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Thickness dependent surface microstructure evolution of bismuth thin film prepared by molecular beam deposition method
Current Applied Physics 12 (2012) 1518e1522
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Thickness dependent surface microstructure evolution of bismuth thin film prepared by molecular beam deposition method Youngkun Ahn a, Young-Hwan Kim a, Seong-Il Kim a, *, Kwang-Ho Jeong b a b
Nano-Materials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-791, Republic of Korea Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 December 2011 Received in revised form 7 March 2012 Accepted 20 April 2012 Available online 4 May 2012
The evolution of surface microstructure on bismuth thin film deposited by molecular beam deposition method is investigated. Morphological, topographical, structural, and electrical property changes of the film with various thicknesses are studied by means of AFM, XRD, XRR, and 4-point probe. Drastic change of surface grain in shape, which transforms from round shape to polyhedral shape, is detected around 13e18 nm film thickness. Abrupt horizontal profile change of surface grain is verified with power spectral density (PSD) function. At this threshold thickness, the film shows very low roughness value and surface area ratio. Then both increase steeply as the film thickness surpasses the thickness. As the bismuth film is deposited thicker, it has textured structure and high roughness on surface. With increment of the thickness, the electrical sheet resistance of the films is significantly decreased. We explain this surface microstructure evolution on the bismuth film with the evolutionary selection model. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Bismuth thin film Thickness dependent surface microstructure evolution Evolutionary selection model Molecular beam deposition
1. Introduction Studies of the bismuth surface have been widely studying because of its significantly different statues between surface and bulk, which is insulator on the surface but metal in the bulk, so called topological insulator [1e3]. Moreover it has been studied with other unusual properties, such as small effective masses, lower carrier density, and long carrier mean free path [4]. It has been applying various fields since these peculiar characteristics, such as spin injection devices [5], thermoelectric devices [6,7], superconductor [8], and large magneto-resistance [9]. Moreover, it is known that bismuth thin film shows semimetalesemiconductor transition [10] and quantum confinement effect [11]. Owing to its unique properties and the wide application, it is important to understand of growth and property changing in growth process in cases of various deposition techniques. Various deposition methods have been attempted for depositing bismuth thin film, such as RF magnetron sputtering [12], DC sputtering [13], pulsed laser deposition (PLD) [14], electrodeposition [15], and thermal evaporation method [16,17]. It is known that bismuth thin film deposited by sputtering method has, in general, polycrystalline structure and small grains. On the other hand, depositing by thermal evaporation
* Corresponding author. Tel.: þ82 2 958 5737; fax: þ82 2 958 5739. E-mail address: [email protected] (S.-I. Kim). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.04.031
method usually induces preferred directed crystal structure and large grains in the film. For practical reasons, characteristic of bismuth thin film fabricated by thermal evaporation method including molecular beam deposition (MBD) method has attracted more attention. Since surface microstructure affects film property dominantly, study of surface structure change is considerable. However, surface microstructure evolution depending on film thickness of bismuth has not been studied thoroughly yet. 2. Experimental Bismuth films with various thicknesses were deposited by MBD using Knudsen-cell (K-cell) onto silicon (100) substrates at room temperature in high vacuum (low 108 torr) system. Silicon substrates were cleaned with acetone, methanol and then rinsed with de-ionized water. Deposition rate was extremely low, 0.7 Å/s, to remove inappropriate effects causing by fast deposition, and film thickness was measured with crystal thickness monitor then modified with X-ray reflectivity (XRR) analysis. XRR analysis was also used for obtaining the information of film density as well. Thicknesses of bismuth thin films are represented from 7.0 nm to 66.6 nm. Surface profile of the films was revealed by means of atomic force microscopy (AFM) using tapping mode. It scanned mainly 1 and 3 mm2 area with a resolution of 512 512 pixels. Surface morphology was obtained with root-mean-square (RMS)
Y. Ahn et al. / Current Applied Physics 12 (2012) 1518e1522
roughness and surface area ratio (SAR). Root-mean-square roughness is the standard deviation of the Z values within the given area. The surface area is the sum of the area of all of the triangles formed by three adjacent data point while the surface area ratio is the percentage of the three-dimensional surface area to the twodimensional surface area produced by projecting the surface onto the threshold plane [18]. Although analysis using RMS roughness and surface area ratio of film surface is simple and useful, it offers only vertical information of the film surface. Therefore statistical values such as roughness and surface area ratio have a limit to present complete information for the film surface [19]. Fortunately, raw data of AFM has both vertical and lateral information about detected surface, and power spectral density (PSD) function provides with the lateral structure of given surface on the film. This function gives a representation of the amplitude of a surface’s roughness as a function of the spatial frequency of the roughness. The PSD function is defined in its two dimensional form as [17]
PSD ¼
2 Z Z 1p iðpxþqyÞ ; dx dye zðx; yÞ A2
(1)
where z(x, y) is the image data, p and q the lateral frequencies, and A is the scan area. In order to investigate the electrical property, bismuth thin films were deposited onto fused glass as well. Through comparing the both films on silicon substrate and fused glass, the surface microstructure evolution of bismuth film on fused glass identified the evolution of the film onto silicon substrate. Electrical sheet resistance was measured with the film onto fused glass by using fourpoint probe. X-ray diffraction (XRD), and X-ray reflectivity also provided crystallography and film density. 3. Results and discussion Fig. 1 shows distinct AFM images and its cross sectional profiles. Fig. 1(a)e(d) represents surface topographies of bismuth film with a thickness of 9.3 nm, 12.3 nm, 18.0 nm, and 44.1 nm, while Fig. 1(e) and (f) displays cross sectional images of Fig. 1(a) and (d), respectively. One can see distinctive round shape grains on the surfaces in Fig. 1(a) and (e). Its diameter is less than 50 nm and many voids between the grains are posed randomly on the surface. Since bismuth molecules in vapor phase are emitted from the K-cell, they form nuclei individually when they reach on the substrate. As the depositing proceeds, grain growth is processed and coalescence occurs with neighboring grains (Fig. 1(b)). The round shape surface grains transform into elongated grains by coalescence. Although elongated grains are formed, there are still voids between the surface grains because of insufficient molecules to fill them. As thickness increases, the voids start being filled up. When thickness of the film reaches around 18 nm, the grains on the surface are formed into polyhedral shape. As the elongated grains start to be adjacent, the grains have angled edges and voids have been filled with expanded grains (Fig. 1(c)). Fig. 1(d) and (f) shows that thicker bismuth film comes to have significantly different topography from the thinner films. Shape of the surface grains changes to polyhedral, also some grains are much more grown than neighboring grains. Bright grains indicate taller grains than the neighbors, but they are relatively smaller width than others. The height of the protruded grains is over 50 nm but the width is around 100 nm while the neighbor grains are much smaller but wider; their height is less than 10 nm while the width is over 200 nm. This difference between the neighboring grains on thick film contrasts sharply with the grains on the thinner film; most of the surface grains on the thinner film have similar height and width.
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Since 2D-PSD functions represent period of the surface grains or patterns, one can notice the topologically lateral change from the functions. Fig. 2 is plots of 2D-PSD functions of the bismuth films. Thin films with a thickness below 13.8 nm have hillock posing on below 0.1 um/cycle wavelength. As the film becomes thicker, the hillock moves gradually to higher wavelength. When the film thickness reaches 13e18 nm, the hillock vanished and high face becomes almost flat. This means that the period of grains on the surface moves from short to long period, then the surface has random period grains as the film reaches the threshold thickness. As the film thickness overcomes the threshold thickness, the peak of the graphs moves on the higher wavelength than 0.1 um/cycle and it does not show flat high face anymore. This signifies that the surface grains on the film have undergone abrupt change that turned them from small and dense to large and sparse. It accords with the surface topological change in the AFM images of Fig. 1. Fig. 3 shows surface area ratio (SAR) and RMS roughness of the bismuth thin films. In the figure, SAR of the thin films that are thinner than 13 nm thickness shows over 2%. That is, the bismuth film does not have wet surface at initial state. According to the XRR experiments, densities of the films with 9.3 nm and 10.7 nm have almost same values which are 4.985 g/cm3, 4.979 g/cm3, respectively, and these densities are roughly half of the bulk bismuth density, which are 9.78 g/cm3. This is explained with that since the grains on the film surface are separated by voids from each other, the film’s density is low comparing to the density of the bulk state. As the film reaches the threshold thickness, RMS roughness reaches below 2 nm and SAR falls even below 1%. Although SAR for 9.3 nm thickness shows relatively low value, it also shows very high error value comparing to the values of adjacent thicknesses. While the densities of the thinnest films have half value of bulk bismuth material, they increase dramatically as the film get thicker. They are 7.918 g/cm3, 8.62 g/cm3, and 9.125 g/cm3 as the thickness becomes 12.4 nm, 13.8 nm, and 15.7 nm, respectively. After all, the film density of the film increases up to 9.197 g/cm3 when the film thickness reaches 18 nm. From these succession results of changes of roughness, SAR, surface topography, film density, and 2D-PSD, it concludes that the bismuth film undergoes significant surface microstructure evolution at the threshold thickness; from 13 nm to 18 nm. While the film’s surface grains are changed, crystal structure is also changed. XRD result of these bismuth films reveals in Fig. 4. The results show (003) and (006) crystal peaks of textured bismuth thin films. While (009) crystal peak also appears weakly at 71.8 it is not displayed here because of dominant silicon (001) crystal peak. In the figure, crystal peak of the film increases noticeably starting from 13.8 nm thickness. This clearly matches with surface changes above mentioned. It is worth to note the two facts, first one is that some grains grows high but narrow in Fig. 1 and, secondly, appearance of surface grains with large width and long period in Fig. 2. It is believed that the crystalline grains can develop a preferred orientation with respect to the substrate plane or the normal to this plane when films are grown [20]. In a related study, Van der Drift [21] reported the evolutionary selection model. According to this model, the nuclei are initially randomly oriented on the substrate. But while grains with their faster growing directions that are perpendicular to the surface are preserved, slower growing grains are terminated as they intersect the column walls of taller grains. Faster growth of specific grains that have the out-ofplane crystalline direction leads noticeable change of crystal structure in the film. This phenomenon can be observed by using of X-ray diffraction experiment as appearance of dominant identified peaks. It explains a previous report that bismuth film deposited by thermal evaporation method has preferred oriented direction [22]. Moreover, it is clear that some grains are protruded since SAR increases steeply in Fig. 3. Abrupt increasing of SAR leads to abrupt
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Y. Ahn et al. / Current Applied Physics 12 (2012) 1518e1522
Fig. 1. Atomic force microscopy (AFM) images and cross sectional profiles of bismuth thin films. (a), (b), (c), and (d) show surface morphology of film with 9.3 nm, 12.4 nm, 18.0 nm, and 44.1 nm thickness, respectively. (e) and (f) are cross sectional profiles of part of (a) and (d), respectively.
Y. Ahn et al. / Current Applied Physics 12 (2012) 1518e1522
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Fig. 2. Two-dimensional power spectral density (2D-PSD) functions as a function of the film thickness.
increasing of surface area and, according to our simple calculation, the volume of eminent grains in scanned area agrees with the deposited bismuth quantity that is equivalent to the thickness difference between two films. Therefore most of adsorbed bismuth molecules contribute to the growth of specific grains by attaching on the grains. Roughness of this film increases steeply since, as mentioned above, grains that have normal direction grow faster than the others. Therefore we believes that grains that have normal crystalline direction grow up fast to out-of-plane direction but not horizontally since the grains do not have chance to expand into horizontal direction, however the others, which have in-plane direction, grow relatively horizontal.
Electrical resistance is measured with four-point probe and displayed in Fig. 5. Because of the disconnected surface grains on the thinner thickness film than the percolation threshold thickness, electrical channel is not formed. Therefore 3 thinnest films among them have eight order ohms per square of sheet resistance which is same as fused glass sheet resistance. Nonetheless voids still remain between the grains, the grains are elongated and they form partly electrical channel so that sheet resistance drop to two to three hundreds order when the thickness reaches 12e13 nm. As the thickness comes to 13e18 nm thickness, electrical channel is formed since grains are connected entirely to each other by
Fig. 3. RMS roughness and surface are ratio (SAR) as a function of the film thickness.
Fig. 4. X-ray diffraction (XRD) patterns with various bismuth film thickness.
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Program through the ministry of Education, Science and Technology (2011K000589).
References
Fig. 5. Change of the sheet resistance as a function of the bismuth film thickness. Numerous investigations are conducted and standard deviation shows as error bars.
percolation and the voids between the grains are filled. So sheet resistance decrease to 250 U per square. Note that this thickness is also the threshold thickness and all the results, such as SAR, 2DPSD and roughness, show significant change around this thickness. 4. Conclusions We investigated the evolution of surface microstructure on bismuth thin film deposited by molecular beam deposition method. Bismuth thin film showed drastic surface changes in morphology, topography, crystal structure, and electrical resistance. As bismuth film reaches 13e18 nm thickness, round shape surface grain changes into polyhedral-shape gains, while the surface area ratio and roughness has smallest values. Abrupt horizontal profile changing of surface grain was verified with power spectral density function at this threshold thickness. When bismuth film has a thickness of 13.8 nm, the film starts to have significant XRD peaks of (00l) direction because the difference of the rate of surface grain growth appeared to prefer out-of-plane crystalline direction. That is, the grains that have out-of-plane direction grow up faster than the other grains. Also, sheet resistance decreases steeply and density of the film begins to have similar value as a bulk state. Acknowledgments This work was supported by the Korea Institute of Science and Technology (2E23150) and by the Converging Research Center
[1] H. Zhang, C. Liu, X. Qi, X. Dai, Z. Fang, S. Zhang, Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface, Nat. Phys. 5 (2009) 438e442. [2] J. Sun, J. Wang, J. Wells, Y.M. Koroteev, G. Bihlmayer, E.V. Chulkov, Ph. Hofmann, K. Pohl, Structure and oscillatory multilayer relaxation of the bismuth(100) surface, New J. Phys. 12 (2010) 063016. [3] Ph. Hofmann, J.E. Gayone, G. Bihlmayer, Yu. M. Koroteev, E.V. Chulkov, Electronic structure and fermi surface of Bi(100), Phys. Rev. B. 71 (2005) 195413. [4] C.F. Gallo, B.S. Chandrasekha, P.H. Sutter, Transport properties of bismuth single crystals, J. Appl. Phys. 34 (1963) 144. [5] K. Lee, J.W. Roh, J. Chang, S. Han, K. Shin, W.Y. Jeung, M. Johnson, W. Lee, Electrical spin injection and detection in semimetallic Bi and BiePb films, Phys. Rev. B 79 (2009) 195201. [6] M. Sasaki, G.X. Tai, M. Inoue, H. Bidadi, Transient thermoelectric effect in bismuth single crystals, J. Appl. Phys. 75 (1994) 4520. [7] S. Cho, A. Divenere, G.K. Wong, J.B. Ketterson, J.R. Meyer, C.A. Hoffman, Thermoelectric power of MBE grown Bi thin films and Bi/CdTe superlattice on CdTe substrates, Sol. State Comm. 102 (1997) 673e676. [8] B. Weitzel, H. Micklitz, Superconductivity in granular systems built from welldefined rhombohedral Bi-clusters: evidence for Bi-surface superconductivity, Phys. Rev. Lett. 66 (1991) 385e388. [9] S. Cho, Y. Kim, A.J. Freeman, G.K.L. Wong, J.B. Ketterson, L.J. Olafsen, I. Vurgaftman, J.R. Meyer, C.A. Hoffman, Large magnetoresistance in postannealed Bi thin films, Appl. Phys. Lett. 79 (2001) 3651. [10] C.A. Hoffman, J.R. Meyer, F.J. Bartoli, Semimetal-to-semiconductor transition in bismuth thin films, Phys. Rev. Lett. 48 (1993) 11431e11434. [11] E.I. Rogacheva, S.N. Grigorov, O.N. Nashchekina, S. Lyubchenko, Quantum-size effects in n-type bismuth thin films, Appl. Phys. Lett. 82 (2003) 2628. [12] D.H. Kim, S.H. Lee, J.K. Kim, G.H. Lee, Structure and electrical transport properties of bismuth thin films prepared by RF magnetron sputtering, Appl. Surf. Sci. 252 (2006) 3525e3531. [13] J.H. Hsu, Y.S. Sun, H.X. Wang, P.C. Kuo, T.H. Hsieh, C.T. Liang, Substrate dependence of large ordinary magnetoresistance in sputtered Bi films, J. Magn. Magn. Mater. 272 (2004) 1769e1771. [14] M.O. Boffoue, B. Lenoir, H. Scherrer, A. Dauscher, Pulsed laser deposition of bismuth in the presence of different ambient atmospheres, Thin Solid Films 322 (1998) 132e137. [15] F.Y. Yang, K. Liu, C.L. Chien, P.C. Searson, Large magnetoresistance and finitesize effects in electrodeposited single-crystal Bi thin films, Phys. Rev. Lett. 82 (1999) 3328e3331. [16] L. Kumari, S.J. Lin, J.H. Lin, Y.R. Ma, P.C. Lee, Y. Liou, Effects of deposition temperature and thickness on the structural properties of thermal evaporated bismuth thin films, Appl. Surf. Sci. 253 (2007) 5931e5938. [17] X. Du, A.F. Hebard, Large magnetoresistance of bismuth/gold films thermally deposited onto glass substrates, Appl. Phys. Lett. 82 (2003) 2293. [18] Nanoscope Command Reference Manual, Version 5.12, Digital Instruments, Inc., 2001. [19] S.J. Fang, S. Haplepete, W. Chen, C.R. Helms, Analyzing atomic force microscopy images using spectral methods, J. Appl. Phys. 82 (1997) 5891. [20] J.L. Vossen, W. Kern, Thin Film Processes, Academic Press, New York, 1978. [21] A. Van der Drift, Evolutionary selection, a principle governing growth orientation in vapour-deposited layers, Philips Res. Rep. 22 (1967) 267e288. [22] A.A. Ramadan, A.M. El-Shabiny, N.Z. El-Sayed, Size-dependent structural characteristics of thin bismuth films, Thin Solid Films 209 (1992) 32e37.
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Thickness dependent surface microstructure evolution of bismuth thin film prepared by molecular beam deposition method Youngkun Ahn a, Young-Hwan Kim a, Seong-Il Kim a, *, Kwang-Ho Jeong b a b
Nano-Materials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-791, Republic of Korea Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 December 2011 Received in revised form 7 March 2012 Accepted 20 April 2012 Available online 4 May 2012
The evolution of surface microstructure on bismuth thin film deposited by molecular beam deposition method is investigated. Morphological, topographical, structural, and electrical property changes of the film with various thicknesses are studied by means of AFM, XRD, XRR, and 4-point probe. Drastic change of surface grain in shape, which transforms from round shape to polyhedral shape, is detected around 13e18 nm film thickness. Abrupt horizontal profile change of surface grain is verified with power spectral density (PSD) function. At this threshold thickness, the film shows very low roughness value and surface area ratio. Then both increase steeply as the film thickness surpasses the thickness. As the bismuth film is deposited thicker, it has textured structure and high roughness on surface. With increment of the thickness, the electrical sheet resistance of the films is significantly decreased. We explain this surface microstructure evolution on the bismuth film with the evolutionary selection model. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Bismuth thin film Thickness dependent surface microstructure evolution Evolutionary selection model Molecular beam deposition
1. Introduction Studies of the bismuth surface have been widely studying because of its significantly different statues between surface and bulk, which is insulator on the surface but metal in the bulk, so called topological insulator [1e3]. Moreover it has been studied with other unusual properties, such as small effective masses, lower carrier density, and long carrier mean free path [4]. It has been applying various fields since these peculiar characteristics, such as spin injection devices [5], thermoelectric devices [6,7], superconductor [8], and large magneto-resistance [9]. Moreover, it is known that bismuth thin film shows semimetalesemiconductor transition [10] and quantum confinement effect [11]. Owing to its unique properties and the wide application, it is important to understand of growth and property changing in growth process in cases of various deposition techniques. Various deposition methods have been attempted for depositing bismuth thin film, such as RF magnetron sputtering [12], DC sputtering [13], pulsed laser deposition (PLD) [14], electrodeposition [15], and thermal evaporation method [16,17]. It is known that bismuth thin film deposited by sputtering method has, in general, polycrystalline structure and small grains. On the other hand, depositing by thermal evaporation
* Corresponding author. Tel.: þ82 2 958 5737; fax: þ82 2 958 5739. E-mail address: [email protected] (S.-I. Kim). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.04.031
method usually induces preferred directed crystal structure and large grains in the film. For practical reasons, characteristic of bismuth thin film fabricated by thermal evaporation method including molecular beam deposition (MBD) method has attracted more attention. Since surface microstructure affects film property dominantly, study of surface structure change is considerable. However, surface microstructure evolution depending on film thickness of bismuth has not been studied thoroughly yet. 2. Experimental Bismuth films with various thicknesses were deposited by MBD using Knudsen-cell (K-cell) onto silicon (100) substrates at room temperature in high vacuum (low 108 torr) system. Silicon substrates were cleaned with acetone, methanol and then rinsed with de-ionized water. Deposition rate was extremely low, 0.7 Å/s, to remove inappropriate effects causing by fast deposition, and film thickness was measured with crystal thickness monitor then modified with X-ray reflectivity (XRR) analysis. XRR analysis was also used for obtaining the information of film density as well. Thicknesses of bismuth thin films are represented from 7.0 nm to 66.6 nm. Surface profile of the films was revealed by means of atomic force microscopy (AFM) using tapping mode. It scanned mainly 1 and 3 mm2 area with a resolution of 512 512 pixels. Surface morphology was obtained with root-mean-square (RMS)
Y. Ahn et al. / Current Applied Physics 12 (2012) 1518e1522
roughness and surface area ratio (SAR). Root-mean-square roughness is the standard deviation of the Z values within the given area. The surface area is the sum of the area of all of the triangles formed by three adjacent data point while the surface area ratio is the percentage of the three-dimensional surface area to the twodimensional surface area produced by projecting the surface onto the threshold plane [18]. Although analysis using RMS roughness and surface area ratio of film surface is simple and useful, it offers only vertical information of the film surface. Therefore statistical values such as roughness and surface area ratio have a limit to present complete information for the film surface [19]. Fortunately, raw data of AFM has both vertical and lateral information about detected surface, and power spectral density (PSD) function provides with the lateral structure of given surface on the film. This function gives a representation of the amplitude of a surface’s roughness as a function of the spatial frequency of the roughness. The PSD function is defined in its two dimensional form as [17]
PSD ¼
2 Z Z 1p iðpxþqyÞ ; dx dye zðx; yÞ A2
(1)
where z(x, y) is the image data, p and q the lateral frequencies, and A is the scan area. In order to investigate the electrical property, bismuth thin films were deposited onto fused glass as well. Through comparing the both films on silicon substrate and fused glass, the surface microstructure evolution of bismuth film on fused glass identified the evolution of the film onto silicon substrate. Electrical sheet resistance was measured with the film onto fused glass by using fourpoint probe. X-ray diffraction (XRD), and X-ray reflectivity also provided crystallography and film density. 3. Results and discussion Fig. 1 shows distinct AFM images and its cross sectional profiles. Fig. 1(a)e(d) represents surface topographies of bismuth film with a thickness of 9.3 nm, 12.3 nm, 18.0 nm, and 44.1 nm, while Fig. 1(e) and (f) displays cross sectional images of Fig. 1(a) and (d), respectively. One can see distinctive round shape grains on the surfaces in Fig. 1(a) and (e). Its diameter is less than 50 nm and many voids between the grains are posed randomly on the surface. Since bismuth molecules in vapor phase are emitted from the K-cell, they form nuclei individually when they reach on the substrate. As the depositing proceeds, grain growth is processed and coalescence occurs with neighboring grains (Fig. 1(b)). The round shape surface grains transform into elongated grains by coalescence. Although elongated grains are formed, there are still voids between the surface grains because of insufficient molecules to fill them. As thickness increases, the voids start being filled up. When thickness of the film reaches around 18 nm, the grains on the surface are formed into polyhedral shape. As the elongated grains start to be adjacent, the grains have angled edges and voids have been filled with expanded grains (Fig. 1(c)). Fig. 1(d) and (f) shows that thicker bismuth film comes to have significantly different topography from the thinner films. Shape of the surface grains changes to polyhedral, also some grains are much more grown than neighboring grains. Bright grains indicate taller grains than the neighbors, but they are relatively smaller width than others. The height of the protruded grains is over 50 nm but the width is around 100 nm while the neighbor grains are much smaller but wider; their height is less than 10 nm while the width is over 200 nm. This difference between the neighboring grains on thick film contrasts sharply with the grains on the thinner film; most of the surface grains on the thinner film have similar height and width.
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Since 2D-PSD functions represent period of the surface grains or patterns, one can notice the topologically lateral change from the functions. Fig. 2 is plots of 2D-PSD functions of the bismuth films. Thin films with a thickness below 13.8 nm have hillock posing on below 0.1 um/cycle wavelength. As the film becomes thicker, the hillock moves gradually to higher wavelength. When the film thickness reaches 13e18 nm, the hillock vanished and high face becomes almost flat. This means that the period of grains on the surface moves from short to long period, then the surface has random period grains as the film reaches the threshold thickness. As the film thickness overcomes the threshold thickness, the peak of the graphs moves on the higher wavelength than 0.1 um/cycle and it does not show flat high face anymore. This signifies that the surface grains on the film have undergone abrupt change that turned them from small and dense to large and sparse. It accords with the surface topological change in the AFM images of Fig. 1. Fig. 3 shows surface area ratio (SAR) and RMS roughness of the bismuth thin films. In the figure, SAR of the thin films that are thinner than 13 nm thickness shows over 2%. That is, the bismuth film does not have wet surface at initial state. According to the XRR experiments, densities of the films with 9.3 nm and 10.7 nm have almost same values which are 4.985 g/cm3, 4.979 g/cm3, respectively, and these densities are roughly half of the bulk bismuth density, which are 9.78 g/cm3. This is explained with that since the grains on the film surface are separated by voids from each other, the film’s density is low comparing to the density of the bulk state. As the film reaches the threshold thickness, RMS roughness reaches below 2 nm and SAR falls even below 1%. Although SAR for 9.3 nm thickness shows relatively low value, it also shows very high error value comparing to the values of adjacent thicknesses. While the densities of the thinnest films have half value of bulk bismuth material, they increase dramatically as the film get thicker. They are 7.918 g/cm3, 8.62 g/cm3, and 9.125 g/cm3 as the thickness becomes 12.4 nm, 13.8 nm, and 15.7 nm, respectively. After all, the film density of the film increases up to 9.197 g/cm3 when the film thickness reaches 18 nm. From these succession results of changes of roughness, SAR, surface topography, film density, and 2D-PSD, it concludes that the bismuth film undergoes significant surface microstructure evolution at the threshold thickness; from 13 nm to 18 nm. While the film’s surface grains are changed, crystal structure is also changed. XRD result of these bismuth films reveals in Fig. 4. The results show (003) and (006) crystal peaks of textured bismuth thin films. While (009) crystal peak also appears weakly at 71.8 it is not displayed here because of dominant silicon (001) crystal peak. In the figure, crystal peak of the film increases noticeably starting from 13.8 nm thickness. This clearly matches with surface changes above mentioned. It is worth to note the two facts, first one is that some grains grows high but narrow in Fig. 1 and, secondly, appearance of surface grains with large width and long period in Fig. 2. It is believed that the crystalline grains can develop a preferred orientation with respect to the substrate plane or the normal to this plane when films are grown [20]. In a related study, Van der Drift [21] reported the evolutionary selection model. According to this model, the nuclei are initially randomly oriented on the substrate. But while grains with their faster growing directions that are perpendicular to the surface are preserved, slower growing grains are terminated as they intersect the column walls of taller grains. Faster growth of specific grains that have the out-ofplane crystalline direction leads noticeable change of crystal structure in the film. This phenomenon can be observed by using of X-ray diffraction experiment as appearance of dominant identified peaks. It explains a previous report that bismuth film deposited by thermal evaporation method has preferred oriented direction [22]. Moreover, it is clear that some grains are protruded since SAR increases steeply in Fig. 3. Abrupt increasing of SAR leads to abrupt
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Fig. 1. Atomic force microscopy (AFM) images and cross sectional profiles of bismuth thin films. (a), (b), (c), and (d) show surface morphology of film with 9.3 nm, 12.4 nm, 18.0 nm, and 44.1 nm thickness, respectively. (e) and (f) are cross sectional profiles of part of (a) and (d), respectively.
Y. Ahn et al. / Current Applied Physics 12 (2012) 1518e1522
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Fig. 2. Two-dimensional power spectral density (2D-PSD) functions as a function of the film thickness.
increasing of surface area and, according to our simple calculation, the volume of eminent grains in scanned area agrees with the deposited bismuth quantity that is equivalent to the thickness difference between two films. Therefore most of adsorbed bismuth molecules contribute to the growth of specific grains by attaching on the grains. Roughness of this film increases steeply since, as mentioned above, grains that have normal direction grow faster than the others. Therefore we believes that grains that have normal crystalline direction grow up fast to out-of-plane direction but not horizontally since the grains do not have chance to expand into horizontal direction, however the others, which have in-plane direction, grow relatively horizontal.
Electrical resistance is measured with four-point probe and displayed in Fig. 5. Because of the disconnected surface grains on the thinner thickness film than the percolation threshold thickness, electrical channel is not formed. Therefore 3 thinnest films among them have eight order ohms per square of sheet resistance which is same as fused glass sheet resistance. Nonetheless voids still remain between the grains, the grains are elongated and they form partly electrical channel so that sheet resistance drop to two to three hundreds order when the thickness reaches 12e13 nm. As the thickness comes to 13e18 nm thickness, electrical channel is formed since grains are connected entirely to each other by
Fig. 3. RMS roughness and surface are ratio (SAR) as a function of the film thickness.
Fig. 4. X-ray diffraction (XRD) patterns with various bismuth film thickness.
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Y. Ahn et al. / Current Applied Physics 12 (2012) 1518e1522
Program through the ministry of Education, Science and Technology (2011K000589).
References
Fig. 5. Change of the sheet resistance as a function of the bismuth film thickness. Numerous investigations are conducted and standard deviation shows as error bars.
percolation and the voids between the grains are filled. So sheet resistance decrease to 250 U per square. Note that this thickness is also the threshold thickness and all the results, such as SAR, 2DPSD and roughness, show significant change around this thickness. 4. Conclusions We investigated the evolution of surface microstructure on bismuth thin film deposited by molecular beam deposition method. Bismuth thin film showed drastic surface changes in morphology, topography, crystal structure, and electrical resistance. As bismuth film reaches 13e18 nm thickness, round shape surface grain changes into polyhedral-shape gains, while the surface area ratio and roughness has smallest values. Abrupt horizontal profile changing of surface grain was verified with power spectral density function at this threshold thickness. When bismuth film has a thickness of 13.8 nm, the film starts to have significant XRD peaks of (00l) direction because the difference of the rate of surface grain growth appeared to prefer out-of-plane crystalline direction. That is, the grains that have out-of-plane direction grow up faster than the other grains. Also, sheet resistance decreases steeply and density of the film begins to have similar value as a bulk state. Acknowledgments This work was supported by the Korea Institute of Science and Technology (2E23150) and by the Converging Research Center
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