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STUDY OF DEFECTS IN a-Ge AND a-GeHx USING SURFACE ACOUSTIC WAVE TECHNIQUE
K. Bhatia, M.V. Haumeder, S. Hunklinger
To cite this version:
K. Bhatia, M.V. Haumeder, S. Hunklinger. STUDY OF DEFECTS IN a-Ge AND a-GeHx USING
SURFACE ACOUSTIC WAVE TECHNIQUE. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-
365-C4-368. �10.1051/jphyscol:1981478�. �jpa-00220935�
JOURNAL DE PHY'SIQUE
CoZZoque C4, supp~e'rnent au nO1O, Tome 42, oetobre 1981 page (24-365
STUDY OF DEFECTS I N a-Ge AND a-GeH, U S I N G SURFACE ACOUSTIC WAVE TECHNIQUE
K.L. ~hatia+, Pi. v. ~aumeder
*
and S. HunklingerMm-PZmek-Institut filr Festkiirperforschung, Heisenbergstrasse 1, 0-7000 Stuttgart 80, F.R. G.
Abstract.- The attenuation of surface acoustic waves in reactively sputtered films of a-Ge and a-GeHx (0
2
x < 0.25) at 300 MHz frequency and in the tem- perature range 0.5 K to 475 K has been studied. A strong attenuation peak is observed in a-Ge at 135 K whose position shifts on annealing. Incorporation of hydrogen completely suppresses this maximum and induces some new features in the attenuation. Below 20 K the attenuation varies linearly with tempera- ture in all the films. Results are discussed in terms of structural relaxa- tion of defects.Introduction.- Amorphous germanium films exhibit a variety of structural defects which are associated with the appearance of dangling bonds, vacancies, microvoids etc. in the amorphous network /1,2/. Although these defects have remarkable in- fluence on the structural and electronic properties of these materials, their nature and origin is far from being understood /3,4/. Incorporation of hydrogen in- to the amorphous network passifies the dangling bonds /5/ and thus drastically alters the electronic properties of the semiconductors. In order to gain more in- formation on the dynamical properties of these structural defects we have investi- gated the acoustic behaviour of a-Ge and a-Ge:H.
Experimental.- The acoustic properties of thin films deposited on a suitable sub- strate can be studied by means of acoustic surface waves /6/. We used yz-cut LiNb03 surface wave devices with interdigital transmitting and receiving trans- ducers 10mm apart. Using heated substrates (temperature 80 to 100 OC), thin films
(thickness about 0.3 ll) of GeHx (0
5
x < 0.25) were reactively sputtered in an argon atmosphere of Torr (hydrogen partial pressure ranging from l x l ~ - ~ to 3x10-~ Torr, background pressure 1x10-~ Torr) onto the entire space between the transducers. Simultaneously, reference films were sputtered on a germanium crystal for observation of infrared transmission spectrum. The calibration curve obtained from the 6.4 MeV resonance in the 'H(~~N,CLY)~~C nuclear reaction was used to deter- mine the absolute hydrogen concentration /6/. On the substrate a reference path without a film was left over to subtract the attenuation of the surface wave due to the substrate itself. Measurements were made at temperatures between 0.5 K and 475 K at a frequency of 300 MHz using standard ultrasonic and low temperature tech- niques. The attenuation CLF of an acoustic surface wave propagating on a substrate which is cgvered by a thin film, is related to the attenuation a of the surface wave on a bulk sample of the film material by the relation: CLF = CLIkh, if the pene-tration depth of the surface wave is much larger than the film thickness h (i. e.
kh << 1, where k is the wave vector) /7/. The constant I depends only on the elastic properties of the substrate and the film. From our measured absorption we have sub- tracted the "residual" attenuation estimated from the leveling off of the absorption at the lowest temperatures. Furthermore we have extrapolated this value to "in- finite film thickness" i. e. we have plotted a = aF/Ikh.
Results and ~iscussi0n.- In Fig. 1 the normalized ultrasonic absorption o'f an a-Ge film is shown versus temperature. Curve I was obtained for the as-prepared film.
At the lowest temperature (up to 30 K), an absorption linear in T is observed. Above 200 K a steep rise in the attenuation is found. Such a behaviour at higher tempera- ture is well known in the case of ordinary glasses /8/ and is attributed to the re-
+ On leave from : Department of Physics, Maharshi Dayanand University, Rohtak-124001, India
*
Present address : Industrie-Anlagen Betriebsgesellschaft IABG, Einsteinstr. 20, D-8012 Ottobrunn, F.R.G.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981478
C4-366 JOURNAL DE PHYSIQUE
duced viscosity in the vicinity of the glass transition temperature. But in the absence of a glass transition temperature in a-Ge, a similar explanation may not be possible in this case. The dominant feature, however, is the absorption maximum centered at 135 K. As in a-Si, where such a peak was observed at 270 K /7,9/, we ascribe this absorption to the thermally activated relaxation of defect states.
For such a process the attenuation is given by
where N is the number density of relaxing defects, p the mass density of the film, W/27 the frequency of the surface wave, v~ the velocity of longitudinal sound waves, and K the coupling constant between sound wave and the defect states. The relaxa- tion time T and the activation energy V for the process are related by the Arrhenius relationship: 'T = Tg exp(V/kg~)
,
where the constant T O is of the order of 10-13 s.We deduce the value V = 100 meV from the position of the absorption maximum. Using K = 1 eV (the usual value in dielectric glasses) we estimate that roughly 1019 centers/cm3 take part in the attenuation process.
In general a defect can undergo structural relaxation if it can occupy at least two different positions by a rearrangement of local configuration. Such a motion is not very probable for four-fold and three-fold coordinated atoms
/lo/
whereas one-fold coordinated Ge-atoms are too rare to account for such a strong ab- sorption /11/. Therefore, we attribute the strong absorption peak to the struc- tural relaxation of two-fold coordinated Ge atoms.
Curve I1 of Fig. (1) represents the absorption in the same film after anneal- ing to 475 K. The annealing behaviour of a-Ge differs considerably from that of a-Si / 9 / . In a-Ge, the heat treatment shifts the position of the maximum from 135 K to 190 K without significantly altering its intensity. This corresponds to a change in activation energy from 100 meV to 120 meV. A similar increase of V has been observed in the measurement of ac-conductivity after annealing at 530 K /12/.
Surprisingly, the absorption at low temperature is not influenced by the heat treatment indicating that the absorption maximum and the low temperature tail have different origin.
In Fig. 2 the ultrasonic attenuation of a-Ge:H films of different compositions is shown. Now the maximum at 135 K in pure a-Ge is suppressed. Instead, some new contributions to the attenuation around 100 K are observed, which become quite prominent in the film with 25 at% hydrogen (see Fig. 3). A steeply rising contribu- tion to the absorption appears above 200 K, which shifts towards higher temperatures with increasing hydrogen content of the film. The magnitude of this steep rise of
300 MHz a - G e
UNANNEALED ANNEALED
Fig. 1: Effect of annealing on the ultra- sonic absorption in Curve (I) (A) a-Ge films
as-prepared
Curve (11) ( 0 ) film after being annealed to 475 K
01 1 10 100 lo00
TEMPERATURE [Kl
Fig. 2: Ultrasonic absorption versus temperature in three a-Ge:H films with hydrogen concen- tration :
o a-Ge:H
9 at%
Curve (I) ( 0 ) 1.5 at%
A a-Ge:H 19
at%
Curve (11) ( 0 ) 9 at%
Curve (111) (A) 19 at%
Absorption in a-Ge film is also plotted as a dotted curve for comparison. The logarithmic ordinate scale is shifted by one decade in case of each film of a-Ge:H for clarity.
01 1 10 100 1000
TEMPERATURE [ K I
the absorption is higher by more than an order of magnitude com2ared with that one observed in pure a-Ge. In order to demonstrate that the attenuation features around
100 K are induced by hydrogen, we have substituted hydrogen by deuterium. In Fig.
( 3 ) , the comparison of such films is shown for a concentration of 25 at% hydrogen or deuterium. In the a-Ge:D film the position of the absorption maximum as well as of the steep rise is shifted towards higher temperatures. This indicates that the motion of hydrogen atoms gives rise to the relaxation effects. The two-fold co- ordinated Ge-atoms, thought to be responsible for the strong absorption peak in pure a-Ge may be correlated to the T! centers located on (100)-like surEaces of micro- voids /5,11,14/. In a-Ge:H, the hydrogen induced features around 100 K are likely to originate from centers like GeH, GeH2 etc. incorporated in the network during film formation /13/. These centers are formed from the interaction of hydrogen with T:, T~~ or the reconstructed bonds / 5,11/. Although the steep rise of the ab- sorption observed in a-Ge:H above room temperature is clearly related with the presence of hydrogen, no plausible explanation is available so far. The absorption at the lowest tempkrature (T < 20 K) remains unchanged by incorporation of hydrogen or deuterium, and by annealing. It would be tempting to correlate this low tempera- ture absorption with the presence of low energy excitations characteristic for ordinary glasses /9/. In such a case the velocity of sound should exhibit the characteristic logarithmic increase with temperature below a few Kelvin. Measure- ment of velocity shows variation neither in a-Si /7,9/, nor in our samples of a-Ge.
Nevertheless the existence of such low energy excitations cannot be excluded com- pletely since their observation in acoustic experiments also depends upon the coupling constant.
In summary, we have studied the ultrasonic properties of sputtered films of a-Ge and a-Ge:H. In a-Ge a strong absorption peak occurs which is probably due to the two-fold coordinated Ge-atoms. A strong influence of annealing and incorpora- tion of hydrogen on the ultrasonic properties has been observed. Incorporation of hydrogen suppresses the strong absorption peak in a-Ge and induces new features in the attenuation. Substitution of hydrogen by deuterium supports this interpretation.
JOURNAL DE PHYSIQUE
Fig. 3: Ultrasonic absorption versus temperature in
a-Ge:H (A) with 25 at%
hydrogen, and in
( 0 ) a-Ge:D with 25 at % deuterium.
0.l 1 10 100 1000
TEMPERATURE
[K l
Below 20 K the absorption varies 1inearly.with temperature in all samples and its magnitude depends neither on hydrogenation nor on annealing.
We wish to thank K. Dransfeld, M. Cardona, and L. Ley for helpful discussions, M. Bulst (Siemens, Miinchen) for supply of LiNb03 samples, R. Gibis for technical assistance. One of us (KLB) is grateful to the Alexander-von Humboldt-Foundation for the award of a Humboldt Fellowship.
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