Rubrene on mica: from the early growth stage to late crystallization

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Rubrene on mica: from the early growth stage to late crystallization

Gregor Hlawacek¹, Shaima Abd-al Baqi², Xiao Ming He¹, Helmut Sitter² and Christian Teichert¹

1 Institute of Physics, University of Leoben, Leoben, Austria E-mail: gregor.hlawacek@unileoben.ac.at, teichert@unileoben.ac.at

2 Institute of Semiconductor and Solid State Physics, University of Linz, Linz, Austria

Abstract. The fabrication of Rubrene thin films is of interest because of the high mobility observed for Rubrene single crystals. Here, we report on an atomic force microscopy (AFM) investigation of the growth of Rubrene thin films by Hot Wall Epitaxy on mica(001). During the initial formation of amorphous islands, a non-constant growth rate is observed due to temperature dependent changes in the sticking coefficient. Furthermore, the contact angle of these islands – also measured by AFM – depends on temperature. With continuous deposition, island coalescence starts resulting in ramified surface aggregates.

The final growth stage is characterized by the formation of crystalline spherulites which also analyzed by AFM.

1. Introduction

Rubrene is known for its large carrier mobility in single crystal form. Values of up to 20 cm²/Vs have been reported [1,2]. Although this would make it an interesting candidate for various devices, thin films grown from Rubrene have shown hole mobilities lower by 7 orders of magnitude [3]. Unfortunately, very often these thin films are amorphous. Recent studies have shown that in crystalline or poly- crystalline films grown with a large overpressure of Rubrene or by utilizing het- erostructures, mobilities of up to 0.2 cm²/Vs can be reached [4-6]. Recent work using the weakly interacting substrate SiO2 and an OTS layer to tune the surface energy of the substrate demonstrated mobilities between 0.1 cm²/Vs and 2.5 cm²/Vs [7].

Here, thin films of Rubrene are grown on mica(001) by means of hot wall epitaxy (HWE). The initially formed amorphous islands as well as the morphologies in crystalline spherulites, observed in thick films, are characterized by atomic force microscopy (AFM). The behavior of the growth rate, the contact angle of the Rub- rene islands, and their fractal dimension are analyzed in dependence on growth temperature and film thickness.

2. Experimental Methods

Mica(001). The (001) surface used is a cleavage plane of 2M1 muscovite. It allows easy ex situ cleavage offering large (>100 μm) atomically flat terraces. Fur-

Rubrene on mica: from the early growth stage to late crystallization

GregorHHHHHHllllallawawawawawawacek¹, Shaima Abd-al Baqi², Xiao Ming He¹, Helmut Sitter² and Christian TeTeTeTeTeTeicicicicicichhhehhh rt¹

1 Institute of fffffPhPhPhPhPhPhysysysysysysicicicicicics,ss University of Leoben, Leoben, Austria E-mail:gregoooooorooooooooooo.hhhhhhhhhhhhhhhlalalalalalalalaaaaaaaaaaawawwwwwwwwwwwwwwwwwwwwwcek@unileoben.ac.at, teichert@unileoben.ac.at

2 Institute of Semicocococococondndndndndnducucucucucuctor and Solid State Physics, University of Linz, Linz, Austria

Abstract. The fabbbbbbririririririccccatitititititiononononnn of Rubrene thin films is of interest because of the high mobility observed for Rubreeeeeenenenenenene ssssssinininininingle crystals. Here, we report on an atomic force microscopy (AFM) investigatioooooonnnnn ofofofofofof ttttthhhehhh growth of Rubrene thin films by Hot Wall Epitaxy on mica(001). During ththththththeeeee eininininininititititittiaiaiaiaiaiallllllformation of amorphous islands, a non-constant growth rate is observed due to temperatatatatataturururrrre e ee e e deddddd pendent changes in the sticking coefficient. Furthermore, the contact angle of theseeeeeeiiiiiislslslslslslananananananddddsdd – also measured by AFM – depends on temperature. With continuous deposition, islaaaandndndndndnd ccoaoaoaoaoaoalescence starts resulting in ramified surface aggregates.

The final growth stage is chchchchchcharaccccccteteteteteterized by the formation of crystalline spherulites which also analyzed by AFM.

1. Introduction

Rubrene is known for iiiiiitstststststsllllllararararararge carrrririririririerererererermmmmmmobility in single crystal form. Values of up to 20 cm²/Vs have been² n n n nn rererrererepopopopopoportrrrrred [1,2]2]2]2]2]2]. AlAlAlAlAlAlthough this would make it an interesting candidate for various deeeeeeviviviviviviccccececes,s,s,s,s,s, thin fifififififlmlmlmlmll ssssss grown from Rubrene have shown hole mobilities lower by 7 ordedededededersrsssssoooooofff fffmagnitududududududeeeeee[3[3[3[3[3[3]. Unfortunately, very often these thin films are amorphous. Rececececececenenenenenentttttt ssssstsudies hahahahahahaveveveveveve shown that in crystalline or poly- crystalline films grown wiwiwiwiwiwiththththththaaaaaa lararararararge oveerprppppprrererreressssssssssssuruuuuu e of Rubrene or by utilizing het- erostructures, mobilities offffff uuuuupppppp to 0.2 cm²²²²²²/V²/V/V/V/V/Vssssss ccccacc n be reached [4-6]. Recent work using the weakly interacting sssssububububububstrate SiOOOOOO2222aaaaaandndndndndndaaaaaan OTS layer to tune the surface energy of the substrate demonstrrrrrratatatatatatedeeeee mmmmmmooboboboo iiililililititititititieieieieieiesssss between 0.1 cm²/Vs and² 2.5 cm²/Vs [7].²

Here, thin films of Rubrene are growowowowowown ononononononmmmica((((((0000000000001)1)1)1)1)1) bbbbby means of hot wall epitaxy (HWE). The initially formed amorphphphphphphououououououss isisisisisislandddddssssss asasasasass wwwwwwell as the morphologies in crystalline spherulites, observed in tttttthihihihihihickckckckckck ffffffiililiililms, arrrreeeeeechchchchchchaaaaraaacterized by atomic force microscopy (AFM). The behavior of the e e ee e grgrgrgrgrgrowowowowowowthttttt ratattttte,e,e,e,e,e,ttttttheheheheheheccontact angle of the Rub- rene islands, and their fractal dimensioooooonnnnnn ararararararee analalalalalalyzyzyzyzyzyzedededededed iiiiiinnnnnn dependence on growth temperature and film thickness.

2. Experimental Methods

Mica(001). The (001) surface used is a cleavage plane of 2MMMMMM111111mmmmmmususususususcovite. It allows easy ex situ cleavage offering large (>100 μm) atomicallyyyyyy flflflflflflatatatatatat terraces. Fur-

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thermore it has shown the capability to align organic molecules by the a strong surface dipole [8]. Standard quality mica was cleaved before insertion into the HV chamber, thus a fresh cleaved surface with only a few cleavage steps is used.

Hot Wall Epitaxy (HWE). HWE is a high vacuum variant of physical vapor deposition with a base pressure of 10^-6 mbar [9]. In contrast to many other growth techniques it utilizes the near field of the molecular beam by moving the sample close or even into the hot wall tube that holds the film material. The walls of the tube can be heated separately and are held on a higher temperature than the sample and the source. This prevents deposition on the tube wall and helps to create a uniform flux of molecules. The main advantages of HWE are that the films are grown close to the thermodynamic equilibrium. The main drawback is that the position of the sample close to the evaporator makes an in situ characterization of the film growth impossible.

Rubrene. This organic semiconductor consists of a tetracene core with four addi- tional phenyl groups connected by single bonds. In contrast to the desired crystalline phase the amorphous one is not stable against oxidation [10]. The two states can be distinguished easily, since the amorphous phase lacks the typical red color found for crystalline films.

Atomic Force Microscopy (AFM). We used an Digital Instruments MultiMode IIIa AFM in intermittant mode to avoid damage to the organic thin film. Conventional Si probes with opening angles of 20° and tip radii of less then 10 nm have been employed. The typical resonance frequency of the used canti- levers is 300 kHz and the force constant is about 40 N/m.

3. Results and Discussion

Figure 1 shows two series of AFM images obtained from amorphous Rubrene (or more likely oxidized Rubrene [10,11]) thin films grown with a substrate 363 K (a- d) and 393 K (e-h). The deposition times for both image sequences ranged between 2 min and 24 h. First, small circular islands of rather uniform size are formed which then grow (a,b,e,f) and start to coalesce (c,g,h).

In fig. 2 the distribution of island height and island base area for the case of 363 K is presented. The island height changes from 50 nm after 2 min to 120 nm after 60 min. While the width of the height distribution remains constant (20 nm) with ongoing deposition, the width of the island area distribution, however, broadens dramatically as soon as coalescence starts. The same holds for the films grown at higher temperature (lower row of fig. 1). However, in this case the island density is much lower which can be related to the increased mobility of the molecules on the surface at higher temperatures.

Figure 3(a) shows the nominal film thickness (f) vs. deposition time. These thickness values are obtained by calculating the total volume of Rubrene divided by the image area of at least three independent images. For the sample grown at higher temperature, f ranges significantly below the film thickness obtained under identical conditions at lower temperature. We can explain this by a change in sticking coefficient for Rubrene on mica(001). From the power law fits indicated we obtain exponents of nearly unity (linear dependence as expected) for the higher temperature growth and 1/2 for the low temperature growth. We can understand thermore it has shown the capability to align organic molecules by the a strong surface dipole [8]. Standard quality mica was cleaved before insertion into the HV chamber, thus a fresh cleaved surface with only a few cleavage steps is used.

Hot Wall EEEEEEppppppitaxy (HWE). HWE is a high vacuum variant of physical vapor de- positionononnnn wwwwwwitttttthhhhhh aa aaaa base pressure of 10^-6mbar [9]. In contrast to many other growth techniquququuuuesesesesesesiiiiiit utilizes the near field of the molecular beam by moving the sample close or eeeeevevevevevevennnn iininiini to the hot wall tube that holds the film material. The walls of the tube cannnnnnbbbbbbe eeeeeheheheheheheatatatatatated separately and are held on a higher temperature than the sample and the soururururururcececececece.... ThTTTTT is prevents deposition on the tube wall and helps to create a uniform fluxxxxxx ooooooffffff momomomomomolelelelelelecules. The main advantages of HWE are that the films are grown close toooooo tttttthehehehehehe tttttthhhehhh rmodynamic equilibrium. The main drawback is that the position of the sasaaaaammmmpmpmplelelelelele close to the evaporator makes an in situ characterization of the film growth imimimimimimpopopopopoposssssssible.

Rubrene. This orrrrrrgagagagagaganinininininicccccc sessss miconductor consists of a tetracene core with four addi- tional phenyl grouuuuuupppppspscononononononnennnnn cted by single bonds. In contrast to the desired crystalline phase the amomomomomomorprprprprprphohohohohohouususususu one is not stable against oxidation [10]. The two states can be distinguished eeeasasasasasasilililililily,y,y,y,y,y, since the amorphous phase lacks the typical red color found for crystalline filmlmlmlmlmlmss.ssss

Atomic Force Microooooossssssccccccoppppppyyyyyy (AFM). We used an Digital Instruments MultiMode IIIa AFM in iiiinii termrmrmrmrmrmittant mode to avoid damage to the organic thin film. Conventional Si probes witititithtthhhhh opening angles of 20° and tip radii of less then 10 nm have been employed.d.d.d.d.d. TTTTTThehehehehehe tyyyypyy ical resonance frequency of the used canti- levers is 300 kHz and the force cocococococonsnsnsnsnsnsttttatt nt is about 40 N/m.

3. Results and DDDDDDiiiiiissssccccccuuuuuusssssssionnnnnn

Figure 1 shows two seririririririesesesesessoooooof f f f f fAFM imimimimimagmagagagagageseseseseses obtained from amorphous Rubrene (or more likely oxidized Rububbbbbrrerererr nenenenenene[[[[10,11]1]1]]]]))))))thththththinininininin films grown with a substrate 363 K (a- d) and 393 K (e-h). Thehehehehehe ddddddepepepepepeposo ition titititititimemememememessssss for both image sequences ranged between 2 min and 24 h... FFFFFFiriririrrrstststststst,,,, smssss all cicicicicicircrcrcrcrcrculululululular islands of rather uniform size are formed which then grow ((((((a,a,a,a,a,a,bbbb,bbe,,,,,,f)f)f)f)f)f)aaaaaand starttttttttttto cocococococoalesce (c,g,h).

In fig. 2 the distribution of isiii lalalalaaandndndnnnd height annnnnnd dd d dd isisisisisislalalalalaland base area for the case of 363 K is presented. The island heighhhhhhttttt t changes fromomomomomom 555555000000nm after 2 min to 120 nm after 60 min. While the width of the heighthththththtddddddistribbbbbbututututututioioioioioionnnnnnrerererereremains constant (20 nm) with ongoing deposition, the width offffffttttttheheheheheheiiiiiislllllland arararararareaeaeaeaeaea ddddddisisisisisistrtrtrtrtrtribution, however, broadens dramatically as soon as coalescencccccceeee eesttttttarararararartststststss...Theeeeeesasasasasasamemememememe holds for the films grown at higher temperature (lower row of figggggg...1)1)1)1)1)1). HHHoHHH wevevevevevever,r,r,r,r,r iiiiiin n n n n n this case the island density is much lower which can be relatedtttttto ooooo ththththththeeeeee inininincreaseseseseseseddddd d mmmmomm bility of the molecules on the surface at higher temperatures.

Figure 3(a) shows the nominal film thiccccccknknknknknkneesesesesesss s s ss (f(( ) vsfff vsvsvsvsvs..dedededededepopopopopoposissssstion time. These thickness values are obtained by calculating the e e e ee tototototototatatatatatallllll vvvvovv lulululululumemememememeooooooffffff RuRRRRR brene divided by the image area of at least three independent iiiiiimamamamamamagegegegegeges.s.s.s.s. For t. ttttthehehehehehessssssamamamamamample grown at higher temperature, ff ranges significantly belowwwwww tttthtt e film tttttthihihihihihickness obtained under identical conditions at lower temperature. We can explaiiiiiinnnnnn ththththththisiiiii by a change in sticking coefficient for Rubrene on mica(001). From the pff pppppowowowowowowereeeee law fits indicated we obtain exponents of nearly unity (linear dependence as exexxxxxpepepepepepectctctctctctedeeeee ) for the higher temperature growth and 1/2 for the low temperature growth...WWWWWWeeeeeecaccccc n understand

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this behavior if we assume different sticking coefficients, namely a high one for Rubrene on mica(001) and a low one for Rubrene on Rubrene.

The observed change in lateral shape of the islands can be evaluated by the fractal dimension D. Figure 3(b) shows the change in D vs. deposition time, where D is calculated by applying a linear fit to the power spectrum [12] of the corresponding 10 μm x10 μm AFM images. Again, the growth of the amorphous Rubrene islands can be divided in three stages. First, the compact islands are formed and the highest fractal dimension is obtained. Then coalescence starts, and the fractal dimension is reduced as more one-dimensional aggregates are formed. The last stage shown in fig. 1(d) leads to a further reduction of the dimensionality as the islands become more ramified.

From the islands three dimensional shape we can make qualitative estimations on the surface free energy. As the Rubrene islands in the initial stage have the shape of a droplet it is assumed that they are amorphous which is confirmed by several techniques [3,13]. Thus, AFM allows measuring the contact angle , like for li - quids, by analyzing cross sections through the island center [3]. Figure 4 shows two cross sections through islands grown at different temperatures. Both sections are taken from samples with deposition times shorter than the deposition time re-

Figure 2: Island height (left) and island size histograms for Rubrene films grown at 363 K.

Figure 1: AFM images of Rubrene thin films grown by HWE on mica(001). Top row samples grown at 363 K, bottom row sample temperature 393 K. Deposition times are (a,e) 2 min, (b,f) 15 min, (c,g) 60 min, and (d,h) 24 h.

this behavior if we assume differeneeee t sticking coefficients, namely a high one for Rubrene on mica(001) and a llllowowowowowowoooooonenenennene for Rubrene on Rubrene.

The observed change in laterrrrrralalalalalalsssssshahahahahahapepepepepepe of the islands can be evaluated by the fractal dimension D. Figure 3(b) shows tttttthehehehehehe change in Dvs. deposition time, where Dis calculated by applying a linear fifififififitttt tttotototototo tttttthehhhhh power spectrum [12] of the corresponding 10 μm x10 μm AFM M imimimimimimagagagagagageseseseseses. Agaiiiiiin,n,nn,n,n,tttttthehehehehehegrowth of the amorphous Rubrene islands can be divided in thhhhhhrererererereeeeeee ststststststaagagagagages. FiFiFiFiFiFirsrsrsrsrsrsttt,t,tt the compact islands are formed and the highest fractal dimensioioioioioionnnnnniiiiis obtaineeeed.d.d.d.d.d. TTTTTThehhhhh n coalescence starts, and the fractal dimension is reduced as momomomomoreorerereeeoooooone-dimmenenenenenensiiiononononononal aggregates are formed. The last stage shown in fig. 1(d) leadsttttttooooooaaaaa afufufufufufurther rerererereredududududuductctctctctctioioioioioion of the dimensionality as the islands become more ramified.

From the islands three dimimmmmmenenenenenensisisisisisiononononononalaaaaa shapepepepepepewwwwwweeeeee cacc n make qualitative estimations on the surface free energy. AAAAAssssss ththththththeeeeeeRRRRRuR brene islslslslslslanananananandsdsdsdsdsds in the initial stage have the shape of a droplet it is assumed that t t t ttththththththey are amomomomomomorprprprprprphohohohohohous which is confirmed by several techniques [3,13]. Thus, AFM allowsssssmmmmmmeasuuuuuuriririririringngngngngng tttttthehehehehehe contact angle , like for li - quids, by analyzing cross sectioonsnsnsnsnsns tttttthrhrhrouououououough tttttthehehehehehe iiiiislslslslslslanananananand ddddd center [3]. Figure 4 shows two cross sections through islandsdsdsdsdsdsggggggrorororororowwwwwnw at didididididiffffffffffffeeererereenenenenenentttt tttemperatures. Both sections are taken from samples with deposiiiiiitititititioniononononon ttttttimimimimimimes sshohohohohohortrtrtrtrtrterererererer than the deposition time re-

Figure 2: Island height (left) and island size histograms for Rubrene fifififififilmlmlmlmlmlms grgrgrgrgrgrown at 363 K.

Figure 1: AFM images oooof fff RuRuRuRuRuRubrene thin films grown by HWE on mica(001). Top row samples grown at 363 K, bobobobobobottttttttttttomomomomomom row sample temperature 393 K. Deposition times are (a,e) 2 min, (b,f) 15 min, (c,g) 6666660 0 0 0 00 mimimimimimin,n,n,n,n, and (d,h) 24 h.,

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quired for coalescence to allow the largest possible drop volume but before changes in island shape might influence the contact angle. Besides the obvious size difference, the contact angle for the film grown at elevated temperature is lar- ger than the one grown at lower temperature. This is contrary to what would be expected at first glance, since surface energy in general decreases with increasing temperature. The observed behavior can be explained when considering different temperature dependencies of surface energies for Rubrene and mica(001). It seems that the surface energy of Rubrene decreases slower with increasing temperature as the one of mica. As a result the surface becomes more rubrenophobic with increasing temperature.

The later growth stage is characterized by the formation of large spherulites [14].

Figure 5 shows AFM images obtained from different parts of a spherulite typically found after 1 hour of Rubrene deposition with a source temperature of 508 K and a sample temperature of 363 K. Three areas can be distinguished optically: a dark red center region, a lighter iris region, and a transparent matrix. Already from the red color impression we conclude that the spherulites are crystalline. The matrix, however, has turned transparent when the sample was removed from the high vacuum system because the amorphous Rubrene film is not stable against oxidation. The center (fig. 5(a)) is formed by a rough (rms-roughness: 33 nm), highly crystalline and faceted area that exhibits a slight radial orientation. The surrounding iris area (fig. 5(b,c) rms-roughness 10 nm) shows branched, strongly radial orientated structures, typical for spherulitic growth. The edge of the spherulite is separated from the amorphous matrix by a deep trench (fig. 5(c)). Furthermore, there is a significant change in height between the spherulite and the matrix region.

Both observations are indications of a large mass transport towards the spherulite since it is fed from the amorphous matrix. Fig. 5(d) finally represents the amorphous matrix (rms-roughness 0.7 nm) which is covered by many small holes of a few nanometer depth. These holes could be either due to the mentioned Figure 3: (a) Nominal film thickness f from AFM images vs. deposition time. (b) Fractal dimension of the Rubrene islands vs. deposition time. Dashed lines separate the regimes.

Figure 4: AFM cross section through Rubrene islands. The contact angle is 27° for 393 K and 22° for 363 K.

quired for coalelelelelelescscscscscscenenenenenencccccec to allow the largest possible drop volume but before changes in islandddddd shshshshshshaapapapapape might influence the contact angle. Besides the obvious size difference, the ecocococococontntntntntntaaacaaa t angle for the film grown at elevated temperature is lar- ger than the one ggggggrorororororownwnwnwnwnwn aaaaaat lower temperature. This is contrary to what would be expected at first glllllanananananancececececece, sisisisisisincnnnnn e surface energy in general decreases with increasing temperature. The obseererererervevevevevevedddd dd bbbbebb havior can be explained when considering different temperature dependenciiiiiieseseseseses ooooooffffff ssussss rface energies for Rubrene and mica(001). It seems that the surface energy ofofofofofof RRRRRRububububububrene decreases slower with increasing temperature as the one of mica. As a result the surface becomes more rubrenophobic with increasing temperature.

The later growth stage is chhhhararararararacacacacaccteteteteteterrrirrrzzzezzz d by the formation of large spherulites [14].

Figure 5 shows AFM images obtaiaiaiaiaiainenenenenened from different parts of a spherulite typically found after 1 hour of Rubrene dedededededepopopopopoposisisisisisitititititition with a source temperature of 508 K and a sample temperature oof f f 363636363636333333K.KK Thrhrhrhrhrreeeeeee aaaaaareas can be distinguished optically: a dark red center region, a lllllligigigigigighthththththtererererereriiiiiirrrirrrs regigigigigigionononononon, aaanaaa d a transparent matrix. Already from the red color impression wwwwwweeeeeecccccoc ncludetttttthahahahahahat ttttt ththththththe spherulites are crystalline. The matrix, however, has turned trrananananananspspspspspspaarararararent whhhhhheeeenee tttttthhhehhh sample was removed from the high vacuum system becauseeeeee ththththththeeeeee aaamaaa orphhhhhhououououououssssss RuRuRRRR brene film is not stable against oxidation. The center (fig. 5((((((a)a)a)a)a)a))))))) iiiisisisfffffformedd d d d dbybybybybybyaaaaaarrrrrrough (rms-roughness: 33 nm), highly crystalline and faceted arararararareaeaeaeaeaea tttttthahahahahahatttttt exhiibibibibibibitstststststs aaaaaa slight radial orientation. The surrounding iris area (fig. 5(b,b,bbbb,c)c)c)c)c)c)rrrrrrmmmmmms-roughneeeeeessssssssssss111111000000nm) shows branched, strongly radial orientated structures, typiccccccalalalalalal for spherululululululititititititiciciciciccggggggrowth. The edge of the spherulite is separated from the amorphous matrrrrrixixixixixix by aaaa aadedededededeepepepepepepttttttrerrrrr nch (fig. 5(c)). Furthermore, there is a significant change in heieieieieieighghghghghght bebebebebebetttwtwttween nn n nn ththththththe eee ee spspspspspspheheheheheherulite and the matrix region.

Both observations are indications ofofofofofofaaaaaallllllaaaaarage masasasasasassssss strtrtrtrtrtraananananansport towards the spherulite since it is fed from the amorphohohohohohouuuuuus mmmmmmataaaaarix. FFFFFFigigigigigig.5(d) finally represents the amorphous matrix (rms-roughness 000.0.00.777777nmnmnmnmnmnm) ) )) ) ) whichhhhhhiiiisii ccccccooooovo ered by many small holes of a few nanometer depth. These hohohohohoholelelelelelessssss cocococococouluuuuud bebebebebebe eeeeeeitititititithehhhhh r due to the mentioned Figure 3: (a) NoNoNoNoNoNomimimimimiminannanananallllllfiffffflm thickness f from AFM images vs. deposition time. (b) Fractalf dimension of theeeeeeRRRRRRububububububrrerererer ne islands vs. deposition time. Dashed lines separate the regimes.

Figure 4: AFM cross section through Rubrene islands. The contactaangngngngngnglelelelelele is 27° for 393 K and 22° for 363 K.

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massive mass transport but are more likely a result of the oxidation process, since they exit all over the surface.

4. Conclusions

Quantitative morphological AFM analysis of HWE growth of Rubrene on mica(001) allowed to draw the following conclusions with respect to sticking coefficient and surface energies: Material and temperature depended sticking coefficients lead to non-linear growth rates for amorphous Rubrene on mica. The sensit- ivity of Rubrene growth with respect to the growth temperature is also reflected in the observed increase of the contact angle for Rubrene on mica(001) with increasing deposition temperature.

Acknowledgments. Funding by Austrian Science Fund Projects S9707, S9706.

References

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Figure 5: Details of a Rubrene spherulite. (a) Center, (b) Iris, (c) spherulite rim, and (d) surrounding amorphous matrix. Z-scale: (a) 300 nm, (b) 50 nm, (c) 30 nm and (d) 5 nm.

The insets show the cantilever position relative to the spherulite center.

massive mass transnsnsnsnsnspopopopopoporrrtrtrr but are more likely a result of the oxidation process, since they exit all over tttttthehehehehehe ssssssururururrrfafffff ce.

4. Conclusions

Quantitative morphologgggggicicicicicicalalalalalal AAAAAAFM analysis of HWE growth of Rubrene on mica(001) allowed to draw the following conclusions with respect to sticking coefficient and surface energies: MaMaMaMaMaMateteteteteterirrrrral and temperature depended sticking coefficients lead to non-linear growwwwwwthththththhrrrrrratatatatatateeesees for amorphous Rubrene on mica. The sensit- ivity of Rubrene growth with respepepepepepecccctctc to the growth temperature is also reflected in the observed increase of theeeeee ccccconononononontatatatatatact angle for Rubrene on mica(001) with increasing deposition temppppppererature.

Acknowledgments. Funununununundidididididingngnngng by AuAuAuAuuuststststststririririririanananananan Science Fund Projects S9707, S9706.

References

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Figure 5: Detatatatatatailililililils ofofofofofof a Rubrene spherulite. (a) Center, (b) Iris, (c) spherulite rim, and (d) surrounding amorphphphphphphouououououousssss matrix. Z-scale: (a) 300 nm, (b) 50 nm, (c) 30 nm and (d) 5 nm.

The insets show ththththththeeeeee cacacacacacantnnnnnilever position relative to the spherulite center.