UHV Surface Preparation Methods

UHV Surface Preparation Methods C Becker, Aix Marseille Université, CNRS, Marseille, France © 2018 Elsevier Inc. All rights reserved.

Introduction Why Do We Need UHV? Preparation of the Surfaces of Bulk Materials in UHV Heating and Chemical Treatment Ion Bombardment Cleavage in UHV Preparation of Thin Films Metal Films Oxide Films Summary References

580 580 582 582 583 585 586 587 588 589 590

Introduction The rapid development of surface science since the beginning of the 1960s is intimately related to ability to prepare, conserve, and investigate clean surfaces in ultrahigh vacuum (UHV). Progress in surface science is in fact closely related to its potential of providing model systems for real-world applications such as microelectronics, heterogeneous catalysis, and thin film technology just to name a few. In this context, perfectly ordered and atomically clean surfaces provide us with models of reduced complexity, a concept that was already put forward by Taylor in 1950,1 for which basic processes in the domains cited earlier can be elucidated and understood. In an early publication on the preparation of clean surfaces in high vacuum, Roberts reviewed in 1963 six major techniques that were then available to produce atomically clean surfaces.2 At that time, a surface was considered as atomically clean if it contained not more than a few percent of a single monolayer of foreign atoms. This definition was certainly owed to the unavailability of analysis methods with higher sensitivity. Since the advent of scanning tunneling microscopy (STM),3 which provided us with the means for detecting single atoms on solid surfaces, this limit has been pushed down well below one-tenth of a percent of a monolayer. However, even nowadays, for many applications, a contamination level below one percent is still considered to be acceptable since it corresponds to the detection limit of a variety of commonly applied surface science techniques. When talking about the preparation of clean surfaces in UHV, we have, thus, to keep this in mind that the cleaning procedures have to be adapted to the investigation techniques that will be used to characterize the surface. Of course, literature concerning the preparation of clean surfaces is abundant, and even today, it is good practice to give a detailed description of surface preparation in the experimental section of a scientific publication. Therefore, we will limit ourselves to a presentation of the major preparation methods, mainly for metal surfaces, which are illustrated by typical examples. For an overview of preparation methods for elemental surfaces, the reader is referred to the work of Musket et al. who published a rather complete review for a great number of materials in 1982.4

Why Do We Need UHV? Let us consider the concept of a perfectly clean surface. This concept, which may appear very trivial at first glance, becomes a real challenge when considering environmental constraints to which technological surfaces are subject. The first and most important constraint is that these surfaces are in general surrounded by a gas phase, which is rather ill-defined and contains reactive gases that readily react with solid surfaces and will, thus, contaminate them. In the view of the preparation of a clean surface, an important challenge will consequently be protecting the surface from the surrounding gas phase. This can in fact be easily accomplished by placing the surface in vacuum. But what are the vacuum conditions needed in order to keep a surface clean during an appreciable lapse of time? In order to answer this question, we have to investigate the interaction of gases with a solid surface. The first and most important figure we need to know is the number of gas phase atoms or molecules that hit a surface per second as a function of the surrounding pressure. This number is easily accessible using elementary kinetic theory of gases. The incident flux F of a gas on a surface is given by eqn [1]: 1 F ¼ rc 4

[1]

It depends on the molecular gas density r and the average molecular speed c. If we consider an ideal gas, the density r is defined as number of particles per unit volume

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UHV Surface Preparation Methods

r¼

N p ¼ V kB T

581

[2]

and the average molecular speed c can be derived from the Maxwell–Boltzmann distribution to equal rffiffiffiffiffiffiffiffiffiffiffi 8kB T c¼ mp

[3]

Substituting eqns [2] and [3] into eqn [1] leads to the Hertz–Knudsen equation, which expresses the flux F as a function of the molecular mass m, the temperature T, and the pressure p: p F ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pmkB T

[4]

We can immediately see that the flux F is proportional to the pressure p. If we consider, for example, an ambient temperature of T ¼ 298 K and the molecular mass of N2, which is the most abundant gas in our atmosphere, m(N2) ¼ 4.65  10 26 kg, we can establish the following table (kB ¼ 1.3806  10 23 J K 1) illustrating the proportionality of F and p. In order to estimate the impact of impinging gas molecules on the surface cleanliness, we have to correlate this flux F to density of atoms n in a solid surface. Let us for simplicity use the example of a Cu(100) surface. The schematic structure of this surface is shown in Figure 1. It possesses a quadratic unit cell with a lattice parameter of a ¼ 2.5  10 10 m. The area of the surface unit cell, which contains one Cu atom, is thus A ¼ a2 ¼ 6.25  10 20 m2. The number density n of the Cu atoms is then the reciprocal of this value and equals to n ¼ 1.6  1019 m 2. When comparing n to the values of the impinging flux F in Table 1, we can easily see that under low vacuum conditions, which are typically obtained with standard laboratory membrane pumps, more than 106 gas particles per second will on average impinge on a single surface atom. It is quite obvious that under these conditions, the surface will be contaminated in a very short time by the surrounding gas. An important value in the context is of course the number of impinging gas molecules, which are actually trapped on the surface. We can quantify this value by introducing a sticking coefficient s that is defined by the ratio of gas particles, which are trapped (absorb) on the surface nads, compared to the total number of impinging gas particles N: s ¼ nads =N

[5]

Sticking is of course very dependent on the nature of the interaction between the molecule in the gas phase and the surface, but generally speaking, reactive gases such as CO stick more readily to surfaces than rather inert gases such as N2 or CO2. Using the concept of the sticking factor, we can introduce the concept of exposure of a solid surface to gases. In this context, the unit langmuir (L) is often used. One langmuir corresponds to an exposure 10 6 torr during 1 s (1 L ¼ 1.33  10 4 Pa s). If we assume a unity sticking factor, an exposure of 1 L would roughly lead to a monolayer coverage of the surface. This exposure value is thus a rather convenient concept for evaluating the pollution of solid surfaces by impinging gases. Let us consider that for a typical experiment in surface science, we will need a clean surface over a lapse of time of several hours, let us say 104 s. Using the concept of exposure, we can easily estimate the required vacuum to equal 10 10 torr (1.33  10 8 Pa). Even in this case, the surface will be entirely covered by impinging gases if the sticking coefficient were unity s ¼ 1. The sticking coefficient is of course often much smaller, but the need for good UHV conditions (p < 10 8 Pa) becomes obvious from this simple estimation.

a a

Figure 1

Schematic representation of a Cu(100) surface. The square indicates the surface unit cell with a lattice constant of a.

Table 1

Table representing different vacuum regimes and the corresponding flux F of impinging N2 molecules on a surface

Vacuum quality

p (Pa)

p (mbar)

p (Torr)

F (m 2 s 1)

Atmospheric pressure Low vacuum Medium vacuum High vacuum Ultrahigh vacuum (UHV) Typical UHV

1.013  105 103 10 1 10 5 10 7 10 9

1013 101 10 3 10 5 10 9 10 11

760 7.50 7.50  10 4 7.50  10 8 7.50  10 10 7.50  10 12

2.92  1027 2.88  1025 2.88  1021 2.88  1017 2.88  1015 2.88  1013

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UHV Surface Preparation Methods

We should also notice that the sticking coefficient depends on temperature s ¼ s(T) and that it generally for nondissociation adsorption increases when the temperature is lowered. Consequently, experiments where the surface is kept at low temperature require in general better vacuum conditions than experiment at room temperature. From the above said we can draw an important conclusion. The preparation of clean surfaces is, thus, intimately related to the availability of UHV conditions in order to keep the prepared surface clean during the lapse of time, which is required to conduct an experiment.

Preparation of the Surfaces of Bulk Materials in UHV In modern surface science, bulk materials still play an important role when it comes to measurements of the properties of materials with very low complexity. In general, single crystals are used, which are cut along a specific crystallographic direction to expose the surface of interest. Which are the specific problems encountered in the preparation of such a single crystal surface? Firstly, the sample is usually cut from single-crystal rod using a saw or by spark erosion and is then mechanically polished to a roughness of about 0.03 mm. This will inevitably destroy the crystalline order of the single crystal close to the surface. In order to reestablish the order at the surface of the crystal, the sample has to be heated (annealed) in UHV conditions. As a rule of thumb, heating the crystal up to 2/3 the melting temperature (Tm) will accomplish this task. This procedure leads us to the second problem, which is related to the diffusion of impurities (segregation) to the surface during the annealing process. This can lead to an appreciable accumulation of impurities at that surface even though the purity of standard metal single crystals is about 99.999% (see Figure 2).

Heating and Chemical Treatment When it comes to simple and easy to apply cleaning methods, heating in UHV (annealing) has to be mentioned first. This method is certainly not applicable to a great number of materials but can be very efficient if volatile pollutants are concerned. If we consider the composition of the residual gas in a typical UHV system, we can easily see that main components of it are H2, H2O, CO, and CO2. These gases are thus prone to contaminate the clean surface. Of these gases, it is certainly CO2, which causes the least problems because it does not readily chemisorb on solid surfaces due to its chemically inert character. The other gases will however at least on transition metal surface adsorb at room temperature and pollute the surface. The question whether this will lead to an irreversible pollution of the surface in question depends on the details of the interaction and most importantly on a possible dissociation of the molecules. Let us consider a simple example, the Pt(111) surface at room temperature in UHV. On this sample, the three gases in question will behave quite differently. H2 readily dissociates and forms chemisorbed atomic hydrogen. It has been shown that one can get rid of this contamination by heating the surface to temperatures above 400 K,5 which leads to recombination of hydrogen atoms to H2 and subsequent desorption of H2. Likewise, CO, which does not dissociate on the Pt(111) surface, can be desorbed by heating to temperatures above 500 K.6,7 These gases can thus be rather easily desorbed from the surface (see Figure 3). If however traces of unsaturated hydrocarbons (e.g., ethene8) are present in the residual gas, these compounds tend to decompose on transition metal surfaces even at room temperature leaving carbon deposits as contaminant. These traces of carbon can in general be easily removed by oxidation after O2 exposure, which will form either CO or CO2 that often can easily be desorbed. Finally, for water, the problem of surface contamination at room temperature on Pt(111) does not exist since the desorption is completed below 200 K (for an exhaustive review of water interaction with metal surface, see Hodgson and Haq9). Insulating materials and oxides in particular are much less sensitive to contamination by residual gases due to their chemical inertness. Surfaces of this type will thus

T

Figure 2

Schematic representation of the surface segregation of impurities as a result of high-temperature annealing.

T

Figure 3

Schematic representation of the desorption of superficial impurities as a result of annealing.

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583

stay clean in UHV conditions for rather long periods after preparation, and deposits and contaminations on these surfaces can normally easily desorbed by flashing the surface, which consists in rapidly heating the surface to the required temperature.

Ion Bombardment The most important and most frequently applied method for cleaning surfaces in UHV is ion bombardment or sputtering. This technique is based on the interaction of energetic ions with the sample surface. If one uses ions of, for example, rare gases with initial energies in the keV range, a variety of interactions appear, which lead in fine to the ejection of atoms for the surface. In this context, the so-called sputter yield Y plays an important role. This quantity is defined by the ratio of the number atoms ejected from the surface after the impact of a single sputter ion. In an early paper, Seah had summarized the available data for pure element sputter yields after Arþ ion impact,10 which showed important variations of the sputter yield as a function of the chemical identity of the target. Later, Taglauer published an exhaustive review of surface cleaning by sputtering, which treats in great detail the underlying concepts.11 We will here discuss some practical issues, which are related to the sputtering of solid surfaces. In order to illustrate that the choice of the sputter gas and the kinetic energy of the ion has on the sputtering process, the corresponding curves are shown in Figure 4.12,13 Here, the sputter yield is shown for the impact of different noble gases on an Au surface in the energy range from 500 eV to 2 keV. Two major conclusions can be drawn from this graph. Firstly, the sputter yield increases smoothly with the kinetic energy but does not even double in this energy range except for Xe. We can thus conclude that the kinetic energy does not play a crucial role for the process. Secondly, important variations can be seen for the different sputter gases. For He, the sputter yield in this energy range is Y < 0.2, which allows gentle sputtering for the removal superficial contaminants. All other gases are by far more efficient with sputter yields that are superior to unity. We can also see that the heavier the gas, the higher the sputter yield. The difference in sputter yield between Ne, Ar, and Xe is however not very pronounced and thus for most of the practical applications Ar will be the gas of choice because of its rather low cost. In order to get an idea of typical sputtering times and currents, we use Arþ sputtering at 1.6 keV as an example, which corresponds to a sputter yield of roughly Y z 3. On a Au(100) surface, which has a density of NAu(100) ¼ 1.2  1019 m 2 Au atoms, we would need a total flux of NAr ¼ 1.2  1019 m 2/3 ¼ 4  1018 m 2 Arþ ions in order to remove the equivalent of a monolayer. This corresponds to a total charge of 4  1018 C m 2. Given that the typical sample surface in UHV studies hardly ever surpasses 1 cm2 ¼ 1  10 4 m2, the effective charge is on the order of 4  1014 C. If we apply sputter current of 1 mA ¼ 6.24  1012 C s 1, the time required for the removal of a monolayer of Au atoms equals to t ¼ 64 s. Typical sputter times will thus be in the range of 1 min up to some ten minutes depending on the number of layers, which have to be removed. This is of course a rather simplified example, which is based on a pure target. In general, the surface will contain different types of contaminants or the sample itself may be an alloy. In this case, the variation of the sputter yield as a function of atomic

Figure 4 Calculated sputter yields from a Au surface for different noble gases. The simulation was done using the ‘simple sputter yield calculator’ provided by Michael Schmid, IAP/TU Wien Surface Physics Group.12

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UHV Surface Preparation Methods

Figure 5 Calculated sputter yields for various materials after impact of Ar þ ions at a kinetic energy of 1 keV. An important variation of sputter yield as a function of the atomic number can be noticed. The simulation was done using the ‘simple sputter yield calculator’ provided by Michael Schmid, IAP/TU Wien Surface Physics Group.12

number has to be taken into account. The important variation of this yield, which is shown for the case of argon ions impacting on a pure material, is shown in Figure 5.12 When looking at this figure, we can immediately see that if a sample contains at least two components, for which the sputter yields are appreciably different, the phenomenon of preferential sputtering is encountered. This will lead to a depletion of the element with the higher sputter yield in the near-surface region during sputtering. This can be in particular important for bulk alloys for which a bulk composition at the surface has to be reestablished after sputtering. Hofmann and Stepanova had illustrated this nicely for Ni3Al and TaSi2 alloys14 showing that in the range of kinetic energies below 1 keV, the relative sputter yields show important energy dependence but remain rather constant above 2 keV. This example reveals that for alloys surfaces, particular care has to be taken during ion bombardment. We will come back to this later when we refer to the surface treatment after sputtering. After having considered the basics of the sputtering process itself, we have to consider the damage to the surface by sputtering. Since the impact energies of the ions are largely superior to the interatomic binding energies, which finally allows for the ejection of atoms of the surface, important damage to crystalline lattice occurs. Furthermore, sputter gas ions are also implanted in the near-surface regions. As a consequence, we have to anneal the sample in order to reestablish crystalline order and to get rid of the implanted ions. During the annealing, one can observe an initial pressure increase in the UHV chamber, which is due to desorption of the sputter gas from the sample. Then, an often prolonged annealing at elevated temperatures is required in order to increase the mobility of the near-surface atoms. This will then reestablish the crystalline order corresponding to the bulk material. The annealing step has two major disadvantages. Firstly, annealing favors the surface segregation of impurities (see Figure 2) and will thus lead to an enrichment of these impurities on the surface until the near-surface region of the bulk material is sufficiently depleted of impurities. Consequently, the sputter and annealing steps have to be repeated in a cyclic manner until the bulk of the crystal is sufficiently clean in order to avoid surface segregation. Secondly, annealing can be rather delicate for alloys, which possess an unfavorable s diagram. If we have a look at the s diagrams presented in Figure 6, we can see examples of two limiting cases. The first one is the case of NiAl alloys for which the two compositions, which are frequently employed in surface science studies (AlNi and AlNi3), are stable and ordered up to their respective melting points. Thus, annealing does in this case poses no problem at all. However, in the case of CuPt alloys, the high-temperature stability of the ordered phases is limited and annealing temperatures have to be restricted to 850 K in the case of Cu3Pt and 920 K in the case of CuPt3. At these relatively low temperatures, which are far below 60% of the melting point of the respective alloys, annealing may be become a difficult task and does not necessarily lead to perfectly flat surfaces. This is one of the reasons why in many cases thin surface alloys are used as model systems for such alloys, which are often easier to prepare (see succeeding text). The sputtering and annealing behavior of bulk alloys can be nicely illustrated using the case of Ni3Al alloys. Sondericker et al. investigated the surface structure and composition of three different surface terminations of this particular alloy.15–17 They found that, due to the higher sputter yield of Al, an enrichment of Ni in the near-surface region can be observed after ion bombardment. In order to reestablish surface order and surface

UHV Surface Preparation Methods

585

Figure 6 Simplified phase diagrams of Al–Ni and Cu–Pt bulk alloys. Adapted from Hansen, M.; Anderko, K. The Constitution of Binary Alloys, 2nd ed.; McGraw-Hill: New York, 1958.

composition, an annealing step is required. In the case of the Ni3Al(111) surface, it has been shown that the onset of surface segregation for Al is at about 550 K.16 A first plateau, which corresponds roughly to the bulk termination, occurs at 850 K, which is followed by a new increase of the segregation at roughly 1000 K. Finally, slow cooldown even from elevated annealing temperatures leads to the desired surface composition. This example is very instructive since it illustrates the whole complexity of the subject. Two different approaches can obviously be employed in order to reestablish the initial surface stoichiometry: prolonged annealing (10 min) at a temperature of about 900 K and rapid (flash) annealing to temperatures above 1000 K followed by a slow cooldown. In any case, the sputtering of bulk alloys calls for an in-depth investigation of the preferential sputtering and the surface segregation in order to yield reliable and reproducible surface stoichiometries. Let us now reconsider the roughening of surfaces during ion bombardment. This phenomenon can be easily visualized by looking at the work of Kalff et al.18 for the case of Xeþ ion bombardment of the Pt(111) surface. Here, the authors used STM to illustrate the morphological evolution of the Pt(111) surface by ion erosion. The evolution of the surface morphology as a function of sputter time and substrate temperature is particularly interesting. It can be globally observed that at low substrate temperatures, mounds and pits are formed, which tend to create defects that persist even after annealing (see Figure 7). However, during sputtering at elevated temperatures, the crystallinity of the surface is generally preserved and, most importantly, leads to smoother surfaces. This observation confirmed earlier observations of Costantini et al. for Ar þ bombardment of Ag(001).19 In view of the preparation of clean well-ordered surfaces, this result is quite important since it indicates that sputtering at elevated temperatures may yield smoother surfaces, which in turn could diminish annealing time and thus surface segregation of impurities. While cycles of sputtering and annealing of metals reliably yield clean and smooth surfaces, the preparation of ionic solids (e.g., salts and oxides) and covalently bonded solids by this method may pose a problem. This is mainly due to two major limiting factors. Firstly, these solids are often insulators and sputtering would inevitably lead to a charging of the solid, which would inherently interfere with the sputtering process itself. Secondly, as it is often the case for oxides, the stoichiometry of the surface may change during annealing, at least at high temperature, as preferential evaporation of oxygen may be encountered. Particularly, the last point calls for a cleaning method that does not rely on annealing.

Cleavage in UHV As it has been stated earlier, sputtering followed by annealing is often not applicable in the case of ionic or covalent solids. We can however take advantage of the brittle character of some solids by using the technique of cleavage. Brittle materials can actually be easily cleaved by mechanical stress. This concept was originally used by mineralogist in order to expose the different faces of crystalline materials such as salts and oxides and was later adopted by surface scientist as a straightforward way to prepare clean surfaces. The process of cleavage is evidently very dependent on the atomic structure of the solid, and one can imagine that in order to prepare a single-crystal surface with a particular crystallographic orientation, the stress that leads to the cleavage of the crystal has to be applied in a certain crystallographic direction as it is illustrated in Figure 8. This implies that individual members of isostructural groups of crystals usually exhibit cleavage on the same crystal planes, for example, halites (NaCI) can be easily cleaved parallel to {1 0 0} planes, sphalerites (ZnS) parallel to {1 1 0} planes, and fluorites (CaF2) parallel to {1 1 1} planes. In the early days of

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UHV Surface Preparation Methods

650K

700K

750K

800K

Figure 7 STM images of Pt(111) surface after the removal of 170 ML at the indicated temperatures. The image widths are 357 nm. Reproduced from Kalff, M.; Comsa, G.; Michely, T. Surf. Sci., 2001, 486, 103–135.

F

Figure 8

! Schematic representation of the cleavage of an ionic material. The force F is applied parallel to the desired cleavage plane.

surface science, even metal surfaces were prepared at low temperature, which made them brittle, by cleavage (see, e.g., Simon).20 In all these cases, the cleavage plane has to be macroscopically identified, which calls for a particular macroscopic shape of the single crystal. This is nicely illustrated in the work of Tröger et al., who reviewed a variety of sample holders that can be used in UHV.21 Furthermore, a cleavage along the particular crystallographic plane implies that this plane corresponds to a high-symmetry plane of the material. In general, this is the case for neutral planes of ionic crystal containing an equal amount of anions and cations, as it is, for example, the case for the {1 0 0} planes of alkali halides and some oxides like MgO. Even if all these prerequisites are fulfilled, charging of the surface will occur after cleavage, which calls for subsequent gentle annealing (see Barth et al.22 and references therein). But cleavage is not restricted to ionic crystals. Other brittle materials such as elemental semiconductors (e.g., Si and Ge) and compound semiconductors (e.g., GaAs, InP, and GaP) can also be easily cleaved, and recent results suggest that very high-quality crystal planes can be obtained by this method.23 Finally, layered materials such as graphite and layered compounds such as metal chalcogenides (e.g., TaSe2) can be easily cleaved by removing layers.24 In this context, we speak of basal cleavage. Even though this is often done ex situ by the aid of an adhesive tape, which is evidently not possible in UHV, a sharp blade can be used in situ much like in the case of the brittle materials for cleavage in UHV. Cleavage in UHV has the advantage that in general, no high-temperature annealing of the crystal after cleavage is required and therefore, no surface segregation occurs. Thus, the purity of the resulting surface will correspond to the purity of the bulk material. The major disadvantage of the method is that a macroscopic loss of material is encountered during each cleavage process. As a once prepared surface will remain clean for only a rather short period of time, repetitive cleavage experiments rapidly consume important amounts of single-crystalline materials. This has two major drawbacks: In general, single crystals are rather expensive, which will render the use of cleavage expensive in the long run, and a new single crystal has to be introduced into UHV when the old one has been completely consumed. For these reasons, cleavage is nowadays limited to cases where bulk materials are required. In all other cases, thin films of the respective material (see succeeding text) provide an excellent alternative to bulk materials.

Preparation of Thin Films Thin film technology is one of the key technologies of the future, and an ever-increasing amount of manufactured goods such as microelectronics devices, optical coatings, and protective coatings is fabricated using thin films. The advantage of thin films as

UHV Surface Preparation Methods

587

compared to bulk materials is that, especially for precious (rare) metals, the quantity of material used can be dramatically reduced without changing the properties of the surface. This has driven a growing number of fundamental investigations on thin films, and consequently, the preparation of thin films in UHV conditions has become an important topic. In fundamental research, thin films are often used as a model for bulk materials, which are difficult to prepare or investigate. This is, for example, the case for bulk oxides, which due to their insulating properties are difficult to in investigate using experimental techniques based on charged particles such as electrons. Here, we will limit the discussion to two distinct cases: metal films and oxide films, which comprise the basic concepts that can be applied to other classes of thin films.

Metal Films The evaporation of thin metal films is in fact one of the oldest means of preparing clean surfaces in vacuum (see Roberts2 and references therein). The basic concept behind thin film growth of metals still remains simple. It is based on thermal evaporation or vaporization by other means of a material that will then be deposited on a solid surface. In the early days of thin film technology, the substrates were often glass, mica, or cleavage planes of ionic crystal, which were rather easy to clean. Later on, the focus shifted to atomically flat single-crystal metal surfaces. Thin film growth by metal deposition is a rather complex phenomenon due to the important number of elementary steps, which are involved in the growth process. Venables et al.25 had given a very detailed introduction to the thermodynamics and kinetics of thin film growth. They focused on the kinetic and thermodynamic aspects of thin films growth, which are illustrated using a few examples later. Later, Brune gave a very detailed review of epitaxial growth focusing on the atomistic details of thin film growth.26,27 The most important challenge remains controlling the kinetics of growth using two crucial parameters: the impinging flux and the temperature of the substrate, which controls the diffusion length of the deposited particles. The important influence of temperature can be easily concluded from the random walk model of the root-mean-square displacement of a particle on a surface: qffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffi [6] h D r 2 i ¼ a Gt which depends on the hoping rate G, the hoping distance a (often the nearest-neighbor distance in the surface plane), and of course the time interval t:

G ¼ neEdiff =kB T

[7]

The hoping rate itself (eqn [7]) shows an exponential, Boltzmann-type dependence on the temperature implying that important changes in the growth behavior can be expected as a function of temperature for a given energy of diffusion Ediff and attempt frequency n. The action of increasing temperature can easily be seen in Figure 9, which shows the temperature dependence of island shapes and island density for homoepitaxial growth of Pt. As the mean square displacement of the deposited atoms increases, islands become bigger and more compact, and as a consequence, the number of islands decreases. In fine, this is one of the most important kinetic factors that will decide on the smoothness of the film. These results are also valid for heteroepitaxial growth and illustrate the importance of temperature. In heteroepitaxial growth, an important phenomenon can be encountered, which is due to the lattice mismatch between the substrate and the overlayer. This in general leads to the formation of strain-relief pattern, which in some cases leads to the formation of ordered nanostructures that can be used a surface templates (see Becker and Wandelt28). Furthermore, when depositing a metal B on a substrate of type A, mixing or alloy formation can occur. This leads us to a short discussion on the preparation of surface alloys. Ross had given an introduction on the preparation of Pt-based alloy films for catalysis studies29 on glass substrates. He put forward two main methods for the preparation of alloy films. The two

200K

300K

400K

500K

600K

700K

50nm

Figure 9 Morphology of Pt islands after deposition of 0.15 ML on Pt(111). The deposition temperatures are given as insets. The incident flux was F ¼ 7  10 3 ML s 1. Kindly provided by Michely, T. Adapted from Michely, T.; Krug, J. Islands, Mounds and Atoms; Springer: Berlin, 2004.

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UHV Surface Preparation Methods

0.36

100 eV

0.32

I64

I430+I64

0.28 (√3x√3) alloy

0.24 100 eV

0.20 0.16 0.12

(2x2) alloy

400

500

600

700

800

900

1000

1100

1200

temperature [K] Figure 10 Auger electron spectroscopy (AES) peak height ratio corresponding to the Sn deposit versus annealing temperature: the (2  2) and (O33  O33)R30 low energy electron diffraction (LEED) patterns correspond to metastable, ordered surface alloys, which exit in the temperature range of 750–900 and 1000–1050 K, respectively. Reproduced from Breitbach, J.; Franke, D.; Hamm, G.; Becker, C.; Wandelt, K. Surf. Sci. 2002, 507, 18–22.

metals can be either coevaporated or deposited successively on the substrate. In addition to that, the substrate can be heated in order to encourage atom mobility or the two metals and thus alloy formation. This nicely summarizes the concepts, which are still used nowadays for the preparation of alloy films on immiscible substrates. Later on, a third method had been developed, which is based on the evaporation of a metal B on a substrate A followed by annealing. This will, if the two metals are miscible, lead to the formation of a surface alloy. This is evidently extremely depending on the phase diagram of the respective surface alloy. A comprehensive theoretical retreatment of surface alloy formation can be found in the work of Christensen et al.30 and Ruban et al.31 For an in-depth review of experimental preparation methods, the reader is referred to the work of Bardi.32 A particularly well-investigated example is the formation of Pt–Sn surface alloys,33–35 which is presented here as a case study. After depositing Sn onto a Pt(111) surface at room temperature, the formation of surface alloys can be induced by annealing. Figure 10 shows the development of the Auger electron spectroscopy (AES) intensities as a function of annealing temperature and the corresponding low energy electron diffraction (LEED) pattern of two ordered surface alloys. It can be seen that the two ordered phases correspond to metastable regions in Pt–Sn composition, which are due to the metastable character of the Pt3Sn and the Pt2Sn surface alloys, which are prepared at 1000 and 800 K, respectively. Recently, atomically well-characterized surface alloys have drawn much attention due to their unique chemical properties. A number of systems have been investigated in the view of a utilization as model catalyst, which can be characterized with atomic precision in terms of surface stoichiometry and structure by STM. As examples, one can cite the following systems: PtxRu1  x on Ru(0001),36 PdxAg1  x on Pd(111),37 and AuxPt1  x on Pt(111).38 All of the cited examples illustrate that thin film technology possesses an enormous potential for the preparation of clean surfaces in UHV.

Oxide Films As we have seen in the previous paragraph, metal film growth relies in most of the cases on a purely physical process. The growth of oxide films on the contrary relies on the chemical modification (oxidation of the surface). In order to accomplish this, different routes can be imagined. The easiest approach consists in using a bulk material, which is cleaned and then oxidized by exposure to oxygen at low pressures (see Figure 11(a)). This is a rather facile approach but it has a couple of drawbacks. Firstly, the kinetics of growth, which will govern the thickness and the structural quality of the oxide film, are difficult to control because the supply of the metal from the substrate is, at least during the early stages of film growth, unlimited. Secondly, a proper annealing of the oxide film can be difficult if the bulk metal has a rather low melting point as it is the case, for example, for Al. However, good results have been obtained for some systems such as NiO on Ni(100).39 The second approach is based on the deposition of the desired metal in the presence of oxygen (Figure 11(b)). In this case, the kinetic constraints, which exist on pure metal surfaces, do not apply since the flux of the impinging metal and the O2 pressure can easily been adjusted in order to produce the desired stoichiometric oxide layer. Care has however been taken that the substrate (metal A) is not oxidized during the growth. This can be ensured if at least one of the following conditions is satisfied. Either the heat of formation of the oxide of the deposited metal is much higher than that of the substrate and high temperatures are applied during growth (thermodynamic limit), or the substrate does not form stable oxides. Excellent results have also been obtained using this method for TiO2 on Pt(111)40,41 and oxides of vanadium on Pt(111) and other

UHV Surface Preparation Methods

589

O2

metal A

(a)

metal B O2

metal A

(b)

O2

(c)

Figure 11 Schematic representation of different routes for the preparation of thin oxide films. (a) Simple oxidation of the bulk material using a surrounding oxygen pressure. (b) Concurrent deposition and oxidation of a second metal B. (c) Oxidation of one component of an alloy by a surrounding oxygen pressure.

surfaces.42 Alumina films can be prepared on the surfaces of refractory metals such as Re(0001), Ru(0001),43 and Ta(110).44 Finally, a third approach can be used, which employs bulk alloys as a substrate. This case is to some extend a mixture of the two cases, which have been discussed earlier. In fact, surface segregation is used to control the transport of the metal to the surface, which can be easily controlled by the kinetics (surface temperature). An appropriate O2 pressure is then applied in order to oxidize the segregated metal. A particular good example for this approach is provided by the Al2O3 film growth on NiAl alloys. Here, the heat of formation of Al2O3 is four times higher than that of NiO, which thermodynamically favors the formation of alumina. In the case of NiAl(110), oxidation at room temperature and subsequent annealing lead to the formation of a well-ordered Al2O3 layer of about 0.5 nm thickness.45 A similar result can be obtained for the oxidation of Ni3Al(111) at 1000 K, which leads to a nanostructured Al2O3 layer.46 In the latter case, the influence of temperature (Al segregation) has been studied, and it has been established that only at temperatures above 900 K, ordered Al2O3 films can be grown, whereas lower temperatures lead to either mixed NiOx and AlOx structures or chemisorption of atomic oxygen.47,48 Similar results can be obtained for the growth of TiO2 films on Pt3Ti(111),49,50 which resemble to the results obtained for TiO2 on Pt(111) cited earlier. These few examples are only a small fraction of the work on oxide films that has been undertaken. For a more complete review, the reader is referred to the work of Freund and Pacchioni.51

Summary Cleaning of surfaces in UHV conditions plays a key role for the production of well-defined single-crystalline surfaces. We have here presented the major preparation routes, which are nowadays applied in order to achieve this goal. Because of the import variety of materials, which are used in surface science, a considerable number of adapted preparation methods have been developed. There are however a few standard methods, which clearly play a major role. For metal surfaces and a number of semiconductors, ion bombardment (sputtering) followed by annealing is the method of choice in the most of the cases. Ionic materials such as oxides and salts can be in many cases conveniently prepared by cleavage. Finally, thin films receive an ever-growing attention during the past years due to their technological importance. Consequently, the preparation of clean surfaces using deposition techniques has followed this trend. Two major processes can be identified in this context: physical vapor deposition and chemical vapor deposition. Frequently, a combination of these two is also employed. We have to keep in mind that in all cases presented here, UHV conditions are required to keep the once prepared surface clean over an appreciable lapse of time.

590

UHV Surface Preparation Methods

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51.

Taylor, H. S. Discuss. Faraday Soc. 1950, 8, 9–18. Roberts, R. W. Br. J. Appl. Phys. 1963, 14, 537–543. Binning, G.; Rohrer, H.; Geber, C.; Weibel, E. Phys. Rev. Lett. 1982, 49, 57–61. Musket, R. G.; McLean, W.; Colmenares, C. A.; Makowiecki, D. M.; Siekhaus, W. J. Appl. Surf. Sci. 1982, 10, 143–207. Christmann, K.; Ertl, G.; Pignet, T. Surf. Sci. 1976, 54, 365–392. Ertl, G.; Neumann, M.; Streit, K. M. Surf. Sci. 1977, 64, 393–410. Steininger, H.; Lehwald, S.; Ibach, H. Surf. Sci. 1982, 123, 264–282. Baro, A. M.; Ibach, H. J. Chem. Phys. 1981, 74, 4194–4199. Hodgson, A.; Haq, S. Surf. Sci. Rep. 2009, 64, 381–451. Seah, M. P. Thin Solid Films 1981, 81, 279–287. Taglauer, E. Appl. Phys. A 1990, 51, 238–251. A Simple Sputter Yield Calculator, http://www.iap.tuwien.ac.at/www/surface/sputteryield. Matsunami, N.; Yamamura, Y.; Itikawa, Y.; Itoh, N.; Kazumata, Y.; Miyagawa, S.; Morita, K.; Shimizu, R.; Tawara, H. Energy Dependence of the Yields of Ion-Induced Sputtering of Monatomic Solids, Institute of Plasma Physics, Nagoya University: Nagoya, 1983. IPPJ-AM-32. Hofmann, S.; Stepanova, M. G. Appl. Surf. Sci. 1995, 90, 227–233. Sondericker, D.; Jona, F.; Marcus, P. M. Phys. Rev. B 1986, 33, 900–903. Sondericker, D.; Jona, F.; Marcus, P. M. Phys. Rev. B 1986, 34, 6770–6774. Sondericker, D.; Jona, F.; Marcus, P. M. Phys. Rev. B 1986, 34, 6775–6778. Kalff, M.; Comsa, G.; Michely, T. Surf. Sci. 2001, 486, 103–135. Costantini, G.; Rusponi, S.; Gianotti, R.; Boragno, C.; Valbusa, U. Surf. Sci. 1998, 416, 245–254. Simon, R. E. Phys. Rev. 1959, 116, 613–617. Tröger, L.; Schütte, J.; Ostendorf, F.; Kühnle, A.; Reichling, M. Rev. Sci. Instrum. 2009, 80, 063703. Barth, C.; Claeys, C.; Henry, C. R. Rev. Sci. Instrum. 2005, 76, 083907. Paul, W.; Miyahara, Y.; Grütter, P. H. J. Vac. Sci. Technol. A 2003, 31, 023201. Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. PNAS 2005, 102, 10451–10453. Venables, J. A.; Spiller, G. D. T.; Hanbücken, M. Rep. Prog. Phys. 1984, 47, 399–459. Brune, H. Surf. Sci. Rep. 1998, 31, 122–229. Brune, H. In Surface and Interface Science, vol. 4, Wandelt, K., Ed., vol. 4; Wiley-VCH: Weinheim, 2014; pp 421–491. Becker, C.; Wandelt, K. Top. Curr. Chem. 2009, 287, 45–86. Moss, R. L. Platinum Met. Rev. 1973, 17, 90–95. Christensen, A.; Ruban, A. V.; Stoltze, P.; Jacobsen, K. W.; Skriver, H. L.; Nørskov, J. K.; Besenbacher, F. Phys. Rev. B 1997, 56, 5822–5834. Ruban, A. V.; Skriver, H. L.; Nørskov, J. K. Phys. Rev. B 1999, 59, 15990–16000. Bardi, U. Rep. Prog. Phys. 1994, 57, 939–987. Paffett, M. T.; Gebhard, S. C.; Windham, R. G.; Koel, B. E. Surf. Sci. 1989, 223, 449–464. Batzill, M.; Beck, D. E.; Koel, B. E. Surf. Sci. 2000, 466, L821–L826. Breitbach, J.; Franke, D.; Hamm, G.; Becker, C.; Wandelt, K. Surf. Sci. 2002, 507, 18–22. Hoster, H. E.; Bergbreiter, A.; Erne, P. M.; Hager, T.; Rauscher, H.; Behm, R. J. Phys. Chem. Chem. Phys. 2008, 10, 3812–3823. Engstfeld, A. K.; Hoster, H. E.; Behm, R. J. Phys. Chem. Chem. Phys. 2012, 14, 10754–10761. Bergbreiter, A.; Alves, O. B.; Hoster, H. E. ChemPhysChem 2010, 11, 1505–1512. Bäumer, M.; Cappus, D.; Kuhlenbeck, H.; Freund, H.-J.; Wilhelmi, G.; Brodde, A.; Neddermeyer, H. Surf. Sci. 1991, 253, 116–128. Sedona, F.; Agnoli, S.; Granozzi, G. J. Phys. Chem. B 2006, 110, 15359–15367. Zhang, Y.; Giordano, L.; Pacchioni, G.; Vittadini, A.; Sedona, F.; Finetti, P.; Granozzi, G. Surf. Sci. 2007, 601, 3488–3496. Surnev, S.; Ramsey, M. G.; Netzer, F. P. Prog. Surf. Sci. 2003, 73, 117–165. Wu, Y.; Garfunkel, E.; Madey, T. E. J. Vac. Sci. A 1996, 14, 2554–2563. Chen, P. J.; Goodman, D. W. Surf. Sci. Lett. 1994, 312, L767–L773. Jaeger, R. M.; Kuhlenbeck, H.; Freund, H.-J.; Wuttig, M.; Hoffmann, W.; Franchy, R.; Ibach, H. Surf. Sci. 1991, 259, 235–252. Degen, S.; Krupski, A.; Kralj, M.; Langner, A.; Becker, C.; Sokolowski, M.; Wandelt, K. Surf. Sci. 2005, 576, L57–L64. Becker, C.; Kandler, J.; Raaf, H.; Linke, R.; Pelster, T.; Dräger, M.; Tanemura, M.; Wandelt, K. J. Vac. Sci. Technol. A 1998, 16, 1000–1005. Rosenhahn, A.; Schneider, J.; Becker, C.; Wandelt, K. J. Vac. Sci. Technol. A 2000, 18, 1923–1927. Le Moal, S.; Moors, M.; Essen, J. M.; Breinlich, C.; Becker, C.; Wandelt, K. J. Phys. Condens. Matter 2013, 25, 045013. Breinlich, C.; Buchholz, M.; Moors, M.; Le Moal, S.; Becker, C.; Wandelt, K. J. Phys. Chem. C 2014, 118, 6186–6192. Freund, H.-J.; Pacchioni, G. Chem. Soc. Rev. 2008, 37, 2224–2242.