Analysis of a MgO Protective Layer Deposited with Ion-Beam-Assisted Deposition in an AC PDP

Transcription

1 Journal of the Korean Physical Society, Vol. 49, No. 6, December 2006, pp Analysis of a MgO Protective Layer Deposited with Ion-Beam-Assisted Deposition in an AC PDP Zhao Hui Li, Eou Sik Cho and Sang Jik Kwon College of Electronics and Electrical Engineering, Kyungwon University, Kyunggi (Received 15 September 2006, in final form 23 October 2006) MgO layer plays an important role for plasma display panels (PDPs). In this study, a MgO layer was deposited using the oxygen-ion-beam-assisted method while the assisting oxygen-ion-beam energy was varied from 100 ev to 500 ev. In order to investigate the relationship between the electron emission and the defect levels of the MgO layer, we measured the cathodoluminescence (CL) spectra of the MgO thin films, and we analyzed the CL peak intensity and peak transition. We found that the assisting ion-beam energy played an important role in the peak intensity and the peak transition of the CL spectrum. Crystallization of the MgO thin film was also measured using X-ray diffraction (XRD), and the surface quality was inspected using atomic forces microscopy (AFM) in order to analyze the characteristics of the MgO thin films. PACS numbers: d, Sm, Hj Keywords: Plasma display panel (PDP), Ion-beam-assisted deposition (IBAD), Secondary electron emission coefficient, Cathodoluminescence (CL) spectra I. INTRODUCTION these luminescences result from the following transitions: The MgO layer is a key element of AC plasma display panels (PDPs). MgO thin films have been widely used to protect dielectric layers above electrodes from sputtering while at the same time yielding a high ion-induced secondary electron emission coefficient (γ) by impinging Ne + ions under gaseous plasma. Because of its large secondary electron emission coefficient and good resistance to sputtering, the MgO layer plays an essential role in keeping the operating voltage relatively low and in limiting the damage due to energetic ions. Therefore, the MgO layer is important both for the efficiency and the lifetime. During MgO deposition, the MgO is decomposed into Mg and O atoms, and the decomposed Mg and O atoms arrive at the substrate, building up one by one, and are finally re-crystallized as MgO [1]. MgO is a typical ionic compound and has some point defects: vacancies, interstitials, impurities, and so on. The F and the F + centers are two basic point defects of MgO crystals. The F + and the F centers are, respectively, one and two electrons trapped by an O vacancy. The ground levels of the F and the F + centers are located at the 3.0 ev and 2.96 ev above the top of the valence band, respectively [2]. The luminescence from F centers is at 530 nm, and that from F + centers is at 390 nm. It is generally agreed that sjkwon@kyungwon.ac.kr; Fax: F center : 1 T 1u + e 1 A lg + hν(530 nm), F + center : F 2+ + e F + + hν(390 nm), where 1 T lu is one of the excited states of the F centers, 1 A lg is the ground state of the F centers, F 2+ can be thought of as an excited state of the F + center [3,4]. The secondary electron emission from MgO has been noted to be affected mainly by the types and the concentrations of intrinsic and extrinsic defects. The mean ion energy on the surface of a MgO film in a PDP cell is between 10 ev and 20 ev, and that of the fast ions in the tail of the distribution may be up to about 100 ev at most. From Hagstrum s two classical papers, the secondary electron emission is due to Auger emission from the metal or semiconductor surfaces. Secondary electron emission by low-energy ions depends mainly on the ion potential energy of the ions and not on their kinetic energy. Therefore, the secondary electron emission yield due to Auger emission is a combination of 1) Auger neutralization (one step), and 2) Resonance neutralization + Auger de-excitation (two steps) [5,6]. Fig. 1 demonstrates the process of Auger emission. The Auger neutralization process is shown in Fig. 1 (a). Electron 1 in the valence band, is captured and moves to the ground state of an ion. Another electron, electron

2 Analysis of a MgO Protective Layer Deposited with Zhao Hui Li et al Fig. 1. Schematic diagram of Auger ejection from the MgO film, including F and F + bands (a) Auger neutralization of an ion (b) Auger de-excitation of an excited atom. Table 1. Definition of physical parameters. ε: energy of an excited electron ε o: energy of vacuum level ε c: energy of bottom of conduction band ε v: energy of top of valence band ε f : energy of top of F band from the vacuum level ε f +: energy of top of F + band from the vacuum level ε g = ε c ε v: band gad χ = ε o ε c: electron affinity ξ = χ + ε g = ε o ε g E i: E m: ionization energy at a distance S from the solid surface excitation energy at a distance S from the solid surface 2 in the F + band, is excited at the same time by the radiated energy. Auger neutralization can always occur if E i > χ. Fig. 1 (b) shows the schematic diagram of Auger de-excitation of an excited atom at the MgO surface, which includes F and F + centers. When an ion approaches the MgO surface and resonance neutralization occurs, the ion becomes an excited atom. If electron 1 in the F + center is captured and move to the ground state of the excited atom, another electron, electron 2 in the excited atom, is excited by radiated energy. The exchange transition is termed an indirect de-excitation, and the process is indicated by the dashed lines in Fig. 1 (b). Otherwise, electron 2 fills the atom core hole form the excited atomic level. In this case, there is no charge transfer between the MgO and the excited atom, but the radiation energy of electron 2 can be given away to the MgO so that excitation of electron 1 is produced. This process is called direct de-excitation and is indicated by the dashed lines in Fig. 1 (b). The Auger de-excitation process can occur whenever E m > χ [7,8]. Table 1 shows the definitions of physical parameters. The electron escapes from the surface and becomes a secondary electron. The emission of electrons by the Auger neutralization process is non-zero when the parameter G = E i 2(ε g +χ) is positive and increases with increasing E i, and Xe, Kr, Ar, Ne, and He in this order will have increasing γ. However, γ is zero for Kr and Xe ions. The quantity H = E m (ε g + χ) is positive for all the noble gases and the secondary electron due to Auger de-excitation should be non-zero even for Xe and Kr. The γ value has been

3 Journal of the Korean Physical Society, Vol. 49, No. 6, December 2006 Table 2. Specifications of the PDP test panel. Front Panel Rear Panel ITO width 320 µm Address electrode width 200 µm ITO thickness 130 nm Dielectric layer thickness 24 µm Bus electrode width 80 µm Barrier rib height 130 µm Dielectric layer thickness 24 µm Barrier rib pitch 300 µm MgO thickness 500 nm Phosphor thickness 10 µm reported to have a strong correlation with the F and/or the F + centers of MgO [9]. Usually, the MgO thin film is prepared by using an electron beam (e-beam) evaporation method, but some problems are seen in MgO films deposited using e-beam evaporation, such as cracks, which occur in the annealing or packaging process. Ion-beam-assisted deposition is said to be able to improve the characteristics of MgO thin films [10 12]. Some papers report that ion-beam-assisted deposition (IBAD) method is appropriate for depositing MgO thin films as protective layers for AC PDPs for improving their discharge and structural characteristics [13, 14]. In this research, we studied the F and the F + centers of MgO thin films prepared by using oxygen ion-beamassisted deposition (IBAD) and analyzed their relation to the secondary electron emission coefficient. II. EXPERIMENTAL PROCEDURES First, 2-inch tested panels were prepared. Their specifications are shown in Table 1. Moreover, the MgO layer was deposited using an IBAD method that used oxygen assisting ions. The feeding oxygen (99.99 %) was fixed at 10 sccm, and the oxygen ions (O + ) irradiation was carried out using an RF-type ion source during deposition. The power of the oxygen-ion-beam was 200 Watt. The IBAD chamber was evacuated to a base pressure of Torr. The working pressure was Torr. The deposition temperature was 300 C, and the deposition rate was 5 Å/s. The O + beam energies were varied from 100 ev to 500 ev. At the same time, one sample (no IBAD) was prepared by using only e-beam evaporation without ion beam irradiation for comparisons as a reference. Annealing was done at 300 C for 1 hour. In this experiment, the cathodoluminescence (CL) spectra of MgO thin films were measured in order to analyze the F and the F + centers. The acceleration voltage of the electron beam was 3 kv, and the scanned area was about 12 µm 10 µm. The firing voltage (V f ) was measured using a current probe (Tektronit, TCPA300) and an oscilloscope (TDS-540C), and the wall charge was measured. X-ray diffraction (XRD) and atomic force microscopy (AFM) were employed to investigate the crystal orientation and the surface features of the MgO thin films, respectively. Fig. 2. Firing voltage and gamma of MgO thin films as functions of the assisting ion-beam energy (acceleration voltage). III. RESULTS AND DISCUSSION In order to measure the secondary electron yield of the MgO protective layer, there were two different approaches: ion beam measurements (e.g. FIB system) and estimates γ from firing voltage measurements (Paschen s law). In this experiment, we measured V f and calculated the γ values for Ne + according to Paschen s law: V f = Bpd ln[apd/ ln(1 + 1/γ)], (1) where A and B are constants, p is the discharging gas pressure, d is the gap length between the X and the Y electrodes, and γ is the secondary electron emission coefficient. Here, p = 400 Torr, d = 80 µm, A = 4 cm 1 Torr 1, and B = 100 V/cm. However, the A and B values are due to the Ne gas. The secondary electron emission decreases for rare gases in the order He, Ne, Ar, Kr, and Xe, and the secondary electron emission coefficient through Auger neutralization on MgO for Xe is zero. As the results of Fig. 2 shown, the γ value increases as the assisting ion-beam energy increases from 100 ev to 300 ev. However, when the assisting ion-beam energy is over 300 ev, the γ value decreases with increasing assisting ion-beam energy. The minimum V f and the maximum γ were observed when the assisting

4 Analysis of a MgO Protective Layer Deposited with Zhao Hui Li et al Fig. 3. AFM images of the MgO thin films for various assisting oxygen-ion-beam energies: (a) no IBAD, (b) IBAD 100 ev, (c) IBAD 200 ev, (d) IBAD 300 ev, (e) IBAD 400 ev, and (f) IBAD 500 ev. ion-beam energy was around 300 ev. We can see that the assisting ion-beam energy plays an important role in the secondary electron emission yield. Fig. 3 shows AFM images of the MgO thin film as a function of the assisting oxygen-ion-beam energy. Fine needle-shaped grain growth occurs on the surface of the substrate that was prepared at an assisting ion-beam energy of 300 ev, as shown in Fig. 3(d). However, with increasing assisting ion-beam energy, a mountain-shaped crystallite structure comes out as shown in Figs. 3(e) and (f) as a result of aggregation of grains. The surface roughness is shown in Fig. 4. The largest surface roughness (Ra = nm) is obtained for an assisting ion-beam energy of 300 ev. Park et al. reported the strongest correlations between discharge voltages and the roughness of MgO surface [15]. From Fig. 2 and Fig. 4, we also observed a similar correlation. For the MgO thin film deposited at a 300- ev assisting energy, the surface roughness was largest, the discharge voltage was minimum, and the γ was maximum. In addition, we also measured the CL spectra of MgO thin film and found that the F and the F + centers depended on the assisting ion-beam energy. We also investigated the relationship between the CL spectra and the γ values. The densities of the F and the F + centers for MgO are proportional to the CL intensities from F and F + centers. Figure 5 shows the CL spectra of the MgO films deposited with the IBAD method. As Fig. 5 shown, the CL spectra of MgO deposited with IBAD are obviously lower than that of the no IBAD, and the CL intensity decreases with increasing assisting ion-beam

5 Journal of the Korean Physical Society, Vol. 49, No. 6, December 2006 Fig. 4. Surface roughness of the MgO films as a function of the assisting oxygen-ion-beam energy. Fig. 6. Wall charges on the MgO layers deposited with different oxygen-ion-beam energies. Fig. 5. CL spectra of the MgO films for various assisting ion-beam energies. energy. However, the peaks of the F and the F + centers cannot be separated from the CL spectra, the transition and the intensities of the F and the F + centers are observed from Fig. 5. The peaks of the CL spectra of no IBAD and 100-eV IBAD are nearly located at 400 nm, and the CL intensity of 100-eV IBAD is lower than that of no IBAD. When the assisting ion-beam energy further increases to 400 ev, the peaks of the CL spectra move to 530 nm. From 200 ev to 300 ev, the peak intensity of the CL spectrum increases with increasing assisting ionbeam energy, but its intensity decreases with increasing assisting ion-beam energy from 300 ev to 400 ev. Compared with the 200-eV IBAD, the 300-eV IBAD MgO has a little higher CL spectral peak. The CL spectral peak almost disappears for an assisting ion-beam energy of 500 ev. Therefore, we can say that the assisting ionbeam energy plays an important role in creating F and F + center and in the concentrations of F and F + centers. From Fig. 2 and Fig. 5, we can observe the relationship between the secondary electron emission yield and the F/F + centers. As a result, the γ value increases as the CL intensity of the F center increases. The lower γ values are obtained when the peaks of the F and the F + centers disappears. The wall voltage plays an important role in lowering the sustaining voltage through the wall charges accumulated on the dielectric surface, so we measured the wall charges by using the charge-voltage (Q-V) Lissajous analysis method [16,17]. From Fig. 6, the maximum wall charge was obtained at a 300-eV assisting ion-beam energy. The wall charge decreases with increasing ion-beam energy when the assisting ion-beam energy exceeds 400 ev. We think that the wall charge and the surface roughness may have a strong correlation because the tendency of curve in Fig. 4 is similar to that of Fig. 6. In the experiment, the maximum γ value and the most wall charge were obtained for the sample deposited with an assisting ion-beam energy of 300 ev. IV. CONCLUSIONS In this reseach, MgO films were deposited using the oxygen-ion-beam assisted deposition method. We cal-

6 Analysis of a MgO Protective Layer Deposited with Zhao Hui Li et al culated the γ values according to Paschen s law and obtained the maximum γ values at 300 ev. From the results of the CL spectra, the concentrations of F and F + decreased as the O + assisting ion-beam energy increased. The higher the O + ion-beam energy is, the more O + ions arrive at the substrate during deposition, and the fewer the F/F + centers generated. Therefore, the CL spectra of the MgO film deposited with O + IBAD are much lower than that of the film deposited with no IBAD. In addition, the γ value is proportional to the intensity of F centers. Not only the secondary electron emission coefficient but also the wall charge has a strong correlation with the surface roughness. For higher surface roughness, the γ value was higher, and the wall charge was larger, as well. We think that these results will be useful for investigating the mechanism of discharge in a PDP in more detail. ACKNOWLEDGMENTS This research was supported by a Korea Research Foundation Grant funded by the Korean Government (Ministry of Education & Human Resources Development, MOEHRD) (KRF D0032) and by the Brain Korean 21 Project. REFERENCES [1] T. Hirakawa, S. Goto and H. Uchiike, SID 03 Proceeding, 896 (2003). [2] R. I. Eglitis, M. M. Kuklja, E. A. Kotomin, A. Stashans and A. I. Popov, Comp. Mate. Sci. 5, 298 (1996). [3] G. P. Summers, T. M. Wilson, B. T. Jeffries, H. T. Toher, Y. Chen and M. M. Abraham, Phys. Rev. B 27, 1283 (1983). [4] G. H. Rosenblatt, M. W. Rowe, G. P. Williams, Jr., R. T. Williams and Y. Chen, Phys. Rev. B 39, (1989). [5] H. D. Hagstrum, Phys. Rev. 96, 336 (1954). [6] H. D. Hagstrum, Phys. Rev. 22, 83 (1960). [7] Y. Motoyama and F. Sato, IEEE Trans. Plasma Sci. 34, 336 (2006). [8] M. A. Cazalilla, N. Lorente, R. Díez. Mui no. J.-P. Gauyacp, D. Teillet-Billy and P. M. Echenique, Phys. Rev. B 58, (1998). [9] K. H. Park and Y. S. Kim, IMID/IDMC 06 Digest, 375 (2006). [10] K. Oumi, H. Matsumoto, K. Kashiwagi and Y. Murayama, Surf. Coatings Tech , 562 (2003). [11] Z. N. Yu, J. W. Seo, S. J. Yu, D. X. Zhang and J. Sun, Surf. Coatings Tech. 162, 11 (2002). [12] J. K. Kim, E. S. Lee, D. H. Kim and D. G. Kim, Thin Solid Films , 95 (2004). [13] A. I. Ektessabi, H. Nomura, N. Yasui and Y. Tsukuda, Thin Solid Films , 383 (2004). [14] N. Yasui, H. Nomura and A. I. Eetessabi, Thin Solid Films , 377 (2004). [15] C. H. Park, Y. K. Kim, S. H. Lee, W. G. Lee and Y. M. Sung, Thin Solid Films 366, 88 (2000). [16] E. H. Choi, T. S. Cho, D. S. Cho, M. C. Choi, J. G. Kim J. Y. Lim, Y. Jung, J. C. Ahn, T. Y. Kim, S. S. Kim, M. W. Chong, S. H. Choi, Y. G. Kim, J. J. Ko, D. I. Kim, C. W. Lee, Y. H. Seo, G. Cho and H. S. Uhm, Jpn. J. Appl. Phys. 38, 6073 (1999). [17] T. S. Cho, J. J. Ko, D. I. Kim, C. W. Lee, G. Cho and E. H. Choi, Jpn. J. Appl. Phys. 39, 4176 (2000).