Applied Optics Letters to the Editor

Letters to the Editors should be addressed to the Editor, APPLIED OPTICS, 7 Norman Road, Newton Highlands, Mass. 02161, and should be accompanied by a signed

Copyright Transfer Agreement. If authors will state in their covering communications whether they expect their institutions to pay the publication charge, publication time

should be shortened (for those who do).

Hydrogenated carbon films produced by sputtering in argon-hydrogen mixtures

D. R. McKenzie, R. C. McPhedran, L. C. Botten, N. Savvldes, and R. P. Netterfield

R. P. Netterfield is with CSIRO National Measurements Laboratory, Division of Applied Physics, P.O. Box 218, Lindfield, N.S.W. 2070, Australia; L. C. Botten is with New South Wales Institute of Technology, School of Mathematical Sciences, P.O. Box 123, Broadway, N.S.W. 2007, Australia; the other authors are with University of Sydney, School of Physics, Sydney, N.S.W. 2006, Aus­tralia. Received 1 May 1982. 0003-6935/82/203615-03$01.00/0. © 1982 Optical Society of America.

We report on the optical constants and room temperature electrical resistivity of films prepared by sputtering from a graphite target in a dc cylindrical-post cathode magnetron apparatus1 using mixtures of argon and hydrogen as the sputtering gas. For an excellent review of magnetron sput­tering devices see Thornton and Penfold.2 The resistivity of the films can be varied over ~ 9 orders of magnitude by ad­justment of the partial pressure of hydrogen in the sputtering gas. Concurrent with the increase in resistivity, the absorp­tion of the films in the visible and near-infrared regions de­creases markedly. We show that the films contain hydrogen by examination of their infrared transmission spectra and that they are similar to films prepared by the glow discharge de­composition of acetylene.3-5

Hydrogenated carbon films are of technological importance as they form a component of spectrally selective surfaces for solar collectors5,6 and may find use as hard, adherent, insu­lating or semiconducting films in electronic and other appli­cations.7

The sputtering target was grade EY9 graphite in the form of a cylinder 28 mm in diameter by 200 mm long. Water cooling was provided down the center of the target and the longitudinal magnetic field was shaped by steel end caps. The base pressure of the sputtering chamber was 3 X 10 - 5 Pa. The sputtering gases were ultrahigh purity argon and hy­drogen replaced at the rate of 0.15 cm3 sec - 1 at STP. The operating parameters of the system were source-substrate distance 60 mm; total gas pressure 1 Pa; sputtering current densities of 50 and 100 A m - 2 ; magnetic field strength 4 × 10 - 2

T. The films were deposited onto glass substrates (Schott B270) at a temperature close to ambient. The target was degassed before deposition. The deposition rate for films prepared at a current density of 100 A m - 2 was ~0.12 nm sec. - 1 .

The normal incidence transmittance (T) and three reflec­tances {R, R1, Rc) at 20° angle of incidence were determined in the wavelength range from 0.35 to 2.5 μm. Here R and Rc

are, respectively, the front-side reflectances of the film on glass and on a thick film of copper which had been evaporated onto the glass substrate before deposition. R1 is the rear-side re­flectance of the film on the glass substrate. The film thick­ness was measured using a Talystep instrument. A Keithley 616 electrometer was employed to determine electrical resis­tivity. A four-terminal method was used for low resistivity films and a two-terminal method was used for high resistivity films.

Films were made at various partial pressures of hydrogen. Films sputtered in the absence of hydrogen were graphitic in appearance and their resistivity was in the range from 8 X 10 - 3

to 4 Ω cm (see Table I). The addition of hydrogen to the plasma had a profound effect on resistivity, increasing it by over 9 orders of magnitude. Similar changes in resistivity have been reported7 for films prepared by glow discharge decomposition of hydrocarbons, where the amount of hy­drogen incorporated into the films was varied by changing the substrate temperature.

The reflectance and transmittance measurements were corrected for substrate effects, and the corrected values were used to calculate the complex refractive index (n + ik) of the films by a new technique.8 In this technique, a new zero-finding algorithm for analytic functions9 is used to provide trajectories of all allowed n and k values corresponding to the corrected reflectances and transmittance. The correct values of n and k are the coordinates of the centroid of the region of intersection of the four trajectories. The variations of re­fractive index with wavelength for films prepared at the highest partial pressure of hydrogen and with current densities of 50 and 100 A m - 2 are shown in Figs. 1 and 2.

The variation of n with wavelength for both films is slight. The films differ in the details of their variation of k with wavelength, although both show a rapid decrease in the visible region (a behavior typical of semiconductors) followed by near transparency in the infrared. The film prepared at 50 A m~2

current density has k values which are always higher and vary more slowly with wavelength than those of the other film. The experimental errors for n and k shown in Figs. 1 and 2 are small, reflecting the good homogeneity of the films. Further details of the determination of these values will be given in a future paper.8

Table I. Preparation Conditions for Each Film a

α P H is the hydrogen partial pressure in which each film was sput­tered, ρ is the resistivity in Ω cm, j is the current density in A m -2, and t is the film thickness in nm.

15 October 1982 / Vol. 21, No. 20 / APPLIED OPTICS 3615

Fig. 1. Variation of the optical constants n ( ) and k ( • ) with wavelength in μm for film 4 of Table I.

Fig. 3. Quantity √nk/λ0 in (μm) - 1 is plotted against photon energy hω in eV: , film 4 of Table I; •, film 5 of Table I.

Fig. 2. Variation of the optical constants n ( ) and k ( • ) with wavelength λ in μm for film 5 of Table I.

The optical energy gap E0 is determined from the rela­tion10-11

where B is a constant and hω is the photon energy. In Fig. 3 we plot √nk/λ0 , where λ0 is the vacuum wavelength, as a function of photon energy. The high energy regions for both films are reasonably represented by straight lines, whose in­tercepts with the energy axis give values of the optical gap. These (eV) values are 1.55 ± 0.10 for the film prepared at 50 A m - 2 current density and 1.65 ± 0.15 for the film prepared 100 A m - 2 current density. These values are within the range reported3,4,7 for films prepared by the glow discharge de­composition of hydrocarbons.

Infrared transmission spectra were obtained from films deposited onto sodium chloride substrates. For films pre­pared at the highest hydrogen partial pressure and 50 A m - 2

current density absorption features were noted at the fol­lowing wave numbers: 2900, 1600, 1440, 1370, and a broad feature centered at 1000 cm - 1 . These wave numbers corre­spond to carbon-hydrogen bond vibration frequencies and are the same as those observed in films prepared by glow discharge decomposition of acetylene.4,5 We conclude that the manner in which the hydrogen is bonded into the carbon network is the same for the two preparation techniques.

In summary, we have shown that hydrogenated carbon films prepared by sputtering a graphite target in argon-hydrogen mixtures are similar to those produced by glow discharge decomposition of hydrocarbons. The films are of good uni­formity and adhesion and can be deposited over large areas. The properties of the films can be controlled by varying the hydrogen partial pressure. In particular, this parameter determines the optical energy gap. Further studies are under way to elucidate the structure of these films.

We acknowledge the assistance of L. M. Briggs with film preparation and the financial support provided by the New South Wales Government and His Royal Highness Prince Nawaf Bin Abdul Aziz of the Kingdom of Saudi Arabia through the Science Foundation for Physics within the Uni­versity of Sydney.

References 1. H. R. Wilson, D. R. McKenzie, and L. M. Briggs, Thin Solid Films

(1982), in press. 2. J. A. Thornton and A. S. Penfold, in Thin Film Processes, J. L.

Vossen and W. Kern, Eds. (Academic, New York, 1978). 3. D. A. Anderson, Philos. Mag. 35, 17 (1977). 4. D. R. McKenzie and L. M. Briggs, Sol. Energy Mater. 6, 97

(1981). 5. S. Craig and G. L. Harding, Thin Solid Films 00, 000 (1982), in

press. 6. L. M. Briggs, D. R. McKenzie, and R. C. McPhedran, Sol. Energy

Mater. 6, 455 (1982). 7. B. Meyerson and F. W. Smith, J. Non-Cryst. Solids 35, 435

(1980).

3616 APPLIED OPTICS / Vol. 21, No. 20 / 15 October 1982

8. L. C. Botten, R. C. McPhedran, D. R. McKenzie, R. P. Netterfield, and P. J. Martin, Appl. Opt., to be submitted.

9. L. C. Botten, M. S. Craig, and R. C. McPhedran, Comput. Phys. Commun (in press).

10. E. A. Davis and N. F. Mott, Philos. Mag. 22, 903 (1970). 11. G. A. N. Connell, in Amorphous Semiconductors, M. H. Brodsky,

Ed. (Springer, New York, 1979), Vol. 36.

15 October 1982 / Vol. 21, No. 20 / APPLIED OPTICS 3617