Transmission electron microscopy of transparent conductive
oxide films made by atmospheric pressure chemical vapor
depositionh
Citation for published version (APA):
Mannie, G. J. A., Deelen, van, J., Niemantsverdriet, J. W., & Thüne, P. C. (2011). Transmission electron microscopy of transparent conductive oxide films made by atmospheric pressure chemical vapor depositionh. Applied Physics Letters, 98(5), 051907-1/3. [051907]. https://doi.org/10.1063/1.3551523
DOI:
10.1063/1.3551523 Document status and date: Published: 01/01/2011
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Transmission electron microscopy of transparent conductive oxide films
made by atmospheric pressure chemical vapor deposition
G. J. A. Mannie,1,2J. van Deelen,3J. W. Niemantsverdriet,2and P. C. Thüne2,a兲
1Material innovation institute (M2i), P.O. Box 5008, 2600 GA Delft, The Netherlands
2Physical Chemistry of Surfaces, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven,
The Netherlands
3TNO Science and Industry, P.O. Box 6235, 5600 HE Eindhoven, The Netherlands
共Received 19 November 2010; accepted 12 January 2011; published online 31 January 2011兲 The properties of crystalline films are strongly dependent on the morphology development during the first stages of growth, that is, the nucleation and growth of the first crystallites. The ideal method to investigate such films at atomic resolution is transmission electron microscopy共TEM兲. However, this technique usually requires extensive sample preparation. In this study we present a simple method to investigate thin film morphology with TEM. The key to this approach is the use of TEM membranes as growth substrates. As an illustration we describe fine grains of SnO2 deposited on these TEM membranes by atmospheric pressure chemical vapor deposition. © 2011 American Institute of Physics.关doi:10.1063/1.3551523兴
Thin metal oxide films such as F-doped SnO2 and
Al-doped ZnO are the material of choice as transparent conduc-tive oxide 共TCO兲 for the production of cheap solar cell due to their relatively low production costs and easy deposition. The morphology of transparent conductive oxide films is an important factor for their optical and electrical properties.1–5 In the literature TCO morphology is mostly probed using secondary electron microscopy 共SEM兲5–8 and atomic force microscopy共AFM兲.4,5,9Although SEM and AFM give topo-graphical information about deposited films they give limited information about morphology with respect to crystal orien-tation and cannot reveal information on the atomic level. X-ray diffraction共XRD兲 is generally applied as technique to investigate film orientation.9,10A drawback of this technique is that XRD requires relatively thick films in order to get enough signal. Therefore, the investigation of film growth at the early stages is difficult. A technique very suitable to in-vestigate crystalline films at atomic resolution is transmis-sion electron microscopy 共TEM兲. This technique requires very thin samples in order to be transparent for electrons. To achieve suitable samples from deposited films small slices are 共diamond兲 cut and polished by a focused ion beam. The works of Alfonso et al.,11 Sundqvist et al.,12 and Mitchell et al.13,14represent elegant examples of this approach in the field of TCOs. Using this method the morphology of the film-substrate interface can be investigated in cross-sectional view and the overall film morphology determined. However, this approach meets its limits when investigating the earliest stages of TCO film growth, i.e., the nucleation and growth of first TCO crystallites. We have therefore developed a method to achieve plan view TEM images of pristine nanostructures on oxide substrates with TEM employing window etched TEM wafers described below. This methodology has already successfully been applied for model catalysts with iron oxide nanoparticles in morphology studies related to the carbon nanotube growth15and the Fischer-Tropsch catalysis.16
In this paper we present our initial results for tin oxide films that were deposited directly on the TEM wafers. The
early growth stages of these films were directly analyzed by SEM, TEM, and AFM.
The tin oxide films discussed here were deposited on custom-made TEM wafers共Fig.1兲 which feature a relatively
large surface area 共20⫻20 mm2兲 to allow an unperturbed film growth. The TEM wafers consist of 36 individual TEM grids that are arranged into a square pattern and stabilized by a silicon frame. The central part of each TEM grid is etched away to create a 15–20 nm thick silicon nitride “membrane” window 共typical dimensions of 100⫻100 m2兲 through
which the electron beam can pass, enabling imaging of the supported particles on a subnanometer length scale. After calcination in dry air the silicon nitride forms a 3 nm thick surface layer of silicon oxide.
Tin oxide films were deposited directly on the TEM wafers by atmospheric pressure chemical vapor deposition 共APCVD兲 in a stagnant flow reactor. The reactor setup consists of two heated bubblers filled with tin tetrachloride 共SnCl4, TTC兲 at 30 °C and water 共H2O兲 at 40 °C. Nitrogen
共N2兲 was used as carrier gas. The gas flow through the
TTC bubbler was 8 mLnmin−1 and was diluted with
94.4 mLnmin−1 N2. The gas flow through the H2O bubbler
was 123.3 mLnmin−1and was diluted with 1.17 mL nmin−1
N2. The total gas flow was lead into the reactor perpendicular to a heated substrate that was held at 570 ° C during the
a兲Electronic mail: p.c.thuene@tue.nl.
FIG. 1. 共Color online兲 Schematic representation of the CVD substrate. 共a兲 TEM wafer consisting of 36 fused TEM grids with windows.共b兲 Cross-sectional view of an individual TEM grid with the electron transparent membrane suspended in a silicon frame.
APPLIED PHYSICS LETTERS 98, 051907共2011兲
0003-6951/2011/98共5兲/051907/3/$30.00 98, 051907-1 © 2011 American Institute of Physics
deposition. After deposition the TEM wafers were broken into 36 TEM grids for analysis. SEM images were taken using a FEI Quanta 600 SEG environmental scanning elec-tron microscope. The TEM studies were carried out on a FEI Tecnai 20 共type Sphera兲. AFM images were taken with a NT-MDT Solver P47H microscope in tapping mode under atmospheric conditions. X-ray photoelectron spectra 共XPS兲 were taken in a Kratos AXIS ultraspectrometer, equipped with a delay-line detector using the monochromatic alumi-num source共Al K␣= 1486.6 eV兲 operating at 150 W.
Figure 2 shows plan view SEM images of early-stage
film growth of a SnO2film grown with APCVD. Figure2共a兲
shows a SnO2 film after only 1 s on deposition, while Figs.
2共b兲and2共c兲show images of films after 4 and 60 s, respec-tively. The deposited material in all samples is uniformly distributed over the substrate surface. The increase in crystal size is clearly visible starting with crystals of around 5–15 nm in diameter 关Fig.2共a兲兴 up to 100 nm in diameter 关Fig.
2共c兲兴. The tin oxide crystals in Fig.2共a兲appear not to form a closed film on the silica surface. This is confirmed by XPS which give a tin coverage—measured as the Sn/Si+Sn atomic ratio—of 61.9% after 1 s deposition. Since the inelas-tic mean free path of Si 2p photoelectrons through SnO2 is
around 2.3 nm共Ref.17兲 and the size of the smallest particles
is around 5 nm, the Sn coverage will be underestimated slightly as the silica substrate can still be detected through the thinnest SnO2 crystallites. The longer deposition times
give complete tin coverages of 100%共spectra not shown for brevity兲.
For a more detailed view on the morphology of tin oxide nanocrystals at the initial stages of film growth we turn to TEM. Figure 3共a兲 shows an intermediate resolution plan view image of a tin oxide film after 1 s APCVD deposition. The image shows a random distribution of irregular shaped tin oxide crystals ranging between 5 and 20 nm in diameter. The bare silica membrane remains visible and the tin oxide coverage derived from this image is about 64%, which is in excellent agreement with the XPS results. The electron dif-fraction pattern关Fig.3共a兲, inset兴 of the sample shows a typi-cal tetragonal SnO2 pattern with the共110兲, 共101兲, and 共211兲 as most intense diffraction rings. This pattern indicates the high degree of crystallinity of the measured samples.
Figure 3共b兲 shows an AFM image of the same sample and with the same magnification as Fig. 3共a兲. Note the large difference in resolution between the TEM image and the
FIG. 2. Plan view SEM images of early-stage film growth of SnO2film deposited by APCVD with variable deposition times:共a兲 1 s deposition time, 共b兲 4 s deposition time, and 共c兲 60 s deposition time. During growth SnO2 crystals become larger with longer deposition times.
FIG. 3.共Color online兲 Plan view TEM images of early-stage film growth of SnO2film deposited by APCVD.共a兲 Overview with clusters of small SnO2 crystals grown on top of the SiO2membrane with electron diffraction pat-terns the same as the inset,共b兲 AFM image of the sample with a line scan, and共c兲 zoom in on a typical cluster of SnO2grains. The noncrystalline area between the grains is bare SiO2substrate as confirmed by XPS.共d兲 Top-view TEM image of twinning in a SnO2crystal.
051907-2 Mannie et al. Appl. Phys. Lett. 98, 051907共2011兲
AFM image; whereas Fig. 3共a兲 shows small and large par-ticles, AFM only reveals the large particles with particle sizes between 20 and 40 nm. This difference with the TEM image is due to the tip shape, which is a known drawback of AFM. On the other hand, measuring height with AFM is very informative. The line scan of the AFM image indicates an average particle height of 5–10 nm with some larger par-ticles with heights around 25–30 nm. This corresponds to the lateral dimensions of the particles in the TEM image of Fig.
3共a兲and indicates a particle aspect ratio around 1.
In order to reveal more morphological detail of the par-ticles high-resolution TEM images were taken. Figures3共c兲 and3共d兲show high-resolution plan view TEM images of the same sample as Fig. 3共a兲. The rough surface of the SnO2 crystallites is clearly visible. Lattice fringes of individual crystals as well as disturbances in their crystal structure can be visualized in detail. The lattice fringes in Figs. 3共c兲and
3共d兲 have a d spacing of 3.35 Å which corresponds to the most intense共110兲 diffraction signal from SnO2. Figure3共d兲
shows a clear example of twinning in a SnO2 crystal. These
figures show the added value of this TEM method by clearly displaying the nucleation of SnO2 crystals and the
subse-quent growth behavior.
Deposition of microcrystalline thin films on electron transparent membranes offers a convenient means to visual-ize the pristine film morphology at atomic resolution. We have demonstrated this approach on tin oxide films grown by APCVD from tin chloride precursors. We are currently em-ploying this method to systematically investigate the influ-ence of substrate pretreatment and deposition parameters on film morphology and electrical properties.
We would like to thank Dr. I. Volintiru 共TNO兲 for the preparation of the CVD samples and Dr. P. Poodt共TNO兲 for scientific discussions regarding morphology development in SnO2 film growth. This research was carried out under
Project No. MC3.07288a in the framework of the Research Program of the Materials Innovation Institute 共M2i兲 共www.m2i.nl兲.
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