Article pubs.acs.org/Langmuir
Local Diffusion Induced Roughening in Cobalt Phthalocyanine Thin Film Growth Murali Gedda,† Nimmakayala V. V. Subbarao,§ and Dipak K. Goswami*,‡ †
Department of Physics and §Center for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati 781039, India ‡ Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur 721302, India ABSTRACT: We have studied the kinetic roughening in the growth of cobalt phthalocyanine (CoPc) thin films grown on SiO2/Si(001) surfaces as a function of the deposition time and the growth temperature using atomic force microscopy (AFM). We have observed that the growth exhibits the formation of irregular islands, which grow laterally as well as vertically with coverage of CoPc molecules, resulting rough film formation. Our analysis further disclosed that such formation is due to an instability in the growth induced by local diffusion of the molecules following an anomalous scaling behavior. The instability relates the (ln(t))1/2, with t as deposition time, dependence of the local surface slope as described in nonequilibrium film growth. The roughening has been characterized by calculating different scaling exponents α, β, and 1/z determined from the height fluctuations obtained from AFM images. We obtained an average roughness exponent α = 0.78 ± 0.04. The interface width (W) increases following a power law as W ∼ tβ, with growth exponent β = 0.37 ± 0.05 and lateral correlation length (ξ) grows as ξ ∼ t1/z with dynamic exponent 1/z = 0.23 ± 0.06. The exponents revealed that the growth belongs to a different class of universality.
1. INTRODUCTION Organic semiconductor materials are the focus of enhanced research activities due to their potential applications in different electronic and optoelectronic devices.1−4 In most of the cases, organic thin films are used as active layer of the device. The performances of these devices were found to crucially depend on the structure and morphologies of the films.5−7 Therefore, understanding the growth mechanism is essential to predict different morphology formation in the film growth. This understanding can enable one to achieve efficient control on the growth of the films for successful implementation in different applications. In past decades, several theoretical models were established to relate the thin film growth mechanism to a set of scaling exponents.8−11 These models were tested for a large number of inorganic systems and few organic systems.12−16 However, the growth mechanism remains poorly understood because the growth of organic thin films is more complex than inorganic film growth due to the molecular anisotropy and weak intermolecular interaction (van der Waals type) between molecules, and issues like epitaxy and strains in organic film growth are not well-defined. Therefore, most of the organic film growth showed anomalous scaling behavior supporting a different universality class of growth, which requires further theoretical understanding considering the different issues related to organic thin film growth. Hence, the detailed knowledge on the growth of organic thin film is still far from complete. In this work, we have studied the roughening mechanism of cobalt(II) phthalocyanine (CoPc) thin film growth on SiO2 surfaces and observed the experimental evidence of the instability in the growth induced by local diffusion of the © 2014 American Chemical Society
molecules. CoPc has emerged as an important molecule because of its high thermal and chemical stability. Nevertheless, CoPc molecule was found to have highly catalytic activity based on its electrochemical oxidation of thiols or the reduction of molecular oxygen.17−19 These interesting redox properties of CoPc have been exploited in the application of gas sensing.20,21 In order to enhance the sensitivity of CoPc thin film based gas sensors, the films need to be smooth with better molecular packing in order to increase the charge transport properties through the film. However, CoPc was found to form irregular islands at the early stage of the growth.22 As the growth continues, the film becomes rough due to such morphology formation. Therefore, the charge transport properties through such films become significantly poor due to the presence of several grain boundaries within the film. Hence, CoPc molecules draw less attention for the fabrication of electronic devices. Recently, the significant enhancement of carrier mobility is observed on the devices based on CoPc wires where better molecular alignment was observed.23 The performance of CoPc thin film based devices can also be enhanced with the detailed knowledge of the film growth. This requires complete understanding of the growth mechanism that enables one to achieve control over the growth at molecular level. In this work, we have studied the growth of CoPc film and explored the possible reasons for such morphology formation. The scaling exponents were calculated to identify the roughening mechanism in the growth of the film. The Received: February 18, 2014 Revised: June 28, 2014 Published: July 3, 2014 8735
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Figure 1. Representative AFM images of (a) SiO2 surfaces and CoPc films grown on SiO2 surfaces at 120 °C substrate temperature with various deposition times (b) 1 min, (c) 2 min, (d) 4 min, (e) 6 min, and (f) 10 min.
roughening mechanism for organic thin film growth is normally explained in terms of Ehrlich-Schwöbel (ES) barrier, which varies as films grow.24,25 ES barrier is the extra potential barrier that exists at the molecular step edge. This extra potential barrier may restrict the downward diffusion of the molecules depending on the growth temperature. Therefore, the deposited molecules on the molecular steps pile up to form islands. As a result, the film becomes rough as growth continues. Monomer bulk diffusion also was reported to be the origin of the kinetic roughening for the growth of polymer film by vapor deposition technique.12 We have observed that the local diffusion of the CoPc molecules, which exists even at the elevated substrate temperature (120 °C), plays a very crucial role in kinetic roughening of CoPc film on SiO2 surfaces.
3. RESULTS AND DISCUSSION The postgrowth surface morphology of the films was characterized systematically using AFM to study the dynamics of the growth. A typical AFM image of SiO2/Si(001) substrate surface is shown in Figure 1a. The root-mean-square (rms) roughness of this surface is about 0.3 nm. Figure 1b−f shows the representative AFM images of CoPc films grown onto the SiO2 surfaces at 120 °C substrate temperature with 1, 2, 4, 6, and 10 min deposition times, respectively. Formation of CoPc islands is observed for the film with 1 and 2 min deposition time as shown in Figure 1b−c. As the deposition time increases further, these islands grow laterally as well as vertically, forming percolated structures as shown in the subsequent AFM images in Figure 1d−f. However, we did not observe formation of uniform films even after deposition of 10 min. Instead, the formation of large islands on top of the percolated structures is observed. Such islands are marked by arrows as shown in Figure 1f. In order to study the dynamic behavior of this growth process, we have calculated the different scaling exponents by analyzing the surface morphology recorded using AFM. The evolution of local surface slope of the islands during the growth was also studied. These quantities were determined from the height−height correlation function, G(r, t), which is defined as mean square of height difference between two surface positions separated by a distance r for the deposition time t as G(r, t) = ⟨[h(r, t) − h(0, t)]2⟩, where h(r, t) and h(0, t) are the heights of the surfaces at the two different locations separated by a distance r. The brackets signify an average over all available pairs of points obtained from the AFM images.26−28 When r is small, the height−height correlation function varies as G(r, t) = [m(t)r]2α, with r ≪ ξ(t), where ξ(t), is the characteristic inplane length scale, α is the roughness scaling exponent, and m(t) is the local surface slope of the height profile for small
2. EXPERIMENTAL DETAILS The powdered CoPc molecules used in this work were purchased from Alfa Aesar (USA). A custom designed organic molecular beam deposition (OMBD) system was used to grow CoPc thin films on SiO2 substrates. Initially the substrates were treated with piranha solution. The cleaned substrates were then ultrasonicated for several cycles with acetone, methanol, and deionized water and dried with argon gas. CoPc molecules were sublimated at 350 °C. In order to study the effect of diffusion of the molecules on the growth, a series of CoPc films were grown by varying coverage of CoPc molecules and substrate temperature. To understand the growth of the film as a function of coverage, we have varied the deposition times as 1, 2, 4, 6, and 10 min. The growth temperatures were varied as 25, 60, 120, 180, and 220 °C to control the diffusion of the molecules on the substrate surfaces. Chamber pressure during the growth was ∼10−7 mbar with the fixed deposition rate of ∼0.2 Å/s. In order to study the dynamics of the growth, we have recorded postgrowth surface morphology using an AFM (Agilent 5500-SPM) in tapping mode in order to avoid any film damage due to tip−surface interactions. 8736
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correlation length, ξ(t) increases following power law as ξ(t) ∼ t1/z, with 1/z = 0.23± 0.06. The exponent 1/z is called dynamic exponent. In order to understand the dynamic of the growth, we have studied the variation of these parameters with deposition time. Log−log variations of W and ξ versus t are shown in Figure 3. Figure 3a shows the increasing nature of W
length scale.13,27 m(t) was calculated from the fitting of linear portion of log−log variation of G(r, t) vs r as per the equation mentioned earlier. The growth of the CoPc films was studied by monitoring two parameters which describe the island growth. The first one represents the average size of the island, whereas the second parameter represents average separation between the islands. Lateral correlation length, ξ(t), is the measure of the length beyond which surface heights are not significantly correlated. For the surfaces with island formation, this length essentially measures the size of the island.29 Another parameter to characterize surface fluctuations is the wavelength (λ), which signifies the average separation between islands. λ and ξ must satisfy the relation ξ ≤ λ as the islands are separated by at least their size. However, when the islands grow next to each other, it would imply that ξ = λ.30 In the case of percolated structures, λ is not well-defined. Therefore, we calculated ξ to characterize the CoPc film growth. Figure 2 shows the log−log
Figure 3. Log−log variation of (a) interface width (W) and (b) lateral correlation length (ξ) with respect to deposition time t. The estimated values of growth exponent β and dynamic exponent 1/z are also represented in the figure.
as the film grows with t. This clearly indicates the roughening of the film growth. However, the increasing nature of ξ with t, as shown in Figure 3b, indicates the lateral growth of the islands. This confirms that the islands grow vertically as well as laterally and the overall film becomes rough as deposition time increases. In order to quantify the dynamics of the roughening, we have plotted ξ versus W for all the samples. This is shown in Figure 4. From the equations of W versus t and ξ versus t, as
Figure 2. Height−height correlation plot with log−log variation of G1/2(r, t) vs r for the films grown at different deposition time intervals of 1, 2, 4, 6, and 10 min.
variation of G1/2(r, t) vs r. In order to determine ξ, we have calculated G(r, t) from AFM images following the procedure described by Pelliccione et al.27 To avoid sampling induced effect in the G(r, t) calculation from height fluctuations, care has been taken to include many AFM images in the averaging of G(r, t) data. In our analysis we have checked that 6 to 10 AFM images taken from different parts of the sample were sufficient to give statistically reliable data to obtain a G(r, t) plot. An upshift of G(r, t) data is observed as deposition temperature increases, as shown in Figure 2. This confirms that the roughness increases as the films grow, indicating roughening in the growth process. To monitor the roughening process quantitatively, we measured the interface width W(t) as a function of t. Interface width, W(t), is essentially defined as the rms roughness of surface morphology. W(t) (shown by arrow marked in Figure 2) is the value of G1/2(r, t) at the first local maximum, as W(t) = G1/2(ξ/2) where ξ, marked by an downward arrow, is the position of r at the first local minimum of G1/2(r, t).31 This definition of interface width is preferable over the large r limit of G(r, t), as artifacts at large length scales can affect AFM data. The roughness exponent α was determined from a power fit to the linear part of the log-log plot of G1/2(r, t) vs r. α essentially represents the variation of height fluctuation as deposition time increases. It corresponds to the vertical growth of the film. The average α observed in the growth of CoPc film is 0.81± 0.05. We have observed the power law behavior of interface width, W(t), as W(t) ∼ tβ with the exponent β = 0.37 ± 0.05. The exponent β characterizes the dynamics of the roughening process and is called the growth exponent. The lateral
Figure 4. Log−log variation of interface width (W) vs lateral correlation length (ξ) calculated from the films grown at various time intervals to quantify the dynamics of the roughening.
described earlier, one can easily derive that W ∼ ξγ, with the exponent γ = β / (1/z) = 1.67 ± 0.06, which is measured from the fit. In this case, the ratio of β = 0.37 ± 0.05 and 1/z = 0.23 ± 0.06, observed earlier, is 1.61 ± 0.13, which is comparable with the fitted value of γ within the measurement error. The value of γ can predict the competition between lateral and vertical growth. The observed value of γ > 1 clearly indicates the faster vertical growth that corresponds to the roughening mechanism of the film growth as observed. The kinetic process responsible for different morphology formation is interlayer mass transfer through diffusion of the molecules.32 Interlayer mass transport is decided by the diffusion activation energy along the local surface slope and the extra ES barrier at the step edge. The higher ES barrier inhibits the step down process that forms 3D islands. 3D islands can also be formed in the case of limited diffusion of the molecule, which inhibits growth kinetics. In order to identify the origin of the roughening in the growth, we have grown films at different substrate temperatures during the growth. Figure 5a−e shows the representative AFM images of CoPc films grown at 25, 60, 8737
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Figure 5. AFM topography images of CoPc films grown on SiO2 surfaces at different substrate temperatures (a) 25 °C, (b) 60 °C, (c) 120 °C, (d) 180 °C, and (e) 220 °C for 10 min deposition time.
120, 180, and 220 °C substrate temperatures, respectively. As the substrate temperature increases, the diffusion of the molecules is enhanced and forms a smooth layer with improved crystalline quality. Hence, the formation of a flat underlying layer together with larger islands on top can be seen at 220 °C substrate temperature. This can be attributed to the enhanced diffusion of the molecules at higher growth temperature. To understand the evolution of the film morphology, we have calculated G(r, t) for all the samples grown with different substrate temperature. Figure 6 shows the variation G1/2(r, t)
shown in Figure 7a and b. It is interesting to observe similar activation barriers along vertical and horizontal growth of the
Figure 7. Arrhenius behavior of (a) interface width (W) and (b) lateral corrrelation length (ξ) with the function of deposition temperature T.
films. Activation barriers calculated from the variation of W and ξ with temperature are associated with rotational and translational barriers of the molecular diffusion.33 In this case, they correspond to the diffusion of the molecule along the local surface slope of the islands and lateral growth on the terraces, respectively. Therefore, lateral diffusion is equally probable to the vertical diffusion of the molecule that contributes to the upward diffusion current of the molecules. However, the upward diffusion current can be purely controlled by ES barrier height if one considers sufficient diffusion of the molecules at a given growth temperature. In the case of CoPc growth, the formation of islands could be either due to the limited diffusion of the molecules even at higher growth temperature or purely due the ES barrier effect as observed in other molecules.24 In order to identify the origin of the roughening mechanism, we have studied the detailed diffusion process during the growth of CoPc film at 120 °C substrate temperature. Zhong et al. showed that the kinetics of island formation is essentially the interplay of different diffusion mechanism of the molecules.32
Figure 6. Height−height correlation plot with log−log variation of G1/2(r, t) vs r for the films with 10 min deposition time grown at different substrate temperatures 25, 60, 120, 180, and 220 °C.
versus r. The average α observed in this case is 0.78 ± 0.04. This value is close to the value (0.81 ± 0.05) obtained while coverage of the CoPc was varied. The average value of α obtained from both the cases is 0.79 ± 0.06. While increasing the substrate temperature, we observed that both interface width and lateral correlation length closely follow an Arrhenius behavior as W, ξ ∼ exp(−Ea/kBT), with observed surface diffusion activation energy Ea = 0.34 ± 0.03 eV, kB is Boltzmann constant, and T is the substrate temperature. This behavior is 8738
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The variation of local surface slope as the film grows can identify the diffusion mechanism.34 Therefore, we studied the variation of local surface slope, m(t), as the deposition temperature increases. If m(t) is independent of the growth time, t, then it is said to be stationary growth. In such cases, height−height correlation function coincides for r ≪ ξ, whereas the diffusion of the molecules in nonstationary growth is limited. In such a case, the local slope m(t) increases with time and an upshift of G(r, t) can be observed as film thickness increases.13 This represents island growth as we see in CoPc film. There are several theoretical models proposed to study the growth of inorganic thin films.11,27,28 However, organic thin film growth is much more complex than that of inorganic thin films. Therefore, none of these models clearly represent the organic film formation. To understand the growth of CoPc films, we have attempted to use nonequilibrium film growth driven by surface diffusion, when the growth equation can be written as14
However, the linear growth equation predicts α = 1 and β = 1/ 4.37 On the other hand, due to ES barrier, the diffusion could also be limited at the step edge and form uniformly sized pyramids with stationary slope. The predicted scaling exponents for the growth driven by ES barrier are α = 1 and β = 1/4.38 None of these models support the exponents that we observed for the growth of CoPc thin films. Therefore, an understanding of the kinetic roughening in organic film growth is still far from complete.13 Hence, growth models considering the issues related to organic film growth may be required to explain the exponents and belong to a different universality class.
4. CONCLUSION In conclusion, we have reported the origin of roughening in the growth of CoPc film on SiO2 surfaces. We have observed equal diffusion activation energy for lateral and vertical diffusion of the molecules. However, it was found that local surface diffusion, which exists even at 120 °C growth temperature, plays a crucial role in roughening in CoPc film growth. The reported scaling exponents do not belong to the existing models, which holds well for inorganic film growth. Therefore, the roughening behavior observed in the present study appears to belong to a different universality class.
∂h = −κ ∇4 h + λ∇2 (∇h)2 + F + η(r , t ) ∂t
where κ and λ are constants and η is a random fluctuation around the average flux F causing roughening. The linear and nonlinear parts are decided by the constants and represent the detailed growth processes. The linear equation corresponds to the local growth that affects the local surface slope. When the growth is driven by the linear equation, atoms stick to the nearest kink sites irreversibly. If the molecules are able to overcome the intermolecular interaction and hop to the next sites, the intermediate diffusion of the molecules and the growth processes are described by the nonlinear equation.13 In the case of local diffusion, the local surface slope increases with time and can be described as m(t) = ⟨(▽h)2⟩1/2 = (C ln(t/ tc))1/2, where C is constant and tc is transition deposition time to the scaling regime.13,35 Figure 8 shows the variation of m(t)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support by the Department of Science and Technology, Government of India under project DST/TSG/ ME/2008/45 and DST/TSG/PT/2009/23 is fully acknowledged. We acknowledge the support provided by Prof. P. K. Iyer, IIT Guwahati. We would also like to acknowledge Dr. M. Banerjee, DS Kothari Fellow, for providing valuable input while preparing this manuscript.
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REFERENCES
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Figure 8. Variation of local slope (m(t)) with deposition time t for the films grown at different deposition time intervals. Solid line is the fit to the data using m(t) = ⟨(▽h)2⟩1/2 = (C ln(t/tc))1/2.
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