B. Wessler et al.: Textured ZnO thin films on (0001) sapphire produced by chemical solution deposition
In accordance with the findings of Ohyama et al.20 it is
essential first to keep the as-deposited film on the hot
plate at 300 °C. At 200 °C the acetate decomposition is
not complete; at 400 °C, on the other hand, all processes,
the decomposition of the acetate, the evaporation of the
2-methoxyethanol and the decomposition of ethanol-
amine could take place too abruptly. Nevertheless
Ohyama et al.19 reported in their first study that heating
the films directly after deposition at 500 °C yields tex-
tured films. This was not observed in our study. Films
heated directly at 500 °C showed virtually no texture.
The second heating step at 500 °C seems to be the most
essential step for the formation of oriented films. It can be
assumed that at low temperatures only grains with an en-
ergetically favored orientation can grow by consuming
other grains. These grains can then grow larger at higher
temperatures. If a high temperature is immediately applied
to the films after heating on the hot plate, all grains grow at
the same rate and no preferred orientation develops.
Contrary to the findings of Ohyama et al.,20 we did not
observe a significant increase in the intensity of the
(0002) ZnO reflection in the second heating step between
500 and 600 °C. On the other hand, a considerable in-
crease in the (0002) intensity was observed at 850 °C.
We attribute this difference in texture to the formation of
the spinel layer in the system ZnO–sapphire, which does
not occur for the silica substrates used by Ohyama
et al.20 While in a nonreacting system preferentially ori-
ented grains are already able to grow in the medium
temperature range (500–600 °C), competition between
spinel formation and grain growth at the ZnO–sapphire
interface is very likely to occur. Of course spinel forma-
tion consumes the ZnO nuclei/grains at the interface, and
therefore well-oriented ZnO nuclei/grains will be con-
sumed too in an early stage of ZnO grain growth.
Although a spinel layer with thickness of 10 to 20 nm
was observed in the cross-sectional TEM images, the
intensity of the corresponding spinel reflections of type
{hhh} was very low. This is due to the low scattering
power of the {hhh} planes in this cubic system and is not
a result of the small film thickness. This also might be the
reason why the formation of spinel was not reported
earlier despite heating above 700 °C.
epitaxial growth of ZnO thin films on basal plane
sapphire. They obtained high quality epitaxial films
with offset angles above 2° off toward the a axis. For
lower offset angles of about 1°, two in-plane orientations
were observed. Due to these results, it seems to be likely
that in our study the miscut of the substrates is respon-
sible for the deviation in the results as well, even though
the miscut was specified to be 0.5°.
V. CONCLUSIONS
Out-of-plane textured ZnO films were produced on
basal plane sapphire substrates using solutions containing
acetate dihydrate and ethanolamine in 2-methoxyethanol
as the solvent. Other solution chemistries were not effec-
tive. Achievement of the textured and epitaxial films was
highly dependent on the heat treatments schedule. At tem-
peratures above 700 °C, an epitaxial ZnAl2O4 spinel was
observed between the ZnO film and the sapphire substrate.
Its formation is thought to be the cause for the strong tem-
perature dependence required to achieve a textured film.
ZnO–Al2O3 is a system with relatively high misfit,
which we conclude to be the main reason for deterred
epitaxy. This is supported by the studies of successful
ZnO epitaxy on the low misfit substrates LiGaO2 (001)
(1.9%)10 and ScAlMgO4 (0001) (0.3%)31 by the same
method used here.
ACKNOWLEDGMENTS
The authors would like to thank Carlos Levi for valu-
able discussions. Financial support of the Fonds der
Chemischen Industrie is gratefully acknowledged.
F.F. Lange thanks the research support of the Materials
Research Laboratory program of the National Science
Foundation under Award No. DMR00-80034, which also
supported the research of B. Wessler during her year at
University of California—Santa Barbara.
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Epitaxial films were infrequently produced using the
same conditions, which produced only out-of-plane tex-
tured films. Parameters that could not be controlled dur-
ing the growth were the humidity and the miscut of the
substrate surfaces, which might be responsible for this
phenomenon. Adsorbed water molecules at the surface
might change the wetting properties. Films produced un-
der virtually the same conditions indeed showed some
deviation in thickness and also in grain size, which is
likely to be a result of the humidity.
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