Ellipsometric examination of the oxidation of vacuum-deposited bismuth films

Thin Solid Films, 128 (1985) 333-339 PREPARATION

AND

555

CHARACTERIZATION

ELLIPSOMETRIC EXAMINATION DEPOSITED BISMUTH FILMS

OF THE OXIDATION

OF VACUUM-

R. ATKINSON

The Quern’.c University (Ct. Britain)

qf

Belfbs~.

Deprrrtmrnt

of

Pure and Applied

Physics,

Belfast

BT7 INN

E. CURRAN Council for Scient[fic and Industrial Research, NEERI, (Received

July 5, 1984; accepted

February

P.O. Box 395, Pretoria 0001 (South Africa)

25, 1985)

An investigation has been carried out of the surface structure and oxidation of thin films of bismuth vacuum deposited onto heated glass substrates. Temporal variations in the optical properties of thin films of bismuth have been shown to be due to the growth of an extraneous surface layer of bismuth oxide. Detailed ellipsometric measurements have made it possible to monitor the rate of growth of this layer and to determine its refractive index throughout the visible spectrum.

1.

INTRODUCTION

The growth of dielectric films on absorbing substrates is of interest from both a theoretical and a practical point of view’. In the case where such a film is the result of oxidation of an optical surface it is useful to be able to characterize the layer in terms of its refractive index and geometrical thickness. In this paper the results of part of an investigation of the optical properties of vacuum-deposited bismuth are presented. In the past, Atkinson and Lissberger’ have shown that the optical properties of thin films of this material are markedly dependent on the microscopic surface roughness resulting from non-uniform film growth and on misoriented crystallites3. More recent work by Vallely4 on the possible optical anisotropy of bismuth sought to overcome this problem by depositing films onto heated substrates5v6. This technique produces much smoother surfaces and thus makes it possible to obtain reliable values for the optical constants from ellipsometric measurements at the airfilm interface. However, temporal variations in the optical properties of the films have been observed to take place over a period of at least 10 days so that measurements must be made in situ or as soon as the film is removed from the vacuum system. An ellipsometric investigation of these temporal variations has been carried out in the visible region of the spectrum and it has been concluded that they are due to the slow growth of an extraneous layer of bismuth oxide (Bi,O,). The refractive index of this layer has been determined and its growth studied over a 9 day aging period under normal atmospheric conditions. 0040-6090/85/$3.30

x> Elsevier Sequoia/Printed

in The Netherlands

334

2.

EXPERIMENTAL

R. ATKINSON,

E. CURRAN

DETAILS

Bismuth films, approximately 50 nm thick, were vacuum deposited at a rate of 0.5 nm s ’ onto borosilicate glass substrates. The material, of purity 99.999:& was evaporated from an electrically heated tungsten boat at a residual gas pressure of 2 x 10m4 Pa. Film thicknesses were monitored during deposition by means of a quartz crystal oscillator and subsequently confirmed by Talystep measurements. In order to keep surface roughness to a minimum, substrates were heated to 353 K. The optimum temperature of 353 K was determined by depositing a series of films onto substrates at various temperatures in the range 300-450 K. The effect of this on the structure and surface topography of the films was examined by X-ray diffractometry and by electron microscopy using conventional replication techniques. Ellipsometric measurements were carried out on the films immediately after their removal from the vacuum system. These were then repeated at intervals of 24 h for a period of 9 days. During this time, films were stored at room temperature in a dust-free environment in air at atmospheric pressure. Optical measurements were made at six wavelengths in the visible region of the spectrum (400-615 nm) using a fully automatic microprocessor-controlled ellipsometer’ operating at an angle of incidence of 45” to the air-film interface. 3.

STRUCTURAL

OBSERVATIONS

The effects of substrate temperature on the crystal structure and surface topography appear to be in genera1 agreement with those reported by Berty et a/.’ and by Namba and Mori’. Below a critical temperature of about 363 K, films consist of a mosaic of hexagonal single-crystal islands, having their c axes oriented perpendicular to the plane of the film. For films deposited at room temperature (Fig. 1) the crystallite size is of the same order of magnitude as the film thickness, and the surface roughness is determined by non-uniform film growth and misoriented crystallites. As the temperature of the substrate during deposition is increased, the crystallite size increases and the number having misoriented c axes decreases. Moreover, as can be seen from Fig. 2, the genera1 surface quality is significantly improved. Namba and Mori’ have developed a phenomenological analysis of the mechanism of film growth for bismuth in relation to the substrate temperature and the rate of deposition. They point out that increasing the substrate temperature gives increased surface mobility to bismuth adatoms, resulting in a channel-filling process which gives rise to smoother films. The channels referred to here are voids or surface indentations which are a consequence of the early stages of film nucleation and growth. Above the critical temperature the film structure is markedly different. Figure 3 shows that films are characterized by a large number ofclosely packed dome-shaped islands protruding from a continuous background layer of bismuth. X-ray diffractometry reveals that the films are predominantly polycrystalline and show little indication of any preferred direction for the c axis. The dome-shaped protrusions are believed to be the result of coalescence and growth of individual islands formed on the substrate surface. The maximum size to which these islands

ELLIPS~METRIC

Fig.

EXAMINATION

1, Electron micrograph

Fig. 2. Surface of a bismuth

Fig. 3. Structure

ofa bismuth

OF OXIDATION

of the surface of a bismuth film deposited

film deposited

OF VACUUM-DEPOSITED

film deposited

Bi

FILMS

335

at 293 K.

at 353 K.

at 37X K.

grow before they reach the point where they touch and begin the network stage of continuous film formation increases with increasing substrate temperature. Consequently, in Fig. 3 we see a network of randomly oriented crystallites with the spaces between them gradually being filled to form a continuous structure. A more detailed description and interpretation of these observations have been given by Currang. It is clear from Fig. 2 that smooth films are obtained at temperatures of 353 K. Consequently, all optical measurements were carried out on films which were deposited at this temperature. 4. RESULTS OF ELLIPSOMETRY In order to determine the refractive index and absolute thickness of the extraneous oxide layer from the ellipsometric measurements, it is necessary to know the optical constants of the underlying pure bismuth film. Vallely4 has made

336

R. ATKINSON,

E. CURRAN

measurements of the complex refractive index of thin film bismuth with radiation incident at the film-substrate interface (Fig. 4). Since this interface is free from the effects of oxidation, the optical constants obtained show no temporal variations and are in close agreement with those obtained from measurements at the air-film interface, provided that these are made within 30 min after removal of the specimen from the vacuum system. It may be concluded therefore that the thickness of any spontaneously formed oxide layer on the surface of bismuth must be extremely thin and that reliable values for the optical constants of bismuth can be determined by conventional ellipsometry applied to freshly made films.

Fig. 4. Optical measurement.

model for an oxidized bismuth

film: I, air-film

interface

measurement;

II, film-substrate

Ellipsometric measurements were therefore performed on each bismuth film immediately on its removal from the vacuum system. Optical constants were calculated assuming the film to be a single homogeneous isotropic metallic layer. The samples were then allowed to age under normal atmospheric conditions for a period of 9 days. Further measurements were carried out during this time to determine the growth characteristics of the oxide layer. On the assumption of a dielectric-metal system consisting of two homogeneous isotropic plane layers as illustrated in Fig. 4, the refractive index n,, and the thickness d of the oxide were calculated from the ellipsometric measurements using the optical constants determined at a time t < 30 min to characterize the underlying bismuth layer. The values of n,, and d therefore apply to material formed during the subsequent 9 day aging period. The results of the ellipsometric observations are summarized in Figs. 5-7. Figure 5 shows typical dispersion curves obtained for the complex refractive index n’ + in” of a freshly made bismuth film. Figure 6 shows the temporal variations in both the oxide thickness and its refractive index, the latter corresponding to a wavelength of 492.1 nm. The values of n,, and d represent the means obtained for all the films measured. Consequently the error bars represent a combination of the random errors associated with the ellipsometric measurements and the variation in properties from film to film resulting from uncontrollable factors in their preparation. The dispersion of noXis illustrated in Fig. 7. Enumerated below are some points concerning these results that are worth noting. (1) It is evident from Fig. 6 that the oxide film thickness increases monotonically, reaching a value of about 1.6 nm after 9 days. Initially the growth rate is about 0.03 nm h r This decreases gradually to about 0.0 1 nm h I after 4 days.

ELLIPSOMETRIC

EXAMINATION

OF OXIDATION

OF VACUUM-DEPOSITED

Bi

FILMS

ZO_

A/

n’

1e-

A /./:

n” -10

@'

Q

35

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0

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127 LOO

600

500

Fig. 5. Dispersion

)

1,

0.0

1

of the optical constants

2

Fig. 6. Temporal

LOO Fig. 7. Dispersion

3

) i

variations

1 5

) 6

TIME /, 7

ofa freshly made bismuth

(Days1 t 8

in the thickness

500 of the refractive

film.

,I

9

(0) and the refractive

index (A) of the oxide layer.

600

index of bismuth

oxide; -,

after Medernach

and MartinlO.

331

338

R. ATKINSON,

E. CURRAN

(2) Most observations of oxidation of metals indicate either a logarithmic or a parabolic growth law. The results in Fig. 6 can be fitted to both of these laws where d = 0.8 ln( 1 + t) or d2 = 0.371 However, it should be pointed out that the random errors associated with the measurements of d do not make it possible to ascertain which, if either, of these two laws is applicable in this particular case. (3) It is clear that after 24 h the refractive index of the oxide layer reaches a steady value of approximately 2.6 (Fig. 6). In addition, it is worth pointing out that the values determined when the film is only a few hours old are subject to large fluctuations. This is attributed to the fact that the thickness of the layer is such that a three-dimensional continuous structure has not formed at this stage and therefore the optical model used to deduce noXis not valid. Moreover, since the oxide film is very thin, the precision with which noX and d can be simultaneously determined is poor. This is because, in this particular case, the ellipsometric parameters $ and d are only weakly dependent on the refractive index of the oxide layer and because this dependence decreases with decreasing d. Consequently, the errors associated with n are largest for the thinnest films. As a guide it should be noted that, for a Givelength of 492.1 nm, calculations based on random errors of F 0.01’ in $ and n indicate that noXcan be determined to within kO.05 for d = 1.6 nm and that this value increases to kO.1 for d = 0.6 nm. Throughout this thickness range, d may be obtained to within i 0.03 nm. (4) Figure 7 indicates that, within the experimental error limits, there is only a small dispersion over the wavelength range 400-650 nm and that the value of the refractive index lies in the range 2.55-2.8. The low dispersion is confirmed by the results of Medernach and Martin” who studied the optical properties of vacuumdeposited Bi,O,. Measurements of the refractive index of such films gave a value of 2.57 at a wavelength of 500 nm. Clearly, both the dispersion and the absolute values or refractive index compare quite favourably, bearing in mind the difficult measurements being reported here. (5) The thicknesses obtained for the same layer of oxide using measurements at different wavelengths show satisfactory consistency within the experimental errors. 5.

CONCLUSIONS

Thin films of bismuth have been produced by vacuum deposition onto heated glass substrates. The temperature of the substrate which leads to films having a minimal surface roughness has been determined to be 353 K. Ellipsometry carried out at the air-film interface has revealed temporal variations which have been shown to be due to the growth of an oxide layer. The thickness of this layer is about 1.6 nm after 9 days under normal atmospheric conditions and the refractive index reaches a steady value of 2.7 after 24 h. Comparison of the dispersion of the refractive index with results found in the literature” suggests that the layer is Bi,O,.

ELLIPS~METRIC

EXAMINATION

OF OXIDATION

OF VACUUM-DEPOSITED

BiFn_Ms

339

Sharma and Pandey ” have reported that bismuth trioxide has three polymorphs. The CLphase is stable at low temperatures, the p phase is metastable and the y phase is stable at high temperatures. Electron diffraction studies” of thermally oxidized films of bismuth reveal polycrystalline films of a-Bi,O, and Bi,O,,,,. It is worth pointing out that X-ray diffraction studies of films used in this investigation did not indicate the presence of the oxide. However, this is not surprising in view of the fact that the layer is only 1.6 nm thick. REFERENCES

I 2 3

E. Idczak and E. Olesrkiewicr, Thin Solid Films, 77 (198 I) 301, R. Atkinson and P. H. Lissberger. Thin Solid Films. I7 (1973) 207. A. Barna, P. B. Barna. R. Fedorowich, G. Radnoczi and H. Sugawara,

Thin Solid Films, 36

(1976) 75. 4

L. Vallely. Ph.D. Thrsis, The Queen’s University of Belfast. 1981, Y. Nambd and T. Mori, .I. Appl. PhJs., 46 (1975) I 159. 6 K. Abdelmoula, B. Pardo. C. Pariset and D. Renaro, Thin Solid Films, 62 (1979) 273. 7 P. H. Lissberger, I. W. Salter. M. Fitzpatrick and P. L. Taylor. J. Phys. E. 10 (1977) 635. 8 J. Berty, M. Brieu. C. Butto and B. Legros-De Mauduit, Thin SolidFilms. 82 (1981) 321. 9 E. C. Curran. M.Sc. Thesis, The Queen’s University of Belfast, 1982. IO J. W. Medernach and R. C. Martin, J. Vuc,. Sci. Techno/., f2( 1975) 63. I I S.K. Sharma and S. L. Pandey. Thin Solid Films, 62 (1979) 209. 5