Research Article Effect of Silicon Crystal Size on Photoluminescence Appearance in Porous Silicon

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1 Interntionl Sholrly Reserh Network ISRN Nnotehnology Volume 2011, Artile ID , 6 pges doi: /2011/ Reserh Artile Effet of Silion Crystl Size on Photoluminesene Apperne in Porous Silion Pushpendr Kumr Physik der Kondensierten Mterie, TU Brunshweig, Brunshweig, Germny Correspondene should e ddressed to Pushpendr Kumr, pkumr iitd@yhoo.om Reeived 31 Mrh 2011; Aepted 16 My 2011 Ademi Editor: M. Tommsini Copyright 2011 Pushpendr Kumr. This is n open ess rtile distriuted under the Cretive Commons Attriution Liense, whih permits unrestrited use, distriution, nd reprodution in ny medium, provided the originl work is properly ited. The photoluminesene (PL) study in porous silion (PS) with deresing Si rystllites size mong the pores ws reported. The PL pperne is ttriuted to eletroni onfinement in olumnr-like (or dotlike) strutures of porous silion. Three different pore dimeter PS smples were prepred y eletrohemil ething in HF-sed solutions. Chnges in porous silion nd Si rystllite size were studied y oserving n symmetri rodening nd shift of the optil silion phonons in Rmn sttering. Fourier trnsform infrred spetrosopy (FTIR) ws used to study the role of siloxene or other moleulr speies, for exmple, SiH x in the luminesene mehnism. This mehnism ws further studied y therml nneling of PS t different tempertures. The PL of PS smple nneled t 300 C for 1 hr shows tht trp eletroni sttes pper in the energy gp of the smller nno-rystl when Si O Si onds re formed. From the oservtion of PL, Rmn, nd FTIR spetrosopy, the origin of PL in terms of intrinsi nd extrinsi properties of nnorystlline silion ws disussed. 1. Introdution The disovery of photoluminesene in porous silion (PS) y Cnhm in 1990 hs triggered lrge-sle investigtion of its use in tehnologil pplitions, for exmple, lightemitting diodes [1],light testing equipment[2], photoeletri solr tteries [3], gs testing devies[4], mirodevies [5] nd iologil testing equipment [6]. The physil properties of porous silion re fundmentlly determined y the shpe, dimeter of pores, porosity, nd the thikness of the formed porous lyer. Depending on the ething prmeters, for exmple, urrent density, HF onentrtion, or sustrte doping type nd level, the physil properties of PS n e vried [7]. In ddition, when the feture size of the pores of PS is less thn few nnometers, vrious quntum-size effets our, whih mke PS even more fsinting. The well-studied PS morphology n rnge from spongy miroporous (pore size <10 nm) nd rnhy mesoporous silion (pore size nm) to the lssil mroporous silion (pore size 50 nm 20 μm). Severl theories on the formtion mehnisms of PS hve een reported sine its disovery [8, 9]. Different models hve een reported to explin PL in PS. Quntum onfinement is the first model proposed to explin visile photoluminesene from porous silion [10]. Lter, mny other lterntive models were proposed sed on hydrogented morphous silion, surfe hydrides, defets, siloxene, nd surfe sttes [11, 12]. Exept for the quntum onfinement model, ll the others ssume n extrinsi origin of PS luminesene. Brndt et l. [13] hve studied PS nd siloxene y using Fourier nd Rmn spetrosopy nd shown tht the room temperture PL in PS is not n intrinsi property of rystlline silion (quntum onfinement effet) ut depends on speifi hemil retions of silion with hydrogen nd oxygen. These oservtions hve further een supported y Fuhs et l. [14]. Gorh et l. [15] supported the view tht hemil pssivtion in PS, in prtiulr y oxygen, is the mjor ftor whih ontrols the origin of PL y using the simultneous investigtion of PL, FTIR sorption, nd SEM oservtion of PS. Mny theoretil [16] nd experimentl [17, 18] reports suggest tht quntum onfinement is most effetivetoexplinplinps.however, there is still n importnt hllenge to understnd the optil properties of PS mteril in terms of its fundmentl

2 2 ISRN Nnotehnology physis. To disriminte the different pprohes (i.e., quntum onfinement or siloxene or some other speies in PS) to explin PL in PS vrious methods n e used. One of them is the study of the ehvior of PL spetrum under vrition of Si rystllites sizes. To further explore the physis ehind the origin of PL in PS, we hve studied systemtilly the pperne of PL in PS with deresing (or inresing the pore dimeter) the Si nnorystllites size mong the pores y using Rmn, PL, nd FTIR spetrosopy. 2. Experimentl Detils PS lyers were prepred y eletrohemil nodi ething of highly doped p-type 100 -oriented silion sustrte with resistivity of Ω m. The eletrohemil ell ws mde of Teflon nd the se plte ws mde of luminum. A silion wfer ws mounted t the se plte y using n O- ring tht llowed 7.0 m 2 of the Si surfe to e exposed to the eletrolytes. The lter onsist of three different onentrtion of 3 : 7, 4 : 6, nd 6 : 4 y volume 48% hydrofluori id (HF): 98% ethnol, respetively. Prior to eh experiment, the Si wfers were ultrsonilly lened in propnol nd etone. The ething urrent density 14.5 ma/m 2 ws pplied for 30 min. The thiknesses of the porous lyers determined with n optil mirosope were in the rnge of μm. The physil properties (i.e., pore dimeter (D P ) nd the porosity (Φ)) of PS smples were determined y reording volumetri nitrogen sorption isotherms t T = 77 K in homemde ll-metl gs hndling system tht is equipped with memrne pressure guge of 1000 mr full sle [19]. The PL mesurements were done using Hmmtsu TG- SWNIR, C9405CA spetrometer nd lser diode exiting the smple t wvelength of 405 nm. Rmn sttering experiments were performed using the exittion line λ = 532 nm of semiondutor lser in qusiksttering geometry. The lser power of 5 mw ws foused to 0.1 mm dimeter spot on the PS surfe. The sttered spetr were olleted y DILOR-XY triple spetrometer nd nitrogenooled hrge-oupled devie detetor. For the mesurements, the smples were pled in smple hmer whih ws evuted or, lterntively, filled with helium gs in order to suppress the low frequeny Rmn sttering y ir nd to prevent possile lser-indued oxidtion of the PS lyers. Sine the PS provides lrge surfe re [20], therefore it hs een extensively studied y FTIR to study surfe speies [21]. A FTIR (Perkin Elmer, system 2000 FT-IR) spetrosopy ws used to mesure moleulr virtionl exittions in PS. To improve the sttistis, dt of 51 spetr were vergedwithresolutionof4m 1. For the FTIR study, free-stnding PS lyers were used to study the hemil speies, s the highly doped ulk silion wfers re not trnsprent for the IR light. After ething of silion for 30 min, the porous lyers were dethed from the silion sustrte y sudden inrese in pplied urrent for 60 s. 3. Results nd Disussion The physil properties of PS n e vried y hnging the ething prmeters, for exmple urrent density, HF onentrtion, or sustrte doping type nd level [7]. In the present se, we hve vried the HF onentrtion to hnge the pore dimeter nd porosity. The three different HF onentrtion, tht is, 30% HF: 70% ethnol, 40% HF: 60% ethnol nd 60% HF: 40% ethnol during ething of Si were used to prepre the PS smple,, nd respetively. The pore dimeter nd porosity of PS ws found to derese with n inrese of the HF onentrtion in the used ething eletrolyte t onstnt pplied urrent density. Theoretil nd experimentl detils onerning this effet hve een reported y some groups [22, 23]. In our work, we report tht the photoluminesene pperne in PS with inrese in pore dimeter (i.e., derese in Si rystllite size mong the pores) is due to quntum onfinement y using forementioned tehniques Si Nnorystllites Size Determintion nd Phonon Dynmis in PS. Rmn spetrosopy, whih is sensitive proe to the lol tomi rrngements nd virtions, hs een used to hrterize PS [24] nd Si nnostrutures [25]. The shift nd hlf-width of the one-phonon Rmn pek in -Si hs often een used to otin n estimte of the hrteristi dimensions of the Si rystllites. To hrterize nd interpret the Rmn spetr of semiondutor nnostrutures, phenomenologil phonon onfinement model [26, 27] hs een widely used. This model, originlly proposed y Rihter, Wng, nd Ley (RWL model) [26], hs een found to e useful for ounting for the pek-position shift, rodening, nd symmetry of nds oserved in nnorystlline Si, [25] Ge[28], nd GAs [29]. The Rmn shift due to the onfinement n e desried y phenomenologil pproh tht is sed on the negtive dispersion of optil phonons with finite momentum nd the verging nd folding of phonon frequenies in smll prtiles. An nlyti form of this pproh to desrie the Rmn frequeny shift s lso used y Pillrd et l. [30]is ( ) γ Δω = A, (1) L where = nm is the lttie onstnt of Si, L is the rystllites size, the prmeters A = 52.3, nd γ = re used to desrie the virtionl onfinement due to the finite size in nnorystl nd depend on the investigted system. By using this eqution we hve lulted the rystllites size of Si mong the pores. In Figure 1 we show the phonon frequeny of porous silion smples,, nd. A pek shift ws oserved from 519 to 516 m 1 nd rodening of the pek width full width t hlf mximum (FWHM) from 4.2 (symmetri) to 8.3 m 1 (symmetri) with n inrese in pore dimeter (D P ) from 3 nm to 10 nm, respetively. The shift of the phonon pek towrds lower wvenumer nd rodening of the pek width with inrese in pore dimeter nd porosity re ttriuted to the onfinement of optil phonons in nnodimensionl Si rystls mong the pores [27, 31]. The shift of the phonon pek ws used to lulte the rystllites size of Si in smple,, nd y using ove eqution. The lulted rystllites size of Si nd other physil properties of PS smples re listed in Tle 1.With

3 ISRN Nnotehnology 3 : D P = 8 nm, L = 4.3nm : D P = 3 nm, L = 6.6nm : D P = 10 nm, L = 2.8nm S i r e f e r e n e I n t e n s i t y (. u. ) R m n s h i f t (m 1 ) Figure 1: Rmn spetr of ulk Si nd PS smples,, nd. D P is pore dimeter nd L is the rystllites size of Si mong the pores. PL intensity (.u.) Wvelength (nm) : L = 2.8nm : L = 4.3nm : L = 6.6nm Figure 2: Photoluminesene of PS smples,, nd. inrese in pore dimeter from 3 nm to 10 nm the nnorystllites size of Si mong the pores dereses from 6.6 nm to 2.8 nm respetively. When the rystllites size of Si pprohes the dimension of free exiton (i.e., 5 nm) of ulk Si the photoluminesene strts to pper whih is disussed in next setion Photoluminesene Apperne in PS. Photoluminesene nd Rmn spetr of porous silion give the informtion of ross-setionl view of the silion nnorystllites whih remins mong the pores, euse of the lrge optil penetrtion depth. Photoluminesene is diretly relted to the eletroni struture nd trnsitions. The hrteristi hnge of the nd gp with derese in size of the nnostrutures n e studied y the oservtion of the lue shift in the photoluminesene pek position [32]. Thus, T r n s m i s s i o n (. u. ) T r n s m i s s i o n (. u. ) HSi O 3 H 2 Si O 2 S i F x S i H 3 SiH 2 S i H Si O Si S i H 2 S i H S i H W v e n u m e r ( m 1 ) Figure 3: FTIR spetr of PS smples,, nd. S i H S i H 2 S i F x Si O Si S i H 2 W v e n u m e r ( m 1 ) HSi O 3 H 2 Si O 2 S i H 3 S i H S i H Figure 4: FTIR spetr of PS smple () fresh PS nd nneled for 1hrstdifferent temperture, () 100 C, () 200 C, (d) 300 C, (e) 400 C, nd (f) 500 C. photoluminesene is useful in the study of quntum onfinement of eletrons in low-dimensionl mterils [33]. In Figures 2() 2() we show PL spetr of smple,, nd. It n e seen from Figure 2 tht with derese in Si nnorystllites size the photoluminesene strts to pper nd inreses in intensity. In Figure 2() we n see smll intensity of PL pek while in smple s shown in Figure 2() we see strong PL pek round t 735 nm. Bulk silion hs n indiret nd gp nd does not show ny photoluminesene pek while porous silion shows photoluminesene pek t round 750 nm. When the Si rystllites size is redued to 2.8 nm, muh smller thn the dimension of free exitons, strong quntum onfinement in Si nnostruture strts to our leding to the pperne of photoluminesene. Severl mehnisms hve een suggested to explin the pperne of this luminesene signl, suh s defets tht lolize rriers t extrinsi enters or surfe sttes providing rditive reomintion enters, surfe hydrides (SiH x groups) or the formtion of siloxne ( Si O H sed polymer), or quntum-onfinement effetsof the exitons used y their sptil onfinement within Si-nnorystllites. The ltter mehnism ws proven to e the origin of the oserved f e d

4 4 ISRN Nnotehnology Tle 1: Vrition of physil properties s funtion of HF onentrtion nd orresponding Si rystllites size of smple,, nd. Smple detils Eletrolyte Pore di. Porosity Pek width Phonon frequeny shift Si rystllites size (HF : ethnol) (nm) % m 1 m 1 (nm) 3: : : PL intensity (.u.) Wvelength (nm) Figure 5: PL spetr of PS smple () fresh PS nd nneled for 1hrstdifferent temperture, () 100 C, () 200 C, (d) 300 C, (e) 400 C, nd (f) 500 C. F r e s h P S N n o p o r e s N n o r y s t l S i CB V B Bulk Si S o u r e A n n e l e d P S O x i d i z e d p o r e w l l CB VB f e d O x i d e i n t e r f e sttes Figure 6: Shemti digrm of fresh PS nd nneled PS nd their orresponding energy levels. luminesene. There is still the disussion open on the mehnism of PL in PS. The sene of PL from smple indites tht porous silion with the Si rystllites size of 6.6 nm ehves similr to ulk silion. The sene of PL signl from smple nd strong signl from smple indites tht the quntum onfinement effet in the nrrow silion regime etween two pores results in strong PL signl. To exlude n effet of other hemil speies, for exmple, SiH x,siloxene, nd so forth, we hve rried out FTIR mesurement of ll three PS smples Chemil Speies in PS. In Figure 3 we show trnsmission FTIR spetr of ll three PS smples. The spetrum of PS smple exhiits Si H ending mode t 630 m 1, Si H 2 wgging mode t 680 m 1 nd Si H sissor mode t 910 m 1. The presene of n sorption pek round 1100 m 1 whih is ssigned to the Si O Si ntisymmetri strething mode in s prepred PS smple, indites tht the PS lyer is lredy prtilly oxidized while trnsferring the smple from the ething ell to the FTIR setup [34, 35]. The struture in the spetrum loted etween 2050 nd 2150 m 1 orrespond to silion hydride speies while those etween 2150 nd 2300 m 1 orresponds to silion oxyhydride speies. The three strong sorption peks t 2090, 2110, nd 2140 m 1 re ssoited with Si H, Si H 2, nd Si H 3 virtionl strething modes, respetively. The sorption lines t 2195 nd 2250 m 1 re ttriuted to H 2 Si O 2 nd HSi O 3 onding, respetively. The peks ssigned in Figure 3 re in good greement with dt reported in the literture [36, 37]. It is interesting to point out tht no ler differene in the FTIR spetr n e found in Figure 3 etween ll three PS smples. This mens tht the light emission in porous mterils re in no reltion to extrinsi origins or effets, suh s hydrogented morphous Si, surfe hydrides, defets in Si or SiO x,siloxene,orsurfe sttes. To further survey the effet of these speies, we nneled the smple () t different temperture up to 500 C for 1 hrs nd mesured the FTIR nd photoluminesene Therml Anneling of PS. In Figure 4 we show the FTIR spetr of thermlly nneled PS. From the spetr it is lerly seen tht most of the moleulr speies desors t 500 C. The orresponding photoluminesene of thermlly nneled PS t different temperture n e seen in Figure 5. Exept smll hnge in pek position nd intensity the PL is still oserved, whih further proves tht PL in PS with the rystllites size of 2.8 nm is not relted to the externl speies. The initil hnge in pek position of PL till the nneling temperture of 200 C ws lue shifted, whih n e understood tht the surfe-oxidtion-indued redution of the prtile sizes leds to higher PL pek energies. But on further inrese in nneling temperture the red shift in the PL spetr ws oserved. The red s well s lue PL pek position shift in PS with inrese in therml nneling temperture hs een reported y severl reserhers. Roy et l. hs reported the similr oservtions tht the PL pek in PS with inrese in nneling temperture shows first lue shift nd then red shift. They hve ttriuted their oservtion to the hyrid model in whih oth quntum onfinement nd luminesene entres outside the nnosle unit hs een ounted [38]. Reently Hung et l. hve reported theoretil lultion where they showed tht the

5 ISRN Nnotehnology 5 trp eletroni sttes pper in the energy gp of the smller nnorystl Si on therml nneling when Si=O onds or Si O Si onds re formed [39]. In the present study, the red shift in PL pek of PS n e understood s follows. In Figure 4 it n e seen tht the pore surfe initilly sturted y the hydrogen toms, nd during the oxidtion proess they re grdully sustituted y oxygen toms. When the nneling temperture is 300 C the oxides lyer on porous Si nnorystl formed. When nnorystlline Si is oxidized, the Si Si onds re likely to weken or rek euse of lrge stress t the distortion surfe. Then the Si O Si onds is more likely to e formed nd stilized the interfe, sine it requires neither lrge deformtion energy nor n exess element, while the lolized trp sttes re formed in nd gp grdully. A shemti digrm showing oxide formtion on pore wlls nd their orresponding lolized sttes formtion due to Si O Si onds re shown in Figure 6. The present oservtion of the red shift in PL pek position on therml nneling re in greement with the theoretil model out the trp eletroni sttes in nnorystl Si reported y Hung et l. [39]. 4. Conlusions The Si rystllites size ws vried in PS y hnging the HF onentrtion in ething eletrolytes. Photoluminesene ws oserved when the rystllite size of Si ws redued to smller thn the dimension of free exiton of ulk Si. The PL pperne in PS is ttriuted to quntum onfinement nd not to externl moleulr speies. The effet of externl speies ws studied with the help of FTIR nd further verified y the nneling of PS. The red shift in PL pek of thermlly nneled PS ws oserved due to the lolized sttes in nd gp. Our investigtion will furthermore e useful for the reserher to understnd the phenomen of PL in PS. Aknowledgment The uthor grtefully knowledge Tehnishe Physik, Universität des Srlnd for providing the FTIR mesurement filities. Referenes [1] A. Rihter, P. Steiner, F. Kozlowski, nd W. 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