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Optical properties and microcharacteristics of thermally evaporated ytterbium films
Thin Solid Films, 75 (1981) 139-142 ELECTRONICS AND OPTICS
139
OPTICAL PROPERTIES AND MICROCHARACTERISTICS OF T H E R M A L L Y E V A P O R A T E D Y T T E R B I U M FILMS E. IDCZAK AND K. ZUKOWSKA
Institute of Physics, Technical University of Wroc&w, Wybrzeke Wyspiahskiego 27, 50-370 Wroclaw (Poland) (Received May 12, 1980; accepted June 20, 1980)
The results of investigations of the optical properties of ytterbium films over a wide spectral range, from UV to IR, are presented in this paper. Ytterbium films were thermally evaporated onto the substrate in a vacuum at room temperature. The optical constants of the ytterbium films were determined for the wavelength range 0.2-25 grn on the basis of measurements of the reflectivity of the films and using Kramers-Kronig dispersion relations. The concentration and collision frequency of conduction electrons in ytterbium films were determined from the optical constants. 1. INTRODUCTION As has been found in many investigations, the optical properties of thin films depend on the technique used for their preparation. Data relating to the optical properties of ytterbium films are very scarce in the literature 1-4. For this reason the optical properties of ytterbium films were investigated over a wide spectral range, 0.2-25 pm, in the present work. 2. EXPERIMENTAL The ytterbium films examined were thermally evaporated in a vacuum of the order of 10 -4 Pa in an Edwards vacuum apparatus. The films were produced from ytterbium of purity 99.9~o (supplied by Koch-Light Laboratories Ltd.) by evaporation from a tantalum boat and condensation at room temperature onto the substrates, which were piano-parallel plates of BK7 glass. The substrates were chemically purified by a standard laboratory method and were subject to ion bombardment before evaporation in a vacuum of 1 Pa. During evaporation the vacuum was of the order of 10-4 Pa and the rate of deposition of the film reached 0.2 nm s- 1. The thickness of the ytterbium films was measured to be about 350 nm using the multiple-beam interference method. The coefficient of reflection for the ytterbium films obtained in this manner was measured from the air side. The measurements of R(co) were carried out for perpendicular incidence using spectrophotometers, covering the spectral range 0.2-25 lam (Fig. 1). The following spectrophotometers were used: a Specord-UV in the range 0.2-0.8 ~tm,a VSU- 1 in the range 0.4-1.1 ~tm and a UR-10 in the range 2-25 Ixm. All the spectrophotometers 0040-6090/81/0000-0000/$02.50
© Elsevier Sequoia/Printed in the Netherlands
140
E. IDCZAK, K. ZUKOWSKA
employed were equipped with specially constructed reflection attachments 5. The measurements of the ellipsometric angles A and ~Uin the spectral range 0.45-0.65 lam conducted for these films allowed the determination of the optical constants n and k of the ytterbium layers in the visible region of the spectrum 6. 3. RESULTS OF INVESTIGATIONS The optical constants of ytterbium films were determined from the measurements of the coefficient of reflection R(co) using the Kramers Kronig dispersion relations. This method requires the measurement of the reflection over the entire frequency range. The measurements of the coefficient of reflection were made in the limited range 0.2-25 ~tm. Account had to be taken in calculations of the contribution of the unmeasurable spectral range to the reflected wave phase. The calculations were made according to the procedure described in ref. 7. The contribution of measurable frequencies in the range 0.2-25 lam was calculated from the experimentally measured R(~o). Since the optical constants of ytterbium films were known from ellipsometric measurements the contribution of the unmeasurable spectral range to the reflected wave phase could be determined. The experimentally measured value of R(~o) and the calculated phase change at the reflection allowed the optical constants of ytterbium films to be determined for the entire spectral range examined from the known relations 8' 9. As follows from the calculations (Fig. 2), the optical constants of ytterbium films increase monotonically with increasing wavelength of the light, and within the spectral range examined k is always greater than n. The nature of the changes in the optical constants in the long wavelength region of the spectrum permits the determination of important microcharacteristics of the metal examined, i.e. the concentration of conduction electrons and the effective frequency of electron collisions 1°. Microcharacteristics relating to conduction electrons can be obtained from measurements in the spectral range in which the effect of interband transitions on optical properties is negligible provided that the character of the skin effect is known. The concentration of conduction electrons and their effective collision frequency (Figs. 3 and 4) were calculated from R °/4 100
t
90 80 7O 60 50 40 30 2O
8t
. / /
o
s
.............................
10 0
i
½
5
,~
~
6Ethyl
lo
15
20
2~
Fig. 1. Spectral dependence of the reflection factor R for non-transparent ytterbium layers. Fig. 2. Spectral dependence of the refractive index n ( × ) and the absorption coefficienl k (©) for ytterbium films.
PROPERTIES OF THERMALLY EVAPORATED Yb FILMS
141
the relations given in ref. 11 on the assumption that normal skin effect conditions were satisfied for the ytterbium films and that the optical constants are known. As follows from Figs. 3 and 4, a spectral range in which the values of N and v do not depend on ). can be determined for ytterbium films. These values can be used to determine the microcharacteristics of the conduction electrons, which for the ytterbium films are concentration N = 5.5 x 1021 cm-3 and effective frequency v of collisions = 2 x 1014 s- 1.
2.2 ~. 1.8 % ~. 1.4 1,0
0.6
O2
5
10
15
25 ~,~aml
20
Fig. 3. Concentration of conduction electrons vs. wavelength for ytterbium. I
1.6 'to
~" 1.2 {:3 X
"-" 0.8
0.4
o . . . .
~
. . . .
~b
. . . .
~
. . . .
2'0 .
. . .
z~ at~m~
Fig. 4. Effective frequency of electron collisions vs. wavelength for ytterbium. ACKNOWLEDGMENTS
This work was carried out under Research Project MRI5. The authors would like to thank Professor C. Wesotowska for her interest in this work. REFERENCES 1 2 3 4 5 6 7
W.E. Miiller, Phys. Lett., 17 (1965) 82. J.P. Petrakian, Thin Solid Films, 20 (1974) 297. J.P. Petrakian, Thin Solid Films, 38 (1976) 83. J.G. Endriz and W. E. Spicer, Phys. Rev. B, 2(1970) 1466. C. Wesotowska, Acta Phys. Pol., 25 (1964) 323. E. Idczak, Thin Solid Films, 34 (1976) 407. F. Stern, Solid State Phys., 15 (1963) 299.
142
8 9 10 11
Z. IDCZAK, K. ZUKOWSKA
M . M . Kirillova, L. V. Nomerowannaya, G. A. Bolotin, V. M. Maevsky, M. M. Noskox and M. S. Bolotina, Fiz. Met. Metaloved., 25 (1968) 459. E. Idczak, Opt. Appl., 2 (1973) 25. G . P . Motulevic, Usp. bTz. Nauk, 97 (1969) 211.
G.P. Motulevic, TturtvFi-.lnst.,Akad. NaukSSSR, 55(1971)3.
139
OPTICAL PROPERTIES AND MICROCHARACTERISTICS OF T H E R M A L L Y E V A P O R A T E D Y T T E R B I U M FILMS E. IDCZAK AND K. ZUKOWSKA
Institute of Physics, Technical University of Wroc&w, Wybrzeke Wyspiahskiego 27, 50-370 Wroclaw (Poland) (Received May 12, 1980; accepted June 20, 1980)
The results of investigations of the optical properties of ytterbium films over a wide spectral range, from UV to IR, are presented in this paper. Ytterbium films were thermally evaporated onto the substrate in a vacuum at room temperature. The optical constants of the ytterbium films were determined for the wavelength range 0.2-25 grn on the basis of measurements of the reflectivity of the films and using Kramers-Kronig dispersion relations. The concentration and collision frequency of conduction electrons in ytterbium films were determined from the optical constants. 1. INTRODUCTION As has been found in many investigations, the optical properties of thin films depend on the technique used for their preparation. Data relating to the optical properties of ytterbium films are very scarce in the literature 1-4. For this reason the optical properties of ytterbium films were investigated over a wide spectral range, 0.2-25 pm, in the present work. 2. EXPERIMENTAL The ytterbium films examined were thermally evaporated in a vacuum of the order of 10 -4 Pa in an Edwards vacuum apparatus. The films were produced from ytterbium of purity 99.9~o (supplied by Koch-Light Laboratories Ltd.) by evaporation from a tantalum boat and condensation at room temperature onto the substrates, which were piano-parallel plates of BK7 glass. The substrates were chemically purified by a standard laboratory method and were subject to ion bombardment before evaporation in a vacuum of 1 Pa. During evaporation the vacuum was of the order of 10-4 Pa and the rate of deposition of the film reached 0.2 nm s- 1. The thickness of the ytterbium films was measured to be about 350 nm using the multiple-beam interference method. The coefficient of reflection for the ytterbium films obtained in this manner was measured from the air side. The measurements of R(co) were carried out for perpendicular incidence using spectrophotometers, covering the spectral range 0.2-25 lam (Fig. 1). The following spectrophotometers were used: a Specord-UV in the range 0.2-0.8 ~tm,a VSU- 1 in the range 0.4-1.1 ~tm and a UR-10 in the range 2-25 Ixm. All the spectrophotometers 0040-6090/81/0000-0000/$02.50
© Elsevier Sequoia/Printed in the Netherlands
140
E. IDCZAK, K. ZUKOWSKA
employed were equipped with specially constructed reflection attachments 5. The measurements of the ellipsometric angles A and ~Uin the spectral range 0.45-0.65 lam conducted for these films allowed the determination of the optical constants n and k of the ytterbium layers in the visible region of the spectrum 6. 3. RESULTS OF INVESTIGATIONS The optical constants of ytterbium films were determined from the measurements of the coefficient of reflection R(co) using the Kramers Kronig dispersion relations. This method requires the measurement of the reflection over the entire frequency range. The measurements of the coefficient of reflection were made in the limited range 0.2-25 ~tm. Account had to be taken in calculations of the contribution of the unmeasurable spectral range to the reflected wave phase. The calculations were made according to the procedure described in ref. 7. The contribution of measurable frequencies in the range 0.2-25 lam was calculated from the experimentally measured R(~o). Since the optical constants of ytterbium films were known from ellipsometric measurements the contribution of the unmeasurable spectral range to the reflected wave phase could be determined. The experimentally measured value of R(~o) and the calculated phase change at the reflection allowed the optical constants of ytterbium films to be determined for the entire spectral range examined from the known relations 8' 9. As follows from the calculations (Fig. 2), the optical constants of ytterbium films increase monotonically with increasing wavelength of the light, and within the spectral range examined k is always greater than n. The nature of the changes in the optical constants in the long wavelength region of the spectrum permits the determination of important microcharacteristics of the metal examined, i.e. the concentration of conduction electrons and the effective frequency of electron collisions 1°. Microcharacteristics relating to conduction electrons can be obtained from measurements in the spectral range in which the effect of interband transitions on optical properties is negligible provided that the character of the skin effect is known. The concentration of conduction electrons and their effective collision frequency (Figs. 3 and 4) were calculated from R °/4 100
t
90 80 7O 60 50 40 30 2O
8t
. / /
o
s
.............................
10 0
i
½
5
,~
~
6Ethyl
lo
15
20
2~
Fig. 1. Spectral dependence of the reflection factor R for non-transparent ytterbium layers. Fig. 2. Spectral dependence of the refractive index n ( × ) and the absorption coefficienl k (©) for ytterbium films.
PROPERTIES OF THERMALLY EVAPORATED Yb FILMS
141
the relations given in ref. 11 on the assumption that normal skin effect conditions were satisfied for the ytterbium films and that the optical constants are known. As follows from Figs. 3 and 4, a spectral range in which the values of N and v do not depend on ). can be determined for ytterbium films. These values can be used to determine the microcharacteristics of the conduction electrons, which for the ytterbium films are concentration N = 5.5 x 1021 cm-3 and effective frequency v of collisions = 2 x 1014 s- 1.
2.2 ~. 1.8 % ~. 1.4 1,0
0.6
O2
5
10
15
25 ~,~aml
20
Fig. 3. Concentration of conduction electrons vs. wavelength for ytterbium. I
1.6 'to
~" 1.2 {:3 X
"-" 0.8
0.4
o . . . .
~
. . . .
~b
. . . .
~
. . . .
2'0 .
. . .
z~ at~m~
Fig. 4. Effective frequency of electron collisions vs. wavelength for ytterbium. ACKNOWLEDGMENTS
This work was carried out under Research Project MRI5. The authors would like to thank Professor C. Wesotowska for her interest in this work. REFERENCES 1 2 3 4 5 6 7
W.E. Miiller, Phys. Lett., 17 (1965) 82. J.P. Petrakian, Thin Solid Films, 20 (1974) 297. J.P. Petrakian, Thin Solid Films, 38 (1976) 83. J.G. Endriz and W. E. Spicer, Phys. Rev. B, 2(1970) 1466. C. Wesotowska, Acta Phys. Pol., 25 (1964) 323. E. Idczak, Thin Solid Films, 34 (1976) 407. F. Stern, Solid State Phys., 15 (1963) 299.
142
8 9 10 11
Z. IDCZAK, K. ZUKOWSKA
M . M . Kirillova, L. V. Nomerowannaya, G. A. Bolotin, V. M. Maevsky, M. M. Noskox and M. S. Bolotina, Fiz. Met. Metaloved., 25 (1968) 459. E. Idczak, Opt. Appl., 2 (1973) 25. G . P . Motulevic, Usp. bTz. Nauk, 97 (1969) 211.
G.P. Motulevic, TturtvFi-.lnst.,Akad. NaukSSSR, 55(1971)3.