Oxygen codoping of ZnS:Tb,F electroluminescent thin film

ARTICLE IN PRESS

Journal of Luminescence 109 (2004) 75–83

Oxygen codoping of ZnS:Tb,F electroluminescent thin film J.P. Kima, M.R. Davidsonb, M. Puga-Lambersb, E. Lambersa, P.H. Hollowaya,b,* a

Department of Materials Science and Engineering, University of Florida, 202 Rhines Hall, P.O. Box 116400, Gainesville, FL 32611-6400, USA b MICROFABRITECH, University of Florida, Gainesville, FL 32611-6400, USA Received 26 February 2003; received in revised form 31 December 2003; accepted 31 December 2003

Abstract Thin films of ZnS:Tb,F were sputter deposited from a ZnS:TbF3 target in an oxygen-argon ambient. The highest electroluminescent brightness (82 cd/m2 at 60 Hz) was measured from ZnS:Tb,F films with a 3.6 at% oxygen concentration. Oxygen concentrations above or below this concentration resulted in sharp decreases in brightness (56 cd/m2 at 2.2 at% oxygen, and 42 cd/m2 at 8.1 at% oxygen). The brightness improvement by oxygen codoping between 0 and 3.6 at% results from increased conduction charge with increasing oxygen concentrations. The brightness decrease for oxygen >3.6 at% is attributed to decreases of both excitation and radiative efficiencies. Improved electroluminescent brightness from oxygen codoping during sputter deposition of ZnS:Tb,F films was equivalent to the improvement observed in films deposited from a ZnS:TbOF target. r 2004 Elsevier B.V. All rights reserved. Keywords: Electroluminescence; Phosphor; Sputter deposition; ZnS:TbOF; Thin film

1. Introduction Even though thin film electroluminescent (TFEL) devices have been investigated for flat panel displays because of advantages such as ruggedness, high contrast, wide viewing angle and good temperature range, the brightness and efficiency of green emitting phosphor still need to be improved for multicolor and full color devices [1,2]. ZnS:Tb is a promising color display material because it has the highest reported brightness *Corresponding author. Department of Materials Science and Engineering, University of Florida, 202 Rhines Hall, P.O. Box 116400, Gainesville, FL 32611-6400, USA. Tel.: +1-352846-3330; fax: +1-352-392-4911. E-mail address: [email protected]fl.edu (P.H. Holloway).

among the green emitting phosphors [3–6]. Even though ZnS:TbOF shows the highest green luminance, its’ brightness and efficiency are much lower than for yellow emitting ZnS:Mn, and these properties may vary due to accidental incorporation of oxygen from residual gases during deposition. Okamoto et al. [7] studied the effect of oxygen doping in ZnS:TbFx thin film electroluminescent (EL) devices. They obtained the highest brightness when S/Zn, F/Tb, and O/Tb atomic ratios were nearly unity. Based on these data, they suggested that the Tb and O acted as substitutional impurities with the Tb occupying Zn sites and O occupying S sites in the ZnS lattice, while F occupied a interstitial site. The Tb–O–F complex center was reported to increase the EL efficiency over a Tb–F complex.

0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2003.12.054

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J.P. Kim et al. / Journal of Luminescence 109 (2004) 75–83

Noma et al. [8] reported that the oxygen doping in the ZnS:TbOF prepared by electron beam evaporation also improved the luminescence of EL devices and crystallinity of phosphor film. Van Den Bossche et al. [9] reported X-ray photoelectron spectroscopy (XPS) spectra of ZnS:TbF3 and ZnS:TbOF thin films. Two main photoelectron peaks were observed from TbF3 powder at B1242 and 1277 eV from Tb+3 3d5/2 and 3d3/2 energy levels, respectively. These two main peaks are each accompanied by a small peak approximately 10 eV higher in binding energy, which is sometimes interpreted to mean that some Tb4+ is incorporated into the ZnS layer. Wang et al. [10] reported that high oxygen concentrations in the ZnS:TbOF phosphor film (O/Tb>1) reduced the brightness of the EL device. They used deep level transient spectroscopy (DLTS) to show that excess oxygen created deep hole traps. Also, Fourier transform infrared spectroscopy (FTIR) data showing strong H–O–H vibration peaks suggest that water absorption may lead to high oxygen concentrations that reduced luminescent brightness. In this study, the change of brightness of EL films deposited from a ZnS:TbF3 target in Ar+O2 atmospheres will be measured and changes of electrical properties with oxygen concentration will be correlated with brightness changes.

2. Experimental The device structure used in this study is shown in Fig. 1. Corning 7059 glass substrates were coated with 360 nm of indium tin oxide (ITO) as a

Fig. 1. Typical half stack EL device structure. In a half stack device, only a bottom dielectric layer is used; there is no top dielectric layer between the phosphor and top Al contact.

transparent conducting electrode and 160 nm of aluminum titanium oxide (ATO) as a bottom insulator layer. The ZnS:Tb,F B1 mm layer was RF planar magnetron sputter deposited (Ar at 20 m Torr, 120W, B6 W/cm2) onto the ATO and a top Al electrode was deposited by evaporation. No top dielectric layer was used, i.e. the devices were tested in a ‘half stack’ configuration for ease of device fabrication. Both a ZnS:TbOF (1.5 mol%) and a ZnS:TbF3 (1.5 mol%) target were used in this study to deposit ZnS:Tb thin films. The ZnS:TbOF target was made from ZnS powder (99.99% purity) mixed with TbOF powder which was hot pressed to consolidate the 200 diameter target. The TbOF powder was synthesized via a solid state reaction of TbF3 (99.99% purity) and Tb4O7 (99.99% purity). The ZnS:TbF3 target was also prepared by hot pressing 99.99% pure ZnS powder with 1.5 mol% TbF3. The oxygen concentration in the sputter deposited ZnS:Tb,F films was controlled by the oxygen partial pressure during deposition. The ultimate base pressure of the sputtering system was o3.0  106 Torr and the oxygen partial pressure was varied from 6.0  106 to 1.6  105 Torr. An alternating voltage trapezoidal waveform with 5 ms rise and fall times and 30 ms pulse width were applied at a frequency of 60 Hz to the half stack EL devices. The brightness was measured using a calibrated PhotoResearch SpectraScan 650 spectrophotometer, and was typically measured at 40 V above threshold volts (B40 ). Threshold voltage (Vth ) was defined as the intercept on the voltage axis of the largest slope of the B vs. V data. A Tektronix TDS 510A digitizing oscilloscope was used to measure the voltage at each point indicated as V1 ; V2 ; V3 and V4 in Fig. 2, allowing the voltage drop across each element to be calculated, i.e. V2 2V1 is the voltage drop across the resistor, V3 2V2 is the voltage drop across the sense capacitor, and V4 2V3 is the voltage drop across the EL device. The current passing through the EL device can be calculated by dividing the voltage drop across the sense resistor, Rseries ; by the resistance of the sense resistor (100 O, i.e. iðtÞ ¼

V2 ðtÞ  V1 ðtÞ : Rseries

ARTICLE IN PRESS J.P. Kim et al. / Journal of Luminescence 109 (2004) 75–83 V2

V1

where f (Hz) is the alternating current frequency. In this study, all data are collected at a frequency of 60 Hz. Ein (W s) is total energy delivered to the pixels per cycle and can be calculated by time integrating the current and multiplying by voltage across the EL devices. The area (m2) of the pixel is 7.9  106 m2.

V4

V3

EL Device Resistor (100Ω)

Resistor (100Ω)

Capacitor (100nF)

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3. Results

High Voltage Pulse Generator

Fig. 2. Schematic diagram of the electrical circuits used to measure the electrical properties of the EL half stack device. The sense resistor (100 O), sense capacitor (100 nF), EL device and high voltage pulse generator are shown.

The external charge ðQext ðtÞÞ in the EL device was determined by integrating the current over time. For time dependent EL emission ðLðtÞÞ; luminance versus time data were collected on the Tektronix 510A oscilloscope from a Si-diode photomultiplier tube (PMT) manufactured by Oriel by the voltage drop across a 1 kO bridge resistor. Electrical data and time dependent EL emission ðLðtÞÞ are alternately collected with the positive or negative voltage applied to the aluminum electrode, which will be referred to as Al+ and Al. The luminous efficiency, Zðlm=W Þ; of EL devices can be calculated by dividing the luminance of the device, L (cd/m2), by input power density, Pin (W/ m2), as Z½lm=W ¼ p 

L½cd=m2  Pin ½W=m2 

:

ð1Þ

The luminance of the device is directly measured and the input power density can be calculated from the voltage drop across the device, Ein ¼ V3 2V4 and current passing through the device such that f ½1=s  Ein ½W s Pin ½W=m2  ¼ ; ð2Þ area½m2 

Addition of oxygen to the sputter gas during deposition of a ZnS:TbF3 thin film affects the brightness, as shown in Fig. 3. These films were tested after annealing for 60 min at 400 C in high purity nitrogen (99.999%). The relative oxygen concentration was measured by SIMS based upon standards of implanted 18O in ZnS. For corroboration, the oxygen concentration in the brightest film was measured by Auger electron spectroscopy (AES) and also found to be B3.6 at%. These data show that oxygen concentrations above and below B3.6 at% leads to lower EL brightness. The optimum concentration of O is about equal to the optimum concentration of Tb in the films reported earlier by Kim, et al. (4%) for sputter deposition from either two targets or from a single target doped to 1.5% Tb6. This is consistent with reports discussed above of optimum performance for an O/Tb ratio of near unity. The EL brightness (82 cd/m2) of ZnS:Tb,F films sputter deposited and annealed under optimized conditions (400 C, 60 min.) were similar (80 cd/ m2) to films sputter deposited using a single ZnS:TbOF target after annealing under optimized conditions (500 C, 60 min). This means that oxygen incorporation from the sputter gas is equal to that from TbOF complexes for improving the EL brightness. It should be pointed out that the same SIMS and AES analysis of the films from the ZnS:TbOF target oxygen concentrations of 6–8 at%, i.e. higher than the maximum value for O codoped ZnS:Tb,F (Fig. 3). The O concentration in ZnS:TbOF films was not varied, so it is uncertain if this O concentration results in a maximum EL brightness, or quenches the brightness similar to data in Fig. 3.

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Fig. 3. The brightness versus oxygen concentration in ZnS:TbF3 thin films after annealing for 60 min at 400 C in nitrogen.

29.8

1.0

0.9

FWHM

0.8

peak position 29.4

0.7 29.2

FWHM

0.6

29

0.5

0.4 28.8 0.3 28.6 0.2

Peak Position of 28.5 degree XRD Peak

29.6

28.4 0.1

0.0

28.2 0

2

4

6

8

10

12

14

16

18

20

Oxygen concentration (at %) using relative oxygen peak intensity (O/ZnS)

Fig. 4. FWHM (left ordinate) and Bragg peak position (right ordinate) of the 28.5 XRD peak versus oxygen concentration in the film from sputter deposition in Ar+O2 ambients.

To evaluate the effects of oxygen codoping upon the crystallinity of the films, the FWHM and peak position of the 28.5 X-ray diffraction peak (which results from overlapped peaks due to diffraction

from both the cubic (1 1 1) and the hexagonal (0 0 0 2) planes) versus oxygen concentration is shown in Fig. 4. The FWHM does not show much change from 0 to E 17.6 at% in oxygen; at most

ARTICLE IN PRESS J.P. Kim et al. / Journal of Luminescence 109 (2004) 75–83

there is a drift to slightly larger FWHMs, which suggests a slight deterioration of crystallinity with higher oxygen concentrations. Recall however from Fig. 3 that the EL brightness exhibited a pronounced maximum at B3.6% oxygen, therefore there is not a strong correlation between EL brightness increase and changes in crystallinity. Similarly, the monotonic increase in the Bragg angle for this XRD peak from low to high oxygen concentrations (Fig. 4) does not correlate with the EL brightness maximum. The shift to higher 2y angles with increasing oxygen concentration means that the (1 1 1)/(0 0 0 1) interplanar spacing is contracting, consistent with substitution of the ( onto smaller oxygen ion (ionic radius of 1.32 A) ( the sulfur ion site (ionic radius of 1.84 A). Assuming Vegard’s law applies, the change in Bragg angle agrees with the expected contraction of the lattice. The photoluminescence (PL) spectrum from a film deposited from the ZnS:TbOF target is shown in Fig. 5. The spectrum has intensity maxima at both 544 and 552 nm, with relative intensities of 0.9 and 1.0, respectively. Double maxima are also observed in the PL spectra from films deposited from a ZnS:TbF3 target with varying concentra-

79

tions of O2 from the sputter gas, but in all cases the 544 nm peak is more intense than the 552 nm peak (Fig. 5). It has been reported [11–13] that both PL peaks result from the Tb 5D4-7F5 transition, with the double peaks resulting from the crystal field around the Tb+3 ion. Specifically, Okamoto et al. [11] report that the PL emission peak from TbF3 powder has a maximum at 544 nm while the emission peak from ZnS:Tb crystal has a maximum at 552 nm. Kong, et al. [13] report that when the Tb ions are shielded by F ions, the peak at 544 nm is more intense than the peak at 552 nm, but the reverse intensity ratio is observed when the Tb ions are more strongly affected by the ZnS lattice crystal field. Data from the present study are consistent with Kong et al. [13] in that the 544 nm peak is more intense than the 552 nm peak when deposition is from a ZnS:TbF3 target. When the film is deposited from a ZnS:TbOF target, apparently the shielding of the ZnS crystal field is not as complete and the 552 nm peak is more intense than the 544 nm peak. Even when the films are deposited from a ZnS:TbF3 target with oxygen added to the sputter gas, the shielding by the F ion is less effective than in films from the ZnS:TbOF target, even after annealing at

250000 ZnS:TbOF ZnS:TbF3 (Oxygen 3.6 at. %) ZnS:TbF3 (Oxygen 2.2 at. %)

200000

Normalized Intensity (A.U)

ZnS:TbF3 (Oxygen 8.1 at. %)

150000

100000

50000

0 500

510

520

530

540

550

560

570

580

590

600

Wavelenth (nm)

Fig. 5. Photoluminescence spectra of films deposited from a ZnS:TbOF target, or from a ZnS:TbF3 target with various levels of oxygen added to the sputter gas to cause 2.2–8.1 at% oxygen concentrations in the films.

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400 C. This is consistent with the fact that the EL intensity is slightly higher for films from the ZnS:TbOF target versus films from a ZnS:TbF3 target with oxygen incorporated during sputter deposition. To better define the mechanisms by which oxygen codoping of ZnS:Tb,F phosphors improves the brightness, the factors that affect EL emission were evaluated using measurement of electrical parameters. The EL brightness will be affected by the amount of conduction charge, excitation efficiency, radiative efficiency and outcoupling efficiency [14–16]. Samples containing 2.2 at% (less than the optimum O; B40 ¼ 56 cd/m2), 3.6 at% (optimum O concentration; B40 ¼ 82 cd/m2), and 8.1 at% (excess O; B40 ¼ 42 cd/m2) oxygen were studied. The external conduction charge is measured using Q2V data, and internal charge is equal to the stored charge (determined from QðtÞ data) at the point where the applied voltage is zero due to rapid discharge of the device capacitance at the end of pulse [14,15]. The definition of excitation efficiency ðBmax =Qint Þ and radiative efficiency (time integrated EL emission intensity) are taken from Mach et al. [16]. The external charge versus voltage data (Q2V curves) for films doped below, at and above the optimum oxygen concentrations are shown in Fig. 6 at V40 ;

and their external conduction charge is summarized in Table 1 for V10 to V40 : The ZnS:TbF3 film with the optimum oxygen concentration (3.6 at%) shows significantly higher conduction charge density as compared to the film with either higher or lower oxygen concentrations. Internal charge, Qint ; was evaluated from a plot of external charge, Qext ; versus time as illustrated in Fig. 7 and tabulated versus oxygen concentration in Table 2. Each value is the average for Al+ and Al polarity since the variation with polarity was less than 75%. The dependence on oxygen concentration is the same as external charge, i.e. the internal charge for films with 3.6 at% oxygen is larger than for films with higher or lower oxygen concentrations. The difference is larger between films with 3.6%

Table 1 External conduction charge versus voltage above threshold for different oxygen concentrations in ZnS:Tb,F codoped films Voltage above threshold

Qext (2.2 at% O) (mC/cm2)

Qext (3.6 at% O) (mC/cm2)

Qext (8.1 at% O) (mC/cm2)

V10 V20 V30 V40

0.6 1.3 2.9 4.5

2.0 3.3 4.5 5.6

1.4 2.5 4.3 5.2

8

2.2 at. % 6

3.6 at. % Charge (uC/cm^2)

4

8.1 at. % 2 0 -250

-200

-150

-100

-50

0

50

100

150

200

250

-2 -4 -6 -8

Voltage (V)

Fig. 6. External charge versus voltage data at V40 from oxygen codoped ZnS:Tb,F films with different oxygen concentrations.

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0.7 0.6 0.5

Q(uC)

0.4 0.3 0.2

Qint

0.1 0 -0.00005

0

0.00005

0.0001

0.00015

0.0002

-0.1

time (sec) Fig. 7. Plot of charge versus time showing how internal charge, Qint ; was determined.

Table 2 Internal charge versus voltage above threshold for ZnS:Tb,F codoped films with different oxygen concentrations Voltage above threshold

Qint (2.2 at% O) (mC)

Qint (3.6 at% O) (mC)

Qint (8.1 at% O) (mC)

V10 V20 V30 V40

0.050 0.109 0.205 0.306

0.152 0.235 0.313 0.380

0.14 0.220 0.299 0.374

versus 2.2% oxygen, being 53% and 24% larger at V30 and V40 : Since brightness is proportional to conducted charge, both external and internal charge ratios indicate that films with 3.6% oxygen should be brightest. However, based only on transferred charge, films with 8.1% oxygen should have been nearly as bright as those with 3.6 at% O. Since these films were only 53% as bright, additional factors must influence the EL brightness from overdoped films. The excitation efficiency can be calculated from the maximum brightness, Bmax ; from EL emission decay curves divided by the internal charge (Qint

from Table 2), and the values are given in Table 3. In general, the excitation efficiency for Al+ is greater than for Al, however the difference is small for films with 2.2% and 3.6% oxygen. Further, there is no significant difference in excitation efficiency for 2.2 versus 3.6 at% oxygen. At an oxygen concentration of 8.1 at% oxygen, there is a lower average excitation efficiency plus a marked lower efficiency for Al versus Al+. The relative radiative efficiencies are equal to the integrated EL emission intensity for each voltage polarity, and are shown in Table 4. These data show that, similar to excitation efficiencies, there is little dependence of radiative efficiency upon polarity or oxygen concentration for concentrations o3.6%. For oxygen concentrations of 8.1%, the radiative efficiency is lower for Al+ and much lower for Al polarity. This decrease of radiative emission efficiency for high oxygen concentrations in the films is due to a relatively fast emission decay. The characteristic time constants for an exponential fit to the time-resolved intensity data are shown in Table 5. The decay constant for the film with 8.1 at% O is B0.54 ms, compared to B0.7 ms for films with 2.2 at% and 3.6 at% O.

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Table 3 Excitation efficiency versus voltage above threshold of ZnS:TbF3 codoped films with different oxygen concentrations Voltage above threshold

V10 V20 V30 V40

2.2 at% O

3.6 at% O (arb.units)

8.1 at% O (arb.units)

Al+

Al

Al+

Al

Al+

Al

0.031 0.029 0.025 0.020

0.019 0.023 0.019 0.017

0.025 0.024 0.021 0.018

0.022 0.021 0.019 0.018

0.026 0.021 0.019 0.017

0.006 0.008 0.01 0.01

Table 4 Radiative efficiency of ZnS:TbF3 codoped films having different oxygen concentrations Aluminum polarity

2.2 at% O (arb.units)

3.6 at% O (arb.units)

8.1 at% O (arb.units)

Al+ Al

0.50 0.42

0.51 0.45

0.43 0.31

Table 5 Characteristic emission decay time constant versus voltage polarity for ZnS:Tb codoped films with different oxygen concentrations Aluminum polarity

2.2 at% O (ms)

3.6 at% O (ms)

8.1 at% O (ms)

Al+ Al

0.75 0.68

0.78 0.71

0.54 0.55

4. Discussion At 3.6 at% oxygen, the amount of conduction charge is larger (54% at V30 and 24% at V40 ) than for 2.2 at% oxygen in the film. However, the excitation and radiative efficiency are relatively constant as the oxygen content is increased over this oxygen range (Tables 3 and 4). Since the B40 of 3.6 at% film is 46% higher than that of film with 2.2 at% O, the increase of the conduction charge with an increase in oxygen is the main reason for improved brightness below 3.6% oxygen. Increased conduction charge suggests that the interface state densities are increased by oxygen. One possible explanation for such an observation is that oxygen in the sputter gas may be ionized and accelerated to the substrate. The increased fluence of ionized oxygen onto the interface could introduce more defect states that act as a source for injected charge.

On the other hand, oxygen codoping above 3.6% decreases the device brightness even though the conduction charge remained constant, as shown in Table 2. Instead, both the excitation (28% at V30 and 23% at V40 ) and radiative (24% at V40 ) efficiencies were lower for film with 8.1 at% versus 3.6 at% O. The B40 of films with 8.1 at% oxygen is 49% less that of the film with 3.6 at% oxygen (42 cd/m2 versus 82 cd/m2). This 49% decrease in brightness is consistent with the 23% decrease in excitation efficiency and 24% decrease in radiative efficiency shown in Tables 3 and 4. Thus, the data suggest that lower brightness for Oo3.6 at% results from lower charge density, while lower EL brightness for O>3.6 at% results from lower excitation and radiative efficiencies. Even though the FWHM from XRD from the oxygen codoped films do not show significant change over the oxygen concentration range of 0– 8%, the increased FWHM value for 17.6 at% oxygen suggests that codoping does degrade slightly the crystallinity of the ZnS:Tb film. In addition, the increase in the Bragg angle for the 28.5 XRD peak with an increase of oxygen (Fig. 4) indicates lattice distortion of ZnS because ( of the difference in the ionic size of O2 (1.32 A) ( Lattice distortion versus the larger S2 (1.84 A). caused by incorporation of smaller oxygen ion could introduce isoelectronic traps that can be responsible for the decreased efficiencies of both excitation and radiative decay.

5. Summary The effects of oxygen codoping of ZnS:Tb,F films deposited from a ZnS:TbF3 target was

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studied. The oxygen codoping level was controlled by the oxygen fraction in the argon sputter gas ambient. ZnS:Tb,F films show the best brightness (82 cd/m2) at 3.6 at% of oxygen concentration in the deposited film, with a very sharp drop off in brightness from either underdoping (56 cd/m2 at 2.2 at% oxygen) or overdoping (42 cd/m2 at 8.1 at% oxygen). Conduction charge during electroluminescence (EL) was calculated from charge (Q) versus voltage (V ) and time (t) data. A 24% increase in conduction charge at V40 was found when the oxygen concentration was increased from 2.2 to 3.6 at%, but there was no significant increase in conduction charge when the oxygen concentration was increased from 3.6 to 8.1 at% oxygen. On the other hand, the excitation and radiative efficiencies were constant for oxygen concentrations increases between 2.2 and 3.6 at% oxygen, but each decreased by 24% when the oxygen concentration increases from 3.6 to 8.1 at%. The EL brightness decreased by 49% as the oxygen concentration increased to 8.1 at% due to decreased excitation and radiative efficiencies. Increased conduction charge was attributed to increased interfacial charge injection due to energetic oxygen bombardment early in the deposition process. Decreased excitation and radiative efficiencies at high oxygen concentrations were attributed to oxygen-induced isoelectronic traps which cause scattering and non-radiative relaxation in the films. Codoping ZnS:Tb,F sputter deposited films with oxygen during deposition resulted in approximately the same EL brightness as for films deposited from a ZnS:TbOF target with a TbOF complex center existing in the target prior to deposition.

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Acknowledgements This work was supported by DARPA Grant No. MDA972-93-1-0030 through the Phosphor Technology Center of Excellence, and ARO Grant DAAD19-01-1-0603. References [1] [2] [3] [4] [5]

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