EFFECT OF TEMPERATURE ON OPTICAL PROPERTIES OF GLOW DISCHARGE HYDROGENATED AMORPHOUS SILICON FILMS

Transcription

1 EFFECT OF TEMPERATURE ON OPTICAL PROPERTIES OF GLOW DISCHARGE HYDROGENATED AMORPHOUS SILICON FILMS A. Donnadieu, B. Yous, J. Berger, J. Ferraton, J. Robin To cite this version: A. Donnadieu, B. Yous, J. Berger, J. Ferraton, J. Robin. EFFECT OF TEMPERA- TURE ON OPTICAL PROPERTIES OF GLOW DISCHARGE HYDROGENATED AMOR- PHOUS SILICON FILMS. Journal de Physique Colloques, 1981, 42 (C4), pp.c4-655-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1981 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE CoZZoque C4, suppldnient au n070, Tome 42, ocbobre 1981 page C4-655 EFFECT OF TEMPERATURE ON OPTICAL PROPERTIES OF GLOW DISCHARGE HYDROGENATED APiORPHOUS SILICON F I LIIS A. Donnadieu, B. Yous, J.M. Berger, J.P. Ferraton and J. Robin LaboraLoire de Speetroscopie 11, Equipe de recherche assoeibe au C.R.R.S., ilniversit~ des Sciences et l'echniques Ou Lanped~c, 340CO?do~tpeZ?,ier, France Abstract* Optical properties of hydrogenated amorphous silicon films, prepared by glow discharge onto fused quartz substrates held at temperature TS varying between 50 and 350 C, were determined from near normal specular reflectance and transmittance measurements in the energy range 0.5 to 5.5 ev. The measurements were done at various temperature Tm in the range 95 to 750 K. When Tm becomes higher than TS, irreversible variations of optical gap appear. An interpretation in terms of an hydrogen evolution and a rearrangement of the matrix under annealing is proposed. I - Introduction0 Many inveo tigat ions to determine how the hydrogen is injorporated in glow discharge amorphous silicon films were done during the last years. PRITZCIIE, in a synthesis paper [l], indicated the different results. The lav temparature films (TS < 250 C) reveal a large amount of polymeric (SiB2)nas well as monohydride; the high temperature films (TS > 250'~) contain predominantly monohydride SiH and only a trace of dihydride. In order to understand the bonding configurations of hydrogen, same authors studied, at room temperature, the optical properties in function of H-content of samples and others studied post-hydrogenated a-si samples or hydrogen evolution under several annealings. We determined the optical properties of samples from near-normal measurements of reflectance and transmittance in the energy range 0.5 to 5.5 ev. The measurements were performed at temperature T,varying between 95 and 750 K. We found two effects : reversible and irreversible variations of optical gap with Tm. The sign of irreversible effect depends on the preparation temperature Ts. 'I - method* Samples of glow discharge silicon (GD) were deposited on quartz substrates by a capacitively coupled rf discharge in a mixture of.50 % argon - 50 X silane at 0.a torr. The deposition rate increased from 2.5.~/s for deposition at 350 C to 5 A/s at the lowest temperature. We can estimate that the H : Si ratio increases in this temperature range from below 8 at X (TS * 350 C) up to at X (TS - 50 C). The optically derived thicknesses varied from 0.9 )1 to 1.3 fl. The optical measurements of near normal reflectance and transmittance at various temperature Tmwere done in two apparatus which were described previously [2,3]. The optical gap Eg(Tm) was calculated from absorption coefficient a according to the equation (aii)lf2 = y [ ~ - E~(T,)] where E is the photon energy and y the slope of the straigth line. Each eeasurement at T, different of room temperature Ta = 295 K, was prece- ded and followed by a m~easurelnent at room temperature. We call ~:(295~, Hi) the optical gap of sample in Mi state after the pth measurement at T K. The following measurement at Tm # 295 K can change the composition (Rydrogen evolution) or the form (annealing effect) of the matrix and produce irreversible effects. If the heating at Tm produces reversible effect. then Eg(29Srl, Mi) - Eg(295, Mi) ; else if at Tm the matrix changes, the irreversible variat on of optical is Article published online by EDP Sciences and available at

3 C4-656 JOURNAL DE PHYSIQUE we can estimate the value of optical gap at T as if the matrix has not changed by m Eg(Tm, Mi+l) is the measured optical gap, Eg(Tm, Hi) the corrected optical gap. This formula can be used only if the change from Hi etate to Mi+l state does not modify the slope of the curve E (T Mi) = f(tm). g m' I11 - Experimental results. Fig. 1 shows the variation of (ae) = f(~) at various T, for a-sill thin film prepared by GD at TS = 350.C. We observe a shift of optical gap tcuards the lower energies when T, increases vithout sensible modification of the slope y. We found the same phenomenon for a11 studied samples. Pig. 1 - Variation of (a~)l/~ versus photon energy E for different Tm. Fig. 2 - Variation of optical gap Eg(Tm) versus Tm for two GD samples prepared 0t 50% and 350.C. Table I Table I gives the values obtained for aeveral samples. Pig. 2 shows the variation of optical gap Eg(Tm) versus Tm for two samples : the first one, with a lw hydrogen content, prepared at TS = 350'~ (curve a), the other with a high hydrogen content, prepared at TS - 50.C (curbe b). We obrerve for each sample

4 reversible and irreversible effects. IV - Discussion a) Irreversible effects. For high TS films we observe (fig. 2 curve a) irreversible effects above a Tmof 600 to 650 K. The variation of the optical gap AEg(Mi+i, Mi) is negative ; the mean value for all samples is ev after the heating at 673 K and ev after the heating at 723 K. We can attribute these variations to the decrease in the H-content when the temperature increases. Perrin et a1 [4] and BruySre et a1 [5] indicated that the variation of optical gap with the H-content is 0.03 ev (at %)-I for SiB bonds and 0.1 ev (at for (Sill2), bonds. If we use the last value, we find that the total hydrogen which is released is higher than the hydrogen contained before heatings. If we use the value relative to SiH bonds, we find that H-content decreases of about 1.5 at. Z after heating at 673 K and 3 at. X after heating at 723 K. The total variation of H-content corresponds to about 70 X of initial ratio. These observations agree very well vith results and interpretation given by Zellama et a1 [6] and confirm that high TS films contain predominantly monohydride. For low TS f ilma we observe (fig. 2 curve b) irreversible effects from a temperature Tm higher by 50-C than Ts. But here, the variation of the optical gap is positive and equal to about +0,06 ev. This increase is due to an annealing effect which produces a rearrangement of the matrix and a decrease in the density of states in the forbidden band. If there is an hydrogen evolution, it is not observable. All low TS films desintegrate at about Tm= 573 K. b) Reversible effects. Equation 2 allows to calculate the corrected optical gap E (T,, Mi) at high temperature T,. Fin. 4 shows the results obtained x-t fgr GD-samples. For comparison, we plot- ted the values relative to a Chemical Vapor Deposition (CVD) silicon sample prepared at 650.C [3']. For all samples the experimental values are localized between the theoretical curves of Varshni [7] and Ravindra et a1 [8]. For,' '4 t experimental curves, it appears an increase in the slope from CVD curve to the T = 50.C GD curve when the H con- 1.6-T,=350 c GD tent Sncreases. But the slope of curves relative to GD samples prepared at TS - 50-C and TS - 280% seems the same. For high TS samples the monohydride den sity increases and reaches a constant ~~650~cCVD value when TS decreases whereas for low TS samples the monohydride density is constant and the amount of polymeric 1.2 increases. We can then suppose that mo T,&) nohydride phase influences the slope of Fig. 3 - Experimental values (+) and optical gap curves with Tm. theoretical curves of Varshni (-) and c) Global efkcrr. The global Ravindra (--) of optical gap versus Tm. effects when Tm increases are different for the two kinds of samples. They both present a reversible effect until the composition or arrangeent of silicon matrix changes. When irreversible effect appears, its sign depends on TS. For the low TS samples the effect of rearrangement of matrix (alteration of the density of states in the forbidden band) is more *or- tant than the hydrogen evolution and gives rise to an increase in optical gap ; but these saqles desintegrate at about 573 K. For the high TS samples the inverse

5 C4-658 JOURNAL DE PHYSIQUE phenornon appears ; the hydrogen evolution predominates on the arrangement of the matri. and the optical gap decreases. As for CVD samples, for all them samples the optical gap tends twards a c-n values around 1.30 ev at 723 K den T increases (fig. 4). Ibh knowledge of variations of the eptical gap with temperature Tm is important ; watly because of the use of this uterial in solar energy conversion. T,=~SO"C CVD Almowledgement. The authors are grateful to Prof. G. UEISER and to J. BEICHLER from the University of MARBWG, RFA, for 121, providing the GD samples and to Prof ~,k B.O. SERAPEIN and to D. BOOTH from the Optical Sciences Center, University of Fig. 4 - Co~nparison of measured op- Arizona, for providing the CVD samples. tical gap of CVD and high TS GD silicon films. Ref erencea El] E. FRITZCHE Solar Energy Materials 3 (1980) 447. [2] J.P. WRRATDN, C. ANCE and A. WNNADIEU Thin Solid Films 78 (1981) 207. [3] A. DIVBBCEY, B. YOUS, J.M. BERGER, J.P. FERRATON, J. ROBIN and A. DONNADIEU Thin Solid Film 78 (1981) 235. [k] J. PEBBIW, I. SOLOIDN, B. BOURDON, J. FONTENILLE and E. LIGEON Thin Solid Pilw 62 (1979) 327. [5] J.C. BRUYERE, A. DENEWILLE, A. MINI, J. FONTBNILLE and H. DANIEUlU J. Appl. Phys. 51 (1980) K. ZELLAM, P. GERMAIN, S. BQUELARD, B. BOURDON, J. FONTENILLE and R. DANIELOU Phys. Rev. to be published. [7] Y.P. VARSBNI Physica 34 (1967) 149. [8] N. RAVINDRA and V.K. SRIVASTAVA J. Phye. Chem. Solids 40 (1979) 791.