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Effect of substrate temperature on the texture and structure of polycrystalline Si0.7Ge0.3 films deposited on SiO2 by molecular beam deposition
Thin Solid Films 350 (1999) 14±20
Effect of substrate temperature on the texture and structure of polycrystalline Si0.7Ge0.3 ®lms deposited on SiO2 by molecular beam deposition Hong Seung Kim*, Jeong Yong Lee Department of Materials Science and Engineering, KAIST, Taejon 305-701, South Korea Received 27 April 1998; received in revised form 18 February 1999; accepted 26 February 1999
Abstract The evolution of microstructure and texture of molecular beam deposited Si0.7Ge0.3 ®lms on SiO2 at the deposition temperature range of 400±7008C was investigated by X-ray diffraction and transmission electron microscopy. At deposition temperatures between 400 and below 5008C, the ®lms were directly deposited as a mixed-phase on SiO2 and have a inversely cone-shaped structure. In this temperature range deposited as a mixed-phase, the grain size increases as the temperature increases, so that the grains not only grow up by deposition, but also laterally grow by the solid phase crystallization, furthermore, the texture is changed from a {110} texture to mixed {311} and {110} textures. At 5008C, the ®lm was deposited as only a crystalline phase and has a columnar structure with a strong {110} texture. In the temperature range of 500±7008C, as the temperature increases, the {311} and {111} textures develop whereas the {110} texture reduces. The ®lm deposited at 7008C has a random orientation and structure. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Germanium; Molecular beam epitaxy; Structure properties; Transmission electron microscopy (TEM)
1. Introduction Polycrystalline Si12xGex (poly-Si12xGex) alloy is recently an attractive candidate for an alternative to polycrystalline Si (poly-Si) for various applications in microelectronics technology [1±3]. Since the melting point of Si12xGex is lower than that of Si, physical phenomena controlling fabrication processes such as deposition [4], crystallization [1,5,6], and dopant activation [3] occur at lower temperatures for Si12xGex than for Si. Thus, for applications in microelectronics technologies, which have limited thermal budget allowances, poly-Si12xGex is preferable to poly-Si. Particularly, poly-Si12xGex formed at much lower temperatures than the softening points of conventional glass plates are promising as a active layer in thin-®lm transistors (TFTs) for a low-temperature process large area electronics technology [1]. In addition to lowering thermal budget, poly-Si12xGex has higher charge carrier mobility, resulting in lower sheet resistance, and variable work functions according to the Ge mole fraction [3]. Therefore, several potential uses for poly-Si12xGex have recently been investi* Corresponding author.
gated as a dopant-diffusion source for the formation of shallow junctions in Si [2], as a gate material in CMOS technologies for improved tradeoff between n-channel and p-channel doping design [3]. The texture and microstructure of poly-Si12xGex ®lms are important properties like those of poly-Si so that the grain size and orientation affect the electrical properties of the materials as the dopant distribution and charge trapping at grain boundary [7]. Although some device characteristics have been studied [1,3], a systematic study of the texture and the resulting microstructure of as-deposited polySi12xGex alloy ®lms has not been performed. The deposition studies on poly-Si12xGex ®lms prepared by conventional low pressure chemical vapor deposition (LPCVD) and rapid-thermal CVD (RTCVD) on SiO2 have reported [4,8]. However, CVD system of Si12xGex has limitations such as higher oxygen contents [9] and dif®culty nucleating on oxide [4,10]. Our work addresses using a molecular beam deposition (MBD) under ultrahigh vacuum (UHV). This paper is to investigate the texture and the microstructure of as-deposited poly-Si0.7Ge0.3 alloy ®lms prepared by MBD on SiO2 at various substrate temperatures. We discuss
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0022 3-0
H. Seung Kim, J. Yong Lee / Thin Solid Films 350 (1999) 14±20
15
TEM specimens were prepared by the lift-off process [11]: the sandwiched SiO2 layer was etched with a HF : H2 O 1 : 1 solution. Cross-sectional TEM specimens were prepared using mechanical polishing followed by ionmilling using a specimen stage cooled by liquid nitrogen. Plan-view and cross-sectional TEM specimens were examined in a JEOL JEM 2000EX electron microscope operated at 200 kV. 3. Results 3.1. The evolution of texture
Fig. 1. X-ray diffraction patterns of the Si0.7Ge0.3 ®lms deposited on SiO2 at deposition temperatures of 400, 450, 500, 600, and 7008C.
on the growth mechanism in relation to the evolution of the texture and the resulting microstructure. 2. Experiment Samples were grown on thermally oxidized Sik100l wafers using a Riber SIVA45 solid source molecular beam epitaxy (MBE) system. The base pressure of the growth chamber was maintained less than at 1 £ 10210 Torr and did not exceed 1 £ 1029 Torr during the deposition. Both pieces of high-purity ¯oating zone (FZ) single crystal silicon and germanium were used as sources, respectively. The deposition rate was about 0.1 nm/s and the total deposition time of each ®lm was 2000 s, thus the nominal thickness of each ®lm was approximately 200 nm. The growth temperatures were varied in the range of 400± 7008C and the Ge mole fraction which was corrected by Rutherford backscattering spectroscopy (RBS) was ®xed 0.3. The deposited samples were investigated both by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The X-ray texture was measured by observing the diffracted intensity at twice the angle of incidence of the X-ray beam in a RIGAKU D/MAX-RC X-ray diffractometer with 40 kV copper radiation. Plan-view
Fig. 1 shows the XRD results of the Si0.7Ge0.3 alloy ®lms at deposition temperatures (400, 450, 500, 600, and 7008C). In the XRD results of all the ®lms, the diffraction peaks appearing at around 2u 28, 47, and 558 represent the diffraction peaks from the {111}, {220}, and {311} planes, respectively, characteristic of a material with the diamond cubic crystal structure. It shows that poly-Si0.7Ge0.3 ®lms are deposited on amorphous substrates at such low temperature as about 4008C by MBD, whereas poly-Si0.7Ge0.3 were deposited at over 5008C by CVD [4]. In addition, two extra diffraction peaks are exhibited. The one is at around 2u 358 which is diffracted not from Si0.7Ge0.3 ®lm, but from Si (100) substrate. The other appears on the low angle side of the {111} diffraction at around 2u 268. The interplanar spacings, dhkl, can be determined from Bragg's law, nl 2dhkl ´sinu with n 1 and l 0:154 nm for Cu Ka . The value of dhkl at around 2u 268 is approximately 0.335 nm, thus it is larger than d111Ge which is 0.326 nm. This additional peak labeled as (111)dh has originated from the diamond hexagonal (dh) structure of the Si12xGex layer [12,13]. The texture of the ®lms was determined from XRD in Fig. 1. In order to quantify the texture of the samples, the orientation factor Ohkl is de®ned as follows X Ihkl I 1 Ohkl hkl = Fhkl hkl Fhkl where for each diffraction plane {hkl}, Fhkl is the scattering correction factor and Ihkl is the measured intensity corrected by an absorption factor to take into account the fact that the ®lms are not in®nitely thick. The ®nite thickness normalization consisted in dividing the measured intensity by the following calibration factor Gx [14] ÿ 2 Gx 1 2 exp 22mt=sinu where m is the absorption coef®cient, t is the thickness of the ®lm, and u is the diffraction angle of each peak. Both the scattering correction factor and the absorption coef®cient for the Si0.7Ge0.3 ®lm are obtained by weight averaging the correction factors and the absorption coef®cients of Si and Ge according to the composition, respectively. According to our de®nition for Ohkl,, this corresponds directly to the
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volume fraction of material oriented in the khkll direction. Considering that only the three major diffraction peaks from the ®lm, i.e., the {111}, {220}, and {311} were above the noise level, samples whose three Ohkl are close to 0.33 have no preferential orientation, that is, it is a random texture. Fig. 2 shows the variations of the orientation factor Ohkl determined for three directions, i.e., k111l, k220l, and k311l, as a function of the substrate temperature. At all the substrate temperatures, O110 is larger than 0.33, and specially, at 5008C, it is up to 0.7. However, at 4508C, although O111 is only around 0.2, O311 increases almost same value of O110 at around 0.38, respectively. It means that the ®lm deposited at 4508C has a mixed {110} and {311} texture. In addition, at high temperature of 7008C, O311 and O111 increase around 0.33 and 0.31, respectively, whereas O110 decreases around 0.33, therefore, the deposited ®lm has a random texture. 3.2. The results of TEM Fig. 3a,b show a bright-®eld and a dark-®eld plan-view TEM micrographs of a Si0.7Ge0.3 ®lm deposited at 4008C. Amorphous regions, one of which is indicated by an arrow, with uniform gray contrast, are embedded in small grains that show a bright and a dark contrast in Fig. 3a,b. It shows that the ®lm forms a mixed-phase. Dark and bright stripes, which were explained by narrow twin plates [15], are observed in the small grains. Fig. 4a,b show a bright-®led and a dark ®eld cross-sectional TEM micrographs of a Si0.7Ge0.3 ®lm deposited at 4008C. The grains with a bright and a dark contrast have a V appearance starting from the ®lm/SiO2 interface and ending at the ®lm surface, and the amorphous regions, one of which is indicated by an arrow, with uniform gray contrast, are observed between V shaped
Fig. 3. (a) Bright-®eld and (b) dark-®eld plan-view TEM micrographs of the Si0.7Ge0.3 ®lm deposited on SiO2 at deposition temperature 4008C.
Fig. 2. Orientation factor Ohkl versus deposition temperature for the Si0.7Ge0.3 ®lms.
grains in Fig. 4a,b. The width of the grains is very small at near the SiO2 interface and increases until the meeting of the grains as the ®lm thickness increases, whereas the volume of amorphous regions decreases with the increase in the ®lm thickness. As results of Figs. 3 and 4, the grains have inversely cone-shaped structure. Moreover, dark and bright stripes, which are perpendicular to the substrate or incline about more or less 108 to the normal direction of substrate, are observed in the grains of Fig. 4. These stripes are also owing to thin laminar {111} twins. Drosd and Washburn [16] showed that if twinning occurs, k110l and k112l are fast growth directions for diamond cubic materials. Brokman et al. [17] concluded that growth proceeds in the k112l direction, which is along twin boundaries and eventually the k110l growth mechanism proposed by Drosd and Washburn take over. Therefore, the grains with twin boundaries parallel to the normal direction of substrate have a {110} texture. Fig. 5a±c show a bright-®eld plan-view and crosssectional and a dark-®eld cross-sectional TEM micrographs of a Si0.7Ge0.3 ®lm deposited at 4508C. Amorphous regions,
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Fig. 4. (a) Bright-®eld and (b) dark-®eld cross-sectional TEM micrographs of the Si0.7Ge0.3 ®lm deposited on SiO2 at deposition temperature 4008C.
one of which is indicated by an arrow, were still observed and the grains have also a inversely cone-shaped structure. However, the grains in Fig. 5a are dramatically larger and the width and surface curvature of the grains in Fig. 5b,c increase, compared to those of the Si0.7Ge0.3 ®lm deposited at 4008C. Fig. 6a,b show a bright-®eld plan-view and crosssectional TEM micrographs of a Si0.7Ge0.3 ®lm deposited at 5008C. The grains with a bright and a dark contrast were observed, however, no amorphous region was found in Fig. 6a. In addition, in Fig. 6b, the amorphous region was not observed, too, and the grains were grown by columnar structure. Many {111} twins, which are perpendicular to the substrate or incline about 108 to the normal direction of substrate, are observed in Fig. 6b. Fig. 7a,b show a bright-®eld plan-view and crosssectional TEM micrographs of a Si0.7Ge0.3 ®lm deposited at 6008C. Fig. 8a±c show a bright-®eld plan-view, a bright®eld cross-sectional, and a dark-®eld cross-sectional TEM micrographs of a Si0.7Ge0.3 ®lm deposited at 7008C. The
Fig. 5. (a) Bright-®eld plan-view, (b) bright-®eld, and (c) dark-®eld crosssectional TEM micrographs of the Si0.7Ge0.3 ®lm deposited on SiO2 at deposition temperature 4508C.
grains were still grown by columnar structure in Fig. 7b, however, in Fig. 8b,c, the grains were not grown by columnar form, but by random form. Moreover, the density of {111} twins was reduced and the surface curvature was increased, compared to those of the ®lms in Figs. 6b and 7b. The average grain size of each sample obtained by using the grain boundary crossing technique [18] from a pair of the bright-®eld and the dark-®eld plan-view micrographs of each sample is shown in Fig. 9. The average grain sizes of the Si0.7Ge0.3 ®lms deposited at 450 and 7008C are about 150 and 100 nm, respectively, whereas others are about 60 nm.
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Fig. 6. (a) Bright-®eld plan-view and (b) bright-®eld cross-sectional TEM micrographs of the Si0.7Ge0.3 ®lm deposited on SiO2 at deposition temperature 5008C.
Compared to the evolution of the texture in Fig. 2, it is interesting that as the {110} orientation factor, O110, decreases, the average grain size increases. 4. Discussion On the basis of the experimental results in previous section, we can classify four morphologically well-characterized different structures according to the substrate temperature. Firstly, at low temperature, 4008C, the Si0.7Ge0.3 ®lm is deposited by both amorphous phase and crystalline phase, that is, it is composed of a mixed-phase and the crystals are inversely cone-shaped structure with {110} texture. Secondly, the Si0.7Ge0.3 ®lm deposited at around 4508C have also a mixed-phase, and the crystals are inversely cone-shaped structure with {311} and {110} mixed texture. Thirdly, at around 5008C, the Si0.7Ge0.3 ®lm is completely deposited by crystalline phase and the crystals are columnar structure with strongly {110} texture.
Fig. 7. (a) Bright-®eld plan-view and (b) bright-®eld cross-sectional TEM micrographs of the Si0.7Ge0.3 ®lm deposited on SiO2 at deposition temperature 6008C.
Fourthly, the Si0.7Ge0.3 ®lm deposited at around 7008C has a randomly oriented structure. The development of the structure has been related to the different work functions of the differently oriented crystals and to the modi®ed Bravais theory of crystal growth, in which the growth velocity of a particular plane is proportional to the density of surface lattice points in that plane [19]. For silicon and germanium, which have a diamond structure, the crystallographic planes in order of importance are the {110}, {111}, and {100}. In addition, the work function of the {110} plane is higher than that of the {111} plane. Both factors favored the growth of the {110} textured grains in the poly-Si and poly-Ge ®lms deposited by MBD on amorphous substrate at more or less amorphous-polycrystalline transition temperature, respectively [19,20]. For the Si12xGex alloys, which have also a diamond structure, the {110} orientation can be preferential orientation at more or less amorphous-polycrystalline transition temperature. However, it is interesting to note that the ®lm deposited at
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Fig. 9. The variation of the average grain size with deposition temperature.
Fig. 8. (a) Bright-®eld plan-view, (b) bright-®eld, and (c) dark-®eld crosssectional TEM micrographs of the Si0.7Ge0.3 ®lm deposited on SiO2 at deposition temperature 7008C.
4508C had the {311} texture as well as the {110} texture and the grain size was largest compared to those deposited at any other substrate temperatures. Bisaro et al. [21] reported that for the case of Si deposited by LPCVD, the appearance of the {110} texture is explained by a nucleation of crystal occurring during deposition, whereas the observed the {311} texture is supposed to be due to a nucleation mechanism occurring during the ®rst stages of deposition followed by a solid phase crystallization mechanism. Moreover, Hwang et al. [6] reported that the {311} textured polySi12xGex formed by the solid phase crystallization. Therefore, their results reveal that the {311} texture is closely related to the solid phase crystallization. We suppose that
the grains deposited at 4508C can be grown not only by deposition, but also by the solid phase crystallization. When a crystallite deposits at initial deposition process, the crystallite is grown in the lateral direction by consuming surrounding amorphous as well as in the vertical direction by deposition. Film deposited at about the amorphous-crystalline transition temperature is associated with higher nucleation rate, therefore, the nucleus density increases rapidly and then the crystallites cover all over substrate surface. In this case, the grain growth competition occurs during which those crystallites preferentially oriented for fast, vertical growth survive at the expense of misoriented, slowly growing grains, thus the ®nal structure of the ®lms become columnar structure [22]. In addition, on the explanation in previous section, the preferentially orientation for fast, vertical growth is k110l, so the ®lm has a {110} textured columnar structure. However, even after reaching temperature formed a continuous polycrystalline ®lm, the developing grain structure could be strongly in¯uenced by the amount of thermal energy available for surface migration. Deposition conditions, which allow the adsorbed atoms to diffuse farther on the surface before being immobilized by subsequently arriving atoms lead to larger grains and a better de®ned structure. Surface migration increases with increasing substrate surface temperature because of the greater random motion associated with the increased thermal energy. This implies that at much higher temperature than amorphouspolycrystalline transition temperature, all oriented crystallites can form and grow without any limitation. Although the texture of the poly-Si ®lms deposited by CVD on SiO2 depends on the partial pressure, at the atmospheric pressure and amorphous-polycrystalline transition temperature of near 6008C, {110} textured ®lms are formed, however, as the temperature increases over 8008C, poly-Si ®lms become a random texture [23,24].
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In our MBD system, the amorphous-polycrystalline transition temperature was about 5008C, so poly-Si0.7Ge0.3 ®lm deposited at 5008C has a strong {110} texture. As the deposition temperature increases, the {311} and {111} textures develop at the expense of the {110} texture and the grain size increases, so that surface migration of adsorbed atoms increases. At 7008C, it is a suf®cient temperature to form all oriented crystallites and to grow without any limitation. Therefore, poly-Si0.7Ge0.3 ®lm deposited at 7008C has a random orientation and structure. 5. Conclusion The evolution of microstructure and texture of molecular beam deposited Si0.7Ge0.3 ®lms on SiO2 in the temperature range of 400±7008C have been studied with use of X-ray diffraction and transmission electron microscopy. In the temperature range of 400 to below 5008C, the ®lm was deposited both as an amorphous phase and a crystalline phase, simultaneously, and the grains have a inversely cone-shaped structure. At 5008C, the ®lm was deposited as only a crystalline phase and the grains have a columnar structure with strong {110} texture. The ®lm deposited at 7008C has a random texture and structure. In both temperature ranges deposited as a mixed-phase and as a crystalline phase, as the deposition temperature increases, the grain size increases and the {311} and {111} textures develop at the expense of the {110} texture, respectively. Acknowledgements This work was supported by the Institute of Information Technology Assessment and the Korean Science and Engineering Foundation (KOSEF) through the Center for Interface Science and Engineering of Materials. The authors acknowledge Dr. S.C. Lee at Electronics and Telecommunications Research Institute (ETRI) for the preparation of MBD samples.
References [1] T.J. King, K.C. Saraswat, IEEE Trans, Electron Devices 41 (1994) 1581. È zturk, Y. Zhong, D.T. Grider, M. Sanganeria, J.J. Wortman, [2] M.C. O M.A. Littlejhon, Proc. SPIE: The Int. Soc. for Opt. Eng. (1990) 260. [3] T.J. King, J.P. McVittie, K.C. Saraswat, IEEE Trans. Electron Devices 41 (1994) 228. [4] T.J. King, K.C. Saraswat, J. Electrochem. Soc. 141 (1994) 2235. [5] F. Edelman, Y. Komem, S.S. Lyer, J. Heydenreich, D. Baither, Thin Solid Films 222 (1992) 57. [6] C.W. Hwang, M.K. Ryu, K.B. Kim, S.C. Lee, C.S. Kim, J. Appl. Phys. 77 (1995) 3042. [7] T.I. Kamins, Polycrystalline Silicon for Integrated Circuit Application, Chapter 2, Kluwer Academic, Boston, 1988 p. 174. È zturk, Y. Zhong, S.P. Ashburn, M. Sanganeria, [8] D.T. Grider, M.C. O J.J. Wortman, Circuit Issues in Semiconductor Materials and Processing Technologies (1992) 55. [9] C.A. King, J.L. Hoyt, C.M. Gronet, J.F. Gibbons, M.P. Scott, J. Turner, Electron Device Lett. 10 (1989) 52. [10] J. Holleman, A.E.T. Kuiper, J.F. Verweij, J. Electrochem. Soc. 140 (1993) 1717. [11] R.B. Iverson, R. Reif, J. Appl. Phys. 57 (1985) 5169. [12] R.H. Wentrof, J.S. Kasper, Science 139 (1963) 338. [13] S.C. Lee, J.Y. Lee, S.J. Yun, J. Cryst. Growth 150 (1995) 974. [14] T.I. Kamins, M.M. Mandurah, K.C. Saraswat, J. Electrochem. Soc. 125 (1978) 927. [15] P. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley, M.J. Whelan, Chapter 6, Electron Microscopy of Thin Crystals, Krieger, New York, 1977. [16] R. Drosd, J. Washburn, J. Appl. Phys. 53 (1982) 397. [17] A. Brokman, R. Gat, Y. Alpern, Appl. Phys. Lett. 49 (1986) 382. [18] F.C. Hull, W.J. Howk, J. Metals 5 (1953) 565. [19] S.S. Chao, J. Gonzalez, D. Martin, R. Tsu, Appl. Phys. Lett. 46 (1985) 1089. [20] M. Matsui, Y. Shiraki, E. Maruyama, J. Appl. Phys. 53 (1982) 995. [21] R. Bisaro, J. MagarinÄo, N. Proust, K. Zellama, J. Appl. Phys. 59 (1986) 1167. [22] E.A. Matson, S.A. Polyakov, Phys. Stat. Sol. A 41 (1977) K93. [23] R.M. Anderson, J. Electrochem. Soc. 120 (1973) 1540. [24] P. Joubert, B. Loisel, Y. Chouan, L. Haji, J. Electrochem. Soc. 134 (1987) 2541.
Effect of substrate temperature on the texture and structure of polycrystalline Si0.7Ge0.3 ®lms deposited on SiO2 by molecular beam deposition Hong Seung Kim*, Jeong Yong Lee Department of Materials Science and Engineering, KAIST, Taejon 305-701, South Korea Received 27 April 1998; received in revised form 18 February 1999; accepted 26 February 1999
Abstract The evolution of microstructure and texture of molecular beam deposited Si0.7Ge0.3 ®lms on SiO2 at the deposition temperature range of 400±7008C was investigated by X-ray diffraction and transmission electron microscopy. At deposition temperatures between 400 and below 5008C, the ®lms were directly deposited as a mixed-phase on SiO2 and have a inversely cone-shaped structure. In this temperature range deposited as a mixed-phase, the grain size increases as the temperature increases, so that the grains not only grow up by deposition, but also laterally grow by the solid phase crystallization, furthermore, the texture is changed from a {110} texture to mixed {311} and {110} textures. At 5008C, the ®lm was deposited as only a crystalline phase and has a columnar structure with a strong {110} texture. In the temperature range of 500±7008C, as the temperature increases, the {311} and {111} textures develop whereas the {110} texture reduces. The ®lm deposited at 7008C has a random orientation and structure. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Germanium; Molecular beam epitaxy; Structure properties; Transmission electron microscopy (TEM)
1. Introduction Polycrystalline Si12xGex (poly-Si12xGex) alloy is recently an attractive candidate for an alternative to polycrystalline Si (poly-Si) for various applications in microelectronics technology [1±3]. Since the melting point of Si12xGex is lower than that of Si, physical phenomena controlling fabrication processes such as deposition [4], crystallization [1,5,6], and dopant activation [3] occur at lower temperatures for Si12xGex than for Si. Thus, for applications in microelectronics technologies, which have limited thermal budget allowances, poly-Si12xGex is preferable to poly-Si. Particularly, poly-Si12xGex formed at much lower temperatures than the softening points of conventional glass plates are promising as a active layer in thin-®lm transistors (TFTs) for a low-temperature process large area electronics technology [1]. In addition to lowering thermal budget, poly-Si12xGex has higher charge carrier mobility, resulting in lower sheet resistance, and variable work functions according to the Ge mole fraction [3]. Therefore, several potential uses for poly-Si12xGex have recently been investi* Corresponding author.
gated as a dopant-diffusion source for the formation of shallow junctions in Si [2], as a gate material in CMOS technologies for improved tradeoff between n-channel and p-channel doping design [3]. The texture and microstructure of poly-Si12xGex ®lms are important properties like those of poly-Si so that the grain size and orientation affect the electrical properties of the materials as the dopant distribution and charge trapping at grain boundary [7]. Although some device characteristics have been studied [1,3], a systematic study of the texture and the resulting microstructure of as-deposited polySi12xGex alloy ®lms has not been performed. The deposition studies on poly-Si12xGex ®lms prepared by conventional low pressure chemical vapor deposition (LPCVD) and rapid-thermal CVD (RTCVD) on SiO2 have reported [4,8]. However, CVD system of Si12xGex has limitations such as higher oxygen contents [9] and dif®culty nucleating on oxide [4,10]. Our work addresses using a molecular beam deposition (MBD) under ultrahigh vacuum (UHV). This paper is to investigate the texture and the microstructure of as-deposited poly-Si0.7Ge0.3 alloy ®lms prepared by MBD on SiO2 at various substrate temperatures. We discuss
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0022 3-0
H. Seung Kim, J. Yong Lee / Thin Solid Films 350 (1999) 14±20
15
TEM specimens were prepared by the lift-off process [11]: the sandwiched SiO2 layer was etched with a HF : H2 O 1 : 1 solution. Cross-sectional TEM specimens were prepared using mechanical polishing followed by ionmilling using a specimen stage cooled by liquid nitrogen. Plan-view and cross-sectional TEM specimens were examined in a JEOL JEM 2000EX electron microscope operated at 200 kV. 3. Results 3.1. The evolution of texture
Fig. 1. X-ray diffraction patterns of the Si0.7Ge0.3 ®lms deposited on SiO2 at deposition temperatures of 400, 450, 500, 600, and 7008C.
on the growth mechanism in relation to the evolution of the texture and the resulting microstructure. 2. Experiment Samples were grown on thermally oxidized Sik100l wafers using a Riber SIVA45 solid source molecular beam epitaxy (MBE) system. The base pressure of the growth chamber was maintained less than at 1 £ 10210 Torr and did not exceed 1 £ 1029 Torr during the deposition. Both pieces of high-purity ¯oating zone (FZ) single crystal silicon and germanium were used as sources, respectively. The deposition rate was about 0.1 nm/s and the total deposition time of each ®lm was 2000 s, thus the nominal thickness of each ®lm was approximately 200 nm. The growth temperatures were varied in the range of 400± 7008C and the Ge mole fraction which was corrected by Rutherford backscattering spectroscopy (RBS) was ®xed 0.3. The deposited samples were investigated both by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The X-ray texture was measured by observing the diffracted intensity at twice the angle of incidence of the X-ray beam in a RIGAKU D/MAX-RC X-ray diffractometer with 40 kV copper radiation. Plan-view
Fig. 1 shows the XRD results of the Si0.7Ge0.3 alloy ®lms at deposition temperatures (400, 450, 500, 600, and 7008C). In the XRD results of all the ®lms, the diffraction peaks appearing at around 2u 28, 47, and 558 represent the diffraction peaks from the {111}, {220}, and {311} planes, respectively, characteristic of a material with the diamond cubic crystal structure. It shows that poly-Si0.7Ge0.3 ®lms are deposited on amorphous substrates at such low temperature as about 4008C by MBD, whereas poly-Si0.7Ge0.3 were deposited at over 5008C by CVD [4]. In addition, two extra diffraction peaks are exhibited. The one is at around 2u 358 which is diffracted not from Si0.7Ge0.3 ®lm, but from Si (100) substrate. The other appears on the low angle side of the {111} diffraction at around 2u 268. The interplanar spacings, dhkl, can be determined from Bragg's law, nl 2dhkl ´sinu with n 1 and l 0:154 nm for Cu Ka . The value of dhkl at around 2u 268 is approximately 0.335 nm, thus it is larger than d111Ge which is 0.326 nm. This additional peak labeled as (111)dh has originated from the diamond hexagonal (dh) structure of the Si12xGex layer [12,13]. The texture of the ®lms was determined from XRD in Fig. 1. In order to quantify the texture of the samples, the orientation factor Ohkl is de®ned as follows X Ihkl I 1 Ohkl hkl = Fhkl hkl Fhkl where for each diffraction plane {hkl}, Fhkl is the scattering correction factor and Ihkl is the measured intensity corrected by an absorption factor to take into account the fact that the ®lms are not in®nitely thick. The ®nite thickness normalization consisted in dividing the measured intensity by the following calibration factor Gx [14] ÿ 2 Gx 1 2 exp 22mt=sinu where m is the absorption coef®cient, t is the thickness of the ®lm, and u is the diffraction angle of each peak. Both the scattering correction factor and the absorption coef®cient for the Si0.7Ge0.3 ®lm are obtained by weight averaging the correction factors and the absorption coef®cients of Si and Ge according to the composition, respectively. According to our de®nition for Ohkl,, this corresponds directly to the
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volume fraction of material oriented in the khkll direction. Considering that only the three major diffraction peaks from the ®lm, i.e., the {111}, {220}, and {311} were above the noise level, samples whose three Ohkl are close to 0.33 have no preferential orientation, that is, it is a random texture. Fig. 2 shows the variations of the orientation factor Ohkl determined for three directions, i.e., k111l, k220l, and k311l, as a function of the substrate temperature. At all the substrate temperatures, O110 is larger than 0.33, and specially, at 5008C, it is up to 0.7. However, at 4508C, although O111 is only around 0.2, O311 increases almost same value of O110 at around 0.38, respectively. It means that the ®lm deposited at 4508C has a mixed {110} and {311} texture. In addition, at high temperature of 7008C, O311 and O111 increase around 0.33 and 0.31, respectively, whereas O110 decreases around 0.33, therefore, the deposited ®lm has a random texture. 3.2. The results of TEM Fig. 3a,b show a bright-®eld and a dark-®eld plan-view TEM micrographs of a Si0.7Ge0.3 ®lm deposited at 4008C. Amorphous regions, one of which is indicated by an arrow, with uniform gray contrast, are embedded in small grains that show a bright and a dark contrast in Fig. 3a,b. It shows that the ®lm forms a mixed-phase. Dark and bright stripes, which were explained by narrow twin plates [15], are observed in the small grains. Fig. 4a,b show a bright-®led and a dark ®eld cross-sectional TEM micrographs of a Si0.7Ge0.3 ®lm deposited at 4008C. The grains with a bright and a dark contrast have a V appearance starting from the ®lm/SiO2 interface and ending at the ®lm surface, and the amorphous regions, one of which is indicated by an arrow, with uniform gray contrast, are observed between V shaped
Fig. 3. (a) Bright-®eld and (b) dark-®eld plan-view TEM micrographs of the Si0.7Ge0.3 ®lm deposited on SiO2 at deposition temperature 4008C.
Fig. 2. Orientation factor Ohkl versus deposition temperature for the Si0.7Ge0.3 ®lms.
grains in Fig. 4a,b. The width of the grains is very small at near the SiO2 interface and increases until the meeting of the grains as the ®lm thickness increases, whereas the volume of amorphous regions decreases with the increase in the ®lm thickness. As results of Figs. 3 and 4, the grains have inversely cone-shaped structure. Moreover, dark and bright stripes, which are perpendicular to the substrate or incline about more or less 108 to the normal direction of substrate, are observed in the grains of Fig. 4. These stripes are also owing to thin laminar {111} twins. Drosd and Washburn [16] showed that if twinning occurs, k110l and k112l are fast growth directions for diamond cubic materials. Brokman et al. [17] concluded that growth proceeds in the k112l direction, which is along twin boundaries and eventually the k110l growth mechanism proposed by Drosd and Washburn take over. Therefore, the grains with twin boundaries parallel to the normal direction of substrate have a {110} texture. Fig. 5a±c show a bright-®eld plan-view and crosssectional and a dark-®eld cross-sectional TEM micrographs of a Si0.7Ge0.3 ®lm deposited at 4508C. Amorphous regions,
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Fig. 4. (a) Bright-®eld and (b) dark-®eld cross-sectional TEM micrographs of the Si0.7Ge0.3 ®lm deposited on SiO2 at deposition temperature 4008C.
one of which is indicated by an arrow, were still observed and the grains have also a inversely cone-shaped structure. However, the grains in Fig. 5a are dramatically larger and the width and surface curvature of the grains in Fig. 5b,c increase, compared to those of the Si0.7Ge0.3 ®lm deposited at 4008C. Fig. 6a,b show a bright-®eld plan-view and crosssectional TEM micrographs of a Si0.7Ge0.3 ®lm deposited at 5008C. The grains with a bright and a dark contrast were observed, however, no amorphous region was found in Fig. 6a. In addition, in Fig. 6b, the amorphous region was not observed, too, and the grains were grown by columnar structure. Many {111} twins, which are perpendicular to the substrate or incline about 108 to the normal direction of substrate, are observed in Fig. 6b. Fig. 7a,b show a bright-®eld plan-view and crosssectional TEM micrographs of a Si0.7Ge0.3 ®lm deposited at 6008C. Fig. 8a±c show a bright-®eld plan-view, a bright®eld cross-sectional, and a dark-®eld cross-sectional TEM micrographs of a Si0.7Ge0.3 ®lm deposited at 7008C. The
Fig. 5. (a) Bright-®eld plan-view, (b) bright-®eld, and (c) dark-®eld crosssectional TEM micrographs of the Si0.7Ge0.3 ®lm deposited on SiO2 at deposition temperature 4508C.
grains were still grown by columnar structure in Fig. 7b, however, in Fig. 8b,c, the grains were not grown by columnar form, but by random form. Moreover, the density of {111} twins was reduced and the surface curvature was increased, compared to those of the ®lms in Figs. 6b and 7b. The average grain size of each sample obtained by using the grain boundary crossing technique [18] from a pair of the bright-®eld and the dark-®eld plan-view micrographs of each sample is shown in Fig. 9. The average grain sizes of the Si0.7Ge0.3 ®lms deposited at 450 and 7008C are about 150 and 100 nm, respectively, whereas others are about 60 nm.
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Fig. 6. (a) Bright-®eld plan-view and (b) bright-®eld cross-sectional TEM micrographs of the Si0.7Ge0.3 ®lm deposited on SiO2 at deposition temperature 5008C.
Compared to the evolution of the texture in Fig. 2, it is interesting that as the {110} orientation factor, O110, decreases, the average grain size increases. 4. Discussion On the basis of the experimental results in previous section, we can classify four morphologically well-characterized different structures according to the substrate temperature. Firstly, at low temperature, 4008C, the Si0.7Ge0.3 ®lm is deposited by both amorphous phase and crystalline phase, that is, it is composed of a mixed-phase and the crystals are inversely cone-shaped structure with {110} texture. Secondly, the Si0.7Ge0.3 ®lm deposited at around 4508C have also a mixed-phase, and the crystals are inversely cone-shaped structure with {311} and {110} mixed texture. Thirdly, at around 5008C, the Si0.7Ge0.3 ®lm is completely deposited by crystalline phase and the crystals are columnar structure with strongly {110} texture.
Fig. 7. (a) Bright-®eld plan-view and (b) bright-®eld cross-sectional TEM micrographs of the Si0.7Ge0.3 ®lm deposited on SiO2 at deposition temperature 6008C.
Fourthly, the Si0.7Ge0.3 ®lm deposited at around 7008C has a randomly oriented structure. The development of the structure has been related to the different work functions of the differently oriented crystals and to the modi®ed Bravais theory of crystal growth, in which the growth velocity of a particular plane is proportional to the density of surface lattice points in that plane [19]. For silicon and germanium, which have a diamond structure, the crystallographic planes in order of importance are the {110}, {111}, and {100}. In addition, the work function of the {110} plane is higher than that of the {111} plane. Both factors favored the growth of the {110} textured grains in the poly-Si and poly-Ge ®lms deposited by MBD on amorphous substrate at more or less amorphous-polycrystalline transition temperature, respectively [19,20]. For the Si12xGex alloys, which have also a diamond structure, the {110} orientation can be preferential orientation at more or less amorphous-polycrystalline transition temperature. However, it is interesting to note that the ®lm deposited at
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Fig. 9. The variation of the average grain size with deposition temperature.
Fig. 8. (a) Bright-®eld plan-view, (b) bright-®eld, and (c) dark-®eld crosssectional TEM micrographs of the Si0.7Ge0.3 ®lm deposited on SiO2 at deposition temperature 7008C.
4508C had the {311} texture as well as the {110} texture and the grain size was largest compared to those deposited at any other substrate temperatures. Bisaro et al. [21] reported that for the case of Si deposited by LPCVD, the appearance of the {110} texture is explained by a nucleation of crystal occurring during deposition, whereas the observed the {311} texture is supposed to be due to a nucleation mechanism occurring during the ®rst stages of deposition followed by a solid phase crystallization mechanism. Moreover, Hwang et al. [6] reported that the {311} textured polySi12xGex formed by the solid phase crystallization. Therefore, their results reveal that the {311} texture is closely related to the solid phase crystallization. We suppose that
the grains deposited at 4508C can be grown not only by deposition, but also by the solid phase crystallization. When a crystallite deposits at initial deposition process, the crystallite is grown in the lateral direction by consuming surrounding amorphous as well as in the vertical direction by deposition. Film deposited at about the amorphous-crystalline transition temperature is associated with higher nucleation rate, therefore, the nucleus density increases rapidly and then the crystallites cover all over substrate surface. In this case, the grain growth competition occurs during which those crystallites preferentially oriented for fast, vertical growth survive at the expense of misoriented, slowly growing grains, thus the ®nal structure of the ®lms become columnar structure [22]. In addition, on the explanation in previous section, the preferentially orientation for fast, vertical growth is k110l, so the ®lm has a {110} textured columnar structure. However, even after reaching temperature formed a continuous polycrystalline ®lm, the developing grain structure could be strongly in¯uenced by the amount of thermal energy available for surface migration. Deposition conditions, which allow the adsorbed atoms to diffuse farther on the surface before being immobilized by subsequently arriving atoms lead to larger grains and a better de®ned structure. Surface migration increases with increasing substrate surface temperature because of the greater random motion associated with the increased thermal energy. This implies that at much higher temperature than amorphouspolycrystalline transition temperature, all oriented crystallites can form and grow without any limitation. Although the texture of the poly-Si ®lms deposited by CVD on SiO2 depends on the partial pressure, at the atmospheric pressure and amorphous-polycrystalline transition temperature of near 6008C, {110} textured ®lms are formed, however, as the temperature increases over 8008C, poly-Si ®lms become a random texture [23,24].
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In our MBD system, the amorphous-polycrystalline transition temperature was about 5008C, so poly-Si0.7Ge0.3 ®lm deposited at 5008C has a strong {110} texture. As the deposition temperature increases, the {311} and {111} textures develop at the expense of the {110} texture and the grain size increases, so that surface migration of adsorbed atoms increases. At 7008C, it is a suf®cient temperature to form all oriented crystallites and to grow without any limitation. Therefore, poly-Si0.7Ge0.3 ®lm deposited at 7008C has a random orientation and structure. 5. Conclusion The evolution of microstructure and texture of molecular beam deposited Si0.7Ge0.3 ®lms on SiO2 in the temperature range of 400±7008C have been studied with use of X-ray diffraction and transmission electron microscopy. In the temperature range of 400 to below 5008C, the ®lm was deposited both as an amorphous phase and a crystalline phase, simultaneously, and the grains have a inversely cone-shaped structure. At 5008C, the ®lm was deposited as only a crystalline phase and the grains have a columnar structure with strong {110} texture. The ®lm deposited at 7008C has a random texture and structure. In both temperature ranges deposited as a mixed-phase and as a crystalline phase, as the deposition temperature increases, the grain size increases and the {311} and {111} textures develop at the expense of the {110} texture, respectively. Acknowledgements This work was supported by the Institute of Information Technology Assessment and the Korean Science and Engineering Foundation (KOSEF) through the Center for Interface Science and Engineering of Materials. The authors acknowledge Dr. S.C. Lee at Electronics and Telecommunications Research Institute (ETRI) for the preparation of MBD samples.
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