Epitaxy of Organic Crystal Films: Phenanthrene on Potassium Acid Phthalate W. Sander Graswinckel, Fieke J. van den Bruele, Willem J. P. van Enckevort,* and Elias Vlieg
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 2 243-249
IMM Department of Solid State Chemistry, Radboud UniVersity Nijmegen, ToernooiVeld 1, NL-6525 ED Nijmegen, The Netherlands ReceiVed April 16, 2006; ReVised Manuscript ReceiVed NoVember 19, 2006
ABSTRACT: The growth of epitaxial layers of phenanthrene from the vapor on cleaved (010) potassium acid phthalate (KAP) single-crystal substrates is reported. Using a growth cell designed for precise control of supersaturation and substrate temperature, the deposition process was followed in situ using optical microscopy. After epitaxial three-dimensional (3D) nucleation at high supersaturation and subsequent growing together of the nuclei at low supersaturation, closed crystalline phenanthrene layers were obtained. The produced crystal film was composed of four orientational domains, being two pairs of enantiomorphs 180° rotated with respect to each other. The epitaxy is explained by a structural match at the KAP (010)-phenanthrene (001) contact faces, which are both terminated by a pseudo-hexagonal pattern of more or less perpendicular hexagonal aromatic rings. 1. Introduction Crystallinity plays an important role in the field of semiconductor technology. In the production of electronic devices, monocrystalline layers are generally obtained by two-dimensional (2D) heteroepitaxial growth. However, this approach imposes high demands on lattice match of the substrate and the grown layer. Another approach is to crystallize thin films by epitaxial three-dimensional (3D) nucleation. Here, 3D islands of a compound are grown from the vapor or solution on a suitable substrate. These islands coalescence and form a closed layer after continued growth. All nuclei must have the same orientation to create a good quality crystal film with few domain boundaries. For this technique, the requirements for lattice match of substrate and layer are much less severe. For atomic compounds, such as GaN, the influence of parameters such as growth rate of the buffer layer,1 layer thickness,2 growth temperature,3 V/III ratio,4 and reactor pressure5 on the 3D nucleation of thin films has been studied in detail. Much less research has been done for organic compounds. However, the epitaxial deposition of these materials is an interesting field, since organic semiconductors are gaining importance. The easy manufacture and tuning of organic semiconductors create a large range of possibilities for electronic devices, such as flexible displays.6,7 To get more insight in the epitaxial growth of organic compounds by a 3D nucleation mechanism, we investigated the growth of phenanthrene on potassium acid phthalate crystals (KAP, o-HOOCC6H4COOK). Phenanthrene is a small aromatic molecule with weak semiconducting behavior and some interesting optical properties.8-10 It is a member of the family of semiconducting polycyclic aromatic hydrocarbons including pentacene and perylene. In its crystal structure, it has a similar herringbone pattern as the other molecules of this group. Phenanthrene has a monoclinic crystal structure with space group P21; its lattice parameters are a ) 8.47 Å, b ) 6.16 Å, c ) 9.46 Å, β ) 97.7° 11 Many different substrates are used for organic semiconductor growth, including graphite, silicon, metals, and organic compounds.12-17 KAP is promising as a * Phone: +31 (0)24 3653433. Fax: +31 (0)24 3653067. E-mail:
[email protected].
substrate because cleavage of this ionic crystal along {010} always results in an apolar surface with aromatic ring structures on top, as shown in Figure 1 .18,19 In addition, large crystals can easily be grown, and, compared to most organic crystals, KAP is relatively hard. The crystal structure of KAP is orthorhombic with space group Pca21 and lattice parameters a ) 9.61 Å, b ) 13.3 Å, c ) 6.47 Å.20 In this study, we have produced epitaxial crystal films of phenanthrene from the vapor on KAP{010}. A special cell was used for accurate control of temperature and vapor pressure. The crystallization process was followed in situ by optical microscopy. After growth, the specimens were examined by differential interference contrast microscopy (DICM), atomic force microscopy (AFM), and X-ray diffraction. 2. Experimental Procedures 2.1. Growth Cell. Figure 2 gives a schematic view of the growth cell that was used for the vapor deposition experiments. The KAP substrate was placed on the central cylindrical stage in the cell. The temperature of the substrate, Tsub, was measured by a thermocouple and controlled using a Peltier heating/cooling element. A large excess of phenanthrene source material was placed in the area around the central stage and was kept at temperature Tsource. The temperatures were controlled with an accuracy of 0.1 °C. The cell was connected to a vacuum pump, which kept the total pressure below ≈0.1 Pa. The vapor pressure of the source material in the cell is determined by the equilibrium pressure of the source material at Tsource. A typical value of the vapor pressure of phenanthrene during our experiments is 0.082 Pa at Tsource ) 35 °C. To ensure that pumping would not lower this partial pressure, a narrow capillary was used to connect the cell with the vacuum pump. The growth cell has a sapphire window, by which the growth of the epitaxial layers can be followed in situ by using a reflection optical microscope (Nikon Microphot FX) equipped with a CCD camera. A more detailed description of the growth cell is given elsewhere.21 In that work, it is also shown that volume diffusion of nutrient material toward the substrate is fast and crystal growth is completely determined by interface kinetics. This implies that the driving force for crystallization
10.1021/cg0602257 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/10/2007
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Figure 2. Cell for in situ observation of crystal growth from the vapor under well-controlled conditions.
Figure 1. KAP crystal structure projected along [001] with the (010) cleavage plane indicated by the dotted line. Cleavage results in an apolar surface with aromatic ring structures on top.
is given by
Peq(Tsource) ∆µ ) ln kT Peq(Tsub) or
(
1 ∆µ ∆Hsub 1 ) kT R Tsub Tsource
)
with ∆Hsub the sublimation enthalpy of phenanthrene, which is 91.3 kJ/mol.22 2.2. Substrate Preparation. The substrate KAP crystals were grown by solvent evaporation. First, an aqueous solution saturated at 35 °C [14 g of KAP (Merck, 99.5% pure) in 100 mL water] was left at room temperature. Upon slow evaporation of the solvent, this produced a large number of small crystals. A number of these crystals were placed in separate 2.5 mL containers. Then, a KAP solution of the same concentration but having a temperature of 50 °C was added. Each container was covered with a lid with a small hole to ensure slow water evaporation. Initially, the crystals dissolved slightly, which resulted in a smooth surface. After the sample was cooled to room temperature and further evaporation over several days, well-formed crystals with a (010) top face of about 10 × 10 mm2 were obtained. For use as a substrate, the KAP crystals were cleaved along the (010) face using a razor blade. In this way, a clean and smooth surface for phenanthrene deposition was created. The KAP substrates were placed in the growth cell immediately after cleavage. 2.3. Crystal Growth. Prior to growth, a cleaved KAP crystal was placed in the cell. The substrate was placed on a small disk of aluminum foil on the central stage to improve heat conduction. For the same reason, the KAP crystal was cleaved as thin as possible (typically, 0.5 mm). Finally, a small thermocouple was clamped onto the substrate to measure the substrate temperature. Phenanthrene (98% pure, Aldrich) was used without further purification. After placing an excess of phenanthrene powder around the central stage, the cell was closed and evacuated. The temperature of the source material
as well as that of the substrate was then raised to the starting value, thereby keeping Tsub > Tsource to avoid preliminary crystallization on the substrate. The growth run was started by decreasing Tsub to a value lower than Tsource. Two approaches were used: (i) keeping Tsub constant during crystal growth; (ii) first decreasing Tsub to a lower temperature to promote epitaxial nucleation and then raising it, still keeping Tsub < Tsource, to stop nucleation and to ensure slower growth. The second procedure gives better crystal quality. At the end of the experiment crystal growth was stopped by raising Tsub until Tsub ) Tsource, and the specimen was removed from the system. Substrate temperatures range from Tsub ) 25-50 °C, and supersaturations up to ∆µ/kT ) 1.7 were used. 2.4. Characterization. The ex situ examination of the layers that were grown was always carried out within 1 or 2 days after cessation of the growth experiment. In this way, specimen surface deterioration due to phenanthrene evaporation during storage was kept at a minimum. The crystallographic orientation of the layers with respect to the substrate as well as their degree of monocrystallinity were verified with the help of a Philips PW 1820 X-ray powder diffractometer using CuKR radiation. Transmission polarization microscopy was used to examine the crystalline perfection of the closed layers as well as the orientation of 3D nuclei prior to coalescence. Calculated Fourier transforms of in situ and ex situ images mapped by optical microscopy were used as a measure of the average orientation of groups of 3D nuclei. The surface structure of the epitaxial layers was examined with the help of reflection DICM (Leica DM RX) and AFM (Nanoscope Dimension D3100, Digital Instruments) operating in intermittant contact (Tapping) mode. To investigate the morphology of the KAP-phenanthrene interface, the grown films were separated from the substrate. This was realized by placing the specimen upside-down in a Petri dish filled with water. After about 1 h, the KAP was dissolved, and the phenantrene crystal, now floating on the water surface, was transferred to a microscope glass slide and dried in air. 3. Results 3.1. General Features. The process of the formation of 3D epitaxial nuclei and their subsequent coalescence into a closed crystal layer is shown in Figure 3. This sequence of in situ optical micrographs was recorded using bright field reflection microscopy. The substrate temperature was 41 °C, and the driving force was kept constant at ∆µ/kT ) 0.44. In first instance, numerous small 3D phenanthrene nuclei were formed at the edges of cleavage steps on the KAP substrate. After a short time, these islands expanded and united into a line pattern (Figure 3a). The nuclei on the terraces between the steps were
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Figure 3. (a-e)Sequence of in situ optical micrographs showing the formation of 3D phenanthrene nuclei and their subsequent coalescence on top of a (010) KAP substrate. The five images were recorded at 30, 70, 220, 360 and 570 s after the start of the experiment. Tsource ) 41 °C, ∆µ/kT ) 0.44. The scale bar represents 100 µm.
less thick than those formed at the step edges; some of the nuclei on the terraces were barely visible, if at all, by the reflection microscope that was used (Figure 3b). In the period that followed, the nuclei expanded and grew thicker until coalescence took place and a closed layer was formed (Figure 3c-e). From the micrographs, it is also evident that not all the nuclei were epitaxial. A part of the crystallites show random orientation and appear black in the micrographs. Ex situ polarization microscopy confirms the epitaxial nature of the majority of the crystallites as well as the arbitrary orientation of the “black” ones. The formation of the non-epitaxial crystallites is a major problem in the growth of monocrystalline phenanthrene layers. Their number can be greatly reduced by careful control of vapor pressure and substrate temperature during nucleation and subsequent crystal growth. 3.2. Orientation. The contact face of the phenanthrene layers on the {010} KAP surface was determined using X-ray diffraction. Figure 4 shows the X-ray diffraction pattern of the substrate-grown layer ensemble as well as of phenanthrene powder. Comparison of both patterns shows that the relative intensities of the 00 l reflections of epitaxial phenanthrene are strongly amplified compared to those in the powder pattern. From this it follows that the phenanthrene face that is in contact with the {010} KAP substrate surface is {001}. The lateral orientation of the layers could be deduced from the morphology of the 3D epitaxial nuclei. Careful measurement of the angles between the different edges of the {001} top faces using optical microscopy shows that the 3D nuclei are bounded by the edges , , and . Using this information and the fact that the phenanthrene contact face is {001}, the orientation of the epitaxial crystallites with respect to the KAP substrate was derived. The result is schematized in Figure 10: the a-axis of phenanthrene coincides with the a-axis of KAP.
Figure 4. X-ray powder diffraction patterns of (a) phenanthrene crystal layer grown on top of a (010) KAP substrate; (b) phenanthrene powder. In (a), the 00l reflections of phenthrene are indicated by their respective indices, and the reflections from KAP are indicated by K.
Figure 5. (a) Optical micrograph showing numerous 3D phenanthrene nuclei on KAP and (b) star-shaped Fourier transform of this image, demonstrating their preferential orientation. The scale bar represents 50 µm.
The preferred orientation of the 3D nuclei could be confirmed by calculating the Fourier transform of micrographs showing patterns of more or less distinct nuclei. As can be seen in Figure 5, the Fourier transform displays a star-like pattern, whereas it would be circular for randomly oriented crystallites. The six directions of the star rays are perpendicular to the , , and edges of the crystallite top faces.
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Figure 6. Closed epitaxial phenanthrene film grown under optimal conditions. (a) Bright field transmission microscopy only reveals slip lines parallel to the phenanthrene a- and b-axis as well as a number of low angle grain boundary patterns. (b) Polarization microscopy shows extinction of the complete layer if the crossed polarizers are parallel and perpendicular to the phenantrene a- and b-axis. The scale bar represents 100 µm.
In most experiments, the side faces of the 3D nuclei were rounded as a result of kinetic roughening. In these cases as well as for the closed monocrystalline layers, polarization microscopy was used to verify the lateral crystal orientation. In all cases, we found the same extinction directions for the phenanthrene layer and the substrate (Figure 6). Extinction occurs if the polarizers are parallel or perpendicular to the substrate’s a and c-axis. Removal of the substrate, yielding free-standing phenanthrene films, did not change the extinction directions. The main axes of the elliptical intersection of the three-dimensional indicatrix parallel to {001} in point group 2 are parallel and perpendicular to the 2-fold axis, which is parallel to b. For our phenanthrene films, this implies that extinction occurs if the crossed polarizers are either parallel or perpendicular to the phenanthrene b-axis. As the extinction directions of the substrate and the overlayer coincide, this implies that the phenanthrene b-axis is either parallel or perpendicular to the KAP a-axis. This observation complies with the conclusion drawn from the morphology of 3D nuclei, that the phenanthrene b-axis is perpendicular to the KAP a-axis. 3.3. Toward a Closed Epitaxial Layer. The growth of a good quality epitaxial phenanthrene crystal film requires good control of the 3D nucleation process as well as of the subsequent growing together of the nuclei into a closed layer. The formation of epitaxial 3D nuclei depends strongly on supersaturation and temperature. Polarization microscopy showed that at lower supersaturations only randomly oriented nuclei are formed. Epitaxial nucleation sets in at driving forces ∆µ/kT > 0.22, the transition being gradual. A supersaturation transition from random to epitaxial 3D nucleus formation was also encountered during our study on the growth of anthraquinone on {001} NaCl substrates.23 At higher ∆µ/kT by far the most nuclei are epitaxial. However, if the driving force is too large, an increasing number of non-epitaxial crystallites form on top of already existing epitaxial nuclei. This implies that nucleation at too high supersaturation for longer periods leads to poor quality films. A possible explanation for the preferential formation of randomly oriented crystallites at low supersaturations might be nucleation on nanocrystalline particles that splintered off during substrate cleavage. At higher supersaturations, epitaxial crystallites dominate the surface as the activation barrier for epitaxial 3D nucleation is overcome. However, an alternative explanation might follow from the fact that critical nuclei are largest at low supersaturation. Because of the large lattice mismatch between the grown crystals and the substrate, a substantial amount of stress builds up, which removes the energy benefit for the
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Figure 7. Optimum conditions for obtaining a closed epitaxial phenanthrene layer with the smallest number of defects.
formation of an epitaxial critical nucleus as compared to a randomly oriented nucleus.23 In addition to supersaturation, temperature plays a decisive role in the formation of 3D nuclei. At higher deposition temperatures (Tsub > 45 °C), the large majority of the nuclei develop along the cleavage steps on the KAP substrate surface, and almost no nuclei are formed on the terraces in between. At lower temperatures, many nuclei also develop on the terraces between the steps. This is probably a consequence of a decrease of the surface diffusion constant of adsorbed phenanthrene molecules on the KAP substrate at lower temperatures, which leads to a decrease of the mean surface diffusion length. At high temperatures, the molecules are able to diffuse over longer distances on the KAP substrate and search for the most favorable places for 3D nucleus formation, which is at the steps. At lower temperatures, the diffusion length is insufficient for the molecules to reach the steps, and nucleation takes place on the terraces in between. Direct evidence for surface diffusion during the epitaxial nucleation and growth of organic crystals on inorganic substrates was found in our studies of alizarin24 and anthraquinone on {001} NaCl.23 As the distribution of nuclei on the substrate has to be more or less uniform to obtain a closed crystal layer by coalescence, the above implies that the temperature during nucleation must be sufficiently low. In this way, preferential nucleation at the steps is reduced, and enough nuclei are formed on the terraces. Figure 7 summarizes the conditions that were found to yield a phenanthrene crystal layer with a minimum number of defects. The source material should be kept constant at Tsource ) 35 °C during the growth run. The experiment is then started by cooling the KAP substrate from its initial value of Tsub ) 35 °C to 30 °C within 60 s. In this period, the driving force increases to ∆µ/kT ) 0.55, and a uniform distribution of epitaxial 3D nuclei is obtained. The nucleation time is kept short to minimize the formation of secondary nuclei, which are randomly oriented. Then, the temperature is raised to 34 °C (∆µ/kT ) 0.12) and kept constant for a period of 3-5 min. Only few additional nuclei are formed in this period, while the existing ones expand, coalesce, and grow to a closed film of 20-30 µm thickness. The low growth rate at this reduced supersaturation also allows for existing defects to heal and retards the development of new defects. At the end of the experiment, growth is stopped by raising the specimen temperature to 35 °C, and the sample is removed from the growth cell. Figure 6 displays a phenanthrene layer grown under the aforementioned circumstances. 3.4. Perfection. The best quality layers show only low-angle grain boundaries and slip lines when examined by bright field
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Figure 8. Top and bottom surfaces of phenanthrene layers grown under optimum conditions: (a) Flat top face, decorated with shallow patterns of low-angle grain boundary outcrops (DICM). The scale bar represents 50 µm. (b) Bottom face of free-standing film showing irregular hole patterns (AFM, 100 × 100 µm).
Figure 9. In situ observation of slip: (a) before slip, (b) after slip. The second micrograph was recorded 5 s after the first one. The vertical stripes on the image are polishing marks on the small disk of aluminum foil below the KAP substrate.
microscopy (Figure 6a). Polarization microscopy shows extinction of the complete layer if the crossed polarizers are parallel and perpendicular to the phenanthrene a- and b-axis (Figure 6b). This proves that the layer is, apart from enantiomorphic and orientational domains as elaborated in section 4.1, single crystalline. In lower quality layers, polarization microscopy always revealed a number of misoriented phenanthrene grains, which appear bright in the extinction image. The top surface of the layers grown under optimal conditions is flat. DICM shows weak patterns that correspond to the outcrops of low-angle grain boundaries between adjacent coalesced nuclei (Figure 8a). The bottom surfaces of freestanding films however show irregular patterns of numerous holes, with diameters ranging from 5 to 50 µm (Figure 8b). We think that these holes are formed after coalescence of 3D nuclei with sloping side faces, leading to closed cavities at the substrate-crystal interface. The thickness of the layer is critical for its perfection. The optimum layer thickness is about 25 µm. If the film is too thin, then its surface is still rough, and the layer is often not completely closed. On the other hand, if the layer thickness becomes larger, the elastic energy due to lattice mismatch accumulates and is released by plastic deformation of the soft phenanthrene film. This leads to the creation of dislocation arrays in the crystal layer and slip lines on its surface. Slip was also observed in situ (Figure 9) and was found to take place almost instantaneously if stress exceeds a critical value. The longest and most well-developed slip lines are parallel to the b-axis of the phenanthrene layer. Weaker and shorter slip lines are parallel to the a-axis. This correlates well with the large lattice mismatch between substrate and overlayer along the phenanthrene a-axis (13.4%) as compared to the b-axis (5.0%).
Figure 10. Orientation of the epitaxially grown phenanthrene crystal layer with respect to the (010) KAP substrate. The edges of the 3D nuclei as well as the two pairs of enantiomorphs AA′ and BB′ 180° rotated with respect to each other are indicated. P denotes the crossed polarizer directions for extinction in the polarization microscope.
As a consequence, a larger tensile stress develops along a, so that the dominant slip lines are parallel to b. 4. Discussion 4.1. Multiple Orientations. Epitaxial 3D nuclei can adopt different orientations, which are imposed by the 2D point group symmetry of both the substrate and the nucleus contact faces. In previous work,25 it was shown that the number of possible orientations is given by
n)
N(Ssub) N(Ssub ∩ Scrys)
with N(Ssub) the number of symmetry operators of the 2D point group of the substrate surface. N(Ssub ∩ Scrys) is the number of coinciding symmetry operators of the 2D point groups of the substrate and the crystal contact faces. The 2D point group of {010} KAP is m, so N(Ssub) ) 2. For {001} phenanthrene the 2D point group is 1, so only the identity operator coincides: N(Ssub ∩ Scrys) ) 1. From this, it follows that the number of symmetry equivalent 3D nucleus orientations is 2. These are enantiomorphs of each other and cannot be distinguished by standard X-ray methods or polarization microscopy. These orientations are indicated by A and A′ in Figure 10. In addition, a pair of enantiomorphs that were rotated 180° about an axis perpendicular to the substrate contact face (B and B′ in Figure 10) was found. Note that AA′ and BB′ are not symmetry equivalent. We were unable to distinguish between
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phenanthrene molecules end at the same height. This implies that both surfaces can be considered as close-packed molecule layers of similar dimensions, which fit together if placed on top of each other as shown in Figure 12. In reality, the phenanthrene layer is expected to be shifted over approximately half a unit cell length. In this way, the phenanthrene molecules fit in the cavities on the KAP surface, by which the distorted 3D close packing of the molecules in the phenanthrene crystal is just continued at the interface. This is favorable from an energetic point of view. Figure 11. Transmission bright field micrograph showing enantiomorphic phenanthrene crystallites on KAP. One enantiomorphic pair is indicated by arrows; the KAP mirror line parallel to [001] is vertical.
Figure 12. Overlay image showing the crystal structure of the KAP (010) face (gray) and one unit cell of the phenanthrene (001) face and the match between both lattices.
both pairs by polarization microscopy and X-ray diffraction. However, we could identify both types and their enantiomorphs by morphological examination of 3D nuclei prior to coalescence. This is a consequence of the inclined side faces, which reveal the actual symmetry rather than the mm pseudo-symmetry of the {001} top and bottom faces. It was found that the number of B and B′ nuclei is much smaller than A and A′. The bright field optical micrograph displayed in figure 11 shows two enantiomorphs, which are related to each other via the KAP (100) mirror plane. The rows of 3D nuclei formed along the KAP cleavage steps are made up of only one type of enantiomorph. Here the KAP mirror symmetry is locally broken and the nucleation of one enantiomorph is preferred. Despite the occurrence of four orientational domains, the best quality phenanthrene films appear uniform when examined by polarization and bright field transmission microscopy. 4.2. Structural Match. An important condition for epitaxial growth is a structural match of the two crystal planes contacting each other at the substrate-grown layer interface. In that case, the critical 3D nuclei form at a preferred orientation with minimal free enthalpy and then grow together as a closed epitaxial layer. The contact faces in the present KAPphenanthrene system are {010} for KAP and {001} for phenanthrene. Figure 12 shows the molecular structure of the two faces with the a-axes oriented parallel. Both faces are apolar and are terminated by more or less perpendicular aromatic rings, which are ordered in a pseudo-hexagonal pattern. The distances between adjacent molecules are similar for both crystal faces. In addition, the two faces are more or less planar at an atomic scale, i.e., the outer rings of the phthalate ions and the
5. Conclusions This study demonstrates that epitaxial phenanthrene crystal layers can be grown on top of cleaved {010} potassium acid phthalate substrates by deposition from the vapor phase. In situ observation using optical microscopy showed that the crystallization process starts with the formation of 3D nuclei, which then expand and coalesce during further growth forming a closed film. The growth of a crystal layer with a minimal amount of defects requires precise control of the growth conditions, which are a short period of high supersaturation to induce 3D nucleation followed by a longer period of subsequent growth at low driving force. The contact plane of the phenanthrene crystal layer is (001) with its in-plane axis [100] parallel to the [100] direction of the KAP substrate surface. This orientation provides an excellent structural match with the apolar KAP (010) substrate, as both are terminated by a pseudo-hexagonal pattern of more or less perpendicular aromatic rings. Grown under optimal conditions, the phenanthrene layer is single crystalline with four orientational domains, being two pairs of enantiomorphs 180° rotated with respect to each other. Further imperfections are slip lines and low-angle grain boundaries, which are induced by the lattice mismatch between substrate and grown layer. The vapor deposition experiments presented in this work show that the apolar (010) KAP surface is promising as a substrate for the epitaxial growth of organic crystal films. Acknowledgment. The authors wish to thank Geert-Jan Janssen for his assistance in the construction of the vapor growth cell. References (1) Kim, K. S.; Oh, C. S.; Lee, K. J.; Yang, G. M.; Hong, C.-H.; Lim, K. Y.; Lee, H. J. J. Appl. Phys. 1999, 85, 8441-8444. (2) Kuznia, J. N.; Asif, K. M.; Olson, D. T. J. Appl. Phys. 1993, 73, 4700. (3) Yi, M. S.; Lee, H. H.; Kim, D. J.; Park, S. J.; Noh, D. Y.; Kim, C. C.; Je, J. H. Appl. Phys. Lett. 1999, 75, 2187-2189. (4) Yang, T.; Uchida, K.; Mishima, T.; Kasai, J.; Gotoh, J. Phys. Status Solidi A 2000, 180, 45-50. (5) Chen, J. Zhang, S. M.; Zhang, B. S.; Zhu, J. J.; Feng, G.; Shen, X. M.; Wang, Y. T.; Yang, H.; Zheng, W. C. J. Cryst. Growth 2003, 254, 348-352. (6) Eder, F.; Klauk, H.; Halik, M.; Zschieschang, U.; Schmid, G.; Dehm, C. Appl. Phys. Lett. 2004, 84, 2673-2675. (7) Reese, C.; Roberts, M.; Ling, M.-M.; Bao, Z. Mater. Today 2004, 7, 20-27. (8) Miller, D. A.; Kauffman, J. W.; Kannewurf, C. R.; Arndt, R. A. Phys. Lett. A 1968, 28, 75-77. (9) Matsui, A.; Tomotika, T.; Tomioka, K. J. Phys. C: Solid State Phys. 1975, 8, 1285-1292. (10) Bhatti, M. T.; Ali, M.; Shahid, G. N.; Saleh, M. Turk. J. Phys. 2000, 24, 673-679. (11) Mason, R. Mol. Phys. 1961, 4, 413. (12) Forrest, S. R. Chem. ReV. 1997, 97, 1793-1896. (13) Ehara, T.; Hirose, H.; Kobayashi, H.; Kotani, M. Synth. Met. 2000, 109, 43-46.
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Crystal Growth & Design, Vol. 7, No. 2, 2007 249 (20) Okaya, Y. Acta Crystallogr. 1965, 19, 879-882. (21) Graswinckel, W. S.; Algra, R. E.; van Enckevort, W. J. P.; Vlieg, E., to be published, 2007. (22) Sabbah, C. R.; Xu-wu, A.; Chickos, J. S.; Leitao, M. L. P.; Roux, M. V.; Torres, L. A. Thermochim. Acta 1999, 331, 93-204. (23) Graswinckel, W. S.; Noorduin, W.; van Enckevort, W. J. P., to be published, 2007. (24) Graswinckel, W. S.; van Enckevort, W. J. P. Leunissen, M. E. Kaminski, D. M.; Shah, I. A. In ’t Veld, M.; Algra, R. E.; Vlieg, E., to be published, 2007. (25) Leunissen, M. E.; Graswinckel, W.S.; van Enckevort, W. J. P.; Vlieg, E. Cryst. Growth Des. 2004, 4, 361-367.
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