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Donor distribution over anodically passivating crystalline and amorphous TiO2 films
Volume
IO. number
MATERIALS
7,8
January
LETTERS
199
1
Donor distribution over anodically passivating crystalline and amorphous Ti02 films Chang-Ha
Kim,
SW11 Pyun
and Eung-Jo
Lee
Departmenf of Materials Science and Engineering, Korea .ddvanced Instlk~e yfkience P.0. Box 1.70, Cheongrymg, Seoul, Korea Received
4 September
and Technology.
1990
The donor distribution in crystalline and amorphous oxide passive films anodically formed on titanium has been determmed by measuring the impedance of a passivated titanium electrode in 0.1 N NaOH solution, The passive films on titanium were prepared gai~anostatically in 1 N HZSO, solution with 5 mA cm-’ at formation potentials ranging between 5 and 30 Vsc,. From the analysis of the non-linear Mott-Schottky plot obtained from the 30 Vsc, passive film (83 nm in thickness), it is concluded that the donor concentration remains constant from the electrolyte/film interface up to about 50% of the total film thickness and then increases considerably with distance toward the film/metal interface. From the 5 Vscr passive !ilm (22 nm in thtckness ), we deduced that the non-linearity in the Mott-Schottky plot results due to the presence of multiple donor levels in the passive
1. Introduction In general, the semiconductive properties of passivating TiOz films are greatly affected by film thickness. The change of donor concentration in the TiOz film has been actually investigated by several workers [ 1.21 as a function of the film thickness. They estimated the donor concentration to be about 10” and 102” cm-’ from the analysis of the linear slope of Mott-Schottky plots obtained from the crystalline film above 30 nm thickness and the amorphous film below 30 nm thickness, respectively. They assumed thereby a uniform distribution of the donor in the oxide film. However, they did not take into account the real donor distribution across the film. In addition, the analysis of the linear Mott-Schottky plots is not actually valid for the amorphous film. In previous works, we suggested the analysis methods of non-linear Mott-Schottky plots obtained from the amorphous [ 31 and crystalline [ 41 TiO, films, respectively. The present work is concerned with the donor distribution in the passivating crystalline and amorphous TiOz films as related to the film thickness on 0167-577x/91/$03.50
0 1991 - Elsevier Science Publishers
the basis of the modified Mott-Schottky analyses reported previously [ 3,4]. For this purpose, the impedance of the TiO:, films was measured as a function of the film thickness, and the crystalline and amorphous structures of the films were substantiated by TEM diffraction analysis.
2. Experimental Specimen preparation and impedance measurement were made at 300 K in a flat cell (EC&G model KG 235 ) which had an exposed surface area of 1 cm’. A platinum foil and a saturated calomel electrode were used as the counter electrode and reference electrode, respectively. The passivated titanium electrode used as a working electrode was prepared from titanium foil of 99.99% purity (Alfa Products). The titanium specimen was etched in a 1: 4: 5 mixture of HF (48%), HNO, (65%) and distilled water. The anodically passivating TiOz films were prepared galvanostatitally with a current density of 5 mA cm-’ until the formation potentials ranging between 5 and 30 Vs(.F. were reached. An aqueous 1 N H,SO, solution used
B.V. (North-Holland)
387
Volume
10, number
7,s
MATERIALS
as electrolyte was previously deaerated by bubbling with purified nitrogen for 24 h. The film thickness was measured as a function of the film formation potential with an ellipsometer (Gaertner Scientific Corp. ) at a beam wavelength of 632 nm. The amorphous and crystalline structures of passivating TiOz films were substantiated by using transmission electron microscopic (JEOL model 2000EX) diffraction analysis. The impedance measurement of the TiOz film was performed in an aqueous 0.1 N NaOH solution with a two-phase lock-in amplifier (EG&G model 5208) and a potentiostat (EG&G model 273) by superimposing an ac voltage of 5 mV amplitude at a frequency of 1O3 Hz on a dc potential. The dc potential ranged between - 1 and 2 VsCE. The NaOH solution used as electrolyte was previously deaerated by bubbling with purified nitrogen for 24 h. A microcomputer was used to control the lock-in amplifier and the potentiostat, and to analyze the measured data.
3. Approach to analysis methods
2 qNd Kc0 A ’
(1)
where C is the space-charge capacitance of the semiconductive electrode, q the electronic charge, Nd the donor concentration, e0 the permittivity of free space, K the relative dielectric constant of the semiconductive electrode, A the area of the specimen, V the applied potential, Vf, the flat-band potential, k the Boltzmann constant, and T the absolute temperature. Several researchers [ 1,2] analyzed also the nonlinear Mott-Schottky plot obtained from the passivating Ti02 films based upon the Mott-Schottky relationship, assuming that the film thickness equals the space-charge width. The analysis gave a uniform distribution of donors over the film. However, they did not take into account the contribution of the deviation from linearity between C2 and V. In order to quantitatively analyze the non-linear Mott-Schottky plot, we employed the modified 388
January
199
I
Mott-Schottky relationship [ 4,6] which is only valid for determining the donor concentration in crystalline semiconductors with a single shallow donor. The relationship is given by N(w)=
1 --?_ qKtoA2 dC-‘/dV’
(2)
where N(W) is the donor concentration at the edge of the space-charge layer and w is the width of the space-charge layer. In order to determine N(w) as a function of w, the following formula is employed, based upon the parallel capacitor model: C= Kt,A/w
.
(3)
w can be calculated from the capacitance measured as a function of V by eq. (3). In an attempt to analyze the deviation from linearity in Mott-Schottky plots obtained from amorphous films, several workers [3,7] suggested theoretically a relationship between the instantaneous slope of the Mott-Schottky plot, dC-‘/d V, and the donor concentration at each applied potential, N( V), dC-2 - dV
A uniform donor concentration profile has been successfully determined from the linear MottSchottky plot [ 51, which is given by c-2=
LETTERS
2 = qN( V)Kt,A2
1 -qKcoA2C-2
dN( V) dV
. (4)
dC2/d V of eq. (2) is given by only one term including N(w) representing the concentration of a single shallow donor at the edge of a space-charge layer, w, whereas dC-‘/dV of eq. (4) is expressed by two terms including N( V) and dN( V)/dV, respectively. N( V) represents the concentration of ionized multiple donors at an applied potential, V. In the present work, N( V) was calculated from numerical analysis of the Mott-Schottky plots by using the Euler method.
4. Results and discussion Fig. 1 shows the thickness of the passivating Ti02 film as a function of the film formation potential. The film thickness is observed to increase almost linearly with increasing formation potential. The film thickness/potential ratio yields approximately 2.9 nm V-l. In order to identify the amorphous and crystalline structures of the passivating Ti02 films, we analyzed
Volume
IO. number
7.8
M.~TERtALS
100 ,
I
Film
Fig. 2. TEM diffraction
Formation
patterns
I
Potential
obtained
LETTERS
January
199
1
1
/
Vse,
Fig. 1. Formation potential dependence of the thi Icknc :ss for the passivating TiOz film anodically formed in a I N H 604 , solution.
from (a) 5 VSCE, (b) 10 V sCE, (c) 20 V,,,
and (d) 30 V,,,
passivatingTiOz
films.
389
Volume
10, number
7,8
MATERIALS
the TEM diffraction pattern. Fig. 2 shows the TEM diffraction pattern obtained from TiOz films formed at various formation potentials of 5 to 30 VSCE.With increasing formation potential, the diffraction patterns change from a diffuse halo pattern, i.e. a typical diffraction pattern representing an amorphous structure, to a ring-shaped pattern representing a polycrystalline structure. The 5 Vs,, passive film is in an amorphous state, but the higher than 10 VsCEpassive films show polycrystalline structure. Fig. 3 shows non-linear Mott-Schottky plots from passive films anodically formed at various potentiais. A linear relationship between inverse square of the space-charge capacitance, C-‘, and applied potential, V , is found to hold only in the applied potential range from C-‘= 0 to the inflection point, and the slopes of Cd2 versus i/ decrease gradually with increasing applied potential in the range of the applied potential more positive than that at the inflection point, irrespective of the film formation potential. As the formation potential decreases, the slope of the C-* versus V plot decreases.
LETTERS
January
I
-0.6 Applied
-0.2 Potential
0.2 /
0.6
V,,
Fig. 3. Mott-Schottky plots obtained from the passivating Ti02 film anodically formed in a 1 N HISO solution at various potentials ranging between 5 and 30 V,,.
390
I
We obtained the shallow donor concentration in the 30 V’s,, passivating ~~stal~ine film from eq. (2) by taking A(C-2)/AV from the non-linear MottSchottky plot. The shallow donor concentration profile across the passive film is shown in fig. 4. The donor concentration is calculated to be 2.1 X 1Or8cmw3 which remains constant from the electrolyte/film interface up to about 50% of the total film thickness and then increases markedly with distance toward the film/metal interface. According to the growth mechanism of anodic oxide films on valve metals suggested by Fromhold [ 8 1, oxygen vacancies and/or excess metal ions which act as donors are injected at the ~lm/metal interface during the film growth. As a result, the oxygen vacancies and/or excess metal ions are more abundant in the region near the film/metal interface as compared to the electrolyte/film interface. Therefore, the oxygen vacancy and/or excess metal ion concentration profile corresponds to that of the donor concentration across the TiOz film. This is valid for the thin or thick film, regardless of the film formation potential, i.e. the film thickness. The donor concentration in the 5 V,- amorphous film is plotted against applied potential in fig. 5. The donor concentration increases with increasing ap-
trolyte/!3m iface
-1.0
199
0
20
Film/Metal Interfact I
40
I
I
60
Distance from Electrolyte/Film interface
60 /
nm
Fig. 4. Donor concentration in the passivating TiOZ film versus distance from the electroiyte/~~m interface. The film was anodically formed at a potential of 30 Vs,-- in a 1 N H,SO, solution.
Volume
IO. number
7,8
I
-
-1.0
MATERIALS
I
I
1
I
1
I
I
-0.6
-0.2
0.2
0.6
Applied
Potential
/
1.0
LETTERS
January
199
I
of 30 VsCE gives a non-linear relationship between the inverse square of the space-charge capacitance and the applied potential. As a result of the analysis of the non-linear Mott-Schottky plot, the donor concentration remains nearly constant from the electrolyte/film interface up to about 50% of the total film thickness and then increases markedly with distance into the semiconductor, i.e. a non-uniform shallow donor concentration profile across the film. (2) The passive film (22 nm in thickness) formed at a low potential of 5 VscE is in an amorphous state. From the experimental result, we infer that the nonlinearity in the Mott-Schottky plot is caused by the presence of multiple donor levels in the passive film.
V,
Fig. 5. Donor concentration in the passivating TiOz film versus distance from the electrolyte/film interface. The film was anodically formed at a potential of 5 VSCE in a I N H,SOI solution.
plied potential. This is interpreted as follows. Multiple donor levels exist in the amorphous film. Some of them are not yet ionized at an applied potential, I’. The higher the applied potential, the larger the number of donor levels which become ionized and contribute to the capacitance. As a result, the amount of ionized multiple donors increases markedly with increasing applied potential.
5. Conclusions The present experimental results permit us to draw the following conclusions on the donor distribution in anodic crystalline and amorphous TiOz films in 0.1 N NaOH solution as related to the film thickness. ( 1) The Mott-Schottky plot of the crystalline TiOz film (83 nm in thickness) formed at a high potential
Acknowledgement The authors acknowledge the receipt of a research grant under the program “Hydrogen Energy Production 1989” from the Ministry of Science and Technology, Korea.
References [ I ] R.M. Torresi, O.R. Camara and C.P. de Pauli, Electrochim. Acta (1987) 1291. [2] R.M. Torresi, O.R. Camara and C.P. de Pauli. Electrochim. Acta 32 (1987) 1357. [ 31 E.-J. Lee and S.-I. Pyun, Electrochim. Acta ( 1990). submitted for publication. [4] C.-H. Kim and S.-I. Pyun, Electrochim. Acta (1990), submitted for publication. [5] J.F. McCann and S.P.S. Badwal, J. Electrochem. Sot. 129 (1982) 551. [6] W.C. Johnson and P.T. Panousis. IEEE Trans. Electron Devices ED-18 (1971) 965. [ 71 M.H. Dean and U. Stimming, Corros. Sci. 29 ( 1989) 199. [ 81 A.T. Fromhold Jr.. J. Electrochem. Sot. I24 ( 1977) 538.
391
IO. number
MATERIALS
7,8
January
LETTERS
199
1
Donor distribution over anodically passivating crystalline and amorphous Ti02 films Chang-Ha
Kim,
SW11 Pyun
and Eung-Jo
Lee
Departmenf of Materials Science and Engineering, Korea .ddvanced Instlk~e yfkience P.0. Box 1.70, Cheongrymg, Seoul, Korea Received
4 September
and Technology.
1990
The donor distribution in crystalline and amorphous oxide passive films anodically formed on titanium has been determmed by measuring the impedance of a passivated titanium electrode in 0.1 N NaOH solution, The passive films on titanium were prepared gai~anostatically in 1 N HZSO, solution with 5 mA cm-’ at formation potentials ranging between 5 and 30 Vsc,. From the analysis of the non-linear Mott-Schottky plot obtained from the 30 Vsc, passive film (83 nm in thickness), it is concluded that the donor concentration remains constant from the electrolyte/film interface up to about 50% of the total film thickness and then increases considerably with distance toward the film/metal interface. From the 5 Vscr passive !ilm (22 nm in thtckness ), we deduced that the non-linearity in the Mott-Schottky plot results due to the presence of multiple donor levels in the passive
1. Introduction In general, the semiconductive properties of passivating TiOz films are greatly affected by film thickness. The change of donor concentration in the TiOz film has been actually investigated by several workers [ 1.21 as a function of the film thickness. They estimated the donor concentration to be about 10” and 102” cm-’ from the analysis of the linear slope of Mott-Schottky plots obtained from the crystalline film above 30 nm thickness and the amorphous film below 30 nm thickness, respectively. They assumed thereby a uniform distribution of the donor in the oxide film. However, they did not take into account the real donor distribution across the film. In addition, the analysis of the linear Mott-Schottky plots is not actually valid for the amorphous film. In previous works, we suggested the analysis methods of non-linear Mott-Schottky plots obtained from the amorphous [ 31 and crystalline [ 41 TiO, films, respectively. The present work is concerned with the donor distribution in the passivating crystalline and amorphous TiOz films as related to the film thickness on 0167-577x/91/$03.50
0 1991 - Elsevier Science Publishers
the basis of the modified Mott-Schottky analyses reported previously [ 3,4]. For this purpose, the impedance of the TiO:, films was measured as a function of the film thickness, and the crystalline and amorphous structures of the films were substantiated by TEM diffraction analysis.
2. Experimental Specimen preparation and impedance measurement were made at 300 K in a flat cell (EC&G model KG 235 ) which had an exposed surface area of 1 cm’. A platinum foil and a saturated calomel electrode were used as the counter electrode and reference electrode, respectively. The passivated titanium electrode used as a working electrode was prepared from titanium foil of 99.99% purity (Alfa Products). The titanium specimen was etched in a 1: 4: 5 mixture of HF (48%), HNO, (65%) and distilled water. The anodically passivating TiOz films were prepared galvanostatitally with a current density of 5 mA cm-’ until the formation potentials ranging between 5 and 30 Vs(.F. were reached. An aqueous 1 N H,SO, solution used
B.V. (North-Holland)
387
Volume
10, number
7,s
MATERIALS
as electrolyte was previously deaerated by bubbling with purified nitrogen for 24 h. The film thickness was measured as a function of the film formation potential with an ellipsometer (Gaertner Scientific Corp. ) at a beam wavelength of 632 nm. The amorphous and crystalline structures of passivating TiOz films were substantiated by using transmission electron microscopic (JEOL model 2000EX) diffraction analysis. The impedance measurement of the TiOz film was performed in an aqueous 0.1 N NaOH solution with a two-phase lock-in amplifier (EG&G model 5208) and a potentiostat (EG&G model 273) by superimposing an ac voltage of 5 mV amplitude at a frequency of 1O3 Hz on a dc potential. The dc potential ranged between - 1 and 2 VsCE. The NaOH solution used as electrolyte was previously deaerated by bubbling with purified nitrogen for 24 h. A microcomputer was used to control the lock-in amplifier and the potentiostat, and to analyze the measured data.
3. Approach to analysis methods
2 qNd Kc0 A ’
(1)
where C is the space-charge capacitance of the semiconductive electrode, q the electronic charge, Nd the donor concentration, e0 the permittivity of free space, K the relative dielectric constant of the semiconductive electrode, A the area of the specimen, V the applied potential, Vf, the flat-band potential, k the Boltzmann constant, and T the absolute temperature. Several researchers [ 1,2] analyzed also the nonlinear Mott-Schottky plot obtained from the passivating Ti02 films based upon the Mott-Schottky relationship, assuming that the film thickness equals the space-charge width. The analysis gave a uniform distribution of donors over the film. However, they did not take into account the contribution of the deviation from linearity between C2 and V. In order to quantitatively analyze the non-linear Mott-Schottky plot, we employed the modified 388
January
199
I
Mott-Schottky relationship [ 4,6] which is only valid for determining the donor concentration in crystalline semiconductors with a single shallow donor. The relationship is given by N(w)=
1 --?_ qKtoA2 dC-‘/dV’
(2)
where N(W) is the donor concentration at the edge of the space-charge layer and w is the width of the space-charge layer. In order to determine N(w) as a function of w, the following formula is employed, based upon the parallel capacitor model: C= Kt,A/w
.
(3)
w can be calculated from the capacitance measured as a function of V by eq. (3). In an attempt to analyze the deviation from linearity in Mott-Schottky plots obtained from amorphous films, several workers [3,7] suggested theoretically a relationship between the instantaneous slope of the Mott-Schottky plot, dC-‘/d V, and the donor concentration at each applied potential, N( V), dC-2 - dV
A uniform donor concentration profile has been successfully determined from the linear MottSchottky plot [ 51, which is given by c-2=
LETTERS
2 = qN( V)Kt,A2
1 -qKcoA2C-2
dN( V) dV
. (4)
dC2/d V of eq. (2) is given by only one term including N(w) representing the concentration of a single shallow donor at the edge of a space-charge layer, w, whereas dC-‘/dV of eq. (4) is expressed by two terms including N( V) and dN( V)/dV, respectively. N( V) represents the concentration of ionized multiple donors at an applied potential, V. In the present work, N( V) was calculated from numerical analysis of the Mott-Schottky plots by using the Euler method.
4. Results and discussion Fig. 1 shows the thickness of the passivating Ti02 film as a function of the film formation potential. The film thickness is observed to increase almost linearly with increasing formation potential. The film thickness/potential ratio yields approximately 2.9 nm V-l. In order to identify the amorphous and crystalline structures of the passivating Ti02 films, we analyzed
Volume
IO. number
7.8
M.~TERtALS
100 ,
I
Film
Fig. 2. TEM diffraction
Formation
patterns
I
Potential
obtained
LETTERS
January
199
1
1
/
Vse,
Fig. 1. Formation potential dependence of the thi Icknc :ss for the passivating TiOz film anodically formed in a I N H 604 , solution.
from (a) 5 VSCE, (b) 10 V sCE, (c) 20 V,,,
and (d) 30 V,,,
passivatingTiOz
films.
389
Volume
10, number
7,8
MATERIALS
the TEM diffraction pattern. Fig. 2 shows the TEM diffraction pattern obtained from TiOz films formed at various formation potentials of 5 to 30 VSCE.With increasing formation potential, the diffraction patterns change from a diffuse halo pattern, i.e. a typical diffraction pattern representing an amorphous structure, to a ring-shaped pattern representing a polycrystalline structure. The 5 Vs,, passive film is in an amorphous state, but the higher than 10 VsCEpassive films show polycrystalline structure. Fig. 3 shows non-linear Mott-Schottky plots from passive films anodically formed at various potentiais. A linear relationship between inverse square of the space-charge capacitance, C-‘, and applied potential, V , is found to hold only in the applied potential range from C-‘= 0 to the inflection point, and the slopes of Cd2 versus i/ decrease gradually with increasing applied potential in the range of the applied potential more positive than that at the inflection point, irrespective of the film formation potential. As the formation potential decreases, the slope of the C-* versus V plot decreases.
LETTERS
January
I
-0.6 Applied
-0.2 Potential
0.2 /
0.6
V,,
Fig. 3. Mott-Schottky plots obtained from the passivating Ti02 film anodically formed in a 1 N HISO solution at various potentials ranging between 5 and 30 V,,.
390
I
We obtained the shallow donor concentration in the 30 V’s,, passivating ~~stal~ine film from eq. (2) by taking A(C-2)/AV from the non-linear MottSchottky plot. The shallow donor concentration profile across the passive film is shown in fig. 4. The donor concentration is calculated to be 2.1 X 1Or8cmw3 which remains constant from the electrolyte/film interface up to about 50% of the total film thickness and then increases markedly with distance toward the film/metal interface. According to the growth mechanism of anodic oxide films on valve metals suggested by Fromhold [ 8 1, oxygen vacancies and/or excess metal ions which act as donors are injected at the ~lm/metal interface during the film growth. As a result, the oxygen vacancies and/or excess metal ions are more abundant in the region near the film/metal interface as compared to the electrolyte/film interface. Therefore, the oxygen vacancy and/or excess metal ion concentration profile corresponds to that of the donor concentration across the TiOz film. This is valid for the thin or thick film, regardless of the film formation potential, i.e. the film thickness. The donor concentration in the 5 V,- amorphous film is plotted against applied potential in fig. 5. The donor concentration increases with increasing ap-
trolyte/!3m iface
-1.0
199
0
20
Film/Metal Interfact I
40
I
I
60
Distance from Electrolyte/Film interface
60 /
nm
Fig. 4. Donor concentration in the passivating TiOZ film versus distance from the electroiyte/~~m interface. The film was anodically formed at a potential of 30 Vs,-- in a 1 N H,SO, solution.
Volume
IO. number
7,8
I
-
-1.0
MATERIALS
I
I
1
I
1
I
I
-0.6
-0.2
0.2
0.6
Applied
Potential
/
1.0
LETTERS
January
199
I
of 30 VsCE gives a non-linear relationship between the inverse square of the space-charge capacitance and the applied potential. As a result of the analysis of the non-linear Mott-Schottky plot, the donor concentration remains nearly constant from the electrolyte/film interface up to about 50% of the total film thickness and then increases markedly with distance into the semiconductor, i.e. a non-uniform shallow donor concentration profile across the film. (2) The passive film (22 nm in thickness) formed at a low potential of 5 VscE is in an amorphous state. From the experimental result, we infer that the nonlinearity in the Mott-Schottky plot is caused by the presence of multiple donor levels in the passive film.
V,
Fig. 5. Donor concentration in the passivating TiOz film versus distance from the electrolyte/film interface. The film was anodically formed at a potential of 5 VSCE in a I N H,SOI solution.
plied potential. This is interpreted as follows. Multiple donor levels exist in the amorphous film. Some of them are not yet ionized at an applied potential, I’. The higher the applied potential, the larger the number of donor levels which become ionized and contribute to the capacitance. As a result, the amount of ionized multiple donors increases markedly with increasing applied potential.
5. Conclusions The present experimental results permit us to draw the following conclusions on the donor distribution in anodic crystalline and amorphous TiOz films in 0.1 N NaOH solution as related to the film thickness. ( 1) The Mott-Schottky plot of the crystalline TiOz film (83 nm in thickness) formed at a high potential
Acknowledgement The authors acknowledge the receipt of a research grant under the program “Hydrogen Energy Production 1989” from the Ministry of Science and Technology, Korea.
References [ I ] R.M. Torresi, O.R. Camara and C.P. de Pauli, Electrochim. Acta (1987) 1291. [2] R.M. Torresi, O.R. Camara and C.P. de Pauli. Electrochim. Acta 32 (1987) 1357. [ 31 E.-J. Lee and S.-I. Pyun, Electrochim. Acta ( 1990). submitted for publication. [4] C.-H. Kim and S.-I. Pyun, Electrochim. Acta (1990), submitted for publication. [5] J.F. McCann and S.P.S. Badwal, J. Electrochem. Sot. 129 (1982) 551. [6] W.C. Johnson and P.T. Panousis. IEEE Trans. Electron Devices ED-18 (1971) 965. [ 71 M.H. Dean and U. Stimming, Corros. Sci. 29 ( 1989) 199. [ 81 A.T. Fromhold Jr.. J. Electrochem. Sot. I24 ( 1977) 538.
391