Wall-Voltage Stability in AC-PDP Dielectric Barrier ... - IEEE Xplore

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 41, NO. 1, JANUARY 2013

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Wall-Voltage Stability in AC-PDP Dielectric Barrier Discharges Yuxiang Chen, Qing Li, Kai Hu, Wenjian Kuang, and Harm Tolner

Abstract—The wall-voltage loss in PDPs with four different types of protective layers, i.e., undoped MgO, Si-doped MgO, Sc-doped MgO, and MgCaO (15%) plasma, are compared. Using a two-electrode opposed-discharge-type PDP, the change in external panel voltage needed to get a new discharge is measured as a function of the waiting time from 5 μs to 50 ms, the number of sustain cycles (1–1024), and the sustain frequency. The results shows that the wall-voltage loss cannot only be caused by amplified exoemission but that dielectric relaxation effects might also play an important role. Index Terms—Dielectric barrier, dielectric-plasma interaction, dielectric relaxation, exoemission, low-temperature plasma, neon xenon, wall voltage.

I. I NTRODUCTION

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N ORDER to reduce the power consumption in a PDP, the undoped MgO thin-film protective layer is being replaced by mixed-oxide layers consisting of, e.g., MgO and CaO. This results in the reduction in sustain voltage. It also enables the use of much higher xenon levels in the PDP. As the efficiency of the PDP discharge is almost proportional to the xenon content, this strongly reduces the power consumption. Until recently, undoped MgO layers are used in PDPs to collect the wall charge, while pure MgO nanocrystals of about 0.2–2.0-μm size are sprayed on top of that layer to generate priming electrons and initiate the addressing discharge after a certain addressing waiting time of up to 20 ms (50 Hz). During that waiting time, the loss in wall voltage should be as low as possible to obtain a wide driving margin. The new MgCaO layers have a much lower discharge voltage than that of MgO, but we find that the loss in wall voltage is about three times higher than that of undoped MgO. In this paper, we systematically compare the wall-voltage losses of four different types of protective layers, namely, undoped MgO, Si-doped MgO, Sc-doped MgO, and MgCaO (15%), and investigate how the loss is dependent on the number

Manuscript received September 11, 2012; revised November 1, 2012; accepted November 3, 2012. Date of publication December 11, 2012; date of current version January 4, 2013. This work was supported in part by the National Natural Science Foundation of China under Grant 61271053, by the 863 Program of China under Grant 2012AA03A302, and by the 973 Program of China under Grant 2013CB328803. (Corresponding author: Q. Li.) Y. Chen, Q. Li, K. Hu, and W. Kuang are with the Display Center, Southeast University, Nanjing 210096, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]). H. Tolner is with the Display Center, Southeast University, Nanjing 210096, China, and also with Tolner Technology (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2012.2227829

Fig. 1. (a) Exploded view and (b) single-cell cross section of the SM-PDP used to measure the loss of wall voltage versus time. The metal shadow mask is kept floating.

of sustain cycles and waiting time. For the tests, we use an SM-PDP, a two-electrode type of PDP, in which the discharge occurs between two opposed MgO surfaces, as shown in Fig. 1. It contains a metal shadow mask to separate the two glass plates, coated with a 30-μm dielectric layer and a surface layer of 700-nm MgO or one of the other protective layers. Contrary to the usual surface-type sustain discharge, in the SM-PDP, both the sustain and addressing discharges occur between the two opposed MgO surfaces. In a three-electrode-type PDP, where two planar sustain electrodes are used, in principle, the loss in wall voltage can occur by conduction through the protective layer. Upon using this two-opposed-electrode structure, we observe that the additional leakage path is not present. The sustain discharge voltage of such an opposed discharge is much smaller than that of a surface-type discharge [1]. The 20% Xe panels that are used for analyzing the protective layer properties have a minimum sustain voltage of ±135 V (MgO) at 200 kHz. By using the shadow mask also as a support for the phosphor layer, as shown in Fig. 1, we do not damage the phosphor with the plasma.

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Fig. 3. Wall-voltage change Vf (tw ) for a 20% Xe panel with undoped MgO after being excited by a different number of sustain cycles from 1 to 1024 at 200 kHz, Vsus = ±220 V, and panel voltage Vw = 0 V during the waiting time tw from 5 μs to 20 ms. Fig. 2. Waveform used to measure the loss of wall voltage with waiting time. (Note: The signal at −Vsus is not on the scale.)

II. E XPERIMENTAL M ETHOD All panels have a total pressure of 500 torr and contain neon gas mixed with 20% Xe. The distance between the MgO surfaces is 120 μm. We used the total number of 128 ∗ 128 pixels in the panel. The panel size is 9.2 in, and the capacitance values are as follows: COFF (the capacitance value of the panel when it is off; no discharge) = 1.4 nF and CON (the capacitance value of the panel when its on; discharge) = 6.4 nF. Fig. 2 shows the waveform applied to the scan electrode and used to measure the loss of wall charge. The address electrode is kept at 0 V. In Fig. 2, after the last sustain pulse, we wait for 10 μs with the panel voltage kept at −Vsus in order to collect all of the space charges [2]. Because the Xe∗ metastables in the discharge space have a lifetime of a few microseconds [3], they combine and produce ions and electrons during that decay time (stepwise ionization). We have measured the time that is needed to collect all of these space charges by changing the initial waiting time from 2 to 20 μs and found that a value of 10 μs is sufficient. Only then (after 10 μs) is the external voltage changed and set at a value +Vw . We carefully checked that no discharge occurs during this step from −Vsus to +Vw . The panel voltage is kept at Vw during the whole addressing waiting time tw of up to 50 ms. Different values for Vw can be used, but in general, we use Vw = 0 V in combination with a sustain-voltage amplitude of 220 V. Using this waveform and voltage level, we get a condition at the beginning of the waiting time, where the internal gas voltage is about 50–70 V below the voltage needed for a discharge. Immediately after the 10-μs waiting time at −220 V, we switch the external voltage to the value Vw and keep this voltage constant during the whole waiting time. The internal gas voltage then is high, typically on the order of 200 V, and therefore, the priming electrons, which are emitted spontaneously by the protective layer, are strongly amplified, causing a gradual loss of wall voltage. To determine the new firing voltage, we apply a fast ramp voltage at the end of the waiting time tw , similar to the method used by Yoshino et al. [4]. Yoshino et al. also used an opposed-

type discharge rather than a three-electrode-type discharge, but the panel structure and the waveform are different. They used a waveform that resulted in a zero internal electrical field; also, they used only a fixed value of tw = 5.6 ms, while we varied the value of tw over a range from 5 μs to 20 ms or more. The panel voltage Vf needed to get a discharge is then determined by measuring the voltage at the discharge timing moment, indicated by the IR pulse, as detected by a Hamamatsu C6386. The values of Vf are plotted as a function of the waiting time tw . A ramp rate of 11.6 V/μs was used; such a ramp rate was found to result in maximum accuracy and reproducibility, with an error in Vf of only ±2 V, including the effect of the statistical jitter ts . In case of only a few sustain cycles, the error is larger. III. E XPERIMENTAL R ESULTS A. Undoped MgO After sustaining at 200 kHz and Vsus = 220 V, the panel with undoped MgO (Fig. 3) shows a wall-voltage loss of about 30 V for a waiting time of 20 ms; the change with time is independent of the number of sustain pulses. The initial wall-voltage loss (at tw = 5 μs), however, is slightly different, shifting the whole curve up and down. The maximum change of Vf with tw occurs for tw > 1 ms, with an increase of about 10 V/decade. A similar behavior was found by Awaji et al. in a three-electrode-type PDP, both in the cases of undoped and scandium-doped MgO [5]. Above tw = 100 μs, Awaji et al. found a strict Vf ∼ ln(tw ) for the loss of wall voltage with a slope that was three times larger than that of undoped MgO. B. MgO:Si The wall-voltage loss of MgO:Si is similar to that of undoped MgO, but instead of a vertical shift in Vf for different numbers of sustain cycles, we observe a crossover point at tw = 1 ms (Fig. 4). Below 1 ms, an increased number of sustain pulses decreases Vf , while above tw = 1 ms, the wall voltage increases with the number of sustain cycles. From 10–100 μs,

CHEN et al.: WALL-VOLTAGE STABILITY IN AC-PDP DIELECTRIC BARRIER DISCHARGES

Fig. 4. Wall-voltage loss versus ln(tw ) of MgO:Si at different numbers of sustain pulses from 1 to 1024 at 200 kHz and Vsus = 220 V. The voltage during the waiting time was Vw = 0.

Fig. 5. Wall-voltage loss versus ln(tw ) of MgO:Sc at different numbers of sustain pulses from 1 to 1024 at 200 kHz and Vsus = 220 V. The voltage during the waiting time was Vw = 0.

there is almost no loss of wall voltage. The maximum change in Vw occurs for n = 1024 with Vf proportional to ln(tw ) above 200 μs.

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Fig. 6. Change in wall voltage versus waiting time for MgCaO (15% CaO) and 20% Xe at different numbers of sustain pulses from 1 to 1024 at 200 kHz, Vsus = ±220 V, and panel voltage Vw = 0 V for tw = 5 μs to 50 ms.

Fig. 7 Wall-voltage loss of MgO:Si as a function of the sustain frequency (10, 40, 100, and 200 kHz) with Vsus = 220 V and n = 1024.

linearly proportional to ln(tw ). The rate of change is strongly dependent on the number of sustain cycles: about 20 V/decade for n = 1024 cycles and 10 V/decade for n = 1 and n = 2 cycles.

C. MgO:Sc

E. Sustain-Frequency Effect

In the case of MgO:Sc, the voltage loss is more strongly dependent on the number of sustain cycles (Fig. 5). At n = 1024, the wall-voltage loss at tw = 20 ms has increased an extra 20 V compared with that of both undoped MgO and MgO:Si. Below tw = 200 μs, Vf is relatively constant.

The change in wall voltage appears to be independent of the sustain frequency; an example is shown in Fig. 7 for MgO:Si with n = 1024. This is also the case for MgCaO at n = 1024; but, at a lower number of sustain cycles, the wall-voltage change of MgCaO varies strongly with frequency, as shown in Fig. 8 for n = 2.

D. MgCaO The wall voltage of MgCaO is the most sensitive to the number of sustain cycles among those of all the other protective layers (Fig. 6). The initial wall-voltage loss (70 V) is much higher than those of the other layers but constant up to tw = 200 μs. Vf changes in total by 50 V after time tw = 20 ms in the case of n = 1024, which is 10 V more than the 500-ppm Sc-doped MgO. Therefore, in both cases, a special driving waveform will be needed to obtain an adequate addressing margin. Above tw = 1 ms, the change in Vf is essentially

F. Effect of Panel Voltage During Waiting Time When changing the voltage Vw during the waiting time tw , we observe that the Vf (tw ) curves are shifting vertically by an amount of two times the change in Vw . All of the curves for the undoped MgO can be matched by Vf = (Vsus + 2 ∗ Vw − 168) + 4.2 ln(1 + tw /100). For the other panels, the same formula applies, but the value of 168 V is slightly different. The value A is a function of the number of sustain pulses and the type of protective layer. Such a parallel shift of the Vf (tw )

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Fig. 8. Wall-voltage loss Vf for an MgCaO (15% CaO) panel at different sustain frequencies (10, 40, 100, and 200 kHz) for n = 2 sustain cycles.

Fig. 9. Extra panel voltage Vf needed for firing, after the waiting time tw , with the panel kept at Vw for Vw = 0, 5, 10, and 15 V in the case of an undoped MgO panel sustained at 40 kHz (220 V). The full lines correspond to Vf = (Vsus + 2Vw − 168) + 4.2 ln(1 + tw /100).

curves was also observed by Awaji et al. for the three-electrode PDP [5] (Fig. 9). When the internal gas voltage is closer to the firing voltage, the internal amplification factor (gas gain) of exoelectrons is increased. The parallel shift indicates, however, that there is no observable effect on the voltage drift rate. We therefore conclude that under these conditions, the voltage drift is not caused mainly by the amplified exocurrent. Deviations from the vertical shift in Vf are only observed if we start at a very small initial voltage Vf (0). In that case, the gas gain becomes much higher, as described by Weber [6], and only then does the wall-voltage change by amplified exocurrent apparently become significant. IV. D ISCUSSION In several cases, we observed that the wall-voltage loss is proportional to the logarithm of the waiting time Vf = V0 + A ∗ ln(tw ), as shown, for instance, in Fig. 6 in the case of MgCaO for tw > 1 ms. The same logarithmic dependence on waiting time was found by Awaji et al. for doped MgO [5]. As the panel displacement current is proportional to the derivative of this voltage change, this means that the loss of wall voltage induces a capacitive displacement current in the

Fig. 10. Exocurrent decay for the MgCaO (15%) protective layer as a function of the number of sustain cycles at 200 kHz. The exocurrent is measured as 1/ts . The decay roughly follows an Iexo ∼ t0.5 w trend line from 0.5 to 50 ms for all values of n = 1–512.

panel that decreases almost exactly inversely with the waiting time, because in that case, Idis ∼ C.dV /dt ∼ (tw )−1 . As indicated earlier, the loss of wall voltage during the waiting time tw could be caused by the emission of exoelectrons, which are amplified by the gas due to xenon ionization and multiplication. Just below the discharge voltage, this gain can be very large, i.e., on the order of 100 or more. To check this in more detail, we used the Laue diagram method [7], determined the statistical variation ts of the discharge initiation, and plotted the inverse values 1/ts as a function of the waiting time for different values of sustain cycles at 200 kHz. This is shown in Fig. 10 for MgCaO (15%). For almost all values of n, we observe that Iexo ∼ (tw )−0.5 . In the simple model of a charged capacitor, which is decharged by the amplified exocurrent Iexo , the gas voltage Vgas (tw ) after a time tw will become tw Vgas (tw ) = V (0) − {Iexo ∗ Gain(V )/Cpanel .}dt.

(1)

0

The gas gain itself is also a function of the gas voltage and is given by Gain(V ) = exp{α(V ).d}, where α(V ) is the voltagedependent xenon-ionization coefficient. The gas gain can be written in the form originally used by Kruithof and Penning [8] Gain(V ) = exp{η(V − V0 )}.

(2)

The value of 1/η is, in fact, the effective ionization voltage ∗ Vion needed to excite and ionize the xenon atoms (note: the neon atoms play no role in this process). We then find that the gas gain for the discharge cell that was used is given ∗ } with a threshold voltage by Gain(V ) = exp{(V − Vthr )/Vion ∗ Vthr = 58 V and Vion = 29 V, which means that the gas gain increases by a factor of about two for every 20 V of change in gas voltage. Numerical solutions show that the resulting gas voltage is initially constant and then decays further according to Vgas = V0 + A. ln(tw ). This is similar to what is observed experimentally for tw > 1 ms. According to the calculation, the proportionality constant A should be directly related to the coefficient in the power law of Iexo ∼ t−x . If x = 1, then a very small change in gas voltage results; but for instance,

CHEN et al.: WALL-VOLTAGE STABILITY IN AC-PDP DIELECTRIC BARRIER DISCHARGES

in the case of x = 0.5, as often observed for the change in exocurrent from the MgO surface, the gas-voltage change is much faster. By differentiating both sides of (1), it is clear that ∗ should be approximately obeyed. A = (1 − x).Vion The observed changes in Vf , however, do not match those calculated from the observed exocurrents. In the case of MgCaO, for instance, we see (Fig. 10) that a change in the number of sustain cycles (n > 2) does not change the value of x, while the plots of Vf (tw ) clearly show a change in slope A by a factor of two from n = 2 to n = 1024. We therefore conclude that there are also other causes of wall-voltage loss in these devices. One candidate is conductivity. This was discussed in the case of a three-electrode PDP design [4], assuming a leakage current between the two sustain electrodes through the dielectric layer. It has been pointed out by Weber and Qun that the effects of the leakage current have to be eliminated [9] in order to properly measure the loss of wall voltage by amplified exocurrent. One possible solution is to subtract the displacement current from a reference panel, which is identical except that it has no MgO layer, so that it does not fire when using the same waveform [10]. In case of a three-electrode design, the conductivity between the two sustain electrodes is also determined by the dielectric layer, covering the electrodes. A different kind of protective layer, therefore, is not expected to change the conductivity of the stack. Therefore, it is more reasonable to assume that most of the wall-voltage change, after the application of a voltage step, is caused by relaxation effects in the dielectrics, generating a displacement current after applying or removing a step voltage. This effect was already observed by Kohlrausch in 1854 in glass capacitors in the form of Leyden jars and later in 1864 by Siemens. After applying a step voltage to a capacitor with a high-k dielectric, we observe that the displacement current is not only decaying as exp(−t/RC), but long after this current should have died away, there is still a dielectric relaxation current decaying as t−1+α , with α close to zero. The discovery of the 1/t time dependence is credited [11]–[16] to Curie and Von Schweidler. Curie observed in his original study that the dielectric relaxation of porcelain increased with water content. “Anomalous” currents like these are often attributed to the time-dependent trapping of charge in the bulk of the dielectric or at a dielectric/electrode interface. A dielectric relaxation current ∼t−1+α will result in a panel-voltage change Vf ∼ tα , with α 1, which is approximately proportional to ln(t). Therefore, the wall-voltage loss proportional to ln(tw ) might probably also be explained as the result of dielectric relaxation currents in both the dielectric and protective layers.

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The dielectric relaxation mechanism, as already described by Kohlrausch in 1854 for glass capacitors, is proposed as a possible explanation, as this mechanism results, in general, in a relaxation current approximately inversely related to the waiting time. R EFERENCES [1] J. P. Boeuf, “Plasma display panels: Physics, recent developments and key issues,” J. Phys. D, Appl. Phys., vol. 36, no. 6, pp. R53–R79, Mar. 2003. [2] T. Sakashita, T. Shiga, and S. Mikoshiba, “Priming effect of exoelectrons during 10 hours in PDPs,” in Proc. Int. Display Workshop Tech. Dig., Otsu, Japan, Dec. 5–8, 2006, p. 1143. [3] H. S. Uhm, B. H. Hong, P. Y. Oh, and E. H. Choi, “Properties of excited xenon atoms in a plasma display panel,” Thin Solid Films, vol. 517, no. 14, pp. 4023–4026, May 2009. [4] K. Yoshino, “Effects of wall charge on firing voltage and statistical delay time in alternating-current plasma display panels,” Jpn. J. Appl. Phys., vol. 49, no. 4, pp. 040212-1–040212-3, Apr. 2010. [5] N. Awaji, H. Tolner, S. Miyamoto, and H. Kajiyama, “Wall voltage loss by exoemission,” SID Dig., vol. 42, no. 1, pp. 508–509, Jun. 2011. [6] L. F. Weber, “Measurement of wall charge and capacitance variation for a single cell in AC plasma display panel,” IEEE Trans. Electron Devices, vol. ED-24, no. 7, pp. 864–869, Jul. 1977. [7] M. Makino, E. Mizobata, and K. Toki, “Dependence on a horizontally adjacent cell discharge of addressing discharge time lag in AC PDP cell,” in Proc. Asia Display/IDW, 2001, pp. 809–812, PDP3-1. [8] A. A. Kruithof and F. M. Penning, “Determination of the Townsend. Ionization coefficient α for mixtures of neon and argon,” Physica, vol. 3, no. 6, pp. 515–533, Jun. 1936. [9] L. F. Weber and Q. Yan, “Very-sensitive direct measurement of plasmadisplay exoemission,” J. Soc. Inf. Display, vol. 19, no. 2, pp. 212–220, Feb. 2011. [10] H. Tolner, X. Zhang, and C. Wang, “Exoemission properties of oxide protective layers in PDP,” in Proc. Eurodisplay, 2009, pp. 475–477. [11] J. R. Jameson, W. Harrison, P. B. Griffin, J. D. Plummer, and Y. Nishi, “A semiclassical model of dielectric relaxation in glasses,” J. Appl. Phys., vol. 100, no. 12, pp. 124104-1–124104-20, Dec. 2006. [12] A. K. Jonscher, “Dielectric relaxation in solids,” J. Phys. D, Appl. Phys., vol. 32, no. 14, pp. R57–R70, Jul. 1999. [13] R. Kohlrausch, Pogg. Ann. Phys. Chem., vol. 91, p. 56, 1854. [14] J. Curie, “Recherches sur le pouvoir inducteur spécifique et la conductibilité des corps cristallisés,” Annal. Chim. Phys., vol. 17, pp. 385–434, 1889. [15] J. Curie, “Recherches sur la conductibilité des corps cristallisés,” (in French), Annal. Chim. Phys., vol. 18, pp. 203–269, 1889. [16] E. von Schweidler, “Studien der Anomalien im Verhalten der Dielektrika,” (in German), Annalen der Physik, vol. 329, no. 14, pp. 711–770, 1907.

V. C ONCLUSION The loss of wall voltage in AC-PDP-type dielectric barrier discharges causes problems in the addressing margins of panels using either doped MgO or MgCaO as a protective layer. For waiting times tw > 1 ms, the wall-voltage losses have been found to be proportional to the logarithm of tw and therefore induce a panel displacement current ∼(tw )−1 . We conclude that the wall-voltage loss cannot be ascribed only to amplified exoemission but that an additional factor has to be involved.

Yuxiang Chen is currently working toward the M.S. degree in the Department of Electronic Science and Engineering, Southeast University, Nanjing, China. His major is in the electron-emission characteristics of MgO layers in PDPs.

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Qing Li received the B.S. and M.S. degrees from the Nanjing University of Science and Technology, Nanjing, China, and the Ph.D. degree from Southeast University, Nanjing. She was with the Nanjing Electronic Devices Institute, Nanjing, from 1990 to 1995. Since 1995, she has been with the Department of Electronic Science and Engineering, Southeast University. In 2010 to 2011, she was with the University of Central Florida, Orlando, as a Senior Visiting Scholar. Her study field covers flat display technology and optic electronic devices research. Her main research work focuses on plasma devices with correlative materials such as protective layers and on LCDs as well as optical devices using liquid crystal. Currently, she has more than 40 publications and 28 patents. Dr. Li is a Senior Member of the Society for Information Display.

Kai Hu is currently working toward the M.S. degree in the Department of Electronic Science and Engineering, Southeast University, Nanjing, China.

Wenjian Kuang received the B.S. degree in information optics from Jiangnan University, Jiangsu, China, in 2008. He is currently working toward the Ph.D. degree in Southeast University, Nanjing, China. His research at the Display Center, Southeast University, primarily involves the analysis of the MgO material and protective layer. He also works on the improvement of both PDPs and flat plasma lamps.

Harm Tolner received the M.S. and Ph.D. degrees in physics from the University of Groningen, Groningen, The Netherlands, in 1972 and 1977, respectively. He has been active in the research and development of electron- and plasma-based devices at Philips and is currently a consultant in plasma technology and luminescent materials. He is a Visiting Professor at Southeast University, Nanjing, China. His main research interests are atmospheric plasma physics and the emission of electrons and photons. He is also the CTO of Tolner Technology.