X-RAY DIFFRACTION STUDY OF ATOMIC STRUCTURE OF PLASMA DEPOSITED a-Si:H ALLOYS

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X-RAY DIFFRACTION STUDY OF ATOMIC STRUCTURE OF PLASMA DEPOSITED a-Si:H

ALLOYS

K. Tsuji, S. Minomura

To cite this version:

K. Tsuji, S. Minomura. X-RAY DIFFRACTION STUDY OF ATOMIC STRUCTURE OF PLASMA DEPOSITED a-Si:H ALLOYS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-233-C4-236.

�10.1051/jphyscol:1981449�. �jpa-00220906�

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X-RAY DIFFRACTION STUDY OF ATOMIC STRUCTURE OF PLASMA DEPOSITED a-Si : H ALLOYS

K. Tsuji and S. Minomura

Institute for Sotid State Physics, University of Tokyo, Tokyo 106, Japan

Abstract.- The atomic structure and defects have been studied by measurements of the x-ray diffraction, density, infrared spectra and optical absorption in a series of plasma-deposited a-Si:H alloys prepared by glow-discharge decomposition of silane (GD) and reactive sputtering of Si target (SP). The first peak of x-ray diffraction intensity shows the systematic changes in position and intensity with increasing H concentration C

.

The first coordination number of Si atoms in GD a-Si:H decreases to 3.Iff0.1 with increasing C to 20 at.% while in SP a-Si:H it decreases more rapidly to 3.320.1. The First coordination distance in these alloys remains unchanged. The systematic change in the structural data with C are discussed in connection with bonding conformation and defects. H

Introduction.- Plasma-deposited a-Si:H films have attracted much recent attention as a promising material for various photovoltaic applications, in particular solar cell. The electronic properties of these films are very sensitive to the atomic structure and defects which depend critically on the preparation conditions. The information on the atomic structure and defects in plasma-deposited a-Si:H have been derived by a variety of measurements, x-ray, electron or neutron diffractions [I-71, vibrational spectra[8,9], photoemission spectra [lo], ESR [Ill and NMR 1121. The major results show that the H atoms are incorporated into the Si network as various bonding conformations which are associated with voids.

In this paper we report the systematic changes in x-ray diffraction intensity I(@), pair distribution function g(R), density, infrared spectra and optical absorption as a function of H concentration C in a series of a-Si:H prepared by glow- discharge decomposition of silane (GD) and by reactive sputtering of Si target (SP). H Experimental.- A series of a-Si:H films were prepared by glow-discharge decomposition of pure silane with RF diode apparatus and by reactive sputtering of Si target in Ar-H2 gases with RF-DC tetrode apparatus [I]. The films were deposited onto A1 foil, c-Si, glass or saphire substrate at temperatures of 200 to 35OoC. The deposi- tion rate was 0.1 to 0.3 pm/h in GD with RF power of 8 W for diode diameter of 200 mm and 0.5 to 1 pm/h in SP with RF power of 120 W for diode diameter of 80 mm.

The density d of films was measured by a floating method in ZnBr2 aqueous solution. The x-ray diffraction data were taken for compressed tablets (3x1 mm 2 area and 0.3 mm thickness) which were made of the films removed from A1 foil substrate. The x-ray diffraction intensity I(0) was measured with a position sensitive detector for MoKa radiation 1131. The interference function a(K) was obtained by the following corrections: inhomogeneity of detector, x-ray absorption in samples, background counts, polarization factor, incoherent scattering and atomic scattering factor.

Results.- The typical data of I(@) for GD and SP a-Si:H are shown in Fig. 1. Since the atomic scattering factor of H atoms is very smaller than that of Si atoms, the scattering intensity from H atoms is negligibly small. The shift of the first and second peaks of I(0) as a function of CH is shown in Fig. 2. In GD a-Si:H the first

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981449

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JOURNAL DE PHYSIQUE

Fig. 1 : X-ray diffraction intensity I ( O ) for a-Si:H with MoKcl radiation.

(a) SP a-Si.

(b) GD a-Si:H, C = 9 at.%.

(c) GD a-s~:H, _<= 21 at.%.

X-ray diffraction measurements were performed by a transmission method. Typical counts were

15,000 per channel for 4 ksec.

One channel corresponds to 0.027'.

%

20' ⁶⁶ ^SO"

A N G L E 2 8 MoKd

peak shifts rapidly to lower scattering angle 20 in low C region and then it shifts slowly with increasing C

.

On the other hand, in SP a - ~ i y ~ the first peak shifts monotonically to lower 28 with increasing C

.

The first peak shifts by 4% at

20 at.%H. The second and third peaks do nor change appreciably. The intensity ratio of the first to second peak as a function of CH is shown in Fig. 3 . The first peak shows a small sharpnening with increasing C whereas the second peak remains

unchanged. H

The systematic changes in density d, first coordination number CNl, first coordination distance R1 and optical gap E as a function of C are illustrated in Fig. 4. In the GD alloys of 7 to 20 at.%HO~N1 decreases from 7.9k0.1 to 3.7kO.1 with increasing CH while in SP alloys of 0 to 20 at.%H CN1 decreases from 4.0rt0.1 to

3.3f0.1 with increasing CH. In both alloys R remains unchanged. The GD alloys show the higher values of d and E than the

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alloys at the same CH. It is clear that the values of d, CNZ and E ?n GD alloys change rapidly in low CH region.

Discussion.- The optical absorption in the fundamental edge region of plasma- deposited a-Si:H alloys is strongly influenced by the preparation method and its conditions [8,91. As shown in Fig. 4, the optical gap E in the GD films of 7 to 20 at.%H increases from 1.60 to 1.67 eV with increasing

?

while in the SP films of 0 to 20 at.%H it increases from 1.25 to 1.73 eV. In the Itow C region the GD films show the higher values of Eo. It is considered to be due to tffe more effective modification of the valence-band density of states. It has been observed by photoemission spectra measurements in SP films that the H incorporation causes two new peaks in the valence-band 1101. The theoretical calculations have also predicted that the valence-band recedes rapidly with increasing CH, but the conduction-band edge does not change appreciably 1141.

The infrared spectra demonstrate that the H atoms are incorporated into the Si network as predominantly SiH bonding conformations in the GD alloys while in the SP alloys they are incorporated as a mixture of SiH and SiH2. Since the H atoms can be only singly coordinated, the H environments are associated with voids of the low- dimensional character. In particular, the SiH2 environments give rise to larger voids of the one-dimensional character.

A model structure for plasma-deposited a-Si:H alloys are derived for an adequate description of the observed CflI of Si atoms as a function of CH. In the cross-linked network of Si atoms with SiH and Six2 bonding the averaged CHI follows the relations

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2.2 2.0

3.2

C N 1

F i g . 4 : Density d, f i r s t c o o r d i n a t i o n number of S i atoms

CNZ, f i r s t c o o r d i n a t i o n d i s t a n c e R1 and o p t i c a l gap Eo a s a

f u n c t i o n of C f o r GD a-Si:H (0) and SP a-Si:H H ( 0 ) .

The f i r s t c o o r d i n a t i o n number was d e r i v e d from t h e a r e a under t h e f i r s t peak of r a i a l

$!

d i s t r i b u t i o n f u n c t i o n 4 ~ n R g ( R ) . The o p t i c a l gap was determined

1.0

from

175

e x t r a p o l a t i o n of

(ahv) vs. hv p l o t .

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JOURNAL DE PHYSIQUE

where c. represents the Si concentration which is bonded to i H atoms. The

calculated value of CNI for 20 at.%H is 3.75. The contributions of c and c2 to CNl are equal. The observed CNZ for GD a-Si:H with 20 at.%H is 3.7_+0.1 wkich agrees with the calculated value. On the other hand, in SP a-Si:H it shows 3.3k0.1 which is much lower than the calculated value. These results suggest that some other model structure should be devised, probably with Ar atoms associated with voids in the Si network. It has been shown from Rutherford back scattering that the SP a-Si:H contain about 6 at.% of Ar atoms [15]. The electron probe micro analysis

(EPMA) has detected about 8 at.% of Ar [16]. From the results of IR, EPMA and H evolution with heating they proposed that Ar atoms stay in the defects of SiH environments. Since the atomic scattering factor of Ar atoms lies close to tgat of Si atoms, the contribution of Ar atoms to CN1 must be taken into account. If the Ar atoms have zero CN1 with voids, the calculated value of CNZ for a-Si:H of 20 at.%H and 6 at.%Ar is 3.45. From these consideration it is proposed that SiH environments are associated with small voids of a two-dimensional character while the SiH environments are associated with larger voids of a one-dimensional character. 2

The atomic structure and defects in plasma-deposited a-Si:H films are reflected in the pressure-induced transitions to the metallic state and the associated changes in the optical gap and resistivity [1,17]. The GD a-Si:H under pressure become metallic with a discontinuous decrease in resistivity over ten orders of magnitude at about 130 kbar. On the other hand, the SP a-Si:H under pressure become metallic with a continuous decrease in resistivity at about 170 kbar This pressure dependence of resistivity is qualitatively similar to the behavior of lone pair amorphous semiconductors [181.

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