Ultrasonic properties of amorphous SiHx-films

944

ULTRASONIC PROPERTIES OF AMORPHOUS SiH~—FILMS Vol. 37, No. 12 8 within an error of K 10 % from the is assumed to be 10~ sec. To fit Eq. 2 to estimated elastic constants of the LiNbO 3—substrate and the large width of the observed attenuation the of half the measured velocity of the Rayleigh wave of maximum, a Gaussian distribution9:P(v) the ac— all our figures we (film have + normalized the In width turnedenergy out. to be introduced 30 meV, and the mean va— the of composite sample substrate). tivitat,ion was measured absorption to”infinite film thickness~’ lue of V is 110 meV. This distribution is ob— i.e. we have plotted m~iim/Ikh. viously caused by the randomness of the amorphous network. From the relaxation strength we Figure 1 shows the normalized acoustic ab— • can estimate NK 2 where N is the number of resorption of an a—Si film as a function of tern— laxing defects. Replacing an Eq.2 K = 1 eV perature. Curve I was obtained by slowly which is a typical value in the case of dieler— warming up the sample to 600 K with a rate of tric glasses, we estimate N = 10 1~7cm 3 centres 1 K/mm. Curve II was measured during cooling.

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TEMPERATURE [K] Fig.1:

Effect of annealing on ultrasonic absorption in a-Si (I) film as prepared; (II) film after being annealed to 600 K.

In the unannealed film (curve I) the dominant feature is an absorption maximum centered at 270 K. It has been observed previously and was ascribed to thermally activated relaxation of defect states. For such a process the attenuation is given by8 R

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to take part in the attenuation process. The occurence of a strong absorption peak is un— expected for an ideal continuous random net— work. In a four—fold coordinated network all atoms occupy fixed positions, so that no struc— tural rearrangements of atoms should be possible 10 This is in contrast to the case of the two— fold coordinated oxygen atoms in a—Si0 2, where relaxing oxygen atoms in bridging configurations are thought to cause a strong acoustic absorption around 50 K. Similar movements become possible for two—fold coordinated Si—atoms in the nonideal network of sputtered a—Si. Curve II of figure 1 shows the acoustic absorption in the same film after annealing to 600 K. We observe a strong reduction of the ab— sorption below 400 K. Obviously a uniform decrease by a factor of 1.5 occurs in this tempe— rature range. This change can be attributed to an increase of the velocity of sound by about 15 % on annealing (see Eq. 2). In fact such an

Vol.

37, No.

12

ULTRASONIC PROPERTIES OF AMORPHOUS SiH~—FILMS

increase is observed in our experiment. Further consequence of annealing is a strong additional reduction of the attenuation at temperatures close to the relaxation maximum. We assume that the number of relaxing is reduced 1 In centers the annealed film by the the heat treatment.’ absorption shows a peak at about ~450 K. This shift is probably due to the fact that mainly those defects are removed by annealing which have relatively small activation energies and are less stable. Above 1~O0K both curves coin— cite again indicating that defects in this tern— perature range are not affected. Defects which are left over are stable up to the highest temperatures. The ultrasonic attenuation of unannealed a—Si:H films of different composition is shown in Figures 2 and 3, respectively. Comparison with pure a—Si demonstrates significant changes, The absorption maximum at 270 K found in a—Si is not present anymore. Instead, an absorption 10000 _i

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temperature dependence of the absorption exhi— bits a structure which is not seen in pure a—Si. In tnis temperature range the attenuation con— sists of two parts. A background attenuation, which the tail of the strong seen atis higher temperatures, and relaxation a hydrogen peak in— duced contribution. In a—Si:H the background attenuation is much smaller than in a—Si since the attenuation maximum is shifted to higher temperatures and is reduced in height. There is another interesting feature: the size of the hydrogen—induced absorption increases with the hydrogen content up to ~ ~, but a decrease is found between ~4 and 15 at %. (See Fig. 3). The annealing behaviour of a—Si:H film of 15 at % hydrogen is shown in Figure ~. On an— nealing the attenuation is reduced in the whole temperature range but the position of the peaks remain unchanged. As in the case of pure a—Si, annealing increases the sound velocity, and con— sequently lowers the attenuation. This effect

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Ultrasonic absorption versus temperature in two Sj:H films [(I) 1.5 at %; (II)

15 at %I in comparison to a pure a—Si film. peak is found around 500 K. At the lowest hy— drogen content of 1.5 at % shown in Figure 2, this peak has degenerated into a shoulder on the rapidly increasing absorption at still higher temperatures. The shift of the maximum reflects an increase of the activation ener~’ from 170meV to 370meV. Between 50 K and 200 K the

alone, however, cannot explain the strong de— crease of the attenuation around 500 K. The continuous line in the middle indicates the reduc— tion of the attenuation due to the measured we— locity change. It is worth mentioning that the relaxation strength of the hydrogen induced absorption at lower temperatures is not influenced

946

ULTRASONIC PROPERTIES OF AMORPHOUS SiHxFILMS

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Ultrasonic absorption versus temperature in the Si:H film of 15 at % (I) compared to a—Si:H film of 4 at % (II).

by the annealing procedure. The microscopic origin of the relaxation at— sorption is the question of major interest. We attribute this absorption to structural relaxation of defects. In general a defect can undergo struc— tural relaxation if it can occupy at least two different positions by a rearrangement of the 10— cal configuration. Accordingly, the high relaxa— tion absorption in a—Si(exceeding the absorption in silica glass by a factor of 10) has been inter— — threefold preted as structural 5 relaxation of coordinated two—fold coor— Si—atoms exhibit atoms only one stable configuration, dinated silicon and onefold coordinated Si—atoms are too rare12 to account for the high absorption. The magni— tude of the measured absorption requires a high number of strong coupling defects. From our data we estimate a density of 10~ defects/cm3. Un— fortunately, this type of defects is not ober— vable in ERR—experiments because of spin pairing.2 Annealing results in a strong reduction of the low temperature part of the absorption as shown by Figure 1. Apparently the amorphous network undergoes a rearrangement resulting in a removement of those defects which have a re— latively small activation energy. There is, however, an absorption left over around 400 K. It is not clear whether this is caused by those two-fold coordinated Si-atoms which remain

stable up to our highest temperatures or whether this absorption is caused by another type of defect. Incorporation of impurities influences re— laxation centers depending on the coordination 8 Monovalent hydrogen number of the impurity. atoms will be attached to relaxation centers in a—Si. In hydrogenated samples most likely Sifl 2—units will play the role of two—fold coordinated Si—atoms in pure a—Si films. There— of the weabsorption maximum in hydrogenated films. fore, do not expect a complete disappearance Indeed, an absorption maximum is observed which is shifted from 270 K to 500 K, and which is re— duced in height. The increase of the activation energy may be the result of a change in the bonding character of silicon as the hydrogen atoms are attached. It could equally well be caused by the repulsive interaction between by— drogen atoms, which are considered to occur in clusters of 3-4 atoms.’3 The reduction of the magnitude indicates either that there are less SiH 2—units in hydrogenated a—Si films than two— fold coordinated Si-atoms in pure a-Si films or that the deformation potential of the Sill2defect is smaller. With increasing hydrogen content the absorption peak becomes more pro— nounced. It seems, however, that this is mainly due to the fact that the high temperature background is shifted to higher tempera-

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Vol.

37, No.

12

ULTRASONIC PROPERTIES OF AMORPHOUS S1IIfFTLMS

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Effect of annealing on ultrasonic absorption in the Si:H film of 15 at %. (I) film as prepared; (II) film after being annealed to 600 K. The solid line in the middle reflects the decrease of the attenuation due to a change of the sound velocity by annealing.

tures. The strong increase of the background at the highest temperatures is found in all measurements. A similar behaviour is observ— ed in glasses close to the glass transition temperature. Because of our limited tempera— ture range we were not able to investigate this aspect in more detail, Annealing of the 15 at % hydrogenated film results in a further strong reduction of the high temperature absorption. Such a decrease of the absorption without shift in temperature indicates that the number of de— fects is reduced without changing their structure. In our hydrogenated films it is the number of 5iH 2—centers which is reduced by annealing, since annealing to 500 K is known to drive hydrogen out of Sill2 and SiB3—sites. Only Sill—configurations are un— affected. This provides evidence that the re— laxation centers in hydrogenated films are Sill2centers. At lower temperatures incorporation of hy— drogen results in additional contributions to the ultrasonic attenuation, As mentioned be— fore neither their relaxation strength nor the position of this attenuation is influenced by annealing (see Figure 4). Since the hydrogen of the SiH2-units is driven out by the annealing procedure, this absorption cannot be due to SiB2. Most probably it is caused by Sill units at which only the hydrogen performs relaxational movements. Obviously the couplinq is much

smaller than in the case of two—fold coordinat— ed silicon atoms resulting in a smaller magnitude of the absorption. Although theoretical models forSi:H have been proposed which could explain structural rearrangementslk, our data do not allow to distinguish between them. Incorporation of deuterium or fluorire should give more insight into this problem. Summary We have studied the influence of annealing and incorporation of hydrogen on the acoustic properties of sputtered. films of aSi:H. Our results confirm the previous assuxnption that the strong absorption in pure a-Si is due to structural relaxation of two-fold coordinated Si—atoms. The absorption observed in hydrogenated films at higher temperatures can be attributed to SiH2-units. At temperatures between 20 K and 200 K we find new, hydrogen-induced contributions to the attenua— tion which are not seen in pure a—Si. Anneal— ing hydrogenated films to temperatures where hydrogen is driven out of SiH3 and SiH2-sites does not affect these contributions. There— fore, we attribute the hydrogen induced attenuation to Sill-centers. Acknowledgements We wish to thank K. Dransfeld,

L. Ley,

948

ULTRASONIC PROPERTIES OF AMORPHOUS SiH~—FILMS

and H.R. Shanks for helpful and stimulating discussions, M. Bulst (Siemens, MOnchen) for supply of the L1NbO 3 samples, R. Gibis for technical assistance in the preparation of the

films, and S. Kalbitzer of H at % in our films.

Vol.

37, No. 12

for the determination One of us (KLB) is

grateful to the Alexander-von-Humboldt—Stiftung for the award of a Humboldt-Fellowship.

References

1. M.H. Brodsky and R.S. Title, Phys.Rev.Lett. 23, 581 (1969) ; T.D. Moustakas, J. Elect.Mat. 8, 391 (1979) 2. F.L. Galeener, Phys.Rev.Lett. 27, 1716 (1971); J.C. Knights, G. Lucovsky, and R.J. Nemanich, J. Non—Cryst. Solids 32, 393 (1979) 3. W.E. Spear and P.G. Lecomber, Phil. Mag. 33, 935 (1976) 4. .8morphous Semiconductors, Edited by M.H. Brodsky; B. v. Roedern, L. Ley, M. Cardona, and F.W. Smith, Phil. Meg. 5, 40, 433 (1979); W. Paul, Sol. State Commun. 34, 283 (1980) 5. M. v. Haumeder, U. Strom, and S. Hunklinger, Phys. Rev. Lett. 44, 84 (1980) 6. The surface wave devices were supplied by Siemens AG, MOnchen, West Germany 7. C.J. Fang, K.J. Gruntz, L. Ley, N. Cardona, F.J. Demond, G. MOller, and S. Kalbitzer,

J. Non-Cryst. Solids 35, and 36, 255 (1980) 8. M. v, Haumeder, Thesis, Konstanz (1980) T. Nakayama, Phys. Rev. 5, 14, 4670 (1976) 9. S. Hunklinger and W. Arnold in “Physical Acoustics” (R.N. Thurston, W.P. Mason, eds.) 12, 155 (1976) 10. D.A. Smith, Phys. Rev. Lett. 42, 729 (1978) 11. Iwao Ohdomari, M. Ikeda, and H. Yoshimoto, Phys. Lett. 62A, 253 (1977) 12. 0. Adler, Phys. Rev. Lett. 41, 1755 (1978); M. Kastner, D. Adler, and H. Fritzsche, Phys. Rev. Lett. 37, 1504 (1976) 13. 0. Weaire and F. Wooten, J. Non—Cryst. Sol. 35, 495 (1980) 14. N.H. Brodsky, H. Cardona, and J.J. Cuomo, Phys. Rev. B16, 3556 (1977); P.M. Martin and W.T. Pawlewicz, Solar Energy Mat. 2, 143 (1979/1980)