In this work the experimental observation of passivation of silicon surface by ultrathin dielectric film in Al/Si/nematic/Indium Tin Oxide(ITO) structure performed on the base of low resistive (~ several Ωxcm) n-type conductivity silicon sample is presented. It is shown that when the DC voltage with polarity +U on silicon is applied to the structure a passivation effect of silicon surface takes place. It is found that a silicon surface passivated by dielectric film changes an initial alignment of liquid crystal 5CB. In the structure with passivating silicon surface a frequency range of structure photosensitivity extends due to limitation of current leakage through Si/liquid crystal interface.

Keywords: nematic, silicon, passivation, alignment, photosensitivity, light modulator

Gritsenko, M.I.

Kucheev, S.I.

Lytvyn, P.M.

Semiconductor Physics Quantum Electronics & Optoelectronics

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

Semiconductor Physics, Quantum Electronics & Optoelectronics. 2004. V. 7, N 2. P. 154-156.' 2004, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine154PACS: 81.65.RvPassivation of silicon surface by ultrathin dielectric filmin M/Si/nematic/ITO structuresM.I. Gritsenko, S.I. Kucheev, P.M. Lytvyn*Chernigov Pedagogical University, 53, G.Polubotka str., 14038 Chernigov, Ukraine*V. Lashkaryov Institute of Semiconductor Physics, prospect Nauky 45, 03028 Kyiv, UkraineE-mail: kucheev@cn.relc.comAbstract. In this work, the experimental observation of passivation of silicon surface byultrathin dielectric film in Al/Si/nematic/Indium Tin Oxide(ITO) structure performed on thebase of low resistive (~ several Ω×cm) n-type conductivity silicon sample is presented. It isshown that when the DC voltage with polarity +U on silicon is applied to the structure apassivation effect of silicon surface takes place. It is found that a silicon surface passivated bydielectric film changes an initial alignment of liquid crystal 5CB. In the structure withpassivating silicon surface, a frequency range of structure photosensitivity extends due tolimitation of current leakage through Si/liquid crystal interface.Keywords: nematic, silicon, passivation, alignment, photosensitivity, light modulator.Paper received 15.12.03; accepted for publication 17.06.04.1. IntroductionMetal/Si/SiO2/nematic/transparent electrode structures(M/Si/SiO2/nematic/ITO) are used in a broad range ofelectrooptical devices including optically addressed spacelight modulators (OA SLM) [14]. A functional opera-tion of these structure is based on a silicon surface deple-tion effect [5] that is caused by means of external electricfield of ITO electrode. SiO2 oxide film having thicknessup to 0.11 µm possesses a well insulating properties andfrequently playing a role of dielectric light mirror. Suchdielectric films fully exclude a charge transferring froma field electrode into silicon sample. A controlled chargeembedded into oxide and repeatability of properties ofSiSiO2 interface ensure a high stability of such devices [6].However, the structures containing no SiO2 dielec-tric film, i.e. M/Si/nematic/ITO structures, possess moreextended functional possibilities because, in this case, asurface conductivity of silicon can be additionally modu-lated: 1) by electric field of ion cloud, which can be local-ized very close to silicon surface and 2) by means of cur-rent leakage through Si/LC interface. Such structuresbeing exploited as OA SLMs, for example, can separatemoving and stop images [7]. Such type of structures al-lows to easily realize an ion quasi electrode concep-tion when the locally accumulated charge within liquidcrystal layer plays a role of the field electrode [7]. Usinga field effect in silicon induced by ion charges [8], there isa possibility to study the ion processes taking place inliquid crystals.Meantime, the charges flowing through Si/LC inter-face can cause the modification of silicon surface. One ofthe most probable process could be gone on silicon sur-face under the action of DC (as well AC) voltage is apassivation of silicon surface by dielectric film.The aim of the present work was to reveal a passivationeffect on silicon surface in a prototype of liquid crystaldevice under the action of DC voltage. In the work, se-veral independent methods were used to confirm a pas-sivation effect.2. Experimental and resultsThe structure performed on the base of n-type conductiv-ity monocrystalline silicon (see Fig.1) and specific resis-tivity of 4.5 Ω×cm was used in order to reveal the pas-sivation effect of silicon surface in M/Si/nematic/ITOstructures. The upper electrodes were performed as ITOstripes. Before assembling the structure the silicon sur-face was etched in HF solution and then rinsed in distil-late water. Liquid crystal 5CB layer thickness was 20 µm.After this the DC voltage equal to 4.4 V with polarity +UM.I. Gritsenko et al.: Passivation of silicon surface by ultrathin dielectric film ...155SQO, 7(2), 2004on silicon substrate was applied to structure for 5 minu-tes. The current density was equal to ~ 1 µA/cm2. Thenthe structure was disassembled, and the silicon surface iscleaned only by organic dissolvent: dimetilformamideand hexane (so SiO2 was not etched).The investigation of silicon surface with optical mi-croscope showed that both A and B silicon surfacesthat lie opposite the stripes (Fig.1) are optically identi-cal. (A and B surfaces lie opposite the stripes onwhich the voltage U was applied and not applied, respec-tively).The A and B surfaces are well detected by meansof a water dew condensation and evaporation. From Asurface a dew evaporates slower than from B surface,what is well seen by the naked eye. This means that theroughness of A surface is large than roughness of Bsurface. Taking into account that A and B surfacesare optically identical, the difference between their rough-ness must be at a nanoscale level.A and B surfaces were examined by the atomicforce microscope (AFM, Digital Instruments NanoScopeIIIa in tapping mode) in scanning areas of 1×1 µm. Thesurface topography in points A and B is shown asheight maps in Fig. 2. As it is well seen, the morphologyof these fragments of A and B surfaces are different.On A surface the round pores (Fig.2a, black small spots)are observed. A typical profile of single pore on A sur-face is presented in Fig. 2c. The depth of pore is approxi-mately equal to several nanometers and its diameter is ~40 nanometers, respectively. We suppose that the poreshave been appeared on A surface by the following way.It is well known [6] that in the course of anode oxidizingthe silicon surface contacting with electrolyte, silicon ox-ide film grows. Varying electrolyte mixture the oxide filmhaving required porosity can be obtained. In our case aliquid crystal 5CB plays a role of weak electrolyte [9].Therefore, when an external voltage with +U polarity onsilicon is applied to Al/Si/5CB/ITO structure, a dielec-tric film grows on silicon surface. We believe that thisdielectric film is silicon oxide. Taking into account thatthe radius of AFM tip is equal to ~ 510 nm, we assumealso that the minimal value of silicon oxide film thick-ness is equal approximately to depth of a pore, i.e. 34 nm.It should be noted from the results of AFM examina-tion of silicon surface it is followed that both A andB surfaces are clean, i.e. there are no any contamina-tion which could influence on the alignment of liquid crys-tals in the following experiments.On the base of above investigated silicon sample con-taining A and B surfaces the Al/Si/5CB/ITO struc-ture was assembled in the way where the upper ITO elec-trode was unbroken. In order to obtain perfect homoge-neous planar alignment of 5CB liquid crystal on wholesurface of ITO electrode, the surface was covered byrubbed polyimide film. Liquid crystal thickness was20 µm. Fig. 3 shows the fragment of Al/Si/5CB/ITO struc-A B+UULCITOSiAlSFig. 1. The cross-section of investigated Al/Si/5CB/ITO struc-ture. S denotes a spacer. 200nm 200nmFig. 2. AFM images of Si surface in points A (a) and B (b)shown in Fig. 1 after removing of liquid crystal; (c)  the profileof pore on surface (a).a bc0 50 100 150 2000.50.00.51.01.52.02.53.03.54.0Horiz. distance, nmVert. distance, nmFig. 3. Image of A and B fragments of silicon surface inAl/Si/5CB/ITO structure with polarized light.156SQO, 7(2), 2004M.I. Gritsenko et al.: Passivation of silicon surface by ultrathin dielectric film ...ture with silicon surface includes A and B regions.As seen, a liquid crystal layer well decorates A andB surfaces, respectively. It means that there takes placethe difference of liquid crystal alignment on A and Bsurfaces (taking into account that the molecules are ho-mogeneously oriented on upper ITO electrode).Then we studied a photosensitivity of the Al/Si/5CB/ITO structure in A and B areas. The photosensiti-vity of structure is determined by the reaction (yes/no) ofliquid crystal on He-Ne laser illumination of correspon-ding area of silicon surface. When external ÀÑ voltage isapplied to Al/Si/nematic/ITO structure the surface dep-letion layer in silicon may be produced and almost wholevoltage drops across depleted layer. Within a laser spota liquid crystal reorients because depleted layer in sili-con is not formed and the total voltage applied to struc-ture drops across liquid the crystal layer. In the case of n-type silicon, a depletion of silicon surface takes placewhen the polarity of AC voltage is +U on silicon. Be-cause of a liquid crystal layer, in general case, is notisolated by dielectric film, a surface density of major car-riers in silicon depends strongly on the current leakage Jthrough Si/LC interface. In order to obtain a silicon sur-face depletion (in other words a structure photosensitiv-ity), it is required that the major carriers flow from sur-face into bulk induced by external electric field must belarger than the total current consisting of the current leak-age J and the current due to thermo- and light-generatednon-equilibrium carriers. Obviously, the ultrathin dielec-tric film on silicon surface can influence on value of cur-rent leakage J and therefore on the structure photosensi-tivity.Figure 4 shows the dependence of the threshold volt-age of structure photosensitivity appearance on the fre-quency of the applied voltage. As seen, the frequency ofstructure photosensitivity appearance in A area (ar-row on A curve) is more than an order of magnitude lowerthan the frequency of structure photosensitivity appear-ance in B area (arrow on B curve). It means that thecurrent leakage J through Si/LC interface in A surfaceof silicon is limited by a dielectric film grown on siliconsurface.3. ConclusionsIn summary, we have found a passivation effect in theliquid crystal structure as a prototype of OA SLM. Whenthe external voltage with polarity +U on Si is applied toAl/Si/5CB/ITO structure a silicon surface, prior etchedin HF solution, is passivated by an ultrathin dielectricfilm. The passivating film changes an initial alignmentof liquid crystal and limits the transfer of carriers throughSi/LC interface that promotes the formation of depletedsurface layer in silicon under the action of AC voltagewithin more wide range of frequency.References1. J.D. Margerum, L.J. Miller, Electro-optical applications ofliquid crystals // J. Colloid and Interface Science, 58(3), pp.559-580 (1977).2. D.G. Sikharulidse, G.S. Chilaya. Images transducers of MOS-electrooptic material type. Radio i svyaz Moscow (1986)(in Russian).3. N.F. Kovtonyuk, E.N. Salnikov Photosensitive MOS-devicesfor image transducers. Radio i svyaz Moscow (1990) (inRussian).4. A.A. Vasiliev et al., // Optic, 67(3), pp. 223-236(1984).5. R. Muller, T. Kamins. Device Electronics for Integrated Cir-cuits, Wiley (1986).6. R.M. Burger, R.P. Donovan. Oxidation, diffusion and epitaxy.Fundamentals of silicon integrated device technology. V.1Mir Moscow (1969) (in Russian).7. Ì.I. Gritsenko, S.I. Kucheev, Ion controlled liquid crystalspatial light modulator // Proc.of SPIE, 5257, pp.145-151(2003).8. Ì.I. Gritsenko, S.I. Kucheev, Ion induced field effect in sili-con in nematic liquid crystal cell // Semicond. Phys., Quant.Elect. and Optoelect., 6(2), pp.129-133 (2003).9. H. Yokoyama, Handbook of Liquid Crystal Research, Ox-ford Press, New York (1997).Threshold of photosensitivity, V1021031041052468101214A BFrequency, HzFig. 4. Frequency dependence of threshold voltage applied toAl/Si/5CB/ITO when a structure photo-sensitivity appears. Illu-mination by He-Ne laser (0.63 µm, — ~ 2 mW).

Passivation of silicon surface by ultrathin dielectric film in M/Si/nematic/ITO structures

http://dspace.nbuv.gov.ua/bitstream/handle/123456789/118165/07-Gritsenko.pdf?sequence=1

Passivation of silicon surface by ultrathin dielectric film in M/Si/nematic/ITO structures

Abstract

Similar works

Full text

Available Versions

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