Luminescence Efficiency of Erbium-Doped BaTiO 3 Thin Films

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saturated with water was the oxidant. The films were deposited on (1 00)MgO substrates at a growth temperature of 725°C. The films were epitaxial as determined by x-ray diffraction. The films were subsequently annealed under either oxidizing or reducing conditions. A number of films were annealed in an oxygen partial pressure of 1 atmosphere at 700-900'C. Samples were also annealed in a vacuum of 100-200 mTorr at temperatures of 400-800'C. Temperature-dependent photoluminescence measurements were performed over the temperature range of 15-300 K using a 980 nm diode laser as the excitation source at a pump intensity of - 6 W/cm 2 . The characteristic luminescence at 1.54 microns was dispersed using a Zeiss MM 12 monochromator and detected with a Northcoast liquid-nitrogen cooled Ge detector. The photoluminescence transients were measured by exciting the films with a pulsed semiconductor laser at a frequency of 10 Hz (50% duty cycle) with a SRS function generator and averaging the signal on an HP oscilloscope.

RESULTS Annealing under an oxidizing ambient Photoluminescence spectroscopy was performed on the as-grown and the annealed films. The predominant emission observed is the luminescence peak at 0.8 eV as shown in Fig. 1. This is due to the intra-4f shell electronic transition from the first excited state (4113/2) to the ground state (4,15/2), upon excitation to the second excited state (4I11/2). The 0.8 eV peak typically has a nominal full width at half maximum of 40 meV. Fig. 1 shows room-temperature PL spectra for an Er-doped BaTiO 3 film (AT50) ([Er] = 5x10 20/cm 3) that was thermally annealed in flowing oxygen at 1 atmosphere at temperatures of 700'C, 800'C, and 900'C. The integrated intensity

600 ---

as grown 700C 800C -o--900C

S500 ý

400 C)

300

__

200

._1 0 .7

0.75

0.8

0.85

0.9

Emission Energy(eV) Figure 1. Room-temperature PL spectra of the 0.8 eV emission in as-grown and oxygen annealed Er-doped BaTiO 3 films. increased with increasing temperature in all cases as summarized in Figure 1. Increases ranged from a factor of 0.5 to 11. For a sample(AT50) annealed at 900'C the intensity increased by a

16

factor of 11. A summary of the oxygen annealing results for several Er-doped BaTiO 3 films is given in Table 1. While two of the samples (AT50, AT178) showed strong enhancement of the PL upon oxygen annealing, the majority of the samples in this study had only a 50-100 % increase in the intensity. Table I: Change in PL intensity for Er:BaTiO 3 Films Oxygen Annealed at 800'C Sample AT50 AT156 AT178 AT300

As-grown PL Intensity (a.u.) 10 10 10 10

Erbium Concentration 21 /cm 3 ) (X×10 0.5 0.6 1 1

Post-anneal PL Intensity 110 70 15 20

The luminescence decay was measured for several of the annealed samples to determine the radiative lifetime. For sample AT50, even though the efficiency increased by a factor of 11, the decay lifetime only increased from 5 msec to 10 msec. The PL intensity can be described by the equation : IPL - fNEr "[/Trad18 , where f is the fraction of radiatively active ions, NE