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H2 process
Applied Surface Science 136 Ž1998. 137–146
Atomic layer epitaxy of copper: an ab initio investigation of the CuClrH 2 process I. Adsorption of CuCl on Cu ž111 / ) Per Martensson , Karin Larsson 1, Jan-Otto Carlsson ˚
2
˚ The Angstrom ¨ Laboratory, Inorganic Chemistry, Box 538, SE-75121 Uppsala, Sweden Received 3 April 1998; accepted 27 June 1998
Abstract An ab initio investigation of the adsorption and disproportionation of copperŽI.chloride on copperŽ111., two crucial processes in CuClrH 2 copper Atomic Layer Epitaxy is presented. Adsorption of CuCl is energetically most favourable on the two different three-fold adsorption sites with adsorption energies of 152 kJ moly1. Adsorption on the two-fold bridge site is only 10 kJ moly1 lower in adsorption energy than on the three-fold sites, whereas adsorption on the on-top site is endothermic by as much as 88 kJ moly1. As previously noticed in an experimental study, adsorption is not limited by any energy barrier. The disproportionation was investigated for three different arrangements with the composition ŽCuCl. 2 . It was confirmed that the formation of ŽCuCl. 2 in the gas phase is favourable, but contrary to earlier experimental findings, no indications of a disproportionation of CuCl could be found. Adsorption of the ŽCuCl. 2 dimers was found to be energetically unfavourable implying that a dissociation to free CuCl molecules is required prior to adsorption. q 1998 Elsevier Science B.V. All rights reserved. PACS: 31.15 A; 31.15 E; 82.65 Y Keywords: Adsorption; Density functional calculations; Copper; CopperŽI.chloride
1. Introduction Atomic Layer Epitaxy, ALE, first introduced by Suntola and Antson in 1977 w1x is a versatile method which has been used for the deposition of thin films of a vast variety of materials w2–10x. Its versatility ) Corresponding author. Tel.: q46-18-471-3726; Fax: q46-18513-548; E-mail: [email protected] 1 Tel.: q46-18-4713750; Fax: q46-18-513548; E-mail: [email protected]. 2 Tel.: q46-18-4713750; Fax: q46-18-513548.
also includes the possibility for in situ studies of deposition processes as the reaction steps are separated in time by intermediate purging of the deposition chamber with an inert gas. Methods like X-ray Photoelectron Spectroscopy, XPS w11,12x, Fourier Transform Infrared Spectroscopy, FTIR w13,14x, Low-Energy Electron Diffraction, LEED w15,16x, Reflection High-Energy Electron Diffraction, RHEED w17,18x, Low-Energy Ion Scattering, LEIS w19x and Quartz Crystal Micro-balance, QCM w20x have been used, offering insights in the processes involved in the adsorption of precursor molecules
0169-4332r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 3 3 0 - 4
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and their further reaction with other reactants andror the substrate. As a complement to experiments, computational methods have gained increased interest during the last decades. This has been possible with the improvement of both computer hardware as well as of the algorithms used for the calculations and the potentials describing the atoms. However, even though much effort has been put into the understanding of surface processes such as adsorption, migration, surface reconstruction and dissociation w21–23x, the only extensive theoretical investigations related to gas phase deposition techniques, such as ALE or Chemical Vapour Deposition, CVD, have been in processes for diamond deposition w24–26x. This system is very suitable for quantum mechanical calculations because of the low number of electrons in the carbon atoms as well as the relatively simple gaseous species. Recently, Mochizuki et al. reported on a theoretical investigation of the GaClrAs 2 GaAs ALE process w27–30x. The precursors used in their study are small enough to allow for the use of a small surface model – still avoiding interactions between adjacent cells – which is a requirement for Quantum Mechanical, QM, calculations to be performed. As also the process presented in this paper is based on a small precursor, QM calculation was considered a suitable tool to gain a deeper insight in the deposition processes. We have previously reported on the deposition of copper from copperŽI.chloride and H 2 by means of ALE w31x. Experimentally, this process consists of four steps which are repeated until a film of desired thickness has been formed. Ž1. CuCl is first supplied and the molecules are adsorbed on the substrate surface. Ž2. This is followed by a purging with an inert gas to remove excess precursor leaving one monolayer of adsorbed CuCl. Ž3. To remove the chlorine atoms from the surface, hydrogen is added to form HCl which is desorbed and removed from the deposition chamber by pumping. Ž4. Finally, the reactor is purged with an inert gas again to remove the HCl formed as well as excess hydrogen. These steps can be further divided into a number of chemicalrphysical fundamental steps. As the CuCl molecule approaches the surface, CuCl–surface interactions will occur and at a close
enough distance, a chemical bond is formed. Prior to bond formation, the molecule can enter into a physisorbed state, i.e., a local minimum in the total energy surface. To enter the final chemically bonded state, an activation barrier has to be overcome which makes the adsorption process temperature-dependent. Finally, the molecule can undergo many different reactions on the surface, e.g., dissociation, disproportionation, substrate reaction or migration. The processes of interest in this study are primarily the adsorption and disproportionation of the CuCl molecule. The fundamental steps in hydrogen reduction of CuCl are similar to those presented above with the presence of the reaction of hydrogen with chlorine in CuCl and the final desorption of HCl. Previous experimental results w31x suggested that adsorption of CuCl is fast and proceeds without or with very low activation, while the reduction step is rather slow and limited by an energy barrier of approximately 80 kJ moly1 . This can be compared with earlier theoretical results which suggests an activation barrier for the dissociation of hydrogen on copper of 50–120 kJ moly1 depending on the geometry of the dissociation and accuracy of the calculations w32–35x and also with experimental results which suggest an activation barrier of 50–60 kJ moly1 w36,37x. Based on the relatively large difference between the activation barrier found in our process and the results reported for the dissociation of hydrogen on different copper surfaces, we assumed that the ratedetermining step in the reduction process was the reaction between adsorbed hydrogen and the chlorine atom of copper chloride. It may here be argued that an activation barrier of 80 kJ moly1 lies well within the barrier found for the dissociation of H 2 on Cu. We did, however, base our conclusion on the assumption that a process proceeds along the path which has the lowest activation barrier which, in this case, corresponds to the experimental results of approximately 50 kJ moly1 w36,37x. However, as almost all investigations of the adsorption and dissociation of hydrogen so far – theoretical as well as experimental – have been performed using ideal surfaces, it is unclear in what way the activation barrier for hydrogen dissociation is affected if the copper surface is terminated with
P. Martensson et al.r Applied Surface Science 136 (1998) 137–146 ˚
CuCl. It is not unlikely that the saturation of a surface by a species ‘A’ can affect the energy barriers for the adsorption and dissociation of species ‘B’. This means that even though the energy barrier found in Ref. w31x deviates significantly from results found for hydrogen dissociation on ideal surfaces, they might coincide with the energy barriers found when the surface has been modified and in that case, our conclusion that it is the formation of HCl that is the rate-determining step, has to be reconsidered. That the final desorption of HCl is the rate-determin-
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ing step was excluded, since it is commonly accepted that the desorption of volatile reaction products is a fast process which requires low activation at elevated temperatures. In this paper, we present the results from calculations on the adsorption of copperŽI.chloride on the four available adsorption sites on a copperŽ111. surface shown in Fig. 1a–d, i.e., the cubic Ž1a. and hexagonal Ž1b. close-packed three-fold sites, the two-fold bridge site Ž1c. and the on-top site Ž1d.. The results from investigations of energy barriers for
Fig. 1. Models of the structures used for the investigation of the adsorption energy for CuCl on CuŽ111.. The sites shown are the ccp three-fold site Ž1a., the hcp three-fold site Ž1b., the two-fold bridge site Ž1c. and the on-top site Ž1d.. Atoms which were allowed to relax during the calculations are marked with grey, whereas atoms held fixed are black. The Cl atom is represented by a sphere that is larger than the Cu atoms.
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adsorption on the two-fold bridge site and the disproportionation of gaseous and adsorbed CuCl are also presented and discussed. The hydrogen reduction is currently under investigation and will be presented in a future publication.
2. Method The calculations were performed within the framework of Density Functional Theory, DFT, based on the work by Hohenberg and Kohn w38x and Kohn and Sham w39x. We used the CASTEP ŽCambridge Sequential Total Energy Package. computer program package from BiosymrMolecular Simulation Technologies of San Diego. The atoms were represented by nonlocal pseudopotentials in the Kleinman–Bylander fully separable form w40x. These potentials treat as valence the 3s and 3p electrons in Cl and the 3d and 4s electrons in Cu. Moreover, the calculations were performed on periodically repeated unit cells, usually referred to as super-cells. The electronic wave functions were expanded in terms of plane waves and the electronic minimisation was performed using a band-by-band conjugate-gradients minimisation technique w41x. The specific k-points were generated using the Monkhorst–Pack scheme which produces a uniform mesh of k-points in the reciprocal space w42x. Exchange and correlation effects were included within the Local Density Approximation, LDA, using the Perdew–Zunger parametrization w43x as well as through the Generalised Gradient Approximation, GGA, developed by Perdew and Wang w44x. A density gradient expansion has been included in the GGA in order to introduce inhomogeneity in the electron density. The GGA has, therefore, proven to give more accurate adsorption energies than the LDA which tends to overestimate chemisorption energies and underestimate the energy barriers w45,46x. The geometry optimisations were performed with one k-point and a cut-off energy for the plane waves of 5 a.u. Ž136 eV. using the LDA, while the final energies for the relaxed models were calculated with four k-points and a cut-off energy for the plane waves of approximately 12 a.u. Ž320 eV. using the GGA.
The adsorption energy was calculated for the four adsorption sites on copperŽ111. shown in Fig. 1, i.e., the three-fold ccp Ž1a. and hcp Ž1b. sites as well as the two-fold bridge Ž1c. and the on-top Ž1d. sites. The copperŽI.chloride molecule was oriented normal to and with the copper atom closest to the surface. The monomer CuCl was used even though it has been shown that CuCl most often exists as a dimer or a polymer in the gas phase w47–49x. This was motivated by the fact that the polymers and dimers are expected to dissociate on, or in the gas phase in the vicinity of, the surface prior to adsorption. This assumption was also proved during this work. For calculation of the adsorption energy the simple expression was used. Eads s Etot y Ž Esurf q ECuCl .
Ž 1.
Here, Eads is the adsorption energy, Etot is the energy for a surface with a CuCl molecule bonded to it, Esurf is the energy for the clean surface and ECuCl is the energy for gaseous CuCl. For the different templates modelling the surface, with or without an adsorbed CuCl molecule, some atoms were allowed to be relaxed, while all others were kept fixed in order to hold the characteristics of the crystal. In Fig. 1a–d, atoms in the surface model which were allowed to relax are marked with grey, whereas the fixed atoms are black. The CuCl molecule was allowed to fully relax in all calculations unless else mentioned. The energy barrier for adsorption on the two-fold bridge site was also investigated. This site was chosen from the results in the studies of the adsorption energy. From these, the adsorption energy for the two three-fold sites are almost identical, while it is approximately 10 kJ moly1 lower for adsorption on the two-fold site. As the three-fold sites are ‘closepacked sites’ and, thus, the most natural sites, it was assumed that the barrier for adsorption on the twofold bridge site would be larger than on the three-fold sites. The two-fold site was, therefore, chosen for the initial investigation of energy barriers during adsorption. We did not consider it appropriate to use the on-top site as adsorption on that site was found to be endothermic. In this study, the position of the copper atom in CuCl was held fixed, whereas atoms as above were allowed to be fully relaxed.
P. Martensson et al.r Applied Surface Science 136 (1998) 137–146 ˚
Finally, the disproportionation of CuCl in the gas phase was studied for the three different arrangements of ŽCuCl. 2 shown in Fig. 2a–c and for the linear arrangement of ŽCuCl. 2 adsorbed on the copper surface as shown in Fig. 2d. As no indications of a disproportionation could be found in these studies, it was not considered valuable to continue this investigation with the adsorption of the remaining two ŽCuCl. 2 dimers.
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3. Results and discussion 3.1. Test calculations 3.1.1. Size of unit cell and number of atoms in the surface model Preliminary calculations suggested the use of a surface model including three layers of copper atoms
Fig. 2. The three gaseous ŽCuCl. 2 molecules Ž2a–c. and the adsorbed linear ŽCuCl. 2 used for the investigation of the disproportionation of CuCl. Atoms which were allowed to relax during the calculations are marked with grey, whereas atoms held fixed are black. The Cl atoms are represented by spheres that are larger than the Cu atoms.
P. Martensson et al.r Applied Surface Science 136 (1998) 137–146 ˚
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with 3 = 3 atoms in each layer and a 7.68 = 7.68 = ˚ unit cell with a s b s 908 and g s 608. The 17.00 A geometry and inter-atomic distances in the surface model were obtained from bulk values. It was necessary to use a slab with at least three copper layers to make it possible to study the effect of the difference between adsorption on the ccp and hcp sites. Even though 2 = 2 atoms in the a–b plane would have been sufficient to include all adsorption sites, we decided to use 3 = 3 atoms to guarantee that no interactions in that plane would occur. This gave a distance between the CuCl molecules in adjacent ˚ which was considered sufficient. cells of 7.68 A, As a first test, the effect on the calculations of a fourth layer of copper atoms in the surface model was investigated. This test was performed using one k-point and a cut-off energy for the plane waves of 5 a.u. within the LDA. In all calculations, apart from when the model was allowed to relax as shown in Fig. 1c, all atoms were kept fixed with a Cu–Cl ˚ as obtained intra-molecular distance of 2.094 A, from allowing a gaseous CuCl to relax completely. The relative energies, compared to the relaxed model with the same number of copper layers, for templates with the CuCl molecule on different distances from the top layer of the copper surface are summarised in Table 1. The values presented in the second row are those obtained from the geometry optimisations. From Table 1, the final geometry is almost identical independent of whether three and four layers in the copper template are used. The difference in bond length is only 0.28% for the Cu–Cl intra-molecular
distance and 0.09% for the vertical distance from the copper atom in CuCl to the top layer of the surface. The differences in relative energies between the two models are higher, but still very small. The important results here are, however, that the geometry obtained from the optimisations are very similar whether we use three or four layers in the copper surface as the final energies are calculated with a higher level of accuracy than in this test. Hence, it was found sufficient to use three layers in the slab in the present investigation. ˚ was found From the tests, a c-axis of 17 A sufficient for all types of investigations performed in this study. This also includes the energy barrier studies where the CuCl molecule is further away from the surface, i.e., closer to the back of the surface in the adjacent cell in the z-direction. 3.1.2. Number of k-points and cut-off energy for the plane waÕes To investigate the necessary number of k-points and cut-off energy for the plane waves, the adsorption energy for adsorption of CuCl on the two-fold bridge site was compared for different combinations of number of k-points and cut-off energy. These test calculations were performed within the LDA and the results are shown in Table 2. From this table, four k-points and a cut-off energy for the plane waves of 11 a.u. gives sufficient accuracy for the calculations performed in this study. However, to further improve the accuracy of the final energies, those calculations were performed within the GGA.
Table 1 Test calculations showing the influence of the number of layers in the surface model on the bond distance in CuCl and the molecule–surface distance for the relaxed model. The difference in relative energies between the two models are also shown Cu Cu Cl – ˚. Surface ŽA
Relative energy ŽkJ moly1 . Three layers
Four layers
2 Optimised
444 0
415 0
8.5 0
˚ Cu–Cl: 2.125 A
˚ Cu–Cl: 2.131 A
Cu–Cl: 0.28
˚ Cu–Surface: 2.210 A 1100 1254 1341
˚ Cu–Surface: 2.208 A 1081 1283 1302
Cu–Surface: 0.09 1.8 2.6 3.2
3 4 5
Relative energy ŽkJ moly1 .
Difference between three and four layersŽ%.
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Table 2 The adsorption energy for CuCl adsorbed on the two-fold bridge site calculated with different combinations of k-points and cut-off energy for the plane waves. The empty cells correspond to calculations which were not possible to perform with the computer power available Adsorption energy ŽkJ moly1 . Cut-off energy for the plane waves ŽeV.
One k-point
Four k-points
Eight k-points
136 300 500
1669 211 207
1582 206
2094 184
In a final test, the energy of formation and bond distance for CuCl and the dissociation energy for the reaction
˚ and rCu – Cl s 2.271– where rCu – Cu s 2.432–2.784 A ˚ depending on the accuracy of their calcula2.380 A, tions.
Ž CuCl. 2 Ž g . ™ 2CuCl Ž g .
3.2. Final results
were compared with the results obtained by Kolmel ¨ and Ahlrichs w48x and Guido et al. w49x. Excellent agreement was found for molecular CuCl. The val˚ and ues obtained in this study Ž rCu – Cl s 2.094 A D Hf s y384 kJ moly1 . agrees very well with the bond length found by Kolmel and Ahlrichs Ž rCu – Cl ¨ ˚ . and the energy of formation found s 2.074–2.176 A by Guido et al. Ž D Hf s y382.3 kJ moly1 .. For the dissociation of the ŽCuCl. 2 molecule shown in Fig. 2c, Kolmel and Ahlrichs w48x found ¨ that De s 183 kJ moly1 , whereas Guido et al. w49x reported an experimental value of 181 kJ moly1 . Our results suggests an energy for dissociation of 223 kJ moly1 which is approximately 22% higher than those reported above. The geometry and intra-molec˚ ular distances found in this study Ž rCu – Cu s 2.527 A ˚ . agrees, however, very well and rCu – Cl s 2.351 A with the corresponding values found in Ref. w48x,
3.2.1. Adsorption of CuCl The adsorption energies, the Cu–Cl intra-molecular distance and Cu CuCl –Cu surface distance for copperŽI.chloride adsorbed on copperŽ111. are shown in Table 3. From this table, the adsorption energies for the ccp and hcp three-fold sites are identical, both being 152 kJ moly1 , whereas it is 10 kJ moly1 lower for the adsorption on the two-fold site. Adsorption on the on the on-top site is endothermic by 88 kJ moly1 . The vertical distance from the top layer of the surface model to the copper atom in CuCl decreases with increasing adsorption energy. This can be expected as a higher adsorption energy corresponds to a stronger and shorter chemical bond. Interestingly, the distance from the copper atom in CuCl to the nearest neighbours in the copper surface in all cases,
Table 3 Adsorption energies and bond distances for CuCl adsorbed on the available adsorption sites on a copperŽ111. surface. The Cu–surface distance is the vertical distance from the copper atom in CuCl to the top layer of the copper surface Adsorption site
Adsorption energy ŽkJ moly1 .
Bond length of ˚. Cu–Cl ŽA
Bond length of Cu ŽCuCl. – ˚. surface ŽA
Three-fold unfilled Three-fold filled Two-fold bridge On-top
y152 y152 y142 q88
2.098 2.170 2.125 2.145
2.105 2.071 2.210 2.313
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except for adsorption on the on-top site, corresponds to within 0.5% to the inter-atomic distances found in metallic copper. This distance is approximately 10% shorter when CuCl is adsorbed on the on-top site. Upon adsorption of CuCl, a small shift of the positions of the relaxed atoms in the surface model is noticed. Whereas this shift is very small for adsorption on sites a, b and c where the atoms are moved a very short distance out of the copper surface, it is more pronounced for adsorption on the on-top site. Here, the CuCl molecule pushes the copper atom ˚ into the copper vertically below it as much as 0.2 A surface. The findings presented above are also reflected in the electron density plots. From these, the electron density for the copper atom in adsorbed CuCl differs widely from that of gaseous CuCl. When CuCl is adsorbed on sites a–c, the electron density of the copper atom closely resembles that of metallic copper. The copper atom in CuCl is also well-incorporated into the copper surface and the resulting template corresponds closely to a copper surface with an adsorbed chlorine atom. Finally, no changes in the electron density of the chlorine atom in CuCl could be observed upon adsorption on any of the sites studied. The Cu–Cl bond distance for adsorbed CuCl was always longer than for the equilibrium bond distance ˚ ., indicating a in gaseous CuCl Ž rCu – Cl s 2.094 A weakening of the Cu–Cl bond upon adsorption. A possible explanation for this is that, as the copper atom is reduced by the substrate, parts of the ionic contribution in the CuCl bond is lost and it, thus, weakens causing an elongation of the Cu–Cl bond.
The trends in adsorption energy and bond distances as well as what was found in the electron density plots, can be regarded as reflections of the structure of the copperŽ111. surface. When CuCl binds to any of the two three-fold sites, the closepacked structure of the surface is followed and, hence, adsorption on these two sites is the most favourable. Adsorption on the two-fold bridge is also possible even though the copper lattice is not followed. This is also shown by the fact that the nearest neighbour distance from copper in CuCl to the atoms in the surface corresponds to the Cu–Cu distances in metallic copper. The on-top site, finally, is not a possible adsorption site, which also is reflected in the distorted bonds and high endothermic adsorption energy. 3.2.2. ActiÕation barrier for adsorption on the twofold site In Table 4 the Cu–Cl bond distances and the energies relative to the relaxed template for models with different Cu CuCl –Cu surface distances are presented. From this table, the energy for the template, where the distance from the copper atom in CuCl to ˚ is very close to that for the the surface is 2.4 A, relaxed model. This implies that the global minimum is very wide and shallow and can explain difficulties in getting some of the calculations to converge. From electron density plots, no interactions between the surface atoms and the CuCl molecule could be observed at longer Cu CuCl –Cu surface distances than 2.4 ˚ This is in some contradiction to the results in A. Table 4, which shows that there are interactions between the surface and the molecule even at longer
Table 4 The relative energies for CuCl at different distances from a copperŽ111. surface showing the absence of activation barrier in the adsorption of CuCl on the two-fold bridge site
˚. Cu surface . ŽA
Distance ŽCu ŽCuCl. –
Relative energy ŽkJ moly1 .
˚. of Cu–Cl ŽA
Bond length
2.126 2.4 2.8 3.2 3.6 4.0
0 6 665 954 978 1158
2.126 2.170 2.171 2.126 2.131 2.126
P. Martensson et al.r Applied Surface Science 136 (1998) 137–146 ˚
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Table 5 The total energies for different combinations of surface templates and gaseous ŽCuCl. n where n s 1, 2. These values show that CuCl aggregates in the gas phase, but do not support the assumption of a disproportionation of CuCl on the substrate surface Templates
Total energy ŽeV.
2CuClŽg. q CuŽ111. surface CuClŽg. q CuCl adsorbed on CuŽ111. ŽCuCl. 2 Žg. q CuŽ111. surface ŽCuCl. 2 adsorbed on CuŽ111. ŽFig. 2d. 2CuClŽg. ŽCuCl. 2 Žg. Žas in Fig. 2a. ŽCuCl. 2 Žg. Žas in Fig. 2b. ŽCuCl. 2 Žg. Žas in Fig. 2c.
y30722.11359 y30723.68943 y30723.25749 y30721.71152 y2883.59282 y2884.73671 y2883.51431 y2885.91804
distances. These interactions do not, however, seem to affect the electron distribution in the CuCl molecule. As no energy barrier could be detected in this study, it was not found valuable to continue this investigation with the two three-fold sites as also the experimental results had suggested that there was no barrier in the adsorption step. 3.2.3. Disproportionation of the copper(I)chloride molecule As previous XPS studies w31x showed that CuCl 2 was present in the copper film after deposition had been completed and also that the deposition rate per cycle increased abnormally during prolonged precursor pulses, the probability for a disproportionation process was investigated. This reaction is expected to proceed as 2CuCl Ž ads . ™ CuCl 2 Ž ads . q Cu Ž s .
Ž 2.
if the CuCl molecules are adsorbed on adjacent sites, or as
Ž CuCl. 2 Ž ads. ™ CuCl 2 Ž ads. q Cu Ž s .
Eads ŽkJ moly1 . y152 q149
The disproportionation process was studied in the gas phase for three different arrangements with the composition ŽCuCl. 2 as shown in Fig. 2a–c and for ŽCuCl. 2 adsorbed on the hcp site as shown in Fig. 2d. From Table 5, the formation of ŽCuCl. 2 in the gas phase is lower in energy by 110 kJ moly1 and 224 kJ moly1 as compared to two CuCl molecules for the arrangements shown in Figs. 2a and 2c, respectively. However, the energy for the arrangement shown in Fig. 2b is slightly higher than that for two free CuCl molecules. The adsorption of the ŽCuCl. 2 aggregate as shown in Fig. 2d is energetically unfavourable with an endothermic adsorption energy of 149 kJ moly1 and from the electron density plots, no indications of a disproportionation of CuCl on CuŽ111. could be found for this model. With the present results in mind, no further investigations of other arrangements were undertaken as the ŽCuCl. 2 was neither expected to adsorb on the substrate nor showed any signs of undergoing a disproportionation.
Ž 3.
if ŽCuCl. 2 is formed in the gas phase and adsorbs intact. As the volatility of CuCl 2 is lower than for the CuCl, traces of CuCl 2 can be expected to be found in the film if the deposition temperature is low enough. At elevated temperatures, CuCl 2 will desorb, leaving one adsorption site free, while the copper atom will be incorporated into the growing film. Obviously, this process destroys the self-limitation in the ALE process and must be avoided.
4. Summary and conclusions To summarise, we have found that adsorption of copperŽI.chloride on copperŽ111. is energetically most favourable on the two three-fold adsorption sites with adsorption energies of 152 kJ moly1 . The energy for adsorption on the two-fold bridge site is just 10 kJ moly1 lower, while adsorption on the on-top site is endothermic by 88 kJ moly1 . We could not find any indications that adsorption of CuCl on
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copperŽ111. is inhibited by an energy barrier which was in accordance with previous experimental results. We have also confirmed that CuCl aggregates in the gas phase forming different ŽCuCl. 2 clusters. Adsorption of these clusters are, however, energetically very unfavourable. Contrary to what was found in a previous experimental study, no evidence for a disproportionation of the CuCl molecule could be found neither in the gas phase nor on the substrate surface.
Acknowledgements
w17x w18x w19x w20x w21x w22x w23x w24x w25x w26x
The results published were generated using the program Cerius 2 e. This program was developed by BIOSYMrMolecular Simulations. Financial support from the Swedish Research Council for Engineering Sciences ŽTFR. and from the Swedish Natural Science Research Council ŽNFR. is also gratefully acknowledged.
w27x w28x w29x w30x
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A.B. Chuband, V.G. Lifshits, Appl. Surf. Sci. 82r83 Ž1994. 576. J.M. Hartmann, G. Feuillet, M. Charleux, H. Mariette, J. Appl. Phys. 79 Ž1996. 3035. B.Y. Maa, P.D. Dapkus, J. Electron. Mater. 19 Ž1990. 289. R.G. van Welzenis, R.A.M. Bink, H.H. Brongersma, Appl. Surf. Sci. 107 Ž1996. 255. J. Aarik, K. Kukli, A. Aidla, L. Pung, Appl. Surf. Sci. 103 Ž1996. 331. D.M. Bird, P.A. White, Surf. Sci. 377–379 Ž1997. 555. V. Milman, S.J. Pennycook, D.E. Jesson, Thin Solid Films 272 Ž1996. 375. G. Boisvert, L.J. Lewis, Phys. Rev. B 56 Ž1997. 7643. J. Wie, J.T. Yates Jr., Crit. Rev. Surf. Chem. 5 Ž1995. 1. M.I. Heggie, G. Jungnickel, C.D. Latham, Diamond Relat. Mater. 5 Ž1996. 236. K.E. Spear, M. Frenklach, in: K.E. Spears, J.P. Dismukes ŽEds.., Synthetic Diamond: Emerging CVD Science and Technology, Chap. 8, Wiley, 1992. Y. Mochizuki, T. Takada, A. Usui, Phys. Rev. B 47 Ž1993. 13420. Y. Mochizuki, T. Takada, A. Usui, Appl. Surf. Sci. 82r83 Ž1994. 200. Y. Mochizuki, T. Takada, T. Sakuma, S. Handa, C. Saoka, A. Usui, J. Cryst. Growth 135 Ž1994. 259. Y. Mochizuki, T. Takada, C. Saoka, A. Usui, E. Miyoshi, Y. Sakai, Phys. Rev. B 49 Ž1994. 4658. P. Martensson, J.-O. Carlsson, Chem. Vap. Deposition 3 ˚ Ž1997. 45. J. Dai, J.Z.H. Zhang, J. Chem. Phys. 102 Ž1995. 6280. B. Hammer, M. Scheffler, K.W. Jacobsen, J.K. Nørskov, Phys. Rev. Lett. 73 Ž1994. 1400. A. Forni, G. Wiesenekker, E.J. Baerends, G.F. Tantardini, Int. J. Quant. Chem. 52 Ž1994. 1067. A. Gross, B. Hammer, M. Scheffler, W. Brenig, Phys. Rev. Lett. 73 Ž1994. 3121. J.M. Campbell, C.T. Campbell, Surf. Sci. 259 Ž1991. 1. P.B. Rasmussen, P.M. Holmblad, H. Christoffersen, P.A. Taylor, I. Chorkendorff, Surf. Sci. 287r288 Ž1993. 79. P. Hohenberg, W. Kohn, Phys. Rev. 136 Ž1964. 864B. W. Kohn, L.J. Sham, Phys. Rev. 140 Ž1965. 1133A. L. Kleinman, D.M. Bylander, Phys. Rev. Lett. 48 Ž1982. 1425. M.P. Teter, M.C. Payne, D.C. Allan, Phys. Rev. B 40 Ž1989. 12255. H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 Ž1976. 5188. J.P. Perdew, A. Zunger, Phys. Rev. B 23 Ž1981. 5048. J.P. Perdew, Y. Wang, Phys. Rev. B 45 Ž1992. 13244. B. Hammer, K.W. Jacobsen, J.K. Nørskov, Phys. Rev. Lett. 70 Ž1993. 3971. P. Hu, D.A. King, S. Crampin, M.-H. Lee, M.C. Payne, Chem. Phys. Lett. 230 Ž1994. 501. R. Madar, C. Bernard, J. Palleau, J. Torres, Microelectron. Eng. 19 Ž1992. 571. C. Kolmel, R. Ahlrichs, J. Phys. Chem. 94 Ž1990. 5536. ¨ M. Guido, G. Gigli, G. Balducci, J. Chem. Phys. 57 Ž1972. 3731.
Atomic layer epitaxy of copper: an ab initio investigation of the CuClrH 2 process I. Adsorption of CuCl on Cu ž111 / ) Per Martensson , Karin Larsson 1, Jan-Otto Carlsson ˚
2
˚ The Angstrom ¨ Laboratory, Inorganic Chemistry, Box 538, SE-75121 Uppsala, Sweden Received 3 April 1998; accepted 27 June 1998
Abstract An ab initio investigation of the adsorption and disproportionation of copperŽI.chloride on copperŽ111., two crucial processes in CuClrH 2 copper Atomic Layer Epitaxy is presented. Adsorption of CuCl is energetically most favourable on the two different three-fold adsorption sites with adsorption energies of 152 kJ moly1. Adsorption on the two-fold bridge site is only 10 kJ moly1 lower in adsorption energy than on the three-fold sites, whereas adsorption on the on-top site is endothermic by as much as 88 kJ moly1. As previously noticed in an experimental study, adsorption is not limited by any energy barrier. The disproportionation was investigated for three different arrangements with the composition ŽCuCl. 2 . It was confirmed that the formation of ŽCuCl. 2 in the gas phase is favourable, but contrary to earlier experimental findings, no indications of a disproportionation of CuCl could be found. Adsorption of the ŽCuCl. 2 dimers was found to be energetically unfavourable implying that a dissociation to free CuCl molecules is required prior to adsorption. q 1998 Elsevier Science B.V. All rights reserved. PACS: 31.15 A; 31.15 E; 82.65 Y Keywords: Adsorption; Density functional calculations; Copper; CopperŽI.chloride
1. Introduction Atomic Layer Epitaxy, ALE, first introduced by Suntola and Antson in 1977 w1x is a versatile method which has been used for the deposition of thin films of a vast variety of materials w2–10x. Its versatility ) Corresponding author. Tel.: q46-18-471-3726; Fax: q46-18513-548; E-mail: [email protected] 1 Tel.: q46-18-4713750; Fax: q46-18-513548; E-mail: [email protected]. 2 Tel.: q46-18-4713750; Fax: q46-18-513548.
also includes the possibility for in situ studies of deposition processes as the reaction steps are separated in time by intermediate purging of the deposition chamber with an inert gas. Methods like X-ray Photoelectron Spectroscopy, XPS w11,12x, Fourier Transform Infrared Spectroscopy, FTIR w13,14x, Low-Energy Electron Diffraction, LEED w15,16x, Reflection High-Energy Electron Diffraction, RHEED w17,18x, Low-Energy Ion Scattering, LEIS w19x and Quartz Crystal Micro-balance, QCM w20x have been used, offering insights in the processes involved in the adsorption of precursor molecules
0169-4332r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 3 3 0 - 4
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and their further reaction with other reactants andror the substrate. As a complement to experiments, computational methods have gained increased interest during the last decades. This has been possible with the improvement of both computer hardware as well as of the algorithms used for the calculations and the potentials describing the atoms. However, even though much effort has been put into the understanding of surface processes such as adsorption, migration, surface reconstruction and dissociation w21–23x, the only extensive theoretical investigations related to gas phase deposition techniques, such as ALE or Chemical Vapour Deposition, CVD, have been in processes for diamond deposition w24–26x. This system is very suitable for quantum mechanical calculations because of the low number of electrons in the carbon atoms as well as the relatively simple gaseous species. Recently, Mochizuki et al. reported on a theoretical investigation of the GaClrAs 2 GaAs ALE process w27–30x. The precursors used in their study are small enough to allow for the use of a small surface model – still avoiding interactions between adjacent cells – which is a requirement for Quantum Mechanical, QM, calculations to be performed. As also the process presented in this paper is based on a small precursor, QM calculation was considered a suitable tool to gain a deeper insight in the deposition processes. We have previously reported on the deposition of copper from copperŽI.chloride and H 2 by means of ALE w31x. Experimentally, this process consists of four steps which are repeated until a film of desired thickness has been formed. Ž1. CuCl is first supplied and the molecules are adsorbed on the substrate surface. Ž2. This is followed by a purging with an inert gas to remove excess precursor leaving one monolayer of adsorbed CuCl. Ž3. To remove the chlorine atoms from the surface, hydrogen is added to form HCl which is desorbed and removed from the deposition chamber by pumping. Ž4. Finally, the reactor is purged with an inert gas again to remove the HCl formed as well as excess hydrogen. These steps can be further divided into a number of chemicalrphysical fundamental steps. As the CuCl molecule approaches the surface, CuCl–surface interactions will occur and at a close
enough distance, a chemical bond is formed. Prior to bond formation, the molecule can enter into a physisorbed state, i.e., a local minimum in the total energy surface. To enter the final chemically bonded state, an activation barrier has to be overcome which makes the adsorption process temperature-dependent. Finally, the molecule can undergo many different reactions on the surface, e.g., dissociation, disproportionation, substrate reaction or migration. The processes of interest in this study are primarily the adsorption and disproportionation of the CuCl molecule. The fundamental steps in hydrogen reduction of CuCl are similar to those presented above with the presence of the reaction of hydrogen with chlorine in CuCl and the final desorption of HCl. Previous experimental results w31x suggested that adsorption of CuCl is fast and proceeds without or with very low activation, while the reduction step is rather slow and limited by an energy barrier of approximately 80 kJ moly1 . This can be compared with earlier theoretical results which suggests an activation barrier for the dissociation of hydrogen on copper of 50–120 kJ moly1 depending on the geometry of the dissociation and accuracy of the calculations w32–35x and also with experimental results which suggest an activation barrier of 50–60 kJ moly1 w36,37x. Based on the relatively large difference between the activation barrier found in our process and the results reported for the dissociation of hydrogen on different copper surfaces, we assumed that the ratedetermining step in the reduction process was the reaction between adsorbed hydrogen and the chlorine atom of copper chloride. It may here be argued that an activation barrier of 80 kJ moly1 lies well within the barrier found for the dissociation of H 2 on Cu. We did, however, base our conclusion on the assumption that a process proceeds along the path which has the lowest activation barrier which, in this case, corresponds to the experimental results of approximately 50 kJ moly1 w36,37x. However, as almost all investigations of the adsorption and dissociation of hydrogen so far – theoretical as well as experimental – have been performed using ideal surfaces, it is unclear in what way the activation barrier for hydrogen dissociation is affected if the copper surface is terminated with
P. Martensson et al.r Applied Surface Science 136 (1998) 137–146 ˚
CuCl. It is not unlikely that the saturation of a surface by a species ‘A’ can affect the energy barriers for the adsorption and dissociation of species ‘B’. This means that even though the energy barrier found in Ref. w31x deviates significantly from results found for hydrogen dissociation on ideal surfaces, they might coincide with the energy barriers found when the surface has been modified and in that case, our conclusion that it is the formation of HCl that is the rate-determining step, has to be reconsidered. That the final desorption of HCl is the rate-determin-
139
ing step was excluded, since it is commonly accepted that the desorption of volatile reaction products is a fast process which requires low activation at elevated temperatures. In this paper, we present the results from calculations on the adsorption of copperŽI.chloride on the four available adsorption sites on a copperŽ111. surface shown in Fig. 1a–d, i.e., the cubic Ž1a. and hexagonal Ž1b. close-packed three-fold sites, the two-fold bridge site Ž1c. and the on-top site Ž1d.. The results from investigations of energy barriers for
Fig. 1. Models of the structures used for the investigation of the adsorption energy for CuCl on CuŽ111.. The sites shown are the ccp three-fold site Ž1a., the hcp three-fold site Ž1b., the two-fold bridge site Ž1c. and the on-top site Ž1d.. Atoms which were allowed to relax during the calculations are marked with grey, whereas atoms held fixed are black. The Cl atom is represented by a sphere that is larger than the Cu atoms.
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adsorption on the two-fold bridge site and the disproportionation of gaseous and adsorbed CuCl are also presented and discussed. The hydrogen reduction is currently under investigation and will be presented in a future publication.
2. Method The calculations were performed within the framework of Density Functional Theory, DFT, based on the work by Hohenberg and Kohn w38x and Kohn and Sham w39x. We used the CASTEP ŽCambridge Sequential Total Energy Package. computer program package from BiosymrMolecular Simulation Technologies of San Diego. The atoms were represented by nonlocal pseudopotentials in the Kleinman–Bylander fully separable form w40x. These potentials treat as valence the 3s and 3p electrons in Cl and the 3d and 4s electrons in Cu. Moreover, the calculations were performed on periodically repeated unit cells, usually referred to as super-cells. The electronic wave functions were expanded in terms of plane waves and the electronic minimisation was performed using a band-by-band conjugate-gradients minimisation technique w41x. The specific k-points were generated using the Monkhorst–Pack scheme which produces a uniform mesh of k-points in the reciprocal space w42x. Exchange and correlation effects were included within the Local Density Approximation, LDA, using the Perdew–Zunger parametrization w43x as well as through the Generalised Gradient Approximation, GGA, developed by Perdew and Wang w44x. A density gradient expansion has been included in the GGA in order to introduce inhomogeneity in the electron density. The GGA has, therefore, proven to give more accurate adsorption energies than the LDA which tends to overestimate chemisorption energies and underestimate the energy barriers w45,46x. The geometry optimisations were performed with one k-point and a cut-off energy for the plane waves of 5 a.u. Ž136 eV. using the LDA, while the final energies for the relaxed models were calculated with four k-points and a cut-off energy for the plane waves of approximately 12 a.u. Ž320 eV. using the GGA.
The adsorption energy was calculated for the four adsorption sites on copperŽ111. shown in Fig. 1, i.e., the three-fold ccp Ž1a. and hcp Ž1b. sites as well as the two-fold bridge Ž1c. and the on-top Ž1d. sites. The copperŽI.chloride molecule was oriented normal to and with the copper atom closest to the surface. The monomer CuCl was used even though it has been shown that CuCl most often exists as a dimer or a polymer in the gas phase w47–49x. This was motivated by the fact that the polymers and dimers are expected to dissociate on, or in the gas phase in the vicinity of, the surface prior to adsorption. This assumption was also proved during this work. For calculation of the adsorption energy the simple expression was used. Eads s Etot y Ž Esurf q ECuCl .
Ž 1.
Here, Eads is the adsorption energy, Etot is the energy for a surface with a CuCl molecule bonded to it, Esurf is the energy for the clean surface and ECuCl is the energy for gaseous CuCl. For the different templates modelling the surface, with or without an adsorbed CuCl molecule, some atoms were allowed to be relaxed, while all others were kept fixed in order to hold the characteristics of the crystal. In Fig. 1a–d, atoms in the surface model which were allowed to relax are marked with grey, whereas the fixed atoms are black. The CuCl molecule was allowed to fully relax in all calculations unless else mentioned. The energy barrier for adsorption on the two-fold bridge site was also investigated. This site was chosen from the results in the studies of the adsorption energy. From these, the adsorption energy for the two three-fold sites are almost identical, while it is approximately 10 kJ moly1 lower for adsorption on the two-fold site. As the three-fold sites are ‘closepacked sites’ and, thus, the most natural sites, it was assumed that the barrier for adsorption on the twofold bridge site would be larger than on the three-fold sites. The two-fold site was, therefore, chosen for the initial investigation of energy barriers during adsorption. We did not consider it appropriate to use the on-top site as adsorption on that site was found to be endothermic. In this study, the position of the copper atom in CuCl was held fixed, whereas atoms as above were allowed to be fully relaxed.
P. Martensson et al.r Applied Surface Science 136 (1998) 137–146 ˚
Finally, the disproportionation of CuCl in the gas phase was studied for the three different arrangements of ŽCuCl. 2 shown in Fig. 2a–c and for the linear arrangement of ŽCuCl. 2 adsorbed on the copper surface as shown in Fig. 2d. As no indications of a disproportionation could be found in these studies, it was not considered valuable to continue this investigation with the adsorption of the remaining two ŽCuCl. 2 dimers.
141
3. Results and discussion 3.1. Test calculations 3.1.1. Size of unit cell and number of atoms in the surface model Preliminary calculations suggested the use of a surface model including three layers of copper atoms
Fig. 2. The three gaseous ŽCuCl. 2 molecules Ž2a–c. and the adsorbed linear ŽCuCl. 2 used for the investigation of the disproportionation of CuCl. Atoms which were allowed to relax during the calculations are marked with grey, whereas atoms held fixed are black. The Cl atoms are represented by spheres that are larger than the Cu atoms.
P. Martensson et al.r Applied Surface Science 136 (1998) 137–146 ˚
142
with 3 = 3 atoms in each layer and a 7.68 = 7.68 = ˚ unit cell with a s b s 908 and g s 608. The 17.00 A geometry and inter-atomic distances in the surface model were obtained from bulk values. It was necessary to use a slab with at least three copper layers to make it possible to study the effect of the difference between adsorption on the ccp and hcp sites. Even though 2 = 2 atoms in the a–b plane would have been sufficient to include all adsorption sites, we decided to use 3 = 3 atoms to guarantee that no interactions in that plane would occur. This gave a distance between the CuCl molecules in adjacent ˚ which was considered sufficient. cells of 7.68 A, As a first test, the effect on the calculations of a fourth layer of copper atoms in the surface model was investigated. This test was performed using one k-point and a cut-off energy for the plane waves of 5 a.u. within the LDA. In all calculations, apart from when the model was allowed to relax as shown in Fig. 1c, all atoms were kept fixed with a Cu–Cl ˚ as obtained intra-molecular distance of 2.094 A, from allowing a gaseous CuCl to relax completely. The relative energies, compared to the relaxed model with the same number of copper layers, for templates with the CuCl molecule on different distances from the top layer of the copper surface are summarised in Table 1. The values presented in the second row are those obtained from the geometry optimisations. From Table 1, the final geometry is almost identical independent of whether three and four layers in the copper template are used. The difference in bond length is only 0.28% for the Cu–Cl intra-molecular
distance and 0.09% for the vertical distance from the copper atom in CuCl to the top layer of the surface. The differences in relative energies between the two models are higher, but still very small. The important results here are, however, that the geometry obtained from the optimisations are very similar whether we use three or four layers in the copper surface as the final energies are calculated with a higher level of accuracy than in this test. Hence, it was found sufficient to use three layers in the slab in the present investigation. ˚ was found From the tests, a c-axis of 17 A sufficient for all types of investigations performed in this study. This also includes the energy barrier studies where the CuCl molecule is further away from the surface, i.e., closer to the back of the surface in the adjacent cell in the z-direction. 3.1.2. Number of k-points and cut-off energy for the plane waÕes To investigate the necessary number of k-points and cut-off energy for the plane waves, the adsorption energy for adsorption of CuCl on the two-fold bridge site was compared for different combinations of number of k-points and cut-off energy. These test calculations were performed within the LDA and the results are shown in Table 2. From this table, four k-points and a cut-off energy for the plane waves of 11 a.u. gives sufficient accuracy for the calculations performed in this study. However, to further improve the accuracy of the final energies, those calculations were performed within the GGA.
Table 1 Test calculations showing the influence of the number of layers in the surface model on the bond distance in CuCl and the molecule–surface distance for the relaxed model. The difference in relative energies between the two models are also shown Cu Cu Cl – ˚. Surface ŽA
Relative energy ŽkJ moly1 . Three layers
Four layers
2 Optimised
444 0
415 0
8.5 0
˚ Cu–Cl: 2.125 A
˚ Cu–Cl: 2.131 A
Cu–Cl: 0.28
˚ Cu–Surface: 2.210 A 1100 1254 1341
˚ Cu–Surface: 2.208 A 1081 1283 1302
Cu–Surface: 0.09 1.8 2.6 3.2
3 4 5
Relative energy ŽkJ moly1 .
Difference between three and four layersŽ%.
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143
Table 2 The adsorption energy for CuCl adsorbed on the two-fold bridge site calculated with different combinations of k-points and cut-off energy for the plane waves. The empty cells correspond to calculations which were not possible to perform with the computer power available Adsorption energy ŽkJ moly1 . Cut-off energy for the plane waves ŽeV.
One k-point
Four k-points
Eight k-points
136 300 500
1669 211 207
1582 206
2094 184
In a final test, the energy of formation and bond distance for CuCl and the dissociation energy for the reaction
˚ and rCu – Cl s 2.271– where rCu – Cu s 2.432–2.784 A ˚ depending on the accuracy of their calcula2.380 A, tions.
Ž CuCl. 2 Ž g . ™ 2CuCl Ž g .
3.2. Final results
were compared with the results obtained by Kolmel ¨ and Ahlrichs w48x and Guido et al. w49x. Excellent agreement was found for molecular CuCl. The val˚ and ues obtained in this study Ž rCu – Cl s 2.094 A D Hf s y384 kJ moly1 . agrees very well with the bond length found by Kolmel and Ahlrichs Ž rCu – Cl ¨ ˚ . and the energy of formation found s 2.074–2.176 A by Guido et al. Ž D Hf s y382.3 kJ moly1 .. For the dissociation of the ŽCuCl. 2 molecule shown in Fig. 2c, Kolmel and Ahlrichs w48x found ¨ that De s 183 kJ moly1 , whereas Guido et al. w49x reported an experimental value of 181 kJ moly1 . Our results suggests an energy for dissociation of 223 kJ moly1 which is approximately 22% higher than those reported above. The geometry and intra-molec˚ ular distances found in this study Ž rCu – Cu s 2.527 A ˚ . agrees, however, very well and rCu – Cl s 2.351 A with the corresponding values found in Ref. w48x,
3.2.1. Adsorption of CuCl The adsorption energies, the Cu–Cl intra-molecular distance and Cu CuCl –Cu surface distance for copperŽI.chloride adsorbed on copperŽ111. are shown in Table 3. From this table, the adsorption energies for the ccp and hcp three-fold sites are identical, both being 152 kJ moly1 , whereas it is 10 kJ moly1 lower for the adsorption on the two-fold site. Adsorption on the on the on-top site is endothermic by 88 kJ moly1 . The vertical distance from the top layer of the surface model to the copper atom in CuCl decreases with increasing adsorption energy. This can be expected as a higher adsorption energy corresponds to a stronger and shorter chemical bond. Interestingly, the distance from the copper atom in CuCl to the nearest neighbours in the copper surface in all cases,
Table 3 Adsorption energies and bond distances for CuCl adsorbed on the available adsorption sites on a copperŽ111. surface. The Cu–surface distance is the vertical distance from the copper atom in CuCl to the top layer of the copper surface Adsorption site
Adsorption energy ŽkJ moly1 .
Bond length of ˚. Cu–Cl ŽA
Bond length of Cu ŽCuCl. – ˚. surface ŽA
Three-fold unfilled Three-fold filled Two-fold bridge On-top
y152 y152 y142 q88
2.098 2.170 2.125 2.145
2.105 2.071 2.210 2.313
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except for adsorption on the on-top site, corresponds to within 0.5% to the inter-atomic distances found in metallic copper. This distance is approximately 10% shorter when CuCl is adsorbed on the on-top site. Upon adsorption of CuCl, a small shift of the positions of the relaxed atoms in the surface model is noticed. Whereas this shift is very small for adsorption on sites a, b and c where the atoms are moved a very short distance out of the copper surface, it is more pronounced for adsorption on the on-top site. Here, the CuCl molecule pushes the copper atom ˚ into the copper vertically below it as much as 0.2 A surface. The findings presented above are also reflected in the electron density plots. From these, the electron density for the copper atom in adsorbed CuCl differs widely from that of gaseous CuCl. When CuCl is adsorbed on sites a–c, the electron density of the copper atom closely resembles that of metallic copper. The copper atom in CuCl is also well-incorporated into the copper surface and the resulting template corresponds closely to a copper surface with an adsorbed chlorine atom. Finally, no changes in the electron density of the chlorine atom in CuCl could be observed upon adsorption on any of the sites studied. The Cu–Cl bond distance for adsorbed CuCl was always longer than for the equilibrium bond distance ˚ ., indicating a in gaseous CuCl Ž rCu – Cl s 2.094 A weakening of the Cu–Cl bond upon adsorption. A possible explanation for this is that, as the copper atom is reduced by the substrate, parts of the ionic contribution in the CuCl bond is lost and it, thus, weakens causing an elongation of the Cu–Cl bond.
The trends in adsorption energy and bond distances as well as what was found in the electron density plots, can be regarded as reflections of the structure of the copperŽ111. surface. When CuCl binds to any of the two three-fold sites, the closepacked structure of the surface is followed and, hence, adsorption on these two sites is the most favourable. Adsorption on the two-fold bridge is also possible even though the copper lattice is not followed. This is also shown by the fact that the nearest neighbour distance from copper in CuCl to the atoms in the surface corresponds to the Cu–Cu distances in metallic copper. The on-top site, finally, is not a possible adsorption site, which also is reflected in the distorted bonds and high endothermic adsorption energy. 3.2.2. ActiÕation barrier for adsorption on the twofold site In Table 4 the Cu–Cl bond distances and the energies relative to the relaxed template for models with different Cu CuCl –Cu surface distances are presented. From this table, the energy for the template, where the distance from the copper atom in CuCl to ˚ is very close to that for the the surface is 2.4 A, relaxed model. This implies that the global minimum is very wide and shallow and can explain difficulties in getting some of the calculations to converge. From electron density plots, no interactions between the surface atoms and the CuCl molecule could be observed at longer Cu CuCl –Cu surface distances than 2.4 ˚ This is in some contradiction to the results in A. Table 4, which shows that there are interactions between the surface and the molecule even at longer
Table 4 The relative energies for CuCl at different distances from a copperŽ111. surface showing the absence of activation barrier in the adsorption of CuCl on the two-fold bridge site
˚. Cu surface . ŽA
Distance ŽCu ŽCuCl. –
Relative energy ŽkJ moly1 .
˚. of Cu–Cl ŽA
Bond length
2.126 2.4 2.8 3.2 3.6 4.0
0 6 665 954 978 1158
2.126 2.170 2.171 2.126 2.131 2.126
P. Martensson et al.r Applied Surface Science 136 (1998) 137–146 ˚
145
Table 5 The total energies for different combinations of surface templates and gaseous ŽCuCl. n where n s 1, 2. These values show that CuCl aggregates in the gas phase, but do not support the assumption of a disproportionation of CuCl on the substrate surface Templates
Total energy ŽeV.
2CuClŽg. q CuŽ111. surface CuClŽg. q CuCl adsorbed on CuŽ111. ŽCuCl. 2 Žg. q CuŽ111. surface ŽCuCl. 2 adsorbed on CuŽ111. ŽFig. 2d. 2CuClŽg. ŽCuCl. 2 Žg. Žas in Fig. 2a. ŽCuCl. 2 Žg. Žas in Fig. 2b. ŽCuCl. 2 Žg. Žas in Fig. 2c.
y30722.11359 y30723.68943 y30723.25749 y30721.71152 y2883.59282 y2884.73671 y2883.51431 y2885.91804
distances. These interactions do not, however, seem to affect the electron distribution in the CuCl molecule. As no energy barrier could be detected in this study, it was not found valuable to continue this investigation with the two three-fold sites as also the experimental results had suggested that there was no barrier in the adsorption step. 3.2.3. Disproportionation of the copper(I)chloride molecule As previous XPS studies w31x showed that CuCl 2 was present in the copper film after deposition had been completed and also that the deposition rate per cycle increased abnormally during prolonged precursor pulses, the probability for a disproportionation process was investigated. This reaction is expected to proceed as 2CuCl Ž ads . ™ CuCl 2 Ž ads . q Cu Ž s .
Ž 2.
if the CuCl molecules are adsorbed on adjacent sites, or as
Ž CuCl. 2 Ž ads. ™ CuCl 2 Ž ads. q Cu Ž s .
Eads ŽkJ moly1 . y152 q149
The disproportionation process was studied in the gas phase for three different arrangements with the composition ŽCuCl. 2 as shown in Fig. 2a–c and for ŽCuCl. 2 adsorbed on the hcp site as shown in Fig. 2d. From Table 5, the formation of ŽCuCl. 2 in the gas phase is lower in energy by 110 kJ moly1 and 224 kJ moly1 as compared to two CuCl molecules for the arrangements shown in Figs. 2a and 2c, respectively. However, the energy for the arrangement shown in Fig. 2b is slightly higher than that for two free CuCl molecules. The adsorption of the ŽCuCl. 2 aggregate as shown in Fig. 2d is energetically unfavourable with an endothermic adsorption energy of 149 kJ moly1 and from the electron density plots, no indications of a disproportionation of CuCl on CuŽ111. could be found for this model. With the present results in mind, no further investigations of other arrangements were undertaken as the ŽCuCl. 2 was neither expected to adsorb on the substrate nor showed any signs of undergoing a disproportionation.
Ž 3.
if ŽCuCl. 2 is formed in the gas phase and adsorbs intact. As the volatility of CuCl 2 is lower than for the CuCl, traces of CuCl 2 can be expected to be found in the film if the deposition temperature is low enough. At elevated temperatures, CuCl 2 will desorb, leaving one adsorption site free, while the copper atom will be incorporated into the growing film. Obviously, this process destroys the self-limitation in the ALE process and must be avoided.
4. Summary and conclusions To summarise, we have found that adsorption of copperŽI.chloride on copperŽ111. is energetically most favourable on the two three-fold adsorption sites with adsorption energies of 152 kJ moly1 . The energy for adsorption on the two-fold bridge site is just 10 kJ moly1 lower, while adsorption on the on-top site is endothermic by 88 kJ moly1 . We could not find any indications that adsorption of CuCl on
146
P. Martensson et al.r Applied Surface Science 136 (1998) 137–146 ˚
copperŽ111. is inhibited by an energy barrier which was in accordance with previous experimental results. We have also confirmed that CuCl aggregates in the gas phase forming different ŽCuCl. 2 clusters. Adsorption of these clusters are, however, energetically very unfavourable. Contrary to what was found in a previous experimental study, no evidence for a disproportionation of the CuCl molecule could be found neither in the gas phase nor on the substrate surface.
Acknowledgements
w17x w18x w19x w20x w21x w22x w23x w24x w25x w26x
The results published were generated using the program Cerius 2 e. This program was developed by BIOSYMrMolecular Simulations. Financial support from the Swedish Research Council for Engineering Sciences ŽTFR. and from the Swedish Natural Science Research Council ŽNFR. is also gratefully acknowledged.
w27x w28x w29x w30x
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