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Quenching of Si nanocrystal photoluminescence

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Quenching of Si nanocrystal photoluminescence by doping with gold or phosphorousIntroductionModelExperimentalResults and discussionDoping of Si nanocrystals with AuDoping of Si nanocrystals with P atomsConclusionsAcknowledgementsReferencesJournal of Luminescence 114 (2005) 137–144Quenching of Si nanocrystal photoluminescence by dopingwith gold or phosphorousAnna L. Tchebotarevaa,, Michiel J.A. de Dooda, Julie S. Biteenb,Harry A. Atwaterb, Albert Polmana,baFOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The NetherlandsbCalifornia Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USAReceived 15 April 2004; received in revised form 10 November 2004; accepted 20 December 2004Available online 19 February 2005AbstractSi nanocrystals embedded in SiO2doped with P and Au at concentrations in the range of 1  1018–3  1020cm3exhibit photoluminescence quenching. Upon increasing the Au concentration, a gradual decrease in nanocrystalphotoluminescence intensity is observed. Using a statistical model for luminescence quenching, we derive a typicalradius of 3 nm for nanocrystals luminescing around 800 nm. Au doping also leads to a luminescence lifetimereduction, which is attributed to energy transfer between adjacent Si nanocrystals, possibly mediated by the presence ofAu in the form of ions or nanocrystals. Doping with P at concentrations up to 3  1019cm3leads to a luminescenceenhancement, most likely due to passivation of the nanocrystal–SiO2interfaces. Upon further P doping the nanocrystalluminescence gradually decreases, with little change in luminescence lifetime.r 2005 Elsevier B.V. All rights reserved.PACS: 78.67.Bf; 78.67.Hc; 68.55.LnKeywords: Photoluminescence; Quenching1. IntroductionDuring the last decade, a large amount ofresearch was devoted to the creation of alternativemethods of obtaining an efficient light sourcebased on silicon. It was found that Si quantumdots emit light at visible and near-infraredwavelengths. This luminescence was first observedin porous silicon [1,2], a material that consistsof a sponge-like network of partially oxidisedsmall Si crystallites. After this discovery, manyalternative methods, including ion implantation ofSi into SiO2[3–6] and sputter deposition of Si-richARTICLE IN PRESSwww.elsevier.com/locate/jlumin0022-2313/$ - see front matter r 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.jlumin.2004.12.014Corresponding author. Tel.: +31 20 608 1234;fax: +31 20 608 4106.E-mail address: [email protected](A.L. Tchebotareva).SiO2[7–10], were developed to create Si nanocrys-tals embedded in SiO2. These systems allow theeffects of the complex nanostructure and ofambient effects on surface passivation to beeliminated. Independent experiments on samplesprepared by different methods demonstrated thatthe broad-band emission from Si quantum dots atred to near-IR wavelengths (l 4 650 nm) are due toa quantum confinement effect [4,5,11,12]. Forensemble measurements on these indirect bandgap semiconductor quantum dots, the largespectral width (4100 nm) typically observed foremission is due to both inhomogeneous broad-ening and homogeneous broadening [13]. Despitea large amount of work on the optical character-ization of Si nanocrystals, the exact relationbetween size and emission energy remains un-known. This is first of all due to the fact thatoptical measurements are most often performed onensembles of nanocrystals with a broad sizedistribution. The size distribution of such samplescan be analyzed by transmission electron micro-scopy (TEM). However, due to the low contrast ofelectron densities in Si and SiO2, TEM analysiscan give only a rough estimate of the diameter ofthe studied Si nanocrystals. This makes sizemeasurements of nanocrystals inaccurate. Further-more, the total size distribution of nanocrystalsestimated by TEM does not provide any informa-tion about the size distribution of optically activenanocrystals. This quantity depends critically onsample preparation conditions such as annealingand passivation treatment.One way to address the relation between sizeand emission energy is to begin with a fixedreference sample and then change the size dis-tribution of optically active nanocrystals in acontrolled way. By comparing photoluminescence(PL) spectra for different distributions of opticallyactive nanocrystals, one can acquire informationregarding the relation between size and emissionenergy of optically active nanocrystals. To do so,we dope an ensemble of optically active nanocrys-tals with luminescence quenching ions, each ofwhich could quench the luminescence of a singlenanocrystal. For this purpose, we choose gold,which is known to produce an efficient deeprecombination trap state in crystalline silicon[14]. Assuming that Au atoms produce a similardeep trap in nanocrystals, one would expect theluminescence from Si nanocrystals to be quenchedby Au doping. Another scheme to quench Sinanocrystal luminescence, but in a different way,uses the introduction of dopants such as phos-phorous. In bulk Si, the donor ionization energylevel of P is situated close to the conduction band.Hence at room temperature, P provides a freeelectron in Si, leading to its well-known dopingeffect. In an analogous way, P atoms located insidea Si nanocrystal can provide an extra electron.This carrier would quench the PL by the annihila-tion of an optically generated exciton throughAuger recombination [15,16].The aim of this study is to investigate thequenching effect of both Au and P atoms on thePL of Si nanocrystals. We introduce a uniformdistribution of each quencher throughout SiO2films containing Si nanocrystals produced by ionimplantation. We use a statistical model todescribe the probability that a nanocrystal willcontain a quencher, assuming that it depends onlyon the quencher concentration and the nanocrystalsize. We find that the PL can indeed be quenchedin a controlled way, with strikingly different effectsfor Au-doped and P-doped samples. The experi-mental data cannot be fully described by thestatistical model, indicating that homogeneousbroadening and interactions between Si nanocrys-tals may need to be taken into account.2. ModelThe optical emission of Si nanocrystals underoptical pumping occurs via a series of processes.First, an electron is excited from the valence bandto one of the higher-lying electronic levels in theconduction band of the nanocrystal, leaving


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