PACS 78.55.M, 79..Jv, 81..Ef Effect of cetone vpor tretment on photoluminescence of porous nc-si SiO x nnostructures I.Z. Indutnyi, K.V. Michilovsk, V.I. Min ko, P.E. Shepelivyi V. Lshkryov Institute of Semiconductor Physics, NAS of Ukrine 41, prospect Nuky, 03028 Kyiv, Ukrine E-mil: indutnyy@isp.kiev.u Astrct. The effect of tretment in sturted cetone vpors on the spectrl composition nd intensity of photoluminescence (PL) in porous olique deposited SiO x films is studied. As result of this tretment followed y high-temperture nneling t the temperture 930 C, considerle PL intensity growth nd the smll lueshift of PL pek position re oserved in the porous, column-like structure films contining Si nnocrystls. A more intense shortwve nd (pek position 5-5 nm) ppers in the PL spectrum of these structures, in ddition to the longwve nd (7-780 nm). Both PL nds in treted smples re chrcterized y monomoleculr rditive recomintion, which cn e ttriuted to nnihiltion of excitons in silicon nnocrystls emedded into oxide mtrix (longwve nd) nd in cron-enriched mtrix ner surfce of oxide nnocolumns (shortwve nd). The possiility to control the PL chrcteristics of the porous structures in wide spectrl rnge y ove tretment is shown. Keywords: nnocrystls, silicon oxide, photoluminescence, thin film. Mnuscript received 13.01.09; ccepted for puliction 18.03.09; pulished online.03.09. 1. Introduction Recently, friction of Si-sed light emitting mterils hs een very ctive re of reserch stimulted y its potentil ppliction in Si-sed optoelectronic devices. Since the discovery of visile photoluminescence (PL) in porous silicon [1], efficient room-temperture PL hs een relized in severl nnometer-sized Si mterils [2-6]. Recent investigtions were focused on Si-rich dielectric thin films (minly SiO x ) mde y techniques comptile with the microelectronic industry. To fricte these films, few sic techniques re usully used: plsm-enhnced deposition, ion implnttion, lser ltion, mgnetron sputtering, evportion in vcuum, etc. All these methods llow to fricte films with required x, nd then the SiO x lyers re nneled t high temperture. As result, Si nnoclusters 1-5 nm in size re formed in the oxide mtrix. The structure of the nnoprticles depends on the nneling temperture: nneling t tempertures elow 900 C results in formtion of morphous inclusions, wheres t higher tempertures, the Si nnocrystls re formed, with the electronic structure modified y the quntum-confinement effect [2, 4-6]. With decresing dimensions of the nnoclusters, the pek of the emission spectrum shifts to shorter wvelengths. In the previous study [7], we suggested method of controlling the dimensions of Si nnoprticles in the oxide mtrix, nmely, the use of olique deposition of Si monoxide thermlly evported in vcuum. During such deposition, SiO x films with porous (column-like) structure re formed. The thermlly stimulted formtion of Si nnoinclusions in such films occurs in restricted volume of the SiO x columns with smll dimeter. These results in the formtion of smll-sized silicon nnoclusters (nc-si) compred to the clusters formed in continuous films of the sme composition. In this cse, we oserve rodening nd shift of the PL nd to longer wvelengths in the spectrum. One of the most importnt fctors influencing the chrcteristics of PL is the stte of the Si SiO x interfce. The Si SiO x interfce cn e modified y dsorption of the compounds of necessry composition. The dsorption is most efficient in porous structures t enhnced pressure nd temperture. In this pper, we report the results of studying the effect of tretment in cetone vpors (croncontining compounds) on PL of the porous nc-si SiO x structures produced y olique deposition. 105
2. Experiment The smples were produced y therml evportion of Cerc Inc. silicon monoxide (with 99.9 % purity) in vcuum (the residul pressure ws (1 2) 10 3 P). As sustrtes, we used two-sided polished Si wfers. During deposition, the sustrtes were oriented t the ngles β = 0, nd 75 etween the norml to the sustrte surfce nd the direction to the evportor. The thickness of the films were monitored in situ y the qurtz-crystl-oscilltor monitor system (КIТ-1) nd mesured fter deposition y microinterferometer (МII-4). The thickness of normlly deposited films ws equl to 2-3 nm, for olique deposited films 0-700 nm. Some of the SiO x smples (except for the reference ones) were kept for two hours in the pressure-tight utoclve tht contined the certin mount of cetone (CH 3 COCH 3 ) t the temperture 0 С, here the pressure of cetone vpors in utoclve ws 18 tm. Then ll the smples were nneled in vcuum t the temperture 930 C for 15 min to produce nnocrystlline silicon inclusions in the oxide mtrix [3]. Some of the reference smples were treted in cetone vpors fter nneling. The PL spectr were recorded t room temperture using system sed on ZMR-3 monochromtor. The PL signl ws excited y rdition of n N 2 lser t the wvelength 337 nm nd violet light-emitting diode EDEV-1LA1 (wvelength 0 nm) which ws used for mesurements of PL intensity dependence on the excittion power. The signl ws detected using FEU-51 photomultiplier, nd PL spectr were normlized to the spectrl sensitivity of the experimentl system. SEM pprtus (ZEISS EVO XVP) ws used to oserve the cross-section of nneled smples. Selective etching of nc-si SiO x smples ws conducted in wek solution of hydrofluoric cid. 3. Results nd discussion In Fig. 1, cross-sectionl views of SiO x films olique deposited on silicon wfer re shown, where Fig. 1 shows SEM microgrph of the smple deposited t β = 75, nd Fig. 1 smple deposited t β =. As cn e seen in this figure, the investigted SiO x films hve porous (inclined column-like) structure with the column dimeters of 10-100 nm. An ngle of column inclintion, their dimensions nd volume of pores in the structure of films depend on the deposition ngle. The porosity of these smples (volume shre of pores) were mesured in the previous pper [8], nd it ws equl to 34 nd 53 % for the smples deposited t β = nd 75, ccordingly. High-temperture nneling of olique deposited SiO x films do not chnge porosity nd column-like structure of the smples. The composition of the films (the prmeter x) ws determined using compositionl dependence of the position of the sic IR nd pek in spectr of SiO x 106 Fig. 1. SEM microgrphs of SiO x film cross-section: () for the smples deposited t β = 75, nd () for tht deposited t β =. lyers within the rnge 1000-1100 cm 1, s estlished in [9]. This nd corresponds to the trnsverse stretching virtions of the ridge-coordinted oxygen toms (Si O Si stretching mode). Since only the virtions in the silicon-oxygen phse re ctive in this spectrl rnge, while the Si Si ond virtions cnnot e recorded, this method cn e used to determine the composition of the oxide mtrix in oth s-prepred nd nneled smples contining the silicon phse. The stoichiometric prmeter x determined y this method is 1.42 nd 1.51 for the smples deposited t the ngles nd 75, respectively [7]. The difference in chemicl composition is relted to the porosity of the films, s descried previously [7, 8]. During nneling, the SiO x oxide is lyered into Si nd SiO 2, nd the stoichiometric prmeter of the oxide mtrix (determined from the IR spectr) is x 1.9 for ll the smples. Fig. 2 shows the PL spectr of two nneled porous smples deposited t the ngle, nmely, the reference smple (curve ) nd the smple treted in cetone vpors in the utoclve efore nneling (curve ). It cn e seen tht for the untreted (reference) smple two rod PL nds re oserved. One, more intensive with mximum t ~780 nm nd second,
PL intensity, r.units 2 0 1 100 0 0 0 700 800 900 Fig. 2. PL spectr of nneled SiO x smples deposited t the ngle : the reference smple () nd tht treted in cetone vpors in utoclve efore nneling (). Both smples were nneled in vcuum t 930 C for 15 min. PL intensity, r. units 0 0 0 0 700 800 900 c Fig. 3. PL spectr of nneled SiO x smples deposited t the ngle 75 : the reference smple (), the smple treted in cetone vpors in utoclve efore nneling (), nd the smple fter etching in 0.1 % solution of HF during s (c). The smples were nneled in vcuum t 930 C for 15 min. considerly less intensive, is in visile rnge of spectrum (mximum t ~5 nm). The intense longwve nd tht ppers in the PL spectrum is connected with the quntum-confinement effect of nc-si, which ppered in the mtrix of oxide during the high temperture nneling of SiO x films [10, 11]. The shortwve nd is determined, to our opinion, y rditive recomintion of excited chrges on the surfce defects of oxide nnocolumns. In PL spectr of continuous non-porous nc-si SiO x structures deposited t β = 0, this nd in the PL spectrum is sent, which confirms our supposition. After tretment in the utoclve efore nneling (curve ) two nds in PL spectrum re lso oserved. But the intensity of oth nds increse, more considerly for the shortwve one. The pek of the shortwve nd is shifted to shorter wvelengths slightly to 5 nm. This nd is more intense thn the longwve one, nd longwve nd ecomes more wide nd symmetric, with pek in the rnge of 7-770 nm. PL spectr of oliquely deposited t the ngle 75 smples non-treted (curve ), treted in utoclve () nd oth nneled t 930 C re shown in Fig. 3. It cn e seen tht for the untreted (reference) smple rod PL nd is oserved with pek t ~7 nm, which cn e connected lso with the quntum-confinement effect nd exciton nnihiltion in nc-si tht pper during nneling of SiO x film (curve ). The shortwve nd is not oserved y contrst to the spectrum in Fig. 2 (curve ). The smples deposited t the ngle 75 hve higher porosity thn deposited t β =. It cn result in higher concentrtion of non-rditive recomintion centers on the surfce of oxide nnocolumns. The nonrditive recomintion quenches the rditive recomintion of excited chrge crriers ner the surfce of oxide nnocolumns tht explins sence of the shortwve nd. Tretment in the utoclve efore high-temperture nneling cuses noticele chnges in PL spectr of these smples. The PL intensity grows significntly, too. In the visile rnge of spectrum, n intensive nd ppers with mximum ner 5 nm. The unstructured PL spectrum, which cn e interpreted s superposition of few nds corresponding to vrious sizes of nc-si, is oserved in IR region. Anlysis of PL spectr of oliquely deposited t the ngles 75 nd smples shows tht tretment in the utoclve efore nneling t high tempertures cuses similr chnges of the spectr. These chnges re chrcterized y growth of the PL intensity, ppernce of n intense nd in the visile rnge of spectrum nd smll lue shift of the nds. Some of the smples were treted in the utoclve y using cetone vpors fter high-temperture nneling. This tretment does not induce ny noticele chnges in the intensity, shpe, or position of the nds in the PL spectrum. Using light-emitting diode EDEV-1LA1 (0 nm) for PL excittion, we explored the dependence of PL intensity on the intensity of excittion. Fig. 4 shows the dependence of PL intensity tken t the pek (7 nm) of infrred emission (curve ) nd t the pek of shortwve (5 nm) nd (curve ) for the smple oliquely deposited t the ngle 75 nd treted in the utoclve. It cn e seen tht the PL intensity for oth nds grows linerly with incresing in the intensity of excittion. The liner dependence etween intensities of luminescence nd excittion hs een otined theoreticlly in the work [12], where the mechnism of exciton recomintion in nnocrystls ws investigted. It indictes tht oth PL nds of the treted smples cn e relted with the quntum-confinement effect nd exciton nnihiltion in nc-si. 107
PL intensity, r.units 70 30 10 0,5 1,0 1,5 2,0 2,5 Excittion intensity, mw/cm 2 Fig. 4. Dependence of the PL intensity on the excittion intensity for SiO x film oliquely deposited t the ngle 75 nd treted in the utoclve. PL intensity t the pek (7 nm) of the infrred emission () nd t the pek of shortwve (5 nm) nd (). PL intensity, r. units 30 10 0 0 0 700 800 900 Fig. 5. PL spectr of the smple oliquely deposited t the ngle nd treted in the utoclve efore (), nd fter etching in 1 % solution of HF during s () nd s (с). The second confirmtion of this ssumption is the dependence of PL mxim of oth nds on the content of silicon in deposited SiO x films. The smple deposited t the ngle contins much silicon thn deposited t the ngle 75 (x = 1.42 nd 1.51 [7]). During the high temperture nneling, in the first film lrger nc-si re formed, nd PL mximum shifts to longer wvelengths (780 nd 7 nm for the smples, deposited t nd 75, ccordingly). The spectrl position of shortwve PL nds tht pper in the treted films lso correlte with the content of silicon in initil lyers (5 nd 5 nm, ccordingly). With the purpose to more definitively determine the origin of shortwve nds, we investigted the influence of the selective etching on the PL spectr of the utoclve-treted nc-si SiO x smples. The etching of the smples ws performed in wek (0.1-1 %) wter solutions of HF t room temperture. As known, HF c solution dissolves SiO 2, while the rte of SiC nd Si dissolution in HF solutions is very smll [13]. Fig. 5 shows PL spectr of the smple oliquely deposited t the ngle nd treted in the utoclve efore etching (curve ), nd fter etching in 1 % solution of HF for nd s (curves nd с, ccordingly). It cn e seen tht etching of the smple results in grdul lowering the PL intensity in ll the spectrl rnge, more ctively in its shortwve region. The mximum of the longwve PL nd grdully shifts to shorter wvelengths, the shortwve PL nd shifts nd disppers. After etching for s in the PL spectrum, one wide nd is presented with its pek t 680 nm. The similr result hs een otined for the smples oliquely deposited t the ngle 75, treted nd nneled in the sme conditions. In the PL spectrum of these smples fter etching in 0.1 % solution of HF, one wide nd is present with the pek t 670-675 nm (Fig. 3, curve с). The intensity of this PL nd is considerly less thn efore etching (Fig. 3, curve ). The shortwve shift of PL nds nd decrese in their intensity s result of etching is possile to e explined y dissolution of SiO 2 lyer on surfce of oxide nnocolumns nd following oxidtion of silicon toms on nc-si surfce. It results in the decresing nc-si size, nd the PL nd is shifted to the shortwve side. Fst diminishing nd disppernce of the shortwve PL nd shows tht this nd is relted with the rditive centers or nc-si, which locted ner the surfce of oxide nnocolumns nd cn e removed under dissolution of the surfce SiO 2 lyer. Appernce of the intense shortwve nd fter rometric tretment in cetone vpors cn e cused y n effect of cron impurities. It ws shown [14] tht, t high tempertures (7-900 C), cetone rects with the silicon surfce, which results in formtion of the SiC x lyer. Introduction of considerle mount of cron into the oxide mtrix contining Si nnocrystls provides mens for noticele shifting the PL spectrum to shorter wvelengths [15]. Therefore, our results my e interpreted similrly to tht done in [15]: during nneling, the dsored cetone rects with the silicon nnocrystls (in the smll-sized oxide columns nd ner the surfce of the columns) to form SiC x surrounding of these nnocrystls, which results in the shift of the PL spectrum to shorter wvelengths. Since the modifiction of nnocrystls with dsored molecules in the course of nneling occurs t the surfce of the columns, the nc- Si mtrix interfce is modified first for those nnocrystls tht re formed in columns smller in dimeter or locted ner the surfce of the columns. Nnocrystls formed in the ulk of lrger columns remin surrounded y the oxide. Therefore, the PL spectrum exhiits two nds: one, t longer wvelengths, corresponds to nnocrystls in the oxide mtrix, nd the other, t shorter wvelengths, to nc-si in Si-C surrounding. Another possile mechnism of ppernce of the shortwve PL nd is thermostimulted formtion of the 108
locl rditive centers on the surfce of oxide nnocolumns. During nneling, cron cn form the Si=C, Si C or O Si C onds. These onds provide effective luminescent centers nd pssivte the interfce etween Si core nd surrounding oxide. Except for it, the sustntil increse the PL intensity fter rometric tretment of investigted smples is well explined y pssivtion of roken silicon onds on the nc-si surfce. 4. Conclusions In this study, SEM mesurements show tht oliquely deposited t the ngle 75 nd SiO x films hve porous column-like structure, porosity of which depends on the ngle of deposition. The effect of rometric processing these films in cetone vpors efore hightemperture (930 C) nneling on the light-emission chrcteristics of nc-si SiO x structures is exmined. It is scertined tht, s result of this tretment, the increse in the PL intensity, the smll lueshift of PL pek position re oserved, nd more intense shortwve nd (pek position 5-5 nm) ppers in the PL spectrum of these structures, in ddition to longwve nd (7-780 nm). For oth nds the position of mxim depends on the composition of initil lyers (content of silicon in SiO x film), which depends on the ngle of deposition. Both PL nds in treted smples re chrcterized y monomoleculr rditive recomintion, which cn e ttriuted to nnihiltion of excitons in silicon nnocrystls emedded into oxide mtrix (longwve nd), nd in nc-si in cron-enriched mtrix ner the surfce of oxide nnocolumns (shortwve nd). The effect of selective etching on the PL spectr of the utoclve-treted nc-si SiO x smples hs confirmed the ssumption tht the intense shortwve nd ppering fter tretment is relted with the rditive centers or nnocrystls locted ner the surfce of oxide nnocolumns. Thus, the rometric tretment of porous oxide lyers in cetone vpors t elevted tempertures is n efficient method tht provides mens to increse the PL intensity of nc-si SiO x structures nd to vry the spectrl composition of emission over wide spectrl region. References 1. L.T. Cnhm, Silicon quntum wire rry friction y electrochemicl nd chemicl dissolution of wfers // Appl. Phys. Lett. 57, p. 1046-1048 (1990). 2. M. Molinry, H. Rinnert nd H. Vergnt, Visile photoluminescence in morphous SiO x thin films prepred y silicon evportion under moleculr oxygen tmosphere // Appl. Phys. Lett. 82, p. 3877-3879 (03). 3. V.Y. Brtus, V.A. Yukhimchuk, L.I. Berezhinsky et l., Structurl trnsformtions nd silicon nnocrystllite formtion in SiO x films // Semiconductors 35(7), p. 821-826 (01). 4. D. Neshev, C. Rptis, A. Perkis et l., Rmn scttering nd photoluminescence from Si nnoprticles in nneled SiO x thin films // J. Appl. Phys. 92, p. 4678-4683 (02). 5. 4. I.P. Lisovskyy, I.Z. Indutnyy, B.N. Gnennyy et l., Structurl nd phse chnges in SiO x films under therml vcuum tretments// Fizik i Tekhnik Poluprovodnikov 37, p. 98-103 (03) (in Russin) [Semiconductors 37, p. 97 (03)]. 6. B.G. Fernndez, M. Lopez, C. Grci et l., Influence of verge size nd interfce pssivtion on the spectrl emission of Si nnocrystls emedded in SiO x // J. Appl. Phys. 89, p. 237-243 (01). 7. I.Z. Indutnyy, I.Yu. Midnchuk, V.I. Min ko, Visile photoluminescence from nneled porous SiO x films // J. Optoelectron. nd Adv. Mter. 7, p. 1231-1236 (05). 8. V.A. Dn ko, I.Z. Indutnyy, I.Y. Midnchuk et l., Formtion of the photoluminescence structure sed on SiO x porous films // Optoelectronik i poluprovodnikovy tekhnik 39, p. 65-72 (04) (in Ukrinin). 9. M. Nkmur, Y. Mochizuki, K. Usmi et l., Infrred sorption spectr nd compositions of evported silicon oxide (SiO x ) // Solid Stte Communs, p. 1079-1081 (1984). 10. S.V. Svechnikov, E.B. Kgnovich, Luminescent properties of Si nnostructures (review) // Optoelectronik i poluprovodnikovy tekhnik 39, p. 5-26 (04) (in Ukrinin). 11. I.Z. Indutnyy, I.Yu. Midnchuk, V.I. Min ko et l., Effect of chemicl tretment on photoluminescence spectr of SiO x lyers contining Si nnoprticles // Fizik i Tekhnik Poluprovodnikov 41 (10), p. 1265-1271 (07) (in Russin). 12. P.K. Kshkrov, B.V. Kmenev, E.A. Konstntinov et l., Dynmics of noneqilirium chrge crriers in silicon quntum wires // Uspekhi fizicheskikh nuk 168 (5), p. 577-582 (1998). 13. G. Wilieke, K. Kellermnn // Semicond. Sci. Technol. 11, p. 415-4 (1996). 14. T. Tkmi, S. Ishidzuk, Y. Igri, H. Rnge, I. Kusunoki, Rection of Si(111) surfce with cetone // Thin Solid Films 376, p. 89-98 (00). 15. S.Y. Seo, K.S. Cho, J.H. Shin, Intense lue-white luminescence from cron-doped silicon-rich silicon oxide // Appl. Phys. Lett. 84, p. 717-719 (04). 109