- Email: [email protected]
Structural and optical properties of Pd1−xInx thin films
aot~
surface science ELSEVIER
Applied Surface Science 92 (1996) 391-395
Structural and optical properties of Pd _xln x thin films W.T. Wu *, P.E. Schmid, F. L6vy lnstitut de Physique Appliqu~e, Ecole Polytechnique F~d~rale de Lausanne, CH-IOI5 Lausanne, Switzerland Received 12 December 1994; accepted for publication 2 March 1995
Abstract
Pd~ _xlnx (0.4 < x < 0.56) thin films have been prepared by RF sputtering from a multi-zone target. Their structural and optical properties have been studied by X-ray diffractometry, near normal incidence optical reflectivity and ellipsometry. Both structural and optical properties exhibit composition dependent characteristics. Indium-deficient films include Pd in antisite positions. Indium-rich films incorporate Pd vacancies. The optical spectrum shows both free electron-like properties and interband transitions giving rise to absorption peaks around 2.7 and 4.8 eV. A rigid band model is not sufficient to explain the optical properties of the alloys. The effects of Pd antisite atoms and Pd vacancies on the band structure must be taken into account to explain the evolution of the optical parameters.
1. Introduction
Nearly stoichiometric PdIn compounds reflect a characteristic pink to gold color depending on their composition [1]. For this reason, these alloys are interesting candidates for decorative applications. It is also of fundamental interest to understand how these optical properties relate to the chemical composition, morphology and electronic band structure. Like the noble metals, colored alloys owe their properties to a good reflectivity in the low energy part of the visible spectrum coupled with a strong absorption band in the higher part of the spectrum. On the basis of the existing electronic band structure calculations, it is usually proposed that the interband transitions originate from states close to the Fermi level to a set of nearly free electron-like conduction
* Corresponding author.
states [2]. In the case of PdIn alloys, a shift of the Fermi level to higher energy with increasing indium content has been proposed to explain the changes of color with chemical composition [3]. PdIn is an example of an intermetallic compound between a transition metal and a simple sp-metal. Such compounds exhibit a variety of structures, but their electron density of states shows remarkably similar behavior: A relatively narrow band formed from the d-levels of the transition metal intersects a broad set of sp-like states. The width and energy position of the d-bands are fairly well understood in transition metal compounds. Recently, several theoretical and experimental investigations have focused on the properties of the conduction states as well as on the d-band states [4-7]. In particular, a shift to lower energy of both the empty states near the Fermi level and of the d-band states has been observed when the concentration of the sp-metal is increased
[5,6].
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 01 6 9 - 4 3 3 2 ( 9 5 ) 0 0 2 6 2 - 6
W.T. Wu et aL / Applied Surface Science 92 (1996) 391-395
392 Table 1
Lattice parameter, Pd vacancy concentration (c) and In ternary concentration in Pdalnt, X c versus chemical concentration x x in P d l _ x l n x
Lattice parameter
c in P d a l n b X c
b in P d a l n b X c
(at%)
(,~)
(at%)
(at%)
43 44.5 46.5 48 50 51.5 53.5
3.228 3.236 3.243 3.246 3.25 3.248 3.24
4 7
43 44.5 46.5 48 50 49.5 49.8
In the present paper, we report on the optical properties of well-characterized Pd t_ ~In x (0.40 < x < 0.56). The optical properties will be discussed in terms of intraband and interband transitions. The mechanisms by which composition and structure changes modify the optical properties will be analyzed.
2. Experimental Pdl_xIn x (0.40 < x < 0.56) films were deposited onto (100) silicon wafers covered with a 100 nm layer of silicon dioxide. The alloys were sputtered off a multi-zone, planar magnetron at a total pressure of 0.5 Pa of argon. The RF power was 40-70 W at 13.56 MI-Iz, and the substrate temperature was kept below 350 K. The composition was changed by varying the surface ratio of the Pd to In targets between 0.5 to 1. The actual composition was determined by electron probe microanalysis (EPMA). The structure of the films was characterized by X-ray diffractometry at both grazing incidence (0 -- 4°) and in the 0 - 2 0 geometry. The near normal incidence optical reflectance spectra were measured between 0.5 and 6 eV using a CARY 5 spectrophotometer with 2 nm resolution in the visible range. The reflectivity was normalized by using a freshly evaporated AI film as a reference. Spectroscopic ellipsometry measurements were also performed in the photon energy range between 1.5 to 5.5 eV using a UIVSL ellipsometer. The incidence angle of the light beam was 70 °. The optical conTM
stants were determined from the ellipsometric angles assuming infinitely thick, perfectly flat samples.
3. Results
The films crystallized in a fl CsCl-type, body centered cubic structure, as previously observed in bulk samples [8]. This is the structure prescribed by the Hume-Rothery rule for electron concentrations of 1.5 valence electron per atom. It has been shown that for the parent Nil_xAl x alloys, alloys with x < 50% contain transition metal atoms in antisite positions, while alloys with x > 50% present transi-
Pdl_xlnx
i I_ ][~ t l /
,o
.
.
....... x -- .535 x = .50 ............. ,3 .
.
.
.
.
.
.
.
.
.
.
40 3O 20
m
0
!
1
i
!
2
|
.
.
3
rb~on , ~
.
.
4
.
.
,
5
-
6
[e~
Fig. 1. Three examples of P d - l n reflectance measured at near normal incidence. The positions of the shallow m i n i m a are indicated by an arrow mark.
393
W.T. Wu et al. / A p p l i e d Surface Science 92 (1996) 391-395
tion metal vacancies [9]. In our X-ray diffraction measurements on Pd~_~Inx films, a similar behavior has been observed. The lattice parameter and the vacancy concentrations of the Pd~_ xIn~ films have been evaluated [10]. The results of Table 1 show that nominally In-rich alloys can be considered as ternary alloys of Pd, In and Pd vacancies, in which the ternary In concentration does not exceed 50%, unlike the chemical, or binary, concentration. This limit on the In concentration will have important consequences for the interpretation of the optical properties. Changes in composition away from perfect stoichiometry lead to modifications of the lattice constant and to the appearance of two specific defects: Transition metal atoms in antisite position in Pd-rich alloys and transition metal vacancies in In-rich alloys. Fig. 1 shows the reflectivity spectra for stoichiometric PdIn together with the spectra corresponding to films with the lowest and the highest Pd concentrations. The details of the reflectivity modifications as a function of composition are shown in Fig. 2. For x < 0.5, the normalized difference reflectivity curves ( A R / R ~ = ( R - R ~ ) I R ~ , where R~ is the reflectivity of Pd0.50In0.50) exhibit positive peaks. The intensity of these peaks increases when the concentration of In decreases. Negative peaks are observed in In-rich
.430
\ x = .535
J •
?. :'
~
.
2
f
"" .
.e ~'':
3
..................
4
5
6
Photon Energy [eV] Fig. 3. Imaginary part of the dielectric susceptibility of Pd-In thin films. For the first film, the free-carrier and interband contributions are also shown separately.
films. The imaginary part of the complex dielectric susceptibility e 2 determined from ellipsometry is shown in Fig. 3. The two broad interband peaks show a shift to lower photon energy when the In concentration is increased.
4. Discussion 3 0
,
.
i
-
,
Pdl-xlnx
-
°
.
°
: x =.43
20
~
~. '~
~- 10
~-..... / /
i
7".. ":~. " , /
P%,-( / 1 ......----":.1 ~ .," ~"-~ I x=.515 / -10
.
1.8
.
2.0
.
.
.
2.2
.
2.4
2.6
1
Photon Energy [eV] Fig. 2. Pdl_~In ~ normalized difference reflectivity curves AR/R r =(R-R,)/R r, where R r is the reflectivity of
Pdo.5olno.5o.
The reflectance spectra of Pd0.4~sln0.535, stoichiometric Pdln, and Pd0.57In0.43 are shown in Fig. 1. The reflectivity spectra exhibit the following general characteristics: Below 1.5 eV, the reflectivity is high and featureless. At about 1.5 eV, a sharp decrease of the reflectivity is observed followed by a shallow minimum at about 2.3 eV. At higher energies, a second minimum at 4.4 eV is observed. The sharp reflectivity drop is a feature of the intrahand transitions that can be modelled by a Drude term. At energies above the reflectivity drop, the reflectivity remains low: This is caused by the presence of two absorption peaks located around 2.7 and 4.8 eV. These peaks correspond to interband transitions. The actual color of the film is very sensitive to the features of the absorption peak centered around 2.7 eV (see Fig. 3). The position of the second reflectivity minimum observed at 4.4 eV is practically independent of chemical composition.
394
W.T. Wu et aL / Applied Surface Science 92 (1996) 391-395 7 6
GrKmsize
•
24
4 ai 8.0 7.5,
4. _
.........
~'""~'........a.....~-..........,,
.
zo[. 1.6t : . , 42
44
:
7,,
:-.v.: 46 48 50 52 In at.% in Pd-In
54
Fig. 4. Intraband transition parameters: tOp and Fp- ~. For comparison, the grain size determined from the width of the (100) X-ray diffraction line. Interband parameters: d-band to conduction-band peak Ec_ d and sp-valence to conduction-band peak Ec_ v. The difference curve Ev-d shows the increasing energy separation between the d-bands and the Fermi level.
4.1. lntraband transitions
As suggested in the literature [11], the optical properties of the Pdln films can be described by a Drude model for the high reflectivity, intraband part, and by a set of Lorentz oscillators for the interband transitions. A single Drude term and a set of 25 oscillators positioned every 0.2 eV have been used to fit the measurements. Fig. 4 presents the results of the fit for the free-carrier response and also for the region of the spectrum corresponding to the two important absorption peaks located between 2 and 6 eV. The free-carrier and the interband contributions to the imaginary part of the dielectric susceptibility are shown separately for one of the alloys. The plasma frequency tOp remains close to 7.7 eV for In concentrations less than 48%. For In concentrations higher than 50%, tOp decreases and reaches a value of 7.1 eV for x-- 53.5%. In the rigid, nearly free, electron-band scheme proposed earlier [3], one would expect a rise of COp.In view of our results for In-rich alloys (Table 1), which show that the In ternary concentration does not exceed 50%, there should be no change for COp.We propose that the decrease of COp results from the presence of Pd vacancies. Like
in many other near noble metal compounds there is little charge transfer between Pd and In atoms, and Pd vacancies behave essentially as neutral defects. The compensation of the excess In concentration by a corresponding amount of Pd vacancies prevents any Fermi level change and thereby enforces the Hume-Rothery rule. Around each vacancy, however, Pd-Pd bonds are missing. As a result, the d-like bands become narrower [6] and some d-electron density of states moves away from the Fermi level. In alloys with x < 50%, one would expect a similar trend, i.e. an increase of top for lower x values because of the stronger Pd-Pd interactions of Pd atoms positioned as antisite defects. On the other hand, the decreasing In concentration in Pd-rich alloys should lower the sp-electron density near the Fermi level, thereby lowering the value of tOp. The experimental results indicate that tOp is nearly constant in Pd-rich alloys, which to say, both effects tend to cancel each other. Changes in the environment of the In atoms clearly play little role in determining COp,as otherwise one would expect tOp to remain nearly constant for x > 50%. The Drude relaxation time parameter Fp ~ is also composition dependent, it shows a striking correlation with the grain size of the films deduced from X-ray diffraction. This result indicates that the optical properties, in the region of the reflectivity drop, are sensitive to the film morphology. 4.2. Interband transitions
In Fig. 4, we also present the energy positions of the maxima of the E2 peaks located around 2.7 and 4.8 eV. Both peaks can be described as a fast rising shoulder followed by a long, high energy tail. This shape is typical for peaks originating from M l saddle points in the joint density of states. The 2.7 eV peak shifts almost linearly to lower energies for increasing x values. On the basis of a band structure calculation [2], this peak has been assigned to M n-like transitions from "~4 valence states to Zn conduction states [12], near the M-point of the simple cubic Brillouin zone. If the Z4 states involved in the peak of the 2.7 eV absorption feature were situated fight at the Fermi level, the absorption peak should display a more step-like onset. The experimental width of the
W.T. Wu et al./ Applied Surface Science 92 (1996) 391-395
onset suggests that the valence states involved in the peak of the feature are located within about 0.5 eV of the Fermi level. According to our previous discussion, this region of the band structure should hardly change when the In concentration is varied. The red shift of the 2.7 eV peak is caused, therefore, by the downward movement of the final states in the conduction band, of about 30 meV when the In concentration increases by 1%. Such a downward movement of the conduction p-like states near the Fermi energy has been described over an extensive range of AI concentration in experiments on Pdl_xAl x [5]. The 4.8 eV peak has been assigned to transitions involving non-bonding d-bands and conduction states around the M-point of the Brillouin zone. Assuming that these final states follow the same downward trend as the conduction states involved in the 2.7 eV peak, one can deduce that the non-bonding d-states move down in energy by some 20 meV when the In concentration increases by 1%. This trend confirms the conclusions drawn for the change of top with In concentration.
5. Conclusions
The optical properties of P d - I n intermetallic thin films exhibit both intraband and interband transitions in the visible. The interband transitions have been discussed on the basis of a previous band structure calculation. The parameters of the interband features as well as tOp are affected essentially by changes in the environment of the Pd atoms caused by antisite defects and vacancies. X-ray diffraction reveals that the ternary concentration of In atoms does not exceed 50% in In-rich alloys. The electron plasma frequency tOp and inverse relaxation time take values between 7.1-7.75 eV and 0.18-0.26 eV depending on composition. The first interband peak, around 2.7 eV, is associated with transitions from "~4 states within about 0.5 eV of the Fermi level and a set of conduction states not far from the M-point of the
395
Brillouin zone. The sp-part of the band structure below E F appears to be stable with respect to In concentration changes. On the other hand, conduction states near the M-point and non-bonding d-bands move downward in energy when the concentration of Pd antisite defects decreases or when the concentration of Pd vacancies increases.
Acknowledgements
The authors would like to thank Dr. F. Bussy for his help with the EPMA measurements, Dr. T. Gerfin (Institut de Chimie Physique II, EPFL) for his help with the ellipsometry measurements, and to acknowledge the financial support of the Swiss National Science Foundation and of the Commission pour l'Encouragement de la Recherche Scientifique.
References
[1] L.V. Nomerovannaya,M.M. Kiriliova, E.M. Savitskii, V.P. Polyakova, N.B. Gorina and N.L. Korenovskii, Soy. Phys. Dokl. 24 (1979) 379. [2] SJ. Cho, Phys. Status Solidi 41 (1970) 179. [3] L.V. Nomerovannaya, M.M. Kiriilova and E.M. Savitskii, Phys. Status Solidi 102 (1980) 715. [4] C.D. Gelau, Jr., A.R. Williamsand V.L. Moruzzi,Phys. Rev. B 27 (1983) 2005. [5] L. Du~, M. Sancrotti, G. CurrY, A. Ruocco, S'Addatu, R. Cosso, P. Unsworth and P. Weightman, Phys. Rev. B 47 (1993) 6937. [6] J.F. van Acker, E.W. Lindeyer and J.C. Fuggle, J. Phys.: Condens. Matter 3 (1991) 9579. 17] K. Tanaka,T. Satio, K. Suzuki and R. Hasegawa,Phys. Rev. B 32 (1985) 6853.
[8] D. Fort, R.E. Smaliman and I.R. Harris, J. Less-Common Met. 31 (1973) 263. [9] J.N. Pratt and J.M. Bird, J. Less-CommonMet. 48 (1976) 167. [10] W.T. Wu, P.E. Schmid and F. l_~vy,to be published. [II] F. Wooten, Optical Properties of Solids (Academic Press, New York, 1972) ch. 3. [I 2] J.P. Jan and S.S. Vishnubhatla, Can. J. Phys. 45 (1967) 2505.
surface science ELSEVIER
Applied Surface Science 92 (1996) 391-395
Structural and optical properties of Pd _xln x thin films W.T. Wu *, P.E. Schmid, F. L6vy lnstitut de Physique Appliqu~e, Ecole Polytechnique F~d~rale de Lausanne, CH-IOI5 Lausanne, Switzerland Received 12 December 1994; accepted for publication 2 March 1995
Abstract
Pd~ _xlnx (0.4 < x < 0.56) thin films have been prepared by RF sputtering from a multi-zone target. Their structural and optical properties have been studied by X-ray diffractometry, near normal incidence optical reflectivity and ellipsometry. Both structural and optical properties exhibit composition dependent characteristics. Indium-deficient films include Pd in antisite positions. Indium-rich films incorporate Pd vacancies. The optical spectrum shows both free electron-like properties and interband transitions giving rise to absorption peaks around 2.7 and 4.8 eV. A rigid band model is not sufficient to explain the optical properties of the alloys. The effects of Pd antisite atoms and Pd vacancies on the band structure must be taken into account to explain the evolution of the optical parameters.
1. Introduction
Nearly stoichiometric PdIn compounds reflect a characteristic pink to gold color depending on their composition [1]. For this reason, these alloys are interesting candidates for decorative applications. It is also of fundamental interest to understand how these optical properties relate to the chemical composition, morphology and electronic band structure. Like the noble metals, colored alloys owe their properties to a good reflectivity in the low energy part of the visible spectrum coupled with a strong absorption band in the higher part of the spectrum. On the basis of the existing electronic band structure calculations, it is usually proposed that the interband transitions originate from states close to the Fermi level to a set of nearly free electron-like conduction
* Corresponding author.
states [2]. In the case of PdIn alloys, a shift of the Fermi level to higher energy with increasing indium content has been proposed to explain the changes of color with chemical composition [3]. PdIn is an example of an intermetallic compound between a transition metal and a simple sp-metal. Such compounds exhibit a variety of structures, but their electron density of states shows remarkably similar behavior: A relatively narrow band formed from the d-levels of the transition metal intersects a broad set of sp-like states. The width and energy position of the d-bands are fairly well understood in transition metal compounds. Recently, several theoretical and experimental investigations have focused on the properties of the conduction states as well as on the d-band states [4-7]. In particular, a shift to lower energy of both the empty states near the Fermi level and of the d-band states has been observed when the concentration of the sp-metal is increased
[5,6].
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 01 6 9 - 4 3 3 2 ( 9 5 ) 0 0 2 6 2 - 6
W.T. Wu et aL / Applied Surface Science 92 (1996) 391-395
392 Table 1
Lattice parameter, Pd vacancy concentration (c) and In ternary concentration in Pdalnt, X c versus chemical concentration x x in P d l _ x l n x
Lattice parameter
c in P d a l n b X c
b in P d a l n b X c
(at%)
(,~)
(at%)
(at%)
43 44.5 46.5 48 50 51.5 53.5
3.228 3.236 3.243 3.246 3.25 3.248 3.24
4 7
43 44.5 46.5 48 50 49.5 49.8
In the present paper, we report on the optical properties of well-characterized Pd t_ ~In x (0.40 < x < 0.56). The optical properties will be discussed in terms of intraband and interband transitions. The mechanisms by which composition and structure changes modify the optical properties will be analyzed.
2. Experimental Pdl_xIn x (0.40 < x < 0.56) films were deposited onto (100) silicon wafers covered with a 100 nm layer of silicon dioxide. The alloys were sputtered off a multi-zone, planar magnetron at a total pressure of 0.5 Pa of argon. The RF power was 40-70 W at 13.56 MI-Iz, and the substrate temperature was kept below 350 K. The composition was changed by varying the surface ratio of the Pd to In targets between 0.5 to 1. The actual composition was determined by electron probe microanalysis (EPMA). The structure of the films was characterized by X-ray diffractometry at both grazing incidence (0 -- 4°) and in the 0 - 2 0 geometry. The near normal incidence optical reflectance spectra were measured between 0.5 and 6 eV using a CARY 5 spectrophotometer with 2 nm resolution in the visible range. The reflectivity was normalized by using a freshly evaporated AI film as a reference. Spectroscopic ellipsometry measurements were also performed in the photon energy range between 1.5 to 5.5 eV using a UIVSL ellipsometer. The incidence angle of the light beam was 70 °. The optical conTM
stants were determined from the ellipsometric angles assuming infinitely thick, perfectly flat samples.
3. Results
The films crystallized in a fl CsCl-type, body centered cubic structure, as previously observed in bulk samples [8]. This is the structure prescribed by the Hume-Rothery rule for electron concentrations of 1.5 valence electron per atom. It has been shown that for the parent Nil_xAl x alloys, alloys with x < 50% contain transition metal atoms in antisite positions, while alloys with x > 50% present transi-
Pdl_xlnx
i I_ ][~ t l /
,o
.
.
....... x -- .535 x = .50 ............. ,3 .
.
.
.
.
.
.
.
.
.
.
40 3O 20
m
0
!
1
i
!
2
|
.
.
3
rb~on , ~
.
.
4
.
.
,
5
-
6
[e~
Fig. 1. Three examples of P d - l n reflectance measured at near normal incidence. The positions of the shallow m i n i m a are indicated by an arrow mark.
393
W.T. Wu et al. / A p p l i e d Surface Science 92 (1996) 391-395
tion metal vacancies [9]. In our X-ray diffraction measurements on Pd~_~Inx films, a similar behavior has been observed. The lattice parameter and the vacancy concentrations of the Pd~_ xIn~ films have been evaluated [10]. The results of Table 1 show that nominally In-rich alloys can be considered as ternary alloys of Pd, In and Pd vacancies, in which the ternary In concentration does not exceed 50%, unlike the chemical, or binary, concentration. This limit on the In concentration will have important consequences for the interpretation of the optical properties. Changes in composition away from perfect stoichiometry lead to modifications of the lattice constant and to the appearance of two specific defects: Transition metal atoms in antisite position in Pd-rich alloys and transition metal vacancies in In-rich alloys. Fig. 1 shows the reflectivity spectra for stoichiometric PdIn together with the spectra corresponding to films with the lowest and the highest Pd concentrations. The details of the reflectivity modifications as a function of composition are shown in Fig. 2. For x < 0.5, the normalized difference reflectivity curves ( A R / R ~ = ( R - R ~ ) I R ~ , where R~ is the reflectivity of Pd0.50In0.50) exhibit positive peaks. The intensity of these peaks increases when the concentration of In decreases. Negative peaks are observed in In-rich
.430
\ x = .535
J •
?. :'
~
.
2
f
"" .
.e ~'':
3
..................
4
5
6
Photon Energy [eV] Fig. 3. Imaginary part of the dielectric susceptibility of Pd-In thin films. For the first film, the free-carrier and interband contributions are also shown separately.
films. The imaginary part of the complex dielectric susceptibility e 2 determined from ellipsometry is shown in Fig. 3. The two broad interband peaks show a shift to lower photon energy when the In concentration is increased.
4. Discussion 3 0
,
.
i
-
,
Pdl-xlnx
-
°
.
°
: x =.43
20
~
~. '~
~- 10
~-..... / /
i
7".. ":~. " , /
P%,-( / 1 ......----":.1 ~ .," ~"-~ I x=.515 / -10
.
1.8
.
2.0
.
.
.
2.2
.
2.4
2.6
1
Photon Energy [eV] Fig. 2. Pdl_~In ~ normalized difference reflectivity curves AR/R r =(R-R,)/R r, where R r is the reflectivity of
Pdo.5olno.5o.
The reflectance spectra of Pd0.4~sln0.535, stoichiometric Pdln, and Pd0.57In0.43 are shown in Fig. 1. The reflectivity spectra exhibit the following general characteristics: Below 1.5 eV, the reflectivity is high and featureless. At about 1.5 eV, a sharp decrease of the reflectivity is observed followed by a shallow minimum at about 2.3 eV. At higher energies, a second minimum at 4.4 eV is observed. The sharp reflectivity drop is a feature of the intrahand transitions that can be modelled by a Drude term. At energies above the reflectivity drop, the reflectivity remains low: This is caused by the presence of two absorption peaks located around 2.7 and 4.8 eV. These peaks correspond to interband transitions. The actual color of the film is very sensitive to the features of the absorption peak centered around 2.7 eV (see Fig. 3). The position of the second reflectivity minimum observed at 4.4 eV is practically independent of chemical composition.
394
W.T. Wu et aL / Applied Surface Science 92 (1996) 391-395 7 6
GrKmsize
•
24
4 ai 8.0 7.5,
4. _
.........
~'""~'........a.....~-..........,,
.
zo[. 1.6t : . , 42
44
:
7,,
:-.v.: 46 48 50 52 In at.% in Pd-In
54
Fig. 4. Intraband transition parameters: tOp and Fp- ~. For comparison, the grain size determined from the width of the (100) X-ray diffraction line. Interband parameters: d-band to conduction-band peak Ec_ d and sp-valence to conduction-band peak Ec_ v. The difference curve Ev-d shows the increasing energy separation between the d-bands and the Fermi level.
4.1. lntraband transitions
As suggested in the literature [11], the optical properties of the Pdln films can be described by a Drude model for the high reflectivity, intraband part, and by a set of Lorentz oscillators for the interband transitions. A single Drude term and a set of 25 oscillators positioned every 0.2 eV have been used to fit the measurements. Fig. 4 presents the results of the fit for the free-carrier response and also for the region of the spectrum corresponding to the two important absorption peaks located between 2 and 6 eV. The free-carrier and the interband contributions to the imaginary part of the dielectric susceptibility are shown separately for one of the alloys. The plasma frequency tOp remains close to 7.7 eV for In concentrations less than 48%. For In concentrations higher than 50%, tOp decreases and reaches a value of 7.1 eV for x-- 53.5%. In the rigid, nearly free, electron-band scheme proposed earlier [3], one would expect a rise of COp.In view of our results for In-rich alloys (Table 1), which show that the In ternary concentration does not exceed 50%, there should be no change for COp.We propose that the decrease of COp results from the presence of Pd vacancies. Like
in many other near noble metal compounds there is little charge transfer between Pd and In atoms, and Pd vacancies behave essentially as neutral defects. The compensation of the excess In concentration by a corresponding amount of Pd vacancies prevents any Fermi level change and thereby enforces the Hume-Rothery rule. Around each vacancy, however, Pd-Pd bonds are missing. As a result, the d-like bands become narrower [6] and some d-electron density of states moves away from the Fermi level. In alloys with x < 50%, one would expect a similar trend, i.e. an increase of top for lower x values because of the stronger Pd-Pd interactions of Pd atoms positioned as antisite defects. On the other hand, the decreasing In concentration in Pd-rich alloys should lower the sp-electron density near the Fermi level, thereby lowering the value of tOp. The experimental results indicate that tOp is nearly constant in Pd-rich alloys, which to say, both effects tend to cancel each other. Changes in the environment of the In atoms clearly play little role in determining COp,as otherwise one would expect tOp to remain nearly constant for x > 50%. The Drude relaxation time parameter Fp ~ is also composition dependent, it shows a striking correlation with the grain size of the films deduced from X-ray diffraction. This result indicates that the optical properties, in the region of the reflectivity drop, are sensitive to the film morphology. 4.2. Interband transitions
In Fig. 4, we also present the energy positions of the maxima of the E2 peaks located around 2.7 and 4.8 eV. Both peaks can be described as a fast rising shoulder followed by a long, high energy tail. This shape is typical for peaks originating from M l saddle points in the joint density of states. The 2.7 eV peak shifts almost linearly to lower energies for increasing x values. On the basis of a band structure calculation [2], this peak has been assigned to M n-like transitions from "~4 valence states to Zn conduction states [12], near the M-point of the simple cubic Brillouin zone. If the Z4 states involved in the peak of the 2.7 eV absorption feature were situated fight at the Fermi level, the absorption peak should display a more step-like onset. The experimental width of the
W.T. Wu et al./ Applied Surface Science 92 (1996) 391-395
onset suggests that the valence states involved in the peak of the feature are located within about 0.5 eV of the Fermi level. According to our previous discussion, this region of the band structure should hardly change when the In concentration is varied. The red shift of the 2.7 eV peak is caused, therefore, by the downward movement of the final states in the conduction band, of about 30 meV when the In concentration increases by 1%. Such a downward movement of the conduction p-like states near the Fermi energy has been described over an extensive range of AI concentration in experiments on Pdl_xAl x [5]. The 4.8 eV peak has been assigned to transitions involving non-bonding d-bands and conduction states around the M-point of the Brillouin zone. Assuming that these final states follow the same downward trend as the conduction states involved in the 2.7 eV peak, one can deduce that the non-bonding d-states move down in energy by some 20 meV when the In concentration increases by 1%. This trend confirms the conclusions drawn for the change of top with In concentration.
5. Conclusions
The optical properties of P d - I n intermetallic thin films exhibit both intraband and interband transitions in the visible. The interband transitions have been discussed on the basis of a previous band structure calculation. The parameters of the interband features as well as tOp are affected essentially by changes in the environment of the Pd atoms caused by antisite defects and vacancies. X-ray diffraction reveals that the ternary concentration of In atoms does not exceed 50% in In-rich alloys. The electron plasma frequency tOp and inverse relaxation time take values between 7.1-7.75 eV and 0.18-0.26 eV depending on composition. The first interband peak, around 2.7 eV, is associated with transitions from "~4 states within about 0.5 eV of the Fermi level and a set of conduction states not far from the M-point of the
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Brillouin zone. The sp-part of the band structure below E F appears to be stable with respect to In concentration changes. On the other hand, conduction states near the M-point and non-bonding d-bands move downward in energy when the concentration of Pd antisite defects decreases or when the concentration of Pd vacancies increases.
Acknowledgements
The authors would like to thank Dr. F. Bussy for his help with the EPMA measurements, Dr. T. Gerfin (Institut de Chimie Physique II, EPFL) for his help with the ellipsometry measurements, and to acknowledge the financial support of the Swiss National Science Foundation and of the Commission pour l'Encouragement de la Recherche Scientifique.
References
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