ISSN 10637834, Physics of the Solid State, 2014, Vol. 56, No. 11, pp. 2286–2293. © Pleiades Publishing, Ltd., 2014. Original Russian Text © G.M. Mikheev, A.S. Saushin, O.Yu. Goncharov, G.A. Dorofeev, F.Z. Gil’mutdinov, R.G. Zonov, 2014, published in Fizika Tverdogo Tela, 2014, Vol. 56, No. 11, pp. 2212–2218.
SURFACE PHYSICS AND THIN FILMS
Effect of the Burning Temperature on the Phase Composition, Photovoltaic Response, and Electrical Properties of Ag/Pd Resistive Films G. M. Mikheeva, *, A. S. Saushina, O. Yu. Goncharovb, G. A. Dorofeevb, F. Z. Gil’mutdinovb, and R. G. Zonova a
Institute of Mechanics, Ural Branch of the Russian Academy of Sciences, ul. T. Baramzinoi 34, Izhevsk, 426067 Russia * email:
[email protected] b PhysicalTechnical Institute, Ural Branch of the Russian Academy of Sciences, ul. Kirova 132, Izhevsk, 426000 Russia Received April 23, 2014
Abstract—Silver–palladium (Ag/Pd) films were grown by thickfilm technology using a resistive paste con sisting of Pd, Ag2O, and glass on ceramic substrates at burning temperatures of 878, 1013, and 1113 K. The effect of the burning temperature and Pd content in the initial paste on the phase composition, resistivity, photovoltaic properties of films, free carrier concentration, and mobility was studied. It was found that the films grown at a burning temperature of 878 K have the greatest factor of conversion of the pulsed laser power to the photovoltaic signal, which depends on the direction of the incident radiation wave vector. Using Xray diffraction, Xray photoelectron spectroscopy, and thermodynamic modeling, it was shown that the AgPd alloy and PdO oxide are the main components of the Ag/Pd film with photovoltaic properties. DOI: 10.1134/S1063783414110195
1. INTRODUCTION Recently, a significant number of studies devoted to the photovoltaic effects sensitive to the polarization and direction of the incident radiation wave vector were published. Such effects are observed in different lowdimensional structures fabricated based on, e.g., GaN [1], metal [2] and carbon [3, 4] films, in graphene [5], and in films of singlewalled carbon nanotubes [6]. These effects are caused by physical mechanisms of different nature [7], whose study, in addition to purely scientific interest, is urgent for developing highspeed hightemperature photodetectors [8, 9], angle sensors [10], and laser polarization analyzers [11, 12]. In previous studies [13, 14], we first showed that the photovoltaic effect sensitive to the direction of the wave vector of incident nanosecond pulsed laser radi ation can be observed in conductive (resistive) Ag/Pd films. Further studies [15, 16] showed that the photo voltaic signal appearing in such films in the form of unipolar photovoltage in the case of oblique laser beam incidence on the surface depends strongly on polarization, including the circular polarization sign. The high sensitivity to polarization and wave vector direction allows the consideration of resistive Ag/Pd films as a promising material for developing optoelec tronic angle sensors and laser polarization analyzers.
Resistive Ag/Pd films have stable electrical param eters [17] and are already widely applied in electron ics. They are used to obtain hybrid integrated circuits, multicrystal modules, integrated circuit assemblies, and passive electronic components such as resistors, multilayer capacitors, and inductors [18]. The Ag/Pd films are grown using technologies [18, 19] based on burning of a special paste, which contains silver oxide and palladium, at high temperatures on the insulating substrate surface. The applied paste layer is dried and fired, i.e., is subjected to heat treat ment which imparts certain electrical and mechanical properties to the film. The paste includes functional, structural, and technological components. The func tional component is the principal one controlling the basic properties of obtained films and consists of silver oxide (Ag2O) and palladium. The structural compo nent represents finely divided particles of glass, e.g., STs273 glass, whose melting temperature is lower than the burning temperature. During burning, mol ten glass wets functional component particles with the suspension formation. After cooling and hardening, a mechanically strong film is formed, within which the quasihomogeneous distribution of functional phase particles takes place. The technological component is a “binder” which imparts certain viscosity and plastic ity to the paste. This component contains organic sub stances (e.g., lanolin, colophony) and a certain sol
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Table 1. Composition of the PR5 and PR50 pastes Composition (wt %) Paste functional component
structural component
Ag2O (36.0) Pd (15.3) Ag2O (24.2) Pd (30.8)
PR5 PR50
STs273 glass (48.7) STs273 glass (45.0)
vent. The latter is evaporated during drying; during burning, organic materials decompose and burn down, i.e., the technological component is removed. The objective of this work is to study the phase composition of resistive Ag/Pd films synthesized under different conditions and its effect on their elec trical and photovoltaic properties. This is necessary to control the physicochemical characteristics of films during their synthesis, in particular, to determine the phases responsible for the appearance of the photovol taic signal. 2. SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE The samples of resistive Ag/Pd films 20 × 20 mm in size and ~20 μm thick were grown on ceramic sub strates of VK94 aluminum oxide ceramics using a standard technique at three burning temperatures Tbur = 878, 1013, and 1113 K [13]. The paste heating rate on the substrate was 20–25 K/min. The heat treatment time at the given Tbur was 13–16 min. Two different pastes (PR5 and PR50) were used in the experiments, which are intended to grow resistive films with sheet resistances of 5 and 50 Ω/䊐, respec
1 6
4
n0
x
–
A
k
E
y
5
2
B 80 ns
II I
3
Fig. 1. Experimental scheme for studying the photovoltaic properties of grown films: (1) silver–palladium film, (2) measuring electrodes, (3) substrate, (4) halfwave plate, (5) digital oscilloscope, and (6) laser beam incidence plane; k is the wave vector, E is the electric field vector, and n0 is the normal to the film surface. The inset on the oscil loscope screen: (I) shape of incident laser pulses and (II) shape of photovoltaic pulses appearing between electrodes. PHYSICS OF THE SOLID STATE
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technological component LS1 organic binder (25.0) LS1 organic binder (25.0)
tively. The composition of these pastes is given in Table 1. The technological component of the paste completely burns down at high temperatures; there fore, only the sum of weight parts of functional and structural components is taken as hundred percents in Table 1. The photovoltaic properties of grown films were studied using a pulsed laser generating light pulses 16 ns long at a wavelength of 532 nm, by the technique described in [14]. To increase the conversion factor due to the size factor [20], samples 8 × 8 mm in size were cut out from the grown films, which were pressed between two parallel measuring electrodes A and B connected to an oscilloscope input 5 (Fig. 1). The angle of radiation incidence on the film was 45°; the measuring electrodes were perpendicular to the inci dence plane 6. Using a halfwave plate 4, linearly polarized laser radiation was converted to s or p polarized light. The photovoltaic signal U (pulsed volt age amplitude) appearing between two electrodes upon exposure to light pulses was recorded using a broadband digital oscilloscope (Tektronix TDS7704B) with an input resistance of 50 Ω. The typical shape of photovoltaic pulses observed on the oscilloscope screen is shown in Fig. 1. In the experiments with films grown from different pastes and at different values of Tbur, the conversion factor η was determined by the formula η = Uτ/Ep, where Ep and τ are the energy and duration of laser pulses. The carrier concentration n, mobility μ, and sign in grown films were determined by the Hall effect using an ECOPIA HMS 3000/1T automated system. The resistivity was determined by the Van der Pauw method according to the fourpointprobe scheme using a Lucas Labs Pro4 automated device. The film morphology was studied using an XL30 ESEM TMP (FEI Co., Netherlands) scanning elec tron microscope (SEM). The chemical composition of surface layers of films was studied by the Xray photoelectron spectroscopy (XPS) method using a SPECS spectrometer with MgKα excitation of photoelectron spectra. The chem ical state of elements was identified using reference data and reference spectra certified by the Xray dif fraction (XRD) method. The experimental data were processed using the CasaXPS software package. The relative error of the element concentration determina tion using empirical elemental sensitivity factors was ±3% of the measured value. 2014
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Xi 1.0
Xi 0.6 3 1
0.8 0.4
1 0.6
0.2
3 0.4
2 4 0 473
0.2 2 4 0
0.2
0.4
0.6
0.8
573
673
773 873 T, K
973
1073 1173
Fig. 3. Temperature dependences of the equilibrium con tents of the components (1) Ag, (2) Pd, (3) PdO, and (4) Ag2O in the Pd–Ag2O system, corresponding to exper imental PR50 compositions.
1.0
g Fig. 2. Dependences of the contents of the components (1) Ag, (2) Pd, (3) PdO, and (4) Ag2O on the palladium mass fraction g in the initial paste at T = 873 K and pres sure P = 0.1 MPa.
The XPS depth profiling was performed using sur face sputtering by a scanning argon ion beam with an energy of 4 keV and a current density of 30 μA/cm2. The experimentally measured average etch rate was 1 nm/min. Xray photoelectron spectra were mea sured after ion etching to depths of 1 and 10 nm. The XRD analysis was performed using a DRON 6 diffractometer with monochromatized CuKα radia tion. XRD patterns were processed a software package for Xray analysis of polycrystals [21]. The equilibrium composition formed in the Pd– Ag2O system in air, i.e., the functional component of the initial paste, was estimated and analyzed using the thermodynamic technique [22]. The effect of struc tural and technological components on the film com position was also studied. Thermodynamic calcula tions were performed using the algorithm [23] based on the solution of the system of equations obtained from the entropy maximum condition. This approach is usually applied to calculate equilibrium concentra tions of components at specific parameters (tempera ture, pressure, component concentrations) in multi component heterogeneous systems. Equilibrium com positions formed in the system under study were estimated using the ASTRA program [23] for the tem perature range T = 473–1173 K at atmospheric pres sure P = 0.1 MPa and a gasphase composition approximately corresponding to the air composition, N54O14. The content of system components was set in palladium mass fractions, g = m1/(m1 + m2), where m1
and m2 are Pd and Ag2O masses, respectively. Further more, the possibility of forming two solid solutions was assumed: (i) one consisting of PdO and Ag2O oxides and (ii) another consisting of Pd and Ag. Thus, the possibility of forming intermetallic compounds was taken into account. The data on thermodynamic properties of components formed in the system, given in the handbook [24] and contained in the ASTRA program database were used. 3. THERMODYNAMIC ANALYSIS OF THE COMPOSITION OF GROWN FILMS Figure 2 shows the results of calculations of the equilibrium content of components Xi in the mole fractions (subscripts i = 1, 2, 3, 4 correspond to Ag, Pd, PdO, Ag2O) in the Pd–Ag2O–air system, depend ing on the palladium mass fraction g in the initial mix ture (in the functional paste component) at T = 873 K. It is easily seen from this figure that the metal silver content X1 decreases linearly in a wide g variations range (Fig. 2, curve 1). At the same time, the increase in the metal palladium mole fraction X2 occurs abruptly with a sharp increase at g ~ 0.6 and ~0.8 (Fig. 2, curve 2). As the Pd content increases in the initial mixture, X3 (the PdO oxide content) also increases, reaching a maximum at g ~ 0.56. The com position of the PR50 sample corresponds to this max imum palladium oxide content in the equilibrium mixture. The second maximum of the PdO content is observed at g ~ 0.75; as the Pd fraction in the initial mixture further increases, the PdO content in the grown films decreases. Thus, the dependence of X3 on g is complex with a two maxima at g ~ 0.56 and ~0.75.
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affect the systematic features of transformations in the functional component.
3 μm Fig. 4. SEM image of the resistive Ag/Pd film surface.
The equilibrium Ag2O content (Fig. 2, curve 4) is insignificant; at g > 0.6, its weight fraction does not exceed 0.001. The equilibrium content of components was esti mated for other temperatures in the range T = 473– 1173 K as well. It was shown that the PdO content decreases with increasing temperature at all concen trations g. Figure 3 shows the temperature depen dences of the content of components in the Pd–Ag2O system, corresponding to experimental PR50 compo sitions. We can see that the metal palladium content in films increases with temperature due to palladium oxide dissociation. Silver oxide exhibits similar behav ior, whose decomposition with increasing temperature leads to an increase in the metal silver content. We note that the established features are characteristic of not only the PR50 and PR5 compositions but also the compositions with the other ratio of primary compo nents g. To estimate the effect of the structural and techno logical components on the film composition, a ther modynamic analysis was carried out, in which not only functional components (Ag2O and Pd) were taken into account in the paste composition, but also STs273 glass and organic binder components. It was shown that the formation of pure carbon in the film can be expected due to hydrocarbon decomposition at the burning temperature T = 878 K, and lead oxide dissociation in glass will result in the formation of pure lead. Furthermore, the formation of Al2SiO5 silicates and (below 473 K) PbAl2O4 lead aluminate is possible. Thus, the formation of new components in the structural and technological components will do not Table 2. Composition of the STs273 glass (component contents are given in wt %) SiO2
Al2O3
TiO2
PbO
ZnO
31.65
2.91
4.75
42.42
18.25
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4. EXPERIMENTAL RESULTS AND DISCUSSION Figure 4 shows the SEM image of the resistive Ag/Pd film surface. It is clearly seen that the film is a porous material. An analysis of the micrographs at dif ferent magnifications showed that the pore radius is from 25 to 500 nm. In this case, the characteristic size of solid particles of this material is from 50 to 200 nm. The porous structure of resistive Ag/Pd films was observed by other researchers as well (see, e.g., [25]). The XPS study of the elemental composition of the surface of the grown films, performed after ion etching to a depth of 10 nm, showed that they contain O, Ag, Pd, C, Si, Pb, and Zn. The presence of Si, Pb, and Zn in the film composition is explained by the presence of the structural component of the initial paste being a STs273 glass and consisting of oxides of these ele ments (Table 2). As predicted above by the thermody namic analysis, carbon formation is caused by decom position of hydrocarbons of the technological compo nent. In the Pd3d5/2 Xray photoelectron spectra, in addition to the peak corresponding to metal palladium with binding energy Eb = 335.4 eV, the presence of PdO with binding energy Eb = 336.5 eV is clearly observed for all films grown from PR50 and PR5 pastes at Tbur = 878, 1013, and 1113 K (Fig. 5). However, the samples of the films grown from PR5 paste exhibit a lowered PdO content. Furthermore, it is clearly seen in Fig. 5 that the PdO content decreases with increas ing Tbur. Palladium oxide is thermodynamically stable only at temperatures T < 1023 K (Fig. 3). However, the film cooling rate is not too high from Tbur > 1023 K to room temperature, which promotes partial PdO for mation on the film surface at Tbur > 1023 K. Figure 6 shows the XRD patterns of the films grown at different Tbur from PR50 paste. The film phase composition determined by analyzing XRD patterns by the Rietveld method is given in Table 3. We can see that the film at Tbur = 878 K is mostly com posed by the crystalline phases: the AgPd and PdO solid solution with a small Ag2O admixture. This is in good agreement with the thermodynamic calculation results shown in Fig. 3. The XRD pattern of the film grown at Tbur = 878 K, (Fig. 6, XRD pattern 1), in addition to diffraction reflections of the indicated crystalline phases, contains the diffuse halo in the angular range 2θ = 24°–48°, which indicates the pres ence of an amorphous phase. Thus, the absence of some phases (SiO2, PbO, TiO2, and others) in the XRD pattern 1, contained in the structural compo nent of the initial paste, can be explained by their amorphous state. The XRD pattern of the film grown at the burning temperature of 1013 K contains reflec tions of additional oxide phases Al2SiO5, PbTiO3, 2014
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MIKHEEV et al. Pd(met) PdO I II III IV
3
3' 2
2'
1 Ag2O AgPd PdO Al2SiO5
1'
Zn2SiO4 PbTiO3 Ti3O5 20
3
30
40
50 60 2θ, deg
70
80
90
Fig. 6. Xray diffraction patterns of the films grown from the PR50 paste at Tbur = (1) 878, (2) 1013, and (3) 1113 K and bar diffraction patterns of detected phases (CuKα).
the film grown at 1113 K (XRD pattern 3 in Fig. 6 and Table 3) is similar to that of the film grown at Tbur = 1013 K, but has no Al2SiO5 phase. At the same time, we can see in the diffraction spectra that the main film component is the solid solution of AgPd with the fcc lattice. In this case, for the films grown at different Tbur, the solid solution lattice parameters a are differ ent: at Tbur = 878, 1013, and 1113 K, a is 0.4036, 0.3992, and 0.3966 nm, respectively.
2
1 332
333
334
335
336 337 Eb, eV
338
339
340
Fig. 5. Pd3d5/2 Xray photoelectron spectra recorded at a depth of 10 nm from the surface of resistive films grown from PR50 (1–3) and PR5 (1'–3') pastes. Tbur = (1, 1') 878, (2, 2') 1013, and (3, 3') 1113 K. (I) Experimental data; (II, III) Pd and PdO spectra obtained by the decomposi tion of the experimental data, respectively; experimental data, and (IV) background level.
Zn2SiO4, and Ti3O5 (see XRD pattern 2 in Fig. 6 and Table 3), whereas reflections of the Ag2O phase disap pear. It is clear that additional crystalline oxides result from the amorphous phase crystallization and the interaction of all components of the initial paste at ele vated burning temperatures. The phase composition of
These values are between the lattice parameters of pure Ag and Pd metals, which are 0.40862 and 0.38902 nm, respectively. Based on Vegard’s law for the concentration dependence of the solid solution lattice parameter [26], the Pd content in the AgPd solid solution can be approximately estimated for dif ferent burning temperatures. Estimations yield 26, 48, and 61 at % Pd for Tbur = 878, 1013, and 1113 K, respectively. The increase in the Pd content in the solid solution with Tbur is obviously associated with PdO oxide decomposition within the film, which was pre dicted by thermodynamic calculations (Fig. 3). Table 3 also shows the significant change in the pal ladium oxide content with increasing Tbur. It is maxi mum for the film grown at Tbur = 878 K and minimum for the film grown at Tbur = 1113 K. These results are
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Table 3. Phase composition of the films according to the Xray diffraction analysis Phase content, wt % Tbur, K 878 1013 1113
AgPd
PdO
Ag2O
Al2SiO5
PbTiO3
Zn2SiO4
Ti3O5
80.3 46.8 62.8
18.7 3.8 0.1
1.0 0 0
0 7.0 0
0 15.9 18.0
0 13.9 17.8
0 12.7 1.3
Table 4. Electrical and photovoltaic properties of ptype films based on the PR50 paste Tbur, K
n, cm–3
878 1013 1113
9.2 × 1020 4.5 × 1020 1 × 1021
μ, cm V–1 s–1
ρ, Ω cm
R, Ω
1 × 10–1 2.2 × 10–2 2.5
6.6 × 10–2 6.1 × 10–1 2.4 × 10–3
35 350 1
2
Conversion factor η, mV MW–1 ppolarization
spolarization
146.6 28.2 ~0.52
178 35.8 ~0.68
Table 5. Electrical and photovoltaic properties of ntype films based on the PR5 paste Tbur, K
n, cm–3
878 1013 1113
–1.7 × 1021 –5.1 × 1021 –7.4 × 1020
μ, cm2 V–1 s–1
ρ, Ω cm
R, Ω
4.2 × 10–1 7.2 × 10–1 4.2
8.6 × 10–3 1.7 × 10–3 2 × 10–3
4.3 1 1
in agreement with the XPS data and thermodynamic calculations presented above. The XRD line width provides information about the average size D of coherently scattering domains. The value of D allows judging the minimum size of crystallites of a particular phase component [27]. For the AgPd and PdO phases fixed at Tbur = 878, 1013, 1113 K, the parameter D is 39 and 28 nm, 65 and 27 nm, 98 and 36 nm, respectively. This means that the minimum diameter of AgPd and PdO phase grains increases with Tbur. Thus, the study shows that the films grown at different burning temperatures differ significantly in composition and structure. Tables 4 and 5 list the electrical and photovoltaic properties of the grown films. The results presented show that all films made of PR50 paste at different Tbur are of ptype conductivity. The films made of PR5 paste are of ntype conductivity independently of burning temperature. Tables 4 and 5 list the measured interelectrode electrical resistances R of the studied samples and the determined conversion factors η for s and ppolariza tions of incident light. The results obtained show that the phase composi tion, electrical and photovoltaic properties of resistive Ag/Pd films depend strongly on the initial paste com PHYSICS OF THE SOLID STATE
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Conversion factor η, mV MW–1 ppolarization
spolarization
25.2 ~0.63 ~0.3
27.8 ~0.76 ~0.39
position and Tbur. For the same PR50 paste, an increase in Tbur from 878 to 1113 K results in a mono tonic decrease in the semiconductor phase component of palladium oxide (Table 3). However, the decrease in the semiconductor component is not accompanied by monotonic changes in electrical properties (Table 4). For example, for the sample of the film made of PR50 paste at Tbur = 1013 K, the carrier concentration and mobility are lower than those for the sample made of the same paste, but at the burning temperature of 878 K. One the explanations of this feature can be the change in the ratio of palladium oxide contents within and between crystallite grains of the AgPd conductive phase with the burning temperature. The results of the study of the photovoltaic response, presented in Tables 4 and 5 show that the largest conversion factor of the laser power to the pho tovoltaic signal amplitude is observed for the film made of PR50 paste at Tbur = 878 K. According to Table 3, among all grown films, this film contains the largest amount of palladium oxide. Therefore, it is rea sonable to associate the photovoltaic signal nature in the films under study to palladium oxide which is a p type semiconductor (see, e.g., [28]). Along with this, the following characteristic feature of films should be borne in mind. It follows from the XPS and XRD stud ies presented above that palladium oxide and AgPd 2014
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solid solution phases are simultaneously arranged on the film surface. This means the possible formation of a microstructure with p–n junctions (Schottky barri ers) appearing at interfaces of semiconductor and metal phases. According to the data given in Tables 3 and 4, the most photosensitive film made of PR50 paste at the burning temperature of 878 K consists of PdO and AgPd. Hence, the microstructure with Schottky barriers in AgPd films can play an important role in photovoltaic signal generation during pulsed laser irradiation.
2. 3.
4.
5.
5. CONCLUSIONS The results of the study of resistive Ag/Pd films obtained from a mixture of Ag2O, Pd, and STs273 glass at burning temperatures Tbur = 878, 1013, and 1113 K lead to the following conclusions. The main phase components of the films grown at Tbur = 878 K is the semiconductor PdO and solid solu tion AgPd consisting mostly of silver. An increase in Tbur to 1113 K results in almost total disappearance of the phase PdO and the formation of PbTiO3, Zn2SiO4, and Ti3O5 phases. An increase in Tbur is accompanied by an increase in the Pd percentage in the AgPd solid solution. For Tbur = 878 K, we determined the opti mum ratio between the Ag2O and Pd components in the initial paste providing the maximum PdO content. The conductivity type of the film depends on the Ag2O and Pd component ratio in the initial paste and is independent of Tbur. It is noteworthy that the depen dences of the resistivity and free carrier concentration of the grown films on Tbur are nonmonotonic func tions. The photovoltaic signal is maximum in the films obtained at Tbur = 878 K. The films obtained at Tbur = 1113 K almost have no photovoltaic properties. The porous structure consisting of PdO and solid solution AgPd containing mostly Ag is responsible for the pho tovoltaic effect in resistive Ag/Pd films. Thus, as a result of this study, it was shown that the electrical and photovoltaic properties of silver–palla dium resistive films depend strongly on the phase composition of the film, which is controlled by the burning temperature and initial paste components.
6. 7.
8.
9. 10. 11.
12. 13.
14. 15. 16. 17. 18.
ACKNOWLEDGMENTS We are grateful to L.M. Russkikh for film growth, V.A. Aleksandrov for preparing photomasks for mesh screens, and O.A. Novodvorskii and E.V. Khaidukov for the assistance in electrical measurements. This study was supported by the Russian Founda tion for Basic Research (project no. 130801031).
19.
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Translated by A. Kazantsev
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