Article pubs.acs.org/crystal
Nucleation and Growth Modes of ALD ZnO§ Zsófia Baji,*,† Zoltán Lábadi,† Zsolt E. Horváth,† György Molnár,† János Volk,† István Bársony,†,‡ and Péter Barna† †
Research Centre for Natural Sciences, Institute for Technical Physics and Materials Science, Konkoly Thege M. út 29-33, H-1121 Budapest, Hungary ‡ Faculty of Information Technology, University of Pannonia, Egyetem u.10, Veszprém, H-8200 Hungary
ABSTRACT: The initial phases of the ALD growth of ZnO have been examined. It is shown that ZnO exhibits an island-like growth on Si, layer-by-layer on GaN, whereas on sapphire, the growth mode can be tuned by the deposition temperature. A new method for depositing ultrathin smooth polycrystalline films is presented. The growth rates on different substrates and at different deposition temperatures were analyzed, and the possibility of epitaxial growth was also examined.
1. INTRODUCTION Atomic layer deposition (ALD) is nowadays considered as one of the most promising thin film and nanostructure fabrication methods. The operation principle is based on the introduction of precursor gases/vapors into the vacuum chamber, and their subsequent chemisorption on a heated substrate surface. The theory is somewhat similar to that of CVD growth except that, here, the precursors are consecutively introduced into the reactor, and between the precursor pulses, the reactor is purged by an inert gas. Therefore, the precursors can only react with the substrate surface and never with one another in the gas phase, which prevents the formation of particles in the gas phase. The chemisorption on the heated substrate surface ensures a uniform and conformal coverage independent of the surface morphology.1−5 In chemisorption processes, the growth rate is determined by the number of connecting sites on the surface, and the partial pressure of the precursor vapor At saturation, that is, with the temperature high enough for the surface reactions to take place, but not for initiating desorption (“ALD window”), the only limiting process is the chemisorption. The maximum density of the adsorbed molecules is defined by their size and the availability of bonding sites. The latter also depends on the crystalline structure of the surface. The precursors used in ALD processes are often rather large molecules; therefore, the steric hindrance is a crucial factor in the deposition process. On the other hand, the surface coverage may not be complete if the surface chemistry also hinders the process, or bulky ligands of a chemisorbed precursor molecule get adsorbed on the surface engaging bonding sites, further © 2012 American Chemical Society
inhibiting the growth by their steric hindrances. Because of the above-mentioned effects, the growth per cycle in atomic layer deposition is always considerably less than a monolayer.6 The nucleation is also pivotal in the initial phase of the growth, as the ALD process can only start if there are adequate chemical species on the substrate surface, to which the precursors may connect. If the surface is inert, the reaction may only start at defect sites. Therefore, in the initial stages of the film growth, an island-like growth takes place. The agglomeration of the adsorbed molecules can also occur at sufficiently high deposition temperatures, if the mobility of the adsorbed species is high enough. In this case, following the growth of a monolayer, the system tries to minimize the interface energy between the two materials and the adsorbates can migrate and form islands. In fact, according to Ritala and Leskela,7 this is a more likely explanation of the island-like growth in atomic layer deposition. They found no clear correlation between the low growth rate and the rough surfaces and island-like initial growth; quite the contrary, smooth films often grow at lower growth rates. At low temperatures with moderate surface mobility, smooth amorphous films are often achieved, but at higher temperatures, both crystallization and agglomeration formation start.8 Another possible explanation for an island-like growth is that certain intermediate species have a higher mobility on the Received: August 7, 2012 Revised: September 26, 2012 Published: October 8, 2012 5615
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Figure 1. AFM micrographs of the growth of ZnO on Si at 150 °C with 0.1 s long precursor pulses: (a) after 5 cycles, (b) the profile of the surface in (a), (c) after 10 cycles, (d) after 15 cycles, (e) the layer after 30 cycles, and (f) the profile of the surface in (e).
surface during film growth, and they are responsible for the migration and the agglomeration7 After the islands have grown together, a stable growth rate sets in. This phenomenon has been reported in the case of Al2O3 growth on Si and carbon surfaces, for example, graphene.1 Agglomerates on ALD-grown ZnS layers were also found9 after the first few (∼10) growth cycles, which then grew in size and then formed a continuous film with a very rough surface. During the first few cycles of growth, the adsorption of the precursors is completely different from the later ones as the reaction with the substrate surface can be different from the reaction with the material itself. Between these two, there is a transient regime, where both the substrate surface and the ALD-grown material surfaces are present. The growth may be independent of the substrate surface or, if the substrate offers more reactive sites than the material surface, a substrateenhanced growth is possible. Another often experienced case is the substrate-inhibited growth, when an island-like growth is possible.8 ZnO has recently attracted considerable attention because of its versatility in a number of applications, such as sensors and photovoltaic devices.10,11 It can be doped with aluminum to increase its conductivity and be used as a transparent conductive oxide layer.12−14 The ALD method for ZnO deposition is widely known and has a great variety of applications.15−19 Atomic layer deposition of ZnO is commonly used on SiOx and glass, where the ZnO growth is polycrystalline, and the structure and orientation depend on the deposition parameters. The deposition temperature determines the dominant crystalline orientation of the layers: At lower temperatures (under 200 °C), the (100) orientation is dominant; that is, the columnar crystallites lie parallel to the surface. At higher temperatures, the (002) peak becomes the dominant orientation; that is, the crystallites stand perpendicular to the surface.20−22 An epitaxial growth of ALD ZnO has been shown possible on GaN, sapphire, and YSZ.23−26 A great
number of other substrates have also been used,27−29 but the initial growth mechanisms, the exact structure, and the nucleation of ALD-deposited ZnO films are unknown. The nucleation issues of Al2O3 have been studied in great detail,1 but the same questions are yet unanswered in the case of the ZnO growth process. The aim of our work has been to study these details.
2. EXPERIMENTAL SECTION ZnO layers were deposited in a Picosun SUNALE R-100 type ALD reactor. The zinc precursor was diethyl-zinc (DEZn), and the oxidant was H2O vapor. The precursors were electronic grade purity and kept at room temperature. The carrier and purging gas was 99.999% purity nitrogen. During deposition, the pressure in the chamber was 15 mbar. The flow rates were 150 sccm. The pulse time of all precursors was 0.1 s; the purging times were 3 s after each DEZn, and 4 s after the water pulses. To study the initial phases of film growth on different substrates, ZnO layers have been deposited on GaN, graphene sapphire, and 10− 15 Ω·cm resistivity p-type Si(100) wafers. The latter was covered with native oxide. The substrates were cleaned with acetone, ethylene, and high-purity water, except for the Si substrates, which were cleaned with concd HNO3 and high-purity water. ZnO layers with 5, 10, 15, and 30 ALD cycles were grown on all substrates at 150, 210, and 300 °C deposition temperatures. The resulting thicknesses and morphologies were studied by spectroscopic ellipsometry (SE) with a Woollam M-2000DI spectroscopic ellipsometer, and an AIST-NT, SmartSPM 1010 type atomic force microscope (AFM). A Budgetsensors Tap300-G type tip was used in tapping mode. The crystal structure and orientation were examined by X-ray diffraction (XRD) with Cu Kα radiation, using a Bruker AXS D8 Discover diffractometer equipped with Göbel mirrors, using a scintillation counter.
3. RESULTS AND DISCUSSION 3.1. Nucleation of ALD ZnO on Different Substrates. Figure 1 shows the morphologies of the ZnO layers on Si surfaces grown at 150 °C. It is evident from all the images that 5616
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Figure 2. Growth of ZnO on Si at 150 °C with extra long initial exposures: (a) 1 s first cycle, then four regular ones, (b) the profile of the morphology shown in (a), and (c) the profile of the morphology of the sample with 10 s long first cycle, then four regular ones.
the ZnO exhibits an island-like growth on Si. The sample with five cycles of ALD-deposited ZnO can be seen in Figure 1a. There are ZnO islands scattered over the surface. Their height is around 10 nm, and they are around 60 nm in diameter. With a quick estimation, we can calculate the amount of ZnO that this means on the surface. Taking a 10 μm × 10 μm area on the surface, this means about 100 sphere sections. Adding up their volumes, we get a volume in the range of 106 nm3. On the other hand, assuming a layer-by-layer growth with a growth rate of 0.19 nm/cycle (which is the typical growth rate at this temperature), a ZnO layer with a thickness of 0.95 nm would cover the surface. This would mean a volume in the order of magnitude of 108 nm3 ZnO. This quick estimation is in agreement with the spectroscopic ellipsometry results. For the evaluation of the measurements, a Bruggeman effective medium approximation was used, supposing a 10 nm thick film, which is a mixture of ZnO and air. The model fitting gave the result that 1.57% of the film volume is ZnO in the case of the samples with five cycles of ZnO deposited at 150 °C. In the case of the sample with 30 cycles of ZnO at the same deposition temperature, the fitting gave a 6.7 nm layer with 90% ZnO, which is in agreement with the continuous layer also shown in Figure 1. At a 300° deposition temperature, according to the ellipsometric results, 1.06% of the10 nm thick film is ZnO, which is also in agreement with the AFM results. This leads to the conclusion that the process behind this island-like growth is not the particles adsorbing on the surface, followed by island formation due to surface diffusion. Instead, as there is much less material on the surface, it seems that the nucleation itself occurs slower than expected, and a nucleation issue must be in the background of the phenomenon, probably due the lack of bonding sites on the surface, as the SiOx surface does not have as many OH groups as presumed. Haukka and Roots in ref 30 found that, depending on the temperature, between 200 and 400 °C, there are about 1−2 OH groups on every nm2 of the SiOx surface. After 10 cycles, there is a lot more islands on the surface, but their size is approximately the same as in the five-cycle case. After 15 cycles, the islands have started to grow together, and a full coverage is achieved. After 30 cycles, a full and uniform coverage of the surface can be seen. The roughness of the layer is around 1 nm. The other deposition temperatures resulted in very similar morphologies. At 220 °C, the nucleation is even slower; the coverage is still not complete after 30 cycles. At 300 °C, the grown layer only becomes uniform after 60 deposition cycles.
The above-mentioned results were also verified with scanning electron microscopy. The islands could be seen on the secondary electron micrographs as well, and the EDS elemental analysis confirmed a higher ZnO concentration in the islands than on the surface between them. From the changing of the surface morphology with the increasing number of deposition cycles, it can be seen that, after the first cycle, not all bonding sites have been occupied as the precursors in the later cycles fill in further nucleation sites at the surface. This can be seen from how further islands with the same size are formed instead of the growth of the alreadyexisting ones in height and diameter. To verify this model, the following deposition methods were tested: (a) A 1 s long pulse of DEZ, followed by a 30 s purge, then a 1 s long water pulse with 40 s purging, then four cycles of the regular pulse lengths. (b) Ten-second-long first pulses were used from both precursors, followed by 1 min purging steps, then four 0.1 s long pulses. The idea was that this extra long exposure in the first step might fill in all the nucleation sites; then the islands would start to grow. The resulting morphologies are shown in Figure 2. It can be seen from Figure 2 that the 1 s long exposure in the first deposition cycle increased the number of islands considerably. After this nucleation period, in the further cycles, the islands grew. The resulting islands are smaller both in diameter and in height than those deposited with the traditional (uniform 0.1 s long) pulse lengths. In the case of the 10 s long first pulse, the result of the five cycles is a continuous layer with a much smaller surface roughness than that usually experienced on ALD-deposited polycrystalline layers. This is the method to deposit ultrathin and smooth polycrystalline ZnO films. According to the literature on the ALD method, this is an unknown phenomenon, as theoretically, if saturation has been achieved, all bonding sites available on the surface become occupied, and there is no need for further exposure. The 0.1 s exposures are sufficient at all temperatures for a saturation of the surface; therefore, this exposure results in the required saturation and partial pressure of the precursors at all temperatures. It is, therefore, a completely unexpected result that a longer exposure only in the case of the first deposition cycle results in higher coverage of the surface. Although there are still available bonding sites on the surface, it takes several cycles for the nucleation to complete. Longer exposures were previously only used in the case of high-aspect-ratio structures, not on smooth surfaces. 5617
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Figure 3. Growth of ZnO on GaN: (a) GaN reference surface, (b) ZnO layer after five (0.1 s long) pulses, and (c) the profile of the layer shown in (b).
Figure 4. Growth of ZnO on sapphire: (a) sapphire substrate surface, (b) the layer after 5 cycles of growth at 150 °C, (c) after 15 cycles at 150 °C, (d) after 5 cycles at 300 °C, and (e) the XRD results of the layer.
already after five deposition cycles. The XRD results revealed highly oriented, almost epitaxial layers. The orientation was mainly (002), with only a small (101) peak visible, whereas in a powdered ZnO sample, the (101) is the highest-intensity peak. A ZnO deposition on graphene at 300 °C was attempted, but even after 500 cycles, there was no ZnO growth at all. The graphene substrate was completely inert. Further experiments were not tried, as this result was by no means unexpected, as on such inert substrates, there are no connection sites for the ALD growth. To succeed in ZnO growth on graphene, further experiments are required, including some functionalization of the graphene surface.31,32 3.2. Growth of ALD ZnO. In our previous work,33 we determined the average growth rates of ∼70 nm thick ZnO layers deposited on Si. We found that it was a function of the deposition temperature: it is 1.07 Å at 120 °C, then increases. The maximal growth rate is at 150 °C: 1.9 Å, then it decreases at higher deposition temperatures. At 300 °C, the growth rate was only 0.9 Å. From this, we may calculate the number of monolayers grown in one deposition cycle, but for this, one must also consider that the orientations are different at the different deposition temperatures. As ZnO grows in a
Figure 3 shows the layers deposited on GaN. On this substrate, the layers grow layer-by-layer at all deposition temperatures. Figure 3a shows the surface of the reference GaN layer. As Figure 3b shows, the morphology of the ZnO layer follows the substrate morphology exactly. The coverage was full and complete already after five cycles at every substrate temperature. The profile of the layer (see Figure 3c) shows the atomic terraces of the layer. If the right temperature is chosen, even an epitaxial growth is possible on GaN substrates. The comparison of the results with SEM and EDS measurements showed a uniform ZnO elemental distribution on the surfaces of all the GaN substrates, and the ZnO concentration increased with the pulse lengths. The ZnO layers grown on sapphire substrates showed a very interesting feature: the growth mode could be controlled by the temperature. At 150 °C, the growth is very similar to that observed on Si. The extremely smooth surface of the sapphire substrate (Figure 4a) is scattered with islands after five deposition cycles (see Figure.4b). After 15 cycles, the layer was already continuous. At temperatures at and above 220 °C, on the other hand, the growth was completely different. A layer-by-layer growth occurred: continuous layers can be seen 5618
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hexagonal wurtzite structure, with the lattice constants a = 3.5 Å and c = 5.25 Å, the distances of the atomic monolayers are d = 2.85 Å if the layer grows perpendicular to the c axis, and d = 2.5 Å if it grows parallel to the c axis. At lower temperatures, the layers grow in the (100) crystallographic direction, that is, perpendicular to the c axis; at higher temperatures, it grows in the (002) direction, parallel to the c axis. Therefore, the number of monolayers (ML) deposited in one deposition cycle can be calculated considering the atomic spacings: at 150 °C, the growth rate is 0.67 ML/cycle; at 220 °C, 0.59 ML/cycle; and at 300 °C, only 0.36 ML/cycle. If we consider that, during chemisorption, the bonding sites are the OH groups on the surface, in the (100) case, the sites are at a 5.2 Å distance from each other, whereas in the (002) case, this spacing is 3.25 Å. In the diethyl-zinc precursor molecules, the Zn−C bond length is 1.95 Å. Theoretically, after the DEZ molecule connects to the surface, it releases one ethyl group; therefore, the radius of the remaining specimen is around 2 Å. This means that, in the (100) case, the molecules definitely have ample space to connect to each site, and no steric hindrance has to be taken into account. The packing density calculated merely from the molecular structure should be one, and so the growth rate should be a monolayer/cycle. In the (002) case, the atomic spacing is close to the size of the adsorbates. Depending on the repulsion of the neighboring molecules, it is also likely that they reconfigure in the next-closest packing structure. This may be one of the hindering processes that decrease the growth rate at the higher temperatures. The other one is that, with increasing the temperatures, the number of OH groups decreases, and that, at higher deposition temperatures, the adsorbed specimen may desorb once again. The growth rates on Si were 1.2 times that on GaN at all deposition temperatures. Obviously, the mechanism of the adsorption, and the chemical processes, must be the same at a given temperature, independent of the surface (in case full coverage has already been achieved). The morphologies of the layers grown on Si and GaN are very different. As seen in Figure 1, the layers grown on Si have a surface roughness of a few nanometers, whereas the surface of the ZnO layers grown on GaN are extremely smooth, except for the small roughness that the substrate already possessed as well. Therefore, the roughness results in different surface areas on nominally equal substrate sizes. It is fair to suppose that the larger growth rate measured on the nominal surface of Si samples is then the result of the larger surface area resulting from the larger roughness. The very small roughness of the GaN surface can be approximated with a surface covered by cylinder sections with a 10 nm height and a 1000 nm width lying parallel to the surface. The actual surface area of this morphology results in an only 1.0004 times larger surface than an ideally flat one. On the other hand, the morphology of the ZnO surface can be approximated with a surface covered with domes, or sphere sections that can be seen in Figure 1a. The domes are 10 nm in height and 60 nm in diameter. It is reasonable to assume that the full coverage means that these same domes cover the whole surface. A close-packed ordering can be achieved, assuming a square symmetry ordering of the domes, and adding an extra one in every gap between four others, so that the top of each sphere section is 10 nm high from the substrate surface. This layout is shown in Figure 5, which is a representation of this surface by the 3D Studio Max program. The surface area of this morphology can be calculated, and it increases the surface by a
Figure 5. ZnO surface resulting from the islands coalescing.
factor of 1.08. The increase of the surface area accounts for a part of the increasing growth rate. The other effect enhancing the growth rate is probably the increased number of defect sites provided by the edges and kinks in this morphology. It can also be seen in the figure that, as the spheres cover each other partly, this assumption results in the typical ALD-deposited ZnO film seen in the AFM micrographs with a surface roughness of a few nanometers. Therefore, the coalescing of 10 nm high islands also accounts for the roughness and morphology of the ALDgrown polycrystalline layers on Si.
4. CONCLUSIONS The initial phases of the ALD growth of ZnO have been examined. It has been shown that ZnO exhibits an island-like growth on Si due to nucleation issues. The coalescing islands give the relatively rough layers known in the ALD literature. An extra long first pulse can solve the nucleation problem. In this case, the nucleation already saturates in the first cycle with smaller islands and the resulting films have a smaller surface roughness. Using this novel method, the growth of extremely thin continuous layers is also possible by ALD. On GaN, the growth always occurs layer-by-layer, and when appropriate deposition parameters are selected, an epitaxial growth is also possible. The growth type on sapphire can be tuned by the deposition temperature. It has been found that the growth rates on Si are always higher than those on GaN under the same deposition conditions. This can be explained by the different surface morphologies and the different growth modes on these two substrates.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: baji.zsofi
[email protected]. Tel: 06-1-3922222. Fax: 06-13922226. Author Contributions
§ The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The athors wish to thank Attila Lajos Tóth and Miklós Fried for the SEM and ellipsometry measurements, and Kata Zih and 5619
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Tamás Mezey for the 3D imaging of the surface. This work was supported by the Hungarian National Science Fund OTKA grant NK 73424 and by the National Development Agency grant TÁ MOP-4.2.2/B-10/1-2010-0025.
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REFERENCES
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