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Surface electromigration of Au ultrathin film on MoS2
Applied Surface Science 169±170 (2001) 485±488
Surface electromigration of Au ultrathin ®lm on MoS2 Nan-Jian Wu*, S. Shimizu, M.T. Hermie, K. Sakamoto, A. Natori, H. Yasunaga The University of Electro-Communications, Chofu-shi, Tokyo 182-8585, Japan Received 2 August 1999; accepted 28 September 1999
Abstract The mass transport of Au ultrathin ®lm on a semiconductor MoS2 was investigated by atomic force microscopy (AFM) and scanning Auger microscopy (SAM). The surface electromigration of the Au ®lm was found when a dc current was passed through the MoS2 substrate. The Au ultrathin ®lm on MoS2 grew in a typical Volmer±Weber (V±W) growth mode, The AFM measurements indicated that the distribution of the Au islands exhibited clearly a preferential lateral spread towards the cathode, that is, the surface electromigration took place. The direction of the surface electromigration on MoS2 is opposite to that of the Au electromigration on Si. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Surface electromigration; Au; MoS2; Mass transport
1. Introduction Surface electromigration is a preferential mass transport towards either the cathode or the anode on a clean semiconductor surface heated by a dc current. It attracts special interest because it plays an important role in dynamic processes on semiconductor surfaces and it is fundamentally different from the conventional electromigration in bulk metals. So far the surface electromigration of metals on Si and Ge has been widely studied [1±3]. The metals on Si and Ge exhibited the Stranski±Krastanov (S±K) growth mode: the adatoms form an intermediate layer at ®rst and then congregate into small islands on the layer. Because the electromigration of the thin ®lm in the S±K growth mode is related with the intermediate layer, it is more dif®cult to clarify the mechanism of the surface electromigration. * Corresponding author. E-mail address: [email protected] (N.-J. Wu).
This paper deals with a new metal/semiconductor system in Volmer±Weber (V±W) growth mode for studying the surface electromigration. We employed a semiconductor MoS2 as the substrate and Au as the migrating species. The MoS2 crystal has layer structure and is composed of a number of S±Mo±S sandwich layers which are bonded weakly by van der Waals force. Hence, cleaving MoS2 between the layers results in a chemically inert surface which exposes only S atoms. As is well known, the Au ®lm on MoS2 grows in V±W growth mode because the interaction between the Au adatom and the surface S atom is weaker than that between the Au adatoms [4,5]. We investigated the surface electromigration of Au on a natural MoS2 substrate and here report the results. 2. Experimental The natural MoS2 crystal was cleaved in air. We made Hall effect measurements at room temperature.
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 4 2 - X
486
N.-J. Wu et al. / Applied Surface Science 169±170 (2001) 485±488
It was obtained that the natural MoS2 is n-type and its resistivity is 7 O cm (n 8 1015 /cm3, m 110 cm2/V s). The cleaved sample (3 mm3 6 mm3 0:2 mm3 ) was mounted on a ceramic sample holder and was inserted into an ultra-high vacuum apparatus. At ®rst the surfaces were cleaned by passing dc current through the substrates at 920 K for 30 min. No impurity peak was observed in the Auger electron spectrum after the thermal treatment. The surface structure was observed by in situ low-energy electron diffraction (LEED) and ex situ atomic force microscope (AFM). The LEED pattern consists of a 1 1 structure. The AFM images shows clearly the atomic Ê in spacing. arrangement of a hexagonal lattice of 3.2 A From the above results, it can be considered that a clean surface of MoS2 was obtained and the reconstruction did not occur on the MoS2 surface. Then the Au ®lm was evaporated on MoS2 through a metal mask with a slit of 5 mm 100 mm. The thickness of the ®lm was monitored by a quart microbalance. The rate of the evaporation is about 0.75 ML/min, where 1 ML corresponds to 1:2 1015 atoms/cm2. When a dc current was applied to the MoS2 through the tantalum electrodes, the accompanying temperature rise of the substrate surface was measured by irradiation thermometer IR308. The evolution of the pro®les of the Au ®lm was recorded intermittently by scanning Auger microscopy (SAM). After the mentioned procedures were made in situ under ultrahigh vacuum with a typical base pressure less than 2 10ÿ7 Pa, the samples were at once taken out in air and transferred to the AFM chamber with a base pressure of 10ÿ3 Pa. The topography of the Au ®lm were investigated by a contact-mode AFM. In order to avoid the movement of the Au islands induced by scanning tip or cantilever [4,5], we selected a soft Si3N4 cantilever with a small spring constant of 0.02 N/m to scan the samples. 3. Results and discussion The time evolution of the mass transport and growth of the Au ®lms was investigated by intermittently recording the Auger line analysis. Fig. 1 shows the results of the Auger line pro®le of 1 ML Au ®lm on MoS2. The con®guration of the Au ®lm, dc current direction and the scanning line is indicated in the inset of Fig. 1. The applied dc current density and the
Fig. 1. The time evolution of Auger line pro®le of 1 ML Au ultrathin ®lm on MoS2 at 873 K. The inset shows the con®guration of the Au ®lm, dc current direction and scanning line of the electron beam.
sample temperature are 105 A/m2 and 873 K, respectively. The Auger line pro®les exhibited that the Auger peak height (APH) of Au was reduced initially to about half of APH of the as-deposited Au ®lm after application of dc current through the substrate. Then the APH deceased slowly with the time of the application of dc current. The width of the region, in which the Au peak can be detected, is almost the same as that of the mask slit. The spread of Au ®lm on MoS2, as observed in metals in S±K growth mode on Si and Ge, was not observed by Auger line analysis. This implies that measuring the mass transport of the ®lm in V±W growth mode is more dif®cult than that in S±K growth mode because there is not the Au intermediate layer on MoS2. The ex situ AFM observation of the Au/MoS2 samples was carried out. Because the scanning range (x, y) of the AFM piezoelectric tube scanner used is less than 80 mm, we had to manipulate the precise x±y stage of the scanner to scan wider area. In the method, a lot of the AFM images of the Au/MoS2 are successively taken along the direction of the dc current. Fig. 2 shows only the typical AFM images of the 1 ML Au/ MoS2 samples, corresponding to the small rectangular areas at coordinates: ÿ20 mm, 0 mm, 50 mm, 100 mm, 200 mm and 300 mm in Fig. 2(c). The AFM images, as shown in Fig. 2(a), of the as-deposited Au/MoS2 sample indicates that the as-deposited Au ®lm exhibits a grainy structure and that the width of the Au ®lm is
N.-J. Wu et al. / Applied Surface Science 169±170 (2001) 485±488
487
Fig. 2. AFM images of (a) the as-deposited 1 ML Au/MoS2 surface and (b) the 1 ML Au/MoS2 surface heated at 873 K by a supply of dc current for 180 min. The Fig. 2(c) indicates the scanning areas of Au/MoS2, corresponding to the AFM images.
about 120 mm, that is the same as that obtained by the SAM line analysis. The AFM images of the Au/MoS2 sample, as shown in Fig. 2(b), exhibit the following results. At ®rst the grainy-structure Au ®lm transformed into Au islands initially (the atomic arrangement of MoS2 was observed between the Au islands) when the substrate MoS2 was heated by the supply of the dc current. The initial reduction of the Au APH can be explained by the transformation from the amorphous Au ®lm into the islands. This is because that the fraction of surface area covered by the Au islands is much smaller than that covered by the amorphous Au ®lm. Secondly the Au ®lm spread towards both the cathode and anode. However, the length of the spread towards the cathode was longer than that towards the anode, that is, the Au ®lm spread preferentially towards the cathode. This demonstrates that the surface electromigration of Au ®lm on MoS2 towards the cathode took place due to the supply of the dc current. To our knowledge, this is
the ®rst measurement succeeding in observing the electromigration of the metal ®lm in V±W growth mode on the semiconductor surface. In addition, the direction of the Au electromigration on MoS2 is opposite to that of the Au electromigration on Si(1 1 1). To quantitatively analyze the Au electromigration and compare the results with that of the Auger line analysis, we approximately estimated the Au island coverage on the MoS2 surface and the shape of the islands. The height (about 3.5 nm) of the Au island in the deposited region is larger than that of the island out of the deposited region. The height of the island out of the deposited region decreased from the boundary of the deposited region to the edge of the spreaded Au islands in the cathode. The heights of the island near the deposited region and at the edge of the spreaded Au islands are about 1.5 and 0.3 nm, respectively. But, the base area of the Au island out of the deposited region is larger than that within the deposited region.
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N.-J. Wu et al. / Applied Surface Science 169±170 (2001) 485±488
and holds the positive charge(s), the electrostatic force acts on the ionic charge and pushes the Au adatom or island to migrate towards the cathode. Although we observed the Au electromigration on MoS2, the driving force that prevails in the system is not yet certain. In order to clarify the driving force, we will carry out further work on the temperature dependence of the electromigration, the surface conductivity at higher temperature and the charge transfer between the Au atom and MoS2. Fig. 3. The distance dependence of the Au coverage on MoS2 at temperatures of 873 and 948 K. The coverage is de®ned as the ratio of the sum of the base areas of the Au islands to the area of each AFM image.
Fig. 3 shows the distance dependence of the Au coverage (the height of the Au islands is not taken into account) on MoS2 at different temperatures. Although the Au island coverage decreased rapidly from the inside to the outside of the deposited region, the distribution of the Au island exhibited clearly a preferential lateral spread toward the cathode. The fraction of surface area covered by the Au islands is smaller than 10% out of the deposited region so that the Au Auger peak was dif®cultly observed, as shown in Fig. 1. Furthermore, the Au electromigration on MoS2 at higher temperature (948 K) is greater than that at 873 K. This enhancement is due to the increase of electric current for heating. As in the conventional electromigration in bulk and thin-®lm metals, the surface electromigration of Au on MoS2 is considered to be driven by the carrier (hole) wind force or electrostatic force. The wind force transfers the momentum by collision from current carriers to mobile Au adatoms or islands. On the other hand, assuming that the Au adatom or island ionizes
4. Summary The mass transport of Au ultrathin ®lm on a semiconductor MoS2 was investigated by AFM and SAM. The surface electromigration of the Au ®lm was found when a dc current was passed through the MoS2 substrate. The AFM observations clari®ed that the Au ultrathin ®lm on MoS2 grows in a typical Volmer±Weber (V±W) growth mode and the distribution of the Au islands exhibited clearly a preferential lateral spread towards the cathode. The direction of the surface electromigration on MoS2 is opposite to that of the Au electromigration on Si.
References [1] H. Yasunaga, A. Natori, Surf. Sci. Rep. 15 (1992) 205, and references cited therein. [2] K. Sakamoto, A. Natori, H. Yasunaga, in: Proceedings of the 13th International Vacuum and 9th International Conference on Solid Surface, SS-WeP, 1995, p. 61. [3] S. Kono, T. Goto, T. Ogura, T. Abukawa, Surf. Sci. 420 (1998) 200. [4] K. Uozumi, J. Microsc. 152 (1988) 193. [5] T. Ichinokawa, T. Ichinose, M. Tohyama, H. Itoh, J. Vac. Sci. Technol. A8 (1990) 500.
Surface electromigration of Au ultrathin ®lm on MoS2 Nan-Jian Wu*, S. Shimizu, M.T. Hermie, K. Sakamoto, A. Natori, H. Yasunaga The University of Electro-Communications, Chofu-shi, Tokyo 182-8585, Japan Received 2 August 1999; accepted 28 September 1999
Abstract The mass transport of Au ultrathin ®lm on a semiconductor MoS2 was investigated by atomic force microscopy (AFM) and scanning Auger microscopy (SAM). The surface electromigration of the Au ®lm was found when a dc current was passed through the MoS2 substrate. The Au ultrathin ®lm on MoS2 grew in a typical Volmer±Weber (V±W) growth mode, The AFM measurements indicated that the distribution of the Au islands exhibited clearly a preferential lateral spread towards the cathode, that is, the surface electromigration took place. The direction of the surface electromigration on MoS2 is opposite to that of the Au electromigration on Si. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Surface electromigration; Au; MoS2; Mass transport
1. Introduction Surface electromigration is a preferential mass transport towards either the cathode or the anode on a clean semiconductor surface heated by a dc current. It attracts special interest because it plays an important role in dynamic processes on semiconductor surfaces and it is fundamentally different from the conventional electromigration in bulk metals. So far the surface electromigration of metals on Si and Ge has been widely studied [1±3]. The metals on Si and Ge exhibited the Stranski±Krastanov (S±K) growth mode: the adatoms form an intermediate layer at ®rst and then congregate into small islands on the layer. Because the electromigration of the thin ®lm in the S±K growth mode is related with the intermediate layer, it is more dif®cult to clarify the mechanism of the surface electromigration. * Corresponding author. E-mail address: [email protected] (N.-J. Wu).
This paper deals with a new metal/semiconductor system in Volmer±Weber (V±W) growth mode for studying the surface electromigration. We employed a semiconductor MoS2 as the substrate and Au as the migrating species. The MoS2 crystal has layer structure and is composed of a number of S±Mo±S sandwich layers which are bonded weakly by van der Waals force. Hence, cleaving MoS2 between the layers results in a chemically inert surface which exposes only S atoms. As is well known, the Au ®lm on MoS2 grows in V±W growth mode because the interaction between the Au adatom and the surface S atom is weaker than that between the Au adatoms [4,5]. We investigated the surface electromigration of Au on a natural MoS2 substrate and here report the results. 2. Experimental The natural MoS2 crystal was cleaved in air. We made Hall effect measurements at room temperature.
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 4 2 - X
486
N.-J. Wu et al. / Applied Surface Science 169±170 (2001) 485±488
It was obtained that the natural MoS2 is n-type and its resistivity is 7 O cm (n 8 1015 /cm3, m 110 cm2/V s). The cleaved sample (3 mm3 6 mm3 0:2 mm3 ) was mounted on a ceramic sample holder and was inserted into an ultra-high vacuum apparatus. At ®rst the surfaces were cleaned by passing dc current through the substrates at 920 K for 30 min. No impurity peak was observed in the Auger electron spectrum after the thermal treatment. The surface structure was observed by in situ low-energy electron diffraction (LEED) and ex situ atomic force microscope (AFM). The LEED pattern consists of a 1 1 structure. The AFM images shows clearly the atomic Ê in spacing. arrangement of a hexagonal lattice of 3.2 A From the above results, it can be considered that a clean surface of MoS2 was obtained and the reconstruction did not occur on the MoS2 surface. Then the Au ®lm was evaporated on MoS2 through a metal mask with a slit of 5 mm 100 mm. The thickness of the ®lm was monitored by a quart microbalance. The rate of the evaporation is about 0.75 ML/min, where 1 ML corresponds to 1:2 1015 atoms/cm2. When a dc current was applied to the MoS2 through the tantalum electrodes, the accompanying temperature rise of the substrate surface was measured by irradiation thermometer IR308. The evolution of the pro®les of the Au ®lm was recorded intermittently by scanning Auger microscopy (SAM). After the mentioned procedures were made in situ under ultrahigh vacuum with a typical base pressure less than 2 10ÿ7 Pa, the samples were at once taken out in air and transferred to the AFM chamber with a base pressure of 10ÿ3 Pa. The topography of the Au ®lm were investigated by a contact-mode AFM. In order to avoid the movement of the Au islands induced by scanning tip or cantilever [4,5], we selected a soft Si3N4 cantilever with a small spring constant of 0.02 N/m to scan the samples. 3. Results and discussion The time evolution of the mass transport and growth of the Au ®lms was investigated by intermittently recording the Auger line analysis. Fig. 1 shows the results of the Auger line pro®le of 1 ML Au ®lm on MoS2. The con®guration of the Au ®lm, dc current direction and the scanning line is indicated in the inset of Fig. 1. The applied dc current density and the
Fig. 1. The time evolution of Auger line pro®le of 1 ML Au ultrathin ®lm on MoS2 at 873 K. The inset shows the con®guration of the Au ®lm, dc current direction and scanning line of the electron beam.
sample temperature are 105 A/m2 and 873 K, respectively. The Auger line pro®les exhibited that the Auger peak height (APH) of Au was reduced initially to about half of APH of the as-deposited Au ®lm after application of dc current through the substrate. Then the APH deceased slowly with the time of the application of dc current. The width of the region, in which the Au peak can be detected, is almost the same as that of the mask slit. The spread of Au ®lm on MoS2, as observed in metals in S±K growth mode on Si and Ge, was not observed by Auger line analysis. This implies that measuring the mass transport of the ®lm in V±W growth mode is more dif®cult than that in S±K growth mode because there is not the Au intermediate layer on MoS2. The ex situ AFM observation of the Au/MoS2 samples was carried out. Because the scanning range (x, y) of the AFM piezoelectric tube scanner used is less than 80 mm, we had to manipulate the precise x±y stage of the scanner to scan wider area. In the method, a lot of the AFM images of the Au/MoS2 are successively taken along the direction of the dc current. Fig. 2 shows only the typical AFM images of the 1 ML Au/ MoS2 samples, corresponding to the small rectangular areas at coordinates: ÿ20 mm, 0 mm, 50 mm, 100 mm, 200 mm and 300 mm in Fig. 2(c). The AFM images, as shown in Fig. 2(a), of the as-deposited Au/MoS2 sample indicates that the as-deposited Au ®lm exhibits a grainy structure and that the width of the Au ®lm is
N.-J. Wu et al. / Applied Surface Science 169±170 (2001) 485±488
487
Fig. 2. AFM images of (a) the as-deposited 1 ML Au/MoS2 surface and (b) the 1 ML Au/MoS2 surface heated at 873 K by a supply of dc current for 180 min. The Fig. 2(c) indicates the scanning areas of Au/MoS2, corresponding to the AFM images.
about 120 mm, that is the same as that obtained by the SAM line analysis. The AFM images of the Au/MoS2 sample, as shown in Fig. 2(b), exhibit the following results. At ®rst the grainy-structure Au ®lm transformed into Au islands initially (the atomic arrangement of MoS2 was observed between the Au islands) when the substrate MoS2 was heated by the supply of the dc current. The initial reduction of the Au APH can be explained by the transformation from the amorphous Au ®lm into the islands. This is because that the fraction of surface area covered by the Au islands is much smaller than that covered by the amorphous Au ®lm. Secondly the Au ®lm spread towards both the cathode and anode. However, the length of the spread towards the cathode was longer than that towards the anode, that is, the Au ®lm spread preferentially towards the cathode. This demonstrates that the surface electromigration of Au ®lm on MoS2 towards the cathode took place due to the supply of the dc current. To our knowledge, this is
the ®rst measurement succeeding in observing the electromigration of the metal ®lm in V±W growth mode on the semiconductor surface. In addition, the direction of the Au electromigration on MoS2 is opposite to that of the Au electromigration on Si(1 1 1). To quantitatively analyze the Au electromigration and compare the results with that of the Auger line analysis, we approximately estimated the Au island coverage on the MoS2 surface and the shape of the islands. The height (about 3.5 nm) of the Au island in the deposited region is larger than that of the island out of the deposited region. The height of the island out of the deposited region decreased from the boundary of the deposited region to the edge of the spreaded Au islands in the cathode. The heights of the island near the deposited region and at the edge of the spreaded Au islands are about 1.5 and 0.3 nm, respectively. But, the base area of the Au island out of the deposited region is larger than that within the deposited region.
488
N.-J. Wu et al. / Applied Surface Science 169±170 (2001) 485±488
and holds the positive charge(s), the electrostatic force acts on the ionic charge and pushes the Au adatom or island to migrate towards the cathode. Although we observed the Au electromigration on MoS2, the driving force that prevails in the system is not yet certain. In order to clarify the driving force, we will carry out further work on the temperature dependence of the electromigration, the surface conductivity at higher temperature and the charge transfer between the Au atom and MoS2. Fig. 3. The distance dependence of the Au coverage on MoS2 at temperatures of 873 and 948 K. The coverage is de®ned as the ratio of the sum of the base areas of the Au islands to the area of each AFM image.
Fig. 3 shows the distance dependence of the Au coverage (the height of the Au islands is not taken into account) on MoS2 at different temperatures. Although the Au island coverage decreased rapidly from the inside to the outside of the deposited region, the distribution of the Au island exhibited clearly a preferential lateral spread toward the cathode. The fraction of surface area covered by the Au islands is smaller than 10% out of the deposited region so that the Au Auger peak was dif®cultly observed, as shown in Fig. 1. Furthermore, the Au electromigration on MoS2 at higher temperature (948 K) is greater than that at 873 K. This enhancement is due to the increase of electric current for heating. As in the conventional electromigration in bulk and thin-®lm metals, the surface electromigration of Au on MoS2 is considered to be driven by the carrier (hole) wind force or electrostatic force. The wind force transfers the momentum by collision from current carriers to mobile Au adatoms or islands. On the other hand, assuming that the Au adatom or island ionizes
4. Summary The mass transport of Au ultrathin ®lm on a semiconductor MoS2 was investigated by AFM and SAM. The surface electromigration of the Au ®lm was found when a dc current was passed through the MoS2 substrate. The AFM observations clari®ed that the Au ultrathin ®lm on MoS2 grows in a typical Volmer±Weber (V±W) growth mode and the distribution of the Au islands exhibited clearly a preferential lateral spread towards the cathode. The direction of the surface electromigration on MoS2 is opposite to that of the Au electromigration on Si.
References [1] H. Yasunaga, A. Natori, Surf. Sci. Rep. 15 (1992) 205, and references cited therein. [2] K. Sakamoto, A. Natori, H. Yasunaga, in: Proceedings of the 13th International Vacuum and 9th International Conference on Solid Surface, SS-WeP, 1995, p. 61. [3] S. Kono, T. Goto, T. Ogura, T. Abukawa, Surf. Sci. 420 (1998) 200. [4] K. Uozumi, J. Microsc. 152 (1988) 193. [5] T. Ichinokawa, T. Ichinose, M. Tohyama, H. Itoh, J. Vac. Sci. Technol. A8 (1990) 500.