© Copyright 2000 by the American Chemical Society
VOLUME 104, NUMBER 50, DECEMBER 21, 2000
LETTERS Structure of Heteroepitaxial Thin Films of Hafnium Diboride Grown on a Hf(0001) Surface As Determined by Scanning Tunneling Microscopy Michael Belyansky† and Michael Trenary* Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7061 ReceiVed: July 19, 2000
Scanning tunneling microscopy has been used to characterize the initial stages of epitaxial growth of HfB2 on a Hf(0001) surface. The film was grown on the substrate at 780 °C through chemical vapor deposition using diborane gas. The growth mechanism appears to combine 2D layer by layer growth with 3D island formation. The HfB2 film first appears as multilayered islands on top of large unreacted Hf(0001) terraces with the island edges aligned with the substrate step edges. For thicker films only hexagonally shaped multilayered HfB2 islands with an average size of 100 Å are seen.
The growth of epitaxial films is of great interest as it provides a way to create crystalline materials with novel properties that are not accessible by bulk crystal growth methods. While it has long been recognized that the macroscopic properties of epitaxial thin films depend critically on the initiation of the growth process, it is only with the advent of recent experimental techniques that the initial stages of film growth can be experimentally investigated. Foremost among these newer techniques is scanning tunneling microscopy (STM). The combination of experimental STM studies combined with theoretical simulations have provided atomic scale understanding of both homoepitaxial1 and heteroepitaxial2 growth of metal thin films on singlecrystal metal substrates under well-controlled ultrahigh vacuum (UHV) conditions. Even a seemingly simple process such as the homoepitaxial growth of Ag on a Ag(100) substrate shows a variety of different structures depending on growth conditions.1 The STM images often reveal richly detailed structural features that are often not observable with any other technique. In many, if not most, cases a full understanding of the observed STM images is still lacking. It is of general interest to extend such * Corresponding author. E-mail:
[email protected]. Fax: 1 312 996 0431. † Present address: IBM Microelectronics Division, 1580 Route 52, Z/33A, Hopewell Junction, NY 12533.
studies of heteroepitaxial growth to cases where the substrate and film material have distinctly different properties. It is also important to use STM to gain a better understanding of epitaxial growth through chemical vapor deposition (CVD) as this method is of great practical importance.3 Here we report STM images of the structure of an epitaxial thin film of HfB2 grown on the surface of a Hf(0001) single crystal under UHV conditions through chemical vapor deposition using B2H6(g). The transition metal diborides are members of a broad class of materials known as the boron-rich solids,4 which consist of extended networks of covalently bonded boron atoms stabilized through donation of electrons from the metal atoms. While all of the boron-rich solids are extremely hard, this is especially true of the group IV (Ti, Zr, Hf) diborides. These compounds have a simple hexagonal structure in which close-packed layers of metal atoms alternate with planar honeycomb nets of covalently linked boron atoms. The topmost atomic layers of both the Hf(0001) and HfB2(0001) surfaces consist of a closepacked hexagonal lattice of Hf atoms with lattice constants parallel to the (0001) planes that differ by only 1.7%, a mismatch well below the minimum needed for epitaxial growth. Although the close-packed Hf planes in the metal and in the diboride are nearly identical structurally, Hf metal and HfB2 have vastly
10.1021/jp002559r CCC: $19.00 © 2000 American Chemical Society Published on Web 11/28/2000
11834 J. Phys. Chem. B, Vol. 104, No. 50, 2000 different properties. Hafnium diboride is a hard brittle material with a very high melting point5 of 3380 °C while hafnium has the hardness and ductility of a typical metal with a melting point of 2200 °C. Hard coatings of early transition metal carbides and nitrides are in wide use for various applications and yet the corresponding diborides are generally harder.6 In addition to potential hard coating applications, HfB2 is used as a thin film resistor7 and has been shown to have desirable properties as a diffusion barrier in microelectronics.8 While these factors alone would suffice to motivate studies of diboride thin film growth, the HfB2/Hf system is unusual in possessing a crystalline interface between a metal and a material with ceramic-like properties. In previous work9 we have found that epitaxial thin films of HfB2 on a Hf(0001) single-crystal substrate can be produced through the reaction of diborane with the substrate at temperatures of 700-900 °C. The formation of the epitaxial films was verified by LEED and XPS measurements within the UHV chamber and by subsequent analysis in air with X-ray diffraction (XRD). To unambiguously characterize the evolution of the surface from Hf(0001) to HfB2(0001), we have carried out extensive studies of the surface properties of several different single crystals of Hf(0001)10 and HfB2(0001).11 This has allowed us, for example, to show that the LEED, XPS, and chemical reactivity characteristics of the HfB2 epitaxial film were essentially the same as those of a clean, well-ordered HfB2(0001) single crystal. Here we compare the structural characteristics of the HfB2 film with those of the (0001) surfaces of Hf and HfB2 as determined by STM. The UHV chamber and associated instruments have been described in detail elsewhere.12,13 In brief, the system consists of a stainless steel UHV chamber equipped with instruments for LEED, XPS, and STM. The STM tips were made from 0.010" diameter W wire that was electrochemically etched to a fine point and then dipped in HF for 1-2 s before placement in the UHV chamber. The Hf(0001) crystal was purchased from Metal Crystals and Oxides, Ltd. as a 6 mm diameter, 1 mm thick disk oriented to within 0.5° of the (0001) plane and polished to a mirror finish. The crystal was mounted on a Mo sample holder and was heated by electron bombardment from behind and the temperature was measured with an optical pyrometer. A separate Hf(0001) crystal was used in the earlier HfB2 epitaxial growth studies and the same cleaning procedure was used for the present study.9 As we have noted earlier, it proved impossible to obtain a completely oxygen-free substrate as indicted by XPS. The STM images of individual terraces of the Hf(0001) surface also show features indicative of contamination, presumably due to patches of oxide. The work reported here was performed when XPS showed the sample to be free of all contaminants except oxygen and when a simple (1 × 1) LEED pattern was observed. These are the same conditions that prevailed in our previous work9 establishing that an epitaxial HfB2 film could be grown on Hf(0001) and our main intent here is to gain further insight into the epitaxial growth mechanism. The semiconductor purity diborane gas was obtained from Matheson Gas Products as a 1% mixture in 99.999% pure argon. The HfB2 films were formed by exposing the sample held at 780 ( 10 °C to a background pressure of (3 ( 1) × 10-5 Torr of the B2H6/Ar mixture for various times followed by a quick anneal of the sample to 850 °C. After the sample was allowed to cool back to room temperature, which took ∼30 min, it was transferred to the STM stage. A further 2 h were needed for the sample and STM to reach thermal equilibrium so as to minimize thermal drift. The STM images were obtained
Letters
Figure 1. STM image (a, top) of a HfB2 film prepared by a 30 min exposure of a 1% B2H6 in Ar mixture at 3 × 10-5 Torr to a Hf(0001) surface held at 780 °C and profile (b, bottom) of the line indicated in (a).
with a sample bias of -10 mV and a constant tunneling current of 1 nA. The displayed images were not filtered except for plane removal. In our earlier study9 in which the epitaxial nature of the HfB2/ Hf(0001) system was established, exposure times of up to 6 h were used. The thickness of the resulting films was estimated by XPS to be at least 40-60 Å. The thickest films studied with STM were obtained with a 3 h exposure, which presumably gave a film thickness of at least 20-30 Å. Although XPS and LEED showed the film to be epitaxial HfB2, the STM images showed a very rough surface with features on the order of 50 Å. A much shorter exposure time of only 30 min yielded the STM image shown in Figure 1. Although the XPS spectrum corresponding to this image showed a B 1s peak at the correct binding energy for HfB2, the B/Hf ratio was much smaller than for the HfB2(0001) single crystal, or for the thicker films, indicating that the thickness in this case was much lower than the electron escape depth. The image shows up to seven different hexagonally shaped layers. The height difference between terraces is 3.5 Å, which is the step height observed on the HfB2(0001) surface and corresponds to the Hf layer spacing in bulk HfB2. In contrast, the step height of the Hf(0001) substrate was observed to be only 2.5 Å, the distance between adjacent Hf atom planes. Moreover, the Hf(0001) surface shows long terraces with an average width of 600 Å. The individual terraces in Figure 1 are much rougher with a corrugation of 0.5 Å compared to less than 0.1 Å found on the HfB2(0001) surface. Figure 2 shows an STM image of an even thinner film obtained from a 20 min exposure. This image shows two distinct regions. In the upper right corner are multiple layers similar to what is seen in Figure 1, whereas toward the bottom of the image are two long broad terraces separated by a narrow terrace. The broader terraces have a corrugation and step shape typical of the Hf(0001) substrate. This suggests that the region in the
Letters
Figure 2. STM image of a HfB2 film after a 20 min exposure of a 1% B2H6 in Ar mixture at 3 × 10-5 Torr to a Hf(0001) surface held at 780 °C.
Figure 3. Profiles corresponding to the three lines marked on the image in Figure 2.
upper right corner is covered by the HfB2 film whereas the rest of the image is of the bare substrate. This interpretation is confirmed in Figure 3, which shows profiles of the lines indicated in Figure 2. Profile B shows a high density of terraces separated by 3.5 Å indicating that this is indeed a HfB2 region while profile C shows a step height of 2.5 Å characteristic of the Hf(0001) surface. The corrugations within the individual terraces of the two regions are also characteristic of the bare Hf surface and of the HfB2 film. The largest Hf(0001) terrace
J. Phys. Chem. B, Vol. 104, No. 50, 2000 11835 also shows some accumulation of material at the upper step edge that is not so apparent on the narrower middle terrace. Accumulation of extra material at step edges is commonly observed in STM images and is also seen in the images of the film-free Hf(0001) substrate. Regardless of the identity of the step-edge material in Figure 2, the fact that it appears more prominently at the edge of the larger terrace than on the narrower terrace suggests that intra-terrace diffusion of the material is much faster than inter-terrace diffusion. Otherwise all step-edges would be decorated to the same extent. Profile A shows that in the HfB2 region the density of terraces is so high that double steps of 7 Å are also present. The alignment of the steps in the two regions is indicated by the dotted lines which show that at least one edge of the HfB2 terraces is oriented parallel to the steps of the substrate and therefore that the film grows in a way that preserves azimuthal alignment with the substrate. The structure of thin films as determined by STM reflects the kinetics of various microscopic rate processes as well as thermodynamics, which dictates the equilibrium structure. Great insights into the kinetics of film growth have been obtained through temperature dependent STM studies in which metastable structures can be created and then observed to evolve to more stable ones as a function of either time or annealing temperature. However, for the most part such detailed studies have been confined to simple cases of one element deposited on another in which chemical reactions play no role. The case considered here is more complicated in that it involves two important chemical reactions: (1) the decomposition of diborane gas to deposit elemental boron and (2) the reaction of the deposited boron with hafnium to form the diboride. Furthermore, the temperature was chosen so as to give the best quality films as determined by techniques other than STM. The use of even higher temperatures leads to the loss of boron from the surface region through diffusion into the bulk. Because the films were subjected to temperatures as high at 850 °C, we will assume that the observed structure reflects the most energetically favored configuration. In interpreting the images it is useful to first consider a highly idealized hypothetical growth process. Since the HfB2(0001) surface is Hf terminated, the deposited boron must eventually penetrate below the topmost Hf layer of the Hf(0001) crystal. With the high temperatures used here, boron deposition and penetration may occur simultaneously. However, we have shown that the epitaxial HfB2 film can also be formed by low temperature deposition of boron, which then reacts to form the diboride upon annealing to higher temperatures. It is therefore useful to consider boron deposition and reaction to form the diboride as sequential events. At a boron coverage of 2 monolayers, the boron could have the exact graphite-like structure of the boron layer in the diboride, provided it were expanded by 1.7 %. Such a case would be akin to a pseudomorphic layer in which the first monolayer conforms to the structure of the underlying surface. This would present a surface corresponding very closely to a boron-terminated surface, provided it had the structure of a bulk-like (0001) boron plane of the diboride. This is the structure of the boron-terminated TaB2 (0001) surface.14 Assuming the first step is the formation of a topmost boron layer, the next step would be for this layer to diffuse below the topmost Hf layer to occupy the positions between the first and second Hf layers. At the same time the separation between the first and second Hf layers would have to increase from 2.5 to 3.5 Å and rearrange to form an AA type stacking as opposed to the AB stacking found in hcp Hf. The stacking of layers of HfB2 on top of regions of unreacted
11836 J. Phys. Chem. B, Vol. 104, No. 50, 2000 Hf terraces as revealed in Figure 2 indicates that the reaction to form the diboride occurs before the surface is covered with boron and that the deposited boron tends to segregate to patches where diboride formation has already occurred. Consideration of the evolution of the step-structure of the Hf substrate to that of HfB2 is also instructive. Two adjacent terraces of Hf(0001) are separated by 2.5 Å and the second Hf layer of the upper terrace is coplanar with the topmost Hf layer of the lower terrace. Inserting a complete diboride-like boron layer just below the topmost Hf layer of each terrace should result in a Hf layer separation directly below each terrace of 3.5 Å. However, if the step height remained at 2.5 Å, then the topmost Hf layer of the lower terrace could no longer be coplanar with the second Hf layer of the upper terrace. Since the step height of the HfB2(0001) single-crystal surface is invariably 3.5 Å, it is necessarily the case that in HfB2 the Hf layer below the topmost Hf layer of the upper terrace must be coplanar with the topmost Hf layer of the lower terrace. Thus, the fact that the first HfB2 layer in Figure 2 is 3.5 Å above the Hf terrace suggests that there is no boron layer beneath the Hf layers of the unreacted Hf(0001) region. This also supports the idea that further HfB2 growth tends to occur in regions of the surface where the reaction to form the diboride has already occurred. The form of heteroepitaxial growth is determined not only by the degree of lattice matching but also by the relative surface energies of film and substrate.15 If the film material has a higher surface energy, 3D island formation is favored. Even at the earliest stages of film growth when submonolayers of film material are deposited, 2D island formation due to attractive interactions in the film material is commonly observed. Twodimensional aggregates are also observed when the film consists of a compound produced through a reaction between the deposited material and the substrate. A well studied example is the formation of Al2O3 on Al(111) in which the oxide develops from dense 1 × 1 islands of chemisorbed oxygen.16 Another example similar in complexity to the present case is the epitaxial growth of β-Ga2O3 on CoGa(001) in which long rectangular islands oriented in the [100], [010] directions are observed on top of large areas of the unreacted substrate.17 In this case Ga atoms diffuse from the bulk to the surface where they react with the oxygen to form the gallium oxide. A multilayered island structure of the sort seen in Figure 1 was reported18 for Cu on Ru(1000), although the island sizes were somewhat smaller than what we observe for the HfB2/Hf(0001) system in Figure 1. The Cu/Ru(0001) images were explained as indicating a combination of layer by layer growth and 3D island formation. Such
Letters a growth mechanism was also reported19 for epitaxial layers of EuTe grown on PbTe where the lattice mismatch is 2.1%. A similar mechanism presumably occurs for the HfB2/Hf(0001) case. Thus, although our understanding of the atomic scale processes that determine heteroepitaxial film structure is still incomplete, it appears that a general understanding of the phenomena will apply to many different interfaces including the HfB2/Hf(0001) system. Acknowledgment. We acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. This work was also supported by the National Science Foundation, DMR941037. We thank Mr. Rasdip Singh for help in preparing the figures and for a critical reading of the manuscript. References and Notes (1) Thiel, P. A.; Evans, J. W. J. Phys. Chem. B 2000, 104, 1663. (2) Brune, H. A. Surf. Sci. Rep. 1998, 31, 121. (3) Shaw, D. W. in Epitaxial Growth; Matthews, J. W., Ed.; Academic: New York, 1982. (4) Hoard, J. L.; Hughes, R. E. In The Chemistry of Boron and Its Compounds; Muetterties, E. L., Ed.; Wiley: New York, 1967. (5) Massalski, T. B.; Murray, J. L.; Bennett, L. H.; Baker, H. Binary Alloy Phase Diagrams; American Society for Metals: Metals Park, OH, 1986. (6) Samsonov, G. V.; Vinitskii, I. M. Handbook of Refractory Compounds; translated by K. Shaw; Plenum: New York, 1980. (7) Wuu, D. S.; Lee, M. L.; Lin, T. Y.; Horng, R. H. Mater. Chem. Phys. 1996, 45, 163. (8) Shappirio, J. R.; Finnegan, J. J.; Lux, R. A. J. Vac. Sci. Technol. B 1986, 4, 1409. Choi, C. S.; Ruggles, G. A.; Shah, A. S.; Xing, G. C.; Osburn, C. M.; Hunn, J. D. J. Electrochem. Soc. 1991, 138, 3062. Sade, G.; Pelleg, J. Appl. Surf. Sci. 1995, 91, 263. Kolwa, E.; Molarius, J. M.; Flick, W.; Nieh, C. W.; Tran, L.; Nicolet, M.-A.; So, F. C. T.; Wei, J. C. S. Thin Solid Films 1988, 166, 29. (9) Belyansky, M.; Trenary, M. Chem. Mater. 1997, 9, 203. (10) Belyanksy, M. Ph.D. Thesis, University of Illinois at Chicago, 1998. (11) Perkins, C. L.; Singh, R.; Trenary, M.; Tanaka, T.; Paderno, Yu. Surf Sci., in press. (12) Foo, W. C.; Ozcomert, J. S.; Trenary, M. Surf. Sci. 1991, 255, 245. (13) Ozcomert, J. S. Ph.D. Thesis, University of Illinois at Chicago, 1992. (14) Kawanowa, H.; Souda, R.; Otani, S.; Gotoh, Y. Phys. ReV. Lett. 1998, 81, 2264. (15) Henzler, M. Surf. Sci. 1996, 357-358, 809. (16) Brune, H.; Wintterlin, J.; Trost, J.; Ertl, G. J. Chem. Phys. 1993, 99, 2128. Trost, J.; Brune, H.; Wintterlin, J.; Behm, R. J.; Ertl, G. J. Chem. Phys. 1998, 108, 1740. (17) Eumann, M.; Schmitz, G.; Franchy, R. Appl. Phys. Lett. 1998, 72, 3440. (18) Po¨tschke, G.; Schro¨der, J.; Gunther, C.; Hwang, R. Q.; Behm, R. J. Surf. Sci. 1991, 251/252, 592. (19) Springholz, G.; Bauer, G. Appl. Surf. Sci. 1996, 104/105, 637.
