- Email: [email protected]
Direct observation of atomic exchange during surface self-diffusion
Surface Science 695 (2020) 121564
Contents lists available at ScienceDirect
Surface Science journal homepage: www.elsevier.com/locate/susc
Direct observation of atomic exchange during surface self-diffusion Matthew A. Koppa , P.R. Schwoebel ⁎
T
Department of Physics and Astronomy, University of New Mexico, 210 Yale Blvd. NE, Albuquerque, NM 87131, United States
ARTICLE INFO
ABSTRACT
Keywords: Surface diffusion Exchange diffusion
The first direct observation of atomic exchange during the surface self-diffusion of single adatoms is reported. Iridium adatoms from a source enriched to ≥ 93% 191Ir was deposited onto an atomically clean and smooth Ir (100) plane as observed in a field ion microscope. Following thermally activated surface self-diffusion the adatom was field desorbed and mass analyzed. Detection of the 193Ir isotope in one-half of the cases demonstrates conclusively that atomic exchange can occur during surface self-diffusion.
1. Introduction The vapor phase growth of crystalline materials is widely used in many technological applications, ranging from the microfabrication of microprocessors to the development of biological sensors. Over the last decade such fabrication processes have been pushed to the near atomic scale where continuous several nanometers-thick films are required and self-organization is of interest in creating features such as quantum dots and nanometer scale surface templates for patterning. Clearly technology is working toward manipulating atoms and creating functional devices such as transistors and memory at the atomic scale [1,2]. Because of this, an understanding of the processes that occur during atom transport and the growth of crystalline materials at the atomic level has become of great technological importance. The dynamics of the surface diffusion of adatoms, the nucleation and diffusion of clusters, and the impact of adatom-step interactions are several key phenomena governing mass transport and the resulting crystal growth. This importance has led to their study at the atomic level for over 50 years [3]. The majority of atomic level studies seeking to address the most basic processes, such as the surface diffusion of adatoms and whether atomic exchange with the substrate might occur during surface diffusion of an adatom, have been conducted using the field ion microscope (FIM) to study heterogeneous systems, i.e. an adatom of a different element than the substrate atoms. This is because the FIM, even when compared to the more recent scanning tunneling microscope and its cousins, remains adept at such atomistic studies [4]. However, like the scanning tunneling microscope, the FIM, in general, has difficulty distinguishing the chemical nature of individual atoms and thus is insensitive to detecting adatom exchange with the substrate if it were to occur in the case of surface self-diffusion. Thus, the first and only direct ⁎
evidence of diffusion by exchange is in a heterogeneous system as reported by Wrigley and Ehrlich [5] for W on Ir(110). In these studies a W adatom was deposited on the channeled Ir(110) surface. If hard sphere hopping was the only diffusion process available, then the expected diffusion would be one dimensional, i.e. along the channels. However, cross-channel diffusion was observed in several instances using both Ir and W adatoms. W adatoms that had undergone cross-channel diffusion were mass analyzed using the atom-probe field ion microscope (APFIM) and identified as Ir indicating that the W adatom had exchanged location with an Ir substrate atom from the channel wall. Atomic exchange during surface self-diffusion has been inferred from previous theoretical studies and FIM-based experiments. In a theoretical model of Al adatoms diffusing on Al(100), Feibelman predicted that an adatom and a substrate atom could undergo a “concerted displacement” in which the adatom descends into the lattice and the substrate atom is displaced upward to the surface at a second nearneighbor site [6]. Experimental evidence for surface self-diffusion by exchange of Ir and Pt adatoms on their respective (100)-type planes was provided by mapping the visitation sites of a single adatom [7–10]. If adatoms diffuse by hopping over substrate adatoms, then the expected site visitation map would form a (1x1) net of the lattice sites. However, the sites visited by a single adatom always formed a c(2x2) net, consistent with a model of diffusion by exchange. Additional evidence was inferred from the activation energies for the diffusion of different elemental adatoms on the Ir(100) surface. Fu and Tsong [11] found that for Ir and Rh adatoms on the Ir(100) plane the activation energy for diffusion of Ir (0.74 ± 0.02 eV) was lower than that for Rh (0.80 ± 0.08 eV). These activation energies are reversed to the order expected for hard sphere hopping given that on Ir(100) the cohesive energy of a Rh atom (5.75 eV/atom) is less that that of an Ir atom(6.94 eV/atom). The authors suggest that the existence of exchange diffusion
Corresponding author. E-mail address: [email protected] (M.A. Koppa).
https://doi.org/10.1016/j.susc.2020.121564 Received 4 October 2019; Received in revised form 28 December 2019; Accepted 9 January 2020 Available online 10 January 2020 0039-6028/ © 2020 Elsevier B.V. All rights reserved.
Surface Science 695 (2020) 121564
M.A. Koppa and P.R. Schwoebel
for Ir/Ir(100) may explain the lower activation energy. Although the c (2x2) map of visitation sites and measurement of activation energies for different elemental adatoms provides strong inferential evidence of exchange diffusion, there remained no direct evidence for atomic exchange in surface self-diffusion. To study atomic exchange during adatom surface self-diffusion we are using a source of adatoms that has been highly enriched in a particular isotope of the elemental substrate. This preserves the homogeneous chemical nature of the system and makes it possible to directly detect exchange diffusion. For example, if adatoms undergo exchange diffusion, then regardless of which elemental isotope is deposited, mass analysis of the adatoms following surface self-diffusion should show the natural isotopic abundance of the substrate. However, if adatoms undergo diffusion by hopping, then mass analysis of the adatoms following surface self-diffusion should show the isotopic abundance of the enriched adatom source. In this paper we show by mass analysis of the adatoms that the diffusion of Ir adatoms on Ir(100) proceeds by atomic exchange.
the exact temperature reached by the tip is difficult to determine without using the conventional elemental resistance thermometry of the tip support loop, adatoms on the plane and step-edge atoms of the plane were both observed to begin to diffuse, which indicates that the temperature was ~ 250 K [10]. This is consistent with estimates of the tip temperature from rough measurements of the mean square displacement, ⟨r2⟩, of the adatom and using the activation energy (0.74 ± 0.02 eV) and prefactor for diffusion (7.3 × 10 3 ± 0.5 cm2 /s) from [11], which yield T ~ 230 K. Following diffusion the adatom was mass analyzed in the atomprobe. Subsequent removal of substrate atoms and their analysis in the atom-probe provided a mass calibration. As a baseline check of the atom-probe and operation of the deposition source, isotope adatoms were deposited onto the substrate while it remained at 77 K and mass analyzed without initiating surface diffusion. Such studies consistently showed that the deposited adatom was 191Ir, as expected. 3. Results and discussion
2. Material and methods
Fig. 1 shows a clean Ir(100)-oriented FIM specimen just prior to the deposition of 191Ir adatoms. Fig. 1b shows the same surface following the deposition of ~ 30 191Ir adatoms, including a single adatom on the central Ir(100) plane. Lastly, Fig. 1c shows the image of the specimen and the displacement of the adatom on the Ir(100) plane following thermally activated surface self-diffusion. The appearance of the adatom and its subsequent motion on the plane are consistent with other observations and detections of Ir on the Ir(100) plane. Diffusion of a few atoms on the step edge of the plane can also be seen, which is expected because these atoms begin to diffuse along the step edge at lower temperatures than those required for adatom diffusion across the Ir(100) plane [10]. Following diffusion, the adatom was aligned with the chevron MCP at the end of the atom probe drift tube. A probe hole was not used as the alignment of adatoms with the drift tube was visually confirmed as shown in Fig. 2a. The adatom was then field evaporated and mass analyzed. Fig. 2b shows boundary atoms of the Ir (100) plane remain unchanged indicating that only the evaporated adatom reached the detector. Measurements of six individual adatoms in six separate experiments proceeding as described above show that following surface self-diffusion on Ir(100) one-half of the adatoms were 191 Ir and one-half were 193Ir, which approximately matches the natural abundances of the substrate. Because there are two outcomes for adatom deposition, either 191Ir or 193Ir, the binomial distribution can be used to determine the probability of depositing k = 3 193Ir atoms in n = 6 trials from the enriched source
Experiments were carried out in an all-metal, ultrahigh vacuum APFIM, constructed in our lab and similar in design to that of Chambers and Ehrlich [12]. The FIM capability provides atomic resolution spatial imaging of the surface. The atom-probe capability allows for removal and mass analysis of the adatom following various adatom-surface interactions, such as possible exchange with the substrate during surface self-diffusion. The background pressure of the system was in the mid 10 10 Torr range. Helium was used as the imaging gas at a pressure of 5 × 10 5 Torr. During atom-probe mass analysis the helium pressure was reduced to 5 × 10 6 Torr in order to decrease detector noise. Ir tip substrates were electrochemically etched from polycrystalline wire, spot welded to a Mo support loop, and outgassed prior to each experiment by thermal annealing at ~ 1200 K. To initiate surface selfdiffusion of an adatom, the tip substrate was heated by a 1064 nm continuous wave laser. The laser was used instead of the conventional loop-Joule-heating method [13] to enhance mass resolution by preserving the shape of the voltage pulse applied to the tip [14]. A single microchannel plate (MCP) provided image intensification for FIM observation of the adatoms. The MCP was mounted to a rotary motion feedthrough and was rotated 90∘ to allow for field desorbed atoms to enter the atom-probe section of the instrument for mass analysis [12]. The detector that the end of the time-of-flight drift tube was a chevron MCP. This detector also allowed us to visualize the position of the adatom of interest and its removal by field evaporation, and to correlate this event with its detection by the atom probe. The adatom deposition source was 191Ir powder enriched to ≥ 93% [15], and suspended on a tungsten-wire support loop. The deposition source was cleaned by heating to ~ 1800 K as verified using an optical pyrometer. The two naturally occurring isotopes of Ir, 191Ir and 193Ir, have relative atomic abundances of ~ 0.37 and 0.63 respectively. 191Ir adatoms were vapor deposited onto the Ir(100) surface by heating the enriched powder with the 1064 nm laser beam to ~ 2200 K for 10 s. Typically of the order of 10 to 100 adatoms were deposited on the visible portion of the specimen ( 10 11 cm2), which corresponds to roughly 10 3 10 2 monolayers. The techniques used in single adatom diffusion studies are widely reported in the literature [13]. In our studies a 191Ir adatom was deposited onto an atomically clean Ir(100) surface prepared by field evaporating several layers at 77 K, as verified by imaging in the FIM. This leaves the (100) surface in its terminated bulk-like state (less some possible relaxations). If more than one adatom was deposited onto the (100) surface field evaporation was used to remove adatoms until only one remained. The tip was then heated for 10 s intervals with the imaging field off, allowed to cool to 77 K, and reimaged to observe the new location of the adatom following surface self-diffusion. Although
P (3) =
n! p k (1 k! (n k )!
p)n
k
=
6! 0. 06863·0. 93143 = .0052. 3!3!
(1)
Thus, due to the high enrichment of the deposition source ( ≥ 93%), the confidence level that atomic exchange occurred during surface selfdiffusion is 100[1 P (3)] = 99.5%. Fig. 3 shows both the adatom mass spectrum with the noise subtracted and the calibration spectrum used to identify the adatom mass. Because the evaporation of adatoms was done in the presence of the helium imaging gas, a significant fraction of the mass spectra consist of the doubly charged helide compound IrHe 2 + for each isotope as expected based on studies by Krishnaswamy et al. of Ir field evaporation in the presence of He imaging gas [16]. Although the three 191Ir atoms were detected as helide compounds, the helide compounds still have a well defined mass-to-charge ratio allowing for the identification of adatoms. The previously discussed site mapping studies provided strong evidence for exchange diffusion in homogeneous systems, but the current results show directly that exchange diffusion occurs in homogeneous systems, ruling out possibilities that, for example, adatoms diffuse diagonally over substrate atoms to form the c(2x2) net. It is noteworthy 2
Surface Science 695 (2020) 121564
M.A. Koppa and P.R. Schwoebel
Fig. 1. Field ion image showing (a) a clean Ir(100)-oriented FIM specimen prepared by thermal heating followed by field evaporation of several atomic layers. The location of the central Ir(100) plane is indicated. The circular shadow on the left is the deposition source. (b) The specimen following the deposition of 191Ir adatoms, including a single adatom on the Ir(100) plane (indicated by an arrow). (c) The specimen after the initiation of thermally activated surface self-diffusion and displacement of the adatom on the Ir(100) plane. Fig. 3. (a) Mass spectrum of the six adatoms observed following deposition of 191 Ir and subsequent surface self-diffusion in six separate experiments. The detections of 193Ir show that the deposited 191Ir adatoms underwent atomic exchange during surface self-diffusion. (b) Mass spectrum used to calibrate the atom-probe.
enables such atomistic level studies in many systems. As an example, for many years the dynamics of steps on crystal surfaces has been recognized as one of the most important process that governs mass transport and the resulting growth of a crystal. From an atomistic view point there are open questions regarding the most fundamental aspects of step dynamics, particularly in homogeneous systems, that can now be addressed, such as: 1) Does a diffusing adatom approaching a lattice step from an upper terrace descend over the edge or does it exchange positions with a step-edge substrate atom? While the Ir(100) plane is not well suited to adatom step descent studies because the boundary atoms become mobile before the adatom on the top of the plane begins to diffuse [10], exchange mechanisms have been inferred in step descent for Ir/Ir(111)[17] and this can now be directly tested; 2) Can adatom-substrate atom exchange processes occur even during diffusion across a terrace?; 3) Can an atom diffusing along the edge of a step move by exchange with atoms in the step edge?; and 4) Does the proximity of other steps, i.e. terrace width, impact such processes?
Fig. 2. (a) The image on the chevron MCP at the end of the drift tube in the atom probe section of the instrument. The adatom on the Ir(100) plane in Fig. 1c is the central bright spot. (b) The image after the adatom was evaporated for mass analysis. The boundary spots are unchanged indicating that only the adatom from the central plane was analyzed.
that exchange diffusion in homogeneous systems has, as yet, only been observed on bulk-like terminated metal surfaces that reconstruct at elevated temperatures, i.e. the fcc (110) and fcc (100) surfaces of Ir and Pt. Exchange diffusion may thus be an indication of the inherent instability of the bulk-like terminated structure of these surfaces and provide an insight to the dynamics involved in their reconstruction. The lack of knowledge of crystal growth dynamics is a significant gap in our knowledge of solid state surface physics. Identifying and understanding such dynamical processes in homogeneous systems is the prototypic case. The technique of using isotopically enriched material as the source of adatoms in the study of homogeneous crystal growth
Declaration of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 3
Surface Science 695 (2020) 121564
M.A. Koppa and P.R. Schwoebel
CRediT authorship contribution statement
[7] C. Chen, T.T. Tsong, Displacement distribution and atomic jump direction in diffusion of Ir atoms on the Ir(001) surface, Phys. Rev. Lett. 64 (1990) 3147–3150, https://doi.org/10.1103/PhysRevLett.64.3147. https://link.aps.org/doi/10.1103/ PhysRevLett.64.3147 [8] G.L. Kellogg, P.J. Feibelman, Surface self-diffusion on Pt(001) by an atomic exchange mechanism, Phys. Rev. Lett. 64 (1990) 3143–3146, https://doi.org/10. 1103/PhysRevLett.64.3143. https://link.aps.org/doi/10.1103/PhysRevLett.64. 3143 [9] A. Friedl, O. Schütz, K. Müller, Self-diffusion on iridium (100). a structure investigation by field-ion microscopy, Surf. Sci. 266 (1992) 24–29, https://doi.org/ 10.1016/0039-6028(92)90992-F. http://www.sciencedirect.com/science/article/ pii/003960289290992F [10] T.-Y. Fu, H.-T. Wu, T.T. Tsong, Energetics of surface atomic processes near a lattice step, Phys. Rev. B 58 (1998) 2340–2346, https://doi.org/10.1103/PhysRevB.58. 2340. https://link.aps.org/doi/10.1103/PhysRevB.58.2340 [11] T.-Y. Fu, T.T. Tsong, Structure and diffusion of small Ir and Rh clusters on Ir(001) surfaces, Surface Science 421 (1999) 157–166, https://doi.org/10.1016/S00396028(98)00847-4. http://www.sciencedirect.com/science/article/pii/ S0039602898008474 [12] R.S. Chambers, G. Ehrlich, Chemical identification of individual surface atoms in the atom probe, J. Vac. Sci. Technol. 13 (1976) 273–276, https://doi.org/10.1116/ 1.568868. [13] G. Kellogg, Field ion microscope studies of single-atom surface diffusion and cluster nucleation on metal surfaces, Surf. Sci. Rep. 21 (1994) 1–88, https://doi.org/10. 1016/0167-5729(94)90007-8. http://www.sciencedirect.com/science/article/pii/ 0167572994900078 [14] J.A. Panitz, R.J. Walko, Determination of nanosecond highâ voltage pulse shapes at the surface of needle emitters, Rev. Sci. Instrum. 47 (1976) 1251–1254, https://doi. org/10.1063/1.1134520. [15] 2016, Supplied by Isoflex USA, [email protected]. [16] S.V. Krishnaswamy, S.B. McLane, E.W. Müller, Field desorption of helium and neon, J. Vac. Sci. Technol. 13 (1976) 665–669, https://doi.org/10.1116/1.568966 arXiv:10.1116/1.568966. [17] S.C. Wang, G. Ehrlich, Atom incorporation at surface clusters: an atomic view, Phys. Rev. Lett. 67 (1991) 2509–2512, https://doi.org/10.1103/PhysRevLett.67.2509. https://link.aps.org/doi/10.1103/PhysRevLett.67.2509
Matthew A. Koppa: Investigation, Writing - original draft. P.R. Schwoebel: Writing - review & editing. Acknowledgment We would like to thank J.A. Panitz for guidance regarding atomprobe operation and design, and Dave Dunlap for productive discussions of diffusion processes. This work was supported by the National Science Foundation Grant no. 1507837. References [1] M. Fuechsle, J.A. Miwa, S. Mahapatra, H. Ryu, S. Lee, O. Warschkow, L.C.L. Hollenberg, G. Klimeck, M.Y. Simmons, A single-atom transistor, Nat. Nanotechnol. 7 (2012) 242. https://doi.org/10.1038/nnano.2012.21 [2] F.E. Kalff, M.P. Rebergen, E. Fahrenfort, J. Girovsky, R. Toskovic, J.L. Lado, J. Fernández-Rossier, A.F. Otte, A kilobyte rewritable atomic memory, Nat. Nanotechnol. 11 (2016) 926. https://doi.org/10.1038/nnano.2016.131 [3] G. Antczak, G. Ehrlich, Surface Diffusion: Metals, Metal Atoms, and Clusters, Cambridge University Press, 2010, https://doi.org/10.1017/CBO9780511730320. [4] G. Ehrlich, Atomic events at lattice steps and clusters: a direct view of crystal growth processes, Surf. Sci. 331-333 (1995) 865–877, https://doi.org/10.1016/ 0039-6028(95)00075-5. http://www.sciencedirect.com/science/article/pii/ 0039602895000755 proceedings of the 14th European Conference on Surface Science [5] J.D. Wrigley, G. Ehrlich, Surface diffusion by an atomic exchange mechanism, Phys. Rev. Lett. 44 (1980) 661–663, https://doi.org/10.1103/PhysRevLett.44.661. https://link.aps.org/doi/10.1103/PhysRevLett.44.661 [6] P.J. Feibelman, Diffusion path for an Al adatom on Al(001), Phys. Rev. Lett. 65 (1990) 729–732, https://doi.org/10.1103/PhysRevLett.65.729. https://link.aps. org/doi/10.1103/PhysRevLett.65.729
4
Contents lists available at ScienceDirect
Surface Science journal homepage: www.elsevier.com/locate/susc
Direct observation of atomic exchange during surface self-diffusion Matthew A. Koppa , P.R. Schwoebel ⁎
T
Department of Physics and Astronomy, University of New Mexico, 210 Yale Blvd. NE, Albuquerque, NM 87131, United States
ARTICLE INFO
ABSTRACT
Keywords: Surface diffusion Exchange diffusion
The first direct observation of atomic exchange during the surface self-diffusion of single adatoms is reported. Iridium adatoms from a source enriched to ≥ 93% 191Ir was deposited onto an atomically clean and smooth Ir (100) plane as observed in a field ion microscope. Following thermally activated surface self-diffusion the adatom was field desorbed and mass analyzed. Detection of the 193Ir isotope in one-half of the cases demonstrates conclusively that atomic exchange can occur during surface self-diffusion.
1. Introduction The vapor phase growth of crystalline materials is widely used in many technological applications, ranging from the microfabrication of microprocessors to the development of biological sensors. Over the last decade such fabrication processes have been pushed to the near atomic scale where continuous several nanometers-thick films are required and self-organization is of interest in creating features such as quantum dots and nanometer scale surface templates for patterning. Clearly technology is working toward manipulating atoms and creating functional devices such as transistors and memory at the atomic scale [1,2]. Because of this, an understanding of the processes that occur during atom transport and the growth of crystalline materials at the atomic level has become of great technological importance. The dynamics of the surface diffusion of adatoms, the nucleation and diffusion of clusters, and the impact of adatom-step interactions are several key phenomena governing mass transport and the resulting crystal growth. This importance has led to their study at the atomic level for over 50 years [3]. The majority of atomic level studies seeking to address the most basic processes, such as the surface diffusion of adatoms and whether atomic exchange with the substrate might occur during surface diffusion of an adatom, have been conducted using the field ion microscope (FIM) to study heterogeneous systems, i.e. an adatom of a different element than the substrate atoms. This is because the FIM, even when compared to the more recent scanning tunneling microscope and its cousins, remains adept at such atomistic studies [4]. However, like the scanning tunneling microscope, the FIM, in general, has difficulty distinguishing the chemical nature of individual atoms and thus is insensitive to detecting adatom exchange with the substrate if it were to occur in the case of surface self-diffusion. Thus, the first and only direct ⁎
evidence of diffusion by exchange is in a heterogeneous system as reported by Wrigley and Ehrlich [5] for W on Ir(110). In these studies a W adatom was deposited on the channeled Ir(110) surface. If hard sphere hopping was the only diffusion process available, then the expected diffusion would be one dimensional, i.e. along the channels. However, cross-channel diffusion was observed in several instances using both Ir and W adatoms. W adatoms that had undergone cross-channel diffusion were mass analyzed using the atom-probe field ion microscope (APFIM) and identified as Ir indicating that the W adatom had exchanged location with an Ir substrate atom from the channel wall. Atomic exchange during surface self-diffusion has been inferred from previous theoretical studies and FIM-based experiments. In a theoretical model of Al adatoms diffusing on Al(100), Feibelman predicted that an adatom and a substrate atom could undergo a “concerted displacement” in which the adatom descends into the lattice and the substrate atom is displaced upward to the surface at a second nearneighbor site [6]. Experimental evidence for surface self-diffusion by exchange of Ir and Pt adatoms on their respective (100)-type planes was provided by mapping the visitation sites of a single adatom [7–10]. If adatoms diffuse by hopping over substrate adatoms, then the expected site visitation map would form a (1x1) net of the lattice sites. However, the sites visited by a single adatom always formed a c(2x2) net, consistent with a model of diffusion by exchange. Additional evidence was inferred from the activation energies for the diffusion of different elemental adatoms on the Ir(100) surface. Fu and Tsong [11] found that for Ir and Rh adatoms on the Ir(100) plane the activation energy for diffusion of Ir (0.74 ± 0.02 eV) was lower than that for Rh (0.80 ± 0.08 eV). These activation energies are reversed to the order expected for hard sphere hopping given that on Ir(100) the cohesive energy of a Rh atom (5.75 eV/atom) is less that that of an Ir atom(6.94 eV/atom). The authors suggest that the existence of exchange diffusion
Corresponding author. E-mail address: [email protected] (M.A. Koppa).
https://doi.org/10.1016/j.susc.2020.121564 Received 4 October 2019; Received in revised form 28 December 2019; Accepted 9 January 2020 Available online 10 January 2020 0039-6028/ © 2020 Elsevier B.V. All rights reserved.
Surface Science 695 (2020) 121564
M.A. Koppa and P.R. Schwoebel
for Ir/Ir(100) may explain the lower activation energy. Although the c (2x2) map of visitation sites and measurement of activation energies for different elemental adatoms provides strong inferential evidence of exchange diffusion, there remained no direct evidence for atomic exchange in surface self-diffusion. To study atomic exchange during adatom surface self-diffusion we are using a source of adatoms that has been highly enriched in a particular isotope of the elemental substrate. This preserves the homogeneous chemical nature of the system and makes it possible to directly detect exchange diffusion. For example, if adatoms undergo exchange diffusion, then regardless of which elemental isotope is deposited, mass analysis of the adatoms following surface self-diffusion should show the natural isotopic abundance of the substrate. However, if adatoms undergo diffusion by hopping, then mass analysis of the adatoms following surface self-diffusion should show the isotopic abundance of the enriched adatom source. In this paper we show by mass analysis of the adatoms that the diffusion of Ir adatoms on Ir(100) proceeds by atomic exchange.
the exact temperature reached by the tip is difficult to determine without using the conventional elemental resistance thermometry of the tip support loop, adatoms on the plane and step-edge atoms of the plane were both observed to begin to diffuse, which indicates that the temperature was ~ 250 K [10]. This is consistent with estimates of the tip temperature from rough measurements of the mean square displacement, ⟨r2⟩, of the adatom and using the activation energy (0.74 ± 0.02 eV) and prefactor for diffusion (7.3 × 10 3 ± 0.5 cm2 /s) from [11], which yield T ~ 230 K. Following diffusion the adatom was mass analyzed in the atomprobe. Subsequent removal of substrate atoms and their analysis in the atom-probe provided a mass calibration. As a baseline check of the atom-probe and operation of the deposition source, isotope adatoms were deposited onto the substrate while it remained at 77 K and mass analyzed without initiating surface diffusion. Such studies consistently showed that the deposited adatom was 191Ir, as expected. 3. Results and discussion
2. Material and methods
Fig. 1 shows a clean Ir(100)-oriented FIM specimen just prior to the deposition of 191Ir adatoms. Fig. 1b shows the same surface following the deposition of ~ 30 191Ir adatoms, including a single adatom on the central Ir(100) plane. Lastly, Fig. 1c shows the image of the specimen and the displacement of the adatom on the Ir(100) plane following thermally activated surface self-diffusion. The appearance of the adatom and its subsequent motion on the plane are consistent with other observations and detections of Ir on the Ir(100) plane. Diffusion of a few atoms on the step edge of the plane can also be seen, which is expected because these atoms begin to diffuse along the step edge at lower temperatures than those required for adatom diffusion across the Ir(100) plane [10]. Following diffusion, the adatom was aligned with the chevron MCP at the end of the atom probe drift tube. A probe hole was not used as the alignment of adatoms with the drift tube was visually confirmed as shown in Fig. 2a. The adatom was then field evaporated and mass analyzed. Fig. 2b shows boundary atoms of the Ir (100) plane remain unchanged indicating that only the evaporated adatom reached the detector. Measurements of six individual adatoms in six separate experiments proceeding as described above show that following surface self-diffusion on Ir(100) one-half of the adatoms were 191 Ir and one-half were 193Ir, which approximately matches the natural abundances of the substrate. Because there are two outcomes for adatom deposition, either 191Ir or 193Ir, the binomial distribution can be used to determine the probability of depositing k = 3 193Ir atoms in n = 6 trials from the enriched source
Experiments were carried out in an all-metal, ultrahigh vacuum APFIM, constructed in our lab and similar in design to that of Chambers and Ehrlich [12]. The FIM capability provides atomic resolution spatial imaging of the surface. The atom-probe capability allows for removal and mass analysis of the adatom following various adatom-surface interactions, such as possible exchange with the substrate during surface self-diffusion. The background pressure of the system was in the mid 10 10 Torr range. Helium was used as the imaging gas at a pressure of 5 × 10 5 Torr. During atom-probe mass analysis the helium pressure was reduced to 5 × 10 6 Torr in order to decrease detector noise. Ir tip substrates were electrochemically etched from polycrystalline wire, spot welded to a Mo support loop, and outgassed prior to each experiment by thermal annealing at ~ 1200 K. To initiate surface selfdiffusion of an adatom, the tip substrate was heated by a 1064 nm continuous wave laser. The laser was used instead of the conventional loop-Joule-heating method [13] to enhance mass resolution by preserving the shape of the voltage pulse applied to the tip [14]. A single microchannel plate (MCP) provided image intensification for FIM observation of the adatoms. The MCP was mounted to a rotary motion feedthrough and was rotated 90∘ to allow for field desorbed atoms to enter the atom-probe section of the instrument for mass analysis [12]. The detector that the end of the time-of-flight drift tube was a chevron MCP. This detector also allowed us to visualize the position of the adatom of interest and its removal by field evaporation, and to correlate this event with its detection by the atom probe. The adatom deposition source was 191Ir powder enriched to ≥ 93% [15], and suspended on a tungsten-wire support loop. The deposition source was cleaned by heating to ~ 1800 K as verified using an optical pyrometer. The two naturally occurring isotopes of Ir, 191Ir and 193Ir, have relative atomic abundances of ~ 0.37 and 0.63 respectively. 191Ir adatoms were vapor deposited onto the Ir(100) surface by heating the enriched powder with the 1064 nm laser beam to ~ 2200 K for 10 s. Typically of the order of 10 to 100 adatoms were deposited on the visible portion of the specimen ( 10 11 cm2), which corresponds to roughly 10 3 10 2 monolayers. The techniques used in single adatom diffusion studies are widely reported in the literature [13]. In our studies a 191Ir adatom was deposited onto an atomically clean Ir(100) surface prepared by field evaporating several layers at 77 K, as verified by imaging in the FIM. This leaves the (100) surface in its terminated bulk-like state (less some possible relaxations). If more than one adatom was deposited onto the (100) surface field evaporation was used to remove adatoms until only one remained. The tip was then heated for 10 s intervals with the imaging field off, allowed to cool to 77 K, and reimaged to observe the new location of the adatom following surface self-diffusion. Although
P (3) =
n! p k (1 k! (n k )!
p)n
k
=
6! 0. 06863·0. 93143 = .0052. 3!3!
(1)
Thus, due to the high enrichment of the deposition source ( ≥ 93%), the confidence level that atomic exchange occurred during surface selfdiffusion is 100[1 P (3)] = 99.5%. Fig. 3 shows both the adatom mass spectrum with the noise subtracted and the calibration spectrum used to identify the adatom mass. Because the evaporation of adatoms was done in the presence of the helium imaging gas, a significant fraction of the mass spectra consist of the doubly charged helide compound IrHe 2 + for each isotope as expected based on studies by Krishnaswamy et al. of Ir field evaporation in the presence of He imaging gas [16]. Although the three 191Ir atoms were detected as helide compounds, the helide compounds still have a well defined mass-to-charge ratio allowing for the identification of adatoms. The previously discussed site mapping studies provided strong evidence for exchange diffusion in homogeneous systems, but the current results show directly that exchange diffusion occurs in homogeneous systems, ruling out possibilities that, for example, adatoms diffuse diagonally over substrate atoms to form the c(2x2) net. It is noteworthy 2
Surface Science 695 (2020) 121564
M.A. Koppa and P.R. Schwoebel
Fig. 1. Field ion image showing (a) a clean Ir(100)-oriented FIM specimen prepared by thermal heating followed by field evaporation of several atomic layers. The location of the central Ir(100) plane is indicated. The circular shadow on the left is the deposition source. (b) The specimen following the deposition of 191Ir adatoms, including a single adatom on the Ir(100) plane (indicated by an arrow). (c) The specimen after the initiation of thermally activated surface self-diffusion and displacement of the adatom on the Ir(100) plane. Fig. 3. (a) Mass spectrum of the six adatoms observed following deposition of 191 Ir and subsequent surface self-diffusion in six separate experiments. The detections of 193Ir show that the deposited 191Ir adatoms underwent atomic exchange during surface self-diffusion. (b) Mass spectrum used to calibrate the atom-probe.
enables such atomistic level studies in many systems. As an example, for many years the dynamics of steps on crystal surfaces has been recognized as one of the most important process that governs mass transport and the resulting growth of a crystal. From an atomistic view point there are open questions regarding the most fundamental aspects of step dynamics, particularly in homogeneous systems, that can now be addressed, such as: 1) Does a diffusing adatom approaching a lattice step from an upper terrace descend over the edge or does it exchange positions with a step-edge substrate atom? While the Ir(100) plane is not well suited to adatom step descent studies because the boundary atoms become mobile before the adatom on the top of the plane begins to diffuse [10], exchange mechanisms have been inferred in step descent for Ir/Ir(111)[17] and this can now be directly tested; 2) Can adatom-substrate atom exchange processes occur even during diffusion across a terrace?; 3) Can an atom diffusing along the edge of a step move by exchange with atoms in the step edge?; and 4) Does the proximity of other steps, i.e. terrace width, impact such processes?
Fig. 2. (a) The image on the chevron MCP at the end of the drift tube in the atom probe section of the instrument. The adatom on the Ir(100) plane in Fig. 1c is the central bright spot. (b) The image after the adatom was evaporated for mass analysis. The boundary spots are unchanged indicating that only the adatom from the central plane was analyzed.
that exchange diffusion in homogeneous systems has, as yet, only been observed on bulk-like terminated metal surfaces that reconstruct at elevated temperatures, i.e. the fcc (110) and fcc (100) surfaces of Ir and Pt. Exchange diffusion may thus be an indication of the inherent instability of the bulk-like terminated structure of these surfaces and provide an insight to the dynamics involved in their reconstruction. The lack of knowledge of crystal growth dynamics is a significant gap in our knowledge of solid state surface physics. Identifying and understanding such dynamical processes in homogeneous systems is the prototypic case. The technique of using isotopically enriched material as the source of adatoms in the study of homogeneous crystal growth
Declaration of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 3
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CRediT authorship contribution statement
[7] C. Chen, T.T. Tsong, Displacement distribution and atomic jump direction in diffusion of Ir atoms on the Ir(001) surface, Phys. Rev. Lett. 64 (1990) 3147–3150, https://doi.org/10.1103/PhysRevLett.64.3147. https://link.aps.org/doi/10.1103/ PhysRevLett.64.3147 [8] G.L. Kellogg, P.J. Feibelman, Surface self-diffusion on Pt(001) by an atomic exchange mechanism, Phys. Rev. Lett. 64 (1990) 3143–3146, https://doi.org/10. 1103/PhysRevLett.64.3143. https://link.aps.org/doi/10.1103/PhysRevLett.64. 3143 [9] A. Friedl, O. Schütz, K. Müller, Self-diffusion on iridium (100). a structure investigation by field-ion microscopy, Surf. Sci. 266 (1992) 24–29, https://doi.org/ 10.1016/0039-6028(92)90992-F. http://www.sciencedirect.com/science/article/ pii/003960289290992F [10] T.-Y. Fu, H.-T. Wu, T.T. Tsong, Energetics of surface atomic processes near a lattice step, Phys. Rev. B 58 (1998) 2340–2346, https://doi.org/10.1103/PhysRevB.58. 2340. https://link.aps.org/doi/10.1103/PhysRevB.58.2340 [11] T.-Y. Fu, T.T. Tsong, Structure and diffusion of small Ir and Rh clusters on Ir(001) surfaces, Surface Science 421 (1999) 157–166, https://doi.org/10.1016/S00396028(98)00847-4. http://www.sciencedirect.com/science/article/pii/ S0039602898008474 [12] R.S. Chambers, G. Ehrlich, Chemical identification of individual surface atoms in the atom probe, J. Vac. Sci. Technol. 13 (1976) 273–276, https://doi.org/10.1116/ 1.568868. [13] G. Kellogg, Field ion microscope studies of single-atom surface diffusion and cluster nucleation on metal surfaces, Surf. Sci. Rep. 21 (1994) 1–88, https://doi.org/10. 1016/0167-5729(94)90007-8. http://www.sciencedirect.com/science/article/pii/ 0167572994900078 [14] J.A. Panitz, R.J. Walko, Determination of nanosecond highâ voltage pulse shapes at the surface of needle emitters, Rev. Sci. Instrum. 47 (1976) 1251–1254, https://doi. org/10.1063/1.1134520. [15] 2016, Supplied by Isoflex USA, [email protected]. [16] S.V. Krishnaswamy, S.B. McLane, E.W. Müller, Field desorption of helium and neon, J. Vac. Sci. Technol. 13 (1976) 665–669, https://doi.org/10.1116/1.568966 arXiv:10.1116/1.568966. [17] S.C. Wang, G. Ehrlich, Atom incorporation at surface clusters: an atomic view, Phys. Rev. Lett. 67 (1991) 2509–2512, https://doi.org/10.1103/PhysRevLett.67.2509. https://link.aps.org/doi/10.1103/PhysRevLett.67.2509
Matthew A. Koppa: Investigation, Writing - original draft. P.R. Schwoebel: Writing - review & editing. Acknowledgment We would like to thank J.A. Panitz for guidance regarding atomprobe operation and design, and Dave Dunlap for productive discussions of diffusion processes. This work was supported by the National Science Foundation Grant no. 1507837. References [1] M. Fuechsle, J.A. Miwa, S. Mahapatra, H. Ryu, S. Lee, O. Warschkow, L.C.L. Hollenberg, G. Klimeck, M.Y. Simmons, A single-atom transistor, Nat. Nanotechnol. 7 (2012) 242. https://doi.org/10.1038/nnano.2012.21 [2] F.E. Kalff, M.P. Rebergen, E. Fahrenfort, J. Girovsky, R. Toskovic, J.L. Lado, J. Fernández-Rossier, A.F. Otte, A kilobyte rewritable atomic memory, Nat. Nanotechnol. 11 (2016) 926. https://doi.org/10.1038/nnano.2016.131 [3] G. Antczak, G. Ehrlich, Surface Diffusion: Metals, Metal Atoms, and Clusters, Cambridge University Press, 2010, https://doi.org/10.1017/CBO9780511730320. [4] G. Ehrlich, Atomic events at lattice steps and clusters: a direct view of crystal growth processes, Surf. Sci. 331-333 (1995) 865–877, https://doi.org/10.1016/ 0039-6028(95)00075-5. http://www.sciencedirect.com/science/article/pii/ 0039602895000755 proceedings of the 14th European Conference on Surface Science [5] J.D. Wrigley, G. Ehrlich, Surface diffusion by an atomic exchange mechanism, Phys. Rev. Lett. 44 (1980) 661–663, https://doi.org/10.1103/PhysRevLett.44.661. https://link.aps.org/doi/10.1103/PhysRevLett.44.661 [6] P.J. Feibelman, Diffusion path for an Al adatom on Al(001), Phys. Rev. Lett. 65 (1990) 729–732, https://doi.org/10.1103/PhysRevLett.65.729. https://link.aps. org/doi/10.1103/PhysRevLett.65.729
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