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Synthesis of Silicon Nanowires Using Microwave Energy for Optical Devices

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Synthesis of Silicon Nanowires Using Microwave Energy for Optical Devices
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   PLEASE SCROLL DOWN FOR ARTICLE This article was downloaded by:On: 1 September 2009  Access details: Access Details: Free Access  Publisher Taylor & Francis  Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK International Journal of Green Nanotechnology: Materials Science &Engineering Publication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t906024405 Synthesis of Silicon Nanowires Using Microwave Energy for Optical Devices Charu Lata Dube a ; Subhash C. Kashyap a ; D. C. Dube b ; D. K. Agarwal ca Department of Physics, Indian Institute of Technology Delhi, New Delhi, India b Electronics andCommunication Department, Institute of Technology and Management, Gurgaon, India c Materials ResearchInstitute, Pennsylvania State University, University Park, Pennsylvania, USAOnline Publication Date: 01 January 2009 To cite this Article Dube, Charu Lata, Kashyap, Subhash C., Dube, D. C. and Agarwal, D. K.(2009)'Synthesis of Silicon NanowiresUsing Microwave Energy for Optical Devices',International Journal of Green Nanotechnology: Materials Science &Engineering,1:1,M11 — M15 To link to this Article: DOI: 10.1080/19430840902931533 URL: http://dx.doi.org/10.1080/19430840902931533 Full terms and conditions of use:http://www.informaworld.com/terms-and-conditions-of-access.pdfThis article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.   International Journal of Green Nanotechnology: Materials Science & Engineering , 1:M11–M15, 2009Copyright c  Taylor & Francis Group, LLCISSN: 1943-0841 print / 1943-0833 onlineDOI: 10.1080/19430840902931533 Synthesis of Silicon Nanowires Using Microwave Energyfor Optical Devices Charu Lata DubeSubhash C. KashyapD. C. DubeD. K. Agarwal ABSTRACT. Inthepresentwork,siliconnanowireshavebeengrowninaTE 011 single-modecylindricalmicrowave resonant cavity operating at 2.45 GHz. The microstructural, structural, elemental, and roomtemperature photoluminiscence studies have been carried out of the grown nanowires. The wires areencapsulated by an ultrathin layer of silicon oxynitride. The observed strong visile photoluminescencis ascribed to ballistic transport and radiative recombination (at Si/SiON interface) of the carriersgenerated due to light absorption. KEYWORDS. microwave processing, Si nanowires, photoluminiscence, optical devices  INTRODUCTION  An efficient room temperature light emis-sion from porous silicon has motivated itsapplication as light-emitting devices compati-ble with silicon-based optoeletronics–integratedcircuits. [1] The large interest in Si nanostruc-tures is due to the hope that the radiation ef-ficiency of indirect optical transitions may besignificantly increased in these nanostructures.The light emission efficiency of silicon can beimproved bylifting up thelattice periodicity and Received 27 February 2009; accepted 17 March 2009.Charu Lata Dube and Subhash C. Kashyap are affiliated with the Department of Physics, Indian Instituteof Technology Delhi, New Delhi, India.D. C. Dube is affiliated with the Electronics and Communication Department, Institute of Technology andManagement, Gurgaon-122017, India.D. K. Agarwal is affiliated with Materials Research Institute, Pennsylvania State University, UniversityPark, Pennsylvania, USA.AddresscorrespondencetoCharuLataDube,DepartmentofPhysics,IndianInstituteofTechnologyDelhi,New Delhi 110016, India. thereby inducing an uncertainty in the k-space.Various attempts have been made to disruptlattice periodicity, thus allowing no-phonon-assistedopticaltransition,whichenhanceslight-emitting properties of silicon, for, e.g., alloyingof silicon with germanium, [2] synthesis of sil-icon nanostructures, [3] and growth of Si/SiO x structures, [4] etc.Nanowiresofdifferentmaterialsaregrownbyvapour-liquid-solid (VLS) process, [5] laser abla-tionmethod, [6 , 7] physicalthermalevaporation, [3] molecular beam epitaxy, [8] solution growth, [9] M11  D o w nl o ad ed  A t : 03 :13 1  S e p t e mb e r 2009   M12 International Journal of Green Nanotechnology: Materials Science and Engineering FIGURE 1. SEM picture of MW-processed silicon powder. and microwave processing technique. [10] In thepresentstudy,microwave(MW)energyhasbeensuccessfully utilized for the growth of siliconnanowires. The microwave processing has manyadvantages over existing growth techniques. Itis an environment friendly, fast, and energy-efficient process of materials synthesis. [11 , 12]  RESULTS AND DISSCUSSION  The scanning electron micrograph (SEM), asshown in Figure 1, reveals the formation of nanowiresinaMW-processedsiliconpellet.Thenanowires are of nearly uniform diameter (lessthan 100 nm). Most of the Si nanowire are ex-tremely long ( > 10 µ m) and randomly oriented.The elemental composition, as determined byin situ energy dispersive spectroscopy (EDS)in SEM, of nanowires is found to be primar-ily consisting of silicon, with small amounts of nitrogen and oxygen. The presence of nitrogenand oxygen (Figure 3) is due to the formationof some oxide and oxynitride of silicon on thesurface of the nanowires grown in an ambientatmosphere.The x-ray diffractograms (XRD) of startingmixture and microwave-processed sample areshown in Figure 2. The XRD peaks have beenidentified by the second-derivative method andfitted by pseudo-Voigt functions. The XRD scanof starting mixture shows the prominent reflec-tions corresponding to the starting constituentsilicon.TheXRDpatternofMW-processedsam-ple confirms the diamond lattice structure of synthesized nanowires. The full width at half maximum (FWHM) of the peak correspond-ing to (111) diffracting plane for MW-processedsample is found to be two times wider than thatfor starting mixture. This increase in FWHM  D o w nl o ad ed  A t : 03 :13 1  S e p t e mb e r 2009  C. L. Dube et al. M13 FIGURE 2. The XRD of starting and MW-processed silicon powder. can be ascribed to the smaller dimension of nanowires.The photoluminescence (PL) spectrumof MW synthesized nanowires, as shown inFigure 4, shows two strong emission peak cen-tered at 2.2 and 3.0 eV. The excitation energy of 3.8 eV is enough to induce direct optical tran-sitions and therefore the absorption of radiationwill be due to both direct and phonon-assistedindirect optical transition in nanowires. Thesmalldimensionofnanowires(lessthan100nm)will encourage the generated free hot carriers toundergoballistictransport. [13] Mostofthesecar-rierswillreachtheSi/SiONinterface,andwillbetrappedbytheluminescentcenterspresentattheinterface. Nishikawa et al. [14] observed severalluminescencebandswithdifferentpeakenergiesrangingfrom1.9to4.3eV.Itwasmentionedthatthe 3.0-eV band corresponds to some intrinsicdefect center. These structural defects act asradiative recombination centers. The observedintensivebluelightemissionat3.0eVcanthere-fore be attributed to the above-mentioned defectcenters.Furthermore,asreportedbyItohetal. [15] the green light emission at 2.2 eV is attributed toradiativedecayofself-trappedexcitonsinsilicondioxide.In summary, a large quantity of siliconnanowires has been obtained by MW processingof polycrystalline silicon by effectively utilizingthe H field of microwaves in a very short dura-tion of 10 min. The nanowires emit strong blueand green lights at room temperature, makingthese potentially useful for optical devices suchas LEDs.  EXPERIMENTAL To synthesize silicon nanowires the siliconpowderwaspalletized.Thepelletwasprocessedfor 10 min in a single-mode MW resonant cav-ity (designed to sustain TE 011 mode provid-ing predominantly H-field with high intensityat the work piece location). The MW power fedto the cavity was nearly 350 W. The temper-ature acquired by the sample is measured us-ing an optical pyrometer (Raytek model numberRAYMA2SCSF).The morphology of the grown nanowireshas been investigated by scanning electron mi-croscopy,recordedbyemployingtheZeissEVO50 electron microscope. The phase analysis of the starting and MW-processed pellets were  D o w nl o ad ed  A t : 03 :13 1  S e p t e mb e r 2009   M14 International Journal of Green Nanotechnology: Materials Science and Engineering FIGURE 3. EDS spectrum of grown nanowires.FIGURE 4. Photoluminescence spectra from as-grown nanowires.  D o w nl o ad ed  A t : 03 :13 1  S e p t e mb e r 2009
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