Characterization and Application of Nanomaterials 2025, 8(2), 11021. https://doi.org/10.24294/can11021 1 Article Tracking the thermostimulated transformations of silicon suboxide film through absorption edge characterization Mykola Sopinskyy*, Ivan Indutnyi, Katerina Michailovska, Volodymyr Yukhymchuk V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kyiv 03028, Ukraine * Corresponding author: Mykola Sopinskyy, sopinskyy@ua.fm, sopinsky@isp.kiev.ua Abstract: In this work, the structural transformations of a suboxide vacuum-deposited film of SiO1.3 composition annealed in an inert atmosphere in a wide temperature range of 100 °C– 1100 °C were characterized by the reflection-transmission spectroscopy technique. The experimental spectroscopic data were used to obtain the spectra of the absorption coefficient α(hν) in the absorption edge region of the film. Based on their processing, the dependences of Urbach energy EU and optical (Tauc) bandgap Eo on the annealing temperature were obtained. An assessment of the electronic band gap (mobility gap) Eg was also carried out. Analysis of these dependences allowed us to trace dynamics of thermally stimulated disproportionation of the suboxide film and the features of the formation of nanocomposites consisting of amorphous and/or crystalline silicon nanoparticles in an oxide matrix. Keywords: non-stoichiometric silicon oxide; SiOx; nanocomposite; silicon nanoparticles; absorption edge; optical band gap; electronic band gap; mobility gap; Urbach energy 1. Introduction Silicon suboxide (SiOx, x < 2) films have been the subject of intensive research for several decades. At an early stage, these studies were mainly driven by the widespread use of such films as passive, insulating, and anti-reflective coatings [1,2]. In recent years, non-stoichiometric silicon oxides, both amorphous and crystalline, have attracted increasing attention for both scientific and technological reasons [3–5]. One of these reasons is that high-temperature phase separation of non-stoichiometric SiOx films is a key method for the formation of composite structures consisting of nanoscale silicon inclusions (amorphous or crystalline) embedded in a silicon oxide matrix [4–9]. The interest in structures with nanoparticles is due to the unique properties of these structures, which can be radically different from the properties of massive homogeneous materials. And silicon nanostructures are one of the three most convenient building blocks of nanotechnology, along with gold and carbon nanostructures [10]. Silicon oxide—nanosilicon composites are of particular interest [11–16] because of their promising use in modern photonic, optoelectronic, and photovoltaic devices; flash memory devices; as field emission cathodes; etc. that are compatible with the mainstream microelectronics technology. Furthermore, non-stoichiometric SiOx films show great potential for applications in nonvolatile resistive random access memory (RRAM, ReRAM) [17,18]. These advanced devices operate by switching between high and low resistance states when a voltage is applied to a metal-insulator-metal (M-I-M) memory cell. The development of RRAM devices based on SiOx films could potentially lead to the low-cost integration of such cells into chips fabricated using silicon-based complementary CITATION Sopinskyy M, Indutnyi I, Michailovska K, Yukhymchuk V. Tracking the thermostimulated transformations of silicon suboxide film through absorption edge characterization. Characterization and Application of Nanomaterials. 2025; 8(2): 11021. https://doi.org/10.24294/can11021 ARTICLE INFO Received: 23 December 2024 Accepted: 18 March 2025 Available online: 6 May 2025 COPYRIGHT Copyright © 2025 by author(s). Characterization and Application of Nanomaterials is published by EnPress Publisher, LLC. This work is licensed under the Creative Commons Attribution (CC BY) license. https://creativecommons.org/licenses/ by/4.0/ Characterization and Application of Nanomaterials 2025, 8(2), 11021. 2 metal-oxide-semiconductor (CMOS) technology. The possibility of using RRAM devices based on SiOx films as electronic synapses in neuromorphic computing has also been demonstrated [19]. Large variations in the structural topology of SiOx films with low-energy topological defects provide the structural complexity necessary for the implementation of a new, relatively simple, and reliable method for information encryption, storage, and user identification. This method uses a physical key known as the ‘physical unclonable function’ [20]. The results of theoretical calculations performed in [20] indicate that SiOx films with a stoichiometry index of x ~ 1.3–1.5 are the most promising in this regard. A review of the above works allows us to conclude that understanding the structure and properties of silicon suboxide is, on the one hand, a scientific problem of some complexity and, on the other hand, of great practical importance. In this work, the near-edge absorption spectra of vacuum-deposited films of SiO1.3 composition, both as-deposited and annealed at temperatures from 100 C to 1100 C, are studied in more detail using the reflection-transmission technique and analytical processing of the experimental results. We chose the specific temperature range for annealing the samples because structural transformations occur in non-stoichiometric SiOx films at these temperatures. Specifically, detailed studies using XRD, HRTEM, Raman, IR, and ESR spectral techniques [21−24] have shown that during low-temperature annealing, the broken bonds of silicon and oxygen are healed, and the film becomes compacted. When the annealing temperature exceeds 500 C−600 °C, amorphous Si nanoinclusions are formed and then enlarged, with subsequent crystallization occurring at temperatures above 1000 °C. These structural transformations in the SiOx layers lead to the observed changes in their optical properties, which are studied in detail in this paper. 2. Materials and methods SiOx films with a ~0.5 m thickness were simultaneously deposited at a rate of 8 nm/s on ten 10  10  1 mm polished fused quartz substrates heated to Td = 100 C by thermal evaporation of 99.9% pure silicon monoxide SiO powder (Cerac Inc., Milwaukee, WI, USA) in vacuum at a residual pressure of (1–2) × 10−5 Torr. The deposition rate and thickness of the deposited films were controlled using a quartz- crystal microbalance thickness gauge (KIT-1) calibrated with an MII-4 micro- interferometer. The films were annealed for 15 min in a nitrogen atmosphere at temperatures Tan = 100 °C–1100 °C. The thicknesses of the resulting films were measured with an accuracy of 5 nm using a LEF-3-M1 laser ellipsometer. The details of the used methodology can be found in the works [6,25]. The transmission T and reflection R spectra of these samples before and after annealing were measured in the wavelength range  = 400–750 nm (photon energy hν = 1.65–3.10 eV) using an apparatus based on the MDR-23 diffraction monochromator. The stoichiometry index x of the deposited films was evaluated using infrared spectroscopy performed on films deposited in the same vacuum cycle on double- polished silicon wafers. The method for determining x using IR spectral measurements is described in [26]; for our samples, the value x = 1.3 was obtained. Characterization and Application of Nanomaterials 2025, 8(2), 11021. 3 3. Results and discussion Figure 1 illustrates the spectral evolution of the absorptance, A = (1 − R − T), both prior to and following the annealing. There is an increase in A from 0.014−0.12 at the long-wave edge of the measuring range to 0.84−0.92 at its short-wave edge. The difference between A and 1 at the short-wave edge of the range is almost entirely due to the value of R, i.e., there is almost no transmission here. Figure 1 also shows that the growth of Tan in the range 300 °C–500 °C gradually shifts the absorption edge to shorter wavelengths. However, annealing at 600 °C changes the direction of the absorption edge shift to the opposite. The long-wavelength shift in the absorption edge is augmented by annealing at 770 °C and is further enhanced by annealing at 910 °C. Annealing at 1010 °C shifts the absorption edge of the film to the short-wavelength side compared to annealing at 910 °C. Further increase in the annealing temperature to 1100 °C results in a shift of the absorption edge to a position that is in close proximity to that of the as-deposited film. Such a non-monotonic excursion of the absorption band edge indicates the complex nature of the structural changes undergone by the film during annealing. This is also evident in the shape of the absorption curves, which differs significantly for different annealing temperatures. Figure 1. Absorptance spectra of A = 1 − R − T of the as-deposited SiOx film (1) and after 15-minute annealing at 300 (2), 500 (3), 600 (4), 770 (5), 910 (6), 1010 (7), and 1100 °C (8). The absorption coefficient α of the films was determined by the formula [27]: 𝛼 = 1 𝑑 ln 𝑇s(1 − 𝑅) 𝑇 (1) where Ts is the transmittance of the substrate without film. In amorphous, disordered, heterogeneous, nanostructured semiconductor and dielectric materials, there are usually regions of the spectra described by the Urbach dependence [28]: 𝛼(ℎ𝑣) = 𝐴 ∙ exp⁡(ℎ𝑣/𝐸U) (2) 1.8 2.1 2.4 2.7 3.0 0.0 0.2 0.4 0.6 0.8 Photon energy h, eV A b so r p ta n c e A = 1 -R -T 1 2 3 4 5 6 7 8 Characterization and Application of Nanomaterials 2025, 8(2), 11021. 4 The absorption in the region described by Equation (2) is caused by the superposition of transitions from localized states in the valence band tail to non- localized states in the conduction band and transitions from non-localized states in the valence band to localized states in the conduction band tail. In Equation (2), A is a numerical constant and EU (Urbach energy) characterizes the width of the exponential absorption tail. It correlates with the widths of the tails of the localized valence band states γv and conduction band states γc and is mainly determined by the larger of the two, i.e., EU ~ Max(γc, γv) [29]. Figure 2. Spectra of the absorption coefficient α of the as-deposited SiOx film (1) and after 15-minute annealing at 300 (2), 500 (3), 600 (4), 770 (5), 910 (6), 1010 (7), and 1100 °C (8), plotted in semi-logarithmic coordinates. Figure 2 presents the absorption coefficient spectra plotted in semi-logarithmic coordinates. The relative error of the α values is 2%, which is only slightly greater than the thickness of the lines in the figure. As can be seen, the exponential dependence of the absorption coefficient on the photon energy in the as-deposited film and in the one annealed at 300 C and 400 C is quite well fulfilled in a significant part of the measured spectral range, including its high-energy edge. That is, this dependence is still observed at α values up to 8.46 × 104 cm−1 for the as-deposited film, 7.05 × 104 cm−1 after annealing at 300 C. The corresponding ЕU values were 0.42, 0.35 eV. After annealing at 500 C and 600 °C, the Urbach region extends to 2.8 eV, with α ≤ 3.3 × 104 and 5 × 104 cm−1, respectively. EU continues to decrease, reaching 0.26 and 0.23 eV, respectively. After annealing at 770 C the corridor of fulfillment of the Urbach dependence narrows much more significantly both in terms of photon energy (to 2.1 eV) and absorption coefficient (α ≤ 1.0 × 104 cm−1), however, no further decrease in ЕU is observed (ЕU = 0.245 eV). The value of ЕU after annealing at 910 C is almost the same as that after annealing at 770 C, but the straight-line section in semi- logarithmic coordinates extends to 2.4 eV (α ≤ 4.3 × 104 cm−1). The spectral range of the Urbach dependence and the value of the Urbach energy after annealing at 1010 C are similar: hν ≤ 2.48 eV, α ≤ 3.7 × 104 cm−1, ЕU = 0.22 eV. After annealing at 1100 1.8 2.1 2.4 2.7 3.0 10 3 10 4 10 5 Photon energy h, eV A b so rp ti o n c o ef fi c ie n t   c m -1 1 3 4 2 5 7 6 8 Characterization and Application of Nanomaterials 2025, 8(2), 11021. 5 C, the region of fulfillment of the Urbach dependence extends to 2.7 eV, with α ≤ 3.4 × 104 cm−1, ЕU = 0.36 eV. Thus, the absorption behavior in this deposited film, which is an alloy of silicon with oxygen, differs significantly from the behavior of amorphous homogeneous stoichiometric materials. First, in such materials, the Urbach dependence is observed at α  104 cm−1. Secondly, the Urbach energy values themselves are much smaller. For example, in high-quality amorphous silicon films ЕU = 0.042 eV [28], and in pure silica glass ЕU = 0.075 eV [30]. For amorphous and composite materials, whose energy structure is characterized by the presence of localized states in the band gap (and, accordingly, the “blurring” of the valence band top and the conduction band bottom), several methods for determining the band gap width are used in the literature. For SiOx, the formula proposed by Tauc et al. [31] is most widely used: 𝛼(𝐸) = 𝐵 ∙ (ℎ𝑣 − 𝐸𝑜) 2/ℎ𝑣 (3) In Equation (3), B is a numerical constant, and Eo is the Tauc optical band gap. Equation (3) describes the absorption due to indirect allowed transitions from non- localized states in the valence band to non-localized states in the conduction band. This postulates a quadratic dispersion law in both bands and the independence of the matrix element of optical transitions from the photon energy. Figure 3 shows the absorption coefficient spectra plotted in the Tauc coordinates hν − (αhν)1/2. The Tauc optical bandgap Eo has been defined as the intersection of the linear approximation of the values of (αhν)1/2 in the high energy part of the spectrum with the abscissa. Comparing Figures 2 and 3, it is easy to see that the less the exponential dependence region extends into the high energy region, the further the observed Tauc dependence region extends into the low energy region. Although the Urbach energy ЕU is almost halved by annealing, it is still at least three times higher than the Urbach energy in perfect SiO2. This indicates a much greater structural disorder (which may also imply nanoheterogeneity) of these films both before and after annealing compared to the structure of amorphous silica in glassy form. Such large ЕU values and the length of the exponential region up to α  104 cm−1 indicate that the Urbachian edge and interband transitions of different (nano)regions may overlap in these samples. In general, for a heterogeneous medium, the ЕU and Eo values should be considered effective, since the heterogeneous medium differs in its structure from the “classical” amorphous medium. Characterization and Application of Nanomaterials 2025, 8(2), 11021. 6 Figure 3. Spectra of the absorption coefficient α of the as-deposited SiOx film (1) and after annealing for 15 min at 300 (2), 500 (3), 600 (4), 770 (5), 910 (6), 1010 (7), and 1100 °C (8), plotted in Tauc coordinates hν − (αhν)1/2. Given the uncertainty involved in determining Eo, Tauc et al. [31] considered Eo to be an empirical quantity. In view of this, an alternative empirical measure of the optical band gap for amorphous semiconductors has been proposed: the isoabsorption band gap Eα. It corresponds to the photon energy at which the absorption coefficient α is equal to a specific value [32,33]. By analyzing the absorption spectra of a-SiHx films with different degrees of disorder achieved by different degrees of hydrogenation, Cody et al. [34] found a linear anticorrelation between the optical band gap Eo and EU in amorphous silicon: 𝐸o = 𝐸g − 𝐶o ∙ 𝐸U (4) where Eg is the optical band gap limit at zero width of the tails of localized states (mobility gap). From the approximation Equation (4), a value of Eg ~ 2.1 eV was obtained with a constant Co of 6.2. Thus, for defect-free amorphous silicon, the mobility gap is about 1 eV larger than for pure crystalline silicon. Grein and Johnn [35] found that the linear anticorrelation between Eo and EU also occurs for a-As2S3 and a-As2Se3. A linear anticorrelation between Eα and EU was observed for amorphous Ge in [33]. In this case, the approximation used to estimate the mobility band gap Eg was: 𝐸𝛼 = 𝐸g − 𝐶𝛼 ∙ 𝐸U (5) It is expected that linear anticorrelations of the type Equations (4) and (5) will be fulfilled for a wide range of amorphous and disordered semiconductors. 1.8 2.1 2.4 2.7 3.0 0 150 300 450 600 Photon energy h, eV ( h  )1 /2 , cm -1 /2 .e V 1 /2 ) 1 2 3 4 5 6 7 8 Characterization and Application of Nanomaterials 2025, 8(2), 11021. 7 Figure 4. Dependence of the Urbach energy EU, Tauc optical band gap Eo, and E04— the isoabsorption band gap at α = 104 cm−1—on the annealing temperature of the SiO1.3 film in a nitrogen atmosphere for 15 min. Figure 4 shows the temperature dependence of E04, Eo, and EU for the investigated samples. The isoabsorption band gap E04 corresponds to the photon energy at which α = 104 cm−1. The error in determining the obtained values of EU, Eo and E04 in our case was ±0.01 eV. Different temperature regions can be distinguished in these dependences. First, there is a clear anti-correlation between the value of EU, on the one hand, and the values of E04 and Eo, on the other hand, at annealing temperatures Tan ≤ 500 °C. By analogy with the results of the above works, it is logical to assume that changes in the parameters E04, Eo, and EU in the range of annealing temperatures up to 500 °C are due to a decrease in the degree of structural disorder of the film with increasing Tan. The linear approximation of the E04(ЕU) dependence in this annealing temperature range gave the Eg = E04(ЕU = 0) = 3.00 ± 0.16 eV, and the linear approximation of the Eo(ЕU) dependence gave the value Eg = Eo(ЕU = 0) = 2.63 ± 0.13 eV. Based on these results, the mobility gap Eg of amorphous SiO1.3 can be estimated to be Eg = 2.50–3.16 eV. In [36], the value of Eg for SiOx alloys in the bulk glassy state was calculated using density functional theory. These calculations gave a value of Eg = 2.5 eV for x = 1.3. Although this value, as the authors point out, is an underestimate due to the peculiarities of the method, it correlates better with the Eg value obtained using the Tauc optical band gap. With an increase in Tan from 500 °C to 600 °C and further to 910 °C a significant decrease in the values of E04 and Eo is observed compared to their values at Tan = 500 °C. This takes place against the background of a slight decrease in the value of the ЕU. This shows that, at these annealing temperatures, the prevailing processes are fundamentally different from the processes of structural ordering of the SiO1.3 film that take place at Tan ≤ 500 °C. It is known that, at high annealing temperatures, SiOx films undergo disproportionation with the formation of amorphous [21,37–40], and, at Ta ≥ 800 °C−900 °C, amorphous, amorphous-crystalline, and crystalline silicon nanoinclusions [37–40] in the matrix of the SiOy (y > x) composition. In suboxide SiOx 0.20 0.25 0.30 0.35 0.40 0 300 600 900 1200 1.8 2.0 2.2 2.4 2.6 E U E o ( e V ), E 0 4 ( e V ) Annealing temperature T an , o C E 04 E o E U , e V Characterization and Application of Nanomaterials 2025, 8(2), 11021. 8 films with stoichiometry indices x ≤ 1.5, the optical band gap Eo grows very rapidly with increasing x [3]. Thus, the main contribution to the absorption of the nanocomposite ‘Si nanoinclusions—SiOy’ after annealing at 600 °C ≤ Tan ≤ 800 °C−900 °C should be attributed to the amorphous silicon nanoparticles. As can be seen from the graph, they already begin to play a noticeable role at Tan = 600 °C. The decrease in the values of the empirical parameters E04 and Eo with further growth of Tan can, obviously, be interpreted as a manifestation of the size effect in amorphous silicon nanoparticles. The ЕU values for annealing temperatures of 600 °C–910 °C do not demonstrate any pronounced trend depending on the sizes of amorphous inclusions: The ЕU almost does not change, remaining at the level of values slightly lower than after annealing at 500 °C. This distinguishes the behavior of the ‘а-Si nanoinclusions—SiOy’ (y > 1.3) nanocomposites from the behavior of the ‘c-Si nanoinclusions—SiO2’ nanocomposites. The latter are characterized by the growth of the ЕU with decreasing nanocrystallite size due to the increasing influence of the c-Si/SiO2 interface [41]. The values of ЕU = 0.23–0.246 eV obtained by us in this range of annealing temperatures are very close to the values of ЕU obtained in [42] for non-hydrogenated a-Si films with thicknesses of 769–1174 nm, grain sizes of 20–30 nm, and porosities of 0.14– 0.24 (i.e., with natural oxide-coated grains). The ЕU values in these films were 0.230– 0.258 eV, and the Eo values were 1.32–1.38 eV. Almost identical ЕU values and higher Eo values for the nanocomposite films ‘а-Si nanoinclusions—SiOy’ obtained by annealing SiO1.3 films at temperatures of 600 °C−910 °C compared to the non- hydrogenated a-Si films can be explained by the size effect. It causes an increase in the band gap of amorphous silicon inclusions with decreasing size. If we assume that for the non-hydrogenated partially porous oxidized a-Si films there is the anticorrelation (Equation 4) with the same coefficient Co = 6.2, then we obtain a mobility gap Еg ~ 2.8 eV. Assuming the validity of Equation (4) with Co = 6.2 for the SiO1.3 films annealed at 600 °C−910 °C, we obtain estimates of the mobility gap Еg ≈ 3.4–3.75 eV. If we assume that Еg for non-hydrogenated partially porous oxidized a-Si films is the same as for hydrogenated films, i.e., 2.1 eV, and that for them Equation (4) is fulfilled, then the coefficient of such an anticorrelation is Co = 3.0. If we accept this coefficient for the films obtained by annealing SiO1.3 films at 600 °C−910 °C, we obtain an estimate of the mobility gap Еg ≈ 2.61−2.98 eV. In [43], the values of Eo were determined for non-hydrogenated a-Si nanoparticles (nanodots) in a solution of highly purified ethanol. Unfortunately, the authors did not determine the value of ЕU in their samples. Therefore, the obtained values of the optical band gap Eo, which increased from 2.58 eV for particles with an average diameter of 5.15 nm to 3.22 eV for particles with an average diameter of 1.15 nm, are lower estimates of Eg. As can be seen, these estimates of Eg for nanoparticles in ethanol are still in better agreement with the estimates of Eg for nanoparticles in our suboxide matrix at a value of the coefficient Co = 3.0 in Equation (4), which is not surprising. The application of annealing at 1010 °C in comparison to annealing at 910 °C has been observed to result in an increase in E04 and Eo, while EU exhibited a decrease from 0.24 to 0.22 eV. In works [44,45] the nucleation of crystalline inclusions in amorphous a-Si:Н films was studied using the Tauc-Lorentz parametric model. It was Characterization and Application of Nanomaterials 2025, 8(2), 11021. 9 determined that this results in an increase in the optical (Tauc) band gap Eo and a decrease in the parameter Γ (half-width of the Lorentz oscillator function), which serves as a measure of the film disorder. These results provide a rationale for associating the observed increase in Eo and decrease in EU following annealing at 1010 °C with an increase in the ordering of the amorphous nanoparticles’ structure, including the formation of a quasi-crystalline core. Annealing at 1100 °C maintains the tendency of increasing E04, Eo. The value of Eo = 2.11 eV is very close to the value of Eo = 2.1 eV for the SiO1.3 film obtained by molecular beam deposition after its annealing at 1100 °C [46]. After such annealing, the film contains both crystalline and amorphous nanoparticles, and Eo should be considered as the effective (averaged) optical band gap. Following annealing at 1100 °C, a pronounced increase in EU for our film is observed, which is indicative of the presence of silicon nanoparticles exhibiting diverse structural and size characteristics. In their investigation of ‘silicon nanocrystallites—SiO2’ superlattices, the authors [41] identified a direct correlation between the concentration of so-called Pb centers—dangling bonds of silicon atoms at the Si/SiO2 boundary—and the Urbach energy, EU. Given that the appearance of Pb centers was recorded following annealing of the films with a similar composition to ours at 1100 °C [37], it is reasonable to conclude that the formation of these centers also contributes to the observed increase in ЕU. 4. Conclusion Non-stoichiometric SiOx (x ≈ 1.3) films exhibit significant variations in the behavior of the fundamental absorption edge as a function of the annealing temperature. At annealing temperatures Ta ≤ 500 С, a short-wavelength shift of the absorption edge occurs, which is then replaced by a long-wavelength shift in the range 600 С ≤ Ta ≤ 900 С. The short-wavelength shift is accompanied by an increase in the optical band gap Eo and a decrease in the Urbach energy EU, indicating structural ordering of the film. In this case, defect annealing, healing of broken silicon and oxygen bonds, film densification, and a corresponding decrease in defect absorption take place. The long-wavelength shift is due to film disproportionation and the formation of amorphous silicon inclusions, which exhibit significant indirect interband absorption. Eo decreases with increasing amorphous silicon nanoparticle size and their volume fraction in the annealed film, while EU remains almost unchanged. By further increasing the annealing temperature of the SiOx film, the absorption edge shifts to shorter wavelengths again, and both Eo and EU increase. The increase in Eo in this case is due to the increasing fraction of the crystalline silicon phase (c-Si nanocrystals have a larger band gap) and a decrease in the fraction of amorphous silicon, while the increase in EU is attributed to the corresponding increase in the area of the c-Si/SiO2 interface, where interface centers (Pb centers) form. Author contributions: Conceptualization, MS and II; methodology, MS, II and VY; validation, II and KM; investigation, II, VY and MS; writing—original draft preparation, MS and KM; writing—review and editing, MS, II, KM and VY; Characterization and Application of Nanomaterials 2025, 8(2), 11021. 10 visualization, KM; supervision, II. All authors have read and agreed to the published version of the manuscript. Acknowledgments: The authors would like to thank Jeffrey Monastyrsky (USA) for his help with the English edition of this article. Institutional review board statement: Not applicable. Informed consent statement: Not applicable. Dedication: This article is dedicated to the memory of our colleague and friend, Dr Petro Shepeliavyi, who recently passed away. He was a prolific author of many sophisticated technological developments that significantly advanced both fundamental research and the creation of specific devices. 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