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Preparation of oxide thin films by controlled diffusion of oxygen atoms
Solid State Ionics 136–137 (2000) 921–926 www.elsevier.com / locate / ssi
Preparation of oxide thin films by controlled diffusion of oxygen atoms Z. Rosenstock*, I. Riess Physics Department, Technion, Haifa 32000, Israel
Abstract A new method for preparing thin oxide films is described. It makes use of slow diffusion of oxygen through a permeable layer, towards the metal to be oxidized. We have used this method to oxidize copper. The permeable layer is a dense silver film. The formation of the oxide was followed in situ in an attempt to measure the resistance of the cell. 2000 Elsevier Science B.V. All rights reserved. Keywords: Oxide thin films; Controlled diffusion; Oxygen permeability; Cu 2 O
1. Introduction Thin oxide films are of great interest in electrochemistry and electronic components. Those films are used for example in fuel cells [1] and photovoltaic solar cells [2,3]. There are several methods for fabricating thin oxide films, including oxidation [3] and electrochemical reaction (anodization) on a conducting substance [2,4]. The film thickness which is produced in these methods is about 1 2 10 mm [5]. At least one electrode is applied after the oxide film growth is completed. This paper offers a new method to prepare thin oxide films based on oxidation and measuring the film thickness in situ. The end product of our process is the cell: Metal(A)uOxideuMetal(B)
*Corresponding author. E-mail address: [email protected] (Z. Rosenstock).
(1)
We are interested, in particular, in a Cu 2 O film between Cu and Ag, forming the cell CuuCu 2 OuAg. Metal(A) is the metallic part of the (copper) oxide. Metal(B) is silver. Contrary to the common method which is mainly based on producing oxide film on top of metal substrate, and adding a second metallic layer on top of the oxide, the new method is based on forming the oxide between two metals. One of the metals allows diffusion of oxygen atoms, while the other one blocks oxygen diffusion. This forces oxygen to accumulate at the interface and form an oxide layer there. The expected advantages of this method are the following: 1. control of oxygen diffusion; 2. detection of film formation; 3. obtaining two metal layers that can be used as electrodes ready on the oxide; 4. the possibility to control the film growth by applying an external dc voltage on the metal layers used as electrodes.
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00570-1
922
Z. Rosenstock, I. Riess / Solid State Ionics 136 – 137 (2000) 921 – 926
The reaction is basically a chemical reaction being limited by diffusion of one reactant (of oxygen). At first the diffusion limitation is in the silver layer. As the reaction proceeds and the Cu 2 O layer becomes thicker the rate-determining step is diffusion in the Cu 2 O layer. When the reaction takes place under an applied electric potential of a few volts it is an electrochemical reaction. The Cu 2 O oxide film thickness was planned to be less than 0.1 mm, to limit its resistance and to avoid peeling off. We report below on our preliminary results.
2. Experimental
2.1. General Ag on Cu samples were prepared from commercial copper, plated with silver films from one side. The samples were oxidized at a temperature of 5008C . Two leads made of Pt110%Rh were attached to each sample for the current / voltage measurement. A Pt / Pt110%Rh thermocouple was used to determine the sample temperature.
2.2. Preparing thin oxide films. The preparation method is as follows: silver is evaporated and condenses on a copper plate forming the cell CuuAg. The copper plate thickness is |1 mm and the thickness of the silver film is |5 mm. The resistance of this cell is negligible. The cell is put swiftly on top of a hot furnace with the silver facing down, towards the hot air stream. Hot air rising in the furnace oxidizes the copper plate at 5008C . The rise time of the temperature of the film is less than 20 s. The heat treatment lasted up to several hours. The oxide film is formed in between the two metal layers. A small constant voltage (of 10 mV) is applied during the oxidation to the Cu and Ag layers, and the current is measured in situ. The purpose of this measurement is to follow the cell resistance in an attempt to detect the film formation. At the end of the oxidation period, the sample is quenched to room temperature. Ie 2V relations are then measured. We also applied a high voltage of 1.5 V with difference polarity to examine the effect on the oxide film
growth. This will be discussed in a separate publication.
3. Background
3.1. Conductivity in Cu2 O The electronic conductivity of Cu 2 O is dominated by the hole conductivity [6,7]. This depends on oxygen stoichiometry. It is a mixed ionic electronic conductor, MIEC, with low ionic conductivity, and an ionic transference number t i #3.5310 24 [8]. The 9 , ionic conduction is mainly via copper vacancies V Cu ¨ and oxygen interstitials O i99 . (We use Kroger–Vink notation [9].) The local neutrality condition is [7]: p 5 fV 9Cu g 1 2 f O 99 i g
(2)
3.2. Ie 2 V relationship The current through the cell CuuAg is at first electronic, accompanied by diffusion of oxygen through the Ag layer. During the formation of the oxide, the conduction mechanism may change, and include also an ionic component. It can be shown [10] that the combined current through the oxide is, in the steady or quasisteady state: Vth 2V V Istd 5 Iistd 1 Iestd 5 ]] 2 ]] R istd R estd
(3)
where Vth is the theoretical Nernst voltage, the difference of the chemical potential, m (O 2 ), of the oxygen on the oxide, expressed in volts Vth 5 Dm (O 2 ) / 4q, q is the elementary charge, V is the applied voltage, R istd and R estd are the resistances of the oxide to ionic and electronic current, respectively. As the film thickness and stoichiometry depend on time, therefore R istd and R estd depend on time as well. Iistd and Iestd are the ionic and electronic currents, respectively. At room temperature both the copper and silver electrodes are blocking for oxygen. The Ie 2V relationship depends on the applied voltage. In the steady state the Ie 2V relationships are exponential for ion-blocking electrodes [10].
Z. Rosenstock, I. Riess / Solid State Ionics 136 – 137 (2000) 921 – 926
The current is dominated by the electronic (hole) one. Eq. (47) of Ref. [10] yields for the steady state:
923
where n 0 , n L are the charge carrier concentrations at the edges of the oxide of thickness L, i.e. at x 5 0 and x 5 L, ne is the charge mobility, k B is the Boltzman constant, T is the temperature and S is the sample cross-sectional area. Under ion blocking electrodes, Vth 5V, i.e. the oxygen redistributes within the oxide until Vth becomes equal to V, unless the sample undergoes decomposition, i.e. electrolyses. Assuming that V is lower than the decomposition voltage and using [10]:
pressurized until it opens pores in the nearby thin films. A break in the film structure will allow rapid decomposition of the film as the voltage now exceeds the decomposition voltage. The saturation of the Ie 2V relationships given in Eq. (10) is unique for the MIEC which obey the defect model n 5 N defined in Ref. [10], i.e. there is one type of mobile ionic defects which are donors or acceptors and they are fully compensated by electrons or holes, respectively. When both native donors and acceptors can be generated in the non-uniform composition of the oxide under the applied voltage, no saturation should occur, up to a few volts. The Ie 2V relationship should be [11]
n 0 5 n L e ( b qVth / 2 )
Ie 5 I0se b qV/ 2) 2 1d,
n 0 2 nL V Ie 5 2 2Sne k B T ]] ] L Vth
(4)
(5)
V .0
(11)
Which can be written approximately as
then Eq. (4) yields,
se ( b qVth / 2) 2 1d Ie 5 2 2Sne k B Tn L ]]]] L
(6)
For two blocking electrodes n 0 and n L depend on V. The total amount of electrons S e0L nsxd dx 5 C is constant. (Equal the integral on the compensating ionic defects S e0L Nisxd dx 5 C.) nsxd is linear in x [10]. x nsxd 5sn 0 2 n Ld] 1 n 0 (7) L Integration yields,
S D
b qV Ie 5 2I0 sinh ]] 2
(12)
for both V . 0 and V , 0. We prepared ultra-thin Cu 2 O films that should allow oxygen anions redistributed at room temperature taking advantage of the mixed conductivity properties of Cu 2 O [11]. It might be expected that at a certain voltage a breakdown of the film will happen due to decomposition of the oxide. This may cause a current jump and a voltage drop in the cell.
L
E
n 0 1 nL C ] ] 5 nsxd dx 5 ]] L ;nL S 2
(8)
4. Results and discussion
0
4.1. General
Combining Eqs. (5) and (8) yields:
S
b qV
th ]] n L 5 2n] e 2 1 1
D
21
(9)
Substituting into Eq. (6) yields: S] se b qV Ie 5 2 4]k B T tanh ]] , ] se 5 qne]n q 4
S D
(10)
Ie saturates for large applied voltage since the total amount of defects is fixed, as no decomposition occurs. The onset of decomposition depends on the history of the sample through the value of ]n. Decomposition will form free oxygen that may accumulate and be
The measurements of resistance to detect in situ the point when the film is formed were not successful, due to the very low resistance of the film. We believe that with an improved four-point arrangement we will be able to detect it. We therefore prepared the film, stopping the heat treatment by trial and error.
4.2. Ie 2 V relationship in Cuu Cu2 Ou Ag The Ie 2V results measured at room temperature are shown in Figs. 1–3. The voltage on the cell
924
Z. Rosenstock, I. Riess / Solid State Ionics 136 – 137 (2000) 921 – 926
Fig. 1. I–V relations of CuuCu 2 OuAg at 300 K; first run.
Fig. 2. I–V relations of CuuCu 2 OuAg at 3000 K; second run, 3 h after breakdown.
CuuCu 2 OuAg is defined as a positive bias when the Cu electrode is positive and the silver electrode is negative. From Fig. 1 it can be seen that the current is low (less than 65 mA) up to 5 V, and at |5 V there is a breakdown where the current increases to |1 A and the voltage falls to |0.8 V. The Ie 2V relations just after the breakdown seem linear with a resistance of 1 V. Hysteresis is observed. Then the current was switched off. Fig. 2 shows the Ie 2V relations 3 h after the first measurement. The voltage was
increased in the opposite direction to V| 24 V, and switched off. The measurement was restarted at V 5 0 and the voltage was increased from 0 to 5 V. The Ie 2V relations are non-linear. The resistivity is about 1 kV. Fig. 3 represents the Ie 2V relations on the same setup 1 week after the first measurement has been done. Both non-linearity and hysteresis are observed. The Ie 2V relations are not linear as one would expect. However, the hysteresis observed indicates
Z. Rosenstock, I. Riess / Solid State Ionics 136 – 137 (2000) 921 – 926
925
Fig. 3. I 2V relations of CuuCu 2 OuAg at 300 K; third run; 1 week after breakdown.
that a steady state has not been reached. This conclusion is also supported by differences of |4 orders of magnitude in the current measured for the same applied voltage at different times, as shown in Figs. 1–3. We attribute these differences to relaxation and redistribution of the internal defects. We therefore cannot make a direct comparison with the theory which assumes steady-state conditions. The breakdown at 5 V (Fig. 1) may indicate partial sample decomposition and oxygen release. We cannot exclude completely the possibility that the Ie 2V relations were affected also by an oxide film of Ag 2 O. This oxide might be formed on the Ag electrode during quenching, before contacting it. The Ie 2V relations of such a film would be similar to that of a Cu 2 O film with ion-blocking electrodes.
2. The changes in the resistivity measurement of the Cu 2 O film reflect a change in the point defects 9 , O 99i , and h ? distributions. V Cu 3. The Ie 2V relations of the cell CuuCu 2 OuAg at room temperature are not linear and show hysteresis. We are currently engaged in further measurements using a four-point arrangement which should enable detection of the point when the ultra-thin oxide is formed and the exclusion of a contribution coming from Ag 2 O, if any. We are continuing the measurements to look for conditions of steady state at room temperature, in order to allow direct comparison with the steady-state theory.
Acknowledgements 5. Conclusion A new method for preparing oxide thin films is presented. Preliminary Ie 2V relations of the cell CuuCu 2 OuAg at room temperature are presented. We find that: 1. Using this method of thin film preparation, one can form extremely thin Cu 2 O films by controlling the diffusion of oxygen through the silver at elevated temperature. These films are so thin that they: (a) do not peel off; (b) have a low resistance at room temperature.
This research was supported by the Israel Science Foundation.
References [1] S.J. Visco, L. Wang, S. Souza, L.C. De Jonghe, Mater. Res. Soc. Symp. Proc. 369 (1995) 683. [2] R.N. Briskman, Solar Energy Mater. Solar Cells 27 (1992) 361–368. [3] E. Fortin, W.M. Sears, Can. J. Phys. 60 (1982) 901. [4] A.K. Mukhopadhyay, A.K. Chakraborty, A.P. Chatterjee, S.K. Lahiri, Thin Solid Films 209 (1992) 92.
926
Z. Rosenstock, I. Riess / Solid State Ionics 136 – 137 (2000) 921 – 926
[5] A.E. Rakhshani, J. Varghese, Solar Energy Mater. 15 (1987) 237. [6] O. Porat, I. Riess, Solid State Ionics 74 (1994) 229. [7] O. Porat, I. Riess, Solid State Ionics 81 (1995) 29.
[8] [9] [10] [11]
Y. Tsur, I. Riess, Z. Phys. Chem. Bd. 207 (1998) S181–213. ¨ F.A. Kroger, H.J. Vink, Solid State Phys. 3 (1956) 307. I. Riess, J. Phys. Chem. Solids 47 (2) (1986) 12. I. Riess, D. Cahen, J. Appl. Phys. 82 (6) (1997) 3147.
Preparation of oxide thin films by controlled diffusion of oxygen atoms Z. Rosenstock*, I. Riess Physics Department, Technion, Haifa 32000, Israel
Abstract A new method for preparing thin oxide films is described. It makes use of slow diffusion of oxygen through a permeable layer, towards the metal to be oxidized. We have used this method to oxidize copper. The permeable layer is a dense silver film. The formation of the oxide was followed in situ in an attempt to measure the resistance of the cell. 2000 Elsevier Science B.V. All rights reserved. Keywords: Oxide thin films; Controlled diffusion; Oxygen permeability; Cu 2 O
1. Introduction Thin oxide films are of great interest in electrochemistry and electronic components. Those films are used for example in fuel cells [1] and photovoltaic solar cells [2,3]. There are several methods for fabricating thin oxide films, including oxidation [3] and electrochemical reaction (anodization) on a conducting substance [2,4]. The film thickness which is produced in these methods is about 1 2 10 mm [5]. At least one electrode is applied after the oxide film growth is completed. This paper offers a new method to prepare thin oxide films based on oxidation and measuring the film thickness in situ. The end product of our process is the cell: Metal(A)uOxideuMetal(B)
*Corresponding author. E-mail address: [email protected] (Z. Rosenstock).
(1)
We are interested, in particular, in a Cu 2 O film between Cu and Ag, forming the cell CuuCu 2 OuAg. Metal(A) is the metallic part of the (copper) oxide. Metal(B) is silver. Contrary to the common method which is mainly based on producing oxide film on top of metal substrate, and adding a second metallic layer on top of the oxide, the new method is based on forming the oxide between two metals. One of the metals allows diffusion of oxygen atoms, while the other one blocks oxygen diffusion. This forces oxygen to accumulate at the interface and form an oxide layer there. The expected advantages of this method are the following: 1. control of oxygen diffusion; 2. detection of film formation; 3. obtaining two metal layers that can be used as electrodes ready on the oxide; 4. the possibility to control the film growth by applying an external dc voltage on the metal layers used as electrodes.
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00570-1
922
Z. Rosenstock, I. Riess / Solid State Ionics 136 – 137 (2000) 921 – 926
The reaction is basically a chemical reaction being limited by diffusion of one reactant (of oxygen). At first the diffusion limitation is in the silver layer. As the reaction proceeds and the Cu 2 O layer becomes thicker the rate-determining step is diffusion in the Cu 2 O layer. When the reaction takes place under an applied electric potential of a few volts it is an electrochemical reaction. The Cu 2 O oxide film thickness was planned to be less than 0.1 mm, to limit its resistance and to avoid peeling off. We report below on our preliminary results.
2. Experimental
2.1. General Ag on Cu samples were prepared from commercial copper, plated with silver films from one side. The samples were oxidized at a temperature of 5008C . Two leads made of Pt110%Rh were attached to each sample for the current / voltage measurement. A Pt / Pt110%Rh thermocouple was used to determine the sample temperature.
2.2. Preparing thin oxide films. The preparation method is as follows: silver is evaporated and condenses on a copper plate forming the cell CuuAg. The copper plate thickness is |1 mm and the thickness of the silver film is |5 mm. The resistance of this cell is negligible. The cell is put swiftly on top of a hot furnace with the silver facing down, towards the hot air stream. Hot air rising in the furnace oxidizes the copper plate at 5008C . The rise time of the temperature of the film is less than 20 s. The heat treatment lasted up to several hours. The oxide film is formed in between the two metal layers. A small constant voltage (of 10 mV) is applied during the oxidation to the Cu and Ag layers, and the current is measured in situ. The purpose of this measurement is to follow the cell resistance in an attempt to detect the film formation. At the end of the oxidation period, the sample is quenched to room temperature. Ie 2V relations are then measured. We also applied a high voltage of 1.5 V with difference polarity to examine the effect on the oxide film
growth. This will be discussed in a separate publication.
3. Background
3.1. Conductivity in Cu2 O The electronic conductivity of Cu 2 O is dominated by the hole conductivity [6,7]. This depends on oxygen stoichiometry. It is a mixed ionic electronic conductor, MIEC, with low ionic conductivity, and an ionic transference number t i #3.5310 24 [8]. The 9 , ionic conduction is mainly via copper vacancies V Cu ¨ and oxygen interstitials O i99 . (We use Kroger–Vink notation [9].) The local neutrality condition is [7]: p 5 fV 9Cu g 1 2 f O 99 i g
(2)
3.2. Ie 2 V relationship The current through the cell CuuAg is at first electronic, accompanied by diffusion of oxygen through the Ag layer. During the formation of the oxide, the conduction mechanism may change, and include also an ionic component. It can be shown [10] that the combined current through the oxide is, in the steady or quasisteady state: Vth 2V V Istd 5 Iistd 1 Iestd 5 ]] 2 ]] R istd R estd
(3)
where Vth is the theoretical Nernst voltage, the difference of the chemical potential, m (O 2 ), of the oxygen on the oxide, expressed in volts Vth 5 Dm (O 2 ) / 4q, q is the elementary charge, V is the applied voltage, R istd and R estd are the resistances of the oxide to ionic and electronic current, respectively. As the film thickness and stoichiometry depend on time, therefore R istd and R estd depend on time as well. Iistd and Iestd are the ionic and electronic currents, respectively. At room temperature both the copper and silver electrodes are blocking for oxygen. The Ie 2V relationship depends on the applied voltage. In the steady state the Ie 2V relationships are exponential for ion-blocking electrodes [10].
Z. Rosenstock, I. Riess / Solid State Ionics 136 – 137 (2000) 921 – 926
The current is dominated by the electronic (hole) one. Eq. (47) of Ref. [10] yields for the steady state:
923
where n 0 , n L are the charge carrier concentrations at the edges of the oxide of thickness L, i.e. at x 5 0 and x 5 L, ne is the charge mobility, k B is the Boltzman constant, T is the temperature and S is the sample cross-sectional area. Under ion blocking electrodes, Vth 5V, i.e. the oxygen redistributes within the oxide until Vth becomes equal to V, unless the sample undergoes decomposition, i.e. electrolyses. Assuming that V is lower than the decomposition voltage and using [10]:
pressurized until it opens pores in the nearby thin films. A break in the film structure will allow rapid decomposition of the film as the voltage now exceeds the decomposition voltage. The saturation of the Ie 2V relationships given in Eq. (10) is unique for the MIEC which obey the defect model n 5 N defined in Ref. [10], i.e. there is one type of mobile ionic defects which are donors or acceptors and they are fully compensated by electrons or holes, respectively. When both native donors and acceptors can be generated in the non-uniform composition of the oxide under the applied voltage, no saturation should occur, up to a few volts. The Ie 2V relationship should be [11]
n 0 5 n L e ( b qVth / 2 )
Ie 5 I0se b qV/ 2) 2 1d,
n 0 2 nL V Ie 5 2 2Sne k B T ]] ] L Vth
(4)
(5)
V .0
(11)
Which can be written approximately as
then Eq. (4) yields,
se ( b qVth / 2) 2 1d Ie 5 2 2Sne k B Tn L ]]]] L
(6)
For two blocking electrodes n 0 and n L depend on V. The total amount of electrons S e0L nsxd dx 5 C is constant. (Equal the integral on the compensating ionic defects S e0L Nisxd dx 5 C.) nsxd is linear in x [10]. x nsxd 5sn 0 2 n Ld] 1 n 0 (7) L Integration yields,
S D
b qV Ie 5 2I0 sinh ]] 2
(12)
for both V . 0 and V , 0. We prepared ultra-thin Cu 2 O films that should allow oxygen anions redistributed at room temperature taking advantage of the mixed conductivity properties of Cu 2 O [11]. It might be expected that at a certain voltage a breakdown of the film will happen due to decomposition of the oxide. This may cause a current jump and a voltage drop in the cell.
L
E
n 0 1 nL C ] ] 5 nsxd dx 5 ]] L ;nL S 2
(8)
4. Results and discussion
0
4.1. General
Combining Eqs. (5) and (8) yields:
S
b qV
th ]] n L 5 2n] e 2 1 1
D
21
(9)
Substituting into Eq. (6) yields: S] se b qV Ie 5 2 4]k B T tanh ]] , ] se 5 qne]n q 4
S D
(10)
Ie saturates for large applied voltage since the total amount of defects is fixed, as no decomposition occurs. The onset of decomposition depends on the history of the sample through the value of ]n. Decomposition will form free oxygen that may accumulate and be
The measurements of resistance to detect in situ the point when the film is formed were not successful, due to the very low resistance of the film. We believe that with an improved four-point arrangement we will be able to detect it. We therefore prepared the film, stopping the heat treatment by trial and error.
4.2. Ie 2 V relationship in Cuu Cu2 Ou Ag The Ie 2V results measured at room temperature are shown in Figs. 1–3. The voltage on the cell
924
Z. Rosenstock, I. Riess / Solid State Ionics 136 – 137 (2000) 921 – 926
Fig. 1. I–V relations of CuuCu 2 OuAg at 300 K; first run.
Fig. 2. I–V relations of CuuCu 2 OuAg at 3000 K; second run, 3 h after breakdown.
CuuCu 2 OuAg is defined as a positive bias when the Cu electrode is positive and the silver electrode is negative. From Fig. 1 it can be seen that the current is low (less than 65 mA) up to 5 V, and at |5 V there is a breakdown where the current increases to |1 A and the voltage falls to |0.8 V. The Ie 2V relations just after the breakdown seem linear with a resistance of 1 V. Hysteresis is observed. Then the current was switched off. Fig. 2 shows the Ie 2V relations 3 h after the first measurement. The voltage was
increased in the opposite direction to V| 24 V, and switched off. The measurement was restarted at V 5 0 and the voltage was increased from 0 to 5 V. The Ie 2V relations are non-linear. The resistivity is about 1 kV. Fig. 3 represents the Ie 2V relations on the same setup 1 week after the first measurement has been done. Both non-linearity and hysteresis are observed. The Ie 2V relations are not linear as one would expect. However, the hysteresis observed indicates
Z. Rosenstock, I. Riess / Solid State Ionics 136 – 137 (2000) 921 – 926
925
Fig. 3. I 2V relations of CuuCu 2 OuAg at 300 K; third run; 1 week after breakdown.
that a steady state has not been reached. This conclusion is also supported by differences of |4 orders of magnitude in the current measured for the same applied voltage at different times, as shown in Figs. 1–3. We attribute these differences to relaxation and redistribution of the internal defects. We therefore cannot make a direct comparison with the theory which assumes steady-state conditions. The breakdown at 5 V (Fig. 1) may indicate partial sample decomposition and oxygen release. We cannot exclude completely the possibility that the Ie 2V relations were affected also by an oxide film of Ag 2 O. This oxide might be formed on the Ag electrode during quenching, before contacting it. The Ie 2V relations of such a film would be similar to that of a Cu 2 O film with ion-blocking electrodes.
2. The changes in the resistivity measurement of the Cu 2 O film reflect a change in the point defects 9 , O 99i , and h ? distributions. V Cu 3. The Ie 2V relations of the cell CuuCu 2 OuAg at room temperature are not linear and show hysteresis. We are currently engaged in further measurements using a four-point arrangement which should enable detection of the point when the ultra-thin oxide is formed and the exclusion of a contribution coming from Ag 2 O, if any. We are continuing the measurements to look for conditions of steady state at room temperature, in order to allow direct comparison with the steady-state theory.
Acknowledgements 5. Conclusion A new method for preparing oxide thin films is presented. Preliminary Ie 2V relations of the cell CuuCu 2 OuAg at room temperature are presented. We find that: 1. Using this method of thin film preparation, one can form extremely thin Cu 2 O films by controlling the diffusion of oxygen through the silver at elevated temperature. These films are so thin that they: (a) do not peel off; (b) have a low resistance at room temperature.
This research was supported by the Israel Science Foundation.
References [1] S.J. Visco, L. Wang, S. Souza, L.C. De Jonghe, Mater. Res. Soc. Symp. Proc. 369 (1995) 683. [2] R.N. Briskman, Solar Energy Mater. Solar Cells 27 (1992) 361–368. [3] E. Fortin, W.M. Sears, Can. J. Phys. 60 (1982) 901. [4] A.K. Mukhopadhyay, A.K. Chakraborty, A.P. Chatterjee, S.K. Lahiri, Thin Solid Films 209 (1992) 92.
926
Z. Rosenstock, I. Riess / Solid State Ionics 136 – 137 (2000) 921 – 926
[5] A.E. Rakhshani, J. Varghese, Solar Energy Mater. 15 (1987) 237. [6] O. Porat, I. Riess, Solid State Ionics 74 (1994) 229. [7] O. Porat, I. Riess, Solid State Ionics 81 (1995) 29.
[8] [9] [10] [11]
Y. Tsur, I. Riess, Z. Phys. Chem. Bd. 207 (1998) S181–213. ¨ F.A. Kroger, H.J. Vink, Solid State Phys. 3 (1956) 307. I. Riess, J. Phys. Chem. Solids 47 (2) (1986) 12. I. Riess, D. Cahen, J. Appl. Phys. 82 (6) (1997) 3147.