aC:H composite films

Vacuum/volume Printed in Great

39/number Britain

Physical films H Biederman,

1 /pages

13 to 15/l

989

properties I ChudZiEek,

Charles University,

D Slavinsk&

0042-207X/89$3.00+.00 Pergamon Press plc

of metal/a-C L MartinO

and J David,

:

Faculty

H composite of Mathematics

and Physics,

V Holes’oviEka’ch 2, 780 00 Praha 8, Czechoslovakia

and S NeSptirek,

Institute

of Macromolecular

Chemistry,

Czechoslovak

Academy

of Sciences,

Praha, Czechoslovakia

Hydrogenated amorphous carbon (a-C: H) films were deposited in an rf glow discharge onto negatively biased substrates using benzene/argon or butane/argon mixtures as a working gas. Metals (Ag, Au or Cu) were simultaneously incorporated into the growing films by co-evaporation from a resistively heated boat. Increased negative bias voltage on the substrates (from - 700 to -800 V) caused a decrease in the optical gap from 2.5 to 1.0 eV in the case of metal-free carbon layers. The development of an anomalous absorption with the increase of metal concentration in the films is reported. The film absorbance in the visible region was measured during the annealing of the composite films up to 200°C and revealed considerably better stability than the metal-containing plasma polymers studied formerly.

1. Introduction Hydrogenated amorphous carbon (a-C : H) films have attracted considerable attention in the last decade’. Characteristics of these films strictly depend on the major parameter-the energy flux carried by the species impacting on the growing insulating layer. It has been concluded* that the structure of as-deposited films comprises several phases : amorphous diamond-like, amorphous graphite and polymeric components. Upon annealing, the amorphous diamond-like and polymeric components decrease, the graphitic part grows and a void component appears. The hypothesis has been proposed that the film properties depend on the amount of hydrogen incorporated into the films. It has been demonstrated that the concentration of H decreases with the energy of the impinging ions3. A similar effect was observed when a halocarbon gas was admixed to the hydrocarbon monomer“, exploiting in this way the scavenging effect of HF formation to reduce the amount of hydrogen incorporated into the films. In recent years several attempts have been made to modify further the film characteristics by incorporating metals into the hard carbon films. In these composite layers metals such as Cr or Al’, Cu or stainless Steele, Ti or Sn’ or Ni*, MO or Pt, Au, Cu9 and finally Au or Ag” were used. The effect of metal incorporation on the microhardness has been previously studied5s7.“. In the present work we have investigated the effect of the amount of the metal incorporated (Ag, Au, Cu) on the resulting optical properties and their stability upon annealing. 2. Experimental details Hard carbon (a-C : H) films were deposited in an rf (13.56 MHz) glow discharge operated in a high vacuum stainless steel system”‘. The substrates were placed on an electrode, 80 mm in diameter capacitively coupled to the rf power supply. Bombardment by energetic ions was caused by a negative self bias. Benzene/argon or butane/argon mixtures were used as the working gases at the

starting total pressure of 3 Pa and a total flow rate of 5 x 10m3 Pa m3 s-’ in all experiments with a power level of 40 W and - 500 V dc negative self bias. The growing a-C : H layers were modified by the incorporation of metals (Ag, Au, Cu) evaporated simultaneously from a resistively heated tungsten boat situated 100 mm above the top of the excitation electrode. A rotating substrate holder was used to enable the preparation of 6 samples without breaking vacuum. This combined deposition process was monitored by an emission spectroscopy assembly consisting of an SPM-1 (Zeiss) monochromator, a 1 P 28 (RCA) photomultiplier and a chart recorder (Zeiss). In the first series of experiments the transmission and reflection of the films deposited onto polished quartz substrates were measured in a Perkin Elmer-330 double beam spectrophotometer. In the second series the absorbance in the visible region and its stability upon annealing up to 200°C at the ambient atmosphere were measured in a Hewlett Packard 8451A double beam spectrometer. Selected samples deposited on evaporated carbon foils fixed onto copper grids were observed in a BS 613 (Tesla) transmission electron microscope (TEM). 3. Results and discussion 3.1. Deposition process. In the first series of experiments the metalfree a-C: H films were deposited from benzene (40%)-argon (60%) mixture at the total pressure of 3 Pa. The film structure was controlled by the power delivered to the discharge and thus by the self developed negative bias voltage V, on the excitation electrode where the substrates were placed. In our experiments the variation of power between 3 W and 100 W caused changes in U, between - 100 V and - 800 V. In the case of metal-containing carbon films a butane (70%)-argon (30%) mixture was used and the power of 50 W evoked lJ, = 500 V. During the incorporation of a metal from the resistively heated boat the light emission spectra were recorded”. 13

H Biederman et al: Physical properties of metal/a-C

: H composite films

The plasma parameters were kept constant (characterized by the emission intensity of argon line at 418.0 nm) and the intensities of the metal most intensive emission lines were monitored : Ag328.0 nm, Au-267.6 nm and Cu-324.7 nm. The ratio of the respective metal spectral line and the argon spectral line was then used as a measure of the amount of metal incorporated in the carbon layer and was kept constant during each deposition cycle.

1001 Ag /a-CIH) 90. T

3.2. Optical properties of a-C : H films. The absorption coefficient c( of the a-C : H films was determined from the film transmission T and reflection R data using a formula’ ’ :

l--R2

exp (-2ad)

It was supposed (aE)“2

(1)

’

that the Taut expression

= A(E-J?&,)

(2)

holds for the amorphous materials of the a-C : H type as proved already by a number of authors4,‘2. The optical gap Eapt was determined by plotting (c&)‘~* vs photon energy E. The dependante of E,,, on the bias voltage U, used during the deposition is shown in Figure l.which shows that changes in U, from - 100 to - 800 V caused a decreased in E,,, from 2.5 eV to 1 eV due to the enhanced bombardment of the growing layers by positive ions from the discharge region. This bombardment influen‘ces the decrease in the hydrogen concentration in the films similarly to reference 3. In further experiments, when metals were incorporated, the parameters characterized by U, = -500 V were used. 3.3. Optical properties of metal/carbon composite films. In metaldielectric composites formed by small metal grains (much smaller than the wavelength of light) an optical anomaly due to optical resonance usually occurs. Such behaviour can be illustrated also by a-C : H films in which the amount of silver is subsequently increased (see Figure 2). The transmission in the visible region decreases and an absorption maximum develops around 0.4 pm and shifts to higher wavelengths. Similarly, a pronounced absorption anomaly developed around 0.55 pm when gold was incorporated”‘. In the case when transparent dielectrics (e.g.

0.2

03

Ok

0.5

06 Xl

r

03 ml

Qa

Figure 2. Optical transmission spectra of a-C : H films containing subsequently increased amount of silver characterized by a ratio of the emission line intensities Z(Ag)/Z(Ar). The film thickness is 40 nm.

0,5 A

I a.u.1 0,4

3 Eopt IeVl 2

1

I

0

200

400

600

800

-ug IV1 Figure 1. Optical gap as a function of the negative self-bias voltage for a-C : H films deposited from a benzene (40%))argon (60%) mixture at the total pressure of 3 Pa. 14

400

I

5al

1

600

700

800 A [nml

Figure 3. Absorbance in the visible region of a metal-free a-C : H carbon film (C) and a-C: H carbon doped by silver (Ag/C), gold (Au/C) and copper (Cu/C).

et al: Physical

H Biederman

properties

of metal/a-C

0.20,l -

films

650 nm

A [a u.1

: H composite

l

z,:

t

: .

. l

Au/C

T

550 nm ;

01;

8’

i

:

0,l

,I

1 450 nm

OS2 .$,r O,l L-----J 0

i

50

100

:

150

200

T [“Cl Figure 4. The absorbance at three different wavelengths as a function of anneal temperature for a gold-containing carbon film.

is oxidesI or po1ymers’4.‘5) are used a coloured appearance obtained. In our case the colour is slightly affected by higher overall absorption of a-C : H in near uv and visible regions. The effect of long term and temperature stability is very important for the study of the deposition parameters on the actual film structure and possible applications. The optical absorbance in the visible region has been measured on as-deposited samples of carbon a-C: H and a-C: H containing silver (Ag/C), copper (CL@) and gold (Au/C). Selected examples are shown in Figure 3. A development of an absorption maximum can be clearly seen. The temperature stability of the optical properties was tested by repeated measurement of the absorbance at elevated temperatures. The absorbance was measured subsequently three times after 30, 60 and 90 min annealing at each subsequent temperature: 40, 60, 80, 100, 120, 160 and 200°C. To estimate the changes in absorbance A with temperature, the value of A at selected wavelengths (450, 550 and 650 nm) is plotted vs temperature in Figure 4 for the gold-containing sample from Figure 3 as an example. It may be concluded within the scatter of the experimental values of A which rarely exceeds 5% that the absorbance does

not change considerably with temperature up to 200°C. The thermal stability is much better than in the case of metal containing plasma polymers studied formerlyI where the changes in the region of the absorption anomaly reached S&100% after annealing the films at 80°C or storing them for 5 days in ambient atmosphere. The optical properties were found to be relatively stable when annealed up to 200°C. Independent measurements of electrical with the aim of finding the effect of structural properties” rearrangement on the film resistivity have shown that the changes become irreversible when the samples are heated to more than 240°C. 4. Conclusions The results of this work may be summarized

as follows :

(a) the optical gap of the carbon layers was found to decrease from 2.5 eV to 1.0 eV when the negative bias voltage varied from -1OOto -800V; (b) the optical anomaly absorption was observed when the concentration of the metals in the carbon matrix was subsequently increased ; (c) the stability of the optical properties represented by the film absorbance is very good when annealing the films to 200°C and it is much better than the stability of the metal-doped plasma polymers studied formerly.

References ’ J C Angus, P Koidl and S Domitz, Plasma Deposition of Thin Films (Edited by J Mort and F Jansen). CRC Press (1986). ’ F W Smith. J aDDI Phvs. 55, 764 (1984). 3B Dischler,‘A &benz&‘and P Koidl, &I$ Phys Lett, 42,636 (1983). 4H Biederman, L Martin6 and J Zemek, Vacuum, 35,447 (1985). ‘Ch Weissmantel, K Brenner and B Winde, Thin Solid Films, 100, 383 (1983). 6 S Craig and G L Harding, Thin Solid Films, 97, 345 (1982). ‘Ch Weissmantel, E Ackerman, K Bewilogua, G Hecht, H Kupfer and B Rau, J Vat Sci Technol A4(6), 2892 (1986). *M Sikkens, Solar Energy Mafer, 6,403 (1982). 9 H Biederman, R P Howson and I McCabe, Proc IPAT 1987, Brighton, p 152 (1987). ‘OH Biederman, Hong jon Chjok, L Martinb, J David, S Kadlec and P LukBE, Proc E-MRS Conf 1987, Strasbourg, June 2-5 (1987). ” T S Moss, Optical Properties of Semiconductors. Butterworths, London (1959). “D A Anderson and S Berg, Phil Maq, 35, 17 (1977). I3 B Abeles, Appl Solid State Sci, 6, 1 (1976). 14E Kav. 2 Phvs D-Atoms. Molecules and Clusters. 3.251 (1986). “H Biederman, L Mart&d, D Slavinski and I ch;dZek, P&e Appl Chem, 60,607 (1988). 16H Biederman, L Martind and S NeSpbrek, Proc ISPC-8, Tokyo, September 14, p 1364 (1987).

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