Surface termination of YBa2Cu3O7−x systems

Journal of Electron Spectroscopy and Related Phenomena, 66 (1994) 453-461 0368-2048/94/$07.00@ 1994 - ElsevierScienceB.V. All rights reserved

453

Surface termination of YBa2Cu307_x systems C. Calandra*, F. Manghi Dipartimento

di Fisica, Universitd di Modena, Via Campi 213/A, I-41100 Modena, Italy

(Received 15 March 1993) Abstract Owing to the complexity of their structure, YBa2Cu307_x systems can have several different terminations of the crystal. Even considering basal plane surfaces only, six possible terminations are found for an ideal truncation of the crystal, having different electronic properties. Tbe characterization of the real surfaces produced by fracturing sintered materials, by cleaving a mono-crystal or by growing a thin film is a very difficult task, due to the occurrence of processes such as contaminant segregation, oxygen loss from the external planes and surface reconstruction. Here we review the experimental work on this subject and critically compare the outcomes of different spectroscopical experiments. The data are discussed in the light of the available theoretical results for the surface band structure of the ideal basal plane surfaces. The role of the surface in determining a non-superconducting surface phase is illustrated. Possible morphological and structural models for the observed surfaces are presented.

1. Introduction One of the interesting features of the l-2-3 systems is the extreme sensitivity of their superconducting properties to modifications in the composition and structure. If one accepts the current view that the superconducting properties are related to the hole doping of the CuOz planes, the rest of the crystal acting as a charge reservoir, then any perturbation that produces a modification or imbalance in the electronic charge distribution of these systems is expected to affect the superconducting properties to some extent. The interest is usually focussed on the influence of the oxygen concentration on the superconducting critical temperature T,. A number of experimental [l-4] as well as theoretical [5-81 studies have shown that the change in doping that takes place on passing from YBa$u30,, the superconductor with the highest T,, to YBazCuj06, an antiferromagnetic insulator, is approximately 0.2 holes per CuOz * Correspondingauthor. SSDI 0368-2048(93)01852-6

plane. Although some significant structural modifications (changes in bond length and crystal symmetry) take place during the deoxygenation process, the hole behaviour is primarily a consequence of the change in the number of neighbours of Cul atoms, i.e. of the creation of vacant oxygen sites in the CuO chains. It should be noticed that a charge transfer of 0.2 holes per unit cell is very small in comparison with the charge modifications that can be produced by creating a surface in the crystal. Indeed the truncation of the crystal gives rise to a high number of broken bonds, which can modify the local electronic structure more deeply than any point defect or vacancy created by removing chain oxygen atoms in the bulk. Moreover changes in composition caused by the removal or the segregation of specific atomic species can occur during the surface creation. Since in a material both external and internal surfaces are present, the superconducting behaviour is likely to be largely affected by the variation of charge near the boundaries, rather than being a direct consequence of its bulk

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properties. This implies that the accurate description of the electronic behaviour near the surface is of primary importance for the understanding of any real system. Indeed one of the reasons that up to now have restricted the applications of high T, l-2-3 materials is the low transport critical current density J,. Experience derived from many investigations is that surface impurities and insulating boundaries are the main sources of the low J, value [9-l 11. Auger spectra obtained from intergranular and transgranular areas exposed on fractured surfaces of sintered YBa&u@_, specimens have shown that most internal surfaces are oxygen deficient compared to the bulk, indicating that the grain boundary layer is likely to be non-superconducting [ 121.The formation of an insulating surface layer of about 1 nm thickness in deposited films has been attributed to the presence of surface impurities or barium carbonates [13,14]. However even clean surfaces of nearly stoichiometric YBa2Cu307, obtained by in situ annealing of ceramic pellets in an oxygen atmosphere, have been shown to be intrinsically non-metallic, despite the bulk metallic behaviour, due to the absence of clear Fermi edges in their valence photoemission spectra [15]. Low temperature (4.2 K) tunnelling spectra from thin film samples show a peculiar tip-sample dependence [ 161:the tunnelling conductance curve changes from semiconductor to superconductor behaviour as the distance decreases and the spectrum turns out to be the superposition of a contribution from a semiconductor at the surface and s-wave superconductor in the bulk. This result indicates that the film is covered by an insulating layer with a thickness of about 1 nm. The role of this layer is to stabilize the film against structural distortions and chemical adsorption. One important conclusion that can be drawn from these studies is that the application of surface sensitive spectroscopical techniques, like photoemission or other electron spectroscopies, on these kinds of samples often provides spectra which are not representative of the superconducting crystal, but of surface phases, which may be largely

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453-467

contaminated. It is also clear from these experimental findings that, in order to improve the quality of the l-2-3 materials for the application, it is crucially important to achieve a complete understanding of the surface behaviour, in particular to explain the reason of the surface instabilities, to find the conditions under which stable metallic surfaces can be produced from a stoichiometric sample and to single out the specific surface features in the electronic structure. These considerations have stimulated experimental work on high quality single crystals cleaved in ultra-high vacuum [17-301 or on epitaxially grown thin films cleaned by chemical etching [3l341or by sputtering and vacuum annealing [35-371. In most cases the above mentioned issues have been addressed with X-ray or ultraviolet photoemission measurements of valence band and core level binding energies, but direct structural information has also been obtained by LEED [18,30,35-31, surface X-ray diffraction [29], ion channelling [3841] and scanning tunnelling microscopy 142,431.The general conclusion which may possibly be drawn from these studies is that the high quality surfaces of superconducting YBa2Cu307 _-xsamples do show a Fermi edge and therefore are metallic in character. Concerning all the other important problems such as the nature of the exposed surface produced by cleavage, the surface stability, the occurrence of reconstructed phases and their attribution to specific crystal terminations, the presence of significant changes in the hole counting at the surface compared to the bulk, the extrinsic or intrinsic nature of the insulating phase, there are still considerable uncertainties and controversial results. The aim of this paper is to review these studies in the light of the theoretical results of surface electronic calculations and to point out some possible interpretation of the available data. 2. Nature of the c-oriented surfaces Basal plane surfaces are produced when the crystal is ideally terminated by an atomic plane normal to the c-axis, i.e. along the [OOl]direction.

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F. iUanghi/J. Electron Spectrosc. Relat. Phenom. 66 (1994) 453-467

Table 1 Different crystal truncations and corr&ponding atomic layer stackings in YBazCu907 BaO(I)

Cu02(I)

BaO(I1)

CuO2(11)

cue

Y

BaO Cuo BaO cuoz Y cuoz BaO

cue, Y Cu02 BaO cue Bao CuOz

BaO cuoz Y cuoz BaO cue BaO

cue* BaO CUO BaO cue* Y cue*

cue BaO cuoz Y cuoz BaO cue

Y cuoz BaO cue BaO cuoz Y

Given the complex structure of YBa2Cu307, six different ideal terminations are possible. The situation is illustrated in Table 1. One can truncate the crystal either by breaking one of the two Cu-0 bonds along the c-axis (the long Cur--O4 bond or the short Cut-O4 bond) or by separating the Y plane from one of its neighbouring Cu02 planes. The two surfaces formed by breaking the Cu2-O4 bond are composed of Ba-O4 and Cu,-O,-O, atoms respectively, corresponding to nominal BaO and Cu02 compositions. One can have an idea of the electrostatic stability of these surfaces using a purely ionic description of the chemical bond, based on the standard valence states of the atoms. According to this model the BaO surface should be neutral, having the same number of positive Ba2+ and negative 02ions. Such a non-polar character is usually taken as an indication that the surface is electrostatically stable. The same is not true for the CuOa surface, which should have a negative charge corresponding to approximately 1.5 electrons, i.e. half of the positive charge located on the Y sites (in the model Y is supposed to be in a completely ionized state) and therefore a strong electrostatic dipole in the outermost planes. In the following we shall indicate these surfaces as BaO(I) and Cu02(I). Another Cu02 termination is obtained by cutting the crystal between the Y plane and its neighbouring layer. This case will be indicated as Cu02(II). The main difference with respect to the previous case is that the negative charge of the top layer now has to be compensated by the positive charge of the underlying Ba plane. As to the Y

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surface, it shows a large positive charge that is only half compensated by the CuOz sublayer. In both cases strong electrostatic instability is expected. Other possible crystal terminations can be achieved by breaking the Cui-O4 bond. Because of the strength of the bond (it is the shortest Cu-0 bond in the crystal) the formation of these surfaces during the cleavage should be more difficult than in the case of BaO(I) and Cu02(I). In this case the surface corresponds either to a BaO plane, hereafter referred to as BaO(II), or to a copper oxygen layer, formed by Cui-0, atoms, with nominal composition CuO. The latter can be considered non-polar if one assumes Cu to be in a divalent state. Some structural instability is expected in this case, since the ideal surface is an array of equally spaced one dimensional chains, which could couple to lead to an energetically more favourable situation with different two-dimensional periodicity. In the following we shall discuss the electronic structure of these surfaces obtained by a single particle theory. The details of the calculations are given in Ref. 44 and we refer the reader to this paper for a complete description of the theoretical results. 3. Charge transfer and hole behaviour Before analyzing the results of the calculations for specific surfaces it is convenient to recall the main features of the bulk electronic charge distri-

Table 2 Calculated atomic occupancies for YBa&h30,_x Atom

x 0

Cuz

5.53 9.02 5.86 9.40

02

5.51

01 Cul 04

03

Y

bulk systems

5.61 0.62

1

-

9.99 5.96 9.44 5.63 5.66 0.62

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bution. Table 2 shows a comparison between the calculated atomic occupancies in the bulk unit cell for the superconducting (X = 0) and the insulating (x = 1) phase of YBa2Cu307_x, assuming the orthorhombic structure for both phases (the actual structure of the insulating material is tetragonal, but this would introduce only minor modifications in the present discussion). It allows illustration of the effects of the charge rearrangement that takes place when the Oi oxygens are removed from the chains. It is interesting to notice how Y and Ba electrons are distributed among the oxygens in the superconducting phase. One can evaluate from Table 2 that Y gives 1.19 electrons to the O2 and O3 oxygens of the neighbouring CuO2 planes. For each plane the charge provided by the Cuz sites amounts to 1.60 electrons per planar cell, giving a total of 2.79 electrons. This value is 0.36 electrons less than the excess charge given in Table 1 for the oxygens. The missing charge is provided by the neighbouring Ba sites, which, in addition, transfer 1.63 electrons per Ba atom to the chain oxygens. As a consequence every single CuO2 plane has an excess negative charge equal to 1.55 electrons per plane. It is customary, in the discussion of superconducting properties, to take as a reference configuration that given by the divalent Cu2+ and d- ions. With respect to this purely ionic description of the chemical bond the calculated distribution corresponds to 1.45 holes per CuOz plane. Turning to the comparison with the x = 1 configuration, we notice that the Cui ions, which have a divalent character in YBazCu301, acquire a 3d” configuration in the oxygen poor phase, while the apical oxygens, which have a significant hole number in the semiconductor, show the 2p6 configuration of the O- ion. Also we have a reduction in the hole counting of the CuO2 planes (from 1.45 to 1.27 holes per cell), caused by an increased charge transfer from the Ba sites. Indeed, due to the removal of 0, atoms, part of the electronic charge, which in YBa2Cus07 is transferred from Ba sites to the CuO chain, is now given to planes, in such a way that the hole counting is significantly reduced. If we assume, as it is commonly believed,

Electron Spectrosc. Relat. Phenom. 66 (1994) 453-467

that the superconductivity in these systems is mainly due to the presence of positive oxygen carriers, then the change in the hole numbers appears to be responsible for the lack of superconductivity in the oxygen poor phases. Notice that the electronic charge that is given to the planes on passing from x = 0 to x = 1 is 0.18 electrons per plane unit cell. This value is in agreement with the value of 0.2 holes that is considered the hole doping corresponding to the highest critical temperature

1414 Table 3 presents the calculated atomic site occupancies for the various basal plane surfaces discussed in the previous section. The data refer to the occupancies of the atoms belonging to the outermost bulk unit cell, since the deviation from bulk behaviour in the internal cells is very small. The first observation we can make is that surfaces which have the same nominal composition, but correspond to a different crystal termination, show strong differences in the surface charge distribution. With reference to the BaO planes we notice that the BaO(I) surface has an excess of electronic charge, while the BaO(I1) surface is characterized by a strong decrease in the occupancy of the Cua3d and the oxygen 2p shells, the chain atoms showing only a slight increase in the atomic population compared to the bulk case. This behaviour is due to the different charge imbalance created in the two cases. For BaO(I) the removal of the CuO2 neighbouring plane makes available an amount of electronic charge equal to 0.36 electrons per Ba atom. This is given to the other atoms of the cell and leads to a

Table 3 Calculated atomic occupancies of the atoms external cell for different surface terminations Atom (32 02

03 04

hl 01 Y

belonging to the of YBa2Cu307

BaO(1)

BaO(I1)

COW)

CuO2(II)

Cuo

9.43 5.61 5.65 5.96 9.07 5.55 0.63

9.29 5.46 5.54 5.89 9.03 5.53 0.62

9.36 5.55 5.61 5.86 8.95 5.46 0.62

9.52 5.49 5.48 5.76 8.65 5.28 0.61

9.39 5.54 5.60 5.88 9.07 5.54 0.63

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slight increase in their occupancies with respect to the bulk. In the case of BaO(II) the breaking of the Cu, -04 bond causes a charge transfer from Ba to O4 surface atoms, which, due to the absence of their copper neighbour, would be left with considerably less electronic charge than in the bulk. This means that part of the Ba electronic charge, that in the bulk is available for the underlying Cu02 plane, is now given to the surface oxygens. As a consequence the hole number in the CuO2 plane close to the surface increases dramatically. A somewhat similar situation occurs for the Cu02 terminations. In the CuOz(I) case the removal of the Ba neighbour causes a slight decrease of the charge available for the surface oxygens. To eliminate this imbalance, charge is taken from the copper atoms belonging to the chains in the underlying crystals. In the Cu02(II) case the cutting of the oxygen-yttrium bonds creates a dramatic imbalance and an equilibrium configuration can be achieved only by transferring electrons from the underlying Ba atoms to the surface plane. This process occurs at the expenses of the chain copper atoms and causes a significant change in their valence state. Although the production of a CuO surface seems to be energetically less favoured since it involves the breaking of the strong Cu-0 bond, the resulting charge distribution represents a small perturbation to the bulk. This is primarily a consequence of the fact that, unlike the previous cases, no cationoxygen bond is cut in the cleavage. While the occurrence of these charge modifications can be traced back to the presence of surface features in the single particle density of states [44-473, it is interesting to see how they can affect the superconducting behaviour. The most straightforward observation concerns the changes in the CuOz hole counting near the surface. Table 4 gives the calculated values for the outermost CuOp plane in the various cases. It is seen that the hole number closest to the value of the superconducting bulk phase is obtained for the CuO surface. Both the CuOz surfaces are slightly ovcrdoped, although the Cu02(I) has a hole counting

pretty close to the bulk case. The situation is totally different for the BaO cases: the BaO(1) termination leads to a significant underdoping of the CuOz external plane and, in this respect, is very close to the bulk x = 1 case, while th BaO(I1) termination shows a large overdoping. As it is well known that s?zperconductivity occurs only in a limited range of doping, the high hole number of BaO(I1) is likely to correspond to a non-superconducting phase. We are thus led to the conclusion that the Ba termination of YBa2Cuj07 might not support a bulklike behaviour concerning superconductivity. This conclusion applies to the case of an ideal termination of the crystal and it is not clear how far one can go in extrapolating these results to the real surface. It is likely that the actual hole counting would be somewhat affected by structural distortions, such as relaxation or perhaps reconstruction, but in the absence of significant change in compositions caused by the removal of oxygens during the surface preparation, the uncontaminated surface should behave as predicted by the present theory. The situation for the CuOz surfaces is less clear: the hole counting is maintained close to the bulk value but rather strong modifications occur in the valence states of the chain atoms, especially in the case of Cu02(II). In these conditions it is not clear whether any reference to the bulk is meaningful. If we assume the superconductivity to bc determined by the hole counting only, then both surfaces would be able to support the superconducting state even in the external planes. The CuO case Table 4 Hole numbers in CuOs planes for YBa&uSO, _ x bulk systems and for different surfaceterminations of YBazCuJ07 System

Hole count

YBa&O, YBazCu306 BaO(I) BaO(I1)

1.45 1.27 1.31 2.11 1.48 1.51 1.47

CuO20)

CuO2(11) cue

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C. Cola&a,

appears to be very clear: there is no reason, in the light of the present discussion, to believe that any change of superconductivity might occur near this surface. A more difficult question to be answered is whether those surfaces which do not show the proper hole number to support superconductivity are metallic or not in the normal phase. This issue is relevant in order to understand the nature of the thin insulating layer, that has been observed in several cases on top of superconducting samples, and to establish if the insulating behaviour is caused by intrinsic or extrinsic factors, such as contaminations or segregation. In the bulk the disappearance of superconductivity as a conse quence of the deoxygenation is accompanied by the formation of an antiferromagnetic insulator. The phase transition cannot be predicted on the basis of single particle calculations, since such theories give a metallic ground state for all the range of compositions [6]. The same is true of the surface calculations [44,47], which invariably give a metallic density of states. There is, however, a substantial difference between the theoretical results for oxygen poor YBaaCu,O-r_, and the surfaces with a low hole number. In YBCO systems the increase of the electronic charge of the CuOz planes is accompanied by a transition of Cur from the divalent to the monovalent state. In the ideal stoichiometric surface case, since no 0, atoms are removed, the chain atoms can have their occupancies decreased when the Cu02 population increases_ This is what happens for the CuO;! terminations. In the other cases we find some slight increase in the Cul occupancy, but this is not accompanied by the presence of additional extracharges in all the atoms, except for BaO(I). In the absence of a more sophisticated theory, these observations can be taken as an indication that the surfaces we have considered are likely to be metallic rather than insulators, even when their hole density does not appear to support superconductivity- Obviously these conclusions apply to the case of an ideal termination of the stoichiometric crystal: one cannot exclude that, as a conse-

l? ManghijJ. Electron Spectrosc. Relat. Phenom. 66 (1994) 453-467

quence of structural rearrangements occurring at the surface, a semiconductor phase is formed in the outermost planes. However this would be a different kind of instability, that in principle can be predicted by single particle theory. We shall come back to this point after discussing the experimental information of the surface properties. 4. Experimental characterization of surface structure and morphology The question of which surface is observed in the experiments does not have a simple answer. Although significant efforts have been made to achieve a structural and compositional characterization of the surfaces, some basic questions concerning the most stable crystal termination or the configuration of the cleavage surface have not yet been answered. In the last few years experimental work has pointed out that the surface properties depend upon the nature of the sample and upon the procedures followed to prepare and clean the surface. Early work on sintered samples based on high resolution electron microscopy (HREM) and computer simulated image matching [48,49] has shown that the external surfaces are formed mainly by BaO layers, the plane sequence near the surface being bulk Y-CuOz-BaO-CuOBaO. Later studies on surfaces produced by fracturing at (001) grain boundaries have shown that the sequence is either bulk Y-Cu02-BaO-CuOBaO or bulk Y-CuO*-BaO-CuO or a mixture of the two [50-521. Spectroscopic observation of BaO terminations in YBa$u@_, sintered samples has been reported by Parmigiani et al. [53] in a study of the behaviour of XP spectra as a function of the take-off angle. Important issues are the composition of the surfaces and the role of contamination or degradation by the environment in determining the observed surface phase. The external surfaces of sintered samples have been found to suffer some oxygen depletion [49] and to decompose quickly at temperatures above 150°C by insertion of extra

C. Calandra, F. ManghilJ. Electron Spectrosc. Relat. Phenom. 66 (1994) 453-467

CuO planes into the original structure, resulting in (CUO)~ double layers between BaO planes [50]. Internal surfaces obtained by fracturing the samples in a high vacuum environment have been analyzed by Auger electron spectroscopy [12,54] and found to be rich in copper and deficient in oxygen compared to the bulk, the thickness of the region with altered composition being in the range 15-20 A from the boundary. In agreement with the observation for the external surfaces, the composition and morphology change with time after the fracture at room temperature. The reactivity of the surfaces turns out to be very high. Sintered samples of YBa2Cu307 react rapidly with the surrounding atmosphere [49] and in the presence of COZ and water, the reaction produces an insulating layer containing BaC02, YzBaCuOS and CuO. The same is true for the fracture surface, which is normally very active and decomposes both in air and in argon. It has been observed [55] that the surface coating formed in air is thick and is left unchanged after exposure to an electron beam. However the coating formed by fracture in argon atmosphere is thin and can be recrystallized after electron beam irradiation. HREM images show that the surfaces produced in this way are clean and free of disorder. The observed surface structures for (OOl), (llO), (111) and (103) terminations do not show any reconstruction. For the (001) surface the HREM images indicate that the crystal terminates with a CuO layer, originally belonging to a CuO chain layer of YBazCu30,. This seems to be a distinct feature of l-2-3 materials, since for the (001) surface of La or Ba based materials the topmost atomic layer was invariably found to be La-O or Ba-0 respectively [56,57]. These experimental results seem to indicate that the CuO termination is particularly stable: An alternative method that has been proposed for eliminating contamination is a cleaning procedure based on in situ annealing under pure oxygen at atmospheric pressure [SS]. Ultraviolet photoemission spectra from samples treated in this way have been compared with early spectra of cleaved

459

samples [17] and found to be very similar. This finding has been taken as experimental evidence that the cleaning process leads to a surface that is in equilibrium with the bulk of the material and has the same composition and the same electronic structure. No attempt to achieve a direct structural characterization of these surfaces has been made. It is interesting to note that, in agreement with other authors [59], no direct evidence of a metallic Fermi edge is reported in photoemission from these surfaces [60]. The lack of significant structures near the Fermi level has been attributed either to changes in the composition of the external planes, such as the oxygen loss or the presence of contaminated or reacted species, or to intrinsic surface effects, such as the modifications in the local density of states of the external planes (band narrowing, surface states, etc.). It should be noted, however, that photoemission spectra similar to those obtained from polycrystalline samples have been observed in bad quality single crystals cleaved in ultra-high vacuum [18]. In particular it has been pointed out that the absence of significant emission from the Fermi level in monocrystals is accompanied by the presence of a large amount of contaminants. This indicates that for sintercd samples a surface insulating phase is always produced even after elaborate cleaning procedures. The work on sintered samples has pointed out four important aspects of the surface behaviour in YBaZCuj07_, systems: (i) the most likely terminations are BaO and CuO; (ii) the Y plane is the ccmmon interface between the surface and bulk phase; (iii) surfaces are often poor in oxygen; (iv) the observed insulating phase is determined by the surface properties. These conclusions have been confirmed and clarified by later studies on good quality cleaved crystals and thin films. It has been observed that the cleaved surface is unstable at room temperature even in ultra-high vacuum. The instability leads to a loss of emission at the Fermi energy and therefore has been attributed to the formation of an insulating phase due to the deoxygenation of the CuO chains near the surface [ 191.Although stable phases have been observed by XPS at room temperature

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[18], it is generally accepted that by cleaving and keeping the sample at very low temperatures, typically around or below 40K [61], one can have surfaces in chemical equilibrium with the bulk. A common feature of such surfaces is a significant photoemission from the Fermi energy, indicating that the stability is deeply related to the presence of a metallic phase. The structural characterization of cleaved surfaces has not led yet to a universally accepted model for the crystal termination. By using scanning tunnelling microscopy Edwards et al. [43] have been able to obtain atomic images from freshly cleaved surfaces of superconducting Y BazCu307 _ r kept at low temperatures in ultra-high vacuum. From the observatioh that the atoms line up along parallel rows separated by the a-direction lattice constant the authors argue that the CuO surface is produced by cleavage. They also observed steps of various heights, some of which were shorter than the size of the unit cell along the c-direction, a feature that indicates the possibility that several different terminations coexist at the surface. From core level photoemission spectra Liu et al. [22] detected two different Ba chemical states, which were attributed to bulk and surface emission and taken as a direct evidence of a BaO termination with some significant structural rearrangement. The results have been confhmed by the later work of Fowler et al, [ 181,who assigned the peaks to surface and bulk phase but in a reverse binding energy order, in agreement with previous authors [62]. Fowler et al. [18] also found that the surface peak is not present for all the cleaved surfaces. The fact that a BaO surface causes a significant core level shift is not surprising, since Ba atoms would have a coordination number of six, significantly different from the bulk and likely to account for the observed shift even in the absence of any reconstruction. Alternatively two Ba states may result from the presence of different surface terminations: a cleaved surface with steps could be composed by BaO(I) planes in some areas and CuOZ planes in others. The absence of the surface peak for some cleavage could be due to the reduced

size, of the BaO areas. In any case the surface Ba peak provides evidence that the surface has a large BaO component, as also obsei-ved in angle resolved XPS data from polycrystalline samples [53]. Edwards et al. [43] argue that the STM observation can be made compatible with the Ba core level data if one assumes that the cleavage takes place between BaO(I) and CuO. planes, leaving a surface with regions of CuO chains and regions where chains have been peeled away to reveal BaO covering the CuOZ planes. LEED data from cleaved crystals show both unreconstructed and reconstructed ~(2 x 2) patterns [30]. The half-order spots are usually broad and weaker, indicating that the reconstruction is strongly disturbed. Although most of the snrfaces obtained by cleavage do not show such a reconstructed pattern, its presence is accompanied by changes in the valence band spectra, the most significant ones being the absence of emission from the region 1eV below the Fermi energy, where a surface state is commonly observed [26,27], an increased emission at EF and a BCS-like gap in the energy distribution curves below the critical temperature T,. The fact that a BCS gap is not observed in the unreconstructed samples, where a surface peak is found at 1 eV binding energy, is a strong indication that the reconstructed surface is the one whose electronic properties are closer to the bulk and, therefore, it is likely to be the most stable crystal termination. Schroeder et al. [30] propose the following interpretation of their data. The surface that is usually obtained by cleaving is made of regions of BaO(1) and CuOZ planes and does not show superconducting behaviour. Instead the reconstructed surface is formed by a Y plane, where half of the Y atoms have been removed to ensure charge neutrality (an unreconstructed Y termination would be highly polar and very unstable). It is argued that this sort of structural configuration leaves the CuOr plane below the surface almost undisturbed compared to the situation of the bulk, i.e. with the same electronic structure and hole counting. The argument relies on the fact that in the bulk, half of the electronic

C. Calandra, F. ManghijJ. Electron Spectrosc. Relat. Phenom. 66 (19941 453-467

charge missing from Y atoms is given to each of the two Cu02 planes that sandwich the Y layer. A Y surface has also been observed in X-ray diffraction experiments from a terminated surface of a three-dimensional single crystal, in conditions very different from those obtained in studies carried out in ultra-high vacuum [29]. The authors conclude that this is the nature of the terminating layer during the growth and that oxygens are likely to be adsorbed for charge neutralization. The importance of thin films for applications has stimulated the experimental work aimed at the characterization of epitaxially grown (001) thin film samples. High quality films can be grown on a single crystal substrate using several deposition techniques, including co-evaporation, off-axis magnetron sputtering and laser ablation. The results that have been obtained depend upon a number of factors such as, (i) the particular substrate and deposition technique, (ii) the process used to clean the surface, (iii) the experimental probe adopted to characterize the surface. In spite of this, significant information has been obtained which allows better specification of the nature of the possible crystal terminations. The composition of the outermost film layers has been studied by angle resolved XPS. Freshly grown films introduced into the UHV chamber without any cleaning procedure turn out to be covered by contaminants, which are responsible for the surface insulating phase [ 14,631.The interface between this non-superconducting surface layer and the superconducting thin film seems to be a layer of Y atoms [14]. Annealing in UHV significantly changes the spectra and leads to different surface compositions, which are consistent with BaO or CuOr terminations, depending upon the thermal treatment: the BaO(1) surface is observed after gentle annealing; moderate annealing leads to a composition consistent with CuO,(II), whilst after strong annealing the BaO(II) surface seems to be the preferred one [63]. Comparison with the crystallographic data rules out the possibility of a separate surface compound BaCuOl, which would also be consistent with the spectroscopic data.

461

The effect of vacuum annealing has been studied by several authors [35,36]. It is generally accepted that the annealing up to about 400-500°C causes the decomposition of carbonates and their removal from the surface region. A 1 x 1 LEED pattern has been observed in these surfaces, apparently in good agreement with previous LEED measurements on cleaved crystals [36]. Based on the study of atomic blocking effects in low energy ion-scattering spectroscopy the surface structure is identified as an unreconstructed CuO surface [41]. The surfaces produced by annealing are extremely reactive to gas exposure and degrade quickly in air. Similar results have been obtained for thin films grown by d.c.-magnetron sputtering on warm SrTiOs substrate [37]. The in situ LEED analysis, without exposing the films to air, reveals the formation of YBCO (2 x 2) and (4 x 1) superstructures, apparently missed by previous investigators [17] due to the diffuse background of their LEED pattern. Apart from the work of Schroeder et al. [30], Reference 37 is the only paper that provides evidence that the stable surface phase is reconstructed. The authors interpret their data in terms of a model obtained by creating vacancies in a CuO terminated crystal, suggesting that the stable surface phase can only be obtained by decreasing the oxygen content in the outermost atomic plane. In agreement with the previous results no LEED pattern has been observed for the air exposed surfaces, although the (2 x 2) super-structure could be obtained by annealing at 500°C. Chemical etching of single crystal films, grown by laser ablation on different substrates, by Brr in absolute ethanol has proved to be a very efficient method to clean the sample and to prepare a stable surface free from defects [31-331. Of particular relevance to the present work is the fact that the XPS energy distribution curves for valence emission from these samples show a well defined Fermi edge, which remains clearly detectable in vacuum at room temperature on a time scale of days [34]. .This means that the surface produced in this way is considerably more stable than the cleaved surfaces, which show a loss of intensity at

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the Fermi level on a time scale of hours. The reduced chemical reactivity of chemically etched YBCO must be a consequence of a different surface morphology with respect to the cleaved surface case. Support for this conclusion comes from the study of the core level binding energy, in particular from the absence of the surface component in the Ba core level spectra. This has been taken as an indication that the etched thin films do not expose stepped surfaces with a predominant BaO(I) contribution but rather smooth surfaces with CuOz(I) character [34]. In other words the etching seems to act in a selective way: it removes the BaO planes easily, but not the rather stable Cu02-Y-Cu02 unit. We will come back to this point after a more detailed comparison between the spectroscopic experiments and the theoretical results. 5. Ekctronic properties of surfaces It is well known that the structure of the valence electrons is deeply affected by the crystal truncation, due to the occurrence of surface states and resonances, that modify the density of states locally, and due to the termination of bulk states, which causes a drastic change of the charge density and, therefore, of the potential in the external part of the crystal. A number of experimental tools have been devised to detect such features, such as photoemission, electron energy loss and Auger spectroscopy and several others. In many cases the observation of surface sensitive features in the valence electron spectra allows discrimination between different possible terminations of surface phases. For this identification theoretical calculations can be of great help since they provide information about the energy location of surface states, the nature of the atomic orbitals contributing to a specific surface feature and the sensitivity of the surface density of states to stmctural or compositional modifications. We have shown previously that, depending upon the atomic layer that is chosen to terminate the crystal, the atomic occupancies are modified in such a way that the

hole distribution in the outermost region of the crystal turns out to be very different from the bulk and we have suggested that this result may be important in determining the superconducting properties of the surface. It is therefore interesting to see whether some information on the specific surface termination of YBa2Cu307 can be achieved by a more detailed comparison between theoretical surface band structure calculations and spectroscopic data. To our knowledge the only data that are available for such a comparison are the energy distribution curves obtained either from XPS or from ultra-violet photoemission spectroscopy. Since the photoelectron escape depth is comparable to the size of the unit cell along the c-axis, both techniques sample essentially the electronic structure of the outermost cell. Therefore the information they give can be compared with surface electronic structure calculations. However, the comparison is not straightforward and free of difficulties when, as in our case, one has to discriminate between several different surfaces and when reliable information on the surface composition and structure is lacking. We have already pointed out that many investigations have led to results which refer to contaminated phases rather than to real YBaaCu301_, and that in many cases oxygen loss from the surface can occur even in cleaved samples. MOEOVer the only calculations performed up to now [M-47] refer to ideal crystal surfaces and do not take into account the possibility of structural rearrangements, such as relaxation or reconstruction, that may occur at the surface. In spite of that in the following we will show that a number of issues concerning the real surfaces can be addressed on the basis of a comparison between the outcomes of single-particle calculations and experimental findings. First we notice that in all the XPS spectra taken from cleaned undamaged surfaces of good quality crystals the observed Cu2p3iz lineshape has to be assigned mainly to divalent copper Cu”, since it is very close to the one reported for CuO and easily distinguishable from those observed in systems where copper is present in a monovalent or tri-

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valent state [18,34,64,65]. This definitely rules out the possibility that substantial changes in Cu occupancy can occur at the surface. If we look at Tables 2 and 3, we notice that theory predicts Cu in a divalent state in the bulk, such a feature being more or less maintained for all the crystal terminations except in the case of the CuO2(11) surface, where a substantial contribution from trivalent Cum is predicted. This is also important for the other terminations with copper and oxygen atoms at the surface: the fact that a single core level is observed indicates that any structural rearrangement which may occur at the surface does not affect the Cu occupancy significantly. As we have pointed out before, the results for Ba core levels are somewhat more controversial, since for some cleaves two distinct Ba levels are observed [ 18,221,while for others and for the etched surfaces only one core level is found. Although the assignment of the two peaks to surface and bulk component is not unanimous, the very presence of two lines is an indication that inequivalent Ba sites are present in the region sampled by photoemission. Liu et al. [22] argue that if the crystal cleaves at BaO(I), then it should also cleave at BaO(II), since the two Ba atoms in the bulk structure occupy equivalent sites. However, as pointed out in Section 2, there are reasons to believe that the two surfaces are not produced with the same probability by the cleavage, since different bonds are cut in the two cases. In particular the formation of the BaO(I1) surface should be less. likely because the Cut-04 bond is very strong. Since the Madelung potentials calculated for the two Ba sites at the surface have opposite sign and show quite a large difference compared to the bulk potential, Liu et al. [22] conclude that the surface is highly unstable and some reconstruction takes place. While the electrostatic potential calculation is based on a purely ionic description of the chemical bond, which is certainly not appropriate for the largely covalent Cu-0 bond, the fact that Ba atoms of the two BaO surface feel a very different potential is supported by single particle surface band structure calculations. To illustrate this point we display in

-7 -6 -5 -4 -3 -2 -1 Energy

0

1

2

- J% (ev)

Fig. 1.Calculated contributions of the chain (Cul , 01,OJ and plane (Cu,, Olt 0,) atoms to the local density of states of the external cell for the BaO(I) termination. The partial bulk densities of states are reported for comparison [44].

Figs. 1 and 2 the calculated local density of states (DOS) for the topmost copper and oxygen atoms in the two cases and compare them with the corresponding bulk DOS. This comparison shows the occurrence of many significant changes in the location and shape of the main structures with respect to the bulk. In BaO(1) we notice that, (i)

E cb

I

I

0, site

%

I

h

x %

a” -7

-6

-5

-4

-3

-2

-1

0

1

2

Energy - Ef (eV) Fig. 2. Calculated contribution of the 0, surface atoms and of the CuOz sublayer for the BaO(II) termination [44].

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C. Calandra, F. ManghilJ. Electron Spechosc. R&t.

the structures near the Fermi level EF are considerably attenuated, (ii) a pronounced bulk peak at -0.7 is replaced at the surface by two structures at about -0.5 and -0.9eV, (iii) the structure found in the bulk at -3.2 eV has been removed, while new pronounced features appear below -3.6 eV. The changes found in BaO(I1) are rather different. For example one can notice that the DOS at the Fermi level is increased for the CuOz sublayer compared to the bulk, while a substantial loss of structures is observed near -1 eV. The calculated atomic populations for the two cases reflect these variations in the local electronic structure, the difference between the charge on the CuOz sublayer and the bulk CuO2 plane having opposite sign (see Table 3). This conclusion does not imply that the argument given by Liu et al. [22] is correct. Rather we believe that if the CQ-O4 bond is cut during the cleavage then it is likely that both BaO(1) and CuO?(I) terminations coexist in the same surface. This is consistent with the presence of a significant Ba surface core level shift, since the Ba atoms would have reduced coordination in the BaO regions and a bulk-like environment in the CuOz areas. This description is also consistent with the results of Schroeder et al. [30], who have performed ultraviolet photoemission experiments from both the lower and the upper surface generated by cleavage, finding that the spectra taken in a point of one surface and in the corresponding mirror image of the other agree much better than the spectra taken in different points of the same surface. While it cannot be excluded that the terraces derive from different cleavage planes, the stepped BaO-CuOz surface appears to be the structural model that agrees better with both kinds of data. A more detailed theoretical analysis attributes the surface induced modifications for these surfaces as well as for the other basal plane terminations to the presence of surface states and resonances. For a detailed description of this work we refer the reader to our paper [44]. What is relevant to the purpose of the present review is that in some cases it is possible to identify surface bands with

Phenom. 66 (1994) 453-467

2 1 0 s4 ‘=ZO mu

-1 9 $ -2 w”

I _3

B !$ -4

W -5 -6 -1 -8 l-

X

M

Y

r

Fig. 3. Surface band structure for the BaO(1) basal plane surface

WI.

-

l-

X

M

cl cuo~osudaee

Y

I-

Fig. 4. Surface band structure for the Cu02(I) basal plane surface [44].

C. Calandra, F. Manghi/J. Electron Spectrosc. Relai. Phenom. 66 (1994) 453-467

l

211

sample sample

t* 7..

I 6 binding

1 3 energy

1 _ E, (eV)

100 binding

% energy

(meV)

Fig. 5. Energy distribution curves for the reconstructed c(2 x 2) (sample 1) (o), and the unreconstructed (sample 2) (0) surfaces of YBa2Cu,07_, superconducting materials [30].

dangling bond character running along symmetry directions of the two dimensional Brillouin zone. As for many other metallic systems where surface states have been identified experimentally [66-691, the presence of the surface bands in certain energy ranges and their dispersion as a function of the two dimensional lattice wave-vector can allow a more precise characterization of the surface. Figures 3 and 4 display the calculated surface band structure for the BaO(1) and CuO2(1) surfaces respectively. The notation Si indicates different surface bands. Note that these bands are present only in limited portions of the Brillouin zone, out of which they mix with the continuum of bulk states and cannot be identified as surface states any more, Experimental observation of surface bands has been reported in a number of studies based on angle resolved photoemission spectroscopy [2628,301. This technique is particularly suitable to investigate the surface electronic structure because it follows the dispersion of surface states, thus providing a very severe test of any theoretical model of the surface structure. Figure 5 reports the measured energy distribution curves for ultraviolet photoemission from cleaved YBCO surfaces [30]. The left part covers a 6eV binding energy range, while the right panel gives the details of

465

the Fermi edge behaviour. Notice that the measurements have been taken at 10K to eliminate the possibility of oxygen loss. The two curves refer to the 1 x 1 (sample 1) and (2x2) reconstructed surface (sample 2). It is seen that the curve for the 1 x 1 sample shows a pronounced peak at about 1eV binding energy, not present in the energy distribution curve of the reconstructed sample. Claessen and co-workers [26,28], who first identified the peak as a surface structure, have carried out a systematic analysis of its behaviour as a function of the photon energy, i.e. varying the perpendicular component k, of the photoelectron momentum and have concluded that it does not show any dispersion as a function of k,. Such behaviour, typical of a surface state, combined with the extraordinarily small full width at half maximum and with the sensitivity to adsorbates is a strong indication that the 1 eV peak is a surface state. Comparison of these experimental results with the theoretical surface band structures given in Figs. 3 and 4 allows identification of the observed peak with one of the surface bands found in the Cu02(I) surface at about the same energy and running along the F-X and F-Y directions of the two dimensional Brillouin zone. In view of the fact that this band is not present in the BaO(1) case, this assignment implies that the surface has a large CuOz component, a result that is consistent with the Ba core level data if we assume the stepped surface model with both BaO(1) and Cu02(II) areas. This theoretical picture seems to be supported by more recent work of Gerhardt and coworkers, who have been able to identify both the S6 and the S7 bands in their data [70]. Another interesting aspect of the angular resolved photoemission data reported in Fig. 5 is the different behaviour of the EDC at the Fermi level for the reconstructed surface compared to the 1 x 1 form. The curve for the 2 x 2 surface has a shape that strongly suggests the presence of a BCS-like gap near EF. No such evidence is provided by the curve for the unreconstructed surface case. This result can have an explanation in terms of the mixed stepped BaO(I) and CuO,(I) model of the

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1 x 1 surface Indeed, as shown in Table 4 and discussed in detail in Section 3, the BaO(I) surface is considerably underdoped compared to the bulk, which can be taken as an indication that superconductivity is absent or strongly reduced. The same does not hold for the Cu02 surface, which has a hole count quite close to the bulk value. The previous assignments and the arguments in favour of the stepped BaO(I)-Cu02(I) surface model cannot be considered as conclusive. At present we do not have a similar description for the reconstructed 2 x 2 phase, which seems to show the smallest deviations from the bulk behaviour, although interesting suggestions have been put forward [30,37]. Also the nature of the stable phases found in thin films has to be clarified, although the suggestion of Vasquez et al. [34] that the surfaces of the etched films are mainly CuO&e would be consistent with the stepped surface model. It seems wise at this stage of the research to conclude that the subject is still in its infancy and much work is needed before arriving at firm conclusions about the properties of the YBCO surfaces. Acknowledgements We thank T. Minerva, F. Parmigiani and U. Gerhardt for fruitful discussions. The research was supported by C.N.R.-Progetto Finalizzato Sistemi Informatici e Cal&o Parallelo, Contract No. 92.01598.PF69. The calculations have been performed at the CICAIA of Modena University, whose technical assistance is acknowledged.

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13

14 15 16

17

18 19

20

21

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