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In-situ resistance measurements during pulsed laser deposition of ultrathin films
ii;i~iiiii~i~iiii~i~i~i~ii~i~i~i~i~i~!~i~i~ applied surface science ELSEVIER
Applied Surface Science 106 (1996) 51-54
In-situ resistance measurements during pulsed laser deposition of ultrathin films X.W. Sun, H.C. Huang, H.S. K w o k * Department c~[Electrical and Electronic Engineering, Hong Kong Unit'ersiO' of Science and Technology, Clearwater Bay, Hong Kong Received 17 September 1995; accepted 1 November 1995
Abstract
In-situ resistance measurement is a powerful technique for monitoring the growth and properties of thin films. In this paper, we showed that it can be applied to study the initial growth mechanisms of indium tin oxide on glass. An interesting transition from a 3D growth mode at low temperature to 2D growth at higher temperature was observed. The transition from the Volmer-Weber mechanism to the Stranski-Krastanov mechanism occurs at 200°C. The density of nucleation sites for the deposition of ITO on glass is also shown to increase as a function of substrate temperature. This result is consistent with thermally activated nucleation on the glass substrate.
1. Introduction
A considerable body of literature exists on the investigation of the initial growth dynamics of thin films. Clearly, the initial growth mode is important in determining the ultimate properties of the thin films. Many sophisticated techniques such as high resolution TEM, STM and AFM have been used [1,2]. These techniques are especially useful for lattice matched systems with epitaxial growth. Layerby-layer growth of the lattice can be seen in real time. On the other hand, less expensive techniques are needed to monitor the growth dynamics. Here, we describe an indirect technique to study the initial growth mode of ultrathin films based simply on in-situ resistance measurement. This technique has been applied in the past for studying the properties
* Corresponding author. Tel.: +852-23587056: fax: +85223581485; e-mail: [email protected].
of thin metallic films leading to important results on crystallization [3]. It has also been applied by us previously to study the dynamics of oxidation of high temperature superconducting oxide films [4] and their interface reactions with various substrates [5]. Such studies are possible because the resistivities are either dependent on the oxygen content or the crystalline structure of the thin films. This technique, though quite simple, is very powerful in obtaining detailed information of the growth process. Recently, in-situ resistance has been used to show that the growth of ultrathin indium tin oxide (ITO) films goes through different modes of tunneling, percolation and linear growth [6]. In this paper, we apply the in-situ resistance technique to examine the very early stage of ITO thin film growth on glass. ITO on glass is an important system to study as such films are used in huge quantities in various flat panel displays such as LCD (liquid crystal displays). It is well-known that for
0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S01 6 9 - 4 3 3 2 ( 9 6 ) 0 0 3 76-5
52
X.W. Sun et al./Applied Surface Science 106 (1996) 51-54
such grossly mismatched systems, the initial growth is via island formation. How the islands grow and coalesce is the subject of the present study. We shall show that below a substrate temperature of 200°C, the islands grow in 3 dimensions similar to the Volmer-Weber mechanism. Above that temperature, the growth is 2 dimensional according to a Stranski-Krastinov mechanism. Moreover, it will be shown that the density of nucleation sites for such island growth increases with substrate temperature. This latter result is intuitively obvious as nucleation is enhanced by the activation energy of the site.
IN-SITU RESISTMTY MEASUREMENT
COMPUTER
T VOLTMETER
Rotating target
Substrate with two silver strips Laser
2. Experimental The thin films were deposited by excimer pulsed laser deposition (PLD). We have previously shown that this technique is capable of growing high quality ITO films at room temperature [7]. Essentially, an ArF laser was focused onto an ITO target with a fluence of about ! J / c m 2 in a vacuum chamber. The film was deposited onto a heated substrate at a distance of 6.8 mm from the target. The rotating target was a 25 mm diameter sintered ceramic disk with 10% SnO 2 and 90% InO 2. The chamber was first pumped down to 7.6 × 10 6 Torr and then backfilled with oxygen at the proper pressure. For PLD of oxides, we have shown previously that the optimum ambient oxygen pressure is dependent on the substrate temperature [7]. For example, at 20°C, the 02 pressure was 20 mTorr. The resistance of the film was measured using a 4-probe technique during deposition as shown in Fig. 1. A pair of aluminum strips were deposited on the substrate prior to deposition. The area monitored, i.e. the gap between the aluminum pads, was typically 1 mm long and 5 mm wide. This geometry allowed higher resistivity and hence thinner films to be measured. A current source was used to bias the substrate and the voltage produced across the film was measured and recorded by a computer. The laser repetition rate was typically 10 Hz, allowing ample time for the data acquisition system to digitize the results. It is estimated that our system is capable of measuring the resistance of a 1 nm thick film with a resistivity of 1 f~ cm or less. This sensitivity is more than sufficient for studying many materials including
Current source Fig. I. Experimental setup for in-situ resistance measurements.
semiconductors and metal films. It is definitely sufficient to study the initial growth of ITO with effective thickness less than 0.1 nm.
3. Results and discussions Fig. 2 shows the composite result of measurements at several substrate temperatures ranging from 20°C to 350°C. It can be seen that there is an initial period where the 'film' does not conduct. There is an exponential increase in conductivity after a certain Film thickness (A)
108 0.5 E o i-
5
50
101
10 2
500
106 104
u}
102 100
10 3
Time (sec.) Fig. 2. Resistance of ITO films as a function of film thickness for various substrate temperatures. The curves are in successive order: 20, 50, 100, 150, 200, 250, 300 and 350°C.
X.W. Sun et al./Applied Surface Science 106 (1996) 51-54
amount of material is deposited. It is obvious that the initial region of no conductivity is due to island formation. The islands of ITO are sufficiently isolated so that there is no current possible, by tunneling or otherwise. Ref. [5] has discussed the region of exponential conductance growth carefully and showed that it can be fitted with a theory of tunneling conduction, followed by percolation of the islands and eventually leading to ohmic conduction. In this paper, we are concerned mainly with the earlier regime before the onset of conductivity. The onset of conductivity is related to how the individual islands grow initially. The critical thickness can be defined as the effective thickness of the film before the onset of conductivity. The effective thickness is simply the volume of the material deposited divided by the area of the film. Fig. 3 plots the critical thickness as a function of substrate temperature of the film. It can be seen that below 200°C, the critical thickness decreases monotonically. Above 200°C, the critical thickness for conduction approaches a constant of 1.2 rim. The interpretation is as follows. At any temperature, the initial stage is always island formation on the glass substrate. These islands will grow and coalesce as usual. Conductivity is measured when the islands are close enough either to provide a tunneling current or forms a conductive path. Below 200°C, these islands grow in 3D according to the Volmer-Weber mechanism. Above 200°C, these islands become typically 1 unit cell thick and than grow laterally in a 2D manner according to the Stranski-Krastonov mechanism. The argument is quite straight-forward. In 2D
50 .<40o 30
o
~_ .1=
o 0
0
0
O
"~ 10 0
0
1O0
i
200
300
400
Substrate temperature (°C)
Fig. 3. Critical thickness for conducting films as a function of substrate temperature.
ii
II
I
f
OOO OO Fig. 4. Illustrative models for nucleated growth of ITO thin films. Upper: 2D growth; lower: 3D growth.
growth, the area of the effective film scales as the total area of the film. Hence the onset of conductivity does not depend on the separation of the nucleation sites. This argument can be illustrated with a simple model for nucleated growth as shown in Fig. 4. It is assumed that the nucleation sites are distributed in a hexagonal close pack manner with a site separation distance of d. Furthermore, it is assumed that the islands are of thickness d o before they grow laterally. Then it is easy to show that when the islands touch, the effective thickness of the system is equal to and is independent of d. Hence the critical thickness is independent of d. The near constancy of the critical thickness above 200°C in Fig. 3 implies that the growth of the island in this region is 2D. On the other hand, if the islands grow 3 dimensionally, then it should take more materials deposited for the islands to be connected. That is, the critical thickness should be larger. Moreover, the critical thickness will now depend on the nucleation site separation. Again, using the simple nucleation pattern as shown in the lower part of Fig. 4, and assuming that the islands are half spherical in shape when they touch, then the effective thickness will be equal to If the density of nucleation site changes, the measured critical thickness will also change. This is the case for substrate temperatures below 200°C. One can also infer from Fig. 3 that the number of nucleation site increases with temperature. This is intuitively obvious since nucleation is a thermally activated process. Higher temperature will imply that more sites will be activated.
7rdo/2f3
~d/6~/~.
o 20
53
54
X.W. Sun et a l . / Applied Sur~tce Science 106 (1996) 51-54
4. Conclusions In the present study, in-situ resistance is employed to study the onset of conduction during the early growth stage of ITO films. It is found that growth of ITO on glass always starts with the formation of islands on activated nucleation sites on the substrate. These islands grow laterally as well as in height (3D Volmer-Weber mechanism) when the substrate temperature is low. At higher temperatures, the islands grow only laterally in a 2D StranskiKrastonov manner. The transition temperature occurs at 200°C. Such behavior is consistent with the general notion of thin film growth on unlattice-matched substrates. Our results demonstrate that in-situ resis-
tance is a powerful technique that can be used to study the growth of thin films.
References [1] R.Q. Hwang, J. Schroder, C. Gunther and R.J. Behm, Phys. Rev. Lett. 67 (1991) 3279. [2] W.M. Tong, E.J. Snyder, R.S. Williams, A. Yanase, Y. Segawa and M.S. Anderson, Surf. Sci. 277 (1992) L63. [3] A. Hoareau, J.X. Hu, P. Jensen, P. Melinon. M. Treilleux and B. Cabaud, Thin Solid Films 209 (1992) 161. [4] Q.Y. Ying, H.S. Kim, D.T. Shaw and H.S. Kwok, Appl. Phys. Lett. 55 (1989) 1041. [5] H.S. Kwok and Q.Y. Ying, Physica C 177 (1991) 122. [6] K. Korobov, M. Leibovitch and Y. Shapira, Appl. Phys. Lett. 65 (1994) 2290. [7] J.P. Zheng and H.S. Kwok, Appl. Phys. Lett. 63 (1993) I.
Applied Surface Science 106 (1996) 51-54
In-situ resistance measurements during pulsed laser deposition of ultrathin films X.W. Sun, H.C. Huang, H.S. K w o k * Department c~[Electrical and Electronic Engineering, Hong Kong Unit'ersiO' of Science and Technology, Clearwater Bay, Hong Kong Received 17 September 1995; accepted 1 November 1995
Abstract
In-situ resistance measurement is a powerful technique for monitoring the growth and properties of thin films. In this paper, we showed that it can be applied to study the initial growth mechanisms of indium tin oxide on glass. An interesting transition from a 3D growth mode at low temperature to 2D growth at higher temperature was observed. The transition from the Volmer-Weber mechanism to the Stranski-Krastanov mechanism occurs at 200°C. The density of nucleation sites for the deposition of ITO on glass is also shown to increase as a function of substrate temperature. This result is consistent with thermally activated nucleation on the glass substrate.
1. Introduction
A considerable body of literature exists on the investigation of the initial growth dynamics of thin films. Clearly, the initial growth mode is important in determining the ultimate properties of the thin films. Many sophisticated techniques such as high resolution TEM, STM and AFM have been used [1,2]. These techniques are especially useful for lattice matched systems with epitaxial growth. Layerby-layer growth of the lattice can be seen in real time. On the other hand, less expensive techniques are needed to monitor the growth dynamics. Here, we describe an indirect technique to study the initial growth mode of ultrathin films based simply on in-situ resistance measurement. This technique has been applied in the past for studying the properties
* Corresponding author. Tel.: +852-23587056: fax: +85223581485; e-mail: [email protected].
of thin metallic films leading to important results on crystallization [3]. It has also been applied by us previously to study the dynamics of oxidation of high temperature superconducting oxide films [4] and their interface reactions with various substrates [5]. Such studies are possible because the resistivities are either dependent on the oxygen content or the crystalline structure of the thin films. This technique, though quite simple, is very powerful in obtaining detailed information of the growth process. Recently, in-situ resistance has been used to show that the growth of ultrathin indium tin oxide (ITO) films goes through different modes of tunneling, percolation and linear growth [6]. In this paper, we apply the in-situ resistance technique to examine the very early stage of ITO thin film growth on glass. ITO on glass is an important system to study as such films are used in huge quantities in various flat panel displays such as LCD (liquid crystal displays). It is well-known that for
0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S01 6 9 - 4 3 3 2 ( 9 6 ) 0 0 3 76-5
52
X.W. Sun et al./Applied Surface Science 106 (1996) 51-54
such grossly mismatched systems, the initial growth is via island formation. How the islands grow and coalesce is the subject of the present study. We shall show that below a substrate temperature of 200°C, the islands grow in 3 dimensions similar to the Volmer-Weber mechanism. Above that temperature, the growth is 2 dimensional according to a Stranski-Krastinov mechanism. Moreover, it will be shown that the density of nucleation sites for such island growth increases with substrate temperature. This latter result is intuitively obvious as nucleation is enhanced by the activation energy of the site.
IN-SITU RESISTMTY MEASUREMENT
COMPUTER
T VOLTMETER
Rotating target
Substrate with two silver strips Laser
2. Experimental The thin films were deposited by excimer pulsed laser deposition (PLD). We have previously shown that this technique is capable of growing high quality ITO films at room temperature [7]. Essentially, an ArF laser was focused onto an ITO target with a fluence of about ! J / c m 2 in a vacuum chamber. The film was deposited onto a heated substrate at a distance of 6.8 mm from the target. The rotating target was a 25 mm diameter sintered ceramic disk with 10% SnO 2 and 90% InO 2. The chamber was first pumped down to 7.6 × 10 6 Torr and then backfilled with oxygen at the proper pressure. For PLD of oxides, we have shown previously that the optimum ambient oxygen pressure is dependent on the substrate temperature [7]. For example, at 20°C, the 02 pressure was 20 mTorr. The resistance of the film was measured using a 4-probe technique during deposition as shown in Fig. 1. A pair of aluminum strips were deposited on the substrate prior to deposition. The area monitored, i.e. the gap between the aluminum pads, was typically 1 mm long and 5 mm wide. This geometry allowed higher resistivity and hence thinner films to be measured. A current source was used to bias the substrate and the voltage produced across the film was measured and recorded by a computer. The laser repetition rate was typically 10 Hz, allowing ample time for the data acquisition system to digitize the results. It is estimated that our system is capable of measuring the resistance of a 1 nm thick film with a resistivity of 1 f~ cm or less. This sensitivity is more than sufficient for studying many materials including
Current source Fig. I. Experimental setup for in-situ resistance measurements.
semiconductors and metal films. It is definitely sufficient to study the initial growth of ITO with effective thickness less than 0.1 nm.
3. Results and discussions Fig. 2 shows the composite result of measurements at several substrate temperatures ranging from 20°C to 350°C. It can be seen that there is an initial period where the 'film' does not conduct. There is an exponential increase in conductivity after a certain Film thickness (A)
108 0.5 E o i-
5
50
101
10 2
500
106 104
u}
102 100
10 3
Time (sec.) Fig. 2. Resistance of ITO films as a function of film thickness for various substrate temperatures. The curves are in successive order: 20, 50, 100, 150, 200, 250, 300 and 350°C.
X.W. Sun et al./Applied Surface Science 106 (1996) 51-54
amount of material is deposited. It is obvious that the initial region of no conductivity is due to island formation. The islands of ITO are sufficiently isolated so that there is no current possible, by tunneling or otherwise. Ref. [5] has discussed the region of exponential conductance growth carefully and showed that it can be fitted with a theory of tunneling conduction, followed by percolation of the islands and eventually leading to ohmic conduction. In this paper, we are concerned mainly with the earlier regime before the onset of conductivity. The onset of conductivity is related to how the individual islands grow initially. The critical thickness can be defined as the effective thickness of the film before the onset of conductivity. The effective thickness is simply the volume of the material deposited divided by the area of the film. Fig. 3 plots the critical thickness as a function of substrate temperature of the film. It can be seen that below 200°C, the critical thickness decreases monotonically. Above 200°C, the critical thickness for conduction approaches a constant of 1.2 rim. The interpretation is as follows. At any temperature, the initial stage is always island formation on the glass substrate. These islands will grow and coalesce as usual. Conductivity is measured when the islands are close enough either to provide a tunneling current or forms a conductive path. Below 200°C, these islands grow in 3D according to the Volmer-Weber mechanism. Above 200°C, these islands become typically 1 unit cell thick and than grow laterally in a 2D manner according to the Stranski-Krastonov mechanism. The argument is quite straight-forward. In 2D
50 .<40o 30
o
~_ .1=
o 0
0
0
O
"~ 10 0
0
1O0
i
200
300
400
Substrate temperature (°C)
Fig. 3. Critical thickness for conducting films as a function of substrate temperature.
ii
II
I
f
OOO OO Fig. 4. Illustrative models for nucleated growth of ITO thin films. Upper: 2D growth; lower: 3D growth.
growth, the area of the effective film scales as the total area of the film. Hence the onset of conductivity does not depend on the separation of the nucleation sites. This argument can be illustrated with a simple model for nucleated growth as shown in Fig. 4. It is assumed that the nucleation sites are distributed in a hexagonal close pack manner with a site separation distance of d. Furthermore, it is assumed that the islands are of thickness d o before they grow laterally. Then it is easy to show that when the islands touch, the effective thickness of the system is equal to and is independent of d. Hence the critical thickness is independent of d. The near constancy of the critical thickness above 200°C in Fig. 3 implies that the growth of the island in this region is 2D. On the other hand, if the islands grow 3 dimensionally, then it should take more materials deposited for the islands to be connected. That is, the critical thickness should be larger. Moreover, the critical thickness will now depend on the nucleation site separation. Again, using the simple nucleation pattern as shown in the lower part of Fig. 4, and assuming that the islands are half spherical in shape when they touch, then the effective thickness will be equal to If the density of nucleation site changes, the measured critical thickness will also change. This is the case for substrate temperatures below 200°C. One can also infer from Fig. 3 that the number of nucleation site increases with temperature. This is intuitively obvious since nucleation is a thermally activated process. Higher temperature will imply that more sites will be activated.
7rdo/2f3
~d/6~/~.
o 20
53
54
X.W. Sun et a l . / Applied Sur~tce Science 106 (1996) 51-54
4. Conclusions In the present study, in-situ resistance is employed to study the onset of conduction during the early growth stage of ITO films. It is found that growth of ITO on glass always starts with the formation of islands on activated nucleation sites on the substrate. These islands grow laterally as well as in height (3D Volmer-Weber mechanism) when the substrate temperature is low. At higher temperatures, the islands grow only laterally in a 2D StranskiKrastonov manner. The transition temperature occurs at 200°C. Such behavior is consistent with the general notion of thin film growth on unlattice-matched substrates. Our results demonstrate that in-situ resis-
tance is a powerful technique that can be used to study the growth of thin films.
References [1] R.Q. Hwang, J. Schroder, C. Gunther and R.J. Behm, Phys. Rev. Lett. 67 (1991) 3279. [2] W.M. Tong, E.J. Snyder, R.S. Williams, A. Yanase, Y. Segawa and M.S. Anderson, Surf. Sci. 277 (1992) L63. [3] A. Hoareau, J.X. Hu, P. Jensen, P. Melinon. M. Treilleux and B. Cabaud, Thin Solid Films 209 (1992) 161. [4] Q.Y. Ying, H.S. Kim, D.T. Shaw and H.S. Kwok, Appl. Phys. Lett. 55 (1989) 1041. [5] H.S. Kwok and Q.Y. Ying, Physica C 177 (1991) 122. [6] K. Korobov, M. Leibovitch and Y. Shapira, Appl. Phys. Lett. 65 (1994) 2290. [7] J.P. Zheng and H.S. Kwok, Appl. Phys. Lett. 63 (1993) I.