Spectroscopic ellipsometry investigation of the optical anisotropy in organic thin films

Transcription

1 Spectoscopic ellipsomety investigation of the optical anisotopy in oganic thin films A. M. Paaian, G. Salvan, T. U. Kampen, M. Fiedich and D. R. T. Zahn Institut fü Physik, Technische Univesität Chemnitz, D Chemnitz, Gemany Abstact. The optical popeties of thin films of peylene deivatives gown on sulfu passivated GaAs(001) substates wee investigated ex situ in the enegy ange fom 300 nm (4.13 ev) to 1700 nm (0.73 ev) by means of spectoscopic ellipsomety. In thin films of 3,4,9,10-peylenetetacaboxylic dianhydide (PTCDA) the molecules ae aligned with thei molecula plane paallel to the substate suface. The in-plane efactive indices and extinction coefficients ae found to be lage than the espective out-of-plane optical constants. In thin films of N,N`-dimethyl-3,4,9,10- peylenetetacaboxylic diimide (DiMe-PTCDI) the molecules ae found to be oiented with the molecula plane tilted with espect to the substate suface. Fo DiMe-PTCDI films of 20 nm the oveall shape of the imaginay pat of the dielectic function is vey much alike with the absoption spectum of DiMe-PTCDI film gown on quatz. One can easily identify the tansition fom the highest occupied molecula obital (HOMO) to the lowest unoccupied molecula obital (LUMO) at 2.16 ev. The imaginay pat of the effective dielectic function shows a stonge dependence with the angle of incidence of the light compaed to PTCDA. Togethe with symmety consideations this poves a stong optical anisotopy of these oganic films. As in the case of PTCDA films, the optical constants paallel to the molecula plane ae found to be lage than the espective out-of-plane optical constants. Keywods: oganic thin films, PTCDA, DiMe-PTCDI, spectoscopic ellipsomety, optical anisotopy. Coesponding autho: Andeea M. Paaian Institution: Technische Univesität Chemnitz Depatment: Institut fü Physik Addess: D Chemnitz, Gemany Phone: Fax: andeea.paaian@physik.tu-chemnitz.de

2 1. Intoduction Thin oganic films of the achetype molecule 3,4,9,10-peylenetetacaboxylic dianhydide (PTCDA) and N,N -dimethyl-3,4,9,10-peylenetetacaboxylic diimide (DiMe-PTCDI) gown on passivated semiconducto substates i.e. sulfu passivated GaAs ae the subject of the pesent contibution. (a) (b) Figue 1. The two peylene deivatives: (a) PTCDA and (b) DiMe-PTCDI. Both oganic mateials cystallize in the monoclinic system with a C 2h symmety having two molecules in the unit cell [1]. Due to thei cystalline symmety, anisotopy in the physical popeties of these oganic films is expected [2]. PTCDA has aleady been used as mateial fo the deposition of films on vaious substates anging fom van-de-waals cystals [3] to metals such as Ag and Au [4] on which quasi-epitaxial gowth is obseved with the molecules lying almost flat and molecula plane being paallel to the substate suface. It has ecently been demonstated that the gowth of oganic molecules on modified semiconducto substates (e.g. Se-passivated GaAs(001) [5]) is consideably impoved with espect to the gowth on the coesponding bae substate. Adding the imide goups instead of the anhydide ones dastically modifies the gowth mode of the oganic film. It has been obseved that fo DiMe-PTCDI thin films gown on KCl sufaces the molecula planes ae stacked paallel to the substate [6] wheeas the films gown on passivated semiconducto substates have the molecula planes tilted with at an angle highe than 45 with espect to the substate suface [7]. As it will be shown hee the oientation of the oganic cystallites has a deep impact on the optical popeties of the films. In this wok spectoscopic ellipsomety is employed fo the investigation of optical anisotopy of PTCDA and DiMe-PTCDI thin films gown on sulfu passivated GaAs and the advantages of genealized ellipsomety fomalism ae used fo a quantitative analysis of the expeimental esults. 2. Expeimental All the samples being subject of this study wee pepaed using n-type GaAs(001) wafe pieces fom Feibege Compound Mateials as substate. The fist step in the sample pepaation consists of degeasing the GaAs substate using acetone, ethanol and de-ionized wate in a ultasonic bath fo 5 minutes each followed by dying of the substates in a dy nitogen flux. The next step, the sulfu passivation, consists of dipping the substates into a mixed solution of S 2 Cl 2 and CCl 4 (1:3) fo 10 seconds, the substate is insed subsequently in CCl 4, acetone, ethanol and de-ionized wate. The esult of the sulfu teatment is a passivation of the suface with monolaye thick sulfides [8]. Immediately afte the chemical teatment the substates ae 2

3 tansfeed into the UHV system whee annealing at 450 C takes place leading to a 2x1 econstucted, sulfu teminated GaAs(001) suface, fom now on efeed to as S-GaAs(001). The oganic mateial, i.e. PTCDA and DiMe-PTCDI puified by double themal gadient sublimation is evapoated by means of Oganic Molecula Beam Deposition onto the sulfu passivated substates. Duing the oganic mateial evapoation the substates ae kept at oom tempeatue. The evapoation ate as monitoed with the quatz micobalance was ~ 0.3 nm fo both mateials the esulting samples having thicknesses 120 nm fo the PTCDA film and 20 nm, 120 nm espectively in the case of DiMe-PTCDI film. The ellipsomety measuements wee pefomed ex situ using a commecially available vaiable angle spectoscopic ellipsomete (WVASE fom J. A. Woollam Co., Inc.) equipped with vetical sample mounting stage, continuously otating analyse, autoetade and a Xe lamp souce. The vetical sample mounting stage is a high pecision sample otato allowing us to measue multiple azimuthal oientations. The angles of incidence of light used hee ae 20, 45 and 70. The measued ellipsometic data ae expessed in tems of the effective dielectic function[9]: ρ < ε>=sin Φ+sin Φtan Φ (1) 1-ρ p i whee ρ= tan( Ψ )e (2) s is the complex eflectance atio, Φ is the angle of incidence of light, Ψ and ae socalled ellipsometic angles, and p and s ae the Fesnel coefficients fo light polaized paallel and pependicula to the plane of incidence, espectively Genealized ellipsomety Genealized Ellipsomety (GE), intoduced by Azzam and Bashaa [10], is a vey useful appoach fo chaacteization of the optical and stuctual popeties of anisotopic layeed mateials. This technique povides nomalized Jones eflection matix elements, which in the case of an anisotopic system can be expesed as: p p E pp ps E = i (3) s s E sp ss Ei whee i and indices efe to the light incident onto and eflected fom the sample, espectively. Fo isotopic mateials the off-diagonal tems vanish and the elationship between Ψand and the mateial dielectic function is given by equation (2). In geneal howeve, incident and eflected polaization states ae elated by the following expessions: pp i pp = tan( Ψ pp )e, ss ps pp sp ss ps = tan( Ψ )e, ps = tan( Ψ )e sp i i sp (4) 3

4 Fo optical investigations on anisotopic samples it is necessay to specify the in-plane oientation of the sample with espect to the plane of incidence which is done using the Eule angle fomalism. The Eule angles (ϕ, θ and ψ) used by the WVASE package [11] coespond to otations aounf the z-axis, then aound the new x-axis, and then again aound the new z-axis. Figue 2. Definition of the Eule angles ϕ, θ and ψ; (x, y, z ) and (x, y, z) efe to the oganic films axes, and substate coodinate systems, espectively. 3. Results and discussion The anisotopic optical behaviou of thin films of PTCDA gown on sulfu passivated GaAs(001) was thooughly discussed elsewhee [12, 13]. Hee evidence fo the optical anisotopy of such films is povided by new esults obtained with the help of genealized ellipsomety measuements. The left-hand side panel of fig. 3 shows the esults of an genealized ellipsomety scan of a 120 nm thick PTCDA film at an angle of incidence of light equal to 70. As mentioned in the subsection above the measued values ae not longe expessed in tems of the equation (2) but athe by the moe complicated elations (eq. (3)). One can clealy obseve that the change of state of polaization fom p to s (Ψ ps ) and also fom s to p (Ψ sp ) gives quite an impotant contibution to the measued spectum thus giving stong evidence fo the anisotopic chaacte since fo an isotopic mateial one should expect null values fo Ψ ps and Ψ sp. In the ight-hand side panel the imaginay pat of the effective dielectic function fo the same angle of incidence, 70, as a function of the sample azimuthal oientation is shown. Hee, 0 and 90 signifies that the plane of incidence contains [110] and [-110] diections of the substate, espectively, and 45 means an azimuthal oientation of the sample such that the plane of incidence makes a 45 angle with any of the substate axes. Although the azimuthal dependence of Im<ε> is not vey well visible the in-plane optical anisotopy of PTCDA films deposited on sulfu passivated GaAs substates is suppoted also by eflectance anisotopy spectoscopy esults [13]. 4

5 Figue.3. Results of genealized ellipsomety measuements of 120 nm PTCDA film gown on S-GaAs(001) substate. Fo thin films of DiMe-PTCDI the oveall shape of the imaginay pat of the dielectic function is vey simila with the absoption spectum of a DiMe-PTCDI film gown on quatz as it can be seen in fig. 4. Enegy positions of the optical tansitions wee detemined by cuve fitting of the absoption spectum. In the spectum of imaginay pat of the effective dielectic function the optical tansitions of DiMe-PTCDI ae vey well esolved such as the tansition fom the highest unoccupied molecula obital (HOMO) to the lowest occupied molecula obital (LUMO) located aound 2.16 ev. The highe obseved tansitions ae also in a vey good ageement with those in the absoption spectum maked by dashed-dotted line. Figue 5 shows the azimuthal dependence of Im<ε> fo a 120 nm DiMe-PTCDI film which compaed to the case of PTCDA has changed damatically. Figue 4. Imaginay pat of the effective dielectic function fo a 20 nm DiMe-PTCDI film on sulfu passivated GaAs(001) (left) and absoption spectum of 50 nm DiMe- PTCDI film gown on quatz substate (ight). 5

6 Figue 5. Azimuthal dependence of Im<ε> fo 120nm DiMe-PTCDI film on S- GaAs(001); 0, 45 and 90 (legend) ae the same as in fig. 3. When the plane of incidence contains the [-110] diection of the substate the imaginay pat of the dielectic function is dominated by a stong featue aound 3.1 ev. This featue can be attibuted to E citical point of GaAs and it is stong enhanced due to intefeence effects. Fo quantitative analysis of the genealized ellipsomety esults fo thicke (~120 nm) DiMe-PTCDI films a fitting pocedue was constucted as follows: a GaAs laye with tabulated optical constants was used as substate while fo descibing the DiMe-PTCDI laye a so-called biaxial laye was employed taking into consideation the anisotopic natue of the oganic laye. This means that diffeent optical constants with espect to all thee catesian axes ae consideed, along with the oientation of the optical axis of the film which is descibed by the tilting angle θ in espect to the z axes (nomal to the film plane). In the spectal egion below 2 ev, whee the oganic film is tanspaent, the dispesion of the efactive indices along the x, y and z diections was descibed by a Cauchy elation and a fit was pefomed fo detemining the laye thickness, θ angle and the Cauchy paametes. The fit esults ae 122 nm ± 1 nm fo the laye thickness and 51 ±2 fo θ. The value obtained fo the θ angle is in good ageement with those obtained fom the analysis of infaed and NEXAFS specta which clealy show that the oientation of the molecules with espect to the substate suface is neithe paallel no pependicula to the GaAs suface. The optical constants of the oganic film along the x, y and z diections ove the entie measued spectal egion ae descibed by oscillatos (Gaussians) having enegy positions coesponding to the optical tansition enegies obseved in the absoption spectum. The obtained optical constants ae displayed in fig. 6 (a) and (b). 6

7 Figue 6. Refactive index (a) and extinction coefficient (b) of DiMe-PTCDI film as detemined by fitting the genealized ellipsomety data. 4. Conclusion Damatic changes ae obseved in the ellipsomety specta when compaing thin films of two peylene deivatives i.e. PTCDA and DiMe-PTCDI. The fome is known to lie almost flat with espect to the substate (GaAs) wheeas the late is poved to have an tilt angle with espect to the substate suface when the films ae gown on S-GaAs(001) substates. The oientation of the optical axis fo the DiMe-PTCDI film has been detemined by fitting the ellipsomety (a) data and the obtained value is in ageement with the esults obtained by othe optical and electon spectoscopy techniques. 7

8 Acknowledgements The authos gatefully acknowledge the financial suppot povided by the EU DIODE netwok (HPRN-CT ). We thank D. T. Wagne (LOT-Oiel GmbH, Damstadt, Gemany) fo useful discussions. Refeences [1] Hädicke E. and Gase F Acta Cyst. C [2] Zang D. Y., So F. F. and Foest S. R Appl. Phys. Lett. 59 (7) 823 [3] Buows P. E., Zhang Y., Haskal E. I. and Foest S. R Appl. Phys. Lett. 61 (20) 2417 [4] Glöckle K., Seidel C., Soukopp A., Sokolowski M., Umbach E, Böhinge M., Bendt R.and Schneide W. D Suf. Sci [5] Kendick C. and Kahn A Appl. Suf. Sci. 123/ [6] Yanagi H, Toda Y. and Noguchi T Jpn. J. Appl. Phys [7] Kampen T. U., Salvan G., Paaian A., Himcinschi C., Kobitski A. Yu., Fiedich M. and Zahn D. R. T Appl. Suf. Sci submitted [8] Li Z. S., Cai W. Z., Su R. Z., Dong G. S., Huang D. M., Ding X. M., Hou X. Y. and Wang Xun 1994 Appl. Phys. Lett. 64 (25) 3425 [9] Azzam R. M. A. and Bashaa N. M Ellipsomety and Polaized Light (Noth-Holland Amstedam) [10] Azzam R. M. A. and Bashaa N. M J. Opt. Soc. Am [11] J. A. Woollam Co., Inc. Guide to Using WVASE32 [12] Kampen T. U., Paaian A. M., Rossow U., Pak S., Salvan G., Wagne Th., Fiedich M. and Zahn D. R. T Phys. Stat. Sol. (a) 188 (4) 1307 [13] Paaian A. M., Rossow U., Pak S., Salvan G., Fiedich M., Kampen T. U. and Zahn D. R. T J. Vac. Sci. Technol. B 19 (4)