In this work, we studied comparative characteristics of the SiO₂/SiC

heterostructures. The following two techniques were used for SiO₂ formation: thermal

oxidation in water vapor (i) and oxidation in solution (ii). According to experimental

results obtained from optical absorption and photoluminescence spectra as well as from

measurements of internal mechanical stresses, one can conclude that the thin SiO₂ films

prepared using the technique (ii) possess SiO₂/SiC interface with a less number of

defective states than that for SiO₂ films prepared using the technique (i)

Bacherikov, Yu.Yu.

Boltovets, N.S.

Kolyadina, E.Yu.

Konakova, R.V.

Ledn’ova, T.M.

Okhrimenko, O.B.

Semiconductor Physics Quantum Electronics & Optoelectronics

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

 Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 1. P. 13-16.  PACS 07.10.Lw, 81.10.Bk, Dn, 77.84.Bw, 78.40, 78.55, 81.16.Pr Interface features of SiO /SiC2  heterostructures according to methods for producing the SiO  thin films2Yu.Yu. Bacherikov1, N.S. Boltovets2, R.V. Konakova1, E.Yu. Kolyadina1,  T.M. Ledn’ova2, O.B. Okhrimenko11V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine 41, prospect Nauky, 03028 Kyiv, Ukraine Phone: (380-44)525-61-82; e-mail: olga@isp.kiev.ua  2State Enterprise Research Institute “Orion”, 8a, Eugene Pottier str., 03057 Kyiv, Ukraine  Abstract. In this work, we studied comparative characteristics of the SiO2/SiC heterostructures. The following two techniques were used for SiO2 formation: thermal oxidation in water vapor (i) and oxidation in solution (ii). According to experimental results obtained from optical absorption and photoluminescence spectra as well as from measurements of internal mechanical stresses, one can conclude that the thin SiO2 films prepared using the technique (ii) possess SiO2/SiC interface with a less number of defective states than that for SiO2 films prepared using the technique (i).    Keywords: SiO2/SiC, thermal oxidation, oxidation in solution, optical absorption, photoluminescence, internal mechanical stresses.  Manuscript received 27.07.11; revised manuscript received 16.11.11; accepted for publication 26.01.12; published online 29.02.12.          1. Introduction  Development of new methods for preparing the structured thin films on silicon carbide compatible with integrated-microcircuit technology offers the challenge for making new devices and equipment with improved parameters. For instance, decrease of the gate insulator thickness provides in CMOS GSI speedup [1, 2]. One of the most preferable insulators for instrumental structures based on silicon carbide is SiO2 due to its dielectric properties. This choice isn’t due to only its insulators properties, but SiO2 can be grown using thermal oxidizing techniques which are compatible with microelectronics technology [3-7]. The main problem of such technology still is preparation of a high-quality interface SiO2/SiC with a minimal quantity of impurities. Though dielectric properties of the SiO2 layer grown on the silicon carbide are similar to SiO2 on Si, but SiO2/SiC interface has different from SiO2/Si one-electronic properties. These properties are the result of interface impurities formed by high-temperature oxidation. Silicon-carbide-based device structures are intended for operation at higher temperatures, higher dissipated power, and these structures possess higher radiation resistance than that of silicon-based device structures [2, 8]. In this relation, search for further ways of the SiO2/SiC interface enhancement seems to be a topical problem. The defect concentration in this interface depends on heating temperature, duration of thermal oxidation, SiC substrates quality and also on growth conditions [5, 7]. 2. Sample preparation and investigation technique In this work, we studied thin SiO2 films based on n-type silicon carbide substrate (6H-SiC polytype) grown using the Lely technique. The free electron concentration was ~ . SiO318 cm10 − 2 films were formed on the Si side of the silicon carbide substrate. The following two techniques were used: thermal oxidation in water vapor for 5 hours at the temperature T = 1150 °C and oxidation in solution where films from film-forming solution were deposited on silicon carbide substrate at room temperature in  © 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine  13  Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 1. P. 13-16.  centrifuge. The solution is composed of butyl alcohol (C4H9OH), ethyl alcohol (C2H5OH), solution of hydrochloric acid (HCl), tetraethoxysilane ((C2H5O)4Si) in the ratio C4H9OH : C2H5OH : HCl : ((C2H5O)4Si) = 5:1.5:1:2.5. Then SiO2/SiC structures were annealed in air for 3 min at T = 800 °C. The thickness of SiO2 layer reached the values dSiO2 ≈ 70 nm (i) and dSiO2 ≈ 170 nm (ii). We studied the optical absorption and photoluminescence (PL) spectra of SiO2/SiC structures, measured also was the value of internal mechanical stresses (IMS) in these films. 3. Experimental results and discussion The absorption spectra for both types of heterostructures SiO2/SiC practically did not differ in the 400 to 800 nm wavelength range (Fig. 1). The absorption band observed at 630 nm is typical to absorption spectra of silicon carbide doped with nitrogen [9-12]. Since thin SiO2 film was transparent in the observable wavelength range, absorption in the structure was the result of silicon carbide substrate and SiO2/SiC interface [13, 14], so differences in absorption spectra were not observed. According to the literature data [9-12], the wide band at 630 nm (Fig. 1) is related to the ground state of donor centers resulted from presence of nitrogen impurities in silicon carbide crystals. This band is observed on wide general background and is in coincidence with [12]. Existence of background absorption is explained by partial overlapping of the wide band (maximum at 630 nm) and two absorption bands: the more intense and wide boundary band and short-wave tail of the infrared absorption band. The bands mentioned above are determined by photoionization of the nitrogen with electron transition to the minimum of conduction band.   4 0 0 5 0 0 6 0 0 7 0 0 8 0 001 02 03 04 05 06 07 0k, cm-1nm 1 2 Fig. 1. Absorption spectra of the SiO2/SiC heterostructures: 1 – SiO2 layer was deposited using the thermal oxidation technique, 2 – SiO2 layer was deposited using oxidation in solution technique. The PL spectra of the SiO2/SiC heterostructures (PL was excited from the side of SiO2 film) have been shown in Fig. 2. As can be seen from this figure, the weak PL band is observed in 450…500 nm region of PL spectra. Since SiO2 films are transparent in the observable wavelength range and the penetration depth of exciting radiation (λexc = 370 nm) for n-SiC single crystals are approximately 10 μm (silicon carbide substrate thickness ~ 460 μm), it can be considered that the PL spectrum is mainly related with the contribution of silicon carbide at the SiO2/SiC interface. Consequently, particular changes in the PL spectra of the whole structure is primarily caused by changes in the properties of the oxide film and silicon carbide at the interface SiO /SiC,2  as well as by localization of substrate structural defects at this interface and the presence of IMS. The appearance of silicon carbide additional bands in the PL spectra within the range 400 to 500 nm is associated in literature with the presence of luminescence centers related to intrinsic defects or breach of stoichiometric silicon carbide crystals [15-17]. There is also evidence that centers providing contribution to the short-wave PL band (λmax ≈ 500 nm) can be considered as point-defect complexes [15-17]. A slight change in the peak position of PL band for the samples with different ways of forming the thin SiO2 film is the result of redistribution of defects localized at the interface SiO /SiC,2  apparently due to different values of the IMS at the interface SiO /SiC2  and depends on the method of film preparation on the substrate, which correlates with data from the heterostructure curvature (see Table). Localization of the defects caused by the presence of IMS at the interface SiO /SiC2  can be judged by the changes observed only in the PL spectrum that is mainly characterized by the state of the sample surface and the interface SiO /SiC. At the same2  time, the value of the band gap determined from optical absorption spectra that characterize the entire sample bulk remains unchanged.The value of IMS for double-layer SiO2/SiC structures can be calculated by Stoney’s formula: ( ) 1216 dRdE⋅⋅−⋅⋅= νσ ,                         (1) where Е and ν are Young modulus and Poisson’s ratio of the substrate, d is the substrate thickness, d1 – film thickness, R – radius of heterostructure curvature. The radius of curvature was measured using the profilometer 201P −  and calculated with the formulalmR82= ,                             (2) where l is bending deflection of the film-substrate structure, m – chord connecting the ends of the arc of a circle. In the film, compressive stresses will be, if it is on the convex side of the substrate, and expanding, if the film is on the concave side heterosystem. Results of experimental studies are presented in Table. © 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine  14  Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 1. P. 13-16.  Table. Comparative characteristics of the SiO2 films obtained using different methods (rav – average value of the deflection radius; T – oxidation temperature; t – oxidation time).  Oxidation regime Deposition method d1, nm Sign of deflection l, mm m, mm rav, m σ, GPa T, °C t, min Thermal oxidation 70 – 79 3 40.63 7.79 1150 300 Oxidation in solution 170 – 86 2.5 30.82 4.41 800 3  400 500 600 7005010015020025030035021PL intensity, arb.un.Wavelength, nm   Fig. 2. SiO2/SiC photoluminescence spectrum. 1 – SiO2 layer was deposited using the thermal oxidation technique, 2 – SiO2 layer was deposited using oxidation in solution technique.  According to these data, SiO2 film is on convex side of the substrate. The value of compressive stresses in the sample obtained using the technique (i) is σ = 7.79·109 N/m2, which correlates with the data [14] for IMS values of the samples obtained by conventional thermal oxidation, and for the sample obtained using the technique (ii) σ = 4.41·109 N/m2. In addition, the stresses in the film caused by the difference of thermal expansion coefficients can be estimated using the formula [18] TE Δ⋅αΔν−=σ αΔ 1 ,                     (3) where Е and ν are Young modulus and Poisson’s ratio of the film, Δα is the difference in thermal expansion coefficients of the epitaxial film and substrate, ΔT – temperature difference between the temperature of the film grown at room temperature.As seen from (3), in this case the ratio of the stresses arising in SiO2 films due to the difference between the thermal expansion coefficients of films grown using various techniques, will be determined by the ratio σΔα1/σΔα2 = ΔT1/ ΔT2 ≈ 1.5 where the index 1 corresponds to the technique (i), and 2 – to the technique (ii). Thereof, it follows that the film SiO2 obtained by thermal oxidation is more stressful.Thus, the experimental results show that thin SiO2 films obtained by oxidation in solution allows one to create the interface SiO /SiC2  with fewer defect states than the film SiO2 obtained by thermal oxidation. References 1. Kuan Yew Cheong, Wook Bahng, Nam-Kyun Kim, Analysis of charge conduction mechanisms in nitrided SiO2 film on 4H SiC // Phys. Lett. A, 372(4), pp. 529-532 (2008). 2. O.A. Ageev, A.E. Beliaev, N.S. Boltovets et al., Silicon Carbide: Technology, Properties, Applications. ISMA, Kharkiv, 2010 (in Russian). 3. M. Schürmann, S. Dreiner, U. Berges et al., Ultrathin SiO2-films on 4H-SiC(0001) studied by angle-scanned photoelectron diffraction // J. Electron Spectroscopy and Related Phenomena, 156-158, pp. 119-123 (2007). 4. S.H. Choi, D. Wang, J.R. Williams et al., Nitridation of the SiO2/4H–SiC interface studied by surface-enhanced Raman spectroscopy // Appl. Surf. Sci., 253(12), p. 5411-5414. 5. C. Virojanadara, L.I. Johansson, Photoemission study of Si-rich 4H-SiC surfaces and initial SiO2 /SiC interface formation // Phys. Rev. B, 71(19), p. 195335-195345 (2005). 6. C. Virojanadara, L.I. Johansson, Oxidation studies of 4H–SiC(0001) and (000 1 ) // Surf. Sci., 505, p. 358-366 (2002). 7. Ya.Yu. Guseinov, A.M. Svetlichnyi, V.V. Poliakov, A.N. Kocherov, Photostimulated Processes of Oxidation of Silicon Carbide Oxidation. Mutardzhim, Baku-Taganrog, 2005 (in Russian). 8. Sokrates T. Pantelides, Sanwu Wang, A. Franceschetti et al., Si/SiO2 and SiC/SiO2 interfaces for MOSFETs – challenges and advances // Mater. Sci. Forum, 527-529, p. 935-948 (2006). 9. G.B. Dubrovskyi, E.I. Radovanova, Optical absorption within the range around 0.6 μm and the structure of α(6H)-SiC conduction band // Fizika tverdogo tela, 11(3), p. 680-684 (1969), in Russian. 10. I.S. Gorban’, V.P. Zavada, A.S. Skirda, Photoionization spectra of impurity centers in α- © 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine  15  Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 1. P. 13-16.  SiC(6H) under high temperatures // Fizika tverdogo tela, 14(10), p. 3095-3097 (1972), in Russian. 11. I.S. Gorban’, A.S. Skirda, Photoionization spectra of impurity centers in α-SiC(6H) under high temperatures // Ukrainskii fizich. zhurnal, 26(2), p. 228-232 (1981), in Russian. 12. I.S. Gorban’, A.P. Krokhmal’, Impurity optical absorption and the structure of conduction band in 6H-SiC // Fizika tekhnika poluprovodnikov, 35(11), p. 1299-1305 (2001), in Russian. 13. Yu.Yu. Bacherikov, R.V. Konakova, A.N. Kocherov, P.M. Litvin, O.S. Litvin, O.B. Okhrimenko, A.M. Svetlichnyi, Influence of super-high frequency annealing on the structures silicon dioxide – silicon carbide // Zhurnal tekhnich. fiziki, 73(5) p. 75-78 (2003), in Russian. 14. Yu.Yu. Bacherikov, R.V. Konakova, E.Yu. Kolya-dina, A.N. Kocherov, O.B. Okhrimenko, A.M. Svetlichnyi, Effect of microwave radiation on optical transmission spectra in SiO2/SiC structures // Semiconductor Physics, Quantum Electronics & Optoelectronics, 5(4), p. 391-394 (2002). 15. V.V. Makarov, Luminescent and optical properties of silicon carbide irradiated with fast neutrons // Fizika tverdogo tela, 13(8), p. 2357-2362 (1971), in Russian. 16. V.I. Levin, Yu.M. Tairov, V.F. Tsvetkov, Silicon carbide luminescence and its relation with variances of stoichiometry // Fizika tekhnika poluprovodnikov, 18(7), p. 1194-1198 (1984), in Russian. 17. A.O. Konstantinov, A. Herny, C.I. Harris, E. Janzen, Photoluminescence studies of porous silicon carbide // Appl. Phys. Lett., 66(17), p. 2250-2252 (1995). 18. I.I. Vyrovets, V.I. Gritsyna, S.F. Dudnik, O.A. Opalev, Ye.N. Reshetniak, V.Ye. Strel’nitskyi, X-ray studies of the structure and stress state of diamond coatings prepared in glowing discharge // Voprosy atomnoi nauki i tekhniki. Ser.: Vacuum, pure materials, superconductors, 17(1), p. 142-146 (2008), in Russian.  © 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine  16 

Interface features of SiO₂/SiC heterostructures according to methods for producing the SiO₂ thin films

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Interface features of SiO₂/SiC heterostructures according to methods for producing the SiO₂ thin films

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)