+
+
1428
Langmuir 1996, 12, 1428-1441
ACS Award in Colloid or Surface Chemistry Lecture Toward a Strategic Surface Science: Progress and Pitfalls Thomas Engel† Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700 Received June 19, 1995. In Final Form: August 23, 1995X Current discussion on how science is being practiced has focused on the issue of whether fundamental research is sufficiently relevant to societal goals. In this article, it is demonstrated that fundamental surface science, which seeks an atomic level understanding of surface processes, is to a significant extent strategic in nature. Examples of research in the areas of nucleation and growth, fundamentals of environmental remediation, semiconductor processing chemistry, and the development of new analytical tools are used to support this thesis. The argument is advanced that a broader implementation of strategic research is desirable and could be achieved though an articulation of societal goals which is clearer and more detailed. A systems approach to coordinating education and research at universities and in industry would promote strategic research as well as lead to a smoother transition for young scientists from the university to the workplace.
1. Introduction The relationship between scientists and our society is in a period of rapid change. Looking back over the four decades in which research and development based on fundamental science has been heavily funded by the federal government, it is clear that the future will be quite different from the past. In the post-World War II era, the relationship between the general population and scientists was not unlike that between a toddler and a parent; little was thought to be beyond the capabilities of the scientific elite. At present, the relationship is more similar to that between an adolescent and a parent. The scientist is consistently challenged, his expertise is called into question, and previously accepted values are critically examined. Much of this is for the good in that the rationale for large scale science needs a reexamination. As has been pointed out in a number of reviews,1,2 the driver for R&D funding for the past four decades has been national defense, which since 1949 has consumed on average 60% of the total R&D budget. It is no longer clear that this is either wise or necessary. However, alternative long range blueprints linking science to societal goals have not been widely discussed. A leading politician has suggested that science provide support for an industrial ecology which encompasses the integration of economics, technology, and environment.1 He states that the development of new materials will be particularly important to the achievement of industrial ecology. An articulate view of how chemists can contribute to the development of new materials has been laid out in a recent National Academy publication.3 At a time when such plans are being discussed, scientists can play an important role in their formulation. This suggests that a discussion of appropriate goals for science is both timely and necessary. In place of this † E-mail address
[email protected]; FAX (206) 6858665; telephone (206) 685-2330. X Abstract published in Advance ACS Abstracts, February 15, 1996.
(1) Brown, G. E., Jr. Phys. Today 1994, September, 31. (2) Schmitt, R. W. Phys. Today 1994, September, 29. (3) Advancing Materials Research; Psaras, P. A., Langford, H. D., Eds.; National Academy Press: Washington, DC, 1987.
larger discussion on how to structure a science policy that focuses research and development on widely accepted societal goals, much recent attention has been focused on less central issues. One example is the move toward an increased emphasis on strategic research, which is supposedly diametrically opposed to curiosity-driven research. This detracts from the broad discussion which is needed to formulate the societal goals toward which science can work in the future. To frame a policy in which science should be more responsive to societal needs in terms of catchwords like basic and applied research is fundamentally flawed. A broad effort to address important issues such as environmental remediation, global warming, and the molecular design of new materials will involve scientists and nonscientists in a coordinated effort in which both long range and short range research and development need to be pursued. Excellent science will always produce ideas which have the potential for changing society for the better. The real questions which we need to ask are what issues should scientists address, what resources are we willing to commit to science, and what is the framework within which universities, national laboratories, and industrial laboratories can work in an effective and timely manner. It is important to have a wide ranging exchange of ideas so that a broad and stable consensus on the role of science emerges. If this happens, a debacle such as that which occurred with the superconducting supercollider will be avoided. Scientists actively engaged in research can and should have a greater role in formulating science policy. This article is an attempt to begin such a participation within the surface science community. The purpose of this article is to illustrate that fundamental surface science based on an atomic level understanding of materials has important implications for future technologies. In the absence of a (much needed) widely accepted set of goals, this is a good first step toward a strategic surface science which will function as a technology driver. With a definition of strategic surface science established by example, some ideas which might be useful in developing an improved framework for expanding this kind of work will be discussed. Due to the limitations of space, no attempt is made at completeness in this survey and
+
+
Strategic Surface Science
Langmuir, Vol. 12, No. 6, 1996 1429
Figure 1. STM images of thiophene, 2,5-dimethylthiophene, and 2,2′-bithiophene adsorbed on Ag(111) at 120 K. All molecules show preferential adsorption at step edges and demonstrate rotational alignment due to specific chemical interactions between the molecule and the step edge. Image sizes are approximately 60 Å × 60 Å for the first two panels and 25 Å × 25 Å for the third.10
apologies are offered in advance for the many fine examples of strategic surface science which have been overlooked. 2. Examples of Current Strategic Surface Science Surfaces are the interface for our interaction with the world around us. A wide range of technologies has emerged which relies on the optimization of surface properties such as corrosion resistance, specific chemical activity, catalytic selectivity, and electrical conductivity. The appropriate role for fundamental surface science in this endeavor is to search for the atomic level factors which influence surface properties. A measure of success in this venture is the ability to predict material properties and to assemble new materials based on atomic level models. Due to the combined work of many researchers over the last few decades, surface scientists have made significant first steps in understanding the properties of surfaces on an atomic level. This review will focus on very recent experiments in which atomic level characterization has led to a deeper understanding of surface processes. For the most part, the experiments have been carried out on model surfaces which reflect rather than duplicate real world situations. Further advances in experimental methods and analysis are needed to allow atomic level studies of surface phenomena which take place at more complex interfaces such as the liquid-membrane interface or in an electrochemical environment. The work described below falls broadly into several categories which highlight the materials research side of surface science rather than the whole field. They include nucleation and growth, environmental surface science, fundamentals of semiconductor processing, and the development of new analytical tools. 2.1. Nucleation and Growth. In the coming era of nanofabrication, devices such as microelectronics components and sensors will in many applications require assembly at the molecular level. This in turn necessitates an understanding of how molecules attach to a substrate and bind to one another to create an organized assembly. This issue has been approached from a number of different starting points including experimental studies of the selfassembly process,4,5 simulations of growth using calculated interaction potentials,6,7 and spectroscopic and structural studies of the growth process.8,9 At this stage of our
knowledge, it is useful to examine a number of systems which exhibit different interactions in order to provide a basis for further modeling studies. It has become clear that control over the kinetic parameters involving adsorption, diffusion, and reaction are vital in generating useful surface films and solid-solid interfaces. Thermodynamic considerations play an important role, but kinetic control allows metastable configurations to be prepared which for many practical applications are sufficiently stable to be as useful as equilibrium films. How do molecular films form on a solid substrate? This question has been addressed in recent years using atomic resolution scanning tunneling microscopy (STM). Figure 1 shows work from the laboratory of Robert Hamers10 for the initial growth of thiophene, bithiophene, and 2,5dimethylthiophene films on Ag(111). It is clearly seen that the initial growth occurs at step edges where the adsorbed molecules experience a stronger binding due to the higher coordination number at step sites. These images also show a preferential molecular orientation of the biothiophene with respect to the step edge which influences further film growth because this initial adsorption acts as a template for the rest of the monolayer film. The initial film growth is determined by the interaction of an individual molecule with the surface. This leads to rotational ordering by the steps as well as an orientation of the molecular plane to the surface. At high coverages, intermolecular forces become important. In this regime, the molecular plane is no longer parallel to the surface, and the out of plane tilt accommodates a higher packing density. As is seen in Figure 2, this film reorganization process can be imaged in real time using STM. Although the step edges act as the initial nucleation sites for the film, they are the last sites to be converted (4) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380. (5) Ulman, A. Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (6) Villarba, M.; Jo´nsson, H. Surf. Sci. 1994, 317, 15. (7) Jacobsen, J.; Jacobsen, K. W.; Stoltze, P.; Norskov, J. K. Phys. Rev. Lett. 1995, 74, 2295. (8) Bauer, E. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1984. (9) Campbell, C. T. Annu. Rev. Phys. Chem. 1990, 41, 775. (10) Frank, E. R.; Chen, X. X.; Hamers, R. J. Surf. Sci. 1995, 334, L709.
+
1430
+
Langmuir, Vol. 12, No. 6, 1996
Engel
Figure 2. Sequential STM images showing nucleation and growth of thiophene on Ag(111) in the herringbone phase at 120 K. The time after initiation of adsorption is indicated in each image.10
to the herringbone structure characteristic of the compressed film. As it is not feasible to experimentally characterize all possible growth processes with the sophistication of these experiments, experiments such as these are needed to provide the basis for modeling studies necessary to predict film growth in related systems. This is an important goal of surface scientists. The key to an understanding which can be transferred to other filmsubstrate combinations is to examine the influence of such important parameters as step site density, molecular asymmetry, and interactions within the film at high packing densities and to integrate this knowledge into the computer simulation environment. Another interface which is of particular importance for technology is the metal-oxide interface. In heterogeneous gas catalysis, active metals are often supported on high area oxide substrates in order to expose as many metal atoms as possible to the reactive environment. Metalceramic materials are widely used in materials science and microelectronics are dependent for their utility on the specific properties of the metal-oxide interface. Energetic considerations can be used to predict that in general metal films will not wet oxide surfaces. However, this information is insufficient to predict the film configuration in a specific case, and the kinetics of adsorption, diffusion, and desorption will determine whether flat islands, spherical metal particles, or even needle-like spires will develop in growing a metal film on an oxide substrate. Atomic resolution studies using STM are also particularly useful in exploring the film morphology under different growth conditions. In Figure 3, STM images are shown for gold grown on an oxygen covered ruthenium
single crystal surface.11 Significant differences are seen in the film morphology for the two coverages shown. At low coverages, most of the surface is covered with small gold islands which show a shape characteristic of Au single crystals. At high coverages, very flat mesa-like islands are formed whose height is dictated by the coverage. Most importantly, a significant fraction of the oxide surface is not covered by the metal film. Although well-known, this behavior has not been understood. A model which explains the major features of film growth for metals on oxide substrates has recently been developed in the group of Charles Campbell.12,13 The essential elements of the model are depicted in Figure 4. Even at temperatures on the order of 150 K, individual metal atoms can diffuse rapidly on the oxide substrate. As the attractive forces between metal atoms are stronger than their attraction to the substrate, isolated monolayer islands with an interatomic spacing characteristic of the metal will form rather than a continuous film with an interatomic spacing characteristic of the oxide. Since the binding energy of a metal atom is greatest at the island edge, initially all atoms incident from the gas phase which impinge on the island will be incorporated at the edges, leading to island growth. However, as the island size increases, more diffusion steps are required for a metal atom in the second layer of a metal island to reach the island boundary. Therefore, it will become increasing probable that nucleation will occur in the second layer. As more metal atoms are deposited, additional (11) Huang, R. Q.; Gu¨nther, C.; Schroeder J.; Gu¨nther, S.; Kopatzi, E.; Behm, R. J. J. Vac. Sci. Technol. 1992, A10, 1970. (12) Ernst, K. H.; Ludviksson, A.; Zhang, R.; Yoshihara, J.; Campbell, C. T. Phys. Rev. 1993, B47, 13782. (13) Campbell, C. T.; Ludviksson, A. J. Vac. Sci. Technol. 1994, A12, 1825.
+
Strategic Surface Science
+
Langmuir, Vol. 12, No. 6, 1996 1431
Figure 4. (a) Schematic side view of an oxide surface with a one atom thick metal island. The simplified potential energy diagram shows the activation barriers that would be experienced by an incoming diffusing metal atom. With these barriers, diffusion across the oxide surface and over the island is rapid at 130 K except at the island boundary. (b) Diagram similar to that in (a) for coverages at which second layer nucleation has occurred. Note the decreased barrier at the right edge of the island which allows metal atoms diffusing across the bare oxide surface to be incorporated into higher layers of the metal island.12,13 Figure 3. (a, top) STM image (1500 Å × 1500 Å) of monolayer thick Au islands on a oxygen precovered Ru(0001) surface. The film has been annealed to 600 K before imaging. This led to a coalescence of smaller irregularly shaped islands into larger well-ordered islands. (b, bottom) STM image (10500 Å × 10500 Å) of a 5 monolayer average thickness Au film on oxygen-covered Ru(0001) after annealing to 800 K. The Au has been redistributed into large flat-topped islands which are 20 to 30 monolayers thick.11
layers grow sequentially, and the mesa-like islands shown in Figure 3 are predicted by the model. There is also an important feature of this model which explains why a significant fraction of the substrate remains uncovered by the metal film even when the average coverage is many monolayers. When the islands are more than a few monolayers high, the presence of metal atoms in the second and higher layers significantly reduces the barrier for a metal atom diffusing over the oxide to be incorporated into higher layers of the metal island. Because of this, even those atoms which initially impinge on the oxide substrate are incorporated into the growing metal islands. These experiments, which were carried out for copper deposition on ZnO, show the importance of the relative rates of various kinetics steps in determining film
morphology. The model which emerged in the explanation of the results offers a way of understanding film growth which should be applicable to many metal-oxide systems. One of the goals in the growth of materials is to produce a particular film morphology. The desired end result might be a uniform coherent film, but could also be quite different. An example of this is the production of nanoparticle assemblies which exhibit quantum confinement. As the energy levels characteristic of these particles depend on their size, they can be very useful in the development of new optoelectronic devices. One- and two-dimensional quantum confinement leads to quantum wires and quantum dots, respectively. From a fundamental point of view, it is important to understand what atomic level processes govern the growth of such particles and under what conditions a narrow distribution of sizes can be achieved. Some interesting insights into these issues have emerged from atomic force microscopy (AFM) and electron microscopy experiments in the group of W. Henry Weinberg.14 These authors have studied the growth of InP islands by metalorganic chemical vapor deposition on InGaP/ (14) DenBaars, S. P.; Reaves, C. M.; Bressler-Hill, V.; Varma, S.; Weinberg, W. H.; Petroff, P. M. J. Cryst. Growth 1994, 145, 721.
+
1432
+
Langmuir, Vol. 12, No. 6, 1996
Engel
Figure 6. Schematic sketch of the principle of cross-sectional STM on heterostructures. After growth, the wafer is cleaved exposing a surface normal to the growth direction. Electron microscopy is used to locate the tip near the growth interface.16,17
Figure 5. AFM images (3 × 104 Å × 3 × 104 Å) of the InP/ InGaAs surface after deposition of InP. The three different types of islands are indicated with arrows. The plane view image in (a) (top) shows the lateral extent of the islands whereas the three-dimensional plot in (b) (bottom) illustrates the marked difference in height between the island types as well as the uniform size distribution of the B islands.14
GaAs(001). They obtain a remarkably uniform distribution of island sizes as shown in Figure 5. Using a combination of AFM and transmission electron microscopy, they are able to show that the A and B type islands are crystalline and grow coherently on the substrate in a strained layer epitaxy and that the C type islands are incoherent and defected. An analysis of the growth for various combinations of flux and temperature reveals that the B islands are formed through coalescence of the A islands and also reveals that there is a considerable jump in height between the tallest A islands and the shortest B islands. The electron microscopy studies show that the initial growth is in the form of two-dimensional islands or the Stranski-Krastanov mode. Due to the strain induced by the lattice mismatch, growth in this mode ceases above a critical island diameter and the islands become more three-dimensional. This leads to the formation of type B islands. The discontinuity in island height between the A and B islands is due to fortuitous kinetics.
At a further critical island size, the continued growth of B islands is no longer energetically favorable because of the strain, and dislocations form, leading to type C islands. This work shows that it is the interplay of energetic factors in which the lattice strain is critical and kinetic growth factors which finally lead to the nearly uniform size population of the B islands. It is this balance which is needed to produce quantum dots. These examples have been taken from studies of film growth in which the film surface must take on a defined form. Often it is the interface between two solids which must be controlled. Examples of this are found in semiconductor technology where, for instance, the interface between Si and SiO2 is of critical importance for gate oxide performance.15 Atomic level analysis of these buried interfaces is also possible with the technique of crosssectional STM shown in Figure 6. After epitaxial layers are grown on a single crystal GaAs substrate, the wafer is cleaved along a 〈001〉 direction which exposes a (110) surface normal to the initial wafer surface. Using in situ electron microscopy, the STM tip is moved to the interfacial growth region and the interface can be scanned with atomic resolution. The usefulness of this method has been shown by Huub Salemink and co-workers16,17 in a study of the GaAs-AlGaAs interface. The Al atoms reside at Ga lattice sites, but it is not clear a priori whether a random alloy or a preferential islanding of the Al takes place. A crosssectional STM image is shown in Figure 7 together with individual line scans. It is seen that the Al and the Ga sites can be distinguished on the basis of their corrugation amplitude. This allows a statistical analysis of the probability of finding Al sites adjacent to Ga sites or other Al sites. The result, which is also shown in Figure 7, is that it is more probable to find Al atoms next to occupied Al sites. With an analytical method of this sensitivity, it has now become possible in many systems to analyze the structure and chemical composition of buried interfaces with atomic resolution. A final example in the area of nucleation and growth is noteworthy because it goes beyond the ultrahigh vacuum conditions under which fundamental surface science is usually done. Whereas much work in nanoscale fabrication is concerned with control of lateral dimensions, it is (15) Hirose, M.; Hiroshima, M.; Yasaka, T.; Miyazaki, S. J. Vac. Sci. Technol. 1994, A12, 1864. (16) Salemink, H. W. M.; Johnson, M. B.; Albrektsen, O. J. Vac. Sci. Technol. 1994, B12, 362. (17) Pfister, M.; Johnson, M. B.; Alvarado, S. F.; Salemink, H. W. M.; Marti, U.; Martin, D.; Morier-Genond, F.; Reinhart, F. K. Appl. Phys. Lett. 1994, 65, 1168.
+
+
Strategic Surface Science
a
Langmuir, Vol. 12, No. 6, 1996 1433
b
c
Figure 7. (a) Empty state STM image of the group III (Ga, Al) sublattice on the (110) plane showing the GaAs-AlGaAs-GaAs growth interface. The uniform and nonuniform corrugations within the GaAs and AlGaAs regions, respectively, can be seen. The enhanced reactivity of the Al for residual gas species is responsible for the irregular features seen in the center of the image. (b) Detail image and line scans across the growth interface. These scans show that it is possible to distinguish between Ga and Al in the alloy on the basis of corrugation heights. (c) Coordination number for Al sites obtained from analyses of images such as that shown in part b (solid circles) with what would be expected from a random alloy (open circles). This shows that there is a tendency for Al atoms to cluster in the alloy.16,17
also important to control dimensions perpendicular to the growth direction with atomic precision. Atomic layer epitaxy18 is a useful method which relies on self-limiting reactions to produce a single layer of a growing film. If a repeating sequence of self-limiting reactions can be combined, they can be interleaved to produce film growth one atomic layer at a time. Layer-by-layer growth of SiO2 has been achieved by the group of Steven George19 by dividing the overall reaction (18) Hankka, S.; Lakomaa, E. L.; Suntola, T. Thin Solid Films 1993, 225, 280.
SiCl4 + 2H2O f SiO2 + 4HCl into the two half reactions
Si-OH* + SiCl4 f SiO-Si-Cl3* + HCl SiCl* + H2O f SiOH* + HCl In each half reaction, the asterisks indicate the surface species. The reaction conditions and the half reaction sequence are shown in Figure 8. Although relatively high pressures are needed to take each reaction to completion, it would not be possible to verify that film growth was
+
1434
+
Langmuir, Vol. 12, No. 6, 1996
Figure 8. Reaction conditions and schematic picture of atomic layer epitaxy in the growth of SiO2 layers using the sequential deposition of SiCl4 (A) and H2O (B).19
Figure 9. Illustration of the changes in the surface coverage of Cl and O during the first three deposition cycles.19
controlled with monolayer precision under reaction conditions. The authors combined the high-pressure cell with an ultrahigh vacuum chamber in which the uptake could be measured with the necessary accuracy. Figure 9 shows the results for the first three cycles and the oxygen uptake is shown for up to ten cycles in Figure 10. These results show that after an initial phase of three cycles, very reproducible amounts of SiO2 are formed in each cycle. The film thickness is fully consistent with atomic layer epitaxy produced by half reactions which become selflimiting when each successive monolayer is filled. This combination of high-pressure reaction and low-pressure spectroscopic studies provides an example of how the basis for atomic layer epitaxy can be established through atomic level studies for a particular reaction system of interest. 2.2. Fundamental Studies Relevant to Environmental Remediation. Environmental remediation is an area of great importance for basic science because of the great need for technology development. Current estimates put the costs for remediation at contaminated Department of Energy sites at 250 billion dollars. Analysis and in situ remediation are areas in which new techniques could substantially reduce these costs. A potentially very useful means of purifying contaminated water and air is photocatalytic oxidation using ultraviolet radiation to(19) Sneh, O.; Wise, M. L.; Ott, A. W.; Okada, L. A.; George, S. M. Surf. Sci. 1995, 334, 135.
Engel
Figure 10. Oxygen coverage and SiO2 film thickness as a function of the number of AB cycles. The oxygen coverage obtained from thermal oxidation in H2O only is shown to demonstrate that this process cannot be responsible for film growth.19
Figure 11. Schematic picture of the local geometry of various defect sites on TiO2(110) resulting from oxygen vacancy formation. Vacancies result in a local nonstoichiometry.21,22
gether with TiO2 photocatalysts.20 In order to devise suitable catalyst materials, it is necessary to establish reaction mechanisms and quantify reaction pathways for well-characterized systems. To date, nearly all fundamental studies have been carried out on colloidal particles which do not allow a detailed characterization of surface sites. A beginning of an atomic level picture of photocatalytic oxidation which incorporates surface properties has emerged from work done in the laboratory of John T. Yates, Jr.21,22 Work that this group has carried out on TiO2(110) shows that adsorbed molecular rather than dissociated or lattice bound atomic oxygen is necessary to photooxidize CO. Moreover, molecular oxygen is only bound to the surface at defect sites produced by the removal of oxygen ions from lattice sites. The types of sites believed to be active for binding O2 are shown in Figure 11. The concentration of these sites can be controlled by heating the surface under vacuum as oxygen is preferentially desorbed because it is less strongly bound than the Ti. It is expected that defect production is a function of both temperature and oxygen partial pressure, which in an ultrahigh vacuum (20) Aquatic and Surface Photochemistry; Helz, G. R., Zepp, R. G., Eds.; Lewis Publishers: Boca Raton, FL, 1994. (21) Lu, G.; Linsebigler, A.; Yates, J. T., Jr. J. Chem. Phys. 1994, 98, 11733. (22) Linsebigler, A.; Lu, G.; Yates, J. T., Jr. To be published.
+
Strategic Surface Science
Figure 12. Photoformation of CO2 from preadsorbed isotopically labeled CO and O2. The TiO2 surface has been heated in vacuum before adsorption of the reactants to produce oxygen vacancies. The upper panel shows that the reaction only proceeds upon illumination with band gap radiation. The lower panel shows that the oxygen-saturated surface, which is believed to be free of oxygen vacancies, is unreactive.21,22
experiment is essentially zero. Figure 12 shows the type of experiment which allowed the authors to develop their reaction model. After adsorption of CO and O2 on an oxygen-deficient surface, the CO2 product is evolved immediately upon illumination with UV light. As can be seen by the low baseline before illumination, the thermal reaction is negligibly slow in comparison with the photocatalytic reaction. The Yates group has also carried out experiments which have addressed the differences between ideal single crystal and powder samples for the same reaction. As can be seen from Figure 13, transmission infrared absorption measurements can determine the time-dependent coverage of the CO and O2 species which are intermediates in the reaction. By simultaneously monitoring the spectral region in which OH groups absorb radiation, they were able to show that for these reaction conditions, the reaction proceeds via molecularly adsorbed oxygen rather than OH as the reactive intermediate. The identification of surface sites remains a central issue in the investigation of surface reactions, because the design of new catalytic materials requires a knowledge of the relationship between structure and reactivity. Whereas for most reactions on metal surfaces there are many reactive sites and reactivity is in general not a strong function of the local geometry, the issue is more complex in surfaces with directional bonding. For example, Avouris and co-workers have shown that the local geometry significantly affects the reactivity of Si atoms at different sites on Si(111).23 The structure of oxide surfaces is not known, with the exception of a few cases,24 and the importance of these materials in understanding environmental processes makes it important that structure(23) Avouris, P. Acc. Chem. Res. 1995, 28, 95. (24) Henrich, V. E.; Cox P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, 1994.
+
Langmuir, Vol. 12, No. 6, 1996 1435
reactivity relationships on oxides are explored. Andras Szabo and Michael Sander have begun such a study in our laboratory by investigating the structure of TiO2(110) as a function of oxygen coverage.25,26 A surprising structural complexity was found. In addition to the expected (1 × 1) phase, a (1 × 2) phase with appreciable displacement of the surface species from their normal sites and several more complex surface phases were observed. When a TiO2(110) surface is heated in vacuum above 1000 K, the surface undergoes reconstruction to a (1 × 2) phase and oxygen is desorbed. If the surface is exposed to O2 at the same temperature, the (1 × 1) phase is not restored. Instead, highly anisotropic islands of TiO2 are formed, presumably from the reaction of gas phase oxygen with Ti diffusing out from the bulk. This can be seen in Figure 14a. As this process is carried to completion, the surface becomes covered with poorly ordered rows whose spacing is about 35 Å (on the order of 12 lattice spacings), as is seen in Figure 14b. These rows can be better ordered and partially desorbed by gentle heating, leading to the phase seen in Figure 14c. This well-ordered phase has an unusually large unit cell of 13 Å × 35 Å and consists of several structural elements. The smaller spacing arises through the (1 × 2) phase and the larger spacing is due to cross-linking rows. The cross-linking rows consist of two identical strands as can be seen from the antiphase relationship of the two double strand segments joined by a single strand. This unanticipated structural complexity shows that we do not fully understand how local site geometries are formed in the balance of equilibrium and kinetic driving forces. It remains to be seen whether such complex structures are of significance in chemical reactivity. As a high resolution image of the cross-linking regions shows, a local c(2 × 2) arrangement is formed, suggesting the model shown in Figure 14d. If such metastable films of TiO2 involving sublayers of the three-dimensional lattice can be grown and stabilized, new materials not present under equilibrium conditions can be synthesized. This is one of the main goals of surface scientists. Both the kinetics studies discussed above and atomic resolution STM studies are needed in order to identify the sites which are active for photochemical detoxification. With a clearer picture of the surfaces required for optimum catalytic activity, serious efforts to prepare practical working photocatalysts can begin. 2.3. Semiconductor Processing Chemistry. The gap between fundamental surface science and manufacturing technology is probably smaller in microelectronics than in any other mature technology. The techniques of ultrahigh vacuum, surface-sensitive spectroscopies, and single crystal substrates are common to researchers and device fabricators. For this reason, there is a close working relationship between industry and fundamental scientists which continuously leads to the development of new tools for technology. Several examples have been given in section 2.1 in which the growth of quantum dots and the degree of ordering in AlGaAs alloys were discussed. In this section, the atomic level science of silicon etching and electron beam lithography will be discussed. Pattern definition on wafers is accomplished by the removal of substrate material in a controlled fashion. As chemical etching in an equilibrium environment is generally isotropic, additional features such as reactant directionality must be incorporated to maintain lateral pattern definition while removing material normal to the surface. This is usually referred to as anisotropic etching and (25) Sander, M.; Engel, T. Surf. Sci. 1994, 302, L263. (26) Szabo, A.; Engel, T. Surf. Sci. 1995, 329, 241.
+
+
1436
Langmuir, Vol. 12, No. 6, 1996
Engel
Figure 13. Infrared absorbance measurements of powdered TiO2 taken in the transmission mode. Spectral regions characteristic of different reactants, products, and surface intermediates are indicated.21,22
excellent fundamental science has been done to elucidate plasma etching27 which is the current industry standard. Although this is a highly successful method, there are reasons to believe that ion-based techniques may be limited in their application as devices take on smaller and smaller dimensions. This is due to the lattice damage induced by momentum transfer from the relatively energetic ions and by dielectric breakdown due to charge buildup on ultrathin insulator films. A method which may avoid these drawbacks is neutral beam etching, in which anisotropy enters though the directed beam of reactant species. In order to assess this potential technology, the mechanism for silicon etching at an atomic level must be understood. Of particular importance in this regard is the spatial dependence of etching. If the reactant, which is delivered uniformly over the surface, forms islands through rapid diffusion prior to reaction, the etching front morphology will be different than if the reactant reacts where it initially impinges on the surface. This issue has been addressed by the group of Andrew Kummel.28 They investigated the etching of Si(111) by varying the energy of incident chlorine molecules, and Figure 15 shows their results. It is seen that the critical parameter in determining the uniformity of the reaction is the translational energy of the incident molecule. Slow molecules diffuse rapidly over the surface before dissociating to form surface reaction intermediates. This leads to a nonuniform distribution, whereas fast molecules dissociate upon impact and lead to a uniform distribution of reaction intermediates. Studies such as these are important in laying the groundwork for a future technology based on atomic layer etching. A further example of how the energy of the incoming species can influence the reaction outcome emerged from (27) Winters, H. F.; Coburn, J. W. Surf. Sci. Rep. 1992, 14, 161. (28) Jensen, J. A.; Yan, C.; Kummel, A. C. Science 1995, 267, 493.
the work of Andras Szabo and Paul Farrall in our laboratory.29 Figure 16 shows the results of molecular beam studies in which a chopped beam of fast chlorine atoms impinged on a Si(100) surface. The SiCl4 reaction product was detected synchronously using a line of sight mass spectrometer. At the lowest surface temperatures, the product waveform is a square wave, showing that the surface residence time is negligible. This leads to the surprising conclusion that the reaction is thermally activated above 180 K but shows no activation energy below this temperature. These experiments show that there are two channels for SiCl4 production by hyperthermal beams of chlorine atoms, only one of which is thermally activated. The nonthermal channel results from direct momentum transfer between an incident chlorine and a surface intermediate present only at Cl coverages in excess of one monolayer. This channel is of particular interest with respect to anisotropic etching because the reaction crosssection shows normal energy scaling. This is illustrated in Figure 17 in which it is shown that only the normal component of the incident momentum influences the reaction probability. This enhances the anisotropy of the etching beyond that of the directionality of the beam since a Cl atom incident on a trench sidewall will have an appreciably lower reaction probability than one which impinges on the bottom of the trench. Under our experimental conditions, the highest energy was limited to 1 eV, and it is seen that the reaction probability is still increasing beyond a conversion of 3% of the incident Cl into SiCl4. This suggests that if more energetic atoms are delivered to the surface at a higher flux, this nonthermal etching channel could be accessible at higher temperatures which would make it more compatible with wafer processing conditions. (29) Szabo, A.; Farrall, P. D.; Engel, T. J. Appl. Phys. 1994, 75, 3623.
+
Strategic Surface Science
+
Langmuir, Vol. 12, No. 6, 1996 1437
Figure 14. STM images of TiO2(110) after different thermal and oxygen adsorption treatments. (a, top left) Anisotropic island formation on the 1 × 2 phase generated by annealing in an O2 pressure of 5 × 10-7 Torr at 1020 K. The image shows a 1000 Å × 1000 Å area. (b, top right) Ordering observed after growth of the islands shown in (a) is continued until saturation is achieved. The image shows a 1000 Å × 1000 Å area. (c, middle left) Further ordering of the structure shown in (b) is observed after gentle annealing to 1000 K. The long period structure is the same as in (b), but a local c(2 × 2) structure is now obvious. (d, middle right) Structure observed after partial desorption at 1020 K starting from the structure shown in (c). The 1 × 2 reconstruction is stabilized by cross-linking rows with local c(2 × 2) periodicity. Note the antiphase relationship between the two strands of the cross-linking rows near the center of the 250 Å × 250 Å image. (e, bottom) Model of the relationship between the structural elements of the 1 × 2 phase and the cross-linking rows. The small circles are the Ti and the large circles are the O ions.25,26
+
1438
+
Langmuir, Vol. 12, No. 6, 1996
Engel
Figure 15. STM images of the distribution of Cl atoms produced by the dissociation of Cl2 molecules with different translational energies. The slower molecules are able to diffuse laterally on the surface prior to dissociation, leading to island formation. The faster molecules dissociate upon impact with the surface, leading to a more uniform Cl coverage.28
Figure 18. STM image showing a pattern of lines written by using the tunneling current to desorb hydrogen atoms from a hydrogen-terminated Si(100) surface. The spacing between lines is 30 Å and the line is approximately two atoms wide. The arrows indicate the transition between regions in which one or both H atoms on a dimer pair have been removed.30
Figure 16. SiCl4 evolution as a function of time for different surface temperatures. A hyperthermal Cl flux is incident on a Si(100) surface. The incident beam is square wave modulated with a frequency of 50 Hz. If the product is formed in a nonthermal channel, the SiCl4 waveform should be identical to the incident waveform. This is the case for temperatures below 180 K. For a normal thermally activated reaction, the waveform should be distorted by a residence time which decreases with increasing temperature. This is seen for temperatures above 170 K.29
Figure 17. Steady state etching rate in the nonthermal channel at 130 K as a function of the normal component of the energy for an incident hyperthermal beam. The fact that the data points for different incident angles lie on the same line shows that the surface potential is not strongly corrugated. This property of the reaction system will enhance the etching anisotropy.29
A final example concerns the definition of patterns using electron beams. In a collaboration between Phaedon (30) Shen, T.-C.; Wang, C.; Abeln, G. C.; Tucker, J. R.; Lyding, J. W.; Avouris, P.; Walkup, R. E. Science 1995, 268, 5217.
Avouris and J. W. Lyding,30 electron impact desorption of hydrogen from a hydrogen-covered Si(100) surface was studied using a STM. In the field emission mode, normal electron impact desorption was observed with a threshold energy which was independent of the electron current. Lines written by desorbing the hydrogen were relatively broad. In the STM mode, it was observed that there is no well-defined threshold for electron impact desorption and that the cross section depends on both the electron current and energy. These results are consistent with vibrational pumping of the Si-H bond facilitated by the long lifetime of the vibrationally excited species and the high current density in the STM current filament. This new mechanism for electron impact desorption is chemically specific as vibrational lifetimes differ substantially for different species. Because of the close proximity of the tip and surface, the tunnel current filament is very narrow. As Figure 18 shows, it is possible to write lines which are only a few atoms wide. The set of lines which are spaced only 30 Å apart are clearly distinguishable. If e-beam lithography were carried out with a single STM, it would clearly be much too slow to be useful. However, a microfabricated array of STMs could scan a wafer in a parallel mode much more quickly. The extremely high resolution attainable would make this a very attractive possibility for the near atomic length scales needed in future devices. 2.4. New Analytical Tools. In the past, surface scientists have been actively involved in the development of new methods for determining the properties of surfaces. ESCA, also known as X-ray photoelectron spectroscopy, Auger electron spectroscopy, low-energy electron diffraction, reflection high-energy electron diffraction, molecular beam epitaxy, and scanning tunneling and atomic force microscopy, to name just a few, originated from fundamental studies in surface science. The current challenge for surface scientists is to develop techniques which provide atomic level information and which are compatible with the complex surfaces and interfaces found in the real world outside of an ultrahigh vacuum chamber. An important example of such work is the practical development of ac-
+
+
Strategic Surface Science
Figure 19. STM image taken with an ac bias of 7.73 GHz. The sample is a portion of a microchannel plate consisting of lead silicate glass. The precipice at the bottom of the image is the edge of one of the 10 µm channels and the structured area in the center of the image is a defect in the glass surface. Note the high resolution seen in the 4800 Å × 4800 Å image.31
STM which can image insulating surfaces with near atomic resolution. Improving on an idea first implemented by Kochanski,32 the group of Paul Weiss31 has obtained a resolution on the order of 10 Å by scanning an STM using a microwave frequency ac rather than the usual dc bias voltage between the sample and tip. An example of this work is shown in Figure 19. Features of 10 Å can be seen in this image. If these techniques can be sufficiently refined to scan insulating surfaces such as polymers under ambient conditions, an enormous expansion of our ability to describe and quantify surface will occur. 3. The Current Discussion on How to Support Science The examples cited above are characterized by their interdisciplinary nature and by the fact that they have considerable potential for integration into technology development. Is this a straightforward process? The function of scientific research and development in our society is changing from a focus on military needs to a focus on improving our way of life and in competing successfully in a global economy. It is clear that fundamental science can have an enormous effect on the economy and, through it, on us all. Examples such as the transistor, radar, lasers, and molecular biology come to mind readily. High-temperature superconductors, molecular recognition, and tools for manipulating and combining individual atoms may play similar roles in the coming decades. As science has become more sophisticated, it has also become more expensive. The potential payoffs may be high, but considerable up-front investments are required. Under the current climate of budget cutting, it is important to examine the purpose and expense of scientific research, and it is equally important to ask the right questions in doing so. Unfortunately, the ongoing discussion is dominated by three issues which are framed in adversarial terms. A closer examination shows that this is not necessary and that it masks the real debate which needs to take place. These issues are curiosity-driven vs strategic research, (31) Stranick, S. J.; Weiss, P. S. J. Phys. Chem. 1994, 98, 1762. (32) Kochanski, G. P. Phys. Rev. Lett. 1989, 62, 2285.
Langmuir, Vol. 12, No. 6, 1996 1439
basic vs applied research, and teaching vs research at universities. All of these need a reframing to emphasize the important underlying issues. As there are no easy answers, this article will raise questions rather than provide answers. This is still useful because asking good questions is often more valuable than providing correct answers to the wrong questions. The issues listed above are better formulated as follows: What is the appropriate balance between the advancement of science on all fronts and a coordinated focus on a timely issue? What is the appropriate balance in allocating funds between short-term and long-term goals? How should universities allocate their scarce teaching resources between educating all students at the introductory level and in training the next generation of scientists, engineers, and other specialists? When framed in this way, there are no opposing views, rather difficult issues of coordination and the allocation of limited resources among equally laudable possibilities. All those who have looked for the simple way to harness science in the service of technology have learned humility; in this activity, one size does not fit all. Consider a worthwhile goal such as the construction of a sensor-based analytical instrument which will allow real-time, on-site quantitative determination of carcinogenic contaminants in soil samples. It is clear that the realization of such an instrument could save millions if not billions of dollars. However, it is an expensive undertaking involving longterm (basic) and short-term (applied) research in an interactive and synergistic way. It would also require the involvement of university, national laboratory, and industry based researchers in a highly interactive mode in order to succeed. To apply simple categories like “strategic” and “applied” in decision making on initiating projects like this is simplistic and counterproductive. The last of these three issues deals with the most critical part of technology development, namely, a well-educated and creative workforce. By addressing this issue in the form of how many PhDs are needed simply asks the wrong question. Surely the increasing technological orientation of our society will require more rather than less scientific expertise from all citizens. The real question which we need to address is what form should this education take? It may well be that scientist-engineers or scientistengineer-economists will be more useful in the future than research oriented PhDs. This would require a rethinking of university education. This discussion will help us to decide how much of our resources should be spent in what at universities is commonly called research but which should be called graduate education and how much should be spent on education at the undergraduate level. 4. Toward a Strategic Surface Science As the preceding examples show, there is a great deal of research activity in the surface science community which is strategic in nature. It is much more difficult to harvest the fruits of strategically oriented fundamental research done in the single investigator mode than that carried out within the framework of a multiinvestigator institution like a national laboratory. However, since so many good ideas and impulses for technology have come from scientists working as single investigators, it is worth thinking how such work can be supported. In order to facilitate strategic science in the single investigator mode, more needs to be done to clearly define a set of strategic goals and to make scientists aware of them. The need for a widely accepted set of goals becomes
+
+
1440
Langmuir, Vol. 12, No. 6, 1996
clear when the termination of long range projects like the SSC and the shaky foundations for the space station and the advanced technology program at NIST are considered. The National Academies of Science and Engineering and professional societies like the American Chemical and American Physical Societies, industry groups, national laboratories, and the public at large should all be involved in setting up goals for scientific research. However, a list of goals without some degree of prioritization is not useful since it neglects a basic reality: there are not enough funds available to do everything we would like to do. It is essential that the setting of priorities not be left to the political process alone, since this has in the past led to such practices as earmarked projects which bypass the peer review system and the claim that engineering activities such as the space station are basic research. To some extent, goal setting and prioritization activities such as those outlined above already exist. However, they lack broad stakeholder involvement and the prioritization between such different programs as the human genome project, the program on advanced materials, and processing and the development of science in areas of critical technologies. In addition, there is a critical piece missing, namely, how can individual researchers help to make these goals a reality? There is a pressing need for a clearer definition of how the long-term goals translate into action at the individual laboratory scale. For instance, a mature technology for the production of flat panel displays is an important strategic goal. How can one chemist, physicist, or materials scientist help to make this possible? A means to set up an effective dialogue between the funding sources and the scientific community on how broad goals translate into action would do much to draw scientists into a problem-solving mode around societal goals. We also need a more effective way to fund innovate science. To do this largely in the framework of peer-reviewed proposals which take several months to write and an additional 6 months to review, with a final success rate for funding of less than 40%, stifles rather than enhances creative problem solving. 5. Is There a Better Way To Link Unversities with Industry? It is remarkable that research ideas conceived in universities make their way into industry given the somewhat haphazard coupling between these two major players in research and development. However, since such transfer does occur, one should be very careful in coming up with a new structure. However, it is likely that major benefits would come from an improved culture of cooperation between industry and university researchers on the basis of strategic goals. A closer cooperation is already important because of the substantial cutbacks in basic research which have taken place in industry laboratories in the past decade. Industry will come to depend more on universities to carry out research with a long payoff time. Cooperation will become increasingly important in the future as science becomes more expensive and complex and because of the strong interdisciplinary character of the work needed to achieve societal goals. This interdisciplinary character makes new demands on scientists who have been educated in the compartmentalized departmental structure of universities. It increasingly makes sense to view research and development which occurs separately at universities and in industry as two complementary parts of the same system and to attempt to maximize their interaction. Minisabbaticals as short as one week annually would be extremely useful in increasing cooperation. Industry scientists could broaden the undergraduate experience
Engel
by bringing their perspective to the teaching of fundamental science while university scientists would bring new ideas into a system more intent on turning out a product than improving upon it. Industries could also join together to support a staff of liaison scientists who would work toward establishing areas of fruitful cooperation between individual universities and companies. They could also provide an important expansion in the education of graduate students by establishing a network of internships to be taken midway through the doctoral program. This would be of great benfit to graduate students who would directly experience the environment for which they are preparing themselves at a time in which they can make course corrections. Industries could be of much help in donating research materials and services to university research groups. For instance, it is much less expensive for a financially strapped surface science group at a university to work on silicon rather than group III-V compounds and to dispense with such methods as electron microscopy and atomic force microscopy to analyze their samples. However, with the donation of materials and services from industrial partners, the range of problems the students could work on would be considerably expanded. This would be in the interests of industrial laboratories since their access to trained personnel would be much greater. It would also make it much easier for university scientists to involve their research groups in state of the art science. One of the casualties in the attempt of American industry to reinvent itself seems to be broad-based and long-range industrial research. It is characteristic that in the era of the focus on quarterly performance research is viewed as unprofitable. Although AT&T has invested heavily in research through Bell Laboratories, this has not kept MCI from becoming a formidable competitor. In the area of surface science, IBM and AT&T heavily supported the fundamental science of silicon processing technology. This is no longer the case. Will this look like a good decision 20 years from now? If university and industrial research are treated as two complementary parts of a single system, it makes sense to decide first what kind of science should be supported and only then to decide where in the system it can best be done. Such an approach will lead to an increase in joint industryuniversity projects in which each sector can best utilize its capabilities in a joint effort. In the end, it is less important in which sector important science is done than that it is done at all. Viewing universities, industrial labs, and national labs as parts of an interactive system would also lead to savings as unnecessary duplication is eliminated. 6. What Kind of a Scientific Training Would Facilitate Strategic Research? Universities have an important role in the scientific establishment which is not duplicated elsewhere, namely, the training of the next generation of scientists. Recently, the traditional training leading to the PhD degree has been subjected to intense scrutiny. Critics maintain that training in narrow fields of expertise is excellent but that students are insufficiently prepared to enter the workforce.33,34 There is general agreement that the heart of the doctoral program should continue to be an education in solving problems skills using state of the art science. However, it is increasingly important in an interdisciplinary world for scientists to be able to view their specific (33) Reshaping the Graduate Education of Scientists and Engineers; National Academy Press: Washington, DC, 1995. (34) Kostiner, E. Chem. Eng. News 1994, December 19, 52.
+
+
Strategic Surface Science
project in the broader context of related fields. They should also be conversant with the tools and methodologies of disciplines which border on their field of study. The demands of an interdisciplinary approach to science also increasingly require that young scientists learn to communicate effectively. The best PhDs fulfill these criteria and are immediately valuable in the workplace. However, it is not clear that our educational system provides sufficient guidance for the majority of future scientists. In part this is due to an unfamiliarity of university and industry scientists with the culture in which their counterparts operate. For instance, universities have been given the task of evaluating whether students have “learned enough”. In a system in which the teacher to student ratio is continually reduced in order to become more “economical”, education takes on the form of impersonal exams and lectures in which students become passive rather than active participants in learning. The ability to work in groups is not taught. Given this context, it is understandable that young scientists often do not understand the big picture and are not accustomed to working in a team. The case for technical education in science as an investment in the future economy is strong. In an era of budget cutting, it is important to realize that competition in the global marketplace requires a well-trained and creative workforce. This should guide our thinking in taking a new look at graduate training in science and engineering. A fine-tuning of graduate and undergraduate education in science can occur naturally if the increased interaction between universities and industry described above is
Langmuir, Vol. 12, No. 6, 1996 1441
implemented. What we need is a willingness to try to bridge gaps and to address the concerns of each of the two sectors. This will lead to a science more in the interest of societal goals and an education which is better suited to a seamless transition from the university to the workplace for graduating students. 7. Summary The examples outlined above show that surface scientists are making significant progress in obtaining an atomic scale understanding of phenomena closely related to technology. As this has occurred in the current system, large scale changes in how fundamental science is carried out may not be necessary. However, a better climate to carry out strategic science can be created. It would include a clearer articulation of societal goals for science and a clearer formulation on how individual investigators can participate in their realization. It would also include a systems approach to coordinate education and research in industry and at universities. Acknowledgment. A critical reading of this manuscript by Alvin Kwiram is gratefully acknowledged. Research from our laboratory cited above has been supported by the Air Force Office of Scientific Research, the Chemistry Division of the National Science Foundation, and and the donors of the Petroleum Research Fund, administered by the American Chemical Society. LA950479M
