Optical properties of thin metal films

Optical constants of metallic thin films made from: Ag, Au, Hf, Ir, Mo, Nb, Os, Pd, Pt, Re, Rh, Ru, Ta, W, Zr were determined on the basis of measured index of refraction in region of wavelength λ = 241216 Å. Two types of relations were used for the calculation. Some of them were obtained, with tak...

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Дата:1999
Автор: Kovalenko, S.A.
Формат: Стаття
Мова:English
Опубліковано: Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України 1999
Назва видання:Semiconductor Physics Quantum Electronics & Optoelectronics
Онлайн доступ:http://dspace.nbuv.gov.ua/handle/123456789/119882
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Цитувати:Optical properties of thin metal films / S.A. Kovalenko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 3. — С. 13-20. — Бібліогр.: 25 назв. — англ.

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spelling irk-123456789-1198822017-06-11T03:02:31Z Optical properties of thin metal films Kovalenko, S.A. Optical constants of metallic thin films made from: Ag, Au, Hf, Ir, Mo, Nb, Os, Pd, Pt, Re, Rh, Ru, Ta, W, Zr were determined on the basis of measured index of refraction in region of wavelength λ = 241216 Å. Two types of relations were used for the calculation. Some of them were obtained, with taking into account that refractive index of absorbing medium can be presented in the form ñ = n ± iæ. Other were obtained from Maxwell boundary condition. Both approaches give rise to very close results for æ, however the dependences n = f(λ) for λ > 200 Å are essentially different. The reasons of such differences are discussed. 1999 Article Optical properties of thin metal films / S.A. Kovalenko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 3. — С. 13-20. — Бібліогр.: 25 назв. — англ. 1560-8034 PACS: 78.66; 78.20.C http://dspace.nbuv.gov.ua/handle/123456789/119882 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection DSpace DC
language English
description Optical constants of metallic thin films made from: Ag, Au, Hf, Ir, Mo, Nb, Os, Pd, Pt, Re, Rh, Ru, Ta, W, Zr were determined on the basis of measured index of refraction in region of wavelength λ = 241216 Å. Two types of relations were used for the calculation. Some of them were obtained, with taking into account that refractive index of absorbing medium can be presented in the form ñ = n ± iæ. Other were obtained from Maxwell boundary condition. Both approaches give rise to very close results for æ, however the dependences n = f(λ) for λ > 200 Å are essentially different. The reasons of such differences are discussed.
format Article
author Kovalenko, S.A.
spellingShingle Kovalenko, S.A.
Optical properties of thin metal films
Semiconductor Physics Quantum Electronics & Optoelectronics
author_facet Kovalenko, S.A.
author_sort Kovalenko, S.A.
title Optical properties of thin metal films
title_short Optical properties of thin metal films
title_full Optical properties of thin metal films
title_fullStr Optical properties of thin metal films
title_full_unstemmed Optical properties of thin metal films
title_sort optical properties of thin metal films
publisher Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
publishDate 1999
url http://dspace.nbuv.gov.ua/handle/123456789/119882
citation_txt Optical properties of thin metal films / S.A. Kovalenko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 3. — С. 13-20. — Бібліогр.: 25 назв. — англ.
series Semiconductor Physics Quantum Electronics & Optoelectronics
work_keys_str_mv AT kovalenkosa opticalpropertiesofthinmetalfilms
first_indexed 2025-07-08T16:50:24Z
last_indexed 2025-07-08T16:50:24Z
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fulltext 13© 1999, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine Semiconductor Physics, Quantum Electronics & Optoelectronics. 1999. V. 2, N 3. P. 13-20. PACS: 78.66; 78.20.C Optical properties of thin metal films S.A. Kovalenko Institute of Semiconductor Physics, NAS of Ukraine 45, prospect Nauki, 252650 Kyiv, Ukraine Abstract. Optical constants of metallic thin films made from: Ag, Au, Hf, Ir, Mo, Nb, Os, Pd, Pt, Re, Rh, Ru, Ta, W, Zr � were determined on the basis of measured index of refraction in region of wave- length λ = 24�1216 Å. Two types of relations were used for the calculation. Some of them were ob- tained, with taking into account that refractive index of absorbing medium can be presented in the form ñ = n ± iæ. Other were obtained from Maxwell boundary condition. Both approaches give rise to very close results for æ, however the dependences n = f(λ) for λ > 200 Å are essentially different. The reasons of such differences are discussed. Keywords: thin layers, refraction coefficient, transmission coefficient, refraction index, absorption index Paper received 02.09.99; revised manuscript received 29.09.99; accepted for publication 18.10.99. 1. Introduction Investigations of optical properties of solids, including these of thin films, are aimed on: first, to obtain data for ascer- taining their band structure; second, to determine such im- portant parameters as refractivities and absorptivities nec- essary for designing devices and their elements. Thin films of transparent and light absorbing materials are widely used in manufacturing interferential [1,2] and absorption [3,4] filters, in production of multi-layer mirrors [5,6], polarizers [7,8], light detectors [9,10], in making anti- reflection coatings [11,12]. In all enumerated cases one needs to have exact data upon optical constants, n and æ, of con- sidered materials. For many years respective calculations were based on relationships that took into account repeated reflections and interference in a thin layer, and measured in experiment were intensities of light beams reflected and transmitted by the thin layer. Formulae for intensities of a light wave reflected and transmitted by the layer include Fresnel coefficients of re- flection and transmission for separate interfaces, namely: air (vacuum) � thin layer and thin layer � substrate. In the case of absorbing media a refraction index is complex value n + iæ. Therefore, transition from transpar- ent substance to absorbing one is usually postulated by the change ñ → n + iæ, which cannot be considered as fully justified. It is this change that leads to meaningless results. For example, when a thickness of metal [13-15] and semi- conductor [16-18] films tends to zero value, the refraction index increases to infinity, though it must approach to unity. Such monotonic thickness dependence of the refraction in- dex for thin absorbing films all the more cast doubt, be- cause it must have at least one resonance maximum on the curve n = f (d) that corresponds to a wedgeshaped layer thickness of which changes from zero to the value when this layer becomes absolutely transparent. Calculations ful- filled in accordance with above formulae [19] do not give such a maximum. As it is known, investigations of metal optical proper- ties are based on using ellipsometric method and measuring reflection coefficients for different angles of light incidence on mirror surface of a bulk sample. In these cases work formulae for n and æ estimations are usually obtained by changing ñ → n + iæ. As a result, obtained values of n in wide ranges of the optical spectrum are less than unity. As an example, we can give data [20,21] for gold. Authors of the first paper considered the range of wavelength 0.5-1 µm where n monotonically decreases from 0.84 to 0.19. In the second paper, measurements were performed in the range of 1.0-4 µm that supplements the previous interval. n-values less than unity, were obtained in the region of 1.0-2.5 µm. With increasing wavelength, these raise from 0.224 to 0.82. Thus, almost in the whole visible part of the spectrum, and in a considerable part of the infra-red region joining to it, the refraction index of gold is less than unit. We shall not give more numeric data by other authors for metals, semi- conductors and dielectrics where values of n < 1 are fixed in wide ranges of the optical spectrum. Moreover, in this paper we cite plots for n = f (λ) dependences from [22] for 16 metals, and each of them has respective ranges of the spectrum. The inequality n < 1 is not alone to generate surprise. It S.A. Kovalenko: Optical properties of thin metal films 14 SQO, 2(3), 1999 is difficult to imagine why it is intrinsic for wide spectral intervals while minimum n-values are usually met in the narrow range of frequencies corresponding to band half- widths of electron or vibration transitions, that is, in the range of anomalous dispersion. As known, in this region, the group velocity with which light wave energy is trans- fered, is connected with refraction index dispersion through known relationship: λ λ ϑ ⋅− = d dn n c gr , (1) were c – is the light velocity in vacuum. Since a phase velocity ncph /=ϑ , then at frequencies where the sign of dn/dλ is changed, i.e., dn/dλ = 0, n c gr == ϑϑ ph > c , (2) taking into consideration that n < 1. This result contradicts the special theory of relativity. By the way, if one takes for example the n-value of silver for the wavelength close to 0.5 µm that is equal to 0.05 then the respective group velocity must exceed the light velocity in vacuum by 20 times! It is a fantastic result, there is nothing to say. One example more [23] where authors cast doubt on legitimateness of using known Fresnel formulae. The authors had the task to measure depth of X- ray penetration into on oxide film on a silicon surface. Thicknesses of such films are small, so for increasing losses of an energy of an X- ray beam at the cost of absorption and scattering, authors directed it at a grazing angle of incidence, and the depth of penetration was determined by the formula:      −= 0 2cosIm2 1 θεω cD . (3) Here, ω is an angular frequency, c is the light velocity in vacuum, ε is the dielectric permittivity of a SiO2 – layer, θ is an angle of grazing incidence. A large deviation of theo- retically estimated and measured D-values is explained by authors as stemmed from non-Fresnel character of Si-SiO2 system behavior. In our opinion, above examples are enough to motivate a necessity of searching new relationships which could ful- fil the same functions as Fresnel formulae did for many years. The first steps along this line are made by T.O.Kudykina [25] who, using boundary conditions by Maxwell, obtained analogs to Fresnel formulae. Using these formulae and experimental data upon transmission and reflection by thin layers of atomic semiconductors obtained earlier [16-18], authors of [25] calculated thickness depen- dencies of optical constants n and æ. As it was expected, each curve has a resonance maximum, and n- values ap- pear to tend to unity when d goes to zero. It would seem that such brilliant confirmation of the Kudykina formulae workability might interpose all on their places. However, as it will be seen below in discussing the problem, with these new opportunities some new questions arose. In this paper, using reflection spectra of thin films for 16 metals optical data of which in the range of λ = 24 - 1216 Å were obtained in [22], we calculated n- and æ- values in accordance to old [19] and new [25] theories. Investigated in [22] metal films were prepared by elec- tron-beam sputtering in vacuum ~ 5·10-6 Torr on silicon substrates temperature of which was kept to be equal to 300 ºC. The thickness of the films was approximately 1000 Å. 2. Work formulae Fig.1 represents a scheme of a sample like those investigated in [22]. Let us consider that the substrate and air are transparent for wavelengths used in the work, i.e. æ1 = æ3=0. It is also accounted that at thicknesses close to 1000Å the film−substrate boundary and, all the more, the substrate− air one give negligable contribution into total−reflectivity. It is in such conditions this new theory is used for calculation n and æ. Necessary relationships see below. These have the following appearance:       ′− += ′− − = , 1 , 1 1 *34 2 *34 *14 * *34 34 14 RRR TR RR T RRR R T (3) where T14, R14 are transmission and reflection coefficients of the system shown in Fig.1, respectively, 2 3 3 34 1 1     + − = n n R is the Fresnel reflection coefficient of the substrate−air boundary; R* = C/A; Ŕ* = B/A; AnnT /)(16 2 2 2 23* ea+= ; γγγδσγρτ 222 sin2cos2)exp()exp( ntnsA ++−+= eaea ; γγγδτγσρ 2222 sin2cos2)exp()exp( nrnqB −+−+= eaea ; r34 n4 t 344 d2 t12 t 23 r23 r12 n 3 n 2 22 n1 3 1 Fig.1. S.A. Kovalenko: Optical properties of thin metal films 15SQO, 2(3), 1999 γγγσργδτ 22 sin2cos2)exp()exp( nrnqC ++−+= eaea ; q n n n n n= + + − + − −( æ )( ) ( æ ) æ2 2 2 2 3 2 2 2 2 2 2 3 2 3 2 21 4 ; ρ = + +( ) æn1 2 2 21 ; s n n n n n= + + − + − +( æ )( ) ( æ ) æ2 2 2 2 3 2 2 2 2 2 2 3 2 3 2 21 4 ; σ = − +( ) æn n2 3 2 2 2 ; r n n n= − + +2 12 3 2 2 2 2 2 3æ ( ) (æ ) ; τ = + +( ) æn n2 3 2 2 2 ; t n n n= + + −2 12 3 2 2 2 2 3æ ( ) (æ ); γ = 4πd2/λ, where λ is the light wavelength in vacuum. Both the traditional and the new theory start from the same relationships for reflection and transmission coeffi- cients. These are as follows: ψαα ψαα cos)exp(2)2exp(1 cos)exp(2)2exp( 2312 2 23 2 12 2312 2 23 2 12 drrdrr drrdrr R −−−+ −−−+= , (4) ψαα α cos)exp(2)2exp(1 )exp( 2312 2 23 2 12 2312 drrdrr dTT T −−−+ −= , (5) where α π λ = 4 2æ , (6) , 4 2dn λ πψ = (7) Contrary to the old theory, in the new one, coefficients of reflection and transmission for separate boundaries air− film and film−substrate do not include absorption indices at the normal angle of light beam incidence: 23 23 23 12 12 12 ; nn nn r nn nn r + − = + − = , (8) 23 1 12 1 2 23 2 12 ; nn n nn n tt ++ == , (9) 32 3 21 2 2 32 2 21 ; nn n nn n tt ++ == , (10) It is worth to note that R12= (r12)2 , R23 = (r23)2 , T12 = t12t21, T23= t23t32. Substituting Zi = exp (-αd), instead of Eqs (4) and (5), one can obtain quadratic equations relatively to Zi : ii ii ZrrZrr ZrrZrr R ⋅−+ ⋅−+= ψ ψ cos21 cos2 2312 22 2312 2312 22 23 2 12 , (11) ψcos21 2312 22 23 2 12 2312 ii i ZrrZrr ZTT T −+ = . (12) Solving Eq. (12) yields roots for Z: Z b a b a T a1 2 2 22 4, / ,= ± − (13) where cos2 , 23122312 2 23 2 12 rTrTTb rTra += = ψ. Substituting Zi into Eq. (11), one can find n2- value as a root of the function F R r r Z r r Z r r Z r r Z i i i i = − + − ⋅ + − ⋅ 12 2 23 2 2 12 23 12 23 2 2 12 23 2 1 2 cos cos ψ ψ . (14) Zi – value must satisfy the condition 0 < Zi < 1. The sign before the root of the solution (13) is chosen in accordance with it. After calculation of the n2- value we can find the æ2- value using the next relation: æ d Z ñ i π λ 4 ln 2 ⋅−= , (15) which follows from (6) accounting the change Zi = exp(-ad). Spectral dependencies of n and æ calculated by using both the traditional formulae and those of the new theory are shown in Figs 2-17. 3. Discussion The analysis of curves testifies that only for short wave- lengths taken from the range of λ = 24 - 100Å refractive index values calculated using both theories practically co- incide. At λ > 100 Å these diverge, moreover, the new theory leads to considerably larger n- values. For gold, silver, iridi- um, platinum and rhenium a maximum difference corresponds to wavelength taken from the interval λ = 530 - 550 Å. But for molibdenium, osmium, palladium, rhutenium and rhod- ium it is shifted up to λ ≈ 600 Å. The largest shift can be observed in cases of hafnium, tantalum (λ ≈ 1000Å), tung- sten, zirconium and titanium (λ ≈ 1100 - 1200Å). It is most frequently at these wavelength that the new theory describes the n- maximum which can be explained by the quantum- sized resonance. As for absolute values of refraction indices, the new theory leads only to n > 1 in all range of wavelength used. At the same time, the old one leads to n < 1 for all metals in wide ranges of the spectrum without any exception. If we take into account that in the range of wavelengths λ = 24 - 1200Å quantum transitions realize between inner electron shells polarizabilities of which are considerably less than those of the outer ones, then n- values exceeding unity can cast doubts. A very interesting result was obtained for absorption indices. These almost coincide in all range of wavelength used, moreover, their absolute meanings do not cast doubts. Coincidence can be explained by respective estimations. If æ2/n2 = 0.05 then there is no place for any divergence be- tween old and new theories, but when æ2/n2 ≥ 0.25 such difference will be considerable. S.A. Kovalenko: Optical properties of thin metal films 16 SQO, 2(3), 1999 Au B D n 2.5 2.0 1.5 1.0 0.5 Ag C E 0 200 400 600 800 1000 1200 1400 0 5 0 Ag B D n 2.0 1.5 1.0 æ 1.0 0.5 0.0 Au C E æ 1.0 0.5 0.0 Hf B D Hf C E B D Ir Ir C E 0 200 400 600 800 1000 1200 1400 λ λ λ λ λ , , , , , Å 0 200 400 600 800 1000 1200 1400 λ λ λ λ λ , , , , , Å 3.0 2.5 2.0 1.5 1.0 0.5 n 3.0 2.5 2.0 1.5 1.0 0.5 n 1.5 1.0 0.5 0.0 æ 1.5 1.0 0.5 0.0 æ Figs 2-5. S.A. Kovalenko: Optical properties of thin metal films 17SQO, 2(3), 1999 Mo B D Mo C E Nb B D Nb C E B D Os C E Os Pd B D Pd C E 0 200 400 600 800 1000 1200 1400 λ λ λ λ λ , , , , , Å 0 200 400 600 800 1000 1200 1400 λ λ λ λ λ , , , , , Å 2.5 2.0 1.5 1.0 0.5 n 2.5 2.0 1.5 1.0 n 3.0 2.5 2.0 1.5 1.0 0.5 n 2.5 2.0 1.5 1.0 0.5 n 1.0 0.5 0.0 æ 1.0 0.5 0.0 æ 1.0 0.5 0.0 æ 1.0 0.5 0.0 æ Figs 6-9. S.A. Kovalenko: Optical properties of thin metal films 18 SQO, 2(3), 1999 B D Pt C E Pt B D Re C E Re B D Rh C E Rh B D Ru C E Ru 0 200 400 600 800 1000 1200 1400 λ λ λ λ λ , , , , , Å 2.5 2.0 1.5 1.0 0.5 n 2.5 2.0 1.5 1.0 0.5 n 2.5 2.0 1.5 1.0 0.5 n 2.5 2.0 1.5 1.0 0.5 n 1.0 0.5 0.0 æ 1.0 0.5 0.0 æ 1.0 0.5 0.0 æ 1.0 0.5 0.0 æ 0 200 400 600 800 1000 1200 1400 λ λ λ λ λ , , , , , Å Figs 10-13. S.A. Kovalenko: Optical properties of thin metal films 19SQO, 2(3), 1999 B D Zr B D Ta C E Ta B D Ti C E Ti B D W C E W C E Zr 0 200 400 600 800 1000 1200 1400 λ λ λ λ λ , , , , , Å 2.5 2.0 1.5 1.0 0.5 n 3.0 2.5 2.0 1.5 1.0 0.5 n n 2.5 2.0 1.5 1.0 0.5 n 3.5 3.0 2.5 2.0 1.5 1.0 0.5 1.5 1.0 0.5 0.0 æ 1.0 0.5 0.0 æ 1.5 1.0 0.5 0.0 æ 1.0 0.5 0.0 æ 0 200 400 600 800 1000 1200 1400 λ λ λ λ λ , , , , , Å Figs 14-17. S.A. Kovalenko: Optical properties of thin metal films 20 SQO, 2(3), 1999 Conclusions 1. Optical constants n and æ for 16 metals in the spectral region of λ = 24 - 1216Å are estimated. Experimental de- pendences of reflection coefficients on the wavelength are considered using two theories: the traditional one, where energetic coefficients of transmission and reflection on the boundary between two substances are calculated using sub- stitutions ñ → n + iæ in the Fresnel formulae for transpar- ent media, and the new theory in which the same coefficients are determined using formulae obtained from the Maxwell boundary conditions. 2. It is shown that coincidence of results takes place only for absorption indexes and for all metals in the whole spectral region. It is reasonable to believe that this coincidence has a random character and can be explained by a small æ-value in comparison with an n-value. 3. The accordance of n = f (λ) curves with predictions of the new theory points on noticeable increase in n- values with wavelength. However, an absolute n- magnitude does not correlate with low polarizability of inner electron shells of atoms in the range of deep ultraviolet and X-rays. Therefore, the new theory predicts higher n-values. 4. Contrary to the previous conclusion, the traditional theory leads to lower n-values. There are wide spectral regions where n < 1, i.e., one has the result deprived of real physical sense. The author would like to emphasize his gratitude to Academician M.P.Lisitsa who is the supervisor and the author of the main idea of this work. References 1. S.I. Gorban, S.V. Orlov, U.A. Pervak, I.V. Fekeshgasi. Multi ply kovering with broad zone reflection and transmition. // Ukrainskii fizicheskii zhurnal, 31, N 10, p.1494-96, (1986) (in Russian). 2. A.A. Aliseev, I.V. Ravadina. Prinsip of criation tunable interferation filter // Izvestiya vuzov. Ser. fizich., N 3, p.3, (1997) (in Russian) 3. L.I. Suslikov, V.P. Slivka, I.P. Lisitsa. Hard optical filter on the hirotropic crystals. 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