Vibrational spectroscopy of hydrogenated evaporated amorphous silicon films

Journal of Non-Crystalline Solids 58 (1983) 153-163 North-Holland Publishing Company

153

VIBRATIONAL SPECTROSCOPY OF HYDROGENATED EVAPORATED A M O R P H O U S SILICON FILMS * N. KNIFFLER, B. SCHRODER and J. GEIGER Fachbereich Physik, Universiti~t Kaiserslautern, D- 6750 Kaiserslautern, FRG Received 17 January 1983

The infrared absorption spectra of amorphous silicon films prepared by evaporation in a hydrogen beam are measured between 400 and 4800 c m - 1. The dependence of the spectra on the temperature of the substrate and on the annealing temperature is investigated. A rearrangement of hydrogen to Si-H sites at intermediate annealing temperatures ( = 600 K) is observed, which parallels to the steep decrease of the electric room temperature dark conductivity. Impurity vibrations are also studied and a comparison is made with gd-a-Si and magnetron-sputtered a-Si films, where the Sill stretching mode near 2000 c m - 1 predominates.

1. Introduction

Hitherto a great number of papers dealing with infrared absorption of amorphous silicon films (a-Si films) prepared by glow discharge technique [1-7] or by reactive sputtering in a H 2 / A r atmosphere [1,8-10] have been published. IR absorption measurements of evaporated a-Si films were investigated less frequently. According to our knowledge two papers have to be mentioned: electron energy loss spectra of evaporated a-Si, which are essentially equivalent to infrared absorption spectra, were measured [11] and recently Dellafera et al. [12] reported on IR absorption spectra of hydrogenated films. More accurate infrared absorption measurements of evaporated a-Si films, which were prepared under high-vacuum conditions or in a H : atmosphere are still lacking. This gap will be filled by this paper. Since the electrical and other properties of some of these films were measured previously [13], they can be brought into relation to the changes of the infrared absorption spectrum. A comparison with films prepared by other methods is made, too.

This paper is also thought to be a contribution to enlighten the question, in which way molecular hydrogen will be incorporated in vacuum-evaporated films in order to saturate dangling bonds. Concerning this matter a controversy exists between Malhotra and Neudeck [14] and Miller et al. [15], whether a molecular-hydrogen atmosphere during evaporation may or may not affect the * This paper includes parts of the thesis by N. Kniffler.

N. Kniffler et al. / Hydrogenated evaporated sificon films

154

film properties. According to our experience an operating ionization gauge or mass spectrometer will influence the properties of the resulting films considerably.

2. Experimental The evaporation set-up was installed in a bakeable metal-sealed vacuum system. The evaporation source was equipped with a 10 kW electron beam gun. The deposition rate could be varied between 0.1 and some 10 A / s , and the temperature of the substrate (KBr) between 10 and 800 K. During the evaporation process a gas beam produced by a collimated hole structure could be directed onto the substrate. The absorption spectra were recorded between 400 and 4800 cm-1 by a Fourier spectrometer (Nicolet MX 1). The spectra shown in the following sections were obtained after subtraction of the underlying interference pattern, which was due to the reflection of light at the interfaces of the film (film thickness 1000-10000 ,~). Fig. 1 presents an overview about the absorption spectrum of an evaporated a-Si film, which was prepared at a pressure of 1 x 10 -6 Torr during evaporation. Since the surface covering rate by molecules arriving at the substrate out of the volume surrounding it at this ambient pressure is comparable with the deposition rate, the resulting film is expected to be contaminated by the typical residual gas molecules such as H 2 0 and hydrocarbons and their decomposition products. As known from electron energy loss spectra the localized vibrations of these impurities characterize the a-Si film and its previous history. The Sill x groups have stretching modes in the range around 2000 c m - l and bending modes near 620, 845 and 890 c m - i [1]. The stretching modes of the CHx groups appear between 2800 and 3100 cm -~, and the bending modes

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N. Kniffler et aL

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Hydrogenated evaporated silicon films

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between 600 and 900 cm-1 and between 1300 and 1500 cm-~ [16]. Other important vibrational modes are the stretching modes of the S i - O - S i group at 1000 and 800 cm-1 (weak) and the bending mode at 450 c m - t [17]. O H stretching modes are found between 3200 and 3350 c m - ~.

3. The stretching vibration of the S i l l x group

In this and the following section absorption spectra of samples, which were prepared in a H 2 beam, are reported and the change of the Sill x absorption bands during a stepwise annealing process is studied. Fig. 2 shows the absorption spectrum in the range of the Sill x stretching modes. The spectrum varies considerably when the sample is heated, in particular the absorption strength of the bands at 2090 and at 2000 c m - ] , which are as generally accepted attributed to the stretching modes of the Sill 2 group and of the Sill group, respectively. Fig. 3 shows the characteristic features of the annealing behaviour of the two vibrational modes more clearly. The absorption of the band at 2090 c m - ], which dominates at low temperatures increases first slowly with increasing annealing temperature but decreases rapidly above 550 K. The absorption of the band around 2000 cm-~ first increases and then, after the annealing

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N. Kniffler et aL / Hydrogenated evaporated silicon films

156

temperature has reached 630 K, decreases like the 2090 c m - ~ absorption band. The annealing behaviour below 600 K can be explained as a rearrangement process, where one hydrogen atom splits off from a Sill 2 group, and diffuses to a silicon site with a dangling bond, where it forms a Sill group. So one Sill 2 group vanishes and two additional Sill groups appear. It seems also possible that molecular hydrogen, which is dissolved in the network or in voids, interacting with a single dangling bond is thermically dissociated and immediately saturates the dangling bond [18]. These two processes lead to an increase of the absorption of the 2000 cm-1 band, the first one would also cause a decrease of the absorption of the 2090 c m - 1 band, in contradiction to the observation. Yet in a third process the incorporated hydrogen can diffuse to unsaturated double (and multiple) dangling Si bonds, where the hydrogen molecule can easily be dissociated to form Sill z groups. This should be the reason why in the average a weak increase of the absorption of the Sill 2 stretching mode is still observed up to 500 K. At higher temperatures the hydrogen bonds become gradually unstable and eventually the hydrogen evolves out of the film. In the spectral range of the stretching modes this process manifests itself at first necessarily by a reduction of the number of Sill 2 groups and correspondingly of the absorption at 2090 cm-~ and an increase of the number of Sill groups and of the absorption at 2000 c m - L With further increasing temperature the Sill as well as the remaining Sill 2 groups decompose finally. The annealing process is accompanied and controlled by structural relaxations and ordering processes, which

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N. Kniffler et al. / Hydrogenated evaporated silicon films

157

may lead to clustering of the Si atoms and finally to crystallization as can be proven by electron diffraction. Mattes [19] predicts a transition from a two-shell silicon cluster to a six-shell cluster at 600 K. Fig. 3 shows also that the temperature for maximum absorption of the 2000 cm-1 band coincides with the annealing temperature for which the room-temperature dark conductivity OD,RT has its minimum [11,13]. From this we conclude that hydrogen atoms from decomposing Sill 2 groups and, possibly, thermal activated dissociated hydrogen can saturate electrically active defects. The other bands observed in fig. 2 are explained as follows: Ginley and Haaland [20] have studied the Sill x stretching modes in silicon single crystals and in polycrystalline silicon, which were treated by the plasma of a hydrogen glow discharge. By etching they were able to distinguish between the two lines at 2019 and 2120 c m - l , which were attributed to a Sill stretching mode on the grain boundaries in the bulk and to a Sill 2 stretching mode on the crystal surfaces, respectively. Keeping this paper in mind, an interpretation of our annealed spectra seems to be possible in the sense of the model quoted by Mattes [19]. At a temperature of about 600 K six-shell silicon clusters are formed and hydrogen atoms will diffuse towards the boundaries of the cluster. At the boundaries they give rise to the 2020 and 2120 cm-1 absorption features observed in our spectrum. A further increase of the annealing temperature reduces the number of the Sill 2 groups as well as that of the Sill groups at the cluster boundaries. The peaks at 2193 and 2247 are attributed to the hydrogen stretching vibrations in OzSi-SiH and O3-SiH groups, possibly located at the surface oxide layer or near internal surfaces. The vibrational frequency of these groups is shifted to higher frequencies relative to that of the Si 3-Sill group. This shift was explained by the electro-negativity of the attached oxygen atom [21 ]. Since these samples were annealed in air the vibrations of the OvSi3_FSiH groups appear particularly clear. The absorption of the 2247 cm-1 band decreases with increasing annealing temperature, but a weak absorption is still observed up to rather high temperatures.

4. T h e deformation vibrations of the S i l l x groups

Fig. 4 shows the infrared absorption spectrum of evaporated a-Si in the region of the Sill x deformation vibrations (500 to 900 cm -1) as a function of the annealing temperature. In this region the spectrum is dominated by strong absorption at 630 and 1075 cm-1 attributed to the Sill x bending and rocking modes [1,4,21] and to the SiOSi asymmetric stretching mode. The rather weak structure at 845 cm-1 could be associated with the wagging vibration of the Sill 2 group. The mode around 870 c m - l may be due to a SiO vibration (875 cm-1, [7]) or a SiN vibration (880 cm-1, [27]). The SiO vibrations at 870 and 1050 cm-1 depend in a characteristic way on the annealing temperature and on the degree of oxidation [17]. While the absorption band around 1050 cm-1

N. Kniffler et a L / Hydrogenated evaporated silicon films

158

substrate temperature KBr substrate

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strongly rises with the annealing temperature the weak band at 870 cm-1 seems to disappear. The band at 450 cm 4 at the low-energy end of the spectrum is at least for the highest annealing temperatures, when the oxidation of the film is strongest, mainly due to the bending vibration of SiO. The amount of hydrogen contained in the a-Si film was determined from the integrated absorption of the 630 c m - i band according to the paper of Shanks et al. [10]. The result is shown in fig. 5 as a function of the annealing temperature TA and of the temperature of the substrate Ts. The concentration of hydrogen in the sample prepared at room temperature increases after annealing up to 370 K. This might be explained by reactions of hydrogen incorporated in the film with defect structures f18]. This is the same chemical reaction which should be also responsible at least partly for the increase of the absorption of the two stretching modes and for the decrease of the electric conductivity in fig. 3. At about 600 K the concentration of hydrogen in the film decreases strongly. Following Mattes [19] this may be explained by the formation of six-shell silicon clusters under the release of hydrogen. For still higher annealing temperatures the hydrogen effusion finally ceases and the 630 c m - l absorption band vanishes. The hydrogen concentration as a function of the substrate temperature of samples prepared in a H E beam shows up a similar but concerning the temperature scale contracted behaviour. Films

N. Kniffler et a L / Hydrogenated evaporated silicon films

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Fig. 5. The hydrogen content of a-Si:H films (PH2 ~- 1 0 - 7 Torr during evaporation with = 1 A / s ) as the function of the substrate temperature Ts. For Ts = 300 K the hydrogen content is also plotted versus the annealing temperature TA.

deposited on a room temperature substrate contain most hydrogen bound to silicon. This temperature seems to be an optimum in relation to the processes involved in. Higher temperatures lead to a decrease of the sticking coefficient of hydrogen and an increasing probability of a thermal decomposition of the hydrogen silicon bond. At lower temperatures less hydrogen is incorporated in bound states, since the surface diffusion of hydrogen decreases and a reaction of hydrogen with silicon becomes less likely. The incorporation of hydrogen in a-Si vapor-deposited films as a function of the substrate temperature was studied by Dellafera et al. [12], too. Caused by different preparation conditions the content of hydrogen in their films is higher than in ours. In any case, for temperatures above 300 K the character of the dependence of the hydrogen concentration on the substrate temperature observed by these authors is quite similar to that shown in fig. 5.

5. Comparison of absorption spectra of a-Si prepared by different methods Fig. 6 gives a comparison of the absorption spectra of an evaporated a-Si film, of a gd-a-Si film deposited by a glow discharge in silane, and of sputtered films. Most hydrogen is incorporated in the gd-a-Si film as estimated from the area under the 630 c m - ] absorption band. The content of hydrogen in the film sputtered in a conventional diode sputter system is smaller, but it cannot be determined very accurately since the background due to impurity vibrations is

Hydrogenatedevaporatedsilicon films

N. Kniffler et aL /

160

rather high. The latter follows from the appearance of very strong absorption near 850 and 1050 cm-1 due to SiO and from the presence of the 2250 c m band. An other sputtered sample was prepared in an unbaked ultra-high vacuum magnetron sputter system. In this case absorption between 700 and 900 c m - t is also observed but weaker. The peak at 770 cm-~ is caused by

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N. Kniffler et al. / Hydrogenated evaporated silicon films

161

S i - C v i b r a t i o n s of the S i - C H 3 group. The a b s o r p t i o n between 800 a n d 900 c m - i m a y be explained b y the S i - O stretching v i b r a t i o n (with b o u n d hydrogen) a n d b y the S i l l 2 scissors or wagging modes. N e a r the SiO stretching m o d e at 1060 c m - t a shoulder or a secondary m a x i m u m is found, which following Wieder et al. [5] m a y be a t t r i b u t e d to a C H x rocking or wagging mode. Fig. 6b illustrates that the U H V m a g n e t r o n sputtering technique enables to operate u n d e r p r e p a r a t i o n c o n d i t i o n s similar to the glow discharge method. Characteristic of both these m e t h o d s is a d o m i n a t i n g a b s o r p t i o n of the S i l l - s t r e t c h i n g v i b r a t i o n a r o u n d 2000 c m - 1 as c o m p a r e d to evaporated films a n d to samples sputtered in a c o n v e n t i o n a l diode system, for which the a b s o r p t i o n m a i n l y occurs b y the SiH2-stretching m o d e at 2090 c m - i. The high c o n t r i b u t i o n of the SiH2-stretching m o d e to the a b s o r p t i o n in evaporated films was explained by Dellafera et al. [12]. The two atoms of the h y d r o g e n molecule after its dissociation will r e m a i n close together, a n d therefore they will chemiTable 1 Positions of the absorption bands measured in the a-Si films, assignment, and comparison with the results of other authors Position of the band (cm- ~) As m e a s u r e d

Litterature

3200-3350

3225

2954 2923 2870 2853 2247 2219 2193 2145 2120 2080-2090 2020

2962 2926 2872 2853 2250 2220 2188 2153 2120 2090 / 2100 2019

1985(2000) 1600 1380

2000 1600-1670 1377

1350 1040

1365 1000-1110 880 880 878 840-850 780 630 480 455

865-875 835 775-780 630 480 455

Assignment

Ref.

O-H stretching vibrations in crystalline (SiOH)2 compounds asymmetric stretching in CH 3 asymmetric stretching in CH 2 symmetric stretching in CH 3 symmetric stretching in CH 2 Sill stretching in 03 -Sill Sill stretching in 02 -Sill 2 Sill stretching in 02 Si-SiH Sill stretching in OSi-SiH 2 Sill stretching on Si surface Sill 2 stretching in Si2-SiH 2 Sill stretching in OSi 2-Sill Sill stretching on Si grain boundaries Sill stretching in Si3Sill HOH bending symmetric deformation of CH 3 group bend waggingof the CH 2 group Si-O-Si asymmetric stretching Sill 2 bend-scissors(?) SiN vibration SiO stretching CH 3 rocking in OSi-CH 3 rocking or waggingof C H 3 o n Si Sill, Sill 2 bending, rocking Si-Si network vibration Si-O bending

22 16 16 16 16 21 24 24 24 20 21 21 20 21 16 4 4 16 4 27 16 25 16 21 17

162

N. Kniffler et al. / Hydrogenated evaporated silicon films

cally react to the same silicon atom with a high probability. The reason for the preponderance of the Sill 2 groups in films prepared by a conventional diode sputter system is not that clear. Oguz et al. [23] observed for samples sputtered in a diode system with decreased distances between target and substrate an increase of the 2000 cm-l band absorption. The photovoltaic properties of these films, however, were found to get worse. The target-substrate distance for the UHV magnetron sputtering system is rather large ( = 20 cm). Apparently, the hydrogen molecules are dissociated more efficiently by the magnetron source, and under appropriate discharge conditions hydrogen is capable for the formation of the desired binary alloy Si:H. The excellent photo-conductivity of these films proves the effective saturation of the dangling bonds [26]. Table 1 collects all the data for the positions of absorption bands obtained in the present paper. The authors would like to express their thanks to Dr. H. Mell, University of Marburg, for supplying them with gd-a-Si films. This work was supported by a grant from the Bundesministerium for Forschung und Technologie, which is gratefully acknowledged. References [1] M.H. Brodsky, M. Cardona and J.J. Cuomo, Phys. Rev. B16 (1977) 3556. [2] P. John, T.M. Odeh, M.J.K. Thomas, M.J. Tricker, F. Riddoch and J.I.B. Wilson, Phil. Mag. B42 (1980) 671. [3] J.C. Knights, G. Lucovsky and R.J. Nemanich, Phil. Mag. B37 (1978) 467. [4] G. Lucovsky, R.J. Nemanich and J.C. Knights, Phys. Rev. BI9 (1979) 2064. [5] H. Wieder, M. Cardona and C.R. Guarnieri, Phys. Stat. Sol. (b) 92 (1979) 99. [6] S.C. Shen and M. Cardona, Phys. Rev. B23 (1981) 5322. [7] J.C. Knights, R.A. Street and G. Lucovsky, J. Non-Crystalline Solids 35&36 (1980) 279. [8] E.C. Freeman and W. Paul, Phys. Rev. B18 (1978) 4288. [9] M.A. Paesler, D.A. Anderson, E.C. Freeman, G. Moddel and W. Paul, Phys. Rev. Lett. 41 (1978) 1492. [10] H. Shanks, C.J. Fang, L. Ley, M. Cardona, F.J. Demond and S. Kalbitzer, Phys. Stat. Sol. (b) 100 (1980) 43. [11] B. SchrOder, J. Geiger, N. Kniffler and H.W. Mialler, Statusbericht Sonnenenergie (VDI, D~isseldorf, 1980) p. 991, and to be published. [12] P. Dellafera, R. Labusch and H.H. Roscher, Phil. Mag. B45 (1982) 607. [13] N. Kniffler, W.W. Miiller, J.M. Pirrung, N. H~inisch, B. SchrOder and J. Geiger, J. de Phys. Suppl. 42 (1981) C4-811. [14] A.K. Malhotra and G.W. Neudeck, Appl. Phys. Letters 28 (1976) 47. [15] D.L Miller, H. Lutz, H. Wiesmann, E. Rock, A.K. Ghosh, S. Ramamoorthy and M. Strongin, J. Appl. Phys. 49 (1978) 6192. [16] C.N.R. Rao, Chemical Application of Infrared Spectroscopy, (Academic Press, New York, 1963). [17] A. Cachard, J.A. Roger, J. Pivot and C.H.S. Dupuy, Phys. Stat. Sol. (a) 5 (1971) 637. [18] J.A. Reimer, R.W. Vaughan and J.C. Knights, Solid State Comm. 37 (1981) 161. [19] B.L. Mattes in: Polycrystalline and Amorphous Thin Films and Devices ed. L.L. Kazmerski, (Academic Press, New York, 1980) p. 1.

N. Kniffler et al. / Hydrogenated evaporated silicon films [20] [21] [22] [23] [24] [25] [26] [27]

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D.S. Ginley and D.M. Haaland, Appl. Phys. Lett. 39 (1981) 271. G. Lucovsky, Solar Cells 2 (1980) 431. M. Kakudo, P.N. Kasai and T. Watase, J. Chem. Phys. 21 (1953) 1894. S. Oguz, D.K. Paul, J. Blake, R.W. Collins, A. Lachter, B.G. Yakobi and W. Paul, J. de Phys. Suppl. 42 (1981) C4-679. G. Lucovsky, Solid State Comm. 29 (1979) 571. L.J. Bellamy, The Infrared Spectra of Complex Molecules (Chapman and Hall, London, 1975). W. MOiler, J. Pirrung, S. Iselborn, H. Riibel, B. Schr6der und J. Geiger, Statusbericht Sonnenenergie 1982 (VDI, Dtisseldorf)p. 211. T.D. Moustakas, Solar Energy Materials 8 (1982) 187.