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1 Title Author() Kobe Univerity Repoitory : Kernel Coexitence of two different energy tranfer procee in SiO2 film containing Si nanocrytal and Er Fujii, Minoru / Imakita, Kenji / Watanabe, Kei / Hayahi, Shinji Citation Journal of Applied Phyic, 95( 1): Iue date Reource Type Reource Verion URL Journal Article / 学術雑誌論文 publiher Create Date:

2 JOURNAL OF APPLIED PHYSICS VOLUME 95, NUMBER 1 1 JANUARY 2004 Coexitence of two different energy tranfer procee in SiO 2 film containing Si nanocrytal and Er Minoru Fujii, a) Kenji Imakita, Kei Watanabe, and Shinji Hayahi Department of Electrical and Electronic Engineering, Faculty of Engineering, Kobe Univerity, Rokkodai, Nada, Kobe , Japan Received 3 July 2003; accepted 14 October 2003 The mechanim of energy tranfer from ilicon nanocrytal nc-si to erbium ion (Er 3 ) in SiO 2 film containing nc-si and Er wa tudied by analyzing delayed infrared luminecence from Er 3. It wa found that, to theoretically reproduce the riing part of the time-dependent luminecence intenity, two different energy tranfer procee, i.e., fat and low procee, hould be conidered. From the fitting of the delayed luminecence to a model, the ratio of the two energy tranfer procee and the energy tranfer rate of the low proce were etimated. The ratio exhibited a clear dependence on the luminecence peak energy of Si nanocrytal, which act a photoenitizer for Er 3, indicating that the ratio depend on the ize of nc-si. The ratio of low to fat procee increaed with the decreae in ize; thi obervation i a trong indication that the fat proce i the direct inheritance of the proce in bulk Si:Er ytem, and the low proce i a characteritic proce occurring only in nc-si:er ytem. The energy tranfer rate of the low proce wa found to depend on the recombination rate of exciton in nc-si American Intitute of Phyic. DOI: / I. INTRODUCTION a Author to whom correpondence hould be addreed; electronic mail: fujii@eedept.kobe-u.ac.jp The electronic tructure of ilicon nanocrytal nc-si i trongly modified from that of bulk Si crytal due to the confinement of electron and hole in a pace maller than the exciton Bohr radiu of bulk Si crytal. 1,2 The modification of the electronic tructure provide nc-si with function which do not occur in bulk Si crytal. For example, one function i a a photoenitizer for optically inactive material. Recently, nc-si were demontrated to act a photoenitizer for molecular oxygen 3,4 and everal kind of rare earth ion In particular, erbium ion (Er 3 ) can be excited very efficiently by the energy tranfer from nc-si; the effective aborption cro ection of the intra-4 f hell tranition of Er 3 i enhanced by 2 4 order of magnitude becaue of the large aborption cro ection of nc-si in a viible range and becaue of the efficient energy tranfer from nc-si to Er Thi enitized excitation of Er 3 provide large benefit in a number of field which utilize near-infrared emiion of Er 3. One application which take advantage of the enhanced aborption cro ection i a planer-waveguide optical amplifier operating at 1.54 m. With a imple ridgetype waveguide tructure 1 cm long, 9 m wide, and 0.5 m high coniting of SiO 2 film containing nc-si and Er excited from the top, a net gain of 7 db/cm at m ha been obtained. 19,20 By employing imilar material a a gate oxide of a metal oxide emiconductor tructure, electroluminecence device operating at room temperature with the quantum efficiency of 1% 21 and 10% 22 have been reported. Becaue of thee poible application, great effort have been devoted to undertanding the mechanim of the interaction between nc-si and Er 3. One tandard approach in analyzing the interaction i to contruct a et of rate equation by taking into account all poible tranition in and between nc-si and Er 3, fitting the experimental data to the model, and extracting parameter decribing the energy exchange procee. Thi approach ha worked very well, and all experimental reult have been explained conitently. 16 Furthermore, by extending thi approach, a guideline to realizing an optical amplifier from nc-si:er ytem ha been given. 16 In thee previou tudie, the coupling between nc-si and Er i aumed to be very trong. However, thi aumption i not alway applicable. In our previou work on SiO 2 film containing nc-si and Er, 14 we demontrated that, after excitation of nc-si by a pule laer with a 5 n pule width, the photoluminecence PL intenity of Er 3 rie very fat up to about 70% of the maximum intenity, and then tart to rie lowly until reaching the maximum. The oberved delay time wa ditributed from everal microecond to everal hundredth of a microecond, depending on the ample preparation condition. The obervation of the long PL delay ugget that, in addition to the trong coupling that i uually conidered, a weak coupling proce exit, although it might play a minor part in the whole energy tranfer proce. Another point not dicued in detail o far i the ize dependence of the interaction propertie. In previou model, nc-si:er ytem have been treated a an independent ytem that i eentially different from Er-doped bulk Si bulk- Si:Er ytem, that i, no continuity from bulk-si:er to nc- Si:Er ytem ha been dicued. Thi ditinction eem to be reaonable by conidering different exciton tranport propertie between the two ytem; in nc-si:er ytem, exciton can interact only with Er 3 taying nearby each nanocrytal, while in bulk-si:er ytem, exciton can migrate /2004/95(1)/272/9/$ American Intitute of Phyic Downloaded 13 Jul 2007 to Reditribution ubject to AIP licene or copyright, ee

3 J. Appl. Phy., Vol. 95, No. 1, 1 January 2004 Fujii et al. 273 and interact with any Er 3. However, it i not plauible that the mechanim of the energy-exchange itelf i completely different between the two ytem. The electronic tructure of nc-si i modified only lightly from that of bulk Si crytal, and the electronic tructure change continuouly from the bulk tructure with decreaing ize. 1,2 A a reult, the optical property of nc-si i a mall modification of thoe of bulk Si crytal, and all parameter e.g., band gap energy, exciton lifetime, degree of electron-hole exchange interaction, and o on change continuouly from the bulk value. It i well-etablihed from experimental and theoretical tudie that all thee change are the reult of patial confinement of electron and hole, or exciton in a mall pace i.e., the quantum ize effect. 1,2 The exciton Bohr radiu of a bulk Si crytal i about 5 nm. The quantum ize effect thu become prominent for nanocrytal maller than about 10 nm in diameter, and larger nanocrytal can be regarded a bulk from the point of view of electronic tructure. Therefore, if the trong interaction between nc-si and Er 3 i a conequence of quantum ize effect, we hould oberve continuou change of the interaction propertie from bulk-si:er-like to nc-si:er-like in the ize range le than 10 nm. The purpoe of thi work i to have experimental acce to thee problem. To do o, we extended our previou work on SiO 2 film containing nc-si and Er. 14 We have tudied the time development of Er 3 PL intenity after puled excitation of nc-si hot. The riing part of Er 3 PL intenity contain valuable information concerning the energy tranfer mechanim. We will demontrate that, in order to quantitatively reproduce the oberved riing part, two different energy tranfer procee fat and low hould be conidered. From the fitting of oberved curve to a model, the ratio of the two procee and the energy tranfer rate of the low proce are extracted. It will be hown that the ratio depend greatly on the ize of nc-si and change continuouly in a wide ize range; thi continuou change indicate that there i no ditinct border from bulk Si to nc-si in the ene of energy exchange with Er 3. The oberved ize dependence implie that the low energy tranfer proce i a characteritic proce occurring only in nc-si:er ytem, thu becoming more important with decreaing ize. It will be hown that the energy tranfer rate of the low proce i determined by the recombination rate of exciton in nc-si. II. EXPERIMENTAL DETAILS FIG. 1. Photoluminecence pectra of SiO 2 film containing Si nanocrytal and Er at room temperature. A broadband at about 1.55 ev i due to recombination of exciton in Si nanocrytal and a harp peak at 0.81 ev arie from intra-4 f hell tranition of Er 3. SiO 2 film containing nc-si and Er (SiO 2 :nc-si:er) were prepared by a coputtering method. Detail of the preparation procedure are decribed in our previou paper. 6,7 Si, SiO 2, and Er 2 O 3 were imultaneouly putter depoited in argon Ar ga, and the depoited film about 1 m in thickne were annealed in a nitrogen (N 2 ) ga % atmophere for 30 min at temperature between 1100 and 1250 C. Si nanocrytal were grown in film of the mixture of SiO 2 and Er 2 O 3 during the annealing. In thi method, the ize of nc-si can be controlled by changing the concentration of Si in the film or changing the annealing temperature. The average ize of nc-si wa etimated by cro-ectional tranmiion electron microcopic obervation. In thi work, the average ize of nc-si (d Si ) wa changed from 2.7 to 5.5 nm. Er concentration wa fixed to 0.11 at. % for all the ample. PL pectra were meaured uing a ingle grating monochromator and a near-infrared photomultiplier with an InP/InGaA photocathode. The excitation ource for taking the pectra wa the nm line of an Ar-ion laer 0.3 W/cm 2. In thi wavelength, Er 3 i not directly excited. For all the pectra, the pectral repone of the detection ytem wa corrected by the reference pectrum of a tandard tungten lamp. For the time repone meaurement, a nm line of a Nd:Yttritium aluminum garnet YAG laer wa ued a the excitation ource pule energy 0.65 mj, pule width 5 n, repetition frequency 20 Hz, and pot ize 2 mm. A multichannel calar wa ued in obtaining the decay curve. The overall time reolution of the ytem wa better than 100 n. All meaurement were performed at room temperature. III. RESULTS AND DISCUSSION A. Analyi of PL decay curve Figure 1 how the typical PL pectra of SiO 2 film containing Si nanocrytal and Er at relatively low Er concentration range 0.11 at. %. Two emiion band due to the exciton recombination in nc-si and intra-4 f hell tranition of Er 3 are oberved. The intenitie of thee band compete with each other; with increaing Er concentration, the Er 3 PL band become tronger, while the exciton PL band i quenched. The ratio of the increae in Er 3 PL intenity integrated intenity to the lo of nc-si PL one wa about 0.7 at low excitation power. The excitation pectrum of the Er 3 PL i very imilar to that of the exciton PL, indicating that excitation of Er 3 i due mainly to the energy tranfer from Si nanocrytal. 7 More evidence of the energy tranfer i obtained in the time tranient of Er 3 PL. 14 Figure 2 how the time dependence of PL intenity jut after excitation by the frequencydoubled puled Nd:YAG laer with a pule width of 5 n. The time reolution of the detection ytem i better than 100 Downloaded 13 Jul 2007 to Reditribution ubject to AIP licene or copyright, ee

4 274 J. Appl. Phy., Vol. 95, No. 1, 1 January 2004 Fujii et al. n. We can clearly ee that PL intenity rie lowly after finihing the excitation, and the delay time depend trongly on the ample. The obervation of the delayed PL i the direct evidence that Er 3 i excited indirectly by the energy tranfer from nc-si. It i worth noting that, although excitation of Er 3 i due mainly to the energy tranfer, at the preent excitation wavelength 532 nm, the direct excitation of Er 3 i alo partially poible. In order to know the extent of the contribution of direct excitation, we compared PL intenitie of the preent ample to other ample without nc-si but with the ame amount of Er 3 (SiO 2 :Er). The PL intenity obtained for the SiO 2 :Er ample wa le than 5% of the PL intenity of the SiO 2 :nc-si:er ample, uggeting that the contribution of direct excitation i negligibly mall in SiO 2 :nc-si:er ample. In the preent ample preparation method, the ize of nc-si can be controlled by two independent parameter, i.e., exce Si concentration and annealing temperature. In Fig. 2 a, the Er concentration 0.11 at. % and annealing temperature 1150 C are fixed and the exce Si concentration i changed. By changing the exce Si concentration from 2.43 to 11.1 vol %, the PL peak energy i hifted from 1.52 to 1.36 ev ee Table I, correponding to the ize change of nm. We can ee that with increaing the ize, the PL delay become lightly longer. In contrat to Fig. 2 a, in Fig. 2 b the annealing temperature i changed and the other parameter are fixed. In thi cae, the PL peak energy i changed in a maller energy range, i.e., from 1.41 to 1.30 ev, correponding to the ize change of nm. Although the change in ize i maller than that in Fig. 2 a, the variation of the delay time i much larger. It i clear that a higher annealing temperature reult in a longer PL delay, even if the ize of nc-si, i.e., the PL peak energy, i the ame. Therefore, in addition to a ize, there exit another parameter that affect the energy tranfer rate. In order to dicu thi in more detail, we quantitatively analyze the oberved data. In the preent Er and nc-si concentration range, we can aume that one nanocrytal interact with one Er 3. Under thi aumption, in the implet model, Er 3 PL intenity i expreed a I t exp w Er t exp w Si t, 1 FIG. 2. PL time tranient at 0.81 ev jut after 5 n puled excitation. Symbol are experimental reult, and curve are reult of model fitting. a Annealing temperature and Er concentration are fixed at 1150 C and 0.11 at. %, repectively, and exce Si concentration i changed. b Exce Si concentration and Er concentration are fixed at 9.28 vol % and 0.11 at. %, and annealing temperature i changed. where w Si, w Er, and are the recombination rate of exciton in nc-si and Er 3, and the energy tranfer rate, repectively. In thi equation, both the riing and decaying part of the curve are expreed with a ingle exponential function. However, a can be een in Fig. 2 b, the intenity rie very fat until reaching about 70% of the maximum intenity and then tart to rie lowly. Thi behavior i apparently different from Eq. 1. To reproduce the oberved decay curve, we have to conider at leat two different energy tranfer procee occurring imultaneouly; one proce i fater than our time reolution 100 n, and the other i lower and depend trongly on the ample. We define the number of Er 3 ion which interact very trongly with nc-si and contribute to the f fat proce a N Er, and the number of Er 3 ion which interact weakly and contribute to the low proce a N Er. f The energy tranfer rate of each proce i and w Tr, repectively. Under the aumption that one nanocrytal interact with one Er 3 f, N Er i equal to the number of nc-si interacting trongly with Er 3 (N f Si ), and N Er i equal to thoe weakly interacting with Er 3 (N Si ). After puled excitation, the number of excited nc-si that can tranfer energy to Er 3 i f N Si t N Si0 exp w f Tr w Si t N Si0 exp w Tr w Si t, where w Si i the recombination rate of exciton in nc-si, and f N Si0 and N Si0 denote the number of nc-si excited at t 0. If we ignore direct excitation of Er 3, the excitation rate of Er 3 can be G t f f N Si0 w Tr N Si0 exp w f Tr w Si t exp w Tr w Si t, 2 3 Downloaded 13 Jul 2007 to Reditribution ubject to AIP licene or copyright, ee

5 J. Appl. Phy., Vol. 95, No. 1, 1 January 2004 Fujii et al. 275 TABLE I. Lit of ample tudied in thi work. Er concentration i fixed to 0.11 at. % for all the ample. Recombination rate of nc-si (w Si ) and Er (w Er ) are obtained from PL decay curve. The ratio of low to fat energy tranfer procee (N f Er /N Er ) and the energy tranfer rate of the low proce (w Tr ) are obtained by fitting experimental decay curve to a model. w Tr * i obtained by ubtracting the time neceary for Er 3 to relax from higher excited tate to the firt excited tate from 1/ ee Ref. 23. Ta C Si vol % PL peak energy ev w Si 1 w Er 1 N f Er /N Er ( ) w Tr * ( ) and the number of Er 3 ion in the excited tate i N Er t w f f Tr N Si0 exp w w f Er t Tr w Si w Er exp w f Tr w Si t w Tr N Si0 w Tr w Si w Er exp w Er t exp w Tr w Si t. A can be een in Fig. 2 b, the fat proce i fater than our time reolution, and i alo fater than the lifetime of nc-si PL m and Er 3 PL 5 m. Therefore, Eq. 4 can be approximated a N Er t N Si0 N f Si 0 N Si0 exp w w Tr Tr w Si w Er w Si w Er exp w Ert w Si t. Equation 5 multiplied by w Er reult in the PL intneity. Since the excitation rate of nc-si doe not depend on whether f the energy tranfer i fat or low, N Si0 /N Si0 N f Si /N Si, and from the aumption that one nanocrytal interact with one Er 3, N f Si /N f Si N Er /N Er. Therefore, PL intenity will be 4 5 I t N f Er N Er w Si w Er exp w Er t exp w w Tr Tr w Si w Er w Si t. f In Eq. 6, N Er /N Er and are unknown parameter and w Er and w Si can be obtained experimentally from the decay curve of Er 3 and nc-si PL. The decay time of nc-si PL wa obtained by fitting the tail part of the decay curve with f /N Er a ingle exponential function. By uing N Er and a fitting parameter, we fitted all of the experimental curve and extracted the value. The olid curve in Fig. 2 are the reult of the fitting. The model can reproduce all the oberved data with very good accuracy. The parameter ued for and obtained from the fitting are ummarized in Table I. B. Ratio of low to fat energy tranfer procee In Fig. 3, the ratio of low to fat energy tranfer procee (N f Er /N Er ) i plotted a a function of the peak energy of nc-si PL. Since the PL pectrum of nc-si i determined by it ize ditribution, the abcia of Fig. 3 can be converted to the average ize of nc-si. We can ee a good correlation between the two parameter, allowing u to conclude that N f Er /N Er i the function of nc-si ize. In Fig. 3, in all the PL peak energy range tudied, N f Er /N Er i maller than 1, indicating that the fat energy tranfer proce alway dominate the whole energy tranfer proce. The traight line in Fig. 3 6 Downloaded 13 Jul 2007 to Reditribution ubject to AIP licene or copyright, ee

6 276 J. Appl. Phy., Vol. 95, No. 1, 1 January 2004 Fujii et al. FIG. 3. The ratio of low to fat procee etimated from curve fitting a a function of luminecence peak energy of Si nanocrytal. The dahed line i the reult of leat-quare fitting of the data. Exce Si concentration are 2.43, 4.08, 6.66, 9.28, and 11.1 vol %. i obtained by the leat-quare fitting of all the data. The intercept of the line to the x axi i about 1.13 ev, which i very cloe to the band gap energy of bulk Si crytal. Although thi coincidence might be accidental, it i clear that, for the ample coniting of very large nc-si, only the fat energy tranfer proce occur, uggeting that the fat tranfer proce i a direct inheritance of the proce in bulk-si:er ytem, and the low proce i a characteritic proce occurring only in nc-si:er ytem. C. Fat energy tranfer proce In the above model, the energy tranfer time include pure energy tranfer time and the time neceary for excited Er 3 to relax to the firt excited tate 2.5, 23 if it i excited to higher excited tate. The tranfer time of the fat proce, which i horter than our time reolution, i much horter than the relaxation time. Thi ugget that the energy tranfer i made only to the firt excited tate of Er 3 ( 4 I 13/2 ), independent of the ize of the nanocrytal. The fact that the energy tranfer i made to the firt excited tate depite the enlargement of the band gap of nc-si upport the above concluion that the fat energy tranfer proce i bulk-si:er-like, i.e., a trap-mediated energy tranfer. In bulk-si:er ytem, energy tranfer i conidered to be mediated by Er-related trap level at about 150 mev below the conduction band of nc-si; exciton are trapped at that level and the recombination energy of exciton i tranferred to Er 3 by an Auger-like proce. 24,25 In general, the energy level of localized tate in nc-si are not trongly modified by quantum ize effect becaue they are localized in pace. 26,27 In fact, the ize-dependent hift of the dangling-bond-related PL of nc-si oberved at low temperature i about half of that of the exciton PL, uggeting that the trap level i almot unaffected by ize and the oberved mall hift reflect that of either a conduction or valence band edge. 2,28 30 Similarly, the energy of the Errelated trap level meaured from the vacuum level i not expected to be trongly affected by the quantum ize effect. A a reult, the energy of exciton trapped at Er-related center will not be high enough to excite Er 3 to higher excited tate, and the energy tranfer i made only to the firt excited tate of Er 3. In bulk-si:er ytem, diociation of exciton from Errelated trap level and energy back-tranfer from excited Er 3 to the level i conidered to be reponible for the oberved trong temperature quenching of the PL. Both quenching procee require aborption of phonon to meet the energy conervation rule. In nc-si:er ytem, even if the Er-related trap level are not trongly affected by ize, both conduction and valence band edge are hifted to higher and lower energie. Thee hift prevent the diociation of exciton, and alo lightly increae the energy of the trapped exciton, although it i not high enough to excite the econd excited tate of Er 3, reulting in very mall temperature quenching. D. Slow energy tranfer proce A the ize decreae, the contribution of the low energy tranfer proce become important, uggeting that the proce i a characteritic one in nc-si:er ytem. Furthermore, conidering the very mall energy tranfer rate of thi proce, Er-related center are not expected to be involved and exciton may interact directly with Er 3. If exciton in nc-si directly interact with Er 3, the energy tranfer hould be made mainly to higher excited tate of Er 3 becaue the nc-si PL peak energy in the preent ample i alway above 1.26 ev. The evidence that Er 3 i excited to higher excited tate i obtained in the PL pectra of nc-si, which are partially quenched due to the energy tranfer to Er At room temperature, by doping Er, nc-si PL i quenched without changing it pectral hape very much and the pectra are featurele Fig. 1. On the other hand, at low temperature, periodic feature can clearly be ditinguihed, i.e., the pectra are periodically uppreed, and dip appear periodically. In particular, energy higher than the third excited tate of Er 3 ( 4 I 9/2 ) i trongly uppreed. Periodic feature appear at about the energie of the econd ( 4 I 11/2 ) and third ( 4 I 9/2 ) excited tate of Er 3 meaured from the ground tate, and the eparation between the feature i about 63 mev. Since the energy level of the Er 3 4 f hell are dicrete, only nc-si with pecific band gap energie can tranfer energy reonantly; the energy tranfer rate i the highet at the reonant energy. At other energie, phonon hould be emitted in nc-si to meet the energy conervation rule. Thi proce i the mot probable for phonon having the highet denity of tate, which in bulk Si are tranvere optical phonon almot at the center of the Brillioun zone with an energy of 63 mev. If the band gap energy of nc-si doe not coincide with the excitation energy of Er 3 plu an integer number of the energy of thoe phonon, additional emiion of acoutic phonon i required to conerve energy. Thi proce ha a maller probability and the efficiency of the energy tranfer i reduced. Conequently, equiditant feature can be een in quenched pectra that validate the phononaited energy tranfer. It i worth noting that, although the Downloaded 13 Jul 2007 to Reditribution ubject to AIP licene or copyright, ee

7 J. Appl. Phy., Vol. 95, No. 1, 1 January 2004 Fujii et al. 277 FIG. 4. Energy tranfer rate of low proce i plotted a a function of a PL peak energy of nc-si, i.e., the ize of nc-si, and b invere of PL lifetime of nc-si at PL peak energy, i.e., average exciton recombination rate in nc-si. Exce Si concentration are 2.43, 4.08, 6.66, 9.28, and 11.1 vol %. In the ame ymbol, higher-annealing temperature reult in lower PL peak energy and maller exciton recombination rate. FIG. 5. a PL pectra of nc-si prepared at different annealing temperature and fixed exce Si concentration. PL decay curve detected at b 1.4 and c 1.2 ev for the ame ample in a. energy tranfer rate i larger for ome pecific energie than other, energy tranfer i poible in all energy range above 0.81 ev by emitting a erie of phonon during the proce. Very imilar but more ditinct PL feature have been oberved in nc-si on which oxygen molecule are adorbed. 3,4 If the energy tranfer i made to higher excited tate, the energy tranfer time we etimated include the time for excited Er 3 to relax to the firt excited tate. In order to extract only the time neceary to tranfer energy, we ubtract the relaxation time 2.5 ; from 4 I 11/2 to 4 I 13/2 tate 23 from 1/ and obtained 1/w Tr *. The relaxation time from further higher tate, e.g., 4 I 9/2 4 I 11/2 i conidered to be much fater and negligible. The etimated value of w Tr * are ummarized in Table I. Firt, we plotted 1/w Tr * a a function of the nc-si PL peak energy Fig. 4 a. The energy tranfer rate increae with increaing PL energy, i.e., with the decreaing ize of nanocrytal. However, the data are largely cattered, and there eem to be another factor that alo trongly affect the energy tranfer rate. Among the largely cattered data, we can find a tendency. By comparing the energy tranfer rate of the ample exhibiting a PL peak at nearly the ame energy, the energy tranfer rate i maller for the ample annealed at higher temperature ee Table I. E. Quality of nc-si To undertand the implication of the data of Fig. 4, we will conider the quality of nc-si. The word quality i rather ambiguou. However, it i ometime ueful to ditinguih nc-si grown from Si uboxide by annealing. Figure 5 how one of the example. In Fig. 5 a, PL pectra of pure Downloaded 13 Jul 2007 to Reditribution ubject to AIP licene or copyright, ee

8 278 J. Appl. Phy., Vol. 95, No. 1, 1 January 2004 Fujii et al. FIG. 6. PL lifetime v detection energy for four nc-si ample annealed at different temperature. nc-si in SiO 2 prepared with different annealing temperature are plotted. The exce Si concentration i fixed for all ample. We can ee that higher temperature annealing reult in a lower energy hift of the PL. Thi hift i due to the increae in ize. In addition to the peak hift, the annealing temperature affect the quality of the nanocrytal. In Fig. 5 b and 5 c, PL decay curve meaured at 1.4 and 1.2 ev are plotted. If the PL lifetime i determined only by ize, the decay curve obtained at the ame energy hould be the ame for all ample. However, the oberved lifetime depend trongly on the annealing temperature, and become longer at higher annealing temperature. Furthermore, the hape of the decay curve depend trongly on the annealing temperature. The decay curve of the ample annealed at higher temperature are nearly a ingle exponential function, but thoe at lower temperature largely deviated from the function. In Fig. 6, PL lifetime obtained for four ample prepared with different annealing temperature are plotted a a function of the PL detection energy. In low-temperature annealed ample, the PL lifetime i almot independent of the detection energy, while in high-temperature annealed ample, it depend heavily on the detection energy. In a quantum confinement model, the lifetime hould depend heavily on the detection energy, becaue tronger confinement of exciton reult in the enhancement of ocillator trength a well a the higher energy hift of the band gap. Therefore, higher-temperature annealed ample obey the prediction of the quantum ize effect, and in thi ene are high quality. The different quality in ample annealed at different temperature can be proved by reonant PL meaurement. The PL pectra of nc-si aemblie are alway featurele and broad, if they are excited by blue or green light, becaue of the inhomogeneou broadening caued by ize ditribution. The ize ditribution can be lifted if the PL i excited in the low-energy tail of the luminecence band. In thi method, only a mall portion of nc-si in the ize ditribution i excited, and a nearly homogenou pectrum can be obtained. In porou Si, with thi method, PL peak correponding to the emiion of momentum-conerving phonon can be oberved, and from the ratio of the phonon-aited and nonphonon PL band, the degree of the breakdown of the momentum conervation rule ha been etimated. 1 The ame technique can be applied to yntheized nc-si. In the reonant PL tudie of the preent ample, we found that higher-temperature annealed ample how clearer phonon-related tructure, indicating that the indirect band gap character of bulk Si i held. On the other hand, in lowertemperature annealed ample, the nonphonon PL band wa dominant and phonon tructure were ill-defined, although the pectral hape under nonreonant excitation wa nearly the ame. Thi reult implie that in low-temperature annealed ample, the tranlational ymmetry of the crytal lattice i broken not only by the ize effect but alo by the diorder remaining in or on the urface of the nanocrytal, making the quaidirect optical tranition more probable. Thi explanation i conitent with the horter PL lifetime oberved for lower-temperature annealed ample Fig. 6. The imultaneou occurrence of the mearing of phonon-related tructure in reonant PL pectra and the hortening of PL lifetime wa oberved by doping germanium Ge in nc-si (Si 1 x Ge x alloy nanocrytal. 31 Breakdown of the tranlational ymmetry by randomly ubtituting Si with Ge i conidered to be reponible for the oberved effect. It i important to note here that low quality nc-si mean that bulk Si character i more meared and the PL propertie cannot be explained by a imple quantum ize effect. It doe not alway mean lower PL quantum efficiency. In fact, in the preent ample, PL intenity i not the highet for the ample annealed at 1250 C, although the quality i the highet. Therefore, in nc-si ample, one poible approach to obtaining higher quantum efficiency may be to introduce proper diorder if it doe not lead to the formation of nonradiative recombination channel. The other poible explanation of the data in Fig. 5 and 6 i that the trength of particle particle interaction i changed by annealing temperature. 15 The interaction between nc-si reult in an energy tranfer from maller toward larger nc-si, leading to a deviation of the decay curve from a ingle-exponential function and a redhift of the PL peak. 15 In thi model, the deviation hould be larger if we detect the PL at the higher-energy part of the PL pectra, becaue the energy tranfer i made only from maller to larger particle. However, a can be een in Fig. 5 c, the deviation become larger for lower-temperature annealed ample, depite the fact that PL detection i made further from the PL peak energy for lower-temperature annealed ample. Furthermore, in our ample, the PL lifetime doe not trongly depend on the fraction of exce Si, a can be found by comparing w Si of ample with different exce Si and the ame annealing temperature in Table I. Thee are evidence that different PL decay propertie oberved for ample annealed at different temperature are not due to the change of the degree of particle particle interaction, but reflect change of the quality of individual nc-si. Downloaded 13 Jul 2007 to Reditribution ubject to AIP licene or copyright, ee

9 J. Appl. Phy., Vol. 95, No. 1, 1 January 2004 Fujii et al. 279 F. What i the factor determining the energy tranfer rate of the low proce A can be een in Fig. 6, the PL lifetime of nc-si depend both on the ize and quality of nc-si. The PL lifetime obtained at a nc-si PL peak energy contain information of both ize ditribution and quality. In Fig. 4 b, the energy tranfer rate of the low proce (w *) i plotted a a function of the invere of nc-si PL lifetime, i.e., the recombination rate of exciton in nc-si (w Si ). In contrat to the plot a a function of PL peak energy hown in Fig. 4 a, the correlation i very good. It hould be treed that the energy tranfer rate ordinate i etimated from the fitting of the oberved PL decay curve of Er-doped ample, while the exciton recombination rate abcia i obtained from the exciton PL of pure nc-si ample prepared with the ame exce Si concentration and annealing temperature of the Erdoped ample. From Fig. 4 a and 4 b, we can conclude that the energy tranfer rate of the low proce i determined by the recombination rate of exciton in nc-si, and the reduction of ize i jut one approach to realizing a larger recombination rate of exciton. The recombination rate of exciton in nc-si exhibit trong temperature dependence. 1,2 At low temperature exciton are populated in optically inactive pin-triplet tate, which are everal milli-electron-volt lower in energy than optically allowed pin-inglet tate. The lifetime of the triplet tate i 5 10 m, which i almot independent of the ize of nc-si. On the other hand, at high temperature, both tate are nearly equally populated and the recombination rate i determined by the lifetime of the inglet tate. The lifetime of the inglet tate i between 1 m to everal microecond, depending on the ize of nc-si. In the ize range of the preent ample, the recombination rate change more than one order of magnitude from room temperature to liquid helium temperature, indicating that the energy tranfer rate will be dratically reduced at very low temperature. Unfortunately, we did not ucceed in experimentally proving thi phenomena. One of the obtacle i a defect-related infrared emiion from nc-si that i alway oberved for almot all kind of nc-si ample at low temperature. 2,28 30 The lifetime of the emiion i on the order of microecond. Therefore, even if the intenity of the defect-related PL under cw excitation i very mall, under puled excitation, the intenity jut after excitation i much larger than that of Er 3 PL, completely covering the time range we are intereted in, i.e., the riing part. It i important to note that, although the energy tranfer rate of the low proce i expected to be decreaed at low temperature, energy tranfer efficiency will not depend trongly on temperature becaue the maller energy tranfer rate will be compenated by the maller radiative recombination rate of exciton (T)/w Si (T) contant. In fact, the intenity ratio of Er 3 PL and nc-si PL doe not trongly depend on temperature. Up to now, we avoided dicuing how the fat and the low procee are related to the ite of Er 3. Unfortunately, we have no clear clue for dicuing thi point. However, the oberved trong ize dependence of the ratio of the two procee ugget that the bulk-si:er-like very fat energy tranfer proce i related to Er 3 that tay in or very cloe to nc-si becaue it i neceary to form an Er-related level in the band gap of nc-si. On the other hand, the low proce come from Er 3 that i more eparated from nc-si. One of the promiing method to identify the ite reponible for the fat and low procee i ite-elective x-ray aborption pectrocopy. Thi method ha been uccefully applied to Er-doped Si thin film. 32,33 By applying the method to the two extreme ample with the larget and mallet ize of nc-si, it may poible to identify independently the ite reponible for the fat and low procee. IV. CONCLUSION By analyzing the time tranient of PL from Er 3 doped into SiO 2 film containing nc-si, two energy tranfer procee fat and low procee were found to coexit in each ample. The ratio of the two procee exhibited a clear dependence on the PL peak energy of Si nanocrytal, indicating that the ratio depend on the ize of the nc-si enitizer. The oberved ize dependence ugget that the fat proce i the direct inheritance of the proce in bulk Si:Er ytem, and the low proce i a characteritic proce occurring only in nc-si:er ytem. The energy tranfer rate of the low proce wa found to depend on the exciton recombination rate in nc-si. Therefore, we can conclude that, in order to realize a higher energy tranfer rate, the exciton recombination rate in nc-si hould be enhanced. Thi can be achieved by breaking down the tranlational ymmetry of crytal lattice by reducing the ize of nc-si or by introducing diorder. The preent reult imply that, from the point of view of application a an amplifier and a light-emitting device, perfect nc-si that poe the indirect band gap character of bulk Si crytal i not uitable. Defective nc-si:er ytem uch a ilicon-rich ilicon dioxide, in which Si cluter maller than 2 nm in diameter are conidered to be dipered, are more uitable. Thi concluion i already empirically known and many report concerning the application of imilar ytem utilize defective nc-si aemblie ACKNOWLEDGMENTS Thi work i upported by a Grant-in-Aid for Scientific Reearch from the Minitry of Education, Culture, Sport, Science and Technology, Japan, and in part by the Indutrial Technology Reearch Grant Program in 2002 from the New Energy and Indutrial Technology Development Organization NEDO, Japan. 1 D. Kovalev, H. Heckler, G. Poliki, and F. Koch, Phy. Statu Solidi B 215, S. Takeoka, M. Fujii, and S. Hayahi, Phy. Rev. B 62, D. Kovalev, E. Gro, N. Künzner, F. Koch, V. Yu. Timohenko, and M. Fujii, Phy. Rev. Lett. 89, E. Gro, D. Kovalev, N. Künzner, F. Koch, V. Yu. Timohenko, and M. Fujii, Phy. Rev. B 68, A. J. Kenyon, P. F. Trwoga, M. Federighi, and C. W. Pitt, J. Phy.: Conden. Matter 6, L M. Fujii, M. Yohida, Y. Kanzawa, S. Hayahi, and K. Yamamoto, Appl. Phy. Lett. 71, Downloaded 13 Jul 2007 to Reditribution ubject to AIP licene or copyright, ee

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