- Email: [email protected]
Preparation and properties of vacuum deposited AgBr thin films
Vacuum 69 (2003) 327–331
Preparation and properties of vacuum deposited AgBr thin films R. Georgieva*, D. Karashanova, N. Starbov Central Laboratory of Photoprocesses, Bulgarian Academy of Sciences, Acad. Georgy Bonchev str., bl. 109, 1113 Sofia, Bulgaria
Abstract It is demonstrated that porous silver bromide films could be obtained via variation of one basic parameter of the PVD technique: the vapour incidence angle. This leads to the preparation of thin AgBr films with different surface-tovolume ratio. Relationship between the film microstructure and microhardness as well as dark ionic conductivity is found, thus revealing an opportunity for designing of novel AgBr thin film materials. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: PVD; Thin films; AgBr; Microstructure; Electrical and mechanical properties
1. Introduction Porous thin films are of great importance for development of new technologies in micro- and optoelectronics, sensor techniques, electrochemistry and catalysis. The applications of these materials depend on their specific mechanical, electrical, optical and chemical properties due to their surface-to-volume ratio. The porous thin films with high specific surface are applied in absorbers [1], in membranes for separation and purification of liquids and gases [2], in catalysts for oxidation and reduction of pollutive gases from the chemical industry and automotive engines [3]. Porous Al2O3, ZrO2, TiO2, SiC and other ceramic materials are used as active media for monitoring and control of pressure, humidity and gas composition [4]. *Corresponding author. Tel.: +359-2-979-3523; fax: +3592-72-24-65. E-mail address: [email protected] (R. Georgieva).
Silver halides have also a potential to be used as a functional media for sensor devices. Recently, Lauer et al. [5] proposed a new sensor principle based on substantial decrease of the dark ionic conductivity when the free surface of AgCl thin films is in contact with ionophilic gases such as ammonia and cyan. Besides, Zhelev [6] has shown that the dark ionic conductivity of thin AgBr films is very sensitive to the humidity and bromine in the ambient atmosphere. In both cases however, the experiments are carried out using dense silver halide films with relatively low surface-to-volume ratio. It is possible to increase this morphological parameter via modification of the columnar structure revealed in vacuum deposited AgBr films [7]. Varying only one of the condensation conditions, i.e. the vapour incidence angle, one can deposit porous films with high specific surface area [8]. Therefore, the aim of the present paper is to obtain porous AgBr thin films as well as to investigate some of their basic properties.
0042-207X/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 3 5 3 - 6
328
R. Georgieva et al. / Vacuum 69 (2003) 327–331
2. Experimental The vacuum deposition of the samples is performed in a standard vacuum unit with oil diffusion pump maintaining residual gas pressure lower than 5 104 Pa. Thin films (500–1500 nm) are obtained via thermal evaporation of high purity silver bromide (99,99 999%) from indirect heated Pt crucible mounted in water cooled massive silver block. Pre-cleaned glass plates at room temperature are used as a substrates positioned under different vapour incidence angles a ¼ 01, 151, 301, 401, 601, 701 and 801 between the substrate normal and the vapour beam, both defining the so-called ‘‘vapour incidence plane’’. The growth morphology of the AgBr films is studied by means of a Phillips 515 Scanning Electron Microscope. Standard microfractographic technique is applied to prepare the samples for the SEM study [9]. This technique requires cutting notches in the backside of the glass substrate after evaporation of AgBr. During the fracture procedure precautions are taken to obtain as much as possible perfect profiles. Further, thin conductive carbon and gold coatings are consecutively sputtered on the examined surfaces. The microhardness (Mh) of AgBr thin films is evaluated by the Knoop prism indentation method [10]. The load value is 1.25 g for all AgBr samples studied. For satisfying the requirements for high precision indentation measurements samples with film thickness is about 1000 nm are used. At least ten prism indentations are measured for calculating the mean Mh value. Dark ionic conductivity measurements are carried out in dry pure nitrogen at atmospheric pressure using a specially designed thermostat [11]. The sample temperature is maintained between 201C and 1001C with an accuracy 70.51C. Gold stripe electrodes evaporated onto the substrate prior to the film preparation are used as contacts with the measurement circuit. The electrical resistance of the films is measured by four-arm Wheatstone bridge build up of high quality resistors. As a zero point indicator a dynamic condenser electrometer is used measuring currents as low as 1011 A. Short electric pulses are passed through the sample, each of alternating direction,
thus keeping minimal AgBr polarization and electrolysis. The experimental set-up provides a great accuracy of the measured resistance and the maximal error never exceeds 5% in the whole temperature region studied. It was found that reproducible values for film conductivity are obtained only after 1 h annealing at 1001C.
3. Results and discussion AgBr films grown on pre-cleaned glass substrates possess a polycrystalline structure with preferred orientation along the zone axis [1 1 1] [12]. Fig. 1 shows scanning electron micrographs of growth profiles of about 1000 nm thick AgBr films deposited at angles a ¼ 01, 401, 601 and 801 (Fig. 1a–d). It is clearly seen that the AgBr films are built up of individual columns inclined in the vapour incidence plane under angle b (also measured to the substrate normal). The columns are separated by free space (voids), thus forming pores through the entire film thickness. Simple measurements of the column inclination using SEM micrographs show that b is smaller than a and obeys the empirical ‘‘tangent rule’’: 2tan b ¼ tan a: The plot of b vs. a is shown in Fig. 2, where literature data [13] for other crystalline and amorphous thin films are also included for comparison. As seen the present results for AgBr films fit the ‘‘tangent rule’’ within the limits of the experimental error. It should be noted here that the divergence in the measured b values with respect to the ‘‘tangent rule’’ is related to the precision of the experimental techniques, i.e. the not ‘‘ideal’’ collimating of the vapour beam and the fracture of sample’s substrate not being exactly in the vapour incidence plane when growth profiles are prepared. Nevertheless, the results obtained show that the AgBr films thus prepared follow the general dependence between the vapour incidence angle and column inclination established for numerous obliquely deposited amorphous or crystalline thin films of different materials [13–15]. Fig. 3 presents the Mh values measured as a function of a when the indentations are produced
R. Georgieva et al. / Vacuum 69 (2003) 327–331
329
Fig. 1. Scanning electron micrographs of growth profiles of 1000 nm thick AgBr films deposited at angles: a ¼ 01 (a), 401 (b), 601 (c) and 801 (d). (all diags to same magnification).
Fig. 2. Column inclination b as dependent on the vapour incidence angle a as published in Ref. [13] for various thin films. Squares relate to AgBr films studied.
perpendicularly to the vapour incidence plane. It is evident from the figure that the Mh decreases to about one third when the vapour incidence angle rises from 01 to 801. The relatively low Mh values at high incidence angles are similar to the results obtained earlier for ZrO2 [14] and GeS2 [15] and corresponds to the increase of the film free volume, i.e. porosity, going from low to high vapour incidence angles. This is a proof that the method used is suitable for preparation of AgBr films with high specific surface area. It was shown earlier [11] that the DC-conductivity of epitaxial AgBr thin films similarly to the massive silver bromide have a pure ionic conductivity. The present electrical measurements show that at room temperature the polycrystalline AgBr films deposited at 01 vapour incidence angle have DC conductivity as low as 4 105 O1 cm1. Its dependence on a for different temperatures between 201C and 1001C is
R. Georgieva et al. / Vacuum 69 (2003) 327–331
330
0.55 0.50
Microhardness [GPa]
0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 −10
0
10
20
30
40
50
60
70
80
90
80
90
α [deg] Fig. 3. Mh of AgBr films vs. vapour incidence angle a:
t = 100˚C t = 70˚C
Conductivity [Ohm-1/ ]
10−7
t = 50˚C t = 30˚C
10−8
10−9 −10
0
10
20
30
40
50
60
70
α [deg] Fig. 4. Conductivity per unit area of AgBr films vs. vapour incidence angle a:
presented in Fig. 4 using, for simplicity, the values of the conductivity per unit area. It is clearly seen that with a increase the electrical conductivity lowers gradually by about a factor of 1.5. This means that the conductivity on the very surface of AgBr columns is lower than that in the bulk since at higher a the surface-to-volume ratio rises. In addition, the electrical charge transport between the individual columns compared to their volume is suppressed due to the increased porosity, which directly influences the overall film conductivity. However, this will not hinder the application of
AgBr films for sensor development since the conductivity values of highly porous samples deposited at a ¼ 801 are about 1.5 times lower than the conductivity of the dense films obtained at a ¼ 01:
4. Conclusion The present results demonstrate that, using the method of oblique physical vapour deposition, it is possible to obtain porous AgBr films. It is shown
R. Georgieva et al. / Vacuum 69 (2003) 327–331
that the sample properties (microhardness and DC conductivity) are strongly dependent on the vapour incident angle and closely related to the film microstructure.
References [1] Fain DE. MRS Bull 1994;19:40. [2] Brave RR. Inorganic membranes: synthesis, characteristics and applications. New York: Van Nostrand Reinhold, 1991. [3] Baiker A. Chimia 1996;50:65. [4] Haussone FJ-M. Technique de l’Enginieures 1985;1E(E1820):1. [5] Lauer U, Maier J, Goepel W. Sensors and Actuators 1990;B2:125.
331
[6] Zhelev V. Comm Dept Chem 1975;8(4):756. [7] Assa J, Starbova K. J Imag Technol 1985;11(4):180. [8] Robbie K, Sit JC, Brett MJ. J Vac Sci Technol 1998;B16(3):1115. [9] Nieuwenhizen JM, Haanstra HB. Philips Tech Rev 1966;27:87. [10] Knoop F, Peter CG, Emerson B. Natl Bureau Stand 1939;23:39. [11] Starbov N, Buroff A, Malinowski J. Phys Stat Sol A 1976;38:161. [12] Eneva J, Malinowski J. J Phot Sci 1974;22(6):273. [13] Dirks AG, Leamy HJ. Thin Solid Films 1977;47:219. [14] Levichkova M, Mankov V, Starbov N, Karashanova D, Mednikarov B, Starbova K. Surf Coat Technol 2001;141:70. [15] Starbova K, Mankov V, Dikova J, Starbov N. Vacuum 1999;53:441.
Preparation and properties of vacuum deposited AgBr thin films R. Georgieva*, D. Karashanova, N. Starbov Central Laboratory of Photoprocesses, Bulgarian Academy of Sciences, Acad. Georgy Bonchev str., bl. 109, 1113 Sofia, Bulgaria
Abstract It is demonstrated that porous silver bromide films could be obtained via variation of one basic parameter of the PVD technique: the vapour incidence angle. This leads to the preparation of thin AgBr films with different surface-tovolume ratio. Relationship between the film microstructure and microhardness as well as dark ionic conductivity is found, thus revealing an opportunity for designing of novel AgBr thin film materials. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: PVD; Thin films; AgBr; Microstructure; Electrical and mechanical properties
1. Introduction Porous thin films are of great importance for development of new technologies in micro- and optoelectronics, sensor techniques, electrochemistry and catalysis. The applications of these materials depend on their specific mechanical, electrical, optical and chemical properties due to their surface-to-volume ratio. The porous thin films with high specific surface are applied in absorbers [1], in membranes for separation and purification of liquids and gases [2], in catalysts for oxidation and reduction of pollutive gases from the chemical industry and automotive engines [3]. Porous Al2O3, ZrO2, TiO2, SiC and other ceramic materials are used as active media for monitoring and control of pressure, humidity and gas composition [4]. *Corresponding author. Tel.: +359-2-979-3523; fax: +3592-72-24-65. E-mail address: [email protected] (R. Georgieva).
Silver halides have also a potential to be used as a functional media for sensor devices. Recently, Lauer et al. [5] proposed a new sensor principle based on substantial decrease of the dark ionic conductivity when the free surface of AgCl thin films is in contact with ionophilic gases such as ammonia and cyan. Besides, Zhelev [6] has shown that the dark ionic conductivity of thin AgBr films is very sensitive to the humidity and bromine in the ambient atmosphere. In both cases however, the experiments are carried out using dense silver halide films with relatively low surface-to-volume ratio. It is possible to increase this morphological parameter via modification of the columnar structure revealed in vacuum deposited AgBr films [7]. Varying only one of the condensation conditions, i.e. the vapour incidence angle, one can deposit porous films with high specific surface area [8]. Therefore, the aim of the present paper is to obtain porous AgBr thin films as well as to investigate some of their basic properties.
0042-207X/03/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 3 5 3 - 6
328
R. Georgieva et al. / Vacuum 69 (2003) 327–331
2. Experimental The vacuum deposition of the samples is performed in a standard vacuum unit with oil diffusion pump maintaining residual gas pressure lower than 5 104 Pa. Thin films (500–1500 nm) are obtained via thermal evaporation of high purity silver bromide (99,99 999%) from indirect heated Pt crucible mounted in water cooled massive silver block. Pre-cleaned glass plates at room temperature are used as a substrates positioned under different vapour incidence angles a ¼ 01, 151, 301, 401, 601, 701 and 801 between the substrate normal and the vapour beam, both defining the so-called ‘‘vapour incidence plane’’. The growth morphology of the AgBr films is studied by means of a Phillips 515 Scanning Electron Microscope. Standard microfractographic technique is applied to prepare the samples for the SEM study [9]. This technique requires cutting notches in the backside of the glass substrate after evaporation of AgBr. During the fracture procedure precautions are taken to obtain as much as possible perfect profiles. Further, thin conductive carbon and gold coatings are consecutively sputtered on the examined surfaces. The microhardness (Mh) of AgBr thin films is evaluated by the Knoop prism indentation method [10]. The load value is 1.25 g for all AgBr samples studied. For satisfying the requirements for high precision indentation measurements samples with film thickness is about 1000 nm are used. At least ten prism indentations are measured for calculating the mean Mh value. Dark ionic conductivity measurements are carried out in dry pure nitrogen at atmospheric pressure using a specially designed thermostat [11]. The sample temperature is maintained between 201C and 1001C with an accuracy 70.51C. Gold stripe electrodes evaporated onto the substrate prior to the film preparation are used as contacts with the measurement circuit. The electrical resistance of the films is measured by four-arm Wheatstone bridge build up of high quality resistors. As a zero point indicator a dynamic condenser electrometer is used measuring currents as low as 1011 A. Short electric pulses are passed through the sample, each of alternating direction,
thus keeping minimal AgBr polarization and electrolysis. The experimental set-up provides a great accuracy of the measured resistance and the maximal error never exceeds 5% in the whole temperature region studied. It was found that reproducible values for film conductivity are obtained only after 1 h annealing at 1001C.
3. Results and discussion AgBr films grown on pre-cleaned glass substrates possess a polycrystalline structure with preferred orientation along the zone axis [1 1 1] [12]. Fig. 1 shows scanning electron micrographs of growth profiles of about 1000 nm thick AgBr films deposited at angles a ¼ 01, 401, 601 and 801 (Fig. 1a–d). It is clearly seen that the AgBr films are built up of individual columns inclined in the vapour incidence plane under angle b (also measured to the substrate normal). The columns are separated by free space (voids), thus forming pores through the entire film thickness. Simple measurements of the column inclination using SEM micrographs show that b is smaller than a and obeys the empirical ‘‘tangent rule’’: 2tan b ¼ tan a: The plot of b vs. a is shown in Fig. 2, where literature data [13] for other crystalline and amorphous thin films are also included for comparison. As seen the present results for AgBr films fit the ‘‘tangent rule’’ within the limits of the experimental error. It should be noted here that the divergence in the measured b values with respect to the ‘‘tangent rule’’ is related to the precision of the experimental techniques, i.e. the not ‘‘ideal’’ collimating of the vapour beam and the fracture of sample’s substrate not being exactly in the vapour incidence plane when growth profiles are prepared. Nevertheless, the results obtained show that the AgBr films thus prepared follow the general dependence between the vapour incidence angle and column inclination established for numerous obliquely deposited amorphous or crystalline thin films of different materials [13–15]. Fig. 3 presents the Mh values measured as a function of a when the indentations are produced
R. Georgieva et al. / Vacuum 69 (2003) 327–331
329
Fig. 1. Scanning electron micrographs of growth profiles of 1000 nm thick AgBr films deposited at angles: a ¼ 01 (a), 401 (b), 601 (c) and 801 (d). (all diags to same magnification).
Fig. 2. Column inclination b as dependent on the vapour incidence angle a as published in Ref. [13] for various thin films. Squares relate to AgBr films studied.
perpendicularly to the vapour incidence plane. It is evident from the figure that the Mh decreases to about one third when the vapour incidence angle rises from 01 to 801. The relatively low Mh values at high incidence angles are similar to the results obtained earlier for ZrO2 [14] and GeS2 [15] and corresponds to the increase of the film free volume, i.e. porosity, going from low to high vapour incidence angles. This is a proof that the method used is suitable for preparation of AgBr films with high specific surface area. It was shown earlier [11] that the DC-conductivity of epitaxial AgBr thin films similarly to the massive silver bromide have a pure ionic conductivity. The present electrical measurements show that at room temperature the polycrystalline AgBr films deposited at 01 vapour incidence angle have DC conductivity as low as 4 105 O1 cm1. Its dependence on a for different temperatures between 201C and 1001C is
R. Georgieva et al. / Vacuum 69 (2003) 327–331
330
0.55 0.50
Microhardness [GPa]
0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 −10
0
10
20
30
40
50
60
70
80
90
80
90
α [deg] Fig. 3. Mh of AgBr films vs. vapour incidence angle a:
t = 100˚C t = 70˚C
Conductivity [Ohm-1/ ]
10−7
t = 50˚C t = 30˚C
10−8
10−9 −10
0
10
20
30
40
50
60
70
α [deg] Fig. 4. Conductivity per unit area of AgBr films vs. vapour incidence angle a:
presented in Fig. 4 using, for simplicity, the values of the conductivity per unit area. It is clearly seen that with a increase the electrical conductivity lowers gradually by about a factor of 1.5. This means that the conductivity on the very surface of AgBr columns is lower than that in the bulk since at higher a the surface-to-volume ratio rises. In addition, the electrical charge transport between the individual columns compared to their volume is suppressed due to the increased porosity, which directly influences the overall film conductivity. However, this will not hinder the application of
AgBr films for sensor development since the conductivity values of highly porous samples deposited at a ¼ 801 are about 1.5 times lower than the conductivity of the dense films obtained at a ¼ 01:
4. Conclusion The present results demonstrate that, using the method of oblique physical vapour deposition, it is possible to obtain porous AgBr films. It is shown
R. Georgieva et al. / Vacuum 69 (2003) 327–331
that the sample properties (microhardness and DC conductivity) are strongly dependent on the vapour incident angle and closely related to the film microstructure.
References [1] Fain DE. MRS Bull 1994;19:40. [2] Brave RR. Inorganic membranes: synthesis, characteristics and applications. New York: Van Nostrand Reinhold, 1991. [3] Baiker A. Chimia 1996;50:65. [4] Haussone FJ-M. Technique de l’Enginieures 1985;1E(E1820):1. [5] Lauer U, Maier J, Goepel W. Sensors and Actuators 1990;B2:125.
331
[6] Zhelev V. Comm Dept Chem 1975;8(4):756. [7] Assa J, Starbova K. J Imag Technol 1985;11(4):180. [8] Robbie K, Sit JC, Brett MJ. J Vac Sci Technol 1998;B16(3):1115. [9] Nieuwenhizen JM, Haanstra HB. Philips Tech Rev 1966;27:87. [10] Knoop F, Peter CG, Emerson B. Natl Bureau Stand 1939;23:39. [11] Starbov N, Buroff A, Malinowski J. Phys Stat Sol A 1976;38:161. [12] Eneva J, Malinowski J. J Phot Sci 1974;22(6):273. [13] Dirks AG, Leamy HJ. Thin Solid Films 1977;47:219. [14] Levichkova M, Mankov V, Starbov N, Karashanova D, Mednikarov B, Starbova K. Surf Coat Technol 2001;141:70. [15] Starbova K, Mankov V, Dikova J, Starbov N. Vacuum 1999;53:441.