Combinatorial approach to morphology studies of epitaxial thin films

Described is the application of a combinatorial physical vapor deposition (CPVD) method for studying the growth dynamics of epitaxial films. The CPVD method takes advantage of the angle-dependent evaporation rate from a point source to produce thin film libraries whose deposition rate changes continuously for a factor of 50 across a 70-mm long-substrate. The link between the deposition rate and the resulting thin film morphology was made by spatially correlated absorption and atomic force microscopy measurements. It is shown that the growth of tryphenyldiamine derivate on a silica surface proceeds by three-dimensional growth of isolated islands which, at some critical coverage, coalesce to form uniform amorphous film. While the critical coverage of such films depends on the deposition rate in the 0.015-0.4 nms region, the particle size distribution function does not. © 2006 American Institute of Physics.National Science Foundation: NSF-CHE0098240 National Council for Scientific Research American Chemical Society Petroleum Research Fund American Chemical SocietySuljovrujic E. iQUEST, University of California , Santa Barbara, California 93106 and Vinca Institute of Nuclear Sciences , P.O. Box 522, 11001 Belgrade, Serbia and Montenegro Micic M. MP Biomedicals LLC , 15 Morgan, Irvine, California 92618 Demic S. a) Srdanov V. I. b) iQUEST, University of California , Santa Barbara, California 93106 a) Present address: Ege University, Solar Energy Institute, 35100-Bornova, Izmir, Turkey. b) Author to whom correspondence should be addressed; electronic mail: [email protected] 20 03 2006 88 12 121902 09 08 2005 21 02 2006 20 03 2006 2006-03-20T09:32:20 2006 American Institute of Physics 0003-6951/2006/88(12)/121902/3/ $23.00 Described is the application of a combinatorial physical vapor deposition (CPVD) method for studying the growth dynamics of epitaxial films. The CPVD method takes advantage of the angle-dependent evaporation rate from a point source to produce thin film libraries whose deposition rate changes continuously for a factor of 50 across a 70-mm long-substrate. The link between the deposition rate and the resulting thin film morphology was made by spatially correlated absorption and atomic force microscopy measurements. It is shown that the growth of tryphenyldiamine derivate on a silica surface proceeds by three-dimensional growth of isolated islands which, at some critical coverage, coalesce to form uniform amorphous film. While the critical coverage of such films depends on the deposition rate in the 0.015 – 0.4 nm / s region, the particle size distribution function does not. NSF NSF-CHE0098240 Understanding the nucleation mechanism and growth dynamics of molecular thin films is pivotal for controlling their morphology and the film interfacial properties. While a great deal of effort has already been done in the area of inorganic thin films, 1 comprehensive studies of epitaxial organic thin films for electronic applications are more recent. 2–4 Owing it to the weak heterogeneous interactions, the growth of organic thin films on inorganic substrates frequently proceeds by Volmer–Weber mechanism. Such growth is characterized by initial formation of molecular agglomerates whose size and number density increase during the deposition until percolation is reached. To capture details of the morphological evolution at various stages of the film growth, a large number of samples needs to be prepared where the deposition time or the deposition rate are systematically varied. Lately, progress was made in applying combinatorial physical vapor deposition (CPVD) methods for optimization of organic thin films 5 and devices. 6 In this letter we describe the application of a CPVD method by which different thin film morphologies can be prepared in a single evaporation. The same CPVD method was applied recently in studying Förster energy transfer in a donor-acceptor mixture of small organic molecules. 7 We focus here on N,N'-Bis(3-methylphenyl) - N,N' diphenylbenzidine (TPD), well known hole transport material widely used in organic opto-electronic devices. 8 Despite several studies addressing the morphology of TPD films 9–11 no coherent picture connecting the thin film morphology with the coverage and/or deposition rate has emerged so far. The evaporation of TPD (American Dye Source, Inc.) was conducted in a high-vacuum chamber ( 2 × 10 - 7 Torr of background pressure) using temperature-stabilized spherical tantalum boats with a 2 mm pinhole. The temperature of evaporator was regulated by an active feedback using a homemade LABVIEW program controlling current-stabilized output from a dc power supply (Sorensen DCS8-125 E). The TPD films were evaporated on a rectangular ( 2 cm × 7 cm ) clean silica substrate whose end was placed a short distance above the evaporator. As a result, the thickness of deposited films varied considerably along the substrate length. Spatially resolved absorbance measurements of such films were obtained by a UV-visible spectrophotometer (HP-8452A) whose aperture was reduced to 2 mm in diameter. Spatially correlated atomic force microscopy (AFM) images of the deposited TPD films were made by DI Dimension 5000 AFM (Veeco Instruments). Imaging was performed in air using noncontact tapping mode with the soft silicon cantilever. All measurements were made immediately after the film deposition. The evaporation rate from a Knudsen source has a cosine-like angular dependence. 12 As a result, spatially resolved optical density of the deposited films has the following functionality: A = A 0 • cos n [ arctan ( r / d ) ] . (1) A 0 is the absorbance of deposited film at r = 0 (which is the point on the substrate directly above the evaporator), r is the substrate spatial coordinate and d is the distance between the evaporator and the substrate. The latter, in turn, determines the value of n , which can be obtained by the least squares fit of the absorbance values as a function of r . The nominal thickness of the film was calculated from the absorbance measurements using TPD absorption coefficient ? = 1.6 × 10 4 cm - 1 at 352 nm . 13 Spatially resolved absorbance measurements of a TPD film evaporated with the source-substrate distance d = 30 mm , for which the deposition rate varied between 0.01 and 0.56 nm / s , are shown in Fig. 1 . A digital photograph of such film reveals the presence of two visually distinct sections separated by a sharp front. The section of the film above the evaporator was transparent while the other one was opaque. Consistent with this was an extended long-wavelength tail found in the absorption spectra of the opaque section of the film, which is characteristic of a light scattering process (see the inset of Fig. 1 ). The light scattering was caused by microscopic TPD aggregates whose AFM images obtained at different substrate coordinates are shown inFig. 2 . Starting at the substrate end with smallest deposition rate, the TPD morphology gradually evolves from sparsely populated spherical droplets (1) through a domain phase composed of packed droplets (2), which evolves into separated striplets (3). Further increase of the deposition rate (i.e., the coverage) leads to a percolated striplet phase (4), which is followed by a tiny section of a flat film with randomly distributed pores (5). Such morphology is known as inverse droplet phase or the holes phase. Further increase in the deposition rate gives rise to uniform flat film. Some details accompanying the transition between the holes phase and the flat film are worth noting. While the surface roughness of the flat film was less than 10% of the film thickness, the thickness variations in the holes phase exceeded 100% of the average thickness. This implies a facile reconstruction of the TPD surface during the morphological phase transition, which is induced by only a slight change in the coverage. The forces governing such surface restructuring have been recently studied in detail by Friesen and co-workers. 14 Similar changes in thin film morphology as a function of the nominal coverage have been reported earlier for several systems. 15–19 The advantage here is that different morphology phases are all present on a single substrate and are obtained under identical experimental conditions. The only variable is the deposition rate whose functionality is precisely known along the substrate length. This allows for studying the influence of the deposition rate on thin film morphology in a rather efficient manner. Detailed studies of that kind are less frequent in the literature 20 as they normally require a series of sequential depositions at different source temperatures. Below we describe a procedure by which such a study can be performed using our simple combinatorial method. If illuminated by a white light during the deposition, one observes a sharp front that separates the transparent from the opaque section of the film, moving slowly along the substrate. Initially, the transparent film forms at the substrate end close to evaporator and continues to expand towards the other end. By taking a series of photographs at certain times during the deposition, one can extract the time required for formation of the flat film at different deposition rates. The product of the two gives the critical (minimal) thickness of the flat film whose dependence on the deposition rate is plotted in Fig. 3 . We see that slow deposition rates lead to large film thicknesses, which exceed 50 nm for deposition rates smaller than 0.01 nm / s . On the other hand, the critical thickness for obtaining flat uniform films decreases with increase of the deposition rate, yet no uniform film can be made thinner than 10 nm regardless of the deposition rate. This implies a positive (nonwetting) Hamaker constant for fused-silica-TPD-air interface. Hamaker constants have been calculated for several commonly used substrates including fused silica 21 thus allowing one to predict trends in the surface aggregation kinetics as a function of the substrate surface energy. We plan on verifying these trends with our combinatorial method. Note that different morphological phases in Fig. 2 were obtained by a simultaneous increase of both deposition rate and the surface coverage. To untangle their influences on the TPD morphology, we deposited two structured TPD films with the same nominal thickness of 2.7 nm using different deposition rates. Their AFM images are shown in the inset of Fig. 3 . The TPD film deposited with the rate of 0.001 nm / s gave rise to 17 droplets per 100 µ m 2 on average, and the surface coverage factor of 0.12. The relevant numbers for the deposition rate of 0.015 nm / s were 250 and 0.28. The average height and base diameter of TPD droplets, whose shape was approximated by a sphere segment, were 60 and 930 nm for the slow deposition versus 25 and 380 nm for the fast one. The particle size distribution functions for the two films, shown by an inset of Fig. 3 , are quite similar yet they somewhat deviate from a distribution function expected for homogeneous thin film growth. 22 This is not surprising as the abundant surface imperfections present on an ordinary silica substrate could easily influence heterogeneous growth of the TPD islands. Nevertheless, the similar distribution functions and the unchanged aspect ratio of the TPD droplets indicate that intrinsic growth mechanism for individual TPD islands is independent of the deposition rate. The particle number density scales inversely with the deposition rate in the 0.001 – 0.015 nm / s region but this should not hold for very high deposition rates. As shown in Fig. 3 the critical film thickness is almost rate independent for very high deposition rates. We suspect in this region that the average size of the capture zone 23 has reached its minimum; hence further increase in the deposition rate/coverage does not lead to new nucleation sites. In summary, described is a simple combinatorial method that allows for quick insight into the thin film morphology evolution and the dynamics of the thin film growth. The method can be applied with different levels of sophistication. It can be equally useful to a device scientist interested in finding conditions for depositing uniform flat films of desired thickness, but also to a surface scientist concerned with fine details of the growth mechanism. The authors thank the National Science Foundation (NSF-CHE0098240) and the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. S.D. thanks The Scientific and Research Council of Turkey (TUBITAK) for financial support. The authors also acknowledge Dr. Sergei Magonov from Veeco Instruments Inc. and Maja Micic for their help with AFM imaging studies. FIG. 1. Digital photograph of a TPD thin film (top) deposited for 3 min from a point source at 461 K yielding a 100-nm-thick film at r = 0 . Visible by naked eye was a border line at r = 29 mm separating uniform from structured portion of the film. The representative absorption spectra of the two film segments are shown in the inset of Fig. 1 . Below, the spatially resolved absorbance measurements are plotted as a function of the substrate coordinate. The solid line is the least squares fit to Eq. (1) ( R = 0.9995 ) from which n = 5.0 ± 0.2 was extracted. FIG. 2. Digital photograph of a TPD thin film (top), deposited for 45 min from a point source at 427 K , yielding a 100-nm-thick film at r = 0 and the border line at r = 19 mm . Shown below are AFM images of various morphology phases that have been identified, linked by a number to the spatial position on the substrate. The following distinct morphology phases were found: spherical droplets (1 and 2), isolated striplets (3), percolated striplets (4), inverse droplets/holes (5) uniform film (6). FIG. 3. The thickness of a TPD film at the border line between structured and uniform regions plotted as a function of the deposition rate. Shown by the insets are AFM images of the two TPD films of the same nominal thickness ( 2.7 nm ) , deposited with 0.015 nm / s (a) and 0.001 nm / s (b). Also shown is the particle distribution function for the two films normalized to the average particle size ? A ? . -- -- -- -- -- -

Ionic diffusion in iPP: DC electrical conductivity

This study provides a new insight into the relationships between absorption and adsorption processes that occur during the treatment of iPP in aqueous solutions of metal-chloride salts, as well as the impact of these processes on the electrical conductivity of this nonpolar polymer. The polypropylene films (0.5 mm) were exposed to three-day treatments in aqueous solutions of chlorine salts of some alkali and transition metals at temperatures of 22 C and 80 C. The treatments induced an increase in the electrical conductivity of iPP, up to 800%. DC conductivity is not directly proportional to the concentrations of metals in the treated films due to the complex relationships between diffusion and adsorption processes. The experiment was set up to simulate the real-world conditions and the study provides practical knowledge on the stability of the electrical conductivity of iPP under exposure to aqueous solutions. The influence of electric aging on the electrical conductivity of the treated films was also examined.This work was supported in part by the Ministry of Education, Science and Technological Development of the Republic of Serbia, by the Ministry of Scientific and Technological Development, Higher Education and Information Society of the Republic of Srpska (project: 19.032/961-112/19) and by the National Research Foundation, South Africa.Scopu