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Atomic Assembly of Thin Film Materials
X. W. Zhou1,a, D. A. Murdick1,b, B. Gillespie1,c, J. J. Quan1,d, H. N. G. Wadley1,e, R. Drautz2,f, and D. Pettifor2,g
1Department of Materials Science and Engineering, 116 Engineer’s Way, Charlottesville, VA
22904-4745, USA
2Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK
Keywords: Molecular dynamics, vapor deposition, interatomic potentials.
Abstract. The atomic-scale structures and properties of thin films are critically determined by the various kinetic processes activated during their atomic assembly. Molecular dynamics simulations of growth allow these kinetic processes to be realistically addressed at a timescale that is difficult to reach using ab initio calculations. The newest approaches have begun to enable the growth simulation to be applied for a wide range of materials. Embedded atom method potentials can be successfully used to simulate the growth of closely packed metal multilayers. Modified charge transfer ionic + embedded atom method potentials are transferable between metallic and ionic materials and have been used to simulate the growth of metal oxides on metals. New analytical bond order potentials are now enabling significantly improved molecular dynamics simulations of semiconductor growth. Selected simulations are used to demonstrate the insights that can be gained about growth processes at surfaces.
Introduction
Many electronic and optical devices are constructed using nanoscale multilayers composed of metals, metal oxides, or semiconductors. For instance, metal multilayers that use a pair of thin (~50 Å) ferromagnetic metal layers sandwiching a thin (~20 Å) conductive metal layer can exhibit giant magnetoresistive (GMR) properties [1]. They are used for high performance readhead sensors for hard disk drives [1]. Magnetic tunnel junction (MTJ) multilayers that use a pair of thin (~50 Å) ferromagnetic metal layers sandwiching a thin (~20 Å) oxide layer exhibit tunneling magnetoresistive (TMR) properties [2,3]. They can be used to build magnetic random access memory (MRAM) [2,3]. Semiconductor thin films are widely used in cellular phones, global positioning systems, light-emitting diodes, lasers, infrared detectors, and solar cells [4,5,6,7].
The performance of all these various devices critically depends on the atomic-scale structures of the thin films. The conductive layer in the GMR multilayers and the oxide tunnel barrier layer in the TMR multilayers must be continuous, have uniform thickness, and form sharp unmixed interfaces with the adjacent ferromagnetic layers. It is a challenge to grow these layers, especially when their thickness is required to be very thin. Semiconductor films must have a high crystallinity and a low defect concentration. This can be difficult to achieve when a low growth temperature is utilized to increase dopant concentrations. Because of these difficulties, optimization of the growth using experimental approaches alone can be a prolonged process. Molecular dynamics (MD) simulations of vapor deposition track the evolution of film structures by solving for the positions of deposited and substrate atoms. They use Newton’s equations of motion and interatomic forces and can provide accurate insights to help improve the growth processes.
There are two challenges to the MD approach. First, the material systems of interest involve metallic, ionic, and covalent bonding. Each requires a different type of high fidelity interatomic potential to calculate the corresponding interatomic forces. Secondly, growth occurs by random
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condensation of atoms on the surface under kinetically constrained conditions. Complex surfaces and lattice defects are therefore encountered. Accordingly, the interatomic potential must accurately predict properties of all the atomic configurations that can possibly form on or below a surface. The newest MD approaches have begun to address these challenges, enabling the growth simulation to be applied for a wide range of materials. Here we illustrate three developments by considering growth simulations of the metallic multilayers, metal/metal oxide multilayers, and covalent semiconductor thin films.
Growth of Giant Magnetoresistive Multilayers
Interatomic Potential. GMR multilayers are constructed only from metals. The embedded atom method (EAM) potential initially developed by Daw and Baskes [8] has provided a sufficiently good potential format for the metals that are used in the GMR multilayers. The EAM potentials for 16 metals and their alloys have been parameterized in our EAM potential database [9,10]. This potential database was used in the growth simulation of the Co90Fe10/Cu /Co90Fe10 multilayer [10]. Results. The MD simulation approach [10,11,12] was used to grow a 5.8 nm Co90Fe10/1.5 nm
Cu/4.0 nm Co90Fe10 multilayer unit on a 1 nm Ni80Fe20 substrate at an adatom energy of 3.0 eV, a normal adatom incident angle, a substrate temperature of 300 K, and a deposition rate of 1 nm/ns. The use of the accelerated deposition rate enabled the films to be grown with existing computers. It inhibited various surface diffusion processes as surface atoms were rapidly buried into the bulk of the films. Because high temperature accelerates the diffusion, the simulated films can be better related to the experimental films obtained at reduced temperatures.
The detailed atomic structure of the simulated multilayers is shown in Fig. 1(a), where Ni, Fe, Co, and Cu atoms are marked by heavy gray, white, light gray, and dark balls, respectively. This image clearly shows that the Cu-on-CoFe interface is relatively sharp, whereas the CoFe-on-Cu interface is quite diffuse. Cu atoms are incorporated in subsequently deposited CoFe layer more significantly than in the underlying CoFe layer. These observations from the simulated film have all been verified by a three-dimensional atom probe (3DAP) experiment [10].
Fig. 1 MD simulated Co90Fe10/Cu/Co90Fe10 multilayers grown on a Ni80Fe20 substrate. (a) Constant adatom energy; and (b) Modulated energy.
The results discussed above showed that MD simulations can provide reliable structural
information about the GMR multilayers. More importantly, these simulations enable the mechanisms of structure formation to be revealed. Further MD simulations revealed that because Cu has a larger atomic size and smaller surface energy than either Co or Fe, Cu tends to segregate onto
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the CoFe surface to release the surface tension under equilibrium conditions. In addition, Co or Fe atoms impacting a Cu surface are more likely to penetrate into the Cu surface, because the Cu lattice interstices are large and the relatively weakly bonded Cu atoms can be easily displaced. This penetration causes the exchange of the impacting Co or Fe atoms and the surface Cu surface atoms, resulting in continuous Cu diffusion into the subsequently deposited CoFe layer. The Cu-on-CoFe interface is relatively sharp because the bigger Cu atoms impacting a more strongly bonded CoFe surface are much less likely to penetrate the lattice. The understanding of this mixing mechanism indicates that a reduction in the impact energy of the Co and Fe adatoms during their deposition on Cu can reduce the exchange probability and therefore sharpens the CoFe-on-Cu interface.
Simulations, however, indicated that a reduction of the adatom energy increased the surface and interfacial roughness, because any nucleated surface asperities could not be flattened by the energetic impacts. The tradeoff between interlayer mixing and interfacial roughness then defines an intermediate adatom energy that optimizes the GMR properties, in good agreement with experiments [13,14]. The analyses also indicated that due to the asymmetric interfacial structures, using a lower adatom energy to deposit the CoFe layer on Cu than to deposit the Cu layer on CoFe (interlayer energy modulation) is better than using a single adatom energy to deposit the entire multilayer stack. This strategy has been found experimentally to improve the GMR properties [15]. Based on the insights revealed by simulations, the new idea of modulating the energy during deposition of each layer (interlayer energy modulation) was explored. In this scheme, the first few atomic layers of Cu (or CoFe) were deposited on CoFe (or Cu) using a low adatom energy to avoid the impact-induced mixing. After the interface was buried well below the surface, the deposition was shifted to higher adatom energy to flatten the surface. The result of the same multilayer structure deposited using this modulated energy scheme is shown in Fig. 1(b). It can be seen that compared to the result shown in Fig. 1(a), the interlayer mixing is greatly reduced while the interfacial roughness remains almost unchanged. In this case, the simulations helped improve the deposition process.
Growth of Magnetic Tunneling Junction Multilayers
Interatomic Potentials. Charges are induced on atoms as metals are oxidized and hence an ionic component to a potential must be considered during simulations of metal oxides. For heterostructures involving both metals and metal oxides, an additional complexity arises because the charge on atoms varies from zero in local metal regions to the maximum value in the oxides. Such problems can only be addressed by advanced variable charge potentials [16,17]. However, early versions of variable charge potentials [16,17] did not incorporate the physics of electron valence. As a result, their parameterization is overly constrained, and they can only be used for binary systems involving oxygen and one metal element. More recent modified charge transfer ionic potentials (CTIPs) [18] overcame this problem and can be applied for systems involving oxygen and any number of metal elements. One such modified CTIP has been integrated with EAM for a quinternary O-Al-Ni-Co-Fe system [19]. It has been applied to study the growth of an AlOx tunnel barrier layer on a Ni65Co20Fe15 surface [20]. Results. The MD simulation approach [20] was used to deposit first a Ni65Co20Fe15 layer on an
initial (111) Ni65Co20Fe15 substrate surface and then an Al layer on a Ni65Co20Fe15 surface. Various Al layer thicknesses were explored. Structures of two selected films, one with about five atomic Al layers (~12 Å) and a second with only a single atomic Al layer (~2.5 Å) are shown on the left of Fig. 2(a) and 2(b) respectively, where white and gray balls represent Al and O atoms and the darker balls refer to Ni, Co, and Fe atoms. Accelerated oxidation of the Al surface was simulated by introducing an atomic oxygen vapor above the Al/Ni65Co20Fe15 multilayer using a high vapor temperature of 8000 K and a high vapor density of 0.0003 oxygen atoms/Å3 (~12 atmospheres). The corresponding film structures after oxidation are shown on the right of Fig. 2(a) and 2(b), respectively.
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Fig. 2 MD simulated AlOx layer grown on a Ni65Co20Fe15 substrate. (a) Oxidation of an ~12.0 Å
aluminum layer deposited on a Ni65Co20Fe15 underlayer; and (b) Oxidation of an ~2.5 Å aluminum layer deposited on a Ni65Co20Fe15 underlayer.
The formation of amorphous AlOx layers is seen in Fig. 2, in agreement with experimental
observations [21]. Fig. 2(a) indicates that oxidation of the thicker aluminum layer results in the formation of a continuous and flat AlOx film. However, when the aluminum layer thickness is reduced to 2.5 Å, Fig. 2(b), the AlOx film is highly discontinuous even though the Al layer prior to oxidation has been continuous and relatively smooth. Oxidation of the thin Al layer is hence found to be the cause for the formation of holes in the AlOx layer that expose areas of the underlying Ni65Co20Fe15 layer. Experiments revealed that the TMR properties initially improved as the thickness of the Al layer prior to the oxidation was reduced from 50 Å [22,23], but they abruptly disappeared when the Al layer thickness was reduced below ~8 Å. This was thought to arise from the formation of discontinuous AlOx layers [23,24]. Simulations account for this observation. They further suggest that the inhibition of the nucleation and growth of holes in the gradually oxidized thin Al layer is the key to grow a continuous AlOx layer at reduced layer thickness.
Growth of Semiconductor Thin Films
Interatomic Potentials. The bonding of semiconductor materials requires angular-dependent interatomic potentials. Recent analysis [25] has indicated that the best interatomic potential approach capable of realistic simulations of the growth of the stoichiometric semiconductor compound films using arbitrary vapor flux ratio and vapor particles (atoms or molecules) is the bond order potential [26,27]. A parameterization of a GaAs bond order potential [28] has been successfully used to simulate the growth of the GaAs films in the [001] direction [29]. Results. The MD simulation method [29] was used to grow GaAs films on an initial (001) GaAs
substrate from atomic Ga and molecular As2 vapor fluxes using an adatom energy ~0.17 eV/atom, a vapor flux direction normal to the surface, a growth rate between 0.14 – 0.22 nm/ns, and various As:Ga vapor flux ratios and temperatures, T. Selected atomic configurations of the simulated films are shown in Fig. 3(a) – 3(d), where As and Ga atoms are represented by the dark and light balls, respectively, and the substrate prior to the deposition is shown as the shaded area.
Fig. 3 indicates that at approximately the same flux ratio, increased temperature improves the crystallinity and reduces the defects. At the low simulated temperature of 700 K, increasing the As:Ga vapor flux ratio causes the extra As atoms to be incorporated in the films, resulting in a decrease in the film crystallinity and an increase in the defect concentration. At the relatively high simulated temperature of 1100 K, increasing the flux ratio causes the excess As adatoms to
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evaporate, and hence is beneficial to maintain the stoichiometric composition, the high crystallinity, and the low As vacancy concentration in the buried films, except that the topmost surface layer became more As rich. Experiments indicated that at the high temperatures, the As dimers could not remain stuck to an As-rich surface [30], and crystalline stoichiometric films could only be grown under an As:Ga vapor flux ratio >> 1 [4,31,32,33]. At low temperatures, the As dimers were more likely to stick to the surface [4,34,35,36,37] and the crystalline stoichiometric films could only be grown at an As:Ga vapor flux ratio ≈ 1 [38,39]. These experimental observations for both low- and high-temperature growth regimes were well predicted in Fig. 3.
Fig. 3 MD simulated growth of a GaAs layer as a function of temperature, T, and As:Ga vapor flux
ratio. (a) T = 700 K, As:Ga = 1.24; (b) T = 700 K, As:Ga = 2.97; (c) T = 1100 K, As:Ga = 1.20; and (d) T = 1100 K, As:Ga = 2.93.
Summary
Recent MD approaches for vapor deposition simulation have opened new windows into the growth of a wide range of materials including metallic multilayers, ionic metal oxide multilayers, and covalent semiconductor compound layers. The simulations can reveal insights to improve the growth processes that cannot be obtained using experiments alone.
Acknowledgments
We are grateful to the Defense Advanced Research Projects Agency and Office of Naval Research (C. Schwartz and J. Christodoulou, Program Managers) for support of this work through grant N00014-03-C-0288. We also thank S. A. Wolf for numerous helpful discussions.
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