Here we report the feasibility of using cheap and environmentally friendly RF-magnetron sputtering of Si3N4 and formation of Si3N4 nanostructured films with developed surface. p-Si(100) polished plates commonly used in photovoltaic structures were chosen as a substrate. Si3N4 film surface morphology was studied using atomic force microscop

Goncharov, I. Yu.

Kolesnikov, D. A.

Kudryavtsev, E. A.

Piluk, E. A.

Zakhvalinskii, V. S.

DSpace at Belgorod State University

RF-Magnetron Sputtering of Si3N4 and Study of Si3N4/p-Si HeterostructuresV. S. Zakhvalinskii1 *, I. Yu. Goncharov1, E. A. Kudryavtsev1, E. A. Piluk1, D. A. Kolesnikov1, V. G. Rodriges1, Z. A. Kabilov1, S. V. Taran1, and A. P. Kuzmenko21 Belgorod National Research University. Belgorod. 308015, Russia 2 South-West Stale University. Kursk, 305040. RussiaHere we report the feasibility of using cheap and environmentally friendly RF-magnetron sputtering of S i3N4 and formation of Si3N4 nanostructured films with developed surface. p-Si(100) polished plates commonly used in photovoltaic structures were chosen as a substrate. Si3N4 film surface morphology was studied using atomic force microscopy. It has been established that the specific surface area of the silicon nitride increases more than 10 times by increasing substrate temperature from Ts =  40 "C to Ts =  800 "C. At the same time, growth of substrate temperature changed the  average size of nano-patterns. Cross section of the heterostructure Si3N4/p-Si(100) was investi­gated by high resolution transmission electron microscopy. Based on electron diffraction studies, it was found that the obtained silicon nitride film  is a mixture of cubic and amorphous phases Si3N4.The Si3N4/p-Si(100) heterostructure is found to be similar to previously studied SiC/p-Si(100).Keywords: RF Magnetron Sputtering, Si3N4 Thin Film, Nano-Patterns, Solar Cells.1. INTRODUCTIONSilicon nitride (Si3N4) is one of the main dielectrics that are used in silicon electronics. As compared with silicon oxide (e &  3 ,9 ) nitride has a higher permittivity (e  я» 7) and is used as an insulating layer. Besides, Si3N4 possesses the ability to localize (capture traps) injected into electrons and holes with a giant lifetime in a localized state, more than 10 years at 85 41. Memory effect in silicon nitride is used in the non-volatile devices (Hash memory).1'2 l;or the silicon devices most commonly used amorphous sili­con nitride. In photovoltaic structures amorphous silicon nitride is used as an anti-reflection layer with excellent surface and hulk passivation properties.3«Silicon nitride Si3N4 are yellowish crystals; polycrys­talline SijN , color varies from white to gray. It is known in three modifications: a  and ft are of hexagonal syn- gony, for a-S i3N4 а  =  0.7765 nm, c =  0.5622 nut, space group 1*3 lc; lor £-SUN4: а =  0.7606 nm, (' =  0.2909 nm, space group P63/m: «-Si3N4 turns into /3 above 1400 41, /3-SijN4 stable up to ~16(X) 41. (lubic у  modification usu­ally formed at high pressures, for y-Si,N 4 а =  7.7418 nm, space group l ’d-3m. Si3N4 bandgap is 4.0 cV. Silicon nitride does not react with nitric acid, sulfuric acid andhydrochloric acid, slightly react with II.,P04 and inten­sively with hydrofluoric acid. Oxidation Si,N4 in air starts above 900 4 !.4 Unique chemical, mechanical, electrical and optical properties of silicon nitride make it promising material for electronics, including phol.ovoll.aics.Layers of Si3N4 or related silicon nitride solid solu­tions are used hi photovoltaic devices for the surface passivation5 and as anti-reficclivc layers.6 lllcctrical passi­vation is used to reduce the surface charge carriers' recom­bination in silicon wafers or films. Use of silicon nitride trap leads to a sharp reduction of carriers’ concentration in the absorbing layer of the silicone wafer and decreases the probability of its recombination. As an antireflection layer Si3N4 increases light absorption.3Particularly influential passivation effects should be pro­nounced on thin-film solar cells due to the fact that its surface is close lo the space charge region of the p -n  junc­tion where charge separation occurs.5In complex p - i - n  solar cells there were used layers of amorphous Si3N4 or related silicon nitrides SiN, and solid solutions based on Si3N4_r , which were deposited on the surface of the absorbing layer of Si emitter. In some cases, passivation layers were deposited before applying a back electrode.3' 5 There arc approved perfomance improvement of industrial photovoltaic cells after Si3N4 application pas­sivation layers of hydrogenated amorphous silicon nitridea-SiNt :H using plasma-cnhanccd chemical vapor depo­sition (PEC'VI)). In many eases the addition of small amounts of hydrogen, Ihai. is ollcn used in the passivation layers in solar cells did not significantly affect the crystal structure and electronic properties of silicon nitride.9For preparation of amorphous hydrogenised films there are knmvn varieties of chemical vapor deposition technol­ogy (CVD): hot wire CVI) (HWCVD), hot filament CVl) (1IFCVD) or catalytic CVD (Cat-CVD),10 CVD tech­nologies are environmentally harmful because of the loxic chemicals used. Environmentally friendly alternatives are high frequency magnetron sputtering (R l’-magnetron sput­tering) or laser ablation (LA).Moreover, in order to reduce the cost of solar panels it is reasonable to use the simplest single-junction structures.7 Efficiency of solar cells could be improved by increased absorption of the upper layer of the ltcterosl.ruei.ure pro­vided by SijN4 films. The high frequency magnetron sput­tering allows obtaining a homogeneous, developed surface(a)(b)Fig. I. Findings tif invesLigalinn tif Lhin foil cross section of heL- crostructiirc Si3M4/Sif 100) in transmission electron microscope fTLiM), fa) Interface overall view of hcterostriicture SijN4/Si(i00), (b) Composite morphology nf Si3N1 111 in cross section.of the absorbing layer. Regime of sputtering can control surface morphology and device structure,In this work we report the obtaining of Si3N4/Si(100) layer with a developed surface and its properties as a func­tion of preparation procedure.2. EXPERIMENTAL DETAILS2.1. Preparation of Si3N4 NanolayersThin films of partially amorphous nanocryslallinc Si,N4 were prepared by the high-lrequency non-rcaelivc mag­netron sputtering in an Ar atmosphere using an improved Ukrrospribor VN-2000 setup. A previously synthesized sil­icon nitride was used as a target. Deposition was carried out on a polished (100) plane of />Si with a resistiv­ity of 2 Ohm -cm . Optical heating allowed to control the substrate temperature in the range from 40 to 800 C  It has been found that on unintentional substrate heating(а)film Si3l\l4 (a + цс)<3 1 1> cubic(4 0 0)(b)substrate Si (100)Mg. 2. likctrou diffraction from Si.N4/Si(100) hcterosmicturc obtained in transmission electron microscope ( 1 Щ . (a) Diffusional rings around гс II с с Li on dilTusional center indicate dominant amorphous nature ot the film, availability of ill-defined concentric rings indicates minor phase оГ nanocrvstallic or line-grained nature, (b) Гог comparison there is shown electron diffraction image obtained for Si(lOO) monocrystallic substrate in the same heterostruclure.40 "С300 "С500 "СA1Uо о800 "ГИй- 3. Л1;М results for Si;N,| film surfaces obtained during high frequency magnetron deposition with temperatures of SKI00) substrate of 300, 500  И 800 9 С. respectively.Z. nml)’ig. 4. Involution of altitude assignment function AfZI of thin films Si 800 C.its temperature was about 40 ' ( '  due to interaction with the plasma. The layer of silicon oxide was removed from Si substrate by chemical etching in HI’ before the Si,N4 film deposition. Si,N4 nanolayers with the thickness of about 20 nm have been deposited at 40. 300. 500 and 800 CC.Si ,n4Height0 200 400 600 800 1000X Axis, nmZ. nmN, obtained for temperatures Si substrate =  40 4.'. 300 500 С and2.2. Characterization of Si,N4 Nanolayers and Si3N4/Si(100) HeterostructureThe vibration structure of deposited layers was charac­terized by Raman spectroscopy using co-local nanomcl- ric resolution Omega Scope A1ST-NT Raman microscope excited with an 532 nm Ar~ laser. The Raman spectra[S = 40"CGrains0 200 400 600 800 1000X Axis, nmFig. 5. On the loft part of the figure it is shown the Al'M image of a Si;N4 film obtained during high-frequency magnetron deposition on the surface under tempera Lure = 4 0  C, On I lie right parL of figure it is shown Lhe image o f  surface, ready for sLali.slical Lrealmenl procedure.AreaunI я£ s2 3nm'nm, 103Average Size(c)- я1  яЧ зJSp* u>• LMaxZ1 2 3 4 5 6 7 *nm,Г)яв» “<§яжI  s  !**» Q10 20 30 40 50 60 70nmMeanZ1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5nm1'lg. 6. Diagrams that shows the calculation of S- ami 3D-parameters describing structure o f the .S i,» , (jfj = 4 0  Ч.Т) him  in the three-dimensional space.show a dominant band at 982 cm in die spectral region characteristic for Si,N4.nHie layer surface morphology was studied by the con­tact mode atomic force microscopy (AFM) on Ntegra Aura (NT-MDT, Russia), using cantilevers CSC 11 series in a controlled atmosphere or low vacuum. The Si,N4 lilm lliickness was esl.imal.cd by the step height at the edge of the tilm.The interlace between Si3N4 lilm and silicon substrate was investigated on cross section foils of the hctcrostruc- turc SijN4/Si(100) using the images and electron diffrac­tion patterns obtained on a Jeol JEM 2100 transmission electron microscope.3. RESULTS AND DISCUSSIONinvestigation of the cross section o f Si3N4/Si( 100) hel- crosl.ruci.urc obtained at a substrate temperature Vj = 4 0  С reveal a distinct boundary between the Si-N4 layer andSi(lOO) (Fig, 1). From the Figure 1(b) you can see a little ~1 nm texture that looks like parallel rows in the Si,N4 film directed perpendicularly to the Si substrate. Figure 2 show's the results of electron beam diffraction in a trans­mission electron microscope. Diffusion regions around the central diffuse reflex are typical for tire amorphous mate­rial and tire presence of ill-defined concentric rings indi­cating the presence of secondary nanocrysiaiiinc phase (Mg. 2(a)). Hence, the Si-,N4 film is in a mixed amorphous and nanocrysiaiiinc state ( a  +  jur). Calculation of inter- planar distances, which were made on the basis of elec­tron diffraction, suggests that Si3N4 nanocrvstals belongs to cubic system, space group Fd-3m. On the Figure 2(b) there is a d ea r picture of symmetrical reflexes spots cor­responding to the single-crystal substrate Si(100).Surface morphology of Si3N4 films obtained al. substrate temperature r s =  40, 3(X). 500 and 800 ‘C was studied using AFM (Fig. 3). Distribution function of profile heightTiihlc T. Statistical analysts and processing of AFM-imaaes I nr S i,N , films obtained aL a Si(IOO) substrate al llie temperature T% = 4 0  C.AreaAveragesi/e T.ength Mean width Aspect ratio Volume Range 7, Max 7, Min 7, Mean 7. T axial mean 7, Perimeter Diametert"nil nin*nm nm nm Nm nm * nm * nm nm nm nm nm nrn nm nmAverage 192? .081 ST) 1204.08741.27614.87558.02617.81129.67412.5472.1350.669345i.2?33152.8732.7371.4154.2601.4181.5230.3622.9770.7021.453 171.599 0.664 64.20546.57616.785Volume12 14 ISnm‘nm‘nm, 103Table U. Statistical treatment of Al'M-imagcs of SijN4 film obtained at die Si( 100) substrate with different temperatures Ts =  40 C. 300 C. 500 ‘ C ami 800 ‘Г..Parameters 40 С 300 сA500 e 800 V•Sz. Maximum peak height, nm 9.1 22.3 43.6 36.3S,. Average roughness, nm 1.1 2.2 4.9 3.9,Sq, RooL mean square (RMS), nm 1.3 2.8 6.3 5.1Average diameter, urn 46.5 39.3 60.6 38.8Л’(». Surface area ratio. 4i 0.8 4.1 7.6 8.9A(Z) was used for the mathematical description o f the sur­face topogr aphy. This function gives the probability that a randomly selected point has a height Z. Figure 4 shows the evolution of the distribution function of altitude profile coating Si,N4 at substrate temperatures of 40, 300, 500 and 800 cC.Statistical processing of ihe A l’M images was per­formed using the software package "Image Analysis P9“ (NT-MDT). The software package allows to calculate S- parameters (a surface area) or 3D-parameters,'4 charac­terizing die structure in three dimensions, lixamplc and results of statistical processing of the obtained SisN4 film at a substrate temperature 7S =  40 4! arc presented in Figures 5 and 6 and Table I. A  similar statistical analysis and processing of AFM-images was performed for Si,N4 films obtained at higher substrate temperatures.From the Al'M image analysis it was found lhai Ihe increment of the surface area (Sdr) increased from 0.8%  at=  40 С lo 4.1, 7.6 and S.9%' at 300, 500 and 800 С. correspondingly (Table 11). However, the average grain diameter 60 nm is maximal for the temperature of 500 :C. The minimum diameter of the grains is about 40  nm, which have die layers deposited at 300 and 800 41. float­ings at a substrate temperature of 40 °C have the grain diameter of 45 nm. The maximum value o f 6 nm of the mean-square roughness (5q) was found for a substrate tem­perature o f 500 41. The minimum Sq value of 1 nm was obtained for the Si-N4 film deposited at 40 41.4. CONCLUSIONIn diis article, the dependence of the surface morphol­ogy of mixed (a  +  nc) amorphous and nanocrystalline silicon nitride films SisN4 depend on temperature were investigated by atomic force microscopy method. It was found that the growth substrate temperature up to '/;s =  800 41 increases the surface area of the film (mean-squareroughness (.Sq)) is more than 10 limes. In ihe case of sil­icon nitride films as absorbing and antireflection layers in photovoltaic structures very large impact on the effi­ciency o f solar energy conver sion has both area and surface morphology of the outer layer. Structure of cross section (u  +  nc) of die Si,NVp-Si(l(X)) heterostructure was inves­tigated by transmission electron microscopy. It has been found that the Si,N 4 film has a mixed character (amor­phous and nanocrysialline phase). On the basis o f electron diffraction studies it was found that the SisN4 nanocryslals belonged lo cubic system, tl was found that a cross section of Si,N 4 film was complex texture, composed of layers of about 1 nm, located perpendicular to die plane o f the Si substrate and larger objects up to tens o f nanometers, which is consistent with the results of AFM microscopy.Acknowledgments: Research was accomplished with Ihe support of Ministry of Hducalion and Science of Ihe Russian Federation, Reference number 367.References and Notes1. Г.-Н. L ee , К.-Г. Pilrk. and X . K im . Appl. Pkys. fjHt. 87. 0 7 3 5 )0  (2005)2. K . A. N asyrov, S . S- Shajm eev. V. A. Gricenko. G . X . Han, S . V. K im . ami С .  V, L i. X h E lT  129. 926  (2006).3. IX-II. Neuhaus anti A . Mitnzcr. Advances in Opto Electronics, in special issue Volume, Article ID 24521, 15 pages (2007).4. V. A . Gritsenko, Physics - Uspekhi 55. 408 (2012).5. J .  L. Cruz-Campa. ct al.. Sol. Energy Mater. Sol. Cells 95. 551 (2011).6. A. T jennie, H . Abdullah. S . Shaari. and K . Sopian. American Journal of Applied Sciences 6, 2Й43 (2009).7. V. Zakhvalinskii. 1!. Filink, L  Goncharov, Л . V. Sim ashkevicli. D . A . Sherban. L . 1. Brue. N . C urm ci. аш.1 M . R usu, p-Si/rt-SiC  nanolayer photovoltaic cell. Proceedings of ihe 28lh Fat rope an Pholmoilaic Solar Energy Conference, Paris. France, SepLember—OeLnber (2013).8. J .  Schmidt, J . D. M oschncr. J . llcn zc . S . D auw e, ami K. Ilczcl. Recent progress in the surface passivation o f  silicon  solar1 cells using silicon nitride, IQlh F.umpeun Photovoltaic Solar Energy Coher­ence. Paris. France. .Tune (2004).9. L . L .  Hintzschc. С . M , Lang. T, Watts. M . M arsm an. Ci. Jordan. M . W. P. l i .  Larners. A . W. Wccber. and Ci. K resse , Physical Renew В  86. 235204 (2012).10. Y. Taw ada and H. Yam agishi. Sol. Energy Staler Sal. Cells 66, 95 (2001).11. 11. Rcch. T. R o sd ie k . J .  Muller. S . Wicder. and II. Wagner. .Sol. Energy Staler. Sol. Cells 66. 267 (2001).12. Y. Hamakaw a. Journal de Pfrysicijue 42(Suppl 10), C 4-1131. (1981).13. R . D . M undo, R . d 'A gostino. I t  Fracassi. and It  Palumbo. Plasma Process. Polym. 2. 612  (2005).14. I ,.  Blunt and X. Jiang, Advanced Techniques for A ssessm ent Sur- Гасе Topography: Developm ent 0 Г a  B a sis  for 3D  Surface Texture Standards "Surfstan d ," K ogan  Page Science, London (2003).

RF-Magnetron sputtering of Si3N4 and study of Si3N4/p-Si heterostructures

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RF-Magnetron sputtering of Si3N4 and study of Si3N4/p-Si heterostructures

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