Annealing temperature dependence of the optical properties of sputtered hydrogenated amorphous silicon carbide

Transcription

1 Journl of Non-Crystlline Solids 128 (1991) North-Hollnd Anneling temperture dependence of the opticl properties of sputtered hydrogented morphous silicon cride A. Crone, F. Demichelis nd G. Knidkis Diprtimento di Fisic del Politecnico di Torino, Corso Duc degli Aruzzi, 24, 1129 Turin, Itly Received 22 Octoer 199 Opticl mesurements (UV-visile-NlR) hve een performed t room temperture on sputtered -SiC:H smples fter nneling t tempertures, T, etween 673 nd 973 K. The complex dielectric constnt in the rnge of ev nd, using the Wemple-Di Domenico model, the dispersion energy, the Penn gp, the vlence electron density, the plsmon energy nd the Fermi energy were deduced. The energy gp ws determined y mens of vrious models. The decrese of energy gp nd the increse of the vlence electron density on one hnd, nd the shift of the exponentil sorption edge on the other hnd, indicte the coupling of the vlence nd the conduction nd sttes y opticl photon s the nneling temperture increses. Moreover, s T exceeds 773 K, the polycrystlline phse cn e oserved to germinte inside the pre-existing morphous phse. 1. Introduction Studies on the physicl properties of morphous silicon cride -SiC:H t room temperture fter therml nneling nd on hydrogen effusion hve een performed recently [1-3]. It hs een shown tht, for -SiC:H films deposited y glow-dischrge, most of the hydrogen leves the smples during nneling elow 773 K nd tht the trnsition from the morphous stte (-SiC:H) to the polycrystlline stte (poli-sic) occurs t out 923 K. Aove this temperture, the smples ecome mixed phse mteril consisting of su-micron size crystllites emedded into morphous mtrix. This new-phse mteril is of interest ecuse of its potentil ppliction in thin film devices. In this pper, we report on systemtic study of the opticl properties of nneled -SiC:H films produced y rf mgnetron Sputtering. The opticl constnts were deduced y mens of mesurements t room temperture in the UV-visile- NIR regions. The nneling tempertures rnge ( K) hs een chosen to scn the hydro- gen effusion nd phse trnsition from morphous to crystlline form. A creful nlysis of the opticl constnts llowed us the determintion of some chrcteristic prmeters of the mteril. 2. Experimentl detils The films were deposited y rf mgnetron sputtering. The deposition prmeters were: Ar flow rte 1 sccm, CH 4 flow rte 1.5 sccm, pressure 1.6 P, sustrte temperture 2 C. The films hve thickness vrying in the rnge ~m. The only vrile ws the power density which ws vried from.6 to 1.6 W cm-2. Isocronl nneling ws crried out for 1 h in vcuum (1-2 P) t nneling tempertures, T, vrying etween 673 nd 973 K. The refrctive index, the sorption coefficient, nd the rel nd imginry prt of the dielectric constnt were otined from trnsmittnce nd reflectnce mesurements performed with Perkin-Elmer UV-visile NIR Lmd 9 spectropho /91/$ Elsevier Science Pulishers B.V. (North-Hollnd)

2 14 A. Crone et l. / Sputtered hydrogented -SiC Z K nneled SP25 o 2r K, e e %. 6t K, eoo.. Oo o o Ihs deposited "~"o ~o~q, ~t iiiii I o..o~. ",, "-- C ~o'.s A A i.o~ ZOO..:,, `...- o'. ojll AI o.%,." % o%*oo~o,~/ **, ) JJJ fo~o Jl~JJ i * g~%o. Mtt," 3 S27 K nnelsd o 827 K 727 K * 627 K S depoz;ted..'o'o" " "o..,-oo " " 2** t1 I ii I Ii1~O* eo el laa eeoo O AA AA Oe.***?~**o. o o..%.._ "'IS" ";" o E[eV] E[eV] Fig. 1. c I vs. photon energy, E, for () SP25 smple nd for () SP1 smple. tometer. The extrction of the opticl constnts ws mde following procedure descried elsewhere [4]. A Hitchi $27 scnning electron microscopy (SEM) t vrile mgnifiction ws used to oserve the film structure. The smples re leled SP followed y numer indicting the totl power in wtt. 3. Results In figs. l() nd () the rel prt of the dielectric constnt, ~1, for the smples SP25 nd SP1, respectively, is plotted vs. photon energy for vrious nneling tempertures, T. The vrition of c 1 = n 2- k 2 with photon energy nd nneling tempertures essentilly reflects the ehviour of n 2 since n >> k for these films. We oserve tht the vlues of ~] re higher for the smple richer in silicon nd, for oth, they increse with T. As the rtio Si/C (otined y mens of nucler techniques) increses, shift of the energy corresponding to Clmx towrds lower energies is oserved. This energy is 4.6 ev nd 5.3 ev, respectively, for smples SP25 nd SP1. The vlues of Clmx re reported in tle 1. The Tle 1 Opticl prmeters Smple T Si/C (lmx (2mx Em Ed hgdp (ev) n v EF (K) (ev) (ev) (ev) (123 e/cm 3) (ev) SP25 s deposited SP1 s deposited

3 A. Crone et l. / Sputtered hydrogented -SiC 141 corresponding vlues for -Si, c-si nd c-sic re, respectively, 2.7 ev [5,6], 3.3 ev [5,7] nd 6.4 ev [8]. From the experimentl vlues of the ~](E), we resort to n evlution of the spectroscopic prmeters: the dispersion energy, E, nd the verge excittion energy, E d, lmost equl to the Penn gp [4]. 4. Discussion According to the model of Wemple-Di Domenico [1] we hve EdEm el(e) = E~" (1) E m - A plot of (el(e) - 1) -] vs E 2 yields the vlues of E~ nd E m reported in tle 1. We oserve tht our experimentl results for E d re in greement with Wemples's contention of the dependence of E d on the short rnge order nd re comprle to tht of crystlline silicon for the smple rich in silicon t tempertures elow the crystlliztion temperture, T~. Aove this temperture, the vlues increse. By contrst, the results on the smple less rich in silicon show vlues of E d lower thn tht of c-c (49.7 ev). These lower vlues cn e explined y the presence, in the smple, of cron in grphitic-like rther thn in dimond-like form. However E d increses with T pproching the vlues (42 ev) of c-sic t tempertures ove T~. The verge excittion energy, Em, is relted to the plsm frequency, h~p, nd then to the plsmon energy y the reltionship [9] e(o) - 1 = ( h~op)2/e 2 (2) nd (ho)p) 2 is directly relted to the density of vlence electrons n~ y (hcp) 2 = h2e2n,,/meo, (3) where the symols hve their usul menings. From eqs. (1)-(3) we deduce me E~ nv- h2e 2 ~()- 1" (4) Mking use of vlues of Er, we clculte the vlues of n~ nd the plsmon energy hwp reported in tle 1. It is known tht opticl trnsitions in morphous semiconductors, s in their crystlline counterprt, cn e treted s one electron excittion. Such trnsitions require the sorption of photon nd contriute to the imginry prt of the complex dielectric constnt, ~2, since it represents the sorptive prt of such constnt. In figs. 2() nd () the dependence of ~2 on photon energy for the sme smples s in figs. l() nd () is plotted. The vlues of e 2 re reported in tle K innlllld $P25 o l? K 727 ~t er K. il depoll lqd iloa -'2." o %.... % ~ooo" o. o ol.m-.. e.%, *-.'... -'z ".o'. o;'.;- ti:.o~. e% 1 e'-oe o, :..." - re -:.:.- {O K Nne~(I o 827 K 727 K 627 K l deposi te.oo.~%* o-. E[eV I E[eVl O~*to~ ooo oo%..; -. O-o e o eoo. o % % eo% AAAA o o ~ o m I AAm % ~ e m A A Am $oo.i ~o * m A A AA ~o* ma $ ola oem A %* Fig. 2. ( z vs. photon energy, E, for () SP25 smple nd for () SPI smple.

4 142 A. Crone et L / Sputtered hydrogented -SiC Tle 2 Energy gp nd slope Smple T~ (K) ~2 (c2e2) 1/2 (c2e) 1/2 SP1 SP25 Eg (ev) B (ev)-' Eg (ev) B E~ (ex /3 (ev) -'/2 s-deposited s-deposited Since the re under the curves of c vs. E is mesure of the totl sorption, the increse of c2 nd of the quoted re s T, increses indicte n increse of the vlence nd density of sttes. This increse is in greement with the vlues of n v otined y the reltionship 8'rrm~,'Em. x n~ - h2e T- Jo %(E)E de, (5) where Em x is the energy corresponding to ~2mx- The vlues of n v s clculted from eq. (5) re the sme, within experimentl errors, s those otined from eq. (4) nd reported in tle I. When n v is known, the Fermi energy, E F, cn e evluted from the reltionship. k2 [-, 2 ~2/3 E F = ~ ~-rr n v ). (6) The clculted vlues of E r re reported in tle 1. The Fermi energy increses slowly to vlue out constnt for tempertures corresponding to the polycrystlline form of the more silicon rich smple. In the smple with less silicon, E F increses shrply due to the effect of cron content. 4xl 5 '7 E o o 4 xl o 927 K nneled 827 K 727 K 627 K s deposited,,.~;.; II. A oo m iiiiiii 1 It 13ll II AI~A~ i A gl o'* f" = =,t/. p II = 4+~+ A Oil= w +~ Zl = o~, == & o o. A if;::" o ~, e m=a. ~ A A~ SP25 4 xl s 7 E g3 4~I 3 927K nneled [] o = 827 K = o = = '== 727 K ~ =1= tl ==" ==.~== =A.o & 627 K = []." AA.. * [] l AAA e s deposited o z.= ++." +*1 O o = ~ e eee O A& o ee [] l I e DO = A ~++. ==.." :;.;." gzl~: /~2/ ~ *." 2 ;..,,,"'i -!i ooono, ~::.;2** SP1 1.2 E [ev] 3.2 E [ev] 3.2 Fig. 3. Asorption coefficient,, vs. photon energy, E, for () SP25 smple nd for () SP1 smple.

5 A. Crone et L / Sputtered hydrogented -SiC 143 In figs. 3() nd () the sorption coefficient,, is plotted vs. energy. Since in the fundmentl sorption region the sorption is due to trnsitions from the top of vlence nd to the ottom of the conduction nd, n increse of density of stte in the vlence nd with increse in nneling tempertures (see tle 1) shifts the sorption edge to lower energies. Such shift to lower energies with the nneling temperture is similr to tht of c-si sorption edge [11,12]. It is significnt to note the shrp vrition of when T is greter thn the crystlliztion temperture, T c. The experimentl vlues of c 2 nd led us to evlute the opticl gp. It is found [5,13,14] from theoreticl considertions tht function of energy of the form f(e) =,2(E)'E (7) cn e used to determine the opticl gp for morphous semiconductors. The nd o prmeters depend on the choice of the form of the nds nd of the trnsitions mtrix element. The opticl gp is deduced from the intersection of the stright line of the liner trend of the function with the energy xis. Tking into ccount our results we hve exmined the following expressions for f(e): f,(e) =c2(e ), (8) f2(e) = [E2(E)E] 1/2, (9) f (E) = (1) 3.6 SPIO SP Eg levi 2.2 Fig. 5. Sttic refrctive index, n o, vs energy gp, Eg. The first model ssumes liner nds, constnt dipole mtrix element nd direct trnsitions etween extended sttes in the vlence nd conduction nds. The model represented y eq. (9) considers indirect llowed trnsitions under the ssumption of prolic nds. Finlly eq. (1) is the well known Tuc formul. The model ssumes prolic nds nd constnt momentum mtrix element. The vlues of the extrpolted gps nd the reltive slopes re reported in tle 2. The gps of our s deposited films re comprle to those otined y mny uthors for rf-sputtered films. After nneling, we note decrese in the opticl gp t tempertures ove 673 K which indictes n irreversile process nd my e minly ttriuted to the loss of hydrogen. At temper SPIO,=, ~e SP25 SPlO il/i T [K] 1273 Fig. 4. () Energy gp, E~, vs. nneling temperture, T, for SP25 nd SP1 smples. () Slope, /3, vs. nnefing temperture, T, for SP25 nd SP1 smples.

6 144 A. Crone et l. / Sputtered hydrogented -SiC tures ove T c decrese of Eg is oserved, wheres the slopes, fl, ecome constnt. The trends of Eg vs. T, otined y mens of the three exmined models, re plotted in fig. 4(). In fig. 4() the slope fl vs. T otined y the Tuc model is plotted. From the Krmers-Kronig reltionship [15,16] written for E =, we hve c1() = f '2(E) ~ d E. (11) Since for E < Eg, ~(E)=, the inferior limit of the integrl ecomes Eg. Tking into ccount tht c1( ) = n~, where n o is the sttic refrctive index, eq. (1) reltes n o nd Eg. In fig. 5 n o vs. Eg is plotted. The interpoltion of experimentl dt gives liner reltionship, the nlytic expression of which is n o = Eg (smple SP25), (12) n o = 8-2.5Eg (smple SP1). (13) The slope, dno/deg= 2.5 ev -1, is the sme for the two smples. A scnning electron microgrph for the smple SP25 nneled t 973 K, in which microcrystls distriuted in n morphous mtrix cn e oserved is shown in fig. 6. The sumicron crystls present t 673 K tend to conglomerte giving rise to microcrystls distriuted in the morphous mtrix. Fig. 6. Scnning electron microgrph for SP25 smple nneled t 973K. 5. Conclusions Incresing T results in the coupling of the vlence nd conduction nd sttes y opticl photons s well s in the progressive evolution to phse chrcterized y the presence of polycrystlline stte in n morphous mtrix. These considertions cn e justified y the following experimentl results: (1) the continuous increse of the rel nd imginry prt of the dielectric constnt through out the exmined spectrl rnge. (2) the increse of the dispersion energy, the Penn gp, the plsmon energy, the Fermi energy nd the vlence electron density. (3) the trend of the opticl energy gp vs. T (light decrese nd sturtion ove 873 K independently of the theoreticl model used for its clcultion). (4) the shift of the sorption edge. References [1] R.A. Street, C.C. Tsi, J. Kklios nd W.B. Jckson, Philos. Mg. B56 (1987) 35. [2] I. Solomon, in: Proc. 1st Int. Symp. on Physics nd Applictions of Amorphous Semiconductors, ed. F. Demichelis (World Scientific, Singpore, 1987) p [3] D.A. Anderson nd W.E. Sper, Philos. Mg. B35 (1977) 1. [4] F. Demichelis, G. Knidkis, A. Tgliferro nd E. Tresso, Appl. Opt. 26 (1987) [5] G. Cody, in: Semiconductors nd Semimetls, Vol. 21, Prt B, ed. J.I. Pnkove (Acdemic Press, New York, 1984) p. 11. [6] H. Piller, in: Hndook of Opticl Constnts of Solids, ed. E. Plik (Acdemic Press, New York, 1985) p [7] D.F. Edwrds, in: Hndook of Opticl Constnts of Solids, ed. P. Plik (Acdemic Press, New York, 1985) p [8] W.J. Choyke nd E.D. Plik, Hndook of Opticl Constnts of Solids, ed. E. Plik (Acdemic Press, New York, 1985) p [9] D.R. Penn, Phys. Rev. 128 (1962) 293. [1] S.H. Wemple nd M. Di Domenico Jr., Phys. Rev. B3 (1971) [11] H.A. Wekliem nd D. Redfield, J. Appl. Phys. 5 (1979) [12] G.E. Jellison Jr. nd F.A. Mdine, Appl. Phys. Lett. 41 (1982) 18.

7 A. Crone et l. / Sputtered hydrogented -SiC 145 [13] J. Tuc, R. Grigorovici nd A. Vncu, Phys. Sttus Solidi 15 (1966) 627. [14] G. Knidkis, in: Properties of Amorphous Silicon (IN- SPEC, Institution of Electricl Engineers, London, 1989) p [15] H.A. Krmers, in: Atti del Congresso Internzionle dei Fisici (Proc. Int. Cong. on Physics), Como, ed. N. Znichelli, Bologn (1927). [16] R. Kronig, Ned. Tijdschr. Ntuurk. 9 (1942) 42.