Plasma deposition of amorphous As-S films

Journal of Non-Crystalline Solids 137&138 (1991) 1001-1004 North-Holland

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PLASMA DEPOSITION OF AMORPHOUS As-S FILMS P. NAGELS, R. CALLAERTS, M. VAN ROY RUCA, University of Antwerp, B-2020 Antwerpen, Belgium M. VLI2EK University of Chemical Technology, 532 10 Pardubice, Czechoslovakia Hydrogenated ASxSa_x thin layers have been prepared by plasma decomposition of the constituent hydrides. Microprobe analysis shows that the chemical composition and homogeneity depend not only on the AsH3/H2S ratio but also on the gas pressure. Infrared and Raman spectra reveal the presence of As4S 4 and As4S 3 molecular units in the structure of As rich samples. On light exposure photobleaching is observed in samples with overstoichiometry of As (x > 0.5).

1. INTRODUCTION In recent years, amorphous chalcogenides have

red in a plasma discharge stainless steel reactor using a 13.56 MHz r.f. power of 15 W, capacitively coupled be-

received considerable attention because of their photo-

tween parallel plate electrodes. The reactant gases, 15

induced structural changes. The main interest lies in their use as inorganic photoresists. Most previous work was

vol.% AsH3 in H2 and pure H2S, were admitted into the reaction chamber at a total flow rate of 10 sccm. The

devoted to thin films of the binary As-S and Ge-Se

AsHJH2S gas ratio was varied in the range from 1/0 to 1/99. The gas inlet and outlet were opposite to each

systems prepared by vacuum evaporation. In this paper we report on the preparation of hydrogenated AsxSl_x thin films (0 ~ x < 1) by plasma-enhanced chemical vapour deposition, using the hydrides as precursor gases. In amorphous silicon the incorporation of hydrogen

other in the middle of the reaction chamber. The electrodes, 8 cm in diameter, were positioned at the same height, so that the gases flowed parallel to their surface. In all experiments the separation between the powered

plays a very important role in saturating the dangling

and grounded electrode was fixed at 3 cm. Depositions

bonds responsible for high densities of electrically active gap states. The defect chemistry in chalcogenide glasses

were made at a gas pressure of 0.025, 0.25 and 1 mbar, automatically controlled by a baratron pressure device

differs greatly from that of the tetrahedrally coordinated

and a butterfly valve regulating the pumping speed. The

amorphous semiconductors) It is generally accepted that

sample substrates were not heated during the deposition.

neutral dangling bonds are transformed into positively and negatively charged dangling bonds pinning the Fermi level close to the middle of the gap. It is expected that

3. RESULTS AND DISCUSSION

hydrogen incorporation in these materials will not drasti-

3.1. Microprobe analysis The influence of the AsHJHzS gas ratio and of the

cally change most of their basic physical properties. Nevertheless, a comparison of the structure and photo-

total pressure on the chemical composition of samples

induced effects of amorphous As-S films prepared either

of electron microprobe analysis. In agreement with previous work by Fritzsche et al.2, we observed that the

by plasma decomposition or by vacuum evaporation might help to get some further insight into the origin of these structure-related phenomena. 2. EXPERIMENTAL PROCEDURES Amorphous AGS~_x thin films (0 ~ x <- 1) were prepa-

deposited on both electrodes was investigated by means

chemical composition of the film deposited at a given AsHJHzS ratio changed with the position z on the substrate along the gas flow direction. This is shown in Fig.l, where the As content expressed in at. % was measured from the middle of the powered electrode (z=o)

0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.

1002

P. Nagels et aL /Plasma deposition of amorphous As-S films AsH3/H2S

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As content of AsxSl_x layers vs. sample position z On the powered electrode for various AsH~rlzS ratios (z=o middle of the electrode).

FIGURE 2 As content of ASxS~_X layers vs. sample position z deposited at various pressures (z=o middle of powered (RF) and grounded (G) electrode).

to its outer circumference for samples deposited at an

(less than 1 at. % variation at the lowest pressure of

AsH~rI2S ratio varying from 1:3 to 1:99.

0.025 mbar). This means that at low pressure the con-

The gas pressure was kept constant at 0.25 mbar. The

centration of the chemically reactive species generated by

layers deposited on the powered electrode always had a

the plasma from AsH 3 remains nearly constant along the

higher As content than those on the grounded electrode.

gas flow direction.

The chemical homogeneity was better in the samples with the highest As content (less than 2 at. % for an AsHJH2S

3.2. Infrared and Raman spectra IR transmission in the range 4000-200 cm -1 of films

ratio equal to 1:3). From Fig.1 it can also be seen that

deposited on polished crystalline Si wafers showed that

the incorporation of S is not very effective : for an AsH 3

A%Sl_x samples with overstoichiometry of either As or of

/H2S ratio equal to 1:99 the resulting chemical composi-

S as compared to As2S3 were hydrogenated. Hydrogen

tion is about As30S70. This indicates that at a pressure of

atoms in these samples were preferentially bonded to the

0.25 mbar, the electron-impact dissociation of AsH 3 is

elements present in excess of stoichiometry : A s - H and

much higher than that of H2S, and also explains why the

S - H stretching modes were observed at 1980 and 2500

As content is higher close to the gas inlet Starting from

cm -j, respectively. The concentration of bonded hydro-

pure H2S, no film was deposited either on the powered

gen atoms in the A s - S layers decreased by decreasing the

electrode or on the grounded electrode.

total pressure during deposition.

We also observed that a change in pressure affects to a

Information concerning the microstructure of the layers

large extent, not only the chemical composition of the

was gained from the IR transmission in the 500-200 cm -a

grown layers, but also the homogeneity along the gas

range (Fig.3). The IR spectra were recorded on layers

flow. Figure 2 shows the microprobe analysis of the As

deposited on the powered electrode at 0.25 mbar using

content in at. % for layers deposited on the powered and

various AsHJH2S ratios. The corresponding chemical

grounded electrodes at the pressures of 0.025, 0.25, and 1

compositions were already given in Fig.1.

mbar, starting from the same gas ratio AsHJHzS = 1:19.

A strong absorption mode at 311 cm -1 for AsHJH2S

A decrease in pressure yielded layers with much higher

< 1:19 (x < 0.4) and two strong modes at 341 cm -1 and

As content. At the pressure of 1 mbar the variation of the composition is considerable, yielding an As content

374 cm < for AsH~/Ho_S > 1:14 (x > 0.4) were dominant in the IR spectra. The absorption band at 311 cm -1 cor-

much higher near the gas inlet. The decrease of pressure

responds to an asymmetric stretching vibration in a pyra-

improved the chemical homogeneity of the A s - S layers

midal AsS 3 unit 3. The assignment of the 341 cm -1

P. Nagels et aL / Plasma deposi.gon of amorphous As-S films

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FIGURE 3 Infrared transmission (500-200 cm -~) of AsxS~_x layers deposited on the RF electrode using AsH~IzS : (a) 1:3; (b) 1:4; (c) 1:5; (d) 1:8; (e) 1:11; (f) 1:14; (g) 1:19.

FIGURE 4 Infrared spectra of AsxS~_~ layers prepared at various pressures (in mbar) with the same AsHJH~S ratio (1:8): (a) 1; (b) 0.5; (c) 0.25; (d) 0.025.

and 374 cm -1 bands to one or two molecular units is not

occurs over a broad wavenumber region between 300 and

straight forward. The infrared spectrum of the crystalline

380 cm -1, indicating that all the above mentioned vibra-

modifications of As4S4 and As4S 3 shows absorption bands

tional modes contribute to the spectrum.

close to those observed in our spectra 4 : 346 and 374 cm -1 in an AsaS 4 crystal; 341 and 370 cm -~ in an ASaS 3 crystal. Considering the small differences in

Raman spectra gave further evidence for the presence of As4S 4 and As4S 3 molecular units in layers with an As content higher than 40 at. % (Fig.5). They showed a

wavenumber observed in the crystals, the 341 and 374 cm -~ bands in our spectra may be due to vibrations either of As4S 4 units or of As4S 3 units or a mixture of both. From the infrared structural study we may conclude that increasing overstoichiometry of As causes the breaking of A s - S bonds with the formation of structural units of the As4S 4 or As4S 3 type containing As-As bonds. The sharpness of the absorption bands in these spectra

D r8 r~ >-

suggests a quasi-molecular like structure. In Fig.4 the IR spectra of layers prepared with the same AsH~-IsS ratio (1:8) but at different pressures are represented. It has already been shown (see Fig.2) that lowering the pressure increases the As content. From the

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spectr4 of Fig.4 it can be noticed that the 341 and 374 cm -a bands become predominant when the pressure decreases (0.5 and 0.25 mbar). The IR activity of the sample prepared at the lowest pressure (0.025 mbar)

FIGURE 5 Ramau spectra of As~SI_~ layers prepared at p = 0.25 mbar with AsH3/H2S = 1:19 (a) and 1:2 (b).

1004

B Nagels et al. /Plasma deposition of amorphous As-S films

70

strong peak at 340 cm -a attributed to vibrational modes of p y r a m i d a l A s S 3 units. Two weaker bands, located at 232 cm -a and 187 cm -1, indicated the presence of ASgS 4 units. In As-rich samples (e.g. AsH3/H2S = 1:5) a strong band appeared at 273 cm -1, which is the most intense band in the Raman spectrum of crystalline As4S 3. The IR and Raman spectra confirm the hypothesis, already put forward by other authors for bulk glasses 5, of a micro-

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heterogeneous structure composed of mixtures of ASS3, A s 4 S 4 and As4S 3 units in our plasma deposited films with

x > 0.4.

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The Raman spectra also indicated that samples with

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Moreover, the presence of hydrogen atoms in their struc-

FIGURE 6 Photoinduced changes in the IR spectrum of an AsxSl_X layer prepared with AsH3/H2S = 1:5; (a) unexposed; (b) after 60' illumination; (c) after 300' illumination.

ture was also detected by the appearance of an 831 cm -~ mode (see spectrum b).

ACKNOWLEDGEMENT

very high As content (x > 0.9) contained pure As clusters (sharp mode at 200 cm -~ in spectrum b of Fig.5).

The authors are grateful to Professor B. Van der Veken 4. PHOTOSENSITIVITY

for performing the Raman measurements.

Compared with results reported by other authors 6 on A s - S layers prepared by evaporation, our plasma-

REFERENCES

deposited films were less photosensitive for compositions below x = 0.5. But samples with higher As overstoichio-

1. M.Kastner and H. Fritzsche, Phil.Mag. B37 (1978) 199.

metry (x ~- 0.6), which are practically impossible to pre-

2. H. Fritzsche, V. Smld, H. Ugur and P.J. Gaczi, J. Phys., (1981) C4 - 699.

pare by vacuum evaporation, were photobleached upon light exposure. Infrared measurements revealed evidence for the photoinduced structure change accompanying the photobleaching effect (Fig.6). The intensity of the absorption bands located at 341 and 374 cm -1 increased after 60 and 300 min. of illumination. We suppose that in As-rich samples, either partial polymerization of [3 -- As4S 4 or transformation of [3 - As4S 4 into other chemical species richer in As takes place. More details

about the photo-induced phenomena in our plasma deposited AsxSl_X films will be published in the near future.

3. G. Lucovsky, Phys.Rev. B6 (1972) 1480. 4. HJ. Whitfield, Austr. J. Chem. 24 (1971) 697. 5. A. Bertoluzza, C. Fagnano, P. Monti and G. Semerano, J. Non-Cryst.Solids 29 (1978) 49. 6. M. Frumar, M. Vl~ek and J. Klikorka, Reactivity of Solids 5 (1988) 341.