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Characterization of a Novel Europium Doped Gallium Oxide Electroluminescent Device P. Wellenius, A. Suresh, J. F. Muth Electrical and Computer Engineering Dept, NC State University Raleigh, NC 27695
[email protected] Cathodo- (CL) and electroluminescence (EL) spectra were obtained using an Oxford Instruments MonoCL monochromator dispersing light into a water cooled photomultiplier tube (PMT). The monochromator is attached to a JEOL JSM-6400 thermionic emission scanning electron microscope, operated at 5 kV for CL measurements. Integrated intensity data was captured by measuring the output voltage from a PMT located directly underneath the sample. Time dependent light emission data was obtained using a Tektronix TDS540B 500 MHz digitizing oscilloscope.
Abstract – Thin film electroluminescent devices based on the red were fabricated by pulsed laser emission of Ga2O3:Eu deposition. Time dependent electrical and optical measurements were used to characterize device performance and behavior.
I. INTRODUCTION Beta phase gallium oxide has been demonstrated as a successful host matrix for several rare earth and transition metals [1,2]. It is also an attractive material for thin film electroluminescent devices due to its large band gap and crystalline qualities which are believed to facilitate impact ionization [3]. Thin film ac electroluminescent devices are planar light sources that operate based on the impact ionization of dopant species, emitting light as electrons deexcite. This study reports on the electronic and optical properties of thin film electroluminescent devices based on europium doped gallium oxide thin film phosphors.
III. RESULTS Transparent thin film electroluminescent devices were successfully produced by pulsed laser deposition. In Fig. 2, CL and EL spectra show similar emission with the most intense peaks around 611 nm, corresponding to the 5D0 to 7 F2 transition in the Eu3+ ion. The devices produced in this study demonstrated an emission threshold voltage near 50 volts ac, independent of frequency, as shown in Fig._3. The device physics of the observed discontinuity for low frequency light emission between 60 and 70 volts is not known. This behavior is unlike other devices where hysteresis effects are observable due to charge trapping [6]. The ac I-V measurements in Fig. 4 show a slight change in slope above the voltage where the discontinuity occurs indicating that more current is flowing through the device per cycle above the discontinuity. More detailed measurements are in progress to determine if charge trapping mechanisms are responsible for the step increase in emission. Fig. 5 shows the time dependence of the light emitted for a 1 kHz driving voltage. During these measurements, the IGZO top contact was grounded while an ac voltage was applied to the ITO bottom contact. Millisecond range carrier lifetimes have been previously observed for Ga2O3:Eu films, which is consistent with the observed decay in the PMT output signal [1]. The PMT output demonstrates that the device only emits light during one half the period of the driving voltage. This suggests electron injection into the Ga2O3 phosphor layer occurs from the IGZO layer only, and relatively few carriers are injected across the ATO dielectric.
II. METHOD Thin film electroluminescent (TFEL) devices were fabricated by pulsed laser depositing a Ga2O3:Eu (2.4 mol% Eu2O3) phosphor layer and a novel indium gallium zinc oxide (IGZO) contact layer [4] onto commercially obtained substrates. These substrates are composed of an aluminum oxide-titanium oxide (ATO) dielectric deposited on top of an indium tin oxide (ITO) transparent contact on Corning 7059 glass. The Ga2O3 layer was deposited at 850oC to encourage formation of the beta phase. The IGZO transparent contact layer is a high mobility, amorphous, wide bandgap material deposited at room temperature also used for transparent thin film transistors [5]. The final device structure is illustrated in Fig. 1.
Fig. 1. TFEL device schematic.
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Fig. 2. CL and EL spectra. Inset is a picture of a device in operation taken under normal room lighting conditions. Fig. 5. Time dependence of light emission and the driving voltage. The charge injection is asymmetric.
IV. CONCLUSIONS In summary, ac thin film electroluminescent devices were fabricated using Ga2O3:Eu thin films deposited by PLD. The strongest emission occurs at 611 nm, corresponding to the 5D0 to 7F2 transition with an emission threshold near 50 volts ac. It was also shown that light emission only occurs during half the sinusoidal excitation and suggests that carriers are injected only from the IGZO contact layer. The efficiency of the devices has not yet been quantified, but these preliminary efforts have produced devices that are bright enough to be seen in a well lit room which is sufficient for some applications. V. ACKNOWLEDGMENTS The authors wish to acknowledge support from NSF, DARPA, and the Office of Naval Research Young Investigator Program.
Fig. 3. EL emission as a function of drive voltage. Note the light output increase significantly above the discontinuity.
VI. REFERENCES [1] P. Gollakota, A. Dhawan, P. Wellenius, Y. N. Saripalli, H. Y Peng, H. O Everitt, L. M. Lunardi and J. F Muth, Appl. Phys. Lett. 88 (2006) 221906 [2] T. Miyata, T. Nakatani and T. Minami, J. Lumin. 87 (2000) 1183-1185 [3] T. Xiao, A. H. Kitai, G. Liu, A. Nakua, and J. Barbier, Appl. Phys. Lett. 72 (1998) 3356-3358. [4] A. Suresh, P. Gollakota, P. Wellenius, A. Dhawan and J. F. Muth, Thin Sol. Films (2007) doi:10.1016/j.tsf.2007.03.153 [5] A.Suresh, P. Wellenius, A. Dhawan, J. Muth, Appl. Phys. Lett. 90 (2007) 123512
Fig. 4. RMS Current-voltage relationship for the TFEL device at several drive frequencies. There is a slight increase in slope above 60-70 volts where the discontinuity was observed.
[6] W. E. Howard, O.Sanhi, P.M. Alt, J. of Appl. Phys. 53: 639 1982
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