- Email: [email protected]
Characterization of organic thin films using transmission electron microscopy and Fourier Transform Infra Red spectroscopy
Thin Solid Films 517 (2009) 5825–5829
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Characterization of organic thin films using transmission electron microscopy and Fourier Transform Infra Red spectroscopy Unnat S. Bhansali, M.A. Quevedo Lopez, Huiping Jia, H.N. Alshareef, DongKyu Cha, M.J. Kim, Bruce E. Gnade ⁎ Department of Materials Science and Engineering, Erik Jonsson School of Engineering and Computer Science, University of Texas at Dallas, Richardson, Texas 75080, USA
a r t i c l e
i n f o
Article history: Received 17 October 2008 Received in revised form 26 February 2009 Accepted 3 March 2009 Available online 10 March 2009 Keywords: Organic films Characterization Transmission Electron Microscopy (TEM) Fourier Transform InfraRed Spectroscopy (FTIR)
a b s t r a c t Organic Light Emitting Diodes (OLEDs) have received much attention for use in display and solid-state lighting applications. Consequently, evaluating materials analyses techniques to better understand potential issues between the different films constituting the OLED device structure becomes important. In particular, film thickness monitoring and control is essential for reproducible and reliable OLED performance. Typically, Quartz Crystal Microbalances (QCMs) are used to monitor the thicknesses in-situ. While QCMs can provide thickness information, they do not provide information about the composition or quality of the deposited films. To overcome these issues, in this paper, we have used Fourier Transform InfraRed Spectroscopy (FT-IR) to measure film thicknesses and compositions in individual as well as stacked organic layers relevant to OLED structures and used cross-sectional Transmission Electron Microscopy imaging to correlate the physical thickness of the organic films to their IR (infrared) absorption peak intensities from FT-IR. We demonstrate that this technique can be used to precisely measure film thicknesses within 5% of the nominal thickness and provide information about film composition. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Although electroluminescence in organic crystals was observed in the early 1960s [1–3], it was the work of Tang and Van Slyke [4] that triggered research in the area of organic electroluminescence. Simple device architectures and efficient Organic Light Emitting Diodes (OLEDs) based on small molecules often consist of an emissive layer sandwiched between a hole transport layer and an electron transport layer [5–7], which are organic-based. Hole and electron injection layers and/or exciton blocking layers have also been used in more complex device structures to enhance efficiency and optimize the charge-balance ratio [8–10]. N, N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′biphenyl 4,4′-diamine (NPB) is often used as a stable and an efficient hole transport molecule, since Van Slyke et al. showed improved stability over other aromatic diamines [11]. Tris (8-hydroxyquinoline) aluminum (Alq3) has been the most commonly used molecule for electron transport and emission in simple bilayer OLEDs [12]. For performance, reproducibility and reliability of OLEDs, it is critical that the thicknesses and compositions of the constituent organic layers be carefully controlled. Little effort has been invested in the characterization of the organic thin films constituting these devices. Normally, Quartz Crystal Microbalances (QCMs) are used to monitor film thicknesses. These devices measure the frequency shift of a quartz crystal resonator, which is directly proportional to the mass
deposited on the sample. These devices are easy to install and use. When QCMs are used for depositing thick films or multiple layers, the mass accumulated on the crystal resonator can result in erroneous readings, necessitating periodic calibration of the QCMs. Recently, structural and morphological characterization of Alq3 using Fourier Transform InfraRed (FT-IR) and Scanning Tunneling Microscopy was reported by Gavrilko et al. [13,14], whereas optical characterization using spectroscopic ellipsometry was demonstrated by Celii et al. [15]. IR band assignments for NPB, the hole transporter, have been reported [16]. However, none of these reports demonstrated a quantitative determination of the thickness of any of the molecular layers. SEM (Secondary Electron Microscopy) and TEM (Transmission Electron Microscopy) have been used for detailed defect analysis and thickness measurement [17]. However, there is no reliable characterization technique that can be used for routine thickness and composition verification of the organic layers forming the OLED stack. Tighter process control will result in more consistent device performance. In this paper we demonstrate that FT-IR can be used to carefully monitor film thicknesses and compositions. We calibrate our results using a dual beam Focused Ion Beam — Transmission Electron Microscopy (FIB-TEM) technique to measure organic film thicknesses on silicon substrates. While we demonstrate the technique for NPB and Alq3, it should be applicable for other molecules. 2. Experimental details
⁎ Corresponding author. E-mail address: [email protected] (B.E. Gnade). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.002
OLEDs were fabricated using NPB and Alq3 from H. W. Sands Corp without further purification. Thin films of NPB and Alq3 with different
5826
U.S. Bhansali et al. / Thin Solid Films 517 (2009) 5825–5829
thicknesses were deposited in a thermal evaporator (Cooke Vacuum Systems Inc) at a base pressure of b2 × 10− 4 Pa on pre-cleaned double-side polished silicon substrates for TEM and FT-IR studies. To determine the physical thickness of the organic films, crosssectional TEM samples were prepared using a focused ion beam milling technique. In this approach, a thin layer of Au is evaporated on the organic film to avoid sample charging in the TEM. A FEI-Nova NanoLab 200 dual column SEM/FIB system is used to prepare the cross-sectional samples for TEM imaging. To prevent degradation of the organic material from the ion beam during the ion milling process, an approximately 1.5 μm thick sacrificial layer of Pt is deposited in-situ on top of the Au. A 0.5 μm deep trench is created on both sides of the region of interest with the ion beam. The sample is then attached to a copper grid in the FIB using a Zyvex S-100 nanomanipulator for further thinning. The sample is carefully thinned down to b80 nm using the FIB. FT-IR spectra were measured using a Nicolet 4700 FT-IR from Thermoelectron Corp. equipped with a KBr beamsplitter and a Liquid N2 cooled Mercury–Cadmium–Telluride (MCT) detector with a resolution of 4 cm− 1. For all the samples, the IR signal is collected in the transmission mode over a spectral range of 650 cm− 1–4000 cm− 1 for 500 scans. A double side polished silicon wafer is used as the background. Sample and background spectra were obtained in the single beam configuration and converted to absorbance using the built-in software application of the FT-IR apparatus using the relationship: Absorbance = − log10
B A
ð1Þ
where, B = Sample spectrum and A = Background spectrum. We use the OMNIC software package to calculate the areas under the selected peaks. The area that is below the selected peak and above the baseline is defined as the corrected peak area. We use these corrected areas to determine the film thicknesses. 3. Results and discussion The molecular structures of Alq3 and NPB are shown in Fig. 1a and b, respectively. Structurally, NPB has terminal phenyl amines and naphthyl groups connected by a biphenyl bridging group. Because of the various structural conformations of naphthyl groups there are 228
Fig. 2. FT-IR absorption spectra ranging from 650 cm− 1 to 1700 cm− 1and the corresponding cross-sectional TEM images (insets) of thermally evaporated thin organic films on a silicon substrate (a) 33 nm Alq3 (b) 39 nm NPB.
Fig. 1. Structures of organic molecules (a) Alq3 (b) NPB.
vibrational degrees of freedom [16], suggesting that the material has a rather complicated FT-IR spectrum. In the molecular structure of Alq3, the three hydroxyquinoline ligands are attached to the central Al atom, giving rise to over 300 vibrational degrees of freedom [13] which also suggest a complicated FT-IR spectrum. Fig. 2a and b shows IR spectra ranging from 650 to 1700 cm− 1 for a 33 nm Alq3 film and a 39 nm NPB film, respectively. In Fig. 2a, for Alq3, the band centered at 1115 cm− 1 is assigned to a CH/CCN bending plus CN stretching vibration and the peaks at 784 cm− 1 and 749 cm− 1 are attributed to the out-of-plane CH wagging vibrations. Bands at 1500 cm− 1, 1468 cm− 1, 1383 cm− 1 and 1327 cm− 1 are attributed to a CC/CN stretching plus CH bending vibration associated with the pyridyl, phenyl groups and the quinoline fragments of Alq3 [16]. In Fig. 2b, for NPB, the peak at 1592 cm− 1 is attributed to the CC stretch of the t-phenyl group. The peaks at 1492 cm− 1 and 1392 cm− 1 are attributed to the CC stretch plus CH bend plus CN stretch of the phenyl
U.S. Bhansali et al. / Thin Solid Films 517 (2009) 5825–5829
5827
Table 1 IR Absorption band assignments for NPB and Alq3. NPB
Alq3
Bands (cm− 1)
Assignment10
Bands (cm− 1)
Assignment7
697
CC torsion (t-phenyl)
749
774 1392
CH wag (naphthyl) CC stretch + CH bend + CN stretch (naphthyl) CC stretch (t-phenyl)
1115 1327
Out-of-plane CH wag (quinoline) CH/CCN bend + CN stretch CC/CN stretch + CH bend (quinoline) CC/CN stretch + CH bend (quinoline)
1592
1383
and naphthyl group, respectively. The bands recorded at 774 cm− 1 and 697 cm− 1 are due to the CH wag in the naphthyl and CC torsion in the t-phenyl group, respectively [13]. Table 1 summarizes the in-plane (heavy atom stretches and bends) and out-of-plane (vibrational modes) IR absorption bands for NPB and Alq3 [13,16] selected for this study. The bands of NPB and Alq3 are observed in our FT-IR spectra. To verify the physical thicknesses of the deposited films, we performed cross-sectional TEM. To minimize experimental variation, the films for FT-IR and TEM measurements were deposited simultaneously. The insets in Fig. 2a and b show the corresponding cross-sectional TEM images with Au and Pt used as capping layers. The IR absorption data are in excellent agreement with previous literature reports indicating the composition of the film [16]. The TEM images show that the organic films are completely amorphous, which is favorable for efficient charge transport through the film in an OLED device [18]. Similar TEM analyses were performed on the other film thicknesses evaluated by FT-IR.
Fig. 4. Plot showing absorbance intensities for the characteristic bands in NPB and Alq3 films as a function of film thickness (a) NPB (b) Alq3.
Fig. 3a and b shows a monotonic increase in absorption intensity of the characteristic bands of NPB and Alq3 as a function of film thickness. Thicker films have higher IR absorbance, which is manifested as an increase in intensity of the typical bands, as seen in Fig. 3a and b. The areas under the selected absorbance bands in Alq3 and NPB increase almost linearly, as shown in Fig. 4a and b, respectively.
Table 2 Baseline range selection, bands and areas under the peaks with standard deviation for various NPB film thicknesses.
Fig. 3. Characteristic FT-IR absorption spectra for increasing film thicknesses of NPB and Alq3. (a) NPB — 39 nm, 70 nm and 100 nm (b) Alq3 — 33 nm, 70 nm and 100 nm.
Area under the peak — NPB film
Baseline/region (cm− 1–cm− 1)
Bands (cm− 1)
39 nm
70 nm
100 nm
709.6–682.6 779.1–767.5 1415.4–1375.9 1604.4–1579.4
697 774 1392 1592
0.0278 ± 0.0011 0.0240 ± 0.0002 0.0359 ± 0.0008 0.0488 ± 0.0003
0.0427 ± 0.0008 0.0391 ± 0.0003 0.0571 ± 0.0015 0.0744 ± 0.0005
0.0664 ± 0.0022 0.0594 ± 0.0008 0.0878 ± 0.0025 0.1100 ± 0.0035
5828
U.S. Bhansali et al. / Thin Solid Films 517 (2009) 5825–5829
Table 3 Baseline range selection, bands and areas under the peaks with standard deviation for various Alq3 film thicknesses. Area under the peak — Alq3 film
Baseline/region (cm− 1–cm− 1)
Bands (cm− 1)
33 nm
70 nm
100 nm
767.5–727.0 1130.1–1103.1 1351.8–1307.5 1409.7–1351.8
749 1115 1327 1383
0.1860 ± 0.0066 0.0941 ± 0.0026 0.1481 ± 0.0014 0.3225 ± 0.0149
0.3604 ± 0.0165 0.1858 ± 0.0060 0.2839 ± 0.0067 0.5970 ± 0.0030
0.4812 ± 0.0370 0.2476 ± 0.0159 0.3848 ± 0.0204 0.7896 ± 0.0229
This monotonic increase in the intensity with thickness indicates that the infrared signal can be used to predict film thicknesses. Tables 2 and 3 list the absorption band areas of NPB and Alq3, and their corresponding standard deviations. Each data point is an average of 5 measurements over an area of 5 cm2. The repeatability and reproducibility of the data is an indication of FT-IR's reliability for routine thickness and film quality control measurements. We note that some of the dominant peaks of NPB at 1292 cm− 1, 1392 cm− 1, 1492 cm− 1 and 1592 cm− 1 overlap with certain Alq3 features. To accurately measure film thicknesses for a multilayer stack it is necessary to select absorption regions that do not interfere with each other. Fig. 5 compares IR spectra in the region from 650 cm− 1 to 1700 cm− 1 for: a) 40 nm NPB film, b) 70 nm Alq3 film and c) a stacked NPB(40 nm)/Alq3 (70 nm) film thermally evaporated on a double-side polished silicon substrate. These thicknesses were selected because they are relevant to a real device structure. This comparison allows us to identify characteristic peaks for NPB and Alq3 that can be used to determine film thicknesses and compositions in a stacked structure. In the IR spectrum for 40 nm NPB, the absorption peaks at 697 cm− 1 and 774 cm− 1 are used as the unique, identifying peaks. These peaks are labeled as peaks (a) and (c) respectively in the IR spectra for the stacked NPB/Alq3 film (Fig. 5). We note that the overall absorption intensities are low since the NPB film is only 40 nm thick as compared to the 70 nm thick Alq3 film. Similarly for Alq3, peaks at 749 cm− 1 and 1115 cm− 1 were selected and are labeled as (b) and (d) respectively, in Fig. 5. Fig. 6 shows a comparison of the absorbance (area under the peak) of individual films and films in a stacked configuration i.e. 40 nm NPB
Fig. 6. Plot showing the absorbance intensities of NPB (39 nm) and Alq3 (70 nm) in isolated individual films (white bars) compared to the stacked structure (gray bars).
followed by 70 nm Alq3. The total thickness of the stack was determined to be 110 nm by TEM. Since the absorbance (area under the peak) of specific bands for an isolated 39 nm NPB film and a 70 nm Alq3 film are known from Tables 2 and 3, we can correlate the thickness of these films from the IR spectrum. The correlation between the absorbance (area under the peak) and the thickness of the films is within the standard deviation, which validates the use of FT-IR for determining thicknesses of organic thin films in isolated as well as multilayered structures. 4. Conclusions We have demonstrated the use of Fourier Transform InfraRed Spectroscopy as an easy, accurate and reliable characterization technique to measure thicknesses and composition of individual or stacked organic films. Using FT-IR offers multiple advantages: a) IR signal is more sensitive to thickness and hence higher accuracy as compared to a stylus profiler for calibrating the QCMs, b) an IR spectrum can indicate if the film is amorphous or crystalline and c) an IR spectrum can also give information about degradation, contamination or decomposition of the organic material being sublimed. We have also used cross-sectional TEM methods to calibrate the thicknesses of the organic films and the measurements match closely with the FT-IR intensities, indicating high accuracy. We believe FT-IR and TEM serve as good complementary techniques for measuring the thickness and composition of organic films. Acknowledgment This research was partially funded by Dept. of Energy Contract # DE-FC26-06NT42856. References [1] [2] [3] [4] [5]
Fig. 5. FT-IR absorption spectra of a 39 nm NPB film, 70 nm Alq3 film and a stacked NPB/ Alq3 film on silicon. Peaks (a), (c) and peaks (b), (d) are unique identification peaks for NPB and Alq3 respectively.
R.G. Kepler, Phys. Rev. 119 (1960) 1226. M. Pope, H.P. Kallmann, P. Magnante, J. Chem. Phys. 38 (1963) 2042. W. Helfrich, W.G. Schneider, Phys. Rev. Lett. 14 (1965) 7. C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. B.W. D Andrade, M.A. Baldo, C. Adachi, J. Brooks, M.E. Thompson, S.E. Forrest, Appl. Phys. Lett. 79 (2001) 1045. [6] H. Aziz, Z.D. Popovic, Appl. Phys. Lett. 80 (2002) 2180. [7] J.P. Duan, P.P. Sun, C.H. Cheng, Adv. Mater. 15 (2003) 224. [8] G. He, M. Pfeiffer, K. Leo, Appl. Phys. Lett. 85 (2004) 3911.
U.S. Bhansali et al. / Thin Solid Films 517 (2009) 5825–5829 [9] S.H. Kim, J. Jan, J.Y. Lee, Appl. Phys. Lett. 90 (2007) 223505. [10] J.W. Long, D. Gui-ying, W. Jin, W. Jing, W. Li-zhong, C. Xi, H. Qiang, W. Hong-mei, Z.X. Hong, Optoelectron. Lett. 4 (2008) 0026. [11] S.A. Van Slyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 15 (1996) 2160. [12] P. Lane, G.P. Kushto, Z. Kafafi, Appl. Phys. Lett. 90 (2007) 023511. [13] T. Gavrilko, R. Fedorovich, G. Dovbeshko, A. Marchenko, A. Naumovets, V. Nechytaylo, G. Puchkovska, L. Viduta, J. Baran, H. Ratajczak, J. Mol. Struct. 704 (2004) 163.
5829
[14] S.Y. Kim, S.Y. Ryu, J.M. Choi, S.J. Kang, S.P. Park, S. Im, C.N. Whang, D.S. Choi, Thin Solid Films 398-399 (2001) 78. [15] F.G. Celii, T.B. Harton, O.F. Phillips, J. Electron. Mater. 26 (1997) 4. [16] M.D. Halls, C.P. Tripp, H.B. Schlegel, Phys. Chem. Chem. Phys. 3 (2001) 2131. [17] Z.G. Li, Microsc. Microanal. 10 (Supplement S02) (2004) 1352. [18] C.Y. Kwong, A.B. Djurisic, V.A.L. Roy, P.T. Lai, W.K. Chan, Thin Solid Films 458 (2004) 281.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Characterization of organic thin films using transmission electron microscopy and Fourier Transform Infra Red spectroscopy Unnat S. Bhansali, M.A. Quevedo Lopez, Huiping Jia, H.N. Alshareef, DongKyu Cha, M.J. Kim, Bruce E. Gnade ⁎ Department of Materials Science and Engineering, Erik Jonsson School of Engineering and Computer Science, University of Texas at Dallas, Richardson, Texas 75080, USA
a r t i c l e
i n f o
Article history: Received 17 October 2008 Received in revised form 26 February 2009 Accepted 3 March 2009 Available online 10 March 2009 Keywords: Organic films Characterization Transmission Electron Microscopy (TEM) Fourier Transform InfraRed Spectroscopy (FTIR)
a b s t r a c t Organic Light Emitting Diodes (OLEDs) have received much attention for use in display and solid-state lighting applications. Consequently, evaluating materials analyses techniques to better understand potential issues between the different films constituting the OLED device structure becomes important. In particular, film thickness monitoring and control is essential for reproducible and reliable OLED performance. Typically, Quartz Crystal Microbalances (QCMs) are used to monitor the thicknesses in-situ. While QCMs can provide thickness information, they do not provide information about the composition or quality of the deposited films. To overcome these issues, in this paper, we have used Fourier Transform InfraRed Spectroscopy (FT-IR) to measure film thicknesses and compositions in individual as well as stacked organic layers relevant to OLED structures and used cross-sectional Transmission Electron Microscopy imaging to correlate the physical thickness of the organic films to their IR (infrared) absorption peak intensities from FT-IR. We demonstrate that this technique can be used to precisely measure film thicknesses within 5% of the nominal thickness and provide information about film composition. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Although electroluminescence in organic crystals was observed in the early 1960s [1–3], it was the work of Tang and Van Slyke [4] that triggered research in the area of organic electroluminescence. Simple device architectures and efficient Organic Light Emitting Diodes (OLEDs) based on small molecules often consist of an emissive layer sandwiched between a hole transport layer and an electron transport layer [5–7], which are organic-based. Hole and electron injection layers and/or exciton blocking layers have also been used in more complex device structures to enhance efficiency and optimize the charge-balance ratio [8–10]. N, N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′biphenyl 4,4′-diamine (NPB) is often used as a stable and an efficient hole transport molecule, since Van Slyke et al. showed improved stability over other aromatic diamines [11]. Tris (8-hydroxyquinoline) aluminum (Alq3) has been the most commonly used molecule for electron transport and emission in simple bilayer OLEDs [12]. For performance, reproducibility and reliability of OLEDs, it is critical that the thicknesses and compositions of the constituent organic layers be carefully controlled. Little effort has been invested in the characterization of the organic thin films constituting these devices. Normally, Quartz Crystal Microbalances (QCMs) are used to monitor film thicknesses. These devices measure the frequency shift of a quartz crystal resonator, which is directly proportional to the mass
deposited on the sample. These devices are easy to install and use. When QCMs are used for depositing thick films or multiple layers, the mass accumulated on the crystal resonator can result in erroneous readings, necessitating periodic calibration of the QCMs. Recently, structural and morphological characterization of Alq3 using Fourier Transform InfraRed (FT-IR) and Scanning Tunneling Microscopy was reported by Gavrilko et al. [13,14], whereas optical characterization using spectroscopic ellipsometry was demonstrated by Celii et al. [15]. IR band assignments for NPB, the hole transporter, have been reported [16]. However, none of these reports demonstrated a quantitative determination of the thickness of any of the molecular layers. SEM (Secondary Electron Microscopy) and TEM (Transmission Electron Microscopy) have been used for detailed defect analysis and thickness measurement [17]. However, there is no reliable characterization technique that can be used for routine thickness and composition verification of the organic layers forming the OLED stack. Tighter process control will result in more consistent device performance. In this paper we demonstrate that FT-IR can be used to carefully monitor film thicknesses and compositions. We calibrate our results using a dual beam Focused Ion Beam — Transmission Electron Microscopy (FIB-TEM) technique to measure organic film thicknesses on silicon substrates. While we demonstrate the technique for NPB and Alq3, it should be applicable for other molecules. 2. Experimental details
⁎ Corresponding author. E-mail address: [email protected] (B.E. Gnade). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.002
OLEDs were fabricated using NPB and Alq3 from H. W. Sands Corp without further purification. Thin films of NPB and Alq3 with different
5826
U.S. Bhansali et al. / Thin Solid Films 517 (2009) 5825–5829
thicknesses were deposited in a thermal evaporator (Cooke Vacuum Systems Inc) at a base pressure of b2 × 10− 4 Pa on pre-cleaned double-side polished silicon substrates for TEM and FT-IR studies. To determine the physical thickness of the organic films, crosssectional TEM samples were prepared using a focused ion beam milling technique. In this approach, a thin layer of Au is evaporated on the organic film to avoid sample charging in the TEM. A FEI-Nova NanoLab 200 dual column SEM/FIB system is used to prepare the cross-sectional samples for TEM imaging. To prevent degradation of the organic material from the ion beam during the ion milling process, an approximately 1.5 μm thick sacrificial layer of Pt is deposited in-situ on top of the Au. A 0.5 μm deep trench is created on both sides of the region of interest with the ion beam. The sample is then attached to a copper grid in the FIB using a Zyvex S-100 nanomanipulator for further thinning. The sample is carefully thinned down to b80 nm using the FIB. FT-IR spectra were measured using a Nicolet 4700 FT-IR from Thermoelectron Corp. equipped with a KBr beamsplitter and a Liquid N2 cooled Mercury–Cadmium–Telluride (MCT) detector with a resolution of 4 cm− 1. For all the samples, the IR signal is collected in the transmission mode over a spectral range of 650 cm− 1–4000 cm− 1 for 500 scans. A double side polished silicon wafer is used as the background. Sample and background spectra were obtained in the single beam configuration and converted to absorbance using the built-in software application of the FT-IR apparatus using the relationship: Absorbance = − log10
B A
ð1Þ
where, B = Sample spectrum and A = Background spectrum. We use the OMNIC software package to calculate the areas under the selected peaks. The area that is below the selected peak and above the baseline is defined as the corrected peak area. We use these corrected areas to determine the film thicknesses. 3. Results and discussion The molecular structures of Alq3 and NPB are shown in Fig. 1a and b, respectively. Structurally, NPB has terminal phenyl amines and naphthyl groups connected by a biphenyl bridging group. Because of the various structural conformations of naphthyl groups there are 228
Fig. 2. FT-IR absorption spectra ranging from 650 cm− 1 to 1700 cm− 1and the corresponding cross-sectional TEM images (insets) of thermally evaporated thin organic films on a silicon substrate (a) 33 nm Alq3 (b) 39 nm NPB.
Fig. 1. Structures of organic molecules (a) Alq3 (b) NPB.
vibrational degrees of freedom [16], suggesting that the material has a rather complicated FT-IR spectrum. In the molecular structure of Alq3, the three hydroxyquinoline ligands are attached to the central Al atom, giving rise to over 300 vibrational degrees of freedom [13] which also suggest a complicated FT-IR spectrum. Fig. 2a and b shows IR spectra ranging from 650 to 1700 cm− 1 for a 33 nm Alq3 film and a 39 nm NPB film, respectively. In Fig. 2a, for Alq3, the band centered at 1115 cm− 1 is assigned to a CH/CCN bending plus CN stretching vibration and the peaks at 784 cm− 1 and 749 cm− 1 are attributed to the out-of-plane CH wagging vibrations. Bands at 1500 cm− 1, 1468 cm− 1, 1383 cm− 1 and 1327 cm− 1 are attributed to a CC/CN stretching plus CH bending vibration associated with the pyridyl, phenyl groups and the quinoline fragments of Alq3 [16]. In Fig. 2b, for NPB, the peak at 1592 cm− 1 is attributed to the CC stretch of the t-phenyl group. The peaks at 1492 cm− 1 and 1392 cm− 1 are attributed to the CC stretch plus CH bend plus CN stretch of the phenyl
U.S. Bhansali et al. / Thin Solid Films 517 (2009) 5825–5829
5827
Table 1 IR Absorption band assignments for NPB and Alq3. NPB
Alq3
Bands (cm− 1)
Assignment10
Bands (cm− 1)
Assignment7
697
CC torsion (t-phenyl)
749
774 1392
CH wag (naphthyl) CC stretch + CH bend + CN stretch (naphthyl) CC stretch (t-phenyl)
1115 1327
Out-of-plane CH wag (quinoline) CH/CCN bend + CN stretch CC/CN stretch + CH bend (quinoline) CC/CN stretch + CH bend (quinoline)
1592
1383
and naphthyl group, respectively. The bands recorded at 774 cm− 1 and 697 cm− 1 are due to the CH wag in the naphthyl and CC torsion in the t-phenyl group, respectively [13]. Table 1 summarizes the in-plane (heavy atom stretches and bends) and out-of-plane (vibrational modes) IR absorption bands for NPB and Alq3 [13,16] selected for this study. The bands of NPB and Alq3 are observed in our FT-IR spectra. To verify the physical thicknesses of the deposited films, we performed cross-sectional TEM. To minimize experimental variation, the films for FT-IR and TEM measurements were deposited simultaneously. The insets in Fig. 2a and b show the corresponding cross-sectional TEM images with Au and Pt used as capping layers. The IR absorption data are in excellent agreement with previous literature reports indicating the composition of the film [16]. The TEM images show that the organic films are completely amorphous, which is favorable for efficient charge transport through the film in an OLED device [18]. Similar TEM analyses were performed on the other film thicknesses evaluated by FT-IR.
Fig. 4. Plot showing absorbance intensities for the characteristic bands in NPB and Alq3 films as a function of film thickness (a) NPB (b) Alq3.
Fig. 3a and b shows a monotonic increase in absorption intensity of the characteristic bands of NPB and Alq3 as a function of film thickness. Thicker films have higher IR absorbance, which is manifested as an increase in intensity of the typical bands, as seen in Fig. 3a and b. The areas under the selected absorbance bands in Alq3 and NPB increase almost linearly, as shown in Fig. 4a and b, respectively.
Table 2 Baseline range selection, bands and areas under the peaks with standard deviation for various NPB film thicknesses.
Fig. 3. Characteristic FT-IR absorption spectra for increasing film thicknesses of NPB and Alq3. (a) NPB — 39 nm, 70 nm and 100 nm (b) Alq3 — 33 nm, 70 nm and 100 nm.
Area under the peak — NPB film
Baseline/region (cm− 1–cm− 1)
Bands (cm− 1)
39 nm
70 nm
100 nm
709.6–682.6 779.1–767.5 1415.4–1375.9 1604.4–1579.4
697 774 1392 1592
0.0278 ± 0.0011 0.0240 ± 0.0002 0.0359 ± 0.0008 0.0488 ± 0.0003
0.0427 ± 0.0008 0.0391 ± 0.0003 0.0571 ± 0.0015 0.0744 ± 0.0005
0.0664 ± 0.0022 0.0594 ± 0.0008 0.0878 ± 0.0025 0.1100 ± 0.0035
5828
U.S. Bhansali et al. / Thin Solid Films 517 (2009) 5825–5829
Table 3 Baseline range selection, bands and areas under the peaks with standard deviation for various Alq3 film thicknesses. Area under the peak — Alq3 film
Baseline/region (cm− 1–cm− 1)
Bands (cm− 1)
33 nm
70 nm
100 nm
767.5–727.0 1130.1–1103.1 1351.8–1307.5 1409.7–1351.8
749 1115 1327 1383
0.1860 ± 0.0066 0.0941 ± 0.0026 0.1481 ± 0.0014 0.3225 ± 0.0149
0.3604 ± 0.0165 0.1858 ± 0.0060 0.2839 ± 0.0067 0.5970 ± 0.0030
0.4812 ± 0.0370 0.2476 ± 0.0159 0.3848 ± 0.0204 0.7896 ± 0.0229
This monotonic increase in the intensity with thickness indicates that the infrared signal can be used to predict film thicknesses. Tables 2 and 3 list the absorption band areas of NPB and Alq3, and their corresponding standard deviations. Each data point is an average of 5 measurements over an area of 5 cm2. The repeatability and reproducibility of the data is an indication of FT-IR's reliability for routine thickness and film quality control measurements. We note that some of the dominant peaks of NPB at 1292 cm− 1, 1392 cm− 1, 1492 cm− 1 and 1592 cm− 1 overlap with certain Alq3 features. To accurately measure film thicknesses for a multilayer stack it is necessary to select absorption regions that do not interfere with each other. Fig. 5 compares IR spectra in the region from 650 cm− 1 to 1700 cm− 1 for: a) 40 nm NPB film, b) 70 nm Alq3 film and c) a stacked NPB(40 nm)/Alq3 (70 nm) film thermally evaporated on a double-side polished silicon substrate. These thicknesses were selected because they are relevant to a real device structure. This comparison allows us to identify characteristic peaks for NPB and Alq3 that can be used to determine film thicknesses and compositions in a stacked structure. In the IR spectrum for 40 nm NPB, the absorption peaks at 697 cm− 1 and 774 cm− 1 are used as the unique, identifying peaks. These peaks are labeled as peaks (a) and (c) respectively in the IR spectra for the stacked NPB/Alq3 film (Fig. 5). We note that the overall absorption intensities are low since the NPB film is only 40 nm thick as compared to the 70 nm thick Alq3 film. Similarly for Alq3, peaks at 749 cm− 1 and 1115 cm− 1 were selected and are labeled as (b) and (d) respectively, in Fig. 5. Fig. 6 shows a comparison of the absorbance (area under the peak) of individual films and films in a stacked configuration i.e. 40 nm NPB
Fig. 6. Plot showing the absorbance intensities of NPB (39 nm) and Alq3 (70 nm) in isolated individual films (white bars) compared to the stacked structure (gray bars).
followed by 70 nm Alq3. The total thickness of the stack was determined to be 110 nm by TEM. Since the absorbance (area under the peak) of specific bands for an isolated 39 nm NPB film and a 70 nm Alq3 film are known from Tables 2 and 3, we can correlate the thickness of these films from the IR spectrum. The correlation between the absorbance (area under the peak) and the thickness of the films is within the standard deviation, which validates the use of FT-IR for determining thicknesses of organic thin films in isolated as well as multilayered structures. 4. Conclusions We have demonstrated the use of Fourier Transform InfraRed Spectroscopy as an easy, accurate and reliable characterization technique to measure thicknesses and composition of individual or stacked organic films. Using FT-IR offers multiple advantages: a) IR signal is more sensitive to thickness and hence higher accuracy as compared to a stylus profiler for calibrating the QCMs, b) an IR spectrum can indicate if the film is amorphous or crystalline and c) an IR spectrum can also give information about degradation, contamination or decomposition of the organic material being sublimed. We have also used cross-sectional TEM methods to calibrate the thicknesses of the organic films and the measurements match closely with the FT-IR intensities, indicating high accuracy. We believe FT-IR and TEM serve as good complementary techniques for measuring the thickness and composition of organic films. Acknowledgment This research was partially funded by Dept. of Energy Contract # DE-FC26-06NT42856. References [1] [2] [3] [4] [5]
Fig. 5. FT-IR absorption spectra of a 39 nm NPB film, 70 nm Alq3 film and a stacked NPB/ Alq3 film on silicon. Peaks (a), (c) and peaks (b), (d) are unique identification peaks for NPB and Alq3 respectively.
R.G. Kepler, Phys. Rev. 119 (1960) 1226. M. Pope, H.P. Kallmann, P. Magnante, J. Chem. Phys. 38 (1963) 2042. W. Helfrich, W.G. Schneider, Phys. Rev. Lett. 14 (1965) 7. C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 51 (1987) 913. B.W. D Andrade, M.A. Baldo, C. Adachi, J. Brooks, M.E. Thompson, S.E. Forrest, Appl. Phys. Lett. 79 (2001) 1045. [6] H. Aziz, Z.D. Popovic, Appl. Phys. Lett. 80 (2002) 2180. [7] J.P. Duan, P.P. Sun, C.H. Cheng, Adv. Mater. 15 (2003) 224. [8] G. He, M. Pfeiffer, K. Leo, Appl. Phys. Lett. 85 (2004) 3911.
U.S. Bhansali et al. / Thin Solid Films 517 (2009) 5825–5829 [9] S.H. Kim, J. Jan, J.Y. Lee, Appl. Phys. Lett. 90 (2007) 223505. [10] J.W. Long, D. Gui-ying, W. Jin, W. Jing, W. Li-zhong, C. Xi, H. Qiang, W. Hong-mei, Z.X. Hong, Optoelectron. Lett. 4 (2008) 0026. [11] S.A. Van Slyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 15 (1996) 2160. [12] P. Lane, G.P. Kushto, Z. Kafafi, Appl. Phys. Lett. 90 (2007) 023511. [13] T. Gavrilko, R. Fedorovich, G. Dovbeshko, A. Marchenko, A. Naumovets, V. Nechytaylo, G. Puchkovska, L. Viduta, J. Baran, H. Ratajczak, J. Mol. Struct. 704 (2004) 163.
5829
[14] S.Y. Kim, S.Y. Ryu, J.M. Choi, S.J. Kang, S.P. Park, S. Im, C.N. Whang, D.S. Choi, Thin Solid Films 398-399 (2001) 78. [15] F.G. Celii, T.B. Harton, O.F. Phillips, J. Electron. Mater. 26 (1997) 4. [16] M.D. Halls, C.P. Tripp, H.B. Schlegel, Phys. Chem. Chem. Phys. 3 (2001) 2131. [17] Z.G. Li, Microsc. Microanal. 10 (Supplement S02) (2004) 1352. [18] C.Y. Kwong, A.B. Djurisic, V.A.L. Roy, P.T. Lai, W.K. Chan, Thin Solid Films 458 (2004) 281.