Study of ion bombardment effect for Alq3 films

Applied Surface Science 252 (2006) 6375–6378 www.elsevier.com/locate/apsusc

Study of ion bombardment effect for Alq3 films Yu-Hung Cheng a,*, Kang-Yi Lin b, Yang-Che Hung b, Chien-Hong Cheng c, Kao-Chih Syao a,b, Ming-Chang Lee b a

Department of Electrical Engineering, National Tsing Hua University, Institution of Electrical Engineering 101, Section 2 Kuang Fu road, Hsinchu 300, Taiwan b Institute of Photonics Technologies, National Tsing Hua University, Taiwan c Department of Chemistry, National Tsing Hua University, Taiwan Received 5 September 2005; accepted 20 November 2005 Available online 18 January 2006

Abstract This paper investigated the blue shift of photoluminescence and the changes of surface morphology of Alq3 films by ionic argon plasma bombardment. Plasma with different conditions was applied to bombard thin Alq3 films, modifying both the physical and chemical properties of the films. After characterizing Alq3 films treated with different RF power by XPS, PL and AFM, we proposed the mechanisms to explain the absence of PL blue shift and chemical shift after the films were exposed in the air for more than 3 h. Experimental results showed that molecular structure damages would affect the bandgap of Alq3, leading to the blue shift effect. XPS results also showed that binding energy shifts are caused by enriched oxygen covalent bonds formed inside the films after plasma treatment. Also, surface roughness improves as RF power is increased. # 2005 Elsevier B.V. All rights reserved. Keywords: Blue shift; Binding energy; Ion bombardment

1. Introduction Organic light-emitting devices (OLEDs) have attracted extensive attention due to their superior performance of high brightness, low power consumption, wide viewing angle, fast response time and potentially low cost. Since Jyh-Jier Ho [1] reported a low-voltage drive electroluminescent (EL) device using tri-(8-hydroxyquinnoline) aluminum (Alq3) as both the emitting material and electron transport material that emits strong electroluminescence in the green spectrum, Alq3 has been one of the most widely used materials as an emitting layer for OLEDs due to its excellent stability and luminescent properties. The plasma surface treatment process was widely used on organic light emitting diode to modify the work function. High power plasma ion bombardment often breaks chemical bonds and modifies surface bonds [1,2]. Furthermore, plasma was used to improve the injection efficiency from electrodes to transport layers [3,4]. Since the performance of any thin films

* Corresponding author. Tel.: +886 3 5715131 4127. E-mail address: [email protected] (Y.-H. Cheng). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.11.080

should depend strongly on the quality of the thin films, it is important to explore other surface treatment techniques for optimizing the properties of organic thin films. However, plasma treatment for the light-emitting layer has not been investigated in the past. After external excitation, the electrons of atoms gain enough energy and jump from the highest occupied molecular orbit (HOMO) to the lowest unoccupied molecular orbit (LUMO). The energy separation between HOMO and LUMO is determined by the material bonding structure. Therefore, observations of PL shift indicate that the material bonding structure was altered after plasma bombardment. We designed a series of experiments and characterizations to examine the influences imposed by argon (Ar) plasma treatment on Alq3 surfaces. 2. Experiments Before characterizing the impact of plasma treatment of tris(8-hydro-quinoline) aluminum films, we deposited 100 nm Alq3 films on silicon wafers by thermal evaporation technique. The chamber pressure was hold at 10 6 Torr while deposition ˚ /s. After 20 min of deposition, the samples rate was set at 1 A were removed from the evaporator and then loaded into a

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Spectral meter (Hitachi F-4500) was used to measure the photoluminescence (PL) spectrum before and after plasma treatment. Following the treatment, we conclude how binding energy spectrum and composition are changed from the results of X-ray photoelectron spectroscopy (XPS) under different plasma conditions. The correlation between the measurements from PL and XPS was therefore established. Surface roughness was characterized by atomic force microscope (AFM) to explain the improvement of surface morphology after increasing RF power. 3. Experimental results We found blue shift phenomena on the main peaks of the photoluminescence spectrum when the applied RF power was increased to 200 W (Fig. 1). It is well-known that the self-bias increases as the substrate RF power increases. In this study, RF power, 150 and 200 W, induce self-bias of 500 and 630 V under 20 mTorr pressure, respectively [8]. From PL analyses, as shown in Fig. 2(a–c), the 3.2 and 7.8 nm blue shifts result from 150 to 200 W treatments. The impact of plasma treatments on Alq3 surface is discussed below. To eliminate the influence that caused by the elevated temperature during the 10-min-long ionic bombardment, we actively applied cooling water around the holder to maintain the substrate at 20 8C. Blue shift of PL spectrum indicates that the molecules in plasma treatment films are highly stressed [5]. The stress is caused by breaking the chemical bonds during plasma treatment. Comparing the samples with different storage time, 0, 3 and 180 h, we found that the PL peak gradually red shifts back to where it started after long exposure in the air. It takes several hours for oxygen atoms to repair the damaged bonds and release the stress. Recently, a new route to design and synthesize small bandgap polymers has been initiated by Havinga et al., who proposed that lower bandgap polymer could be achieved by molecular design bringing electron-donating and withdrawing

Fig. 1. Photoluminescence results showed the emission spectrums of (a) asdeposition, (b) 3 h and (c) 180 h.

reactive ionic etching (RIE) system. The plasma treatment was then done by RIE under the pressure of 10 2 Torr for 10 min. The argon plasma ion bombardment modified the Alq3 surfaces with 150 and 200 W RF power at 13.56 MHz and the substrate holder was constantly kept at 20 8C.

Fig. 2. Compositional changes of Alq3 films before and after plasma treatments.

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Fig. 3. Molecular structure formula of Alq3.

groups together along the conjugated backbone [9–11]. Toussaint et al. also states that lower bandgap is a result of the strong stabilizing interaction occurring between the LUMO of conjugated skeleton associated to the polymer and the LUMO of the electron-withdrawing group [12]. In this study, Alq3 is a metal chelate composed of one metal aluminum ion (Al3+) and three 8-hydroxyquinoline in which three nitrogen atoms and three oxygen atoms of 8-hydroxyquinoline form eighthedron structure with Al3+ through coordinate bonds. In Alq3 molecular, the electron structure of Al3+ is 1S22S22P6, a metal ion structure similar to inert gas atom, so Alq3 has rather good stability in dry atmosphere. Fig. 2 shows that the surface compositions have significant variations after plasma treatment. The plasma bombarded surface was damaged by high energy Ar+ plasma, leading to the restructuring of the conjugated molecular. Because the electron energy state of LUMO in Alq3 was not in the most stable state, the recombination energy of electron-hole pair is affected by the dangling bonding atoms inside the film or on the film surface. Therefore, blue shift is resulted from the intrinsic bandgap changes of Alq3 (Fig. 3). Next, we characterized the films by XPS to analyze the compositional changes of Alq3 thin films before and after plasma treatment. As shown in Fig. 2, composition analysis revealed that after plasma bombardment, the composition of carbon and nitrogen greatly reduces, whereas the composition of oxygen and aluminum increases. Because the 600 V substrate self-bias was applied at the Alq3 films, argon ions acquire enough energy from electric field to break the molecular bonds. In this study, argon ions could acquire over 100 eV of kinetic energy to break the covalent bonds of Alq3 [6,7]. Al–O and Al–N have lower covalent bond energy, 2.01 and 2.74 eV, respectively, while C–C bond energy is 3.6 eV

Fig. 4. XPS results of C(1S) energy level before and after plasma treatments.

[15]. During plasma treatment, the Al–O and Al–N bonds are easily broken and may form gases such as CO, CO2, N2 or other organic compounds that evaporate out of the films. Such evaporations are partially responsible for the compositional reduction of carbon and nitrogen. The reduction is further enhanced by incorporating oxygen into the film. Compositional reduction of carbon can also be corroborated from the change of intensity and the shift of peak binding energy, as shown in Fig. 4 [13]. Since the oxygen atoms cannot fully compensate the missing carbon and nitrogen atoms, the composition of aluminum relatively increases. On the other hand, Fig. 4, XPS data of C1s energy level proved that the binding energy of carbon shifted to higher energy level. Binding energy change is a result of the displacement of chemical bonding or increased oxidation bonds inside the films [13,14]. Therefore, the chemical shifts followed by the O/C ratio increased as RF power was raised. Substantial increase of Al atoms, as shown in Fig. 2, confirms that Al atoms oxide in the air easily. Also, as shown in Fig. 4, we compared the XPS results before and after plasma processes. One hundred and fifty and 200 W treatments cause chemical shifts of 1.3 and 1.6 eV, respectively. The C1s binding energy increases from 286.2 to 287.8 eV after a 200 W treatment. Therefore, binding energy shifts are caused by enriched oxygen composition inside the films after plasma treatments. There are two etching mechanisms, the ion-enhanced etching and the chemical etching [16,17] in RIE process. In

Fig. 5. AFM analysis on 100 nm Alq3 films after different RF powers was applied (a) as-deposition, (b) 150 W and (c) 200 W.

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4. Conclusions

Fig. 6. Trends of changes of rms roughness and etching rate as RF power was increased.

our study, only argon gas is applied so that the chemical etching is minimized, where as the ion-enhanced etching is maximized. Based on physical bombardment of ions or atoms, plasma is used to energize a chemically inert projectile so that it moves at high velocity when it strikes the substrate. Momentum is then transferred during the collision and substrate atoms are dislodged if projectile energy exceeds bonding energy. As shown in Fig. 5(a), because of isotropic etching process surface morphology was substantially altered after using plasma etching technique. AFM analysis revealed the differences for as-deposition, 150 and 200 W samples. The surface roughness in root mean square (rms) is 3.32 nm after thermal evaporation deposition. With 150 W ion bombardment, the surface is smoothed out and the roughness is greatly reduced to 0.732 nm. With 200 and 300 W plasma treatment, the surface roughness further reduced down to 0.473 and 0.367 nm, respectively. As shown in Fig. 6, surface roughness decreases as RF power increases while etching rate increases as RF power increases. Etching rate increased to around 4.5 nm/min as RF power reached 300 W, which provided 750 V self-bias at the substrate. Lower melting point and bonding energy of Alq3 film lead to enhanced etching rate. Such high etching rate may be due to the higher power that elevates the surface temperature of Alq3 films to somewhere close to the transition temperature (Tg) of Alq3, therefore, increasing the evaporation rate of Alq3 films. However, surface roughness gradually saturated to around 0.4 nm when RF power was over 200 W.

In this study, different plasma power conditions were applied to bombard thin Alq3 films to modify the bonding energy and composition of Alq3 film. PL results show 7.8 nm blue shift after 200 W treatment, and the blue shift effect was enhanced as we raised plasma bombardment power. From the XPS results, 150 and 200 W treatments have chemical shifts of about 1.3 and 1.6 eV, respectively. After plasma treatments, enriched oxygen composition inside the films increases the binding energy and leads to such chemicals shifts. The enriched oxygen composition also manifests the disappearance of PL blue shifts after the films were exposed in air for more than 3 h. After 200 and 300 W plasma treatments, the surface roughness reduced to 0.473 and 0.367 nm, respectively. Etching rate increased to around 4.5 nm/min as RF power applied at 300 W, which provided 750 V self-bias at the substrate and the etching rate increase faster as the power was more than 200 W.

Acknowledgement The authors would like to thank professor Chien-Hong Cheng for his technical assistance.

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