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Studies on surface structures and growth of In ultrathin films on Mo(1 1 0) surface
Surface Science 566–568 (2004) 181–185 www.elsevier.com/locate/susc
Studies on surface structures and growth of In ultrathin films on Mo(1 1 0) surface A. Katoh, H. Miwa, Y. Maehara, H. Kawanowa, Y. Gotoh
*
Department of Materials Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Available online 4 June 2004
Abstract The surface structures and growth mode of In ultrathin films on Mo(1 1 0) surface have been studied by reflection high energy electron diffraction (RHEED). Three kinds of two-dimensional superstructures a, b and c were observed in nearly monolayer region of In thickness. The a structure exists stably in wide temperature range from RT to 600 C. This pffiffiffi surface structure is expressed as [1 0 1 4] in matrix notation and has an atomic arrangement of bcc(1 1 0) layer of 2 3=3 times as large as Mo(1 1 0) plane rotated by 54.7 resulting in inducing compression of In atom by 3.0% in ½0 0 1Mo orientation. The b structure expressed as [1 0 )2 7] in matrix notation appears from 600 to 700 C and has a smaller surface density than that of a structure. The c structure expressed as [1 )1 1 2] in matrix notation appears as high temperature phase from 700 to 800 C. SEM observation reveals that room temperature condensation takes place in accordance with Stranski–Krastanov growth mode. 2004 Elsevier B.V. All rights reserved. Keywords: Indium; Molybdenum; Scanning electron microscopy (SEM); Surface structure, morphology, roughness, and topography; Growth
1. Introduction According to the interfacial energy calculation of fcc/bcc, it has been known that the atomic diameter ratio of both materials plays an important role to the epitaxial orientation at the interface [1]. Indium has a comparatively large atomic diameter and has a specific face-centered tetragonal structure in bulk phase. Although many researches on structures of fcc/bcc interface are made, there are few studies on growth of deposit materials with specific structure such as In on bcc *
Corresponding author. Tel.: +81-47-124-1501x4329; fax: +81-47-123-9362. E-mail address: [email protected] (Y. Gotoh).
substrate [2–4]. In the present paper, the initial stage of epitaxial growth of In on Mo(1 1 0) surface investigated by RHEED is reported. 2. Experiment This experimental work was performed using an ultra high vacuum chamber (UHV) with RHEED apparatus whose pressure was in the order of 1010 Torr. The surface treatment of Mo(1 1 0) substrate was given by machine polish and electrolysis polish in solution 97%H2 SO4 + 3%H2 O. The method of surface cleaning of the substrate was made by annealing in oxygen. First, Mo substrate was annealed at 600 C in an oxygen atmosphere with a pressure of 106 Torr in order to form MoO2
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.05.042
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crystallites and then was heated at 1500 C for a few second in a vacuum of 1010 Torr to evaporate the oxide with impurities on the surface [5,6]. The clean surface of Mo structure was confirmed by the RHEED pattern of 1 · 1 structure with low background. In was deposited on the Mo(1 1 0) substrate by ohmic heating of W basket. After deposition on the Mo surface various monolayer thicknesses, followed by annealing at various temperatures from room temperature to 800 C for 20 s, the specimen was observed by RHEED. Thus, the surface phase diagram for temperature variation versus film thickness was obtained. SEM observation on deposited specimen was carried out to clarify the growth mode.
3. Results and discussion Three surface structures a, b and c were formed in an In/Mo(1 1 0) system in the temperature range from RT to 800 C. Fig. 1 shows the two-dimensional phase diagram of the surface structures for various In deposition thickness versus substrate annealing temperature. The value at 1 ML thickness which means the same number of In atom as Mo of substrate surface is 3.69 A.
Fig. 1. Diagram of the surface structures of the Mo(1 1 0) surface induced by the In deposition.
The a structure exists stably in wide temperature range from RT to 600 C as shown in Fig. 1. Fig. 2(a) shows the RHEED pattern with ½0 0 1Mo electron incidence due to the a structure with In deposition of 0.3 ML at 150 C. The 1/4 ordered reflections between 0 0 and 0 1 Mo reflections in the zeroth Laue Zone (L0 ) are due to In. The intensity distribution of each reflection suggest that the position 3/4 reflections in the zeroth Laue Zone correspond to that of the reciprocal rod of the a structure. The other streaks between 0 0 and 0 1 reflections of Mo in the zeroth Laue Zone are due to the multiple scattering. There are no reflections between zeroth Laue Zone and first one. The basic unit vector of the reciprocal lattice of the a structure is drawn in Fig. 2(b), in which open and dark circles show Mo and In twodimensional reciprocal rods, respectively. From this reciprocal unit mesh, a real unit mesh is obtained whose unit vectors aa and ba are 3.15 and and the angle is 109.5. Atomic model of 3.15 A, the a structure is shown in Fig. 5(a), where In and Mo atoms are superposed at the origin as on-top site. The atomic distance of In along ½0 0 1Mo direction is equal to that of Mo. The atomic density of the a structure is 10.7 · 1014 atom/cm2 . This structure forms a coincident site lattice (CSL) with Mo surface atoms, which is expressed as [1 0 1 4] in matrix notation, where Mo unit vectors are defined by a [0 0 1] and a [1 1 1], a being lattice constant. The same lengths of aa and aMo , and the specific angle in the a structure lead to the fact that the atomic configuration of In has an enlarged bcc(1 1 0) structure compared with Mo(1 1 0) surface. Using new unit vectors aX and bX , the a structurepisffiffiffi rotated from Mo(1 1 0) plane by 54.7 and is 2 3=3 times as large as Mo(1 1 0) plane. The In atoms in aa and ba directions are compressed by 3.0% respectively, which means that In atomic diameter is nearly kept. Such CSL structure with same atomic row distances of In and Mo along ½1 1 1Mo orientation may decrease interfacial energy. Because if the origin of In atom is moved to bridge site from on-top site, all In atoms will fall down to potential valley in Mo atomic groove. The b structure exists stably in a temperature range from 600 to 700 C as shown in Fig. 1. Fig. 3 shows the RHEED pattern with ½0 0 1Mo electron
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Fig. 2. (a) RHEED pattern of a structure taken from Mo(1 1 0) surface after In deposition. The electron incidence is parallel to the ½0 0 1Mo direction. (b) Reciprocal rod arrangement in a structure.
incidence parallel to the direction due to the a structure with In deposition of 0.4 ML at 600 C. The 1/7 ordered reflections between 0 0 and 0 1 Mo reflections in the zeroth Laue Zone (L0 ) are observed. There are no reflections between zeroth Laue Zone and first one. The unit vectors of this structure are 4/7 reflection in the zeroth Laue Zone and 1 0 reflection in the first Laue Zone. The other streaks between 0 0 and 0 1 reflections of Mo in the zeroth Laue Zone and between 1 0 and 1 1 reflections of Mo in the first Laue Zone are due to multiple scattering. From this reciprocal unit mesh, real unit mesh is obtained whose unit vec-
(¼ aMo ), 3.91 A and the tors ab and bb are 3.15 A angle is 95.8. Atomic model of the b structure is shown in Fig. 5(b). Each atom of In and Mo is superposed at the origin as on-top site. The atomic distance of Mo along ½0 0 1Mo direction is equal to that of In in the b structure just like the a structure. This structure also forms a CSL with Mo surface atoms, which is expressed as [1 0 )2 7] in matrix notation. The atomic density of the b structure is 8.16 · 1014 atom/cm2 , which is smaller than that of a structure. The c structure was observed in a temperature range from 700 to 800 C as shown in Fig. 1. Fig. 4 shows the RHEED pattern with ½0 0 1Mo electron
Fig. 3. RHEED pattern of b structure from Mo(1 1 0) surface induced by In. The electron incidence is parallel the ½0 0 1Mo orientation.
Fig. 4. RHEED pattern of c structure from Mo(1 1 0) surface induced by In. The electron incidence is parallel the ½0 0 1Mo orientation.
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incidence with In deposition of 0.3 ML at 700 C. The 1/3 ordered reflections between 0 1 Mo reflections in the zeroth Laue Zone (L0 ) and 1 1 Mo reflections in the first Laue Zone (L1 ), P and Q are due to the superstructure of In. From the reciprocal unit mesh shown by two vectors P and Q, real unit mesh is obtained whose unit vectors ac respectively and the angle is and bc are 4.45, 4.72 A 115.2. This structure is expressed as [1 )1 1 2] in matrix notation. The atomic arrangement of c structure is shown in Fig. 5(c). The atomic density of the c structure is 4.76 · 1014 atom/cm2 . The c structure has atomic density less than the b structure. With increasing of substrate temperature, In atoms composing b structure evaporate gradually from the Mo surface, c structure of more stable phase at this temperature will arise. Finally In evaporation is completed at 800 C on the Mo surface resulting in emergence of clean surface. Fig. 6 shows a SEM image of the specimen deposited with a 20 ML In thick at room temperature. The diameter of each island is nearly 1 lm. The average of distance between 3D In islands is 3– 10 lm. The shape of 3D islands is hexagonal and one side of the island is parallel to ½0 0 1Mo orientation. The observed angles between two ridge lines of In crystal are 61, 61 and 58. The crystal structure of In is face-centered-tetragonal c ¼ 4:94 A). From this geometri(a ¼ b ¼ 4:59 A, cal relation it is found that In crystallites grow with a stable shape with {1 1 1} upper face and {1 1 1}
Fig. 6. SEM image of the In crystallites grown on Mo(1 1 0) substrate at room temperature. The deposition thickness is 20 ML of In.
and {1 0 0} lateral faces, which reals the epitaxial orientation ð1 1 1ÞIn kð1 1 0ÞMo , ½0 0 1Mo k½1 1 0In . According to the RHEED observation In forms two-dimensional ordered phase of a structure on Mo substrate in wide temperature range from room temperature to 600 C and thickness range from submonolayer to 20 ML. On the other hand In crystallites were observed with the specimen deposited with 20 ML. After the a structure covers the Mo surface, 3D islands of In grow epitaxially on the a structure. Namely, In deposited on the Mo(1 1 0) surface grows in accordance with the Stranski–Krastanov growth mode in temperature range between room temperature and 150 C. The shape of 3D islands of In deposited on Mo(1 1 0) surface at temperatures above 200 C is spherical. As the substrate temperature is over the melting point of In, 3D crystals melt and solidify with nearly spherical shape with decreasing of substrate temperature.
4. Conclusions
Fig. 5. Real-space atomic models of the a, b and c structures shown in (a), (b) and (c) respectively. Open and dark circles indicate Mo and In atoms, respectively.
In the present study, three kinds of twodimensional ordered phases a, b and c of In on the Mo(1 1 0) surface are found at nearly 1 ML thickness, which are expressed as [1 0 0 4], [1 0 )2 7] and [1 )1 1 2] in matrix notation, respectively. The atomic distance of In along ½0 0 1Mo in a
A. Katoh et al. / Surface Science 566–568 (2004) 181–185
structure is consistent with that of Mo, and distance of In atomic rows along Æ1 1 1æ in these structures are consistent with that of MoÆ1 1 1æ orientation. The a structure has a bcc(1 1 0) layer pffiffiffi of 2 3=3 times as large as Mo(1 1 0) plane rotated by 54.7. Room temperature condensation of In on the Mo surface takes place in accordance with Stranski–Krastanov growth mode.
185
References [1] Y. Gotoh, S. Entani, H. Kawanowa, Surf. Sci. 507/510 (2002) 401. [2] Y. Gotoh, E. Yanokura, Surf. Sci. 269/270 (1992) 707. [3] S. Jo, Y. Gotoh, Surf. Sci. 454/456 (2000) 729. [4] S. Jo, Y. Gotoh, Surf. Sci. 464 (2000) 145. [5] T.W. Hass, A.G. Jackson, J. Chem. Phys. 44 (1966) 2921. [6] Y. Gotoh, E. Yanokura, Surf. Sci. 287/288 (1993) 979.
Studies on surface structures and growth of In ultrathin films on Mo(1 1 0) surface A. Katoh, H. Miwa, Y. Maehara, H. Kawanowa, Y. Gotoh
*
Department of Materials Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Available online 4 June 2004
Abstract The surface structures and growth mode of In ultrathin films on Mo(1 1 0) surface have been studied by reflection high energy electron diffraction (RHEED). Three kinds of two-dimensional superstructures a, b and c were observed in nearly monolayer region of In thickness. The a structure exists stably in wide temperature range from RT to 600 C. This pffiffiffi surface structure is expressed as [1 0 1 4] in matrix notation and has an atomic arrangement of bcc(1 1 0) layer of 2 3=3 times as large as Mo(1 1 0) plane rotated by 54.7 resulting in inducing compression of In atom by 3.0% in ½0 0 1Mo orientation. The b structure expressed as [1 0 )2 7] in matrix notation appears from 600 to 700 C and has a smaller surface density than that of a structure. The c structure expressed as [1 )1 1 2] in matrix notation appears as high temperature phase from 700 to 800 C. SEM observation reveals that room temperature condensation takes place in accordance with Stranski–Krastanov growth mode. 2004 Elsevier B.V. All rights reserved. Keywords: Indium; Molybdenum; Scanning electron microscopy (SEM); Surface structure, morphology, roughness, and topography; Growth
1. Introduction According to the interfacial energy calculation of fcc/bcc, it has been known that the atomic diameter ratio of both materials plays an important role to the epitaxial orientation at the interface [1]. Indium has a comparatively large atomic diameter and has a specific face-centered tetragonal structure in bulk phase. Although many researches on structures of fcc/bcc interface are made, there are few studies on growth of deposit materials with specific structure such as In on bcc *
Corresponding author. Tel.: +81-47-124-1501x4329; fax: +81-47-123-9362. E-mail address: [email protected] (Y. Gotoh).
substrate [2–4]. In the present paper, the initial stage of epitaxial growth of In on Mo(1 1 0) surface investigated by RHEED is reported. 2. Experiment This experimental work was performed using an ultra high vacuum chamber (UHV) with RHEED apparatus whose pressure was in the order of 1010 Torr. The surface treatment of Mo(1 1 0) substrate was given by machine polish and electrolysis polish in solution 97%H2 SO4 + 3%H2 O. The method of surface cleaning of the substrate was made by annealing in oxygen. First, Mo substrate was annealed at 600 C in an oxygen atmosphere with a pressure of 106 Torr in order to form MoO2
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.05.042
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A. Katoh et al. / Surface Science 566–568 (2004) 181–185
crystallites and then was heated at 1500 C for a few second in a vacuum of 1010 Torr to evaporate the oxide with impurities on the surface [5,6]. The clean surface of Mo structure was confirmed by the RHEED pattern of 1 · 1 structure with low background. In was deposited on the Mo(1 1 0) substrate by ohmic heating of W basket. After deposition on the Mo surface various monolayer thicknesses, followed by annealing at various temperatures from room temperature to 800 C for 20 s, the specimen was observed by RHEED. Thus, the surface phase diagram for temperature variation versus film thickness was obtained. SEM observation on deposited specimen was carried out to clarify the growth mode.
3. Results and discussion Three surface structures a, b and c were formed in an In/Mo(1 1 0) system in the temperature range from RT to 800 C. Fig. 1 shows the two-dimensional phase diagram of the surface structures for various In deposition thickness versus substrate annealing temperature. The value at 1 ML thickness which means the same number of In atom as Mo of substrate surface is 3.69 A.
Fig. 1. Diagram of the surface structures of the Mo(1 1 0) surface induced by the In deposition.
The a structure exists stably in wide temperature range from RT to 600 C as shown in Fig. 1. Fig. 2(a) shows the RHEED pattern with ½0 0 1Mo electron incidence due to the a structure with In deposition of 0.3 ML at 150 C. The 1/4 ordered reflections between 0 0 and 0 1 Mo reflections in the zeroth Laue Zone (L0 ) are due to In. The intensity distribution of each reflection suggest that the position 3/4 reflections in the zeroth Laue Zone correspond to that of the reciprocal rod of the a structure. The other streaks between 0 0 and 0 1 reflections of Mo in the zeroth Laue Zone are due to the multiple scattering. There are no reflections between zeroth Laue Zone and first one. The basic unit vector of the reciprocal lattice of the a structure is drawn in Fig. 2(b), in which open and dark circles show Mo and In twodimensional reciprocal rods, respectively. From this reciprocal unit mesh, a real unit mesh is obtained whose unit vectors aa and ba are 3.15 and and the angle is 109.5. Atomic model of 3.15 A, the a structure is shown in Fig. 5(a), where In and Mo atoms are superposed at the origin as on-top site. The atomic distance of In along ½0 0 1Mo direction is equal to that of Mo. The atomic density of the a structure is 10.7 · 1014 atom/cm2 . This structure forms a coincident site lattice (CSL) with Mo surface atoms, which is expressed as [1 0 1 4] in matrix notation, where Mo unit vectors are defined by a [0 0 1] and a [1 1 1], a being lattice constant. The same lengths of aa and aMo , and the specific angle in the a structure lead to the fact that the atomic configuration of In has an enlarged bcc(1 1 0) structure compared with Mo(1 1 0) surface. Using new unit vectors aX and bX , the a structurepisffiffiffi rotated from Mo(1 1 0) plane by 54.7 and is 2 3=3 times as large as Mo(1 1 0) plane. The In atoms in aa and ba directions are compressed by 3.0% respectively, which means that In atomic diameter is nearly kept. Such CSL structure with same atomic row distances of In and Mo along ½1 1 1Mo orientation may decrease interfacial energy. Because if the origin of In atom is moved to bridge site from on-top site, all In atoms will fall down to potential valley in Mo atomic groove. The b structure exists stably in a temperature range from 600 to 700 C as shown in Fig. 1. Fig. 3 shows the RHEED pattern with ½0 0 1Mo electron
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Fig. 2. (a) RHEED pattern of a structure taken from Mo(1 1 0) surface after In deposition. The electron incidence is parallel to the ½0 0 1Mo direction. (b) Reciprocal rod arrangement in a structure.
incidence parallel to the direction due to the a structure with In deposition of 0.4 ML at 600 C. The 1/7 ordered reflections between 0 0 and 0 1 Mo reflections in the zeroth Laue Zone (L0 ) are observed. There are no reflections between zeroth Laue Zone and first one. The unit vectors of this structure are 4/7 reflection in the zeroth Laue Zone and 1 0 reflection in the first Laue Zone. The other streaks between 0 0 and 0 1 reflections of Mo in the zeroth Laue Zone and between 1 0 and 1 1 reflections of Mo in the first Laue Zone are due to multiple scattering. From this reciprocal unit mesh, real unit mesh is obtained whose unit vec-
(¼ aMo ), 3.91 A and the tors ab and bb are 3.15 A angle is 95.8. Atomic model of the b structure is shown in Fig. 5(b). Each atom of In and Mo is superposed at the origin as on-top site. The atomic distance of Mo along ½0 0 1Mo direction is equal to that of In in the b structure just like the a structure. This structure also forms a CSL with Mo surface atoms, which is expressed as [1 0 )2 7] in matrix notation. The atomic density of the b structure is 8.16 · 1014 atom/cm2 , which is smaller than that of a structure. The c structure was observed in a temperature range from 700 to 800 C as shown in Fig. 1. Fig. 4 shows the RHEED pattern with ½0 0 1Mo electron
Fig. 3. RHEED pattern of b structure from Mo(1 1 0) surface induced by In. The electron incidence is parallel the ½0 0 1Mo orientation.
Fig. 4. RHEED pattern of c structure from Mo(1 1 0) surface induced by In. The electron incidence is parallel the ½0 0 1Mo orientation.
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incidence with In deposition of 0.3 ML at 700 C. The 1/3 ordered reflections between 0 1 Mo reflections in the zeroth Laue Zone (L0 ) and 1 1 Mo reflections in the first Laue Zone (L1 ), P and Q are due to the superstructure of In. From the reciprocal unit mesh shown by two vectors P and Q, real unit mesh is obtained whose unit vectors ac respectively and the angle is and bc are 4.45, 4.72 A 115.2. This structure is expressed as [1 )1 1 2] in matrix notation. The atomic arrangement of c structure is shown in Fig. 5(c). The atomic density of the c structure is 4.76 · 1014 atom/cm2 . The c structure has atomic density less than the b structure. With increasing of substrate temperature, In atoms composing b structure evaporate gradually from the Mo surface, c structure of more stable phase at this temperature will arise. Finally In evaporation is completed at 800 C on the Mo surface resulting in emergence of clean surface. Fig. 6 shows a SEM image of the specimen deposited with a 20 ML In thick at room temperature. The diameter of each island is nearly 1 lm. The average of distance between 3D In islands is 3– 10 lm. The shape of 3D islands is hexagonal and one side of the island is parallel to ½0 0 1Mo orientation. The observed angles between two ridge lines of In crystal are 61, 61 and 58. The crystal structure of In is face-centered-tetragonal c ¼ 4:94 A). From this geometri(a ¼ b ¼ 4:59 A, cal relation it is found that In crystallites grow with a stable shape with {1 1 1} upper face and {1 1 1}
Fig. 6. SEM image of the In crystallites grown on Mo(1 1 0) substrate at room temperature. The deposition thickness is 20 ML of In.
and {1 0 0} lateral faces, which reals the epitaxial orientation ð1 1 1ÞIn kð1 1 0ÞMo , ½0 0 1Mo k½1 1 0In . According to the RHEED observation In forms two-dimensional ordered phase of a structure on Mo substrate in wide temperature range from room temperature to 600 C and thickness range from submonolayer to 20 ML. On the other hand In crystallites were observed with the specimen deposited with 20 ML. After the a structure covers the Mo surface, 3D islands of In grow epitaxially on the a structure. Namely, In deposited on the Mo(1 1 0) surface grows in accordance with the Stranski–Krastanov growth mode in temperature range between room temperature and 150 C. The shape of 3D islands of In deposited on Mo(1 1 0) surface at temperatures above 200 C is spherical. As the substrate temperature is over the melting point of In, 3D crystals melt and solidify with nearly spherical shape with decreasing of substrate temperature.
4. Conclusions
Fig. 5. Real-space atomic models of the a, b and c structures shown in (a), (b) and (c) respectively. Open and dark circles indicate Mo and In atoms, respectively.
In the present study, three kinds of twodimensional ordered phases a, b and c of In on the Mo(1 1 0) surface are found at nearly 1 ML thickness, which are expressed as [1 0 0 4], [1 0 )2 7] and [1 )1 1 2] in matrix notation, respectively. The atomic distance of In along ½0 0 1Mo in a
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structure is consistent with that of Mo, and distance of In atomic rows along Æ1 1 1æ in these structures are consistent with that of MoÆ1 1 1æ orientation. The a structure has a bcc(1 1 0) layer pffiffiffi of 2 3=3 times as large as Mo(1 1 0) plane rotated by 54.7. Room temperature condensation of In on the Mo surface takes place in accordance with Stranski–Krastanov growth mode.
185
References [1] Y. Gotoh, S. Entani, H. Kawanowa, Surf. Sci. 507/510 (2002) 401. [2] Y. Gotoh, E. Yanokura, Surf. Sci. 269/270 (1992) 707. [3] S. Jo, Y. Gotoh, Surf. Sci. 454/456 (2000) 729. [4] S. Jo, Y. Gotoh, Surf. Sci. 464 (2000) 145. [5] T.W. Hass, A.G. Jackson, J. Chem. Phys. 44 (1966) 2921. [6] Y. Gotoh, E. Yanokura, Surf. Sci. 287/288 (1993) 979.