variation parameters) as the thinner sapphire substrate sample. Even more interesting is the revealed influence of the buffer layer thickness on the temperature overheating values. For substrates with low thermal conductance made from sapphire, we found that overheating in the active region of the device could be reduced using a thicker buffer layer. The experimentally obtained results are in good agreement with the simulation data. We expect that this effect will be even more significant in gated HEMT structures. Kuball et al. [23] experimentally show that the self-heating is strongly nonuniform at the gate edge close to the drain. The structures in this paper have only one thickness buffer layer. An increase in the buffer thickness should result in an improvement in the device performance. The simulation of the overheating effects in gated structures with a different buffer thickness is underway. Any degradation in AlGaN/GaN-based device performance is directly reflected in low-frequency noise features, as shown in [24]. Our investigations of low-frequency noise spectra reveal features directly related to thermal effects in the structure. These features allow the study of the physical origin of the device performance changes under self-heating effect.

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Fig. 6. Spectra of the current noise of the AlGaN/GaN HEMT TNS structure with channel L = 25 μm and a width of 100 μm measured for different electric fields E (in kilovolts per centimeter) of 0.04, 0.09, 0.18, 0.35, 0.75, 1.52, 2.73, and 3.78.

Additionally, these investigations are useful for the analysis of traps studied at room temperature, as will be shown here. Low-frequency noise spectra were measured at different applied voltages for the TNS sample with the highest temperature rise. Deviation from conventional 1/f flicker noise was observed at some transient frequency fT , with a tendency toward noise-level suppression in a wide frequency range below fT (Fig. 6). It was found that the transient frequency strongly depends on the dissipated power. An analysis of the high-frequency interval of the noise spectra showed that the normalized noise level at this frequency interval was independent of the applied voltage. It should be noted that the observed features in the low-frequency interval are caused by heat dissipation. In view of the fact that selfheating occurs, the analysis of the noise spectra in both the lowfrequency and transition intervals demands that the temperature increase in the channel of the AlGaN/GaN heterostructures be estimated. We estimated temperature rise ΔT [Fig. 7(a)] versus dissipated power Pdis based on the theoretical model with 1) heat dissipation and heat transfer modeling in the device and 2) self-consistent solution of coupled nonlinear equations for channel current I and channel temperature rise ΔT . The latter result allowed us to plot time parameter τ , which corresponds to the frequency separating the low-frequency 1/f range and transition region as a function of ΔT . An exponential dependence of τ can clearly be shown in Fig. 7(b). This reflects the activation process of the hopping conductivity, which results in an increase in the low-frequency component of the noise. The estimated value of the trap energy is about 0.6 eV. This energy is associated with a nitrogen vacancy, which is in good agreement with literature data. Transport and noise characteristics improved after a small dose of gamma radiation treatment of 106 rad. In order to gain an insight into the structural transformation of the heterostructures during irradiation, the angular distribution of the XRD corresponding to the (0002), (0004), (10¯12), and (¯ 1¯ 124) reflections of the GaN and AlGaN layers was measured by means of triple-crystal differential diffractometry (PANalytical X’Pert MRD) under the conditions of symmetrical and asymmetrical Bragg–Laue geometry. A shift in the peak corresponding to the (0004) plane of the AlGaN layer to a lower angle was observed after irradiation. The changes in the

Fig. 7. (a) Temperature rise as a function of the dissipated power in the channel of the TNS HEMT with L = 25 μm and T = 300 K. (b) Dependence of the characteristic time on the channel overheating.

spectra clearly indicate a strain relaxation in the AlGaN layer after irradiation. To verify this statement, direct measurements of the elastic strain in the heterostructures were performed. An integrated method for measuring the surface curvature was used. The strain relaxation in the HEMT structure could lead to the generation of additional defects or the structural ordering of native defects. The observed improvement in room-temperature mobility from 1380 cm2 /V · s (in the nonirradiated sample) to 1460 cm2 /V · s (in the irradiated sample at a dose of 106 rad) indicates that the latter process is dominant in the radiationstimulated relaxation of the TNS sample. Similar changes in mobility were also registered for two other structures. For example, in the sample labeled BFR, the observed improvement in room-temperature mobility from 1850 cm2 /V · s (in the nonirradiated sample) to 2000 cm2 /V · s at low-temperature mobility exhibited a greater increase after gamma radiation treatment at a dose of 106 rad (from 11 500 to 19 600 cm2 /V · s). The results of the electrical characterization of the structures obtained before and after irradiation can be summarized as follows: The carrier concentration is slightly decreased after an irradiation dose of 106 rad, whereas the 2DEG mobility measured at low temperatures exhibited a considerable increase. The latter is consistent with the aforementioned decrease in strain accompanied by the structural ordering of defects. The changes should improve the conditions for charge transfer. This is reflected by an increase in mobility. The most striking result obtained for the BFR sample is the amplification of the oscillation amplitude after irradiation with

VITUSEVICH et al.: AlGaN/GaN HEMT STRUCTURES: SELF-HEATING EFFECT AND PERFORMANCE DEGRADATION

a dose of 106 rad. Moreover, the spin splitting of Shubnikov-de Haas (SdH) oscillations became well resolved at a magnetic field B > 8 T with decreasing temperature. The onset of oscillations and their amplitude, which is determined by the degree of disorder broadening of the developing Landau levels, can be used in addition to zero-field mobility to estimate material quality. Thus, the observed amplification of SdH oscillation amplitude, together with the increase in the average time between scattering events (known as the quantum scattering time), strongly supports our conclusion concerning the enhancement of carrier mobility caused by native defect ordering, as also shown by the structural characterization. After a dose of 106 rad, it was shown that the dispersion of the transfer characteristics considerably decreased on a set of AlGaN/GaN HEMTs patterned on the same wafer. V. C ONCLUSION In summary, the transport and noise properties of AlGaN/ GaN heterostructures were studied with respect to reliability. Heterostructures grown on sapphire and SiC substrates show an opposite dependence of self-heating on the buffer layer thickness. It was demonstrated that the noise spectra can be used to determine the activation energy of the traps of the AlGaN/GaN heterostructures at room temperature. We revealed an improvement in the transport properties of 2DEG in AlGaN/ GaN heterostructures after treatment with small doses of gamma irradiation. The effect was confirmed by the results of a structure analysis and by well-resolved spin splitting in SdH oscillations, which were observed after this dose of gamma radiation treatment. The samples displayed an increase in mobility and quantum lifetime at T = 0.3 K. The revealed improvement in the transport properties of AlGaN/GaN HEMTs after gamma irradiation is irreversible in time. The results suggest that the improvements in the transport characteristics can be achieved by using gamma-quanta irradiation in processing technology. ACKNOWLEDGMENT The authors would like to thank H. Hardtdegen of Forschungszentrum Juelich, Juelich, Germany, and Z. Bougrioua of CRHEA, Valbonne, France, for providing us with epitaxial material and reviewers for valuable comments and suggestions. R EFERENCES [1] S. J. Pearton, F. Ren, A. Z. Zhang, and K. P. Lee, “Fabrication and performance of GaN electronic devices,” Mater. Sci. Eng., vol. R30, no. 3–6, pp. 55–212, Dec. 2000. [2] U. K. Mishra, P. Parikh, and Y.-F. Wu, “AlGaN/GaN HEMTs—An overview of device operation and applications,” Proc. IEEE, vol. 90, no. 6, pp. 1022–1031, Jun. 2002. [3] U. K. Mishra, Y.-F. Wu, B. P. Keller, S. Keller, and S. P. Denbaars, “GaN microwave electronics,” IEEE Trans. Microw. Theory Tech., vol. 46, no. 6, pp. 756–761, Jun. 1998. [4] L. Chen, R. Cofie, D. Buttari, S. Heikman, A. Chakraborty, A. Chini, S. Keller, S. D. DenBaars, and U. K. Mishra, “High-power polarizationengineered GaN/AlGaN/GaN HEMTs without surface passivation,” IEEE Electron Device Lett., vol. 25, no. 1, pp. 7–9, Jan. 2004.

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Svetlana A. Vitusevich received the M.Sc. degree in radiophysics and electronics from Kiev State University, Kiev, Ukraine, in 1981 and the Ph.D. degree in physics and mathematics from the Institute of Semiconductor Physics (ISP), Kiev, in 1991. From 1981 to 1997, she was with ISP, where she became a Researcher in 1981, a Scientific Researcher in 1992, and a Senior Scientific Researcher in 1994. From 1997 to 1999, she was an Alexander von Humboldt Research Fellow with the Institute of Thin Films and Interface, Forschungszentrum Juelich (FZJ), Juelich, Germany. Since 1999, she has been a Senior Scientific Researcher with FZJ. She is the author of more than 100 papers in refereed scientific journals. She is also the holder of seven patents, two of which are for semiconductor functional transformers. Her research interests include the transport and noise properties of different kinds of materials for advanced electronic devices and circuits.

Andrey M. Kurakin received the Ph.D. degree in physics from the Technical University of Dortmund, Dortmund, Germany, in 2007. He is currently a Research Scientist with the Institute of Bio- and Nanosystems, Research Center Juelich, Juelich, Germany. He was also a Researcher with the Department for Semiconductor Heterostructures, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kiev. Since 2000, he has been studying the radiation hardness of semiconductor devices. He has also been involved in the development of measurement and automation systems. He is the author of more than 20 journal and conference proceeding papers.

Norbert Klein, photograph and biography not available at the time of publication.

Mykhailo V. Petrychuk received the Ph.D. degree in semiconductor physics from the Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine, in 1993. He is currently a Senior Research Scientist with the Department of Radio Engineering, National Taras Shevchenko University of Kiev, Kiev. Since 2002, he has been involved in the investigation of noise in low-dimensional quantum structures. He has also made a significant contribution to the discovery and understanding of nonequilibrium low-frequency noise in GaN-based structures with a 2-D conducting channel. He is the author of more than 70 journal and conference proceeding papers. His primary area of expertise is the physics of low-frequency noise in semiconductors and devices. His research interests include the study of the fundamental origin of 1/f noise.

Andrey V. Naumov was born in Kiev, Ukraine, in 1980. He received the B.S. and M.S. degrees in physics from the National Taras Shevchenko University of Kiev, Kiev, in 2003. He is currently a Junior Researcher with the V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kiev. His research interests include the physics of semiconductor nanostructures, electronic transport, and the electrical and optical properties of quantum heterostructures and devices based on the structures.

Alexander E. Belyaev received the M.S. degree in X-ray and metal physics from Kiev State University, Kiev, Ukraine, in 1972 and the Ph.D. degree and the Doctor of Science degree in the physics of semiconductors and insulators from the Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kiev, in 1980 and 1991, respectively From 1972 to 1974, he was engaged in research and development of ultrasound techniques for material characterization at the Faculty of Physics, Kiev State University. He is currently the Vice-Director and a Head of Department with the V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine. Since 1999, he has been a Scientific Advisor with the Institute of Fundamental Problems for High Technology, National Academy of Sciences of Ukraine, Kiev. In addition, he has collaborated with the Research Institute “Orion” in the study of micromachined circuits for microwave and millimeter-wave applications. He is a member of the Editorial Board of the journal Semiconductors, Quantum Electronics and Optoelectronics. He is the author of more than 250 publications, including three books. Prof. Belyaev is a member of the Physical Society of Ukraine (1992), The International Society for Optical Engineering (1993), The American Physical Society (1996), the Council of the National Academy of Sciences of Ukraine on the Problem “Physics of Semiconductors” (1985), the Scientific Council on Awarding of the Ph.D. degrees (1989), and the Scientific Council of the National Program “Nanotechnology” (2000). He was elected as Deputy Academician—Secretary of Physics and Astronomy Division, National Academy of Sciences of Ukraine, in April 2004. Since 2006, he has been a Fellow with the National Academy of Sciences of Ukraine.